Drug experience epigenetically primes Fosb gene inducibility in rat nucleus accumbens.

ΔFosB, a Fosb gene product, is induced in nucleus accumbens (NAc) and caudate-putamen (CPu) by repeated exposure to drugs of abuse such as cocaine. This induction contributes to aberrant patterns of gene expression and behavioral abnormalities seen with repeated drug exposure. Here, we assessed whether a remote history of cocaine exposure in rats might alter inducibility of the Fosb gene elicited by subsequent drug exposure. We show that prior chronic cocaine administration, followed by extended withdrawal, increases inducibility of Fosb in NAc, as evidenced by greater acute induction of ΔFosB mRNA and faster accumulation of ΔFosB protein after repeated cocaine reexposure. No such primed Fosb induction was observed in CPu; in fact, subsequent acute induction of ΔFosB mRNA was suppressed in CPu. These abnormal patterns of Fosb expression are associated with chromatin modifications at the Fosb gene promoter. Prior chronic cocaine administration induces a long-lasting increase in RNA polymerase II (Pol II) binding at the Fosb promoter in NAc only, suggesting that Pol II "stalling" primes Fosb for induction in this region upon reexposure to cocaine. A cocaine challenge then triggers the release of Pol II from the gene promoter, allowing for more rapid Fosb transcription. A cocaine challenge also decreases repressive histone modifications at the Fosb promoter in NAc, but increases such repressive marks and decreases activating marks in CPu. These results provide new insight into the chromatin dynamics at the Fosb promoter and reveal a novel mechanism for primed Fosb induction in NAc upon reexposure to cocaine.

Essential role of the cAMP-cAMP response-element binding protein pathway in opiate-induced homeostatic adaptations of locus coeruleus neurons.

Excessive inhibition of brain neurons in primary or slice cultures can induce homeostatic intrinsic plasticity, but the functional role and underlying molecular mechanisms of such plasticity are poorly understood. Here, we developed an ex vivo locus coeruleus (LC) slice culture system and successfully recapitulated the opiate-induced homeostatic adaptation in electrical activity of LC neurons seen in vivo. We investigated the mechanisms underlying this adaptation in LC slice cultures by use of viral-mediated gene transfer and genetic mutant mice. We found that short-term morphine treatment of slice cultures almost completely abolished the firing of LC neurons, whereas chronic morphine treatment increased LC neuronal excitability as revealed during withdrawal. This increased excitability was mediated by direct activation of opioid receptors and up-regulation of the cAMP pathway and accompanied by increased cAMP response-element binding protein (CREB) activity. Overexpression of a dominant negative CREB mutant blocked the increase in LC excitability induced by morphine- or cAMP-pathway activation. Knockdown of CREB in slice cultures from floxed CREB mice similarly decreased LC excitability. Furthermore, the ability of morphine or CREB overexpression to up-regulate LC firing was blocked by knockout of the CREB target adenylyl cyclase 8. Together, these findings provide direct evidence that prolonged exposure to morphine induces homeostatic plasticity intrinsic to LC neurons, involving up-regulation of the cAMP-CREB signaling pathway, which then enhances LC neuronal excitability.

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.

Epigenetic basis of opiate suppression of Bdnf gene expression in the ventral tegmental area.

Brain-derived neurotrophic factor (BDNF) has a crucial role in modulating neural and behavioral plasticity to drugs of abuse. We found a persistent downregulation of exon-specific Bdnf expression in the ventral tegmental area (VTA) in response to chronic opiate exposure, which was mediated by specific epigenetic modifications at the corresponding Bdnf gene promoters. Exposure to chronic morphine increased stalling of RNA polymerase II at these Bdnf promoters in VTA and altered permissive and repressive histone modifications and occupancy of their regulatory proteins at the specific promoters. Furthermore, we found that morphine suppressed binding of phospho-CREB (cAMP response element binding protein) to Bdnf promoters in VTA, which resulted from enrichment of trimethylated H3K27 at the promoters, and that decreased NURR1 (nuclear receptor related-1) expression also contributed to Bdnf repression and associated behavioral plasticity to morphine. Our findings suggest previously unknown epigenetic mechanisms of morphine-induced molecular and behavioral neuroadaptations.

Enhancing depression mechanisms in midbrain dopamine neurons achieves homeostatic resilience.

Typical therapies try to reverse pathogenic mechanisms. Here, we describe treatment effects achieved by enhancing depression-causing mechanisms in ventral tegmental area (VTA) dopamine (DA) neurons. In a social defeat stress model of depression, depressed (susceptible) mice display hyperactivity of VTA DA neurons, caused by an up-regulated hyperpolarization-activated current (I(h)). Mice resilient to social defeat stress, however, exhibit stable normal firing of these neurons. Unexpectedly, resilient mice had an even larger I(h), which was observed in parallel with increased potassium (K(+)) channel currents. Experimentally further enhancing Ih or optogenetically increasing the hyperactivity of VTA DA neurons in susceptible mice completely reversed depression-related behaviors, an antidepressant effect achieved through resilience-like, projection-specific homeostatic plasticity. These results indicate a potential therapeutic path of promoting natural resilience for depression treatment.

Neurobiological sequelae of witnessing stressful events in adult mice.

BACKGROUND: It is well known that exposure to severe stress increases the risk for developing mood disorders. However, most chronic stress models in rodents involve at least some form of physically experiencing traumatic events. METHODS: This study assessed the effects of a novel social stress paradigm that is insulated from the effects of physical stress. Specifically, adult male C57BL/6J mice were exposed to either emotional (ES) or physical stress (PS) for 10 minutes per day for 10 days. The ES mice were exposed to the social defeat of a PS mouse by a larger, more aggressive CD-1 mouse from the safety of an adjacent compartment. RESULTS: Like PS mice, ES mice exhibited a range of depression- and anxiety-like behaviors both 24 hours and 1 month after the stress. Increased levels of serum corticosterone, part of the stress response, accompanied these behavioral deficits. Based on previous work that implicated gene expression changes in the ventral tegmental area (a key brain reward region) in the PS phenotype, we compared genome-wide mRNA expression patterns in this brain region of ES and PS mice using RNA-seq. We found significant overlap between these conditions, which suggests several potential gene targets for mediating the behavioral abnormalities observed. CONCLUSIONS: These findings demonstrate that witnessing traumatic events is a potent stress in adult male mice capable of inducing long-lasting neurobiological perturbations.

Hippocampal-dependent antidepressant-like activity of histone deacetylase inhibition.

Chronic social defeat stress in mice significantly decreases subsequent social interactions and induces other depression-like behaviors. Here we measured and manipulated levels of acetylated histone H3 (acH3), a chromatin mark of transcriptional activation, in the hippocampus and amygdala after ten continuous days of social defeat stress in male C57/Bl6J mice. This form of social stress causes a transient increase, followed by a persistent decrease, in the levels of acH3 in hippocampus. By comparison, increased acH3 in amygdala was more robust but also highly transient. The persistent decrease in acH3 in hippocampus may be pathological, since it is reversed by chronic fluoxetine administration. Consistent with this hypothesis, infusion of a histone deacetylase (HDAC) inhibitor MS-275 (100 µM) into hippocampus reverses a defeat-induced deficit in sucrose preference, although it does not restore social interaction behavior. Next, different forms of social enrichment were examined with or without hippocampal infusion of MS-275. After social stress, simple pair-housing with another male C57, or female C57, mouse does not reverse social avoidance. However, when HDAC inhibitors are infused into hippocampus during social housing with another male, social avoidance is attenuated. Interestingly, social avoidance is reversed when MS-275 is infused directly into amygdala. Together, these findings further support the antidepressant potential of HDAC inhibitors, and indicate that temporally overlapping environmental and molecular events are required to optimally reverse specific stress-induced behavioral symptoms.