Phospho-acetylation of histone H3 in the amygdala after acute lithium chloride

Disruption of relapse to cocaine and morphine seeking by LiCl-induced aversive counterconditioning following memory retrieval

Substance use disorder is conceptualized as a form of maladaptive learning, whereby drug-associated memories, elicited by the presence of stimuli related to drug contexts or cues, contribute to the persistent recurrence of craving and the reinstatement of drug-seeking behavior. Hence, use of pharmacology or non-pharmacology way to disrupt drug-related memory holds promise to prevent relapse. Several studies have shown that memories can be unstable and susceptible to modification during the retrieval reactivation phase, termed the "reconsolidation time window". In this study, we use the classical conditioned place preference (CPP) model to investigate the role of aversive counterconditioning on drug-related memories during reconsolidation. Specifically, we uncovered that reconditioning drug cues through counterconditioning with LiCl-induced aversive outcomes following drug memory retrieval reduces subsequent drug-seeking behavior. Notably, the recall of cocaine- or morphine-CPP was eliminated when LiCl-induced aversive counterconditioning was performed 10 min, but not 6 h (outside the reconsolidation time window) after cocaine or morphine memory retrieval. In addition, the effect of LiCl-induced aversive counterconditioning could last for about 14 days. These results suggest that aversive counterconditioning during the reconsolidation of cocaine or morphine memory can prevent the re-seeking of cocaine or morphine, presumably by updating or replacing cocaine or morphine memories with aversive information.

Sodium butyrate does not protect spinal motor neurons from AMPA-induced excitotoxic degeneration in vivo

Motor neuron (MN) loss is the primary pathological hallmark of amyotrophic lateral sclerosis (ALS). Histone deacetylase 4 (HDAC4) is one of several factors involved in nerve-muscle communication during MN loss, hindering muscle reinnervation, as shown in humans and in animal models of ALS, and may explain the differential progression observed in patients with ALS - rapid versus slow progression. In this work, we inhibited HDAC4 activity through the administration of a pan-histone deacetylase inhibitor, sodium butyrate, in an in vivo model of chronic spinal MN death induced by AMPA-mediated excitotoxicity. We infused AMPA into the spinal cord at low and high doses, which mimic the rapid and slow progression observed in humans, respectively. We found that muscle HDAC4 expression was increased by high-dose infusion of AMPA. Treatment of animals with sodium butyrate further decreased expression of muscle HDAC4, although non-significantly, and did not prevent the paralysis or the MN loss induced by AMPA infusion. These results inform on the role of muscle HDAC4 in MN degeneration in vivo and provide insights for the search for more suitable therapeutic strategies.

Epigenetic mechanisms regulate cue memory underlying discriminative behavior

The burgeoning field of neuroepigenetics has introduced chromatin modification as an important interface between experience and brain function. For example, epigenetic mechanisms like histone acetylation and DNA methylation operate throughout a lifetime to powerfully regulate gene expression in the brain that is required for experiences to be transformed into long-term memories. This review highlights emerging evidence from sensory models of memory that converge on the premise that epigenetic regulation of activity-dependent transcription in the sensory brain facilitates highly precise memory recall. Chromatin modifications may be key for neurophysiological responses to transient sensory cue features experienced in the "here and now" to be recapitulated over the long term. We conclude that the function of epigenetic control of sensory system neuroplasticity is to regulate the amount and type of sensory information retained in long-term memories by regulating neural representations of behaviorally relevant cues that guide behavior. This is of broad importance in the neuroscience field because there are few circumstances in which behavioral acts are devoid of an initiating sensory experience.

Behavior and Fos activation reveal that male and female rats differentially assess affective valence during CTA learning and expression

Females are more affected by psychiatric illnesses including eating disorders, depression, and post-traumatic stress disorder than males. However, the neural mechanisms mediating these sex differences are poorly understood. Animal models can be useful in exploring such neural mechanisms. Conditioned taste aversion (CTA) is a behavioral task that assesses how animals process the competition between associated reinforcing and aversive stimuli in subsequent task performance, a process critical to healthy behavior in many domains. The purpose of the present study was to identify sex differences in this behavior and associated neural responses. We hypothesized that females would value the rewarding stimulus (Boost®) relative to the aversive stimulus (LiCl) more than males in performing CTA. We evaluated behavior (Boost® intake, LiCl-induced behaviors, ultrasonic vocalizations (USVs), CTA performance) and Fos activation in relevant brain regions after the acute stimuli [acute Boost® (AB), acute LiCl (AL)] and the context-only task control (COT), Boost® only task (BOT) and Boost®-LiCl task (BLT). Acutely, females drank more Boost® than males but showed similar aversive behaviors after LiCl. Females and males performed CTA similarly. Both sexes produced 55 kHz USVs anticipating BOT and inhibited these calls in the BLT. However, more females emitted both 22 kHz and 55 kHz USVs in the BLT than males: the latter correlated with less CTA. Estrous cycle stage also influenced 55 kHz USVs. Fos responses were similar in males and females after AB or AL. Females engaged the gustatory cortex and ventral tegmental area (VTA) more than males during the BOT and males engaged the amygdala more than females in both the BOT and BLT. Network analysis of correlated Fos responses across brain regions identified two unique networks characterizing the BOT and BLT, in both of which the VTA played a central role. In situ hybridization with RNAscope identified a population of D1-receptor expressing cells in the CeA that responded to Boost® and D2 receptor-expressing cells that responded to LiCl. The present study suggests that males and females differentially process the affective valence of a stimulus to produce the same goal-directed behavior.

Mini review: Recent advances on epigenetic effects of lithium

Lithium (Li) remains the first line long-term treatment of bipolar disorders notwithstanding a high inter-individual variability of response. Significant research effort has been undertaken to understand the molecular mechanisms underlying Li cellular and clinical effects in order to identify predictive biomarkers of response. Li response has been shown to be partly heritable, however mechanisms that do not rely on DNA variants could also be involved. In recent years, modulation of epigenetic marks in relation with the level of Li response has appeared increasingly plausible. Recent results in this field of research have provided new insights into the molecular processes involved in Li effects. In this review, we examined the literature investigating the involvement of three epigenetic mechanisms (DNA methylation, noncoding RNAs and histone modifications) in Li clinical efficacy in bipolar disorder.

Implications of Gut-Brain axis in the pathogenesis of Psychiatric disorders

<abstract> <p>Psychiatric disorders may extremely impair the quality of life with patients and are important reasons of social disability. Several data have shown that psychiatric disorders are associated with an altered composition of gut microbiota. Dietary intake could determine the microbiota, which contribute to produce various metabolites of fermentation such as short chain fatty acids. Some of the metabolites could result in epigenetic alterations leading to the disease susceptibility. Epigenetic dysfunction is in fact implicated in various psychiatric and neurologic disorders. For example, it has been shown that neuroepigenetic dysregulation occurs in psychiatric disorders including schizophrenia. Several studies have demonstrated that the intestinal microbiome may influence the function of central nervous system. Furthermore, it has been proved that the alterations in the gut microbiota-composition might affect in the bidirectional communication between gut and brain. Similarly, evidences demonstrating the association between psychiatric disorders and the gut microbiota have come from preclinical studies. It is clear that an intricate symbiotic relationship might exist between host and microbe, although the practical significance of the gut microbiota has not yet to be determined. In this review, we have summarized the function of gut microbiota in main psychiatric disorders with respect to the mental health. In addition, we would like to discuss the potential mechanisms of the disorders for the practical diagnosis and future treatment by using bioengineering of microbiota and their metabolites.</p> </abstract>

In vivo human brain expression of histone deacetylases in bipolar disorder

The etiology of bipolar disorder (BD) is unknown and the neurobiological underpinnings are not fully understood. Both genetic and environmental factors contribute to the risk of BD, which may be linked through epigenetic mechanisms, including those regulated by histone deacetylase (HDAC) enzymes. This study measures in vivo HDAC expression in individuals with BD for the first time using the HDAC-specific radiotracer [¹¹C]Martinostat. Eleven participants with BD and 11 age- and sex-matched control participants (CON) completed a simultaneous magnetic resonance - positron emission tomography (MR-PET) scan with [¹¹C]Martinostat. Lower [¹¹C]Martinostat uptake was found in the right amygdala of BD compared to CON. We assessed uptake in the dorsolateral prefrontal cortex (DLPFC) to compare previous findings of lower uptake in the DLPFC in schizophrenia and found no group differences in BD. Exploratory whole-brain voxelwise analysis showed lower [¹¹C]Martinostat uptake in the bilateral thalamus, orbitofrontal cortex, right hippocampus, and right amygdala in BD compared to CON. Furthermore, regional [¹¹C]Martinostat uptake was associated with emotion regulation in BD in fronto-limbic areas, which aligns with findings from previous structural, functional, and molecular neuroimaging studies in BD. Regional [¹¹C]Martinostat uptake was associated with attention in BD in fronto-parietal and temporal regions. These findings indicate a potential role of HDACs in BD pathophysiology. In particular, HDAC expression levels may modulate attention and emotion regulation, which represent two core clinical features of BD.

Lithium acts to modulate abnormalities at behavioral, cellular, and molecular levels in sleep deprivation-induced mania-like behavior

BACKGROUND

Ample amount of data suggests role of rapid eye movement (REM) sleep deprivation as the cause and effect of mania. Studies have also suggested disrupted circadian rhythms contributing to the pathophysiology of mood disorders, including bipolar disorder. However, studies pertaining to circadian genes and effect of lithium treatment on clock genes are scant. Thus, we wanted to determine the effects of REM sleep deprivation on expression of core clock genes and determine whether epigenetics is involved. Next, we wanted to explore ultrastructural abnormalities in the hippocampus. Moreover, we were interested to determine oxidative stress, tumor necrosis factor-α (TNF-α), and brain-derived neurotrophic factor levels in the central and peripheral systems.

METHODS

Rats were sleep deprived by the flower pot method and were then analyzed for various behaviors and biochemical tests. Lithium was supplemented in diet.

RESULTS

We found that REM sleep deprivation resulted in hyperactivity, reduction in anxiety-like behavior, and abnormal dyadic social interaction. Some of these behaviors were sensitive to lithium. REM sleep deprivation also altered circadian gene expression and caused significant imbalance between histone acetyl transferase/histone deacetylase (HAT/HDAC) activity. Ultrastructural analysis revealed various cellular abnormalities. Lipid peroxidation and increased TNF-α levels suggested oxidative stress and ongoing inflammation. Circadian clock genes were differentially modulated with lithium treatment and HAT/HDAC imbalance was partially prevented. Moreover, lithium treatment prevented myelin fragmentation, disrupted vasculature, necrosis, inflammation, and lipid peroxidation, and partially prevented mitochondrial damage and apoptosis.

CONCLUSIONS

Taken together, these results suggest plethora of abnormalities in the brain following REM sleep deprivation, many of these changes in the brain may be target of lithium's mechanism of action.

General Anesthesia and Young Brain: What is New?

Considering that growing population of very young children is exposed to general anesthesia every year, it is of utmost importance to understand how and whether such practice may affect the development and growth of their very immature and vulnerable brains. Compelling evidence from animal studies suggests that an early exposure to general anesthesia is detrimental to normal brain development leading to structural and functional impairments of neurons and glia, and long-lasting impairments in normal emotional and cognitive development. Although the evidence from animal studies is overwhelming and confirmed across species examined from rodents to non-human primates, the evidence from human studies is inconsistent and not conclusive at present. In this review we focus on new developments in animal studies of anesthesia-induced developmental neurotoxicity and summarize recent clinical studies while focusing on outcome measures and exposure variables in terms of their utility for assessing cognitive and behavioral development in children.

General Anesthesia Causes Epigenetic Histone Modulation of c-Fos and Brain-derived Neurotrophic Factor, Target Genes Important for Neuronal Development in the Immature Rat Hippocampus

BACKGROUND

Early postnatal exposure to general anesthesia (GA) may be detrimental to brain development, resulting in long-term cognitive impairments. Older literature suggests that in utero exposure of rodents to GA causes cognitive impairments in the first-generation as well as in the second-generation offspring never exposed to GA. Thus, the authors hypothesize that transient exposure to GA during critical stages of synaptogenesis causes epigenetic changes in chromatin with deleterious effects on transcription of target genes crucial for proper synapse formation and cognitive development. They focus on the effects of GA on histone acetyltransferase activity of cAMP-responsive element-binding protein and the histone-3 acetylation status in the promoters of the target genes brain-derived neurotrophic factor and cellular Finkel-Biskis-Jinkins murine sarcoma virus osteosarcoma oncogene (c-Fos) known to regulate the development of neuronal morphology and function.

METHODS

Seven-day-old rat pups were exposed to a sedative dose of midazolam followed by combined nitrous oxide and isoflurane anesthesia for 6 h. Hippocampal neurons and organotypic hippocampal slices were cultured in vitro and exposed to GA for 24 h.

RESULTS

GA caused epigenetic modulations manifested as histone-3 hypoacetylation (decrease of 25 to 30%, n = 7 to 9) and fragmentation of cAMP-responsive element-binding protein (two-fold increase, n = 6) with 25% decrease in its histone acetyltransferase activity, which resulted in down-regulated transcription of brain-derived neurotrophic factor (0.2- to 0.4-fold, n = 7 to 8) and cellular Finkel-Biskis-Jinkins murine sarcoma virus osteosarcoma oncogene (about 0.2-fold, n = 10 to 12). Reversal of histone hypoacetylation with sodium butyrate blocked GA-induced morphological and functional impairments of neuronal development and synaptic communication.

CONCLUSION

Long-term impairments of neuronal development and synaptic communication could be caused by GA-induced epigenetic phenomena.

Histone Posttranslational Modifications in Schizophrenia

Schizophrenia is a complex neuropsychiatric disorder with high heritability; however, family and twin studies have indicated that environmental factors also play important roles in the etiology of disease. Environmental triggers exert their influence on behavior via epigenetic mechanisms. Epigenetic modifications, such as histone acetylation and methylation, as well as DNA methylation, can induce lasting changes in gene expression and have therefore been implicated in promoting the behavioral and neuronal behaviors that characterize this disorder. Importantly, because epigenetic processes are potentially reversible, they might serve as targets in the design of novel therapies in psychiatry. This chapter will review the current information regarding histone modifications in schizophrenia and the potential therapeutic relevance of such marks.

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.

Search for common targets of lithium and valproic acid identifies novel epigenetic effects of lithium on the rat leptin receptor gene

Epigenetics may have an important role in mood stabilizer action. Valproic acid (VPA) is a histone deacetylase inhibitor, and lithium (Li) may have downstream epigenetic actions. To identify genes commonly affected by both mood stabilizers and to assess potential epigenetic mechanisms that may be involved in their mechanism of action, we administered Li (N = 12), VPA (N = 12), and normal chow (N = 12) to Brown Norway rats for 30 days. Genomic DNA and mRNA were extracted from the hippocampus. We used the mRNA to perform gene expression analysis on Affymetrix microarray chips, and for genes commonly regulated by both Li and VPA, we validated expression levels using quantitative real-time PCR. To identify potential mechanisms underlying expression changes, genomic DNA was bisulfite treated for pyrosequencing of key CpG island 'shores' and promoter regions, and chromatin was prepared from both hippocampal tissue and a hippocampal-derived cell line to assess modifications of histones. For most genes, we found little evidence of DNA methylation changes in response to the medications. However, we detected histone H3 methylation and acetylation in the leptin receptor gene, Lepr, following treatment with both drugs. VPA-mediated effects on histones are well established, whereas the Li effects constitute a novel mechanism of transcriptional derepression for this drug. These data support several shared transcriptional targets of Li and VPA, and provide evidence suggesting leptin signaling as an epigenetic target of two mood stabilizers. Additional work could help clarify whether leptin signaling in the brain has a role in the therapeutic action of Li and VPA in bipolar disorder.

An update on the epigenetics of psychotic diseases and autism

The examination of potential roles of epigenetic alterations in the pathogenesis of psychotic diseases have become an essential alternative in recent years as genetic studies alone are yet to uncover major gene(s) for psychosis. Here, we describe the current state of knowledge from the gene-specific and genome-wide studies of postmortem brain and blood cells indicating that aberrant DNA methylation, histone modifications and dysregulation of micro-RNAs are linked to the pathogenesis of mental diseases. There is also strong evidence supporting that all classes of psychiatric drugs modulate diverse features of the epigenome. While comprehensive environmental and genetic/epigenetic studies are uncovering the origins, and the key genes/pathways affected in psychotic diseases, characterizing the epigenetic effects of psychiatric drugs may help to design novel therapies in psychiatry.

Histone modifications contribute to cellular replicative and hydrogen peroxide-induced premature senescence in human embryonic lung fibroblasts

Histone modifications are major post-translational mechanisms responsible for regulation of gene transcription involved in cellular senescence. By using immunofluorescence and Western blot, we showed that the global acetylated levels of histone H3 and H4 were significantly reduced in both replicative and premature senescence of human embryonic lung fibroblasts. However the whole trimethylated level of histone H4 lysine 20 was higher in senescent cells. The alterations in the mRNA and protein levels of histone acetyltransferases (HATs), histone methyltransferase (HMT), and histone deacetylases (HDACs) indicate that differential expression exists between replicative and premature senescent cells. Meanwhile, the reduced activity of HDACs was accompanied by cellular senescence. By employing the quantitative chromatin immunoprecipitation assay in detecting specific histone modifications in senescence-related genes including p53 and p16, it was demonstrated that the mRNA expression of p53 was associated with increased H4 acetylation in replicative senescence and increased H4 acetylation and trimethylation of histone H3 at lysine 4 (H3K4me3) in premature senescence. Both acetylation and trimethylation of H3 were involved in replicative senescence, while the acetylation of histone H3 and H4 was predominant in premature senescence, contributing to the mRNA expression of p16. In summary, the global hypoacetylation of histone H3 and H4 and the hypertrimethylation of histone H4 lysine 20 account for epigenetic characteristics in senescence, controlled by HATs, HMT, and HDACs differentially between replicative and premature senescence. Taken together, these findings suggest that the specific histone modifications are involved in regulating the expression of genes related to senescence of human embryonic lung fibroblasts.

A randomized, phase 2 clinical trial of lithium carbonate in Machado-Joseph disease

BACKGROUND

Because lithium exerts neuroprotective effects in preclinical models of polyglutamine disorders, our objective was to assess the safety and efficacy of lithium carbonate (0.5-0.8 milliequivalents per liter) in patients with Machado-Joseph disease (spinocerebellar ataxia type 3 [MJD/SCA3]).

METHODS

For this phase 2, single-center, double-blind, parallel, placebo-controlled trial (ClinicalTrials.gov identifier NCT01096082), 62 patients who had MJD/SCA3 with a disease duration ≤10 years and an independent gait were randomly assigned (1:1) to receive either lithium or placebo.

RESULTS

After 24 weeks, 169 adverse events were reported, including 50.3% in the lithium group (P = 1.00; primary safety outcome). Sixty patients (31 in the placebo group and 29 in the lithium group) were analyzed for efficacy (intention-to-treat analysis). Mean progression between groups did not differ according to scores on the Neurological Examination Score for the Assessment of Spinocerebellar Ataxia (NESSCA) after 48 weeks (-0.35; 95% confidence interval, -1.7 to 1.0; primary efficacy outcome). The lithium group exhibited minor progression on the PATA speech-rate (P = 0.002), the nondominant Click Test (P = 0.023), the Spinocerebellar Ataxia Functional Index (P = 0.003), and the Composite Cerebellar Functional Score (P = 0.029).

CONCLUSIONS

Lithium was safe and well tolerated, but it had no effect on progression when measured using the NESSCA in patients with MJD/SCA3. This slowdown in secondary outcomes deserves further clarification.

Horizons of psychiatric genetics and epigenetics: where are we and where are we heading?

Today multinational studies using genome-wide association scan (GWAS) for >1000,000 polymorphisms on >100,000 cases with major psychiatric diseases versus controls, combined with next-generation sequencing have found ~100 genetic polymorphisms associated with schizophrenia (SCZ), bipolar disorder (BD), autism, attention deficit and hyperactivity disorder (ADHD), etc. However, the effect size of each genetic mutation has been generally low (<1%), and altogether could portray a tiny fraction of these mental diseases. Furthermore, none of these polymorphisms was specific to disease phenotypes indicating that they are simply genetic risk factors rather than causal mutations. The lack of identification of the major gene(s) in huge genetic studies increased the tendency for reexamining the roles of environmental factors in psychiatric and other complex diseases. However, this time at cellular/molecular levels mediated by epigenetic mechanisms that are heritable, but reversible while interacting with the environment. Now, gene-specific or whole-genome epigenetic analyses have introduced hundreds of aberrant epigenetic marks in the blood or brain of individuals with psychiatric diseases that include aberrations in DNA methylation, histone modifications and microRNA expression. Interestingly, most of the current psychiatric drugs such as valproate, lithium, antidepressants, antipsychotics and even electroconvulsive therapy (ECT) modulate epigenetic codes. The existing data indicate that, the impacts of environment/nurture, including the uterine milieu and early-life events might be more significant than genetic/nature in most psychiatric diseases. The lack of significant results in large-scale genetic studies led to revise the bolded roles of genetics and now we are at the turning point of genomics for reconsidering environmental factors that through epigenetic mechanisms may impact the brain development/functions causing disease phenotypes.

The future of neuroepigenetics in the human brain

Complex mechanisms shape the genome of brain cells into transcriptional units, clusters of condensed chromatin, and many other features that distinguish between various cell types and developmental stages sharing the same genetic material. Only a few years ago, the field's focus was almost entirely on a single mark, CpG methylation; the emerging complexity of neuronal and glial epigenomes now includes multiple types of DNA cytosine methylation, more than 100 residue-specific posttranslational histone modifications and histone variants, all of which superimposed by a dynamic and highly regulated three-dimensional organization of the chromosomal material inside the cell nucleus. Here, we provide an update on the most innovative approaches in neuroepigenetics and their potential contributions to approach cognitive functions and disorders unique to human. We propose that comprehensive, cell type-specific mappings of DNA and histone modifications, chromatin-associated RNAs, and chromosomal "loopings" and other determinants of three-dimensional genome organization will critically advance insight into the pathophysiology of the disease. For example, superimposing the epigenetic landscapes of neuronal and glial genomes onto genetic maps for complex disorders, ranging from Alzheimer's disease to schizophrenia, could provide important clues about neurological function for some of the risk-associated noncoding sequences in the human genome.

Epigenetic regulation of memory formation and maintenance

Understanding the cellular and molecular mechanisms underlying the formation and maintenance of memories is a central goal of the neuroscience community. It is well regarded that an organism's ability to lastingly adapt its behavior in response to a transient environmental stimulus relies on the central nervous system's capability for structural and functional plasticity. This plasticity is dependent on a well-regulated program of neurotransmitter release, post-synaptic receptor activation, intracellular signaling cascades, gene transcription, and subsequent protein synthesis. In the last decade, epigenetic markers like DNA methylation and post-translational modifications of histone tails have emerged as important regulators of the memory process. Their ability to regulate gene transcription dynamically in response to neuronal activation supports the consolidation of long-term memory. Furthermore, the persistent and self-propagating nature of these mechanisms, particularly DNA methylation, suggests a molecular mechanism for memory maintenance. In this review, we will examine the evidence that supports a role of epigenetic mechanisms in learning and memory. In doing so, we hope to emphasize (1) the widespread involvement of these mechanisms across different behavioral paradigms and distinct brain regions, (2) the temporal and genetic specificity of these mechanisms in response to upstream signaling cascades, and (3) the functional outcome these mechanisms may have on structural and functional plasticity. Finally, we consider the future directions of neuroepigenetic research as it relates to neuronal storage of information.

Epigenetics in the human brain

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

A multi-tissue analysis identifies HLA complex group 9 gene methylation differences in bipolar disorder

Epigenetic studies of DNA and histone modifications represent a new and important activity in molecular investigations of human disease. Our previous epigenome-wide scan identified numerous DNA methylation differences in post-mortem brain samples from individuals affected with major psychosis. In this article, we present the results of fine mapping DNA methylation differences at the human leukocyte antigen (HLA) complex group 9 gene (HCG9) in bipolar disorder (BPD). Sodium bisulfite conversion coupled with pyrosequencing was used to interrogate 28 CpGs spanning ∼700 bp region of HCG9 in 1402 DNA samples from post-mortem brains, peripheral blood cells and germline (sperm) of bipolar disease patients and controls. The analysis of nearly 40 000 CpGs revealed complex relationships between DNA methylation and age, medication as well as DNA sequence variation (rs1128306). Two brain tissue cohorts exhibited lower DNA methylation in bipolar disease patients compared with controls at an extended HCG9 region (P=0.026). Logistic regression modeling of BPD as a function of rs1128306 genotype, age and DNA methylation uncovered an independent effect of DNA methylation in white blood cells (odds ratio (OR)=1.08, P=0.0077) and the overall sample (OR=1.24, P=0.0011). Receiver operating characteristic curve A prime statistics estimated a 69-72% probability of correct BPD prediction from a case vs control pool. Finally, sperm DNA demonstrated a significant association (P=0.018) with BPD at one of the regions demonstrating epigenetic changes in the post-mortem brain and peripheral blood samples. The consistent multi-tissue epigenetic differences at HCG9 argue for a causal association with BPD.

Mitogen-activated protein kinase in the amygdala plays a critical role in lithium chloride-induced taste aversion learning

The intracellular mitogen-activated protein kinase (MAPK) pathway in the brain is necessary for the formation of a variety of memories including conditioned taste aversion (CTA) learning. However, the functional role of MAPK activation in the amygdala during lithium chloride (LiCl)-induced CTA learning has not been established. In the present study, we investigated if local microinjection of SL327, a MAPK kinase inhibitor, into the rat amygdala could alleviate LiCl-induced CTA learning. Our results revealed that acute administration of a high dose of LiCl (0.15M, 12 ml/kg, i.p.) rapidly increased the level of phosphorylated MAPK (pMAPK)-positive cells in the central nucleus of the amygdala (CeA) and nucleus of the solitary tract (NTS) of rats as measured by immunohistochemistry. Local microinjection of SL327 (1 μg/0.5 μl/hemisphere) into the CeA 10 min before LiCl administration decreased both the strength of LiCl-induced CTA paired with 0.125% saccharin and the level of LiCl-induced pMAPK-positive cells in the CeA, but not in the NTS. Our data suggest that the intracellular signaling cascade of the MAPK pathway in the CeA plays a critical role in the processing of visceral information induced by LiCl for CTA learning.

Current status and future prospects for epigenetic psychopharmacology

Mounting evidence suggest that epigenetic regulation of brain functions is important in the etiology of psychiatric disorders. These epigenetic regulatory mechanisms, such as DNA methylation and histone acetylation, are influenced by many pharmaceutical compounds including psychiatric drugs. It is therefore of interest to investigate how psychiatric drugs are of influence and what the potential is of new epigenetic drugs for psychiatric disorders. With this targeted review we summarize the current state of knowledge in order to provide insight in this developing field. Several traditional psychiatric drugs have been found to alter the epigenome and in a variety of animal studies, experimental compounds with epigenetic targets have been investigated as potential psychiatric drugs. After discussion of the most relevant epigenetic mechanisms we present the evidence for epigenetic effects for the most relevant classes of drugs.

CA. Histone deacetylases and mood disorders: epigenetic programming in gene-environment interactions

Epigenetics involves molecular mechanisms related to gene expression independent of DNA sequence, mostly mediated by modification of chromatin histones. It has recently been suggested that these transcriptional changes may be implicated in the pathophysiology of mood disorders. In addition, histone deacetylase (HDAC) inhibitors have been shown to control epigenetic programming associated with the regulation of cognition and behavior, and may reverse dysfunctional epigenetic regulation associated with early life events in preclinical models. In this context, the active and continuous adaptation of chromatin, and the access of gene promoters to transcription factor mechanisms may represent a potential therapeutic target in the treatment of mood disorders such as bipolar disorder (BD) and major depressive disorder (MDD). Notably, the standard mood stabilizer valproate (VPA) has been shown to modulate the epigenome by inhibiting HDACs. However, several potential limitations are associated with this class of agents, including lack of selectivity for specific HDAC isoforms as well as risk of potentially serious side effects. Further studies regarding the potential role of chromatin remodeling in the mechanism of action of antidepressants and mood stabilizers are necessary to clarify the potential role of this class of agents as therapeutics for mood disorders.

Is an epigenetic switch the key to persistent extinction?

Many studies of learning have demonstrated that conditioned behavior can be eliminated when previously established relations between stimuli are severed. This extinction process has been extremely important for the development of learning theories and, more recently, for delineating the neurobiological mechanisms that underlie memory. A key finding from behavioral studies of extinction is that extinction eliminates behavior without eliminating the original memory; extinguished behavior often returns with time or with a return to the context in which the original learning occurred. This persistence of the original memory after extinction creates a challenge for clinical applications that use extinction as part of a treatment intervention. Consequently, a goal of recent neurobiological research on extinction is to identify potential pharmacological targets that may result in persistent extinction. Drugs that promote epigenetic changes are particularly promising because they can result in a long-term molecular signal that, combined with the appropriate behavioral treatment, can cause persistent changes in behavior induced by extinction. We will review evidence demonstrating extinction enhancements by drugs that target epigenetic mechanisms and will describe some of the challenges that epigenetic approaches face in promoting persistent suppression of memories.

Intra-amygdalar okadaic acid enhances conditioned taste aversion learning and CREB phosphorylation in rats

Protein phosphatases (PPs) regulate many substrates implicated in learning and memory. Conditioned taste aversion (CTA) learning, in which animals associate a novel taste paired with a toxin and subsequently avoid the taste, is dependent on several serine/threonine phosphatase substrates and the PP1-binding protein spinophilin. In order to examine the effects of PP1/2A blockade on CTA acquisition and extinction, rats received bilateral infusions of okadaic acid (OA) (100nM, 1microl/hemisphere) or vehicle (0.15M NaCl) into the amygdala either 5min prior to, or 5min after, a single pairing of sodium saccharin (0.125%, 10-min access) and LiCl or NaCl (0.15M, 3ml/kg i.p.). Two-bottle, 24-h preference tests were conducted for 13days to measure CTA expression and extinction. Rats conditioned with saccharin and LiCl showed a decreased preference for saccharin, and OA administered before (but not after) the pairing of saccharin and LiCl resulted in a significantly stronger CTA that did not extinguish over 13days. The enhancement of the CTA was not due to aversive effects of OA, because rats given OA and a pairing of saccharin and NaCl did not acquire a CTA. Finally, OA administration increased levels of phosphorylated CREB immunoreactivity following a CTA trial. Together, these results suggest a critical role for PP1/2A during normal CTA learning. Because CTA learning was enhanced only when OA was given prior to conditioning, phosphatase activity may be a constraint on learning during the taste-toxin interval but not during acquisition and consolidation processes that occur after toxin administration.