mGlu5 receptors are involved in the discriminative stimulus effects of self-administered ethanol in rats

Regulation of glutamate signaling in the extended amygdala by adolescent alcohol exposure

Adolescence is a critical period for brain development and behavioral maturation, marked by increased risk-taking behavior and the initiation of drug use. There are significant changes in gray matter volume and pruning of synapses along with a shift in excitatory to inhibitory balance which marks the maturation of cognition and decision-making. Because of ongoing brain development, adolescents are particularly sensitive to the detrimental effects of drugs, including alcohol, which can cause long-lasting consequences into adulthood. The extended amygdala is a region critically implicated in withdrawal and negative affect such as anxiety and depression. As negative affective disorders develop during adolescence, the effects of adolescent alcohol exposure on extended amygdala circuitry needs further inquiry. Here we aim to provide a framework to discuss the existing literature on the extended amygdala, the neuroadaptations which result from alcohol use, and the intersection of factors which contribute to the long-lasting effects of this exposure.

Allosteric modulation of metabotropic glutamate receptors in alcohol use disorder: Insights from preclinical investigations

Metabotropic glutamate (mGlu) receptors are family C G protein-coupled receptors (GPCRs) that modulate neuronal excitability and synaptic transmission throughout the nervous system. Owing to recent advances in development of subtype-selective allosteric modulators of mGlu receptors, individual members of the mGlu receptor family have been proposed as targets for treating a variety of neurological and psychiatric disorders, including substance use disorders. In this chapter, we highlight preclinical evidence that allosteric modulators of mGlu receptors could be useful for reducing alcohol consumption and preventing relapse in alcohol use disorder (AUD). We begin with an overview of the preclinical models that are used to study mGlu receptor involvement in alcohol-related behaviors. Alcohol exposure causes adaptations in both expression and function of various mGlu receptor subtypes, and pharmacotherapies aimed at reversing these adaptations have the potential to reduce alcohol consumption and seeking. Positive allosteric modulators (PAMs) of mGlu₂ and negative allosteric modulators of mGlu₅ show particular promise for reducing alcohol intake and/or preventing relapse. Finally, this chapter discusses important considerations for translating preclinical findings toward the development of clinically useful drugs, including the potential for PAMs to avoid tolerance issues that are frequently observed with repeated administration of GPCR agonists.

MGluR5 activity is required for the induction of ethanol behavioral sensitization and associated changes in ERK MAP kinase phosphorylation in the nucleus accumbens shell and lateral habenula

Metabotropic glutamate receptor subtype-5 (mGluR5) activity regulates a variety of behavioral pathologies associated with alcohol addiction. The main goal of this study was to determine if mGluR5 regulates the induction of ethanol-induced locomotor sensitization, which is a model of experience-dependent plasticity following initial exposure to drugs of abuse. The extracellular signal-regulated kinase (ERK_1/2) pathway is downstream of mGluR5 and implicated in alcohol addiction; however, its role in sensitization remains unexplored. We sought to determine if mGluR5-mediated changes in ethanol-induced sensitization are associated with changes in ERK_1/2 phosphorylation (pERK_1/2) in specific brain regions. Adult male DBA/2 J mice were tested for acute locomotor response to ethanol (0 or 2 g/kg, IP) followed by a 9-day induction period in which the mGluR5 antagonist MPEP (0 or 30 mg/kg, IP) was administered prior to ethanol (0 or 2.5 g/kg, IP). One day later, ethanol (2 g/kg) produced a robust within- and between-group increase in locomotor activity, indicating sensitization in mice that received MPEP (0 mg/kg) during induction. MPEP (30 mg/kg) treatment during induction resulted in locomotor response to ethanol (2 g/kg) challenge that was equivalent to an acute response, indicating full blockade of sensitization. Sensitization was associated with increased pERK_1/2 immunoreactivity (IR) in nucleus accumbens shell (AcbSh) and a reduction in lateral habenula (LHb), both of which were blocked by MPEP treatment during induction. Sensitization was also associated with mGluR5-independent increases in pERK_1/2 IR in the nucleus accumbens core and decreases in the dentate gyrus and lateral septum. These data indicate that mGluR5 activity is required for the induction of ethanol locomotor sensitization and associated changes in ERK_1/2 phosphorylation in the AcbSh and LHb, which raises the hypothesis that mGluR5-mediated cell signaling in these brain regions may mediate the induction of sensitization. Elucidating mechanisms of sensitization may increase understanding of how ethanol hijacks behavioral functions during the development of addiction.

Cross-Species Translational Findings in the Discriminative Stimulus Effects of Ethanol

The progress on understanding the pharmacological basis of ethanol's discriminative stimulus effects has been substantial, but appears to have plateaued in the past decade. Further, the cross-species translational efforts are clear in laboratory animals, but have been minimal in human subject studies. Research findings clearly demonstrate that ethanol produces a compound stimulus with primary activity through GABA and glutamate receptor systems, particularly ionotropic receptors, with additional contribution from serotonergic mechanisms. Further progress should capitalize on chemogenetic and optogenetic techniques in laboratory animals to identify the neural circuitry involved in mediating the discriminative stimulus effects of ethanol. These infrahuman studies can be guided by in vivo imaging of human brain circuitry mediating ethanol's subjective effects. Ultimately, identifying receptors systems, as well as where they are located within brain circuitry, will transform the use of drug discrimination procedures to help identify possible treatment or prevention strategies for alcohol use disorder.

Potentiation of amygdala AMPA receptor activity selectively promotes escalated alcohol self-administration in a CaMKII-dependent manner

Growing evidence indicates that drugs of abuse gain control over the individual by usurping glutamate-linked mechanisms of neuroplasticity in reward-related brain regions. Accordingly, we have shown that glutamate α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) activity in the amygdala is required for the positive reinforcing effects of alcohol, which underlie the initial stages of addiction. It is unknown, however, if enhanced AMPAR activity in the amygdala facilitates alcohol self-administration, which is a kernel premise of glutamate hypotheses of addiction. Here, we show that low-dose alcohol (0.6 g/kg/30 minutes) self-administration increases phosphorylation (activation) of AMPAR subtype GluA1 S831 (pGluA1 S831) in the central amygdala (CeA), basolateral amygdala and nucleus accumbens core (AcbC) of selectively bred alcohol-preferring P-rats as compared with behavior-matched (non-drug) sucrose controls. The functional role of enhanced AMPAR activity was assessed via site-specific infusion of the AMPAR positive modulator, aniracetam, in the CeA and AcbC prior to alcohol self-administration. Intra-CeA aniracetam increased alcohol-reinforced but not sucrose-reinforced responding and was ineffective following intra-AcbC infusion. Because GluA1 S831 is a Ca2+/calmodulin-dependent protein kinase II (CaMKII) substrate, we sought to determine if AMPAR regulation of enhanced alcohol self-administration is dependent on CaMKII activity. Intra-CeA infusion of the cell-permeable CaMKII peptide inhibitor myristolated autocamtide-2-related inhibitory peptide (m-AIP) dose-dependently reduced alcohol self-administration. A subthreshold dose of m-AIP also blocked the aniracetam-induced escalation of alcohol self-administration, demonstrating that AMPAR-mediated potentiation of alcohol reinforcement requires CaMKII activity in the amygdala. Enhanced activity of plasticity-linked AMPAR-CaMKII signaling in the amygdala may promote escalated alcohol use via increased positive reinforcement during the initial stages of addiction.

Glutamate plasticity woven through the progression to alcohol use disorder: a multi-circuit perspective

Glutamate signaling in the brain is one of the most studied targets in the alcohol research field. Here, we report the current understanding of how the excitatory neurotransmitter glutamate, its receptors, and its transporters are involved in low, episodic, and heavy alcohol use. Specific animal behavior protocols can be used to assess these different drinking levels, including two-bottle choice, operant self-administration, drinking in the dark, the alcohol deprivation effect, intermittent access to alcohol, and chronic intermittent ethanol vapor inhalation. Importantly, these methods are not limited to a specific category, since they can be interchanged to assess different states in the development from low to heavy drinking. We encourage a circuit-based perspective beyond the classic mesolimbic-centric view, as multiple structures are dynamically engaged during the transition from positive- to negative-related reinforcement to drive alcohol drinking. During this shift from lower-level alcohol drinking to heavy alcohol use, there appears to be a shift from metabotropic glutamate receptor-dependent behaviors to N-methyl-D-aspartate receptor-related processes. Despite high efficacy of the glutamate-related pharmaceutical acamprosate in animal models of drinking, it is ineffective as treatment in the clinic. Therefore, research needs to focus on other promising glutamatergic compounds to reduce heavy drinking or mediate withdrawal symptoms or both.

Chronic intermittent alcohol disrupts the GluN2B-associated proteome and specifically regulates group I mGlu receptor-dependent long-term depression

N-Methyl-d-aspartate receptors (NMDARs) are major targets of both acute and chronic alcohol, as well as regulators of plasticity in a number of brain regions. Aberrant plasticity may contribute to the treatment resistance and high relapse rates observed in alcoholics. Recent work suggests that chronic alcohol treatment preferentially modulates both the expression and subcellular localization of NMDARs containing the GluN2B subunit. Signaling through synaptic and extrasynaptic GluN2B-NMDARs has already been implicated in the pathophysiology of various other neurological disorders. NMDARs interact with a large number of proteins at the glutamate synapse, and a better understanding of how alcohol modulates this proteome is needed. We employed a discovery-based proteomic approach in subcellular fractions of hippocampal tissue from chronic intermittent alcohol (CIE)-exposed C57Bl/6J mice to gain insight into alcohol-induced changes in GluN2B signaling complexes. Protein enrichment analyses revealed changes in the association of post-synaptic proteins, including scaffolding, glutamate receptor and PDZ-domain binding proteins with GluN2B. In particular, GluN2B interaction with metabotropic glutamate (mGlu)1/5 receptor-dependent long-term depression (LTD)-associated proteins such as Arc and Homer 1 was increased, while GluA2 was decreased. Accordingly, we found a lack of mGlu1/5 -induced LTD while α1 -adrenergic receptor-induced LTD remained intact in hippocampal CA1 following CIE. These data suggest that CIE specifically disrupts mGlu1/5 -LTD, representing a possible connection between NMDAR and mGlu receptor signaling. These studies not only demonstrate a new way in which alcohol can modulate plasticity in the hippocampus but also emphasize the utility of this discovery-based proteomic approach to generate new hypotheses regarding alcohol-related mechanisms.

Metabotropic and ionotropic glutamate receptors as potential targets for the treatment of alcohol use disorder

Emerging evidence indicates that dysfunctional glutamate neurotransmission is critical in the initiation and development of alcohol and drug dependence. Alcohol consumption induced downregulation of glutamate transporter 1 (GLT-1) as reported in previous studies from our laboratory. Glutamate is the major excitatory neurotransmitter in the brain, which acts via interactions with several glutamate receptors. Alcohol consumption interferes with the glutamatergic signal transmission by altering the functions of these receptors. Among the glutamate receptors involved in alcohol-drinking behavior are the metabotropic receptors such as mGluR1/5, mGluR2/3, and mGluR7, as well as the ionotropic receptors, NMDA and AMPA. Preclinical studies using agonists and antagonists implicate these glutamatergic receptors in the development of alcohol use disorder (AUD). Therefore, the purpose of this review is to discuss the neurocircuitry involving glutamate transmission in animals exposed to alcohol and further outline the role of metabotropic and ionotropic receptors in the regulation of alcohol-drinking behavior. This review provides ample information about the potential therapeutic role of glutamatergic receptors for the treatment of AUD.

Gabapentin potentiates sensitivity to the interoceptive effects of alcohol and increases alcohol self-administration in rats

Gabapentin, a drug used in the treatment of epileptic seizures and neuropathic pain, has shown efficacy in the treatment of alcohol dependence. Moreover, given that gabapentin is used in the general population (e.g., non-dependent individuals, social drinkers), we sought to utilize preclinical assessments to examine the effects of gabapentin on sensitivity to moderate alcohol doses and alcohol self-administration in rats with a history of moderate drinking. To this end, we assessed whether gabapentin (0, 10, 30, 120 mg/kg, IG) pretreatment alters sensitivity to experimenter- and self-administered alcohol, and whether gabapentin alone has alcohol-like discriminative stimulus effects in rats trained to discriminate alcohol dose (1 g/kg, IG) vs. water. Second, we assessed whether gabapentin (0, 10, 30, 60 mg/kg, IG) would alter alcohol self-administration. Gabapentin pretreatment potentiated the interoceptive effects of both experimenter-administered and self-administered alcohol in discrimination-trained rats. Additionally, the highest gabapentin doses tested (30 and 120 mg/kg) were found to have partial alcohol-like discriminative stimulus effects when administered alone (e.g., without alcohol). In the self-administration trained rats, gabapentin pretreatment (60 mg/kg) resulted in an escalation in alcohol self-administration. Given the importance of interoceptive drug cues in priming and maintaining self-administration, these data define a specific behavioral mechanism (i.e., potentiation of alcohol effects) by which gabapentin may increase alcohol self-administration in non-dependent populations.

Stress hormone exposure reduces mGluR5 expression in the nucleus accumbens: functional implications for interoceptive sensitivity to alcohol

Escalations in alcohol drinking associated with experiencing stressful life events and chronic life stressors may be related to altered sensitivity to the interoceptive/subjective effects of alcohol. Indeed, through the use of drug discrimination methods, rats show decreased sensitivity to the discriminative stimulus (interoceptive) effects of alcohol following exposure to the stress hormone corticosterone (CORT). This exposure produces heightened elevations in plasma CORT levels (eg, as may be experienced by an individual during stressful episodes). We hypothesized that decreased sensitivity to alcohol may be related, in part, to changes in metabotropic glutamate receptors-subtype 5 (mGluR5) in the nucleus accumbens, as these receptors in this brain region are known to regulate the discriminative stimulus effects of alcohol. In the accumbens, we found reduced mGluR5 expression (immunohistochemistry and Western blot) and decreased neural activation (as measured by c-Fos immunohistochemistry) in response to a moderate alcohol dose (1 g/kg) following CORT exposure (7 days). The functional role of these CORT-induced adaptations in relation to the discriminative stimulus effects of alcohol was confirmed, as both the systemic administration of 3-Cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB) an mGluR5 positive allosteric modulator and the intra-accumbens administration of (R,S)-2-Amino-2-(2-chloro-5-hydroxyphenyl)acetic acid sodium salt (CHPG) an mGluR5 agonist restored sensitivity to alcohol in discrimination-trained rats. These results suggest that activation of mGluR5 may alleviate the functional impact of the CORT-induced downregulation of mGluR5 in relation to sensitivity to alcohol. Understanding the contribution of such neuroadaptations to the interoceptive effects of alcohol may enrich our understanding of potential changes in subjective sensitivity to alcohol during stressful episodes.

Ovarian hormones and the heterogeneous receptor mechanisms mediating the discriminative stimulus effects of ethanol in female rats

Past studies have suggested that progesterone-derived ovarian hormones contribute to the discriminative stimulus effects of ethanol, particularly via progesterone metabolites that act at γ-aminobutyric acid type A (GABA(A)) receptors. It is unknown whether loss of ovarian hormones in women, for example, after menopause, may be associated with altered receptor mediation of the effects of ethanol. The current study measured the substitution of allopregnanolone, pregnanolone, pentobarbital, midazolam, dizocilpine, TFMPP, and RU 24969 in female sham and ovariectomized rats trained to discriminate 1.0 g/kg ethanol from water. The groups did not differ in the substitution of GABA(A)-positive modulators (barbiturates, benzodiazepines, neuroactive steroids) or the N-methyl-D-aspartate receptor antagonist dizocilpine. Similarly, blood-ethanol concentration did not differ between the groups, and plasma adrenocorticotropic hormone, progesterone, pregnenolone, and deoxycorticosterone were unchanged 30 min after administration of 1.0 g/kg ethanol or water. However, substitution of neuroactive steroids and RU 24969, a 5-hydroxytryptamine (5-HT)(1A/1B) receptor agonist, was lower than observed in previous studies of male rats, and TFMPP substitution was decreased in ovariectomized rats. Ovarian hormones appear to contribute to 5-HT receptor mediation of the discriminative stimulus effects of ethanol in rats.

Assessment of the interoceptive effects of alcohol in rats using short-term training procedures

In the present study, we sought to determine whether the interoceptive effects of alcohol (1 g/kg, IG) could be assessed using a Pavlovian discrimination method, in which the alcohol drug state sets the occasion for which an environmental stimulus (e.g., light) will be followed by a sucrose reward. This procedure takes advantage of a naturally occurring behavior (i.e., food-seeking) which can be trained rapidly prior to the initiation of discrimination training. Given that the interoceptive effects of alcohol are routinely assessed using operant drug discrimination methods, another group of rats was trained using standard two-lever operant drug discrimination procedures in an effort to compare the Pavlovian procedure to a known behavioral benchmark. The results from this work show that, in addition to operant discrimination procedures, a Pavlovian discrimination task can be used to evaluate the interoceptive effects of alcohol. In addition to the brief behavioral sucrose access training (3 days) required prior to the initiation of the Pavlovian discrimination, the alcohol discrimination was acquired relatively rapidly (i.e., 8 training sessions), shortening the overall duration of the experiment. These features of the Pavlovian procedure make it a valuable method by which to assess the interoceptive effects of alcohol if a short experimental time frame is required, such as assessing the interoceptive effects of alcohol during a brief developmental window (e.g., adolescence) or determining the effects of a pretreatment (i.e., chronic stress, chronic drug pretreatment) on the acquisition of the alcohol discrimination. As such, this initial characterization confirms the feasibility of using this Pavlovian discrimination training method as an additional tool by which to assess the interoceptive effects of alcohol, as there may be experimental situations that necessitate short term discrimination training.

The effects of repeated corticosterone exposure on the interoceptive effects of alcohol in rats

RATIONALE AND OBJECTIVE

Repeated and/or heightened elevations in glucocorticoids (e.g., repeated stress) can promote escalated drug-taking behaviors and induce compromised HPA axis function. Given that interoceptive/subjective drug cues are a fundamental factor in drug-taking behavior, we sought to determine the effects of exposure to repeated elevations in the glucocorticoid corticosterone (CORT) on the interoceptive effects of alcohol in rats using drug discrimination techniques.

METHODS

Male Long Evans rats trained to discriminate alcohol (1 g/kg, IG) vs. water were exposed to CORT (300 μg/ml) in the home cage drinking water for 7 days. The interoceptive effects of experimenter- and self-administered alcohol were assessed and HPA axis function was determined.

RESULTS

The interoceptive effects of experimenter- and self-administered alcohol were blunted following CORT. Control experiments determined that this decreased sensitivity was unrelated to discrimination performance impairments or decreased CORT levels at the time of testing and was dependent on repeated CORT exposure. Susceptibility to compromised HPA axis function following CORT exposure was suggested by an altered pattern of CORT secretion and blunted CORT response following injection of the synthetic glucocorticoid dexamethasone.

CONCLUSIONS

These findings present a possible behavioral mechanism for escalated alcohol drinking during episodes of heightened elevations in glucocorticoids (e.g., stress). That is, during these episodes, individuals may consume more alcohol to achieve the desired interoceptive effects. Understanding these behavioral mechanisms may lead to a better understanding of factors that promote alcoholism and alcohol abuse in at risk populations.

Pharmacological modulation of mGluR7 with AMN082 and MMPIP exerts specific influences on alcohol consumption and preference in rats

Growing evidence supports a role for the central nervous system (CNS) neurotransmitter L-glutamate and its metabotropic receptors (mGluRs) in drug addiction in general and alcohol-use disorders in particular. Alcohol dependence, for instance, has a genetic component, and the recent discovery that variations in the gene coding for mGluR7 modulate alcohol consumption further validates involvement of the L-glutamate system. Consequently, increasing interest emerges in developing L-glutamatergic therapies for the treatment of alcohol abuse and dependence. To this end, we performed a detailed behavioral pharmacology study to investigate the regulation of alcohol consumption and preference following administration of the mGluR7-selective drugs N,N'-dibenzyhydryl-ethane-1,2-diamine dihydrochloride (AMN082) and 6-(4-Methoxyphenyl)-5-methyl-3-(4-pyridinyl)-isoxazolo[4,5-c]pyridin-4(5H)-one hydrochloride (MMPIP). Upon administration of the allosteric agonist AMN082 (10 mg/kg, i.p.) in rats, there was a significant decrease in ethanol consumption and preference, without affecting ethanol blood metabolism. In contrast, mGluR7 blockade with MMPIP (10 mg/kg, i.p.) showed an increase in alcohol intake and reversed AMN082's effect on ethanol consumption and preference. Both mGluR7-directed pharmacological tools had no effect on total fluid intake, taste preference, or on spontaneous locomotor activity. In conclusion, these findings support a specific regulatory role for mGluR7 on alcohol drinking and preference and provide evidence for the use of AMN082-type drugs as potential new treatments for alcohol-use disorders in man.

Intra-amygdala inhibition of ERK(1/2) potentiates the discriminative stimulus effects of alcohol

Extracellular signal-regulated kinase (ERK(1/2)) has been implicated in modulating drug seeking behavior and is a target of alcohol and other drugs of abuse. Given that the discriminative stimulus (subjective/interoceptive) effects of drugs are determinants of abuse liability and can influence drug seeking behavior, we examined the role of ERK(1/2) in modulating the discriminative stimulus effects of alcohol. Using drug discrimination procedures, rats were trained to discriminate a moderate intragastric (IG) alcohol dose (1g/kg) versus water (IG). Following an alcohol (1g/kg) discrimination session phosphorylated ERK(1/2) (pERK(1/2)) immunoreactivity (IR) was significantly elevated in the amygdala, but not the nucleus accumbens. Therefore, we hypothesized that intra-amygdala inhibition of ERK(1/2) would disrupt expression of the discriminative stimulus effects of alcohol. However, intra-amygdala or accumbens administration of the MEK/ERK(1/2) inhibitor U0126 (1 and 3μg) had no effect on the discriminative stimulus effects of the training dose of alcohol (1g/kg). Contrary to our hypothesis, intra-amygdala infusion of U0126 (3μg) potentiated the discriminative stimulus effects of a low alcohol dose (0.5g/kg) and had no effect following nucleus accumbens infusion. Importantly, site-specific inhibition of pERK(1/2) in each brain region was confirmed. Therefore, the increase in pERK(1/2) IR in the amygdala following systemic alcohol administration may be reflective of the widespread effects of alcohol on the brain (activation/inhibition of brain circuits), whereas the site specific microinjection studies confirmed functional involvement of intra-amygdala ERK(1/2). These findings show that activity of the ERK signaling pathway in the amygdala can influence the discriminative stimulus effects of alcohol.

Potent mGluR5 antagonists: pyridyl and thiazolyl-ethynyl-3,5-disubstituted-phenyl series

We report the synthesis of four series of 3,5-disubstituted-phenyl ligands targeting the metabotropic glutamate receptor subtype 5: (2-methylthiazol-4-yl)ethynyl (1a-j,), (6-methylpyridin-2-yl)ethynyl (2a-j), (5-methylpyridin-2-yl)ethynyl (3a-j,), and (pyridin-2-yl)ethynyl (4a-j,). The compounds were evaluated for antagonism of glutamate-mediated mobilization of internal calcium in an mGluR5 in vitro assay. All compounds were found to be full antagonists and exhibited low nanomolar to subnanomolar activity.

The metabotropic glutamate receptor subtype 5 mediates sensitivity to the sedative properties of ethanol

OBJECTIVE

Inbred long-sleep and short-sleep mice (ILS and ISS) were selectively bred for differential sensitivity to the sedative effects of ethanol. Lines of mice derived from these progenitors have been used to identify several quantitative trait loci (QTLs) mediating loss of the righting reflex due to ethanol (LORE). This study investigated the metabotropic glutamate receptor subtype 5 (mGluR5) as a candidate gene underlying Lore7, a QTL mediating differential LORE sensitivity.

METHODS

We used knockout mice, a quantitative complementation test, pharmacological antagonism of mGluR5, real-time quantitative PCR, radioligand binding, DNA sequencing, and bioinformatics to examine the role of mGluR5 in ethanol-induced sedation.

RESULTS

mGluR5 knockout mice had a significantly longer LORE duration than wildtype controls. Administration of the mGluR5 antagonist 2-methyl-6-(phenylethyl)-pyridine (MPEP) had differential effects on LORE in ILS and ISS mice. A quantitative complementation test also supported mGluR5 mediating LORE. Two intronic single-nucleotide polymorphisms in mGluR5 were highly correlated with LORE in recombinant inbred mice derived from a cross between ILS and ISS (LXS RIs). Differences in mGluR5 mRNA level and receptor density were observed between ILS and ISS in distinct brain regions. Finally, data from WebQTL showed that mGluR5 expression was highly correlated with several LORE phenotypes in the LXS RIs.

CONCLUSION

Altogether, this data provides convincing evidence that mGluR5 mediates differential sensitivity to the sedative effects of ethanol. Studies from the human literature have also identified mGluR5 as a potential candidate gene for ethanol sensitivity.

Chronic alcohol neuroadaptation and stress contribute to susceptibility for alcohol craving and relapse

Alcoholism is a chronic relapsing disorder. Major characteristics observed in alcoholics during an initial period of alcohol abstinence are altered physiological functions and a negative emotional state. Evidence suggests that a persistent, cumulative adaptation involving a kindling/allostasis-like process occurs during the course of repeated chronic alcohol exposures that is critical for the negative symptoms observed during alcohol withdrawal. Basic studies have provided evidence for specific neurotransmitters within identified brain sites being responsible for the negative emotion induced by the persistent cumulative adaptation following intermittent-alcohol exposures. After an extended period of abstinence, the cumulative alcohol adaptation increases susceptibility to stress- and alcohol cue-induced negative symptoms and alcohol seeking, both of which can facilitate excessive ingestion of alcohol. In the alcoholic, stressful imagery and alcohol cues alter physiological responses, enhance negative emotion, and induce craving. Brain fMRI imaging following stress and alcohol cues has documented neural changes in specific brain regions of alcoholics not observed in social drinkers. Such altered activity in brain of abstinent alcoholics to stress and alcohol cues is consistent with a continuing ethanol adaptation being responsible. Therapies in alcoholics found to block responses to stress and alcohol cues would presumably be potential treatments by which susceptibility for continued alcohol abuse can be reduced. By continuing to define the neurobiological basis of the sustained alcohol adaptation critical for the increased susceptibility of alcoholics to stress and alcohol cues that facilitate craving, a new era is expected to evolve in which the high rate of relapse in alcoholism is minimized.

The mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) potentiates conditioned place preference induced by various addictive and non-addictive drugs in rats

We have recently reported that the metabotropic glutamate receptor 5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) potentiates acquisition of conditioned place preference (CPP) induced by heroin and ketamine. The present study investigated to what extent this effect of MPEP can be generalized to other classes of drugs, such as the stimulants nicotine and cocaine, and to drugs that produce CPP in the rat despite a lack of abuse potential in humans, such as buspirone and clonidine. Adult male Sprague Dawley rats were subjected to a standard unbiased CPP protocol (six conditioning sessions lasting 20 minutes for nicotine and 40 minutes for the other compounds). Rats were conditioned with either nicotine (0.05-0.2 mg/kg, subcutaneously), cocaine [1-10 mg/kg, intraperitoneally (i.p.)], buspirone (0.3-3 mg/kg, i.p.) or clonidine (0.2-0.6 mg/kg, i.p.) in combination with MPEP (0 or 10 mg/kg, i.p.). For nicotine and cocaine, the minimal effective dose to induce CPP was lowered by pre-treatment with MPEP. While buspirone and clonidine did not induce CPP when given alone (i.e. combined with MPEP vehicle), both compounds induced CPP after pre-treatment with MPEP. It is concluded that MPEP consistently potentiates acquisition of drug-induced reward, independent of the mechanism of action of the co-administered drug. We suggest that the proposed anti-abuse effect of MPEP may be due to a substitution-like effect.

Metabotropic glutamate receptor 5 activity in the nucleus accumbens is required for the maintenance of ethanol self-administration in a rat genetic model of high alcohol intake

BACKGROUND

Systemic modulation of Group I and II metabotropic glutamate receptors (mGluRs) regulate ethanol self-administration in a variety of animal models. Although these receptors are expressed in reward-related brain regions, the anatomical specificity of their functional involvement in ethanol self-administration remains to be characterized. This study sought to evaluate the functional role of Group I (mGluR5) and Group II (mGluR2/3) in mesocorticolimbic brain regions in ethanol self-administration.

METHODS

Alcohol-preferring (P) rats, a genetic model of high alcohol drinking, were trained to self-administer ethanol (15% v/v) versus water in operant conditioning chambers. Effects of brain site-specific infusion of the mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP) and the mGluR2/3 agonist were then assessed on the maintenance of self-administration.

RESULTS

Microinjection of the mGluR5 antagonist MPEP in the nucleus accumbens reduced ethanol self-administration at a dose that did not alter locomotor activity. By contrast, infusion of the mGluR2/3 agonist LY379268 in the nucleus accumbens reduced self-administration and produced nonspecific reductions in locomotor activity. The mGluR5 involvement showed anatomical specificity as evidenced by lack of effect of MPEP infusion in the dorsomedial caudate or medial prefrontal cortex on ethanol self-administration. To determine reinforcer specificity, P-rats were trained to self-administer sucrose (.4% w/v) versus water, and effects of intra-accumbens MPEP were tested. The MPEP did not alter sucrose self-administration or motor behavior.

CONCLUSIONS

These results suggest that mGluR5 activity specifically in the nucleus accumbens is required for the maintenance of ethanol self-administration in individuals with genetic risk for high alcohol consumption.

Cognitive effects of Group I metabotropic glutamate receptor ligands in the context of drug addiction

Glutamate plays a pivotal role in regulating drug self-administration and drug-seeking behavior, and the past decade has witnessed a substantial surge of interest in the role of Group I metabotropic glutamate receptors (mGlu(1) and mGlu(5) receptors) in mediating these behaviors. As will be reviewed here, Group I mGlu receptors are involved in normal and drug-induced synaptic plasticity, drug reward, reinforcement and relapse-like behaviors, and addiction-related cognitive processes such as maladaptive learning and memory, behavioral inflexibility, and extinction learning. Animal models of addiction have revealed that antagonists of Group I mGlu receptors, particularly the mGlu(5) receptor, reduce self-administration of virtually all drugs of abuse. Since inhibitors of mGlu5 receptor function have now entered clinical trials for other medical conditions and appear to be well-tolerated, a key question that remains unanswered is - what changes in cognition are produced by these compounds that result in reduced drug intake and drug-seeking behavior? Finally, in contrast to mGlu(5) receptor antagonists, recent studies have indicated that positive allosteric modulation of mGlu(5) receptors actually enhances synaptic plasticity and improves various aspects of cognition, including spatial learning, behavioral flexibility, and extinction of drug-seeking behavior. Thus, while inhibition of Group I mGlu receptor function may reduce drug reward, reinforcement, and relapse-related behaviors, positive allosteric modulation of the mGlu5 receptor subtype may actually enhance cognition and potentially reverse some of the cognitive deficits associated with chronic drug use.

Metabotropic glutamate receptor ligands as potential therapeutics for addiction

There is now compelling evidence that the excitatory amino acid neurotransmitter glutamate plays a pivotal role in drug addiction and alcoholism. As a result, there has been increasing interest in developing glutamate-based therapies for the treatment of addictive disorders. Receptors for glutamate are primarily divided into two classes: ionotropic glutamate receptors (iGluRs) that mediate fast excitatory glutamate transmission, and metabotropic glutamate receptors (mGluRs), which are G-protein coupled receptors that mediate slower, modulatory glutamate transmission. Most iGluR antagonists, while showing some efficacy in animal models of addiction, exhibit serious side effects when tested in humans. mGluR ligands, on the other hand, which have been advanced to testing in clinical trials for various medical conditions, have demonstrated the ability to reduce drug reward, reinforcement, and relapse-like behaviors in animal studies. mGluR ligands that have been shown to be primarily effective are Group I (mGluR1 and mGluR5) negative allosteric modulators and Group II (mGluR2 and mGluR3) orthosteric presynaptic autoreceptor agonists. In this review, we will summarize findings from animal studies suggesting that these mGluR ligands may be of potential benefit in reducing on-going drug self-administration and may aid in the prevention of relapse. The neuroanatomical distribution of mGluR1, mGluR2/3, and mGluR5 receptors and the pharmacological properties of Group I negative allosteric modulators and Group II agonists will also be overviewed. Finally, we will discuss the current status of mGluR ligands in human clinical trials.

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

BACKGROUND

Homer proteins are constituents of scaffolding complexes that regulate the trafficking and function of central Group1 metabotropic glutamate receptors (mGluRs) and N-methyl-d-aspartate (NMDA) receptors. Research supports the involvement of these proteins in ethanol-induced neuroplasticity in mouse. In this study, we examined the effects of short versus long-term withdrawal from chronic ethanol consumption on Homer and glutamate receptor protein expression within striatal and amygdala subregions of selectively bred, alcohol-preferring P rats.

METHODS

For 6 months, male P rats had concurrent access to 15% and 30% ethanol solutions under intermittent (IA: 4 d/wk) or continuous (CA: 7 d/wk) access conditions in their home cage. Rats were killed 24 hours (short withdrawal: SW) or 4 weeks (long withdrawal: LW) after termination of ethanol access, subregions of interest were micropunched and tissue processed for detection of Group1 mGluRs, NR2 subunits of the NMDA receptor and Homer protein expression.

RESULTS

Within the nucleus accumbens (NAC), limited changes in NR2a and NR2b expression were detected in the shell (NACsh), whereas substantial changes were observed for Homer2a/b, mGluRs as well as NR2a and NR2b subunits in the core (NACc). Within the amygdala, no changes were detected in the basolateral subregion, whereas substantial changes, many paralleling those observed in the NACc, were detected in the central nucleus (CeA) subregion. In addition, most of the changes observed in the CeA, but not NACc, were present in both SW and LW rats.

CONCLUSIONS

Overall, these subregion specific, ethanol-induced increases in mGluR/Homer2/NR2 expression within the NAC and amygdala suggest changes in glutamatergic plasticity had taken place. This may be a result of learning and subsequent memory formation of ethanol's rewarding effects in these brain structures, which may, in part, mediate the chronic relapsing nature of alcohol abuse.

Interoceptive effects of alcohol require mGlu5 receptor activity in the nucleus accumbens

The interoceptive effects of alcohol are major determinants of addiction liability. Metabotropic glutamate (mGlu) receptors are widely expressed in striatal circuits known to modulate drug-seeking. Given that the interoceptive effects of drugs can be important determinants of abuse liability, we hypothesized that striatal mGlu receptors modulate the interoceptive effects of alcohol. Using drug discrimination learning, rats were trained to discriminate alcohol (1 g/kg, i.g.) versus water. We found that systemic antagonism of metabotropic glutamate subtype 5 (mGlu5) receptors [10 mg/kg 2-methyl-6-(phenylethynyl)pyridine (MPEP) and 3 mg/kg 3-((2-methyl-1,3-thiazol-4-yl)ethynyl)pyridine], but not mGlu1 receptors ([0.3-3 mg/kg JNJ16259685) (3,4-dihydro-2H-pyrano[2,3]beta-quinolin-7-yl)(cis-4-methoxycyclohexyl) methanone)], inhibited the discriminative stimulus effects of alcohol. Furthermore, mGlu5 receptor antagonism (10 mg/kg MPEP) significantly inhibited neuronal activity in the nucleus accumbens core as levels of the transcription factor c-Fos were significantly reduced. Accordingly, targeted inhibition of mGlu5 receptors (20 microg of MPEP) in the nucleus accumbens core blunted the discriminative stimulus effects of alcohol (1 g/kg). Anatomical specificity was confirmed by the lack of effect of inhibition of mGlu5 receptors (10-30 microg of MPEP) in the dorsomedial caudate-putamen and the similar cytological expression patterns and relative density of mGlu5 receptors between the brain regions. Functional involvement of intra-accumbens mGlu5 receptors was confirmed as activation of mGlu5 receptors [10 microg of (RS)-2-amino-2-(2-chloro-5-hydroxyphenyl)acetic acid sodium salt] enhanced the discriminative stimulus effects of a low alcohol dose (0.5 g/kg), and mGlu5 receptor inhibition (20 microg of MPEP) prevented the agonist-induced enhancement. These results show that mGlu5 receptor activity in the nucleus accumbens is required for the expression of the interoceptive effects of alcohol.

The mGlu5 receptor antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP) supports intravenous self-administration and induces conditioned place preference in the rat

We recently reported that the mGlu5 receptor antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP) reduces intravenous self-administration of ketamine and, to a lesser extent, heroin in rats. We also found that MPEP potentiates conditioned place preference induced by these drugs, suggesting that the reduction of self-administration results from an MPEP-induced potentiation of the rewarding effect of the self-administered drug. The aim of the present study was to examine whether MPEP has intrinsic positive reinforcing and rewarding effects. In experiment 1, rats were trained to self-administer either ketamine [0.5 mg/kg/infusion, 2 h sessions, fixed-ratio (FR) 3] or heroin (0.05 mg/kg/infusion, 1 h sessions, FR 10), followed by a number of substitution sessions with MPEP (1 mg/kg/infusion) or saline. In experiment 2, drug-naïve rats were allowed to acquire intravenous self-administration of MPEP (1 mg/kg/infusion, 2 h sessions, FR 3) or saline. In experiment 3, rats were subjected to a single-trial unbiased conditioned place preference protocol with MPEP (0.3-10 mg/kg i.v., 20 min conditioning). It was found that (1) substitution with MPEP in rats which had learned to self-administer ketamine or heroin resulted in stable self-administration behavior, whereas substitution with saline resulted in a typical extinction profile, (2) drug-naïve rats learned to self-administer MPEP, but not saline, and self-administration remained stable for at least 7 sessions, and (3) MPEP induced dose-dependent place preference with a minimal effective dose of 3 mg/kg. These data clearly demonstrate that MPEP has (weak) positive reinforcing and rewarding effects when administered i.v.

Increased operant responding for ethanol in male C57BL/6J mice: specific regulation by the ERK1/2, but not JNK, MAP kinase pathway

RATIONALE

Extracellular signal-regulated protein kinase (ERK(1/2)) is a member of the mitogen-activated protein kinase (MAPK) signaling pathway and a key molecular target for ethanol (EtOH) and other drugs of abuse.

OBJECTIVE

The aim of the study was to assess the role of two MAPK pathways, ERK(1/2) and c-Jun N-terminal kinase (JNK), on the modulation of EtOH and sucrose self-administration.

MATERIALS AND METHODS

C57BL/6J mice were trained to lever press on a fixed-ratio 4 schedule with 9% EtOH/2% sucrose, or 2% sucrose, as the reinforcer. In experiments 1 and 2, mice were injected with the MEK(1/2) inhibitor SL 327 (0-100 mg/kg) and the JNK inhibitor AS 6012452 (0-56 mg/kg) prior to self-administration. In experiment 3, SL 327 (0-100 mg/kg) was administered prior to performance on a progressive ratio (PR) schedule of EtOH reinforcement. In experiment 4, SL 327 and AS 601245 were injected 2 h before a locomotor test.

RESULTS

SL 327 (30 mg/kg) significantly increased EtOH self-administration without affecting locomotion. Higher doses of SL 327 and AS 601245 reduced EtOH-reinforced responding and locomotor activity. Reductions of both ligands on sucrose self-administration were due to decreases in motor activity. SL 327 pretreatment had no effect on PR responding.

CONCLUSIONS

ERK(1/2) activity is more directly involved in modulating the reinforcing properties of EtOH than JNK activity due to its selective potentiation of EtOH-reinforced responding. The specificity of this effect to EtOH self-administration, rather than sucrose self-administration, suggests that the mechanism by which ERK(1/2) increases EtOH-reinforced responding does not generalize to all reinforcing solutions and is not due to increased motivation to consume EtOH.

Acute effects of acamprosate and MPEP on ethanol Drinking-in-the-Dark in male C57BL/6J mice

BACKGROUND

Recently, a simple procedure in mice, Drinking-in-the-Dark (DID), was hypothesized to have value for medication development for human alcoholism. In DID, mice are offered intermittent, limited access to ethanol over a series of days during the dark phase that results in rapid drinking to intoxication in predisposed genotypes.

METHODS

We measured the effects of acamprosate or MPEP, metabotropic glutamate 5 receptor (mGluR5) antagonist, on intake of 20% ethanol, plain tap water or 10% sugar water using the DID procedure in male C57BL/6J mice.

RESULTS

Acamprosate (100, 200, 300, or 400 mg/kg) dose dependently decreased ethanol drinking with 300 mg/kg reducing ethanol intake by approximately 20% without affecting intake of plain water or 10% sugar water. MPEP (1, 3, 5, 10, 20, or 40 mg/kg) was more potent than acamprosate with 20 mg/kg reducing ethanol intake by approximately 20% and for longer duration without affecting intake of plain water or 10% sugar water.

CONCLUSIONS

These results support the hypothesis that mGluR5 signaling plays a role in excessive ethanol intake in DID and suggest DID may have value for screening novel compounds that reduce overactive glutamate signaling for potential pharmaceutical treatment of excessive ethanol drinking behavior.

Metabotropic glutamate receptor 5 (mGluR5) regulation of ethanol sedation, dependence and consumption: relationship to acamprosate actions

Recent studies have demonstrated that metabotropic glutamate receptor 5 (mGluR5) antagonists decrease alcohol self-administration and suggest that the anti-craving medication, acamprosate, may also act to decrease mGluR5 function. To address the role of mGluR5 in behavioural actions of ethanol and acamprosate, we compared mutant mice with deletion of the mGluR5 gene and mice treated with a mGluR5 antagonist (MPEP) or acamprosate. Lack of mGluR5 or administration of MPEP reduced the severity of alcohol-induced withdrawal (AW), increased the sedative effect of alcohol (duration of loss of righting reflex; LORR), and increased basal motor activity. The motor stimulation produced by ethanol was blocked by deletion of mGluR5, but not by injection of MPEP. Both acamprosate and MPEP increased ethanol-induced LORR and reduced AW. Importantly, the protective effects of both MPEP and acamprosate on AW were found when the drugs were injected before, but not after, injection of ethanol. This indicates that the drugs prevented development of dependence rather than merely producing an anticonvulsant action. No effects of acamprosate or MPEP on ethanol-induced LORR and AW were found in mGluR5 knockout mice, demonstrating that mGluR5 is required for these actions. mGluR5 null mutant mice showed decreased alcohol consumption in some, but not all, tests. These data show the importance of mGluR5 for several actions of alcohol and support the hypothesis that some effects of acamprosate require mGluR5 signalling.

Ethanol-induced alterations of c-Fos immunoreactivity in specific limbic brain regions following ethanol discrimination training

The discriminative stimulus properties of ethanol are functionally regulated by ionotropic GABA(A) and NMDA receptors in specific limbic brain regions including the nucleus accumbens, amygdala, and hippocampus, as determined by microinjection studies. The purpose of the present work was to further investigate potential neural substrates of ethanol's discriminative stimulus effects by examining if ethanol discrimination learning produces changes in brain regional response to ethanol. To accomplish this goal, immunohistochemistry was used to assess the effects of ethanol (2 g/kg) on c-Fos immunoreactivity (Fos-IR). Comparisons in ethanol-induced Fos-IR were made between a group of rats that was trained to discriminate the stimulus properties of ethanol (2 g/kg, IG) from water (IG) and a drug/behavior-matched control group that did not receive differential reinforcement for lever selection, which precluded acquisition of discriminative stimulus control by ethanol. In some brain regions discrimination training had no effect on ethanol-induced Fos-IR changes (caudate putamen, bed nucleus of the stria terminalis, and CA1 region of the hippocampus). In contrast, discrimination training altered the pattern of ethanol-induced Fos-IR in the nucleus accumbens (core), medial septum, and the hippocampus (dentate and CA3). These results indicate that having behavior under the stimulus control of ethanol can change ethanol-induced Fos-IR in some brain regions. This suggests that learning about the subjective properties of ethanol produces adaptive changes in how the brain responds to acute ethanol exposure.

Cue-induced reinstatement of alcohol-seeking behavior is associated with increased ERK1/2 phosphorylation in specific limbic brain regions: blockade by the mGluR5 antagonist MPEP

Relapse to alcohol use after periods of abstinence is a hallmark behavioral pathology of alcoholism and a major clinical problem. Emerging evidence indicates that metabotropic glutamate receptor 5 (mGluR5) antagonists attenuate relapse to alcohol-seeking behavior but the molecular mechanisms of this potential therapeutic effect remain unexplored. The extracellular signal-regulated kinase (ERK1/2) pathway is downstream of mGluR5 and has been implicated in addiction. We sought to determine if cue-induced reinstatement of alcohol-seeking behavior, and its reduction by an mGluR5 antagonist, is associated with changes in ERK1/2 activation in reward-related limbic brain regions. Selectively-bred alcohol-preferring (P) rats were trained to lever press on a concurrent schedule of alcohol (15% v/v) vs. water reinforcement. Following 9 days of extinction, rats were given an additional extinction trial or injected with the mGluR5 antagonist MPEP (0, 1, 3, or 10mg/kg) and tested for cue-induced reinstatement. Brains were removed 90-min later from the rats in the extinction and MPEP (0 or 10mg/kg) conditions for analysis of p-ERK1/2, total ERK1/2, and p-ERK5 immunoreactivity (IR). Cue-induced reinstatement of alcohol-seeking behavior was associated with a three to five-fold increase in p-ERK1/2 IR in the basolateral amygdala and nucleus accumbens shell. MPEP administration blocked both the relapse-like behavior and increase in p-ERK1/2 IR. p-ERK1/2 IR in the central amygdala and NAcb core was dissociated with the relapse-like behavior and the pharmacological effect of mGluR5 blockade. No changes in total ERK or p-ERK5 were observed. These results suggest that exposure to cues previously associated with alcohol self-administration is sufficient to produce concomitant increases in relapse-like behavior and ERK1/2 activation in specific limbic brain regions. Pharmacological compounds, such as mGluR5 antagonists, that reduce cue-induced ERK1/2 activation may be useful for treatment of relapse in alcoholics that is triggered by exposure to environmental events.

Nonselective suppression of operant ethanol and sucrose self-administration by the mGluR7 positive allosteric modulator AMN082

Emerging evidence indicates that specific metabotropic glutamate receptors (mGluRs) modulate ethanol self-administration. In general, inhibition of glutamate transmission through blockade of postsynaptic mGluRs, or activation of presynaptic mGluRs, inhibits ethanol self-administration. The goal of this preclinical study was to further characterize mGluR regulation of ethanol self-administration by examining effects of AMN082, an allosteric positive modulator of presynaptic mGluR7 activity. Separate groups of C57BL/6J male mice were trained to self-administer ethanol or sucrose on a fixed-ratio 4 schedule of reinforcement during 1 h sessions. On test days, mice were pretreated with AMN082 (0, 1.0, 3.0, 5.6, or 10 mg/kg) 30 min prior to self-administration sessions. Functional specificity and activity was examined by testing the effects of AMN082 (0-10 mg/kg) on open-field locomotor activity and HPA axis function as measured by plasma corticosterone levels. AMN082 (10 mg/kg) produced a significant reduction in ethanol and sucrose reinforced responding, and inhibited locomotor activity. Plasma corticosterone levels were significantly increased following AMN082 (5.6 and 10 mg/kg) suggesting a dose-dependent dissociation between the behavioral and hormonal effects of the compound. These data suggest that activation of mGluR7 by AMNO82 produces nonspecific reductions in motivated behavior that are associated with negative effects on motor activity.

G protein-coupled receptor dimers: functional consequences, disease states and drug targets

With an ever-expanding need for reliable therapeutic agents that are highly effective and exhibit minimal deleterious side effects, a greater understanding of the mechanisms underlying G protein-coupled receptor (GPCR) regulation is fundamental. GPCRs comprise more than 30% of all therapeutic drug targets and it is likely that this will only increase as more orphan GPCRs are identified. The past decade has seen a dramatic shift in the prevailing concept of how GPCRs function, in particular the growing acceptance that GPCRs are capable of interacting with one another at a molecular level to form complexes, with significantly different pharmacological properties to their monomeric selves. While the ability of like-receptors to associate and form homodimers raises some interesting mechanistic issues, the possibility that unlike-receptors could heterodimerise in certain tissue types, producing a functionally unique signalling complex that binds specific ligands, provides an invaluable opportunity to refine and redefine pharmacological interventions with greater specificity and efficacy.

Physiology and pharmacology of alcohol: the imidazobenzodiazepine alcohol antagonist site on subtypes of GABAA receptors as an opportunity for drug development?

Alcohol (ethanol, EtOH) has pleiotropic actions and induces a number of acute and long-term effects due to direct actions on alcohol targets, and effects of alcohol metabolites and metabolism. Many detrimental health consequences are due to EtOH metabolism and metabolites, in particular acetaldehyde, whose high reactivity leads to nonspecific chemical modifications of proteins and nucleic acids. Like acetaldehyde, alcohol has been widely considered a nonspecific drug, despite rather persuasive evidence implicating inhibitory GABA(A) receptors (GABA(A)Rs) in acute alcohol actions, for example, a GABA(A)R ligand, the imidazobenzodiazepine Ro15-4513 antagonizes many low-to-moderate dose alcohol actions in mammals. It was therefore rather surprising that abundant types of synaptic GABA(A)Rs are generally not responsive to relevant low concentrations of EtOH. In contrast, delta-subunit-containing GABA(A)Rs and extrasynaptic tonic GABA currents mediated by these receptors are sensitive to alcohol concentrations that are reached in blood and tissues during low-to-moderate alcohol consumption. We recently showed that low-dose alcohol enhancement on highly alcohol-sensitive GABA(A)R subtypes is antagonized by Ro15-4513 in an apparently competitive manner, providing a molecular explanation for behavioural Ro15-4513 alcohol antagonism. The identification of a Ro15-4513/EtOH binding site on unique GABA(A)R subtypes opens the possibility to characterize this alcohol site(s) and screen for compounds that modulate the function of EtOH/Ro15-4513-sensitive GABA(A)Rs. The utility of such drugs might range from novel alcohol antagonists that might be useful in the emergency room, to drugs for the treatment of alcoholism, as well as alcohol-mimetic drugs to harness acute positive effects of alcohol.

Effects of mGlu1-receptor blockade on ethanol self-administration in inbred alcohol-preferring rats

The Group I family of metabotropic glutamate receptors includes subtype 1 (mGlu1) and subtype 5 (mGlu5) receptors. This family of receptors has generated interest as potential targets for different areas of therapeutic development, including intervention for alcohol and drug abuse. Most of this interest is driven by findings showing involvement of mGlu5 receptors in the regulation of drug self-administration; however, studies examining the role of mGlu1 receptors in drug self-administration are limited. The purpose of this work was to examine the role of mGlu1-receptor antagonism in the maintenance of ethanol self-administration and the self-administration of an alternate nondrug reward, sucrose. Male alcohol-preferring inbred rats were trained to self-administer ethanol (15% vol/vol) versus water on a concurrent schedule of reinforcement, and the effect of the mGlu1-receptor antagonist JNJ16259685 (0.1-1.0mg/kg intraperitoneal [IP]) was evaluated on self-administration. The rats were then trained to self-administer sucrose (0.4% wt/vol) versus water, and the same dose range of JNJ16259685 was tested. Locomotor activity was tested in a separate assessment to evaluate potential nonspecific motor effects of the antagonist. Ethanol self-administration was dose dependently reduced by JNJ16259685. This reduction was likely due to a motor impairment as the lowest effective dose (0.1mg/kg) significantly reduced locomotor behavior. Sucrose self-administration was reduced by the highest JNJ16259685 dose (1.0mg/kg), and this reduction was also likely due to a motor impairment. Interestingly, ethanol self-administration was more sensitive to mGlu1-receptor antagonism than sucrose self-administration as lower JNJ16259685 doses reduced ethanol-reinforced responding and motor behavior. Together, these results suggest that mGlu1 receptors do not play a specific role in modulating ethanol self-administration or the self-administration of an alternate nondrug reward (i.e., sucrose).

Glutamatergic substrates of drug addiction and alcoholism

The past two decades have witnessed a dramatic accumulation of evidence indicating that the excitatory amino acid glutamate plays an important role in drug addiction and alcoholism. The purpose of this review is to summarize findings on glutamatergic substrates of addiction, surveying data from both human and animal studies. The effects of various drugs of abuse on glutamatergic neurotransmission are discussed, as are the effects of pharmacological or genetic manipulation of various components of glutamate transmission on drug reinforcement, conditioned reward, extinction, and relapse-like behavior. In addition, glutamatergic agents that are currently in use or are undergoing testing in clinical trials for the treatment of addiction are discussed, including acamprosate, N-acetylcysteine, modafinil, topiramate, lamotrigine, gabapentin and memantine. All drugs of abuse appear to modulate glutamatergic transmission, albeit by different mechanisms, and this modulation of glutamate transmission is believed to result in long-lasting neuroplastic changes in the brain that may contribute to the perseveration of drug-seeking behavior and drug-associated memories. In general, attenuation of glutamatergic transmission reduces drug reward, reinforcement, and relapse-like behavior. On the other hand, potentiation of glutamatergic transmission appears to facilitate the extinction of drug-seeking behavior. However, attempts at identifying genetic polymorphisms in components of glutamate transmission in humans have yielded only a limited number of candidate genes that may serve as risk factors for the development of addiction. Nonetheless, manipulation of glutamatergic neurotransmission appears to be a promising avenue of research in developing improved therapeutic agents for the treatment of drug addiction and alcoholism.

Differential modulation of ethanol-induced sedation and hypnosis by metabotropic glutamate receptor antagonists in C57BL/6J mice

BACKGROUND

Emerging evidence implicates metabotropic glutamate receptor (mGluR) function in the neurobiological effects of ethanol. The recent development of subtype specific mGluR antagonists has made it possible to examine the roles of specific mGluRs in biochemical and behavioral responses to ethanol. The purpose of the present study was to determine if mGluRs modulate the acute sedative-hypnotic properties of ethanol in mice.

METHODS

C57BL/6J mice were tested for locomotor activity (sedation) and duration of loss of the righting reflex (hypnosis) following acute systemic administration of ethanol alone or in combination with the mGluR5-selective antagonist, 2-methyl-6-(phenylethynyl)pyridine (MPEP), the mGluR1-selective antagonist, 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt), or the mGluR2/3-selective antagonist (2S)-2-Amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid (LY341495)).

RESULTS

MPEP (10 and 30 mg/kg) significantly enhanced both the sedative and hypnotic effects of ethanol, while LY341495 (10 and 30 mg/kg) significantly reduced the sedative-hypnotic effects of ethanol. CPCCOEt had no effect at any concentration tested. Further loss of righting reflex experiments revealed that LY341495 (30 mg/kg) significantly reduced hypnosis induced by the gamma-aminobutyric acid type A (GABAA) positive modulators, pentobarbital (50 mg/kg) and midazolam (60 mg/kg), and the N-methyl-d-aspartate (NMDA) receptor antagonist, ketamine (150 mg/kg), while MPEP (30 mg/kg) only significantly enhanced the hypnotic properties of ketamine (150 mg/kg).

CONCLUSIONS

These findings suggest that specific subtypes of the metabotropic glutamate receptor differentially modulate the sedative-hypnotic properties of ethanol through separate mechanisms of action, potentially involving GABA(A) and NMDA receptors.

GABAA receptor subtypes: the "one glass of wine" receptors

This review discusses evidence for and apparent controversy about, gamma-aminobutyric acid type A (GABAA) receptor (GABAAR) subtypes that mediate alcohol effects experienced during social drinking. GABAARs that contain the beta3 and delta subunits were shown to be enhanced by alcohol concentrations that mirror the concentration dependence of alcohol responses in humans. A mutation (alpha6R100Q) previously found in alcohol nontolerant rats in the cerebellar GABAAR alpha6 subunit is sufficient for increased alcohol-induced ataxia in rats homozygous for this mutation (alpha6-100QQ) and further increases alcohol sensitivity of tonic GABA currents (mediated by alpha6betadelta receptors) in cerebellar granule cells of alpha6-100QQ rats and in recombinant alpha6R100Qbeta3delta receptors. This provided the first direct evidence that these types of receptors mediate behavioral effects of ethanol. Furthermore, the behavioral alcohol antagonist Ro15-4513 specifically reverses ethanol enhancement on alpha4/6beta3delta receptors. Unexpectedly, native and recombinant alpha4/6beta3delta receptors bind the behavioral alcohol antagonist Ro15-4513 with high affinity and this binding is competitive with EtOH, suggesting a specific and mutually exclusive (competitive) ethanol/Ro15-4513 site, which explains the puzzling activity of Ro15-4513 as a behavioral alcohol antagonist. Our conclusion from these findings is that alcohol/Ro15-4513-sensitive GABAAR subtypes are important alcohol targets and that alcohol at relevant concentrations is more specific than previously thought. In this review, we discuss technical difficulties in expressing recombinant delta subunit-containing receptors in oocytes and mammalian cells that may have contributed to negative results and confusion. Not only because we have reproduced detailed positive results numerous times, and we and many others have built extensively on basic findings, but also because we explain and combine many previously puzzling results into a coherent and highly plausible paradigm on how alcohol exerts an important part of its action in the brain, we are confident about our findings and conclusions. However, many important open questions remain to be answered.