Role Of Hippocampal Neurogenesis In The Etiology And Treatment Of Mood & Anxiety Disorders

142 Pages·2016·4.23 MB·English

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Publicly Accessible Penn Dissertations
Summer 2011
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Jennifer L. Onksen
[email protected]
Follow this and additional works at: https://repository.upenn.edu/edissertations
Part of the Behavioral Neurobiology Commons, Behavior and Behavior Mechanisms Commons,
Mental Disorders Commons, and the Pharmacology Commons
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Onksen, Jennifer L., "Role Of Hippocampal Neurogenesis In The Etiology And Treatment Of Mood &
Anxiety Disorders" (2011). Publicly Accessible Penn Dissertations. 387.
https://repository.upenn.edu/edissertations/387
This paper is posted at ScholarlyCommons. https://repository.upenn.edu/edissertations/387
For more information, please contact [email protected].
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Mood disorders are associated with deficits in hippocampal size and function. Neurogenesis in the
dentate gyrus of the hippocampus has been implicated in the behavioral efficacy of some
antidepressants. However, the precise function of newborn hippocampal neurons in the etiology and
treatment of mood disorders remains unclear. Anxiety, which is highly comorbid with mood disorders,
also appears to be influenced by neurogenesis. The intricacies of this influence are poorly understood.
Here, the role of neurogenesis in the pathophysiology and treatment of mood and anxiety disorders is
investigated in a transgenic mouse exhibiting partial suppression of hippocampal neurogenesis. This
reduction is induced by hippocampal microinjection of a Cre-expressing adeno-associated virus to delete
ATR, a cell cycle checkpoint kinase, from the hippocampus of adult mice. Subsequent to hippocampal
ATR deletion, behavior is examined in mouse models of anxiety, depressive-like behavior, antidepressant
efficacy, and susceptibility to stress. In addition, the necessity of heightened neurogenesis in exercise-
induced anxiety is examined. ATR deletion resulted in an anhedonic phenotype in a sucrose drinking task
but did not increase the likelihood of developing behavioral despair in the learned helplessness paradigm.
Furthermore, mice lacking hippocampal ATR exhibited reduced anxiety in a number of behavioral tests. In
the novelty-induced hypophagia task, the ability of chronic antidepressants to alleviate hyponeophagia
was absent in ATR-deleted mice, and this effect was linked to an attenuation of the ability of chronic
antidepressants to stimulate dendritic spine density. Finally, ATR deletion attenuated the neurogenic
effect of voluntary wheel running, and concurrently blocked some of the anxiogenic effects of wheel
running. In conclusion, neurogenesis appears to be an essential regulator of behaviors indicative of mood
and anxiety disorders. These observations contribute to the body of work investigating the etiology and
treatment of mood disorders for the purpose of guiding pharmaceutical drug discovery towards the
eventual identification of antidepressant compounds with greater efficacy and fewer side effects.
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Dissertation
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Doctor of Philosophy (PhD)
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Pharmacology
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Dr. Julie A. Blendy
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neurogenesis, antidepressants, depression, anxiety, stress, exercise
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Behavioral Neurobiology | Behavior and Behavior Mechanisms | Mental Disorders | Pharmacology
This dissertation is available at ScholarlyCommons: https://repository.upenn.edu/edissertations/387
ROLE OF HIPPOCAMPAL NEUROGENESIS IN THE ETIOLOGY AND
TREATMENT OF MOOD & ANXIETY DISORDERS

Jennifer L Onksen
A DISSERTATION
in
Pharmacology

Presented to the Faculties of the University of Pennsylvania
in
Partial Fulfillment of the Requirements for the
Degree of Doctor of Philosophy
2011

Supervisor of Dissertation

Signature ________________________________
Julie A Blendy, Ph.D
Professor of Pharmacology

Graduate Group Chairperson

Signature_________________________________
Vladimir Muzykantov, MD, Ph.D
Professor of Pharmacology

Dissertation Committee
Steven Thomas, MD, Ph.D, Associate Professor of Pharmacology
Eric Brown, Ph.D, Assistant Professor of Cancer Biology
Irwin Lucki, Ph.D, Professor of Pharmacology
Brenda Porter, MD, Ph.D, Assistant Professor of Pediatrics and Neurology
ABSTRACT

ROLE OF HIPPOCAMPAL NEUROGENESIS IN THE ETIOLOGY AND TREATMENT
OF MOOD & ANXIETY DISORDERS

Jennifer L Onksen
Advisor: Julie A Blendy, Ph.D

Mood disorders are associated with deficits in hippocampal size and function.
Neurogenesis in the dentate gyrus of the hippocampus has been implicated in the
behavioral efficacy of some antidepressants. However, the precise function of newborn
hippocampal neurons in the etiology and treatment of mood disorders remains unclear.
Anxiety, which is highly comorbid with mood disorders, also appears to be influenced by
neurogenesis. The intricacies of this influence are poorly understood. Here, the role of
neurogenesis in the pathophysiology and treatment of mood and anxiety disorders is
investigated in a transgenic mouse exhibiting partial suppression of hippocampal
neurogenesis. This reduction is induced by hippocampal microinjection of a Cre-
expressing adeno-associated virus to delete ATR, a cell cycle checkpoint kinase, from
the hippocampus of adult mice. Subsequent to hippocampal ATR deletion, behavior is
examined in mouse models of anxiety, depressive-like behavior, antidepressant efficacy,
and susceptibility to stress. In addition, the necessity of heightened neurogenesis in
exercise-induced anxiety is examined. ATR deletion resulted in an anhedonic phenotype
in a sucrose drinking task but did not increase the likelihood of developing behavioral
despair in the learned helplessness paradigm. Furthermore, mice lacking hippocampal
ATR exhibited reduced anxiety in a number of behavioral tests. In the novelty-induced
hypophagia task, the ability of chronic antidepressants to alleviate hyponeophagia was
absent in ATR-deleted mice, and this effect was linked to an attenuation of the ability of
ii
chronic antidepressants to stimulate dendritic spine density. Finally, ATR deletion
attenuated the neurogenic effect of voluntary wheel running, and concurrently blocked
some of the anxiogenic effects of wheel running. In conclusion, neurogenesis appears to
be an essential regulator of behaviors indicative of mood and anxiety disorders. These
observations contribute to the body of work investigating the etiology and treatment of
mood disorders for the purpose of guiding pharmaceutical drug discovery towards the
eventual identification of antidepressant compounds with greater efficacy and fewer side
effects.

iii
Table of Contents

I. Introduction ..................................................................................................1
A. Mood and anxiety disorders.................................................................................. 1
i. Mood disorders
ii. Anxiety disorders
B. Rodent models..................................................................................................... 3
i. Modeling depression
ii. Modeling anxiety
C. Hippocampal neurogenesis ................................................................................... 8
i. Characterization of hippocampal neurogenesis
ii. Neurogenesis in the etiology of mood and anxiety disorders
iii. Neurogenesis in the treatment of mood and anxiety disorders
D. Manipulating neurogenesis in rodents...................................................................12
i. Cytotoxic chemicals
ii. X-ray irradiation
iii. Genetic manipulation
E. Crelox-conditional ATR knockout..........................................................................15
i. ATR, a cell cycle checkpoint kinase
ii. ATR deletion in the adult mouse
iii. Targeting ATR in the hippocampus
F. References..........................................................................................................17

II. Selective deletion of ATR reduces neurogenesis and alters responses in
rodent models of behavioral affect.................................................................29
A. Abstract..............................................................................................................30
B. Introduction........................................................................................................31
C. Materials and Methods.........................................................................................33
D. Results...............................................................................................................42
E. Figures...............................................................................................................47
F. Discussion...........................................................................................................58
G. References.........................................................................................................64
iv
III. Low neurogenesis does not increase susceptibility to stress-induced or
corticosterone-induced depressive behaviors in mice....................................96
A. Abstract..............................................................................................................70
B. Introduction........................................................................................................71
C. Materials and Methods.........................................................................................73
D. Results...............................................................................................................80
E. Figures...............................................................................................................83
F. Discussion...........................................................................................................87
G. References.........................................................................................................91

IV. Mice deficient in hippocampal neurogenesis do not develop exercise-
induced anxiety...............................................................................................96
A. Abstract..............................................................................................................97
B. Introduction........................................................................................................98
C. Materials and Methods.........................................................................................99
D. Results.............................................................................................................105
E. Figures.............................................................................................................110
F. Discussion.........................................................................................................117
G. References.......................................................................................................122

V. Conclusions and future directions ............................................................126
A. Discussion........................................................................................................126
B. References .......................................................................................................132

v
List of Illustrations

Chapter II
Figure 1. Hippocampal microinjection of AAV.Cre deletes ATR....................................47
Figure 2. Hippocampal ATR deletion inhibits hippocampal neurogenesis......................48
Figure 3. Hippocampal ATR deletion is associated with reduced anxiety in the
elevated zero maze (EZM).........................................................................50
Figure 4. ATR deletion is associated with reduced anxiety..........................................51
Figure 5. In the NIH paradigm, ATR-deleted mice exhibit altered
antidepressant response............................................................................52
Figure 6. Hippocampal CA1 dendritic spine density following DMI treatment................54
Figure s1. Hippocampal ATR deletion did not affect general locomotor activity
or HPA axis activation following an acute stress.........................................55
Figure s2. Fear conditioning is not altered by hippocampal ATR deletion.....................56
Figure s3. ATR deletion does not induce impairments in spatial learning
and memory in the Morris water maze......................................................57

Chapter III
Figure 1. Sucrose preference and fluid intake............................................................83
Figure 2. Learned helplessness.................................................................................84
Figure 3. Behavioral and endocrine consequences of chronic corticosterone................85

Chapter IV
Figure 1. Experimental design................................................................................110
Figure 2. ATRΔHipp mice exhibit deficits in running-induced neurogenesis....................111
Figure 3. Running-induced anxiety in the NIH test is attenuated in ATRΔHipp mice.......112
Figure 4. Running-induced anxiety in an open field is attenuated in ATRΔHipp mice......113
Figure 5. Running-induced anxiety in the light-dark box is attenuated in
ATRΔHipp mice..........................................................................................115
vi
CHAPTER I. INTRODUCTION

A. Mood and anxiety disorders
Mood and anxiety disorders are highly prevalent mental illnesses afflicting a
significant proportion of adults in the US each year. Among affected individuals, these
disorders are highly variable in their etiology and treatment outcome. A better
understanding of the pathophysiology of mood and anxiety disorders will not only
improve disease prognosis but also ease the large financial burden these illnesses impart
due to high healthcare and economic costs (Greenberg et al., 2003).

Mood disorders
Approximately 10% of adults in the US suffer from a mood disorder each year
(Kessler et al., 2005). The majority suffer from dysthymic or major depressive disorder,
both of which are characterized by persistent, depressive-like symptoms and differ in
regards to severity. The DSM-IV further subdivides depressive disorders into numerous
clinical subtypes, supporting the hypothesis that depression may represent a diverse set
of pathophysiologies presenting with similar, overlapping symptoms (Akiskal, 1989;
Fagiolini and Kupfer, 2003). Depression is defined as the presence of at least one two-
week episode of dysphoria and/or anhedonia plus a multitude of additional symptoms
including altered sleep/appetite, fatigue, agitation, inability to concentrate, and recurrent
thoughts of death or suicide (American Psychiatric Association. and American Psychiatric
Association. Task Force on DSM-IV., 2000). Depression is thought to be brought on by
an interaction between stressful life experiences and genetic susceptibility factors
(Goldberg, 2006), including alterations in the serotonin transporter gene (Caspi et al.,
1
2003). Treatment for depression includes cognitive behavioral therapy,
pharmacotherapy, and electroconvulsive therapy for severe, treatment resistant
depression. Current pharmacotherapies act to increase levels of monoamine transmitters
in the brain, either through blocking their reuptake at the synapse or through inhibiting
their degradation. Because these drugs require 4-6 weeks to relieve symptoms of
depression, they are thought to induce downstream changes in gene transcription and
neuronal plasticity following chronic administration (Hyman and Nestler, 1996). Current
pharmacotherapies often produce unwanted side effects and less than half of patients
show full remission of their disease, even following trials of multiple drugs (Berton and
Nestler, 2006).

Anxiety Disorders
Approximately 18% of adults in the US experience an anxiety disorder each year
(Kessler et al., 2005), including general anxiety, panic disorder, social anxiety, post-
traumatic stress disorder, and obsessive-compulsive disorder. Anxiety disorders are
marked by pervasive worry, tension and avoidance (Allgulander, 2006) and as such,
components of fear circuitry in the brain, including the amygdala, nucleus accumbens,
hippocampus and insular cortex are implicated in animal models (Kent and Rauch, 2003;
Davis, 2006; Maren, 2008). Treatment for anxiety includes benzodiazepines,
anticonvulsants, and the serotonin and norepinephrine reuptake inhibitors commonly
used in the treatment of depression (Baldwin et al., 2011). Depression and anxiety
disorders are highly comorbid; up to 60% of depressed individuals also suffer from
anxiety symptoms (Kessler et al., 2003). This comorbidity, in addition the ability of
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