Histone H3 phosphoacetylation is critical for heroin-induced place preference

An Investigation of the RNA Modification m6A and Its Regulatory Enzymes in Rat Brains Affected by Chronic Morphine Treatment and Withdrawal

N6-methyladenosine (m6A) is one of the most prevalent methylated modifications of mRNA in eukaryotes. This reversible alteration can directly or indirectly influence biological functions, including RNA degradation, translation, and splicing. This study investigates the impact of chronic morphine administration and varying withdrawal durations (1 day, 1 week, 4 weeks, and 12 weeks) on the m6A modification levels in brain regions critical to addiction development and persistence. Our findings indicate that in the prefrontal cortex, the m6A levels and METTL3 expression decrease, accompanied by an increase in FTO and ALKBH5 expression, followed by fluctuating, but statistically insignificant changes in methylation-regulating enzymes over prolonged withdrawal. In the striatum, reductions in m6A levels and METTL3 expression are observed at 4 weeks of withdrawal, preceded by non-significant fluctuations in enzyme expression and the m6A modification levels. In contrast, no changes in the m6A modification levels or the expression of related enzymes are detected in the hippocampus and the cerebellum. Our data suggest that m6A modification and its regulatory enzymes undergo region-specific and time-dependent changes in response to chronic morphine exposure and subsequent withdrawal.

Elucidating the molecular symphony: unweaving the transcriptional & epigenetic pathways underlying neuroplasticity in opioid dependence and withdrawal

The persistent use of opioids leads to profound changes in neuroplasticity of the brain, contributing to the emergence and persistence of addiction. However, chronic opioid use disrupts the delicate balance of the reward system in the brain, leading to neuroadaptations that underlie addiction. Chronic cocaine usage leads to synchronized alterations in gene expression, causing modifications in the Nucleus Accumbens (NAc), a vital part of the reward system of the brain. These modifications assist in the development of maladaptive behaviors that resemble addiction. Neuroplasticity in the context of addiction involves changes in synaptic connectivity, neuronal morphology, and molecular signaling pathways. Drug-evoked neuroplasticity in opioid addiction and withdrawal represents a complicated interaction between environmental, genetic, and epigenetic factors. Identifying specific transcriptional and epigenetic targets that can be modulated to restore normal neuroplasticity without disrupting essential physiological processes is a critical consideration. The discussion in this article focuses on the transcriptional aspects of drug-evoked neuroplasticity, emphasizing the role of key transcription factors, including cAMP response element-binding protein (CREB), ΔFosB, NF-kB, Myocyte-enhancing factor 2 (MEF2), Methyl-CpG binding protein 2 (MeCP2), E2F3a, and FOXO3a. These factors regulate gene expression and lead to the neuroadaptive changes observed in addiction and withdrawal. Epigenetic regulation, which involves modifying gene accessibility by controlling these structures, has been identified as a critical component of addiction development. By unraveling these complex molecular processes, this study provides valuable insights that may pave the way for future therapeutic interventions targeting the mechanisms underlying addiction and withdrawal.

How life events may confer vulnerability to addiction: the role of epigenetics

Substance use disorder (SUD) represents a large and growing global health problem. Despite the strong addictive potency of drugs of abuse, only a minority of those exposed develop SUDs. While certain life experiences (e.g., childhood trauma) may increase subsequent vulnerability to SUDs, mechanisms underlying these effects are not yet well understood. Given the chronic and relapsing nature of SUDs, and the length of time that can elapse between prior life events and subsequent drug exposure, changes in SUD vulnerability almost certainly involve long-term epigenetic dysregulation. To validate this idea, functional effects of specific epigenetic modifications in brain regions mediating reinforcement learning (e.g., nucleus accumbens, prefrontal cortex) have been investigated in a variety of animal models of SUDs. In addition, the effects of epigenetic modifications produced by prior life experiences on subsequent SUD vulnerability have been studied, but mostly in a correlational manner. Here, we review how epigenetic mechanisms impact SUD-related behavior in animal models and summarize our understanding of the relationships among life experiences, epigenetic regulation, and future vulnerability to SUDs. Despite variations in study design, epigenetic modifications that most consistently affect SUD-related behavior are those that produce predominantly unidirectional effects on gene regulation, such as DNA methylation and histone phosphorylation. Evidence explicitly linking environmentally induced epigenetic modifications to subsequent SUD-related behavior is surprisingly sparse. We conclude by offering several directions for future research to begin to address this critical research gap.

Disrupted Circadian Rhythms and Substance Use Disorders: A Narrative Review

Substance use disorder is a major global health concern, with a high prevalence among adolescents and young adults. The most common substances of abuse include alcohol, marijuana, cocaine, nicotine, and opiates. Evidence suggests that a mismatch between contemporary lifestyle and environmental demands leads to disrupted circadian rhythms that impair optimal physiological and behavioral function, which can increase the vulnerability to develop substance use disorder and related problems. The circadian system plays an important role in regulating the sleep-wake cycle and reward processing, both of which directly affect substance abuse. Distorted substance use can have a reciprocal effect on the circadian system by influencing circadian clock gene expression. Considering the detrimental health consequences and profound societal impact of substance use disorder, it is crucial to comprehend its complex association with circadian rhythms, which can pave the way for the generation of novel chronotherapeutic treatment approaches. In this narrative review, we have explored the potential contributions of disrupted circadian rhythms and sleep on use and relapse of different substances of abuse. The involvement of circadian clock genes with drug reward pathways is discussed, along with the potential research areas that can be explored to minimize disordered substance use by improving circadian hygiene.

Pre-mating administration of theophylline could prevent the transgenerational effects of maternal morphine dependence on offspring anxiety behavior: The role of dopamine receptors

Opioid addiction causes some molecular alterations in the brain reward pathway, such as changes in gene expression that may be transferred to the next generation via epigenetic mechanisms such as histone acetylation. This study aimed to evaluate the effect of theophylline as an HDAC (Histone deacetylases) activator on D1 and D2 dopamine receptor expression in the nucleus accumbens (NAc) and anxiety behavior in the offspring of morphine-dependent female rats. Female rats were exposed to escalating doses of morphine for six days and were then treated with theophylline (20 mg/kg) or saline for 10 days before mating with normal male rats. Male and female offspring were tested for anxiety behavior using an elevated plus maze apparatus. Besides, the expression of D1 and D2 dopamine receptors in the NAc was evaluated by real-time PCR (polymerase chain reaction). Results showed that offspring of morphine-dependent female rats had increased expression of both D1 and D2 receptors in the NAc, as well as decreased anxiety behavior, compared to control offspring. However, the mentioned effects were returned to normal levels in the offspring whose morphine-dependent mothers had received theophylline for 10 days before mating. It is concluded that theophylline may be therapeutically effective in minimizing the adverse consequences of maternal morphine dependence on offspring behavior by restoring normal dopamine receptor expression levels and modulating anxiety. To completely comprehend the underlying mechanisms of this phenomenon, more research is required.

Morphine-withdrawal aversive memories and their extinction modulate H4K5 acetylation and Brd4 activation in the rat hippocampus and basolateral amygdala

Chromatin modification is a crucial mechanism in several important phenomena in the brain, including drug addiction. Persistence of drug craving and risk of relapse could be attributed to drug-induced epigenetic mechanisms that seem to be candidates explaining long-lasting drug-induced behaviour and molecular alterations. Histone acetylation has been proposed to regulate drug-seeking behaviours and the extinction of rewarding memory of drug taking. In this work, we studied the epigenetic regulation during conditioned place aversion and after extinction of aversive memory of opiate withdrawal. Through immunofluorescence assays, we assessed some epigenetic marks (H4K5ac and p-Brd4) in crucial areas related to memory retrieval -basolateral amygdala (BLA) and hippocampus-. Additionally, to test the degree of transcriptional activation, we evaluated the immediate early genes (IEGs) response (Arc, Bdnf, Creb, Egr-1, Fos and Nfkb) and Smarcc1 (chromatin remodeler) through RT-qPCR in these nuclei. Our results showed increased p-Brd4 and H4K5ac levels during aversive memory retrieval, suggesting a more open chromatin state. However, transcriptional activation of these IEGs was not found, therefore suggesting that other secondary response may already be happening. Additionally, Smarcc1 levels were reduced due to morphine chronic administration in BLA and dentate gyrus. The activation markers returned to control levels after the retrieval of aversive memories, revealing a more repressed chromatin state. Taken together, our results show a major role of the tandem H4K5ac/p-Brd4 during the retrieval of aversive memories. These results might be useful to elucidate new molecular targets to improve and develop pharmacological treatments to address addiction and to avoid drug relapse.

Histone modifications in cocaine, methamphetamine and opioids

Cocaine, methamphetamine and opioids are leading causes of drug abuse-related deaths worldwide. In recent decades, several studies revealed the connection between and epigenetics. Neural cells acquire epigenetic alterations that drive the onset and progress of the SUD by modifying the histone residues in brain reward circuitry. Histone modifications, especially acetylation and methylation, participate in the regulation of gene expression. These alterations, as well as other host and microenvironment factors, are associated with a serious of negative neurocognitive disfunctions in various patient populations. In this review, we highlight the evidence that substantially increase the field's ability to understand the molecular actions underlying SUD and summarize the potential approaches for SUD pharmacotherapy.

Phosphorylation of Neurofilament Light Chain in the VLO Is Correlated with Morphine-Induced Behavioral Sensitization in Rats

Neurofilament light chain (NF-L) plays critical roles in synapses that are relevant to neuropsychiatric diseases. Despite postmortem evidence that NF-L is decreased in opiate abusers, its role and underlying mechanisms remain largely unknown. We found that the microinjection of the histone deacetylase (HDAC) inhibitor Trichostatin A (TSA) into the ventrolateral orbital cortex (VLO) attenuated chronic morphine-induced behavioral sensitization. The microinjection of TSA blocked the chronic morphine-induced decrease of NF-L. However, our chromatin immunoprecipitation (ChIP)-qPCR results indicated that this effect was not due to the acetylation of histone H3-Lysine 9 and 14 binding to the NF-L promotor. In line with the behavioral phenotype, the microinjection of TSA also blocked the chronic morphine-induced increase of p-ERK/p-CREB/p-NF-L. Finally, we compared chronic and acute morphine-induced behavioral sensitization. We found that although both chronic and acute morphine-induced behavioral sensitization were accompanied by an increase of p-CREB/p-NF-L, TSA exhibited opposing effects on behavioral phenotype and molecular changes at different addiction contexts. Thus, our findings revealed a novel role of NF-L in morphine-induced behavioral sensitization, and therefore provided some correlational evidence of the involvement of NF-L in opiate addiction.

Normalization of the H3K9me2/H3K14ac-ΔFosB pathway in the nucleus accumbens underlying the reversal of morphine-induced behavioural and synaptic plasticity by Compound 511

BACKGROUND

Although opioid agonist-based treatments are considered the first-line treatment for opioid use disorders, nonopioid alternatives are urgently needed to combat the inevitable high relapse rates. Compound 511 is a formula derived from ancient traditional Chinese medical literature on opiate rehabilitation. Previously, we observed that Compound 511 could effectively prevent the acquisition of conditioned place preference (CPP) during early morphine exposure. However, its effects on drug-induced reinstatement remain unclear.

PURPOSE

This study aims to estimate the potential of Compound 511 for the therapeutic intervention of opioid relapse in rodent models and explore the potential mechanisms underlying the observed actions.

STUDY DESIGN/METHODS

The CPP and locomotor sensitization paradigm were established to evaluate the therapeutic effect of Compound 511 treatment on morphine-induced neuroadaptations, followed by immunofluorescence and western blot (WB) analysis of the synaptic markers PSD-95 and Syn-1. Furthermore, several addiction-associated transcription factors and epigenetic marks were examined by qPCR and WB, respectively. Furthermore, the key active ingredients and targets of Compound 511 were further excavated by network pharmacology approach and experimental validation.

RESULTS

The results proved that Compound 511 treatment during abstinence blunted both the reinstatement of morphine-evoked CPP and locomotor sensitization, accompanied by the normalization of morphine-induced postsynaptic plasticity in the nucleus accumbens (NAc). Additionally, Compound 511 was shown to exert a selectively repressive influence on morphine-induced hyperacetylation at H3K14 and a reduction in H3K9 dimethylation as well as ΔFosB activation and accumulation in the NAc. Finally, two herbal ingredients of Compound 511 and six putative targets involved in the regulation of histone modification were identified.

CONCLUSION

Our findings indicated that Compound 511 could block CPP reinstatement and locomotor sensitization predominantly via the reversal of morphine-induced postsynaptic plasticity through epigenetic mechanisms. Additionally, 1-methoxy-2,3-methylenedioxyxanthone and 1,7-dimethoxyxanthone may serve as key ingredients of Compound 511 by targeting specific epigenetic enzymes. This study provided an efficient nonopioid treatment against opioid addiction.

The translational genetics of ADHD and related phenotypes in model organisms

Attention-deficit/hyperactivity disorder (ADHD) is a highly prevalent neurodevelopmental disorder resulting from the interaction between genetic and environmental risk factors. It is well known that ADHD co-occurs frequently with other psychiatric disorders due, in part, to shared genetics factors. Although many studies have contributed to delineate the genetic landscape of psychiatric disorders, their specific molecular underpinnings are still not fully understood. The use of animal models can help us to understand the role of specific genes and environmental stimuli-induced epigenetic modifications in the pathogenesis of ADHD and its comorbidities. The aim of this review is to provide an overview on the functional work performed in rodents, zebrafish and fruit fly and highlight the generated insights into the biology of ADHD, with a special focus on genetics and epigenetics. We also describe the behavioral tests that are available to study ADHD-relevant phenotypes and comorbid traits in these models. Furthermore, we have searched for new models to study ADHD and its comorbidities, which can be useful to test potential pharmacological treatments.

Epigenetic Effects of Addictive Drugs in the Nucleus Accumbens

Substance use induces long-lasting behavioral changes and drug craving. Increasing evidence suggests that epigenetic gene regulation contributes to the development and expression of these long-lasting behavioral alterations. Here we systematically review extensive evidence from rodent models of drug-induced changes in epigenetic regulation and epigenetic regulator proteins. We focus on histone acetylation and histone methylation in a brain region important for drug-related behaviors: the nucleus accumbens. We also discuss how experimentally altering these epigenetic regulators via systemically administered compounds or nucleus accumbens-specific manipulations demonstrate the importance of these proteins in the behavioral effects of drugs and suggest potential therapeutic value to treat people with substance use disorder. Finally, we discuss limitations and future directions for the field of epigenetic studies in the behavioral effects of addictive drugs and suggest how to use these insights to develop efficacious treatments.

Genomic and Personalized Medicine Approaches for Substance Use Disorders (SUDs) Looking at Genome-Wide Association Studies

Drug addiction, or substance use disorder (SUD), is a chronic, relapsing disorder in which compulsive drug-seeking and drug-taking behaviour persist despite serious negative consequences. Drug abuse represents a problem that deserves great attention from a social point of view, and focuses on the importance of genetic studies to help in understanding the genetic basis of addiction and its medical treatment. Despite the complexity of drug addiction disorders, and the high number of environmental variables playing a role in the onset, recurrence, and duration of the symptoms, several studies have highlighted the non-negligible role of genetics, as demonstrated by heritability and genome-wide association studies. A correlation between the relative risk of addiction to specific substances and heritability has been recently observed, suggesting that neurobiological mechanisms may be, at least in part, inherited. All these observations point towards a scenario where the core neurobiological factors of addiction, involving the reward system, impulsivity, compulsivity, stress, and anxiety response, are transmitted, and therefore, genes and mutations underlying their variation might be detected. In the last few years, the development of new and more efficient sequencing technologies has paved the way for large-scale studies in searching for genetic and epigenetic factors affecting drug addiction disorders and their treatments. These studies have been crucial to pinpoint single nucleotide polymorphisms (SNPs) in genes that affect the reaction to medical treatments. This is critically important to identify pharmacogenomic approaches for substance use disorder, such as OPRM1 SNPs and methadone required doses for maintenance treatment (MMT). Nevertheless, despite the promising results obtained by genome-wide association and pharmacogenomic studies, specific studies related to population genetics diversity are lacking, undermining the overall applicability of the preliminary findings, and thus potentially affecting the portability and the accuracy of the genetic studies. In this review, focusing on cannabis, cocaine and heroin use, we report the state-of-the-art genomics and pharmacogenomics of SUDs, and the possible future perspectives related to medical treatment response in people that ask for assistance in solving drug-related problems.

Alterations in microbiome composition and metabolic byproducts drive behavioral and transcriptional responses to morphine

Recent evidence has demonstrated that the gut microbiome has marked effects on neuronal function and behavior. Disturbances to microbial populations within the gut have been linked to myriad models of neuropsychiatric disorders. However, the role of the microbiome in substance use disorders remains understudied. Here we show that male mice with their gut microbiome depleted by nonabsorbable antibiotics (Abx) exhibit decreased formation of morphine conditioned place preference across a range of doses (2.5-15 mg/kg), have decreased locomotor sensitization to morphine, and demonstrate marked changes in gene expression within the nucleus accumbens (NAc) in response to high-dose morphine (20 mg/kg × 7 days). Replacement of short-chain fatty acid (SCFA) metabolites, which are reduced by microbiome knockdown, reversed the behavioral and transcriptional effects of microbiome depletion. This identifies SCFA as the crucial mediators of microbiome-brain communication responsible for the effects on morphine reward caused by microbiome knockdown. These studies add important new behavioral, molecular, and mechanistic insight to the role of gut-brain signaling in substance use disorders.

Epigenetic Alterations in Prescription Opioid Misuse: New Strategies for Precision Pain Management

Prescription opioids are used for some chronic pain conditions. However, generally, long-term therapy has unwanted side effects which may trigger addiction, overdose, and eventually cause deaths. Opioid addiction and chronic pain conditions have both been associated with evidence of genetic and epigenetic alterations. Despite intense research interest, many questions about the contribution of epigenetic changes to this typology of addiction vulnerability and development remain unanswered. The aim of this review was to summarize the epigenetic modifications detected in specific tissues or brain areas and associated with opioid prescription and misuse in patients who have initiated prescribed opioid management for chronic non-cancer pain. The review considers the effects of opioid exposure on the epigenome in central and peripheral tissues in animal models and human subjects and highlights the mechanisms in which opioid epigenetics may be involved. This will improve our current understanding, provide the basis for targeted, personalized pain management, and thus balance opioid risks and benefits in managing chronic pain.

Psychostimulants and opioids differentially influence the epigenetic modification of histone acetyltransferase and histone deacetylase in astrocytes

Illicit drugs are known to affect central nervous system (CNS). Majorly psychostimulants such as cocaine, methamphetamine (METH) and opioids such as morphine are known to induce epigenetic changes of histone modifications and chromatin remodeling which are mediated by histone acetyltransferase (HAT) and histone deacetylase (HDAC). Aberrant changes in histone acetylation-deacetylation process further exacerbate dysregulation of gene expression and protein modification which has been linked with neuronal impairments including memory formation and synaptic plasticity. In CNS, astrocytes play a pivotal role in cellular homeostasis. However, the impact of psychostimulants and opioid mediated epigenetic changes of HAT/HADCs in astrocytes has not yet been fully elucidated. Therefore, we have investigated the effects of the psychostimulants and opioid on the acetylation-regulating enzymes- HAT and HDACs role in astrocytes. In this study, Class I and II HDACs and HATs gene expression, protein changes and global level changes of acetylation of H3 histones at specific lysines were analyzed. In addition, we have explored the neuroprotective “nootropic” drug piracetam were exposed with or without psychostimulants and opioid in the human primary astrocytes. Results revealed that psychostimulants and opioid upregulated HDAC1, HDAC4 and p300 expression, while HDAC5 and GCN5 expression were downregulated. These effects were reversed by piracetam coexposure. Psychostimulants and opioid exposure upregulated global acetylation levels of all H3Ks, except H3K14. These results suggest that psychostimulants and opioids differentially influence HATs and HDACs.

Epigenetics of addiction

Substance use disorders are complex biopsychosocial disorders that have substantial negative neurocognitive impact in various patient populations. These diseases involve the compulsive use of licit or illicit substances despite adverse medicolegal consequences and appear to be secondary to long-lasting epigenetic and transcriptional adaptations in brain reward and non-reward circuits. The accumulated evidence supports the notion that repeated drug use causes changes in post-translational histone modifications and in DNA methylation/hydroxymethylation processes in several brain regions. This review provides an overview of epigenetic changes reported in models of cocaine, methamphetamine, and opioid use disorders. The accumulated data suggest that future therapeutic interventions should focus on the development of epigenetic drugs against addictive diseases.

Oxycodone self-administration activates the mitogen-activated protein kinase/ mitogen- and stress-activated protein kinase (MAPK-MSK) signaling pathway in the rat dorsal striatum

To identify signaling pathways activated by oxycodone self-administration (SA), Sprague–Dawley rats self-administered oxycodone for 20 days using short—(ShA, 3 h) and long-access (LgA, 9 h) paradigms. Animals were euthanized 2 h after SA cessation and dorsal striata were used in post-mortem molecular analyses. LgA rats escalated their oxycodone intake and separated into lower (LgA-L) or higher (LgA-H) oxycodone takers. LgA-H rats showed increased striatal protein phosphorylation of ERK1/2 and MSK1/2. Histone H3, phosphorylated at serine 10 and acetylated at lysine 14 (H3S10pK14Ac), a MSK1/2 target, showed increased abundance only in LgA-H rats. RT-qPCR analyses revealed increased AMPA receptor subunits, GluA2 and GluA3 mRNAs, in the LgA-H rats. GluA3, but not GluA2, mRNA expression correlated positively with changes in pMSK1/2 and H3S10pK14Ac. These findings suggest that escalated oxycodone SA results in MSK1/2-dependent histone phosphorylation and increases in striatal gene expression. These observations offer potential avenues for interventions against oxycodone addiction.

Epigenetic Neuropharmacology: Drugs Affecting the Epigenome in the Brain

This review explores how different classes of drugs, including those with therapeutic and abuse potential, alter brain functions and behavior via the epigenome. Epigenetics, in its simplest interpretation, is the study of the regulation of a genes' transcriptional potential. The epigenome is established during development but is malleable throughout life by a wide variety of drugs, with both clinical utility and abuse potential. An epigenetic effect can be central to the drug's therapeutic or abuse potential, or it can be independent from the main effect but nevertheless produce beneficial or adverse side effects. Here, I discuss the various epigenetic effects of main pharmacological drug classes, including antidepressants, antiepileptics, and drugs of abuse.

Opioid-induced structural and functional plasticity of medium-spiny neurons in the nucleus accumbens

Opioid Use Disorder (OUD) is a chronic relapsing clinical condition with tremendous morbidity and mortality that frequently persists, despite treatment, due to an individual's underlying psychological, neurobiological, and genetic vulnerabilities. Evidence suggests that these vulnerabilities may have neurochemical, cellular, and molecular bases. Key neuroplastic events within the mesocorticolimbic system that emerge through chronic exposure to opioids may have a determinative influence on behavioral symptoms associated with OUD. In particular, structural and functional alterations in the dendritic spines of medium spiny neurons (MSNs) within the nucleus accumbens (NAc) and its dopaminergic projections from the ventral tegmental area (VTA) are believed to facilitate these behavioral sequelae. Additionally, glutamatergic neurons from the prefrontal cortex, the basolateral amygdala, the hippocampus, and the thalamus project to these same MSNs, providing an enriched target for synaptic plasticity. Here, we review literature related to neuroadaptations in NAc MSNs from dopaminergic and glutamatergic pathways in OUD. We also describe new findings related to transcriptional, epigenetic, and molecular mechanisms in MSN plasticity in the different stages of OUD.

Sex-specific transgenerational effects of morphine exposure on reward and affective behaviors

Current estimates indicate that millions of people in the United States abuse opioid drugs, which may also affect their offspring. To determine whether parental exposure to morphine alters reward and affective behaviors in subsequent generations we exposed male and female C57BL/6NTac mice to morphine (75 mg) or placebo pellets for 4 weeks. Naïve mice were used as mating partners to create subsequent generations (F1 and F2). Adult male and female F1 and F2 mice were tested in the morphine conditioned place preference paradigm (CPP), marble burying (MB), acoustic startle response (ASR), and open field tests (OFT). Paternal morphine exposure resulted in significantly attenuated preference scores amongst F1 male offspring, but significantly higher preference scores amongst F1 female offspring at the lowest CPP dose tested (5 mg/kg). In contrast, maternal exposure to morphine did not affect morphine reward in the F1 generation; however, the F2 male offspring of morphine-exposed F0 females displayed significantly higher CPP preference scores. Preference scores in F2 females were not affected by F0 male or female morphine exposure. Sex-specific alterations in affective behaviors were observed only in the offspring of F0 males exposed to morphine with F1 males spending less time in the center of the open field and F1 females spending more time in the center of the open field. One generation later, affective behaviors were no longer altered in F2 males but F2 females from the F0 male morphine exposure buried more marbles in the MB test. In summary, early exposure to morphine in males and females causes lineage-specific inheritance of reward and affective behaviors.

Epigenetic Mechanisms of Opioid Addiction

Opioid use kills tens of thousands of Americans each year, devastates families and entire communities, and cripples the health care system. Exposure to opioids causes long-term changes to brain regions involved in reward processing and motivation, leading vulnerable individuals to engage in pathological drug seeking and drug taking that can remain a lifelong struggle. The persistence of these neuroadaptations is mediated in part by epigenetic remodeling of gene expression programs in discrete brain regions. Although the majority of work examining how epigenetic modifications contribute to addiction has focused on psychostimulants such as cocaine, research into opioid-induced changes to the epigenetic landscape is emerging. This review summarizes our knowledge of opioid-induced epigenetic modifications and their consequential changes to gene expression. Current evidence points toward opioids promoting higher levels of permissive histone acetylation and lower levels of repressive histone methylation as well as alterations to DNA methylation patterns and noncoding RNA expression throughout the brain's reward circuitry. Additionally, studies manipulating epigenetic enzymes in specific brain regions are beginning to build causal links between these epigenetic modifications and changes in addiction-related behavior. Moving forward, studies must leverage advanced chromatin analysis and next-generation sequencing approaches combined with bioinformatics pipelines to identify novel gene networks regulated by particular epigenetic modifications. Improved translational relevance also requires increased focus on volitional drug-intake models and standardization of opioid exposure paradigms. Such work will significantly advance our understanding of how opioids cause persistent changes to brain function and will provide a platform on which to develop interventions for treating opioid addiction.

Epigenetic pharmacotherapy for substance use disorder

Identifying novel therapeutics for the treatment of substance use disorder (SUD) is an area of intensive investigation. Prior strategies that have attempted to modify one or a few neurotransmitter receptors have had limited success, and currently there are no FDA-approved medications for the treatment of cocaine, methamphetamine, and marijuana use disorders. Because drugs of abuse are known to alter the expression of numerous genes in reward-related brain regions, epigenetic-based therapies have emerged as intriguing targets for therapeutic innovation. Here, I evaluate potential therapeutic approaches and challenges in targeting epigenetic factors for the treatment of SUD and highlight examples of promising strategies and future directions.

It is a complex issue: emerging connections between epigenetic regulators in drug addiction

Drug use leads to addiction in some individuals, but the underlying brain mechanisms that control the transition from casual drug use to an intractable substance use disorder (SUD) are not well understood. Gene x environment interactions such as the frequency of drug use and the type of substance used likely to promote maladaptive plastic changes in brain regions that are critical for controlling addiction-related behavior. Epigenetics encompasses a broad spectrum of mechanisms important for regulating gene transcription that are not dependent on changes in DNA base pair sequences. This review focuses on the proteins and complexes contributing to epigenetic modifications in the nucleus accumbens (NAc) following drug experience. We discuss in detail the three major mechanisms: histone acetylation and deacetylation, histone methylation, and DNA methylation. We discuss how drug use alters the regulation of the associated proteins regulating these processes and highlight how experimental manipulations of these proteins in the NAc can alter drug-related behaviors. Finally, we discuss the ways that histone modifications and DNA methylation coordinate actions by recruiting large epigenetic enzyme complexes to aid in transcriptional repression. Targeting these multiprotein epigenetic enzyme complexes - and the individual proteins that comprise them - might lead to effective therapeutics to reverse or treat SUDs in patients.

Will Widespread Synthetic Opioid Consumption Induce Epigenetic Consequences in Future Generations?

A growing number of evidence demonstrates that ancestral exposure to xenobiotics (pollutants, drugs of abuse, etc.) can perturb the physiology and behavior of descendants. Both maternal and paternal transmission of phenotype across generations has been proved, demonstrating that parental drug history may have significant implications for subsequent generations. In the last years, the burden of novel synthetic opioid (NSO) consumption, due to increased medical prescription of pain medications and to easier accessibility of these substances on illegal market, is raising new questions first in term of public health, but also about the consequences of the parental use of these drugs on future generations. Besides being associated to the neonatal abstinence syndrome, in utero exposure to opioids has an impact on neuronal development with long-term repercussions that are potentially transmitted to subsequent generations. In addition, recent reports suggest that opioid use even before conception influences the reactivity to opioids of the progeny and the following generations, likely through epigenetic mechanisms. This review describes the current knowledge about the transgenerational effects of opioid consumption. We summarize the preclinical and clinical findings showing the implications for the subsequent generations of parental exposure to opioids earlier in life. Limitations of the existing data on NSOs and new perspectives of the research are also discussed, as well as clinical and forensic consequences.

PI3K/Akt Pathway: A Role in δ-Opioid Receptor-Mediated RGC Neuroprotection

Purpose

This study examines the role of PI3K/Akt pathway in δ-opioid receptor agonist (SNC-121)-induced RGC neuroprotection in a chronic glaucoma rat model.

Methods

Injecting hypertonic saline into the limbal veins of Brown Norway rats elevated IOP. Rats were treated either with 1 mg/kg SNC-121 or 3 mg/kg 2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride (LY-294002; PI3K/Akt inhibitor) plus SNC-121 once daily for 7 days. Pattern ERGs were recorded in response to contrast reversal of patterned visual stimuli. Retinal ganglion cells (RGC) were visualized by Fluorogold retrograde labeling. Optic nerve head (ONH) astrocytes were pretreated with PI3K/Akt inhibitors for 30 minutes followed by 1-μM SNC-121 treatment. Changes in matrix metalloproteinases (MMP-1, -2, and -3) production and PI3K/Akt activation in optic nerve and TNF-α treated ONH astrocytes were measured by immunohistochemistry and Western blotting.

Results

SNC-121 activates the PI3K/Akt pathway in ONH astrocytes and the retina. In ONH astrocytes, SNC-121–induced Akt activation was fully inhibited by PI3K/Akt inhibitors. A sustained decline (7–42 days post injury) in Akt activation was seen in the ocular-hypertensive retina and optic nerve. This decline is reversed to normal levels by 1-mg/kg intraperitoneally (i.p.) SNC-121 treatment. Both pattern ERG amplitudes and RGC numbers were reduced in ocular hypertensive eyes, which were significantly increased in SNC-121–treated animals. Interestingly, SNC-121–induced increase in pattern-ERG amplitudes and RGC numbers were inhibited in LY-294002 pretreated animals. Additionally, SNC-121 treatment inhibited MMP-1, -2, and -3 production from the optic nerve of ocular hypertensive rats and TNF-α–treated ONH astrocytes.

Conclusions

PI3K/Akt pathway plays a crucial role in SNC-121–mediated RGC neuroprotection against glaucomatous injury.

Epigenetic Priming in Drug Addiction

Drug addiction is a chronic relapsing brain disorder that is characterized by compulsive drug seeking and continued use despite negative outcomes. Current pharmacological therapies target neuronal receptors or transporters upon which drugs of abuse act initially, yet these treatments remain ineffective for most individuals and do not prevent disease relapse after abstinence. Drugs of abuse, in addition to their acute effects, cause persistent plasticity after repeated use, involving dysregulated gene expression in the brain's reward regions, which are thought to mediate the persistent behavioral abnormalities that characterize addiction. Emerging evidence implicates epigenetic priming as a key mechanism that underlies the long-lasting alterations in neuronal gene regulation, which can remain latent until triggered by re-exposure to drug-associated stimuli or the drug itself. Thus, to effectively treat drug addiction, we must identify the precise epigenetic mechanisms that establish and preserve the drug-induced pathology of the brain reward circuitry.

Neuroepigenetics and addiction

Drug addiction involves long-term behavioral abnormalities that arise in response to repeated exposure to drugs of abuse in vulnerable individuals. It is a multifactorial syndrome involving a complex interplay between genes and the environment. Evidence suggests that the underlying mechanisms regulating these persistent behavioral abnormalities involve changes in gene expression throughout the brain's reward circuitry, in particular, in the mesolimbic dopamine system. In the past decade, investigations have begun to reveal potential genes involved in the risk for addiction through genomewide association studies. Additionally, a crucial role for epigenetic mechanisms, which mediate the enduring effects of drugs of abuse on the brain in animal models of addiction, has been established. This chapter focuses on recent evidence that genetic and epigenetic regulatory events underlie the changes throughout the reward circuitry in humans, as well as animal models of addiction. While further investigations are necessary, a picture of genetic and epigenetic mechanisms involved in addiction is beginning to emerge and the insight gained from these studies will be key to the identification of novel targets for improved diagnosis and treatment of addiction syndromes in humans.

Epigenetic regulation of motivated behaviors by histone deacetylase inhibitors

Growing evidence has begun to elucidate the contribution of epigenetic mechanisms in the modulation and maintenance of gene expression and behavior. Histone acetylation is one such epigenetic mechanism, which has been shown to profoundly alter gene expression and behaviors. In this review, we begin with an overview of the major epigenetic mechanisms including histones acetylation. We next focus on recent evidence about the influence of environmental stimuli on various motivated behaviors through histone acetylation and highlight how histone deacetylase inhibitors can correct some of the pathologies linked to motivated behaviors including substance abuse, feeding and social attachments. Particularly, we emphasize that the effects of histone deacetylase inhibitors on motivated behaviors are time and context-dependent.

Pharmacoepigenomics of opiates and methadone maintenance treatment: current data and perspectives

Current treatments of opioid addiction include primarily maintenance medications such as methadone. Chronic exposure to opiate and/or long-lasting maintenance treatment induce modulations of gene expression in brain and peripheral tissues. There is increasing evidence that epigenetic modifications underlie these modulations. This review summarizes published results on opioid-induced epigenetic changes in animal models and in patients. The epigenetic modifications observed with other drugs of abuse often used by opiate abusers are also outlined. Specific methadone maintenance treatment induced epigenetic modifications at different treatment stages may be combined with the ones resulting from patients’ substance use history. Therefore, research comparing groups of addicts with similar history and substances use disorders but contrasting for well-characterized treatment phenotypes should be encouraged.

Striatal H3K27 Acetylation Linked to Glutamatergic Gene Dysregulation in Human Heroin Abusers Holds Promise as Therapeutic Target

BACKGROUND

Opiate abuse and overdose reached epidemic levels in the United States. However, despite significant advances in animal and in vitro models, little knowledge has been directly accrued regarding the neurobiology of the opiate-addicted human brain.

METHODS

We used postmortem human brain specimens from a homogeneous European Caucasian population of heroin users for transcriptional and epigenetic profiling, as well as direct assessment of chromatin accessibility in the striatum, a brain region central to reward and emotion. A rat heroin self-administration model was used to obtain translational molecular and behavioral insights.

RESULTS

Our transcriptome approach revealed marked impairments related to glutamatergic neurotransmission and chromatin remodeling in the human striatum. A series of biochemical experiments tracked the specific location of the epigenetic disturbances to hyperacetylation of lysine 27 of histone H3, showing dynamic correlations with heroin use history and acute opiate toxicology. Targeted investigation of GRIA1, a glutamatergic gene implicated in drug-seeking behavior, verified the increased enrichment of lysine-27 acetylated histone H3 at discrete loci, accompanied by enhanced chromatin accessibility at hyperacetylated regions in the gene body. Analogous epigenetic impairments were detected in the striatum of heroin self-administering rats. Using this translational model, we showed that bromodomain inhibitor JQ1, which blocks the functional readout of acetylated lysines, reduced heroin self-administration and cue-induced drug-seeking behavior.

CONCLUSIONS

Overall, our data suggest that heroin-related histone H3 hyperacetylation contributes to glutamatergic transcriptional changes that underlie addiction behavior and identify JQ1 as a promising candidate for targeted clinical interventions in heroin use disorder.

Effects of histone deacetylase inhibitor sodium butyrate on heroin seeking behavior in the nucleus accumbens in rats

Histone acetylation and other modifications of the chromatin are important regulators of gene expression and may contribute to drug-induced behaviors and neuroplasticity. Inhibition of histone deacetylases (HDAC) activity results in the change of some drug-induced behaviors，however, relatively little is known about the effects of HDAC inhibitors on heroin-seeking behavior. In the present study, male rats were trained to self-administer heroin under a FR1 schedule for consecutive 14 days, followed by 14 daily 2h extinction session in the operant chamber. After training, the heroin priming (250μg/kg) was introduced for the reinstatement of heroin-seeking behavior. Pretreatment with sodium butyrate (NaB) (200 or 400mg/kg, i.p.), an inhibitor of HDAC, failed to affect heroin self-administration. Additionally，systemic administration of NaB (400mg/kg, i.p.)increased significantly the reinstatement of heroin-seeking induced by heroin priming when NaB administered 12h, but not 6h before the reinstatement test. The same effect was observed after the intracerebroventricular injection of NaB (5μL, 100μg/μL). Moreover, the levels of histone H3 acetylation at lysine 18（H3K18）and H4 acetylation at lysine 5 or lysine 8（H4K5 or H4K8）in the accumbens nucleus core and shell were remarkably increased during the reinstatement and were further strengthened after intracerebroventricular injection of NaB. These results demonstrated that activation of histone acetylation may be involved in the heroin-seeking behavior, and identifying these epigenetic changes will be critical in proposing a novel pharmacological strategy for treating heroin addiction.

Epigenetics of Stress, Addiction, and Resilience: Therapeutic Implications

Substance use disorders (SUDs) are highly prevalent. SUDs involve vicious cycles of binges followed by occasional periods of abstinence with recurrent relapses despite treatment and adverse medical and psychosocial consequences. There is convincing evidence that early and adult stressful life events are risks factors for the development of addiction and serve as cues that trigger relapses. Nevertheless, the fact that not all individuals who face traumatic events develop addiction to licit or illicit drugs suggests the existence of individual and/or familial resilient factors that protect these mentally healthy individuals. Here, I give a brief overview of the epigenetic bases of responses to stressful events and of epigenetic changes associated with the administration of drugs of abuse. I also discuss the psychobiology of resilience and alterations in epigenetic markers that have been observed in models of resilience. Finally, I suggest the possibility that treatment of addiction should involve cognitive and pharmacological approaches that enhance resilience in at risk individuals. Similar approaches should also be used with patients who have already succumbed to the nefarious effects of addictive substances.

Epigenetics and memory: causes, consequences and treatments for post-traumatic stress disorder and addiction

Understanding the interaction between fear and reward at the circuit and molecular levels has implications for basic scientific approaches to memory and for understanding the etiology of psychiatric disorders. Both stress and exposure to drugs of abuse induce epigenetic changes that result in persistent behavioral changes, some of which may contribute to the formation of a drug addiction or a stress-related psychiatric disorder. Converging evidence suggests that similar behavioral, neurobiological and molecular mechanisms control the extinction of learned fear and drug-seeking responses. This may, in part, account for the fact that individuals with post-traumatic stress disorder have a significantly elevated risk of developing a substance use disorder and have high rates of relapse to drugs of abuse, even after long periods of abstinence. At the behavioral level, a major challenge in treatments is that extinguished behavior is often not persistent, returning with changes in context, the passage of time or exposure to mild stressors. A common goal of treatments is therefore to weaken the ability of stressors to induce relapse. With the discovery of epigenetic mechanisms that create persistent molecular signals, recent work on extinction has focused on how modulating these epigenetic targets can create lasting extinction of fear or drug-seeking behavior. Here, we review recent evidence pointing to common behavioral, systems and epigenetic mechanisms in the regulation of fear and drug seeking. We suggest that targeting these mechanisms in combination with behavioral therapy may promote treatment and weaken stress-induced relapse.

Histone-mediated epigenetics in addiction

Many of the brain regions, neurotransmitter systems, and behavioral changes that occur after occasional drug use in healthy subjects and after chronic drug abuse in addicted patients are well characterized. An emerging literature suggests that epigenetic processes, those processes that regulate the accessibility of DNA to regulatory proteins within the nucleus, are keys to how addiction develops and how it may be treated. Investigations of the regulation of chromatin, the organizational system of DNA, by histone modification are leading to a new understanding of the cellular and behavioral alterations that occur after drug use. We will describe how, when, and where histone tails are modified and how some of the most recognized histone regulation patterns are involved in the cycle of addiction, including initial and chronic drug intake, withdrawal, abstinence, and relapse. Finally, we consider how an approach that targets histone modifications may promote successful treatment.

Multigenerational and transgenerational inheritance of drug exposure: The effects of alcohol, opiates, cocaine, marijuana, and nicotine

Familial inheritance of drug abuse is composed of both genetic and environmental factors. Additionally, epigenetic transgenerational inheritance may provide a means by which parental drug use can influence several generations of offspring. Recent evidence suggests that parental drug exposure produces behavioral, biochemical, and neuroanatomical changes in future generations. The focus of this review is to discuss these multigenerational and transgenerational phenotypes in the offspring of animals exposed to drugs of abuse. Specifically, changes found following the administration of alcohol, opioids, cocaine, marijuana, and nicotine will be discussed. In addition, epigenetic modifications to the genome following administration of these drugs will be detailed as well as their potential for transmission to the next generation.

Redox-based epigenetic status in drug addiction: a potential contributor to gene priming and a mechanistic rationale for metabolic intervention

Alcohol and other drugs of abuse, including psychostimulants and opioids, can induce epigenetic changes: a contributing factor for drug addiction, tolerance, and associated withdrawal symptoms. DNA methylation is a major epigenetic mechanism and it is one of more than 200 methylation reactions supported by methyl donor S-adenosylmethionine (SAM). Levels of SAM are controlled by cellular redox status via the folate and vitamin B12-dependent enzyme methionine synthase (MS). For example, under oxidative conditions MS is inhibited, diverting its substrate homocysteine (HCY) to the trans sulfuration pathway. Alcohol, dopamine, and morphine, can alter intracellular levels of glutathione (GSH)-based cellular redox status, subsequently affecting SAM levels and DNA methylation status. Here, existing evidence is presented in a coherent manner to propose a novel hypothesis implicating the involvement of redox-based epigenetic changes in drug addiction. Further, we discuss how a “gene priming” phenomenon can contribute to the maintenance of redox and methylation status homeostasis under various stimuli including drugs of abuse. Additionally, a new mechanistic rationale for the use of metabolic interventions/redox-replenishers as symptomatic treatment of alcohol and other drug addiction and associated withdrawal symptoms is also provided. Hence, the current review article strengthens the hypothesis that neuronal metabolism has a critical bidirectional coupling with epigenetic changes in drug addiction exemplified by the link between redox-based metabolic changes and resultant epigenetic consequences under the effect of drugs of abuse.

Neuroepigenetic Regulation of Pathogenic Memories

Our unique collection of memories determines our individuality and shapes our future interactions with the world. Remarkable advances into the neurobiological basis of memory have identified key epigenetic mechanisms that support the stability of memory. Various forms of epigenetic regulation at the levels of DNA methylation, histone modification, and non-coding RNAs (ncRNAs) can modulate transcriptional and translational events required for memory processes. By changing the cellular profile in the brain's emotional, reward, and memory circuits, these epigenetic modifications have also been linked to perseverant, pathogenic memories. In this review, we will delve into the relevance of epigenetic dysregulation to pathogenic memory mechanisms by focusing on two neuropsychiatric disorders perpetuated by aberrant memory associations: substance use disorder (SUD) and post-traumatic stress disorder (PTSD). As our understanding improves, neuroepigenetic mechanisms may someday be harnessed to develop novel therapeutic targets for the treatment of these chronic, relapsing disorders.

Regulation of chromatin states by drugs of abuse

Drug addiction involves long-term behavioral abnormalities and gene expression changes throughout the mesolimbic dopamine system. Epigenetic mechanisms establish/maintain alterations in gene expression in the brain, providing the impetus for investigations characterizing how epigenetic processes mediate the effects of drugs of abuse. This review focuses on evidence that epigenetic events, specifically histone modifications, regulate gene expression changes throughout the reward circuitry. Drugs of abuse induce changes in histone modifications throughout the reward circuitry by altering histone-modifying enzymes, manipulation of which reveals a role for histone modification in addiction-related behaviors. There is a complex interplay between these enzymes, resulting in a histone signature of the addicted phenotype. Insights gained from these studies are key to identifying novel targets for diagnosis and therapy.

Inhibition of histone deacetylase in the basolateral amygdala facilitates morphine context-associated memory formation in rats

Histone acetylation/deacetylation is a crucial mechanism in memory formation and drug addiction. There is evidence suggesting that histone H3 acetylation may contribute to the long-term neural and behavioral responses to addictive drugs. In addition, the basolateral amygdala (BLA) is critically involved in the formation of cue-associated memories. However, the behavioral effect of histone deacetylase (HDAC) inhibition in the BLA and the underlying molecular alterations at different phases of morphine-induced conditioned place preference (CPP) has not been investigated. In this study, we measured the expression, extinction, and reinstatement of morphine-induced place preference in rats pretreated with trichostatin A (TSA), an HDAC inhibitor. Intra-BLA pretreatment with TSA significantly enhanced morphine-induced CPP acquisition and expression, facilitated extinction, and reduced reinstatement of morphine-induced CPP. These behavioral changes were associated with a general increase in histone H3 lysine14 (H3K14) acetylation in the BLA together with upregulation of the brain-derived neurophic factor (BDNF) and ΔFosB and CREB activation. Collectively, our findings imply that HDAC inhibition in the BLA promotes some aspects of the memory that develops during conditioning and extinction training. Furthermore, histone H3 acetylation may play a role in learning and memory for morphine addiction in the BLA.

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.

Role of ERK signaling in activity-dependent modifications of histone proteins

It is well-established that neuronal intracellular signaling governed by the extracellular signal-regulated kinase (ERK/MAPK) plays a crucial role in long-term adaptive changes that occur during cognitive processes. ERK is a downstream component of a conserved signaling module that is activated by the serine/threonine kinase, Raf, which activates the MAPK/ERK kinase (MEK)1/2 protein kinases, which, in turn, activate ERK1/2. This signaling pathway has been reported to be activated in numerous physiological conditions due to a variety of stimuli, ranging from the activation of ionotropic glutamatergic receptors to metabotropic dopaminergic receptors and neurotrophin receptors. Interestingly, activated ERK can have early and late downstream effects at both the nuclear and synaptic levels. Locally, ERK signaling results in transient changes in the efficacy of synaptic transmission by modifying both pre- and post-synaptic targets. Once translocated into the nucleus, ERK signaling may control transcription by targeting several different regulators of gene expression such as transcription factors and histone proteins. ERK function is considered fundamental in processes such as long-term memory storage and drug addiction, by means of its role in activity-dependent epigenetic modifications that occur in the brain. In this review, we summarize the current understanding of ERK action in the neuroepigenetic processes underlying physiological responses, cognitive processes and drug addiction.

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'.

Morphine epigenomically regulates behavior through alterations in histone H3 lysine 9 dimethylation in the nucleus accumbens

Dysregulation of histone modifying enzymes has been associated with numerous psychiatric disorders. Alterations in G9a (Ehmt2), a histone methyltransferase that catalyzes the euchromatic dimethylation of histone H3 at lysine 9 (H3K9me2), has been implicated recently in mediating neural and behavioral plasticity in response to chronic cocaine administration. Here, we show that chronic morphine, like cocaine, decreases G9a expression, and global levels of H3K9me2, in mouse nucleus accumbens (NAc), a key brain reward region. In contrast, levels of other histone methyltransferases or demethylases, or of other methylated histone marks, were not affected in NAc by chronic morphine. Through viral-mediated gene transfer and conditional mutagenesis, we found that overexpression of G9a in NAc opposes morphine reward and locomotor sensitization and concomitantly promotes analgesic tolerance and naloxone-precipitated withdrawal, whereas downregulation of G9a in NAc enhances locomotor sensitization and delays the development of analgesic tolerance. We identified downstream targets of G9a by providing a comprehensive chromatin immunoprecipitation followed by massively parallel sequencing analysis of H3K9me2 distribution in NAc in the absence and presence of chronic morphine. These data provide novel insight into the epigenomic regulation of H3K9me2 by chronic morphine and suggest novel chromatin-based mechanisms through which morphine-induced addictive-like behaviors arise.

Endogenous opiates and behavior: 2011

This paper is the thirty-fourth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2011 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration (Section 16); and immunological responses (Section 17).