Epigenetic mechanisms underlying extinction of memory and drug-seeking behavior

Transcriptomic signature of extinction learning in the brain of the fire-bellied toad, Bombina orientalis

Insight into the molecular and cellular mechanisms of learning and memory from a diverse array of taxa contributes to our understanding of the evolution of these processes. The fire-bellied toad, Bombina orientalis, is a basal anuran amphibian model species who could help us describe shared and divergent characteristics of learning and memory mechanisms between amphibians and other vertebrates, and hence answer questions about the evolution of learning. Utilizing next generation sequencing techniques, we profiled gene expression patterns associated with the extinction of prey-catching conditioning in the brain of the fire-bellied toad. For this purpose, gene expression was at first compared between toads sacrificed after acquisition and extinction of the conditioned response. A second comparison was done between toads submitted to extinction following either short or long acquisition training, which results in toads displaying response extinction or resistance to extinction, respectively. We analyzed brain tissue transcription profiles common to both acquisition and extinction learning, or unique to extinction learning and resistance to extinction, and found significant overlap in gene expression related to molecular pathways involving neuronal plasticity (e.g. structural modification, transcription). However, extinction learning induced a unique GABAergic transcriptomic signal, which may be responsible for suppression of the original response memory. Further, when comparing extinction learning in short- and long-trained groups, short training engaged many pathways related to neuronal plasticity, as expected, but long training engaged molecular pathways related to the suppression of learning through epigenetic mediated transcriptional suppression and inhibitory neurotransmission. Overall, gene expression patterns associated with extinction learning in the fire-bellied toad were similar to those found in mammals submitted to extinction, although some divergent profiles highlighted potential differences in the mechanisms of learning and memory among tetrapods.

Nonhuman animal models of substance use disorders: Translational value and utility to basic science

BACKGROUND

The National Institute on Drug Abuse (NIDA) recently released a Request for Information (RFI) soliciting comments on nonhuman animal models of substance use disorders (SUD).

METHODS

A literature review was performed to address the four topics outlined in the RFI and one topic inspired by the RFI: (1) animal models that best recapitulate SUD, (2) animal models that best balance the trade-offs between resources and ecological validity, (3) animal models whose translational value are frequently misrepresented or overrepresented by the scientific community, (4) aspects of SUD that are not currently being modeled in animals, and (5) animal models that are optimal for examining the basic mechanisms by which drugs produce their abuse-related effects.

RESULTS

Models that employ response-contingent drug administration, use complex schedules of reinforcement, measure behaviors that mimic the distinguishing features of SUD, and use animals that are phylogenetically similar to humans have the greatest translational value. Models that produce stable and reproducible baselines of behavior, lessen the number of uncontrolled variables, and minimize the influence of extraneous factors are best at examining basic mechanisms contributing to drug reward and reinforcement.

CONCLUSIONS

Nonhuman animal models of SUD have undergone significant refinements to increase their utility for basic science and translational value for SUD. The existing literature describes numerous examples of how these models may best be utilized to answer mechanistic questions of drug reward and identify potential therapeutic interventions for SUD. Progress in the field could be accelerated by further collaborations between researchers using animals versus humans.

BDNF rescues BAF53b-dependent synaptic plasticity and cocaine-associated memory in the nucleus accumbens

Recent evidence implicates epigenetic mechanisms in drug-associated memory processes. However, a possible role for one major epigenetic mechanism, nucleosome remodelling, in drug-associated memories remains largely unexplored. Here we examine mice with genetic manipulations targeting a neuron-specific nucleosome remodelling complex subunit, BAF53b. These mice display deficits in cocaine-associated memory that are more severe in BAF53b transgenic mice compared with BAF53b heterozygous mice. Similar to the memory deficits, theta-induced long-term potentiation (theta-LTP) in the nucleus accumbens (NAc) is significantly impaired in slices taken from BAF53b transgenic mice but not heterozygous mice. Further experiments indicate that theta-LTP in the NAc is dependent on TrkB receptor activation, and that BDNF rescues theta-LTP and cocaine-associated memory deficits in BAF53b transgenic mice. Together, these results suggest a role for BAF53b in NAc neuronal function required for cocaine-associated memories, and also that BDNF/TrkB activation in the NAc may overcome memory and plasticity deficits linked to BAF53b mutations.

Sensory Cortical Plasticity Participates in the Epigenetic Regulation of Robust Memory Formation

Neuroplasticity remodels sensory cortex across the lifespan. A function of adult sensory cortical plasticity may be capturing available information during perception for memory formation. The degree of experience-dependent remodeling in sensory cortex appears to determine memory strength and specificity for important sensory signals. A key open question is how plasticity is engaged to induce different degrees of sensory cortical remodeling. Neural plasticity for long-term memory requires the expression of genes underlying stable changes in neuronal function, structure, connectivity, and, ultimately, behavior. Lasting changes in transcriptional activity may depend on epigenetic mechanisms; some of the best studied in behavioral neuroscience are DNA methylation and histone acetylation and deacetylation, which, respectively, promote and repress gene expression. One purpose of this review is to propose epigenetic regulation of sensory cortical remodeling as a mechanism enabling the transformation of significant information from experiences into content-rich memories of those experiences. Recent evidence suggests how epigenetic mechanisms regulate highly specific reorganization of sensory cortical representations that establish a widespread network for memory. Thus, epigenetic mechanisms could initiate events to establish exceptionally persistent and robust memories at a systems-wide level by engaging sensory cortical plasticity for gating what and how much information becomes encoded.

Histone-mediated epigenetics in addiction

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

Epigenetic mechanisms are involved in the regulation of ethanol consumption in mice

BACKGROUND

Repeated alcohol exposure is known to increase subsequent ethanol consumption in mice. However, the underlying mechanisms have not been fully elucidated. One postulated mechanism involves epigenetic modifications, including histone modifications and DNA methylation of relevant genes such as NR2B or BDNF.

METHODS

To investigate the role of epigenetic mechanisms in the development of alcohol drinking behavior, an established chronic intermittent ethanol exposure reinforced ethanol drinking mouse model with vapor inhalation over two 9-day treatment regimens was used. The DNA methyltransferase inhibitor, 5-azacytidine or the histone deacetylase inhibitor, Trichostatin A was administered (intraperitoneally) to C57BL/6 mice 30 min before daily exposure to chronic intermittent ethanol. Changes in ethanol consumption were measured using the 2-bottle choice test.

RESULTS

The results indicated that systemic administration of Trichostatin A (2.5 µg/g) facilitated chronic intermittent ethanol-induced ethanol drinking, but systemic administration of 5-azacytidine (2 µg/g) did not cause the same effect. However, when 5-azacytidine was administered by intracerebroventricular injection, it facilitated chronic intermittent ethanol-induced ethanol drinking. Furthermore, the increased drinking caused by chronic intermittent ethanol was prevented by injection of a methyl donor, S-adenosyl-L-methionine. To provide evidence that chronic intermittent ethanol- or Trichostatin A-induced DNA demethylation and histone modifications of the NR2B promoter may underlie the altered ethanol consumption, we examined epigenetic modifications and NR2B expression in the prefrontal cortex of these mice. Chronic intermittent ethanol or Trichostatin A decreased DNA methylation and increased histone acetylation in the NR2B gene promoter, as well as mRNA levels of NR2B in these mice.

CONCLUSIONS

Taken together, these results indicate that epigenetic modifications are involved in regulating ethanol drinking behavior, partially through altering NR2B expression.

Epigenetic mechanisms of memory formation and reconsolidation

Memory consolidation involves transcriptional control of genes in neurons to stabilize a newly formed memory. Following retrieval, a once consolidated memory destabilizes and again requires gene transcription changes in order to restabilize, a process referred to as reconsolidation. Understanding the molecular mechanisms of gene transcription during the consolidation and reconsolidation processes could provide crucial insights into normal memory formation and memory dysfunction associated with psychiatric disorders. In the past decade, modifications of epigenetic markers such as DNA methylation and posttranslational modifications of histone proteins have emerged as critical transcriptional regulators of gene expression during initial memory formation and after retrieval. In light of the rapidly growing literature in this exciting area of research, we here examine the most recent and latest evidence demonstrating how memory acquisition and retrieval trigger epigenetic changes during the consolidation and reconsolidation phases to impact behavior. In particular we focus on the reconsolidation process, where we discuss the already identified epigenetic regulators of gene transcription during memory reconsolidation, while exploring other potential epigenetic modifications that may also be involved, and expand on how these epigenetic modifications may be precisely and temporally controlled by important signaling cascades critical to the reconsolidation process. Finally, we explore the possibility that epigenetic mechanisms may serve to regulate a system or circuit level reconsolidation process and may be involved in retrieval-dependent memory updating. Hence, we propose that epigenetic mechanisms coordinate changes in neuronal gene transcription, not only during the initial memory consolidation phase, but are triggered by retrieval to regulate molecular and cellular processes during memory reconsolidation.

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.

Methamphetamine-associated memory is regulated by a writer and an eraser of permissive histone methylation

BACKGROUND

Memories associated with drugs of abuse, such as methamphetamine (METH), increase relapse vulnerability to substance use disorder by triggering craving. The nucleus accumbens (NAc) is essential to these drug-associated memories, but underlying mechanisms are poorly understood. Posttranslational chromatin modifications, such as histone methylation, modulate gene transcription; thus, we investigated the role of the associated epigenetic modifiers in METH-associated memory.

METHODS

Conditioned place preference was used to assess the epigenetic landscape in the NAc supporting METH-associated memory (n = 79). The impact of histone methylation (H3K4me2/3) on the formation and expression of METH-associated memory was determined by focal, intra-NAc knockdown (KD) of a writer, the methyltransferase mixed-lineage leukemia 1 (Mll1) (n = 26), and an eraser, the histone lysine (K)-specific demethylase 5C (Kdm5c) (n = 38), of H3K4me2/3.

RESULTS

A survey of chromatin modifications in the NAc of animals forming a METH-associated memory revealed the global induction of several modifications associated with active transcription. This correlated with a pattern of gene activation, as revealed by microarray analysis, including upregulation of oxytocin receptor (Oxtr) and FBJ osteosarcoma oncogene (Fos), the promoters of which also had increased H3K4me3. KD of Mll1 reduced H3K4me3, Fos and Oxtr levels and disrupted METH-associated memory. KD of Kdm5c resulted in hypermethylation of H3K4 and prevented the expression of METH-associated memory.

CONCLUSIONS

The development and expression of METH-associated memory are supported by regulation of H3K4me2/3 levels by MLL1 and KDM5C, respectively, in the NAc. These data indicate that permissive histone methylation, and the associated epigenetic writers and erasers, represent potential targets for the treatment of substance abuse relapse, a psychiatric condition perpetuated by unwanted associative memories.

Unique functional roles for class I and class II histone deacetylases in central nervous system development and function

Non-specific pharmacological inhibition of the histone deacetylase (HDAC) family of enzymes has largely beneficial effects in a variety of diverse contexts including cancer, cognitive function, and neurodegeneration. This review will discuss the role of individual HDAC isoforms in brain function during development and in the adult. Importantly class I and class II HDACs exhibit distinct cellular and subcellular expression patterns and utilize different signaling pathways to influence their substrates. Moreover, dissociable phenotypic outcomes emerge following manipulation of individual HDACs in the brain. To date, pharmacological inhibitors capable of targeting individual HDACs have proven difficult to develop, an obstacle that must be overcome to unlock the substantial clinical promise of manipulating endogenous HDAC isoforms in the central nervous system.

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.

The role of histone acetylation in cocaine-induced neural plasticity and behavior

How do drugs of abuse, such as cocaine, cause stable changes in neural plasticity that in turn drive long-term changes in behavior? What kind of mechanism can underlie such stable changes in neural plasticity? One prime candidate mechanism is epigenetic mechanisms of chromatin regulation. Chromatin regulation has been shown to generate short-term and long-term molecular memory within an individual cell. They have also been shown to underlie cell fate decisions (or cellular memory). Now, there is accumulating evidence that in the CNS, these same mechanisms may be pivotal for drug-induced changes in gene expression and ultimately long-term behavioral changes. As these mechanisms are also being found to be fundamental for learning and memory, an exciting new possibility is the extinction of drug-seeking behavior by manipulation of epigenetic mechanisms. In this review, we critically discuss the evidence demonstrating a key role for chromatin regulation via histone acetylation in cocaine action.

Effects of sodium butyrate on methamphetamine-sensitized locomotor activity

Neuroadaptations associated with behavioral sensitization induced by repeated exposure to methamphetamine (MA) appear to be involved in compulsive drug pursuit and use. Increased histone acetylation, an epigenetic effect resulting in altered gene expression, may promote sensitized responses to psychostimulants. The role of histone acetylation in the expression and acquisition of MA-induced locomotor sensitization was examined by measuring the effect of histone deacetylase inhibition by sodium butyrate (NaB). For the effect on expression, mice were treated repeatedly with MA (10 days of 2mg/kg MA) or saline (10 days), and then vehicle or NaB (630 mg/kg, intraperitoneally) was administered 30 min prior to MA challenge and locomotor response was measured. NaB treatment increased the locomotor response to MA in both acutely MA treated and sensitized animals. For acquisition, NaB was administered 30 min prior to each MA exposure (10 days of 1 or 2mg/kg), but not prior to the MA challenge test. Treatment with NaB during the sensitization acquisition period significantly increased locomotor activation by MA in sensitized mice only. NaB alone did not significantly alter locomotor activity. Acute NaB or MA, but not the combination, increased striatal acetylation at histone H4. Repeated treatment with MA, but not NaB or MA plus NaB, increased striatal acetylation at histone H3. Although increased histone acetylation may alter the expression of genes involved in acute locomotor response to MA and in the acquisition of MA-induced sensitization, results for acetylation at H3 and H4 showed little correspondence with behavior.

Age-related memory impairment is associated with disrupted multivariate epigenetic coordination in the hippocampus

Mounting evidence linking epigenetic regulation to memory-related synaptic plasticity raises the possibility that altered chromatin modification dynamics might contribute to age-dependent cognitive decline. Here we show that the coordinated orchestration of both baseline and experience-dependent epigenetic regulation seen in the young adult hippocampus is lost in association with cognitive aging. Using a well-characterized rat model that reliably distinguishes aged individuals with significant memory impairment from others with normal memory, no single epigenetic mark or experience-dependent modification in the hippocampus uniquely predicted differences in the cognitive outcome of aging. The results instead point to a multivariate pattern in which modification-specific, bidirectional chromatin regulation is dependent on recent behavioral experience, chronological age, cognitive status, and hippocampal region. Whereas many epigenetic signatures were coupled with memory capacity among young adults and aged rats with preserved cognitive function, such associations were absent among aged rats with deficits in hippocampal memory. By comparison with the emphasis in current preclinical translational research on promoting chromatin modifications permissive for gene expression, our findings suggest that optimally successful hippocampal aging may hinge instead on enabling coordinated control across the epigenetic landscape.

Satb1 ablation alters temporal expression of immediate early genes and reduces dendritic spine density during postnatal brain development

Complex behaviors, such as learning and memory, are associated with rapid changes in gene expression of neurons and subsequent formation of new synaptic connections. However, how external signals are processed to drive specific changes in gene expression is largely unknown. We found that the genome organizer protein Satb1 is highly expressed in mature neurons, primarily in the cerebral cortex, dentate hilus, and amygdala. In Satb1-null mice, cortical layer morphology was normal. However, in postnatal Satb1-null cortical pyramidal neurons, we found a substantial decrease in the density of dendritic spines, which play critical roles in synaptic transmission and plasticity. Further, we found that in the cerebral cortex, Satb1 binds to genomic loci of multiple immediate early genes (IEGs) (Fos, Fosb, Egr1, Egr2, Arc, and Bdnf) and other key neuronal genes, many of which have been implicated in synaptic plasticity. Loss of Satb1 resulted in greatly alters timing and expression levels of these IEGs during early postnatal cerebral cortical development and also upon stimulation in cortical organotypic cultures. These data indicate that Satb1 is required for proper temporal dynamics of IEG expression. Based on these findings, we propose that Satb1 plays a critical role in cortical neurons to facilitate neuronal plasticity.

How does the social environment 'get into the mind'? Epigenetics at the intersection of social and psychiatric epidemiology

The social environment plays a considerable role in determining major psychiatric disorders. Emerging evidence suggests that features of the social environment modify gene expression independently of the primary DNA sequence through epigenetic processes. Accordingly, dysfunction of epigenetic mechanisms offers a plausible mechanism by which an adverse social environment gets "into the mind" and results in poor mental health. The purpose of this review is to provide an overview of the studies suggesting that epigenetic changes introduced by the social environment then manifest as psychological consequences. Our goal is to build a platform to discuss the ways in which future epidemiologic studies may benefit from including epigenetic measures. We focus on schizophrenia, major depressive disorder, post-traumatic stress disorder, anorexia nervosa, and substance dependence as examples that highlight the ways in which social environmental exposures, mediated through epigenetic processes, affect mental health.

Dopamine D1 receptor antagonism impairs extinction of cocaine-cue memories

Increasingly, research suggests a role for dopamine D1 receptors in the consolidation of extinction of both appetitive and aversive memories. However, a role for D1 receptors in extinction of memories involving drug reward has yet to be established. Here we show that post-retrieval, but not delayed, systemic administration of the D1 receptor antagonist SCH23390 results in prolonged extinction of cocaine conditioned place preference (CPP), suggesting a critical role for D1 receptors in the consolidation of extinction of cocaine-cue memories.

Histone deacetylases govern cellular mechanisms underlying behavioral and synaptic plasticity in the developing and adult brain

Histone deacetylases (HDACs) are a family of enzymes that alter gene expression patterns by modifying chromatin architecture. There are 11 mammalian HDACs that are classified by homology into four subfamilies, all with distinct expression patterns in the brain. Through the use of pharmacological HDAC inhibitors, and more recently HDAC knockout mice, the role of these enzymes in the central nervous system are starting to be elucidated. We will discuss the latest findings on the specific or redundant roles of individual HDACs in the brain as well as the impact of HDAC function on complex behavior, with a focus on learning, memory formation, and affective behavior. Potential HDAC-mediated cellular mechanisms underlying those behaviors are discussed.