Ami Graham

Molecular adaptations underlying susceptibility and resistance to social defeat in brain reward regions.

While stressful life events are an important cause of psychopathology, most individuals exposed to adversity maintain normal psychological functioning. The molecular mechanisms underlying such resilience are poorly understood. Here, we demonstrate that an inbred population of mice subjected to social defeat can be separated into susceptible and unsusceptible subpopulations that differ along several behavioral and physiological domains. By a combination of molecular and electrophysiological techniques, we identify signature adaptations within the mesolimbic dopamine circuit that are uniquely associated with vulnerability or insusceptibility. We show that molecular recapitulations of three prototypical adaptations associated with the unsusceptible phenotype are each sufficient to promote resistant behavior. Our results validate a multidisciplinary approach to examine the neurobiological mechanisms of variations in stress resistance, and illustrate the importance of plasticity within the brains reward circuits in actively maintaining an emotional homeostasis.

Mania-like behavior induced by disruption of CLOCK

Circadian rhythms and the genes that make up the molecular clock have long been implicated in bipolar disorder. Genetic evidence in bipolar patients suggests that the central transcriptional activator of molecular rhythms, CLOCK, may be particularly important. However, the exact role of this gene in the development of this disorder remains unclear. Here we show that mice carrying a mutation in the Clock gene display an overall behavioral profile that is strikingly similar to human mania, including hyperactivity, decreased sleep, lowered depression-like behavior, lower anxiety, and an increase in the reward value for cocaine, sucrose, and medial forebrain bundle stimulation. Chronic administration of the mood stabilizer lithium returns many of these behavioral responses to wild-type levels. In addition, the Clock mutant mice have an increase in dopaminergic activity in the ventral tegmental area, and their behavioral abnormalities are rescued by expressing a functional CLOCK protein via viral-mediated gene transfer specifically in the ventral tegmental area. These findings establish the Clock mutant mice as a previously unrecognized model of human mania and reveal an important role for CLOCK in the dopaminergic system in regulating behavior and mood.

Histone Deacetylase 5 Epigenetically Controls Behavioral Adaptations to Chronic Emotional Stimuli

Previous work has identified alterations in histone acetylation in animal models of drug addiction and depression. However, the mechanisms which integrate drugs and stress with changes in chromatin structure remain unclear. Here, we identify the activity-dependent class II histone deacetylase, HDAC5, as a central integrator of these stimuli with changes in chromatin structure and gene expression. Chronic, but not acute, exposure to cocaine or stress decreases HDAC5 function in the nucleus accumbens (NAc), a major brain reward region, which allows for increased histone acetylation and transcription of HDAC5 target genes. This regulation is behaviorally important, as loss of HDAC5 causes hypersensitive responses to chronic, not acute, cocaine or stress. These findings suggest that proper balance of histone acetylation is a crucial factor in the saliency of a given stimulus and that disruption of this balance is involved in the transition from an acute adaptive response to a chronic psychiatric illness.

Nuclear factor kB signaling regulates neuronal morphology and cocaine reward

Although chronic cocaine-induced changes in dendritic spines on nucleus accumbens (NAc) neurons have been correlated with behavioral sensitization, the molecular pathways governing these structural changes, and their resulting behavioral effects, are poorly understood. The transcription factor, nuclear factor κ B (NFκB), is rapidly activated by diverse stimuli and regulates expression of many genes known to maintain cell structure. Therefore, we evaluated the role of NFκB in regulating cocaine-induced dendritic spine changes on medium spiny neurons of the NAc and the rewarding effects of cocaine. We show that chronic cocaine induces NFκB-dependent transcription in the NAc of NFκB-Lac transgenic mice. This induction of NFκB activity is accompanied by increased expression of several NFκB genes, the promoters of which show chromatin modifications after chronic cocaine exposure consistent with their transcriptional activation. To study the functional significance of this induction, we used viral-mediated gene transfer to express either a constitutively active or dominant-negative mutant of Inhibitor of κ B kinase (IKKca or IKKdn), which normally activates NFκB signaling, in the NAc. We found that activation of NFκB by IKKca increases the number of dendritic spines on NAc neurons, whereas inhibition of NFκB by IKKdn decreases basal dendritic spine number and blocks the increase in dendritic spines after chronic cocaine. Moreover, inhibition of NFκB blocks the rewarding effects of cocaine and the ability of previous cocaine exposure to increase an animals preference for cocaine. Together, these studies establish a direct role for NFκB pathways in the NAc to regulate structural and behavioral plasticity to cocaine.

Knockdown of Clock in the Ventral Tegmental Area Through RNA Interference Results in a Mixed State of Mania and Depression-Like Behavior

BACKGROUND Circadian rhythm abnormalities are strongly associated with bipolar disorder; however the role of circadian genes in mood regulation is unclear. Previously, we reported that mice with a mutation in the Clock gene (ClockDelta19) display a behavioral profile that is strikingly similar to bipolar patients in the manic state. METHODS Here, we used RNA interference and viral-mediated gene transfer to knock down Clock expression specifically in the ventral tegmental area (VTA) of mice. We then performed a variety of behavioral, molecular, and physiological measures. RESULTS We found that knockdown of Clock, specifically in the VTA, results in hyperactivity and a reduction in anxiety-related behavior, which is similar to the phenotype of the ClockDelta19 mice. However, VTA-specific knockdown also results in a substantial increase in depression-like behavior, creating an overall mixed manic state. Surprisingly, VTA knockdown of Clock also altered circadian period and amplitude, suggesting a role for Clock in the VTA in the regulation of circadian rhythms. Furthermore, VTA dopaminergic neurons expressing the Clock short hairpin RNA have increased activity compared with control neurons, and this knockdown alters the expression of multiple ion channels and dopamine-related genes in the VTA that could be responsible for the physiological and behavioral changes in these mice. CONCLUSIONS Taken together, these results suggest an important role for Clock in the VTA in the regulation of dopaminergic activity, manic and depressive-like behavior, and circadian rhythms.

Environmental Enrichment Produces a Behavioral Phenotype Mediated by Low Cyclic Adenosine Monophosphate Response Element Binding (CREB) Activity in the Nucleus Accumbens

BACKGROUND Previous research has shown that rats reared in an enriched condition (EC) are more sensitive to the acute effects of amphetamine than rats reared in an isolated condition (IC); yet, EC rats self-administer less amphetamine than IC rats. The present study used cocaine to further explore this environmental enrichment behavioral phenotype, as well as the underlying molecular mechanisms involved. METHODS Enriched condition and IC rats were studied in a broad battery of behavioral tests, including cocaine conditioned place preference (CPP) and self-administration and several measures of anxiety- and depression-related behavior. The involvement of the transcription factor, cyclic adenosine monophosphate (cAMP) response element binding protein (CREB), in mediating EC versus IC differences was investigated. RESULTS Enriched condition rats exhibited less cocaine self-administration, despite showing enhanced cocaine CPP. Enriched condition rats also displayed less depression-like behavior but higher levels of anxiety-like behavior. This behavioral phenotype is consistent with low CREB activity in the nucleus accumbens, a key brain reward region. Indeed, EC rats have less phospho-CREB (the transcriptionally active form of the protein) in the nucleus accumbens than IC rats, and a selective knockdown of CREB in this brain region of normally reared rats, by use of a novel viral vector expressing a short hairpin RNA (shRNA) directed against CREB, reproduced the EC behavioral phenotype. CONCLUSIONS These studies identify a potential molecular mechanism for how rearing environment-a nonpharmacological, nonsurgical manipulation-can modify a wide range of complex emotional behaviors.

Tropomyosin-Related Kinase B in the Mesolimbic Dopamine System: Region-Specific Effects on Cocaine Reward

BACKGROUND Previous studies found that brain-derived neurotrophic factor (BDNF) derived from nucleus accumbens (NAc) neurons can mediate persistent behavioral changes that contribute to cocaine addiction. METHODS To further investigate BDNF signaling in the mesolimbic dopamine system, we analyzed tropomyosin-related kinase B (TrkB) messenger RNA (mRNA) and protein changes in the NAc and ventral tegmental area (VTA) in rats following 3 weeks of cocaine self-administration. To study the role of BDNF-TrkB activity in the VTA and NAc in cocaine reward, we used localized viral-mediated Cre recombinase expression in floxed BDNF and floxed TrkB mice to knockdown BDNF or TrkB in the VTA and NAc in cocaine place conditioning tests and TrkB in the NAc in cocaine self-administration tests. RESULTS We found that 3 weeks of active cocaine self-administration significantly increased TrkB protein levels in the NAc shell, while yoked (passive) cocaine exposure produced a similar increase in the VTA. Localized BDNF knockdown in either region reduced cocaine reward in place conditioning, whereas only TrkB knockdown in the NAc reduced cocaine reward. In mice self-administering cocaine, TrkB knockdown in the NAc produced a downward shift in the cocaine self-administration dose-response curve but had no effect on the acquisition of cocaine or sucrose self-administration. CONCLUSIONS Together, these data suggest that BDNF synthesized in either VTA or NAc neurons is important for maintaining sensitivity to cocaine reward but only BDNF activation of TrkB receptors in the NAc mediates this effect. In addition, up-regulation of NAc TrkB with chronic cocaine use could promote the transition to more addicted biological states.

Specific Role of VTA Dopamine Neuronal Firing Rates and Morphology in the Reversal of Anxiety-Related, but not Depression-Related Behavior in the ClockΔ19 Mouse Model of Mania

Lithium has been used extensively for mood stabilization, and it is particularly efficacious in the treatment of bipolar mania. Like other drugs used in the treatment of psychiatric diseases, it has little effect on the mood of healthy individuals. Our previous studies found that mice with a mutation in the Clock gene (ClockΔ19) have a complete behavioral profile that is very similar to human mania, which can be reversed with chronic lithium treatment. However, the cellular and physiological effects that underlie its targeted therapeutic efficacy remain unknown. Here we find that ClockΔ19 mice have an increase in dopaminergic activity in the ventral tegmental area (VTA), and that lithium treatment selectively reduces the firing rate in the mutant mice with no effect on activity in wild-type mice. Furthermore, lithium treatment reduces nucleus accumbens (NAc) dopamine levels selectively in the mutant mice. The increased dopaminergic activity in the Clock mutants is associated with cell volume changes in dopamine neurons, which are also rescued by lithium treatment. To determine the role of dopaminergic activity and morphological changes in dopamine neurons in manic-like behavior, we manipulated the excitability of these neurons by overexpressing an inwardly rectifying potassium channel subunit (Kir2.1) selectively in the VTA of ClockΔ19 mice and wild-type mice using viral-mediated gene transfer. Introduction of this channel mimics the effects of lithium treatment on the firing rate of dopamine neurons in ClockΔ19 mice and leads to a similar change in dopamine cell volume. Furthermore, reduction of dopaminergic firing rates in ClockΔ19 animals results in a normalization of locomotor- and anxiety-related behavior that is very similar to lithium treatment; however, it is not sufficient to reverse depression-related behavior. These results suggest that abnormalities in dopamine cell firing and associated morphology underlie alterations in anxiety-related behavior in bipolar mania, and that the therapeutic effects of lithium come from a reversal of these abnormal phenotypes.

Role of Nuclear Factor κB in Ovarian Hormone-Mediated Stress Hypersensitivity in Female Mice

BACKGROUND The molecular mechanisms of stress-induced depressive behaviors have been characterized extensively in male rodents; however, much less is known about female subjects, despite the fact that human depression is far more prevalent in women. METHODS To gain insight into these mechanisms, we performed microarray analysis in nucleus accumbens (NAc), a key brain reward region implicated in depression, in ovariectomized (OVX) and gonadally intact female mice after chronic unpredictable stress and measured stress-induced depression-like behavior in the forced swim test (FST). Male mice were studied in the FST for comparison. RESULTS We find that stress regulation of genes in NAc of gonadally intact female mice is blunted in OVX mice. This pattern of gene regulation is consistent with behavioral findings on the FST: the pro-depression-like effect of stress in intact female mice is absent in OVX female and gonadally intact male mice. We identified, among many genes regulated by stress, several nuclear factor kappaB (NFkappaB) subunits-a pro-survival transcription factor involved in cellular responses to stress-as being highly upregulated in NAc of OVX mice. Given the role of NFkappaB during stress, we hypothesized that upregulation of NFkappaB by OVX decreases susceptibility to stress. Indeed, we show that inhibition of NFkappaB in NAc of OVX animals increases susceptibility to stress-induced depressive behaviors, whereas activation of NFkappaB in NAc of intact female subjects blocks susceptibility. CONCLUSIONS These results suggest a hormonal mechanism of NFkappaB regulation that contributes to stress-induced depressive behaviors in female subjects and might represent a mechanism for gender differences in prevalence rates of these disorders in humans.

Effects of inhibitor of κb kinase activity in the nucleus accumbens on emotional behavior

Inhibitor of κB kinase (IκK) has historically been studied in the context of immune response and inflammation, but recent evidence demonstrates that IκK activity is necessary and sufficient for regulation of neuronal function. Chronic social defeat stress of mice increases IκK activity in the nucleus accumbens (NAc) and this increase is strongly correlated to depression-like behaviors. Inhibition of IκK signaling results in a reversal of chronic social defeat stress-induced social avoidance behavior. Here, we more completely define the role of IκK in anxiety and depressive-like behaviors. Mice underwent stereotaxic microinjection of a herpes simplex virus expressing either green fluorescent protein, a constitutively active form of IκK (IκKca), or a dominant negative form of IκK into the NAc. Of all three experimental groups, only mice expressing IκKca show a behavioral phenotype. Expression of IκKca results in a decrease in the time spent in the non-periphery zones of an open field arena and increased time spent immobile during a forced swim test. No baseline differences in sucrose preference were observed, but following the acute swim stress we noted a marked reduction in sucrose preference. To determine whether IκK activity alters responses to other acute stressors, we examined behavior and spine morphology in mice undergoing an acute social defeat stress. We found that IκKca enhanced social avoidance behavior and promoted thin spine formation. These data show that IκK in NAc is a critical regulator of both depressive- and anxiety-like states and may do so by promoting the formation of immature excitatory synapses.