Histone deacetylase inhibition decreases preference without affecting aversion for nicotine

Epigenetic mechanisms of nicotine dependence

Smoking continues to be a leading cause of preventable disease and death worldwide. Nicotine dependence generates a lifelong propensity towards cravings and relapse, presenting an ongoing challenge for the development of treatments. Accumulating evidence supports a role for epigenetics in the development and maintenance of addiction to many drugs of abuse, however, the involvement of epigenetics in nicotine dependence is less clear. Here we review evidence that nicotine interacts with epigenetic mechanisms to enable the maintenance of nicotine-seeking across time. Research across species suggests that nicotine increases permissive histone acetylation, decreases repressive histone methylation, and modulates levels of DNA methylation and noncoding RNA expression throughout the brain. These changes are linked to the promoter regions of genes critical for learning and memory, reward processing and addiction. Pharmacological manipulation of enzymes that catalyze core epigenetic modifications regulate nicotine reward and associative learning, demonstrating a functional role of epigenetic modifications in nicotine dependence. These findings are consistent with nicotine promoting an overall permissive chromatin state at genes important for learning, memory and reward. By exploring these links through next-generation sequencing technologies, epigenetics provides a promising avenue for future interventions to treat nicotine dependence.

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.

Sex Differences in Psychostimulant Abuse: Implications for Estrogen Receptors and Histone Deacetylases

Substance abuse is a chronic pathological disorder that negatively affects many health and neurological processes. A growing body of literature has revealed gender differences in substance use. Compared to men, women display distinct drug-use phenotypes accompanied by recovery and rehabilitation disparities. These observations have led to the notion that sex-dependent susceptibilities exist along the progression to addiction. Within this scope, neuroadaptations following psychostimulant exposure are thought to be distinct for each sex. This review summarizes clinical findings and animal research reporting sex differences in the subjective and behavioral responses to cocaine, methamphetamine, and nicotine. This discussion is followed by an examination of epigenetic and molecular alterations implicated in the addiction process. Special consideration is given to histone deacetylases and estrogen receptor-mediated gene expression.

Epigenetic Mechanisms Mediate Nicotine-Induced Reward and Behaviour in Zebrafish

Nicotine induces long-term changes in the neural activity of the mesocorticolimbic reward pathway structures. The mechanisms involved in this process have not been fully characterized. The hypothesis discussed here proposed that epigenetic regulation participates in the installation of persistent adaptations and long-lasting synaptic plasticity generated by nicotine action on the mesolimbic dopamine neurons of zebrafish. The epigenetic mechanisms induced by nicotine entail histone and DNA chemical modifications, which have been described to lead to changes in gene expression. Among the enzymes that catalyze epigenetic chemical modifications, histone deacetylases (HDACs) remove acetyl groups from histones, thereby facilitating DNA relaxation and making DNA more accessible to gene transcription. DNA methylation, which is dependent on DNA methyltransferase (DNMTs) activity, inhibits gene expression by recruiting several methyl binding proteins that prevent RNA polymerase binding to DNA. In zebrafish, phenylbutyrate (PhB), an HDAC inhibitor, abolishes nicotine rewarding properties together with a series of typical reward-associated behaviors. Furthermore, PhB and nicotine alter long- and short-term object recognition memory in zebrafish, respectively. Regarding DNA methylation effects, a methyl group donor L-methionine (L-met) was found to dramatically reduce nicotine-induced conditioned place preference (CPP) in zebrafish. Simultaneous treatment with DNMT inhibitor 5-aza-2’-deoxycytidine (AZA) was found to reverse the L-met effect on nicotine-induced CPP as well as nicotine reward-specific effects on genetic expression in zebrafish. Therefore, pharmacological interventions that modulate epigenetic regulation of gene expression should be considered as a potential therapeutic method to treat nicotine addiction.

Evidence for Modulation of Substance Use Disorders by the Gut Microbiome: Hidden in Plain Sight

The gut microbiome modulates neurochemical function and behavior and has been implicated in numerous central nervous system (CNS) diseases, including developmental, neurodegenerative, and psychiatric disorders. Substance use disorders (SUDs) remain a serious threat to the public well-being, yet gut microbiome involvement in drug abuse has received very little attention. Studies of the mechanisms underlying SUDs have naturally focused on CNS reward circuits. However, a significant body of research has accumulated over the past decade that has unwittingly provided strong support for gut microbiome participation in drug reward. β-Lactam antibiotics have been employed to increase glutamate transporter expression to reverse relapse-induced release of glutamate. Sodium butyrate has been used as a histone deacetylase inhibitor to prevent drug-induced epigenetic alterations. High-fat diets have been used to alter drug reward because of the extensive overlap of the circuitry mediating them. This review article casts these approaches in a different light and makes a compelling case for gut microbiome modulation of SUDs. Few factors alter the structure and composition of the gut microbiome more than antibiotics and a high-fat diet, and butyrate is an endogenous product of bacterial fermentation. Drugs such as cocaine, alcohol, opiates, and psychostimulants also modify the gut microbiome. Therefore, their effects must be viewed on a complex background of cotreatment-induced dysbiosis. Consideration of the gut microbiome in SUDs should have the beneficial effects of expanding the understanding of SUDs and aiding in the design of new therapies based on opposing the effects of abused drugs on the host's commensal bacterial community. SIGNIFICANCE STATEMENT: Proposed mechanisms underlying substance use disorders fail to acknowledge the impact of drugs of abuse on the gut microbiome. β-Lactam antibiotics, sodium butyrate, and high-fat diets are used to modify drug seeking and reward, overlooking the notable capacity of these treatments to alter the gut microbiome. This review aims to stimulate research on substance abuse-gut microbiome interactions by illustrating how drugs of abuse share with antibiotics, sodium butyrate, and fat-laden diets the ability to modify the host microbial community.

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.

The Role of Epigenetics in Addiction: Clinical Overview and Recent Updates

Addiction is an international public health problem. It is a polygenic disorder best understood by accounting for the interplay between genetic and environmental factors. A recent way of perceiving this interaction is through epigenetics, which help grasp the neurobiological changes that occur in addiction and explain its relapsing-remitting nature. It is now known that every cell has a different way of expressing its phenotype, despite a universal DNA sequence. This is particularly true in the central nervous system where environmental factors influence this expression. Three major epigenetic processes have been found to participate in the perpetuation of addiction by changing the state of the chromatin and the degree of gene transcription: histone acetylation and methylation, DNA methylation, and noncoding RNAs. In the animal model literature, substantial evidence exists about the role of these epigenetic changes in the different phases of substance use disorders. This book chapter is a non-systematic literature review of the recent publications tackling the topic of epigenetics in addiction. Even though this evidence remains scarce and relatively poorly systematized, it is a promising foundation for future research of molecules that target specific brain regions and their functions to address core behavioral changes seen in addiction.

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.

Evaluation of the rewarding properties of nicotine and caffeine by implementation of a five-choice conditioned place preference task in zebrafish

The rewarding properties of drugs in zebrafish can be studied using the conditioned place preference (CPP) paradigm. Most devices that have been used for CPP consist of two-half tanks with or without a central chamber. Here we evaluated the rewarding effects of nicotine and caffeine using a tank with five arms distributed radially from a central chamber that we have denoted Fish Tank Radial Maze (FTRM). Zebrafish were trained to associate nicotine or caffeine with a coloured arm. In testing sessions to assess CPP induction, between two and five different arms were available to explore. We found that when offering the two arms, one of them associated to the drug mediating conditioning for 14 days, zebrafish showed nicotine-induced CPP but not caffeine-induced CPP. When zebrafish had the option to explore drug-paired arms together with new coloured arms as putative distractors, the nicotine-CPP strength was maintained for at least three days. The presence of novel environments induced caffeine-CPP, which was still positive after three days of testing sessions. Complementary behavioural data supported these findings. Nicotine-CPP was prevented by the histone deacetylase inhibitor phenylbutyrate administered during conditioning; however, there were no effects on caffeine-CPP. The specific acetylation of lysine 9 in histone 3 (H3-K9) was increased in nicotine-conditioned zebrafish brains. This study suggests that novel environmental cues facilitate drug-environment associations, and hence, the use of drugs of abuse.

New insights on the effects of varenicline on nicotine reward, withdrawal and hyperalgesia in mice

Varenicline, a partial agonist for α4β2* nicotinic acetylcholine receptors (nAChRs) and a full agonist for α3β4 and α7 nAChRs, is approved for smoking cessation treatment. Although, partial agonism at α4β2* nAChRs is believed to be the mechanism underlying the effects of varenicline on nicotine reward, the contribution of other nicotinic subtypes to varenicline's effects on nicotine reward is currently unknown. Therefore, we examined the role of α5 and α7 nAChR subunits in the effects of varenicline on nicotine reward using the conditioned place preference (CPP) test in mice. Moreover, the effects of varenicline on nicotine withdrawal-induced hyperalgesia and aversion are unknown. We also examined the reversal of nicotine withdrawal in mouse models of dependence by varenicline. Varenicline dose-dependently blocked the development and expression of nicotine reward in the CPP test. The blockade of nicotine reward by varenicline (0.1 mg/kg) was preserved in α7 knockout mice but reduced in α5 knockout mice. Administration of varenicline at high dose of 2.5 mg/kg resulted in a place aversion that was dependent on α5 nAChRs but not β2 nAChRs. Furthermore, varenicline (0.1 and 0.5 mg/kg) reversed nicotine withdrawal signs such as hyperalgesia and somatic signs and withdrawal-induced aversion in a dose-related manner. Our results indicate that the α5 nAChR subunit plays a role in the effects of varenicline on nicotine reward in mice. Moreover, the mediation of α5 nAChRs, but not β2 nAChRs are probably needed for aversive properties of varenicline at high dose. Varenicline was also shown to reduce several nicotine withdrawal signs.

Epigenetic mechanisms associated with addiction-related behavioural effects of nicotine and/or cocaine: implication of the endocannabinoid system

The addictive use of nicotine (NC) and cocaine (COC) continues to be a major public health problem, and their combined use has been reported, particularly during adolescence. In neural plasticity, commonly induced by NC and COC, as well as behavioural plasticity related to the use of these two drugs, the involvement of epigenetic mechanisms, in which the reversible regulation of gene expression occurs independently of the DNA sequence, has recently been reported. Furthermore, on the basis of intense interactions with the target neurotransmitter systems, the endocannabinoid (ECB) system has been considered pivotal for eliciting the effects of NC or COC. The combined use of marijuana with NC and/or COC has also been reported. This article presents the addiction-related behavioural effects of NC and/or COC, based on the common behavioural/neural plasticity and combined use of NC/COC, and reviews the interacting role of the ECB system. The epigenetic processes inseparable from the effects of NC and/or COC (i.e. DNA methylation, histone modifications and alterations in microRNAs) and the putative therapeutic involvement of the ECB system at the epigenetic level are also discussed.

Regulation and function of MeCP2 Ser421 phosphorylation in U50488-induced conditioned place aversion in mice

RATIONALE

Phosphorylation of the methyl DNA-binding protein MeCP2 at Ser421 (pMeCP2-S421) is induced in corticolimbic brain regions during exposure to drugs of abuse and modulates reward-driven behaviors. However, whether pMeCP2-S421 is also involved in behavioral adaptations to aversive drugs is unknown.

OBJECTIVES

Our goal was to establish the role and regulation of pMeCP2-S421 in corticolimbic brain regions of mice upon acute treatment with the kappa opioid receptor agonist U50488 and during the expression of U50488-induced conditioned place aversion (CPA).

METHODS

pMeCP2-S421 levels were measured in the nucleus accumbens (NAc), prelimbic cortex, infralimbic cortex (ILC), and basolateral amygdala (BLA) of male mice after intraperitoneal administration of U50488 and upon the expression of U50488-induced CPA. Fos was measured as marker of neural activity in the same brain regions. U50488-induced CPA and Fos levels were compared between knockin (KI) mice that lack pMeCP2-S421 and their wild-type (WT) littermates.

RESULTS

U50488 administration acutely induced pMeCP2-S421 and Fos selectively in the NAc but did not alter MeCP2 levels in any brain region. U50488-induced CPA was associated with decreased pMeCP2-S421 in the ILC and BLA and induced Fos in the BLA. MeCP2 KI mice showed CPA indistinguishable from their WT littermates, but they also showed less BLA Fos induction upon CPA.

CONCLUSION

These data are the first to show that pMeCP2-S421 is induced in the brain acutely after U50488 administration but not upon U50488-induced CPA. Although pMeCP2-S421 is not required for U50488-induced CPA, this phosphorylation event may contribute to molecular plasticities in brain regions that govern aversive behaviors.

Short- and long-term effects of nicotine and the histone deacetylase inhibitor phenylbutyrate on novel object recognition in zebrafish

RATIONALE

Zebrafish have a sophisticated color- and shape-sensitive visual system, so we examined color cue-based novel object recognition in zebrafish. We evaluated preference in the absence or presence of drugs that affect attention and memory retention in rodents: nicotine and the histone deacetylase inhibitor (HDACi) phenylbutyrate (PhB).

OBJECTIVES

The objective of this study was to evaluate whether nicotine and PhB affect innate preferences of zebrafish for familiar and novel objects after short- and long-retention intervals.

METHODS

We developed modified object recognition (OR) tasks using neutral novel and familiar objects in different colors. We also tested objects which differed with respect to the exploratory behavior they elicited from naïve zebrafish.

RESULTS

Zebrafish showed an innate preference for exploring red or green objects rather than yellow or blue objects. Zebrafish were better at discriminating color changes than changes in object shape or size. Nicotine significantly enhanced or changed short-term innate novel object preference whereas PhB had similar effects when preference was assessed 24 h after training. Analysis of other zebrafish behaviors corroborated these results.

CONCLUSIONS

Zebrafish were innately reluctant or prone to explore colored novel objects, so drug effects on innate preference for objects can be evaluated changing the color of objects with a simple geometry. Zebrafish exhibited recognition memory for novel objects with similar innate significance. Interestingly, nicotine and PhB significantly modified innate object preference.

Making Sense of Epigenetics

The gene-environment interactions that underlie development and progression of psychiatric illness are poorly understood. Despite a century of progress, genetic approaches have failed to identify new treatment modalities, perhaps because of the heterogeneity of the disorders and lack of understanding of mechanisms. Recent exploration into epigenetic mechanisms in health and disease has uncovered changes in DNA methylation and chromatin structure that may contribute to psychiatric disorders. Epigenetic changes suggest a variety of new therapeutic options due to their reversible chemistry. However, distinguishing causal links between epigenetic changes and disease from changes consequent to life experience has remained problematic. Here we define epigenetics and explore aspects of epigenetics relevant to causes and mechanisms of psychiatric disease, and speculate on future directions.

Nicotine Suppressed Fetal Adrenal StAR Expression via YY1 Mediated-Histone Deacetylation Modification Mechanism

Steroidogenic acute regulatory (StAR) protein plays a pivotal role in steroidogenesis. Previously, we have demonstrated that prenatal nicotine exposure suppressed fetal adrenal steroidogenesis via steroidogenic factor 1 deacetylation. This study further explored the potential role of the transcriptional repressor Yin Yang 1 (YY1) in nicotine-mediated StAR inhibition. Nicotine was subcutaneously administered (1.0 mg/kg) to pregnant rats twice per day and NCI-H295A cells were treated with nicotine. StAR and YY1 expression were analyzed by real-time PCR, immunohistochemistry, and Western blotting. Histone modifications and the interactions between the YY1 and StAR promoter were assessed using chromatin immunoprecipitation (ChIP). Prenatal nicotine exposure increased YY1 expression and suppressed StAR expression. ChIP assay showed that there was a decreasing trend for histone acetylation at the StAR promoter in fetal adrenal glands, whereas H3 acetyl-K14 at the YY1 promoter presented an increasing trend following nicotine exposure. Furthermore, in nicotine-treated NCI-H295A cells, nicotine enhanced YY1 expression and inhibited StAR expression. ChIP assay showed that histone acetylation decreased at the StAR promoter in NCI-H295A cells and that the interaction between the YY1 and StAR promoter increased. These data indicated that YY1-medicated histone deacetylation modification in StAR promoters might play an important role in the inhibitory effect of nicotine on StAR expression.

Putative Epigenetic Involvement of the Endocannabinoid System in Anxiety- and Depression-Related Behaviors Caused by Nicotine as a Stressor

Like various stressors, the addictive use of nicotine (NC) is associated with emotional symptoms such as anxiety and depression, although the underlying mechanisms have not yet been fully elucidated due to the complicated involvement of target neurotransmitter systems. In the elicitation of these emotional symptoms, the fundamental involvement of epigenetic mechanisms such as histone acetylation has recently been suggested. Furthermore, among the interacting neurotransmitter systems implicated in the effects of NC and stressors, the endocannabinoid (ECB) system is considered to contribute indispensably to anxiety and depression. In the present study, the epigenetic involvement of histone acetylation induced by histone deacetylase (HDAC) inhibitors was investigated in anxiety- and depression-related behavioral alterations caused by NC and/or immobilization stress (IM). Moreover, based on the contributing roles of the ECB system, the interacting influence of ECB ligands on the effects of HDAC inhibitors was evaluated in order to examine epigenetic therapeutic interventions. Anxiety-like (elevated plus-maze test) and depression-like (forced swimming test) behaviors, which were observed in mice treated with repeated (4 days) NC (subcutaneous 0.8 mg/kg) and/or IM (10 min), were blocked by the HDAC inhibitors sodium butyrate (SB) and valproic acid (VA). The cannabinoid type 1 (CB1) agonist ACPA (arachidonylcyclopropylamide; AC) also antagonized these behaviors. Conversely, the CB1 antagonist SR 141716A (SR), which counteracted the effects of AC, attenuated the anxiolytic-like effects of the HDAC inhibitors commonly in the NC and/or IM groups. SR also attenuated the antidepressant-like effects of the HDAC inhibitors, most notably in the IM group. From these results, the combined involvement of histone acetylation and ECB system was shown in anxiety- and depression-related behaviors. In the NC treatment groups, the limited influence of SR against the HDAC inhibitor-induced antidepressant-like effects may reflect the characteristic involvement of histone acetylation within the NC-related neurotransmitter systems other than the ECB system.

The histone deacetylase inhibitor sodium butyrate decreases excessive ethanol intake in dependent animals

Converging evidence indicates that epigenetic mechanisms are involved in drug addiction, and that enzymes involved in chromatin remodeling may represent interesting targets in addiction treatment. No study has addressed whether histone deacetylase (HDAC) inhibitors (HDACi) can reduce excessive ethanol intake or prevent relapse in alcohol-dependent animals. Here, we assessed the effects of two HDACi, sodium butyrate (NaB) and MS-275, in the operant ethanol self-administration paradigm in dependent and non-dependent rats. To characterize some of the epigenetic mechanisms associated with alcohol dependence and NaB treatment, we measured the levels of histone H3 acetylation in different brain areas of dependent and non-dependent rats, submitted or not to NaB treatment. Our results demonstrated that (1) NaB and MS-275 strongly decreased excessive alcohol intake of dependent rats in the operant ethanol self-administration paradigm but not of non-dependent rats; (2) NaB reduced excessive drinking and prevented the escalation of ethanol intake in the intermittent access to 20% ethanol paradigm; and (3) NaB completely blocked the increase of ethanol consumption induced by an alcohol deprivation, thus demonstrating a preventive effect of NaB on relapse. The mapping of cerebral histone H3 acetylation revealed a hyperacetylation in the amygdala and cortical areas in dependent rats. Interestingly, NaB did not exacerbate the hyperacetylation observed in these regions, but instead restored it, specifically in cortical areas. Altogether, our results clearly demonstrated the efficacy of NaB in preventing excessive ethanol intake and relapse and support the hypothesis that HDACi may have a potential use in alcohol addiction treatment.

[Epigenetics and drug addiction: a focus on MeCP2 and on histone acetylation]

Chronic drug exposure alters gene expression in the brain, which is believed to underlie compulsive drug seeking and drug taking behavior. Recent evidence shows that drug-induced long-term neuroadaptations in the brain are mediated in part by epigenetic mechanisms. By remodeling chromatin, this type of regulation contributes to drug-induced synaptic plasticity that translates into behavioral modifications. How drug-induced alterations in DNA methylation regulate gene expression is reviewed here, with a focus on MeCP2, a protein binding methylated DNA. The importance of histone modifications, especially acetylation is also discussed, with an emphasis on the effects of inhibitors of histone deacetylases on drug-induced behavioral changes. The precise identification of the epigenetic mechanisms that are under the control of drugs of abuse may help to uncover novel targets for the treatment of drug seeking and relapse.

Inhibition of histone deacetylases facilitates extinction and attenuates reinstatement of nicotine self-administration in rats

Chromatin remodelling is integral to the formation of long-term memories. Recent evidence suggests that histone modification may play a role in the persistence of memories associated with drug use. The present series of experiments aimed to examine the effect of histone deacetylase (HDAC) inhibition on the extinction and reinstatement of nicotine self-administration. Rats were trained to intravenously self-administer nicotine for 12 days on a fixed-ratio 1 schedule. In Experiment 1, responding was then extinguished through removal of nicotine and response-contingent cues. After each extinction session, the HDAC inhibitor, sodium butyrate (NaB), was administered immediately, or six hours after each session. In Experiment 2, response-contingent cues remained available across extinction to increase rates of responding during this phase, and NaB was administered immediately after the session. Finally, in Experiment 3, the effect of NaB treatment on extinction of responding for sucrose pellets was assessed. Across all experiments reinstatement to the cue and/or the reward itself was then tested. In the first experiment, treatment with NaB significantly attenuated nicotine and nicotine + cue reinstatement when administered immediately, but not six hours after each extinction session. When administered after cue-extinction (Expt. 2), NaB treatment specifically facilitated the rate of extinction across sessions, indicating that HDAC inhibition enhanced consolidation of the extinction memory. In contrast, there was no effect of NaB on the extinction and reinstatement of sucrose-seeking (Expt. 3), indicating that the observed effects are specific to a drug context. These results provide the first demonstration that HDAC inhibition facilitates the extinction of responding for an intravenously self-administered drug of abuse and further highlight the potential of HDAC inhibitors in the treatment of drug addiction.

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.

Vulnerability to opiate intake in maternally deprived rats: implication of MeCP2 and of histone acetylation

We previously showed that maternal deprivation predisposes male rats to anxiety, accompanied with an increase in their opiate consumption. In the present report, we searched for brain epigenetic mechanisms that possibly underlie this increase. For that, we examined the expression of the methyl-CpG-binding protein MeCP2 and of the histone deacetylases HDAC2 and HDAC3, as well as the acetylation status of histone H3 and H4 in mesolimbic structures of adult maternally deprived rats, using immunohistochemistry and Western blot analysis. A long-lasting increase in MeCP2 expression was found throughout the striatum of deprived rats. Enhanced HDAC2 expression and increased nuclear HDAC activity in the nucleus accumbens of deprived rats were associated with lower acetylation levels of histone H3 and H4. Treatment for 3 weeks with the HDAC inhibitor sodium valproate abolished HDAC activation together with the decrease in the acetylation levels of histone H4, and was accompanied with normalized oral morphine consumption. The data indicate that epigenetic mechanisms induced by early adverse environment memorize life experience to trigger greater opiate vulnerability during adult life. They suggest that sodium valproate may lessen vulnerability to opiate intake, particularly in subgroups of individuals subjected to adverse postnatal environments.

Nicotinic, glutamatergic and dopaminergic synaptic transmission and plasticity in the mesocorticolimbic system: focus on nicotine effects

Cigarette smoking is currently the leading cause of preventable deaths and disability throughout the world, being responsible for about five million premature deaths/year. Unfortunately, fewer than 10% of tobacco users who try to stop smoking actually manage to do so. The main addictive agent delivered by cigarette smoke is nicotine, which induces psychostimulation and reward, and reduces stress and anxiety. The use of new technologies (including optogenetics) and the development of mouse models characterised by cell-specific deletions of receptor subtype genes or the expression of gain-of-function nAChR subunits has greatly increased our understanding of the molecular mechanisms and neural substrates of nicotine addiction first revealed by classic electrophysiological, neurochemical and behavioural approaches. It is now becoming clear that various aspects of nicotine dependence are mediated by close interactions of the glutamatergic, dopaminergic and γ-aminobutyric acidergic systems in the mesocorticolimbic system. This review is divided into two parts. The first provides an updated overview of the circuitry of the ventral tegmental area, ventral striatum and prefrontal cortex, the neurotransmitter receptor subtypes expressed in these areas, and their physiological role in the mesocorticolimbic system. The second will focus on the molecular, functional and behavioural mechanisms involved in the acute and chronic effects of nicotine on the mesocorticolimbic system.

The histone deacetylase (HDAC) inhibitor valproic acid reduces ethanol consumption and ethanol-conditioned place preference in rats

Recent evidence suggests that epigenetic mechanisms such as chromatin modification (specifically histone acetylation) may play a crucial role in the development of addictive behavior. However, little is known about the role of epigenetic modifications in the rewarding properties of ethanol. In the current study, we studied the effects of systemic injection of the histone deacetylase (HDAC) inhibitor, valproic acid (VPA) on ethanol consumption and ethanol-elicited conditioned place preference (CPP). The effect of VPA (300 mg/kg) on voluntary ethanol intake and preference was assessed using continuous two-bottle choice procedure with escalating concentrations of alcohol (2.5-20% v/v escalating over 4 weeks). Taste sensitivity was studies using saccharin (sweet; 0.03% and 0.06%) and quinine (bitter; 20 µM and 40 µM) tastants solutions. Ethanol conditioned reward was investigated using an unbiased CPP model. Blood ethanol concentration (BEC) was also measured. Compared to vehicle, VPA-injected rats displayed significantly lower preference and consumption of ethanol in a two-bottle choice paradigm, with no significant difference observed with saccharin and quinine. More importantly, 0.5 g/kg ethanol-induced-CPP acquisition was blocked following VPA administration. Finally, vehicle- and VPA-treated mice had similar BECs. Taken together, our results implicated HDAC inhibition in the behavioral and reinforcement-related effects of alcohol and raise the question of whether specific drugs that target HDAC could potentially help to tackle alcoholism in humans.

Nicotine inhibits the proliferation by upregulation of nitric oxide and increased HDAC1 in mouse neural stem cells

Cigarette smoking (CS) is considered one of the major risk factors to cause neurodegenerative disorders. Nicotine is the main chemical in CS which is responsible for dysfunction of the brain as a neuroteratogen. Also, nicotine dependency is a real mental illness and disease. Recently, chronic nicotine exposure has been shown to cause oxidative/nitrosative stress leading to a deleterious condition to cellular death in different brain regions. However, little is known about the effects of nicotine on mouse neural stem cells (mNSCs). The aim of this study is to investigate the effects of nicotine on mNSCs and elucidate underlying mechanisms involved in expression of a diversity of genes regulated by nicotine. When mNSCs were isolated from the whole brain of embryonic day 16 mice treated with nicotine at vehicle, 100, 400, and 800 μM for 5 d, nicotine significantly decreased the number and size of neurospheres. In immunocytochemistry, nicotine-exposed mNSCs expressing nestin showed the shortened filaments and condensed nuclei. In RT-PCR, messenger RNA (mRNA) levels of proliferating cell nuclear antigen (PCNA) and sirtuin1 (SIRT1) were significantly decreased, while the production of nitric oxide and mRNA levels of cyclooxygenase2 (COX-2), tumor necrosis factor-alpha TNF-α, and histone deacetylase 1 (HDAC1) were increased in a dose-dependent manner. In addition, sodium butyrate and valproic acid, HDAC inhibitors, partially rescue proliferation of mNSCs via inhibition of HDAC1 expression and NO production. Taken together, these data demonstrate that prolonged exposure of nicotine decreased proliferation of mNSCs by increased NO and inflammatory cytokine through increased HDAC1. Furthermore, this study could help in the development of a therapy for nicotine-induced neurodegenerative disorder and drug abuse.

The epigenetic landscape of alcoholism

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

Whole-body pharmacokinetics of HDAC inhibitor drugs, butyric acid, valproic acid and 4-phenylbutyric acid measured with carbon-11 labeled analogs by PET

The fatty acids, n-butyric acid (BA), 4-phenylbutyric acid (PBA) and valproic acid (VPA, 2-propylpentanoic acid) have been used for many years in the treatment of a variety of CNS and peripheral organ diseases including cancer. New information that these drugs alter epigenetic processes through their inhibition of histone deacetylases (HDACs) has renewed interest in their biodistribution and pharmacokinetics and the relationship of these properties to their therapeutic and side effect profiles. In order to determine the pharmacokinetics and biodistribution of these drugs in primates, we synthesized their carbon-11 labeled analogues and performed dynamic positron emission tomography (PET) in six female baboons over 90 min. The carbon-11 labeled carboxylic acids were prepared by using (11)CO2 and the appropriate Grignard reagents. [(11)C]BA was metabolized rapidly (only 20% of the total carbon-11 in plasma was parent compound at 5 min post injection) whereas for VPA and PBA 98% and 85% of the radioactivity were the unmetabolized compound at 30 min after their administration respectively. The brain uptake of all three carboxylic acids was very low (<0.006%ID/cc, BA>VPA>PBA), which is consistent with the need for very high doses for therapeutic efficacy. Most of the radioactivity was excreted through the kidneys and accumulated in the bladder. However, the organ biodistribution between the drugs differed. [(11)C]BA showed relatively high uptake in spleen and pancreas whereas [(11)C]PBA showed high uptake in liver and heart. Notably, [(11)C]VPA showed exceptionally high heart uptake possibly due to its involvement in lipid metabolism. The unique biodistribution of each of these drugs may be of relevance in understanding their therapeutic and side effect profile including their teratogenic effects.

Behavioral and molecular analysis of nicotine-conditioned place preference in zebrafish

Studies using mice and rats have demonstrated that nicotine induces a conditioned place preference (CPP), with more effective results obtained by using biased procedures. Zebrafish have also been used as a model system to identify factors influencing nicotine-associated reward by using an unbiased design. Here, we report that zebrafish exhibited putative nicotine biased CPP to an initially aversive compartment (nicotine-paired group). A counterbalanced nicotine-exposed control group did not show a significant preference shift, providing evidence that the preference shift in the nicotine-paired group was not due to a reduction of aversion for this compartment. Zebrafish preference was corroborated by behavioral analysis of several indicators of drug preference, such as time spent in the drug-paired side, number of entries to the drug-paired side, and distance traveled. These results provided strong evidence that zebrafish may actually develop a preference for nicotine, although the drug was administrated in an aversive place for the fish, which was further supported by molecular studies. Reverse transcription-quantitative real-time PCR analysis depicted a significant increase in the expression of α7 and α6 but not α4 and β2 subunits of the nicotinic receptor in nicotine-paired zebrafish brains. In contrast, zebrafish brains from the counterbalanced nicotine group showed no significant changes. Moreover, CREB phosphorylation, an indicator of neural activity, accompanied the acquisition of nicotine-CPP. Our studies offered an incremental value to the drug addiction field, because they further describe behavioral features of CPP to nicotine in zebrafish. The results suggested that zebrafish exposed to nicotine in an unfriendly environment can develop a preference for that initially aversive place, which is likely due to the rewarding effect of nicotine. Therefore, this model can be used to screen exogenous and endogenous molecules involved in nicotine-associated reward in vertebrates.

The effect of previous exposure to nicotine on nicotine place preference

RATIONALE

Prior exposure to drugs of abuse may increase or decrease the reinforcing effects of the drug in later consumptions. Based on the initial locomotor activity (LA) response to an acute drug administration or to novelty in an open-field arena, animals can be classified as low or high LA responders (LR or HR). Few studies have used this classification with nicotine, and the results are controversial. Some authors suggested that nicotine can induce conditioned-place preference (CPP) following prior nicotine exposure, whereas others suggested that previous nicotine exposure extinguishes nicotine-CPP.

OBJECTIVE

To explore if the administration of nicotine in a novel environment without explicit behavioral consequences to classify animals in low and high nicotine responders (LNR and HNR) could affect the establishment of nicotine CPP in male Sprague-Dawley rats.

RESULTS

Prior exposure to a single dose of nicotine (0.4 mg/kg, subcutaneously) induced CPP in LNR rats after 14 days of conditioning (seven-trial) but not after two or eight conditioning days. In contrast, HNR rats did not show CPP under any condition. In addition, our results indicated that previous exposure to nicotine decreased its rewarding effects in eight conditioning days CPP (four-trial), which can be regularly established without prior exposure to nicotine.

CONCLUSION

The results suggested that response to a single exposure to nicotine predicts the acquisition of nicotine preference in a 14-day conditioning protocol only for LNR rats. Thus, our findings demonstrated the relevance of using LNR and HNR classification when the individual susceptibility to nicotine preference is studied.

The epigenetic effect of nicotine on dopamine D1 receptor expression in rat prefrontal cortex

Nicotine is a highly addictive drug and exerts its effect partially through causing dopamine release, thereby increasing intrasynaptic dopamine levels in the brain reward systems. Dopaine D1 receptor (DRD1) mRNAs and receptors are localized in reward-related brain regions, which receive cholinergic input. The aim of this study is to evaluate whether nicotine administration affects the expression of DRD1s, and if so, whether epigenetic mechanisms, such as histone acetylation, are involved. Twenty Male Sprague Dawley rats received nicotine (0.4 mg/kg/day, s.c.) or saline injections for 15 days. After nicotine/saline treatment, rats were perfused with saline; prefrontal cortex (PFC), corpus striatum (STR), and ventral tegmental area (VTA) were dissected. Homogenates were divided into two parts for total RNA isolation and histone H4 acetylation studies. DRD1 mRNA expression was significantly higher in the PFC of the nicotine-treated group compared with controls; similar trends were observed in the VTA and STR. To study epigenetic regulation, the 2kb upstream region of the DRD1 gene promoter was investigated for histone H4 acetylation in PFC samples. After chromatin immunoprecipitation with anti-acetyl histone H4 antibody, we found an increase in histone acetylation by two different primer pairs which amplified the -1365 to -1202 (P < 0.005) and -170 to +12 (P < 0.05) upstream regions of the DRD1 promoter. Our results suggest that intermittent subcutaneous nicotine administration increases the expression of DRD1 mRNA in the PFC of rats, and this increase may be due to changes in histone H4 acetylation of the 2kb promoter of the DRD1 gene.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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

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

Chromatin remodeling--a novel strategy to control excessive alcohol drinking

Harmful excessive use of alcohol has a severe impact on society and it remains one of the major causes of morbidity and mortality in the population. However, mechanisms that underlie excessive alcohol consumption are still poorly understood, and thus available medications for alcohol use disorders are limited. Here, we report that changing the level of chromatin condensation by affecting DNA methylation or histone acetylation limits excessive alcohol drinking and seeking behaviors in rodents. Specifically, we show that decreasing DNA methylation by inhibiting the activity of DNA methyltransferase (DNMT) with systemic administration of the FDA-approved drug, 5-azacitidine (5-AzaC) prevents excessive alcohol use in mice. Similarly, we find that increasing histone acetylation via systemic treatment with several histone deacetylase (HDAC) inhibitors reduces mice binge-like alcohol drinking. We further report that systemic administration of the FDA-approved HDAC inhibitor, SAHA, inhibits the motivation of rats to seek alcohol. Importantly, the actions of both DNMT and HDAC inhibitors are specific for alcohol, as no changes in saccharin or sucrose intake were observed. In line with these behavioral findings, we demonstrate that excessive alcohol drinking increases DNMT1 levels and reduces histone H4 acetylation in the nucleus accumbens (NAc) of rodents. Together, our findings illustrate that DNA methylation and histone acetylation control the level of excessive alcohol drinking and seeking behaviors in preclinical rodent models. Our study therefore highlights the possibility that DNMT and HDAC inhibitors can be used to treat harmful alcohol abuse.

Overexpression of cyclic GMP-dependent protein kinase reduces MeCP2 and HDAC2 expression

Nitric oxide (NO) and the C-type natriuretic peptide (CNP) exert their action via stimulation of the cyclic GMP (cGMP)-signaling pathway, which includes the activation of cGMP-dependent protein kinases (PKG). The present report shows that the activation of PKG by local application of 8-bromo-cGMP in the caudate-putamen reduced the expression of the epigenetic markers, methyl-CpG-binding protein 2 (MeCP2) and histone deacetylase 2 (HDAC2), in dopaminergic projection areas of cocaine-treated rats. An effect of lesser amplitude was observed when rats were not injected with cocaine. We also studied the effect of PKG overexpression by injecting a plasmid vector containing the human PKG-Iα cDNA in either the caudate-putamen or the ventral tegmental area. Injection in the caudate-putamen reduced the epigenetic parameters with higher amplitude than the cGMP analog. The effect was abolished by the injection of a selective PKG inhibitor, confirming that it was due to PKG-dependent phosphorylation. As MeCP2 and HDAC2 modulate dynamic functions in the adult brain such as memory formation and synaptic plasticity, the downregulation of expression by PKG suggests that the cGMP pathway affects cognitive processes through a mechanism that comprises the MeCP2/HDAC2 complex and the subsequent control of gene silencing.

Development of novel pharmacotherapeutics for tobacco dependence: progress and future directions

INTRODUCTION

The vast majority of tobacco smokers seeking to quit will relapse within the first month of abstinence. Currently available smoking cessation agents have limited utility in increasing rates of smoking cessation and in some cases there are notable safety concerns related to their use. Hence, there is a pressing need to develop safer and more efficacious smoking cessation medications.

METHODS

Here, we provide an overview of current efforts to develop new pharmacotherapeutic agents to facilitate smoking cessation, identified from ongoing clinical trials and published reports.

RESULTS

Nicotine is considered the major addictive agent in tobacco smoke, and the vast majority of currently available smoking cessation agents act by modulating nicotinic acetylcholine receptor (nAChR) signaling. Accordingly, there is much effort directed toward developing novel small molecule therapeutics and biological agents such as nicotine vaccines for smoking cessation that act by modulating nAChR activity. Our increasing knowledge of the neurobiology of nicotine addiction has revealed new targets for novel smoking cessation therapeutics. Indeed, we highlight many examples of novel small molecule drug development around non-nAChR targets. Finally, there is a growing appreciation that medications already approved for other disease indications could show promise as smoking cessation agents, and we consider examples of such repurposing efforts.

CONCLUSION

Ongoing clinical assessment of potential smoking cessation agents offers the promise of new effective medications. Nevertheless, much of our current knowledge of molecular mechanisms of nicotine addiction derived from preclinical studies has not yet been leveraged for medications development.

Methylation, memory and addiction

Dynamic chromatin remodeling is at the heart of most biological processes including gene transcription, DNA replication and repair, cell differentiation and apoptosis. Chromatin remodeling as a result of covalent histone modifications, including histone acetylation, methylation or SUMOylation, play important roles in these processes. Similarly, direct chemical modification of DNA, most notably DNA methylation, also plays a key role in controlling gene expression and basic aspects of cell biology. Memory, one of the most fundamental of all brain functions, is a complex process involving diverse cellular signaling cascades and coordinated regulation of entire networks of genes. Synaptic plasticity, which is defined as activity-dependent changes in synaptic strength between neurons, provides the cellular basis of memory. The role for covalent histone modifications in synaptic plasticity and in learning and memory has been now been firmly established. In contrast, much less had been known concerning DNA methylation in memory formation and storage. Emerging evidence now suggests that DNA methylation plays a central role in these processes, likely by directly influencing the expression of genes involved in synaptic plasticity.