Differential effects of chronic ethanol consumption and withdrawal on homer/glutamate receptor expression in subregions of the accumbens and amygdala of P rats

Amoxicillin and amoxicillin/clavulanate reduce ethanol intake and increase GLT-1 expression as well as AKT phosphorylation in mesocorticolimbic regions

Studies have shown that administration of the β-lactam antibiotic ceftriaxone (CEF) attenuates ethanol consumption and cocaine seeking behavior as well as prevents ethanol-induced downregulation of glutamate transporter 1 (GLT-1) expression in central reward brain regions. However, it is not known if these effects are compound-specific. Therefore, the present study examined the effects of two other β-lactam antibiotics, amoxicillin (AMOX) and amoxicillin/clavulanate (Augmentin, AUG), on ethanol drinking, as well as GLT-1 and phosphorylated-AKT (pAKT) levels in the nucleus accumbens (Acb) and medial prefrontal cortex (mPFC) of alcohol-preferring (P) rats. P rats were exposed to free-choice of ethanol (15% and 30%) for five weeks and were given five consecutive daily i.p. injections of saline vehicle, 100 mg/kg AMOX or 100mg/kg AUG. Both compounds significantly decreased ethanol intake and significantly increased GLT-1 expression in the Acb. AUG also increased GLT-1 expression in the mPFC. Results for changes in pAKT levels matched those for GLT-1, indicating that β-lactam antibiotic-induced reductions in ethanol intake are negatively associated with increases in GLT-1 and pAKT levels within two critical brains regions mediating drug reward and reinforcement. These findings add to a growing literature that pharmacological increases in GLT-1 expression are associated with decreases in ethanol intake and suggest that one mechanism mediating this effect may be increased phosphorylation of AKT. Thus, GLT-1 and pAKT may serve as molecular targets for the treatment of alcohol and drug abuse/dependence.

Neuroplasticity of A-type potassium channel complexes induced by chronic alcohol exposure enhances dendritic calcium transients in hippocampus

RATIONALE

Chronic alcohol-induced cognitive impairments and maladaptive plasticity of glutamatergic synapses are well-documented. However, it is unknown if prolonged alcohol exposure affects dendritic signaling that may underlie hippocampal dysfunction in alcoholics. Back-propagation of action potentials (bAPs) into apical dendrites of hippocampal neurons provides distance-dependent signals that modulate dendritic and synaptic plasticity. The amplitude of bAPs decreases with distance from the soma that is thought to reflect an increase in the density of Kv4.2 channels toward distal dendrites.

OBJECTIVE

The aim of this study was to quantify changes in hippocampal Kv4.2 channel function and expression using electrophysiology, Ca(2+) imaging, and western blot analyses in a well-characterized in vitro model of chronic alcohol exposure.

RESULTS

Chronic alcohol exposure significantly decreased expression of Kv4.2 channels and KChIP3 in hippocampus. This reduction was associated with an attenuation of macroscopic A-type K(+) currents in CA1 neurons. Chronic alcohol exposure increased bAP-evoked Ca(2+) transients in the distal apical dendrites of CA1 pyramidal neurons. The enhanced bAP-evoked Ca(2+) transients induced by chronic alcohol exposure were not related to synaptic targeting of N-methyl-D-aspartate (NMDA) receptors or morphological adaptations in apical dendritic arborization.

CONCLUSIONS

These data suggest that chronic alcohol-induced decreases in Kv4.2 channel function possibly mediated by a downregulation of KChIP3 drive the elevated bAP-associated Ca(2+) transients in distal apical dendrites. Alcohol-induced enhancement of bAPs may affect metaplasticity and signal integration in apical dendrites of hippocampal neurons leading to alterations in hippocampal function.

Targeting glutamate uptake to treat alcohol use disorders

Alcoholism is a serious public health concern that is characterized by the development of tolerance to alcohol's effects, increased consumption, loss of control over drinking and the development of physical dependence. This cycle is often times punctuated by periods of abstinence, craving and relapse. The development of tolerance and the expression of withdrawal effects, which manifest as dependence, have been to a great extent attributed to neuroadaptations within the mesocorticolimbic and extended amygdala systems. Alcohol affects various neurotransmitter systems in the brain including the adrenergic, cholinergic, dopaminergic, GABAergic, glutamatergic, peptidergic, and serotonergic systems. Due to the myriad of neurotransmitter and neuromodulator systems affected by alcohol, the efficacies of current pharmacotherapies targeting alcohol dependence are limited. Importantly, research findings of changes in glutamatergic neurotransmission induced by alcohol self- or experimenter-administration have resulted in a focus on therapies targeting glutamatergic receptors and normalization of glutamatergic neurotransmission. Glutamatergic receptors implicated in the effects of ethanol include the ionotropic glutamate receptors (AMPA, Kainate, and NMDA) and some metabotropic glutamate receptors. Regarding glutamatergic homeostasis, ceftriaxone, MS-153, and GPI-1046, which upregulate glutamate transporter 1 (GLT1) expression in mesocorticolimbic brain regions, reduce alcohol intake in genetic animal models of alcoholism. Given the hyperglutamatergic/hyperexcitable state of the central nervous system induced by chronic alcohol abuse and withdrawal, the evidence thus far indicates that a restoration of glutamatergic concentrations and activity within the mesocorticolimbic system and extended amygdala as well as multiple memory systems holds great promise for the treatment of alcohol dependence.

Homer2 deletion alters dendritic spine morphology but not alcohol-associated adaptations in GluN2B-containing N-methyl-D-aspartate receptors in the nucleus accumbens

Repeated exposure to ethanol followed by withdrawal leads to alterations in glutamatergic signaling and impaired synaptic plasticity in the nucleus accumbens (NAc) in both clinical and preclinical models of ethanol exposure. Homer2 is a member of a family of postsynaptic density (PSD) scaffolding proteins that functions in part to cluster N-methyl-D-aspartate (NMDA) signaling complexes in the PSD, and has been shown to be critically important for plasticity in multiple models of drug and alcohol abuse. Here we used Homer2 knockout (KO) mice and a chronic intermittent intraperitoneal (IP) ethanol injection model to investigate a potential role for the protein in ethanol-induced adaptations in dendritic spine morphology and PSD protein expression. While deletion of Homer2 was associated with increased density of long spines on medium spiny neurons of the NAc core of saline treated mice, ethanol exposure had no effect on dendritic spine morphology in either wild-type (WT) or Homer2 KO mice. Western blot analysis of tissue samples from the NAc enriched for PSD proteins revealed a main effect of ethanol treatment on the expression of GluN2B, but there was no effect of genotype or treatment on the expression other glutamate receptor subunits or PSD95. These data indicate that the global deletion of Homer2 leads to aberrant regulation of dendritic spine morphology in the NAc core that is associated with an increased density of long, thin spines. Unexpectedly, intermittent IP ethanol did not affect spine morphology in either WT or KO mice. Together these data implicate Homer2 in the formation of long, thin spines and further supports its role in neuronal structure.

Mechanisms of Neuroplasticity and Ethanol's Effects on Plasticity in the Striatum and Bed Nucleus of the Stria Terminalis

Long-lasting changes in synaptic function (i.e., synaptic plasticity) have long been thought to contribute to information storage in the nervous system. Although synaptic plasticity mainly has adaptive functions that allow the organism to function in complex environments, it is now clear that certain events or exposure to various substances can produce plasticity that has negative consequences for organisms. Exposure to drugs of abuse, in particular ethanol, is a life experience that can activate or alter synaptic plasticity, often resulting in increased drug seeking and taking and in many cases addiction.Two brain regions subject to alcohol's effects on synaptic plasticity are the striatum and bed nucleus of the stria terminalis (BNST), both of which have key roles in alcohol's actions and control of intake. The specific effects depend on both the brain region analyzed (e.g., specific subregions of the striatum and BNST) and the duration of ethanol exposure (i.e., acute vs. chronic). Plastic changes in synaptic transmission in these two brain regions following prolonged ethanol exposure are thought to contribute to excessive alcohol drinking and relapse to drinking. Understanding the mechanisms underlying this plasticity may lead to new therapies for treatment of these and other aspects of alcohol use disorder.

Habitual alcohol seeking: modeling the transition from casual drinking to addiction

The transition from goal-directed actions to habitual ethanol seeking models the development of addictive behavior that characterizes alcohol use disorders. The progression to habitual ethanol-seeking behavior occurs more rapidly than for natural rewards, suggesting that ethanol may act on habit circuit to drive the loss of behavioral flexibility. This review will highlight recent research that has focused on the formation and expression of habitual ethanol seeking, and the commonalities and distinctions between ethanol and natural reward-seeking habits, with the goal of highlighting important, understudied research areas that we believe will lead toward the development of novel treatment and prevention strategies for uncontrolled drinking.

Amygdalar neuronal plasticity and the interactions of alcohol, sex, and stress

Alcohol abuse and alcoholism are major medical problems affecting both men and women. Previous animal studies reported a difference in c-Fos neuronal activation after chronic alcohol exposure; however, females remain an understudied population. To model chronic alcohol exposure match-pair fed adult male and female rats were administered 14 days of a liquid ethanol containing diet. Analysis focused on the central nucleus of the amygdala (CeA), a region integral to stress sensitivity and substance abuse. Immunocytochemical approaches identified cells containing ΔFosB, a marker of sustained neuronal activation, and activity patterns within the CeA were mapped by subdivision and rostral-caudal extent. Significant interactions were present between all groups, with gender differences noted among control groups, and ethanol exposed animals having the greatest number of ΔFosB immunoreactive cells indicating baseline dysregulation. Compared with c-Fos, a marker of recent neuronal activation, male ethanol treated animals had similar activity to controls, indicating a neuronal habituation not seen in females. Next, a cohort of animals were exposed to the forced swim test (FST), and c-Fos was examined in addition to FST behavior. Neuronal activity was increased in ethanol exposed animals compared to controls, and control females compared to males, indicating a potentiated stress response. Further, a population of activated neurons were shown to contain either corticotropin releasing factor or enkephalin. The present data suggest that dysregulation in the CeA neuronal activity may underlie some of the negative sequelae of alcohol abuse, and may, in part, underlie the distinctive response seen between genders to alcohol use.

Alcohol dependence and free-choice drinking in mice

Alcohol dependence continues to be an important health concern and animal models are critical to furthering our understanding of this complex disease. A hallmark feature of alcoholism is a significant increase in alcohol drinking over time. While several different animal models of excessive alcohol (ethanol) drinking exist for mice and rats, a growing number of laboratories are using a model that combines chronic ethanol exposure procedures with voluntary ethanol drinking with mice as experimental subjects. Primarily, these studies use a chronic intermittent ethanol (CIE) exposure pattern to render mice dependent and a 2-h limited access procedure to evaluate drinking behavior. Compared to non-dependent mice that also drink ethanol, the ethanol-dependent mice demonstrate significant increases in voluntary ethanol drinking. The increased drinking significantly elevates blood and brain ethanol concentrations compared to the non-dependent control mice. Studies report that the increased drinking by dependent mice is driven by neuroadaptations in glutamatergic and corticotropin-releasing factor signaling in different brain regions known to be involved in alcohol-related behaviors. The dysregulation of these systems parallels findings in human alcoholics and treatments that demonstrate efficacy in alcoholics can also reduce drinking in this model. Moreover, preclinical findings have informed the development of human clinical trials, further highlighting the translational potential of the model. As a result of these features, the CIE exposure and free-choice drinking model is becoming more widely used and promises to provide more insight into mechanisms of excessive drinking that may be important for developing treatments for human alcoholics. The salient features and possible future considerations for CIE exposure and free-choice drinking in mice are discussed.

Effects of ceftriaxone on chronic ethanol consumption: a potential role for xCT and GLT1 modulation of glutamate levels in male P rats

Alterations in glutamatergic neurotransmission have been suggested to affect many aspects of neuroplasticity associated with alcohol/drug addiction. We have previously shown that ceftriaxone, a β-lactam antibiotic known to upregulate glutamate transporter 1 (GLT1), reduced ethanol intake after 5 weeks of free choice ethanol drinking paradigm in male alcohol-preferring (P) rats. Evidence suggests that differential effects involving alterations of glutamatergic neurotransmission occur after long-term ethanol consumption. In this study, we tested whether the efficacy of administration of ceftriaxone persists after 14 weeks of free access to 15 and 30 % ethanol in male P rats. After 14 weeks of ethanol consumption, male P rats were administered ceftriaxone (100 mg/kg, intraperitoneal (i.p.)) or saline vehicle for 5 days. We found that ceftriaxone treatment resulted in a significant reduction in ethanol intake starting from day 2 (48 h after the first i.p. injections of ceftriaxone) through day 14, 10 days after final injection. Western blot analysis of brain samples from animals euthanized 24 h after treatment with the last dose of ceftriaxone revealed a significant upregulation of cystine/glutamate exchanger (xCT) and GLT1 levels in prefrontal cortex, nucleus accumbens, and amygdala as compared to saline vehicle-treated group. These findings demonstrated the effectiveness of ceftriaxone in attenuating ethanol intake in a chronic consumption paradigm. These might be due in part through the upregulation of both xCT and GLT1 levels in brain reward regions. Thus, the drug has a potential therapeutic action for the treatment of alcohol dependence.

mGluR1 within the nucleus accumbens regulates alcohol intake in mice under limited-access conditions

Idiopathic or alcohol-induced increases in the expression and function of the Group1 metabotropic glutamate receptor subtype 1 (mGluR1) within the extended amygdala are theorized to contribute to an individual's propensity to consume excessive amounts of alcohol. In the past, the detailed study of the functional relevance of mGluR1 for alcoholism-related behaviors in animal models was hampered by the poor solubility and non-specific side effects of available inhibitors; however, the advent of the highly potent and soluble mGluR1 negative allosteric modulator JNJ-16259685 [(3,4-Dihydro-2H-pyrano[2,3-b]quinolin-7-yl)-(cis-4-methoxycyclohexyl)-methanone] has instigated a re-examination of the role for this mGluR subtype in mediating the behavioral effects of alcohol. In this regard, systemic pretreatment with JNJ-16259685 was proven effective at reducing alcohol reinforcement and motivation for the drug. mGluR1 is a Gαq/o-coupled receptor, the stimulation of which activates phospholipase C (PLC). Thus, the present study investigated potential neuroanatomical substrates and intracellular molecules involved in the ability of JNJ-16259685 to reduce alcohol intake. JNJ-16259685 (0-30 pg/side) was infused into the shell subregion of the nucleus accumbens (NAC) of C57BL/6J and Homer2 knock-out (KO) mice, either alone or in combination with the PLC inhibitor U-73122 (5.8 fg/side). Alcohol intake was then assessed under Drinking-in-the-Dark (DID) procedures. Intra-NAC JNJ-16259685 infusion dose-dependently reduced alcohol consumption by C57BL/6J mice; this effect was not additive with that produced by U-73122, nor was it present in Homer2 KO animals. These data provide novel evidence in support of a critical role for mGluR1-PLC signaling, scaffolded by Homer2, within the NAC shell, in maintaining alcohol consumption under limited access procedures. Such findings have relevance for both the pharmacotherapeutics and pharmacogenetics of risky alcohol drinking and alcoholism.

Binge alcohol drinking by mice requires intact group 1 metabotropic glutamate receptor signaling within the central nucleus of the amygdala

Despite the fact that binge alcohol drinking (intake resulting in blood alcohol concentrations (BACs) 80 mg% within a 2-h period) is the most prevalent form of alcohol-use disorders (AUD), a large knowledge gap exists regarding how this form of AUD influences neural circuits mediating alcohol reinforcement. The present study employed integrative approaches to examine the functional relevance of binge drinking-induced changes in glutamate receptors, their associated scaffolding proteins and certain signaling molecules within the central nucleus of the amygdala (CeA). A 30-day history of binge alcohol drinking (for example, 4-5 g kg(-1) per 2 h(-1)) elevated CeA levels of mGluR1, GluN2B, Homer2a/b and phospholipase C (PLC) β3, without significantly altering protein expression within the adjacent basolateral amygdala. An intra-CeA infusion of mGluR1, mGluR5 and PLC inhibitors all dose-dependently reduced binge intake, without influencing sucrose drinking. The effects of co-infusing mGluR1 and PLC inhibitors were additive, whereas those of coinhibiting mGluR5 and PLC were not, indicating that the efficacy of mGluR1 blockade to lower binge intake involves a pathway independent of PLC activation. The efficacy of mGluR1, mGluR5 and PLC inhibitors to reduce binge intake depended upon intact Homer2 expression as revealed through neuropharmacological studies of Homer2 null mutant mice. Collectively, these data indicate binge alcohol-induced increases in Group1 mGluR signaling within the CeA as a neuroadaptation maintaining excessive alcohol intake, which may contribute to the propensity to binge drink.

Effects of the mGluR5 antagonist MPEP on ethanol withdrawal induced anxiety-like syndrome in rats

Abstinence from chronic ethanol consumption leads to the manifestation of a variety of symptoms attributed to central nervous system hyperexcitability, such as increased irritability, anxiety, and restlessness. Recent studies have demonstrated the importance of metabotropic glutamate receptor 5 (mGluR5) in addictive behaviours. This study investigates the effects of the mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) on ethanol withdrawal induced anxiety using two behavioural paradigms. Male Wistar rats were fed a Modified Liquid Diet (MLD) containing low fat cow milk, sucrose, and maltodextrin with a gradual introduction of 2.4%, 4.8% and 7.2% ethanol for 20 days. Six hours into ethanol withdrawal, the rats were intraperitoneally injected with normal saline and MPEP (2.5, 5.0, 10, 20, 30 mg/kg) and were assessed for ethanol withdrawal induced anxiety-like syndrome using an automated elevated plus maze and an open field. MPEP at 10 mg/kg significantly attenuated ethanol withdrawal induced anxiety without any compromising effects on locomotor activities. Despite reversing several indices of ethanol withdrawal induced anxiety in both the elevated plus maze and the open field, low doses of MPEP (2.5, 5 mg/kg) significantly compromised the locomotor activities of ethanol withdrawn rats. High doses of MPEP (20 and 30 mg/kg) significantly attenuated withdrawal anxiety when tested in the elevated plus maze but not in the open field. Administration of MPEP (2.5, 5, 10, 20, 30 mg/kg) has no significant compromising effect on the locomotor activities of ethanol naïve rats. Despite significantly reducing withdrawal anxiety in both behavioural paradigms at 10 mg/kg, the compromising effects of low and high doses of MPEP must be further explored along with the therapeutic efficiency of this drug for relieving withdrawal induced anxiety.

Glutamatergic targets for new alcohol medications

RATIONALE

An increasingly compelling literature points to a major role for the glutamate system in mediating the effects of alcohol on behavior and the pathophysiology of alcoholism. Preclinical studies indicate that glutamate signaling mediates certain aspects of ethanol's intoxicating and rewarding effects, and undergoes adaptations following chronic alcohol exposure that may contribute to the withdrawal, craving and compulsive drug-seeking that drive alcohol abuse and alcoholism.

OBJECTIVES

We discuss the potential for targeting the glutamate system as a novel pharmacotherapeutic approach to treating alcohol use disorders, focusing on five major components of the glutamate system: the N-methyl-D-aspartate (NMDA) receptor and specific NMDA subunits, the glycineB site on the NMDA receptors (NMDAR), L-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid ionotropic (AMPA) and kainate (KAR) receptors, metabotropic receptors (mGluR), and glutamate transporters.

RESULTS

Chronic alcohol abuse produces a hyperglutamatergic state, characterized by elevated extracellular glutamate and altered glutamate receptors and transporters. Pharmacologically manipulating glutamatergic neurotransmission alters alcohol-related behaviors including intoxication, withdrawal, and alcohol-seeking, in rodents and human subjects. Blocking NMDA and AMPA receptors reduces alcohol consumption in rodents, but side-effects may limit this as a therapeutic approach. Selectively targeting NMDA and AMPA receptor subunits (e.g., GluN2B, GluA3), or the NMDAR glycineB site offers an alternative approach. Blocking mGluR5 potently affects various alcohol-related behaviors in rodents, and mGluR2/3 agonism also suppresses alcohol consumption. Finally, glutamate transporter upregulation may mitigate behavioral and neurotoxic sequelae of excess glutamate caused by alcohol.

CONCLUSIONS

Despite the many challenges that remain, targeting the glutamate system offers genuine promise for developing new treatments for alcoholism.

Sex differences in neuroadaptation to alcohol and withdrawal neurotoxicity

Recent work suggests that sex differences exist with regard to both the nature of neuroadaptation to alcohol during the development of dependence, and possibly, the neurodegenerative consequences of alcohol dependence. Volumetric studies in human samples show that females may demonstrate increased volumetric brain loss with equal or lesser dependence histories than males. Furthermore, animal studies demonstrate sex differences in glutamatergic, GABAergic, and adenosinergic receptor signaling and endocrine responses following prolonged alcohol exposure. These differences may influence the development of dependence, neuronal function, and viability, particularly during alcohol withdrawal. The present review discusses the current state of knowledge in this regard. It is concluded that there exists a clear need for a more extensive examination of potential sex differences in neurodegenerative consequences of alcohol dependence in men and women, particularly with regard to the role that alterations in amino acid signaling and hypothalamic-pituitary-adrenal axis function may play. Furthermore, we note the need for expanded examination of the unique role that alcohol withdrawal-associated neuronal activity may have in the development of dependence-associated neurotoxicity.

mTOR activation is required for the anti-alcohol effect of ketamine, but not memantine, in alcohol-preferring rats

Glutamate NMDA receptors mediate many molecular and behavioral effects of alcohol, and they play a key role in the development of excessive drinking. Uncompetitive NMDA receptor antagonists may, therefore, have therapeutic potential for alcoholism. The first aim was to compare the effects of the NMDA antagonists memantine and ketamine on ethanol and saccharin drinking in alcohol-preferring rats. The second aim was to determine whether the effects of the two NMDA receptor antagonists were mediated by the mammalian target of rapamycin (mTOR). TSRI Sardinian alcohol-preferring rats were allowed to self-administer either 10% w/v ethanol or 0.08% w/v saccharin, and water. Operant responding and motor activity were assessed following administration of either memantine (0-10mg/kg) or ketamine (0-20mg/kg). Finally, ethanol self-administration was assessed in rats administered with either memantine or ketamine but pretreated with the mTOR inhibitor rapamycin (2.5mg/kg). The uncompetitive NMDA receptor antagonists memantine and ketamine dose-dependently reduced ethanol drinking in alcohol-preferring rats; while memantine had a preferential effect on alcohol over saccharin, ketamine reduced responding for both solutions. Neither antagonist induced malaise, as shown by the lack of effect on water intake and motor activity. The mTOR inhibitor rapamycin blocked the effects of ketamine, but not those of memantine. Memantine and ketamine both reduce alcohol drinking in alcohol-preferring rats, but only memantine is selective for alcohol. The effects of ketamine, but not memantine, are mediated by mTOR. The results support the therapeutic potential of uncompetitive NMDA receptor antagonists, especially memantine, in alcohol addiction.

The central amygdala and alcohol: role of γ-aminobutyric acid, glutamate, and neuropeptides

Alcohol dependence is a chronically relapsing disorder characterized by compulsive drug seeking and drug taking, loss of control in limiting intake, and the emergence of a withdrawal syndrome in the absence of the drug. Accumulating evidence suggests an important role for synaptic transmission in the central amygdala (CeA) in mediating alcohol-related behaviors and neuroadaptative mechanisms associated with alcohol dependence. Acute alcohol facilitates γ-aminobutyric acid-ergic (GABAergic) transmission in CeA via both pre- and postsynaptic mechanisms, and chronic alcohol increases baseline GABAergic transmission. Acute alcohol inhibits glutamatergic transmission via effects at N-methyl-d-aspartate (NMDA) and AMPA receptors in CeA, whereas chronic alcohol up-regulates N-methyl-d-aspartate receptor (NMDAR)-mediated transmission. Pro- (e.g., corticotropin-releasing factor [CRF]) and anti-stress (e.g., NPY, nociceptin) neuropeptides affect alcohol- and anxiety-related behaviors, and also alter the alcohol-induced effects on CeA neurotransmission. Alcohol dependence produces plasticity in these neuropeptide systems, reflecting a recruitment of those systems during the transition to alcohol dependence.

Synaptic effects induced by alcohol

Ethanol (EtOH) has effects on numerous cellular molecular targets, and alterations in synaptic function are prominent among these effects. Acute exposure to EtOH activates or inhibits the function of proteins involved in synaptic transmission, while chronic exposure often produces opposing and/or compensatory/homeostatic effects on the expression, localization, and function of these proteins. Interactions between different neurotransmitters (e.g., neuropeptide effects on release of small molecule transmitters) can also influence both acute and chronic EtOH actions. Studies in intact animals indicate that the proteins affected by EtOH also play roles in the neural actions of the drug, including acute intoxication, tolerance, dependence, and the seeking and drinking of EtOH. This chapter reviews the literature describing these acute and chronic synaptic effects of EtOH and their relevance for synaptic transmission, plasticity, and behavior.

New insights on neurobiological mechanisms underlying alcohol addiction

Alcohol dependence/addiction is mediated by complex neural mechanisms that involve multiple brain circuits and neuroadaptive changes in a variety of neurotransmitter and neuropeptide systems. Although recent studies have provided substantial information on the neurobiological mechanisms that drive alcohol drinking behavior, significant challenges remain in understanding how alcohol-induced neuroadaptations occur and how different neurocircuits and pathways cross-talk. This review article highlights recent progress in understanding neural mechanisms of alcohol addiction from the perspectives of the development and maintenance of alcohol dependence. It provides insights on cross talks of different mechanisms and reviews the latest studies on metaplasticity, structural plasticity, interface of reward and stress pathways, and cross-talk of different neural signaling systems involved in binge-like drinking and alcohol dependence.

Animal models for medications development targeting alcohol abuse using selectively bred rat lines: neurobiological and pharmacological validity

The purpose of this review paper is to present evidence that rat animal models of alcoholism provide an ideal platform for developing and screening medications that target alcohol abuse and dependence. The focus is on the 5 oldest international rat lines that have been selectively bred for a high alcohol-consumption phenotype. The behavioral and neurochemical phenotypes of these rat lines are reviewed and placed in the context of the clinical literature. The paper presents behavioral models for assessing the efficacy of pharmaceuticals for the treatment of alcohol abuse and dependence in rodents, with particular emphasis on rats. Drugs that have been tested for their effectiveness in reducing alcohol/ethanol consumption and/or self-administration by these rat lines and their putative site of action are summarized. The paper also presents some current and future directions for developing pharmacological treatments targeting alcohol abuse and dependence.

The genetics of alcohol dependence: advancing towards systems-based approaches

BACKGROUND

Personalized treatment for psychopathologies, in particular alcoholism, is highly dependent upon our ability to identify patterns of genetic and environmental effects that influence a person's risk. Unfortunately, array-based whole genome investigations into heritable factors that explain why one person becomes dependent upon alcohol and another does not, have indicated that alcohol's genetic architecture is highly complex. That said, uncovering and interpreting the missing heritability in alcohol genetics research has become all the more important, especially since the problem may extend to our inability to model the cumulative and combinatorial relationships between common and rare genetic variants. As numerous studies begin to illustrate the dependency of alcohol pharmacotherapies on an individual's genotype, the field is further challenged to identify new ways to transcend agnostic genomewide association approaches. We discuss insights from genetic studies of alcohol related diseases, as well as issues surrounding alcohol's genetic complexity and etiological heterogeneity. Finally, we describe the need for innovative systems-based approaches (systems genetics) that can provide additional statistical power that can enhance future gene-finding strategies and help to identify heretofore-unrealized mechanisms that may provide new targets for prevention/treatments efforts. Emerging evidence from early studies suggest that systems genetics has the potential to organize our neurological, pharmacological, and genetic understanding of alcohol dependence into a biologically plausible framework that represents how perturbations across evolutionarily robust biological systems determine susceptibility to alcohol dependence.

Gene expression in the ventral tegmental area of 5 pairs of rat lines selectively bred for high or low ethanol consumption

The objective of this study was to determine if there are common innate differences in gene expression or gene pathways in the ventral tegmental area (VTA) among 5 different pairs of rat lines selectively bred for high (HEC) or low (LEC) ethanol consumption: (a) alcohol-preferring (P) vs. alcohol-non-preferring (NP) rats; (b) high-alcohol-drinking (HAD) vs. low-alcohol-drinking (LAD) rats (replicate line pairs 1 and 2); (c) ALKO alcohol (AA) vs. nonalcohol (ANA) rats; and (d) Sardinian alcohol-preferring (sP) vs. alcohol-nonpreferring (sNP) rats. Microarray analysis revealed between 370 and 1340 unique named genes that significantly differed in expression between the individual line-pairs. Analysis using Gene Ontology (GO) and Ingenuity Pathways information indicated significant categories and networks in common for up to 3 line-pairs, but not for all 5 line-pairs; moreover, there were few genes in common in these categories and networks. ANOVA of the combined data for the 5 line-pairs indicated 1295 significant (p<0.01) differences in expression of named genes. Although no individual named gene was significant across all 5 line-pairs, there were 22 genes that overlapped in the same direction in 3 or 4 of the line-pairs. Overall, the findings suggest that (a) some biological categories or networks may be in common for subsets of line-pairs; and (b) regulation of different genes and/or combinations of multiple biological systems (e.g., transcription, synaptic function, intracellular signaling and protection against oxidative stress) within the VTA (possibly involving dopamine and glutamate) may be contributing to the disparate alcohol drinking behaviors of these line-pairs.

Coordinated dynamic gene expression changes in the central nucleus of the amygdala during alcohol withdrawal

BACKGROUND

Chronic alcohol use causes widespread changes in the cellular biology of the amygdala's central nucleus (CeA), a GABAergic center that integrates autonomic physiology with the emotional aspects of motivation and learning. While alcohol-induced neurochemical changes play a role in dependence and drinking behavior, little is known about the CeA's dynamic changes during withdrawal, a period of emotional and physiologic disturbance.

METHODS

We used a qRT-PCR platform to measure 139 transcripts in 92 rat CeA samples from control (N = 33), chronically alcohol exposed (N = 26), and withdrawn rats (t = 4, 8, 18, 32, and 48 hours; N = 5, 10, 7, 6, 5). This focused transcript set allowed us to identify significant dynamic expression patterns during the first 48 hours of withdrawal and propose potential regulatory mechanisms.

RESULTS

Chronic alcohol exposure causes a limited number of small magnitude expression changes. In contrast, withdrawal results in a greater number of large changes within 4 hours of removal of the alcohol diet. Sixty-five of the 139 measured transcripts (47%) showed differential regulation during withdrawal. Over the 48-hour period, dynamic changes in the expression of γ-aminobutyric acid type A (GABA(A) ), ionotropic glutamate and neuropeptide system-related G-protein-coupled receptor subunits, and the Ras/Raf signaling pathway were seen as well as downstream transcription factors (TFs) and epigenetic regulators. Four temporally correlated gene clusters were identified with shared functional roles including NMDA receptors, MAPKKK and chemokine signaling cascades, and mediators of long-term potentiation, among others. Cluster promoter regions shared overrepresented binding sites for multiple TFs including Cebp, Usf-1, Smad3, Ap-2, and c-Ets, suggesting a potential regulatory role.

CONCLUSIONS

During alcohol withdrawal, the CeA experiences rapid changes in mRNA expression of these functionally related transcripts that were not predicted by measurement during chronic exposure. This study provides new insight into dynamic expression changes during alcohol withdrawal and suggests novel regulatory relationships that potentially impact the aspects of emotional modulation.

K(Ca)2 channels: novel therapeutic targets for treating alcohol withdrawal and escalation of alcohol consumption

Small-conductance, calcium-activated potassium (K(Ca)2) channels influence neuronal firing properties, intrinsic excitability, and NMDA receptor-dependent synaptic responses and plasticity. In this mini-review, we discuss new evidence that chronic alcohol-associated plasticity critically involves K(Ca)2 channels in hippocampus, ventral tegmental area, and nucleus accumbens. K(Ca)2 channel activity can modulate the magnitude of excitation of midbrain dopamine neurons induced by acute alcohol exposure. Emerging evidence indicates that K(Ca)2 channels regulate neuroadaptations to chronic alcohol that contribute to withdrawal hyperexcitability and escalation of voluntary alcohol consumption. Restoring K(Ca)2 channel activity can attenuate the severity of the alcohol withdrawal syndrome in vivo and withdrawal-associated neurotoxicity in vitro. Pharmacological modulation of K(Ca)2 channels can bi-directionally influence drinking behavior in rat and mouse models of voluntary alcohol consumption. Collectively, these studies using various rodent models have clearly indicated a central role for K(Ca)2 channels in the neuroplasticity of chronic alcohol exposure. In addition, accumulating evidence suggests that K(Ca)2 channels are a novel therapeutic target to alleviate the symptoms of alcohol withdrawal and reduce high amounts of alcohol drinking.

Effects of alcohol on the membrane excitability and synaptic transmission of medium spiny neurons in the nucleus accumbens

Chronic and excessive alcohol drinking lead to alcohol dependence and loss of control over alcohol consumption, with serious detrimental health consequences. Chronic alcohol exposure followed by protracted withdrawal causes profound alterations in the brain reward system that leads to marked changes in reinforcement mechanisms and motivational state. These long-lasting neuroadaptations are thought to contribute to the development of cravings and relapse. The nucleus accumbens (NAcc), a central component of the brain reward system, plays a critical role in alcohol-induced neuroadaptive changes underlying alcohol-seeking behaviors. Here we review the findings that chronic alcohol exposure produces long-lasting neuroadaptive changes in various ion channels that govern intrinsic membrane properties and neuronal excitability, as well as excitatory and inhibitory synaptic transmission in the NAcc that underlie alcohol-seeking behavior during protracted withdrawal.

Rapid temporal changes in the expression of a set of neuromodulatory genes during alcohol withdrawal in the dorsal vagal complex: molecular evidence of homeostatic disturbance

BACKGROUND

Chronic alcohol exposure produces neuroadaptation, which increases the risk of cellular excitotoxicity and autonomic dysfunction during withdrawal. The temporal progression and regulation of the gene expression that contributes to this physiologic and behavioral phenotype is poorly understood early in the withdrawal period. Further, it is unexplored in the dorsal vagal complex (DVC), a brainstem autonomic regulatory structure.

METHODS

We use a quantitative polymerase chain reaction platform to precisely and simultaneously measure the expression of 145 neuromodulatory genes in more than 100 rat DVC samples from control, chronically alcohol-exposed, and withdrawn rats. To gain insight into the dynamic progression and regulation of withdrawal, we focus on the expression of a subset of functionally relevant genes during the first 48 hours, when behavioral symptoms are most severe.

RESULTS

In the DVC, expression of this gene subset is essentially normal in chronically alcohol-exposed rats. However, withdrawal results in rapid, large-magnitude expression changes in this group. We observed differential regulation in 86 of the 145 genes measured (59%), some as early as 4 hours into withdrawal. Time series measurements (4, 8, 18, 32, and 48 hours after alcohol removal) revealed dynamic expression responses in immediate early genes, γ-aminobutyric acid type A, ionotropic glutamate, and G-protein coupled receptors and the Ras/Raf signaling pathway. Together, these changes elucidate a complex, temporally coordinated response that involves correlated expression of many functionally related groups. In particular, the expression patterns of Gabra1, Grin2a, Grin3a, and Grik3 were tightly correlated. These receptor subunits share overrepresented transcription factor binding sites for Pax-8 and other transcription factors, suggesting a common regulatory mechanism and a role for these transcription factors in the regulation of neurotransmission within the first 48 hours of alcohol withdrawal.

CONCLUSIONS

Expression in this gene set is essentially normal in the alcohol-adapted DVC, but withdrawal results in immediate, large-magnitude, and dynamic changes. These data support both increased research focus on the biological ramifications of alcohol withdrawal and enable novel insights into the dynamic withdrawal expression response in this understudied homeostatic control center.

Nucleus accumbens mGluR5-associated signaling regulates binge alcohol drinking under drinking-in-the-dark procedures

BACKGROUND

Alcohol increases the expression of Group 1 metabotropic glutamate receptors (mGluRs) and their associated scaffolding protein Homer2 and stimulates phosphatidylinositol 3-kinase (PI3K) within the nucleus accumbens (NAC). Moreover, functional studies suggest that NAC Group 1 mGluR/Homer2/PI3K signaling may be a potential target for pharmacotherapeutic intervention in alcoholism.

METHODS

Immunoblotting was conducted to examine the effects of alcohol consumption under drinking-in-the-dark (DID) procedures on Group 1 mGluR-associated proteins in C57BL/6J (B6) mice. Follow-up behavioral studies examined the importance of Group 1 mGluR/Homer2/PI3K signaling within the NAC shell for limited-access alcohol drinking. Finally, immunoblotting examined whether the NAC expression of Group 1 mGluR-associated proteins is a genetic correlate of high alcohol drinking using a selectively bred high DID (HDID-1) mouse line.

RESULTS

Limited-access alcohol drinking under DID procedures up-regulated NAC shell Homer2 levels, concomitant with increases in mGluR5 and NR2B. Intra-NAC shell blockade of mGluR5, Homer2, or PI3K signaling, as well as transgenic disruption of the Homer binding site on mGluR5, decreased alcohol consumption in B6 mice. Moreover, transgenic disruption of the Homer binding site on mGluR5 and Homer2 deletion both prevented the attenuating effect of mGluR5 and PI3K blockade upon intake. Finally, the basal NAC shell protein expression of mGluR1 and Homer2 was increased in offspring of HDID-1 animals.

CONCLUSIONS

Taken together, these data further implicate Group 1 mGluR signaling through Homer2 within the NAC in excessive alcohol consumption.

Neuropeptide modulation of central amygdala neuroplasticity is a key mediator of alcohol dependence

Alcohol use disorders are characterized by compulsive drug-seeking and drug-taking, loss of control in limiting intake, and withdrawal syndrome in the absence of drug. The central amygdala (CeA) and neighboring regions (extended amygdala) mediate alcohol-related behaviors and chronic alcohol-induced plasticity. Acute alcohol suppresses excitatory (glutamatergic) transmission whereas chronic alcohol enhances glutamatergic transmission in CeA. Acute alcohol facilitates inhibitory (GABAergic) transmission in CeA, and chronic alcohol increases GABAergic transmission. Electrophysiology techniques are used to explore the effects of neuropeptides/neuromodulators (CRF, NPY, nociceptin, dynorphin, endocannabinoids, galanin) on inhibitory transmission in CeA. In general, pro-anxiety peptides increase, and anti-anxiety peptides decrease CeA GABAergic transmission. These neuropeptides facilitate or block the action of acute alcohol in CeA, and chronic alcohol produces plasticity in neuropeptide systems, possibly reflecting recruitment of negative reinforcement mechanisms during the transition to alcohol dependence. A disinhibition model of CeA output is discussed in the context of alcohol dependence- and anxiety-related behaviors.

Neurobiology of consummatory behavior: mechanisms underlying overeating and drug use

Consummatory behavior is driven by both caloric and emotional need, and a wide variety of animal models have been useful in research on the systems that drive consumption of food and drugs. Models have included selective breeding for a specific trait, manipulation of gene expression, forced or voluntary exposure to a substance, and identification of biomarkers that predict which animals are prone to overconsuming specific substances. This research has elucidated numerous brain areas and neurochemicals that drive consummatory behavior. Although energy homeostasis is primarily mediated by the hypothalamus, reinforcement is more strongly mediated by nuclei outside the hypothalamus, in mesocorticolimbic regions. Orexigenic neurochemicals that control food intake can provide a general signal for promoting caloric intake or a more specific signal for stimulating consumption of a particular macronutrient, fat, carbohydrate, or protein. The neurochemicals involved in controlling fat ingestion--galanin, enkephalin, orexin, melanin-concentrating hormone, and the endocannabinoids--show positive feedback with this macronutrient, as these peptides both increase fat intake and are further stimulated by its intake. This positive association offers some explanation for why foods high in fat are so often overconsumed. Consumption of ethanol, a drug of abuse that also contains calories, is similarly driven by the neurochemical systems involved in fat intake, according to evidence that closely relates fat and ethanol consumption. Further understanding of the systems involved in consummatory behavior will enable the development of effective therapies for the treatment of both overeating and drug abuse.

Cannabinoid exposure in pubertal rats increases spontaneous ethanol consumption and NMDA receptor associated protein levels

Recent evidence suggests an involvement of the endocannabinoid system in the regulation of emotional behaviour and ethanol intake. Here we investigated age-specific acute behavioural effects of the cannabinoid receptor agonist WIN 55,212-2 (WIN) on anxiety-related behaviour and voluntary ethanol consumption in rats. Animals were treated with WIN (1.2 mg/kg)/vehicle at puberty onset on postnatal day (PD) 40, or at adulthood (PD 100). Animals were tested in the elevated plus-maze (EPM) and the light/dark emergence test (EMT) and for the initial response to alcohol in a free-choice ethanol consumption paradigm. Acute WIN treatment increased anxiety-related behaviours, and this effect was found to be partially more pronounced in pubertal than adult rats. Additionally, increased intake of higher ethanol solutions after cannabinoid treatment was only observed in pubertal rats. These drug-induced behavioural changes during puberty are paralleled by induction of the NR1 subunit of the NMDA receptor in the medial prefrontal cortex and the striatum. Moreover, pubertal but not adult WIN administration increased the levels of the scaffold protein Homer in these brain regions. Enhanced CB₁ receptor levels in the reinforcement system were also observed in pubertal compared to adult rats. These data support the notion that puberty is a highly vulnerable period for the aversive effects of cannabinoid exposure. In particular, augmented ethanol intake in pubertal cannabinoid-exposed animals might be related to some extent to increased emotional behaviour and in particular to enhanced NMDA and CB₁ receptor signalling.

The role of metabotropic glutamate receptors in addiction: evidence from preclinical models

Addiction is a chronic disorder characterised by repeated bouts of drug taking, abstinence and relapse. The addicted state may be in part due to drug-induced neuroadaptations in the mesocorticolimbic and corticostriatal pathways. Recently focus has been on the role of aberrant glutamate transmission and its contribution to the hierarchical control over these systems. This review will expand our current knowledge of the most recent advances that have been made in preclinical animal models that provide evidence that implicate metabotropic glutamate receptors (mGluRs) in contributing to the neuroadaptations pertinent to addiction, as well as the role of Homer proteins in regulating these responses. The recent discovery of receptor mosaics will be discussed which add an additional dimension to the complexity of understanding the mechanism of glutamate mediated behaviours. Finally this review introduces a new area related to glutamatergic responses, namely microRNAs, that may become pivotal in directing our future understanding of how to best target intervention strategies to prevent addictive behaviours.

Immunogold electron microscopic evidence of differential regulation of GluN1, GluN2A, and GluN2B, NMDA-type glutamate receptor subunits in rat hippocampal CA1 synapses during benzodiazepine withdrawal

Benzodiazepine withdrawal-anxiety is associated with enhanced α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPAR)-mediated glutamatergic transmission in rat hippocampal CA1 synapses due to enhanced synaptic insertion and phosphorylation of GluA1 homomers. Interestingly, attenuation of withdrawal-anxiety is associated with a reduction in N-methyl-D-aspartate receptor (NMDAR)-mediated currents and subunit expression, secondary to AMPA receptor potentiation. Therefore, in this study ultrastructural evidence for possible reductions in NMDAR GluN1, GluN2A, and GluN2B subunits was sought at CA1 stratum radiatum synapses in proximal dendrites using postembedding immunogold labeling of tissues from rats withdrawn for 2 days from 1-week daily oral administration of the benzodiazepine, flurazepam (FZP). GluN1-immunogold density and the percentage of immunopositive synapses were significantly decreased in tissues from FZP-withdrawn rats. Similar decreases were observed for GluN2B subunits; however, the relative lateral distribution of GluN2B-immunolabeling within the postsynaptic density did not change after BZ withdrawal. In contrast to the GluN2B subunit, the percentage of synapses labeled with the GluN2A subunit antibody and the density of immunogold labeling for this subunit was unchanged. The spatial localization of immunogold particles associated with each NMDAR subunit was consistent with a predominantly postsynaptic localization. The data therefore provide direct evidence for reduced synaptic GluN1/GluN2B receptors and preservation of GluN1/GluN2A receptors in the CA1 stratum radiatum region during BZ withdrawal. Based on collective findings in this benzodiazepine withdrawal-anxiety model, we propose a functional model illustrating the changes in glutamate receptor populations at excitatory synapses during benzodiazepine withdrawal.

Cocaine-induced metaplasticity in the nucleus accumbens: silent synapse and beyond

The neuroadaptation theory of addiction suggests that, similar to the development of most memories, exposure to drugs of abuse induces adaptive molecular and cellular changes in the brain which likely mediate addiction-related memories or the addictive state. Compared to other types of memories, addiction-related memories develop fast and last extremely long, suggesting that the cellular and molecular processes that mediate addiction-related memories are exceptionally adept and efficient. We recently demonstrated that repeated exposure to cocaine generated a large portion of "silent" glutamatergic synapses within the nucleus accumbens (NAc). Silent glutamatergic synapses are synaptic connections in which only N-methyl-D-aspartic acid receptor (NMDAR)-mediated responses are readily detected whereas alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) are absent or highly labile. Extensive experimental evidence suggests that silent synapses are conspicuously efficient plasticity sites at which long-lasting plastic changes can be more easily induced and maintained. Thus, generation of silent synapses can be regarded as a process of metaplasticity, which primes the NAc for subsequent durable and robust plasticity for addiction-related memories. Focusing on silent synapse-based metaplasticity, this review discusses how key brain regions, such as the NAc, utilize the metaplasticity mechanism to optimize the plasticity machineries to achieve fast and durable plastic changes following exposure to cocaine. A summary of recent related results suggests that upon cocaine exposure, newly generated silent synapses may prime excitatory synapses within the NAc for long-term potentiation (LTP), thus setting the direction of future plasticity. Furthermore, because cocaine-generated silent synapses are enriched in NMDARs containing the NR2B subunit, the enhanced NR2B-signaling may set up a selective recruitment of certain types of AMPARs. Thus, silent synapse-based metaplasticity may lead to not only quantitative but also qualitative alterations in excitatory synapses within the NAc. This review is one of the first systematic analyses regarding the hypothesis that drugs of abuse induce metaplasticity, which regulates the susceptibility, the direction, and the molecular details of subsequent plastic changes. Taken together, metaplasticity ultimately serves as a key step in mediating cascades of addiction-related plastic alterations.

Molecular targets of alcohol action: Translational research for pharmacotherapy development and screening

Alcohol abuse and dependence are multifaceted disorders with neurobiological, psychological, and environmental components. Research on other complex neuropsychiatric diseases suggests that genetically influenced intermediate characteristics affect the risk for heavy alcohol consumption and its consequences. Diverse therapeutic interventions can be developed through identification of reliable biomarkers for this disorder and new pharmacological targets for its treatment. Advances in the fields of genomics and proteomics offer a number of possible targets for the development of new therapeutic approaches. This brain-focused review highlights studies identifying neurobiological systems associated with these targets and possible pharmacotherapies, summarizing evidence from clinically relevant animal and human studies, as well as sketching improvements and challenges facing the fields of proteomics and genomics. Concluding thoughts on using results from these profiling technologies for medication development are also presented.

Ethanol modulation of synaptic plasticity

Synaptic plasticity in the most general terms represents the flexibility of neurotransmission in response to neuronal activity. Synaptic plasticity is essential both for the moment-by-moment modulation of neural activity in response to dynamic environmental cues and for long-term learning and memory formation. These temporal characteristics are served by an array of pre- and post-synaptic mechanisms that are frequently modulated by ethanol exposure. This modulation likely makes significant contributions to both alcohol abuse and dependence. In this review, I discuss the modulation of both short-term and long-term synaptic plasticity in the context of specific ethanol-sensitive cellular substrates. A general discussion of the available preclinical, animal-model based neurophysiology literature provides a comparison between results from in vitro and in vivo studies. Finally, in the context of alcohol abuse and dependence, the review proposes potential behavioral contributions by ethanol modulation of plasticity.

Accumbens Homer2-mediated signaling: a factor contributing to mouse strain differences in alcohol drinking?

Alcohol-induced increases in nucleus accumbens glutamate actively regulate alcohol consumption, and the alcohol responsiveness of corticoaccumbens glutamate systems relates to genetic variance in alcohol reward. Here, we extend earlier data for inbred mouse strain differences in accumbens glutamate by examining for differences in basal and alcohol-induced changes in the striatal expression of glutamate-related signaling molecules between inbred C57BL/6J and DBA2/J mice. Repeated alcohol treatment (8 × 2 g/kg) increased the expression of Group1 metabotropic glutamate receptors, the NR2a/b subunits of the N-methyl-D-aspartate receptor, Homer2a/b, as well as the activated forms of protein kinase C (PKC) epsilon and phosphoinositol-3-kinase within ventral, but not dorsal, striatum. Regardless of prior alcohol experience, C57BL/6J mice exhibited higher accumbens levels of mGluR1/5, Homer2a/b, NR2a and activated kinases vs. DBA2/J mice, whereas an alcohol-induced rise in dorsal striatum mGluR1/5 expression was observed only in C57BL/6J mice. We next employed virus-mediated gene transfer approaches to ascertain the functional relevance of the observed strain difference in accumbens Homer2 expression for B6/D2 differences in alcohol-induced glutamate sensitization, as well as alcohol preference/intake. Manipulating nucleus accumbens shell Homer2b expression actively regulated these measures in C57BL/6J mice, whereas DBA2/J mice were relatively insensitive to the neurochemical and behavioral effects of virus-mediated changes in Homer2 expression. These data support the over-arching hypothesis that augmented accumbens Homer2-mediated glutamate signaling may be an endophenotype related to genetic variance in alcohol consumption. If relevant to humans, such data pose polymorphisms affecting glutamate receptor/Homer2 signaling in the etiology of alcoholism.

Down-regulation of synaptic GluN2B subunit-containing N-methyl-D-aspartate receptors: a physiological brake on CA1 neuron α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid hyperexcitability during benzodiazepine withdrawal

A significant link was previously established between benzodiazepine withdrawal anxiety and a progressive increase in α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) potentiation in hippocampal CA1 neurons from rats withdrawn up to 2 days from 1-week oral administration of the benzodiazepine flurazepam (FZP). Despite AMPAR current potentiation, withdrawal anxiety was masked by a 2-fold reduction in CA1 neuron N-methyl-D-aspartate receptor (NMDAR) currents since preinjection of an NMDA antagonist restored NMDAR currents and unmasked anxiety in 2-day FZP-withdrawn rats. In the current study, GluN subunit levels in postsynaptic density (PSD)-enriched subfractions of CA1 minislices were compared with GluN2B-mediated whole-cell currents evoked in CA1 neurons in hippocampal slices from 1- and 2-day FZP-withdrawn rats. GluN1 and GluN2B, although not the phosphoSer1303-GluN2B ratio or GluN2A subunit levels, were decreased in PSD subfractions from 2-day, but not 1-day, FZP-withdrawn rats. Consistent with immunoblot analyses, GluN2B-mediated NMDAR currents evoked in slices from 2-day FZP-withdrawn rats were decreased in the absence, but not the presence, of the GluN2B subunit-selective antagonist ifenprodil. In contrast, ifenprodil-sensitive NMDAR currents were unchanged in slices from 1-day withdrawn rats. Because AMPA (1 μM) preincubation of slices from 1-day FZP-withdrawn rats induced depression of GluN2B subunit-mediated currents, depression of NMDAR currents was probably secondary to AMPAR potentiation. CA1 neuron NMDAR currents were depressed ∼50% after 2-day withdrawal and offset potentiation of AMPAR-mediated currents, leaving total charge transfer unchanged between groups. Collectively, these findings suggest that a reduction of GluN2B-containing NMDAR may serve as a homeostatic feedback mechanism to modulate glutamatergic synaptic strength during FZP withdrawal to alleviate benzodiazepine withdrawal symptoms.

Ethanol acutely modulates mGluR1-dependent long-term depression in cerebellum

BACKGROUND

Acute and chronic ethanol exposure produces profound impairments in motor functioning. Individuals with lower sensitivity to the acute motor impairing effects of ethanol have an increased risk of developing alcohol dependence and abuse, and infants with subtle delays in motor coordination development may have an increased risk for subsequently developing alcoholism. Thus, understanding the mechanism by which ethanol disrupts motor functioning is very important.

METHODS

Parasagittal slices of the cerebellar vermis (250 microM thick) were prepared from P17 to 20 Sprague-Dawley rats. Whole-cell recordings of Purkinje cells were obtained with an Axopatch 200B amplifier. Parallel fiber-Purkinje cell synaptic currents were sampled at 1 kHz and digitized at 10 kHz, and synaptic long-term depression (LTD) was observed in either external or internal application of ethanol for comparison.

RESULTS

We determined whether ethanol acutely affects parallel fiber LTD using whole-cell patch-clamp recordings from Purkinje cells. Application of ethanol both externally (50 mM) and internally (17 and 10 mM) significantly suppressed mGluR-mediate slow currents. Short-term external ethanol exposure (50 but not 17 mM) during tetanus blocked mGluR-dependent parallel fiber LTD. Furthermore, internal 17 and 10 mM ethanol completely inhibited this LTD.

CONCLUSIONS

The results of the current study demonstrate that ethanol acutely suppresses parallel fiber LTD and may influence the mGluR-mediated slow current intracellularly. This study, plus previous evidence by Carta and colleagues (2006) and Belmeguenai and colleagues (2008), suggests significant actions of ethanol on mGluR-mediated currents and its dependent plasticity in brain.