Epigenetic basis of the dark side of alcohol addiction

Alcoholism is a complex brain disease characterized by three distinct stages of the addiction cycle that manifest as neuroadaptive changes in the brain. One such stage of the addiction cycle is alcohol withdrawal and the negative affective states that promote drinking and maintain addiction. Repeated alcohol use, genetic predisposition to alcoholism and anxiety, and alcohol exposure during crucial developmental periods all contribute to the development of alcohol-induced withdrawal and negative affective symptoms. Epigenetic modifications within the amygdala have provided a molecular basis of these negative affective symptoms, also known as the dark side of addiction. Here, we propose that allostatic change within the epigenome in the amygdala is a prime mechanism of the biological basis of negative affective states resulting from, and contributing to, alcoholism. Acute alcohol exposure produces an anxiolytic response which is associated with the opening of chromatin due to increased histone acetylation, increased CREB binding protein (CBP) levels, and histone deacetylase (HDAC) inhibition. After chronic ethanol exposure, these changes return to baseline along with anxiety-like behaviors. However, during withdrawal, histone acetylation decreases due to increased HDAC activity and decreased CBP levels in the amygdala circuitry leading to the development of anxiety-like behaviors. Additionally, innately higher expression of the HDAC2 isoform leads to a deficit in global and gene-specific histone acetylation in the amygdala that is associated with a decrease in the expression of several synaptic plasticity-associated genes and maintaining heightened anxiety-like behavior and excessive alcohol intake. Adolescent alcohol exposure also leads to higher expression of HDAC2 and a deficit in histone acetylation leading to decreased expression of synaptic plasticity-associated genes and high anxiety and drinking behavior in adulthood. All these studies indicate that the epigenome can undergo allostatic reprogramming in the amygdaloid circuitry during various stages of alcohol exposure. Furthermore, opening the chromatin by inhibiting HDACs using pharmacological or genetic manipulations can lead to the attenuation of anxiety as well as alcohol intake. Chromatin remodeling provides a clear biological basis for the negative affective states seen during alcohol addiction and presents opportunities for novel drug development and treatment options. This article is part of the Special Issue entitled "Alcoholism".

The Potential Role of Amygdaloid MicroRNA-494 in Alcohol-Induced Anxiolysis

BACKGROUND

The antianxiety effects of ethanol appear to be a crucial factor in promoting alcohol intake. Regulation of gene expression by microRNA (miRNA) is an important epigenetic mechanism that affects neuronal pathways and behaviors. We investigated the role of miRNAs underlying the mechanisms of ethanol-induced anxiolysis.

METHODS

Acute ethanol-induced anxiolysis was measured in adult rats, and amygdaloid tissues were used for miRNA profiling by microarray analysis. The expression of miR-494 and its target genes in the amygdala was measured using real-time quantitative polymerase chain reaction. The direct role of miR-494 in the anxiety phenotype was also investigated via infusion of a miR-494 antagomir into the central nucleus of amygdala.

RESULTS

Microarray profiling of miRNAs in the amygdala showed significant alteration of several miRNA expression levels by acute ethanol exposure. Expression of miR-494 was significantly decreased, whereas expression of the binding protein of cyclic adenosine monophosphate response element binding protein (CBP), p300, and Cbp/p300-interacting transactivator 2 (Cited2) was increased in the amygdala during ethanol-induced anxiolysis. Inhibition of miR-494 in the central nucleus of amygdala, through infusion of a specific antagomir, provoked anxiolysis, mimicking the action of ethanol. Also, expression of Cited2, CBP, and p300 as well as histone H3-lysine 9 acetylation was significantly increased by miR-494 antagomir infusion, indicating their regulation by miR-494 in the amygdala.

CONCLUSIONS

These novel results suggest that acute ethanol-induced reduction in miR-494 expression in the amygdala can serve as a key regulatory mechanism for chromatin remodeling possibly leading to anxiolysis.

Role of Growth Arrest and DNA Damage-Inducible, Beta in Alcohol-Drinking Behaviors

BACKGROUND

The contribution of epigenetic factors, such as histone acetylation and DNA methylation, to the regulation of alcohol-drinking behavior has been increasingly recognized over the last several years. GADD45b is a protein demonstrated to be involved in DNA demethylation at neurotrophic factor gene promoters, including at brain-derived neurotrophic factor (Bdnf) which has been highly implicated in alcohol-drinking behavior.

METHODS

DNA methyltransferase-1 (Dnmt1), 3a, and 3b, and Gadd45a, b, and g mRNA were measured in the nucleus accumbens (NAc) and ventral tegmental areas of high ethanol (EtOH) consuming C57BL/6J (C57) and low alcohol consuming DBA/2J (DBA) mice using quantitative reverse transcriptase polymerase chain reaction (PCR). In the NAc, GADD45b protein was measured via immunohistochemistry and Bdnf9a mRNA using in situ PCR. Bdnf9a promoter histone H3 acetylated at lysines 9 and 14 (H3K9,K14ac) was measured using chromatin immunoprecipitation, and 5-methylcytosine (5MC) and 5-hydroxymethylcytosine (5HMC) using methylated DNA immunoprecipitation. Alcohol-drinking behavior was evaluated in Gadd45b haplodeficient (+/-) and null mice (-/-) utilizing drinking-in-the-dark (DID) and 2-bottle free-choice paradigms.

RESULTS

C57 mice had lower levels of Gadd45b and g mRNA and GADD45b protein in the NAc relative to the DBA strain. C57 mice had lower NAc shell Bdnf9a mRNA levels, Bdnf9a promoter H3K9,K14ac, and higher Bdnf9a promoter 5HMC and 5MC. Acute EtOH increased GADD45b protein, Bdnf9a mRNA, and histone acetylation and decreased 5HMC in C57 mice. Gadd45b +/- mice displayed higher drinking behavior relative to wild-type littermates in both DID and 2-bottle free-choice paradigms.

CONCLUSIONS

These data indicate the importance of the DNA demethylation pathway and its interactions with histone posttranslational modifications in alcohol-drinking behavior. Further, we suggest that lower DNA demethylation protein GADD45b levels may affect Bdnf expression possibly leading to altered alcohol-drinking behavior.

Gadd45b and N-methyl-D-aspartate induced DNA demethylation in postmitotic neurons

AIM

In nondividing neurons examine the role of Gadd45b in active 5-methylcytosine (5MC) and 5-hydroxymethylcytosine (5HMC) removal at a gene promoter highly implicated in mental illnesses and cognition, Bdnf.

MATERIALS & METHODS

Mouse primary cortical neuronal cultures with and without Gadd45b siRNA transfection were treated with N-methyl-d-aspartate (NMDA). Expression changes of genes reportedly involved in DNA demethylation, Bdnf mRNA and protein and 5MC and 5HMC at Bdnf promoters were measured.

RESULTS

Gadd45b siRNA transfection in neurons abolishes the NMDA-induced increase in Bdnf IXa mRNA and reductions in 5MC and 5HMC at the Bdnf IXa promoter.

CONCLUSION

These results contribute to our understanding of DNA demethylation mechanisms in neurons, and its role in regulating NMDA responsive genes implicated in mental illnesses.

Adolescent Alcohol Exposure: Burden of Epigenetic Reprogramming, Synaptic Remodeling, and Adult Psychopathology

Adolescence represents a crucial phase of synaptic maturation characterized by molecular changes in the developing brain that shape normal behavioral patterns. Epigenetic mechanisms play an important role in these neuromaturation processes. Perturbations of normal epigenetic programming during adolescence by ethanol can disrupt these molecular events, leading to synaptic remodeling and abnormal adult behaviors. Repeated exposure to binge levels of alcohol increases the risk for alcohol use disorder (AUD) and comorbid psychopathology including anxiety in adulthood. Recent studies in the field clearly suggest that adolescent alcohol exposure causes widespread and persistent changes in epigenetic, neurotrophic, and neuroimmune pathways in the brain. These changes are manifested by altered synaptic remodeling and neurogenesis in key brain regions leading to adult psychopathology such as anxiety and alcoholism. This review details the molecular mechanisms underlying adolescent alcohol exposure-induced changes in synaptic plasticity and the development of alcohol addiction-related phenotypes in adulthood.

A role for histone acetylation mechanisms in adolescent alcohol exposure-induced deficits in hippocampal brain-derived neurotrophic factor expression and neurogenesis markers in adulthood

Binge drinking during adolescence is a risk factor for neuropsychiatric disorders that can develop later in life. Histone acetylation is an important epigenetic mechanism that contributes to neurodevelopment. We investigated the effects of adolescent intermittent ethanol (AIE) exposure, as opposed to normal saline (AIS) exposure, on histone acetylation-mediated regulation of brain-derived neurotrophic factor (BDNF) expression and developmental stages of neurogenesis (proliferating and immature neurons) in the hippocampus in adulthood. AIE exposure increased whole hippocampal histone deacetylase (HDAC) activity and decreased binding protein of cyclic adenosine monophosphate response element binding protein (CBP) and histone H3-K9 acetylation levels in the CA1, CA2, and CA3 regions of the hippocampus. BDNF protein and exon IV mRNA levels in the CA1 and CA3 regions of the hippocampus of AIE-exposed adult rats were decreased as compared to AIS-exposed adult rats. AIE-induced anxiety-like behaviors and deficits in histone H3 acetylation at BDNF exon IV promoter in the hippocampus during adulthood, which were reversed by treatment with the HDAC inhibitor, trichostatin A (TSA). Similarly, neurogenesis was inhibited by AIE in adulthood as demonstrated by the decrease in Ki-67 and doublecortin (DCX)-positive cells in the dentate gyrus, which was normalized by TSA treatment. These results indicate that AIE exposure increases HDACs and decreases CBP levels that may be associated with a decrease in histone H3 acetylation in the hippocampus. These epigenetic changes potentially decrease BDNF expression and inhibit neurogenesis in the hippocampus that may be involved in AIE-induced behavioral abnormalities, including anxiety, in adulthood.

Adolescent alcohol exposure alters GABAA receptor subunit expression in adult hippocampus

BACKGROUND

The long-term consequences of adolescent alcohol abuse that persist into adulthood are poorly understood and have not been widely investigated. We have shown that intermittent exposure to alcohol during adolescence decreased the amplitude of GABAA receptor (GABAA R)-mediated tonic currents in hippocampal dentate granule cells in adulthood. The aim of this study was to investigate the enduring effects of chronic intermittent alcohol exposure during adolescence or adulthood on the expression of hippocampal GABAA Rs.

METHODS

We used a previously characterized tissue fractionation method to isolate detergent resistant membranes and soluble fractions, followed by Western blots to measure GABAA R protein expression. We also measured mRNA levels of GABAA R subunits using quantitative real-time polymerase chain reaction.

RESULTS

Although the protein levels of α1-, α4-, and δ-GABAA R subunits remained stable between postnatal day (PD) 30 (early adolescence) and PD71 (adulthood), the α5-GABAA R subunit was reduced across that period. In rats that were subjected to adolescent intermittent ethanol (AIE) exposure between PD30 and PD46, there was a significant reduction in the protein levels of the δ-GABAA R, in the absence of any changes in mRNA levels, at 48 hours and 26 days after the last ethanol (EtOH) exposure. Protein levels of the α4-GABAA R subunit were significantly reduced, but mRNA levels were increased, 26 days (but not 48 hours) after the last AIE exposure. Protein levels of α5-GABAA R were not changed by AIE, but mRNA levels were reduced at 48 hours but normalized 26 days after AIE. In contrast to the effects of AIE, chronic intermittent ethanol (CIE) exposure during adulthood had no effect on expression of any of the GABAA R subunits examined.

CONCLUSIONS

AIE produced both short- and long-term alterations of GABAA R subunits mRNA and protein expression in the hippocampus, whereas CIE produced no long-lasting effects on those measures. The observed reduction of protein levels of the δ-GABAA R, specifically, is consistent with previously reported altered hippocampal GABAA R-mediated electrophysiological responses after AIE. The absence of effects of CIE underscores the emerging view of adolescence as a time of distinctive vulnerability to the enduring effects of repeated EtOH exposure.

Neurogenetic and epigenetic correlates of adolescent predisposition to and risk for addictive behaviors as a function of prefrontal cortex dysregulation

As addiction professionals, we are becoming increasingly concerned about preteenagers and young adults' involvement with substance abuse as a way of relieving stress and anger. The turbulent underdeveloped central nervous system, especially in the prefrontal cortex (PFC), provides impetus to not only continue important neuroimaging studies in both human and animal models, but also to encourage preventive measures and cautions embraced by governmental and social media outlets. It is well known that before people reach their 20s, PFC development is undergoing significant changes and, as such, hijacks appropriate decision making in this population. We are further proposing that early genetic testing for addiction risk alleles will offer important information that could potentially be utilized by their parents and caregivers prior to use of psychoactive drugs by these youth. Understandably, family history, parenting styles, and attachment may be modified by various reward genes, including the known bonding substances oxytocin/vasopressin, which effect dopaminergic function. Well-characterized neuroimaging studies continue to reflect region-specific differential responses to drugs and food (including other non-substance-addictive behaviors) via either "surfeit" or "deficit." With this in mind, we hereby propose a "reward deficiency solution system" that combines early genetic risk diagnosis, medical monitoring, and nutrigenomic dopamine agonist modalities to combat this significant global dilemma that is preventing our youth from leading normal productive lives, which will in turn make them happier.

Potential role of adolescent alcohol exposure-induced amygdaloid histone modifications in anxiety and alcohol intake during adulthood

Binge drinking is common during adolescence and can lead to the development of psychiatric disorders, including alcoholism in adulthood. Here, the role and persistent effects of histone modifications during adolescent intermittent ethanol (AIE) exposure in the development of anxiety and alcoholism in adulthood were investigated. Rats received intermittent ethanol exposure during post-natal days 28-41, and anxiety-like behaviors were measured after 1 and 24 h of the last AIE. The effects of AIE on anxiety-like and alcohol-drinking behaviors in adulthood were measured with or without treatment with the histone deacetylase (HDAC) inhibitor, trichostatin A (TSA). Amygdaloid brain regions were collected to measure HDAC activity, global and gene-specific histone H3 acetylation, expression of brain-derived neurotrophic factor (BDNF) and activity-regulated cytoskeleton-associated (Arc) protein and dendritic spine density (DSD). Adolescent rats displayed anxiety-like behaviors after 24 h, but not 1 h, of last AIE with a concomitant increase in nuclear and cytosolic amygdaloid HDAC activity and HDAC2 and HDAC4 levels leading to deficits in histone (H3-K9) acetylation in the central (CeA) and medial (MeA), but not in basolateral nucleus of amygdala (BLA). Interestingly, some of AIE-induced epigenetic changes such as, increased nuclear HDAC activity, HDAC2 expression, and decreased global histone acetylation persisted in adulthood. In addition, AIE decreased BDNF exons I and IV and Arc promoter specific histone H3 acetylation that was associated with decreased BDNF, Arc expression and DSD in the CeA and MeA during adulthood. AIE also induced anxiety-like behaviors and enhanced ethanol intake in adulthood, which was attenuated by TSA treatment via normalization of deficits in histone H3 acetylation of BDNF and Arc genes. These novel results indicate that AIE induces long-lasting effects on histone modifications and deficits in synaptic events in the amygdala, which are associated with anxiety-like and alcohol drinking behaviors in adulthood.

Molecular mechanisms of synaptic remodeling in alcoholism

Alcohol use and alcohol addiction represent dysfunctional brain circuits resulting from neuroadaptive changes during protracted alcohol exposure and its withdrawal. Alcohol exerts a potent effect on synaptic plasticity and dendritic spine formation in specific brain regions, providing a neuroanatomical substrate for the pathophysiology of alcoholism. Epigenetics has recently emerged as a critical regulator of gene expression and synaptic plasticity-related events in the brain. Alcohol exposure and withdrawal induce changes in crucial epigenetic processes in the emotional brain circuitry (amygdala) that may be relevant to the negative affective state defined as the "dark side" of addiction. Here, we review the literature concerning synaptic plasticity and epigenetics, with a particular focus on molecular events related to dendritic remodeling during alcohol abuse and alcoholism. Targeting epigenetic processes that modulate synaptic plasticity may yield novel treatments for alcoholism.

Effects of acute ethanol exposure on anxiety measures and epigenetic modifiers in the extended amygdala of adolescent rats

Epigenetic mechanisms appear to play an important role in neurodevelopment. We investigated the effects of acute ethanol exposure on anxiety measures and function of histone deacetylases (HDAC) and DNA methyltransferases (DNMT) in the amygdala and bed nucleus of stria terminalis (BNST) of adolescent rats. One hour after ethanol exposure, rats were subjected to anxiety measures. A subset of adolescent rats was exposed to two doses (24 h apart) of ethanol (2 g/kg) to measure rapid ethanol tolerance to anxiolysis. The HDAC and DNMT activities and mRNA levels of DNMT isoforms were measured in the amygdala and BNST. The lower dose of ethanol (1 g/kg) produced neither anxiolysis, nor inhibited the HDAC and DNMT activities in the amygdala and BNST, except DNMT activity in BNST was attenuated. Anxiolysis by ethanol was observed at 2 and 2.25 g/kg, whereas higher doses (2.5 and 3 g/kg) were found to be sedative. DNMT activity in the amygdala and BNST, and nuclear HDAC activity in the amygdala, but not in the BNST were also inhibited by these doses of ethanol. A lack of tolerance was observed on ethanol-induced inhibition of DNMT activity in the amygdala and BNST, and nuclear HDAC activity in the amygdala, as well to anxiolysis produced by ethanol (2 g/kg). The DNMT1, DNMT3a, and DNMT3b mRNA expression in the amygdala was not affected by either 1or 2 doses of 2 g/kg. However, DNMT1 and DNMT3a expression in the BNST was increased, whereas DNMT3l mRNA was decreased in the amygdala, after 2 doses of 2 g/kg ethanol. These results suggest that reduced sensitivity to anxiolysis and the lack of rapid tolerance to the anxiolytic effects of ethanol and inhibition of HDAC and DNMT functions may play a role in engaging adolescents in binge drinking patterns.

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.

Regulation of DNA methylation by ethanol induces tissue plasminogen activator expression in astrocytes

Alcohol exposure affects neuronal plasticity in the adult and developing brain. Astrocytes play a major role in modulating neuronal plasticity and are a target of ethanol. Tissue plasminogen activator (tPA) is involved in modulating neuronal plasticity by degrading the extracellular matrix proteins including fibronectin and laminin and is up-regulated by ethanol in vivo. In this study we explored the hypothesis that ethanol affects DNA methylation in astrocytes thereby increasing expression and release of tPA. It was found that ethanol increased tPA mRNA levels, an effect mimicked by an inhibitor of DNA methyltransferase (DNMT) activity. Ethanol also increased tPA protein expression and release, and inhibited DNMT activity with a corresponding decrease in DNA methylation levels of the tPA promoter. Furthermore, it was observed that protein levels of DNMT3A, but not DNMT1, were reduced in astrocytes after ethanol exposure. These novel studies show that ethanol inhibits DNA methylation in astrocytes leading to increased tPA expression and release; this effect may be involved in astrocyte-mediated inhibition of neuronal plasticity by alcohol.

DNA methylation/demethylation network expression in psychotic patients with a history of alcohol abuse

BACKGROUND

Recent studies suggest that protracted and excessive alcohol use induces an epigenetic dysregulation in human and rodent brains. We recently reported that DNA methylation dynamics are altered in brains of psychotic (PS) patients, including schizophrenia and bipolar disorder patients. Because PS patients are often comorbid with chronic alcohol abuse, we examined whether the altered expression of multiple members of the DNA methylation/demethylation network observed in postmortem brains of PS patients was modified in PS patients with a history of chronic alcohol abuse.

METHODS

DNA-methyltransferase-1 (DNMT1) mRNA-positive neurons were counted in situ in prefrontal cortex samples obtained from the Harvard Brain Tissue Resource Center, Belmont, MA. 10-11-translocation (TETs 1, 2, 3), apolipoprotein B editing complex enzyme (APOBEC-3C), growth and DNA-damage-inducible protein 45β (GADD45β), and methyl-binding domain protein-4 (MBD4) mRNAs were measured by quantitative real-time polymerase chain reaction in inferior parietal cortical lobule samples obtained from the Stanley Foundation Neuropathology Consortium, Bethesda, MD.

RESULTS

We observed an increase in DNMT1 mRNA-positive neurons in PS patients compared with non-PS subjects. In addition, there was a pronounced decrease in APOBEC-3C and a pronounced increase in GADD45β and TET1 mRNAs in PS patients with no history of alcohol abuse. In PS patients with a history of chronic alcohol abuse, the numbers of DNMT1-positive neurons were not increased significantly. Furthermore, the decrease in APOBEC-3C mRNA was less pronounced, while the increase in TET1 mRNA had a tendency to be potentiated in those PS patients that were chronic alcohol abusers. GADD45β and MBD4 mRNAs were not influenced by alcohol abuse. The effect of chronic alcohol abuse on DNA methylation/demethylation network enzymes cannot be attributed to confounding demographic variables or to the type and dose of medication used.

CONCLUSIONS

Based on these results, we hypothesize that PS patients may abuse alcohol as a potential attempt at self-medication to normalize altered DNA methylation/demethylation network pathways. However, before accepting this conclusion, we need to study alterations in the DNA methylation/demethylation pathways and the DNA methylation dynamics in a substantial number of alcoholic PS and non-PS patients. Additional investigation may also be necessary to determine whether the altered DNA methylation dynamics are direct or the consequence of an indirect interaction of alcohol with the neuropathogenetic mechanisms underlying psychosis.

TLR4-MyD88 signalling: a molecular target for alcohol actions

Several studies implicate Toll-like receptors (TLRs) in alcohol-induced neuroinflammatory processes. The work reported by Wu et al., in this issue of the British Journal of Pharmacology, indicates that TLR4 along with its intracellular adaptor protein, MyD88, may play crucial roles in the acute actions of alcohol. The deletions of TLR4 or MyD88 gene or pharmacological inhibition of TLR4 by (+)-naloxone were able to attenuate alcohol-induced sedation, motor impairment and acute alcohol-induced increases in IkBα protein levels in the hippocampus of mice. These results clearly suggest that TLR4-MyD88 signalling may play a causal role in the mediation of the behavioural effects of acute alcohol.

Histone deacetylases (HDAC)-induced histone modifications in the amygdala: a role in rapid tolerance to the anxiolytic effects of ethanol

BACKGROUND

Rapid tolerance to the anxiolytic effects of ethanol appears to be an important factor in the development of alcoholism. Here, we investigated the involvement of amygdaloid histone deacetylases (HDAC)-induced epigenetic changes in rapid ethanol tolerance (RET).

METHODS

RET in rats was induced by 2 ethanol injections administered 24 hours apart. Both ethanol-tolerant and control rats were treated with the HDAC inhibitor, trichostatin A (TSA), and anxiety-like behaviors were measured. HDAC activity, histone (H3 and H4) acetylation, and neuropeptide Y (NPY) expression in the amygdala of these rats were also measured.

RESULTS

A single ethanol exposure was able to produce an anxiolytic response, inhibit amygdaloid HDAC activity, and increase both histone acetylation and NPY expression (mRNA and protein levels) in the central nucleus of amygdala (CeA) and medial nucleus of amygdala (MeA) of rats. In contrast, 2 exposures of the same dose of ethanol (24 hours apart) neither elicited a similar anxiolytic response nor modulated HDAC activity, histone acetylation, or NPY expression in the amygdala. However, exposure to a higher dose of ethanol on the second day was able to produce an anxiolytic response and also inhibit amygdaloid HDAC activity. TSA treatment caused the reversal of RET by inhibiting HDAC activity, thereby increasing histone acetylation and NPY expression in the CeA and MeA.

CONCLUSIONS

Cellular tolerance to the initial acute ethanol-induced inhibition of HDAC activity and the subsequent upregulation of histone acetylation and NPY expression in the amygdala may be involved in the mechanisms underlying rapid tolerance to the anxiolytic effects of ethanol.

The role of amygdaloid brain-derived neurotrophic factor, activity-regulated cytoskeleton-associated protein and dendritic spines in anxiety and alcoholism

Innate anxiety appears to be a robust factor in the promotion of alcohol intake, possibly due to the anxiolytic effects of self-medication with alcohol. Brain-derived neurotrophic factor (BDNF) and its downstream target, activity-regulated cytoskeleton-associated (Arc) protein, play a role in the regulation of synaptic function and structure. In order to examine the role of the BDNF-Arc system and associated dendritic spines in the anxiolytic effects of ethanol, we investigated the effects of acute ethanol exposure on anxiety-like behaviors of alcohol-preferring (P) and -nonpreferring (NP) rats. We also examined changes in the expression of BDNF and Arc, and dendritic spine density (DSD), in amygdaloid brain regions of P and NP rats with or without ethanol exposure. It was found that in comparison with NP rats, P rats displayed innate anxiety-like behaviors, and had lower mRNA and protein levels of both BDNF and Arc, and also had lower DSD in the central amygdala (CeA) and medial amygdala (MeA), but not in the basolateral amygdala (BLA). Acute ethanol treatment had an anxiolytic effect in P, but not in NP rats, and was associated with an increase in mRNA and protein levels of BDNF and Arc, and in DSD in the CeA and MeA, but not BLA. These results suggest that innate deficits in BDNF-Arc levels, and DSD, in the CeA and MeA may be involved in the anxiety-like and excessive alcohol-drinking behaviors of P rats, as ethanol increased these amygdaloid synaptic markers and produced anxiolytic effects in P rats, but not NP rats.

Hetereogeneity of dose and time effects of estrogen on neuron-specific neuronal protein and phosphorylated cyclic AMP response element-binding protein in the hippocampus of ovariectomized rats

Previous studies have shown changes in the cyclic AMP response element-binding protein (CREB) signaling pathway in CA1 and CA3 regions of the rostral hippocampus with 10 μg estrogen treatment for 14 days. It appears that estrogen's action on CREB phosphorylation in brain structures depends on other estrogen doses and lengths of treatment. We therefore examined the effects of moderate regimens [2.5 μg estradiol benzoate (EB) for 4 or 14 days] on mean numbers of neuron-specific neuronal protein (NeuN)-positive cells and phosphorylated CREB (pCREB)-positive cells and subregion volume defined by NeuN and pCREB immunolabeling and compared those results with results from the high regimen (10 μg EB for 14 days) in CA1, CA2, and CA3 regions and dorsal (DDG) and ventral (VDG) dentate gyrus and hilus of the hippocampus of ovariectomized rats by stereology. For whole hippocampus, all regimens increased mean neuronal (NeuN) numbers and pCREB-positive cell and volume compared with sesame oil (SO) in CA1, CA2, and CA3 regions, DDG and VDG, and hilus. In rostral hippocampus, however, some hippocampal subregions were not responsive to the high regimen, and the moderate regimens appear to be more effective for increasing mean number of NeuN-positive neurons and pCREB-positive cells and subregion volume. Heterogeneity in responsiveness to estrogen was mainly seen within rostral, but not whole, hippocampal subregions. Our results indicate that responsiveness of cells expressing NeuN and pCREB to different EB regimens may vary depending on the specific region of the hippocampus.

Neuroscience of alcoholism: molecular and cellular mechanisms

Alcohol use and abuse appear to be related to neuroadaptive changes at functional, neurochemical, and structural levels. Acute and chronic ethanol exposure have been shown to modulate function of the activity-dependent gene transcription factor, cAMP-responsive element binding (CREB) protein in the brain, which may be associated with the development of alcoholism. Study of the downstream effectors of CREB have identified several important CREB-related genes, such as neuropeptide Y, brain-derived neurotrophic factor, activity-regulated cytoskeleton-associated protein, and corticotrophin-releasing factor, that may play a crucial role in the behavioral effects of ethanol and molecular changes in the specific neurocircuitry that underlie both alcohol addiction and a genetic predisposition to alcoholism. Brain chromatin remodeling due to histone covalent modifications may also be involved in mediating the behavioral effects and neuroadaptive changes that occur during ethanol exposure. This review outlines progressive neuroscience research into molecular and epigenetic mechanisms of alcoholism.

Neuropeptide Y signaling in the central nucleus of amygdala regulates alcohol-drinking and anxiety-like behaviors of alcohol-preferring rats

BACKGROUND

The neuropeptide Y (NPY) system of the central nucleus of amygdala (CeA) has been shown to be involved in anxiety and alcoholism. In this study, we investigated the molecular mechanisms by which NPY in the CeA regulates anxiety and alcohol drinking behaviors using alcohol-preferring (P) rats as an animal model.

METHODS

Alcohol-preferring rats were bilaterally cannulated targeting the CeA and infused with artificial cerebrospinal fluid (aCSF) or NPY. Alcohol drinking and anxiety-like behaviors were assessed by the 2-bottle free-choice paradigm and light/dark box (LDB) exploration test, respectively. The levels of NPY and related signaling proteins were determined by the gold immunolabeling procedure. The mRNA levels of NPY were measured by in situ RT-PCR. Double-immunofluorescence labeling was performed to observe the colocalization of NPY and Ca(2+)/calmodulin-dependent protein kinase IV (CaMK IV).

RESULTS

We found that NPY infusion into the CeA produced anxiolytic effects, as measured by the LDB exploration test, and also decreased alcohol intake in P rats. NPY infusion into the CeA significantly increased levels of CaMK IV and phosphorylated cAMP responsive element-binding (pCREB) protein and increased mRNA and protein levels of NPY, but produced no changes in protein levels of CREB or the catalytic alpha-subunit of protein kinase A (PKA-Calpha) in the CeA. We also observed that alcohol intake produced anxiolytic effects in P rats in the LDB test and also increased NPY expression and protein levels of pCREB and PKA-Calpha without modulating protein levels of CREB or CaMK IV, in both the CeA and medial nucleus of amygdala. In addition, we found that CaMK IV-positive cells were co-localized with NPY in amygdaloid structures of P rats.

CONCLUSIONS

These results suggest that NPY infusion may increase the expression of endogenous NPY in the CeA, which is most likely attributable to an increase in CaMK IV-dependent CREB phosphorylation and this molecular mechanism may be involved in regulating anxiety and alcohol drinking behaviors of P rats.

Estrogen affects levels of Bcl-2 protein and mRNA in medial amygdala of ovariectomized rats

The survival factor Bcl-2 is a cyclic AMP response element-binding protein (CREB) gene product implicated in mediating some of estrogen's effects on neuroprotection. Previously, we showed an effect of estradiol benzoate (E) on numbers of neuron-specific protein (NeuN)- and phosphorylated CREB (pCREB)-positive cells in medial (MeA), but not central (CeA), amygdala of ovariectomized rats. To determine whether these effects are accompanied by an E-induced increase in Bcl-2, we examined the effects of E on levels of Bcl-2 protein and mRNA in MeA and CeA of ovariectomized rats treated with E regimens resulting in moderate (2.5 microg E for 4 or 14 days) or high (10 microg E for 14 days) plasma estradiol levels. As a physiological control, we showed that all E treatments increased uterine wet weight relative to vehicle; 10 microg E for 14 days also increased uterine weight compared with that seen with lower E levels. Western blot analysis revealed that all E groups displayed an increase in uterine Bcl-2 protein levels compared with vehicle. We found that 2.5 microg and 10 microg E for 14 days increased levels of Bcl-2 gold immunolabeling compared with vehicle and 2.5 microg E for 4 days in MeA, but not CeA. We measured Bcl-2 mRNA levels in vehicle and 2.5 microg E-treated 14-day groups. There was a significant increase in Bcl-2 mRNA levels in MeA, but not CeA, of E-treated ovariectomized rats compared with vehicle controls. The E-induced increase in protein and mRNA levels of Bcl-2 in MeA may be important for neuroprotection in this region.

Innate differences in the expression of brain-derived neurotrophic factor in the regions within the extended amygdala between alcohol preferring and nonpreferring rats

BACKGROUND

Animal lines such as alcohol-preferring (P) and nonpreferring (NP) rats appear to be suitable animal models to investigate the biological basis of alcohol-drinking behaviors. The extended amygdala serves as a neuroanatomical substrate for alcohol-drinking behaviors. Brain-derived neurotrophic factor (BDNF) in the amygdala has been implicated in alcohol-drinking behaviors; however, its expression in the extended amygdala of P and NP rats is unknown. Therefore, we examined the basal expression of BDNF in the extended amygdala of alcohol naive P and NP rats.

METHODS

We determined the basal mRNA and protein levels of BDNF by in situ RT-PCR and immuno-histochemical procedure, respectively, in the amygdaloid [central nucleus of amygdala (CeA), medial nucleus of amygdala (MeA), and basolateral amygdala (BLA)], nucleus accumbal (NAc shell and core), and bed nucleus of stria terminalis (BNST) [lateral BNST (lBNST), medial BNST (mBNST), and ventral BNST (vBNST)] brain structures of P and NP rats. In addition, we examined the localization of BDNF in neurons using double-immunofluorescence labeling of BDNF with neuron-specific nuclear protein (NeuN) and also determined the number of NeuN-positive neurons in the amygdaloid structures of P and NP rats.

RESULTS

The mRNA and protein levels of BDNF were found to be significantly lower in both the CeA and MeA, but not in the BLA, of P compared with NP rats. We also found that BDNF was expressed in neurons in the amygdaloid structures of P and NP rats. In addition, we found that the number of NeuN-positive neurons was similar in the amygdaloid structures of P and NP rats. Interestingly, the mRNA and protein levels of BDNF were also significantly lower in the lBNST, mBNST, and vBNST of P compared with NP rats. On the other hand, mRNA and protein levels of BDNF were similar in the NAc shell and core structures of P and NP rats.

CONCLUSIONS

P and NP rats are selectively bred for higher and lower alcohol preference, respectively; therefore it is possible that lower BDNF levels in the amygdaloid and BNST structures may be associated with the excessive alcohol-drinking behaviors of P rats.

Dose and time effects of estrogen on expression of neuron-specific protein and cyclic AMP response element-binding protein and brain region volume in the medial amygdala of ovariectomized rats

Although estrogen has been shown to be neuroprotective, studies concerning its effect on some behaviors are contradictory, reporting both ameliorative and detrimental effects. A factor involved in hormone efficacy is the estrogen regimen. We reported an effect of 10 microg estrogen for 14 days on the cyclic AMP response element-binding protein (CREB) pathway, including brain-derived neurotrophic factor, in rat medial amygdala (MeA). To determine the effects of estrogen on neuronal numbers and brain region volume in MeA and central nucleus of the amygdala (CeA), we used stereology to test the effect of various estrogen regimens on the number of neuron-specific protein (NeuN)-labeled neurons and brain region volume of MeA and CeA. Ovariectomized rats were injected with vehicle for 14 days, 2.5 microg estradiol benzoate (E2) for 4 or 14 days, or 10 microg estrogen for 14 days. Because NeuN-labeled neuronal number may be related to neuronal survival and upregulation of CREB signaling, we tested the effect of these regimens on levels of phosphorylated CREB (pCREB) labeling in the MeA and CeA. The 2.5 microg estrogen for 14 days regimen increased the mean number of NeuN-labeled neurons and pCREB-labeled cells in the MeA compared to vehicle or 2.5 microg for 4 days. There was an increase in volume of the MeA with 2.5 microg estrogen for 14 days compared to vehicle or 2.5 microg for 4 days. No differences in these parameters were seen in CeA. These data indicate a neuroanatomical heterogeneity of a time effect of estrogen on cells expressing NeuN and pCREB in the MeA versus CeA.