Sumana Chakravarty

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.

Toll-Like Receptor 4 on Nonhematopoietic Cells Sustains CNS Inflammation during Endotoxemia, Independent of Systemic Cytokines

Inflammatory agonists such as lipopolysaccharide (LPS) induce robust systemic as well as CNS responses after peripheral administration. Responses in the innate immune system require triggering of toll-like receptor 4 (TLR4), but the origin of CNS sequelas has been controversial. We demonstrate expression of TLR4 transcripts in mouse brain in the meninges, ventricular ependyma, circumventricular organs, along the vasculature, and in parenchymal microglia. The contribution of TLR4 expressed in CNS resident versus hematopoietic cells to the development of CNS inflammation was examined using chimeric mice. Reciprocal bone marrow chimeras between wild-type and TLR4 mutant mice show that TLR4 on CNS resident cells is critically required for sustained inflammation in the brain after systemic LPS administration. Hematopoietic TLR4 alone supported the systemic release of acute phase cytokines, but transcription of proinflammatory genes in the CNS was reduced in duration. In contrast, TLR4 function in radiation-resistant cells was sufficient for inflammatory progression in the brains of chimeric mice, despite the striking absence of cytokine elevations in serum. Surprisingly, a temporal rise in serum corticosterone was also dependent on TLR4 signaling in nonhematopoietic cells. Our findings demonstrate a requirement for TLR4 function in CNS-resident cells, independent of systemic cytokine effects, for sustained CNS-specific inflammation and corticosterone rise during endotoxemia.

Genome-wide Analysis of Chromatin Regulation by Cocaine Reveals a Role for Sirtuins

Changes in gene expression contribute to the long-lasting regulation of the brains reward circuitry seen in drug addiction; however, the specific genes regulated and the transcriptional mechanisms underlying such regulation remain poorly understood. Here, we used chromatin immunoprecipitation coupled with promoter microarray analysis to characterize genome-wide chromatin changes in the mouse nucleus accumbens, a crucial brain reward region, after repeated cocaine administration. Our findings reveal several interesting principles of gene regulation by cocaine and of the role of DeltaFosB and CREB, two prominent cocaine-induced transcription factors, in this brain region. The findings also provide comprehensive insight into the molecular pathways regulated by cocaine-including a new role for sirtuins (Sirt1 and Sirt2)-which are induced in the nucleus accumbens by cocaine and, in turn, dramatically enhance the behavioral effects of the drug.

CREB regulation of nucleus accumbens excitability mediates social isolation-induced behavioral deficits

Here, we characterized behavioral abnormalities induced by prolonged social isolation in adult rodents. Social isolation induced both anxiety- and anhedonia-like symptoms and decreased cAMP response element–binding protein (CREB) activity in the nucleus accumbens shell (NAcSh). All of these abnormalities were reversed by chronic, but not acute, antidepressant treatment. However, although the anxiety phenotype and its reversal by antidepressant treatment were CREB-dependent, the anhedonia-like symptoms were not mediated by CREB in NAcSh. We found that decreased CREB activity in NAcSh correlated with increased expression of certain K+ channels and reduced electrical excitability of NAcSh neurons, which was sufficient to induce anxiety-like behaviors and was reversed by chronic antidepressant treatment. Together, our results describe a model that distinguishes anxiety- and depression-like behavioral phenotypes, establish a selective role of decreased CREB activity in NAcSh in anxiety-like behavior, and provide a mechanism by which antidepressant treatment alleviates anxiety symptoms after social isolation.

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

BACKGROUNDnCircadian 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.nnnMETHODSnHere, 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.nnnRESULTSnWe 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.nnnCONCLUSIONSnTaken 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.

The Influence of ΔFosB in the Nucleus Accumbens on Natural Reward-Related Behavior

The transcription factor deltaFosB (ΔFosB), induced in nucleus accumbens (NAc) by chronic exposure to drugs of abuse, has been shown to mediate sensitized responses to these drugs. However, less is known about a role for ΔFosB in regulating responses to natural rewards. Here, we demonstrate that two powerful natural reward behaviors, sucrose drinking and sexual behavior, increase levels of ΔFosB in the NAc. We then use viral-mediated gene transfer to study how such ΔFosB induction influences behavioral responses to these natural rewards. We demonstrate that overexpression of ΔFosB in the NAc increases sucrose intake and promotes aspects of sexual behavior. In addition, we show that animals with previous sexual experience, which exhibit increased ΔFosB levels, also show an increase in sucrose consumption. This work suggests that ΔFosB is not only induced in the NAc by drugs of abuse, but also by natural rewarding stimuli. Additionally, our findings show that chronic exposure to stimuli that induce ΔFosB in the NAc can increase consumption of other natural rewards.

Increased Impulsivity during Withdrawal from Cocaine Self-Administration: Role for ΔFosB in the Orbitofrontal Cortex

Increased impulsivity caused by addictive drugs is believed to contribute to the maintenance of addiction and has been linked to hypofunction within the orbitofrontal cortex (OFC). Recent data indicate that cocaine self-administration induces the transcription factor DeltaFosB in the OFC that alters the effects of investigator-administered cocaine on impulsivity. Here, using viral-mediated gene transfer, the effects of overexpressing DeltaFosB within the OFC were assessed on the cognitive sequelae of chronic cocaine self-administration as measured by the 5-choice serial reaction time task (5CSRT). Cognitive testing occurred in the mornings, and self-administration sessions in the evenings, to enable the progressive assessment of repeated volitional drug intake on performance. Animals self-administering cocaine initially made more omissions and premature or impulsive responses on the 5CSRT but quickly developed tolerance to these disruptive effects. However, withdrawal from cocaine dramatically increased premature responding. When access to cocaine was increased, animals overexpressing DeltaFosB failed to regulate their intake as effectively and were more impulsive during withdrawal. In summary, rats develop tolerance to the cognitive disruption caused by cocaine self-administration and show a deficit in impulse control that is unmasked during withdrawal. Our findings suggest that induction of DeltaFosB within the OFC is one mediator of these effects and, thereby, increases vulnerability to addiction.

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

BACKGROUNDnThe 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.nnnMETHODSnTo 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.nnnRESULTSnWe 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.nnnCONCLUSIONSnThese 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.

Multiple actions of spinophilin regulate mu opioid receptor function.

Spinophilin, a dendritic spine-enriched scaffold protein, modulates synaptic transmission via multiple functions mediated by distinct domains of the protein. Here, we show that spinophilin is a key modulator of opiate action. Knockout of the spinophilin gene causes reduced sensitivity to the analgesic effects of morphine and early development of tolerance but a higher degree of physical dependence and increased sensitivity to the rewarding actions of the drug. At the cellular level, spinophilin associates with the mu opioid receptor (MOR) in striatum and modulates MOR signaling and endocytosis. Activation of MOR by opiate agonists such as fentanyl and morphine promotes these events, which feedback to suppress MOR responsiveness. Our findings support a potent physiological role of spinophilin in regulating MOR function and provide a potential new target for the treatment of opiate addiction.