Histone serotonylation is a permissive modification that enhances TFIID binding to H3K4me3.

Chemical modifications of histones can mediate diverse DNA-templated processes, including gene transcription1-3. Here we provide evidence for a class of histone post-translational modification, serotonylation of glutamine, which occurs at position 5 (Q5ser) on histone H3 in organisms that produce serotonin (also known as 5-hydroxytryptamine (5-HT)). We demonstrate that tissue transglutaminase 2 can serotonylate histone H3 tri-methylated lysine 4 (H3K4me3)-marked nucleosomes, resulting in the presence of combinatorial H3K4me3Q5ser in vivo. H3K4me3Q5ser displays a ubiquitous pattern of tissue expression in mammals, with enrichment observed in brain and gut, two organ systems responsible for the bulk of 5-HT production. Genome-wide analyses of human serotonergic neurons, developing mouse brain and cultured serotonergic cells indicate that H3K4me3Q5ser nucleosomes are enriched in euchromatin, are sensitive to cellular differentiation and correlate with permissive gene expression, phenomena that are linked to the potentiation of TFIID4-6 interactions with H3K4me3. Cells that ectopically express a H3 mutant that cannot be serotonylated display significantly altered expression of H3K4me3Q5ser-target loci, which leads to deficits in differentiation. Taken together, these data identify a direct role for 5-HT, independent from its contributions to neurotransmission and cellular signalling, in the mediation of permissive gene expression.

Reversal of Stress-Induced Social Interaction Deficits by Buprenorphine.

Background: Patients with post-traumatic stress disorder frequently report persistent problems with social interactions, emerging after a traumatic experience. Chronic social defeat stress is a widely used rodent model of stress that produces robust and sustained social avoidance behavior. The avoidance of other rodents can be reversed by 28 days of treatment with selective serotonin reuptake inhibitors, the only pharmaceutical class approved by the U.S. Food and Drug Administration for treating post-traumatic stress disorder. In this study, the sensitivity of social interaction deficits evoked by 10 days of chronic social defeat stress to prospective treatments for post-traumatic stress disorder was examined. Methods: The effects of acute and repeated treatment with a low dose of buprenorphine (0.25 mg/kg/d) on social interaction deficits in male C57BL/6 mice by chronic social defeat stress were studied. Another cohort of mice was used to determine the effects of the selective serotonin reuptake inhibitor fluoxetine (10 mg/kg/d), the NMDA antagonist ketamine (10 mg/kg/d), and the selective kappa opioid receptor antagonist CERC-501 (1 mg/kg/d). Changes in mRNA expression of Oprm1 and Oprk1 were assessed in a separate cohort. Results: Buprenorphine significantly reversed social interaction deficits produced by chronic social defeat stress following 7 days of administration, but not after acute injection. Treatment with fluoxetine for 7 days, but not 24 hours, also reinstated social interaction behavior in mice that were susceptible to chronic social defeat. In contrast, CERC-501 and ketamine failed to reverse social avoidance. Gene expression analysis found: (1) Oprm1 mRNA expression was reduced in the hippocampus and increased in the frontal cortex of susceptible mice and (2) Oprk1 mRNA expression was reduced in the amygdala and increased in the frontal cortex of susceptible mice compared to non-stressed controls and stress-resilient mice. Conclusions: Short-term treatment with buprenorphine and fluoxetine normalized social interaction after chronic social defeat stress. In concert with the changes in opioid receptor expression produced by chronic social defeat stress, we speculate that buprenorphine's efficacy in this model of post-traumatic stress disorder may be associated with the ability of this compound to engage multiple opioid receptors.

Mouse model of OPRM1 (A118G) polymorphism increases sociability and dominance and confers resilience to social defeat.

A single nucleotide polymorphism (SNP) in the human µ-opioid receptor gene (OPRM1 A118G) has been widely studied for its association in drug addiction, pain sensitivity, and, more recently, social behavior. The endogenous opioid system has been shown to regulate social distress and reward in a variety of animal models. However, mechanisms underlying the associations between the OPRM1 A118G SNP and these behaviors have not been clarified. We used a mouse model possessing the human equivalent nucleotide/amino acid substitution to study social affiliation and social defeat behaviors. In mice with the Oprm1 A112G SNP, we demonstrate that the G allele is associated with an increase in home-cage dominance and increased motivation for nonaggressive social interactions, similar to what is reported in human populations. When challenged by a resident aggressor, G-allele carriers expressed less submissive behavior and exhibited resilience to social defeat, demonstrated by a lack of subsequent social avoidance and reductions in anhedonia as measured by intracranial self-stimulation. Protection from social defeat in G-allele carriers was associated with a greater induction of c-fos in a resilience circuit comprising the nucleus accumbens and periaqueductal gray. These findings led us to test the role of endogenous opioids in the A112G mice. We demonstrate that the increase in social affiliation in G carriers is blocked by pretreatment with naloxone. Together, these data suggest a mechanism involving altered hedonic state and neural activation as well as altered endogenous opioid tone in the differential response to aversive and rewarding social stimuli in G-allele carriers.

Prefrontal cortical circuit for depression- and anxiety-related behaviors mediated by cholecystokinin: role of ΔFosB.

Decreased medial prefrontal cortex (mPFC) neuronal activity is associated with social defeat-induced depression- and anxiety-like behaviors in mice. However, the molecular mechanisms underlying the decreased mPFC activity and its prodepressant role remain unknown. We show here that induction of the transcription factor ΔFosB in mPFC, specifically in the prelimbic (PrL) area, mediates susceptibility to stress. ΔFosB induction in PrL occurred selectively in susceptible mice after chronic social defeat stress, and overexpression of ΔFosB in this region, but not in the nearby infralimbic (IL) area, enhanced stress susceptibility. ΔFosB produced these effects partly through induction of the cholecystokinin (CCK)-B receptor: CCKB blockade in mPFC induces a resilient phenotype, whereas CCK administration into mPFC mimics the anxiogenic- and depressant-like effects of social stress. We previously found that optogenetic stimulation of mPFC neurons in susceptible mice reverses several behavioral abnormalities seen after chronic social defeat stress. Therefore, we hypothesized that optogenetic stimulation of cortical projections would rescue the pathological effects of CCK in mPFC. After CCK infusion in mPFC, we optogenetically stimulated mPFC projections to basolateral amygdala or nucleus accumbens, two subcortical structures involved in mood regulation. Stimulation of corticoamygdala projections blocked the anxiogenic effect of CCK, although no effect was observed on other symptoms of social defeat. Conversely, stimulation of corticoaccumbens projections reversed CCK-induced social avoidance and sucrose preference deficits but not anxiogenic-like effects. Together, these results indicate that social stress-induced behavioral deficits are mediated partly by molecular adaptations in mPFC involving ΔFosB and CCK through cortical projections to distinct subcortical targets.

Deep brain stimulation of the nucleus accumbens shell attenuates cocaine reinstatement through local and antidromic activation.

Accumbal deep brain stimulation (DBS) is a promising therapeutic modality for the treatment of addiction. Here, we demonstrate that DBS in the nucleus accumbens shell, but not the core, attenuates cocaine priming-induced reinstatement of drug seeking, an animal model of relapse, in male Sprague Dawley rats. Next, we compared DBS of the shell with pharmacological inactivation. Results indicated that inactivation using reagents that influenced (lidocaine) or spared (GABA receptor agonists) fibers of passage blocked cocaine reinstatement when administered into the core but not the shell. It seems unlikely, therefore, that intrashell DBS influences cocaine reinstatement by inactivating this nucleus or the fibers coursing through it. To examine potential circuit-wide changes, c-Fos immunohistochemistry was used to examine neuronal activation following DBS of the nucleus accumbens shell. Intrashell DBS increased c-Fos induction at the site of stimulation as well as in the infralimbic cortex, but had no effect on the dorsal striatum, prelimbic cortex, or ventral pallidum. Recent evidence indicates that accumbens DBS antidromically stimulates axon terminals, which ultimately activates GABAergic interneurons in cortical areas that send afferents to the shell. To test this hypothesis, GABA receptor agonists (baclofen/muscimol) were microinjected into the anterior cingulate, and prelimbic or infralimbic cortices before cocaine reinstatement. Pharmacological inactivation of all three medial prefrontal cortical subregions attenuated the reinstatement of cocaine seeking. These results are consistent with DBS of the accumbens shell attenuating cocaine reinstatement via local activation and/or activation of GABAergic interneurons in the medial prefrontal cortex via antidromic stimulation of cortico-accumbal afferents.

Raphe GABAergic neurons mediate the acquisition of avoidance after social defeat.

Serotonin (5-HT) modulates neural responses to socioaffective cues and can bias approach or avoidance behavioral decisions, yet the cellular mechanisms underlying its contribution to the regulation of social experiences remain poorly understood. We hypothesized that GABAergic neurons in the dorsal raphe nucleus (DRN) may participate in socioaffective regulation by controlling serotonergic tone during social interaction. We tested this hypothesis using whole-cell recording techniques in genetically identified DRN GABA and 5-HT neurons in mice exposed to social defeat, a model that induces long-lasting avoidance behaviors in a subset of mice responsive to serotonergic antidepressants. Our results revealed that social defeat engaged DRN GABA neurons and drove GABAergic sensitization that strengthened inhibition of 5-HT neurons in mice that were susceptible, but not resilient to social defeat. Furthermore, optogenetic silencing of DRN GABA neurons disinhibited neighboring 5-HT neurons and prevented the acquisition of social avoidance in mice exposed to a social threat, but did not affect a previously acquired avoidance phenotype. We provide the first characterization of GABA neurons in the DRN that monosynaptically inhibit 5-HT neurons and reveal their key role in neuroplastic processes underlying the development of social avoidance.

HDAC6 regulates glucocorticoid receptor signaling in serotonin pathways with critical impact on stress resilience.

Genetic variations in certain components of the glucocorticoid receptor (GR) chaperone complex have been associated with the development of stress-related affective disorders and individual variability in therapeutic responses to antidepressants. Mechanisms that link GR chaperoning and stress susceptibility are not well understood. Here, we show that the effects of glucocorticoid hormones on socioaffective behaviors are critically regulated via reversible acetylation of Hsp90, a key component of the GR chaperone complex. We provide pharmacological and genetic evidence indicating that the cytoplasmic lysine deacetylase HDAC6 controls Hsp90 acetylation in the brain, and thereby modulates Hsp90-GR protein-protein interactions, as well as hormone- and stress-induced GR translocation, with a critical impact on GR downstream signaling and behavior. Pet1-Cre-driven deletion of HDAC6 in serotonin neurons, the densest HDAC6-expressing cell group in the mouse brain, dramatically reduced acute anxiogenic effects of the glucocorticoid hormone corticosterone in the open-field, elevated plus maze, and social interaction tests. Serotonin-selective depletion of HDAC6 also blocked the expression of social avoidance in mice exposed to chronic social defeat and concurrently prevented the electrophysiological and morphological changes induced, in serotonin neurons, by this murine model of traumatic stress. Together, these results identify HDAC6 inhibition as a potential new strategy for proresilience and antidepressant interventions through regulation of the Hsp90-GR heterocomplex and focal prevention of GR signaling in serotonin pathways. Our data thus uncover an alternate mechanism by which pan-HDAC inhibitors may regulate stress-related behaviors independently of their action on histones.

Adult hippocampal neurogenesis is functionally important for stress-induced social avoidance.

The long-term response to chronic stress is variable, with some individuals developing maladaptive functioning, although other "resilient" individuals do not. Stress reduces neurogenesis in the dentate gyrus subgranular zone (SGZ), but it is unknown if stress-induced changes in neurogenesis contribute to individual vulnerability. Using a chronic social defeat stress model, we explored whether the susceptibility to stress-induced social avoidance was related to changes in SGZ proliferation and neurogenesis. Immediately after social defeat, stress-exposed mice (irrespective of whether they displayed social avoidance) had fewer proliferating SGZ cells labeled with the S-phase marker BrdU. The decrease was transient, because BrdU cell numbers were normalized 24 h later. The survival of BrdU cells labeled before defeat stress was also not altered. However, 4 weeks later, mice that displayed social avoidance had more surviving dentate gyrus neurons. Thus, dentate gyrus neurogenesis is increased after social defeat stress selectively in mice that display persistent social avoidance. Supporting a functional role for adult-generated dentate gyrus neurons, ablation of neurogenesis via cranial ray irradiation robustly inhibited social avoidance. These data show that the time window after cessation of stress is a critical period for the establishment of persistent cellular and behavioral responses to stress and that a compensatory enhancement in neurogenesis is related to the long-term individual differences in maladaptive responses to stress.

Induction of deltaFosB in the periaqueductal gray by stress promotes active coping responses.

We analyzed the influence of the transcription factor DeltaFosB on learned helplessness, an animal model of affective disorder wherein a subset of mice exposed to inescapable stress (IS) develop a deficit in escape behavior. Repeated IS induces DeltaFosB in the ventrolateral periaqueductal gray (vlPAG), and levels of the protein are highly predictive of an individual's subsequent behavorial deficit-with the strongest DeltaFosB induction observed in the most resilient animals. Induction of DeltaFosB by IS predominates in substance P-positive neurons in the vlPAG, and the substance P gene, a direct target for DeltaFosB, is downregulated upon DeltaFosB induction. Local overexpression of DeltaFosB in the vlPAG using viral-mediated gene transfer dramatically reduces depression-like behaviors and inhibits stress-induced release of substance P. These results indicate that IS-induced accumulation of DeltaFosB in the vlPAG desensitizes substance P neurons enriched in this area and opposes behavioral despair by promoting active defense responses.

Antidepressant actions of histone deacetylase inhibitors.

Persistent symptoms of depression suggest the involvement of stable molecular adaptations in brain, which may be reflected at the level of chromatin remodeling. We find that chronic social defeat stress in mice causes a transient decrease, followed by a persistent increase, in levels of acetylated histone H3 in the nucleus accumbens, an important limbic brain region. This persistent increase in H3 acetylation is associated with decreased levels of histone deacetylase 2 (HDAC2) in the nucleus accumbens. Similar effects were observed in the nucleus accumbens of depressed humans studied postmortem. These changes in H3 acetylation and HDAC2 expression mediate long-lasting positive neuronal adaptations, since infusion of HDAC inhibitors into the nucleus accumbens, which increases histone acetylation, exerts robust antidepressant-like effects in the social defeat paradigm and other behavioral assays. HDAC inhibitor [N-(2-aminophenyl)-4-[N-(pyridine-3-ylmethoxy-carbonyl)aminomethyl]benzamide (MS-275)] infusion also reverses the effects of chronic defeat stress on global patterns of gene expression in the nucleus accumbens, as determined by microarray analysis, with striking similarities to the effects of the standard antidepressant fluoxetine. Stress-regulated genes whose expression is normalized selectively by MS-275 may provide promising targets for the future development of novel antidepressant treatments. Together, these findings provide new insight into the underlying molecular mechanisms of depression and antidepressant action, and support the antidepressant potential of HDAC inhibitors and perhaps other agents that act at the level of chromatin structure.

Sustained hippocampal chromatin regulation in a mouse model of depression and antidepressant action.

To better understand the molecular mechanisms of depression and antidepressant action, we administered chronic social defeat stress followed by chronic imipramine (a tricyclic antidepressant) to mice and studied adaptations at the levels of gene expression and chromatin remodeling of five brain-derived neurotrophic factor (Bdnf) splice variant mRNAs (I-V) and their unique promoters in the hippocampus. Defeat stress induced lasting downregulation of Bdnf transcripts III and IV and robustly increased repressive histone methylation at their corresponding promoters. Chronic imipramine reversed this downregulation and increased histone acetylation at these promoters. This hyperacetylation by chronic imipramine was associated with a selective downregulation of histone deacetylase (Hdac) 5. Furthermore, viral-mediated HDAC5 overexpression in the hippocampus blocked imipramine's ability to reverse depression-like behavior. These experiments underscore an important role for histone remodeling in the pathophysiology and treatment of depression and highlight the therapeutic potential for histone methylation and deacetylation inhibitors in depression.

An essential role for DeltaFosB in the nucleus accumbens in morphine action.

The transcription factor DeltaFosB is induced in the nucleus accumbens (NAc) and dorsal striatum by the repeated administration of drugs of abuse. Here, we investigated the role of DeltaFosB in the NAc in behavioral responses to opiates. We achieved overexpression of DeltaFosB by using a bitransgenic mouse line that inducibly expresses the protein in the NAc and dorsal striatum and by using viral-mediated gene transfer to specifically express the protein in the NAc. DeltaFosB overexpression in the NAc increased the sensitivity of the mice to the rewarding effects of morphine and led to exacerbated physical dependence, but also reduced their sensitivity to the analgesic effects of morphine and led to faster development of analgesic tolerance. The opioid peptide dynorphin seemed to be one target through which DeltaFosB produced this behavioral phenotype. Together, these experiments demonstrated that DeltaFosB in the NAc, partly through the repression of dynorphin expression, mediates several major features of opiate addiction.

HDAC6-selective inhibitors decrease nerve-injury and inflammation-associated mechanical hypersensitivity in mice.

BACKGROUND: HDAC6 is a class IIB histone deacetylase expressed at many levels of the nociceptive pathway. This study tested the ability of novel and selective HDAC6 inhibitors to alleviate sensory hypersensitivity behaviors in mouse models of peripheral nerve injury and peripheral inflammation. METHODS: We utilized the murine spared nerve injury (SNI) model for peripheral nerve injury and the Complete Freund's Adjuvant (CFA) model of peripheral inflammation. We applied the Von Frey assay to monitor mechanical allodynia. RESULTS: Using the SNI model, we demonstrate that daily administration of the brain-penetrant HDAC6 inhibitor, ACY-738, abolishes mechanical allodynia in male and in female mice. Importantly, there is no tolerance to the antiallodynic actions of these compounds as they produce a consistent increase in Von Frey thresholds for several weeks. We observed a similar antiallodynic effect when utilizing the HDAC6 inhibitor, ACY-257, which shows limited brain expression when administered systemically. We also demonstrate that ACY-738 and ACY-257 attenuate mechanical allodynia in the CFA model of peripheral inflammation. CONCLUSIONS: Overall, our findings suggest that inhibition of HDAC6 provides a promising therapeutic avenue for the alleviation of mechanical allodynia associated with peripheral nerve injury and peripheral inflammation.

Viral vector induction of CREB expression in the periaqueductal gray induces a predator stress-like pattern of changes in pCREB expression, neuroplasticity, and anxiety in rodents.

Predator stress is lastingly anxiogenic. Phosphorylation of CREB to pCREB (phosphorylated cyclic AMP response element binding protein) is increased after predator stress in fear circuitry, including in the right lateral column of the PAG (periaqueductal gray). Predator stress also potentiates right but not left CeA-PAG (central amygdala-PAG) transmission up to 12 days after stress. The present study explored the functional significance of pCREB changes by increasing CREB expression in non-predator stressed rats through viral vectoring, and assessing the behavioral, electrophysiological and pCREB expression changes in comparison with handled and predator stressed controls. Increasing CREB expression in right PAG was anxiogenic in the elevated plus maze, had no effect on risk assessment, and increased acoustic startle response while delaying startle habituation. Potentiation of the right but not left CeA-PAG pathway was also observed. pCREB expression was slightly elevated in the right lateral column of the PAG, while the dorsal and ventral columns were not affected. The findings of this study suggest that by increasing CREB and pCREB in the right lateral PAG, it is possible to produce rats that exhibit behavioral, brain, and molecular changes that closely resemble those seen in predator stressed rats.

Postnatal loss of methyl-CpG binding protein 2 in the forebrain is sufficient to mediate behavioral aspects of Rett syndrome in mice.

BACKGROUND: Mutations in the methyl-CpG binding protein 2 (MeCP2) gene cause Rett syndrome (RTT), a neurodevelopmental disorder that is accompanied by a broad array of behavioral phenotypes, mainly affecting females. Methyl-CpG binding protein 2 is a transcriptional repressor that is widely expressed in all tissues. METHODS: To investigate whether the postnatal loss of MeCP2 in the forebrain is sufficient to produce the behavioral phenotypes observed in RTT, we have generated conditional MeCP2 knockout mice. RESULTS: These mice display behavioral abnormalities similar to RTT phenotypes, including hindlimb clasping, impaired motor coordination, increased anxiety, and abnormal social behavior with other mice. These mice, however, have normal locomotor activity and unimpaired context-dependent fear conditioning, suggesting that the behavioral deficits observed are the result of loss of MeCP2 function in postnatal forebrain and not the result of generalized global deficits. CONCLUSIONS: These data highlight the important role of MeCP2 in the forebrain and suggest that even partial loss of MeCP2 expression in these brain regions is sufficient to recapitulate features of RTT.

Phosphorylation of DARPP-32 at Threonine-34 is required for cocaine action.

Mice lacking DARPP-32, a striatal-enriched phosphoprotein, show abnormal behavioral and biochemical responses to cocaine, but the role of individual phosphorylation sites in DARPP-32 in these responses is unknown. We show here that mutation of Thr-34 in DARPP-32 mimicked the behavioral phenotype of the constitutive DARPP-32 knockout in cocaine-induced place conditioning, locomotor activity, and sensitization paradigms. In contrast, mutations of Thr75 did not affect conditioned place preference or the acute locomotor response to cocaine, but DARPP-32 Thr-75 mutants showed no locomotor sensitization in response to repeated cocaine administration. Consistent with these behavioral findings, we found that cocaine regulation of gene expression in striatum, including the acute induction of the immediate early genes c-fos and arc (activity-regulated cytoskeletal-associated gene), was abolished in DARPP-32 Thr-34 mutants, but not in Thr-75 mutants. Similarly, induction of the transcription factor DeltaFosB in the ventral striatum (nucleus accumbens) by chronic cocaine was diminished by the Thr-34, but not the Thr-75, mutation. These findings highlight distinct roles of the Thr-34 and Thr-75 phosphorylation sites of DARPP-32 in mediating short- and long-term behavioral and biochemical actions of cocaine.

Lasting anxiogenic effects of feline predator stress in mice: sex differences in vulnerability to stress and predicting severity of anxiogenic response from the stress experience.

Previous work in male Swiss Webster (CFW) mice demonstrated a long lasting effect of predator stress on risk assessment in the elevated plus maze (EPM). Most severe effects (increases in risk assessment) were seen following a brief unprotected exposure to a cat. Lesser effects were produced by a brief exposure of mice to the cat exposure room without a cat in the room (room stress). This graded response is analogous to the covariation of symptom severity and severity of the precipitating stressor in posttraumatic stress disorder (PTSD). The present study extended these findings to another strain of mice, C57/BL6, and a broader range of tests of anxiety-like behavior, including EPM, acoustic startle response and light/dark box test. Sex was introduced as a variable to investigate if females might be more susceptible to the effects of stressors than males, as has been suggested in human PTSD. Graded and lasting (7 days) effects of a 10 min exposure to a cat (predator stress) or to the cat exposure room only (room stress) were observed on lighted chamber avoidance in the light/dark box. Room stress was without effect on startle responses, but predator stress enhanced peak startle amplitudes measured in the light or in the dark. There was no evidence of light-enhancement of startle in C57 mice. Female mice were more susceptible to the effects of predator and room stress, depending on the measure. Females only responded to cat exposure with a lasting increase in average startle amplitude. This was due to an increased and more prolonged multipeak response to startle after the first and maximal peak startle response. In addition, in females, room and predator stress were equally anxiogenic in measures of open arm avoidance in the EPM. In contrast, room stress was without effect on open arm avoidance in males, but cat exposure was as anxiogenic in males as it was in females. These findings suggest EPM anxiety in females is affected more by the milder stress of room exposure. Severity of effects of predator stress on anxiety-like behaviors in EPM and startle were well predicted (60% of the variance) by measures of cat behavior and probability of mouse defensive response to particular cat behaviors during the cat exposure. Finally, factor analysis indicated that different tests of anxiety-like behavior may be measuring different and independent aspects of mouse affect. Moreover, stressors had no lasting effects on sugar solution consumption. Implications of these findings for modeling PTSD and using transgenic strains of mice to study lasting effects of stress on affect are discussed.

Cellular mechanisms of deep brain stimulation: activity-dependent focal circuit reprogramming?

Deep brain stimulation (DBS) is a well-established treatment modality for movement disorders. As more behavioral disorders are becoming understood as specific disruptions in neural circuitry, the therapeutic realm of DBS is broadening to encompass a wider range of domains, including disorders of compulsion, affect, and memory, but current understanding of the cellular mechanisms of DBS remains limited. We review progress made during the last decade focusing in particular on how recent methods for targeted circuit manipulations, imaging and reconstruction are fostering preclinical and translational advances that improve our neurobiological understanding of DBS's action in psychiatric disorders.

Top-Down Control of Serotonin Systems by the Prefrontal Cortex: A Path toward Restored Socioemotional Function in Depression.

Social withdrawal, increased threat perception, and exaggerated reassurance seeking behaviors are prominent interpersonal symptoms in major depressive disorder (MDD). Altered serotonin (5-HT) systems and corticolimbic dysconnectivity have long been suspected to contribute to these symptomatic facets; however, the underlying circuits and intrinsic cellular mechanisms that control 5-HT output during socioemotional interactions remain poorly understood. We review literature that implicates a direct pathway between the ventromedial prefrontal cortex (vmPFC) and dorsal raphe nucleus (DRN) in the adaptive and pathological control of social approach-avoidance behaviors. Imaging and neuromodulation during approach-avoidance tasks in humans point to the cortical control of brainstem circuits as an essential regulator of socioemotional decisions and actions. Parallel rodent studies using viral-based connectomics and optogenetics are beginning to provide a cellular blueprint of the underlying circuitry. In these studies, manipulations of vmPFC synaptic inputs to the DRN have revealed bidirectional influences on socioaffective behaviors via direct monosynaptic excitation and indirect disynaptic inhibition of 5-HT neurons. Additionally, adverse social experiences that result in permanent avoidance biases, such as social defeat, drive long-lasting plasticity in this microcircuit, potentiating the indirect inhibition of 5-HT output. Conversely, neuromodulation of the vmPFC via deep brain stimulation (DBS) attenuates avoidance biases by restoring the direct excitatory drive of 5-HT neurons and strengthening a key subset of forebrain 5-HT projections. Better understanding the cellular organization of the vmPFC-DRN pathway and identifying molecular determinants of its neuroplasticity can open fundamentally novel avenues for the treatment of affective disorders.

Enhancement of stress resilience through histone deacetylase 6-mediated regulation of glucocorticoid receptor chaperone dynamics.

BACKGROUND: Acetylation of heat shock protein 90 (Hsp90) regulates downstream hormone signaling via the glucocorticoid receptor (GR), but the role of this molecular mechanism in stress homeostasis is poorly understood. We tested whether acetylation of Hsp90 in the brain predicts and modulates the behavioral sequelae of a mouse model of social stress. METHODS: Mice subjected to chronic social defeat stress were stratified into resilient and vulnerable subpopulations. Hypothalamic-pituitary-adrenal axis function was probed using a dexamethasone/corticotropin-releasing factor test. Measurements of Hsp90 acetylation, Hsp90-GR interactions, and GR translocation were performed in the dorsal raphe nucleus. To manipulate Hsp90 acetylation, we pharmacologically inhibited histone deacetylase 6, a known deacetylase of Hsp90, or overexpressed a point mutant that mimics the hyperacetylated state of Hsp90 at lysine K294. RESULTS: Lower acetylated Hsp90, higher GR-Hsp90 association, and enhanced GR translocation were observed in dorsal raphe nucleus of vulnerable mice after chronic social defeat stress. Administration of ACY-738, a histone deacetylase 6-selective inhibitor, led to Hsp90 hyperacetylation in brain and in neuronal culture. In cell-based assays, ACY-738 increased the relative association of Hsp90 with FK506 binding protein 51 versus FK506 binding protein 52 and inhibited hormone-induced GR translocation. This effect was replicated by overexpressing the acetylation-mimic point mutant of Hsp90. In vivo, ACY-738 promoted resilience to chronic social defeat stress, and serotonin-selective viral overexpression of the acetylation-mimic mutant of Hsp90 in raphe neurons reproduced the behavioral effect of ACY-738. CONCLUSIONS: Hyperacetylation of Hsp90 is a predictor and causal molecular determinant of stress resilience in mice. Brain-penetrant histone deacetylase 6 inhibitors increase Hsp90 acetylation and modulate GR chaperone dynamics offering a promising strategy to curtail deleterious socioaffective effects of stress and glucocorticoids.