Ethanol increases desensitization of recombinant GluR-D AMPA receptor and TARP combinations

Cessation of fluoxetine treatment increases alcohol seeking during relapse and dysregulates endocannabinoid and glutamatergic signaling in the central amygdala

Administration of selective serotonin reuptake inhibitors (SSRIs), typically used as antidepressants, induces long-lasting behavioral changes associated with alcohol use disorder (AUD). However, the contribution of SSRI (fluoxetine)-induced alterations in neurobiological processes underlying alcohol relapse such as endocannabinoid and glutamate signaling in the central amygdala (CeA) remains largely unknown. We utilized an integrative approach to study the effects of repeated fluoxetine administration during abstinence on ethanol drinking. Gene expression and biochemical and electrophysiological studies explored the hypothesis that dysregulation in glutamatergic and endocannabinoid mechanisms in the CeA underlie the susceptibility to alcohol relapse. Cessation of daily treatment with fluoxetine (10 mg/kg) during abstinence resulted in a marked increase in ethanol seeking during re-exposure periods. The increase in ethanol self-administration was associated with (a) reductions in levels of the endocannabinoids N-arachidonoylethanolomine and 2-arachidonoylglycerol in the CeA, (b) increased amygdalar gene expression of cannabinoid type-1 receptor (CB1), N-acyl phosphatidylethanolamine phospholipase D (Nape-pld), fatty acid amid hydrolase (Faah), (c) decreased amygdalar gene expression of ionotropic AMPA (GluA2 and GluA4) and metabotropic (mGlu3) glutamate receptors, and (d) increased glutamatergic receptor function. Overall, our data suggest that the administration of the antidepressant fluoxetine during abstinence dysregulates endocannabinoid signaling and glutamatergic receptor function in the amygdala, facts that likely facilitate alcohol drinking behavior during relapse.

Neurotoxic Agent-Induced Injury in Neurodegenerative Disease Model: Focus on Involvement of Glutamate Receptors

Glutamate receptors play a crucial role in the central nervous system and are implicated in different brain disorders. They play a significant role in the pathogenesis of neurodegenerative diseases (NDDs) such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. Although many studies on NDDs have been conducted, their exact pathophysiological characteristics are still not fully understood. In in vivo and in vitro models of neurotoxic-induced NDDs, neurotoxic agents are used to induce several neuronal injuries for the purpose of correlating them with the pathological characteristics of NDDs. Moreover, therapeutic drugs might be discovered based on the studies employing these models. In NDD models, different neurotoxic agents, namely, kainic acid, domoic acid, glutamate, β-N-Methylamino-L-alanine, amyloid beta, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, 1-methyl-4-phenylpyridinium, rotenone, 3-Nitropropionic acid and methamphetamine can potently impair both ionotropic and metabotropic glutamate receptors, leading to the progression of toxicity. Many other neurotoxic agents mainly affect the functions of ionotropic glutamate receptors. We discuss particular neurotoxic agents that can act upon glutamate receptors so as to effectively mimic NDDs. The correlation of neurotoxic agent-induced disease characteristics with glutamate receptors would aid the discovery and development of therapeutic drugs for NDDs.

Synaptic targets: Chronic alcohol actions

Alcohol acts on numerous cellular and molecular targets to regulate neuronal communication within the brain. Chronic alcohol exposure and acute withdrawal generate prominent neuroadaptations at synapses, including compensatory effects on the expression, localization and function of synaptic proteins, channels and receptors. The present article reviews the literature describing the synaptic effects of chronic alcohol exposure and their relevance for synaptic transmission in the central nervous system. This review is not meant to be comprehensive, but rather to highlight the effects that have been observed most consistently and that are thought to contribute to the development of alcohol dependence and the negative aspects of withdrawal. Specifically, we will focus on the major excitatory and inhibitory neurotransmitters in the brain, glutamate and GABA, respectively, and how their neuroadaptations after chronic alcohol exposure contributes to alcohol reinforcement, dependence and withdrawal. This article is part of the Special Issue entitled "Alcoholism".

Intersubunit interactions at putative sites of ethanol action in the M3 and M4 domains of the NMDA receptor GluN1 and GluN2B subunits

BACKGROUND AND PURPOSE

The NMDA receptor is an important target of alcohol action in the brain. Recent studies in this laboratory have demonstrated that alcohol-sensitive positions in the intersubunit interfaces of the M3 and M4 domains of GluN1 and GluN2A subunits interact with respect to ethanol sensitivity and receptor kinetics and that alcohol-sensitive positions in the M domains of GluN2A and GluN2B subunits differ. In this study, we tested for interactions among alcohol-sensitive positions at the M domain intersubunit interfaces in GluN1/GluN2B NMDA receptors.

EXPERIMENTAL APPROACH

We used whole-cell patch-clamp recording in tsA201 cells expressing tryptophan substitution mutants at ethanol-sensitive positions in the GluN1 and GluN2B NMDA receptor subunits to test for interactions among positions.

KEY RESULTS

Six pairs of positions in GluN1/GluN2B significantly interacted to regulate ethanol inhibition: Gly(638) /Met(824) , Gly(638) /Leu(825) , Phe(639) /Leu(825) , Phe(639) /Gly(826) , Met(818) /Phe(637) and Val(820) /Phe(637) . Tryptophan substitution at Met(824) or Leu(825) in GluN2B did not alter ethanol sensitivity but interacted with positions in the GluN1 M3 domain to regulate ethanol action, whereas tryptophan substitution at Gly(638) , which is the cognate of an ethanol-sensitive position in GluN2A, did not alter ethanol sensitivity or interact with positions in GluN1. Two and three pairs of positions interacted to regulate glutamate steady-state and peak current EC50 , respectively, and one pair interacted with respect to macroscopic desensitization.

CONCLUSIONS

Despite highly-conserved M domain sequences and similar ethanol sensitivity in the GluN2A and GluN2B subunits, the manner in which these subunits interact with the GluN1 subunit to regulate ethanol sensitivity and receptor kinetics differs.

Advances in the pharmacology of lGICs auxiliary subunits

Ligand-gated ion channels (LGICs) are cell surface integral proteins that mediate the fast neurotransmission in the nervous system. LGICs require auxiliary subunits for their trafficking, assembly and pharmacological modulation. Auxiliary subunits do not form functional homomeric receptors, but are reported to assemble with the principal subunits in order to modulate their pharmacological profiles. For example, nACh receptors are built at least by co-assemble of α and β subunits, and the neuronal auxiliary subunits β3 and α5 and muscle type β, δ, γ, and ϵ determine the agonist affinity of these receptors. Serotonergic 5-HT3B, 5-HT3C, 5-HT3D and 5-HT3E are reported to assemble with the 5-HT3A subunit to modulate its pharmacological profile. Functional studies evaluating the role of γ2 and δ auxiliary subunits of GABAA receptors have made important advances in the understanding of the action of benzodiazepines, ethanol and neurosteroids. Glycine receptors are composed principally by α1-3 subunits and the auxiliary subunit β determines their synaptic location and their pharmacological response to propofol and ethanol. NMDA receptors appear to be functional as heterotetrameric channels. So far, the existence of NMDA auxiliary subunits is controversial. On the other hand, Kainate receptors are modulated by NETO 1 and 2. AMPA receptors are modulated by TARPs, Shisa 9, CKAMP44, CNIH2-3 auxiliary proteins reported that controls their trafficking, conductance and gating of channels. P2X receptors are able to associate with auxiliary Pannexin-1 protein to modulate P2X7 receptors. Considering the pharmacological relevance of different LGICs auxiliary subunits in the present work we will highlight the therapeutic potential of these modulator proteins.

A novel alcohol-sensitive position in the N-methyl-D-aspartate receptor GluN2A subunit M3 domain regulates agonist affinity and ion channel gating

Abundant evidence supports a role for N-methyl-d-aspartate (NMDA) receptor inhibition in the behavioral actions of ethanol, but the underlying molecular mechanisms have not been fully elucidated. We recently found that clusters of five positions in the third and fourth membrane-associated domains (M3 and M4) at the intersubunit interfaces form putative sites of alcohol action. In the present study, we found that one of these positions, NMDA receptor subunit, GluN2A(F636), can strongly regulate ethanol sensitivity, glutamate potency, and apparent desensitization: ethanol IC50 values, peak (Ip) and steady-state (Iss) glutamate EC50 values, and steady-state to peak current ratio (Iss:Ip) values differed significantly among the mutants tested. Changes in glutamate affinity among the various mutants were not attributable to agonist trapping due to desensitization, as glutamate peak EC50 values were correlated with values of both steady-state EC50 and Iss:Ip. The mean open times determined in selected mutants could be altered up to 4-fold but did not account for the changes in ethanol sensitivity. Ethanol sensitivity was significantly correlated with glutamate EC50 and Iss:Ip values, but the changes in ethanol IC50 among mutants at this position do not appear to be secondary to changes in ion channel kinetics. Substitution of the isomeric amino acids leucine and isoleucine had markedly different effects on ethanol sensitivity, agonist potency, and desensitization, which is consistent with a stringent structural requirement for ion channel modulation by the side chain at this position. Our results indicate that GluN2A(F636) plays an important role in both channel function and ethanol inhibition in NMDA receptors.

Enhanced AMPA receptor activity increases operant alcohol self-administration and cue-induced reinstatement

Long-term alcohol exposure produces neuroadaptations that contribute to the progression of alcohol abuse disorders. Chronic alcohol consumption results in strengthened excitatory neurotransmission and increased α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptors (AMPA) receptor signaling in animal models. However, the mechanistic role of enhanced AMPA receptor activity in alcohol-reinforcement and alcohol-seeking behavior remains unclear. This study examined the role of enhanced AMPA receptor function using the selective positive allosteric modulator, aniracetam, in modulating operant alcohol self-administration and cue-induced reinstatement. Male alcohol-preferring (P-) rats, trained to self-administer alcohol (15%, v/v) versus water were pre-treated with aniracetam to assess effects on maintenance of alcohol self-administration. To determine reinforcer specificity, P-rats were trained to self-administer sucrose (0.8%, w/v) versus water, and effects of aniracetam were tested. The role of aniracetam in modulating relapse of alcohol-seeking was assessed using a response contingent cue-induced reinstatement procedure in P-rats trained to self-administer 15% alcohol. Aniracetam pre-treatment significantly increased alcohol-reinforced responses relative to vehicle treatment. This increase was not attributed to aniracetam-induced hyperactivity as aniracetam pre-treatment did not alter locomotor activity. AMPA receptor involvement was confirmed because 6,7-dinitroquinoxaline-2,3-dione (AMPA receptor antagonist) blocked the aniracetam-induced increase in alcohol self-administration. Aniracetam did not alter sucrose-reinforced responses in sucrose-trained P-rats, suggesting that enhanced AMPA receptor activity is selective in modulating the reinforcing function of alcohol. Finally, aniracetam pre-treatment potentiated cue-induced reinstatement of alcohol-seeking behavior versus vehicle-treated P-rats. These data suggest that enhanced glutamate activity at AMPA receptors may be key in facilitating alcohol consumption and seeking behavior, which could ultimately contribute to the development of alcohol abuse disorders.

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.

Polyamine modulation of NMDARs as a mechanism to reduce effects of alcohol dependence

Relapse and neurodegeneration are two of the major therapeutic targets in alcoholism. Fortuitously, the roles of glutamate/NMDA receptors (NMDARs) in withdrawal, conditioning and neurotoxicity mean that NMDAR inhibitors are potentially valuable for both targets. Preclinical studies further suggest that inhibitory modulators that specifically reduce the co-agonist effects of polyamines on NMDARs are potential non-toxic medications. Using agmatine as a lead compound, over 1000 novel compounds based loosely on this structure were synthesized using feedback from a molecular screen. A novel series of aryliminoguanidines with appropriate NMDAR activity in the molecular screen were discovered (US patent application filed 2007). The most potent and selective aryliminoguanidine, JR 220 [4- (chlorobenzylidenamino)- guanidine hydrochloride], has now been tested in a screening hierarchy for anti-relapse and neuroprotective activity, ranging from cell-based assay, through tissue culture to animal behavior. This hierarchy has been validated using drugs with known, or potential, clinical value at these targets (acamprosate (N-acetyl homotaurine), memantine and topiramate). JR220 was non-toxic and showed excellent activity in every screen with a potency 5-200x that of the FDA-approved anti-relapse agent, acamprosate. This chapter will present a review of the background and rationale for this approach and some of the findings garnered from this approach as well as patents targeting the glutamatergic system especially the NMDAR.

Ethanol up-regulates nucleus accumbens neuronal activity dependent pentraxin (Narp): implications for alcohol-induced behavioral plasticity

Neuronal activity dependent pentraxin (Narp) interacts with α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) glutamate receptors to facilitate excitatory synapse formation by aggregating them at established synapses. Alcohol is well-characterized to influence central glutamatergic transmission, including AMPA receptor function. Herein, we examined the influence of injected and ingested alcohol upon Narp protein expression, as well as basal Narp expression in mouse lines selectively bred for high blood alcohol concentrations under limited access conditions. Alcohol up-regulated accumbens Narp levels, concomitant with increases in levels of the GluR1 AMPA receptor subunit. However, accumbens Narp or GluR1 levels did not vary as a function of selectively bred genotype. We next employed a Narp knock-out (KO) strategy to begin to understand the behavioral relevance of alcohol-induced changes in protein expression in several assays of alcohol reward. Compared to wild-type mice, Narp KO animals: fail to escalate daily intake of high alcohol concentrations under free-access conditions; shift their preference away from high alcohol concentrations with repeated alcohol experience; exhibit a conditioned place-aversion in response to the repeated pairing of 3 g/kg alcohol with a distinct environment and fail to exhibit alcohol-induced locomotor hyperactivity following repeated alcohol treatment. Narp deletion did not influence the daily intake of either food or water, nor did it alter any aspect of spontaneous or alcohol-induced motor activity, including the development of tolerance to its motor-impairing effects with repeated treatment. Taken together, these data indicate that Narp induction, and presumably subsequent aggregation of AMPA receptors, may be important for neuroplasticity within limbic subcircuits mediating or maintaining the rewarding properties of alcohol.

Acute effects of ethanol on glutamate receptors

Several studies have revealed that acute ethanol inhibits the function of glutamate receptors. Glutamate receptor-mediated synaptic plasticity, such as N-methyl-D-aspartate-dependent long-term potentiation, is also inhibited by ethanol. However, the inhibition seems to be restricted to certain brain areas such as the hippocampus, amygdala and striatum. Ethanol inhibition of glutamate receptors generally requires relatively high concentrations and may therefore explain consequences of severe ethanol intoxication such as impairment of motor performance and memory. Effects of ethanol on glutamate system of developing nervous system may have a role in causing foetal alcohol syndrome. Newly found regulatory proteins of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid AMPA receptors seem to affect ethanol inhibition thus opening new lines of research.