University of Southern California Dissertations and Theses

The role of monoamine oxidase in behavioral plasticity

(USC Thesis Other)

The role of monoamine oxidase in behavioral plasticity

PDF

Download a page range

Download transcript

Copy asset link

Request this asset

Transcript (if available)

Content

THE ROLE OF MONOAMINE OXIDASE IN BEHAVIORAL PLASTICITY

by
Sean C. Godar
_______________________________________________________

A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(MOLECULAR PHARMACOLOGY AND TOXICOLOGY)

December 2010

Copyright 2010 Sean C. Godar
ii

Epigraph

All our knowledge has its origins in our perceptions.
~Leonardo da Vinci
iii

Dedication

For those who believed in me, especially during the times when I didn‟t.

iv

Authorships
Published and in press works by the Author incorporated into the Dissertation

Bortolato M, Godar SC, Davarian, S, Chen K, Shih JC. (2009) Behavioral
disinhibition and reduced anxiety-like behaviors in monoamine oxidase B-
deficient mice. Neuropsychopharmacology. Dec;34(13):2746-57.
Bortolato M, Godar SC. (2010) Animal models of virus-induced neurobehavioral
sequelae: recent advances, methodological issues, and future prospects.
Interdiscip Perspect Infect Dis. 2010:380456.
Godar SC, Bortolato M, Frau R, Dousti M, Chen K, Shih JC. (In press).
Maladaptive defensive behaviors in monoamine oxidase A-deficient mice. Int J
Neuropsychopharm.
v

Table of Contents
Epigraph ii
Dedication iii
Authorships iv
List of Figures viii
Abbreviations xi
Abstract xiii
Chapter 1: Introduction 1
1.1: An introduction to MAO 1
1.2: MAO inhibitors in psychiatry 4
1.3: MAO A-deficiency and low MAO A activity polymorphisms in clinical
populations 5
1.4: Animal studies of MAO A-deficiency 8
1.5: The enigmatic role of MAO B in neuropsychiatry 12
1.6: Animal models of MAO B KO 15
1.7: Why study MAO – Limitations in modern psychiatry
and endophenotypes 17
1.8: MAO in emotional reactivity 18
1.9: MAO in behavioral plasticity and adaptation 21
1.10: MAO confers susceptibility to the emergence of psychiatric
manifestations: role of stress 24
1.11: MAO modulation of other neurotransmitter systems: NMDA
glutamatergic receptor 26
1.12: Major aims and thesis 30
Chapter 2: Maladaptive defensive behaviors in monoamine oxidase
A-deficient mice 33
2.1: Abstract 33
2.2: Introduction 35
2.3: Methods 37
2.4: Results 43
2.5: Discussion 55
Chapter 3: Monoamine oxidase A-deficient mice exhibit alterations
in stress-coping behaviors 61
3.1: Abstract 61
3.2: Introduction 63
3.3: Methods 66
vi

3.4: Results 70
3.5: Discussion 73
Chapter 4: Hypomorphic monoamine oxidase A-deficient mice
exhibit compulsive behaviors and reduced aggression in mice 80
4.1: Abstract 80
4.2: Introduction 81
4.3: Methods 83
4.4: Results 87
4.5: Discussion 93
Chapter 5: Monoamine oxidase A-deficient mice display autistic-
related features 101
5.1: Abstract 101
5.2: Introduction 103
5.3: Methods 106
5.4: Results 111
5.5: Discussion 116
Chapter 6: Behavioral disinhibition and reduced anxiety-like
behaviors in monoamine oxidase B deficient mice 124
6.1: Abstract 124
6.2: Introduction 126
6.3: Methods 128
6.4: Results 133
6.5: Discussion 141
Chapter 7: Transgenic monoamine oxidase mutant mice display
disturbances in NMDA glutamatergic receptor function 152
7.1: Abstract 152
7.2: Introduction 153
7.3: Methods 156
7.4: Results 158
7.5: Discussion 160
Chapter 8: Summary and conclusion 167
8.1: Brief recapitulation of results 167
8.2: Impact of MAO A on emotional reactivity 168
8.3: Effects of MAO A-deficiency on behavioral responses to stress-
inducing stimuli 169
vii

8.4: Low MAO A activity is conducive to behavioral compulsivity 170
8.5: Autistic spectrum disorder-related behavioral disturbances 171
8.6: Role of MAO B in emotional regulation 173
8.7: The role of MAO in the regulation of NMDA receptor function 175
8.8: Conclusion 177
Bibliography 180

viii

List of Figures

Figure 2.1. MAO A KO mice display contextual-specific alterations in
novel object exploration. Values are represented as means ± SEM.
*P<0.05, **P<0.01, and ***P<0.001 compared to WT mice. 45
Figure 2.2. MAO A KO mice exhibit a reduction in risk-assessment and
escape behaviors in the elevated plus-maze and elevated T-maze
paradigms. Values are represented as means ± SEM. *P<0.05 and
**P<0.01 compared to WT mice. Abbrev: SAPs, stretch-attend postures. 47
Figure 2.3. MAO A KO mice display a reduction in fear-related behaviors
in the emergence test with an object imbibed with predator urine. Values
are represented as means ± SEM. ***P<0.001 compared to WT mice. 49
Figure 2.4. MAO A KO mice display a marked reduction in fear-related
behaviors in the presence of an anesthetized rat. Values are represented
as means ± SEM. *P<0.05 and **P<0.01 compared to WT mice. 50
Figure 2.5. MAO A KO mice display maladaptive digging reactions to
unfamiliar objects in the absence and presence of predator urine.
Values are represented as means ± SEM. *P<0.05, **P<0.01, and
***P<0.001 compared to WT mice exposed to the odorless foreign object.
#
P<0.05 and
##
P<0.01 compared to MAO A KO mice exposed to the
odorless novel object. 52
Figure 2.6. MAO A KO mice do not display alterations in sensory function.
Values are represented as means ± SEM. Abbrev. % NEI, percent novelty
exploration index. 54
Figure 3.1. MAO A KO mice display a reduction in behavioral responses
to acute restraint stress. Values are represented as means ± SEM.
*P<0.05, **P<0.01, and ***P<0.001 compared to WT mice.
#
P<0.05
compared to MAO A KO mice not subjected to restraint stress. 72
Figure 4.1. MAO A
neo
mice exhibit a unique spectrum of emotional
alterations. Values are represented as mean ± SEM. *P<0.05,
**P<0.01, ***P<0.001 compared to WT mice and
#
P<0.05,
##
P<0.01,
###
P<0.001 compared to MAO A
neo
mice. 89
ix

Figure 4.2. MAO A
neo
mice exhibit context-dependent alterations in
aggressiveness. Values are represented as mean ± SEM. *P<0.05,
**P<0.01, ***P<0.001 compared to WT mice and
#
P<0.05,
##
P<0.01,
###
P<0.001 compared to MAO A
neo
mice. 91
Figure 4.3. OCD-like traits in MAO A
neo
mice are attenuated by fluoxetine
treatment. Values are represented as mean ± SEM. P<0.05 and
P<0.001 compared with WT mice (main effect for genotype). *P<0.05,
**P<0.01, ***P<0.001 compared to WT mice and
#
P<0.05,
##
P<0.01,
###
P<0.001 compared to MAO A
neo
mice. 92
Figure 5.1. MAO A-deficient mice exhibit deficits in communication and
behavioral flexibility. Values are represented as mean ± SEM. *P<0.05,
**P<0.01, ***P<0.001 compared to WT mice. Abbrev: USV, ultrasonic
vocalizations; HDs, head dips. 113
Figure 5.2. Early developmental inhibition of 5-HT synthesis attenuates
perseverative behaviors in MAO A-deficient mice. (a-b) Early postnatal
treatment with PCPA significantly reduces the compulsive digging
behavior of MAO A KO mice. Values are represented as mean ± SEM.
*P<0.05, **P<0.01, ***P<0.001 compared to WT mice. 115
Figure 6.1. MAO B KO mice exhibit significant reductions in anxiety-like
behaviors in the elevated plus maze under dim (a-g), but not bright (h-n)
light conditions. Values are represented as means ± SEM. *P<0.05,
compared to wild type (WT) controls. 134
Figure 6.2. MAO B KO mice display decreased anxiety-like behaviors in
the defensive withdrawal paradigm. Values are represented as means
± SEM. *P<0.05, compared to WT mice. 135
Figure 6.3. MAO B KO mice exhibit a reduction in marble-burying and
digging behaviors. Values are represented as means ± SEM. *P<0.05,
**P<0.01 compared to WT controls. 136
Figure 6.4. MAO B KO mice display overall locomotor activity and head
dips similar to their WT counterparts in the hole-board, but explore
central holes with longer duration and shorter latency. Values are
represented as means ± SEM. *P<0.05, compared to WT controls. 138
x

Figure 6.5. MAO B KO mice display higher levels of exploration targeting
novel objects and risk-taking behavior in the wire-beam bridge test.
Values are represented as means ± SEM. *P<0.05, ** P<0.01 compared
to WT littermates. 140
Figure 7.1. Partial or full ablation of MAO A disrupts NMDA receptor function.
Locomotor patterns of mice treated with saline (top) or NMDA receptor
antagonist dizocilpine (bottom). 159
xi

Abbreviations

MAO: Monoamine oxidase
MAOI: Monoamine oxidase inhibitor(s)
KO: Knockout
WT: Wild type
5-HT: Serotonin
DA: Dopamine
NE: Norepinephrine
PEA: β-phenylethylamine
NMDA receptor: N-methyl-d-aspartate receptor
GABA: γ-Aminobutyric acid
TAAR: Trace amine-associated receptor
ASD: Autism spectrum disorders
OCD: Obsessive-compulsive disorder
ADHD: Attention-deficit hyperactivity disorder
PTSD: Post-traumatic stress disorder
HPA: Hypothalamic-pituitary-adrenal axis
SAP: Stretch-attend postures
NEIs: Percent novelty exploration index
HDs: Head dips
USVs: Ultrasonic vocalizations
OFC: Orbitofrontal cortex
5-HTT: Serotonin transporter
xii

EEG: Electroencephalography
PCPA: 4-chloro-DL-phenylalanine methyl ester
ANOVA: Analysis of variance
EPSC: Excitatory postsynaptic current
mGluR: Metabotropic glutamate receptor(s)
xiii

Abstract

Monoamine oxidase (MAO) is the primary catabolic enzyme for the oxidative
deamination of monoamines and has been implicated in several neuropsychiatric
disorders. Cogent evidence has documented that both the MAO A and MAO B
isoenzymes play a role in emotional regulation, suggesting that the behavioral
abnormalities associated with MAO deficiency are underpinned by alterations in
emotional processing. Although it has been well-established that MAO A
deficiency leads to aggression and antisocial personality, the neurobiological
substrates underlying these behavioral disturbances are still unclear. Moreover,
little is known about the contribution of the MAO B isoenzyme in modulating
emotional behaviors. To this end, I hypothesized that MAO A and B function
through the regulation of behavioral plasticity, defined as the ability to properly
integrate the perceptual information and emotional processing with appropriate
behavioral outcomes. In order to test this hypothesis, the present set of studies
investigated the behavioral responses of MAO A-deficient mice to foreign
elements and predator-related cues. These experiments were complemented by
testing the changes in behavioral responses of MAO A-deficient mice following
exposure to stress-inducing stimuli. Although MAO A-deficient mice exhibited
aversive defensive responses to foreign inanimate objects, this was
accompanied by a profound reduction in reactivity to both potential threat and
stress-inducing stimuli. To examine the specific contribution of low MAO A
activity on behavioral outcomes, I characterized a novel line of hypomorphic
xiv

MAO A mutant mice. Specifically, the hypomorphic mutant mice were tested for
anxiety-related manifestations, aggression, and perseverative behaviors.
Hypomorphic MAO A mutant mice displayed an increase in compulsive and
anxiety-related behaviors and context-dependent alterations in aggression.
Since clinical reports of MAO A deficiency document describe a constellation of
behavioral abnormalities that are highly reminiscent to autistic spectrum
disorders, I tested whether MAO A-deficient mice exhibited behavioral alterations
relevant to autistic-related symptomatology. Genetic ablation of MAO A activity
resulted in a spectrum of autistic-related manifestations, including antisocial
behaviors, behavioral inflexibility, and disturbances in communication. In order
to investigate the role of MAO B in emotional regulation, I designed a series of
behavioral tasks aimed to capture different facets of behavioral disinhibition.
Abrogation of MAO B activity in mice led to a marked reduction in anxiety-related
parameters, as well as novelty-seeking and risk taking behaviors, signifying
behavioral disinhibition. Finally, I examined the possibility that the MAO
isoenzymes influence behavior through modulation of the NMDA glutamatergic
receptor function by treating transgenic MAO mutant mice with sub-threshold
doses of the NMDA receptor antagonist dizocilpine. NMDA receptor blockade
resulted in profound stereotyped behaviors and severe impairments in locomotor
coordination in MAO A-, but not MAO B-deficient mice. Taken together, these
findings indicate that MAO A deficiency causes marked disturbances in NMDA
receptor function that may contribute to the dramatic behavioral abnormalities in
xv

MAO A- and MAO A/B-deficient lines. The present set of findings show that
MAO plays a critical role in the regulation of behavioral plasticity.
1

Chapter 1 : Introduction

1.1: An introduction to MAO
Monoamine oxidase (MAO) [amine: oxygen oxidoreductase (deaminating) (flavin-
containing); MAO; E.C. 1.4.3.4] is a family of mitochondrial membrane-bound
flavoproteins that catalyze the oxidative deamination of a broad range of
substrates including the dietary amines, biogenic amines, trace amines, and
hormones. MAO is composed of two isoenzymes, A and B, which share ~70 %
homology and follow similar kinetic mechanisms. Conversely, they differ in
substrate affinity, transcriptional regulation and regional distribution. While MAO
A prefers serotonin (5-hydroxytryptamine, 5-HT) and norepinephrine (NE), MAO
B displays a high affinity for β-phenylethylamine (PEA) and other trace amines.
Dopamine (DA) and tyramine are metabolized by different forms across species.
MAO B primarily degrades these neurotransmitters in humans and other
mammals, however, in rodents the oxidative deamination of DA is primarily
served by MAO A (Bortolato et al., 2008). Although both MAO A and MAO B
have may have different substrate preferences, in the absence of either
isoenzyme, the other can partially compensate. This compensatory mechanism
is most pronounced in animal models with genetic inactivation of both MAO A
and MAO B. MAO A/B KO mice exhibit a 15-fold increases in PEA, 8-fold
increase in 5-HT, over 2-fold increase NE, and a 1.5-fold increase in DA (Chen et
al., 2004). Conversely MAO B KO mice have an 8-fold increase in PEA, while
2

MAO A KO mice have 7-fold and 2-fold increases in 5-HT and NE (Cases et al.,
1995; Grimsby et al., 1997).
MAO B is prominently featured in the raphe nuclei, serotonergic neurons,
histaminergic neurons, as well as the granule cells in the dentate gyrus and
astrocytes (Saura et al., 1996; Bortolato et al., 2008). Conversely, MAO A
displays higher expression levels in catacholaminergic neurons (Westlund et al.,
1988; Saura et al., 1996; Vitalis et al., 2002).
MAO A is expressed at early developmental stages in a regional-specific
manner, whereas MAO B shows a gradual increase in expression from early
postnatal development through adulthood (Leung et al., 1993; Vitalis et al.,
2002). Nevertheless, both isoenzymes play important roles in brain
development, as perturbations in MAO function during early critical time periods
can result in long-term deficits in cortical function and disturbances in behavioral
outcomes.
Several congenic and environmental factors have been shown to alter MAO
levels. A large number of studies have shown that the transcriptional activity of
human Maoa gene may be affected by different variants of an extensive repeat
structure located in the promoter, 1.2 kb upstream of the MAO A coding
sequence. The functional significance of this motif and its role in behavioral
regulation will be further discussed in later in this chapter. Similarly, polymorphic
3

variants of the Maob gene have also been reported (Balciuniene et al., 2002) and
will be addressed below.
In addition to congenic elements, a rich body of evidence has documented that
several environmental factors influence MAO expression. Tobacco smoking, for
instance, has been shown to significantly impact MAO levels (Oreland, 2004;
Berlin et al., 2009; Leroy et al., 2009) and impair cortical development (Navarro
et al., 1988; Ernst et al., 2001). Similarly, metals such as cadmium, mercury,
lead, and selenium can alter MAO activity and monoamine turnover (Chakrabarti
et al., 1998; Leret et al., 2002; Stamler et al., 2006). Stress, especially during
early postnatal periods, can also lead to perturbations in brain structure and
developmental functions (Cook and Wellman, 2004; Brown et al., 2005; Kreider
et al., 2005; Liston et al., 2006; Shansky and Morrison, 2009).
This neurochemical divergence in substrate affinity and monoaminergic
localization may indicate that these isoenzymes exert different functions in the
organization of brain activity and neurophysiological processes (Bortolato et al.,
2008). Although MAO A and B have been implicated in an array of psychiatric
phenomena, their specific roles in behavioral organization remain partially
elusive. To this end, this chapter will focus on the function of MAO isoenzymes
in behavioral regulation, including its role in pathophysiology of neuropsychiatric
disorders, the impact of MAO on emotional reactivity, behavioral adaptation and
stress responses, and possible mechanisms of this enzyme in the development
and emergence of neuropsychiatric manifestations.
4

1.2: MAO inhibitors in psychiatry
MAO has been implicated in a wide range of neuropsychiatric abnormalities,
including schizophrenia, aggression, mood and developmental disorders.
Initially, MAO inhibitors (MAOIs) served as one of the first classes of
antidepressant therapies (Blier and de Montigny, 1998; Brunton, 2006). Although
MAOIs exhibited strong clinical efficacy in alleviation of depressive symptoms,
these mood-enhancing properties were accompanied by a constellation of severe
side effects. One of the most well documented of these adverse effects
stemmed from the consumption of dietary products containing the biogenic
amine tyramine, such as wine and cheese. The irreversible nature of these
inhibitors resulted in a wide spread inactivation of intestinal and hepatic
monoaminergic metabolism, leading to the accumulation of dietary amines in the
bloodstream and ultimately a host of hypertensive complications. The
therapeutic success of MAOIs, however, represented an important milestone in
the treatment of mood disorders and led to the development of the tricyclic family
and other newer generations of antidepressants. While the severity of problems
and hazardous interactions with other agents substantially limits the utility of
these compounds in disease treatment, psychiatrists currently employ reversible
MAOIs, which have fewer deleterious complications, for both intermediate- and
long-term treatment of refractory depression.
In line with these studies, MAOIs and MAO isoenzyme selective antagonists
have also been used in the treatment of anxiety and related phobias (Liebowitz et
5

al., 1988; Gelernter et al., 1991; Liebowitz et al., 1992; Versiani et al., 1992) and
panic disorder (Liebowitz, 1993; Brunton, 2006), as well as obsessive-
compulsive disorder (Annesley, 1969; Liebowitz et al., 1990; Jenike et al., 1997).
Moreover, MAO blockade has been shown to reduce specific symptom clusters
in post-traumatic stress disorder patients (Neal et al., 1997; Hageman et al.,
2001), supporting the role of MAO in the modulation of affect.
Therapeutic effects of MAO B inhibitors have been reported in the treatment of
attention-deficit hyperactivity disorder (ADHD) (Bortolato et al., 2008; Verbeeck
et al., 2009), possibly due to the stimulant properties of PEA (Zucchi et al., 2006)
and its actions on potentiating of catecholaminergic signaling responses (Sabelli
et al., 1975; Xie and Miller, 2008). Low doses of MAO B antagonists have also
been used in conjunction with levodopa in the treatment of Parkinson‟s disease
(Brunton, 2006). Although additional details on the roles and mechanisms of
MAOI‟s in psychiatry is beyond the scope of this paper, readers can refer to
Youdim et al, (2006) for an excellent review on MAOI‟s.

1.3: MAO A-deficiency and low MAO A activity polymorphisms in clinical
populations
Insights into the role of MAO A in neuropsychiatric phenomena were afforded by
the discovery of a small Dutch kindred with a nonsense point mutation in the
Maoa gene and congenital monoamine oxidase A-deficiency (Brunner et al.,
6

1993a). Affected males of this X-linked recessive condition, termed Brunner
syndrome, displayed marked increases in urinary 5-HT levels, impulsive
aggression, mild cognitive deficits, and antisocial behavior (Brunner et al.,
1993a). In view of the low prevalence of Brunner syndrome and the elusive
nosographic characterization of this disease, the identification of neurobiological
and phenotypic traits underpinning the neuropsychological abnormalities caused
by MAO A deficiency is essential for the general understanding and conceptual
framework for the role of MAO A in aggression and other psychiatric disorders
(Brunner et al., 1993a; Brunner et al., 1993b; Hebebrand and Klug, 1995).
A host of clinical studies have documented the effects of functional polymorphic
variants of the Maoa gene on behavioral outcomes, such as impulsive
aggression and antisocial personality disorder. In particular, the variable-number
tandem repeat (VNTR) polymorphism, located in the promoter region, 1.2 kb
upstream of the transcriptional initiation site, has been extensively investigated.
Although six forms have been identified, the 3-repeat and 4-repeat haplotypes
are the most well-established as markers for low and high MAO A transcription,
respectively.
Converging evidence has shown a link between low MAO A activity
polymorphisms and antisocial personality, high levels of aggression, and
emotional alterations (Foley et al., 2004; Jacob et al., 2005; Nilsson et al., 2006;
Oreland et al., 2007; Alia-Klein et al., 2008; Buckholtz and Meyer-Lindenberg,
2008). Moreover, this allelic variant has been associated with impairments in the
7

emotional processing of environmental and social cues (Caspi et al., 2002;
Brummett et al., 2008; Buckholtz and Meyer-Lindenberg, 2008; Lee and Ham,
2008; Kumari et al., 2009; Williams et al., 2009; Edwards et al., 2010), stress
response (Jabbi et al., 2007), and decision-making (Ibanez et al., 2000; Perez de
Castro et al., 2002; Meyer-Lindenberg et al., 2006). In particular, low MAO A
activity during critical stages of early development may confer vulnerability to
neuropsychiatric manifestations. Indeed, the interaction between low MAO A
and childhood neglect or maltreatment, during this time period greatly enhances
the susceptibility to develop violent and antisocial behavior (Caspi et al., 2002;
Foley et al., 2004; Kim-Cohen et al., 2006; Edwards et al., 2010). Caution should
be exercised, however, in the general interpretation of these results since the
gene-environment relationship of low MAO A activity polymorphic variants and
early childhood abuse may be oversimplified, and other unknown factors may
play a critical role in the pathogenesis of violent behavior (Haberstick et al., 2005;
Reif et al., 2007). Nevertheless, in vivo imaging methods have shown an inverse
correlation between MAO A activity in the brain and trait aggression, as well as
an association with increased anger reactivity (Alia-Klein et al., 2008; Alia-Klein
et al., 2009). Collectively, these findings heavily suggest that low brain MAO A
activity may serve as a potential biomarker to measure the genetic susceptibility
towards specific psychiatric manifestations.

8

1.4: Animal studies of MAO A-deficiency
A useful experimental tool to evince the phenotypic outcomes of MAO A
deficiency is afforded by MAO A knockout (KO) mice (Cases et al., 1995), which
exhibit elevated brain levels of 5-HT and NE, and several morphological
alterations including the deficits in the auditory and visual pathways (Upton et al.,
1999; Thompson, 2008). In particular, MAO A ablation causes serotonin-
dependent alterations in the barrel fields of the somatosensory cortex, which
function in the coordination of mystacial vibrissae in the rodent snout (Erzurumlu
and Jhaveri, 1990; Cases et al., 1995; Luhmann et al., 2005). The C3H
background strain, however, has well-documented sensory impairments, which
inherently limits the phenotypic characterization of the MAO A KO line. Recently,
our lab has identified a novel line of spontaneous knockout mice with an
amorphic mutation in the maoa gene (MAO A
A863T
KO) (Scott et al., 2008). This
model exhibits high monoaminergic levels and aggressiveness, and provides a
more naturalistic model for Brunner syndrome. Moreover, the genetic
background of these mice (129S6) is well-suited for studies of emotional
reactivity in mutant mice (Paulus et al., 1999; Marques et al., 2008) and does not
result in sensory deficits.
Behavioral analysis of MAO A-deficient mice in different stages of development
have revealed that MAO A KO pups in postnatal stages display trembling, head
bobbing, impaired right reflex, and other locomotor problems (Cases et al.,
1995). This delay in developmental maturation is accompanied by alterations in
9

sensory functions and abnormal social and sexual behaviors (Cases et al.,
1995). In adulthood, MAO A KO mice show a constellation of behavioral
abnormalities including heightened levels of impulsive aggressiveness in the
resident-intruder task, aberrant stress reactivity and exploratory impairments
(Cases et al., 1995; Shih et al., 1999b; Dubrovina et al., 2006). Similar
behavioral disturbances are observed following treatment with MAO inhibitors
during embryonic and early postnatal stages (Whitaker-Azmitia et al., 1994; Mejia
et al., 2002), but not in adulthood (Griebel et al., 1998; Steckler et al., 2001),
suggesting that these alterations are primarily due to neurodevelopmental
effects.
In addition to the overt aggression, MAO A-deficient mice exhibit a reduction in
immobility in the forced swim test (Cases et al., 1995) and a higher response to
foot shock and retention of aversive memories and conditioned passive
avoidance (Kim et al., 1997; Dubrovina et al., 2006). While these traits suggest
that MAO A KO mice may display alterations in anxiety- and fear-related
behavior, data from classical anxiety paradigms, such as the light-dark box, the
elevated plus-maze (Popova et al., 2001; Vishnivetskaya et al., 2007) and the
open field (Agatsuma et al., 2006) have failed to identify differences.
The absence of anxiety-related manifestations is particularly striking in
consideration of the critical role of 5-HT in emotional regulation and the reactive
nature of the aggressiveness in MAO A-deficient mice. In order to examine the
role of MAO A in antisocial behavior and aggression, the neurobiological and
10

neurobehavioral substrates that underpin these behaviors need to be elucidated.
To this end, one of the potential shortcomings of previous investigations in this
line is the heavy reliance on a limited set of paradigms to detect behavioral
abnormalities. Since no single assay can fully recapitulate the array of
heterogeneous behavioral alterations in a psychiatric phenotype, behavioral
characterization based on a multimodal approach with complementary paradigms
may be more appropriate to capture more subtle and complex facets of
emotional domains. Indeed, several reports have documented that the elevated
plus-maze may be limited to a model of generalized anxiety and may be
suboptimal for the detection of other anxiety-related behaviors, such as
neophobia and avoidance (File, 1992; Roy et al., 2009).
Previous studies have also shown that MAO A KO mice exhibit blunted
physiological responses to severe stressors, such as physical restraint and
chronic variable stress (Popova et al., 2006). While the heightened sensitivity
towards fear conditioning and the attenuated endocrine response to severe
stressor may appear as incongruent, an alternative explanation may be that
these behavioral disturbances are a result of abnormal perceptual processes or
maladaptive defensive responses to contextual cues. To this end, further studies
are warranted to investigate emotional reactivity, behavioral adaptation to
external stimuli, stress-coping, and other behavioral outcomes of stress in MAO
A-deficient mice.
11

Another interesting area of research is the developmental role of MAO A in
behavioral outcomes. For instance, while the behavioral disturbances induced
by MAO A-deficiency cannot be recapitulated by treatment with MAOIs in
adulthood, early pharmacological inhibition of MAO A has been shown to elicit
several emotional and aggressive manifestations (Whitaker-Azmitia et al., 1994;
Mejia et al., 2002), thereby highlighting the developmental significance of MAO A
abrogation. In parallel with these findings, early developmental treatment of MAO
A KO mice with para-chlorophenylalanine (PCPA), an inhibitor of serotonin
synthesis, corrects the disturbances in barrel field formation (Cases et al., 1996;
Upton et al., 1999). This manipulation is particularly relevant to emotional
reactivity as barrel field disruption has been shown to result in marked
impairments of perceptual processing, exploratory activity, threat response and
sensory integration of contextual stimuli (Hurwitz et al., 1990; Cases et al., 1996;
Sanders et al., 2001; Dowman and Ben-Avraham, 2008; Straube et al., 2009).
The implication of MAO A in emotional disturbances and behavioral adaptation,
as well as specific developmental functions of MAO A in behavior cannot be
optimally studied in MAO A KO mice, as the complexity of their behavioral
abnormalities are likely to mask other more subtle emotional deficits. In addition,
the low prevalence of Brunner syndrome challenges the translational value of the
MAO A KO model. Since carriers of low MAO A activity polymorphic variants
represent a larger clinical population than their MAO A-deficient counterparts, it
may be advantageous to focus on effects of low MAO A activity in development
12

on behavioral outcomes. One potential method to address these limitations is
through the generation of novel lines of conditional KO and knock-in transgenic
mice, and the production of a hypomorphic mutant model with a higher degree of
isomorphism to clinical samples. Moreover, future studies should address the
impact of MAO A perturbations on other neurobiological substrates and
neurotransmitter systems to elucidate the behavioral disturbances characteristic
of this line. Two intriguing possibilities are the glutamatergic and GABAergic (γ-
aminobutyric acid) systems, which are critical players in the regulation of signal
processing and brain function.

1.5: The enigmatic role of MAO B in neuropsychiatry
Cogent evidence has shown that MAO B plays a role in emotional regulation.
Administration of MAO B inhibitors elicit anxiolytic and mood-enhancing effects
(Mendlewicz and Youdim, 1980; Quitkin et al., 1984; Tariot et al., 1987; Goad et
al., 1991; Tolbert and Fuller, 1996; Robinson et al., 2007) and low platelet MAO
B activity has been correlated with several different domains of behavioral
disinhibition (Buchsbaum et al., 1976; von Knorring et al., 1984; Oreland, 1993),
such as poor impulse control (Skondras et al., 2004b; Paaver et al., 2007),
novelty- and sensation-seeking (Fowler et al., 1980a; Reist et al., 1990; Ruchkin
et al., 2005), and risk-taking behaviors (Blanco et al., 1996).
13

Although a paucity of studies have documented polymorphic variations of the
Maob gene (Balciuniene et al., 2002), patients carrying these variants exhibit
behavioral disturbances including emotional alterations and other mood-related
disorders, attention-deficit hyperactivity disorder, and Parkinson‟s disease (Kurth
et al., 1993; Costa et al., 1997; Lin et al., 2000; Li et al., 2008; Dlugos et al.,
2009).
PEA is an alternative target to study the molecular mechanisms underpinning
MAO B in behavioral regulation. PEA has been implicated in a number of
neuropsychiatric disorders such as depression, mania, and schizophrenia. The
psychostimulant properties of this trace amine have prompted researchers to
view PEA as an endogenous amphetamine (Janssen et al., 1999; Wolinsky et al.,
2007). Like amphetamine, PEA administration has been shown to cause
increased alertness, irritability, insomnia, tremor, and euphoria (Baud et al.,
1985; Zucchi et al., 2006).
Although the mechanism of action is still obscure, PEA may exert some of its
amphetamine-like effects through the activation of the trace amine-associated
receptors (TAARs) (Zucchi et al., 2006). Indeed, both amphetamine and PEA
are potent agonists of TAAR-1 (Bunzow et al., 2001).
PEA also serves as a neuromodulator by altering neuronal responsivity to
monoamines, specifically enhancing catecholaminergic responses (Sabelli et al.,
1975; Wolinsky et al., 2007; Xie and Miller, 2008). Since PEA is maintained in
14

low physiological concentrations, it is possible that PEA elicits behavioral effects
through the fine-tuning of monoaminergic signaling pathways.
MAO B acts as the primary catabolic enzyme for the degradation of DA in
humans, which has been extensively shown to regulate emotions, learning and
cognition, as well as motivation and reward. Moreover, dysfunctions in
dopaminergic signaling mechanisms have been hypothesized to play a key role
in the pathophysiology schizophrenic symptoms in patients (Fang et al., 1995;
Bodkin et al., 1996; Carlsson et al., 2001 1987 - 3447667; Carrera et al., 2009;
Piton et al., 2010). Although the relationship between MAO B and schizophrenia
has not been consistently reported (Fowler et al., 1981), it is likely that high levels
of PEA may lead to the emergence of schizophrenia-related symptoms in specific
patient subsets (Janssen et al, 1999). Similarly, the high parallelism between
dopaminergic hyperactivity in the striatum and the neurobiological localization of
MAO B in this region provides an interesting corollary for the study of MAO B in
mental illness and warrants further investigation (Lewis and Anderson, 1995;
Laruelle and Abi-Dargham, 1999; Meyer-Lindenberg et al., 2002).
Clinical studies have shown that selective MAO B inhibitors improves attention-
impairments in ADHD patients (Jankovic, 1993; Feigin et al., 1996; Akhondzadeh
et al., 2003; Rubinstein et al., 2006), suggesting a role in the modulation of
information processing and attention. This effect is likely due to the structural
and functional similarities between the MAO B substrate PEA and amphetamine,
which shows therapeutic effectiveness in the alleviation of ADHD symptoms
15

(Bunzow et al., 2001; Bortolato et al., 2008). In line with these findings, reports
have documented associations between low platelet MAO B activity with
attention-deficit hyperactivity (Shekim et al., 1986; Coccini et al., 2009; Nedic et
al., 2010) impulsivity (Ward et al., 1987; Carrasco et al., 1994; Blanco et al.,
1996), the latter a key feature in ADHD psychopathology.
Nevertheless, the absence of available pharmacological tools targeting trace
amine-associated receptors, as well as the lack of agents that selectively
manipulate PEA synthesis and neurotransmission, pose difficult future
challenges for researchers. Of note, the recent discovery of a selective TAAR-1
antagonist (Bradaia et al., 2009) may help provide mechanistic insights into the
role of PEA and MAO B in emotional regulation.

1.6: Animal models of MAO B KO
MAO B knockout (MAO B KO) mice display elevated levels of PEA, a reduction
in immobility in the forced swim test, and impaired habituation (Grimsby et al.,
1997; Lee et al., 2004), suggesting possible mnemonic or adaptive impairments.
Although a wealth of evidence has associated low MAO B activity with behavioral
disinhibition, previous studies in our lab were unable to support this link, possibly
due to differences in the role of MAO B on DA metabolism between humans and
rodents. The difficulty in probing the relationship between MAO B and behavioral
outcomes lies in the lack of pharmacological tools to selectively manipulate
16

phenylethylamine synthesis and target receptors. One of the possibilities to
overcome this limitation is to test MAO B KO mice in a set of paradigms designed
to specifically capture behavioral features relevant to behavioral disinhibition,
such as responses to novelty and risk.
In view of the amphetamine-like actions of PEA, high levels of this trace amine in
MAO B KO mice may result in behavioral disturbances similar to alterations
induced by chronic amphetamine treatment. Notably, repeated amphetamine
administration has been shown to increase behavioral disinhibition and reduce
anxiety-related behaviors (Megens et al., 1992; Evenden and Ryan, 1996; Leyton
et al., 2002). These behavioral abnormalities are highly reminiscent with the
phenotypic manifestations associated with low MAO B activity levels in humans.
Moreover, chronic amphetamine use has been associated with long-term
neuropsychological deficits in decision-making, reversal learning, mnemonic
function, and attention (Bechara et al., 2001; Bechara and Damasio, 2002;
Fillmore et al., 2003; Aguilar de Arcos et al., 2005; Ersche et al., 2005; Ersche et
al., 2006).
Although previous studies have also documented that chronic amphetamine
treatment results in maladaptive responses to contextual cues (Ridley et al.,
1981; Ornstein et al., 2000; Featherstone et al., 2008), these findings have not
always been replicated (Kokkinidis and Anisman, 1978; Ersche et al., 2008).
One possibility to explain this discrepancy may be due to the effects of repeated
amphetamine administration on attentional parameters (Crider et al., 1982; Deller
17

and Sarter, 1998; Tenn et al., 2003; Dalley et al., 2005; Martinez et al., 2005;
Sarter et al., 2005; Fletcher et al., 2007; Kozak et al., 2007). Nevertheless,
further studies on the effects of chronic amphetamine and PEA administration on
synaptic plasticity and behavioral responses are needed to elucidate the role of
MAO B and PEA in behavioral regulation.

1.7: Why study MAO – Limitations in modern psychiatry and
endophenotypes
A central limitation in the study of mental disorders is the gap between
psychological processes and molecular pathways. The Diagnostic and Statistical
Manual of Mental Disorders (DSVM-IV-TR) must rely on the subjective analysis
of semeiological features to categorize mental illnesses due to the absence of
biomarkers and other quantitative indices of translational value. This
classification results in nosologic categories of several neuropsychiatric disorders
with overlapping symptomatology, suggesting a common molecular mechanism
underlying specific clusters of behavior. In line with this premise, these diseases
may be incorrectly catalogued as distinct disorders based on an artificial set of
subjective criteria, rather than different subtypes of a single disorder. For
instance, impairments in emotional reactivity, impulse control, informational
processing, and behavioral flexibility are features typically observed in
schizophrenia, autism, OCD, depression, and aggression. In contrast, diseases
such as OCD and aggression are heterogeneous multifactorial disorders and
18

may be composed of several different independent traits, which may interact,
manifesting into a specific neurobehavioral profile. Consequently, patients are
often misdiagnosed and inappropriately treated - therapeutic regimens are
prescribed based on prior effectiveness and probability of success.
In order to bridge the gap between complex psychological disorders and
behavioral disturbances with neurobiological substrates and molecular
mechanisms, researchers developed the endophenotype construct. This
concept allows for the deconstruction of a wide array of complex neuropsychiatric
symptoms into smaller, quantifiable features. Endophenotypes may reflect
psychological, biological, or behavioral traits that are associated with a disease
and adhere to specific criteria: (1) heritable; (2) state-dependent; (3) higher rate
of co-segregation with non-affected relatives than found in normal populations
(Leboyer et al., 1998; Gottesman and Gould, 2003; Gould and Gottesman, 2006;
Viswanath et al., 2009; Bortolato and Godar, 2010). In accordance with this
conceptual framework, the identification of endophenotypes associated with
MAO dysfunction is an essential step to further elucidate the role of these
isoenzymes in the pathophysiology of several neuropsychiatric manifestations.

1.8: MAO in emotional reactivity
Converging evidence has documented that the MAO isoenzymes play a major
role in emotional regulation. Emotions are critical components that influence the
19

behavioral responses of animals. Although the neurobiological and
neurochemical substrates that underpin specific emotional processes are still
partially elusive, the gamut of emotional behaviors has developed through the
evolution of basic drives, including hunger, thirst, predation, survival, and sexual
reproduction. In higher functioning species, these instinctual behaviors, derived
from the brain stem, are further elaborated in subcortical structures, such as the
limbic system, and regulated by cortical processing and feedback mechanisms.
This computational algorithm produces a complex set of behavioral outcomes
based on the integration of intrinsic motivational factors, implicit and explicit
memory and emotional valence of incoming stimuli. Thus, an imbalance
between cortical control mechanisms on the regulation of subcortical emotional
processing may enhance the risk and/or lead to the development of
neuropsychiatric abnormalities (Hare et al., 2008).
One of the key substrates underlying the emotional apparatus is emotional
reactivity (also called emotional information processing, emotionality or
arousability) (Grillon, 2006). Broadly defined, emotional reactivity consists of the
perception, appraisal, and assignment of emotional valence to contextual cues,
as well as the behavioral organization and selection of appropriate coping and
adaptive responses to the stimuli. Since emotional reactivity heavily relies on the
sensitivity, threshold, and integration of informational processing and
responsivity, individuals displaying abnormal and extreme forms of emotional
reactivity to environmental cues and scenarios are typically the most susceptible
20

for neuropsychiatric disorders. Similarly, the restriction or absence of emotional
reactivity (restricted or blunted affect or affective flattening) is a common feature
found in a number of psychiatric diseases including schizophrenia, autism,
PTSD, and depression.
In view of the important modulatory functions of the monoaminergic systems in
emotional processing, disturbances in MAO regulation may impinge on
elaboration of appropriate behavioral responses in accordance with
environmental content. Accordingly, the emotional component of aggressiveness
in MAO A KO mice strongly suggests the existence of alterations emotional
regulation (Neumann et al., 2010), as well as impairments in their evaluation of
ambiguous, potential or actual threat (Blair, 2009). In line with these premises,
pharmacological treatment with MAO inhibitors and patients with Brunner
syndrome have been shown to display abnormal responses to environmental
cues (Murphy and Kalin, 1980) and unprovoked violent behavior (Brunner et al.,
1993a). Moreover, low brain MAO A activity has also been associated with
antisocial personality, impaired decision-making (Ibanez et al., 2000; Perez de
Castro et al., 2002; Meyer-Lindenberg et al., 2006), aberrant reactivity to stress
and poor processing of threat and social cues (Caspi et al., 2002; Alia-Klein et
al., 2008; Brummett et al., 2008; Buckholtz and Meyer-Lindenberg, 2008; Lee
and Ham, 2008; Kumari et al., 2009; Williams et al., 2009). Collectively, these
data suggest that MAO A-deficiency may be underpinned by emotional
21

disturbances and deficits in behavioral adaptation to social and contextual
elements.
In parallel to involvement of MAO A in emotional regulation, several reports have
documented an association between low MAO B activity in platelets and
impulsivity, risk-taking, and novelty- and sensation-seeking behaviors
(Buchsbaum et al., 1976; Fowler et al., 1980a; von Knorring et al., 1984; Reist et
al., 1990; Oreland, 1993; Blanco et al., 1996; Skondras et al., 2004b; Ruchkin et
al., 2005; Paaver et al., 2007). Similarly, changes in urinary PEA levels have
been implicated in a number of mood disorders (Sabelli and Javaid, 1995).
Although previous studies have shown that MAO B KO mice display a decrease
in immobility in the forced swim test (Grimsby et al., 1997), alterations in other
emotional domains in this line have not been found.

1.9: MAO in behavioral plasticity and adaptation
In addition to its role in emotional regulation, the MAO isoenzymes have been
implicated in the modulation of behavioral responses, suggesting a role in
behavioral adaptation. Behavioral adaptation can be defined as the ability to
enact or shift behavioral strategies or patterns in accordance with alterations in
contextual contingencies (Ragozzino, 2007). These mechanisms heavily rely on
sensorimotor gating functions to define and process salient information
necessary for mounting an appropriate behavioral response. Since the integrity
22

of synaptic circuitry and connectivity plays an important role in information
signaling, disturbances in the remodeling and reorganization of neural networks
can be detrimental to neurotransmission. For instance, overabundance of
synaptic arborization and connectivity can slow the processing of the input
stimulus (signal), by impeding the ability to filter out the non-salient elements
(noise) and thereby resulting in poor signal-to-noise contrast. This scenario may
contribute to deficits in the allocation of adaptive behaviors and risk detection, as
well as an overly narrow, but heightened attention to a given stimulus. In a
pathological form, this limited, but enhanced focus may impinge on the brains
ability to shift attention to other sensory stimuli, ultimately manifesting in
behavioral perseveration or rigidity and impairments in mnemonic extinction
(Bryson, 2006). Conversely, the inability to define a specific target signal may
result in a lack of focus and leading to behavioral disinhibition and attention-
related problems. Nevertheless, maladaptive behavioral responses have been
extensively implicated in several neuropsychiatric diseases, such as
schizophrenia, autism-spectrum disorders, depression, aggression, and
obsessive-compulsive spectrum disorders.
Indeed, low MAO A has been linked to a number of psychiatric disturbances
characterized by perseverative behaviors and low flexibility, such as autism-
spectrum disorders and obsessive-compulsive disorder (Cohen et al.; Camarena
et al., 2001; Chugani, 2002; Cohen et al., 2003; Davis et al., 2008; Yoo et al.,
2009). Similarly, patients with MAO A deficiency have been characterized with
23

repetitive stereotypical movements (Sims et al., 1989; Shih and Thompson,
1999; Whitaker-Azmitia, 2001; Whibley et al., 2010) and deficits in social and
functional adaptation (Brunner et al., 1993a; Brunner et al., 1993b). In line with
these findings, MAO A KO mice display deficits in behavioral habituation
(Agatsuma et al., 2006), which may reflect an impairment in environmental
adaptation. Moreover, it is possible that the decreased immobility in the forced
swim and overt aggressiveness may reflect abnormalities in adaptation to
contextual and social elements. Nevertheless, future studies are warranted to
examine the relationship between MAO A and adaptive behaviors.
Converging studies have documented the effects of PEA and amphetamine on
adaptive responses. In addition to inducing mood-enhancing effects, PEA
treatment has also been shown to effect attention (Sabelli et al., 1996), as well as
impair goal-directed behavior (Moja et al., 1976; Greenshaw, 1984; Sabelli and
Javaid, 1995). In parallel to these reports, acute amphetamine elicits
perseverative behaviors (Ridley et al., 1981; Evenden and Robbins, 1983; Bruto
et al., 1984) and improves attention and impulse control problems (McKetin et al.,
1999; Silber et al., 2006; Sagvolden and Xu, 2008). Conversely, high or chronic
administration of amphetamine decreases behavioral rigidity (Kokkinidis and
Anisman, 1978; Bruto et al., 1983; Fletcher et al., 2005) and disrupts attentional
performance (Crider et al., 1982; Deller and Sarter, 1998; Martinez et al., 2005;
Fletcher et al., 2007; Kozak et al., 2007). This deviation in behavioral sequelae
may result from region-specific alterations in cortical morphology and
24

neurotransmission following repeated amphetamine treatment (Crombag et al.,
2005; Homayoun and Moghaddam, 2006). An intriguing possibility to explain
these changes is through a shift in gating control from the identification or
definition of the target stimulus to the cognitive selection of salient cues (Corbetta
and Shulman, 2002; Montague et al., 2004; Featherstone et al., 2007).
Nevertheless, further studies are warranted to investigate the impact of MAO B-
deficiency on attentional performance.

1.10: MAO confers susceptibility to the emergence of psychiatric
manifestations: role of stress
Although genetic elements, such as low MAO A activity, may predispose or
increase the susceptibility of an organism to develop behavioral disturbances, the
interaction between genetic components and environmental challenges,
especially during specific critical time periods, plays a pivotal role in the
emergence of neuropsychiatric symptoms. Indeed, the precipitation of
obsessive-compulsive behaviors, aggression, schizophrenia and depression are
heavily influenced by stress. Stress can be defined as the failure to appropriately
respond to an intense or persistent condition/demand. While stress is an
essential protective mechanism to alert an animal to a potential danger, severe
or chronic stress can cause long-term complications, such as post-traumatic
stress disorder, depression, and even death.
25

Stress activates two overlapping systems: the autonomic nervous system and
the hypothalamic-pituitary-adrenal system. These two systems engage in fast-
acting responses to counteract the potential harmful effects of the stressor.
Recently, the urocortin system has been described as a slow-acting response to
facilitate long-term behavioral adaptation to stress. Although a detailed review of
the stress pathways are beyond the scope of this review (Huether et al., 1999; de
Kloet et al., 2005; Calabrese et al., 2009), it is worthy to note that stress induces
neuronal remodeling and reorganization in emotional processing, as well as other
adaptive functions in order to re-establish homeostasis (Grillon and Davis, 1997).
Moreover, this process of reshaping the synaptic architecture is heavily
influenced by the stressor severity, duration, previous experience and context (de
Kloet et al., 2005). The functional changes in neuronal network connectivity,
especially during early stages when the developing brain is highly plastic,
highlight the importance of stress regulation on emotional reactivity and
behavioral adaptation (Matsumoto et al., 2009).
Stress response is modulated by the monoaminergic system. Broader insights
into the role of MAO A and stress are afforded by reports documenting that
males carrying low MAO A activity polymorphisms and exposed to early
childhood abuse or maltreatment have a significantly higher risk to develop
aggression and violence in adulthood (Caspi et al., 2002; Haberstick et al.,
2005). In line with these findings, MAO inhibitors have been shown to improve
stress-coping behaviors in rodents (Katz et al., 1981).
26

Previous studies on MAO A- and MAO B-deficient mice have shown that both
lines exhibit lower immobility time in the forced swim test (Cases et al., 1995;
Grimsby et al., 1997). While this behavioral readout was interpreted to signify an
increase in stress reactivity (Shih et al., 1999a), the validity of this paradigm for
the assessment of stress has been questioned (Blokland et al., 2002; Popova et
al., 2006). The reduction in immobility duration in the forced swim task may
instead reflect a lower vulnerability to stress-induced depression (Trzctnska et
al., 1999; Strekalova et al., 2004), an outcome that closely mirrors the clinical
antidepressant effects of MAO inhibitors. Indeed, MAO A KO mice display an
attenuated physiological response to restraint, cold, water deprivation, and
chronic variable stress (Popova et al., 2006). In summary, the physiological and
behavioral effects of MAO dysregulation on stress reactivity is still obscure and
further studies warrant the investigation of the effects of MAO B deficiency on
physiological stress indices, as well as the behavioral responses to stressors in
both lines.

1.11: MAO modulation of other neurotransmitter systems: NMDA
glutamatergic receptor
Glutamate is the most prominent excitatory neurotransmitter in the brain and
together with gamma-amino-butyric acid (GABA), the primary inhibitor
neurotransmitter in the brain, the interplay between these two systems has been
proposed to regulate synaptic neurotransmission through modification of the
27

signal-to-noise ratio. One of the critical mechanisms that govern neuronal
network function and re-organization is through the glutamatergic ionotropic N-
methyl-D-aspartate (NMDA) receptor. The NMDA receptor is a non-specific
cation channel that is primarily activated by the binding of glutamate.
Conversely, α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), the
other major ionotropic glutamate receptor is a sodium channel. While the
activation of AMPA receptors produces rapid, transient changes in postsynaptic
potentials involved in the more short-lived changes in plasticity, NMDA receptor
activation potentiates the strength of the postsynaptic potential, producing a
longer and more profound effect on synaptic remodeling than AMPA receptor
activation. Summarily, depolarization of the cell activates the NMDA receptor,
producing an influx of calcium ions into the cell, which initiates a number of
downstream intracellular signaling cascades and results in the release of
anterograde and retrograde messengers to communicate and modify the
interactions with other neurons in the network. Moreover, the NMDA receptor
induces changes in gene expression, which may ultimately lead to long-term
changes in neuronal plasticity. This mechanism is accomplished through the
activation or deactivation of specific networks, and also stimulating neural
network remodeling and reorganization. Taken together, the NMDA receptor
regulates behavior through alterations in the signal-to-noise ratio of information
neurotransmission by the modification of neuronal network connections and
interactions. This conceptual framework is supported by reports showing that
NMDA receptor dysfunction impairs information filtering and processing by
28

disturbing synaptic remodeling processes and thus vitiates the brain‟s ability to
appropriately adapt to the environment (Jackson et al., 2004; Mozhui et al.,
2010).
Cogent evidence has shown that stress initiates changes in neuronal network
organization through the NMDA receptor, which have long-term consequences
on emotional reactivity and adaptive behaviors (Kim et al., 1996; Adamec et al.,
1998; Moghaddam, 2002; Popoli et al., 2002; Susman, 2006). NMDA receptor
dysfunction also contributes to the pathogenesis of several neuropsychiatric
disorders. Similarly, NMDA receptor blockade-induced psychotomimetic
disturbances, such as hallucinations and delusions, as well as aggression, and
cognitive and mnemonic impairments, are surmised to act through alterations in
monoaminergic mechanisms (Martin et al., 1998; Ninan and Kulkarni, 1998;
O'Neill et al., 1998; Varty et al., 1999; Kegeles et al., 2000; Nilsson et al., 2004;
Rung et al., 2005). These effects are likely caused by the convergence of
serotonergic and dopaminergic neurobiological pathways on glutamatergic
neurons (Carlsson, 1998; Carlsson et al., 2001; Carlsson, 2001; Kondziella et al.,
2006; Lopez-Gil et al., 2009).
In consideration of its central role in behavioral regulation, the identification of
indirect mechanisms that can alter glutamatergic neurotransmission is essential.
Glutamatergic signaling is reciprocally modulated by monoamines and a large
number of monoaminergic synaptic processes terminate on glutamatergic
primary neurons. This scenario heavily suggests that these two neurotransmitter
29

systems interact in behavioral regulation. Despite this evidence, the
mechanisms underlying the reciprocal actions of the monoaminergic system on
glutamatergic dysfunction is still highly elusive.
The importance of this elucidating the relationship between the two
neurotransmitter systems is highlighted by the limitations in the treatment of
neuropsychiatric disturbances. Despite the involvement of glutamatergic
dysfunctions in the psychopathophysiology of several neuropsychiatric disorders,
direct pharmacologic manipulation of glutamatergic pathways is problematic due
to the severe neurotoxic complications. Conversely, indirect targeting through
monoamines produces a number of unwanted non-specific side effects. As a
result, intermediate molecules involved in the monoamine-glutamatergic
interaction need to be evinced and utilized for the design of more effective
therapies.
Mice with intrinsically high levels of monoamines, such as monoamine oxidase-
deficient mice, provide an ideal model for investigation of this interaction.
Behavioral characterization of these mice has demonstrated abnormalities
consistent with glutamatergic dysfunction, similar to a cluster of symptoms found
in neuropsychiatric diseases such as schizophrenia, obsessive-compulsive
disorder (OCD), aggression, depression, and autism. Consequently, elucidation
of the mechanisms of monoaminergic modulation of glutamatergic signaling is
critical for deciphering the neuropathology of these disorders, discovering
30

intermediate molecules involved in the monoamine-glutamatergic interaction, and
designing more specific and efficacious therapies.

1.12: Major aims and thesis
In the introduction, I have endeavored to provide a current state-of-the-art review
of MAO in neuropsychiatry. Moreover, I have identified several critical gaps of
knowledge that are necessary to elucidate the role of MAO in the pathogenesis
of mental disorders. Despite the complexity of molecular mechanisms that
contribute to neuropsychiatric manifestations, the dissection of these disorders
into more simplistic endophenotypes will facilitate the identification of common
genetic and environmental factors that underpin disease pathogenesis and lead
to the generation of novel therapeutics that offer greater selectivity and efficacy.
While both MAO A and B are heavily implicated in a wide array of
neuropsychiatric abnormalities, the nosological characterization of these
behavioral disturbances remain obscure. For instance, although the role of
MAO A in aggression and antisocial behavior is well-established, little is known
about the endophenotypes that underpin disease pathophysiology. Similarly,
there are a paucity of studies documenting the function of MAO B in psychiatric
manifestations.
Based on the substantial gaps in knowledge outlined in the introduction, this
thesis will address several important questions relating to the role of MAO in
31

behavioral regulation and identify potential endophenotypes underlying the
function of MAO in neuropsychiatric abnormalities that will contribute to a broader
theoretical framework of disease pathogenetic mechanisms. To this end, I
hypothesize that one of the key roles of MAO is to regulate behavioral
plasticity through alterations in monoamine neurotransmission resulting in
functional changes in NMDA receptors.
Behavioral plasticity can be defined as the integration of sensory input to output
specific behavioral patterns. Accordingly, I hypothesize that one of the
central roles of MAO A and B is to regulate behavioral plasticity through
the proper integration of perceptual information with the execution of
appropriate behavioral outcomes. This effect is mediated by changes in
monoamine regulation that converge on the modulation of NMDA
receptors. Although both MAO A and MAO B isoenzymes have overlapping
functions, they also act on distinct mechanisms to influence behavior. I
hypothesize that a role of MAO A in behavior is to properly integrate
sensory stimuli in accordance with social and environmental cues.
Conversely, I hypothesize that MAO B regulates behavioral control and
affect.
To test the behavioral plasticity in transgenic MAO mutant mice, I will employ
complementary approaches to measure emotional behaviors, including the
reaction to environment, neutral objects and stress-inducing stimuli, as well as
the behavioral flexibility. Furthermore, I will investigate the state of NMDA
32

function by studying the exploratory and stereotyped behavioral responses
following NMDA receptor antagonism.
33

Chapter 2 : Maladaptive defensive behaviors in
monoamine oxidase A-deficient mice

2.1: Abstract
Rich evidence indicates that monoamine oxidase (MAO) A, the major enzyme
catalyzing the degradation of monoamine neurotransmitters, plays a key role in
emotional regulation. Although MAO A deficiency is associated with reactive
aggression in humans and mice, the involvement of this enzyme in defensive
behavior remains controversial and poorly understood. To address this issue, we
tested MAO A knockout (KO) mice in a spectrum of paradigms and settings
associated with variable degrees of threat. The presentation of novel inanimate
objects induced a significant reduction in exploratory approaches and increase in
defensive behaviors, such as tail-rattling, biting and digging. These neophobic
responses were context-dependent and particularly marked in the home cage. In
the elevated plus- and T-mazes, MAO A KO mice and wild-type (WT) littermates
displayed equivalent locomotor activity and time in closed and open arms;
however, MAO A KO mice featured significant reductions in risk assessment, as
well as unconditioned avoidance and escape. No differences between genotypes
were observed in the defensive withdrawal and emergence test. Conversely,
MAO A KO mice exhibited a dramatic reduction of defensive and fear-related
behaviors in the presence of predator-related cues, such as cat urine or an
anesthetized rat, were significantly lower than those observed in their WT
34

littermates. The behavioral abnormalities in MAO A KO mice were not paralleled
by overt alterations in sensory and microvibrissal functions. Collectively, these
results suggest that MAO A deficiency leads to a general inability to appropriately
assess contextual risk and attune defensive and emotional responses to
environmental cues.

35

2.2: Introduction
The nosographic characterization of the endophenotypical abnormalities caused
by MAO A deficiency is still highly elusive, in consideration of the low prevalence
of Brunner syndrome (Hebebrand and Klug, 1995). In particular, the reactive
nature of the aggression associated with MAO A-deficiency strongly supports the
existence of alterations in the appraisal of potential or actual danger (Blair, 2009).
Accordingly, preliminary findings have shown that individuals with polymorphic
variants associated with low MAO A activity display disturbances in the emotional
processing of environmental and social cues (Caspi et al., 2002; Brummett et al.,
2008; Buckholtz and Meyer-Lindenberg, 2008; Lee and Ham, 2008; Kumari et
al., 2009; Williams et al., 2009).
Despite the well-established aggression towards conspecifics, the available
evidence on the defensive and anxiety-like behaviors in MAO A KO mice remains
incomplete and controversial. While few lines of research indicate that MAO A
KO mice may have a heightened sensitivity to fear-inducing stimuli, such as a
mild footshock (Kim et al., 1997), these mutants show reduced endocrine
response to restraint stress (Popova et al., 2006) and lack of anxiety-related
behaviors in several assays (Popova et al., 2001; Agatsuma et al., 2006;
Vishnivetskaya et al., 2007; Scott et al., 2008).
This background prompted us to hypothesize that MAO A deficiency may result
in a generalized alteration of threat assessment. We tested this possibility by
analyzing different domains of defensive and emotional reactivity of MAO A KO
36

mice, including avoidance, neophobia, risk assessment and response to
predator-associated cues and height.
37

2.3: Methods
Animal husbandry. We used 3-4 month old, experimentally naïve male
129S6/SvEvTac mice (n = 247; 118 WT and 129 MAO A
A863T
KO), weighing 25-
30 g. Animals were housed in group cages with ad libitum access to food and
water. The room was maintained at 22°C, on a 12 h: 12 h light/dark cycle. Light
and sound were maintained at 10 lux and 70 dB for all behavioral tests unless
otherwise indicated. Experimental procedures were in compliance with the
National Institute of Health guidelines and approved by the Animal Use
Committees of University of Southern California and University of Cagliari.
Novel object exploration. The experiment was performed as previously
described (Bortolato et al., 2009a). Mice (WT = 9; MAO A KO = 11) were tested
within a grey Plexiglas cubic box (20 x 20 x 20 cm). Mice were acclimated to the
chamber for 15 min. Twenty-four h later, two novel, identical black plastic
cylinders (8 cm tall 3.5 cm in diameter) were symmetrically placed at equal
distance (4 cm) from the center and affixed to the floor of the box. Mice were
placed in a corner, facing the center, and left undisturbed for 15 min. Their start
position was rotated and counterbalanced for each genotype throughout the test.

38

The same experimental protocol was used in two alternative contextual situations
with separate groups of animals:
- in moderately familiar cages (after 2 consecutive days of acclimation, for 15
min/day), to study the impact of contextual adjustment on object exploration (WT
= 8 ; MAO A KO = 8)
- in their home cages (commercial Makrolon mouse caging units; size: 28 17
12 cm; 0.5 cm of bedding layer) (WT = 8; MAO A KO = 8). Twenty-four h
following acclimation to the new home cage, two novel objects were affixed to the
cage floor, at equal distances (7 cm) from the center.
For each trial, we analyzed: locomotor activity (defined as the number of
crossings on a grid superimposed onto the image of each cage in a video
monitor); tail-rattling; latency to first exploratory approach; total duration and
number of exploratory approaches. Exploration was defined as sniffing or
touching objects with the snout; climbing or sitting on the object was not
considered exploration.
Elevated plus-maze and T-maze. The test was performed as previously
described (Bortolato et al., 2009a). Briefly, we used a black Plexiglas apparatus
consisting of two open (25 x 5 cm) and two closed arms (25 x 5 x 5 cm), which
extended from a central platform (5 x 5 cm) at 60 cm from the ground. Mice (WT
= 8; MAO A KO = 8) were individually placed on the central platform facing an
open arm. Behavior was recorded for 5 min. Measures included: entries and
duration in the open and closed arms and the central platform; frequency of
39

stretch-attend postures and head dips (defined as previously described (Rodgers
et al., 1992b); number of fecal boli. In a second experiment, one of the closed
arms was blocked with a black Plexiglas panel, so as to convert the apparatus
into a T-maze. Using a different set of mice (WT = 15; MAO A KO = 15), we
employed a simplified variation of the protocol previously described (Carvalho-
Netto and Nunes-de-Souza, 2004). Mice were initially placed at the end of the
accessible closed arm, facing the central platform, and their latency to exit this
compartment was recorded (with a 3-min time limit). Animals were then
positioned at the end of an open arm, facing the central platform, and their
latency to escape into the closed arm was measured (with a 5-min time limit).
Defensive withdrawal. We used the protocol described by Bortolato et al (2009a).
Mice (WT= 8; MAO A KO= 8) were individually placed inside a cylindrical
aluminum chamber (7 cm diameter x11 cm length) located along one of the four
walls of a dimly lit (10 lux) black Plexiglas open field (40 x 40 x 40 cm), with the
open end facing the center. Mice were allowed to freely explore the environment
for 15 min. Behaviors were recorded and monitored by an observer unaware of
the genotype. Behavioral measures included: latency to exit the chamber;
transitions between the chamber and open field; time spent in the chamber.
Emergence test. The emergence test was performed as previously described
(Holmes et al., 2003; Liu et al., 2007) with minor variations. A black Plexiglas
rectangular arena (40 x 10 x 20 cm) was divided by a guillotine door into two
compartments. The first compartment (start chamber; 10 x 10 x 20 cm) was
40

covered with a black ceiling, while the second compartment (open chamber; 30 x
10 x 20 cm) was left uncovered. Light and sound were maintained at 300 lux
(central compartment and goal chamber) and 70 dB respectively. Mice (WT = 8;
MAO A KO = 9) were individually placed in the start chamber for 10 min of
acclimation. The door was raised at 6 cm from the floor and the animal was
allowed to freely explore the open chamber for 5 min.
Using different groups of animals, the same test was performed to test whether
the emergence behavior of MAO A KO mice may be conditioned by the following
elements: an object (tennis ball, 7 cm in diameter) at 5 cm from the door; the
same object imbibed with 10 mL of diluted bobcat urine (vol:vol, 1:1) (Lexington
Outdoors Inc, Robbinston ME); a male Long-Evans adult rat, previously
anesthetized with pentobarbital (50 mg/kg, i.p.), with the snout at 7 cm from the
guillotine door. For each test, we measured the latency to emerge from the start
chamber, the number of transitions across the guillotine door, the time spent in
each chamber and sniffing the object (or rat) and the number of fecal boli.
Defensive burying. The defensive burying test was based on objects
impregnated with predator urine, as previously described (Campbell et al., 2003).
Mice (WT = 27; MAO A KO = 31) were individually placed into new home cages
filled with 2 cm of sawdust. Following a twenty-four h familiarization period, mice
were exposed to a pair of wooden blocks (3 x 3 x 3 cm), previously imbibed with
1 mL of diluted bobcat urine (vol:vol, 1:1) (Lexington Outdoors Inc). Water-
impregnated objects were used as controls. Blocks were placed in the center of
41

the cage at equal distances apart. The number and duration of digging bouts
were measured for the 5 min periods before (baseline) and after placing the
blocks.
Visual cliff test. The visual cliff test was used to ascertain the presence of overt
visual deficits (Fox, 1965) in MAO A KO mice. The apparatus consisted of two
pedestals (30 x 30 x 36 cm), featuring a black/white checkerboard pattern on
their surface. The pedestals were placed 25 cm apart and connected by a 1 cm-
thick, transparent Plexiglas (85 x 36 cm) platform. Mice (WT = 9; MAO A KO =
11) were placed in the middle of the ledge area at the edge of the apparent cliff,
and their activity was recorded for 5 min by a video camera positioned above the
apparatus. The latency of each mouse to cross the cliff ledge was measured.
Buried food test. Olfactory activity of animals was evaluated as described by
(Yang and Crawley, 2009). Mini chocolate-cereal chips (weigh ~ 1.0g) were used
as the food stimulus. For three consecutive nights, one chip was placed into each
cage to establish odor familiarization. Chip consumption was verified every
morning. Mice (WT = 12; MAO A KO = 11) were deprived of food for 24 h prior
to testing. Mice were individually exposed to a standard clean cage with a 3-cm-
thick layer of clean bedding for a 5 min-acclimation period. The animal was
briefly removed and a familiar food pellet was buried 1 cm beneath the surface.
Food and animal placement were randomized. The latency to retrieve the chip
was measured with a 15 min cut-off time.
42

Object recognition under total darkness. The object recognition test was
performed under total darkness to verify the haptic function of microvibrissae in
exploration (Brecht et al., 1997). Novel object exploration was studied in WT and
MAO A KO mice as described above (with one day of cage acclimation), under
either regular environmental light or total darkness (with an infrared camera to
videotape behavior). Ninety min later, animals were returned to the same cage
for 15 min, under the same light conditions as in the first exploration trials. The
cage contained one object identical to those used in the previous trial (familiar
object) and a different novel object with equivalent odor but different size
(rectangular block, 6 cm x 3 cm x 3cm) and texture. Positions of familiar and
novel objects were counterbalanced throughout the experiment. Each object was
used only once throughout the experiment. A novelty exploration index (NEI) was
calculated as the ratio of the duration of the exploratory approaches targeting the
novel object over the time of exploration of both objects.
Statistical analyses. Normality and homoscedasticity of data distribution were
verified using the Kolmogorov-Smirnov and Bartlett‟s test. Parametric analyses
were performed with one-way or two-way ANOVA, as appropriate, followed by
Tukey‟s test with Spjøtvoll-Stoline correction for post-hoc comparisons.
Nonparametric comparisons were carried out by Mann-Whitney and Kruskal-
Wallis tests as appropriate. Correlation analyses were performed by multiple
regression. Comparisons of categorical data were performed by Fisher‟s exact
test. Significance threshold was set at P = 0.05.
43

2.4: Results
Novel object exploration. We first compared the exploratory and defensive
responses of MAO A KO mice to novel objects in a standard cage, following one
day of acclimatization. Under these conditions, no significant difference was
found in either the exploratory duration (Fig. 2.1a) [F (1, 18) = 1.13; NS; ANOVA]
or in the latency to the first approach (Fig. 2.1c) [H(1) = 0.24; NS; Kruskal-Wallis].
Conversely, the number of approaches was found to be significantly lower in
MAO A KO mice than WT counterparts (Fig. 2.1b) [F(1, 18) = 7.23; P<0.05].
Object presentation also elicited tail-rattling responses in some (33%) MAO A-
deficient mice, but not in WT conspecifics; however, the occurrence of this
phenomenon was not found significantly different between the two genotypes
(Fisher‟s exact test). Crossings were comparable between genotypes (data not
shown) [F(1, 15) = 1.89; NS]. Notably, no significant correlation was found
between locomotor activity and exploratory duration (data not shown).
The same test, conducted in a different group of mice after 2 days of acclimation
to the cage, elicited significant reductions in both duration (Fig. 2.1d) [H(1) =
4.41; P<0.05] and number of exploratory approaches (Fig. 2.1e) [F(1, 14) =
11.09; P<0.01] from MAO A KO mice, as compared to their WT counterparts.
Latency to explore, however, was comparable between genotypes (Fig. 2.1f)
[F(1, 12) = 0.15; NS]. Notably, tail-rattling was observed in a high percentage
(62.5%) of MAO A KO mice, but not in WT animals (P<0.05, Fisher‟s exact test).
Although the number of crossings was reduced in MAO A KO mice (data not
44

shown) [F(1, 14) = 5.74; P<0.05], linear regression analysis on the exploratory
duration showed no significant difference between genotypes (data not shown).
In their home cages, MAO A-deficient mice exhibited significant reductions in
both the duration (Fig. 2.1g) [F(1, 15) = 11.39; P<0.01] and number (Fig. 2.1h)
[F(1, 15) = 22.6; P<0.001] of exploratory approaches in their home cages. MAO
A KO mice also displayed significant increases in the latency to explore (Fig.
2.1i) [H(1) = 8.72; P<0.01] and tail-rattling responses (62.5% of occurrence
among MAO A KO mice; 0% in WT) (P<0.05, Fisher‟s exact test) compared to
WT littermates. The number of crossings (data not shown) [F(1, 15) = 0.00; NS]
was equivalent between genotypes. Moreover, the locomotion and exploratory
activity were not significantly correlated (data not shown).
45

Figure 2.1. MAO A KO mice display contextual-specific alterations in novel object exploration. (a-
c) MAO A-deficient mice exhibit a decrease in exploratory approaches, but not duration or latency
to explore a foreign object under standard conditions. (d-f) Environmental acclimation resulted in
a reduction of exploratory duration and approaches, but not latency in MAO A KO mice. (g-i)
Contextual familiarity produced a decrease in exploratory duration and approaches, and a
significant increase in latency to explore the novel objects. All values are represented as means
± SEM. *P<0.05, **P<0.01, and ***P<0.001 compared to WT mice.
Elevated plus-maze. In agreement with previous studies on other lines of MAO
A-deficient mice (Popova et al., 2001), MAO A KO mice show comparable
percent open arm entries (Fig. 2.2a) [F(1, 14) = 0.32; NS], percent closed arm
entries (data not shown) [F(1, 14) = 0.16; NS], and total arm entries (Fig. 2.2b)
[F(1, 14) = 0.80; NS] to their WT counterparts. Moreover, both genotypes
46

displayed an equivalent duration in the open arms (Fig. 2.2c) [F(1, 14) = 0.07;
NS], closed arms (Fig. 2.2d) [F(1, 14) = 0.78; NS], and on the central platform
(data not shown) [F(1, 14) = 0.80; NS], respectively. We also examined the risk
assessment and exploration by measuring stretch-attend postures and head
dips, respectively (Rodgers and Johnson, 1995). MAO A KO mice displayed
fewer stretch-attend postures (Fig. 2.2e) [F(1, 14) = 8.88; P<0.01] and a lower
number of head dips (Fig. 2.2f) [F(1, 14) = 5.21; P<0.05] than WT mice.
Conversely, the number of fecal boli was comparable between genotypes (data
not shown) [F(1, 14) = 0.05; NS].
47

Figure 2.2. MAO A KO mice exhibit a reduction in risk-assessment and escape behaviors in the
elevated plus-maze and elevated T-maze paradigms. (a-d) Both genotypes show equivalent
measures for anxiety-related parameters in the elevated plus-maze task. (e-f) MAO A-deficient
mice display a significant reduction in stretch-attend postures and head dips compared to WT
littermates. (g-h) MAO A KO mice exhibit a significant increase in the latencies to exit a closed
arm and to escape from an open arm in the elevated T-maze assay. All values are represented
as means ± SEM. *P<0.05 and **P<0.01 compared to WT mice. Abbrev: SAPs, stretch-attend
postures.
48

Elevated T-maze. MAO A KO mice displayed a significantly longer latency to exit
the closed arm (Fig. 2.2g) [U(15, 15) = 49; P<0.01]. Similarly, MAO A-deficient
mice exhibited a significant increase in latency to escape to the closed arm (Fig.
2.2h) [U(14, 13) = 46.5; P<0.05]. These results indicate that MAO A KO mice
display reductions in both exploratory activity and escape behaviors.
Defensive withdrawal. No significant differences were found between MAO A KO
and WT mice in any behavioral parameter (data not shown).
Emergence test. Both genotypes showed similar behavioral responses in the
emergence test under standard conditions and in the presence of a foreign object
in the open chamber (data not shown). In contrast, the introduction of an object
impregnated with predator urine produced an increase in fear-related parameters
in WT, but not in MAO A KO mice. Specifically, MAO A KO mice exhibited a
significant reduction in the latency to exit the start chamber (Fig. 2.3a) [U(8, 8) =
0.00; P<0.001]. No differences were detected between genotypes in the percent
time in the start chamber (Fig. 2.3b) [U(7, 9) =16; NS], percent time in open
chamber (Fig. 2.3c) [U(7, 9) = 16; NS], transitions (Fig. 2.3d) [F(1, 15) = 0.32;
NS], sniffing duration (Fig. 2.3e) [F(1, 15) = 0.56; NS], and sniffing bouts (Fig.
2.3f) [F(1, 15) = 1.94; NS].
49

Figure 2.3. MAO A KO mice display a reduction in fear-related behaviors in the emergence test
with an object imbibed with predator urine. (a) MAO A KO mice exhibit a significant decrease in
the latency to emerge from the start chamber compared to WT mice. (b-f) No significant
differences were detected between genotypes in the percent time in the start chamber, percent
time in the open chamber, number of transitions, object sniffing bouts and duration. All values are
represented as means ± SEM. ***P<0.001 compared to WT mice.
The presence of an anesthetized rat in the open chamber also elicited a marked
increase in fear-related responses in WT mice, but not in MAO A KO littermates.
In particular, the latter displayed a significant decrease in latency to emerge from
the start chamber (Fig. 2.4a) [U(5, 9) = 2.00; P<0.01] compared to WT
counterparts. This reduction in latency was accompanied by a significant
decrease in percent time in the start chamber (Fig. 2.4b) [F(1, 14) = 7.53;
P<0.05] and a significant increase in the percent time spent in the open chamber
(Fig. 2.4c) [F(1, 14) = 7.53; P < 0.05]. Moreover, MAO A KO mice exhibited a
significant increase in sniffing duration (Fig. 2.4e) [F(1, 14) = 5.79; P < 0.05], and
50

sniffing bouts (Fig. 2.4f) [F(5, 10) = 1.94; P < 0.05], but not the number of
transitions (Fig. 2.4d) [F(1, 13) = 1.38; NS].

Figure 2.4. MAO A KO mice display a marked reduction in fear-related behaviors in the presence
of an anesthetized rat. (a-b) MAO A KO mice exhibit a significant decrease in the latency to
emerge and the percent time in the start chamber compared to WT mice. (c-d) MAO A KO mice
showed a significant increase in the percent time in the open chamber, but comparable number of
transitions to their WT littermates. (e-f) Moreover, MAO A-deficient mice engaged in a
significantly more predator sniffing behavior than their WT counterparts. All values are
represented as means ± SEM. *P<0.05 and **P<0.01 compared to WT mice.
Defensive burying. We exposed mice to foreign objects, either odorless or
imbibed with cat urine, in their home cage. Both genotypes showed comparable
baseline digging frequency (Fig. 2.5a) [F(1, 49) = 0.28; NS] and duration (Fig.
2.5c) [F(1, 48) = 0.09; NS]. Although the presentation of odorless objects
induced a significant increase in digging responses (in both digging bouts and
duration) in MAO A KO mice compared to WT littermates, predator urine-imbibed
51

objects elicited higher digging activity in WT than MAO A KO mice, in both
frequency (Fig. 2.5b) [H(3) = 10.09; P<0.05] and overall duration (Fig. 2.5d)
[H(3) = 11.63; P<0.01]. Post-hoc analysis revealed significant differences
between genotypes in digging frequency in the absence (P<0.05), but not in the
presence of urine (P<0.10). Moreover, urine-imbibed object presentation elicited
a significant difference in digging bouts (P<0.01) in MAO A deficient compared to
odorless objects. In line with these findings, significant differences in digging
duration were detected between genotypes exposed to the odorless object
(P<0.01). Differences in digging duration between genotypes in the presence of
predator urine were not significant (P<0.06). The presence of urine-impregnated
objects also induced a significant difference in WT (P<0.05) and MAO A KO mice
(P<0.05) compared to odorless objects.

52

Figure 2.5. MAO A KO mice display maladaptive digging reactions to unfamiliar objects in the
absence and presence of predator urine. (a, c) Both genotypes show equivalent baseline digging
activity. (b, d) In contrast, the presentation of an odorless object induces a robust digging
response in MAO A deficient mice. Impregnation of the object with cat urine elicited a marked
decrease in digging behavior in MAO A KO mice compared to their WT counterparts. All values
are represented as means ± SEM. *P<0.05, **P<0.01, and ***P<0.001 compared to WT mice
exposed to the odorless foreign object.
#
P<0.05 and
##
P<0.01 compared to MAO A KO mice
exposed to the odorless novel object.
Assessment of visual, olfactory and microvibrissal functions. We then
investigated whether some of the exploratory alterations in MAO A KO mice may
be attributed to alterations in visual and olfactory sensitivity, or in the haptic
function of the microvibrissae. In the visual cliff paradigm, both MAO A KO mice
and WT littermates exhibited comparable visual acuity, as measured by their
latency to reach the cliff [F(1, 18) = 0.69; NS] (Fig. 2.6a). Similarly, the
equivalent latency to locate the hidden chocolate chips in the buried food test
53

(Fig. 2.6b) [F(1, 21) = 0.46; NS], signified the lack of differences in olfactory
sensitivity between MAO A KO and WT mice.
In the object recognition test, we did not detect any difference between WT and
MAO A KO mice in exploratory duration (Fig. 2.6c) [F(1, 32) = 0.56; NS] and
novelty exploration index (Fig. 2.6d) [F(1, 28) = 0.00; NS], irrespective of the
light conditions. These results suggest that the behavioral impairments in MAO A
KO mice are not associated to an overt impairment of the microvibrissal function
(Brecht et al., 1997) despite the alterations in barrel fields displayed by these
animals (Cases et al., 1995).

54

Figure 2.6. MAO A KO mice do not display alterations in sensory function. (a-b) MAO A-KO mice
exhibit comparable latencies to reach a cliff and locate buried food in the visual cliff and buried
food tests, respectively. (c-d) In the novel object interaction and recognition task, MAO A KO mice
show comparable exploratory activity and percent novelty exploration indices compared to WT
mice in the absence of light, indicating intact microvibrissae function. All values are represented
as means ± SEM. Abbrev. % NEI, percent novelty exploration index.
55

2.5: Discussion
The results of the present study show that MAO A KO mice exhibit a broad
spectrum of maladaptive defensive responses to contextual cues. Interestingly,
the abnormalities in defensive responsiveness featured by MAO A KO mice were
multidirectional, in relation to the degree of potential danger associated with the
environmental elements. On one hand, novel inanimate objects elicited high
levels of neophobia in MAO A KO mice, manifested as defensive behaviors
(including tail-rattling, biting and digging) and reduced exploratory activity,
particularly if introduced in familiar environments. On the other hand, MAO A KO
mice showed a paradoxical reduction of their unconditioned fear-related and
escape responses across several settings associated with a higher level of
potential danger, such as the open arms of an elevated T-maze or the presence
of predator urine or a rat. In substantial agreement with previous findings
(Popova et al., 2001; Agatsuma et al., 2006; Vishnivetskaya et al., 2007; Scott et
al., 2008), the ethological conflict between protected and unprotected
environments (in the elevated plus-maze, defensive withdrawal and emergence
paradigms) did not induce marked behavioral variations in MAO A KO mice.
These alterations were probably supported by a parallel decrement in both
avoidance/fear and approach/exploration responses, as clearly documented by
the elevated T-maze test (Viana et al., 1994; Torrejais et al., 2008). Interestingly,
the behavioral deficits in MAO A KO mice were not accompanied by overt
alterations in locomotor, visual, olfactory and microvibrissal functions, suggesting
56

that their impairments may result from a general impairment in their emotional
regulation.
Taken together, our findings suggest that MAO A deficiency leads to maladaptive
emotional and defensive reactivity to environmental cues. In particular, MAO A
KO mice exhibited a distinct inability to attune their responses to the situational
content of their milieu, as indicated by the inappropriateness of their defensive
behaviors (For a more comprehensive review on this concept, please refer to:
Blanchard, 2008). Specifically, the reduction in stretch-attend postures in the
elevated plus-maze (Rodgers et al., 1992b) and the manifestation of maladaptive
responses, such as climbing a predator or rattling the tail in response to novel,
innocuous objects, suggest a general impairment in risk assessment and goal-
directed performance in MAO A KO mice. This conceptual framework may
account for the dysregulated aggressive and stress-induced responses in MAO A
KO mice (Cases et al, 1995; Kim et al., 1997; Popova et al., 2006).
The behavioral changes in MAO A KO mice may reflect their limited range of
adaptive responses in comparison with WT mice, possibly in relation to lower
levels of behavioral flexibility. Indeed, MAO A activity has actually been linked to
a number of psychiatric disturbances characterized by perseverative behaviors
and low flexibility, such as autism-spectrum disorders and obsessive-compulsive
disorder (Cohen et al.; Camarena et al., 2001; Chugani, 2002; Cohen et al.,
2003; Davis et al., 2008; Yoo et al., 2009). Based on the level of danger
associated with a given context and/or situation, the behavior of MAO A KO mice
57

may therefore appear more or less defensive and fearful, insofar as it is
compared with the broader, multi-faceted behavioral repertoire of WT mice.
Alternatively, the neurochemical changes induced by MAO A deficiency may lead
to specific alterations for each domain of defensive and emotional reactivity. In
keeping with this possibility, both 5-HT and NE have been extensively shown to
play different roles in the regulation of defensive behaviors (Graeff, 1993; Grahn
et al., 2002; Dringenberg et al., 2003; Bondi et al., 2007).
Our experiments documented a general reduction of exploratory activity in MAO
A KO mice across a broad spectrum of paradigms, which was not associated
with a reduction in locomotor activity. This finding suggests that MAO A KO mice
may display a lower level of inquisitiveness. This view is supported by previous
findings, attesting poor novelty-seeking and reward-dependence traits in male
carriers of the low-activity variant of MAO A polymorphism (Shiraishi et al., 2006).
Interestingly, the gradual familiarization of MAO A KO mice to the external
environment induced a progressive enhancement of the defensive responses
targeting novel objects. This finding may indicate that the sharp contrast between
the relative acquaintance with the context and the novelty of the objects may
enhance the salience (and the anxiogenic valence) of the latter, thereby leading
MAO A KO mice to channel their aversive responses on them.
While novel object exploration in an unfamiliar environment is widely regarded as
a dependable model of state-anxiety (Belzung and Le Pape, 1994), the same
task in a familiar context has been proposed to measure trait-anxiety
58

(Avgustinovich et al., 2000). This premise may suggest that the anxiety-like
behavior of MAO A KO mice may be an innate, enduring characteristic, rather
than a transient alteration of emotional responsiveness. Moreover, high trait-
anxiety may also partially contribute to the poor environmental adaptation and
habituation (Hare et al., 2008) previously observed in MAO A-deficient mice
(Agatsuma et al., 2006).
Previous studies have shown that long-term treatment with MAO A inhibitors in
adult rodents induces a decrease in defensive behavior against predators
(Griebel et al., 1998), but an enhancement in exploratory activity (Steckler et al.,
2001). These findings indicate that the emotional alterations featured by MAO A
KO mice are at least partially due to neurodevelopmental alterations. Indeed,
several studies have shown that the sensorimotor cortex deficits in these animals
are due to neurodevelopmental alterations based on the excessive 5-HT levels
and 5-HT
1B
receptor hyperactivation in the first days of postnatal life (Cases et
al., 1995; Vitalis et al., 1998; Salichon et al., 2001). Additionally, while most
behavioral alterations of MAO A KO mice – such as the elevated aggressiveness
and fear conditioning – cannot be reproduced by pharmacological inhibition of
this enzyme in adulthood, early treatment with MAO A inhibitors has been shown
to induce antisocial behavior and emotional impairments in rodents (Whitaker-
Azmitia et al., 1994; Mejia et al., 2002).
MAO A KO mice have been shown to display alterations of the barrel fields
(Cases et al., 1995), the cortical representations of the mystacial vibrissae in the
59

rodent snout (Erzurumlu and Jhaveri, 1990). These formations play a key role in
the functional coordination of the mystacial vibrissae in the rodent snout
(Luhmann et al., 2005), and their impairment has been shown to result in
profound alterations of perceptual processing, exploratory activity, threat
response and sensory integration of environmental stimuli (Hurwitz et al., 1990;
Cases et al., 1996; Sanders et al., 2001; Dowman and Ben-Avraham, 2008;
Straube et al., 2009). Nevertheless, we did not observe any significant change in
the exploration and recognition of novel objects in the absence of light. This
finding shows that, irrespective of the presence of visual cues or odor
differences, MAO A KO and WT mice did not display differences in object
recognition, thereby ruling out the role of microvibrissae in the exploratory deficits
displayed by the mutant genotype. In line with this concept, we found that
MAOA
A863T
KO did not display any overt impairment of olfactory and visual
perception, the other two sensory modalities used in object exploration by
rodents. In particular, our results on the visual cliff paradigm suggest that the
abnormal development of retinal projections previously documented in C3H MAO
A KO mice (Tg8) (Upton et al., 1999) do not exert profound effects on visual
acuity and discrimination in our line.
Our data expand and complement previous clinical evidence showing a link
between low MAO A activity and impairments in threat processing (Lee and Ham,
2008; Kumari et al., 2009; Williams et al., 2009), stress response (Jabbi et al.,
2007) and decision-making (Ibanez et al., 2000; Perez de Castro et al., 2002;
60

Meyer-Lindenberg et al., 2006). These alterations are likely to account for the
aberrant aggressiveness observed in individuals with low MAO A activity
(Brunner et al., 1993a; Brunner et al., 1993b; Caspi et al., 2002; Foley et al.,
2004; Jacob et al., 2005; Nilsson et al., 2006; Alia-Klein et al., 2008). In spite of
these similarities, several limitations advocate extreme caution in the
interpretation of our findings with respect to their translational validity: first,
psychiatric disorders cannot be fully recapitulated in murine models, and data
from these findings may not fully translate to clinical settings; second, the lack of
molecular bases to the observed alterations does not allow to understand the
mechanisms of MAO A in the regulation of defensive responses.
In conclusion, these findings support the hypothesis that MAO A KO mice display
alterations in several domains of defensive reactivity, including neophobia,
emotional processing of contextual elements, risk-assessment and escape.
Additionally, our study highlights the nodal role of MAO A in the processing of
adaptive, goal-directed responses to contextual cues. Further studies are
warranted to validate the endophenotypic traits identified in MAO A KO mice
throughout our experiments, and elucidate their neurobiological substrates.

61

Chapter 3 : Monoamine oxidase A-deficient mice exhibit
alterations in stress-coping behaviors

3.1: Abstract
Cogent evidence has shown that monoamine oxidase (MAO) A, the primary
catabolic enzyme for the degradation of serotonin (5-HT) and norepinephrine
(NE), plays a critical role in emotional regulation. In accordance with this
conceptual framework, MAO A knockout (KO) mice exhibit aversive reactions to
neutral objects, as well as a marked reduction in risk-assessment and fear-
related behaviors in the presence of predator cues. Although MAO A-deficiency
results in deficits in threat perception, the impact of this enzyme in modulating
behavioral plasticity is still poorly understood. To address this issue, the present
study was aimed at characterizing the physiological and behavioral responses of
MAO A KO mice following exposure to 4 h of restraint stress using several
paradigms designed to capture facets of exploration, neophobia and risk-taking
behaviors. MAO A-deficient mice subjected to restraint showed a dramatic
reduction in stress-induced physiological parameters. Similarly, MAO A KO
mice displayed comparable locomotor activity in the open field as their non-
stressed counterparts. In the novel object interaction test, restraint stress-
induced a significant attenuation in the exploratory activity of wild-type controls,
however, the intensity of object investigation in MAO A-deficient mice was
unaffected. To evaluate whether stress altered subsequent risk-taking behavior,
62

MAO A KO mice were tested in the wire-beam bridge test. Irrespective of pre-
exposure to restraint, MAO A-deficient mice showed a significant attenuation of
risk-assessment, accompanied by an increase in risk-taking behavior. Taken
together, these findings suggest that MAO A-deficiency results in abnormal
responses to stress-inducing stimuli and deficits in stress perception, indicating
that this enzyme plays a critical role in the modulation of behavioral plasticity.
63

3.2: Introduction
As shown in the previous chapter, MAO A KO mice display an array of behavioral
disturbances in threat perception and maladaptive responses to contextual cues.
Interestingly, MAO A-deficient mice exhibited aversive defensive reactions to
inanimate objects, suggesting that these mice perceive the foreign objects as
potentially threatening. In line with these findings, MAO A mutant mice show
overt aggressiveness towards unfamiliar conspecifics. Taken together, these
data strongly suggest that MAO A KO mice display aberrant defensive reactions
to intrusive elements. A corollary to these results is to test the behavioral
outcomes of MAO A-deficient mice following exposure to severe stress.
Accumulating evidence has shown that a number of neuropsychiatric disorders
are unmasked and exacerbated by exposure to stress. Although stress is a
potent modulator of the monoaminergic system, the role of MAO A dysfunction
on behavioral responses to stress is still partially obscure.
Previous reports have documented that early childhood maltreatment or abuse in
males harboring low MAO A activity polymorphisms have a higher susceptibility
to develop aggression and antisocial behavior in adulthood (Caspi et al., 2002;
Foley et al., 2004; Huang et al., 2004; Kim-Cohen et al., 2006; Nilsson et al.,
2006; Taylor and Kim-Cohen, 2007; Edwards et al., 2010). Moreover, several
studies have reported that aggression and antisocial behavior are inversely
correlated with both brain MAO A activity (Alia-Klein et al., 2008) and
hypothalamic-pituitary-adrenal (HPA) activity (McBurnett et al., 2000; Shoal et al.,
64

2003; van Goozen and Fairchild, 2006; Bohnke et al., 2010; Poustka et al.,
2010). These data suggest that MAO A-deficiency may lead to alterations in
stress processing and reactivity.
In line with these findings, the role of MAO A in stress regulation is strongly
supported by several pieces of data. MAO inhibitor treatment has been
extensively used to combat symptoms associated with post-traumatic stress
disorder and depression, reverse learned helplessness induced by repeated
exposure to uncontrollable stress and enhance resistance to the effects of
chronic mild stress (Sherman et al., 1982; Martin et al., 1987; Griebel et al.,
1998; Ward et al., 1998; Menard and Treit, 1999; Holmes and Rodgers, 2003;
Millan, 2003). Although previous studies have shown that MAO A KO mice
exhibit blunted corticosterone responses to severe stressors, the stress-induced
behavioral outcomes remain elusive.
A rich body of evidence indicates that affective manifestations are profoundly
influenced by variations in the representation of the surrounding environment
(Phillips et al., 2003; Rolls, 2004), and the evaluation and response to potential
threats (Lang et al., 2000; Blanchard et al., 2001; File, 2001; Merali et al., 2003;
Ribeiro-Barbosa et al., 2005; Marques et al., 2008). This scenario suggests that
the emotional disturbances and defensive behaviors exhibited by MAO A KO
mice in previous studies may be underpinned by deficits in stress perception and
coping responses. We tested this possibility by investigating the effects of four h
acute restraint stress on the behavioral responses of MAO A KO mice. In
65

particular, we examined the physiological responses as well as the exploratory
responses to novel environments and objects associated with different degrees
of risk.
66

3.3: Methods
Animal husbandry. We used 3-5 month old, experimentally naïve male 129/Sv
mice (n = 48; 24 WT and 24 MAO A
A863T
KO), weighing 26-32 g. The MAO
A
A863T
KO mutation - consisting of an X-linked, single-point nonsense mutation in
exon 8 of Maoa gene - arose spontaneously in one of our 129/Sv in-house
colonies (Scott et al, 2008). The mutant mice were backcrossed to
129S6/SvEvTac for 12 generations to enhance reliability for behavioral testing.
MAOA
A863T
KO sires and heterozygous dams were crossed to generate MAO
A
A863T
KO and wild-type (WT) male littermates. Offspring was genotyped as
previously described (Scott et al., 2008)}. Animals were housed in group cages
with ad libitum access to food and water. The room was maintained at 22°C, on a
12 h: 12 h light/dark cycle. Before behavioral testing, all animals were found to
display equivalent no patent physical and neurological abnormalities. To avoid
potential carryover effects, each animal was used only once throughout the
study. Mice were isolated for four weeks prior to behavioral testing. Litter effects
were minimized by using mice from at least three different litters in each
behavioral test. Experimental procedures were in compliance with the National
Institute of Health guidelines and approved by the University of Southern
California Animal Use Committees.
Stress regimen. Mice were divided into two groups per genotype. In the stress
group, mice were restrained for 4 h in 50 mL plastic conical tubes, with holes
drilled at each end and on the sides to allow air circulation. Immediately
67

following the restraint, mice were removed and their fecal boli was counted. Mice
designated for the non-restraint condition, were briefly placed into the conical
tubes and returned to their cages for 4 h, where they were allowed to withdrawal
from the tube. Both groups were deprived of both water and food during their 4 h
condition.
Stress-induced hyperthermia. Temperature was taken via rectal probe
(Physitemp instruments Clifton, New Jersey) prior to and immediately following
the stress regimen. The overall change in temperature (final temperature – initial
temperature) was used as an index of hyperthermia (Bouwknecht et al., 2007).
Experimental battery.
Behavioral responses to stress were consecutively performed in a battery of
progressively stressful tasks. Mice were briefly returned to their home cages in
between paradigms. To maximize the behavioral analyses following stress, the
total time between the post-stress temperature acquisition and animal euthanasia
never exceeded 45 min (Van der Heyden et al., 1997).
Open-field. Analysis of the open field behaviors was performed in experimentally
naïve, adult male mice, following a modified version of the protocol used in
(Bortolato et al., 2009a). The open field was a Plexiglas square grey arena (40 x
40 cm) surrounded by 4 black walls (40 cm high). On the floor, 2 zones of
equivalent areas were defined: a central square quadrant of 28.28 cm per side,
and a concentric peripheral frame including the area within 11.72 cm from the
68

walls. Mice were placed in the central zone and their behavior was monitored for
5 min. Analysis of locomotor activity was performed using Ethovision (Noldus
Instruments, Wageningen, The Netherlands). Light and background noise in the
room were kept at 10 lux and 70 dB respectively. Behavioral measures included
the distance travelled by the mouse; the duration of the time spent, the distance
travelled and the number of entries in the center quadrant (calculated as
percentage of total distance travelled by the mouse).
Novel cage exploration. Testing was conducted in a dimly lit (9 lux) Makrolon
cage (35 x 28 cm), with 5 cm of fine sawdust. Mice were placed in the center
and allowed to freely explore the cage for 5 min. Behavioral measures include:
frequency and time spent digging; number and duration grooming; rears;
locomotor activity (defined as the number of crossings on a grid superimposed
onto the image of each cage in a video monitor).
Object interaction. We used a modified version of the protocol described by
Godar et al (Manuscript in press). We used a dimly lit (8 lux) gray Plexiglas cubic
box (20 x 20 x 20 cm) containing two novel black plastic cylinders (8 cm tall x 3.5
cm in diameter), affixed to the floor and symmetrically placed at 6 cm from the
two nearest walls. Mice were placed in a corner, facing the center, and at equal
distance from the two objects for 5 min. The start position was rotated and
counterbalanced for each genotype and condition throughout the tests. For each
exploration trial, we analyzed the total duration and number of exploratory
approaches, the latency to the first exploratory approach as well as wall and
69

object-directed rearing behaviors. The locomotor activity (defined as the number
of crossings on a grid superimposed onto the image of each cage in a video
monitor) was also measured. Exploration was defined as sniffing or touching
objects with the snout; climbing or sitting on the object was not considered
exploration.
Wire-beam bridge test. The experiment was conducted with a variation of the
protocol employed in Bortolato et al (2009a), specifically readapted to capture
risk-assessment and emotional responses to an objective risk. The apparatus
consisted of two square platforms (10 x 10 x 20 cm in height) connected by a
horizontal, unrailed wire-beam bridge (1.25 x 40 cm). Mice were individually
placed on the center of the bridge and allowed to freely explore the bridge until
they reached a platform. The latency to initial movement (defined as the first
goal-directed step) and the latency to traverse the bridge were measured.
70

3.4: Results
Restraint stress-induced hyperthermia. MAO A-deficient mice subjected to
restraint displayed a decrease in the number of fecal boli (Fig. 3.1a) [F(1, 27) =
4.34; P<0.05] compared to WT mice under the same conditions. In parallel with
these findings, MAO A KO mice exhibited a significantly different hyperthermic
response to stress (Fig. 3.1b) [main-effect for genotype x condition: F(1, 44) =
6.13; P<0.05]. Post-hoc analysis revealed that while both genotypes had similar
baseline temperatures, restraint stress-induced a significant decrease in
temperature in MAO A KO mice (P<0.05) compared to their non-restraint
counterparts.
Open field. MAO A KO mice displayed significantly less locomotor activity than
their WT counterparts (Fig. 3.1c) [main-effect for genotype: F(1, 44) = 4.28;
P<0.05]. Conversely, the time spent in the center and percent locomotor activity
in the center were comparable between genotypes and treatment conditions.
Novel cage paradigm. Although both genotypes displayed a similar frequency
and duration of grooming and digging activity, the quality of these behaviors were
substantially different and could not be captured by video recording. MAO A KO
mice subjected to restraint engaged in purpose-less grooming bouts, which
appeared to originate as a response to the confinement itself, rather than the
stress associated with restraint. Conversely, WT mice under the same
conditions were wetter and dirtier than their mutant counterparts, following
removal from the restraint tube. This may be a direct result of the differences in
71

hyperthermia between the two genotypes. Nevertheless, WT mice exhibited
grooming behavior until clean.
Object interaction task. Restraint stress induced significantly different
exploratory responses between MAO A KO and WT mice in both overall
exploratory approaches (Fig. 3.1d) [main-effect for genotype x condition: F(1, 43)
= 8.44; P<0.01] and duration (Fig. 3.1e) [main-effect for genotype x condition:
F(1, 41) = 6.55; P<0.05]. Post-hoc analysis revealed that non-restrained MAO A
KO mice showed a decrease in exploratory frequency compared to WT mice
under the same experimental conditions, however, this effect did not reach
significance (P<0.07). Conversely, while WT mice subjected to restraint stress
exhibited a significant reduction in exploratory approaches (P<0.001) and
duration (P<0.01) compared to their non-restrained WT littermates, these
parameters were unaffected by experimental conditions in MAO A KO mice.
Wire-beam bridge test. MAO A KO mice showed a significantly lower latency to
first movement (data not shown) [main-effect for genotype: F(1, 42) = 8.24;
P<0.01] than WT mice. In line with these findings, MAO A-deficient mice
exhibited a significant reduction in latency to escape the bridge (Fig. 3.1f) [H(3) =
19.38; P<0.001] compared to WT littermates. Post-hoc analysis revealed that
non-restrained MAO A KO mice displayed a significantly lower latency to escape
the bridge (P<0.05) than non-restrained WT mice. Moreover, while WT mice
subjected to restraint stress showed a significantly increased latency to escape
72

the bridge (P<0.05) compared to their non-restrained counterparts, MAO A-
deficient mice displayed similar latencies to escape irrespective of conditions.

Figure 3.1. MAO A KO mice display a reduction in behavioral responses to acute restraint stress.
(a) MAO A-deficient mice subjected to restraint exhibit a decrease in the number of fecal boli
compared to WT mice under the same conditions. (b) MAO A KO mice show a reduction in
hyperthermia compared to non-stressed counterparts, indicating a resistance to stress. (c) MAO
A KO mice displayed equivalent locomotor activity in both stress and non-stressed conditions. (d-
e) Although restraint induced a significant reduction in exploratory behaviors in WT mice, MAO A
KO mice showed similar exploratory responses to objects irrespective on stress exposure. (f)
Similarly, MAO A-deficient mice subjected to restraint did not alter their behavioral response in
the wire-beam bridge test. All values are represented as means ± SEM. *P<0.05, **P<0.01, and
***P<0.001 compared to WT mice.
#
P<0.05 compared to MAO A KO mice not subjected to
restraint stress.

73

3.5: Discussion
The major finding of this study is that MAO A KO mice display an attenuated
behavioral response to acute restraint stress. Compared to WT littermates, MAO
A-deficient mice exhibited a significant reduction in hyperthermia - a physiological
index of stress reactivity (Groenink et al., 1994; Groenink et al., 1995; Mitsukawa
et al., 2009). In parallel to these findings, MAO A KO mice did not engage in any
noticeable behavioral alterations between stressed and non-stressed groups.
Conversely, restraint stress induced a significant increase in anxiety-related
parameters (digging and grooming behaviors in the novel cage paradigm) in WT
mice. Of note, while restraint reduced the exploratory behavior in WT mice, MAO
A-deficient mice displayed similar exploratory approaches and duration to novel
objects in the object interaction test. Collectively, MAO A KO mice show an
absence of behavioral modification following severe acute stress, further
supporting a role of this enzyme in stress reactivity and stress-coping. Moreover,
these data heavily suggest that MAO A-deficiency decreases behavioral flexibility
and disturbs adaptive responses to changes in contextual contingencies.
These findings are in substantial agreement with previous reports documenting
that partial or complete MAO A deficiency blunts corticosterone activation in
response to severe stressors (Reul et al., 1994; Popova et al., 2006; Jabbi et al.,
2007). Moreover, reduced autonomic function has been strongly associated with
antisocial aggression (Shoal et al., 2003; Kim and Haller, 2007), a well-
74

established feature accompanying developmental MAO A deficiency (Whitaker-
Azmitia et al., 1994; Cases et al., 1995; Mejia et al., 2002; Scott et al., 2008).
Interestingly, MAO A-deficient mice exhibit several behavioral disturbances that
are highly reminiscent of stress-induced abnormalities, including antisocial
behavior (Scott et al., 2008), disordered sleep (Real et al., 2007), anxiety, poor
decision-making and maladaptive responses to contextual cues (Godar et al,
Manuscript in press), as well as increased compulsive behavior and deficits in
working memory and behavioral flexibility (see Chapter 4; M. Grace Baron,
2006).
Previous studies have shown that MAO A-deficient mice display a reduction in
immobility in the forced swim test (Cases et al., 1995) and an increase in
struggling behavior in the tail-suspension tests (Scott et al., 2008), well-validated
paradigms for depression (Bortolato et al., 2008). Moreover, MAO inhibitors are
mood-enhancing agents typically used to combat moderate to severe depression
in patients. Based on the dissociative effects of anti-depressant therapies, the
reduction of MAO A may contribute to an emotional flattening of behavior, a
phenotypic trait typically found in schizophrenia, as well as PTSD (Danckwerts
and Leathem, 2003; Constans, 2005; Gur et al., 2006; Gur et al., 2007). Taken
together, these findings suggest that MAO A modulates stress perception, as the
absence of this enzyme restricts the behavioral responsiveness to potential
adversity.
75

It should be noted that these findings may initially appear at variance with
previous data, in which MAO A KO mice display enhanced marble burying
activity and digging both prior to and after marble exposure. This increase in
digging behavior may signify a type of perseverative response, or alternatively
may represent emotional reactivity or an active coping strategy to combat the
novelty associated with foreign objects (Sluyter et al., 1996). Indeed, prior studies
have shown that MAO A KO mice exhibit neophobia and deficits in threat
perception (Godar et al, Manuscript in press). Interestingly, active coping
strategies have been correlated with reduced neuroendocrinological activation,
which is mirrored in the MAO A KO line (Sluyter et al., 1996; Popova et al.,
2006). Nevertheless, both sets of results suggest that MAO A may contribute to
stress-related disease pathogenesis, possibly by the increase in reactive
oxidative species generated by MAO A metabolism. Alternatively, MAO A may
prematurely degrade monoamine neurotransmitters prior to the activation of
neuroprotective mechanisms, the initiation of molecular cascades to counteract
potential stress-induced insults, and the re-establishment of cellular homeostasis.
The effects of MAO A on autonomic regulation are likely caused by perturbations
in both 5-HT and NE. One possibility is that high levels of these two
monoamines may serve to alter the threshold of autonomic responsiveness in
MAO A-deficient mice and therefore act as a buffer to combat stress and
depression-related stimuli (Bortolato et al., 2008). Indeed, overactive HPA has
76

been frequently observed in patients with major depression (Nemeroff et al.,
1984; Banki et al., 1987; Axelson et al., 1993; Bale, 2005).
Although a wealth of evidence has implicated 5-HT in major depression, the
relationship is still unclear. For instance, ablation of serotonin transporter in
rodents does not protect against stress, as a large number of reports have found
that these animals display depressive-like responses (Kalueff et al., 2010).
Alternatively, aberrations in 5-HT levels during development may increase the
vulnerability to depression through alterations in emotional circuitry (Ansorge et
al., 2004; Ansorge et al., 2008; Holmes, 2008). Indeed, tonically elevated
serotonergic tone has been associated with a number of behavioral abnormalities
including behavioral adaptation (Kahne et al., 2002), alterations in emotional
responses to stress (Whitaker-Azmitia, 2005), and autistic-like manifestations, as
well as perturbations in cortical functioning (Whitaker-Azmitia, 2005).
MAO may also act on stress responsivity through interactions with the
noradrenergic system (Morilak et al., 2005). NE has also been highly implicated
in the pathophysiology of stress-related disorders, including depression and post-
traumatic stress disorder (Southwick et al., 1993; Southwick et al., 1997; Krystal
and Neumeister, 2009), and several classes of antidepressant therapies also
target noradrenergic neurotransmission (Cryan et al., 2004; Lucki and O'Leary,
2004; Morilak et al., 2005). In consideration of its role in arousal, chronically
elevated NE levels may result in dysfunction between the emotional valence of a
stressor and the behavioral activation of arousal systems (Valentino et al., 1993;
77

Robbins et al., 1998; Koob, 1999; Simmons et al., 2009; Adenauer et al., 2010).
Indeed, reduction in stress reactivity has been reported in animals with elevated
NE levels due to the genetic blockade of the norepinephrine transporter (Xu et
al., 2000; Haller et al., 2002; Keller et al., 2006; Perona et al., 2008).
An alternative interpretation is that MAO A KO mice have abnormal stress
perception. Indeed, MAO A-deficient mice exhibit exaggerated aversive and
defensive responses to inanimate objects, while predator cues fail to elicit
behavioral changes (Godar et al, Manuscript in press). In line with these findings,
MAO A mutants showed a reduction in adrenocortical releasing hormone
following severe or chronic variable stress, indicating that these conditions did
not elicit robust HPA activation in this line in comparison to their WT counterparts
(Popova et al., 2006). Interestingly, both genotypes had equivalent adrenocortical
responses to psychosocial stress (Popova et al., 2006). Collectively, these
results suggest that MAO A KO mice have deficits in stress perception and
further support the contention that this line perceives intrusive elements, such as
foreign conspecifics as stressful.
In parallel to these results, a large number of patients with PTSD display
exaggerated reactions to mild or neutral stimuli (Pannu Hayes et al., 2009;
Brunetti et al., 2010), as well as deficits in response to threat cues (McFarlane et
al., 1993; Felmingham et al., 2003; Weber, 2008; Catani et al., 2009;
Rougemont-Bucking et al., 2010). Moreover, MAO A KO mice may have an
attentional bias towards fear-related stimuli as suggested by their impairment in
78

fear extinction memory (Kim et al., 1997); common traits in PTSD
symptomatology (Milad et al., 2009; Pannu Hayes et al., 2009). These alterations
may be underpinned by perturbations in neural circuitry underlying emotional
regulation (New et al., 2009; Pannu Hayes et al., 2009). Indeed, MAO A KO mice
show morphological disturbances in the somatosensory cortex (Cases et al.,
1995), a region involved in the integration of sensory modalities and perceptual
processing of external stimuli (Hurwitz et al., 1990; Cases et al., 1996; Sanders
et al., 2001; Dowman and Ben-Avraham, 2008; Straube et al., 2009). The
absence of behavioral changes following restraint stress in MAO A-deficient mice
may also signify behavioral rigidity and that this line has a limited range of
behavioral responses to contextual cues.
Although this study does not address the neurobiological underpinnings that
govern the alterations in stress reactivity in MAO A KO mice, these findings
provide further support that MAO A plays an important role in stress-responses.
Future studies should examine the effects of chronic restraint stress on
behavioral regulation, as well as manipulation of 5-HT and NE during critical
developmental time periods. An intriguing possibility is also to test the NMDA
receptor partial agonist cycloserine on the behavioral responses in MAO A KO
mice. Since NMDA receptors are modulated by monoamines and have
reciprocal interactions with the HPA system, it is possible that the deficits in
stress perception are underpinned by alterations in NMDA receptor function. -
Nevertheless, this chapter supports the contention that ablation of MAO A
79

impairs cortical control mechanisms and perceptual functions, as well as
significantly restricts the behavioral repertoire leading to an overall maladaptive
phenotype.
80

Chapter 4 : Hypomorphic monoamine oxidase A-
deficient mice exhibit compulsive behaviors and reduced
aggression in mice

4.1: Abstract
Monoamine oxidase (MAO) A is a key enzyme for the degradation of brain
serotonin (5-hydroxytryptamine, 5-HT) and catecholamines. In humans and mice,
MAO A deficiency results in high levels of 5-HT and elevated reactive
aggression. Although alterations in brain MAO A has been posited to modulate
the pathophysiology of obsessive-compulsive disorder (OCD), evidence in this
respect is still elusive. To this end, we characterized a newly generated line of
hypomorphic MAO A mutants, MAO A
neo
mice. We also tested their behaviors in
well-validated models of anxiety-like behavior, aggression and compulsivity.
MAO A
neo
mice displayed perseverative responses and emotional alterations
reminiscent of OCD-related manifestations, such as compulsive marble-burying
and water mist-induced grooming, open field thigmotaxis and locomotor
tortuosity. These responses were corrected by the 5-HTT blocker fluoxetine (10
mg/kg, i.p.), a benchmark treatment for OCD. In contrast with MAO A knockout
(KO) mice, MAO A
neo
conspecifics did not exhibit high levels of resident-intruder
aggression. Our findings indicate that MAO A hypomorphism results in
compulsive behaviors, and provide insights into the role of MAO A and 5-HTT in
the regulation of emotional reactivity and OCD pathophysiology.

81

4.2: Introduction
Although the previous chapters have shown that MAO A deficiency results in
alterations in threat perception and stress reactivity, the severity and complexity
of their behavioral abnormalities are likely to mask other more subtle emotional
deficits. Indeed, while MAO A KO mice display overt aggressiveness, the
intensity of these antisocial behaviors may obscure other behavioral alterations.
An attractive approach to avert this difficulty is the development of hypomorphic
mutations of the Maoa gene, resulting in a lower penetrance of the abnormal
phenotypes associated with MAO A deficiency. In view of the critical role of this
enzyme in emotional reactivity and impulse control (Manuck et al., 2000; Caspi et
al., 2002; Jacob et al., 2005; Meyer-Lindenberg et al., 2006; Brummett et al.,
2008; Buckholtz and Meyer-Lindenberg, 2008) and the low prevalence of MAO A
deficiency, the generation of a hypomorphic MAO A line may provide a more
optimal model with higher clinical relevance to study the effects of low MAO A on
behavioral outcomes.
Here we report the characterization of MAO A
neo
mice, a novel line of
hypomorphic MAO A transgenic animals. A parallel study showed that MAO A
neo

mice display low levels of MAO A enzymatic activity, normal 5-HT concentrations
and decreased 5-HT transporter (5-HTT) in the prefrontal cortex and amygdala.
This particular neurochemical profile is conducive to a spectrum of behavioral
abnormalities remarkably different from those exhibited by MAO A KO mice.
Specifically, MAO A
neo
mutants show reduced levels of aggression, as well as
82

perseverative manifestations and emotional alterations isomorphic with OCD
symptoms.
83

4.3: Methods
Animal husbandry. Throughout the study we used adult (2-3 months old) male
129S6 mice, weighing between 25 and 35 g. MAO A
neo
mice were compared
with age-matched WT and MAO A KO mice on 129S6 genetic background. In
particular, for MAO A KO mice, we used MAO A
A863T
KO mice, which display a
spontaneous point nonsense mutation strikingly similar to that featured in
Brunner syndrome (Scott et al., 2008). Each line was bred with homogenotypic
pairs and backcrossed to 129S6 every 3 generations.
The colony room was maintained at approximately 22°C with a 12h:12h light:dark
cycle with lights off at 6:00 pm. All procedures used in the present study were in
compliance with the National Institute of Health guidelines, and the protocols
were approved by the University of Southern California Animal Use Committee.
To minimize litter effects, mice from at least three different litters were used in
each test.
Open field. Analysis of locomotor activity was performed as described in Scott et
al (Scott et al., 2008). The open field was a Plexiglas square grey arena (40 x 40
cm) surrounded by 4 black walls (40 cm high). On the floor, two zones of
equivalent areas were defined: a central square quadrant of 28.28 cm per side,
and a concentric peripheral frame including the area within 11.72 cm from the
walls. Light and sound were maintained at 10 lux and 70 dB respectively. On
three consecutive days, mice were placed in the central zone and their behavior
was monitored for 5 min. Analysis of locomotor activity, the duration of the time
84

spent and the distance travelled in both the center quadrant and in the peripheral
frame, and meandering were performed using Ethovision (Noldus Instruments,
Wageningen, The Netherlands). Percent locomotor activity in the center was
calculated by the distance traveled in the center over the total distance traveled.
Tortuosity was defined as the average ratio of the total distance over the
summation of the beelines between the points captured every 0.16 sec (arc-
chord ratio).
Resident-intruder test. Following isolation in single cages for 7 days, both
resident and intruder mice were exposed to an experimental room as previously
described (Chen et al., 2007). Resident mice were then exposed to intruder
males carrying the same genotype, but from different litters, for 5 min. Although
the duration of the resident-intruder test was 300 s for all mice, whenever
animals did not engage in any aggressive behavior within this timeframe, they
were left in the cage until they exhibited a first attack. This protocol variation was
devised to better capture possible differences in attack latency without bias
resulting from “ceiling effects”. Behavior was video-monitored from an adjacent
room, recorded and scored by a trained observer unaware of the genotype.
Measures included: (i) attack latency, (ii) attack duration (total), (iii) number of
aggressive bouts, (iv) total locomotor activity, measured as number of crossings
on a grid superimposed onto the image of each cage in the video monitor).
Marble burying. Marble burying was conducted as previously described
(Bortolato et al., 2009a). Briefly, mice were placed in a Makrolon cage (35 x
85

28 cm), in which fine sawdust was placed to a depth of 5 cm. Light and
background noise levels in the room were kept at 5 lux and 70 dB. Mice were
placed individually in the cages for 30 min for habituation. Subsequently, mice
were briefly removed and 20 marbles (1 cm diameter) were placed in each cage
at even distances apart on top of the sawdust. Mice were placed into the cages
again and left undisturbed for 30 min. Their behavior and the number of buried
marbles were video recorded and monitored by two independent observers blind
to the genotype of the animals. Inter-rater reliability was tested by Cohen‟s kappa
coefficient and always higher than 0.85. A marble was considered buried if at
least two-thirds of its surface area was covered in sawdust. Locomotor activity
was analyzed by counting the crossings of a grid (5 x 4 squares) superimposed
onto the image of each cage in the video monitor.
Water mist-induced grooming. Water mist-induced grooming was analyzed as
described in previously (Hill et al., 2007). Briefly, mice were habituated to a
Makrolon cage (20 x 10 cm) in the experimental room for 30 min. Light and
sound were maintained at 5 lux and 70 dB. Following verification (for 5 min) that
mice did not exhibit any spontaneous grooming, they were subjected to two
squirts of sterile water mist spray. Animals were then left undisturbed in the
experimental room, and their grooming activity was recorded and monitored for a
period of 30 min immediately after spraying. Overall grooming duration and
frequency of grooming initiations were then analyzed by two independent
observers blinded to the mouse genotype, and values were averaged for
86

statistical analysis. Inter-rater reliability, as tested by Cohen‟s kappa coefficient,
was always higher than 0.85.
Elevated plus-maze. The elevated plus-maze test was conducted as previously
described (Bortolato et al., 2009a). The apparatus was constructed from black
Plexiglas with a light grey floor and was comprised of two open arms (25 × 5 cm)
and two closed arms (25 × 5 × 5 cm) that extended from a central platform
(5 × 5 cm) at 60 cm from the ground. Behavioral analysis was performed by an
observer unaware of the genotype. Mice were placed individually in the center
square facing an open arm and allowed to explore the maze for 5 min. Light and
background noise in the room were kept at 10 lux and 70 dB. An arm entry was
counted only when all four paws were inside the arm. Measures scored included:
time spent in open and closed arms and in the central platform; number of open
and closed arm entries.
Statistical analyses. Normality and homoscedasticity of data distribution were
verified by using the Kolmogorov-Smirnov and Bartlett‟s test. Parametric
analyses were performed with One- or Two-way ANOVAs, as appropriate,
followed by Tukey‟s test with Spjotvoll-Stoline correction for post-hoc
comparisons of the means. Non-parametric analyses were performed by Kruskal-
Wallis test, followed by Nemenyi‟s test for post-hoc. Significance threshold was
set at 0.05.
87

4.4: Results
Open field. We analyzed the behavioral responses of WT, MAO A
neo
and MAO A
KO mice in a number of tests aimed at capturing different aspects of their
emotional reactivity. In a novel open field, both MAO A
neo
and MAO A KO mice
exhibited significantly lower locomotor activity than WT counterparts (Fig.
4.1b) (P<0.001, ANOVA) throughout the three days of experiments, although
MAO A KO covered a significantly greater distance than MAO
A
neo
(P<0.05, Tukey‟s). MAO A
neo
mice exhibited an enhancement in anxiety-
related behaviors with relation to the other genotypes, as shown by their
reductions in time (P<0.05) (Fig. 4.1c) relative locomotor activity (P<0.001) (Fig.
4.1d) and entries (P<0.001) in the central quadrant of the arena. In comparison
with both WT and MAO A KO mice, the locomotor performance of MAO
A
neo
mice was characterized by significant enhancements in tortuosity,
meandering and perseverative counter-clockwise turning (Fig. 4.1e, f).
Elevated plus-maze. In contrast with the enhanced thigmotaxis in the open-field
paradigm, MAO A
neo
did not manifest any significant anxiety-related alteration in
the percent time and entries in the open arms of an elevated plus maze, in a
fashion similar to WT and MAO A KO mice (data not shown).
Marble-burying and Water mist-induced grooming. We then tested MAO
A
neo
mice in two well-validated models of compulsive behavior, marble burying
and water-mist induced grooming. MAO A
neo
mice displayed higher marble-
88

burying (Fig. 4.1g) and digging behavior (Fig. 4.1h) than WT and MAO A KO
conspecifics (P<0.001). Similarly, while no difference in baseline grooming
activity was observed across the three genotypes, water mist induced a
significant increase in grooming bouts (Fig. 4.1i) (P<0.05) and duration (Fig. 4.1j)
(P<0.01) in MAO A
neo
, as compared to WT and MAO A KO counterparts.
89

Figure 4.1. MAO A
neo
mice exhibit a unique spectrum of emotional alterations. (a) Locomotor
pathways of WT, MAO A
neo
, and MAO A KO mice in the open field. (b) MAO A
neo
mice spend
less time in the central quadrant than both WT and MAO A KO mice. (c) Although MAO
A
neo
exhibit significantly lower locomotor activity, (d) this reduction did not affect the thigmotactic
behavior as the percent locomotor activity in the central zone was significantly reduced in MAO
A
neo
mice compared to their counterparts. In addition, MAO A
neo
mice showed a significant
increase in (e) tortuosity and (f) meandering in the open field. MAO A
neo
mice engaged in higher
(g) marble-burying and (h) digging activities, as well as a more robust (i-j) grooming response to
water mist compared to their counterparts.
90

Resident-intruder. In the resident-intruder test, MAO A
neo
mice generally failed to
engage in fighting behavior within the first five minutes of observation, with an
actual average latency to attack of 425.30 ±60.53 s. Notably, this measure was
significantly higher (Fig. 4.2a) (P<0.001, Kruskall-Wallis) than their
counterparts. Post-hoc analysis revealed a significant difference between MAO
A
neo
and WT (P<0.05, Nemenyi‟s) and MAO A KO (P<0.001) mice. Furthermore,
MAO A
neo
mice showed significantly lower frequency (Fig. 4.2b) (P<0.001,
ANOVA; Tukey‟s test) and duration (Fig. 4.2c) (P<0.001; ANOVA; Tukey‟s test)
of fighting behavior than their null-allele conspecifics. To verify whether the lower
levels of aggression could reflect the higher levels of anxiety-like responses in
MAO A
neo
mice, we repeated the experiment with a different set of animals under
dim-light (10 lux) conditions (Fig. 4.2d-f), since this condition has been shown to
attenuate the emotional impact of environmental stressors in rodents (Ramos
and Mormede, 1998). In agreement with our hypothesis, MAO A
neo
mice did
exhibit significantly lower latency (P<0.05) (Fig. 4.2d) and higher duration
(P<0.01) (Fig. 4.2f) than WT controls. Nevertheless, they still displayed
significantly lower levels of aggressiveness – higher latency to attack (P<0.05)
and lower fighting bouts and duration (P<0.05) – than MAO A
A863T
KO mice (Fig.
4.2d-f). No differences in locomotor activity were detected during the test, ruling
out the possibility of experimental artifacts.
91

Figure 4.2. MAO A
neo
mice exhibit context-dependent alterations in aggressiveness. MAO
A
neo
mice showed significant increases in (a) latency to attack intruders compared to WT and
MAO A KO mice. Furthermore, MAO A
neo
mice featured a significant reduction in (b) fighting
bouts and (c) fighting duration compared to MAO A KO mice under normal light conditions.
Conversely, MAO A
neo
mice displayed a significant decrease in (d) attack latency, as well as an
increase in (f) fighting duration in dim lighting compared to WT mice. (e) Fighting bouts were
equivalent between MAO A
neo
and WT littermates. Similar to the normal light conditions, MAO A
KO mice exhibited more robust (d-f) aggressiveness than MAO A
neo
and WT counterparts in all
parameters. Values are represented as mean ± SEM. *P<0.05, **P<0.01, ***P<0.001 compared
to WT mice and
#
P<0.05,
##
P<0.01,
###
P<0.001 compared to MAO A
neo
mice.
Fluoxetine corrects the open-field anxiety and compulsive behaviors in MAO
A
neo
mice.
The abnormal behavior of MAO A
neo
mice in the open field was reversed by
fluoxetine, the 5-HTT blocker commonly employed in OCD treatment (Fig. 4.3a).
Specifically, this drug increased the time and the relative distance in the center
by MAO A
neo
mice (P<0.01 in comparison with saline-treated MAO A
neo
mice)
(Fig. 4.3c-d) without affecting their locomotor activity (Fig. 4.3b). The same
treatment ablated locomotor tortuosity in the open field (P<0.05, ANOVA) (Fig.
92

4.3e), marble-burying and digging activity (P<0.01, 2-way ANOVA, Tukey‟s test)
(Fig. 4.3f-h) and water-mist induced grooming in MAO A
neo
mice (P<0.05; 2-way
ANOVA, Tukey‟s test) (Fig. 4.3i).
Figure 4.3. OCD-like traits in MAO A
neo
mice are attenuated by fluoxetine treatment. (a)
Locomotor patterns in the open field following fluoxetine administration. Although (b) locomotor
activity was unaffected by fluoxetine treatment in both genotypes, MAO A
neo
mice showed a
significant reduction in locomotion and the (c) time spent in the center compared to WT mice.
Conversely, fluoxetine treatment significantly increased (d) percent locomotor activity in the
center and attenuated the stereotyped (e) thigmotactic behavior in MAO A
neo
mice. 5-HTT
blockade significantly reduced the perseverative (f) marble burying, (g-h) digging, and (i)
grooming behaviors in MAO A
neo
mice. Values are represented as mean ± SEM. P<0.05 and
  P<0.001 compared with WT mice (main effect for genotype). *P<0.05, **P<0.01, ***P<0.001
compared to WT mice and
#
P<0.05,
##
P<0.01,
###
P<0.001 compared to MAO A
neo
mice.
93

4.5: Discussion
Our lab characterized a newly generated line of MAO A hypomorphic transgenic
mice. Previous studies showed that this line exhibited normal 5-HT levels in the
prefrontal cortex and amygdala, but high 5-HT in the hippocampus, striatum, and
midbrain regions (unpublished data, courtesy of Dr. Jean Shih). Phenotypical
analyses revealed that these hypomorphic mutants show a spectrum of
behavioral alterations and neurochemical changes reminiscent of OCD and
remarkably distinct from those observed in MAO A KO conspecifics. MAO A
neo

mice feature a unique set of perseverative behaviors and emotional
abnormalities in well-validated models of OCD. The isomorphism of these
disturbances with pathognomonic symptoms of OCD and their sensitivity to
fluoxetine, a benchmark therapy for this disorder, provide compelling criteria of
face and predictive validity in support of MAO A
neo
mice as a dependable model
of OCD. Of note, MAO A
neo
mice exhibit a reduction of reactive aggression in the
resident-intruder paradigm compared to MAO A KO mice. Data from a parallel
study showed that these behavioral abnormalities were underpinned by low WT
Maoa mRNA in the brain, in association with low MAO A enzymatic activity,
normal 5-HT levels and reduced 5-HTT expression in the amygdala and
prefrontal cortex. Conversely, they featured a biochemical profile akin to their
null-allele counterparts in the hippocampus and midbrain.
Taken together, these results substantiate and complement previous clinical
evidence suggesting a possible association between low MAO A activity
94

polymorphic variants and OCD (Camarena et al., 2001; Kim and Kim, 2006). In
addition, our findings suggest that the role of low MAO A in the pathophysiology
of OCD may be mediated by dysregulations of 5-HT neurotransmission and
reuptake in specific forebrain regions. Indeed, changes in 5-HTergic signaling
have been implicated in the manifestation of OCD traits (Denys et al., 2003;
Stengler-Wenzke et al., 2004; Hesse et al., 2005; Hasselbalch et al., 2007;
Reimold et al., 2007; van Dijk, 2010).
In line with previous evidence (Moens et al., 1992; Barrow and Capecchi, 1996;
Nagy et al., 1998; Rucker et al., 2000; Lewandoski, 2001; Manley et al., 2001),
the retention of a foreign Neo
R
cassette in the intron of Maoa gene resulted in
altered mRNA splicing and reduced levels of functional transcript. In particular,
our sequencing analysis suggests that the presence of PGK1 promoter in the
Neo
R
cassette may interact with the spliceosome, thereby interfering with the
recognition of the correct intervening sequence between exon 12 and intron 12 of
the Maoa gene. These post-transcriptional alterations originate a non-functional
chimeric MAO A variant, with a truncated C-terminus and thus unable to anchor
to the mitochondrial outer membrane (Rebrin et al., 2001). Small amounts of
Maoa transcript, however, escape the aforementioned splicing aberrances,
resulting in low expression of functional transcript throughout the brain
(unpublished data, courtesy of Dr. Jean Shih). While the suboptimal quality of the
available anti-mouse MAO A antibodies did not allow us to document the
expression of the protein across different brain regions, we detected low amounts
95

of MAO A enzymatic activity in the amygdala and prefrontal cortex (unpublished
data, courtesy of Dr. Jean Shih). The mismatch between the presence of mRNA
and the lack of activity in other regions likely reflects the possibility that the levels
of functional enzyme in most regions of MAO A
neo
mice may have remained
below the detection threshold of our assay. The variations of MAO A transcript
and activity across different regions indicate the diverse ability of each brain area
to modulate splicing and post-transcriptional processing, possibly in relation to
their specific neurochemical milieu and/or their responsiveness to contextual
factors (Meerson et al.; Gattoni et al., 1996; Battaglia and Ogliari, 2005;
Meshorer et al., 2005; Lai and McCobb, 2006). This last hypothesis is particularly
intriguing, in view of the role of environmental stress in the functional modulation
of the amygdala and prefrontal cortex.
The expression of modest amounts of functional MAO A enzymes in the
amygdala and - to a lower degree - prefrontal cortex resulted in normal 5-HT
levels in both regions (unpublished data, courtesy of Dr. Jean Shih). Additionally,
amygdalar NE levels, albeit significantly higher than WT content, were lower than
those in MAO A KO mice (unpublished data, courtesy of Dr. Jean Shih). The
impact of small amounts of MAO A on NE is likely more limited than that on 5-
HT, in view of the much lower affinity (Strolin Benedetti et al., 1983), as well as
the important role of catecholamine-O-methyl-transferase in NE degradation.
Of note, the regions of MAO A
neo
mice with 5-HT levels comparable to WT mice
(amygdala, prefrontal cortex and striatum) also featured a marked down-
96

regulation of 5-HTT (unpublished data, courtesy of Dr. Paola Castelli).
Furthermore, in line with previous findings, MAO A KO mice (Evrard et al., 2002)
showed lower 5-HTT levels in specific compartments of each area. The
reduction in 5-HTT expression may reflect a homeostatic mechanism directed at
avoiding 5-HT accumulation in presynaptic terminals due to the low MAO A
activity. Our present data cannot fully explain the higher degree of 5-HTT
decrease in MAO A
neo
mice as compared to their KO counterparts; nevertheless,
these findings may suggest the greater ability of MAO A
neo
mice to enact
compensatory processes aimed at the modulation of synaptic 5-HT
concentrations. Alternatively, the reduced expression of 5-HTT in MAO A
neo
mice
may represent an impoverishment in 5-HT fibers; this last possibility, however, is
partially challenged by the lack of significant variations in tryptophan hydroxylase
immunoreactivity in the dorsal raphe across the three genotypes (unpublished
data, courtesy of Dr. Rick Lin).
The neurochemical changes of MAO A
neo
mice were accompanied by a set of
unique behavioral alterations, typically distinct from those featured by MAO A KO
mutants. In a novel inescapable open field, MAO A
neo
mice displayed reduced
locomotor activity and enhanced thigmotaxis (signified by the lower time and
percent activity in center). While this last behavior is generally interpreted as
representative of high levels of anxiety (Belzung, 1999; Prut and Belzung, 2003),
this possibility is partially challenged by the lack of overt anxiety-like responses
(such as reduced open arm duration and percent entries) in the elevated plus
97

maze by MAO A
neo
mice . Alternatively, the wall-hugging behavior in this line may
partially reflect a more generalized alteration in their exploratory strategy,
perhaps influenced by a tendency to enact perseverative responses, as shown
by their high levels of meandering and path tortuosity. Notably, the abnormal
responses observed in MAO A
neo
mice in the open field are akin to those featured
by other well-validated murine models of OCD (McGrath et al., 1999; Kalueff et
al., 2010).
The difference between MAO A
neo
and MAO A KO mice was also apparent in
their divergent levels of reactive aggression. Resident MAO A
neo
mice exhibited a
longer latency to attack intruders than both WT and MAO A KO mice, as well as
significantly reduced duration of offensive behaviors in comparison with MAO A
KO conspecifics under normal light conditions. This result represents a
remarkable exception to the well-documented association between low (or no)
MAO A activity and antisocial behavior and extends our previous finding that
restoration of MAO A in forebrain regions normalizes the local monoamine levels
and rescues the aggression in MAO A KO mice (Chen et al., 2007). The
pronounced reduction in aggressive behavior in comparison with WT, however,
was reversed by the attenuation of light intensity, a well-documented anxiogenic
contextual factor, suggesting that the failure to attack intruders in MAO A
neo
mice
may reflect higher levels of anxiety. Taken together, these results suggest that
MAO A
neo
mice display context-dependent alterations in the regulation of
emotional reactivity and responsiveness to potentially threatening situations.
98

MAO A
neo
mice showed significant increases in compulsive responses in the
marble-burying and water-mist-induced grooming tests, two well-validated
behavioral models of pathognomonic OCD-related manifestations (hoarding and
cleaning rituals) (Hill et al., 2007; Thomas et al., 2009b). The abnormalities in
open field and perseverative responses were all corrected by fluoxetine,
highlighting the predictive validity of MAO A
neo
mice as an OCD model.
In keeping with our results, previous studies indicated an association between
low-activity MAO A polymorphic variants and OCD in several populations of
patients (Karayiorgou et al., 1999; Camarena et al., 2001; Kim and Kim, 2006)
and reporting lower 5-HTT expression in several brain regions of OCD patients
(Stengler-Wenzke et al., 2004; Hesse et al., 2005; Hasselbalch et al., 2007;
Voyiaziakis et al., 2009). Although the reduction in 5-HTT levels likely plays a
role in the behavioral abnormalities of MAO A
neo
mice, the mechanisms of this
involvement remain partially elusive, as the blockade of this target by fluoxetine
reversed the OCD-like manifestations in these mutants. Indeed, previous findings
in humans suggest that the downregulation of 5-HTT in OCD patients may not be
a primary pathological factor in this disorder, but rather a compensatory
phenomenon aimed at the attenuation of the dysregulations in 5-HTergic
mechanisms in OCD (Hesse et al., 2005).
The emotional alterations and compulsive behaviors featured in MAO A
neo
mice
may be underpinned by region-specific alterations of monoaminergic
neurotransmission. Specifically, we found that the increase in catecholamines
99

was not paralleled by a commensurate augmentation in 5-HT levels in the
prefrontal cortex, amygdala, and striatum. This imbalance may lead to the unique
set of behavioral abnormalities observed in MAO A
neo
mice; accordingly, previous
evidence showed that OCD is associated with perturbed interactions between 5-
HTergic and catecholaminergic systems in the cortico-limbic circuitry (Szeszko et
al., 1999; Micallef and Blin, 2001; Denys et al., 2004; Szeszko et al., 2004; Kontis
et al., 2008; Fineberg et al., 2010).
Alternatively, the dysregulated responses to anxiogenic environmental stimuli in
MAO A
neo
mice may reflect select region-specific mechanisms. While rich
evidence documents a predominant role of prefrontal cortex, striatum and
amygdala in the pathophysiology of OCD (Baxter et al., 1988; Swedo et al.,
1989; Abbruzzese et al., 1995; Szeszko et al., 1999; Carlsson, 2001; Cannistraro
et al., 2004; van den Heuvel et al., 2004), other regions, such as ventral
hippocampus or midbrain nuclei, have been shown to exert nodal functions in
anxiety-related and emotional responses. Finally, variations in the penetrance
and expressivity of the hypomorphic mutation may depend on a set of numerous
factors, including developmental and environmental components: for example,
the mutated Maoa-neo chimeric transcript (or protein), albeit non-functional, may
still serve other modulatory functions by interacting with other molecular targets,
and modify the phenotypical outcomes of MAO A deficiency.
100

Irrespective of these possibilities, our study provides compelling evidence
supporting a link between low MAO A and compulsivity and other OCD-related
manifestations. Moreover, our results highlight the value of MAO A
neo
mice as a
dependable OCD model. Delineation of the region-specific roles of MAO A in the
regulation of behavioral responses in this model will be critical in defining the
involvement of MAO A in OCD and other related illnesses, such as Tourette
syndrome and autism-spectrum disorders.
101

Chapter 5 : Monoamine oxidase A-deficient mice
display autistic-related features
5.1: Abstract
Rich evidence has documented that early developmental abnormalities of
serotonin play a key role in the pathophysiology of autism-spectrum disorders
(ASD). Specifically, high levels of 5-HT have been reported in a large fraction of
autistic patients. Moreover, low activity in monoamine oxidase (MAO) A, the
primary catabolic enzyme for 5-HT degradation, has been associated with an
increasing severity of ASD disturbances. In order to examine the contributions of
MAO A in the manifestation of autistic phenomena, the present study tested
whether MAO A knockout (KO) mice exhibited autistic-like features in a set of
behavioral tasks aimed at capturing the major symptomatic domains of ASD.
MAO A-deficient mice displayed a marked reduction in maternal separation-
induced ultrasonic vocalizations, suggesting an impairment in communication.
In line with previous results, MAO A KO mice showed a profound increase in
aggression and antisocial responses towards conspecifics. To evaluate whether
MAO A KO mice exhibit a repetitive and rigid behaviors, we tested the
exploratory activity in two well-validated models of behavioral flexibility. MAO A-
deficient mice displayed a significant reduction in spontaneous alternations in the
T-maze, as well as perseverative bouts of exploration in the hole-board
paradigm, signifying a deficit in behavioral plasticity. These behaviors were
accompanied by a decrease in idiosyncratic responses to sensory stimuli and
102

maladaptive reactions to contextual cues. Collectively, these findings show that
MAO A-deficiency leads to the manifestation of behavioral disturbances strikingly
reminiscent to ASD-related features and point to MAO A KOmice as a novel
model of ASD.
103

5.2: Introduction
The studies in the previous chapters show that both MAO A KO and MAO A
neo

lines share a cluster of behavioral abnormalities, including perseveration and
context-dependent alterations in emotional reactivity and responsiveness to
potentially threatening situations, supporting the role of MAO A in emotional
regulation. Moreover, MAO A-deficient mice exhibit a restricted behavioral
repertoire, cortical dysmorphogenesis, deficits in aversive extinction, and delayed
developmental growth and maturation. Collectively, these behavioral features
are highly reminiscent to the behavioral disturbances associated with autistic-
spectrum disorders (ASD).
ASD are set of pervasive developmental disorders comprised of an array of
behavioral abnormalities featuring widespread disturbances in social interactions,
communication deficits, as well as a restricted repertoire of behavioral responses
and marked behavioral perseveration. In addition to these core symptoms,
patients with ASD exhibit a broad range of secondary features, which include
idiosyncratic sensory and emotional alterations, difficulties in mnemonic
extinction, and poor executive function.
Cogent evidence has documented that early developmental perturbations in the
serotonergic system play a critical role in the pathophysiology of ASD.
Specifically, a large percentage of autistic subjects show high levels of serotonin
(5-HT), which may result in abnormal growth patterns, alterations in brain
morphology and region-specific changes in neuronal circuitry in early
104

developmental stages (Anderson et al., 1987; Herault et al., 1996; Chugani et al.,
1999; Carper and Courchesne, 2000; Courchesne et al., 2001; Whitaker-Azmitia,
2001; Betancur et al., 2002; Courchesne et al., 2003; Carper and Courchesne,
2005; Wassink et al., 2007). Although serotonergic-related genes, such as the 5-
HT transporter (5-HTT) have been associated with ASD (Brune et al., 2006;
Wassink et al., 2007), they do not appear to be a major determinant in the
disease pathogenesis (Tordjman et al., 2001; Anderson et al., 2002; Ramoz et
al., 2006), and the molecular mechanisms underpinning the role of 5-HT in the
etiology of ASD are still highly elusive.
MAO A represents an attractive genetic target in ASD since MAO-deficiency in
humans has been shown to produce elevated levels of 5-HT and “autistic-like”
symptoms in patients, including stereotyped movements and cognitive
impairments (Shih and Thompson, 1999; Whibley et al., 2010). In line with these
findings, several preliminary studies have shown an association between low
activity of the MAO A and ASD severity (Cohen et al.; Chugani, 2002; Cohen et
al., 2003; Davis et al., 2008; Voyiaziakis et al., 2009). Taken together, these
data heavily suggest that the reduction or ablation of MAO A may be involved in
the pathophysiology of ASD features.
These findings prompted our group to study whether MAO A KO mice exhibit
autistic-like features in a number of behavioral models designed to capture major
symptomatic domains of ASD. Specifically, MAO A and A/B KO mice were
tested for core features of ASD, including: maladaptive social interactions; the
105

presence of repetitive, ritualistic behaviors; idiosyncratic responses to sensory
stimuli; risk-assessment and environmental anxiety. We also investigated
whether early developmental serotonergic hyperactivity contributes to the
perseverative behaviors exhibited by MAO A-deficient mice.

106

5.3: Methods
Animal husbandry. We used 3-5 month old, experimentally naïve male 129/Sv
mice, weighing 26-32 g. The MAO A
A863T
KO mutation - consisting of an X-
linked, single-point nonsense mutation in exon 8 of Maoa gene - arose
spontaneously in one of our 129/Sv in-house colonies (Perona et al., 2008). The
mutant mice were backcrossed to 129S6/SvEvTac for 12 generations to enhance
reliability for behavioral testing. MAOA
A863T
KO sires and heterozygous dams
were crossed to generate MAO A
A863T
KO and wild-type (WT) male littermates.
Offspring was genotyped as previously described (Scott et al., 2008). Animals
were housed in group cages with ad libitum access to food and water. The room
was maintained at 22°C, on a 12 h: 12 h light/dark cycle. Before behavioral
testing, all animals were found to display equivalent no patent physical and
neurological abnormalities. To avoid potential carryover effects, each animal was
used only once throughout the study. Mice were isolated for four weeks prior to
behavioral testing. Litter effects were minimized by using mice from at least
three different litters in each behavioral test. Experimental procedures were in
compliance with the National Institute of Health guidelines and approved by the
University of Southern California Animal Use Committees.
PCPA treatment. Pups were individually treated with 300 mg/kg of the tryptophan
hydroxylase inhibitor 4-chloro-DL-phenylalanine methyl ester (PCPA) (Sigma-
aldrich, St. Louis, MO) via i.p. injection from postnatal days 8-14.
107

Ultrasonic vocalizations in pups. Vocalizations were tested in pups on postnatal
day 6. Cages were acclimated to the room for 30 min. Light and background
noise were maintained at 5 lux and 70 dB using white noise. Pups were
individually placed on the test platform in a sound-proof cabinet. Ultrasonic
vocalizations were measured for 5 min using an ultrasonic vocalization detector
(Med-associates, St Albans, VT).
Resident-intruder. Resident intruder was performed as previously described
(Chen et al., 2004). Mice were isolated for 14 days prior to treatment. An age-
and weight-matched foreign male intruder was introduced to the cage for 10 min.
Light and background noise were maintained at 10 lux and 70 dB respectively.
Behavioral measures include the latency to attack and the frequency and
duration of fighting behavior.
Conditioned place aversion. To test the emotional responses of MAO A KO mice
to foreign objects, we used a conditioned-place avoidance paradigm, based on
the experimental protocol described in (Kuzmin et al., 2003). While generally
employed for drug-related interoceptive states, this test has also been
successfully used to evaluate aversive responses to environmental stimuli
(DeLorey et al., 2008). We used the same apparatus and contextual cues as
described in the social conditioned place preference task above. During phase I,
mice (WT = 6; MAO A KO = 8) were habituated to the apparatus for 15 min and
their initial side preferences were recorded. Phase II lasted 6 days and consisted
of 3 alternated presentations of two oval-shaped objects (10 cm tall x 6 cm wide
108

x 3 cm deep). Specifically, on odd days mice were positioned in the non-
preferred compartment (separated from the other by a guillotine door) for 30 min,
while on even days mice were positioned in the preferred compartment
containing the objects for the same amount of time. On the test day (phase III),
the animals were placed in the cage with free access to both sides for 15 min.
Placement was counterbalanced for each genotype. The total duration in each
compartment was measured. Object avoidance was calculated as the difference
between the time spent in the object-paired chamber in the test and the pretest
and the time spent in the non-object-paired chamber in the test and the pretest.
T-maze. The apparatus is comprised of a starting box (10 x 40 cm) with two arms
(10 x 20 cm) bifurcating from the top. The T-maze is constructed of black
Plexiglas and the floor of the T-maze was lined with paper towels to minimize
olfactory cues and replaced between trails. Light and sound was kept at 50 lux
and 70 dB respectively. A black plastic cup was used for transferring the mice to
minimize any inter-trial handling and stress. For the first trial of each mouse,
mice (WT = 8; MAO A KO = 5; MAO A/B KO = 7) were individually placed in the
start box facing the back wall away from the arms. The door of the start box was
opened and the mouse was allowed 2 min to enter one of the end arms. Once
an arm is entered (chosen), the door to the arm was quietly closed confining the
animal to that arm. An arm entry was counted only when all four paws are inside
the arm. Following 30 sec, the animal was removed via the plastic cup and the
maze was cleaned. The mouse was placed in the start box facing the arms in
109

each of the subsequent trials. If the mouse did not enter an arm within the 2 min
time limit, then the mouse „failed‟ and was returned to the start box for the next
trial. If the mouse fails on three consecutive trials then the mouse was omitted
from further testing. A total of 7 trials were performed on each mouse per
session. The number of arm alterations and the latency to enter the alternate
arm was analyzed for each mouse.
Hole-board. We used a grey Plexiglas platform (40 x 40 cm) raised to a height of
15 cm from the floor of a transparent Plexiglas box (40 x 40 x 40 cm) in a dimly lit
room (10 lux). The platform consisted of 16 equivalent quadrants (12 peripheral
and 4 central), each featuring a central circular hole (2.5 cm diameter). Mice
were individually placed in the central quadrants and their behavior was recorded
using both a top and an underside video camera for 10 min. Behaviors measured
included the number of repetitive head dips (defined as the number of
consecutive head dips into the same hole), as well as the total number of head
dips.
Sticky tape test. Mice (WT = 9, MAO A KO = 8) were briefly restrained and a
circular piece of tape (5 mm in diameter) was placed on the bottom of each
forepaw. Animals were then released and the latency to remove each piece of
tape was recorded.
110

Hot plate. Mice (WT = 16, MAO A KO = 13) were individually exposed to a hot
plate (IITC Life Science) at 52.5°C. The latency to lick their paws was measured.
40 sec cut-off time was employed to prevent tissue damage.
Novel cage digging. Mice were placed in a Makrolon cage (35 x 28 cm), in which
fine sawdust was placed to a depth of 5 cm. Light and background noise levels in
the room were kept at 5 lux and 70 dB. Mice were placed into the cages again
and left undisturbed for 30 min. Their behavior and the number of buried marbles
were video recorded and monitored by two independent observers blind to the
genotype of the animals. Inter-rater reliability was tested by Cohen‟s kappa
coefficient and always higher than 0.85.
111

5.4: Results
Maternal separation-induced ultrasonic vocalizations. In order to examine
whether MAO A contributes to the manifestation of ASD-related symptoms, we
tested for communication deficits in MAO A KO and MAO B KO pups during 5-
min of maternal separation. Significant differences in the number of emitted
ultrasonic vocalizations (Fig. 5.1a) [F(2, 21) = 9.44; P<0.01] were detected
between genotypes. Post-hoc analysis revealed that MAO A KO pups exhibited
significantly less USVs (P<0.05) than their WT littermates. Similarly, MAO B KO
pups emitted significantly less USVs (P<0.01) than WT pups, but not MAO A KO
pups.
Resident-intruder. Since impairments in reciprocal social behaviors are a core
feature of ASD symptomatology, we tested the behavioral responses of MAO A
KO mice towards an unfamiliar conspecific in the resident-intruder task. In
parallel to our previous data, exposure to a foreign intruder elicited marked
aggression in MAO A KO mice, including a decrease in the latency to attack, as
well as an increase in the number of fighting bouts and overall fighting duration.
These characteristics remained unaltered following the manipulation of
anxiogenic environmental factors (light, noise, etc.).
Hole-board. To investigate whether MAO A KO and MAO A/B KO mice exhibit
repetitive and perseverative behaviors, we tested the exploratory patterns of
transgenic MAO A and MAO A/B mutant mice in the hole-board test. Compared
to WT counterparts, knockout mice displayed a significant reduction in the
112

number of head dips (Fig. 5.1b) [H(2, N = 26) = 6.32; P<0.05]. Post-hoc
analyses revealed that MAO A KO (P<0.05) and MAO A/B KO mice (P<0.05)
exhibited a significant reduction in head dips than WT. Moreover, MAO A KO
and MAO A/B KO mice showed a distinct difference in quality of behavioral
patterns compared to WT mice. In particular, both transgenic lines engaged in
significant repetitive exploration of a select number of holes, which may suggest
behavioral perseveration and rigidity. Conversely, WT mice did display any
specific patterns of hole exploration or bias.
T-maze. In order to verify that the deficits in perseverative exploratory behavior
reflect behavioral rigidity, we measured the spontaneous alternations in the T-
maze paradigm. MAO A/B-deficient mice displayed a significant difference in the
latency to alternate (Fig. 5.1c) [F(3, 22) = 5.50; P<0.01]. Post-hoc analysis
revealed that MAO A/B KO mice exhibit a significant reduction in the latency to
alternate compared to WT (P<0.01) and MAO B KO (P<0.05), but not MAO A KO
counterparts. Similarly, transgenic MAO mutants showed a significant difference
in the number of spontaneous alternations (Fig. 5.1d) [F(3, 22) = 7.64; P<0.01].
Post-hoc testing revealed that MAO A/B KO (P<0.001) and MAO A KO (P < 0.05)
had significantly fewer spontaneous alternations compared to WT mice.
Conversely, MAO B KO mice displayed less spontaneous alternations, although
this parameter did not quite reach statistical significance (P<0.08).
113

Object-associated conditioned avoidance. In comparison with their WT
littermates, MAO A KO mice spent significantly less time in the object-associated
compartment than WT (data not shown) [F(1, 12) = 6.21, P<0.05].

Figure 5.1. MAO A-deficient mice exhibit deficits in communication and behavioral flexibility. (a)
Both MAO A KO and MAO B KO mice display a marked reduction in maternal separation-induced
ultrasonic vocalizations. (b) MAO A- and MAO A/B-deficient mice engage in significantly less
head dip exploration than WT counterparts. (c) MAO A/B KO mice display a significant increase
in the latency to first alternate. (d) Both MAO A and MAO A/B KO mice exhibit markedly less
spontaneous alternations than WT mice. Values are represented as mean ± SEM. *P<0.05,
**P<0.01, ***P<0.001 compared to WT mice. Abbrev: USV, ultrasonic vocalizations; HDs, head
dips.
Assessment of sensory modalities. We evaluated whether the maladaptive
behavioral responses may be attributed to impairments in tactile and nociceptive
sensitivity. To test haptic sensitivity in MAO A KO mice, we subjected both
genotypes to the sticky-tape paradigm. Compared to WT mice, MAO A KO mice
114

exhibited a significantly longer latency to remove the adhesive labels from the
forepaws (data not shown) [U(7, 9) = 2.5; P<0.01], which suggests a lower
responsiveness to haptic stimulation. In the hot plate task, MAO A-deficient mice
displayed a significant increase in the latency to paw lick (data not shown) [F(1,
27) = 8.71; P<0.01], signifying a reduction in nocioceptive sensitivity.
PCPA treatment of perseverative digging behaviors. Early developmental
blockade of 5-HT synthesis significantly attenuated the compulsive digging
frequency (Fig. 5.2a) [H(3, N=38) = 13.99; P<0.01] and duration (Fig. 5.2b) [H(3,
N=37) = 15.19; P<0.01] in MAO A KO mutants compared to their non-treated
MAO A KO counterparts. Post-hoc analysis revealed that saline-treated MAO A
KO mice displayed a significant increase in digging bouts (P<0.001) and duration
(P<0.01) than WT mice under similar treatment conditions. Dizocilpine treatment
significantly increased digging frequency (P<0.05) in WT mice and decreased
digging duration (P<0.05) in MAO A-deficient mice compared to saline-treated
counterparts.

115

Figure 5.2. Early developmental inhibition of 5-HT synthesis attenuates perseverative behaviors
in MAO A-deficient mice. (a-b) Early postnatal treatment with PCPA significantly reduces the
compulsive digging behavior of MAO A KO mice. Values are represented as mean ± SEM.
*P<0.05, **P<0.01, ***P<0.001 compared to WT mice.

116

5.5: Discussion
The major results of the present study are that MAO A KO mice display
communication deficits and marked behavioral rigidity in several different
behavioral paradigms. Specifically, MAO A-deficient mice exhibit a reduction in
maternal separation-induced USVs, signifying communication impairments
during early development. Moreover, both MAO A and MAO A/B KO lines show
a significant reduction in spontaneous alternations in the T-maze and
perseverative hole-board exploration, which may signify behavioral inflexibility.
In line with previous findings documenting emotional reactivity and maladaptive
responses to contextual cues, MAO A-deficient mice exhibited significantly higher
avoidance of an object-associated compartment than their WT counterparts.
These behavioral disturbances were accompanied by a reduction in haptic and
nociceptive sensitivity, as well as a reduction in acoustic startle habituation.
These findings are in substantial agreement with previous clinical studies on
MAO A deficiency, which show maladaptive responses in accordance to changes
in environmental cues (Brunner et al., 1993a). Moreover, these results parallel
clinical reports showing “autistic-like” symptoms, such as stereotyped
movements and cognitive impairments in patients carrying deficiencies in MAO A
or both MAO isoenzymes (Shih and Thompson, 1999; Whibley et al., 2010). The
implication of MAO A in autism is further supported in several preliminary studies
documenting an association between low MAO A activity and ASD severity
117

(Cohen et al.; Chugani, 2002; Cohen et al., 2003; Davis et al., 2008; Yoo et al.,
2009).
Although both MAO A and MAO B mutant pups displayed significantly fewer
ultrasonic vocalizations, this effect in MAO B KO mice may reflective of an overall
reduction in anxiety-like behaviors (see Chapter 6 for more details) (Zimmerberg
et al., 2005). Communication deficits are one of the three core symptoms of
ASD and a decrease in vocalizations were also observed in other rodent models
of pervasive developmental disorders (De Filippis et al., 2010). In parallel with
these results, MAO A KO mice show that an increase in aggressive and
antisocial behaviors (Cases et al., 1995; Scott et al., 2008). While previous
studies have reported that MAO A/B-deficient mice display abnormal social
behaviors and a decreased latency to attack a novel conspecific, surprisingly, no
differences in aggression were concluded (Chen et al., 2004). This discrepancy
may result from suboptimal testing conditions. Despite several early bouts of
fighting, the overall session duration was extended to capture fighting behavior in
WT mice. This extension may ultimately lead to the abatement of aggressive
behavior in MAO A/B KO mice, due to their documented cardiovascular
problems, rather than factors relating to aggression (Holschneider et al., 2002;
Kaludercic et al., 2010).
In order to investigate whether transgenic MAO mutant mice exhibit
perseverative behaviors and deficits in behavioral flexibility, we tested
spontaneous alternations in the T-maze. The reduction in spontaneous
118

alternations in this paradigm has been shown to signify both behavioral rigidity
and spatial working memory impairments (Wenk, 2001; Moustgaard et al., 2008).
Recent preliminary evidence from our lab has suggested that MAO B KO, but not
MAO A KO mice exhibit a significant deficit in long-term memory (unpublished
data). These results suggest that the reduction in the number of spontaneous
alternations in MAO B KO may be due to mnemonic problems, while the
decrease in MAO A-deficient mice may be caused by behavioral rigidity.
Similarly, MAO A/B KO mice display both an increase in latency to first alternate
and a decrease in spontaneous alternations, which may point to behavioral
inflexibility. These results are in agreement with previous behavioral data
showing that MAO A KO mice engage in significantly more marble burying and
digging activity, indices that signify perseverative and repetitive behavior
(Thomas et al., 2009b).
In line with previous findings, MAO A KO mice exhibited an increase in object
avoidance in the object-associated conditioned avoidance test. These data
support previous results showing abnormal aversive reactions to foreign objects,
indicating that MAO A KO mice attribute negative emotional valence to intrusive
elements. Moreover, MAO A-deficient mice exhibit maladaptive responses to
contextual stimuli, including deficits in acoustic startle habituation (data not
shown) and environmental habituation (Agatsuma et al., 2006), as well as
aberrant reactions to intrusive elements.
119

Unsurprisingly, MAO A/B-deficient mice have more profound behavioral deficits
than MAO A-deficient mice, which may be due to the significantly higher levels of
5-HT and NE in comparison to MAO A KO counterparts. This phenomenon is
likely caused by the absence of MAO B, which acts to partially compensate for
the lack of MAO A in MAO A-deficient mice by metabolizing 5-HT and NE, albeit
to a significantly lower degree. In addition to its role in emotional regulation, 5-HT
also functions to activate neurogenesis, dendritic arborization (Lieske et al.,
1999b; Lotto et al., 1999; Ponimaskin et al., 2007), and synaptogenesis (Mazer
et al., 1997; Lieske et al., 1999a), and alterations of its levels have been shown
to perturb developmental processes such as pruning in the cerebral cortex (Durig
and Hornung, 2000), suggesting that high 5-HT levels may also lead to the
cortical dysmorphism and behavioral disturbances in MAO A KO and MAO A/B
KO mice.
The importance of hyperserotonergic activity in MAO A-deficient mice is
underscored by several studies documenting high 5-HT levels in autistic patients
(Anderson et al., 1987; Cook et al., 1993; Singh et al., 1997; McBride et al.,
1998; Veenstra-VanderWeele et al., 2002; Whitaker-Azmitia, 2005; Coutinho et
al., 2007; Hranilovic et al., 2007; Nakamura et al., 2010). Indeed, one group
suggested hyperserotonermia as a potential endophenotype for ASD (McNamara
et al., 2008). To examine the impact of 5-HT on the perseverative behaviors in
MAO A-deficient mice, we blocked 5-HT synthesis during early developmental
stages by treating pups with PCPA. Administration of PCPA significantly
120

attenuated the perseverative digging behaviors in MAO A KO mutants compared
to their non-treated MAO A KO counterparts. These findings support the
contention that high 5-HT in development contributes to the manifestation of
perseverative behaviors. Future experiments are warranted to investigate the
role of developmental serotonergic hyperactivity on other behavioral modalities.
Previous studies have identified a number of dysmorphogenic features in
sensory pathways of MAO A KO mice, including disruption of the somatosensory
cortex and of the auditory circuitry. In a parallel set of experiments, our
collaborators found that MAO A-deficient mice exhibit significant alterations in
dendritic arborization in the pyramidal cells of the orbitofrontal cortex
(unpublished data, courtesy of Dr. Cara Wellman), which may contribute to an
overall cortical enlargement (Courchesne et al., 2003; Carper and Courchesne,
2005; Hazlett et al., 2005; Hazlett et al., 2006; Wassink et al., 2007; Davis et al.,
2008; Schumann et al., 2010) and cortical hyperactivity in this region.
The general reduction tactile and nociceptive responses in MAO A KO mice are
in substantial agreement with several pieces of evidence documenting sensory
dysfunctions in autistic patients (Wiggins et al., 2009; Lane et al., 2010).
Conversely, MAO A-deficient mice did not show any remarkable changes in
visual or olfactory sensitivity as described in previous studies. These alterations
may be due to the disrupted columnar formation of the barrel cortex in this line,
as this region has been shown to play a critical role in the integration of sensory
modalities and perceptual processing. A second set of morphological
121

experiments found that MAO A KO mice exhibited reduced myelination of the
contralateral cortical fibers in the corpus callosum (unpublished data, courtesy of
Dr. Rick Lin), a region highly implicated in autism (Stanfield et al., 2008; Hardan
et al., 2009; Keary et al., 2009; Isler et al., 2010). Since the corpus callosum is
involved in the connection and neurotransmission of information between
hemispheres, the decrease in myelination in this region can profoundly impair
perceptual discrimination and integration between the external environment and
the internal state, as well as brain coherence (Gazzaniga, 2005). Moreover,
these findings support the behavioral data showing abnormal threat and stress
perception, as well as maladaptive behavioral responses to contextual elements.
In order to test for alterations in synaptic activity in the prefrontal cortex, our
collaborators examined the electroencephalography (EEG) rhythms in MAO A-
deficient mice. EEG alterations are a well-documented phenomenon in a large
number of ASD patients. Accordingly, MAO A KO mice display significant
increases in all brain rhythms in the frontal cortex in comparison to WT mice
(unpublished data, courtesy of Dr. Marco Bortolato). This effect likely reflects
neuronal hyperconnectivity in the cortical circuitry, which may lead to a gross
overload of sensory stimulation, impeding neuronal network synchronization, and
ultimately rupturing cortical control. To this end, the overload of sensory input
may retard information processing mechanisms and the integration of different
systems (sensory, emotional, etc.) resulting in the manifestation of inappropriate
behaviors in response to environmental changes.
122

One group of researchers hypothesized that the large number of developmental
and morphological abnormalities in autistic patients results in local and short-
distance hyperconnectivity in the frontal cortex and impaired long-distance
neuronal communication, thereby vitiating the information processing functions
between neural networks and resulting in gross distortions in the integration of
sensory and perceptual inputs (Courchesne and Pierce, 2005). In line with these
premises, our findings show that MAO A KO mice display increased dendritic
arborization in the OFC and reduced myelination of contralateral cortical fibers in
the corpus callosum, as well as increases in all types of EEG rhythms.
These morphological and electrophysiological alterations are accompanied by a
striking array of behavioral changes that closely mirror ASD-related pathologies,
including communication deficits, impaired fear extinction, emotional reactivity,
aberrant sensory processing, antisocial and aggressive manifestations,
behavioral rigidity, and maladaptive behavioral responses to contextual cues.
Interestingly, behavioral and synaptic adaptation appears to be modulated
through NMDA receptors function in the cortico-striatal pathways (Mao et al.,
2009), suggesting that the behavioral abnormalities exhibited by MAO A KO mice
may be mediated by NMDA receptor dysfunction. In view of the theoretical role
of NMDA receptors in the etiology of autism, future experiments should address
the impact of MAO A on NMDA receptor regulation.
Although these results provide compelling evidence implicating MAO A in
autistic-like disturbances, the current study did not address whether MAO A-
123

deficient mice exhibited changes in other ASD-related behavioral modalities,
including social transmission of food preference, reversal learning, social novelty
and preference, and empathy. In order to more accurately assess the different
contributions of each isoenzyme to the behavioral changes, all genotypes must
be tested in each paradigm. Despite the similarities in behavioral and
neurobiological alterations, additional experiments should investigate the
electrophysiological properties in corticolimbic circuitry in the MAO A KO and
MAO A/B KO lines. It is worthy to note that this animal model cannot fully mirror
clinical disorders and caution should be exercised in the interpretation of these
data. Moreover, ASD are a group of heterogeneous psychological conditions
that may be comprised of distinct subsets of genetic abnormalities. In view of
this possibility, the neurobiological substrates underpinning specific clusters may
differ considerably. Finally, although the paucity of studies implicating MAO A
with autistic features limits the overall construct validity of this model, this
consideration should not exclude the possibility that MAO A may contribute to
specific subsets of ASD. Nevertheless, these results confirm that deficiency of
MAO A or both MAO isoenzymes is conducive to behavioral abnormalities
strikingly reminiscent of the key core symptoms of ASD and point to this line as a
possible animal model for ASD. Moreover, the higher severity of disturbances
found in MAO A/B KO mice prompt us to speculate a link between these
phenomena and the amount of brain 5-HT levels, particularly at early
developmental stages.
124

Chapter 6 : Behavioral disinhibition and reduced anxiety-
like behaviors in monoamine oxidase B-deficient mice

6.1: Abstract
Monoamine oxidase (MAO) B catalyzes the degradation of β-phenylethylamine
(PEA), a trace amine neurotransmitter implicated in mood regulation. Although
several studies have shown an association between low MAO B activity in
platelets and behavioral disinhibition in humans, the nature of this relation
remains undefined. To investigate the impact of MAO B deficiency on the
emotional responses elicited by environmental cues, we tested MAO B knockout
(KO) mice in a set of behavioral assays capturing different aspects of anxiety-
related manifestations, such as the elevated plus maze, defensive withdrawal,
marble burying and hole-board. Furthermore, MAO B KO mice were evaluated
for their exploratory patterns in response to unfamiliar objects and risk-taking
behaviors. In comparison to their wild-type (WT) littermates, MAO B KO mice
exhibited significantly lower anxiety-like responses and shorter latency to explore
unfamiliar objects and engage in risk-taking behaviors. To determine the
neurobiological bases of the behavioral differences between WT and MAO B KO
mice, a parallel study was conducted to measure the brain-regional levels of PEA
in both genotypes. Although PEA levels were significantly higher in all brain
regions of MAO B KO in comparison to WT mice, the most remarkable
increments were observed in striatum and prefrontal cortex, two key regions for
the regulation of behavioral disinhibition. Taken together, these results suggest
125

that MAO B deficiency may lead to behavioral disinhibition and decreased
anxiety-like responses partially through regional increases of PEA levels.
126

6.2: Introduction
Although several studies have been performed to investigate the roles of MAO A
in behavioral disturbances, the neurobiological and behavioral implications of
MAO B deficiency remain poorly understood. Nevertheless, a wealth of evidence
has shown that MAO B regulates mood and emotional processes, suggesting
that MAO B KO mice have alterations in emotional behaviors.
Previous studies have documented that MAO B KO mice display high brain
levels of PEA, but not 5-HT, DA or NE. While PEA has been implicated in the
regulation of emotional responses, including exploratory activity, arousal and
behavioral reinforcement (Sabelli and Javaid, 1995), phenotypic analyses of
MAO B-deficient mice have failed to detect significant differences in mood-
related behaviors (Grimsby et al., 1997; Shih and Thompson, 1999). Since PEA
is present in very low brain concentrations, the absence of observed behavioral
disturbances may be due to more subtle phenotypic alterations.
Recently, several lines of investigations have ascertained that some of the
actions of PEA are mediated by the activation of specific receptors, such as
TAAR1 (Borowsky et al., 2001; Lindemann et al., 2005), which has been
implicated in the regulation of DA signaling in the striatum (Lindemann et al.,
2005; Wolinsky et al., 2007; Xie and Miller, 2009).
In keeping with these premises, MAO B KO mice exhibit decreases in behavioral
parameters reflective of stress susceptibility (Bohus et al., 1987; Korte et al.,
127

1996; Louvart et al., 2005), such as forced-swim immobility and locomotor
habituation (Grimsby et al., 1997; Lee et al., 2004). Low MAO B platelet activity
in humans has been consistently correlated with extraversion and novelty-
seeking traits, yet a causal relationship between the two phenomena has not
been established (Oreland, 1993).
This scenario suggests that MAO B deficiency may result in behavioral
disinhibition, a temperamental tendency characterized by novelty- and sensation-
seeking personality and negligence of potential or actual dangers (Fowler et al.,
1980a; Reist et al., 1990; Verkes et al., 1998; Skondras et al., 2004a; Paaver et
al., 2007)To verify this possibility, in the present study we analyzed the
behavioral performances enacted by MAO B KO mice in a number of models
exploring different facets of responsiveness to contextual stimuli, including
anxiety-like responses, exploratory activity and risk-taking behaviors.
128

6.3: Methods
Animals. We used 4-5 months old, experimentally naïve male 129/Sv mice
(n=166; 83/genotype), weighing 30-35 g. MAO B KO mice and WT littermates
were generated as previously described (Grimsby et al., 1997). Animals were
housed in group cages with ad libitum access to food and water. The room was
maintained at 22°C, on a 12 h:12 h light:dark cycle, with lights off at 6:00 pm.
Prior to behavioral testing, all animals were found to display equivalent physical
and neurological characteristics. All experimental procedures were in compliance
with the National Institute of Health guidelines and approved by the University of
Southern California Animal Use Committees. To avoid potential carryover effects,
each animal was used only once throughout the study. Litter effects were
minimized by using mice from at least 3 different litters in each behavioral test.
Elevated plus-maze. The test was performed as previously described (Wall and
Messier, 2000), under either dim (10 lux) or bright (300 lux) environmental light.
Briefly, the apparatus was in black Plexiglas with a light grey floor and consisted
of two open (25 × 5 cm) and two closed arms (25 × 5 × 5 cm), which extended
from a central platform (5 × 5 cm) at 60 cm from the ground. Mice (n =
17/genotype) were individually placed on the central platform facing an open
arm, and their behavior was observed for 5 min by an experimenter unaware of
the genotype. An arm entry was counted when all four paws were inside the arm.
Behavioral measures included: time spent and entries into each partition of the
elevated plus-maze; number of fecal boli.
129

Defensive withdrawal. We used a variation of the protocol described in Bortolato
et al (2006). Mice (WT = 7; MAO B KO = 10) were individually placed inside a
cylindrical aluminum chamber (7 cm diameter x 11 cm length) located along one
of the four walls of a dimly-lit (10 lux) black Plexiglas open field (40 x 40 x 40
cm), with the open end facing the center. Mice were allowed to freely explore the
environment for 15 min. Behaviors were recorded and monitored by an observer
unaware of the genotype. Behavioral measures included: latency to exit the
chamber; transitions between the chamber and open field; time spent in the
chamber; head pokes out of the chamber; crossings (on a 4 x 4 square grid
superimposed onto the video image of the open field); velocity (ratio of crossings
to time spent in the open field).
Marble burying. Testing was performed using a modification of the methods
described in (Hirano et al., 2005). Briefly, mice (WT = 20; MAO B KO = 13) were
individually placed in a dimly-lit (10 lux) Makrolon cages (35 x 28 cm), with 5 cm
of fine sawdust, for a 30-min acclimatization period. Subsequently, mice were
briefly removed and 20 marbles (1 cm diameter) were placed in each cage, on
top of the sawdust. Mice were then returned to the cages, and their behavior was
videorecorded for the following 30 min. Measures included the number of buried
marbles, and the number and total duration of digging bouts. A marble was
considered buried if at least two thirds of its surface area was covered in
sawdust. General activity was analyzed by counting the crossings of a grid (5 x 4
squares), as described above.
130

Hole-board. We used a grey Plexiglas platform (40 x 40 cm) raised to a height of
15 cm from the floor of a transparent Plexiglas box (40 x 40 x 40 cm) in a dimly-lit
room (10 lux). The platform consisted of 16 equivalent square compartments (12
peripheral and 4 central), each featuring a central circular hole (2.5 cm diameter).
Mice (WT = 8; MAO B KO = 12) were individually placed in the center and their
behavior was recorded for 6 min. Behaviors were measured diachronically in 2-
min intervals, and included the number of crossings between compartments, and
the time spent and number of head pokes in the peripheral and central
compartments.
Novel object exploration. We used a modified version of the protocol described in
Bortolato et al (2009b). Mice (WT = 7, MAO B KO = 8) were individually
acclimatized to a dimly-lit (10 lux) grey Plexiglas cubic box (20 x 20 x 20 cm) for
15 min. Twenty-four h later, animals were exposed to two novel black plastic
cylinders (8 cm tall x 3.5 cm in diameter), affixed to the floor and symmetrically
placed at 6 cm from the two nearest walls. Mice were placed in a corner, facing
the center and at equal distance from the two objects. Their start position was
rotated and counterbalanced for each genotype throughout the test. Behaviors
were videotaped for 15 min. Analysis included number and total duration of
exploratory approaches, latency to the first exploration, and the number of
crossings (measured as described in the Defensive Withdrawal section).
Exploration was defined as sniffing or touching either of the two objects with the
snout; sitting on the object was not considered exploration. The time spent in the
131

central 4 squares and the object areas (defined as a 1.75 cm-wide annulus
concentric to the cylinders) was also measured.
Novelty-induced grooming. Mice (WT = 8, MAO B KO = 7) were placed in a
dimly-lit (10 lux) Makrolon cage (35 x 28 cm) for 30 min. Following their removal,
five voluminous objects (different for size, shape and color) were attached to the
bottom of one half of the cage (object area). Animals were returned to the empty
half of the cage and left undisturbed for 20 min. Time spent in the object area
and grooming bouts and duration were recorded and analyzed by an observer
unaware of the genotype.
Wire-beam bridge. The apparatus consisted of a 30-cm high Plexiglas platform
and a 50-cm high Plexiglas wall, oppositely placed at 30 cm distance. The
platform was surmounted by an 8-cm deep enclosure, with a square (13 x 13 cm)
opening facing the wall and placed right above the edge of the platform.
Following 24 h of isolation and food deprivation, mice (WT = 6; MAO B KO = 5)
were individually placed in the enclosure, under dim (10 lux) light conditions, and
returned to their cage after 10 min. The edge of the platform and the wall were
then connected by a horizontal, unrailed bridge (1.25 x 30 cm), made in black
aluminum wire. The bridge consisted of 2 parallel beams (0.01 cm thick)
perpendicularly connected by 24 equally distanced cross-ties (1.25 cm long). It
was modestly flexible, with a downward deflection of 1 cm per 100-g load at the
center point. A circular plastic dish (6 cm in diameter) containing 6 food pellets
(approx. 20 g) was attached onto the end of the bridge adjacent to the wall. Mice
132

were placed in the enclosure and their behavior was recorded for 10 min.
Behavioral measures included the latencies to access the bridge (with all 4 paws
on it) and to reach the food, as well as the sniffing frequency (calculated as the
ratio between the sniffing approaches to the bridge prior to the actual access to it
and the latency to access the bridge).
Statistical analysis. Statistical analyses on behavioral parameters were
performed with one-way or two-way ANOVAs, as appropriate, followed by
Tukey‟s test with Spjøtvoll-Stoline correction for post-hoc comparisons. Normality
and homoscedasticity of data distribution were verified by using the Kolmogorov-
Smirnov and Bartlett‟s test. Non-parametric comparisons were performed by
Mann-Whitney test. Significance threshold was set at P = 0.05.

133

6.4: Results
Elevated plus-maze. In previous reports, we documented that MAO B KO and
WT mice do not display significant differences in anxiety-related parameters in
the elevated plus maze (such as the number of entries and the time spent in the
open arms), under light conditions analogous to those kept in the housing room
(300 lux) (Grimsby et al., 1997). Nevertheless, cogent evidence has shown that
high levels of environmental illuminance exacerbate the anxiogenic properties of
the open arms of the elevated plus-maze (Dawson and Tricklebank, 1995).
These premises prompted us to speculate that dim light conditions (10 lux) may
be more appropriate to unravel subtle behavioral alterations displayed by MAO B
KO mice in this paradigm. Indeed, under these conditions, MAO B KO mice did
exhibit significantly more open arm entries (Fig. 6.1a) [F(1,18) = 4.48, P<0.05]
and spent longer time in the open arms (Fig. 6.1b) [F(1,18) = 4.69, P<0.05] in
comparison to their WT counterparts. MAO B KO mice also spent a significantly
shorter time on the center platform (Fig. 6.1d) [F(1,18) = 4.89, P<0.05].
Conversely, the two genotypes exhibited comparable entries in the center (Fig.
6.1c) [F(1, 18) = 1.63, NS], as well as both entries [F(1,18) = 0.51, NS] and time
spent [F(1,18) = 0.08, NS] in the closed arms (Fig. 6.1e-f). The reduction in
anxiety-like responses in MAO B KO mice was also confirmed by their lower
number of fecal boli excreted during the testing session (Fig. 6.1g) [F(1, 18) =
7.24, P<0.05]. The behavioral variability between genotypes did not reflect
134

differences in activity, as shown by the equivalent number of total entries [F(1,
18) = 1.89, NS] (Fig. 6.1h).
A second set of experiments performed under bright light conditions (300 lux)
with a different set of animals, we confirmed the lack of substantial behavioral
differences between WT and MAO B KO mice (Fig. 6.1i-p). However, the latter
spent a shorter time on the center platform (Fig. 6.1l) [F(1, 14) = 8.66, P<0.05], in
a fashion similar to their homogenotypic counterparts tested under dim light.

Figure 6.1. MAO B KO mice exhibit significant reductions in anxiety-like behaviors in the elevated
plus maze under dim (a-g), but not bright (h-n) light conditions. All values are represented as
means ± SEM. *P<0.05, compared to wild type (WT) controls.
Defensive withdrawal. To further characterize the ethological significance of the
behavioral alterations displayed by MAO B KO mice in the elevated plus-maze,
we tested them in another well-validated model of anxiety, the defensive
withdrawal paradigm. This conflict test harnesses the conflict between the natural
tendency of rodents to explore a novel open arena and to retreat in an enclosed
chamber, according to their degree of threat perception (Takahashi et al., 1989).
135

MAO B KO mice showed a significantly reduced latency to withdraw from the
chamber (Fig. 6.2a) [U(7,10) = 10, P<0.05] and a marked reduction, albeit not
significant, in time spent inside the chamber (Fig. 6.2b) [F(1,15) = 3.17, P<0.10].
The number of transitions between the chamber and the open arena was also
higher for MAO B KO mice (Fig. 6.2c) [F(1,15) = 7.4, P<0.05]. However, no
differences between MAO B KO and WT mice were detected in head pokes (WT
= 23.6 ± 3.6; MAO B KO = 22.3 ± 7.1) [F(1,15) = 0.03, NS], velocity in the open
field (defined as the ratio between the number of crossings and the time spent in
the arena) (WT = 0.15 ± 0.06; MAO B KO = 0.22 ± 0.03) [F(1, 15) = 1.49, NS],
and fecal boli (WT = 3.1 ± 1.0; MAO B KO = 1.6 ± 0.5) [F(1,15) = 1.81, NS].

Figure 6.2. MAO B KO mice display decreased anxiety-like behaviors in the defensive withdrawal
paradigm. All values are represented as means ± SEM. *P<0.05, compared to WT mice.
Marble burying. To further substantiate the hypothesis that MAO B KO mice
show decreased anxiety-like behaviors, we used the marble burying paradigm,
as this model has been recently shown to capture aspects of environmental
reactivity different from those assessed in the conflict-based assays (Thomas et
al., 2009a). Unlike their WT counterparts, MAO B KO mice buried a remarkably
136

low number of marbles (Fig. 6.3a) [U(20,13) = 72, P<0.05], but displayed
equivalent cage activity (Fig. 6.3b) [F(1,24) = 0.27, NS]. The reduction in buried
marbles was paralleled by a significant decrease in digging bouts (Fig. 6.3c)
[F(1,26) = 7.63, P<0.05] and duration (Fig. 6.3d) [U(19,10) = 34, P<0.01]. In spite
of their dramatic reduction in digging and burrowing behaviors, MAO B KO mice
approached and actively explored the marbles, with an average frequency of
4.38 ± 0.01 exploratory bouts/ min. This parameter, however, could not be
efficiently compared across genotypes, as WT mice engaged in significantly
more marble-burying behavior.

Figure 6.3. MAO B KO mice exhibit a reduction in marble-burying and digging behaviors. All
values are represented as means ± SEM. *P<0.05, **P<0.01 compared to WT controls.
Hole-board. Following the detection of reduced anxiety-related manifestations in
MAO B KO mice, we assessed the potential impact of these alterations on the
137

exploratory activity by testing the animals in the hole-board paradigm
(Casarrubea et al., 2008). The total number of head dips was comparable
between MAO B KO and WT (Fig. 6.4a) [F(1, 18) = 0.28, NS]. Nevertheless,
MAO B KO mice performed significantly more head dips in the center than WT
mice (Fig. 6.4b) [F(1, 17) = 4.52, P<0.05], a behavior that has been interpreted
as reflective of anxiolysis (Hranilovic et al., 2005). Peripheral head dips were
comparable between genotypes (Fig.4c) [F(1, 18) = 2.07, NS]. No other
significant differences were found in either locomotor activity [F(1, 16) = 0.27,
NS] or time spent in the central quadrants (WT = 29.2 ± 6.6; MAO B KO = 76.5 ±
29.8) [F(1, 16) = 1.11, NS]. Although the latency to initial head dip was equivalent
across genotypes, (Fig. 6.4d) [F(1, 18) = 0.06, NS] MAO B KO mice exhibited a
significantly decreased latency to the first head dip in one of the central holes
(Fig. 6.4e) [U(8, 12) = 17, P<0.05]. Latency to the first peripheral head dip was
statistically equivalent in WT and MAO B KO mice (Fig. 6.4f) [U(8, 11) = 36, NS].
138

Figure 6.4. MAO B KO mice display overall locomotor activity and head dips similar to their WT
counterparts in the hole-board, but explore central holes with longer duration and shorter latency.
All values are represented as means ± SEM. *P<0.05, compared to WT controls.
Novel object exploration. During both the acclimation phase and the exposure to
novel objects MAO B KO and WT mice exhibited similar magnitude and patterns
of locomotor activity (data not shown). MAO B KO mice approached the objects
with a significantly reduced latency (Fig. 6.5a) [U(7, 8) = 6, P<0.01] and explored
them for a significantly longer time (Fig. 6.5b) [F(1, 13) = 5.17, P<0.05]. In
contrast, the number of exploratory approaches was analogous between
genotypes (WT = 80.9 ± 19.3; MAO B KO = 95.6 ± 8.3) [F(1, 13) = 0.96, NS].
Statistical analysis also showed a trend for MAO B KO mice to spend longer time
in object areas (WT = 161.1 ± 28.5; MAO B KO = 263.6 ± 36.9) [F(1, 13) = 3.97,
P<0.10]. These results suggest that MAO B KO mice may be characterized by a
stronger drive (or a reduced timidity) to explore unfamiliar objects.
139

Novelty-induced grooming. Previous studies (Kalueff and Tuohimaa, 2004) and
preliminary observations conducted by our group have shown that 129/Sv mice
display poor spontaneous grooming activity in response to novel environmental
stimuli. Thus, we optimized our protocol to detect novelty-induced grooming and
avoidance by placing five voluminous objects in one half of the cage. MAO B KO
mice spent significantly less time grooming (Fig. 6.5c) [F(1, 12) = 4.93, P<0.05],
but engaged in a similar number of grooming bouts (WT = 7.71 ± 1.69; MAO B
KO = 5.71 ± 1.26) [F(1, 12) = 0.78, NS] in comparison to their WT counterparts.
Furthermore, MAO B KO mice spent a significantly longer time in the object area
(Fig. 6.5d) [U (8, 7) = 6, P<0.01]. These results confirm that MAO B KO mice
display less environmental and object-related neophobia than WT mice.
Wire-beam bridge test. Low levels of platelet MAO activity have been strongly
associated with features of the behavioral disinhibition spectrum including
impulsivity, sensation-seeking, and risk-taking (Hirshfeld-Becker et al., 2003;
Ruchkin et al., 2005). In order to capture these elements, we measured the
animal‟s proclivity to cross an unrailed flexible bridge suspended over a 30-cm
deep gap, to reach a food reward. MAO B KO mice exhibited a significantly
shorter latency to access the bridge (Fig. 6.5e) [U(6, 5) = 1.5, P<0.01] ] and
latency to touch the food (WT = 483.83 ± 81.64; MAO B KO = 48.00 ± 23.66)
[U(6, 5) = 1, P<0.01] to obtain the food reward in comparison to their WT
counterparts. In the time prior to their access to the novel bridge, MAO B KO
mice engaged in a significantly higher sniffing frequency towards it (Fig. 6.5f)
140

[U(6, 5) = 2, P<0.05] compared to WT mice. These results provide further
support that MAO B KO mice display greater impulsivity, sensation-seeking and
risk-taking behaviors than WT mice.

Figure 6.5. MAO B KO mice display higher levels of exploration targeting novel objects and risk-
taking behavior in the wire-beam bridge test. All values are represented as means ± SEM.
*P<0.05, **P<0.01 compared to WT littermates.
141

6.5: Discussion
The major result of the present study is that MAO B deficiency in mice leads to
behavioral disinhibition in several models of contextual anxiety and risk-taking
behavior, as well as enhanced novelty-seeking responses with respect to
unfamiliar objects. These findings are in substantial agreement with previous
cross-sectional investigations, documenting that low platelet MAO B enzymatic
activity is correlated with several facets of behavioral disinhibition (Buchsbaum et
al., 1976; von Knorring et al., 1984; Oreland, 1993), including novelty- and
sensation-seeking personality (Fowler et al., 1980a; Reist et al., 1990; Ruchkin et
al., 2005), poor impulse control (Fowler et al., 1980a; Reist et al., 1990; Verkes et
al., 1998; Skondras et al., 2004a; Paaver et al., 2007), and proclivity to engage in
risky behaviors (Blanco et al., 1996).
Previous studies indicate that low platelet MAO B activity is highly heritable
(Oxenstierna et al., 1986) and may influence behavior since the neonatal stage
(Sostek et al., 1981), suggesting that this index may be a genetic determinant for
uninhibited personality (Oreland and Hallman, 1995; Blanco et al., 1996).
Notably, MAO B deficiency in Norrie disease patients was reported to result in no
overt physical or mental alterations (Lenders et al., 1996). However, it should be
observed that the severe degree of sensory and perceptual impairments induced
by Norrie disease (early-onset blindness and progressive hearing loss) likely
masked alterations in environmental reactivity in these subjects.
142

The role of MAO B in emotional regulation is further supported by a host of
clinical studies, showing that chronic administration of l-deprenyl exerts mood-
enhancing and anxiolytic effects in depression (Mendlewicz and Youdim, 1980;
Quitkin et al., 1984; Robinson et al., 2007) and other disorders (Tariot et al.,
1987; Goad et al., 1991; Tolbert and Fuller, 1996). Interestingly, l-deprenyl (both
in acute and chronic administration) elicits only minor or no anxiolytic-like effects
in rodents (Commissaris et al., 1995; De Angelis and Furlan, 2000; Nowakowska
et al., 2001). The most likely explanation for the apparent discrepancy between
these reports and our results lies in the genetic nature of MAO B inactivation
examined in this study, which cannot be completely recapitulated by the
outcomes of chronic exposure to enzyme inhibitors (Whitaker-Azmitia et al.,
1994)
In rodents, emotional reactivity and novelty-seeking behavior is measured as a
function of the exploratory activity towards unfamiliar environments and objects
(Oreland, 1993; Robinet et al., 1998; Fornai et al., 1999). Accordingly, the validity
of novelty-induced tasks as animal models of anxiety (Pellow et al., 1985;
Takahashi et al., 1989) is based on the opposition between exploratory drive and
neophobia-derived avoidance (Dellu et al., 2000). This contrast can be influenced
by certain environmental manipulations, such as the variation of light intensity in
the experimental room (Dawson and Tricklebank, 1995).
143

In a dimly lit elevated plus maze, MAO B KO mice displayed a significant
reduction in anxiety-like behavior, signified by an increase in open arm time and
entries, as well as a reduction of defecation frequency (Tarantino and Bucan,
2000). Conversely, a bright illuminance level (300 lux) failed to elicit significant
differences between MAO B KO and WT mice in these anxiety-related
parameters, possibly due to “floor effects”.
Dim light conditions have been extensively used to capture fine modifications in
anxiety-like behaviors (Low et al., 2000; Bourin et al., 2001; Genn et al., 2003;
Bortolato et al., 2006; Rubino et al., 2008). In the present study, low
environmental luminosity provided an optimal setting to reveal the reduction in
anxiety-like behaviors in MAO B KO mice, suggesting that the deficiency of this
enzyme results in subtle, context-dependent changes in anxiety regulation. This
contention is also supported by the observation that the behavioral abnormalities
in MAO B KO mice could not be observed in their home cages (data not shown),
but only in the presence of novel objects or contexts.
In the defensive withdrawal test, MAO B KO mice exhibited reductions in latency
to exit the chamber and in transitions between the chamber and the open arena,
independent of differences in locomotor activity. Both parameters are highly
dependable indices to measure defensive behaviors (Arborelius and Nemeroff,
2002), and their reduction is considered reflective of reduced fearfulness or
deficits in threat detection. This interpretation is also supported by the significant
decline in time spent on the elevated plus-maze central platform, which has been
144

suggested to indicate potential impairments in decision-making or impulse-
control processes (Rodgers et al., 1992a; Trullas and Skolnick, 1993).
The reduction in anxiety-related responses in MAO B KO mice was also
confirmed by the nearly complete abrogation of their marble-burying behavior.
This murine assay has been validated to capture different aspects of anxiety-like
behaviors than conflict-based paradigms (Thomas et al., 2009a), in a fashion
sensitive to anxiolytic and antidepressant drugs (Broekkamp et al., 1986; Njung'e
and Handley, 1991; Borsini et al., 2002; Nicolas et al., 2006). Marble burying has
been shown to reflect digging activity, but is independent from locomotion or
exploration (Gyertyan, 1995; Thomas et al., 2009a). Accordingly, MAO B KO
mice dug significantly less than WT counterparts, but displayed a comparable
number of crossings.
Our findings on the patterns of exploratory activity in the hole-board test are also
supportive of reduced anxiety-like behaviors in MAO B KO mice. Although overall
locomotor activity was comparable between genotypes, MAO B KO mice
manifested a lower level of avoidance towards the central holes, reflecting a
reduction in thigmotactic behavior (Hranilovic et al., 2005) and novelty-related
aversion (Brown and Nemes, 2008). Since animals were initially placed in the
center of the HB, it may be also proposed that the increased number of central
head dips may also reflect an enhancement in perseverative behavior, following
exploration of the first central hole. Nevertheless, this possibility is partially
tempered by the equivalent latency to the first peripheral head dip between the
145

two genotypes, which was accompanied by a similar locomotor activity (and
number of head dips in the external zone) in the first 2-min period of testing. This
phenomenon likely indicates that MAO B KO mice did not exhibit an initial
tendency to neglect holes in the external zone of the hole-board.
The possibility that the behavior enacted by MAO B KO mice may be reflective of
low neophobia is also supported by their reaction to unfamiliar objects. Indeed, in
comparison to their WT counterparts, MAO B KO mice exhibited a stronger
inclination to explore novel objects (with higher duration and lower latency), as
well as lower levels of novelty-induced grooming and reduced avoidance of
object-laden areas. These responses may also signify poor impulse control and
higher drive towards risk-taking behaviors in MAO B KO mice. In rodents,
impulsivity is generally studied by means of go/no-go tests, which measure the
capacity to withhold behavioral reactions (Dalley et al, 2008). However, as these
tasks are based on operant responses, they cannot be dependably used in MAO
B KO mice, due to alterations in mnemonic acquisition in these mice (Bortolato et
al, in preparation). To obviate this pitfall, we tested MAO B KO mice in the wire
beam bridge task, an assay devised to verify their inclination to engage in risk-
taking behaviors (such as walking on a novel, flexible bridge placed above a 30-
cm deep gap) to reach a rewarding goal. The prevalence of motivational drives
over the ability to adjust behavioral responses to contextual elements is
considered a key feature of impulsiveness (Jentsch and Taylor, 1999; Bechara et
al, 2000). Upon exposure to the novel bridge, MAO B KO mice exhibited a
146

significantly shorter latency to both access and cross the bridge to reach the food
reward compared to WT mice. This divergence likely signifies enhanced risk-
taking behavior in MAO B KO mice, and may reflect alterations in decision-
making processes and emotional regulation in this genotype (Llewellyn, 2008).
Interestingly, MAO B KO mice engaged in a significantly higher frequency of
sniffing bouts towards the bridge than WT mice, showing that their shorter
latency to bridge access was not reflective of inadequate exploration of the
bridge itself or lower risk assessment.
In a previous report, we described that MAO B KO mice exhibited lower
immobility in the forced swim test than WT counterparts (Grimsby et al., 1997). In
mutant mice, this behavior has been associated with either enhanced (Parks et
al, 1998) or reduced anxiety-like behavior (Bale and Vale, 2003; Tschenett et al,
2003). Our present findings help define the conceptual framework for the
interpretation of the stress response exhibited by MAO B KO mice, suggesting
that their behavior in the forced swim paradigm may reflect their enhanced ability
to counteract the stress induced by novel contextual factors and hazardous
situations. Substantial evidence has indeed shown that the increased mobility in
forced swim test is associated with a lower vulnerability to stress-induced
anhedonia and depression (Strekalova et al, 2004; Trzctńska et al, 1999), as well
as decreased neophobia (Gundersen et al, 2009). Accordingly, decreased
grooming activity has been shown to be a dependable criterion to measure
increased inclination to cope with stress (Engelmann et al., 1996; Kalueff and
147

Tuohimaa, 2004). The higher resistance to stress in MAO B KO mice is also
corroborated by their significantly lower levels of hyperthermia (Bouwknecht et al,
2007) induced by 2 h of physical restraint, in comparison with WT littermates
(WT: ΔT: 1.09 ± 0.13 ºC; MAO B KO: ΔT: 0.12 ± 0.22 ºC; P<0.01) (unpublished
data).
MAO B KO mice feature a significant elevation in whole-brain levels of PEA, but
not other monoamines (Grimsby et al., 1997). This premise suggests that this
trace amine may play a key role in the behavioral alterations induced by MAO B
genetic deficiency. Indeed, PEA has been shown to enhance mood and sensory
functions in joint administration with MAO B inhibitors (which prevent its
degradation) (Sabelli et al., 1994; Sabelli and Javaid, 1995). Furthermore, the
synthetic PEA analog amphetamine is known to increase novelty-seeking
behaviors and reduce impulse control in both rodents and humans (Evenden and
Ryan, 1996; Williamson et al., 1997; Leyton et al., 2002).
In apparent contrast with our findings, acute administration of PEA has been
shown to induce anxiety in rodents (Lapin, 1990, 1993). However, congenital,
chronic exposure to high PEA levels may reduce anxiety-spectrum responses in
MAO B KO mice, probably via the progressive recruitment of processes
opposing the anxiogenic effects of this trace amine. Similarly, MAO B KO mice
also fail to exhibit other alterations reminiscent of the effects induced by acute
PEA administration, such as hyperlocomotion (Mantegazza and Riva, 1963),
stereotyped behavior (Moja et al., 1976) and anorexia (Dourish and Boulton,
148

1981). Final verification of the involvement of PEA in the behavioral performance
of MAO B KO mice would require pharmacological manipulations to reduce their
PEA levels, such as the inhibition of its synthesis. The accomplishment of this
objective, however, is hindered by the lack of substrate-specificity of the only
PEA-synthesizing enzyme as yet characterized in mice, aromatic l-amino acid
decarboxylase (EC4.1.1.28) (Kubovcakova et al., 2004; Zucchi et al., 2006).
Indeed, this enzyme catalyzes key reactions in the synthesis of all the other
major neurotransmitter systems, such as DA, NE and 5-HT (Allen et al., 2009),
and its inhibition results in a number of non-specific effects on several brain
functions (Fisher et al., 1999).
To partially circumvent these limitations, a parallel study was conducted to
examine the differential expression of PEA levels in several brain regions
associated with emotional reactivity. Although the increase in PEA brain levels
involves several brain regions of MAO B KO mice, the most marked
enhancements in PEA levels were observed in striatum and prefrontal cortex,
two regions extensively implicated in behavioral disinhibition (Johansson and
Hansen, 2000; Winstanley et al., 2006).
The involvement of striatum and prefrontal cortex in behavioral disinhibition has
been linked to the functional activity of DAergic system (Pattij et al., 2007),
suggesting that DA may be implicated in the behavioral alterations in MAO B KO
mice. This possibility is supported by a host of studies underscoring the key role
149

of DA in behavioral disinhibition (Megens et al., 1992; Black et al., 2002; van
Gaalen et al., 2006) and anxiolysis (Shabanov et al., 2005; Picazo et al., 2009).
Previous studies have shown that PEA induces modification of the DA signaling,
which may play a role in the behavioral responses mediated by this trace amine
(Kuroki et al., 1990; Sotnikova et al., 2004). Interestingly, while MAO B KO mice
do not feature alterations of striatal DA synthesis, uptake and release, they do
exhibit alterations in DA receptors in this region (Chen et al., 1999). Furthermore,
structural changes in other monoamine systems (such as 5-HT) may also be
induced by MAO B deficiency (Whitaker-Azmitia et al., 1994; Oreland et al.,
2007).
Emerging evidence points to a role of TAAR1 receptor in the PEA-mediated
modulation of DAergic signaling (Lindemann et al., 2008; Xie and Miller, 2009).
Although no apparent compensatory changes in TAAR1 receptor expression was
detected in either striatum or frontal cortex in a parallel study, these data cannot
fully exclude the functional contribution of this receptor to the behavioral
spectrum of MAO B KO mice (unpublished data, courtesy of Dr. Marco
Bortolato). While pharmacological studies with TAAR1 receptor antagonists may
help elucidate this issue, these agents are currently unavailable (Sotnikova et al.,
2009).
In summary, this study documents that MAO B deficiency in mice results in
behavioral disinhibition and reduced neophobia. These results complement
previous findings on the correlation between low MAO B platelet activity and
150

novelty-seeking personality, suggesting a potential causal link between the two
phenomena. Nevertheless, both the interpretation of the behavioral phenotype in
MAO B KO mice and its translational validity should be considered with caution,
in view of several limiting considerations. First, although the array of behavioral
abnormalities in MAO B KO mice supports the idea that these animals do display
behavioral disinhibition, some of the reported alterations - such as the increased
exploration of novel objects or central holes in the hole-board paradigm, or the
decreased marble burying - may also reflect other disturbances in perceptual,
attentional, emotional and cognitive regulation. Further investigations on the
impact of MAO B deficiency in these behavioral domains (and in female mice)
are necessary to elucidate this possibility and further refine our understanding of
the complex phenotype exhibited by MAO B KO mice. Second, the results
observed in MAO B KO mice may not be directly applicable to clinical
manifestations, in view of the predominance of this isoenzyme in the human
brain (Fowler et al., 1980b; Oreland and Gottfries, 1986; Kalaria et al., 1988),
which contrasts with its relatively poor expression in rodents (Saura et al., 1996).
Third, while DA is degraded by MAO A in mice (Cases et al., 1995; Fornai et al.,
1999), it is mainly metabolized by MAO B in primates (Garrick and Murphy,
1980), suggesting that the reported alterations in murine phenotype may only
partially reproduce the behavioral outcomes of MAO B deficiency in humans.
Irrespective of these considerations, these findings strongly support the role of
MAO B in the modulation of the neural pathways underlying behavioral
disinhibition and emotional reactivity towards contextual stimuli, and warrant
151

further investigations on the function of this enzyme in the regulation of anxiety-
related endophenotypes.

152

Chapter 7 : Transgenic monoamine oxidase mutant mice
show disturbances in NMDA glutamatergic receptor
function

7.1: Abstract
Converging evidence has shown that monoamine oxidase (MAO), the major
catabolic enzyme for the degradation of monoamines and trace amines, is
involved in emotional regulation and behavioral plasticity. In view of the
important role of n-methyl-d-aspartate (NMDA) glutamatergic receptors in
emotional control, the present study explored the possibility that transgenic MAO
mutant mice exhibit disturbances in NMDA receptor function. To this end, the
exploratory and stereotyped behaviors were tested in transgenic MAO mutants
treated with subthreshold doses of the NMDA receptor antagonist dizocilpine.
Dizocilpine induced a significant increase in behavioral stereotypies in
hypomorphic MAO A, MAO A knockout (KO), and MAO A/B KO mice, but not
MAO B KO mice. In particular, MAO A/B KO mice treated with dizocilpine
showed profound disruptions in motor coordination and increased ataxia. These
data suggest that the reduction in MAO A activity results in a hypersensitivity to
NMDA receptor blockade. Collectively, these findings indicate that MAO A
disturbs NMDA receptor function and points to a potential molecular mechanism
underlying behavioral abnormalities associated with MAO A- and MAO A/B-
deficiency.
153

7.2: Introduction
Although the previous chapters examined the behavioral outcomes in MAO
mutant mice, the heterogeneity of these neuropsychiatric manifestations
subsumes a multifactorial etiology. Based on this conceptual framework,
impairments in the reciprocal interactions of the brain monoamines and trace
amines with other neurotransmitter systems may underpin perturbations in brain
morphology and neurobehavioral sequelae in these lines.
Glutamate is the most prominent excitatory neurotransmitter in the brain and has
been implicated in the pathogenesis of a host of neuropsychiatric abnormalities.
In particular, the glutamatergic system plays a key role in learning and adaptation
through synaptic plasticity mechanisms. The balance between glutamatergic and
inhibitory GABAergic signaling in the prefrontal cortex functions in the top-down
inhibitory control over the subcortical emotional processing machinery to regulate
behavior (Jackson et al., 2004; Hare et al., 2008). These mechanisms are
modulated by monoaminergic input to finely tune behavioral responses
appropriate for changes in contextual contingencies.
Several pieces of evidence suggest that MAO deficiency may contribute to
disturbances in glutamatergic neurotransmission. Since glutamatergic pyramidal
cells have robust innervations of serotonergic fibers, serotonergic hyperactivity
may impinge on prefrontal networks. Similarly, glutamtergic function may
modulate dopaminergic signaling in several regions resulting in a number of
behavioral abnormalities, such as psychotomimetic symnptoms, stereotypies,
154

cognitive impairments, and possible attentional disturbances (Adams and
Moghaddam, 1998; Kegeles et al., 2000; Balla et al., 2001b; Balla et al., 2001a;
Miyamoto et al., 2004; Egerton et al., 2008). Moreover, MAO A-deficient mice
exhibit abnormal emotional reactivity and cortical dysmorphogensis, which
strongly suggests a dysregulation in the corticolimbic circuitry and NMDA
receptor dysfunction. NMDA receptor disturbances have been implicated in the
pathophysiology of a number of neuropsychiatric disorders, including
schizophrenia, OCD, aggression, autism, and depression, as well as emotional
regulation (Carlsson, 1998; Popoli et al., 2002; Javitt, 2004; Chakrabarty et al.,
2005; Li et al., 2006; Murai et al., 2007; Szasz et al., 2007; Labrie and Roder,
2010; Marek et al., 2010). Similarly, NMDA receptor dysfunction in animals
results in a constellation of severe behavioral deficits that parallel the behavioral
disturbances manifested in MAO A-deficient mice, such as behavioral rigidity and
perseveration, cognitive and social impairments, affective flattening, and
alterations in emotional and stress-reactivity (Blanchard et al., 1992; Corbett et
al., 1995; Stefani et al., 2003; Carli et al., 2006; Fumagalli et al., 2009; Amitai and
Markou, 2010; Mozhui et al., 2010; Stefani and Moghaddam, 2010).
In line with these findings, serotonergic input modulates NMDA receptor-
mediated alterations in behavioral and synaptic mechanisms in the cortico-striatal
pathways (Gu, 2002; Beique et al., 2004; Mao et al., 2009). This scenario
suggests that the behavioral abnormalities exhibited by MAO A KO mice may be
mediated by NMDA receptor dysfunction. In keeping with this possibility, we
155

tested the locomotor and stereotyped behaviors in MAO A KO mice following
administration of sub-threshold doses of NMDA receptor antagonist dizocilpine.
156

7.2: Methods
Animal husbandry. We used 3-4 month old, experimentally naïve male
129S6/SvEvTac mice (n = 56; 12 WT, 9 MAO A
neo
, 11 MAO B KO, 12 MAO
A
A863T
KO, and 12 MAO A/B KO), weighing 25-30 g. MAO A
neo
, MAO A
A863T
KO
(Scott et al., 2008), and MAO A/B KO (Chen et al., 2004) were generated as
previously described. Animals were housed in group cages with ad libitum
access to food and water. The room was maintained at 22°C, on a 12 h: 12 h
light/dark cycle. Experimental procedures were in compliance with the National
Institute of Health guidelines and approved by the Animal Use Committee of
University of Southern California.
Open field. Analysis of the open field behaviors was performed in experimentally
naïve, adult male mice following a modified version of the protocol used in
(Bortolato et al., 2009a). The open field was a Plexiglas square grey arena (40 x
40 cm) surrounded by 4 black walls (40 cm high). On the floor, 2 zones of
equivalent areas were defined: a central square quadrant of 28.28 cm per side,
and a concentric peripheral frame including the area within 11.72 cm from the
walls. Mice were placed in the central zone and their behavior was monitored for
60 min. Light and background noise in the room were kept at 10 lux and 70 dB
respectively. Behavioral measures included the total locomotor activity (defined
as the number of crossings on a grid superimposed onto the image of each cage
in a video monitor) and the number of fecal boli.
157

Dizocilpine-induced stereotypies. Stereotyped behavior and ataxia were
simultaneously measured during the open field test. Two independent observers
unaware of genotype scored the stereotyped behaviors according to the modified
Creese and Iversen scale (Creese and Iversen, 1973; Hoffman, 1992) at 4-min
intervals. Ataxia scores were rated following the dizocilpine-induced ataxia
scales previously described (Wu et al., 2005).
Drugs. MK-801 hydrogen maleate (dizocilpine) (Sigma-Aldrich, St. Louis, MO)
was dissolved in 0.9% saline solution and administered at a dosage of 0.1 mg/kg
via i.p. injection 30 min prior to behavioral testing.
Statistical analyses. Normality and homoscedasticity of data distribution were
verified using the Kolmogorov-Smirnov and Bartlett‟s test. Parametric analyses
were performed with one-way or two-way ANOVA, as appropriate, followed by
Tukey‟s test with Spjøtvoll-Stoline correction for post-hoc comparisons.
Nonparametric comparisons were carried out by Mann-Whitney and Kruskal-
Wallis tests as appropriate. Significance threshold was set at P = 0.05.

158

7.4: Results
Open field. Locomotor responses to NMDA receptor antagonism were measured
in the open field paradigm (Fig. 7.1). Although dizocilpine induced an increase in
locomotor behavior in WT mice, this effect did not reach statistical significance
[F(1, 10) = 3.78; P<0.08]. Conversely, NMDA blockade elicited a significant
reduction in locomotion in MAO A/B KO mice [U(5, 7) = 5.00; P<0.05]. The
locomotor activity in MAO A
neo
[F(1, 7) = 0.42; NS], MAO A KO [U(6, 6) = 15.00;
NS], and MAO B KO mice [F(1, 9) = 1.56; NS] were unaffected by NMDA
receptor antagonism.
Dizocilpine-induced stereotypies and ataxia. MAO A
neo
, MAO A KO, and MAO
A/B KO mice displayed marked stereotyped behavior. Specifically, while
dizocilpine elicited a significant increase in oral stereotypies and impairment in
motor coordination in MAO A
neo
mice, MAO A-deficient mice exhibited locomotor
sinuosity. MAO A/B KO mice showed the most profound stereotyped behavior
with persistent vacuous chewing and gnawing, as well as a severely restricted
motor deficit. Conversely, MAO B KO mice displayed equivalent stereotyped
behaviors as their WT counterparts.
159

Figure 7.1. Partial or full ablation of MAO A disrupts NMDA receptor function. Locomotor
patterns of mice treated with saline (top) or NMDA receptor antagonist dizocilpine (bottom).

160

7.5: Discussion
The major results of the present study are that MAO A/B KO, MAO A KO and
MAO A
neo
mice display deficits in NMDA receptor function. Specifically, treatment
with sub-threshold doses of dizocilpine elicited marked increases in stereotyped
behavior of these three lines in comparison to WT mice. NMDA receptor
blockade induced perseverative sinuosity in the MAO A KO line and significantly
decreased the number of fecal boli, however, there were no significant
differences in ataxia. Conversely, MAO A
neo
mice exhibited a higher ataxia score
than WT, MAO B KO and MAO A KO mice, but not MAO A/B KO mice. The
overall locomotor activity, however, was similar between treatment groups in the
MAO A
neo
line. In MAO B KO mice, dizocilpine-induced stereotypies and ataxia
ratings were similar to their WT counterparts.
The congenital ablation of both MAO isoenzymes led to a more severe
stereotypy and ataxia than all other lines tested. This profound impairment was
accompanied by a significant decrease in overall locomotion compared the saline
treatment group. Interestingly, MAO A
neo
, MAO A KO, and MAO A/B KO have
increasing levels of stereotypy and decreasing levels of locomotor activity. This
behavioral pattern and the absence of significant changes in dizocilpine-induced
stereotypies in MAO B KO mice, heavily suggest that the NMDA receptor
dysfunction is mediated by the higher levels of 5-HT.
Administration of dizocilpine or other NMDA receptor antagonists in humans elicit
strong auditory and visual hallucinations, as well as disturbances in thought
161

processes. The similarity between the general presentation of symptoms and
schizophrenic manifestations has led to the use of NMDA receptor blockers as
psychotomimetics in animal models of psychosis. In view of these premises, the
hypersensitivity to NMDA blockade may suggest inherent deficits in sensory and
perceptual processing mechanisms.
Stereotypies are a set of purposeless repetitive behaviors, such as excessive
sniffing, head bobbing, gnawing and vacuous chewing, that are typically initiated
by a hyperactivation of the dopaminergic system. Dizocilpine-induced
stereotypies in these lines suggest that this effect is mediated by glutamatergic
mechanisms rather than driven by dopaminergic limbic activation. An intriguing
possibility is that NMDA receptor blockade in MAO A KO and MAO A/B KO mice
may initiate a neurobehavioral loop that manifests as repetitive behavioral
sequences. Dizocilpine-induced exacerbation of the perseverative and inflexible
behavioral responses may be a consequence of their inability to properly
extinguish prior behavior patterns (Kim et al., 1997; Arnsten, 2009).
To verify these findings, our collaborators tested the behavioral responses to
subthreshold doses of dizcilpine in the prepulse inhibition of the acoustic startle,
a well-validated paradigm that measures pre-attentional gating function. Deficits
in the prepulse inhibition of the acoustic startle were found in both MAO A/B KO
and MAO A KO mice, confirming that these lines have NMDA receptor
dysfunction (unpublished data, courtesy of Dr. Marco Bortolato). Since 5-HT2A
antagonists have been shown to abolish dizcilpine treatment, MAO A/B KO and
162

MAO A KO mice were given the 5-HT2A receptor antagonist ketanserin.
Ketanserin effectively attenuated the dizocilpine-induced deficits in the prepulse
inhibition of the acoustic startle paradigm, suggesting that the NMDA receptor
dysfunction is mediated by 5-HT2A (unpublished data, courtesy of Dr. Marco
Bortolato).
Converging studies have documented that low doses of dizocilpine elicit
locomotor hyperactivity, whereas high doses lead to severe ataxia. Accordingly,
the high levels of stereotyped behavior in MAO A/B KO, MAO A KO and MAO
A
neo
mice treated with subthreshold dosages of dizocilpine indicate NMDA
receptor dysfunction. Moreover, the more robust behavioral abnormalities
induced by subthreshold doses heavily suggest that these lines either have a
reduction in the number of NMDA receptors or a hypersensitivity to blockade.
Based on these results, additional experiments were performed to test NMDA
receptor expression through audioradiography.
In a parallel experiment, our collaborators found equivalent levels of NMDA
receptor protein expression levels and glutamate release in MAO A/B KO, MAO
A KO and WT mice (unpublished data, courtesy of Dr. Paola Devoto and Dr.
Paola Castelli), indicating that the alterations in NMDA receptor blockade in the
two transgenic lines were not due to changes in the number of NMDA receptors.
To investigate whether there were functional differences in the NMDA channel
properties in MAO A/B KO and MAO A KO mice, our collaborators tested used
whole cell patch clamp on cortical pyramidal cells (unpublished data, courtesy of
163

Dr. Miriam Melis). Stimulation of pyramidal neurons evoked a significant
decrease in NMDA receptor-mediated excitatory postsynaptic currents (EPSCs)
of MAO A/B KO and MAO A KO mice compared to WT mice. Moreover, both
transgenic lines exhibited an increase in the minimum stimulus intensity needed
to produce a NMDA EPSC, suggesting a higher activation threshold. These
effects were accompanied by a significantly shorter decay, which reflects a
decrease in duration of NMDA EPSCs. Additionally, dizocilpine application to the
pyramidal neuron caused a significantly greater reduction in percent NMDA
EPSCs than their WT counterparts, suggesting a higher sensitivity to NMDA
receptor antagonism. Taken together, these data show that MAO A deficiency
impairs NMDA receptor function through alterations in channel properties,
including a higher activation requirement, a shorter duration of channel
activation, and a significantly greater sensitivity to blockade. Although the
mechanism underlying the change in electrophysiological properties is unknown,
one possibility is that the elevated levels of 5-HT lead to a change in channel
subunit composition or alterations in the intracellular signaling cascade(s).
NMDA receptor dysfunction in MAO A-deficient mice is likely due to
hyperserotonergic activity. Indeed, prefrontal NMDA receptor neurotransmission
is modulated by 5-HT activity through activation of cortical 5-HT2A (Tao and
Auerbach, 1996; Arvanov et al., 1999; Puig et al., 2003). In turn, 5-HT2A
receptor activation induces NMDA cortical excitation and suppresses 5-HT
release from the dorsal raphe (Martin-Ruiz et al., 2001; Yuen et al., 2008; Albert
164

Adell, 2010). This scenario suggests that elevated levels of 5-HT during early
stages may result in the excessive activation of cortical 5-HT2A receptors and
lead to alterations in NMDA receptor-mediated neurotransmission. This
possibility is further supported by a host of studies documenting the impact of 5-
HT2A receptors on cortical NMDA receptor function (Wang and Liang, 1998;
Martin-Ruiz et al., 2001; Goto et al., 2010). Moreover, 5-HT2A blockade
corrected the dizocilpine-induced deficits in prepulse inhibition, as well as
attenuated aggression in MAO A KO mice (Shih et al., 1999b).
In view of the critical role of NMDA receptors in synaptic plasticity, dysfunction
may also contribute to the cortical dysmorphogenesis in MAO A-deficient lines.
While previous studies have shown that high serotonergic tone during early
development disrupt the segregation of thalamocortical afferents into barrel
formations in the primary somatosensory cortex (Cases et al., 1996; Salichon et
al., 2001), this input is mediated by NMDA receptors, which enact on synaptic
plasticity mechanisms to modify cortical maps (Daw et al., 2007). In addition, the
integrity of the barrel cortex plays a critical role in the integration of perceptual
and sensory modalities, suggesting that changes in NMDA receptor
neurotransmission may contribute to the disturbances in perception and
maladaptive responses to contexts in MAO A KO mice.
Similarly, the dendritic hyperconnectivity in the orbitofrontal cortex may impair
several behavioral modalities, including decision-making, sensory integration,
impulse control, and affective processing. Taken together, these findings
165

suggest that the serotonergic hyperactivity in MAO A-deficient mice contribute to
NMDA receptor-mediated changes in cortical circuitry and lead to deficits in
information processing and impairments in the integration of sensory and
perceptual systems. It is worth noting that together with the elevated brain
rhythms, these results heavily suggest that MAO A KO mice may have an
overload of sensory stimulation that severely retards information processing
mechanisms. To evaluate this possibility, future studies should examine the
electrophysiological and behavioral consequences of early postnatal inhibition of
5-HT synthesis in MAO A-deficient mice.
Although MAO B KO mice did not exhibit significant alterations in dizocilpine-
induced behaviors, it is possible that this isoenzyme may interact with different
components of the glutamatergic pathways. An interesting possibility is the
group 1 metabotropic glutamate receptors (mGluRs), and specifically, mGluR5,
which is highly expressed in the striatum and has been shown to modulate
dopaminergic signaling (Paquet and Smith, 2003; Schwendt and McGinty, 2007;
McGinty et al., 2008; Mitrano et al., 2010). Indeed, a recent study has shown
that amphetamine, which is functionally and structurally similar to PEA, regulates
mGluR5 in vivo (Shaffer et al., 2010). Since MAO B exhibit behavioral
disinhibition and reduced anxiety-like behaviors, the phenotypic alterations in this
line may be underpinned by changes in other neurotransmitter systems, such as
GABA. In view of the anxiolytic effects of benzodiazepines and the mood-
166

enhancing actions of PEA administration and MAO B inhibitors, the interactions
of MAO B and PEA with the GABAergic system warrants further study.
In summary, this study documents that MAO A and MAO A/B deficiency result in
disruptions in NMDA receptor function. Moreover, these data point to a possible
a molecular mechanism that may underpin the cortical dysmorphogenesis, as
well as the alterations in perceptual processing of stimuli and aggression in MAO
A-deficient mice. Caution should be exercised, however, in the interpretation of
these results and in consideration of several limiting factors in this report. The
low prevalence of MAO A and MAO B in clinical populations may temper the
translational validity of this study. Additional studies are warranted to investigate
whether the underlying changes in NMDA receptor are due to alterations in
channel subunit composition or intracellular molecular cascades.
Irrespective of these conditions, the present study strongly supports the role of
MAO A in the regulation of NMDA receptor neurotransmission, and warrant
further investigations on the impact of NMDA receptor dysfunction on emotional
reactivity and adaptive behaviors.

167

Chapter 8 : Summary and conclusion

8.1: Brief recapitulation of results
The major findings of the present studies are that MAO A and MAO B exhibit
both overlapping and distinct effects on emotional reactivity and behavioral
plasticity and adaptation. In particular, MAO A-deficiency results in perturbations
in threat perception and affective disturbances, as well as marked behavioral
rigidity. While MAO A KO mice display elevated impulsive aggression,
hypomorphic MAO A mice show decreased aggression and an increase in
anxiety-like behaviors in the presence of different environmental conditions.
Nevertheless, both genotypes engage in compulsive marble-burying and digging
behaviors, as well as disturbances in emotional reactivity, further supporting the
role of MAO A in emotional regulation and maladaptive responses to contextual
cues. Interestingly, the behavioral outcomes of MAO A deficiency are unaffected
by exposure to severe restraint stress, which may reflect a dysregulation in
stress perception and underscore the emotional flattening and the limited
behavioral repertoire in this line.
Similar to their MAO A KO counterparts, MAO B KO mice show a blunted
hyperthermic reaction to restraint stress. Conversely, MAO B-deficient mice also
exhibit a reduction in anxiety-like behaviors and display several different facets of
behavioral disinhibition. Whereas MAO A- and MAO B-deficient mutants have
168

divergent effects on affective domains, genetic abrogation of each isoenzyme
results in the emergence of several overlapping features. Specifically, both
genotypes display a reduction in ultrasonic vocalizations, alterations in emotional
parameters, antisocial behavior to foreign conspecifics, and maladaptive
responses to environmental conditions. The congenital ablation of both
isoenzymes leads to more severe behavioral phenotype that encompasses
several different domains of autistic-like symptoms, such as communication
deficits, elevated aggression and antisocial manifestations, impairments in
aversive extinction, and behavioral rigidity that is highly reminiscent of the
tendency towards sameness. The behavioral anomalies featured in MAO A and
MAO A/B KO mice are accompanied by an increasing degree of NMDA receptor
dysfunction, which may heavily impinge on gating properties and result in the
maladaptive emotional responses to contextual cues observed in both
genotypes.

8.2: Impact of MAO A on emotional reactivity
Although the reactive nature of aggression in MAO A KO mice strongly
suggested that these disturbances were underpinned by alterations in emotional
processing and contextual evaluation, previous studies in rodents were unable
support this link. In agreement with the hypothesis, MAO A-deficient mice exhibit
disturbances in emotional regulation as evidenced by their neophobic and
aversive responses to foreign inanimate objects, as well as the reduction in fear-
169

related behaviors to predator stimuli and overt aggression to unfamiliar
conspecifics. Interestingly, these divergent responses to inanimate objects and
predator may reflect an overall affective flattening in this line. Nevertheless,
future studies should examine whether emotional reactivity is a potential
endophenotypic marker for aggressive behavior.
The cortical dysmorphogenesis and elevated EEG responses suggest a
dysregulation in the corticolimbic circuitry that may result in an overall impairment
of emotional information processing mechanisms. In view of the critical role of
the amygdala in fear and emotional processing, future studies should investigate
whether MAO A KO mice display alterations in corticoamygdalar circuitry.

8.3: Effects of MAO A-deficiency on behavioral responses to stress-
inducing stimuli
In line with the previous findings on emotional reactivity and physiological
response to stressors, MAO A-deficient mice exposed to severe restraint stress
failed to elicit behavioral alterations. Taken together with the absence of reaction
to predator threat, these results suggest that MAO A KO mice exhibit flat affect
and marked deficits in stress and threat perception. Interestingly, previous
studies documented that MAO A KO mice displayed a reduction in corticosterone
response to severe restraint, cold, water deprivation, and chronic variable stress,
but psychosocial stress evoked a similar increase in corticosterone as in their WT
170

counterparts (Popova et al., 2006). This scenario suggests that MAO A KO mice
view foreign conspecifics as more stressful than the other stress-inducing
conditions and may indicate disturbances in stress perception or a maladaptive
responses to contextual cues in this line. One of the primary limitations of this
study, however, is the lack of molecular data to support the role of MAO A in
stress regulation. To this end, future studies are warranted to examine different
components of the stress response, such as the corticotrophin-releasing factor
receptors. Additional experiments may investigate whether acute or chronic
stress alters MAO activity, as well as the role of early developmental stress on
MAO function. Since stress heavily influences synaptic plasticity through NMDA
receptor interactions, further studies are needed to examine the effects of NMDA
receptor manipulations, such as treatment with the NMDA receptor partial
agonist cycloserine, on the behavioral responses following stress in MAO A KO
mice.

8.4: Low MAO A activity is conducive to behavioral compulsivity
The partial ablation of MAO A activity in mice induces perseverative burying
behavior, as well as reduced aggression and an elevated sensitivity to
environmental conditions. These results are in agreement with my hypothesis,
however, hypomorphic MAO A mutant mice show an increase in selective
anxiety-related components, manifested as an increase in thigmotaxis, a
reduction in aggressiveness, and a higher intensity of compulsive responses than
171

their MAO A KO counterparts. Of note, the manipulation of environmental
conditions, such as light intensity, produced marked changes in the
aggressiveness of MAO A
neo
mice. These results suggest that MAO A impacts
aggression through impairments in emotional and perceptual processing of
contextual cues. Thus, while MAO A KO mice display an extremely rigid
phenotype, MAO A
neo
mice display more plastic behavioral responses. Similarly,
MAO A
neo
mice exhibited a comparable number of spontaneous alternations in
the T-maze paradigm as WT mice, suggesting that the partial ablation of MAO A
activity is sufficient to maintain a certain level of behavioral flexibility. Since the
neurochemical profile of MAO A
neo
mice differ from their MAO A KO counterparts
in cortical and amygdalar 5-HT levels, these two neurobiological substrates may
be critical for the development and/or maintenance of behavioral rigidity.
Nevertheless, this is the first study that addresses the role of hypomorphic MAO
A on behavioral outcomes and supports a link between low MAO A activity and
OCD-related traits. Future studies are warranted to investigate the impact of
stress-inducing stimuli on the behavioral outcomes of these mice.

8.5: Autistic spectrum disorder-related behavioral disturbances
The absence of fear-related responses in several different environments
associated with different degrees of threat, as well as the exaggerated aversive
reaction to a foreign object, heavily suggest that MAO A KO mice have deficits in
behavioral adaptation. In line with these premises, the overt aggression may be
172

a combination of emotional reactivity and behavioral perseveration. Indeed, a
typical encounter to an unfamiliar intruder mouse consists of the immediate
initiation of aversive defensive behaviors, signifying an inappropriate emotional
response to intrusive elements (foreign conspecific), while the repetition of
attacks may be mediated by compulsive-related substrates. The deficits in
idiosyncratic sensory modalities may also contribute to the maladaptive
exploratory activity towards environmental, object, and conspecifics.
Alternatively, the absence of behavioral adaptation to stimuli associated with
different intensities of threat may indicate that these mice have a narrow range of
behavioral responses and poor ability to modify their behaviors to changes in
environmental conditions. This latter possibility is supported by the deficits in
spontaneous alternation in the T-maze, as well as the significant reduction in
habituation. Of note, unpublished studies from our lab have found that MAO A
KO mice do not display short or long-term memory impairments, suggesting that
the habituation impairments are not due to mnemonic problems. In order to
verify that the maladaptive responses are caused by behavioral rigidity, future
studies should examine whether MAO A-deficient mice have deficits in reversal
learning.
Interestingly, MAO A KO mice display several other abnormalities that are
strikingly reminiscent of autistic-like behaviors. In addition to the emotional
reactivity, antisocial aggression, and behavioral rigidity, MAO A-deficient pups
show a reduction in maternal separation-induced vocalizations, which suggests a
173

deficit in communication from early postnatal stages. Although these results
heavily suggest the presence of autistic-like behaviors in this line, further
experiments may help extend these findings to other autistic features. To this
end, future studies may test whether MAO A-deficient mice exhibit a preference
for social novelty, empathetic responses to conspecifics in pain, or the engage in
the social transmission of a food preference.
Nevertheless, these data support previous evidence implicating MAO A in
emotional regulation and qualify the role of this enzyme in behavioral flexibility
and adaptation. Moreover, these findings are in substantial agreement with my
hypothesis, showing that MAO A modulates the fine adjustment of behavioral
responses in accordance to contextual cues, as deficits in MAO A severely
restrict the behavioral repertoire, leading to maladaptive responses and
emotional reactivity. Finally, these results provide compelling evidence for MAO
A-deficient mice as a model for autistic traits.

8.6: Role of MAO B in emotional regulation
In line with MAO A, perturbations in MAO B results in alterations in emotional and
adaptive behaviors. Specifically, we showed that MAO B ablation in mice results
in a reduction in anxiety-like behaviors and behavioral disinhibition. These data
are in substantial agreement with my hypothesis and support clinical reports
174

documenting novelty-seeking and risk-taking behaviors in patients with low MAO
B activity.
Unpublished data from the elevated plus-maze paradigm show that MAO B KO
mice display an increase in stretch-attend postures from the closed arms (secure
environment), but a decrease from the open arms (unsecure environment). This
mismatch of behavioral responses in these contexts supports the contention that
MAO B-deficient mice have maladaptive behavioral disturbances.
In addition, MAO B KO mice display a significant reduction in spontaneous
alternations in the T-maze paradigm. While this parameter may represent
behavioral rigidity, it may also signify spatial working memory. Preliminary
studies in our lab have found that MAO B-deficient mice exhibit long-term
mnemonic impairments, suggesting that the reduction in alternations is related to
mnemonic indices. Spatial working memory impairments has been hypothesized
as a core psychological construct underpinning ADHD psychopathology and
thought to contribute to the maladaptive behavior and the motivation to seek out
novel stimulation often documented in patients (Stevens, 2005). Interestingly,
MAO B KO mice exhibit increased novelty-seeking behavior and reduced threat
detection in different environmental conditions, which may suggest that the
behavioral disinhibition and maladaptive behavior reflects underlying impairments
in attention processing.
175

In view of this possibility, the behavioral outcomes of MAO B-deficient mice may
be attributed to increases in distractability and other attentional deficits, which
may impinge on their mnemonic capacity and their inability to properly adapt in
accordance with environmental stimuli. Since low MAO B activity has been
associated with poor impulse control, a core feature of ADHD, the link between
MAO B and attention should be examined in future studies. Specifically,
experiments should be designed to capture impulsive parameters dimensions
and attentional performance, such as the delayed discounting, go-no/go, and 5-
choice serial reaction tests. Future studies should also investigate the mnemonic
function in MAO B KO mice in view of the long-term and working memory
impairments mirrored by chronic amphetamine treatment (Dalley et al., 2005;
Peleg-Raibstein et al., 2009; Selemon et al., 2010). Nevertheless, these findings
expand on the previously elusive role of MAO B in behavioral disinhibition and
provide possible insights in the behavioral sequelae of high levels of PEA.

8.7: The role of MAO in the regulation of NMDA receptor function
In partial support of my hypothesis, MAO A KO and MAO A/B KO mice show
hypersensitivity to subthreshold doses of dizocilpine, indicating disturbances in
NMDA receptor function. Surprisingly, MAO A
neo
mice did not display an
intermediate phenotype between MAO A KO and WT mice, but exhibited a
significant enhancement of stereotyped behavior and ataxia. Conversely, MAO
A KO mice exhibited marked locomotor sinuosity in the open field paradigm.
176

Although this behavior was qualitatively distinct from the typical ataxia attributed
to high doses of dizocilpine, the repetition of this stereotyped behavioral pattern
likely signifying NMDA receptor dysfunction. The differences in movement
quality between MAO A KO and MAO A
neo
mice may reflect alterations in
corticocerebellar circuitry that mediate locomotor coordination. This effect may
be related to the divergent neurochemical profiles in the cortex between the lines.
Nevertheless, the dizocilpine-induced sinuosity in MAO A-deficient mice may
result from an activation of a glutamatergic-mediated perseverative behavioral
loop that is difficult to impede due to the deficits in behavioral extinction and
flexibility in this line.
NMDA receptor blockade in MAO A/B KO mice elicited profound stereotyped
behaviors, consisting of vacuous chewing, gnawing, and other oral stereotypies,
as well as a marked loss in motor coordination. This effect is likely mediated by
the serotonergic hyperactivity in these lines, in view of the reciprocal modulatory
actions between 5-HT and glutamatergic mechanisms. Based on the critical role
of NMDA receptors in synaptic plasticity mechanisms, MAO-induced
perturbations in NMDA receptor neurotransmission may impair the integration of
perceptual information with appropriate behavioral responses through alterations
in cortical networks. Ultimately, these disturbances may contribute to several
different behavioral abnormalities in these lines, including behavioral rigidity,
deficits in extinction, and maladaptive reactions to contextual cues. An intriguing
possibility is that excessive activation of 5-HT2A during early postnatal time
177

periods may initiate genetic modifications of the NMDA receptor composition or
changes in the intracellular cascade to avoid the detrimental effects of chronic 5-
HT2A stimulation.
Conversely, MAO B-deficient mice displayed similar behavioral manifestations to
dizocilpine as WT mice, indicating that MAO B ablation does not influence NMDA
receptor function. Although these results are not in agreement with my
hypothesis, it may be possible that MAO B acts on other glutamatergic-related
mechanisms, such as the mGluRs. Nevertheless, these studies provide an
important conceptual framework on the effect of MAO A in NMDA receptor
function and provide mechanistic insights into the role of MAO A in behavioral
disturbances.

8.8: Conclusion
It is worthy to note that while the ablation of MAO A in mice results in an increase
in neophobia and aversive responses to objects, the loss of MAO B activity leads
to the opposite phenotype – an increase in novelty-seeking behavior.
Conversely, both genotypes show attenuated responses to stress and deficits in
behavioral adaptation.
Taken together, these findings highlight the distinct roles of each isoenzyme in
emotional regulation and behavioral adaptation. Specifically, these isoenzymes
appear to set a balance of emotional reactivity and a sensitivity to change
178

according to contextual cues. While alterations in environmental stimuli lead to
adaptive modifications and subtle changes in the behavioral repertoire of WT
mice, these identical conditions induce a limited set of behavioral responses in
MAO mutant mice. In keeping with these premises, the absence of MAO
severely restricts the capacity to finely-tune behaviors, and instead results in a
proverbial “on/off” switching to different behavioral strategies. Furthermore, the
ability to switch behavioral responses is significantly impaired, as signified by the
marked behavioral rigidity in the MAO A KO and MAO A/B KO lines.
Although these studies expanded the role of MAO in behavioral organization, the
interpretation of these results should be tempered. Importantly, the use of
animals to partially mimic psychiatric disorders may not be translatable to clinical
settings. Moreover, these models cannot fully recapitulate a neuropsychatric
disorder and great care should be employed to avoid anthropomorphizing the
data. Similarly, the absence of molecular data in these studies warrants further
experimental testing to link the behavioral findings with neurobiological
substrates and underlying molecular mechanisms.
Nevertheless, these experiments highlight the role of MAO in modulating the
emotional processing of environmental stimuli and enacting an appropriate
behavioral response. MAO appears to finely tune the emotional valence of
behavioral responses to contextual cues, whereas the loss of MAO function
cripples the ability to adequately modify behaviors in accordance with
179

contingency changes. In summary, these studies may help lead to a
reconceptualization of the role of MAO in behavioral plasticity.
180

Bibliography

Abbruzzese M, Bellodi L, Ferri S, Scarone S (1995) Frontal lobe dysfunction in
schizophrenia and obsessive-compulsive disorder: a neuropsychological
study. Brain Cogn 27:202-212.
Adamec R, Kent P, Anisman H, Shallow T, Merali Z (1998) Neural plasticity,
neuropeptides and anxiety in animals--implications for understanding and
treating affective disorder following traumatic stress in humans. Neurosci
Biobehav Rev 23:301-318.
Adams B, Moghaddam B (1998) Corticolimbic dopamine neurotransmission is
temporally dissociated from the cognitive and locomotor effects of
phencyclidine. J Neurosci 18:5545-5554.
Adenauer H, Pinosch S, Catani C, Gola H, Keil J, Kissler J, Neuner F (2010)
Early processing of threat cues in posttraumatic stress disorder-evidence
for a cortical vigilance-avoidance reaction. Biol Psychiatry 68:451-458.
Agatsuma S, Lee M, Zhu H, Chen K, Shih JC, Seif I, Hiroi N (2006) Monoamine
oxidase A knockout mice exhibit impaired nicotine preference but normal
responses to novel stimuli. Hum Mol Genet 15:2721-2731.
Aguilar de Arcos F, Verdejo-Garcia A, Peralta-Ramirez MI, Sanchez-Barrera M,
Perez-Garcia M (2005) Experience of emotions in substance abusers
exposed to images containing neutral, positive, and negative affective
stimuli. Drug Alcohol Depend 78:159-167.
Akhondzadeh S, Tavakolian R, Davari-Ashtiani R, Arabgol F, Amini H (2003)
Selegiline in the treatment of attention deficit hyperactivity disorder in
children: a double blind and randomized trial. Prog
Neuropsychopharmacol Biol Psychiatry 27:841-845.
Albert Adell AB, Llorenc Diaz-Mataix, Noemi Santana, Pau Celada, Francesc
Artigas (2010) Serotonin Interaction with Other Transmitter Systems. In:
Handbook of the Behavioral Neurobiology of Serotonin (Christian P. Muller
BLJ, ed): Academic Press.
Alia-Klein N, Goldstein RZ, Tomasi D, Woicik PA, Moeller SJ, Williams B, Craig
IW, Telang F, Biegon A, Wang GJ, Fowler JS, Volkow ND (2009) Neural
mechanisms of anger regulation as a function of genetic risk for violence.
Emotion 9:385-396.

181

Alia-Klein N, Goldstein RZ, Kriplani A, Logan J, Tomasi D, Williams B, Telang F,
Shumay E, Biegon A, Craig IW, Henn F, Wang GJ, Volkow ND, Fowler JS
(2008) Brain monoamine oxidase A activity predicts trait aggression. J
Neurosci 28:5099-5104.
Allen GF, Land JM, Heales SJ (2009) A new perspective on the treatment of
aromatic L-amino acid decarboxylase deficiency. Mol Genet Metab 97:6-
14.
Amitai N, Markou A (2010) Disruption of performance in the five-choice serial
reaction time task induced by administration of N-methyl-D-aspartate
receptor antagonists: relevance to cognitive dysfunction in schizophrenia.
Biol Psychiatry 68:5-16.
Anderson GM, Gutknecht L, Cohen DJ, Brailly-Tabard S, Cohen JH, Ferrari P,
Roubertoux PL, Tordjman S (2002) Serotonin transporter promoter
variants in autism: functional effects and relationship to platelet
hyperserotonemia. Mol Psychiatry 7:831-836.
Anderson GM, Freedman DX, Cohen DJ, Volkmar FR, Hoder EL, McPhedran P,
Minderaa RB, Hansen CR, Young JG (1987) Whole blood serotonin in
autistic and normal subjects. J Child Psychol Psychiatry 28:885-900.
Annesley PT (1969) Nardil response in a chronic obsessive compulsive. Br J
Psychiatry 115:748.
Ansorge MS, Morelli E, Gingrich JA (2008) Inhibition of serotonin but not
norepinephrine transport during development produces delayed,
persistent perturbations of emotional behaviors in mice. J Neurosci
28:199-207.
Ansorge MS, Zhou M, Lira A, Hen R, Gingrich JA (2004) Early-life blockade of
the 5-HT transporter alters emotional behavior in adult mice. Science
306:879-881.
Arborelius L, Nemeroff C (2002) Preclinical models of anxiety. In: Textbook of
anxiety disorders (Stein D, Hollander E, eds), pp 29-42. Washington, DC:
American Psychiatric Publishing Inc.
Arnsten AF (2009) Stress signalling pathways that impair prefrontal cortex
structure and function. Nat Rev Neurosci 10:410-422.
Arvanov VL, Liang X, Russo A, Wang RY (1999) LSD and DOB: interaction with
5-HT2A receptors to inhibit NMDA receptor-mediated transmission in the
rat prefrontal cortex. Eur J Neurosci 11:3064-3072.
182

Avgustinovich DF, Lipina TV, Bondar NP, Alekseyenko OV, Kudryavtseva NN
(2000) Features of the genetically defined anxiety in mice. Behav Genet
30:101-109.
Axelson DA, Doraiswamy PM, McDonald WM, Boyko OB, Tupler LA, Patterson
LJ, Nemeroff CB, Ellinwood EH, Jr., Krishnan KR (1993)
Hypercortisolemia and hippocampal changes in depression. Psychiatry
Res 47:163-173.
Balciuniene J, Emilsson L, Oreland L, Pettersson U, Jazin E (2002) Investigation
of the functional effect of monoamine oxidase polymorphisms in human
brain. Hum Genet 110:1-7.
Bale TL (2005) Sensitivity to stress: dysregulation of CRF pathways and disease
development. Horm Behav 48:1-10.
Balla A, Koneru R, Smiley J, Sershen H, Javitt DC (2001a) Continuous
phencyclidine treatment induces schizophrenia-like hyperreactivity of
striatal dopamine release. Neuropsychopharmacology 25:157-164.
Balla A, Hashim A, Burch S, Javitt DC, Lajtha A, Sershen H (2001b)
Phencyclidine-induced dysregulation of dopamine response to
amphetamine in prefrontal cortex and striatum. Neurochem Res 26:1001-
1006.
Banki CM, Bissette G, Arato M, O'Connor L, Nemeroff CB (1987) CSF
corticotropin-releasing factor-like immunoreactivity in depression and
schizophrenia. Am J Psychiatry 144:873-877.
Barrow JR, Capecchi MR (1996) Targeted disruption of the Hoxb-2 locus in mice
interferes with expression of Hoxb-1 and Hoxb-4. Development 122:3817-
3828.
Battaglia M, Ogliari A (2005) Anxiety and panic: from human studies to animal
research and back. Neurosci Biobehav Rev 29:169-179.
Baud P, Arbilla S, Cantrill RC, Scatton B, Langer SZ (1985) Trace amines inhibit
the electrically evoked release of [3H]acetylcholine from slices of rat
striatum in the presence of pargyline: similarities between beta-
phenylethylamine and amphetamine. J Pharmacol Exp Ther 235:220-229.
Baxter LR, Jr., Schwartz JM, Mazziotta JC, Phelps ME, Pahl JJ, Guze BH,
Fairbanks L (1988) Cerebral glucose metabolic rates in nondepressed
patients with obsessive-compulsive disorder. Am J Psychiatry 145:1560-
1563.

183

Bechara A, Damasio H (2002) Decision-making and addiction (part I): impaired
activation of somatic states in substance dependent individuals when
pondering decisions with negative future consequences.
Neuropsychologia 40:1675-1689.
Bechara A, Dolan S, Denburg N, Hindes A, Anderson SW, Nathan PE (2001)
Decision-making deficits, linked to a dysfunctional ventromedial prefrontal
cortex, revealed in alcohol and stimulant abusers. Neuropsychologia
39:376-389.
Beique JC, Chapin-Penick EM, Mladenovic L, Andrade R (2004) Serotonergic
facilitation of synaptic activity in the developing rat prefrontal cortex. J
Physiol 556:739-754.
Belzung C (1999) Handbook of Molecular Genetic Techniques for Brain and
Behavior Research. Amsterdam: Elsevier.
Belzung C, Le Pape G (1994) Comparison of different behavioral test situations
used in psychopharmacology for measurement of anxiety. Physiol Behav
56:623-628.
Berlin I, Heilbronner C, Georgieu S, Meier C, Launay JM, Spreux-Varoquaux O
(2009) Reduced monoamine oxidase A activity in pregnant smokers and
in their newborns. Biol Psychiatry 66:728-733.
Betancur C, Corbex M, Spielewoy C, Philippe A, Laplanche JL, Launay JM,
Gillberg C, Mouren-Simeoni MC, Hamon M, Giros B, Nosten-Bertrand M,
Leboyer M (2002) Serotonin transporter gene polymorphisms and
hyperserotonemia in autistic disorder. Mol Psychiatry 7:67-71.
Black KJ, Hershey T, Koller JM, Videen TO, Mintun MA, Price JL, Perlmutter JS
(2002) A possible substrate for dopamine-related changes in mood and
behavior: prefrontal and limbic effects of a D3-preferring dopamine
agonist. Proc Natl Acad Sci U S A 99:17113-17118.
Blair RJR (2009) The Neurobiology of Aggression. In: Neurobiology of Mental
Illiness, 3rd Edition (Charney DS, Nestler, E.J., ed), pp 1307-1320.
Oxford: Oxford University Press.
Blanchard D, Blanchard, RJ (2008) Defensive behaviors, fear, and anxiety. In:
Handbook of Anxiety and Fear (Blanchard R, Blanchard, DC, Griebel, G,
Nutt, DJ, ed), pp 63-79. Amsterdam: Academic Press.
Blanchard DC, Griebel G, Blanchard RJ (2001) Mouse defensive behaviors:
pharmacological and behavioral assays for anxiety and panic. Neurosci
Biobehav Rev 25:205-218.
184

Blanchard DC, Blanchard RJ, Carobrez Ade P, Veniegas R, Rodgers RJ,
Shepherd JK (1992) MK-801 produces a reduction in anxiety-related
antipredator defensiveness in male and female rats and a gender-
dependent increase in locomotor behavior. Psychopharmacology (Berl)
108:352-362.
Blanco C, Orensanz-Munoz L, Blanco-Jerez C, Saiz-Ruiz J (1996) Pathological
gambling and platelet MAO activity: a psychobiological study. Am J
Psychiatry 153:119-121.
Blier P, de Montigny C (1998) Possible serotonergic mechanisms underlying the
antidepressant and anti-obsessive-compulsive disorder responses. Biol
Psychiatry 44:313-323.
Blokland A, Lieben C, Deutz NE (2002) Anxiogenic and depressive-like effects,
but no cognitive deficits, after repeated moderate tryptophan depletion in
the rat. J Psychopharmacol 16:39-49.
Bodkin JA, Cohen BM, Salomon MS, Cannon SE, Zornberg GL, Cole JO (1996)
Treatment of negative symptoms in schizophrenia and schizoaffective
disorder by selegiline augmentation of antipsychotic medication. A pilot
study examining the role of dopamine. J Nerv Ment Dis 184:295-301.
Bohnke R, Bertsch K, Kruk MR, Naumann E (2010) The relationship between
basal and acute HPA axis activity and aggressive behavior in adults. J
Neural Transm 117:629-637.
Bohus B, Benus RF, Fokkema DS, Koolhaas JM, Nyakas C, van Oortmerssen
GA, Prins AJ, de Ruiter AJ, Scheurink AJ, Steffens AB (1987)
Neuroendocrine states and behavioral and physiological stress responses.
Prog Brain Res 72:57-70.
Bondi CO, Barrera G, Lapiz MD, Bedard T, Mahan A, Morilak DA (2007)
Noradrenergic facilitation of shock-probe defensive burying in lateral
septum of rats, and modulation by chronic treatment with desipramine.
Prog Neuropsychopharmacol Biol Psychiatry 31:482-495.
Borowsky B, Adham N, Jones KA, Raddatz R, Artymyshyn R, Ogozalek KL,
Durkin MM, Lakhlani PP, Bonini JA, Pathirana S, Boyle N, Pu X,
Kouranova E, Lichtblau H, Ochoa FY, Branchek TA, Gerald C (2001)
Trace amines: identification of a family of mammalian G protein-coupled
receptors. Proc Natl Acad Sci U S A 98:8966-8971.
Borsini F, Podhorna J, Marazziti D (2002) Do animal models of anxiety predict
anxiolytic-like effects of antidepressants? Psychopharmacology (Berl)
163:121-141.
185

Bortolato M, Godar SC (2010) Animal models of virus-induced neurobehavioral
sequelae: recent advances, methodological issues, and future prospects.
Interdiscip Perspect Infect Dis 2010:380456.
Bortolato M, Chen K, Shih JC (2008) Monoamine oxidase inactivation: from
pathophysiology to therapeutics. Adv Drug Deliv Rev 60:1527-1533.
Bortolato M, Godar SC, Davarian S, Chen K, Shih JC (2009a) Behavioral
disinhibition and reduced anxiety-like behaviors in monoamine oxidase B-
deficient mice. Neuropsychopharmacology 34:2746-2757.
Bortolato M, Frau R, Piras A, Luesu W, Bini V, Diaz G, Gessa G, Ennas M,
Castelli M (2009b) Methamphetamine Induces Long-term Alterations in
Reactivity to Environmental Stimuli: Correlation with Dopaminergic and
Serotonergic Toxicity. Neurotox Res 15:369-393.
Bortolato M, Campolongo P, Mangieri RA, Scattoni ML, Frau R, Trezza V, La
Rana G, Russo R, Calignano A, Gessa GL, Cuomo V, Piomelli D (2006)
Anxiolytic-like properties of the anandamide transport inhibitor AM404.
Neuropsychopharmacology 31:2652-2659.
Bourin M, Nic Dhonnchadha BA, Claude Colombel M, Dib M, Hascoet M (2001)
Cyamemazine as an anxiolytic drug on the elevated plus maze and
light/dark paradigm in mice. Behav Brain Res 124:87-95.
Bouwknecht JA, Spiga F, Staub DR, Hale MW, Shekhar A, Lowry CA (2007)
Differential effects of exposure to low-light or high-light open-field on
anxiety-related behaviors: relationship to c-Fos expression in serotonergic
and non-serotonergic neurons in the dorsal raphe nucleus. Brain Res Bull
72:32-43.
Bradaia A, Trube G, Stalder H, Norcross RD, Ozmen L, Wettstein JG, Pinard A,
Buchy D, Gassmann M, Hoener MC, Bettler B (2009) The selective
antagonist EPPTB reveals TAAR1-mediated regulatory mechanisms in
dopaminergic neurons of the mesolimbic system. Proc Natl Acad Sci U S
A 106:20081-20086.
Brecht M, Preilowski B, Merzenich MM (1997) Functional architecture of the
mystacial vibrissae. Behavioural Brain Research 84:81-97.
Broekkamp CL, Rijk HW, Joly-Gelouin D, Lloyd KL (1986) Major tranquillizers
can be distinguished from minor tranquillizers on the basis of effects on
marble burying and swim-induced grooming in mice. Eur J Pharmacol
126:223-229.
Brown GR, Nemes C (2008) The exploratory behaviour of rats in the hole-board
apparatus: is head-dipping a valid measure of neophilia? Behav
Processes 78:442-448.
186

Brown SM, Henning S, Wellman CL (2005) Mild, short-term stress alters dendritic
morphology in rat medial prefrontal cortex. Cereb Cortex 15:1714-1722.
Brummett BH, Boyle SH, Siegler IC, Kuhn CM, Surwit RS, Garrett ME, Collins A,
Ashley-Koch A, Williams RB (2008) HPA axis function in male caregivers:
effect of the monoamine oxidase-A gene promoter (MAOA-uVNTR). Biol
Psychol 79:250-255.
Brune CW, Kim SJ, Salt J, Leventhal BL, Lord C, Cook EH, Jr. (2006) 5-HTTLPR
Genotype-Specific Phenotype in Children and Adolescents With Autism.
Am J Psychiatry 163:2148-2156.
Brunetti M, Sepede G, Mingoia G, Catani C, Ferretti A, Merla A, Del Gratta C,
Romani GL, Babiloni C (2010) Elevated response of human amygdala to
neutral stimuli in mild post traumatic stress disorder: neural correlates of
generalized emotional response. Neuroscience 168:670-679.
Brunner HG, Nelen M, Breakefield XO, Ropers HH, van Oost BA (1993a)
Abnormal behavior associated with a point mutation in the structural gene
for monoamine oxidase A. Science 262:578-580.
Brunner HG, Nelen MR, van Zandvoort P, Abeling NG, van Gennip AH, Wolters
EC, Kuiper MA, Ropers HH, van Oost BA (1993b) X-linked borderline
mental retardation with prominent behavioral disturbance: phenotype,
genetic localization, and evidence for disturbed monoamine metabolism.
Am J Hum Genet 52:1032-1039.
Brunton LL, Lazo, John S., Parker, Keith L., ed (2006) Goodman and Gilman's
The Pharmacological Basis of Therapeutics, 11th Edition: McGraw-Hill
Inc.
Bruto V, Kokkinidis L, Anisman H (1983) Attenuation of perseverative behavior
after repeated amphetamine treatment: tolerance or attentional deficits?
Pharmacol Biochem Behav 19:497-504.
Bruto V, Beauchamp C, Zacharko RM, Anisman H (1984) Amphetamine-induced
perseverative behavior in a radial arm maze following DSP4 or 6-OHDA
pretreatment. Psychopharmacology (Berl) 83:62-69.
Bryson SE (2006) The Autistic Mind. In: The Neurobiology of Autism, 2nd Edition
(Bauman ML, Kimper, Thomas L., ed): The Johns Hopkins University
Press.
Buchsbaum MS, Coursey RD, Murphy DL (1976) The biochemical high-risk
paradigm: behavioral and familial correlates of low platelet monoamine
oxidase activity. Science 194:339-341.
187

Buckholtz JW, Meyer-Lindenberg A (2008) MAOA and the neurogenetic
architecture of human aggression. Trends Neurosci 31:120-129.
Bunzow JR, Sonders MS, Arttamangkul S, Harrison LM, Zhang G, Quigley DI,
Darland T, Suchland KL, Pasumamula S, Kennedy JL, Olson SB, Magenis
RE, Amara SG, Grandy DK (2001) Amphetamine, 3,4-
methylenedioxymethamphetamine, lysergic acid diethylamide, and
metabolites of the catecholamine neurotransmitters are agonists of a rat
trace amine receptor. Mol Pharmacol 60:1181-1188.
Calabrese F, Molteni R, Racagni G, Riva MA (2009) Neuronal plasticity: a link
between stress and mood disorders. Psychoneuroendocrinology 34 Suppl
1:S208-216.
Camarena B, Rinetti G, Cruz C, Gomez A, de La Fuente JR, Nicolini H (2001)
Additional evidence that genetic variation of MAO-A gene supports a
gender subtype in obsessive-compulsive disorder. Am J Med Genet
105:279-282.
Campbell T, Lin S, DeVries C, Lambert K (2003) Coping strategies in male and
female rats exposed to multiple stressors. Physiol Behav 78:495-504.
Cannistraro PA, Wright CI, Wedig MM, Martis B, Shin LM, Wilhelm S, Rauch SL
(2004) Amygdala responses to human faces in obsessive-compulsive
disorder. Biol Psychiatry 56:916-920.
Carli M, Baviera M, Invernizzi RW, Balducci C (2006) Dissociable contribution of
5-HT1A and 5-HT2A receptors in the medial prefrontal cortex to different
aspects of executive control such as impulsivity and compulsive
perseveration in rats. Neuropsychopharmacology 31:757-767.
Carlsson A, Waters N, Holm-Waters S, Tedroff J, Nilsson M, Carlsson ML (2001)
Interactions between monoamines, glutamate, and GABA in
schizophrenia: new evidence. Annu Rev Pharmacol Toxicol 41:237-260.
Carlsson ML (1998) Hypothesis: is infantile autism a hypoglutamatergic disorder?
Relevance of glutamate - serotonin interactions for pharmacotherapy. J
Neural Transm 105:525-535.
Carlsson ML (2001) On the role of prefrontal cortex glutamate for the antithetical
phenomenology of obsessive compulsive disorder and attention deficit
hyperactivity disorder. Prog Neuropsychopharmacol Biol Psychiatry 25:5-
26.
Carper RA, Courchesne E (2000) Inverse correlation between frontal lobe and
cerebellum sizes in children with autism. Brain 123 ( Pt 4):836-844.
188

Carper RA, Courchesne E (2005) Localized enlargement of the frontal cortex in
early autism. Biol Psychiatry 57:126-133.
Carrasco JL, Saiz-Ruiz J, Hollander E, Cesar J, Lopez-Ibor JJ, Jr. (1994) Low
platelet monoamine oxidase activity in pathological gambling. Acta
Psychiatr Scand 90:427-431.
Carrera N, Sanjuan J, Molto MD, Carracedo A, Costas J (2009) Recent adaptive
selection at MAOB and ancestral susceptibility to schizophrenia. Am J
Med Genet B Neuropsychiatr Genet 150B:369-374.
Carvalho-Netto EF, Nunes-de-Souza RL (2004) Use of the elevated T-maze to
study anxiety in mice. Behav Brain Res 148:119-132.
Casarrubea M, Sorbera F, Crescimanno G (2008) Structure of rat behavior in
hole-board: I) multivariate analysis of response to anxiety. Physiol Behav
96:174-179.
Cases O, Vitalis T, Seif I, De Maeyer E, Sotelo C, Gaspar P (1996) Lack of
barrels in the somatosensory cortex of monoamine oxidase A-deficient
mice: role of a serotonin excess during the critical period. Neuron 16:297-
307.
Cases O, Seif I, Grimsby J, Gaspar P, Chen K, Pournin S, Muller U, Aguet M,
Babinet C, Shih JC, et al. (1995) Aggressive behavior and altered
amounts of brain serotonin and norepinephrine in mice lacking MAOA.
Science 268:1763-1766.
Caspi A, McClay J, Moffitt TE, Mill J, Martin J, Craig IW, Taylor A, Poulton R
(2002) Role of genotype in the cycle of violence in maltreated children.
Science 297:851-854.
Catani C, Adenauer H, Keil J, Aichinger H, Neuner F (2009) Pattern of cortical
activation during processing of aversive stimuli in traumatized survivors of
war and torture. Eur Arch Psychiatry Clin Neurosci 259:340-351.
Chakrabarti SK, Loua KM, Bai C, Durham H, Panisset JC (1998) Modulation of
monoamine oxidase activity in different brain regions and platelets
following exposure of rats to methylmercury. Neurotoxicol Teratol 20:161-
168.
Chakrabarty K, Bhattacharyya S, Christopher R, Khanna S (2005) Glutamatergic
dysfunction in OCD. Neuropsychopharmacology 30:1735-1740.
Chen K, Holschneider DP, Wu W, Rebrin I, Shih JC (2004) A spontaneous point
mutation produces monoamine oxidase A/B knock-out mice with greatly
elevated monoamines and anxiety-like behavior. J Biol Chem 279:39645-
39652.
189

Chen K, Cases O, Rebrin I, Wu W, Gallaher TK, Seif I, Shih JC (2007) Forebrain-
specific expression of monoamine oxidase A reduces neurotransmitter
levels, restores the brain structure, and rescues aggressive behavior in
monoamine oxidase A-deficient mice. J Biol Chem 282:115-123.
Chen L, He M, Sibille E, Thompson A, Sarnyai Z, Baker H, Shippenberg T, Toth
M (1999) Adaptive changes in postsynaptic dopamine receptors despite
unaltered dopamine dynamics in mice lacking monoamine oxidase B. J
Neurochem 73:647-655.
Chugani DC (2002) Role of altered brain serotonin mechanisms in autism. Mol
Psychiatry 7 Suppl 2:S16-17.
Chugani DC, Muzik O, Behen M, Rothermel R, Janisse JJ, Lee J, Chugani HT
(1999) Developmental changes in brain serotonin synthesis capacity in
autistic and nonautistic children. Ann Neurol 45:287-295.
Coccini T, Crevani A, Rossi G, Assandri F, Balottin U, Nardo RD, Manzo L
(2009) Reduced platelet monoamine oxidase type B activity and
lymphocyte muscarinic receptor binding in unmedicated children with
attention deficit hyperactivity disorder. Biomarkers 14:513-522.
Cohen IL, Liu X, Schutz C, White BN, Jenkins EC, Brown WT, Holden JJ (2003)
Association of autism severity with a monoamine oxidase A functional
polymorphism. Clin Genet 64:190-197.
Cohen IL, Liu X, Lewis ME, Chudley A, Forster-Gibson C, Gonzalez M, Jenkins
E, Brown WT, Holden JJ Autism severity is associated with child and
maternal MAOA genotypes. Clin Genet.
Commissaris RL, Humrich J, Johns J, Geere DG, Fontana DJ (1995) The effects
of selective and non-selective monoamine oxidase (MAO) inhibitors on
conflict behavior in the rat. Behav Pharmacol 6:195-202.
Constans JI (2005) Information-Processing Biases in PTSD. In: Neuropsychology
of PTSD: Biological, Cognitive, and Clinical Perspectives (Jennifer J.
Vasterling CRB, ed): Guilford Publications Inc.
Cook EH, Jr., Arora RC, Anderson GM, Berry-Kravis EM, Yan SY, Yeoh HC,
Sklena PJ, Charak DA, Leventhal BL (1993) Platelet serotonin studies in
hyperserotonemic relatives of children with autistic disorder. Life Sci
52:2005-2015.
Cook SC, Wellman CL (2004) Chronic stress alters dendritic morphology in rat
medial prefrontal cortex. J Neurobiol 60:236-248.

190

Corbett R, Camacho F, Woods AT, Kerman LL, Fishkin RJ, Brooks K, Dunn RW
(1995) Antipsychotic agents antagonize non-competitive N-methyl-D-
aspartate antagonist-induced behaviors. Psychopharmacology (Berl)
120:67-74.
Corbetta M, Shulman GL (2002) Control of goal-directed and stimulus-driven
attention in the brain. Nat Rev Neurosci 3:201-215.
Costa P, Checkoway H, Levy D, Smith-Weller T, Franklin GM, Swanson PD,
Costa LG (1997) Association of a polymorphism in intron 13 of the
monoamine oxidase B gene with Parkinson disease. Am J Med Genet
74:154-156.
Courchesne E, Pierce K (2005) Why the frontal cortex in autism might be talking
only to itself: local over-connectivity but long-distance disconnection. Curr
Opin Neurobiol 15:225-230.
Courchesne E, Carper R, Akshoomoff N (2003) Evidence of brain overgrowth in
the first year of life in autism. JAMA 290:337-344.
Courchesne E, Karns CM, Davis HR, Ziccardi R, Carper RA, Tigue ZD, Chisum
HJ, Moses P, Pierce K, Lord C, Lincoln AJ, Pizzo S, Schreibman L, Haas
RH, Akshoomoff NA, Courchesne RY (2001) Unusual brain growth
patterns in early life in patients with autistic disorder: an MRI study.
Neurology 57:245-254.
Coutinho AM, Sousa I, Martins M, Correia C, Morgadinho T, Bento C, Marques
C, Ataide A, Miguel TS, Moore JH, Oliveira G, Vicente AM (2007)
Evidence for epistasis between SLC6A4 and ITGB3 in autism etiology and
in the determination of platelet serotonin levels. Hum Genet 121:243-256.
Creese I, Iversen SD (1973) Blockage of amphetamine induced motor stimulation
and stereotypy in the adult rat following neonatal treatment with 6-
hydroxydopamine. Brain Res 55:369-382.
Crider A, Solomon PR, McMahon MA (1982) Disruption of selective attention in
the rat following chronic d-amphetamine administration: relationship to
schizophrenic attention disorder. Biol Psychiatry 17:351-361.
Crombag HS, Gorny G, Li Y, Kolb B, Robinson TE (2005) Opposite effects of
amphetamine self-administration experience on dendritic spines in the
medial and orbital prefrontal cortex. Cereb Cortex 15:341-348.
Cryan JF, O'Leary OF, Jin SH, Friedland JC, Ouyang M, Hirsch BR, Page ME,
Dalvi A, Thomas SA, Lucki I (2004) Norepinephrine-deficient mice lack
responses to antidepressant drugs, including selective serotonin reuptake
inhibitors. Proc Natl Acad Sci U S A 101:8186-8191.
191

Dalley JW, Theobald DE, Berry D, Milstein JA, Laane K, Everitt BJ, Robbins TW
(2005) Cognitive sequelae of intravenous amphetamine self-administration
in rats: evidence for selective effects on attentional performance.
Neuropsychopharmacology 30:525-537.
Danckwerts A, Leathem J (2003) Questioning the link between PTSD and
cognitive dysfunction. Neuropsychol Rev 13:221-235.
Davis LK, Hazlett HC, Librant AL, Nopoulos P, Sheffield VC, Piven J, Wassink
TH (2008) Cortical enlargement in autism is associated with a functional
VNTR in the monoamine oxidase A gene. Am J Med Genet B
Neuropsychiatr Genet 147B:1145-1151.
Daw MI, Scott HL, Isaac JT (2007) Developmental synaptic plasticity at the
thalamocortical input to barrel cortex: mechanisms and roles. Mol Cell
Neurosci 34:493-502.
Dawson GR, Tricklebank MD (1995) Use of the elevated plus maze in the search
for novel anxiolytic agents. Trends Pharmacol Sci 16:33-36.
De Angelis L, Furlan C (2000) The anxiolytic-like properties of two selective
MAOIs, moclobemide and selegiline, in a standard and an enhanced
light/dark aversion test. Pharmacol Biochem Behav 65:649-653.
De Filippis B, Ricceri L, Laviola G (2010) Early postnatal behavioral changes in
the Mecp2-308 truncation mouse model of Rett syndrome. Genes Brain
Behav 9:213-223.
de Kloet ER, Joels M, Holsboer F (2005) Stress and the brain: from adaptation to
disease. Nat Rev Neurosci 6:463-475.
Deller T, Sarter M (1998) Effects of repeated administration of amphetamine on
behavioral vigilance: evidence for "sensitized" attentional impairments.
Psychopharmacology (Berl) 137:410-414.
Dellu F, Contarino A, Simon H, Koob GF, Gold LH (2000) Genetic differences in
response to novelty and spatial memory using a two-trial recognition task
in mice. Neurobiol Learn Mem 73:31-48.
DeLorey TM, Sahbaie P, Hashemi E, Homanics GE, Clark JD (2008) Gabrb3
gene deficient mice exhibit impaired social and exploratory behaviors,
deficits in non-selective attention and hypoplasia of cerebellar vermal
lobules: a potential model of autism spectrum disorder. Behav Brain Res
187:207-220.
Denys D, van der Wee N, van Megen HJ, Westenberg HG (2003) A double blind
comparison of venlafaxine and paroxetine in obsessive-compulsive
disorder. J Clin Psychopharmacol 23:568-575.
192

Denys D, van der Wee N, Janssen J, De Geus F, Westenberg HG (2004) Low
level of dopaminergic D2 receptor binding in obsessive-compulsive
disorder. Biol Psychiatry 55:1041-1045.
Dlugos AM, Palmer AA, de Wit H (2009) Negative emotionality: monoamine
oxidase B gene variants modulate personality traits in healthy humans. J
Neural Transm 116:1323-1334.
Dourish CT, Boulton AA (1981) The effects of acute and chronic administration of
beta-phenylethylamine on food intake and body weight in rats. Prog
Neuropsychopharmacol 5:411-414.
Dowman R, Ben-Avraham D (2008) An artificial neural network model of
orienting attention toward threatening somatosensory stimuli.
Psychophysiology 45:229-239.
Dringenberg HC, Dennis KE, Tomaszek S, Martin J (2003) Orienting and
defensive behaviors elicited by superior colliculus stimulation in rats:
effects of 5-HT depletion, uptake inhibition, and direct midbrain or frontal
cortex application. Behav Brain Res 144:95-103.
Dubrovina NI, Popova NK, Gilinskii MA, Tomilenko RA, Seif I (2006) Acquisition
and extinction of a conditioned passive avoidance reflex in mice with
genetic knockout of monoamine oxidase A. Neurosci Behav Physiol
36:335-339.
Durig J, Hornung JP (2000) Neonatal serotonin depletion affects developing and
mature mouse cortical neurons. Neuroreport 11:833-837.
Edwards AC, Dodge KA, Latendresse SJ, Lansford JE, Bates JE, Pettit GS,
Budde JP, Goate AM, Dick DM (2010) MAOA-uVNTR and early physical
discipline interact to influence delinquent behavior. J Child Psychol
Psychiatry 51:679-687.
Egerton A, Reid L, McGregor S, Cochran SM, Morris BJ, Pratt JA (2008)
Subchronic and chronic PCP treatment produces temporally distinct
deficits in attentional set shifting and prepulse inhibition in rats.
Psychopharmacology (Berl) 198:37-49.
Engelmann M, Thrivikraman KV, Su Y, Nemeroff CB, Montkowski A, Landgraf R,
Holsboer F, Plotsky PM (1996) Endocrine and behavioral effects of airpuff-
startle in rats. Psychoneuroendocrinology 21:391-400.
Ernst M, Moolchan ET, Robinson ML (2001) Behavioral and neural
consequences of prenatal exposure to nicotine. J Am Acad Child Adolesc
Psychiatry 40:630-641.
193

Ersche KD, Roiser JP, Robbins TW, Sahakian BJ (2008) Chronic cocaine but not
chronic amphetamine use is associated with perseverative responding in
humans. Psychopharmacology (Berl) 197:421-431.
Ersche KD, Clark L, London M, Robbins TW, Sahakian BJ (2006) Profile of
executive and memory function associated with amphetamine and opiate
dependence. Neuropsychopharmacology 31:1036-1047.
Ersche KD, Fletcher PC, Lewis SJ, Clark L, Stocks-Gee G, London M, Deakin
JB, Robbins TW, Sahakian BJ (2005) Abnormal frontal activations related
to decision-making in current and former amphetamine and opiate
dependent individuals. Psychopharmacology (Berl) 180:612-623.
Erzurumlu RS, Jhaveri S (1990) Thalamic axons confer a blueprint of the sensory
periphery onto the developing rat somatosensory cortex. Brain Res Dev
Brain Res 56:229-234.
Evenden JL, Robbins TW (1983) Increased response switching, perseveration
and perseverative switching following d-amphetamine in the rat.
Psychopharmacology (Berl) 80:67-73.
Evenden JL, Ryan CN (1996) The pharmacology of impulsive behaviour in rats:
the effects of drugs on response choice with varying delays of
reinforcement. Psychopharmacology (Berl) 128:161-170.
Evrard A, Malagie I, Laporte AM, Boni C, Hanoun N, Trillat AC, Seif I, De Maeyer
E, Gardier A, Hamon M, Adrien J (2002) Altered regulation of the 5-HT
system in the brain of MAO-A knock-out mice. Eur J Neurosci 15:841-851.
Fang J, Yu PH, Gorrod JW, Boulton AA (1995) Inhibition of monoamine oxidases
by haloperidol and its metabolites: pharmacological implications for the
chemotherapy of schizophrenia. Psychopharmacology (Berl) 118:206-212.
Featherstone RE, Kapur S, Fletcher PJ (2007) The amphetamine-induced
sensitized state as a model of schizophrenia. Prog
Neuropsychopharmacol Biol Psychiatry 31:1556-1571.
Featherstone RE, Rizos Z, Kapur S, Fletcher PJ (2008) A sensitizing regimen of
amphetamine that disrupts attentional set-shifting does not disrupt working
or long-term memory. Behav Brain Res 189:170-179.
Feigin A, Kurlan R, McDermott MP, Beach J, Dimitsopulos T, Brower CA,
Chapieski L, Trinidad K, Como P, Jankovic J (1996) A controlled trial of
deprenyl in children with Tourette's syndrome and attention deficit
hyperactivity disorder. Neurology 46:965-968.
194

Felmingham KL, Bryant RA, Gordon E (2003) Processing angry and neutral
faces in post-traumatic stress disorder: an event-related potentials study.
Neuroreport 14:777-780.
File S (1992) Behavioural detection of anxiolytic action. In: Experimental
approaches to anxiety and depression (Elliot JM HD, Marsden CA, ed), pp
25-44. Chichester: John Wiley & Sons Ltd.
File SE (2001) Factors controlling measures of anxiety and responses to novelty
in the mouse. Behav Brain Res 125:151-157.
Fillmore MT, Rush CR, Marczinski CA (2003) Effects of d-amphetamine on
behavioral control in stimulant abusers: the role of prepotent response
tendencies. Drug Alcohol Depend 71:143-152.
Fineberg NA, Potenza MN, Chamberlain SR, Berlin HA, Menzies L, Bechara A,
Sahakian BJ, Robbins TW, Bullmore ET, Hollander E (2010) Probing
compulsive and impulsive behaviors, from animal models to
endophenotypes: a narrative review. Neuropsychopharmacology 35:591-
604.
Fisher A, Biggs CS, Eradiri O, Starr MS (1999) Dual effects of -3,4-
dihydroxyphenylalanine on aromatic -amino acid decarboxylase,
dopamine release and motor stimulation in the reserpine-treated rat:
evidence that behaviour is dopamine independent. Neuroscience 95:97-
111.
Fletcher PJ, Tenn CC, Rizos Z, Lovic V, Kapur S (2005) Sensitization to
amphetamine, but not PCP, impairs attentional set shifting: reversal by a
D1 receptor agonist injected into the medial prefrontal cortex.
Psychopharmacology (Berl) 183:190-200.
Fletcher PJ, Tenn CC, Sinyard J, Rizos Z, Kapur S (2007) A sensitizing regimen
of amphetamine impairs visual attention in the 5-choice serial reaction
time test: reversal by a D1 receptor agonist injected into the medial
prefrontal cortex. Neuropsychopharmacology 32:1122-1132.
Foley DL, Eaves LJ, Wormley B, Silberg JL, Maes HH, Kuhn J, Riley B (2004)
Childhood adversity, monoamine oxidase a genotype, and risk for conduct
disorder. Arch Gen Psychiatry 61:738-744.
Fornai F, Chen K, Giorgi FS, Gesi M, Alessandri MG, Shih JC (1999) Striatal
dopamine metabolism in monoamine oxidase B-deficient mice: a brain
dialysis study. J Neurochem 73:2434-2440.
Fowler CJ, von Knorring L, Oreland L (1980a) Platelet monoamine oxidase
activity in sensation seekers. Psychiatry Res 3:273-279.
195

Fowler CJ, Carlsson A, Winblad B (1981) Monoamine oxidase-A and -B activities
in the brain stem of schizophrenics and non-schizophrenic psychotics. J
Neural Transm 52:23-32.
Fowler CJ, Wiberg A, Oreland L, Marcusson J, Winblad B (1980b) The effect of
age on the activity and molecular properties of human brain monoamine
oxidase. J Neural Transm 49:1-20.
Fox MW (1965) The visual cliff test for the study of visual depth perception in the
mouse. Anim Behav 13:232-233.
Fumagalli F, Pasini M, Frasca A, Drago F, Racagni G, Riva MA (2009) Prenatal
stress alters glutamatergic system responsiveness in adult rat prefrontal
cortex. J Neurochem 109:1733-1744.
Garrick NA, Murphy DL (1980) Species differences in the deamination of
dopamine and other substrates for monoamine oxidase in brain.
Psychopharmacology (Berl) 72:27-33.
Gattoni R, Mahe D, Mahl P, Fischer N, Mattei MG, Stevenin J, Fuchs JP (1996)
The human hnRNP-M proteins: structure and relation with early heat
shock-induced splicing arrest and chromosome mapping. Nucleic Acids
Res 24:2535-2542.
Gazzaniga MS (2005) Forty-five years of split-brain research and still going
strong. Nat Rev Neurosci 6:653-659.
Gelernter CS, Uhde TW, Cimbolic P, Arnkoff DB, Vittone BJ, Tancer ME, Bartko
JJ (1991) Cognitive-behavioral and pharmacological treatments of social
phobia. A controlled study. Arch Gen Psychiatry 48:938-945.
Genn RF, Tucci SA, Thomas A, Edwards JE, File SE (2003) Age-associated sex
differences in response to food deprivation in two animal tests of anxiety.
Neurosci Biobehav Rev 27:155-161.
Goad DL, Davis CM, Liem P, Fuselier CC, McCormack JR, Olsen KM (1991) The
use of selegiline in Alzheimer's patients with behavior problems. J Clin
Psychiatry 52:342-345.
Goto Y, Yang CR, Otani S (2010) Functional and dysfunctional synaptic plasticity
in prefrontal cortex: roles in psychiatric disorders. Biol Psychiatry 67:199-
207.
Gottesman, II, Gould TD (2003) The endophenotype concept in psychiatry:
etymology and strategic intentions. Am J Psychiatry 160:636-645.
Gould TD, Gottesman, II (2006) Psychiatric endophenotypes and the
development of valid animal models. Genes Brain Behav 5:113-119.
196

Graeff FG (1993) Role of 5-HT in defensive behavior and anxiety. Rev Neurosci
4:181-211.
Grahn RE, Hammack SE, Will MJ, O'Connor KA, Deak T, Sparks PD, Watkins
LR, Maier SF (2002) Blockade of alpha1 adrenoreceptors in the dorsal
raphe nucleus prevents enhanced conditioned fear and impaired escape
performance following uncontrollable stressor exposure in rats. Behav
Brain Res 134:387-392.
Greenshaw AJ (1984) beta-Phenylethylamine and reinforcement. Prog
Neuropsychopharmacol Biol Psychiatry 8:615-620.
Griebel G, Curet O, Perrault G, Sanger DJ (1998) Behavioral effects of
phenelzine in an experimental model for screening anxiolytic and anti-
panic drugs: correlation with changes in monoamine-oxidase activity and
monoamine levels. Neuropharmacology 37:927-935.
Grillon C, Davis M (1997) Effects of stress and shock anticipation on prepulse
inhibition of the startle reflex. Psychophysiology 34:511-517.
Grillon C, Ameli, Rezvan (2006) Methods of affective clinical psychophysiology.
In: Neurobiology of Mental Illness, 2nd Edition (Charney DS, Nestler, Eric
J., ed). New York: Oxford University Press.
Grimsby J, Toth M, Chen K, Kumazawa T, Klaidman L, Adams JD, Karoum F,
Gal J, Shih JC (1997) Increased stress response and beta-
phenylethylamine in MAOB-deficient mice. Nat Genet 17:206-210.
Groenink L, van der Gugten J, Zethof T, van der Heyden J, Olivier B (1994)
Stress-induced hyperthermia in mice: hormonal correlates. Physiol Behav
56:747-749.
Groenink L, Compaan J, van der Gugten J, Zethof T, van der Heyden J, Olivier B
(1995) Stress-induced hyperthermia in mice. Pharmacological and
endocrinological aspects. Ann N Y Acad Sci 771:252-256.
Gu Q (2002) Neuromodulatory transmitter systems in the cortex and their role in
cortical plasticity. Neuroscience 111:815-835.
Gur RE, Kohler CG, Ragland JD, Siegel SJ, Lesko K, Bilker WB, Gur RC (2006)
Flat affect in schizophrenia: relation to emotion processing and
neurocognitive measures. Schizophr Bull 32:279-287.
Gur RE, Loughead J, Kohler CG, Elliott MA, Lesko K, Ruparel K, Wolf DH, Bilker
WB, Gur RC (2007) Limbic activation associated with misidentification of
fearful faces and flat affect in schizophrenia. Arch Gen Psychiatry
64:1356-1366.
197

Gyertyan I (1995) Analysis of the marble burying response: marbles serve to
measure digging rather than evoke burying. Behav Pharmacol 6:24-31.
Haberstick BC, Lessem JM, Hopfer CJ, Smolen A, Ehringer MA, Timberlake D,
Hewitt JK (2005) Monoamine oxidase A (MAOA) and antisocial behaviors
in the presence of childhood and adolescent maltreatment. Am J Med
Genet B Neuropsychiatr Genet 135B:59-64.
Hageman I, Andersen HS, Jorgensen MB (2001) Post-traumatic stress disorder:
a review of psychobiology and pharmacotherapy. Acta Psychiatr Scand
104:411-422.
Haller J, Bakos N, Rodriguiz RM, Caron MG, Wetsel WC, Liposits Z (2002)
Behavioral responses to social stress in noradrenaline transporter
knockout mice: effects on social behavior and depression. Brain Res Bull
58:279-284.
Hardan AY, Pabalan M, Gupta N, Bansal R, Melhem NM, Fedorov S, Keshavan
MS, Minshew NJ (2009) Corpus callosum volume in children with autism.
Psychiatry Res 174:57-61.
Hare TA, Tottenham N, Galvan A, Voss HU, Glover GH, Casey BJ (2008)
Biological substrates of emotional reactivity and regulation in adolescence
during an emotional go-nogo task. Biol Psychiatry 63:927-934.
Hasselbalch SG, Hansen ES, Jakobsen TB, Pinborg LH, Lonborg JH, Bolwig TG
(2007) Reduced midbrain-pons serotonin transporter binding in patients
with obsessive-compulsive disorder. Acta Psychiatr Scand 115:388-394.
Hazlett HC, Poe MD, Gerig G, Smith RG, Piven J (2006) Cortical gray and white
brain tissue volume in adolescents and adults with autism. Biol Psychiatry
59:1-6.
Hazlett HC, Poe M, Gerig G, Smith RG, Provenzale J, Ross A, Gilmore J, Piven
J (2005) Magnetic resonance imaging and head circumference study of
brain size in autism: birth through age 2 years. Arch Gen Psychiatry
62:1366-1376.
Hebebrand J, Klug B (1995) Specification of the phenotype required for men with
monoamine oxidase type A deficiency. Hum Genet 96:372-376.
Herault J, Petit E, Martineau J, Cherpi C, Perrot A, Barthelemy C, Lelord G, Muh
JP (1996) Serotonin and autism: biochemical and molecular biology
features. Psychiatry Res 65:33-43.
Hesse S, Muller U, Lincke T, Barthel H, Villmann T, Angermeyer MC, Sabri O,
Stengler-Wenzke K (2005) Serotonin and dopamine transporter imaging in
patients with obsessive-compulsive disorder. Psychiatry Res 140:63-72.
198

Hill RA, McInnes KJ, Gong EC, Jones ME, Simpson ER, Boon WC (2007)
Estrogen deficient male mice develop compulsive behavior. Biol
Psychiatry 61:359-366.
Hirano K, Kimura R, Sugimoto Y, Yamada J, Uchida S, Kato Y, Hashimoto H,
Yamada S (2005) Relationship between brain serotonin transporter
binding, plasma concentration and behavioural effect of selective
serotonin reuptake inhibitors. Br J Pharmacol 144:695-702.
Hirshfeld-Becker DR, Biederman J, Calltharp S, Rosenbaum ED, Faraone SV,
Rosenbaum JF (2003) Behavioral inhibition and disinhibition as
hypothesized precursors to psychopathology: implications for pediatric
bipolar disorder. Biol Psychiatry 53:985-999.
Hoffman DC (1992) Typical and atypical neuroleptics antagonize MK-801-
induced locomotion and stereotypy in rats. J Neural Transm Gen Sect
89:1-10.
Holmes A (2008) Genetic variation in cortico-amygdala serotonin function and
risk for stress-related disease. Neurosci Biobehav Rev 32:1293-1314.
Holmes A, Rodgers RJ (2003) Prior exposure to the elevated plus-maze
sensitizes mice to the acute behavioral effects of fluoxetine and
phenelzine. Eur J Pharmacol 459:221-230.
Holmes A, Yang RJ, Lesch KP, Crawley JN, Murphy DL (2003) Mice lacking the
serotonin transporter exhibit 5-HT(1A) receptor-mediated abnormalities in
tests for anxiety-like behavior. Neuropsychopharmacology 28:2077-2088.
Holschneider DP, Scremin OU, Chialvo DR, Chen K, Shih JC (2002) Heart rate
dynamics in monoamine oxidase-A- and -B-deficient mice. Am J Physiol
Heart Circ Physiol 282:H1751-1759.
Homayoun H, Moghaddam B (2006) Progression of cellular adaptations in medial
prefrontal and orbitofrontal cortex in response to repeated amphetamine. J
Neurosci 26:8025-8039.
Hranilovic D, Cicin-Sain L, Bordukalo-Niksic T, Jernej B (2005) Rats with
constitutionally upregulated/downregulated platelet 5HT transporter:
differences in anxiety-related behavior. Behav Brain Res 165:271-277.
Hranilovic D, Bujas-Petkovic Z, Vragovic R, Vuk T, Hock K, Jernej B (2007)
Hyperserotonemia in adults with autistic disorder. J Autism Dev Disord
37:1934-1940.

199

Huang YY, Cate SP, Battistuzzi C, Oquendo MA, Brent D, Mann JJ (2004) An
association between a functional polymorphism in the monoamine oxidase
a gene promoter, impulsive traits and early abuse experiences.
Neuropsychopharmacology 29:1498-1505.
Huether G, Doering S, Ruger U, Ruther E, Schussler G (1999) The stress-
reaction process and the adaptive modification and reorganization of
neuronal networks. Psychiatry Res 87:83-95.
Hurwitz BE, Dietrich WD, McCabe PM, Watson BD, Ginsberg MD, Schneiderman
N (1990) Sensory-motor deficit and recovery from thrombotic infarction of
the vibrissal barrel-field cortex. Brain Res 512:210-220.
Ibanez A, Perez de Castro I, Fernandez-Piqueras J, Blanco C, Saiz-Ruiz J
(2000) Pathological gambling and DNA polymorphic markers at MAO-A
and MAO-B genes. Mol Psychiatry 5:105-109.
Isler JR, Martien KM, Grieve PG, Stark RI, Herbert MR (2010) Reduced
functional connectivity in visual evoked potentials in children with autism
spectrum disorder. Clin Neurophysiol.
Jabbi M, Korf J, Kema IP, Hartman C, van der Pompe G, Minderaa RB, Ormel J,
den Boer JA (2007) Convergent genetic modulation of the endocrine
stress response involves polymorphic variations of 5-HTT, COMT and
MAOA. Mol Psychiatry 12:483-490.
Jackson ME, Homayoun H, Moghaddam B (2004) NMDA receptor hypofunction
produces concomitant firing rate potentiation and burst activity reduction in
the prefrontal cortex. Proc Natl Acad Sci U S A 101:8467-8472.
Jacob CP, Muller J, Schmidt M, Hohenberger K, Gutknecht L, Reif A, Schmidtke
A, Mossner R, Lesch KP (2005) Cluster B personality disorders are
associated with allelic variation of monoamine oxidase A activity.
Neuropsychopharmacology 30:1711-1718.
Jankovic J (1993) Deprenyl in attention deficit associated with Tourette's
syndrome. Arch Neurol 50:286-288.
Janssen PA, Leysen JE, Megens AA, Awouters FH (1999) Does
phenylethylamine act as an endogenous amphetamine in some patients?
Int J Neuropsychopharmacol 2:229-240.
Javitt DC (2004) Glutamate as a therapeutic target in psychiatric disorders. Mol
Psychiatry 9:984-997, 979.
Jenike MA, Baer L, Minichiello WE, Rauch SL, Buttolph ML (1997) Placebo-
controlled trial of fluoxetine and phenelzine for obsessive-compulsive
disorder. Am J Psychiatry 154:1261-1264.
200

Johansson AK, Hansen S (2000) Increased alcohol intake and behavioral
disinhibition in rats with ventral striatal neuron loss. Physiol Behav 70:453-
463.
Kahne D, Tudorica A, Borella A, Shapiro L, Johnstone F, Huang W, Whitaker-
Azmitia PM (2002) Behavioral and magnetic resonance spectroscopic
studies in the rat hyperserotonemic model of autism. Physiol Behav
75:403-410.
Kalaria RN, Mitchell MJ, Harik SI (1988) Monoamine oxidases of the human
brain and liver. Brain 111 ( Pt 6):1441-1451.
Kaludercic N, Takimoto E, Nagayama T, Feng N, Lai EW, Bedja D, Chen K,
Gabrielson KL, Blakely RD, Shih JC, Pacak K, Kass DA, Di Lisa F,
Paolocci N (2010) Monoamine oxidase A-mediated enhanced catabolism
of norepinephrine contributes to adverse remodeling and pump failure in
hearts with pressure overload. Circ Res 106:193-202.
Kalueff AV, Tuohimaa P (2004) Grooming analysis algorithm for
neurobehavioural stress research. Brain Res Brain Res Protoc 13:151-
158.
Kalueff AV, Olivier JD, Nonkes LJ, Homberg JR (2010) Conserved role for the
serotonin transporter gene in rat and mouse neurobehavioral
endophenotypes. Neurosci Biobehav Rev 34:373-386.
Karayiorgou M, Sobin C, Blundell ML, Galke BL, Malinova L, Goldberg P, Ott J,
Gogos JA (1999) Family-based association studies support a sexually
dimorphic effect of COMT and MAOA on genetic susceptibility to
obsessive-compulsive disorder. Biol Psychiatry 45:1178-1189.
Katz RJ, Roth KA, Schmaltz K (1981) Amphetamine and tranylcypromine in an
animal model of depression: pharmacological specificity of the reversal
effect. Neurosci Biobehav Rev 5:259-264.
Keary CJ, Minshew NJ, Bansal R, Goradia D, Fedorov S, Keshavan MS, Hardan
AY (2009) Corpus callosum volume and neurocognition in autism. J
Autism Dev Disord 39:834-841.
Kegeles LS, Abi-Dargham A, Zea-Ponce Y, Rodenhiser-Hill J, Mann JJ, Van
Heertum RL, Cooper TB, Carlsson A, Laruelle M (2000) Modulation of
amphetamine-induced striatal dopamine release by ketamine in humans:
implications for schizophrenia. Biol Psychiatry 48:627-640.
Keller NR, Diedrich A, Appalsamy M, Miller LC, Caron MG, McDonald MP,
Shelton RC, Blakely RD, Robertson D (2006) Norepinephrine transporter-
deficient mice respond to anxiety producing and fearful environments with
bradycardia and hypotension. Neuroscience 139:931-946.
201

Kim-Cohen J, Caspi A, Taylor A, Williams B, Newcombe R, Craig IW, Moffitt TE
(2006) MAOA, maltreatment, and gene-environment interaction predicting
children's mental health: new evidence and a meta-analysis. Mol
Psychiatry 11:903-913.
Kim JJ, Haller J (2007) Glucocorticoid hyper- and hypofunction: stress effects on
cognition and aggression. Ann N Y Acad Sci 1113:291-303.
Kim JJ, Foy MR, Thompson RF (1996) Behavioral stress modifies hippocampal
plasticity through N-methyl-D-aspartate receptor activation. Proc Natl
Acad Sci U S A 93:4750-4753.
Kim JJ, Shih JC, Chen K, Chen L, Bao S, Maren S, Anagnostaras SG, Fanselow
MS, De Maeyer E, Seif I, Thompson RF (1997) Selective enhancement of
emotional, but not motor, learning in monoamine oxidase A-deficient mice.
Proc Natl Acad Sci U S A 94:5929-5933.
Kim SJ, Kim CH (2006) The genetic studies of obsessive-compulsive disorder
and its future directions. Yonsei Med J 47:443-454.
Kokkinidis L, Anisman H (1978) Abatement of stimulus perseveration following
repeated d-amphetamine treatment: absence of behaviorally augmented
tolerance. Pharmacol Biochem Behav 8:557-563.
Kondziella D, Brenner E, Eyjolfsson EM, Markinhuhta KR, Carlsson ML,
Sonnewald U (2006) Glial-neuronal interactions are impaired in the
schizophrenia model of repeated MK801 exposure.
Neuropsychopharmacology 31:1880-1887.
Kontis D, Boulougouris V, Papakosta VM, Kalogerakou S, Papadopoulos S,
Poulopoulou C, Papadimitriou GN, Tsaltas E (2008) Dopaminergic and
serotonergic modulation of persistent behaviour in the reinforced spatial
alternation model of obsessive-compulsive disorder. Psychopharmacology
(Berl) 200:597-610.
Koob GF (1999) Corticotropin-releasing factor, norepinephrine, and stress. Biol
Psychiatry 46:1167-1180.
Korte SM, De Kloet ER, Buwalda B, Bouman SD, Bohus B (1996) Antisense to
the glucocorticoid receptor in hippocampal dentate gyrus reduces
immobility in forced swim test. Eur J Pharmacol 301:19-25.
Kozak R, Martinez V, Young D, Brown H, Bruno JP, Sarter M (2007) Toward a
neuro-cognitive animal model of the cognitive symptoms of schizophrenia:
disruption of cortical cholinergic neurotransmission following repeated
amphetamine exposure in attentional task-performing, but not non-
performing, rats. Neuropsychopharmacology 32:2074-2086.
202

Kreider ML, Aldridge JE, Cousins MM, Oliver CA, Seidler FJ, Slotkin TA (2005)
Disruption of rat forebrain development by glucocorticoids: critical
perinatal periods for effects on neural cell acquisition and on cell signaling
cascades mediating noradrenergic and cholinergic
neurotransmitter/neurotrophic responses. Neuropsychopharmacology
30:1841-1855.
Krystal JH, Neumeister A (2009) Noradrenergic and serotonergic mechanisms in
the neurobiology of posttraumatic stress disorder and resilience. Brain
Res 1293:13-23.
Kubovcakova L, Krizanova O, Kvetnansky R (2004) Identification of the aromatic
L-amino acid decarboxylase gene expression in various mice tissues and
its modulation by immobilization stress in stellate ganglia. Neuroscience
126:375-380.
Kumari V, Barkataki I, Goswami S, Flora S, Das M, Taylor P (2009)
Dysfunctional, but not functional, impulsivity is associated with a history of
seriously violent behaviour and reduced orbitofrontal and hippocampal
volumes in schizophrenia. Psychiatry Res 173:39-44.
Kuroki T, Tsutsumi T, Hirano M, Matsumoto T, Tatebayashi Y, Nishiyama K,
Uchimura H, Shiraishi A, Nakahara T, Nakamura K (1990) Behavioral
sensitization to beta-phenylethylamine (PEA): enduring modifications of
specific dopaminergic neuron systems in the rat. Psychopharmacology
(Berl) 102:5-10.
Kurth JH, Kurth MC, Poduslo SE, Schwankhaus JD (1993) Association of a
monoamine oxidase B allele with Parkinson's disease. Ann Neurol 33:368-
372.
Kuzmin A, Sandin J, Terenius L, Ogren SO (2003) Acquisition, expression, and
reinstatement of ethanol-induced conditioned place preference in mice:
effects of opioid receptor-like 1 receptor agonists and naloxone. J
Pharmacol Exp Ther 304:310-318.
Labrie V, Roder JC (2010) The involvement of the NMDA receptor D-
serine/glycine site in the pathophysiology and treatment of schizophrenia.
Neurosci Biobehav Rev 34:351-372.
Lai GJ, McCobb DP (2006) Regulation of alternative splicing of Slo K+ channels
in adrenal and pituitary during the stress-hyporesponsive period of rat
development. Endocrinology 147:3961-3967.
Lane AE, Dennis SJ, Geraghty ME (2010) Brief Report: Further Evidence of
Sensory Subtypes in Autism. J Autism Dev Disord.
203

Lang PJ, Davis M, Ohman A (2000) Fear and anxiety: animal models and human
cognitive psychophysiology. J Affect Disord 61:137-159.
Lapin IP (1990) Beta-phenylethylamine (PEA): an endogenous anxiogen? Three
series of experimental data. Biol Psychiatry 28:997-1003.
Lapin IP (1993) Anxiogenic effect of phenylethylamine and amphetamine in the
elevated plus-maze in mice and its attenuation by ethanol. Pharmacol
Biochem Behav 44:241-243.
Laruelle M, Abi-Dargham A (1999) Dopamine as the wind of the psychotic fire:
new evidence from brain imaging studies. J Psychopharmacol 13:358-
371.
Leboyer M, Bellivier F, Nosten-Bertrand M, Jouvent R, Pauls D, Mallet J (1998)
Psychiatric genetics: search for phenotypes. Trends Neurosci 21:102-105.
Lee BT, Ham BJ (2008) Monoamine oxidase A-uVNTR genotype affects limbic
brain activity in response to affective facial stimuli. Neuroreport 19:515-
519.
Lee M, Chen K, Shih JC, Hiroi N (2004) MAO-B knockout mice exhibit deficient
habituation of locomotor activity but normal nicotine intake. Genes Brain
Behav 3:216-227.
Lenders JW, Eisenhofer G, Abeling NG, Berger W, Murphy DL, Konings CH,
Wagemakers LM, Kopin IJ, Karoum F, van Gennip AH, Brunner HG
(1996) Specific genetic deficiencies of the A and B isoenzymes of
monoamine oxidase are characterized by distinct neurochemical and
clinical phenotypes. J Clin Invest 97:1010-1019.
Leret ML, Garcia-Uceda F, Antonio MT (2002) Effects of maternal lead
administration on monoaminergic, GABAergic and glutamatergic systems.
Brain Res Bull 58:469-473.
Leroy C, Bragulat V, Berlin I, Gregoire MC, Bottlaender M, Roumenov D, Dolle F,
Bourgeois S, Penttila J, Artiges E, Martinot JL, Trichard C (2009) Cerebral
monoamine oxidase A inhibition in tobacco smokers confirmed with PET
and [11C]befloxatone. J Clin Psychopharmacol 29:86-88.
Leung TK, Lim L, Lai JC (1993) Brain regional distributions of monoamine
oxidase activities in postnatal development in normal and chronically
manganese-treated rats. Metab Brain Dis 8:137-149.
Lewandoski M (2001) Conditional control of gene expression in the mouse. Nat
Rev Genet 2:743-755.
204

Lewis DA, Anderson SA (1995) The functional architecture of the prefrontal
cortex and schizophrenia. Psychol Med 25:887-894.
Leyton M, Boileau I, Benkelfat C, Diksic M, Baker G, Dagher A (2002)
Amphetamine-induced increases in extracellular dopamine, drug wanting,
and novelty seeking: a PET/[11C]raclopride study in healthy men.
Neuropsychopharmacology 27:1027-1035.
Li J, Wang Y, Hu S, Zhou R, Yu X, Wang B, Guan L, Yang L, Zhang F, Faraone
SV (2008) The monoamine oxidase B gene exhibits significant association
to ADHD. Am J Med Genet B Neuropsychiatr Genet 147:370-374.
Li YF, Zhang YZ, Liu YQ, Wang HL, Cao JB, Guan TT, Luo ZP (2006) Inhibition
of N-methyl-D-aspartate receptor function appears to be one of the
common actions for antidepressants. J Psychopharmacol 20:629-635.
Liebowitz MR (1993) Depression with anxiety and atypical depression. J Clin
Psychiatry 54 Suppl:10-14; discussion 15.
Liebowitz MR, Hollander E, Schneier F, Campeas R, Welkowitz L, Hatterer J,
Fallon B (1990) Reversible and irreversible monoamine oxidase inhibitors
in other psychiatric disorders. Acta Psychiatr Scand Suppl 360:29-34.
Liebowitz MR, Gorman JM, Fyer AJ, Campeas R, Levin AP, Sandberg D,
Hollander E, Papp L, Goetz D (1988) Pharmacotherapy of social phobia:
an interim report of a placebo-controlled comparison of phenelzine and
atenolol. J Clin Psychiatry 49:252-257.
Liebowitz MR, Schneier F, Campeas R, Hollander E, Hatterer J, Fyer A, Gorman
J, Papp L, Davies S, Gully R, et al. (1992) Phenelzine vs atenolol in social
phobia. A placebo-controlled comparison. Arch Gen Psychiatry 49:290-
300.
Lieske V, Bennett-Clarke CA, Rhoades RW (1999a) Effects of serotonin on
neurite outgrowth from thalamic neurons in vitro. Neuroscience 90:967-
974.
Lieske V, Bennett-Clarke CA, Rhoades RW (1999b) Effects of serotonin on
neurite outgrowth from thalamic neurons in vitro. Neuroscience 90:967-
974.
Lin S, Jiang S, Wu X, Qian Y, Wang D, Tang G, Gu N (2000) Association
analysis between mood disorder and monoamine oxidase gene. Am J
Med Genet 96:12-14.

205

Lindemann L, Ebeling M, Kratochwil NA, Bunzow JR, Grandy DK, Hoener MC
(2005) Trace amine-associated receptors form structurally and functionally
distinct subfamilies of novel G protein-coupled receptors. Genomics
85:372-385.
Lindemann L, Meyer CA, Jeanneau K, Bradaia A, Ozmen L, Bluethmann H,
Bettler B, Wettstein JG, Borroni E, Moreau JL, Hoener MC (2008) Trace
amine-associated receptor 1 modulates dopaminergic activity. J
Pharmacol Exp Ther 324:948-956.
Liston C, Miller MM, Goldwater DS, Radley JJ, Rocher AB, Hof PR, Morrison JH,
McEwen BS (2006) Stress-induced alterations in prefrontal cortical
dendritic morphology predict selective impairments in perceptual
attentional set-shifting. J Neurosci 26:7870-7874.
Liu GX, Cai GQ, Cai YQ, Sheng ZJ, Jiang J, Mei Z, Wang ZG, Guo L, Fei J
(2007) Reduced anxiety and depression-like behaviors in mice lacking
GABA transporter subtype 1. Neuropsychopharmacology 32:1531-1539.
Lopez-Gil X, Artigas F, Adell A (2009) Role of different monoamine receptors
controlling MK-801-induced release of serotonin and glutamate in the
medial prefrontal cortex: relevance for antipsychotic action. Int J
Neuropsychopharmacol 12:487-499.
Lotto B, Upton L, Price DJ, Gaspar P (1999) Serotonin receptor activation
enhances neurite outgrowth of thalamic neurones in rodents. Neurosci
Lett 269:87-90.
Louvart H, Maccari S, Darnaudery M (2005) Prenatal stress affects behavioral
reactivity to an intense stress in adult female rats. Brain Res 1031:67-73.
Low K, Crestani F, Keist R, Benke D, Brunig I, Benson JA, Fritschy JM, Rulicke
T, Bluethmann H, Mohler H, Rudolph U (2000) Molecular and neuronal
substrate for the selective attenuation of anxiety. Science 290:131-134.
Lucki I, O'Leary OF (2004) Distinguishing roles for norepinephrine and serotonin
in the behavioral effects of antidepressant drugs. J Clin Psychiatry 65
Suppl 4:11-24.
Luhmann HJ, Huston JP, Hasenohrl RU (2005) Contralateral increase in
thigmotactic scanning following unilateral barrel-cortex lesion in mice.
Behav Brain Res 157:39-43.
M. Grace Baron JG, Gerald Groden, Lewis P Lipsitt (2006) Stress and coping in
Autism, 1 Edition: Oxford University Press.
Manley NR, Barrow JR, Zhang T, Capecchi MR (2001) Hoxb2 and hoxb4 act
together to specify ventral body wall formation. Dev Biol 237:130-144.
206

Mantegazza P, Riva P (1963) Amphetamine-like activity of β-phenylethylamine
after a monoamine oxidase inhibitor in vivo. J Pharm Pharmacol 15:472-
478.
Manuck SB, Flory JD, Ferrell RE, Mann JJ, Muldoon MF (2000) A regulatory
polymorphism of the monoamine oxidase-A gene may be associated with
variability in aggression, impulsivity, and central nervous system
serotonergic responsivity. Psychiatry Res 95:9-23.
Mao LM, Wang W, Chu XP, Zhang GC, Liu XY, Yang YJ, Haines M, Papasian
CJ, Fibuch EE, Buch S, Chen JG, Wang JQ (2009) Stability of surface
NMDA receptors controls synaptic and behavioral adaptations to
amphetamine. Nat Neurosci 12:602-610.
Marek GJ, Behl B, Bespalov AY, Gross G, Lee Y, Schoemaker H (2010)
Glutamatergic (N-methyl-D-aspartate receptor) hypofrontality in
schizophrenia: too little juice or a miswired brain? Mol Pharmacol 77:317-
326.
Marques JM, Olsson IA, Ogren SO, Dahlborn K (2008) Evaluation of exploration
and risk assessment in pre-weaning mice using the novel cage test.
Physiol Behav 93:139-147.
Martin-Ruiz R, Puig MV, Celada P, Shapiro DA, Roth BL, Mengod G, Artigas F
(2001) Control of serotonergic function in medial prefrontal cortex by
serotonin-2A receptors through a glutamate-dependent mechanism. J
Neurosci 21:9856-9866.
Martin P, Soubrie P, Simon P (1987) The effect of monoamine oxidase inhibitors
compared with classical tricyclic antidepressants on learned helplessness
paradigm. Prog Neuropsychopharmacol Biol Psychiatry 11:1-7.
Martin P, Waters N, Schmidt CJ, Carlsson A, Carlsson ML (1998) Rodent data
and general hypothesis: antipsychotic action exerted through 5-Ht2A
receptor antagonism is dependent on increased serotonergic tone. J
Neural Transm 105:365-396.
Martinez V, Parikh V, Sarter M (2005) Sensitized attentional performance and
Fos-immunoreactive cholinergic neurons in the basal forebrain of
amphetamine-pretreated rats. Biol Psychiatry 57:1138-1146.
Matsumoto M, Yoshioka M, Togashi H (2009) Early postnatal stress and neural
circuit underlying emotional regulation. Int Rev Neurobiol 85:95-107.

207

Mazer C, Muneyyirci J, Taheny K, Raio N, Borella A, Whitaker-Azmitia P (1997)
Serotonin depletion during synaptogenesis leads to decreased synaptic
density and learning deficits in the adult rat: a possible model of
neurodevelopmental disorders with cognitive deficits. Brain Res 760:68-
73.
McBride PA, Anderson GM, Hertzig ME, Snow ME, Thompson SM, Khait VD,
Shapiro T, Cohen DJ (1998) Effects of diagnosis, race, and puberty on
platelet serotonin levels in autism and mental retardation. J Am Acad Child
Adolesc Psychiatry 37:767-776.
McBurnett K, Lahey BB, Rathouz PJ, Loeber R (2000) Low salivary cortisol and
persistent aggression in boys referred for disruptive behavior. Arch Gen
Psychiatry 57:38-43.
McFarlane AC, Weber DL, Clark CR (1993) Abnormal stimulus processing in
posttraumatic stress disorder. Biol Psychiatry 34:311-320.
McGinty JF, Shi XD, Schwendt M, Saylor A, Toda S (2008) Regulation of
psychostimulant-induced signaling and gene expression in the striatum. J
Neurochem 104:1440-1449.
McGrath MJ, Campbell KM, Veldman MB, Burton FH (1999) Anxiety in a
transgenic mouse model of cortical-limbic neuro-potentiated compulsive
behavior. Behav Pharmacol 10:435-443.
McKetin R, Ward PB, Catts SV, Mattick RP, Bell JR (1999) Changes in auditory
selective attention and event-related potentials following oral
administration of D-amphetamine in humans. Neuropsychopharmacology
21:380-390.
McNamara IM, Borella AW, Bialowas LA, Whitaker-Azmitia PM (2008) Further
studies in the developmental hyperserotonemia model (DHS) of autism:
social, behavioral and peptide changes. Brain Res 1189:203-214.
Meerson A, Cacheaux L, Goosens KA, Sapolsky RM, Soreq H, Kaufer D
Changes in brain MicroRNAs contribute to cholinergic stress reactions. J
Mol Neurosci 40:47-55.
Megens AA, Niemegeers CJ, Awouters FH (1992) Behavioral disinhibition and
depression in amphetaminized rats: a comparison of risperidone,
ocaperidone and haloperidol. J Pharmacol Exp Ther 260:160-167.
Mejia JM, Ervin FR, Baker GB, Palmour RM (2002) Monoamine oxidase
inhibition during brain development induces pathological aggressive
behavior in mice. Biol Psychiatry 52:811-821.
208

Menard J, Treit D (1999) Effects of centrally administered anxiolytic compounds
in animal models of anxiety. Neurosci Biobehav Rev 23:591-613.
Mendlewicz J, Youdim MB (1980) Antidepressant potentiation of 5-
hydroxytryptophan by L-deprenil in affective illness. J Affect Disord 2:137-
146.
Merali Z, Levac C, Anisman H (2003) Validation of a simple, ethologically
relevant paradigm for assessing anxiety in mice. Biol Psychiatry 54:552-
565.
Meshorer E, Bryk B, Toiber D, Cohen J, Podoly E, Dori A, Soreq H (2005) SC35
promotes sustainable stress-induced alternative splicing of neuronal
acetylcholinesterase mRNA. Mol Psychiatry 10:985-997.
Meyer-Lindenberg A, Miletich RS, Kohn PD, Esposito G, Carson RE, Quarantelli
M, Weinberger DR, Berman KF (2002) Reduced prefrontal activity predicts
exaggerated striatal dopaminergic function in schizophrenia. Nat Neurosci
5:267-271.
Meyer-Lindenberg A, Buckholtz JW, Kolachana B, A RH, Pezawas L, Blasi G,
Wabnitz A, Honea R, Verchinski B, Callicott JH, Egan M, Mattay V,
Weinberger DR (2006) Neural mechanisms of genetic risk for impulsivity
and violence in humans. Proc Natl Acad Sci U S A 103:6269-6274.
Micallef J, Blin O (2001) Neurobiology and clinical pharmacology of obsessive-
compulsive disorder. Clin Neuropharmacol 24:191-207.
Milad MR, Pitman RK, Ellis CB, Gold AL, Shin LM, Lasko NB, Zeidan MA,
Handwerger K, Orr SP, Rauch SL (2009) Neurobiological basis of failure
to recall extinction memory in posttraumatic stress disorder. Biol
Psychiatry 66:1075-1082.
Millan MJ (2003) The neurobiology and control of anxious states. Prog Neurobiol
70:83-244.
Mitrano DA, Pare JF, Smith Y (2010) Ultrastructural relationships between
cortical, thalamic, and amygdala glutamatergic inputs and group I
metabotropic glutamate receptors in the rat accumbens. J Comp Neurol
518:1315-1329.
Mitsukawa K, Lu X, Bartfai T (2009) Bidirectional regulation of stress responses
by galanin in mice: involvement of galanin receptor subtype 1.
Neuroscience 160:837-846.

209

Miyamoto S, Snouwaert JN, Koller BH, Moy SS, Lieberman JA, Duncan GE
(2004) Amphetamine-induced Fos is reduced in limbic cortical regions but
not in the caudate or accumbens in a genetic model of NMDA receptor
hypofunction. Neuropsychopharmacology 29:2180-2188.
Moens CB, Auerbach AB, Conlon RA, Joyner AL, Rossant J (1992) A targeted
mutation reveals a role for N-myc in branching morphogenesis in the
embryonic mouse lung. Genes Dev 6:691-704.
Moghaddam B (2002) Stress activation of glutamate neurotransmission in the
prefrontal cortex: implications for dopamine-associated psychiatric
disorders. Biol Psychiatry 51:775-787.
Moja EA, Stoff DM, Gillin JC, Wyatt RJ (1976) Dose-response effects of beta-
phenylethylamine on stereotyped behavior in pargyline-pretreated rats.
Biol Psychiatry 11:731-742.
Montague PR, Hyman SE, Cohen JD (2004) Computational roles for dopamine in
behavioural control. Nature 431:760-767.
Morilak DA, Barrera G, Echevarria DJ, Garcia AS, Hernandez A, Ma S, Petre CO
(2005) Role of brain norepinephrine in the behavioral response to stress.
Prog Neuropsychopharmacol Biol Psychiatry 29:1214-1224.
Moustgaard A, Hau J, Lind NM (2008) Effects of dopamine D4 receptor
antagonist on spontaneous alternation in rats. Behav Brain Funct 4:49.
Mozhui K, Karlsson RM, Kash TL, Ihne J, Norcross M, Patel S, Farrell MR, Hill
EE, Graybeal C, Martin KP, Camp M, Fitzgerald PJ, Ciobanu DC,
Sprengel R, Mishina M, Wellman CL, Winder DG, Williams RW, Holmes A
(2010) Strain differences in stress responsivity are associated with
divergent amygdala gene expression and glutamate-mediated neuronal
excitability. J Neurosci 30:5357-5367.
Murai R, Noda Y, Matsui K, Kamei H, Mouri A, Matsuba K, Nitta A, Furukawa H,
Nabeshima T (2007) Hypofunctional glutamatergic neurotransmission in
the prefrontal cortex is involved in the emotional deficit induced by
repeated treatment with phencyclidine in mice: implications for
abnormalities of glutamate release and NMDA-CaMKII signaling. Behav
Brain Res 180:152-160.
Murphy DL, Kalin NH (1980) Biological and behavioral consequences of
alterations in monoamine oxidase activity. Schizophr Bull 6:355-367.
Nagy A, Moens C, Ivanyi E, Pawling J, Gertsenstein M, Hadjantonakis AK, Pirity
M, Rossant J (1998) Dissecting the role of N-myc in development using a
single targeting vector to generate a series of alleles. Curr Biol 8:661-664.
210

Nakamura K, Sekine Y, Ouchi Y, Tsujii M, Yoshikawa E, Futatsubashi M,
Tsuchiya KJ, Sugihara G, Iwata Y, Suzuki K, Matsuzaki H, Suda S,
Sugiyama T, Takei N, Mori N (2010) Brain serotonin and dopamine
transporter bindings in adults with high-functioning autism. Arch Gen
Psychiatry 67:59-68.
Navarro HA, Seidler FJ, Whitmore WL, Slotkin TA (1988) Prenatal exposure to
nicotine via maternal infusions: effects on development of catecholamine
systems. J Pharmacol Exp Ther 244:940-944.
Neal LA, Shapland W, Fox C (1997) An open trial of moclobemide in the
treatment of post-traumatic stress disorder. Int Clin Psychopharmacol
12:231-237.
Nedic G, Pivac N, Hercigonja DK, Jovancevic M, Curkovic KD, Muck-Seler D
(2010) Platelet monoamine oxidase activity in children with attention-
deficit/hyperactivity disorder. Psychiatry Res 175:252-255.
Nemeroff CB, Widerlov E, Bissette G, Walleus H, Karlsson I, Eklund K, Kilts CD,
Loosen PT, Vale W (1984) Elevated concentrations of CSF corticotropin-
releasing factor-like immunoreactivity in depressed patients. Science
226:1342-1344.
Neumann ID, Veenema AH, Beiderbeck DI (2010) Aggression and anxiety: social
context and neurobiological links. Front Behav Neurosci 4:12.
New AS, Fan J, Murrough JW, Liu X, Liebman RE, Guise KG, Tang CY, Charney
DS (2009) A functional magnetic resonance imaging study of deliberate
emotion regulation in resilience and posttraumatic stress disorder. Biol
Psychiatry 66:656-664.
Nicolas LB, Kolb Y, Prinssen EP (2006) A combined marble burying-locomotor
activity test in mice: a practical screening test with sensitivity to different
classes of anxiolytics and antidepressants. Eur J Pharmacol 547:106-115.
Nilsson KW, Sjoberg RL, Damberg M, Leppert J, Ohrvik J, Alm PO, Lindstrom L,
Oreland L (2006) Role of monoamine oxidase A genotype and
psychosocial factors in male adolescent criminal activity. Biol Psychiatry
59:121-127.
Nilsson M, Carlsson A, Markinhuhta KR, Sonesson C, Pettersson F, Gullme M,
Carlsson ML (2004) The dopaminergic stabiliser ACR16 counteracts the
behavioural primitivization induced by the NMDA receptor antagonist MK-
801 in mice: implications for cognition. Prog Neuropsychopharmacol Biol
Psychiatry 28:677-685.

211

Ninan I, Kulkarni SK (1998) 5-HT2A receptor antagonists block MK-801-induced
stereotypy and hyperlocomotion. Eur J Pharmacol 358:111-116.
Njung'e K, Handley SL (1991) Evaluation of marble-burying behavior as a model
of anxiety. Pharmacol Biochem Behav 38:63-67.
Nowakowska E, Kus K, Chodera A, Rybakowski J (2001) Investigating potential
anxiolytic, antidepressant and memory enhancing activity of deprenyl. J
Physiol Pharmacol 52:863-873.
O'Neill MF, Hicks CA, Shaw G, Parameswaran T, Cardwell GP, O'Neill MJ
(1998) Effects of 5-hydroxytryptamine2 receptor antagonism on the
behavioral activation and immediate early gene expression induced by
dizocilpine. J Pharmacol Exp Ther 287:839-846.
Oreland L (1993) Monoamine oxidase in neuropsychiatric disorders. In:
Monoamine oxidase: basic and clinical aspects (Yasuhara H PS, Sandler
M, Oguchi K, Nagatsu T, ed), pp 219-247. Utrecht: VSP press.
Oreland L (2004) Platelet monoamine oxidase, personality and alcoholism: the
rise, fall and resurrection. Neurotoxicology 25:79-89.
Oreland L, Gottfries CG (1986) Brain and brain monoamine oxidase in aging and
in dementia of Alzheimer's type. Prog Neuropsychopharmacol Biol
Psychiatry 10:533-540.
Oreland L, Hallman J (1995) The correlation between platelet MAO activity and
personality: short review of findings and a discussion on possible
mechanisms. Prog Brain Res 106:77-84.
Oreland L, Nilsson K, Damberg M, Hallman J (2007) Monoamine oxidases:
activities, genotypes and the shaping of behaviour. J Neural Transm
114:817-822.
Ornstein TJ, Iddon JL, Baldacchino AM, Sahakian BJ, London M, Everitt BJ,
Robbins TW (2000) Profiles of cognitive dysfunction in chronic
amphetamine and heroin abusers. Neuropsychopharmacology 23:113-
126.
Oxenstierna G, Edman G, Iselius L, Oreland L, Ross SB, Sedvall G (1986)
Concentrations of monoamine metabolites in the cerebrospinal fluid of
twins and unrelated individuals--a genetic study. J Psychiatr Res 20:19-
29.
Paaver M, Nordquist N, Parik J, Harro M, Oreland L, Harro J (2007) Platelet
MAO activity and the 5-HTT gene promoter polymorphism are associated
with impulsivity and cognitive style in visual information processing.
Psychopharmacology (Berl) 194:545-554.
212

Pannu Hayes J, Labar KS, Petty CM, McCarthy G, Morey RA (2009) Alterations
in the neural circuitry for emotion and attention associated with
posttraumatic stress symptomatology. Psychiatry Res 172:7-15.
Paquet M, Smith Y (2003) Group I metabotropic glutamate receptors in the
monkey striatum: subsynaptic association with glutamatergic and
dopaminergic afferents. J Neurosci 23:7659-7669.
Pattij T, Janssen MC, Vanderschuren LJ, Schoffelmeer AN, van Gaalen MM
(2007) Involvement of dopamine D1 and D2 receptors in the nucleus
accumbens core and shell in inhibitory response control.
Psychopharmacology (Berl) 191:587-598.
Paulus MP, Dulawa SC, Ralph RJ, Mark AG (1999) Behavioral organization is
independent of locomotor activity in 129 and C57 mouse strains. Brain
Res 835:27-36.
Peleg-Raibstein D, Yee BK, Feldon J, Hauser J (2009) The amphetamine
sensitization model of schizophrenia: relevance beyond psychotic
symptoms? Psychopharmacology (Berl) 206:603-621.
Pellow S, Chopin P, File SE, Briley M (1985) Validation of open:closed arm
entries in an elevated plus-maze as a measure of anxiety in the rat. J
Neurosci Methods 14:149-167.
Perez de Castro I, Ibanez A, Saiz-Ruiz J, Fernandez-Piqueras J (2002)
Concurrent positive association between pathological gambling and
functional DNA polymorphisms at the MAO-A and the 5-HT transporter
genes. Mol Psychiatry 7:927-928.
Perona MT, Waters S, Hall FS, Sora I, Lesch KP, Murphy DL, Caron M, Uhl GR
(2008) Animal models of depression in dopamine, serotonin, and
norepinephrine transporter knockout mice: prominent effects of dopamine
transporter deletions. Behav Pharmacol 19:566-574.
Phillips ML, Drevets WC, Rauch SL, Lane R (2003) Neurobiology of emotion
perception I: The neural basis of normal emotion perception. Biol
Psychiatry 54:504-514.
Picazo O, Chuc-Meza E, Anaya-Martinez V, Jimenez I, Aceves J, Garcia-
Ramirez M (2009) 6-Hydroxydopamine lesion in thalamic reticular nucleus
reduces anxiety behaviour in the rat. Behav Brain Res 197:317-322.
Piton A et al. (2010) Systematic resequencing of X-chromosome synaptic genes
in autism spectrum disorder and schizophrenia. Mol Psychiatry.
213

Ponimaskin E, Voyno-Yasenetskaya T, Richter DW, Schachner M, Dityatev A
(2007) Morphogenic signaling in neurons via neurotransmitter receptors
and small GTPases. Mol Neurobiol 35:278-287.
Popoli M, Gennarelli M, Racagni G (2002) Modulation of synaptic plasticity by
stress and antidepressants. Bipolar Disord 4:166-182.
Popova NK, Maslova LN, Morosova EA, Bulygina VV, Seif I (2006) MAO A
knockout attenuates adrenocortical response to various kinds of stress.
Psychoneuroendocrinology 31:179-186.
Popova NK, Skrinskaya YA, Amstislavskaya TG, Vishnivetskaya GB, Seif I, de
Meier E (2001) Behavioral characteristics of mice with genetic knockout of
monoamine oxidase type A. Neurosci Behav Physiol 31:597-602.
Poustka L, Maras A, Hohm E, Fellinger J, Holtmann M, Banaschewski T,
Lewicka S, Schmidt MH, Esser G, Laucht M (2010) Negative association
between plasma cortisol levels and aggression in a high-risk community
sample of adolescents. J Neural Transm 117:621-627.
Prut L, Belzung C (2003) The open field as a paradigm to measure the effects of
drugs on anxiety-like behaviors: a review. Eur J Pharmacol 463:3-33.
Puig MV, Celada P, Diaz-Mataix L, Artigas F (2003) In vivo modulation of the
activity of pyramidal neurons in the rat medial prefrontal cortex by 5-HT2A
receptors: relationship to thalamocortical afferents. Cereb Cortex 13:870-
882.
Quitkin FM, Liebowitz MR, Stewart JW, McGrath PJ, Harrison W, Rabkin JG,
Markowitz J, Davies SO (1984) l-Deprenyl in atypical depressives. Arch
Gen Psychiatry 41:777-781.
Ragozzino ME (2007) The contribution of the medial prefrontal cortex,
orbitofrontal cortex, and dorsomedial striatum to behavioral flexibility. Ann
N Y Acad Sci 1121:355-375.
Ramos A, Mormede P (1998) Stress and emotionality: a multidimensional and
genetic approach. Neurosci Biobehav Rev 22:33-57.
Ramoz N, Reichert JG, Corwin TE, Smith CJ, Silverman JM, Hollander E,
Buxbaum JD (2006) Lack of evidence for association of the serotonin
transporter gene SLC6A4 with autism. Biol Psychiatry 60:186-191.
Real C, Popa D, Seif I, Callebert J, Launay JM, Adrien J, Escourrou P (2007)
Sleep apneas are increased in mice lacking monoamine oxidase A. Sleep
30:1295-1302.
214

Rebrin I, Geha RM, Chen K, Shih JC (2001) Effects of carboxyl-terminal
truncations on the activity and solubility of human monoamine oxidase B.
J Biol Chem 276:29499-29506.
Reif A, Rosler M, Freitag CM, Schneider M, Eujen A, Kissling C, Wenzler D,
Jacob CP, Retz-Junginger P, Thome J, Lesch KP, Retz W (2007) Nature
and nurture predispose to violent behavior: serotonergic genes and
adverse childhood environment. Neuropsychopharmacology 32:2375-
2383.
Reimold M, Smolka MN, Zimmer A, Batra A, Knobel A, Solbach C, Mundt A,
Smoltczyk HU, Goldman D, Mann K, Reischl G, Machulla HJ, Bares R,
Heinz A (2007) Reduced availability of serotonin transporters in
obsessive-compulsive disorder correlates with symptom severity - a
[11C]DASB PET study. J Neural Transm 114:1603-1609.
Reist C, Haier RJ, DeMet E, Chicz-DeMet A (1990) Platelet MAO activity in
personality disorders and normal controls. Psychiatry Res 33:221-227.
Reul JM, Labeur MS, Grigoriadis DE, De Souza EB, Holsboer F (1994)
Hypothalamic-pituitary-adrenocortical axis changes in the rat after long-
term treatment with the reversible monoamine oxidase-A inhibitor
moclobemide. Neuroendocrinology 60:509-519.
Ribeiro-Barbosa ER, Canteras NS, Cezario AF, Blanchard RJ, Blanchard DC
(2005) An alternative experimental procedure for studying predator-related
defensive responses. Neurosci Biobehav Rev 29:1255-1263.
Ridley RM, Baker HF, Haystead TA (1981) Perseverative behaviour after
amphetamine; dissociation of response tendency from reward association.
Psychopharmacology (Berl) 75:283-286.
Robbins TW, Granon S, Muir JL, Durantou F, Harrison A, Everitt BJ (1998)
Neural systems underlying arousal and attention. Implications for drug
abuse. Ann N Y Acad Sci 846:222-237.
Robinet PM, Rowlett JK, Bardo MT (1998) Individual differences in novelty-
induced activity and the rewarding effects of novelty and amphetamine in
rats. Behavioural Processes 44:1-9.
Robinson DS, Gilmor ML, Yang Y, Moonsammy G, Azzaro AJ, Oren DA,
Campbell BJ (2007) Treatment effects of selegiline transdermal system on
symptoms of major depressive disorder: a meta-analysis of short-term,
placebo-controlled, efficacy trials. Psychopharmacol Bull 40:15-28.

215

Rodgers RJ, Lee C, Shepherd JK (1992a) Effects of diazepam on behavioural
and antinociceptive responses to the elevated plus-maze in male mice
depend upon treatment regimen and prior maze experience.
Psychopharmacology (Berl) 106:102-110.
Rodgers RJ, Cole JC, Cobain MR, Daly P, Doran PJ, Eells JR, Wallis P (1992b)
Anxiogenic-like effects of fluprazine and eltoprazine in the mouse elevated
plus-maze: profile comparisons with 8-OH-DPAT, CGS 12066B, TFMPP
and mCPP. Behav Pharmacol 3:621-634.
Rolls ET (2004) Convergence of sensory systems in the orbitofrontal cortex in
primates and brain design for emotion. Anat Rec A Discov Mol Cell Evol
Biol 281:1212-1225.
Rougemont-Bucking A, Linnman C, Zeffiro TA, Zeidan MA, Lebron-Milad K,
Rodriguez-Romaguera J, Rauch SL, Pitman RK, Milad MR (2010) Altered
Processing of Contextual Information during Fear Extinction in PTSD: An
fMRI Study. CNS Neurosci Ther.
Roy V, Chapillon P, Jeljeli M, Caston J, Belzung C (2009) Free versus forced
exposure to an elevated plus-maze: evidence for new behavioral
interpretations during test and retest. Psychopharmacology (Berl)
203:131-141.
Rubino T, Vigano D, Realini N, Guidali C, Braida D, Capurro V, Castiglioni C,
Cherubino F, Romualdi P, Candeletti S, Sala M, Parolaro D (2008)
Chronic delta 9-tetrahydrocannabinol during adolescence provokes sex-
dependent changes in the emotional profile in adult rats: behavioral and
biochemical correlates. Neuropsychopharmacology 33:2760-2771.
Rubinstein S, Malone MA, Roberts W, Logan WJ (2006) Placebo-controlled
study examining effects of selegiline in children with attention-
deficit/hyperactivity disorder. J Child Adolesc Psychopharmacol 16:404-
415.
Ruchkin VV, Koposov RA, af Klinteberg B, Oreland L, Grigorenko EL (2005)
Platelet MAO-B, personality, and psychopathology. J Abnorm Psychol
114:477-482.
Rucker EB, 3rd, Dierisseau P, Wagner KU, Garrett L, Wynshaw-Boris A, Flaws
JA, Hennighausen L (2000) Bcl-x and Bax regulate mouse primordial
germ cell survival and apoptosis during embryogenesis. Mol Endocrinol
14:1038-1052.
Rung JP, Carlsson A, Markinhuhta KR, Carlsson ML (2005) The dopaminergic
stabilizers (-)-OSU6162 and ACR16 reverse (+)-MK-801-induced social
withdrawal in rats. Prog Neuropsychopharmacol Biol Psychiatry 29:833-
839.
216

Sabelli H, Fahrer R, Medina RD, Ortiz Fragola E (1994) Phenylethylamine
relieves depression after selective MAO-B inhibition. J Neuropsychiatry
Clin Neurosci 6:203.
Sabelli H, Fink P, Fawcett J, Tom C (1996) Sustained antidepressant effect of
PEA replacement. J Neuropsychiatry Clin Neurosci 8:168-171.
Sabelli HC, Javaid JI (1995) Phenylethylamine modulation of affect: therapeutic
and diagnostic implications. J Neuropsychiatry Clin Neurosci 7:6-14.
Sabelli HC, Vazquez AJ, Flavin D (1975) Behavioral and electrophysiological
effects of phenylethanolamine and 2-phenylethylamine.
Psychopharmacologia 42:117-125.
Sagvolden T, Xu T (2008) l-Amphetamine improves poor sustained attention
while d-amphetamine reduces overactivity and impulsiveness as well as
improves sustained attention in an animal model of Attention-
Deficit/Hyperactivity Disorder (ADHD). Behav Brain Funct 4:3.
Salichon N, Gaspar P, Upton AL, Picaud S, Hanoun N, Hamon M, De Maeyer E,
Murphy DL, Mossner R, Lesch KP, Hen R, Seif I (2001) Excessive
activation of serotonin (5-HT) 1B receptors disrupts the formation of
sensory maps in monoamine oxidase a and 5-ht transporter knock-out
mice. J Neurosci 21:884-896.
Sanders MJ, Dietrich WD, Green EJ (2001) Behavioral, electrophysiological, and
histopathological consequences of mild fluid-percussion injury in the rat.
Brain Res 904:141-144.
Sarter M, Nelson CL, Bruno JP (2005) Cortical cholinergic transmission and
cortical information processing in schizophrenia. Schizophr Bull 31:117-
138.
Saura J, Bleuel Z, Ulrich J, Mendelowitsch A, Chen K, Shih JC, Malherbe P, Da
Prada M, Richards JG (1996) Molecular neuroanatomy of human
monoamine oxidases A and B revealed by quantitative enzyme
radioautography and in situ hybridization histochemistry. Neuroscience
70:755-774.
Schumann CM, Bloss CS, Barnes CC, Wideman GM, Carper RA, Akshoomoff N,
Pierce K, Hagler D, Schork N, Lord C, Courchesne E (2010) Longitudinal
magnetic resonance imaging study of cortical development through early
childhood in autism. J Neurosci 30:4419-4427.
Schwendt M, McGinty JF (2007) Regulator of G-protein signaling 4 interacts with
metabotropic glutamate receptor subtype 5 in rat striatum: relevance to
amphetamine behavioral sensitization. J Pharmacol Exp Ther 323:650-
657.
217

Scott AL, Bortolato M, Chen K, Shih JC (2008) Novel monoamine oxidase A
knock out mice with human-like spontaneous mutation. Neuroreport
19:739-743.
Selemon LD, Begovic A, Williams GV, Castner SA (2010) Reversal of neuronal
and cognitive consequences of amphetamine sensitization following
chronic treatment with a D1 antagonist. Pharmacol Biochem Behav
96:325-332.
Shabanov PD, Lebedev AA, Meshcherov Sh K, Strel'tsov VF (2005) The effects
of neurochemical lesioning of dopaminergic terminals in early ontogenesis
on behavior in adult rats. Neurosci Behav Physiol 35:535-544.
Shaffer C, Guo ML, Fibuch EE, Mao LM, Wang JQ (2010) Regulation of group I
metabotropic glutamate receptor expression in the rat striatum and
prefrontal cortex in response to amphetamine in vivo. Brain Res 1326:184-
192.
Shansky RM, Morrison JH (2009) Stress-induced dendritic remodeling in the
medial prefrontal cortex: effects of circuit, hormones and rest. Brain Res
1293:108-113.
Shekim WO, Bylund DB, Alexson J, Glaser RD, Jones SB, Hodges K, Perdue S
(1986) Platelet MAO and measures of attention and impulsivity in boys
with attention deficit disorder and hyperactivity. Psychiatry Res 18:179-
188.
Sherman AD, Sacquitne JL, Petty F (1982) Specificity of the learned
helplessness model of depression. Pharmacol Biochem Behav 16:449-
454.
Shih JC, Thompson RF (1999) Monoamine oxidase in neuropsychiatry and
behavior. Am J Hum Genet 65:593-598.
Shih JC, Chen K, Ridd MJ (1999a) Monoamine oxidase: from genes to behavior.
Annu Rev Neurosci 22:197-217.
Shih JC, Ridd MJ, Chen K, Meehan WP, Kung MP, Seif I, De Maeyer E (1999b)
Ketanserin and tetrabenazine abolish aggression in mice lacking
monoamine oxidase A. Brain Res 835:104-112.
Shiraishi H, Suzuki A, Fukasawa T, Aoshima T, Ujiie Y, Ishii G, Otani K (2006)
Monoamine oxidase A gene promoter polymorphism affects novelty
seeking and reward dependence in healthy study participants. Psychiatr
Genet 16:55-58.
218

Shoal GD, Giancola PR, Kirillova GP (2003) Salivary cortisol, personality, and
aggressive behavior in adolescent boys: a 5-year longitudinal study. J Am
Acad Child Adolesc Psychiatry 42:1101-1107.
Silber BY, Croft RJ, Papafotiou K, Stough C (2006) The acute effects of d-
amphetamine and methamphetamine on attention and psychomotor
performance. Psychopharmacology (Berl) 187:154-169.
Simmons A, Strigo IA, Matthews SC, Paulus MP, Stein MB (2009) Initial
evidence of a failure to activate right anterior insula during affective set
shifting in posttraumatic stress disorder. Psychosom Med 71:373-377.
Sims KB, de la Chapelle A, Norio R, Sankila EM, Hsu YP, Rinehart WB, Corey
TJ, Ozelius L, Powell JF, Bruns G, et al. (1989) Monoamine oxidase
deficiency in males with an X chromosome deletion. Neuron 2:1069-1076.
Singh VK, Singh EA, Warren RP (1997) Hyperserotoninemia and serotonin
receptor antibodies in children with autism but not mental retardation. Biol
Psychiatry 41:753-755.
Skondras M, Markianos M, Botsis A, Bistolaki E, Christodoulou G (2004a)
Platelet monoamine oxidase activity and psychometric correlates in male
violent offenders imprisoned for homicide or other violent acts. European
Archives of Psychiatry and Clinical Neuroscience 254:380-386.
Skondras M, Markianos M, Botsis A, Bistolaki E, Christodoulou G (2004b)
Platelet monoamine oxidase activity and psychometric correlates in male
violent offenders imprisoned for homicide or other violent acts. Eur Arch
Psychiatry Clin Neurosci 254:380-386.
Sluyter F, Korte SM, Bohus B, Van Oortmerssen GA (1996) Behavioral stress
response of genetically selected aggressive and nonaggressive wild
house mice in the shock-probe/defensive burying test. Pharmacol
Biochem Behav 54:113-116.
Sostek AJ, Sostek AM, Murphy DL, Martin EB, Born WS (1981) Cord blood
amine oxidase activities relate to arousal and motor functioning in human
newborns. Life Sci 28:2561-2568.
Sotnikova TD, Caron MG, Gainetdinov RR (2009) Trace amine-associated
receptors as emerging therapeutic targets. Mol Pharmacol 76:229-235.
Sotnikova TD, Budygin EA, Jones SR, Dykstra LA, Caron MG, Gainetdinov RR
(2004) Dopamine transporter-dependent and -independent actions of
trace amine beta-phenylethylamine. J Neurochem 91:362-373.
219

Southwick SM, Krystal JH, Morgan CA, Johnson D, Nagy LM, Nicolaou A,
Heninger GR, Charney DS (1993) Abnormal noradrenergic function in
posttraumatic stress disorder. Arch Gen Psychiatry 50:266-274.
Southwick SM, Krystal JH, Bremner JD, Morgan CA, 3rd, Nicolaou AL, Nagy LM,
Johnson DR, Heninger GR, Charney DS (1997) Noradrenergic and
serotonergic function in posttraumatic stress disorder. Arch Gen
Psychiatry 54:749-758.
Stamler CJ, Abdelouahab N, Vanier C, Mergler D, Chan HM (2006) Relationship
between platelet monoamine oxidase-B (MAO-B) activity and mercury
exposure in fish consumers from the Lake St. Pierre region of Que.,
Canada. Neurotoxicology 27:429-436.
Stanfield AC, McIntosh AM, Spencer MD, Philip R, Gaur S, Lawrie SM (2008)
Towards a neuroanatomy of autism: a systematic review and meta-
analysis of structural magnetic resonance imaging studies. Eur Psychiatry
23:289-299.
Steckler T, Rammes G, Sauvage M, van Gaalen MM, Weis C, Zieglgansberger
W, Holsboer F (2001) Effects of the monoamine oxidase A inhibitor
moclobemide on hippocampal plasticity in GR-impaired transgenic mice. J
Psychiatr Res 35:29-42.
Stefani MR, Moghaddam B (2010) Activation of type 5 metabotropic glutamate
receptors attenuates deficits in cognitive flexibility induced by NMDA
receptor blockade. Eur J Pharmacol 639:26-32.
Stefani MR, Groth K, Moghaddam B (2003) Glutamate receptors in the rat medial
prefrontal cortex regulate set-shifting ability. Behav Neurosci 117:728-737.
Stengler-Wenzke K, Muller U, Angermeyer MC, Sabri O, Hesse S (2004)
Reduced serotonin transporter-availability in obsessive-compulsive
disorder (OCD). Eur Arch Psychiatry Clin Neurosci 254:252-255.
Stevens J (2005) Working Memory in Children With ADHD. In: Attention Deficit
Hyperactivity Disorder: From Genes to Patients (Gozal D. MDL, ed).
Totowa: Humana Press.
Straube T, Schmidt S, Weiss T, Mentzel HJ, Miltner WH (2009) Dynamic
activation of the anterior cingulate cortex during anticipatory anxiety.
Neuroimage 44:975-981.
Strekalova T, Spanagel R, Bartsch D, Henn FA, Gass P (2004) Stress-induced
anhedonia in mice is associated with deficits in forced swimming and
exploration. Neuropsychopharmacology 29:2007-2017.
220

Strolin Benedetti M, Boucher T, Fowler CJ (1983) The deamination of
noradrenaline and 5-hydroxytryptamine by rat brain and heart monoamine
oxidase and their inhibition by cimoxatone, toloxatone and MD 770222.
Naunyn Schmiedebergs Arch Pharmacol 323:315-320.
Susman EJ (2006) Psychobiology of persistent antisocial behavior: stress, early
vulnerabilities and the attenuation hypothesis. Neurosci Biobehav Rev
30:376-389.
Swedo SE, Schapiro MB, Grady CL, Cheslow DL, Leonard HL, Kumar A,
Friedland R, Rapoport SI, Rapoport JL (1989) Cerebral glucose
metabolism in childhood-onset obsessive-compulsive disorder. Arch Gen
Psychiatry 46:518-523.
Szasz BK, Mike A, Karoly R, Gerevich Z, Illes P, Vizi ES, Kiss JP (2007) Direct
inhibitory effect of fluoxetine on N-methyl-D-aspartate receptors in the
central nervous system. Biol Psychiatry 62:1303-1309.
Szeszko PR, Robinson D, Alvir JM, Bilder RM, Lencz T, Ashtari M, Wu H,
Bogerts B (1999) Orbital frontal and amygdala volume reductions in
obsessive-compulsive disorder. Arch Gen Psychiatry 56:913-919.
Szeszko PR, MacMillan S, McMeniman M, Lorch E, Madden R, Ivey J, Banerjee
SP, Moore GJ, Rosenberg DR (2004) Amygdala volume reductions in
pediatric patients with obsessive-compulsive disorder treated with
paroxetine: preliminary findings. Neuropsychopharmacology 29:826-832.
Takahashi LK, Kalin NH, Vanden Burgt JA, Sherman JE (1989) Corticotropin-
releasing factor modulates defensive-withdrawal and exploratory behavior
in rats. Behav Neurosci 103:648-654.
Tao R, Auerbach SB (1996) Differential effect of NMDA on extracellular serotonin
in rat midbrain raphe and forebrain sites. J Neurochem 66:1067-1075.
Tarantino LM, Bucan M (2000) Dissection of behavior and psychiatric disorders
using the mouse as a model. Hum Mol Genet 9:953-965.
Tariot PN, Cohen RM, Sunderland T, Newhouse PA, Yount D, Mellow AM,
Weingartner H, Mueller EA, Murphy DL (1987) L-deprenyl in Alzheimer's
disease. Preliminary evidence for behavioral change with monoamine
oxidase B inhibition. Arch Gen Psychiatry 44:427-433.
Taylor A, Kim-Cohen J (2007) Meta-analysis of gene-environment interactions in
developmental psychopathology. Dev Psychopathol 19:1029-1037.
Tenn CC, Fletcher PJ, Kapur S (2003) Amphetamine-sensitized animals show a
sensorimotor gating and neurochemical abnormality similar to that of
schizophrenia. Schizophr Res 64:103-114.
221

Thomas A, Burant A, Bui N, Graham D, Yuva-Paylor LA, Paylor R (2009a)
Marble burying reflects a repetitive and perseverative behavior more than
novelty-induced anxiety. Psychopharmacology (Berl).
Thomas A, Burant A, Bui N, Graham D, Yuva-Paylor LA, Paylor R (2009b)
Marble burying reflects a repetitive and perseverative behavior more than
novelty-induced anxiety. Psychopharmacology (Berl) 204:361-373.
Thompson AM (2008) Serotonin immunoreactivity in auditory brainstem neurons
of the postnatal monoamine oxidase-A knockout mouse. Brain Res
1228:58-67.
Tolbert SR, Fuller MA (1996) Selegiline in treatment of behavioral and cognitive
symptoms of Alzheimer disease. Ann Pharmacother 30:1122-1129.
Tordjman S, Gutknecht L, Carlier M, Spitz E, Antoine C, Slama F, Carsalade V,
Cohen DJ, Ferrari P, Roubertoux PL, Anderson GM (2001) Role of the
serotonin transporter gene in the behavioral expression of autism. Mol
Psychiatry 6:434-439.
Torrejais JC, Rosa CC, Boerngen-Lacerda R, Andreatini R (2008) The elevated
T-maze as a measure of two types of defensive reactions: a factor
analysis. Brain Res Bull 76:376-379.
Trullas R, Skolnick P (1993) Differences in fear motivated behaviors among
inbred mouse strains. Psychopharmacology (Berl) 111:323-331.
Trzctnska M, Tonkiss J, Galler JR (1999) Influence of prenatal protein
malnutrition on behavioral reactivity to stress in adult rats. Stress 3:71-83.
Upton AL, Salichon N, Lebrand C, Ravary A, Blakely R, Seif I, Gaspar P (1999)
Excess of serotonin (5-HT) alters the segregation of ispilateral and
contralateral retinal projections in monoamine oxidase A knock-out mice:
possible role of 5-HT uptake in retinal ganglion cells during development.
J Neurosci 19:7007-7024.
Valentino RJ, Foote SL, Page ME (1993) The locus coeruleus as a site for
integrating corticotropin-releasing factor and noradrenergic mediation of
stress responses. Ann N Y Acad Sci 697:173-188.
van den Heuvel OA, Veltman DJ, Groenewegen HJ, Dolan RJ, Cath DC,
Boellaard R, Mesina CT, van Balkom AJ, van Oppen P, Witter MP,
Lammertsma AA, van Dyck R (2004) Amygdala activity in obsessive-
compulsive disorder with contamination fear: a study with oxygen-15 water
positron emission tomography. Psychiatry Res 132:225-237.
Van der Heyden JA, Zethof TJ, Olivier B (1997) Stress-induced hyperthermia in
singly housed mice. Physiol Behav 62:463-470.
222

van Dijk A, Klompmakers, A., Denys, D. (2010) The serotonergic system in
obsessive-compulsive disorder. In: Handbook of the behavioral
neurobiology of serotonin (Muller CP, Jacobs, B.L., ed), pp 547-563.
London, UK: Academic Press.
van Gaalen MM, Brueggeman RJ, Bronius PF, Schoffelmeer AN, Vanderschuren
LJ (2006) Behavioral disinhibition requires dopamine receptor activation.
Psychopharmacology (Berl) 187:73-85.
van Goozen SH, Fairchild G (2006) Neuroendocrine and neurotransmitter
correlates in children with antisocial behavior. Horm Behav 50:647-654.
Varty GB, Bakshi VP, Geyer MA (1999) M100907, a serotonin 5-HT2A receptor
antagonist and putative antipsychotic, blocks dizocilpine-induced prepulse
inhibition deficits in Sprague-Dawley and Wistar rats.
Neuropsychopharmacology 20:311-321.
Veenstra-VanderWeele J, Kim SJ, Lord C, Courchesne R, Akshoomoff N,
Leventhal BL, Courchesne E, Cook EH, Jr. (2002) Transmission
disequilibrium studies of the serotonin 5-HT2A receptor gene (HTR2A) in
autism. Am J Med Genet 114:277-283.
Verbeeck W, Tuinier S, Bekkering GE (2009) Antidepressants in the treatment of
adult attention-deficit hyperactivity disorder: a systematic review. Adv Ther
26:170-184.
Verkes RJ, Van der Mast RC, Hengeveld MW, Tuyl JP, Zwinderman AH, Van
Kempen GMJ (1998) Reduction by Paroxetine of Suicidal Behavior in
Patients With Repeated Suicide Attempts But Not Major Depression. Am J
Psychiatry 155:543-547.
Versiani M, Nardi AE, Mundim FD, Alves AB, Liebowitz MR, Amrein R (1992)
Pharmacotherapy of social phobia. A controlled study with moclobemide
and phenelzine. Br J Psychiatry 161:353-360.
Viana MB, Tomaz C, Graeff FG (1994) The elevated T-maze: a new animal
model of anxiety and memory. Pharmacol Biochem Behav 49:549-554.
Vishnivetskaya GB, Skrinskaya JA, Seif I, Popova NK (2007) Effect of MAO A
deficiency on different kinds of aggression and social investigation in mice.
Aggress Behav 33:1-6.
Viswanath B, Janardhan Reddy YC, Kumar KJ, Kandavel T, Chandrashekar CR
(2009) Cognitive endophenotypes in OCD: a study of unaffected siblings
of probands with familial OCD. Prog Neuropsychopharmacol Biol
Psychiatry 33:610-615.
223

Vitalis T, Cases O, Callebert J, Launay JM, Price DJ, Seif I, Gaspar P (1998)
Effects of monoamine oxidase A inhibition on barrel formation in the
mouse somatosensory cortex: determination of a sensitive developmental
period. J Comp Neurol 393:169-184.
Vitalis T, Fouquet C, Alvarez C, Seif I, Price D, Gaspar P, Cases O (2002)
Developmental expression of monoamine oxidases A and B in the central
and peripheral nervous systems of the mouse. J Comp Neurol 442:331-
347.
von Knorring L, Oreland L, Winblad B (1984) Personality traits related to
monoamine oxidase activity in platelets. Psychiatry Res 12:11-26.
Voyiaziakis E et al. (2009) Association of SLC6A4 variants with obsessive-
compulsive disorder in a large multicenter US family study. Mol
Psychiatry.
Wall PM, Messier C (2000) Ethological confirmatory factor analysis of anxiety-like
behaviour in the murine elevated plus-maze. Behav Brain Res 114:199-
212.
Wang RY, Liang X (1998) M100907 and clozapine, but not haloperidol or
raclopride, prevent phencyclidine-induced blockade of NMDA responses
in pyramidal neurons of the rat medial prefrontal cortical slice.
Neuropsychopharmacology 19:74-85.
Ward HE, Johnson EA, Goodman IJ, Birkle DL, Cottrell DJ, Azzaro AJ (1998)
Corticotropin-releasing factor and defensive withdrawal: inhibition of
monoamine oxidase prevents habituation to chronic stress. Pharmacol
Biochem Behav 60:209-215.
Ward PB, Catts SV, Norman TR, Burrows GD, McConaghy N (1987) Low platelet
monoamine oxidase and sensation seeking in males: an established
relationship? Acta Psychiatr Scand 75:86-90.
Wassink TH, Hazlett HC, Epping EA, Arndt S, Dager SR, Schellenberg GD,
Dawson G, Piven J (2007) Cerebral cortical gray matter overgrowth and
functional variation of the serotonin transporter gene in autism. Arch Gen
Psychiatry 64:709-717.
Weber DL (2008) Information Processing Bias in Post-traumatic Stress Disorder.
Open Neuroimag J 2:29-51.
Wenk GL (2001) Assessment of spatial memory using the T maze. Curr Protoc
Neurosci Chapter 8:Unit 8 5B.
224

Westlund KN, Denney RM, Rose RM, Abell CW (1988) Localization of distinct
monoamine oxidase A and monoamine oxidase B cell populations in
human brainstem. Neuroscience 25:439-456.
Whibley A, Urquhart J, Dore J, Willatt L, Parkin G, Gaunt L, Black G, Donnai D,
Raymond FL (2010) Deletion of MAOA and MAOB in a male patient
causes severe developmental delay, intermittent hypotonia and
stereotypical hand movements. Eur J Hum Genet 18:1095-1099.
Whitaker-Azmitia PM (2001) Serotonin and brain development: role in human
developmental diseases. Brain Res Bull 56:479-485.
Whitaker-Azmitia PM (2005) Behavioral and cellular consequences of increasing
serotonergic activity during brain development: a role in autism? Int J Dev
Neurosci 23:75-83.
Whitaker-Azmitia PM, Zhang X, Clarke C (1994) Effects of gestational exposure
to monoamine oxidase inhibitors in rats: preliminary behavioral and
neurochemical studies. Neuropsychopharmacology 11:125-132.
Wiggins LD, Robins DL, Bakeman R, Adamson LB (2009) Brief report: sensory
abnormalities as distinguishing symptoms of autism spectrum disorders in
young children. J Autism Dev Disord 39:1087-1091.
Williams LM, Gatt JM, Kuan SA, Dobson-Stone C, Palmer DM, Paul RH, Song L,
Costa PT, Schofield PR, Gordon E (2009) A polymorphism of the MAOA
gene is associated with emotional brain markers and personality traits on
an antisocial index. Neuropsychopharmacology 34:1797-1809.
Williamson S, Gossop M, Powis B, Griffiths P, Fountain J, Strang J (1997)
Adverse effects of stimulant drugs in a community sample of drug users.
Drug Alcohol Depend 44:87-94.
Winstanley CA, Theobald DE, Dalley JW, Cardinal RN, Robbins TW (2006)
Double dissociation between serotonergic and dopaminergic modulation
of medial prefrontal and orbitofrontal cortex during a test of impulsive
choice. Cereb Cortex 16:106-114.
Wolinsky TD, Swanson CJ, Smith KE, Zhong H, Borowsky B, Seeman P,
Branchek T, Gerald CP (2007) The Trace Amine 1 receptor knockout
mouse: an animal model with relevance to schizophrenia. Genes Brain
Behav 6:628-639.
Wu J, Zou H, Strong JA, Yu J, Zhou X, Xie Q, Zhao G, Jin M, Yu L (2005)
Bimodal effects of MK-801 on locomotion and stereotypy in C57BL/6 mice.
Psychopharmacology (Berl) 177:256-263.
225

Xie Z, Miller GM (2008) Beta-phenylethylamine alters monoamine transporter
function via trace amine-associated receptor 1: implication for modulatory
roles of trace amines in brain. J Pharmacol Exp Ther 325:617-628.
Xie Z, Miller GM (2009) Trace amine-associated receptor 1 as a monoaminergic
modulator in brain. Biochem Pharmacol 78:1095-1104.
Xu F, Gainetdinov RR, Wetsel WC, Jones SR, Bohn LM, Miller GW, Wang YM,
Caron MG (2000) Mice lacking the norepinephrine transporter are
supersensitive to psychostimulants. Nat Neurosci 3:465-471.
Yang M, Crawley JN (2009) Simple behavioral assessment of mouse olfaction.
Curr Protoc Neurosci Chapter 8:Unit 8 24.
Yoo HJ, Lee SK, Park M, Cho IH, Hyun SH, Lee JC, Yang SY, Kim SA (2009)
Family- and population-based association studies of monoamine oxidase
A and autism spectrum disorders in Korean. Neuroscience Research
63:172-176.
Youdim MB, Edmondson D, Tipton KF (2006) The therapeutic potential of
monoamine oxidase inhibitors. Nat Rev Neurosci 7:295-309.
Yuen EY, Jiang Q, Chen P, Feng J, Yan Z (2008) Activation of 5-HT2A/C
receptors counteracts 5-HT1A regulation of n-methyl-D-aspartate receptor
channels in pyramidal neurons of prefrontal cortex. J Biol Chem
283:17194-17204.
Zimmerberg B, Brunelli SA, Fluty AJ, Frye CA (2005) Differences in affective
behaviors and hippocampal allopregnanolone levels in adult rats of lines
selectively bred for infantile vocalizations. Behav Brain Res 159:301-311.
Zucchi R, Chiellini G, Scanlan TS, Grandy DK (2006) Trace amine-associated
receptors and their ligands. Br J Pharmacol 149:967-978.

Abstract (if available)

Abstract Monoamine oxidase (MAO) is the primary catabolic enzyme for the oxidative deamination of monoamines and has been implicated in several neuropsychiatric disorders. Cogent evidence has documented that both the MAO A and MAO B isoenzymes play a role in emotional regulation, suggesting that the behavioral abnormalities associated with MAO deficiency are underpinned by alterations in emotional processing. Although it has been well-established that MAO A deficiency leads to aggression and antisocial personality, the neurobiological substrates underlying these behavioral disturbances are still unclear. Moreover, little is known about the contribution of the MAO B isoenzyme in modulating emotional behaviors. To this end, I hypothesized that MAO A and B function through the regulation of behavioral plasticity, defined as the ability to properly integrate the perceptual information and emotional processing with appropriate behavioral outcomes. In order to test this hypothesis, the present set of studies investigated the behavioral responses of MAO A-deficient mice to foreign elements and predator-related cues. These experiments were complemented by testing the changes in behavioral responses of MAO A-deficient mice following exposure to stress-inducing stimuli. Although MAO A-deficient mice exhibited aversive defensive responses to foreign inanimate objects, this was accompanied by a profound reduction in reactivity to both potential threat and stress-inducing stimuli. To examine the specific contribution of low MAO A activity on behavioral outcomes, I characterized a novel line of hypomorphic MAO A mutant mice. Specifically, the hypomorphic mutant mice were tested for anxiety-related manifestations, aggression, and perseverative behaviors. Hypomorphic MAO A mutant mice displayed an increase in compulsive and anxiety-related behaviors and context-dependent alterations in aggression.

Linked assets

University of Southern California Dissertations and Theses

Conceptually similar

PDF

Monoamine oxidase deficiency and emotional reactivity: neurochemical and developmental studies

PDF

Differential regulation of monoamine oxidase A and B genes

PDF

Monoamine oxidase and cancer

PDF

MAO a deficient mice exhibit an altered immune system in the brain and prostate

PDF

Potential therapeutic effect of monoamine oxidase (MAO) inhibitor on human neuroblastoma

PDF

Regulation of ethanol intake by purinergic P2X4 receptors

PDF

Monoamine oxidase A inhibitors and androgen receptor antagonists regulate mitochondrial function in prostate cancer cells

PDF

Monoamine oxidase (MAO) knock-out mouse models: Tools for studying the molecular basis of aggression, anxiety, autism and cancers

PDF

Preclinical investigation of ivermectin as a novel therapeutic agent for treatment of alcohol use disorders

PDF

Role of neuronal nitric oxide synthase in aging and neurodegeneration

PDF

Memory abnormalities in Alzheimer's disease and anxiety models

PDF

Maternal inflammation disrupts fetal blood-brain barrier formation via cyclooxygenase activation

PDF

Assessment of theranostic agent for brain cancer

PDF

Role of purinergic P2X4 receptors in regulation of dopamine homeostasis in the basal ganglia and associated behaviors

PDF

Insulin sensitivity in cognition, Alzheimer's disease and brain aging

PDF

The mitochondrial energy – redox axis in aging and caloric restriction: role of nicotinamide nucleotide transhydrogenase

PDF

Role of steroidogenic acute regulatory protein (STAR) and sterol carrier protein-x (SCPx) in the transport of cholesterol and other lipids

PDF

Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) signaling regulates fatty acid beta-oxidation

PDF

Beneficial effect of antibiotic treatment on alcohol-related liver pathology in mice is not due to reduction in the butyrate-producing gut microbial phyla

PDF

Gene-environment interactions in neurodevelopment

Asset Metadata

Creator Godar, Sean C. (author)

Core Title The role of monoamine oxidase in behavioral plasticity

School School of Pharmacy

Degree Doctor of Philosophy

Degree Program Molecular Pharmacology

Degree Conferral Date 2010-12

Publication Date 12/17/2010

Defense Date 10/26/2010

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag behavior,emotional reactivity,knockout,monoamine oxidase,OAI-PMH Harvest

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Shih, Jean C. (committee chair), Alkana, Ronald (committee member), Bortolato, Marco (committee member)

Creator Email bopnoh10@yahoo.com,godar@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-m3606

Unique identifier UC1477578

Identifier etd-Godar-4133 (filename),usctheses-m40 (legacy collection record id),usctheses-c127-434519 (legacy record id),usctheses-m3606 (legacy record id)

Legacy Identifier etd-Godar-4133.pdf

Dmrecord 434519

Document Type Dissertation

Rights Godar, Sean C.

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Repository Name Libraries, University of Southern California

Repository Location Los Angeles, California

Repository Email cisadmin@lib.usc.edu