Regulation of chromatin states by drugs of abuse

Cell type-specific epigenetic priming of gene expression in nucleus accumbens by cocaine

A hallmark of addiction is the ability of drugs of abuse to trigger relapse after periods of prolonged abstinence. Here, we describe an epigenetic mechanism whereby chronic cocaine exposure causes lasting chromatin and downstream transcriptional modifications in the nucleus accumbens (NAc), a critical brain region controlling motivation. We link prolonged withdrawal from cocaine to the depletion of the histone variant H2A.Z, coupled with increased genome accessibility and latent priming of gene transcription, in D1 dopamine receptor-expressing medium spiny neurons (D1 MSNs) that relate to aberrant gene expression upon drug relapse. The histone chaperone ANP32E removes H2A.Z from chromatin, and we demonstrate that D1 MSN-selective Anp32e knockdown prevents cocaine-induced H2A.Z depletion and blocks cocaine's rewarding actions. By contrast, very different effects of cocaine exposure, withdrawal, and relapse were found for D2 MSNs. These findings establish histone variant exchange as an important mechanism and clinical target engaged by drugs of abuse to corrupt brain function and behavior.

It is not just about transcription: involvement of brain RNA splicing in substance use disorders

Alternative splicing is a co-transcriptional process that significantly contributes to the molecular landscape of the cell. It plays a multifaceted role in shaping gene transcription, protein diversity, and functional adaptability in response to environmental cues. Recent studies demonstrate that drugs of abuse have a profound impact on alternative splicing patterns within different brain regions. Drugs like alcohol and cocaine modify the expression of genes responsible for encoding splicing factors, thereby influencing alternative splicing of crucial genes involved in neurotransmission, neurogenesis, and neuroinflammation. Notable examples of these alterations include alcohol-induced changes in splicing factors such as HSPA6 and PCBP1, as well as cocaine's impact on PTBP1 and SRSF11. Beyond the immediate effects of drug exposure, recent research has shed light on the role of alternative splicing in contributing to the risk of substance use disorders (SUDs). This is exemplified by exon skipping events in key genes like ELOVL7, which can elevate the risk of alcohol use disorder. Lastly, drugs of abuse can induce splicing alterations through epigenetic modifications. For example, cocaine exposure leads to alterations in levels of trimethylated lysine 36 of histone H3, which exhibits a robust association with alternative splicing and serves as a reliable predictor for exon exclusion. In summary, alternative splicing has emerged as a critical player in the complex interplay between drugs of abuse and the brain, offering insights into the molecular underpinnings of SUDs.

Decoding cocaine-induced proteomic adaptations in the mouse nucleus accumbens

Cocaine use disorder (CUD) is a chronic neuropsychiatric condition that results from enduring cellular and molecular adaptations. Among substance use disorders, CUD is notable for its rising prevalence and the lack of approved pharmacotherapies. The nucleus accumbens (NAc), a region that is integral to the brain's reward circuitry, plays a crucial role in the initiation and continuation of maladaptive behaviors that are intrinsic to CUD. Leveraging advancements in neuroproteomics, we undertook a proteomic analysis that spanned membrane, cytosolic, nuclear, and chromatin compartments of the NAc in a mouse model. The results unveiled immediate and sustained proteomic modifications after cocaine exposure and during prolonged withdrawal. We identified congruent protein regulatory patterns during initial cocaine exposure and reexposure after withdrawal, which contrasted with distinct patterns during withdrawal. Pronounced proteomic shifts within the membrane compartment indicated adaptive and long-lasting molecular responses prompted by cocaine withdrawal. In addition, we identified potential protein translocation events between soluble-nuclear and chromatin-bound compartments, thus providing insight into intracellular protein dynamics after cocaine exposure. Together, our findings illuminate the intricate proteomic landscape that is altered in the NAc by cocaine use and provide a dataset for future research toward potential therapeutics.

Neuroepigenetic Editing

Epigenetic regulation is intrinsic to basic neurobiological function as well as neurological disease. Regulation of chromatin-modifying enzymes in the brain is critical during both development and adulthood and in response to external stimuli. Biochemical studies are complemented by numerous next-generation sequencing (NGS) studies that quantify global changes in gene expression, chromatin accessibility, histone and DNA modifications in neurons and glial cells. Neuroepigenetic editing tools are essential to distinguish between the mere presence and functional relevance of histone and DNA modifications to gene transcription in the brain and animal behavior. This review discusses current advances in neuroepigenetic editing, highlighting methodological considerations pertinent to neuroscience, such as delivery methods and the spatiotemporal specificity of editing and it demonstrates the enormous potential of epigenetic editing for basic neurobiological research and therapeutic application.

Epigenetic inheritance of phenotypes associated with parental exposure to cocaine

Parental exposure to drugs of abuse induces changes in the germline that can be transmitted across subsequent generations, resulting in enduring effects on gene expression and behavior. This transgenerational inheritance involves a dynamic interplay of environmental, genetic, and epigenetic factors that impact an individual's vulnerability to neuropsychiatric disorders. This chapter aims to summarize recent research into the mechanisms underlying the inheritance of gene expression and phenotypic patterns associated with exposure to drugs of abuse, with an emphasis on cocaine. We will first define the epigenetic modifications such as DNA methylation, histone post-translational modifications, and expression of non-coding RNAs that are impacted by parental cocaine use. We will then explore how parental cocaine use induces heritable epigenetic changes that are linked to alterations in neural circuitry and synaptic plasticity within reward-related circuits, ultimately giving rise to potential behavioral vulnerabilities. This discussion will consider phenotypic differences associated with gestational as well as both maternal and paternal preconception drug exposure and will emphasize differences based on offspring sex. In this context, we explore the complex interactions between genetics, epigenetics, environment, and biological sex. Overall, this chapter consolidates the latest developments in the multigenerational effects and long-term consequences of parental substance abuse.

Effect of germ-free status on transcriptional profiles in the nucleus accumbens and transcriptomic response to chronic morphine

Opioid use disorder is a public health crisis that causes tremendous suffering for patients as well as substantial social and economic costs for society. There are currently available treatments for patients with opioid use disorder, but they remain intolerable or ineffective for many. Thus the need to develop new avenues for therapeutics development in this space is great. Substantial work in models of substance use disorders, including opioid use disorder, demonstrates that prolonged exposure to drugs of abuse leads to marked transcriptional and epigenetic dysregulation in limbic substructures. It is widely believed that these changes in gene regulation in response to drugs are a key driving factor in the perpetuation of drug taking and seeking behaviors. Thus, development of interventions that could shape transcriptional regulation in response to drugs of abuse would be of high value. Over the past decade there has been a surge in research demonstrating that the resident bacteria of the gastrointestinal tract, collectively the gut microbiome, can have tremendous influence on neurobiological and behavioral plasticity. Previous work from our group and others has demonstrated that alterations in the gut microbiome can alter behavioral responses to opioids in multiple paradigms. Additionally, we have previously reported that depletion of the gut microbiome with antibiotics markedly shifts the transcriptome of the nucleus accumbens following prolonged morphine exposure. In this manuscript we present a comprehensive analysis of the effects of the gut microbiome on transcriptional regulation of the nucleus accumbens following morphine by utilizing germ-free, antibiotic treated, and control mice. This allows for detailed understanding of the role of the microbiome in regulating baseline transcriptomic control, as well as response to morphine. We find that germ-free status leads to a marked gene dysregulation in a manner distinct to adult mice treated with antibiotics, and that altered gene pathways are highly related to cellular metabolic processes. These data provide additional insight into the role of the gut microbiome in modulating brain function and lay a foundation for further study in this area.

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.

Rapid and Chronic Ethanol Tolerance Are Composed of Distinct Memory-Like States in Drosophila

Ethanol tolerance is the first type of behavioral plasticity and neural plasticity that is induced by ethanol intake, and yet its molecular and circuit bases remain largely unexplored. Here, we characterize the following three distinct forms of ethanol tolerance in male Drosophila: rapid, chronic, and repeated. Rapid tolerance is composed of two short-lived memory-like states, one that is labile and one that is consolidated. Chronic tolerance, induced by continuous exposure, lasts for 2 d, induces ethanol preference, and hinders the development of rapid tolerance through the activity of histone deacetylases (HDACs). Unlike rapid tolerance, chronic tolerance is independent of the immediate early gene Hr38/Nr4a Chronic tolerance is suppressed by the sirtuin HDAC Sirt1, whereas rapid tolerance is enhanced by Sirt1 Moreover, rapid and chronic tolerance map to anatomically distinct regions of the mushroom body learning and memory centers. Chronic tolerance, like long-term memory, is dependent on new protein synthesis and it induces the kayak/c-fos immediate early gene, but it depends on CREB signaling outside the mushroom bodies, and it does not require the Radish GTPase. Thus, chronic ethanol exposure creates an ethanol-specific memory-like state that is molecularly and anatomically different from other forms of ethanol tolerance.SIGNIFICANCE STATEMENT The pattern and concentration of initial ethanol exposure causes operationally distinct types of ethanol tolerance to form. We identify separate molecular and neural circuit mechanisms for two forms of ethanol tolerance, rapid and chronic. We also discover that chronic tolerance forms an ethanol-specific long-term memory-like state that localizes to learning and memory circuits, but it is different from appetitive and aversive long-term memories. By contrast, rapid tolerance is composed of labile and consolidated short-term memory-like states. The multiple forms of ethanol memory-like states are genetically tractable for understanding how initial forms of ethanol-induced neural plasticity form a substrate for the longer-term brain changes associated with alcohol use disorder.

Convergent abnormalities in striatal gene networks in human cocaine use disorder and mouse cocaine administration models

Cocaine use disorder (CUD) is an intractable syndrome, and rising overdose death rates represent a substantial public health crisis that exacts tremendous personal and financial costs on patients and society. Sharp increases in cocaine use drive the urgent need for better mechanistic insight into this chronic relapsing brain disorder that currently lacks effective treatment options. To investigate the transcriptomic changes involved, we conducted RNA sequencing on two striatal brain regions that are heavily implicated in CUD, the nucleus accumbens and caudate nucleus, from men suffering from CUD and matched controls. Weighted gene coexpression analyses identified CUD-specific gene networks enriched in ionotropic receptors and linked to lowered neuroinflammation, contrasting the proinflammatory responses found in opioid use disorder. Integration of comprehensive transcriptomic datasets from mouse cocaine self-administration models revealed evolutionarily conserved gene networks in CUD that implicate especially D1 medium spiny neurons as drivers of cocaine-induced plasticity.

Cocaine regulation of Nr4a1 chromatin bivalency and mRNA in male and female mice

Cocaine epigenetically regulates gene expression via changes in histone post-translational modifications (HPTMs). We previously found that the immediate early gene Nr4a1 is epigenetically activated by cocaine in mouse brain reward regions. However, few studies have examined multiple HPTMs at a single gene. Bivalent gene promoters are simultaneously enriched in both activating (H3K4me3 (K4)) and repressive (H3K27me3 (K27)) HPTMs. As such, bivalent genes are lowly expressed but poised for activity-dependent gene regulation. In this study, we identified K4&K27 bivalency at Nr4a1 following investigator-administered cocaine in male and female mice. We applied sequential chromatin immunoprecipitation and qPCR to define Nr4a1 bivalency and expression in striatum (STR), prefrontal cortex (PFC), and hippocampus (HPC). We used Pearson's correlation to quantify relationships within each brain region across treatment conditions for each sex. In female STR, cocaine increased Nr4a1 mRNA while maintaining Nr4a1 K4&K27 bivalency. In male STR, cocaine enriched repressive H3K27me3 and K4&K27 bivalency at Nr4a1 and maintained Nr4a1 mRNA. Furthermore, cocaine epigenetically regulated a putative NR4A1 target, Cartpt, in male PFC. This study defined the epigenetic regulation of Nr4a1 in reward brain regions in male and female mice following cocaine, and, thus, shed light on the biological relevance of sex to cocaine use disorder.

Epigenetic Studies for Evaluation of NPS Toxicity: Focus on Synthetic Cannabinoids and Cathinones

In the recent decade, numerous new psychoactive substances (NPSs) have been added to the illicit drug market. These are synthetized to mimic the effects of classic drugs of abuse (i.e., cannabis, cocaine, etc.), with the purpose of bypassing substance legislations and increasing the pharmacotoxicological effects. To date, research into the acute pharmacological effects of new NPSs is ongoing and necessary in order to provide an appropriate contribution to public health. In fact, multiple examples of NPS-related acute intoxication and mortality have been recorded in the literature. Accordingly, several in vitro and in vivo studies have investigated the pharmacotoxicological profiles of these compounds, revealing that they can cause adverse effects involving various organ systems (i.e., cardiovascular, respiratory effects) and highlighting their potential increased consumption risks. In this sense, NPSs should be regarded as a complex issue that requires continuous monitoring. Moreover, knowledge of long-term NPS effects is lacking. Because genetic and environmental variables may impact NPS responses, epigenetics may aid in understanding the processes behind the harmful events induced by long-term NPS usage. Taken together, “pharmacoepigenomics” may provide a new field of combined study on genetic differences and epigenetic changes in drug reactions that might be predictive in forensic implications.

Combined molecular, structural and memory data unravel the destructive effect of tramadol on hippocampus

Tramadol is a synthetic analogue of codeine and stimulates neurodegeneration in several parts of the brain that leads to various behavioral impairments. Despite the leading role of hippocampus in learning and memory as well as decreased function of them under influence of tramadol, there are few studies analyzing the effect of tramadol administration on gene expression profiling and structural consequences in hippocampus region. Thus, we sought to determine the effect of tramadol on both PC12 cell line and hippocampal tissue, from gene expression changes to structural alterations. In this respect, we investigated genome-wide mRNA expression using high throughput RNA-seq technology and confirmatory quantitative real-time PCR, accompanied by stereological analysis of hippocampus and behavioral assessment following tramadol exposure. At the cellular level, PC12 cells were exposed to 600μM tramadol for 48 hrs, followed by the assessments of ROS amount and gene expression levels of neurotoxicity associated with neurodegenerative pathways such as apoptosis and autophagy. Moreover, the structural and functional alteration of the hippocampus under chronic exposure to tramadol was also evaluated. In this regard, rats were treated with tramadol at doses of 50 mg/kg for three consecutive weeks. In vitro data revealed that tramadol provoked ROS production and caused the increase in the expression of autophagic and apoptotic genes in PC12 cells. Furthermore, in-vivo results demonstrated that tramadol not only did induce hippocampal atrophy, but it also triggered microgliosis and microglial activation, causing upregulation of apoptotic and inflammatory markers as well as over-activation of neurodegeneration. Tramadol also interrupted spatial learning and memory function along with long-term potentiation (LTP). Taken all together, our data disclosed the neurotoxic effects of tramadol on both in vitro and in-vivo. Moreover, we proposed a potential correlation between disrupted biochemical cascades and memory deficit under tramadol administration.

Epigenetic Regulation of Circadian Clocks and Its Involvement in Drug Addiction

Based on studies describing an increased prevalence of addictive behaviours in several rare sleep disorders and shift workers, a relationship between circadian rhythms and addiction has been hinted for more than a decade. Although circadian rhythm alterations and molecular mechanisms associated with neuropsychiatric conditions are an area of active investigation, success is limited so far, and further investigations are required. Thus, even though compelling evidence connects the circadian clock to addictive behaviour and vice-versa, yet the functional mechanism behind this interaction remains largely unknown. At the molecular level, multiple mechanisms have been proposed to link the circadian timing system to addiction. The molecular mechanism of the circadian clock consists of a transcriptional/translational feedback system, with several regulatory loops, that are also intricately regulated at the epigenetic level. Interestingly, the epigenetic landscape shows profound changes in the addictive brain, with significant alterations in histone modification, DNA methylation, and small regulatory RNAs. The combination of these two observations raises the possibility that epigenetic regulation is a common plot linking the circadian clocks with addiction, though very little evidence has been reported to date. This review provides an elaborate overview of the circadian system and its involvement in addiction, and we hypothesise a possible connection at the epigenetic level that could further link them. Therefore, we think this review may further improve our understanding of the etiology or/and pathology of psychiatric disorders related to drug addiction.

Exposure to Running Wheels Prevents Ethanol Rewarding Effects: The Role of CREB and Deacetylases SIRT-1 and SIRT-2 in the Nucleus Accumbens and Prefrontal Cortex

Alcohol use disorder is one of the most prevalent addictions, strongly influenced by environmental factors. Voluntary physical activity (VPA) has proven to be intrinsically reinforcing and we hypothesized that, as a non-drug reinforcer, VPA could mitigate ethanol-induced rewarding effects. The transcriptional factor cAMP response element binding protein (CREB), and deacetylases isozymes sirtuins 1 and 2 (SIRT-1 and SIRT-2) have a complex interplay and both play a role in the rewarding effects of ethanol. To test whether the exposure of mice to running wheels inhibits the development of ethanol-induced conditioned place preference (CPP), mice were assigned into four groups: housed in home cages with locked ("Sedentary") or unlocked running wheels (VPA), and treated with saline or 1.8 g/kg ethanol during the conditioning phase. The groups were referred as Saline-Sedentary, Saline-VPA, Ethanol-Sedentary and Ethanol-VPA. The expression of CREB, SIRT-1 and SIRT-2 were evaluated in the prefrontal cortex (PFC) and nucleus accumbens (NAc). VPA prevented the development of ethanol-induced CPP. VPA, ethanol and the combination of both inhibited pCREB and pCREB/CREB ratio in the NAc, suggesting that both reward stimuli can share similar patterns of CREB activation. However, we have found that ethanol-induced increased CREB levels were prevented by VPA. Both VPA groups presented lower SIRT-1 levels in the NAc compared to the Sedentary groups. Thus, exposure to running wheels prevented ethanol-rewarding effects and ethanol-induced increases in CREB in the NAc. The molecular alterations underlying CPP prevention may be related to a lower expression of CREB in the NAc of Ethanol-VPA compared to the respective Sedentary group, given the positive correlation between CPP and CREB levels in the Ethanol-Sedentary group.

Long read, isoform aware sequencing of mouse nucleus accumbens after chronic cocaine treatment

To better understand the full-length transcriptome of the nucleus accumbens (NAc)-a key brain reward region-in chronic cocaine treatment, we perform the first single molecule, long-read sequencing analysis using the Iso-seq method to detect 42,114 unique transcripts from mouse NAc polyadenylated RNA. Using GENCODE annotation as a reference, we find that over half of the Iso-seq derived transcripts are annotated, while 46% of them harbor novel splicing events in known genes; around 1% of them correspond to other types of novel transcripts, such as fusion, antisense and intergenic. Approximately 34% of the novel transcripts are matched with a compiled transcriptome assembled from published short-read data from various tissues, with the remaining 69% being unique to NAc. These data provide a more complete picture of the NAc transcriptome than existing annotations and can serve as a comprehensive reference for future transcriptomic analyses of this important brain reward region.

Epigenetic Mechanisms in Drug Relapse

A growing body of evidence from the past 15 years implicates epigenetic mechanisms in the behavioral effects of addictive drugs. The main focus of these studies has been epigenetic mechanisms of psychomotor sensitization and drug reinforcement, as assessed by the conditioned place preference and drug self-administration procedures. Some of these studies have documented long-lasting changes in the expression of epigenetic enzymes and molecules that persist for weeks after the last drug exposure. These observations have inspired more recent investigations on the epigenetic mechanisms of relapse to drug seeking after prolonged abstinence. Here, we review studies that have examined epigenetic mechanisms (e.g., histone modifications, chromatin remodeler-associated modifications, and DNA methylation) that contribute to relapse to cocaine, amphetamine, methamphetamine, morphine, heroin, nicotine, or alcohol seeking, as assessed in rodent models. We first provide a brief overview of studies that have examined persistent epigenetic changes in the brain after prolonged abstinence from noncontingent drug exposure or drug self-administration. Next, we review studies on the effect of either systemic or brain site-specific epigenetic manipulations on the reinstatement of drug-conditioned place preference after extinction of the learned preference, the reinstatement of drug seeking after operant drug self-administration and extinction of the drug-reinforced responding, and the incubation of drug craving (the time-dependent increase in drug seeking after cessation of drug self-administration). We conclude by discussing the implications of these studies for understanding mechanisms contributing to persistent relapse vulnerability after prolonged abstinence. We also discuss the implications of these results for translational research on the potential use of systemically administered epigenetic enzyme inhibitors for relapse prevention in human drug users.

From Circuits to Chromatin: The Emerging Role of Epigenetics in Mental Health

A central goal of neuroscience research is to understand how experiences modify brain circuits to guide future adaptive behavior. In response to environmental stimuli, neural circuit activity engages gene regulatory mechanisms within each cell. This activity-dependent gene expression is governed, in part, by epigenetic processes that can produce persistent changes in both neural circuits and the epigenome itself. The complex interplay between circuit activity and neuronal gene regulation is vital to learning and memory, and, when disrupted, is linked to debilitating psychiatric conditions, such as substance use disorder. To develop clinical treatments, it is paramount to advance our understanding of how neural circuits and the epigenome cooperate to produce behavioral adaptation. Here, we discuss how new genetic tools, used to manipulate neural circuits and chromatin, have enabled the discovery of epigenetic processes that bring about long-lasting changes in behavior relevant to mental health and disease.

Regulation of Social Stress and Neural Degeneration by Activity-Regulated Genes and Epigenetic Mechanisms in Dopaminergic Neurons

Transcriptional and epigenetic regulation of both dopaminergic neurons and their accompanying glial cells is of great interest in the search for therapies for neurodegenerative disorders such as Parkinson’s disease (PD). In this review, we collate transcriptional and epigenetic changes identified in adult Drosophila melanogaster dopaminergic neurons in response to either prolonged social deprivation or social enrichment, and compare them with changes identified in mammalian dopaminergic neurons during normal development, stress, injury, and neurodegeneration. Surprisingly, a small set of activity-regulated genes (ARG) encoding transcription factors, and a specific pattern of epigenetic marks on gene promoters, are conserved in dopaminergic neurons over the long evolutionary period between mammals and insects. In addition to their classical function as immediate early genes to mark acute neuronal activity, these ARG transcription factors are repurposed in both insects and mammals to respond to chronic perturbations such as social enrichment, social stress, nerve injury, and neurodegeneration. We suggest that these ARG transcription factors and epigenetic marks may represent important targets for future therapeutic intervention strategies in various neurodegenerative disorders including PD.

From Signaling Molecules to Circuits and Behaviors: Cell-Type-Specific Adaptations to Psychostimulant Exposure in the Striatum

Addiction is characterized by a compulsive pattern of drug seeking and consumption and a high risk of relapse after withdrawal that are thought to result from persistent adaptations within brain reward circuits. Drugs of abuse increase dopamine (DA) concentration in these brain areas, including the striatum, which shapes an abnormal memory trace of drug consumption that virtually highjacks reward processing. Long-term neuronal adaptations of gamma-aminobutyric acidergic striatal projection neurons (SPNs) evoked by drugs of abuse are critical for the development of addiction. These neurons form two mostly segregated populations, depending on the DA receptor they express and their output projections, constituting the so-called direct (D₁ receptor) and indirect (D₂ receptor) SPN pathways. Both SPN subtypes receive converging glutamate inputs from limbic and cortical regions, encoding contextual and emotional information, together with DA, which mediates reward prediction and incentive values. DA differentially modulates the efficacy of glutamate synapses onto direct and indirect SPN pathways by recruiting distinct striatal signaling pathways, epigenetic and genetic responses likely involved in the transition from casual drug use to addiction. Herein we focus on recent studies that have assessed psychostimulant-induced alterations in a cell-type-specific manner, from remodeling of input projections to the characterization of specific molecular events in each SPN subtype and their impact on long-lasting behavioral adaptations. We discuss recent evidence revealing the complex and concerted action of both SPN populations on drug-induced behavioral responses, as these studies can contribute to the design of future strategies to alleviate specific behavioral components of addiction.

Memory and plasticity impairment after binge drinking in adolescent rat hippocampus: GluN2A/GluN2B NMDA receptor subunits imbalance through HDAC2

Ethanol (EtOH) induces cognitive impairment through modulation of synaptic plasticity notably in the hippocampus. The cellular mechanism(s) of these EtOH effects may range from synaptic signaling modulation to alterations of the epigenome. Previously, we reported that two binge-like exposures to EtOH (3 g/kg, ip, 9 h apart) in adolescent rats abolished long-term synaptic depression (LTD) in hippocampus slices, induced learning deficits, and increased N-methyl-d-aspartate (NMDA) receptor signaling through its GluN2B subunit after 48 hours. Here, we tested the hypothesis of EtOH-induced epigenetic alterations leading to modulation of GluN2B and GluN2A NMDA receptor subunits. Forty-two days old rats were treated with EtOH or the histone deacetylase inhibitor (HDACi) sodium butyrate (NaB, 600 mg/kg, ip) injected alone or 30 minutes before EtOH. After 48 hours, learning was tested with novel object recognition while synaptic plasticity and the role of GluN2A and GluN2B subunits in NMDA-fEPSP were measured in CA1 field of hippocampus slices. LTD and memory were impaired 48 hours after EtOH and NMDA-fEPSP analysis unraveled changes in the GluN2A/GluN2B balance. These results were associated with an increase in histone deacetylase (HDAC) activity and HDAC2 mRNA and protein while Ac-H4K12 labelling was decreased. EtOH increases expression of HDAC2 and mRNA level for GluN2B subunit (but not GluN2A), while HDAC2 modulates the promoter of the gene encoding GluN2B. Interestingly, NaB pretreatment prevented all the cellular and memory-impairing effects of EtOH. In conclusion, the memory-impairing effects of two binge-like EtOH exposure involve NMDA receptor-dependent LTD deficits due to a GluN2A/GluN2B imbalance resulting from changes in GluN2B expression induced by HDAC2.

Effect of morphine exposure on novel object memory of the offspring: The role of histone H3 and ΔFosB

It has been demonstrated that alteration in histone acetylation in the regions of the brain involved in the reward which may have an important role in morphine addiction. It is well established that epigenetic changes prior to birth influence the function and development of the brain. The current study was designed to evaluate changes in novel object memory, histone acetylation and ΔFosB in the brain of the offspring of morphine-withdrawn parents. Male and female Wistar rats received morphine orally for 21 following days. After ten days of abstinent, they were prepared for mating. The male offspring of the first parturition were euthanized on postnatal days 5, 21, 30 and 60. The novel object recognition (NOR) test was performed on adult male offspring. The amount of acetylated histone H3 and ΔFosB were evaluated in the prefrontal cortex (PFC) and hippocampus using western blotting. Obtained results indicated that the discrimination index in the NOR test was decreased in the offspring of morphine-withdrawn parents as compared with morphine-naïve offspring. In addition, the level of acetylated histone H3 was decreased in the PFC and hippocampus in the offspring of morphine-withdrawn parents during lifetime (postnatal days 5, 21, 30 and 60). In the case of ΔFosB, it also decreased in these regions in the morphine-withdrawn offspring. These results demonstrated that parental morphine exposure affects NOR memory, and decreased the level of histone H3 acetylation and ΔFosB in the PFC and hippocampus. Taken together, the effect of morphine might be transmitted to the next generation even after stop consuming morphine.

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.

Long-term epigenetic changes in offspring mice exposed to alcohol during gestation and lactation

BACKGROUND

Alcohol exposure impairs brain development and leads to a range of behavioural and cognitive dysfunctions, termed as foetal alcohol spectrum disorders. Although different mechanisms have been proposed to participate in foetal alcohol spectrum disorders, the molecular insights of such effects are still uncertain. Using a mouse model of foetal alcohol spectrum disorder, we have previously shown that maternal binge-like alcohol drinking causes persistent effects on motor, cognitive and emotional-related behaviours associated with neuroimmune dysfunctions.

AIMS

In this study, we sought to evaluate whether the long-term behavioural alterations found in offspring with early exposure to alcohol are associated with epigenetic changes in the hippocampus and prefrontal cortex.

METHODS

Pregnant C57BL/6 female mice underwent a model procedure for binge alcohol drinking throughout both the gestation and lactation periods. Subsequently, adult offspring were assessed for their cognitive function in a reversal learning task and brain areas were extracted for epigenetic analyses.

RESULTS

The results demonstrated that early binge alcohol exposure induces long-term behavioural effects along with alterations in histone acetylation (histone H4 lysine 5 and histone H4 lysine 12) in the hippocampus and prefrontal cortex. The epigenetic effects were linked with an imbalance in histone acetyltransferase activity that was found to be increased in the prefrontal cortex of mice exposed to alcohol.

CONCLUSIONS

In conclusion, our results reveal that maternal binge-like alcohol consumption induces persistent epigenetic modifications, effects that might be associated with the long-term cognitive and behavioural impairments observed in foetal alcohol spectrum disorder models.

Translational Molecular Approaches in Substance Abuse Research

Excessive abuse of psychoactive substances is one of the leading contributors to morbidity and mortality worldwide. In this book chapter, we review translational research strategies that are applied in the pursuit of new and more effective therapeutics for substance use disorder (SUD). The complex, multidimensional nature of psychiatric disorders like SUD presents difficult challenges to investigators. While animal models are critical for outlining the mechanistic relationships between defined behaviors and genetic and/or molecular changes, the heterogeneous pathophysiology of brain diseases is uniquely human, necessitating the use of human studies and translational research schemes. Translational research describes a cross-species approach in which findings from human patient-based data can be used to guide molecular genetic investigations in preclinical animal models in order to delineate the mechanisms of reward circuitry changes in the addicted state. Results from animal studies can then inform clinical investigations toward the development of novel treatments for SUD. Here we describe the strategies that are used to identify and functionally validate genetic variants in the human genome which may contribute to increased risk for SUD, starting from early candidate gene approaches to more recent genome-wide association studies. We will next examine studies aimed at understanding how transcriptional and epigenetic dysregulation in SUD can persistently alter cellular function in the disease state. In our discussion, we then focus on examples from the literature illustrating molecular genetic methodologies that have been applied to studies of different substances of abuse - from alcohol and nicotine to stimulants and opioids - in order to exemplify how these approaches can both delineate the underlying molecular systems driving drug addiction and provide insights into the genetic basis of SUD.

Epigenetic Regulation of Hippocampal Fosb Expression Controls Behavioral Responses to Cocaine

Drug addiction results in part from maladaptive learning, including the formation of strong associations between the drug and the circumstances of consumption. However, drug-induced changes in gene expression underlying the saliency of these associations remain understudied. Consolidation of explicit memories occurs within the hippocampus, and we have shown that spatial learning induces expression of the transcription factor ΔFosB in hippocampus and that this induction is critical for learning. Drugs of abuse also upregulate ΔFosB in hippocampus, but the mechanism of its induction by cocaine and its role in hippocampus-dependent cocaine responses is unknown. We investigated differences in mouse dorsal and ventral hippocampal ΔFosB expression in response to chronic cocaine, because these regions appear to regulate distinct cocaine-related behaviors. We found that cocaine-mediated induction of ΔFosB was subregion-specific, and that ΔFosB transcriptional activity in both the dorsal and ventral hippocampus is necessary for cocaine conditioned place preference. Further, we characterize changes in histone modifications at the FosB promoter in hippocampus in response to chronic cocaine and found that locus-specific epigenetic modification is essential for FosB induction and multiple hippocampus-dependent behaviors, including cocaine place preference. Collectively, these findings suggest that exposure to cocaine induces histone modification at the hippocampal FosB gene promoter to cause ΔFosB induction critical for cocaine-related learning.SIGNIFICANCE STATEMENT Although cocaine addiction is driven in part by the formation of indelible associations between the drug and the environment, paraphernalia, and circumstances of use, and although this type of associative learning is dependent upon changes in gene expression in a brain region called the hippocampus, the mechanisms by which cocaine alters hippocampal gene expression to drive formation of these associations is poorly understood. Here, we demonstrate that chronic cocaine engages locus-specific changes in the epigenetic profile of the FosB gene in the hippocampus, and that these alterations are required for cocaine-dependent gene expression and cocaine-environment associations. This work provides novel insight into addiction etiology and potential inroads for therapeutic intervention in cocaine addiction.

The chromatin landscape of neuronal plasticity

Examining the links between neuronal activity, transcriptional output, and synaptic function offers unique insights into how neurons adapt to changing environments and form memories. Epigenetic markers, such as DNA methylation and histone modifications, have been implicated in the formation of not only cellular memories such as cell fate, but also memories of experience at the organismal level. Here, we review recent advances in chromatin regulation that contribute to synaptic plasticity and drive adaptive behaviors through dynamic and precise regulation of transcription output in neurons. We discuss chromatin-associated proteins, histone variant proteins, the contribution of cis-regulatory elements and their interaction with histone modifications, and how these mechanisms are integrated into distinct behavior and environmental response paradigms.

National Institute on Drug Abuse genomics consortium white paper: Coordinating efforts between human and animal addiction studies

The National Institute on Drug Abuse Genetics and Epigenetics Cross-Cutting Research Team convened a diverse group of researchers, clinicians, and healthcare providers on the campus of the University of California, San Diego, in June 2018. The goal was to develop strategies to integrate genetics and phenotypes across species to achieve a better understanding of substance use disorders through associations between genotypes and addictive behaviors. This conference (a) discussed progress in harmonizing large opioid genetics cohorts, (b) discussed phenotypes that are used for genetics studies in humans, (c) examined phenotypes that are used for genetics studies in animal models, (d) identified synergies and gaps in phenotypic analyses of human and animal models and (e) identified strategies to integrate genetics and genomics data with phenotypes across species. The meeting consisted of panels that focused on phenotype harmonization (Dr. Laura Bierut, Dr. Olivier George, Dr. Dan Larach and Dr. Sesh Mudumbai), translating genetic findings between species (Dr. Elissa Chesler, Dr. Gary Peltz and Dr. Abraham Palmer), interpreting and understanding allelic variations (Dr. Vanessa Troiani and Dr. Tamara Richards) and pathway conservation in animal models and human studies (Dr. Robert Hitzemann, Dr. Huda Akil and Dr. Laura Saba). There were also updates that were provided by large consortia (Dr. Susan Tapert, Dr. Danielle Dick, Dr. Howard Edenberg and Dr. Eric Johnson). Collectively, the conference was convened to discuss progress and changes in genome-wide association studies.

Activity-Dependent Epigenetic Remodeling in Cocaine Use Disorder

Substance use disorder (SUD) is a behavioral disorder characterized by cycles of abstinence, drug seeking, and relapse. SUD is characterized by aberrant learning processes which develop after repeated exposure to drugs of abuse. At the core of this phenotype is the persistence of symptoms, such as craving and relapse to drug seeking, long after the cessation of drug use. The neural basis of these behavioral changes has been linked to dysfunction in neural circuits across the brain; however, the molecular drivers that allow for these changes to persist beyond the lifespan of any individual protein remain opaque. Epigenetic adaptations - where DNA is modified to increase or decrease the probability of gene expression at key genes - have been identified as a mechanism underlying the long-lasting nature of drug-seeking behavior. Thus, to understand SUD, it is critical to define the interplay between neuronal activation and longer-term changes in transcription and epigenetic remodeling and define their role in addictive behaviors. In this review, we discuss the current understanding of drug-induced changes to circuit function, recent discoveries in epigenetic mechanisms that mediate these changes, and, ultimately, how these adaptations drive the persistent nature of relapse, with emphasis on adaptations in models of cocaine use disorder. Understanding the complex interplay between epigenetic gene regulation and circuit activity will be critical in elucidating the neural mechanisms underlying SUD. This, with the advent of novel genetic-based techniques, will allow for the generation of novel therapeutic avenues to improve treatment outcomes in SUD.

Decoding the transcriptional programs activated by psychotropic drugs in the brain

Analysis of drug-induced gene expression in the brain has long held the promise of revealing the molecular mechanisms of drug actions as well as predicting their long-term clinical efficacy. However, despite some successes, this promise has yet to be fulfilled. Here, we present an overview of the current state of understanding of drug-induced gene expression in the brain and consider the obstacles to achieving a robust prediction of the properties of psychoactive compounds based on gene expression profiles. We begin with a comprehensive overview of the mechanisms controlling drug-inducible transcription and the complexity resulting from expression of noncoding RNAs and alternative gene isoforms. Particular interest is placed on studies that examine the associations within drug classes with regard to the effects on gene transcription, alterations in cell signaling and neuropharmacological drug properties. While the ability of gene expression signatures to distinguish specific clinical classes of psychotropic and addictive drugs remains unclear, some reports show that under specific constraints, drug properties can be predicted based on gene expression. Such signatures offer a simple and effective way to classify psychotropic drugs and screen novel psychoactive compounds. Finally, we note that the amount of data regarding molecular programs activated in the brain by drug treatment has grown exponentially in recent years and that future advances may therefore come in large part from integrating the currently available high-throughput data sets.

Neuroepigenetics and addictive behaviors: Where do we stand?

Substance use disorders involve long-term changes in the brain that lead to compulsive drug seeking, craving, and a high probability of relapse. Recent findings have highlighted the role of epigenetic regulations in controlling chromatin access and regulation of gene expression following exposure to drugs of abuse. In the present review, we focus on data investigating genome-wide epigenetic modifications in the brain of addicted patients or in rodent models exposed to drugs of abuse, with a particular focus on DNA methylation and histone modifications associated with transcriptional studies. We highlight critical factors for epigenomic studies in addiction. We discuss new findings related to psychostimulants, alcohol, opiate, nicotine and cannabinoids. We examine the possible transmission of these changes across generations. We highlight developing tools, specifically those that allow investigation of structural reorganization of the chromatin. These have the potential to increase our understanding of alteration of chromatin architecture at gene regulatory regions. Neuroepigenetic mechanisms involved in addictive behaviors could explain persistent phenotypic effects of drugs and, in particular, vulnerability to relapse.

Epigenetic Effects Induced by Methamphetamine and Methamphetamine-Dependent Oxidative Stress

Methamphetamine is a widely abused drug, which possesses neurotoxic activity and powerful addictive effects. Understanding methamphetamine toxicity is key beyond the field of drug abuse since it allows getting an insight into the molecular mechanisms which operate in a variety of neuropsychiatric disorders. In fact, key alterations produced by methamphetamine involve dopamine neurotransmission in a way, which is reminiscent of spontaneous neurodegeneration and psychiatric schizophrenia. Thus, understanding the molecular mechanisms operated by methamphetamine represents a wide window to understand both the addicted brain and a variety of neuropsychiatric disorders. This overlapping, which is already present when looking at the molecular and cellular events promoted immediately after methamphetamine intake, becomes impressive when plastic changes induced in the brain of methamphetamine-addicted patients are considered. Thus, the present manuscript is an attempt to encompass all the molecular events starting at the presynaptic dopamine terminals to reach the nucleus of postsynaptic neurons to explain how specific neurotransmitters and signaling cascades produce persistent genetic modifications, which shift neuronal phenotype and induce behavioral alterations. A special emphasis is posed on disclosing those early and delayed molecular events, which translate an altered neurotransmitter function into epigenetic events, which are derived from the translation of postsynaptic noncanonical signaling into altered gene regulation. All epigenetic effects are considered in light of their persistent changes induced in the postsynaptic neurons including sensitization and desensitization, priming, and shift of neuronal phenotype.

Conduct Disorder Symptoms and Illicit Drug Use in Juvenile Justice Involved Youth: The Reciprocal Relationship Between Positive Illicit Drug-Use Attitudes and Illicit Drug Use

Conduct disorder (CD) symptoms cooccur at high rates with illicit drug use in juvenile justice involved youth, which results in poorer outcomes; however, research has not identified where best to intervene in this relationship, limiting the identification of modifiable risk factors to reduce negative effects of CD symptoms. Two mediation models were examined to investigate the potential for CD symptoms to influence a reciprocal relationship between illicit drug use and positive drug attitudes, controlling for age, gender, and race. Data were examined for 245 juvenile justice involved youth (mean age = 15.46, SD = 1.30, range 12-18, 64.9% Black, 80.4% male) who completed court-ordered psychological assessments. Findings indicate: (1) Positive attitudes toward illicit drug use significantly mediated the relationship between CD symptoms and illicit drug use (β = 0.16, CI 0.09-0.27; test for indirect effect z = 4.17, p < .001) and (2) illicit drug use significantly mediated the relationship between CD symptoms and positive attitudes toward illicit drug use (β = 0.20, CI 0.12-0.32; test for indirect effect z = 4.87, p < .001). Overall, the present study suggests that CD symptoms impart risk for illicit drug use both indirectly, through more positive attitudes toward illicit drug use, and directly, which further strengthens positive attitudes toward illicit drug use.

Environmental, genetic and epigenetic contributions to cocaine addiction

Decades of research on cocaine has produced volumes of data that have answered many important questions about the nature of this highly addictive drug. Sadly, none of this information has translated into the development of effective therapies for the treatment of cocaine addiction. This review endeavors to assess the current state of cocaine research in an attempt to identify novel pathways for therapeutic development. For example, risk of cocaine addiction is highly heritable but genome-wide analyses comparing cocaine-dependent individuals to controls have not resulted in promising targets for drug development. Is this because the genetics of addiction is too complex or because the existing research methodologies are inadequate? Likewise, animal studies have revealed dozens of enduring changes in gene expression following prolonged exposure to cocaine, none of which have translated into therapeutics either because the resulting compounds were ineffective or produced intolerable side-effects. Recently, attention has focused on epigenetic modifications resulting from repeated cocaine intake, some of which appear to be heritable through changes in the germline. While epigenetic changes represent new vistas for therapeutic development, selective manipulation of epigenetic marks is currently challenging even in animals such that translational potential is a distant prospect. This review will reveal that despite the enormous progress made in understanding the molecular and physiological bases of cocaine addiction, there is much that remains a mystery. Continued advances in genetics and molecular biology hold potential for revealing multiple pathways toward the development of treatments for the continuing scourge of cocaine addiction.

Methamphetamine Induces TET1- and TET3-Dependent DNA Hydroxymethylation of Crh and Avp Genes in the Rat Nucleus Accumbens

Methamphetamine (METH) addiction is a biopsychosocial disorder that is accompanied by multiple relapses even after prolonged abstinence, suggesting the possibilities of long-lasting maladaptive epigenetic changes in the brain. Here, we show that METH administration produced time-dependent increases in the expression of corticotropin-releasing hormone (Crh/Crf), arginine vasopressin (Avp), and cocaine- and amphetamine-regulated transcript prepropeptide (Cartpt) mRNAs in the rat nucleus accumbens (NAc). Chromatin immunoprecipitation (ChIP) assays revealed that METH increased the abundance of phosphorylated CREB (pCREB) at the promoter of Cartpt but not at Avp or Crh DNA sequences. In contrast, METH produced DNA hypomethylation at sites near the Crh transcription start site (TSS) and at intragenic Avp sequences. METH also increased DNA hydroxymethylation at the Crh TSS and at intragenic Avp sites. In addition, METH increased the protein expression of ten-eleven-translocation enzymes that catalyze DNA hydroxymethylation. Importantly, METH increased TET1 binding at the Crh promoter and increased TET3 binding at Avp intragenic regions. We further tested the role of TET enzymes in METH-induced changes in gene expression by using the TET inhibitor, 1,5-isoquinolinediol (IQD), and found that IQD blocked METH-induced increases in Crh and Avp mRNA expression. Together, these results indicate that METH produced changes in neuropeptide transcription by both activation of the cAMP/CREB pathway and stimulation of TET-dependent DNA hydroxymethylation. These results provide molecular evidence for epigenetic controls of METH-induced changes in the expression of neuropeptides.

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.

Neuroepigenetic Editing

Studies of the mammalian nervous system have revealed widespread epigenetic regulation underlying gene expression intrinsic to basic neurobiological function as well as neurological disease. Over the past decade, a critical role has emerged for the neural regulation of chromatin-modifying enzymes during both development and adulthood, and in response to external stimuli. These biochemical data are complemented by numerous next generation sequencing (NGS) studies that quantify the extent of chromatin and DNA modifications in neurons. Neuroepigenetic editing tools can be applied to distinguish between the mere presence and functional relevance of such modifications to neural transcription and animal behavior. This review discusses current advances in neuroepigenetic editing, highlighting methodological considerations pertinent to neuroscience, such as delivery methods and the spatiotemporal specificity of editing. Although neuroepigenetic editing is a nascent field, the studies presented in this review demonstrate the enormous potential of this approach for basic neurobiological research and therapeutic application.

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.

Regulation of habit formation in the dorsal striatum

Habits are an essential and pervasive component of our daily lives that allow us to efficiently perform routine tasks. But their disruption contributes to the symptoms that underlie many psychiatric diseases. Emerging data are revealing the cellular and molecular mechanisms of habit formation in the dorsal striatum. New data suggest that in both the dorsolateral and dorsomedial striatum histone deacetylase (HDAC) activity acts as a critical negative regulator of the transcriptional processes underlying habit formation. In this review, we discuss this recent work and draw conclusions relevant to the treatment of diseases marked by maladaptive habits.

Machine Learning Analysis Identifies Drosophila Grunge/Atrophin as an Important Learning and Memory Gene Required for Memory Retention and Social Learning

High-throughput experiments are becoming increasingly common, and scientists must balance hypothesis-driven experiments with genome-wide data acquisition. We sought to predict novel genes involved in Drosophila learning and long-term memory from existing public high-throughput data. We performed an analysis using PILGRM, which analyzes public gene expression compendia using machine learning. We evaluated the top prediction alongside genes involved in learning and memory in IMP, an interface for functional relationship networks. We identified Grunge/Atrophin (Gug/Atro), a transcriptional repressor, histone deacetylase, as our top candidate. We find, through multiple, distinct assays, that Gug has an active role as a modulator of memory retention in the fly and its function is required in the adult mushroom body. Depletion of Gug specifically in neurons of the adult mushroom body, after cell division and neuronal development is complete, suggests that Gug function is important for memory retention through regulation of neuronal activity, and not by altering neurodevelopment. Our study provides a previously uncharacterized role for Gug as a possible regulator of neuronal plasticity at the interface of memory retention and memory extinction.

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.

Cross-talk between the epigenome and neural circuits in drug addiction

Drug addiction is a behavioral disorder characterized by dysregulated learning about drugs and associated cues that result in compulsive drug seeking and relapse. Learning about drug rewards and predictive cues is a complex process controlled by a computational network of neural connections interacting with transcriptional and molecular mechanisms within each cell to precisely guide behavior. The interplay between rapid, temporally specific neuronal activation, and longer-term changes in transcription is of critical importance in the expression of appropriate, or in the case of drug addiction, inappropriate behaviors. Thus, these factors and their interactions must be considered together, especially in the context of treatment. Understanding the complex interplay between epigenetic gene regulation and circuit connectivity will allow us to formulate novel therapies to normalize maladaptive reward behaviors, with a goal of modulating addictive behaviors, while leaving natural reward-associated behavior unaffected.

Drug Addiction and Histone Code Alterations

Acute and prolonged exposure to drugs of abuse induces changes in gene expression, synaptic function, and neural plasticity in brain regions involved in reward. Numerous genes are involved in this process, and persistent changes in gene expression coincide with epigenetic histone modifications and DNA methylation. Histone modifications are attractive regulatory mechanisms, which can encode complex environmental signals in the genome of postmitotic cells, like neurons. Recently, it has been demonstrated that specific histone modifications are involved in addiction-related gene regulatory mechanisms, by a diverse set of histone-modifying enzymes and readers. These histone modifiers and readers may prove to be valuable pharmacological targets for effective treatments for drug addiction.

Compulsive methamphetamine taking in the presence of punishment is associated with increased oxytocin expression in the nucleus accumbens of rats

Methamphetamine addiction is mimicked in rats that self-administer the drug. However, these self-administration (SA) models do not include adverse consequences that are necessary to reach a diagnosis of addiction in humans. Herein, we measured genome-wide transcriptional consequences of methamphetamine SA and footshocks in the rat brain. We trained rats to self-administer methamphetamine for 20 days. Thereafter, lever-presses for methamphetamine were punished by mild footshocks for 5 days. Response-contingent punishment significantly reduced methamphetamine taking in some rats (shock-sensitive, SS) but not in others (shock-resistant, SR). Rats also underwent extinction test at one day and 30 days after the last shock session. Rats were euthanized one day after the second extinction test and the nucleus accumbens (NAc) and dorsal striatum were collected to measure gene expression with microarray analysis. In the NAc, there were changes in the expression of 13 genes in the SRvsControl and 9 genes in the SRvsSS comparison. In the striatum, there were 9 (6 up, 3 down) affected genes in the SRvsSS comparison. Among the upregulated genes was oxytocin in the NAc and CARTpt in the striatum of SR rats. These observations support a regional role of neuropeptides in the brain after a long withdrawal interval when animals show incubation of methamphetamine craving.

Cocaine Administration and Its Withdrawal Enhance the Expression of Genes Encoding Histone-Modifying Enzymes and Histone Acetylation in the Rat Prefrontal Cortex

Chronic exposure to cocaine, craving, and relapse are attributed to long-lasting changes in gene expression arising through epigenetic and transcriptional mechanisms. Although several brain regions are involved in these processes, the prefrontal cortex seems to play a crucial role not only in motivation and decision-making but also in extinction and seeking behavior. In this study, we applied cocaine self-administration and extinction training procedures in rats with a yoked triad to determine differentially expressed genes in prefrontal cortex. Microarray analysis showed significant upregulation of several genes encoding histone modification enzymes during early extinction training. Subsequent real-time PCR testing of these genes following cocaine self-administration or early (third day) and late (tenth day) extinction revealed elevated levels of their transcripts. Interestingly, we found the enrichment of Brd1 messenger RNA in rats self-administering cocaine that lasted until extinction training during cocaine withdrawal with concomitant increased acetylation of H3K9 and H4K8. However, despite elevated levels of methyl- and demethyltransferase-encoded transcripts, no changes in global di- and tri-methylation of histone H3 at lysine 4, 9, 27, and 79 were observed. Surprisingly, at the end of extinction training (10 days of cocaine withdrawal), most of the analyzed genes in the rats actively and passively administering cocaine returned to the control level. Together, the alterations identified in the rat prefrontal cortex may suggest enhanced chromatin remodeling and transcriptional activity induced by early cocaine abstinence; however, to know whether they are beneficial or not for the extinction of drug-seeking behavior, further in vivo evaluation is required.

Targeted Epigenetic Remodeling of the Cdk5 Gene in Nucleus Accumbens Regulates Cocaine- and Stress-Evoked Behavior

Recent studies have implicated epigenetic remodeling in brain reward regions following psychostimulant or stress exposure. It has only recently become possible to target a given type of epigenetic remodeling to a single gene of interest, and to probe the functional relevance of such regulation to neuropsychiatric disease. We sought to examine the role of histone modifications at the murine Cdk5 (cyclin-dependent kinase 5) locus, given growing evidence of Cdk5 expression in nucleus accumbens (NAc) influencing reward-related behaviors. Viral-mediated delivery of engineered zinc finger proteins (ZFP) targeted histone H3 lysine 9/14 acetylation (H3K9/14ac), a transcriptionally active mark, or histone H3 lysine 9 dimethylation (H3K9me2), which is associated with transcriptional repression, specifically to the Cdk5 locus in NAc in vivo We found that Cdk5-ZFP transcription factors are sufficient to bidirectionally regulate Cdk5 gene expression via enrichment of their respective histone modifications. We examined the behavioral consequences of this epigenetic remodeling and found that Cdk5-targeted H3K9/14ac increased cocaine-induced locomotor behavior, as well as resilience to social stress. Conversely, Cdk5-targeted H3K9me2 attenuated both cocaine-induced locomotor behavior and conditioned place preference, but had no effect on stress-induced social avoidance behavior. The current study provides evidence for the causal role of Cdk5 epigenetic remodeling in NAc in Cdk5 gene expression and in the control of reward and stress responses. Moreover, these data are especially compelling given that previous work demonstrated opposite behavioral phenotypes compared with those reported here upon Cdk5 overexpression or knockdown, demonstrating the importance of targeted epigenetic remodeling tools for studying more subtle molecular changes that contribute to neuropsychiatric disease.

SIGNIFICANCE STATEMENT

Addiction and depression are highly heritable diseases, yet it has been difficult to identify gene sequence variations that underlie this heritability. Gene regulation via epigenetic remodeling is an additional mechanism contributing to the neurobiological basis of drug and stress exposure. In particular, epigenetic regulation of the Cdk5 gene alters responses to cocaine and stress in mouse and rat models. In this study, we used a novel technology, zinc-finger engineered transcription factors, to remodel histone proteins specifically at the Cdk5 gene. We found that this is sufficient to regulate the expression of Cdk5 and results in altered behavioral responses to cocaine and social stress. These data provide compelling evidence of the significance of epigenetic regulation in the neurobiological basis of reward- and stress-related neuropsychiatric disease.

Histone posttranslational modifications predict specific alternative exon subtypes in mammalian brain

A compelling body of literature, based on next generation chromatin immunoprecipitation and RNA sequencing of reward brain regions indicates that the regulation of the epigenetic landscape likely underlies chronic drug abuse and addiction. It is now critical to develop highly innovative computational strategies to reveal the relevant regulatory transcriptional mechanisms that may underlie neuropsychiatric disease. We have analyzed chromatin regulation of alternative splicing, which is implicated in cocaine exposure in mice. Recent literature has described chromatin-regulated alternative splicing, suggesting a novel function for drug-induced neuroepigenetic remodeling. However, the extent of the genome-wide association between particular histone modifications and alternative splicing remains unexplored. To address this, we have developed novel computational approaches to model the association between alternative splicing and histone posttranslational modifications in the nucleus accumbens (NAc), a brain reward region. Using classical statistical methods and machine learning to combine ChIP-Seq and RNA-Seq data, we found that specific histone modifications are strongly associated with various aspects of differential splicing. H3K36me3 and H3K4me1 have the strongest association with splicing indicating they play a significant role in alternative splicing in brain reward tissue.

Acetyl-CoA synthetase regulates histone acetylation and hippocampal memory

Metabolic production of acetyl-CoA is linked to histone acetylation and gene regulation, yet the precise mechanisms are largely unknown. Here we show that the metabolic enzyme acetyl-CoA synthetase 2 (ACSS2) is a direct regulator of histone acetylation in neurons and of spatial memory in mammals. In a neuronal cell culture model, ACSS2 increases in nuclei of differentiating neurons and localizes to upregulated neuronal genes near elevated histone acetylation. Reduction of ACSS2 lowers nuclear acetyl-CoA levels, histone acetylation, and responsive expression of the cohort of neuronal genes. In adult mice, attenuation of hippocampal ACSS2 expression impairs long-term spatial memory, a cognitive process reliant on histone acetylation. ACSS2 reduction in hippocampus also leads to defective upregulation of memory-related neuronal genes that are pre-bound by ACSS2. These results reveal a unique connection between cellular metabolism, gene regulation, and neural plasticity, establishing a link between acetyl-CoA generation “on-site” at chromatin for histone acetylation and the transcription of critical neuronal genes.

Alcohol effects on the epigenome in the germline: Role in the inheritance of alcohol-related pathology

Excessive alcohol exposure has severe health consequences, and clinical and animal studies have demonstrated that disruptions in the epigenome of somatic cells, such as those in brain, are an important factor in the development of alcohol-related pathologies, such as alcohol-use disorders (AUDs) and fetal alcohol spectrum disorders (FASDs). It is also well known that alcohol-related health problems are passed down across generations in human populations, but the complete mechanisms for this phenomenon are currently unknown. Recent studies in animal models have suggested that epigenetic factors are also responsible for the transmission of alcohol-related pathologies across generations. Alcohol exposure has been shown to induce changes in the epigenome of sperm of exposed male animals, and these epimutations are inherited in the offspring. This paper reviews evidence for multigenerational and transgenerational epigenetic inheritance of alcohol-related pathology through the germline. We also review the literature on the epigenetic effects of alcohol exposure on somatic cells in brain, and its contribution to AUDs and FASDs. We note gaps in knowledge in this field, such as the lack of clinical studies in human populations and the lack of data on epigenetic inheritance via the female germline, and we suggest future research directions.