Drug-induced activation of dopamine D(1) receptor signaling and inhibition of class I/II histone deacetylase induce chromatin remodeling in reward circuitry and modulate cocaine-related behaviors

Mechanisms of epigenetic memory and addiction

Epigenetic regulation of cellular identity and function is at least partly achieved through changes in covalent modifications on DNA and histones. Much progress has been made in recent years to understand how these covalent modifications affect cell identity and function. Despite the advances, whether and how epigenetic factors contribute to memory formation is still poorly understood. In this review, we discuss recent progress in elucidating epigenetic mechanisms of learning and memory, primarily at the DNA level, and look ahead to discuss their potential implications in reward memory and development of drug addiction.

Epigenetic signaling in psychiatric disorders

Psychiatric disorders are complex multifactorial illnesses involving chronic alterations in neural circuit structure and function. While genetic factors are important in the etiology of disorders such as depression and addiction, relatively high rates of discordance among identical twins clearly indicate the importance of additional mechanisms. Environmental factors such as stress or prior drug exposure are known to play a role in the onset of these illnesses. Such exposure to environmental insults induces stable changes in gene expression, neural circuit function, and ultimately behavior, and these maladaptations appear distinct between developmental and adult exposures. Increasing evidence indicates that these sustained abnormalities are maintained by epigenetic modifications in specific brain regions. Indeed, transcriptional dysregulation and associated aberrant epigenetic regulation is a unifying theme in psychiatric disorders. Aspects of depression and addiction can be modeled in animals by inducing disease-like states through environmental manipulations (e.g., chronic stress, drug administration). Understanding how environmental factors recruit the epigenetic machinery in animal models reveals new insight into disease mechanisms in humans.

D1/D5 receptors and histone deacetylation mediate the Gateway Effect of LTP in hippocampal dentate gyrus

The dentate gyrus (DG) of the hippocampus is critical for spatial memory and is also thought to be involved in the formation of drug-related associative memory. Here, we attempt to test an aspect of the Gateway Hypothesis, by studying the effect of consecutive exposure to nicotine and cocaine on long-term synaptic potentiation (LTP) in the DG. We find that a single injection of cocaine does not alter LTP. However, pretreatment with nicotine followed by a single injection of cocaine causes a substantial enhancement of LTP. This priming effect of nicotine is unidirectional: There is no enhancement of LTP if cocaine is administrated prior to nicotine. The facilitation induced by nicotine and cocaine can be blocked by oral administration of the dopamine D1/D5 receptor antagonist (SKF 83566) and enhanced by the D1/D5 agonist (SKF 38393). Application of the histone deacetylation inhibitor suberoylanilide hydroxamic acid (SAHA) simulates the priming effect of nicotine on cocaine. By contrast, the priming effect of nicotine on cocaine is blocked in genetically modified mice that are haploinsufficient for the CREB-binding protein (CBP) and possess only one functional CBP allele and therefore exhibit a reduction in histone acetylation. These results demonstrate that the DG of the hippocampus is an important brain region contributing to the priming effect of nicotine on cocaine. Moreover, both activation of dopamine-D1 receptor/PKA signaling pathway and histone deacetylation/CBP mediated transcription are required for the nicotine priming effect in the DG.

Epigenetic mechanisms of drug addiction

Drug addiction involves potentially life-long behavioral abnormalities that are caused in vulnerable individuals by repeated exposure to a drug of abuse. The persistence of these behavioral changes suggests that long-lasting changes in gene expression, within particular regions of the brain, may contribute importantly to the addiction phenotype. Work over the past decade has demonstrated a crucial role for epigenetic mechanisms in driving lasting changes in gene expression in diverse tissues, including brain. This has prompted recent research aimed at characterizing the influence of epigenetic regulatory events in mediating the lasting effects of drugs of abuse on the brain in animal models of drug addiction. This review provides a progress report of this still early work in the field. As will be seen, there is robust evidence that repeated exposure to drugs of abuse induces changes within the brain's reward regions in three major modes of epigenetic regulation-histone modifications such as acetylation and methylation, DNA methylation, and non-coding RNAs. In several instances, it has been possible to demonstrate directly the contribution of such epigenetic changes to addiction-related behavioral abnormalities. Studies of epigenetic mechanisms of addiction are also providing an unprecedented view of the range of genes and non-genic regions that are affected by repeated drug exposure and the precise molecular basis of that regulation. Work is now needed to validate key aspects of this work in human addiction and evaluate the possibility of mining this information to develop new diagnostic tests and more effective treatments for addiction syndromes. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

The epigenetic landscape of alcoholism

Alcoholism is a complex psychiatric disorder that has a multifactorial etiology. Epigenetic mechanisms are uniquely capable of accounting for the multifactorial nature of the disease in that they are highly stable and are affected by environmental factors, including alcohol itself. Chromatin remodeling causes changes in gene expression in specific brain regions contributing to the endophenotypes of alcoholism such as tolerance and dependence. The epigenetic mechanisms that regulate changes in gene expression observed in addictive behaviors respond not only to alcohol exposure but also to comorbid psychopathology such as the presence of anxiety and stress. This review summarizes recent developments in epigenetic research that may play a role in alcoholism. We propose that pharmacologically manipulating epigenetic targets, as demonstrated in various preclinical models, hold great therapeutic potential in the treatment and prevention of alcoholism.

Gene expression in the addicted brain

Addiction is due to changes in the structure and function of the brain, including neuronal networks and the cells that comprise them. Within cells, gene expression changes can track and help explain their altered function. Transcriptional changes induced by addictive agents are dynamic and divergent and range from signal pathway-specific perturbations to widespread molecular and cellular dysregulation that can be measured by "omic" methods and that can be used to identify new pathways. The molecular effects of addiction depend on timing of exposure or withdrawal, the stage of adaptation, the brain region, and the behavioral model, there being many models of addiction. However, the molecular neural adaptations across different drug exposures, conditions, and regions are to some extent shared and can reflect common actions on pathways relevant to addiction. Epigenetic studies of DNA methylation and histone modifications and studies of regulatory RNA networks have been informative for elucidating the mechanisms of transcriptional change in the addicted brain.

Neurotrophins: transcription and translation

Neurotrophins are powerful molecules. Small quantities of these secreted proteins exert robust effects on neuronal survival, synapse stabilization, and synaptic function. Key functions of the neurotrophins rely on these proteins being expressed at the right time and in the right place. This is especially true for BDNF, stimulus-inducible expression of which serves as an essential step in the transduction of a broad variety of extracellular stimuli into neuronal plasticity of physiologically relevant brain regions. Here we review the transcriptional and translational mechanisms that control neurotrophin expression with a particular focus on the activity-dependent regulation of BDNF.

Changes in microRNA expression profile in hippocampus during the acquisition and extinction of cocaine-induced conditioned place preference in rats

Background

MicroRNA (miRNA) emerges as important player in drug abuse. Yet, their expression profile in neurological disorder of cocaine abuse has not been well characterized. Here, we explored the changes of miRNA expression in rat hippocampus following repeated cocaine exposure and subsequent abstinence from cocaine treatment.

Results

Conditioned place preference (CPP) procedure was used to assess the acquisition and extinction of cocaine-seeking behavior in rats. MiRNA microarray was performed to examine miRNAs levels in rat hippocampus. Quantitative RT-PCR was conducted to further confirm results in microarray study. Finally, bioinformatic predictions were made to suggest potential target genes of cocaine-responsive miRNA in this study. MiRNA array found that 34 miRNA levels were changed in rat hippocampus while acquiring cocaine CPP and 42 miRNAs levels were altered after the cocaine-induced CPP were extinguished, as compared to normal controls. The findings from qRT-PCR study support results from microarray analysis.

Conclusions

The current study demonstrated dynamic changes in miRNA expression in rat hippocampus during the acquisition and extinction of cocaine-induced CPP. Some miRNAs which have been previously reported to be involved in brain disorders and drug abuse, including miR-133b, miR-134, miR-181c, miR-191, miR-22, miR-26b, miR-382, miR-409-3p and miR-504, were found to be changed in their expression following repeated cocaine exposure and subsequent abstinence from cocaine treatment. These findings may extend our understanding of the regulatory network underlying cocaine abuse and may provide new targets for the future treatment of drug abuse.

Epigenetics, drugs of abuse, and the retroviral promoter

Drug abuse alone has been shown to cause epigenetic changes in brain tissue that have been shown to play roles in addictive behaviors. In conjunction with HIV-1 infection, it can cause epigenetic changes at the viral promoter that can result in altered gene expression, and exacerbate disease progression overall. This review entails an in-depth look at research conducted on the epigenetic effects of three of the most widely abused drugs (cannabinoids, opioids, and cocaine), with a particular focus on the mechanisms through which these drugs interact with HIV-1 infection at the viral promoter. Here we discuss the impact of this interplay on disease progression from the point of view of the nature of gene regulation at the level of chromatin accessibility, chromatin remodeling, and nucleosome repositioning. Given the importance of chromatin remodeling and DNA methylation in controlling the retroviral promoter, and the high susceptibility of the drug abusing population of individuals to HIV infection, it would be beneficial to understand the way in which the host genome is modified and regulated by drugs of abuse.

FDG-PET imaging reveals local brain glucose utilization is altered by class I histone deacetylase inhibitors

The purpose of this work--the first of its kind--was to evaluate the impact of chronic selective histone deacetylase (HDAC) inhibitor treatment on brain activity using uptake of the radioligand (18)F-fluorodeoxyglucose and positron emission tomography ((18)FDG-PET). HDAC dysfunction and other epigenetic mechanisms are implicated in diverse CNS disorders and animal research suggests HDAC inhibition may provide a lead toward developing improved treatment. To begin to better understand the role of the class I HDAC subtypes HDAC 1, 2 and 3 in modulating brain activity, we utilized two benzamide inhibitors from the literature, compound 60 (Cpd-60) and CI-994 which selectively inhibit HDAC 1 and 2 or HDACs 1, 2 and 3, respectively. One day after the seventh treatment with Cpd-60 (22.5 mg/kg) or CI-994 (5 mg/kg), (18)FDG-PET experiments (n=11-12 rats per treatment group) revealed significant, local changes in brain glucose utilization. These 2-17% changes were represented by increases and decreases in glucose uptake. The pattern of changes was similar but distinct between Cpd-60 and CI-994, supporting that (18)FDG-PET is a useful tool to examine the relationship between HDAC subtype activity and brain activity. Further work using additional selective HDAC inhibitors will be needed to clarify these effects as well as to understand how brain activity changes influence behavioral response.

On the nature of extraversion: variation in conditioned contextual activation of dopamine-facilitated affective, cognitive, and motor processes

Research supports an association between extraversion and dopamine (DA) functioning. DA facilitates incentive motivation and the conditioning and incentive encoding of contexts that predict reward. Therefore, we assessed whether extraversion is related to the efficacy of acquiring conditioned contextual facilitation of three processes that are dependent on DA: motor velocity, positive affect, and visuospatial working memory. We exposed high and low extraverts to three days of association of drug reward (methylphenidate, MP) with a particular laboratory context (Paired group), a test day of conditioning, and three days of extinction in the same laboratory. A Placebo group and an Unpaired group (that had MP in a different laboratory context) served as controls. Conditioned contextual facilitation was assessed by (i) presenting video clips that varied in their pairing with drug and laboratory context and in inherent incentive value, and (ii) measuring increases from day 1 to Test day on the three processes above. Results showed acquisition of conditioned contextual facilitation across all measures to video clips that had been paired with drug and laboratory context in the Paired high extraverts, but no conditioning in the Paired low extraverts (nor in either of the control groups). Increases in the Paired high extraverts were correlated across the three measures. Also, conditioned facilitation was evident on the first day of extinction in Paired high extraverts, despite the absence of the unconditioned effects of MP. By the last day of extinction, responding returned to day 1 levels. The findings suggest that extraversion is associated with variation in the acquisition of contexts that predict reward. Over time, this variation may lead to differences in the breadth of networks of conditioned contexts. Thus, individual differences in extraversion may be maintained by activation of differentially encoded central representations of incentive contexts that predict reward.

Peripheral toxicity in crack cocaine use disorders

A growing body of evidence suggests that crack cocaine misuse has widespread systemic and cognitive consequences, but little attention has been given to its systemic pathophysiology. We report here changes in inflammation markers, oxidative damage and brain derived neurotrophic factor in a sample of outpatients with crack cocaine use disorders. Fifty-three outpatients were recruited for this cross-sectional study and matched with fifty control subjects. The focus of this report is in between group differences in cytokines, oxidative damage and brain-derived neurotrophic factor (BDNF). Crack cocaine use was associated with higher BDNF levels when compared to controls, present only in those who used crack cocaine in the last month. Patients also had higher circulating levels of IL-1β, TNF-α and IL-10 when compared to controls. There were no significant differences in oxidative damage between patients and controls. These results represent a first demonstration that crack cocaine use disorders entail an activation of the reward, immune and inflammatory systems.

Blockade of ethanol-induced behavioral sensitization by sodium butyrate: descriptive analysis of gene regulations in the striatum

BACKGROUND

Behavioral sensitization induced by repeated ethanol (EtOH) exposure may play a critical role in the development of alcohol dependence. Because recent data demonstrate that histone deacetylase inhibitor (HDACi) may be of interest in the treatment of addiction, we explored the effect of the HDACi sodium butyrate (NaB) on EtOH-induced behavioral sensitization (EIBS) in DBA/2J mice. We also investigated gene regulations in the striatum of sensitized mice using epigenetic- and signal transduction-related PCR arrays.

METHODS

Mice were injected with saline or EtOH (0.5 to 2.5 g/kg) once a day for 10 days. Mice received NaB (200 to 600 mg/kg) 30 minutes before each injection (prevention protocol) or once daily between days 11 and 16 (reversal protocol). At day 17, brains were removed 30 minutes after a saline or EtOH challenge to assess gene and proteins levels.

RESULTS

Only the intermediate EtOH doses (1.0 and 2.0 g/kg) were effective in inducing EIBS, and both doses were associated with specific gene regulations in the striatum. The induction of sensitization by 1.0 g/kg (but not 2.0 g/kg) EtOH was dose-dependently prevented or reversed by NaB. Among the 168 studied genes, EIBS blockade was associated with specific gene regulations (bcl-2, bdnf, hdac4, pak1, penk, tacr1, vip…) and changes in brain-derived neurotrophic factor in both striatum and prefrontal cortex.

CONCLUSIONS

These results indicate that EIBS is associated with specific gene regulations in the striatum depending on the EtOH dose and that NaB can be useful in blocking some long-lasting neuro-adaptations to repeated EtOH administrations.

Class I HDAC inhibition blocks cocaine-induced plasticity by targeted changes in histone methylation

Induction of histone acetylation in the nucleus accumbens (NAc), a key brain reward region, promotes cocaine-induced alterations in gene expression. Histone deacetylases (HDACs) tightly regulate the acetylation of histone tails, but little is known about the functional specificity of different HDAC isoforms in the development and maintenance of cocaine-induced plasticity, and prior studies of HDAC inhibitors report conflicting effects on cocaine-elicited behavioral adaptations. Here, we demonstrate that specific and prolonged blockade of HDAC1 in NAc of mice increased global levels of histone acetylation, but also induced repressive histone methylation and antagonized cocaine-induced changes in behavior, an effect mediated in part via a chromatin-mediated suppression of GABA_A receptor subunit expression and inhibitory tone on NAc neurons. Our findings suggest a novel mechanism by which prolonged and selective HDAC inhibition can alter behavioral and molecular adaptations to cocaine and inform the development of novel therapeutics for cocaine addiction.

Epigenetics and psychostimulant addiction

Chronic drug exposure alters gene expression in the brain and produces long-term changes in neural networks that underlie compulsive drug taking and seeking. Exactly how drug-induced changes in synaptic plasticity and subsequent gene expression are translated into persistent neuroadaptations remains unclear. Emerging evidence suggests that complex drug-induced neuroadaptations in the brain are mediated by highly synchronized and dynamic patterns of gene regulation. Recently, it has become clear that epigenetic mechanisms contribute to drug-induced structural, synaptic, and behavioral plasticity by regulating expression of gene networks. Here we review how alterations in histone modifications, DNA methylation, and microRNAs regulate gene expression and contribute to psychostimulant addiction with a focus on the epigenetic mechanisms that regulate brain-derived neurotrophic factor (BDNF) expression following chronic cocaine exposure. Identifying epigenetic signatures that define psychostimulant addiction may lead to novel, efficacious treatments for drug craving and relapse.

The histone deacetylase inhibitor sodium butyrate modulates acquisition and extinction of cocaine-induced conditioned place preference

Despite decades of research on treatments for cocaine dependence, relapse rates following many behavioral and drug-based therapies remain high. This may be in part because cocaine-associated cues and contexts can invoke powerful drug cravings years after quitting. Recent studies suggest that drugs that promote cognitive function can enhance the formation of memories involving cocaine and other substances. One target of these drugs is facilitating histone acetylation to promote learning by increasing gene transcription that supports memory formation. Here, we investigate the effects of the histone deacetylase (HDAC) inhibitor sodium butyrate (NaBut) on cocaine-induced conditioned place preference (CPP) in C57BL/6 mice. After establishing a graded dose-response curve (2, 5, & 20 mg/kg) for cocaine-induced CPP, we examined the effects of different doses of NaBut (0, 0.3, 0.6, & 1.2 g/kg) on conditioning, extinction, and post-extinction reconditioning of CPP. A high dose of NaBut (1.2 g/kg) enhanced initial acquisition of cocaine CPP, but there were no effects of NaBut on reconditioning of extinguished CPP. Effects of NaBut on extinction were more complex, with a low-dose (0.3 g/kg) facilitating extinction and a high dose (1.2 g/kg) weakening extinction evident by preference at a retention test. These findings suggest that HDAC inhibition may have dose dependent effects on different components of cocaine CPP, with implications for (1) involvement of histone acetylation in context-drug learning, (2) interpretation of acute and chronic drug effects, and (3) the targeting of different types of learning in therapeutic application of HDAC inhibitors.

Epigenetics in the human brain

Many cellular constituents in the human brain permanently exit from the cell cycle during pre- or early postnatal development, but little is known about epigenetic regulation of neuronal and glial epigenomes during maturation and aging, including changes in mood and psychosis spectrum disorders and other cognitive or emotional disease. Here, we summarize the current knowledge base as it pertains to genome organization in the human brain, including the regulation of DNA cytosine methylation and hydroxymethylation, and a subset of (altogether >100) residue-specific histone modifications associated with gene expression, and silencing and various other functional chromatin states. We propose that high-resolution mapping of epigenetic markings in postmortem brain tissue or neural cultures derived from induced pluripotent cells (iPS), in conjunction with transcriptome profiling and whole-genome sequencing, will increasingly be used to define the molecular pathology of specific cases diagnosed with depression, schizophrenia, autism, or other major psychiatric disease. We predict that these highly integrative explorations of genome organization and function will provide an important alternative to conventional approaches in human brain studies, which mainly are aimed at uncovering group effects by diagnosis but generally face limitations because of cohort size.

Regulation of BDNF expression by cocaine

Brain-derived neurotrophic factor (BDNF) is a member of the neurotrophin family of growth factors. It is expressed throughout the nervous system. A unique feature of the BDNF gene is the existence of multiple mRNA transcripts, all of which are translated into BDNF protein, suggesting a multilevel regulation of expression. In particular, the BDNF exon IV promoter region is a preferential target for epigenetic alterations, as it contains binding sites for CREB and MeCP2, two transcriptional regulators known to mediate epigenetic changes. Exposure to drugs of abuse is known to modulate epigenetic regulation of BDNF gene expression. This review will discuss how exposure to cocaine, one of the most addictive drugs known to mankind, can produce alterations in BDNF gene expression, especially in the mesolimbic dopaminergic system, which lead to alterations in the reward-mediated behaviors involved in addiction.

Upregulation of nerve growth factor in central amygdala increases sensitivity to opioid reward

The rewarding properties of opioids are essential driving force for compulsive drug-seeking and drug-taking behaviors in the development of opioid-mediated drug addiction. Prior drug use enhances sensitivity to the rewarding effects of subsequently used drugs, increasing vulnerability to relapse. The molecular mechanisms underlying this reward sensitization are still unclear. We report here that morphine that induced reward sensitization, as demonstrated by reinstatement of the behavior of conditioned place preference (CPP) with sub-threshold priming morphine, epigenetically upregulated the output activity of Ngf encoding the nerve growth factor (NGF) by increasing histone H4 acetylation in the rat central nucleus of the amygdala (CeA). NGF locally infused into the CeA mimicked the morphine effect in inducing new functional delta-opioid receptor (DOR) that was required for the reward sensitization, and morphine-induced reward sensitization was inhibited by blocking NGF receptor signaling in the CeA. Histone deacetylase inhibitors that increased the acetylation level at the Ngf promoter and NGF expression in the CeA also induced reward sensitization in a CeA NGF signaling- and DOR-dependent manner. Furthermore, CeA-applied NGF substituted prior morphine to induce reward sensitization in naive rats and also substituted priming morphine to reinstate the CPP induced by prior morphine. Thus, epigenetic upregulation of NGF activity in the CeA may promote the behavior of opioid reward and increase the sensitivity to the rewarding effect of subsequent opioids, a potentially important mechanism in drug addiction.

Current perspectives on the neurobiology of drug addiction: a focus on genetics and factors regulating gene expression

Drug addiction is a chronic, relapsing disorder defined by cyclic patterns of compulsive drug seeking and taking interspersed with episodes of abstinence. While genetic variability may increase the risk of addictive behaviours in an individual, exposure to a drug results in neuroadaptations in interconnected brain circuits which, in susceptible individuals, are believed to underlie the transition to, and maintenance of, an addicted state. These adaptations can occur at the cellular, molecular, or (epi)genetic level and are associated with synaptic plasticity and altered gene expression, the latter being mediated via both factors affecting translation (epigenetics) and transcription (non coding microRNAs) of the DNA or RNA itself. New advances using techniques such as optogenetics have the potential to increase our understanding of the microcircuitry mediating addictive behaviours. However, the processes leading to addiction are complex and multifactorial and thus we face a major contemporary challenge to elucidate the factors implicated in the development and maintenance of an addicted state.

Antidepressant-like effect of sodium butyrate (HDAC inhibitor) and its molecular mechanism of action in the rat hippocampus

OBJECTIVES

Epigenetic mechanisms, such as changes in gene expression resulting from chromatin remodeling through histone acetylation, have been implicated in the pathophysiology of depression. However, the antidepressant-like effect of the histone deacetylase inhibitor sodium butyrate (SB) has been inconclusive. The aim of this study was to examine the antidepressant-like effect of SB and elucidate its molecular mechanisms.

METHODS

We examined the antidepressant-like effect of SB in a forced swim test (FST) and a tail suspension test (TST). Hippocampal gene expression analyses using DNA microarray and real-time PCR were undertaken. Western blotting and ChIP assay were undertaken to examine whether histone acetylation was associated with changes in gene expression by SB.

RESULTS

Repeated administration of SB significantly reduced immobility on the FST and the TST, and significantly altered the levels of mRNA for several genes; e.g., upregulation of transthyretin (Ttr) and downregulation of serotonin 2A receptor (Htr2a). Western blotting and ChIP assay revealed selective increases in histone H4 acetylation at the promoter of the Ttr gene with a significant increase in Ttr immunoreactivity 24 h after the final administration of SB.

CONCLUSION

These findings suggest the possibility that alterations in gene expression, including upregulation of Ttr and downregulation of several other genes, including Htr2a, may be involved in antidepressant-like effect of SB.

The histone deacetylase inhibitor, sodium butyrate, alleviates cognitive deficits in pre-motor stage PD

Parkinson's disease (PD) patients often times experience impairment in their cognitive abilities early on in the progression of the disease. The reported deficits appear to mainly involve functions that are associated with frontal lobe and frontal-striatal pathways subserving attentional set-shifting, working memory and executive function. The current study explored executive function deficits in a rat model of PD in the pre-motor deficit stage. The rats were lesioned with 12 μg of 6-hydroxydonpamine (6-OHDA) in the striatum in a two step process (10 μg/μl followed by 2 μg/μl) 48 hours apart. Executive function was tested at 3 weeks post-surgery using a rat analogue of Wisconsin card sorting test called the Extra Dimensional/Intra Dimensional (ED/ID) set-shifting task. The results demonstrated that performance by the pre-motor rat model of PD was equivalent to that of the control groups in the simple and the compound discriminations as well as the intra-dimensional set-shifting. However the PD group exhibited attentional set-shifting deficits similar to those observed in PD patients. Additionally, sodium butyrate, a short chain fatty acid derivative and inhibitor of class I and II histone deacetylase (HDACi), was tested as a potential therapeutic agent to mitigate the pre-motor cognitive deficits in PD. The results indicated that the sodium butyrate treatment not only effectively alleviated the set-shifting deficits, but also improved the attentional set formation in the treated rats.

CBP in the nucleus accumbens regulates cocaine-induced histone acetylation and is critical for cocaine-associated behaviors

Cocaine exposure triggers molecular events that lead to long-lasting changes in brain structure and function. These changes can lead to the development of persistent and robust behavioral adaptations that characterize addiction. Recent evidence suggests the regulation of transcription via chromatin modification, such as histone acetylation, has an important role in the development of addictive behavior. Histone acetylation is regulated by histone acetyltransferases (HATs), which acetylate histones and promote transcription, and histone deacetylases (HDACs), which remove acetyl groups and silence transcription. Studies have demonstrated that HDACs may negatively regulate cocaine-induced behaviors, but very little is known about the role of specific HATs in long-lasting drug-induced plasticity. The histone acetyltransferase CREB-binding protein (CBP) mediates transcriptional activation by recruiting basal transcription machinery and acetylating histones. CBP is a critically important chromatin-modifying enzyme involved in regulating gene expression required for long-term plasticity and memory. However, the role of CBP in cocaine-induced behaviors remains largely unknown. We examined the role of CBP in drug-induced plasticity using CBP-FLOX genetically modified mice in combination with adeno-associated virus expressing Cre-recombinase to generate focal homozygous deletions of Cbp in the nucleus accumbens (NAc). A complete loss of CBP in NAc neurons results in decreased histone acetylation and significantly altered c-fos expression in response to cocaine. Furthermore, the deletion of CBP in the NAc correlates with significant impairments in cocaine sensitivity and context-cocaine associated memory. This is the first study to demonstrate a definitive role for CBP in modulating gene expression that may subserve drug-seeking behaviors.

Increased brain-derived neurotrophic factor (BDNF) expression in the ventral tegmental area during cocaine abstinence is associated with increased histone acetylation at BDNF exon I-containing promoters

Recent evidence suggests that the persistence of cocaine seeking during periods of protracted drug abstinence following chronic cocaine exposure is mediated, in part, by neuroadaptations in the mesolimbic dopamine system. Specifically, incubation of cocaine-seeking behavior coincides with increased brain-derived neurotrophic factor (BDNF) protein expression in the ventral tegmental area (VTA). However, the molecular mechanisms that regulate time-dependent changes in VTA BDNF protein expression during cocaine abstinence are unclear. The goal of these experiments was to determine whether VTA BDNF transcript levels are altered following cocaine abstinence and identify the molecular mechanisms regulating cocaine-induced changes in VTA BDNF transcription. Rats were allowed to self-administer cocaine (0.25 mg/infusion, i.v.) for 14 days on a fixed-ratio schedule of reinforcement followed by 7 days of forced drug abstinence. BDNF protein and exon I-containing transcripts were significantly increased in the VTA of cocaine-experienced rats following 7 days of forced drug abstinence compared to yoked saline controls. Cocaine-induced changes in BDNF mRNA were associated with increased acetylation of histone 3 and binding of CREB-binding protein to exon I-containing promoters in the VTA. Taken together, these results suggest that drug abstinence following cocaine self-administration remodels chromatin in the VTA resulting in increased expression of BDNF, which may contribute to neuroadaptations underlying cocaine craving and relapse.

A role for repressive histone methylation in cocaine-induced vulnerability to stress

Substance abuse increases an individual's vulnerability to stress-related illnesses, which is presumably mediated by drug-induced neural adaptations that alter subsequent responses to stress. Here, we identify repressive histone methylation in nucleus accumbens (NAc), an important brain reward region, as a key mechanism linking cocaine exposure to increased stress vulnerability. Repeated cocaine administration prior to subchronic social defeat stress potentiated depressive-like behaviors in mice through decreased levels of histone H3 lysine 9 dimethylation in NAc. Cre-mediated reduction of the histone methyltransferase, G9a, in NAc promoted increased susceptibility to social stress, similar to that observed with repeated cocaine. Conversely, G9a overexpression in NAc after repeated cocaine protected mice from the consequences of subsequent stress. This resilience was mediated, in part, through repression of BDNF-TrkB-CREB signaling, which was induced after repeated cocaine or stress. Identifying such common regulatory mechanisms may aid in the development of new therapies for addiction and depression.

Transcriptional and epigenetic mechanisms of addiction

Investigations of long-term changes in brain structure and function that accompany chronic exposure to drugs of abuse suggest that alterations in gene regulation contribute substantially to the addictive phenotype. Here, we review multiple mechanisms by which drugs alter the transcriptional potential of genes. These mechanisms range from the mobilization or repression of the transcriptional machinery - including the transcription factors ΔFOSB, cyclic AMP-responsive element binding protein (CREB) and nuclear factor-κB (NF-κB) - to epigenetics - including alterations in the accessibility of genes within their native chromatin structure induced by histone tail modifications and DNA methylation, and the regulation of gene expression by non-coding RNAs. Increasing evidence implicates these various mechanisms of gene regulation in the lasting changes that drugs of abuse induce in the brain, and offers novel inroads for addiction therapy.

The epigenetic landscape of addiction

Drug-induced alterations in gene expression throughout the reward circuitry of the brain are likely components of the persistence of the drug-addicted state. Recent studies examining the molecular mechanisms controlling drug-induced transcriptional, behavioral, and synaptic plasticity have indicated a direct role for chromatin remodeling in the regulation and stability of drug-mediated neuronal gene programs, and the subsequent promulgation of addictive behaviors. In this review, we discuss recent advances in our understanding of chromatin phenomena--or epigenetics, by one definition--that contribute to drug addiction, with the hope that such mechanistic insights may aid in the development of novel therapeutics for future treatments of addiction.

Attenuation of vascular dementia by sodium butyrate in streptozotocin diabetic rats

RATIONALE

Vascular dementia is the second leading cause of dementia, which is strongly associated with diabetes. Diabetes and dementia have become a major public health concern worldwide. At this point of time, it is very important to find the possible pharmacological agents which may be useful in management and therapy of dementia including Alzheimer's disease, vascular dementia, etc.

OBJECTIVES

To investigate the effect of sodium butyrate on streptozotocin (STZ) diabetes induced vascular dementia in rats.

METHODS

Diabetes and subsequent endothelial dysfunction and dementia were induced in rats by administration of single dose of STZ. Drug treatment was started after 1 month of STZ administration and treatment was continued until the end of the study. Morris water maze (MWM) test was employed for testing learning and memory. Endothelial function was measured on isolated aortic rings using student physiograph. Serum glucose, body weight, serum nitrite/nitrate, aortic superoxide anion generation, brain thiobarbituric acid reactive species (TBARS), reduced glutathione (GSH) levels, and acetylcholinesterase activity were also tested.

RESULTS

STZ treatment produced endothelial dysfunction, impairment of learning and memory, reduction in body weight and serum nitrite/nitrate, and increase in serum glucose, aortic and brain oxidative stress (increased superoxide anion, TBARS, and decreased GSH levels), and brain acetylcholinesterase activity. Treatment of sodium butyrate attenuated diabetes induced impairment of learning, memory, endothelial function, and various biochemical parameters.

CONCLUSIONS

Sodium butyrate may be considered as potential pharmacological agent for the management of diabetes induced vascular dementia.

Chronic high-fat diet drives postnatal epigenetic regulation of μ-opioid receptor in the brain

Opioid system dysregulation has been observed in both genetic and high-fat diet (HFD)-induced models of obesity. An understanding of the molecular mechanisms of MOR transcriptional regulation, particularly within an in vivo context, is lacking. Using a diet-induced model of obesity (DIO), mice were fed a high-fat diet (60% calories from fat) from weaning to >18 weeks of age. Compared with mice fed the control diet, DIO mice had a decreased preference for sucrose. MOR mRNA expression was decreased in reward-related circuitry (ventral tegmental area (VTA), nucleus accumbens (NAc), and prefrontal cortex (PFC)) but not the hypothalamus, important in the homeostatic regulation of feeding. DNA methylation is an epigenetic modification that links environmental exposures to altered gene expression. We found a significant increase in DNA methylation in the MOR promoter region within the reward-related brain regions. Methyl CpG-binding protein 2 (MeCP2) can bind methylated DNA and repress transcription, and DIO mice showed increased binding of MeCP2 to the MOR promoter in reward-related regions of the brain. Finally, using ChIP assays we examined H3K9 methylation (inactive chromatin) and H3 acetylation (active chromatin) within the MOR promoter region and found increased H3K9 methylation and decreased H3 acetylation. These data are the first to identify DNA methylation, MeCP2 recruitment, and chromatin remodeling as mechanisms leading to transcriptional repression of MOR in the brains of mice fed a high-fat diet.

Sensitization to cocaine is inhibited after intra-accumbal GR103691 or rimonabant, but it is enhanced after co-infusion indicating functional interaction between accumbens D(3) and CB1 receptors

RATIONALE

Dopamine D(3) receptors and cannabinoid CB(1) receptors are both expressed in the nucleus accumbens, and they have been involved in motor sensitization to cocaine. The objectives were: (1) to study the effects of blockade of these receptors on sensitization to repeated cocaine, by using GR103691, D(3) receptor blocker, and rimonabant, CB(1) receptor ligand, and (2) to discern if both receptors interact by co-infusing them.

MATERIALS AND METHODS

Cocaine (10 mg/kg) was injected daily for 3 days (induction phase) and later on day 8 (expression phase), and locomotor activity was measured during 2 h after cocaine. GR103691 and rimonabant were bilaterally injected (0.5 μl volume of each infusion) in the nucleus accumbens through cannulae (GR103691, 0, 4.85, and 9.7 μg/μl; rimonabant, 0, 0.5, and 1.5 μg/μl), before cocaine, during either induction or expression phases of sensitization.

RESULTS

The findings indicated that sensitizing effects of cocaine were abolished after D(3) receptor blocking during both induction and expression phases, as well as rimonabant infusion during the expression (not induction) phase. A functional interaction between both receptors was also observed, because if GR103691 was injected during induction and rimonabant during expression, sensitizing effects of cocaine were observed to be normal or further enhanced.

CONCLUSION

Dopamine D(3) receptors within the nucleus accumbens are critical for the development and consolidation of sensitization, and cannabinoid CB(1) receptors are critical for the expression of sensitization. Co-blockade of D(3) and CB(1) receptors exert opposite effects to blockade of these receptors separately, revealing the existence of a functional interaction between them.

Dnmt3a regulates emotional behavior and spine plasticity in the nucleus accumbens

Despite abundant expression of DNA methyltransferases (Dnmt’s) in brain, the regulation and behavioral role of DNA methylation remain poorly understood. We find that Dnmt3a expression is regulated in mouse nucleus accumbens (NAc) by chronic cocaine and chronic social defeat stress. Moreover, NAc specific manipulations that block DNA methylation potentiate cocaine reward and exert antidepressant-like effects, whereas NAc specific Dnmt3a overexpression attenuates cocaine reward and is pro-depressant. On a cellular level, we show that chronic cocaine selectively increases thin dendritic spines on NAc neurons and that DNA methylation is both necessary and sufficient to mediate these effects. These data establish the importance of Dnmt3a in the NAc in regulating cellular and behavioral plasticity to emotional stimuli.

How the epigenome contributes to the development of psychiatric disorders

Epigenetics commonly refers to the developmental process by which cellular traits are established and inherited without a change in DNA sequence. These mechanisms of cellular memory also orchestrate gene expression in the adult brain and recent evidence suggests that the "epigenome" represents a critical interface between environmental signals, activation, repression and maintenance of genomic responses, and persistent behavior. We here review the current state of knowledge regarding the contribution of the epigenome toward the development of psychiatric disorders.

Mechanisms involved in the neurotoxic, cognitive, and neurobehavioral effects of alcohol consumption during adolescence

Studies over the last decade demonstrate that adolescence is a brain maturation period from childhood to adulthood. Plastic and dynamic processes drive adolescent brain development, creating flexibility that allows the brain to refine itself, specialize, and sharpen its functions for specific demands. Maturing connections enable increased communication among brain regions, allowing greater integration and complexity. Compelling evidence has shown that the developing brain is vulnerable to the damaging effects of ethanol. It is possible to infer, therefore, that alcohol exposure during the critical adolescent developmental stages could disrupt the brain plasticity and maturation processes, resulting in behavioral and cognitive deficits. Recent neuroimaging studies have provided evidence of the impact of human adolescent drinking in brain structure and functions. Findings in experimental animals have also given new insight into the potential mechanisms of the toxic effects of ethanol on both adolescent brain maturation and the short- and long-term cognitive consequences of adolescent drinking. Adolescence is also characterized by the rapid maturation of brain systems mediating reward and by changes in the secretion of stress-related hormones, events that might participate in the increasing in anxiety and the initiation pattern of alcohol and drug consumption. Studies in human adolescents demonstrate that drinking at early ages can enhance the likelihood of developing alcohol-related problems. Experimental evidence suggests that early exposure to alcohol sensitizes the neurocircuitry of addiction and affects chromatin remodeling, events that could induce abnormal plasticity in reward-related learning processes that contribute to adolescents' vulnerability to drug addiction. In this article, we review the potential mechanisms by which ethanol impacts brain development and lead to brain impairments and cognitive and behavioral dysfunctions as well as the neurobiological and neurochemical processes underlying the adolescent-specific vulnerability to drug addiction.

Maladaptive striatal plasticity in L-DOPA-induced dyskinesia

Dopamine (DA) replacement therapy with l-DOPA remains the most effective treatment for Parkinson's disease, but causes dyskinesia (abnormal involuntary movements) in the vast majority of the patients. The basic mechanisms of l-DOPA-induced dyskinesia (LID) have become the object of intense research focusing on neurochemical and molecular adaptations in the striatum. Here we review this vast literature and highlight trends that converge into a unifying pathophysiological interpretation. We propose that the core molecular alteration of striatal neurons in LID consists in an inability to turn down supersensitive signaling responses downstream of DA D1 receptors (where supersensitivity is primarily caused by DA denervation). The sustained activation of intracellular signaling pathways induced by each dose of l-DOPA leads to abnormal cellular plasticity and high bioenergetic expenditure. The over-exploitation of signaling pathways and energy reserves during treatment impairs the ability of striatal neurons to dynamically gate cortically driven motor commands. LID thus exemplifies a disorder where 'too much' molecular plasticity leads to plasticity failure in the striatum.

Selective boosting of transcriptional and behavioral responses to drugs of abuse by histone deacetylase inhibition

Histone acetylation and other modifications of the chromatin are important regulators of gene expression and, consequently, may contribute to drug-induced behaviors and neuroplasticity. Earlier studies have shown that a reduction in histone deacetylase (HDAC) activity results in the enhancement of some psychostimulant-induced behaviors. In this study, we extend those seminal findings by showing that the administration of the HDAC inhibitor sodium butyrate enhances morphine-induced locomotor sensitization and conditioned place preference. In contrast, this compound has no effects on the development of morphine tolerance and dependence. Similar effects were observed for cocaine and ethanol-induced behaviors. These behavioral changes were accompanied by a selective boosting of a component of the transcriptional program activated by chronic morphine administration that included circadian clock genes and other genes relevant to addictive behavior. Our results support a specific function for histone acetylation and the epigenetic modulation of transcription at a reduced number of biologically relevant loci on non-homeostatic, long-lasting, drug-induced behavioral plasticity.

Chromatin regulation in drug addiction and depression

Alterations in gene expression are implicated in the pathogenesis of several neuropsychiatrie disorders, including drug addiction and depression, increasing evidence indicates that changes in gene expression in neurons, in the context of animal models of addiction and depression, are mediated in part by epigenetic mechanisms that alter chromatin structure on specific gene promoters. This review discusses recent findings from behavioral, molecular, and bioinformatic approaches that are being used to understand the complex epigenetic regulation of gene expression in brain by drugs of abuse and by stress. These advances promise to open up new avenues for improved treatments of these disorders.

Epigenetic mechanisms underlying extinction of memory and drug-seeking behavior

An increasing body of evidence shows that structural modifications of chromatin, the DNA-protein complex that packages genomic DNA, do not only participate in maintaining cellular memory (e.g., cell fate), but they may also underlie the strengthening and maintenance of synaptic connections required for long-term changes in behavior. Accordingly, epigenetics has become a central topic in several neurobiology fields such as memory, drug addiction, and several psychiatric and mental disorders. This interest is justified as dynamic chromatin modifications may provide not only transient but also stable (or even potentially permanent) epigenetic marks to facilitate, maintain, or block transcriptional processes, which in turn may participate in the molecular neural adaptations underlying behavioral changes. Through epigenetic mechanisms the genome may be indexed in response to environmental signals, resulting in specific neural modifications that largely determine the future behavior of an organism. In this review we discuss recent advances in our understanding of how epigenetic mechanisms contribute to the formation of long-term memory and drug-seeking behavior and potentially how to apply that knowledge to the extinction of memory and drug-seeking behavior.

Cocaine-induced metabolic activation in cortico-limbic circuitry is increased after exposure to the histone deacetylase inhibitor, sodium butyrate

Drug-induced inhibition of histone deacetylase (HDAC) results in the modification of many behavioral changes resulting from exposure to cocaine and other stimulant drugs-of-abuse, but a comprehensive map of the neuronal circuitries involved is lacking. The present study used blood-oxygen-level-dependent functional magnetic resonance imaging (BOLD fMRI) in awake rats to determine the effects of the HDAC inhibitor, sodium butyrate (SBt) on brain metabolic activation patterns during the initial stage of repeated cocaine administration. Three groups of rats received cocaine during BOLD fMRI, (i) acutely for the first time, or pretreated for 2 days with either (ii) saline or (iii) SBt 30 min prior to cocaine. Acute but not repeated exposure to cocaine resulted in widespread BOLD activation in fore- and mid-brain. Pretreatment with SBt restored BOLD signals in the forebrain after repeated cocaine exposure, including a pronounced activation in the anterior thalamus, the hippocampus/amygdala and various portions of limbic and sensory cortex. Mesocorticolimbic areas showed a similar trend, but did not reach statistical significance. These findings suggest that HDACi modulation after repeated stimulant exposure involves cortico-limbic circuitry regulating emotion, motivation and memory.

Effects of the histone deacetylase inhibitor sodium butyrate in models of depression and anxiety

Histone modification, which affects the rate of transcription without altering DNA sequence, occurs in response to various psychiatric drugs and in several models of psychiatric disease. As increases in histone acetylation have been seen after treatment with antidepressants, we investigated whether directly increasing histone acetylation using a histone deacetylase inhibitor would have antidepressant effects. We administered sodium butyrate (NaB, 100 mg/kg, i.p.) to mice acutely (3 injections over 24 h) or chronically (twice daily for 21 days) and subjected them to a number of behavioral tests of antidepressant response. This dose of NaB had no effect on overall locomotor activity after either acute or chronic treatment. Acutely treated mice showed an increase in immobility in the forced-swim test (FST) and an increase in latency to consume in the novel environment of the novelty-induced hypophagia (NIH) paradigm, an anxiogenic effect. The effect of NaB on anxiety did not generalize to another test, the elevated zero maze, where it had no effect. Chronic treatment with NaB had no effect on latency to consume in the NIH or immobility in the FST. However, this dose did alter histone acetylation in the hippocampus. While H4 acetylation increased in the hippocampus 30 min following acute NaB, chronic treatment caused a decrease in AcH4. There were no changes in AcH3 following either treatment. While changes in chromatin structure may be involved in the mechanism of action of antidepressant drugs, these data suggest that increasing histone acetylation pharmacologically is not sufficient to produce antidepressant effects.

Histone acetylation in drug addiction

Regulation of chromatin structure through post-translational modifications of histones (e.g., acetylation) has emerged as an important mechanism to translate a variety of environmental stimuli, including drugs of abuse, into specific changes in gene expression. Since alterations in gene expression are thought to contribute to the development and maintenance of the addicted state, recent efforts are aimed at identifying how drugs of abuse alter chromatin structure and the enzymes which regulate it. This review discusses how drugs of abuse alter histone acetylation in brain reward regions, through which enzymes this occurs, and ultimately what role histone acetylation plays in addiction-related behaviors.

Repeated alcohol administration during adolescence causes changes in the mesolimbic dopaminergic and glutamatergic systems and promotes alcohol intake in the adult rat

Adolescence is a developmental period which the risk of drug and alcohol abuse increases. Since mesolimbic dopaminergic system undergoes developmental changes during adolescence, and this system is involved in rewarding effects of drugs of abuse, we addressed the hypothesis that ethanol exposure during juvenile/adolescent period over-activates mesolimbic dopaminergic system inducing adaptations which can trigger long-term enduring behavioural effects of alcohol abuse. We treated juvenile/adolescent or adult rats with ethanol (3 g/kg) for two-consecutive days at 48-h intervals over 14-day period. Here we show that intermittent ethanol treatment during the juvenile/adolescence period alters subsequent ethanol intake. In vivo microdialysis demonstrates that ethanol elicits a similar prolonged dopamine response in the nucleus accumbens of both adolescent and adult animals pre-treated with multiple doses of ethanol, although the basal dopamine levels were higher in ethanol-treated adolescents than in adult-treated animals. Repeated ethanol administration also down-regulates the expression of DRD2 and NMDAR2B phosphorylation in prefrontal cortex of adolescent animals, but not of adult rats. Finally, ethanol treatment during adolescence changes the acetylation of histones H3 and H4 in frontal cortex, nucleus accumbens and striatum, suggesting chromatin remodelling changes. In summary, our findings demonstrate the sensitivity of adolescent brain to ethanol effects on dopaminergic and glutamatergic neurotransmission, and suggest that abnormal plasticity in reward-related processes and epigenetic mechanisms could contribute to the vulnerability of adolescents to alcohol addiction.

Epigenetic and pharmacoepigenomic studies of major psychoses and potentials for therapeutics

Individuals with neuropsychiatric diseases have epigenetic programming disturbances, both in the brain, which is the primary affected organ, and in secondary tissues. Epigenetic modulations are molecular modifications made to DNA, RNA and proteins that fine-tune genotype into phenotype and do not include DNA base changes. For instance, gene-expression modulation is linked to epigenetic codes in chromatin that consist of post-replication DNA methylation and histone protein modifications (e.g., methylation, acetylation and so on), particularly in gene-promoter regions. Epigenetic coding is modulated globally, and in a gene-specific manner by environmental exposures that include nutrition, toxins, drugs and so on. Analysis of epigenetic aberrations in diseases helps to identify dysfunctional genes and pathways, establish more robust cause–effect relationships than genetic studies alone, and identify new pharmaceutical targets and drugs, including nucleic acid reagents such as inhibitory RNAs. The emerging science of pharmacoepigenomics can impact the treatment of psychiatric and other complex diseases. In fact, some therapeutics now in use target epigenetic programming. In the near future, epigenetic interventions should help stabilize affected individuals and lead to prevention strategies.

Endocrine and gene expression changes following forced swim stress exposure during cocaine abstinence in mice

RATIONALE

Stress can reinstate previous cocaine-seeking long after drug is no longer present. However, little is known regarding the effect of chronic drug exposure and subsequent drug abstinence on responsivity to stress.

OBJECTIVE

To determine the effect of acute (24-h) and prolonged (14-day) drug-free periods in cocaine-experienced mice on behavioral, endocrine, and molecular outputs following stress exposure.

MATERIALS AND METHODS

Mice were administered a cocaine binge (15 mg/kg, every hour for 3h) for 2 weeks. Following a 24-h or 14-day drug-free period, stress responsivity, along with levels of anxiety, were measured using the forced swim test and elevated zero maze, respectively. In addition, alterations in the levels of plasma corticosterone, corticotrophin-releasing factor (CRF) mRNA, brain-derived neurotrophic factor (BDNF) mRNA, and histone acetylation at their respective promoters were examined following stress exposure.

RESULTS

At both acute and prolonged abstinence time points, behavioral measures were essentially unaltered; however, cocaine-experienced mice exhibited an augmented corticosterone response to the forced swim stress compared to saline-treated mice. Stress exposure increased BDNF mRNA levels in the ventral tegmental area (VTA) and nucleus accumbens (NAc) only in cocaine-experienced mice following a prolonged, but not acute, drug-free period. Increased BDNF mRNA in the NAc was associated with an increase in acetylated histone 3 (AcH3) at the BDNF I promoter. CRF mRNA levels were increased in the amygdala (AMYG); however, this was not associated with alterations in histone acetylation at the promoter.

CONCLUSION

These results demonstrate that drug history and prolonged abstinence can alter the endocrine and molecular responses to stress, which may facilitate the reinstatement of drug-seeking behaviors.

CNS genes implicated in relapse

Drug abuse is a condition that impacts not only the individual drug user, but society as a whole. Although prevention of initial drug use is the most effective way to prevent addiction, avoiding relapse is a crucial component of drug addiction recovery. Recent studies suggest that there is a set of genes whose expression is robustly and stably altered following drug use and ensuing abstinence. Such stable changes in gene expression correlate with ultrastructural changes in brain as well as alterations in behavior. As persistent molecular changes, these genes may provide targets for the development of therapeutics. Developing a list of well-characterized candidate genes and examining the effect of manipulating these genes will contribute to the ultimate goal of developing effective treatments to prevent relapse to drug use.

Epigenetic mechanisms in drug addiction

Changes in gene expression in brain reward regions are thought to contribute to the pathogenesis and persistence of drug addiction. Recent studies have begun to focus on the molecular mechanisms by which drugs of abuse and related environmental stimuli, such as drug-associated cues or stress, converge on the genome to alter specific gene programs. Increasing evidence suggests that these stable gene expression changes in neurons are mediated in part by epigenetic mechanisms that alter chromatin structure on specific gene promoters. This review discusses recent findings from behavioral, molecular and bioinformatic approaches being used to understand the complex epigenetic regulation of gene expression by drugs of abuse. This novel mechanistic insight might open new avenues for improved treatments of drug addiction.