Ethanol reduces GABAA alpha1 subunit receptor surface expression by a protein kinase Cgamma-dependent mechanism in cultured cerebral cortical neurons

GABAergic mechanisms in alcohol dependence

The target of alcohol's effect on the central nervous system has been sought for more than 50 years in the brain's GABA system. The behavioral and emotional effects of alcohol in humans and rodents are very similar to those of barbiturates and benzodiazepines, and GABAA receptors have been shown to be one of the sites of alcohol action. The mechanisms of GABAergic inhibition have been a hotspot of research but have turned out to be complex and controversial. Genetics support the involvement of some GABAA receptor subunits in the development of alcohol dependence and in alcohol use disorders (AUD). Since the effect of alcohol on the GABAA system resembles that of a GABAergic positive modulator, it may be possible to develop GABAergic drug treatments that could substitute for alcohol. The adaptation mechanisms of the GABA system and the plasticity of the brain are a big challenge for drug development: the drugs that act on GABAA receptors developed so far also may cause adaptation and development of additional addiction. Human polymorphisms should be studied further to get insight about how they affect receptor function, expression or other factors to make reasonable predictions/hypotheses about what non-addictive interventions would help in alcohol dependence and AUD.

Salicylate Induced GABAAR Internalization by Dopamine D1-Like Receptors Involving Protein Kinase C (PKC) in Spiral Ganglion Neurons

BACKGROUND Sodium salicylate (SS) induces excitotoxicity of spiral ganglion neurons (SGNs) by inhibiting the response of γ-aminobutyric acid type A receptors (GABAARs). Our previous studies have shown that SS can increase the internalization of GABAARs on SGNs, which involves dopamine D1-like receptors (D1Rs) and related signaling pathways. In this study, we aimed to explore the role of D1Rs and their downstream molecule protein kinase C (PKC) in the process of SS inhibiting GABAARs. MATERIAL AND METHODS The expression of D1Rs and GABARγ2 on rat cochlear SGNs cultured in vitro was tested by immunofluorescence. Then, the SGNs were exposed to SS, D1R agonist (SKF38393), D1R antagonist (SCH23390), clathrin/dynamin-mediated endocytosis inhibitor (dynasore), and PKC inhibitor (Bisindolylmaleimide I). Western blotting and whole-cell patch clamp technique were used to assess the changes of surface and total protein of GABARγ2 and GABA-activated currents. RESULTS Immunofluorescence showed that D1 receptors (DRD1) were expressed on SGNs. Data from western blotting showed that SS promoted the internalization of cell surface GABAARs, and activating D1Rs had the same result. Inhibiting D1Rs and PKC decreased the internalization of GABAARs. Meanwhile, the phosphorylation level of GABAARγ2 S327 affected by PKC was positively correlated with the degree of internalization of GABAARs. Moreover, whole-cell patch clamp recording showed that inhibition of D1Rs or co-inhibition of D1Rs and PKC attenuated the inhibitory effect of SS on GABA-activated currents. CONCLUSIONS D1Rs mediate the GABAAR internalization induced by SS via a PKC-dependent manner and participate in the excitotoxic process of SGNs.

Regulation of GABAA Receptor Subunit Expression in Substance Use Disorders

The modulation of neuronal cell firing is mediated by the release of the neurotransmitter GABA (γ-aminobuytric acid), which binds to two major families of receptors. The ionotropic GABAA receptors (GABA_ARs) are composed of five distinct subunits that vary in expression by brain region and cell type. The action of GABA on GABA_ARs is modulated by a variety of clinically and pharmacologically important drugs such as benzodiazepines and alcohol. Exposure to and abuse of these substances disrupts homeostasis and induces plasticity in GABAergic neurotransmission, often via the regulation of receptor expression. Here, we review the regulation of GABA_AR subunit expression in adaptive and pathological plasticity, with a focus on substance use. We examine the factors influencing the expression of GABA_AR subunit genes including the regulation of the 5′ and 3′ untranslated regions, variations in DNA methylation, immediate early genes and transcription factors that regulate subunit expression, translational and post-translational modifications, and other forms of receptor regulation beyond expression. Advancing our understanding of the factors regulating GABA_AR subunit expression during adaptive plasticity, as well as during substance use and withdrawal will provide insight into the role of GABAergic signaling in substance use disorders, and contribute to the development of novel targeted therapies.

Chronic Ethanol Exposure and Withdrawal Impair Synaptic GABAA Receptor-Mediated Neurotransmission in Deep-Layer Prefrontal Cortex

BACKGROUND

The prefrontal cortex (PFC) acts as an integrative hub for the processing of cortical and subcortical input into meaningful efferent signaling, permitting complex associative behaviors. PFC dysfunction is consistently observed with ethanol (EtOH) dependence and is a core component of the pathology of alcohol use disorders in current models of addiction. While intracortical gamma-aminobutryric acid (GABA)ergic neurotransmission is understood to be essential for maintaining coordinated network activity within the cortex, relatively little is known regarding functional GABAergic adaptations in PFC during EtOH dependence.

METHODS

In the present study, male and female (> postnatal day 60) Sprague-Dawley rats were administered EtOH (5.0 g/kg; intragastric gavage) for 14 to 15 consecutive days. Twenty-four hours after the final administration, animals were sacrificed and brains extracted for electrophysiological recordings of isolated GABA_A receptor-mediated currents or analysis of GABA_A receptor subunit protein expression in deep-layer PFC neurons.

RESULTS

Chronic EtOH exposure significantly attenuated activity-dependent spontaneous GABA_A receptor-mediated inhibitory postsynaptic current (IPSC) frequency with no effect on amplitude. Furthermore, analysis of IPSC decay kinetics revealed a significant enhancement of IPSC decay time that was associated with decrements in expression of the α1 GABA_A receptor subunit, indicative of further impaired phasic inhibition. These phenomena occurred irrespective of neuron projection destination and sex. Based on previous observations by our laboratory of an epigenetic mechanism for EtOH-induced changes in cortical GABA_A receptor subunit expression, the histone deacetylase inhibitor Trichostatin A was administered to water- and EtOH-exposed animals, and prevented EtOH-induced changes in spontaneous IPSC frequency, IPSC decay kinetics, and GABA_A receptor subunit expression.

CONCLUSIONS

Taken together, these results demonstrate that chronic EtOH exposure impairs synaptic inhibitory neurotransmission in deep-layer pyramidal neurons of the medial PFC in both male and female rats. These maladaptations occur in neurons projecting to numerous regions implicated in the sequelae of EtOH dependence, offering a mechanistic link between the manifestation of PFC dysfunction and negative affective states observed with extended consumption.

Taurine modulates acute ethanol-induced social behavioral deficits and fear responses in adult zebrafish

Ethanol (EtOH) is a central nervous system (CNS) depressant drug that modifies various behavioral domains (i.e., sociability, aggressiveness, and memory) by promoting disinhibition of punished operant behavior and neurochemical changes. Taurine (TAU) is a β-amino sulfonic acid with pleiotropic roles in the brain. Although exogenous TAU is found in energy drinks and often mixed with alcohol in beverages, the putative risks of mixing TAU and EtOH are poorly explored. Here, we investigated whether TAU modulates social and fear responses by assessing shoaling behavior, preference for conspecifics, and antipredatory behavior of adult zebrafish acutely exposed to EtOH. Zebrafish shoals (4 fish per shoal) were exposed to water (control), TAU (42, 150, and 400 mg/L), 0.25% (v/v) EtOH alone or in association with TAU for 1 h, and their behaviors were analyzed at different time intervals (0-5 min, 30-35 min, and 55-60 min). The effects of TAU and EtOH were further tested in a social preference test and during exposure to a predator. Both EtOH and TAU co-treated fish showed a higher shoal dispersion, while TAU 400/EtOH group shoal area had a similar profile when compared to control. However, in the social preference test, TAU 400/EtOH impaired the seeking for conspecifics. Regarding fear-like behaviors, TAU-cotreated fish showed a prominent reduction in risk assessments when compared to EtOH alone. Overall, we demonstrate that TAU modulates EtOH-induced changes in different behavioral domains, suggesting a complex relationship between social and fear-like responses.

Histone deacetylases mediate GABAA receptor expression, physiology, and behavioral maladaptations in rat models of alcohol dependence

Alcohol use disorders are chronic debilitating diseases characterized by severe withdrawal symptoms that contribute to morbidity and relapse. GABA_A receptor (GABA_AR) adaptations have long been implicated in the chronic effects of alcohol and contribute to many withdrawal symptoms associated with alcohol dependence. In rodents, GABA_AR hypofunction results from decreases in Gabra1 expression, although the underlying mechanism controlling Gabra1 expression after chronic ethanol exposure is still unknown. We found that chronic ethanol exposure using either ethanol gavage or two-bottle choice voluntary access paradigms decreased Gabra1 expression and increased Hdac2 and Hdac3 expression. Administration of the HDAC inhibitor trichostatin A (TSA) after chronic ethanol exposure prevents the decrease in Gabra1 expression and function as well as the increase in Hdac2 and Hdac3 expression in both the cortex and the medial prefrontal cortex (mPFC). Chronic ethanol exposure and withdrawal, but not acute ethanol exposure or acute withdrawal, cause a selective upregulation of HDAC2 and HDAC3 associated with the Gabra1 promoter that accompanies a decrease in H3 acetylation of the Gabra1 promoter and the reduction in GABA_AR α1 subunit expression. TSA administration prevented each of these molecular events as well as behavioral manifestations of ethanol dependence, including tolerance to zolpidem-induced loss of righting reflex, reduced open-arm time in the elevated plus maze, reduced center-time and locomotor activity in the open-field assay, and TSA reduced voluntary ethanol consumption. The results show how chronic ethanol exposure regulates the highly prominent GABA_AR α1 subunit by an epigenetic mechanism that represents a potential treatment modality for alcohol dependence.

Single Ethanol Withdrawal Regulates Extrasynaptic δ-GABAA Receptors Via PKCδ Activation

Alcohol (ethanol, EtOH) is one of the most widely abused drugs with profound effects on brain function and behavior. GABA_A receptors (GABA_ARs) are one of the major targets for EtOH in the brain. Temporary plastic changes in GABA_ARs after withdrawal from a single EtOH exposure occur both in vivo and in vitro, which may be the basis for chronic EtOH addiction, tolerance and withdrawal symptoms. Extrasynaptic δ-GABA_AR endocytosis is implicated in EtOH-induced GABA_AR plasticity, but the mechanisms by which the relative abundance and localization of specific GABA_ARs are altered by EtOH exposure and withdrawal remain unclear. In this study, we investigated the mechanisms underlying rapid regulation of extrasynaptic δ-GABA_AR by a single EtOH withdrawal in cultured rat hippocampal neurons. Thirty-minutes EtOH (60 mM) exposure increased extrasynaptic tonic current (I_tonic) amplitude without affecting synaptic GABA_AR function in neurons. In contrast, at 30 min after withdrawal, I_tonic amplitude and responsiveness to acute EtOH were both reduced. Similar results occurred in neurons with okadaic acid (OA) or phorbol 12,13-dibutyrate (PDBu) exposure. Protein kinase C (PKC) inhibition prevented the reduction of I_tonic amplitude and the tolerance to acute EtOH, as well as the reduction of GABA_AR-δ subunit abundance induced by a single EtOH withdrawal. Moreover, EtOH withdrawal selectively increased PKCδ level, whereas PKCδ inhibition specifically rescued the EtOH-induced alterations in GABA_AR-δ subunit level and δ-GABA_AR function. Together, we provided strong evidence for the important roles of PKCδ in the rapid regulation of extrasynaptic δ-GABA_AR induced by a single EtOH withdrawal.

GABA type a receptor trafficking and the architecture of synaptic inhibition

Ubiquitous expression of GABA type A receptors (GABAA R) in the central nervous system establishes their central role in coordinating most aspects of neural function and development. Dysregulation of GABAergic neurotransmission manifests in a number of human health disorders and conditions that in certain cases can be alleviated by drugs targeting these receptors. Precise changes in the quantity or activity of GABAA Rs localized at the cell surface and at GABAergic postsynaptic sites directly impact the strength of inhibition. The molecular mechanisms constituting receptor trafficking to and from these compartments therefore dictate the efficacy of GABAA R function. Here we review the current understanding of how GABAA Rs traffic through biogenesis, plasma membrane transport, and degradation. Emphasis is placed on discussing novel GABAergic synaptic proteins, receptor and scaffolding post-translational modifications, activity-dependent changes in GABAA R confinement, and neuropeptide and neurosteroid mediated changes. We further highlight modern techniques currently advancing the knowledge of GABAA R trafficking and clinically relevant neurodevelopmental diseases connected to GABAergic dysfunction. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 238-270, 2018.

Maternal care-related differences in males and females rats' sensitivity to ethanol and the associations between the GABAergic system and steroids in males

This study investigated the effect of maternal care on adolescent ethanol consumption, sensitivity to ethanol-induced hypnosis, as well as gonadal hormones and γ-aminobutyric acid type-A (GABA_A ) systems. Long Evans rat dams were categorized by maternal licking/grooming (LG) frequency into High- and Low-LG mothers. Both female and male offspring from Low-LG rats demonstrated a greater sensitivity to ethanol-induced hypnosis in the loss-of-righting-reflex test at ethanol doses of 3.0 and 3.5 g/kg during late-adolescence (postnatal Day 50) but not at mid-adolescence (postnatal Day 42). However, we found no effect of maternal care on consumption of a 5% ethanol solution in a two-bottle choice test. We further investigated the association between the observed variations in sensitivity to ethanol-induced hypnosis and baseline hormonal levels in males. In male offspring from Low-LG mothers compared to High-LG mothers, baseline plasma corticosterone and progesterone levels were higher. GABA_A α1 and δ subunit expressions were also higher in the cerebral cortex of Low-LG males but lower in the cerebellar synaptosomal fraction. Early environmental influences on adolescent sensitivity to ethanol-induced hypnosis, consumption, and preference may be mediated by gonadal hormones and possibly through GABAergic functions.

A versatile optical tool for studying synaptic GABAA receptor trafficking

Live-cell imaging methods can provide critical real-time receptor trafficking measurements. Here, we describe an optical tool to study synaptic γ-aminobutyric acid (GABA) type A receptor (GABA_AR) dynamics through adaptable fluorescent-tracking capabilities. A fluorogen-activating peptide (FAP) was genetically inserted into a GABA_AR γ2 subunit tagged with pH-sensitive green fluorescent protein (γ2^pHFAP). The FAP selectively binds and activates Malachite Green (MG) dyes that are otherwise non-fluorescent in solution. γ2^pHFAP GABA_ARs are expressed at the cell surface in transfected cortical neurons, form synaptic clusters and do not perturb neuronal development. Electrophysiological studies show γ2^pHFAP GABA_ARs respond to GABA and exhibit positive modulation upon stimulation with the benzodiazepine diazepam. Imaging studies using γ2^pHFAP-transfected neurons and MG dyes show time-dependent receptor accumulation into intracellular vesicles, revealing constitutive endosomal and lysosomal trafficking. Simultaneous analysis of synaptic, surface and lysosomal receptors using the γ2^pHFAP-MG dye approach reveals enhanced GABA_AR turnover following a bicucculine-induced seizure paradigm, a finding not detected by standard surface receptor measurements. To our knowledge, this is the first application of the FAP-MG dye system in neurons, demonstrating the versatility to study nearly all phases of GABA_AR trafficking.

Ethanol Exposure Regulates Gabra1 Expression via Histone Deacetylation at the Promoter in Cultured Cortical Neurons

γ-Aminobutyric acid A receptors (GABAA-Rs) mediate the majority of inhibitory neurotransmission in the adult brain. The α1-containing GABAA-Rs are the most prominent subtype in the adult brain and are important in both homeostatic function and several disease pathologies including alcohol dependence, epilepsy, and stress. Ethanol exposure causes a decrease of α1 transcription and peptide expression both in vivo and in vitro, but the mechanism that controls the transcriptional regulation is unknown. Because ethanol is known to activate epigenetic regulation of gene expression, we tested the hypothesis that ethanol regulates α1 expression through histone modifications in cerebral cortical cultured neurons. We found that class I histone deacetylases (HDACs) regulate ethanol-induced changes in α1 gene and protein expression as pharmacologic inhibition or knockdown of HDAC1-3 prevents the effects of ethanol exposure. Targeted histone acetylation associated with the Gabra1 promoter using CRISPR (clustered regularly interspaced palindromic repeat) dCas9-P300 (a nuclease-null Cas9 fused with a histone acetyltransferase) increases histone acetylation and prevents the decrease of Gabra1 expression. In contrast, there was no effect of a mutant histone acetyltransferase or generic transcriptional activator or targeting P300 to a distant exon. Conversely, using a dCas9-KRAB construct that increases repressive methylation (H3K9me3) does not interfere with ethanol-induced histone deacetylation. Overall our results indicate that ethanol deacetylates histones associated with the Gabra1 promoter through class I HDACs and that pharmacologic, genetic, or epigenetic intervention prevents decreases in α1 expression in cultured cortical neurons.

Differential Molecular Targets for Neuroprotective Effect of Chlorogenic Acid and its Related Compounds Against Glutamate Induced Excitotoxicity and Oxidative Stress in Rat Cortical Neurons

The present study has been designed to explore the molecular mechanism and signaling pathway targets of chlorogenic acid (CGA) and its main hydrolysates, caffeic (CA) and quinic acid in the protective effect against glutamate-excitotoxicity. For this purpose 8-DIV cortical neurons in primary culture were exposed to 50 μM L-glutamic acid plus 10 µM glycine, with or without 10-100 μM tested compounds. Chlorogenic acid and caffeic acid via their antioxidant properties inhibited cell death induced by glutamate in dose depended manner. However, quinic acid slightly protects neurons at a higher dose. DCF, JC-1 and Ca²⁺sensitive fluorescent dye fura-2, were used to measure intracellular ROS accumulation, mitochondrial membrane potential integration and intracellular calcium concentration [Ca²⁺] _i . Results indicate that similarly, CGA acts as a protective agent against glutamate-induced cortical neurons injury by suppressing the accumulation of endogenous ROS and restore the mitochondrial membrane potential, activate the enzymatic antioxidant system by the increase levels of SOD activity and modulate the rise of intracellular calcium levels by increasing the rise of intracellular concentrations of Ca²⁺caused by glutamate overstimulation. PKC signaling cascade appear to be engaged in this protective mechanism. Interseling, CGA and CA also exhibit antiapoptotic properties against glutamate-induced cleaved activation of pro-caspases; caspase 1,8 and 9 and calpain (PD 150606,Calpeptin and MDL 28170).These data suggest that neuroprotective activity of CGA ester may occurs throught its hydrolysate,the caffeic acid and its interaction with intracellular molecules suggesting that CGA exert its neuroprotection via its caffeoly acid group that might potentially be used as a therapeutic agent in neurodegeneratives disorders associated with glutamate excitotoxicity.

The allostatic impact of chronic ethanol on gene expression: A genetic analysis of chronic intermittent ethanol treatment in the BXD cohort

The transition from acute to chronic ethanol exposure leads to lasting behavioral and physiological changes such as increased consumption, dependence, and withdrawal. Changes in brain gene expression are hypothesized to underlie these adaptive responses to ethanol. Previous studies on acute ethanol identified genetic variation in brain gene expression networks and behavioral responses to ethanol across the BXD panel of recombinant inbred mice. In this work, we have performed the first joint genetic and genomic analysis of transcriptome shifts in response to chronic intermittent ethanol (CIE) by vapor chamber exposure in a BXD cohort. CIE treatment is known to produce significant and sustained changes in ethanol consumption with repeated cycles of ethanol vapor. Using Affymetrix microarray analysis of prefrontal cortex (PFC) and nucleus accumbens (NAC) RNA, we compared CIE expression responses to those seen following acute ethanol treatment, and to voluntary ethanol consumption. Gene expression changes in PFC and NAC after CIE overlapped significantly across brain regions and with previously published expression following acute ethanol. Genes highly modulated by CIE were enriched for specific biological processes including synaptic transmission, neuron ensheathment, intracellular signaling, and neuronal projection development. Expression quantitative trait locus (eQTL) analyses identified genomic loci associated with ethanol-induced transcriptional changes with largely distinct loci identified between brain regions. Correlating CIE-regulated genes to ethanol consumption data identified specific genes highly associated with variation in the increase in drinking seen with repeated cycles of CIE. In particular, multiple myelin-related genes were identified. Furthermore, genetic variance in or near dynamin3 (Dnm3) on Chr1 at ∼164 Mb may have a major regulatory role in CIE-responsive gene expression. Dnm3 expression correlates significantly with ethanol consumption, is contained in a highly ranked functional group of CIE-regulated genes in the NAC, and has a cis-eQTL within a genomic region linked with multiple CIE-responsive genes.

Role of interleukin-10 (IL-10) in regulation of GABAergic transmission and acute response to ethanol

Mounting evidence indicates that ethanol (EtOH) exposure activates neuroimmune signaling. Alterations in pro-inflammatory cytokines after acute and chronic EtOH exposure have been heavily investigated. In contrast, little is known about the regulation of neurotransmission and/or modulation by anti-inflammatory cytokines in the brain after an acute EtOH exposure. Recent evidence suggests that interleukin-10 (IL-10), an anti-inflammatory cytokine, is upregulated during withdrawal from chronic EtOH exposure. In the present study, we show that IL-10 is increased early (1 h) after a single intoxicating dose of EtOH (5 g/kg, intragastric) in Sprague Dawley rats. We also show that IL-10 rapidly regulates GABAergic transmission in dentate gyrus neurons. In brain slice recordings, IL-10 application dose-dependently decreases miniature inhibitory postsynaptic current (mIPSC) area and frequency, and decreases the magnitude of the picrotoxin sensitive tonic current (Itonic), indicating both pre- and postsynaptic mechanisms. A PI3K inhibitor LY294002 (but not the negative control LY303511) ablated the inhibitory effects of IL-10 on mIPSC area and Itonic, but not on mIPSC frequency, indicating the involvement of PI3K in postsynaptic effects of IL-10 on GABAergic transmission. Lastly, we also identify a novel neurobehavioral regulation of EtOH sensitivity by IL-10, whereby IL-10 attenuates acute EtOH-induced hypnosis. These results suggest that EtOH causes an early release of IL-10 in the brain, which may contribute to neuronal hyperexcitability as well as disturbed sleep seen after binge exposure to EtOH. These results also identify IL-10 signaling as a potential therapeutic target in alcohol-use disorders and other CNS disorders where GABAergic transmission is altered.

Ethanol Regulation of Synaptic GABAA α4 Receptors Is Prevented by Protein Kinase A Activation

Ethanol alters GABAA receptor trafficking and function through activation of protein kinases, and these changes may underlie ethanol dependence and withdrawal. In this study, we used subsynaptic fraction techniques and patch-clamp electrophysiology to investigate the biochemical and functional effects of protein kinase A (PKA) and protein kinase C (PKC) activation by ethanol on synaptic GABAA α4 receptors, a key target of ethanol-induced changes. Rat cerebral cortical neurons were grown for 18 days in vitro and exposed to ethanol and/or kinase modulators for 4 hours, a paradigm that recapitulates GABAergic changes found after chronic ethanol exposure in vivo. PKA activation by forskolin or rolipram during ethanol exposure prevented increases in P2 fraction α4 subunit abundance, whereas inhibiting PKA had no effect. Similarly, in the synaptic fraction, activation of PKA by rolipram in the presence of ethanol prevented the increase in synaptic α4 subunit abundance, whereas inhibiting PKA in the presence of ethanol was ineffective. Conversely, PKC inhibition in the presence of ethanol prevented the ethanol-induced increases in synaptic α4 subunit abundance. Finally, we found that either activating PKA or inhibiting PKC in the presence of ethanol prevented the ethanol-induced decrease in GABA miniature inhibitory postsynaptic current decay τ1, whereas inhibiting PKA had no effect. We conclude that PKA and PKC have opposing effects in the regulation of synaptic α4 receptors, with PKA activation negatively modulating, and PKC activation positively modulating, synaptic α4 subunit abundance and function. These results suggest potential targets for restoring normal GABAergic functioning in the treatment of alcohol use disorders.

Differential regulation of synaptic and extrasynaptic α4 GABA(A) receptor populations by protein kinase A and protein kinase C in cultured cortical neurons

The GABAA α4 subunit exists in two distinct populations of GABAA receptors. Synaptic GABAA α4 receptors are localized at the synapse and mediate phasic inhibitory neurotransmission, while extrasynaptic GABAA receptors are located outside of the synapse and mediate tonic inhibitory transmission. These receptors have distinct pharmacological and biophysical properties that contribute to interest in how these different subtypes are regulated under physiological and pathological states. We utilized subcellular fractionation procedures to separate these populations of receptors in order to investigate their regulation by protein kinases in cortical cultured neurons. Protein kinase A (PKA) activation decreases synaptic α4 expression while protein kinase C (PKC) activation increases α4 subunit expression, and these effects are associated with increased β3 S408/409 or γ2 S327 phosphorylation respectively. In contrast, PKA activation increases extrasynaptic α4 and δ subunit expression, while PKC activation has no effect. Our findings suggest synaptic and extrasynaptic GABAA α4 subunit expression can be modulated by PKA to inform the development of more specific therapeutics for neurological diseases that involve deficits in GABAergic transmission.

Regulation of Extrasynaptic GABAA α4 Receptors by Ethanol-Induced Protein Kinase A, but Not Protein Kinase C Activation in Cultured Rat Cerebral Cortical Neurons

Ethanol produces changes in GABAA receptor trafficking and function that contribute to ethanol dependence symptomatology. Extrasynaptic γ-aminobutyric acid A receptors (GABAA-R) mediate inhibitory tonic current and are of particular interest because they are potentiated by physiologically relevant doses of ethanol. Here, we isolate GABAA α4δ receptors by western blotting in subsynaptic fractions to investigate protein kinase A (PKA) and protein kinase C (PKC) modulation of ethanol-induced receptor trafficking, while extrasynaptic receptor function is determined by measurement of tonic inhibition and responses evoked by 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP). Rat cerebral cortical neurons were grown for 18 days in vitro and exposed to ethanol and/or PKA/PKC modulators. Ethanol exposure (1 hour) did not alter GABAA α4 receptor abundance, but it increased tonic current amplitude, an effect that was prevented by inhibiting PKA, but not PKC. Direct activation of PKA, but not PKC, increased the abundance and tonic current of extrasynaptic α4δ receptors. In contrast, prolonged ethanol exposure (4 hours) reduced α4δ receptor abundance as well as tonic current, and this effect was also PKA dependent. Finally, PKC activation by ethanol or phorbol-12,13-dibutyrate (PdBu) had no effect on extrasynaptic α4δ subunit abundance or activity. We conclude that ethanol alters extrasynaptic α4δ receptor function and expression in cortical neurons in a PKA-dependent manner, but ethanol activation of PKC does not influence these receptors. These results could have clinical relevance for therapeutic strategies to restore normal GABAergic functioning for the treatment of alcohol use disorders.

Receptor Reserve Moderates Mesolimbic Responses to Opioids in a Humanized Mouse Model of the OPRM1 A118G Polymorphism

The OPRM1 A118G polymorphism is the most widely studied μ-opioid receptor (MOR) variant. Although its involvement in acute alcohol effects is well characterized, less is known about the extent to which it alters responses to opioids. Prior work has shown that both electrophysiological and analgesic responses to morphine but not to fentanyl are moderated by OPRM1 A118G variation, but the mechanism behind this dissociation is not known. Here we found that humanized mice carrying the 118GG allele (h/mOPRM1-118GG) were less sensitive than h/mOPRM1-118AA littermates to the rewarding effects of morphine and hydrocodone but not those of other opioids measured with intracranial self-stimulation. Reduced morphine reward in 118GG mice was associated with decreased dopamine release in the nucleus accumbens and reduced effects on GABA release in the ventral tegmental area that were not due to changes in drug potency or efficacy in vitro or receptor-binding affinity. Fewer MOR-binding sites were observed in h/mOPRM1-118GG mice, and pharmacological reduction of MOR availability unmasked genotypic differences in fentanyl sensitivity. These findings suggest that the OPRM1 A118G polymorphism decreases sensitivity to low-potency agonists by decreasing receptor reserve without significantly altering receptor function.

Reduction of α1GABAA receptor mediated by tyrosine kinase C (PKC) phosphorylation in a mouse model of fragile X syndrome

Fragile X syndrome (FXS) caused by lack of fragile X mental retardation protein (Fmr1) is the most common cause of inherited intellectual disability and characterized by many cognitive disturbances like attention deficit, autistic behavior, and audiogenic seizure and have region-specific altered expression of some gamma-aminobutyric acid (GABAA) receptor subunits. Quantitative real-time polymerase chain reaction and western blot experiments were performed in the cultured cortical neurons and forebrain obtained from wild-type (WT) and Fmr1 KO mice demonstrate the reduction in the expression of α1 gamma-aminobutyric acid (α1GABAA) receptor, phospho-α1GABAA receptor, PKC and phosphor-PKC in Fmr1 KO mice comparing with WT mice, both in vivo and in vitro. Furthermore, we found that the phosphorylation of the α1GABAA receptor was mediated by PKC. Our results elucidate that the lower phosphorylation of the α1GABAA receptor mediated by PKC neutralizes the seizure-promoting effects in Fmr1 KO mice and point to the potential therapeutic targets of α1GABAA agonists for the treatment of fragile X syndrome.

Regulation of GABAARs by phosphorylation

γ-Aminobutyric acid type A receptors (GABAARs) are the principal mediators of fast synaptic inhibition in the brain as well as the low persistent extrasynaptic inhibition, both of which are fundamental to proper brain function. Thus unsurprisingly, deficits in GABAARs are implicated in a number of neurological disorders and diseases. The complexity of GABAAR regulation is determined not only by the heterogeneity of these receptors but also by its posttranslational modifications, the foremost, and best characterized of which is phosphorylation. This review will explore the details of this dynamic process, our understanding of which has barely scratched the surface. GABAARs are regulated by a number of kinases and phosphatases, and its phosphorylation plays an important role in governing its trafficking, expression, and interaction partners. Here, we summarize the progress in understanding the role phosphorylation plays in the regulation of GABAARs. This includes how phosphorylation can affect the allosteric modulation of GABAARs, as well as signaling pathways that affect GABAAR phosphorylation. Finally, we discuss the dysregulation of GABAAR phosphorylation and its implication in disease processes.

Ethanol induced impairment of glucose metabolism involves alterations of GABAergic signaling in pancreatic β-cells

Alcohol overindulgence is a risk factor of type 2 diabetes mellitus. However, the mechanisms by which alcohol overindulgence damages glucose metabolism remain unclear. Pancreatic islet β-cells are endowed with type-A γ-aminobutyric acid receptor (GABAAR) mediated autocrine signaling mechanism, which regulates insulin secretion and fine-tunes glucose metabolism. In neurons GABAAR is one of the major targets for alcohol. This study investigated whether ethanol alters glucose metabolism by affecting GABAAR signaling in pancreatic β-cells. Blood glucose level of test mice was measured using a blood glucose meter. Insulin secretion by the pancreatic β-cell line INS-1 cells was examined using a specific insulin ELISA kit. Whole-cell patch-clamp recording was used to evaluate GABA-elicited current in INS-1 cells. Western blot and immunostaining were used to measure the expression of GABAAR subunits in mouse pancreatic tissues or in INS-1 cells. Intraperitoneal (i.p.) administration of ethanol (3.0g/kg body weight) to mice altered glucose metabolism, which was associated with decreased expression of GABAAR α1- and δ- subunits on the surface of pancreatic β-cells. Acute treatment of cultured INS-1cells with ethanol (60mM) decreased the GABA-induced current and reduced insulin secretion. In contrast, treating INS-1 cells with GABA (100μM) largely prevented the ethanol-induced reduction of insulin release. Importantly, pre-treating mice with GABA (i.p., 1.5mg/kg body weight) partially reversed ethanol-induced impairment of glucose homeostasis in mice. Our data suggest a novel role of pancreatic GABA signaling in protecting pancreatic islet β-cells from ethanol-induced dysfunction.

Plasticity of GABA(A) receptor-mediated neurotransmission in the nucleus accumbens of alcohol-dependent rats

Chronic alcohol exposure-induced changes in reinforcement mechanisms and motivational state are thought to contribute to the development of cravings and relapse during protracted withdrawal. The nucleus accumbens (NAcc) is a key structure of the mesolimbic dopaminergic reward system and plays an important role in mediating alcohol-seeking behaviors. Here we describe the long-lasting alterations of γ-aminobutyric acid type A receptors (GABA(A)Rs) of medium spiny neurons (MSNs) in the NAcc after chronic intermittent ethanol (CIE) treatment, a rat model of alcohol dependence. CIE treatment and withdrawal (>40 days) produced decreases in the ethanol and Ro15-4513 potentiation of extrasynaptic GABA(A)Rs, which mediate the picrotoxin-sensitive tonic current (I(tonic)), while potentiation of synaptic receptors, which give rise to miniature inhibitory postsynaptic currents (mIPSCs), was increased. Diazepam sensitivity of both I(tonic) and mIPSCs was decreased by CIE treatment. The average magnitude of I(tonic) was unchanged, but mIPSC amplitude and frequency decreased and mIPSC rise time increased after CIE treatment. Rise-time histograms revealed decreased frequency of fast-rising mIPSCs after CIE treatment, consistent with possible decreases in somatic GABAergic synapses in MSNs from CIE rats. However, unbiased stereological analysis of NeuN-stained NAcc neurons did not detect any decreases in NAcc volume, neuronal numbers, or neuronal cell body volume. Western blot analysis of surface subunit levels revealed selective decreases in α1 and δ and increases in α4, α5, and γ2 GABA(A)R subunits after CIE treatment and withdrawal. Similar, but reversible, alterations occurred after a single ethanol dose (5 g/kg). These data reveal CIE-induced long-lasting neuroadaptations in the NAcc GABAergic neurotransmission.

Molecular and behavioral characterization of adolescent protein kinase C following high dose ethanol exposure

RATIONALE

Ethanol is commonly used and abused during adolescence. Although adolescents display differential behavioral responses to ethanol, the mechanisms by which this occurs are not known. The protein kinase C (PKC) pathway has been implicated in mediating many ethanol-related effects in adults, as well as gamma-aminobutyric acid (GABA(A)) receptor regulation.

OBJECTIVES

The present study was designed to characterize cortical PKC isoform and GABA(A) receptor subunit expression during adolescence relative to adults as well as assess PKC involvement in ethanol action.

RESULTS

Novel PKC isoforms were elevated, while PKCγ was lower during mid-adolescence relative to adults. Whole-cell lysate and synaptosomal preparations correlated for all isoforms except PKCδ. In parallel, synaptosomal GABAA receptor subunit expression was also developmentally regulated, with GABA(A)R δ and α4 being lower while α1 and γ2 were higher or similar, respectively, in adolescents compared to adults. Following acute ethanol exposure, synaptosomal novel and atypical PKC isoform expression was decreased only in adolescents. Behaviorally, inhibiting PKC with calphostin C, significantly increased ethanol-induced loss of righting reflex (LORR) in adolescents but not adults, whereas activating PKC with phorbol dibutyrate was ineffective in adolescents but decreased LORR duration in adults. Further investigation revealed that inhibiting the cytosolic phospholipase A2/arachidonic acid (cPLA2/AA) pathway increased LORR duration in adolescents, but was ineffective in adults.

CONCLUSIONS

These data indicate that PKC isoforms are variably regulated during adolescence and may contribute to adolescent ethanol-related behavior. Furthermore, age-related differences in the cPLA2/AA pathway may contribute to ethanol's age-related effects on novel and atypical PKC isoform expression and behavior.

The neurobiology of alcohol consumption and alcoholism: an integrative history

Studies of the neurobiological predisposition to consume alcohol (ethanol) and to transition to uncontrolled drinking behavior (alcoholism), as well as studies of the effects of alcohol on brain function, started a logarithmic growth phase after the repeal of the 18th Amendment to the United States Constitution. Although the early studies were primitive by current technological standards, they clearly demonstrated the effects of alcohol on brain structure and function, and by the end of the 20th century left little doubt that alcoholism is a "disease" of the brain. This review traces the history of developments in the understanding of ethanol's effects on the most prominent inhibitory and excitatory systems of brain (GABA and glutamate neurotransmission). This neurobiological information is integrated with knowledge of ethanol's actions on other neurotransmitter systems to produce an anatomical and functional map of ethanol's properties. Our intent is limited in scope, but is meant to provide context and integration of the actions of ethanol on the major neurobiologic systems which produce reinforcement for alcohol consumption and changes in brain chemistry that lead to addiction. The developmental history of neurobehavioral theories of the transition from alcohol drinking to alcohol addiction is presented and juxtaposed to the neurobiological findings. Depending on one's point of view, we may, at this point in history, know more, or less, than we think we know about the neurobiology of alcoholism.

Altered GABAA receptor expression and seizure threshold following acute ethanol challenge in mice lacking the RIIβ subunit of PKA

Ethanol causes pathological changes in GABAA receptor trafficking and function. These changes are mediated in part by ethanol activation of protein kinase A (PKA). The current study investigated the expression of the GABAA α1 and α4 subunits and the kinase anchoring protein AKAP150, as well as bicuculline-induced seizure threshold, at baseline and following acute injection of ethanol (3.5 g/kg IP) in a mouse line lacking the regulatory RIIβ subunit of PKA. Whole cerebral cortices were harvested at baseline, 1 h, or 46 h following injection of ethanol or saline and subjected to fractionation and western blot analysis. Knockout (RIIβ-/-) mice had similar baseline levels of PKA RIIα and GABAA α1 and α4 subunits compared to wild type (RIIβ+/+) littermates, but had deficits in AKAP150. GABAA α1 subunit levels were decreased in the P2 fraction of RIIβ-/-, but not RIIβ+/+, mice following 1 h ethanol, an effect that was driven by decreased α1 expression in the synaptic fraction. GABAA α4 subunits in the P2 fraction were not affected by 1 h ethanol; however, synaptic α4 subunit expression was increased in RIIβ+/+, but not RIIβ-/- mice, while extrasynaptic α4 and δ subunit expression were decreased in RIIβ-/-, but not RIIβ+/+ mice. Finally, RIIβ knockout was protective against bicuculline-induced seizure susceptibility. Overall, the results suggest that PKA has differential roles in regulating GABAA receptor subunits. PKA may protect against ethanol-induced deficits in synaptic α1 and extrasynaptic α4 receptors, but may facilitate the increase of synaptic α4 receptors.

Ethanol activation of protein kinase A regulates GABAA α1 receptor function and trafficking in cultured cerebral cortical neurons

Ethanol exposure produces alterations in GABAergic signaling that are associated with dependence and withdrawal. Previously, we demonstrated that ethanol-induced protein kinase C (PKC) γ signaling selectively contributes to changes in GABAA α1 synaptic receptor activity and surface expression. Here, we demonstrate that protein kinase A (PKA) exerts opposing effects on GABAA receptor adaptations during brief ethanol exposure. Cerebral cortical neurons from day 0-1 rat pups were tested after 18 days in culture. Receptor trafficking was assessed by Western blot analysis, and functional changes were measured using whole-cell patch-clamp recordings of evoked and miniature inhibitory postsynaptic current (mIPSC) responses. One-hour ethanol exposure increased membrane-associated PKC and PKA, but steady-state GABAA α1 subunit levels were maintained. Activation of PKA by Sp-adenosine 3',5'-cyclic monophosphothioate triethylamine alone increased GABAA α1 subunit surface expression and zolpidem potentiation of GABA responses, whereas coexposure of ethanol with the PKA inhibitor Rp-adenosine 3',5'-cyclic monophosphothioate triethylamine decreased α1 subunit expression and zolpidem responses. Exposure to the PKC inhibitor calphostin-C with ethanol mimicked the effect of direct PKA activation. The effects of PKA modulation on mIPSC decay τ were consistent with its effects on GABA currents evoked in the presence of zolpidem. Overall, the results suggest that PKA acts in opposition to PKC on α1-containing GABAA receptors, mediating the GABAergic effects of ethanol exposure, and may provide an important target for the treatment of alcohol dependence/withdrawal.

Ethanol promotes clathrin adaptor-mediated endocytosis via the intracellular domain of δ-containing GABAA receptors

Pharmacological and genetic evidence reveals that GABA(A) receptor (GABA(A)-R) expression and localization are modulated in response to acute and chronic ethanol (EtOH) exposure. To determine molecular mechanisms of GABA(A)-R plasticity in response to in vivo acute EtOH, we measured early time changes in GABA(A)-R subunit localization. Single doses of EtOH (3 g/kg via i.p. injection in rats) produced decreases in surface levels of GABA(A)-R α4 and δ subunits at 5-15 min post-EtOH in hippocampus CA1 and dentate gyrus, verifying our earlier report (Liang et al., 2007). Here we also examined the β3 subunit and its phosphorylation state during internalization. β3 also was internalized during 5-15 min after EtOH exposure, while phosphorylation of β3 was increased, then decreased at later times, ruling out β3 dephosphorylation-dependent endocytosis. As early as 5 min post-EtOH, there is an initial increase in association between the δ subunits with clathrin adaptor proteins AP2-μ2 revealed by coimmunoprecipitation, followed by a decrease in association 15 min post-EtOH. In vitro studies using glutathione S-transferase fused to the δ subunit intracellular domain (ICD) show that two regions, one containing a classical YxxΦ motif and the other an atypical R/K-rich motif, directly and differentially bind to AP2-μ2, with the former YRSV exhibiting higher affinity. Mutating both regions in the δ-ICD abolishes μ2 binding, providing a possible mechanism that can explain the rapid downregulation of extrasynaptic α4βδ-GABA(A)-R following in vivo EtOH administration, in which the δ-ICD increases in affinity for clathrin AP2-μ2 leading to endocytosis.

Ethanol inhibition of a T-type Ca²+ channel through activity of protein kinase C

BACKGROUND

T-type calcium channels (T-channels) are widely distributed in the central and peripheral nervous system, where they mediate calcium entry and regulate the intrinsic excitability of neurons. T-channels are dysregulated in response to alcohol administration and withdrawal. We therefore investigated acute ethanol (EtOH) effects and the underlying mechanism of action in human embryonic kidney (HEK) 293 cell lines, as well as effects on native currents recorded from dorsal root ganglion (DRG) neurons cultured from Long-Evans rats.

METHODS

Whole-cell voltage-clamp recordings were performed at 32 to 34°C in both HEK cell lines and DRG neurons. The recordings were taken after a 10-minute application of EtOH or protein kinase C (PKC) activator (phorbol 12-myristate 13-acetate [PMA]).

RESULTS

We recorded T-type Ca²⁺ currents (T-currents) from 3 channel isoforms (CaV3.1, CaV3.2, and CaV3.3) before and during administration of EtOH. We found that only 1 isoform, CaV3.2, was significantly affected by EtOH. EtOH reduced current density as well as producing a hyperpolarizing shift in steady-state inactivation of both CaV3.2 currents from HEK 293 cell lines and in native T-currents from DRG neurons that are known to be enriched in CaV3.2. A myristoylated PKC peptide inhibitor (MPI) blocked the major EtOH effects, in both the cell lines and the DRG neurons. However, PMA effects were more complex. Lower concentration PMA (100 nM) replicated the major effects of EtOH, while higher concentration PMA (1 μM) did not, suggesting that the EtOH effects operate through activation of PKC and were mimicked by lower concentration of PMA.

CONCLUSIONS

EtOH primarily affects the CaV3.2 isoform of T-type Ca²⁺ channels acting through PKC, highlighting a novel target and mechanism for EtOH effects on excitable membranes.

Surgical incision induces phosphorylation of AMPA receptor GluR1 subunits at Serine-831 sites and GluR1 trafficking in spinal cord dorsal horn via a protein kinase Cγ-dependent mechanism

Spinal α-amino-3-hydroxy-5-methy-4-isoxazole propionate (AMPA) receptor plays an important role in acute pain induced by surgical tissue injuries. Our previous study has shown that the enhanced phosphorylation of AMPA receptor GluR1 subunits at Serine-831 sites by protein kinase C (PKC) in the spinal cord dorsal horn is involved in post-surgical pain hypersensitivity. However, which isoforms of PKC are responsible for the phosphorylation of AMPA receptor GluR1 subunits at Serine-831 sites remains to be established. In the present study, using an animal model of postoperative pain, we found that surgical tissue injuries enhanced the membrane translocation level of PKCγ, but not PKCα, βI, and βII, and induced the trafficking of GluR1, but not GluR2 into neuronal plasma membrane. Intrathecal (i.t.) pretreatment of small interfering RNA targeting PKCγ to reduce the PKCγ expression in the spinal cord significantly attenuated the pain hypersensitivity and inhibited the phosphorylation of AMPA receptor GluR1 subunits at Serine-831 sites as well as GluR1 membrane trafficking. Our study indicates that the surgical incision-induced phosphorylation of AMPA receptor GluR1 subunits at Serine-831 sites and GluR1 trafficking are regulated by a PKCγ-dependent mechanism.

Signaling pathways mediating alcohol effects

Ethanol's effects on intracellular signaling pathways contribute to acute effects of ethanol as well as to neuroadaptive responses to repeated ethanol exposure. In this chapter we review recent discoveries that demonstrate how ethanol alters signaling pathways involving several receptor tyrosine kinases and intracellular tyrosine and serine-threonine kinases, with consequences for regulation of cell surface receptor function, gene expression, protein translation, neuronal excitability and animal behavior. We also describe recent work that demonstrates a key role for ethanol in regulating the function of scaffolding proteins that organize signaling complexes into functional units. Finally, we review recent exciting studies demonstrating ethanol modulation of DNA and histone modification and the expression of microRNAs, indicating epigenetic mechanisms by which ethanol regulates neuronal gene expression and addictive behaviors.

The biology of protein kinase C

This review gives a basic introduction to the biology of protein kinase C, one of the first calcium-dependent kinases to be discovered. We review the structure and function of protein kinase C, along with some of the substrates of individual isoforms. We then review strategies for inhibiting PKC in experimental systems and finally discuss the therapeutic potential of targeting PKC. Each aspect is covered in summary, with links to detailed resources where appropriate.

Zolpidem withdrawal delirium

The Z-category hypnotics are promoted for their relative safety. However, this view is challenged by the emerging clinical evidence in the form of zolpidem related intoxication delirium and seizures, and dependence and complicated withdrawal. We report the case of a zolpidem-naive alcohol-dependent inpatient that, while undergoing alcohol de-addiction, was prescribed zolpidem for insomnia and developed delirium during taper-off. He was successfully detoxified for alcohol, treated for delirium and put on disulfiram prophylaxis. The case highlights the need for being cautious while using zolpidem for insomnia in alcohol dependent subjects.

Ethanol Activation of Protein Kinase A Regulates GABA(A) Receptor Subunit Expression in the Cerebral Cortex and Contributes to Ethanol-Induced Hypnosis

Protein kinases are implicated in neuronal cell functions such as modulation of ion channel function, trafficking, and synaptic excitability. Both protein kinase C (PKC) and A (PKA) are involved in regulation of γ-aminobutyric acid type A (GABA_A) receptors through phosphorylation. However, the role of PKA in regulating GABA_A receptors (GABA_A-R) following acute ethanol exposure is not known. The present study investigated the role of PKA in the effects of ethanol on GABA_A-R α1 subunit expression in rat cerebral cortical P2 synaptosomal fractions. Additionally, GABA-related behaviors were examined. Rats were administered ethanol (2.0–3.5 g/kg) or saline and PKC, PKA, and GABA_A-R α1 subunit levels were measured by western blot analysis. Ethanol (3.5 g/kg) transiently increased GABA_A-R α1 subunit expression and PKA RIIβ subunit expression at similar time points whereas PKA RIIα was increased at later time points. In contrast, PKC isoform expression remained unchanged. Notably, lower ethanol doses (2.0 g/kg) had no effect on GABA_A-R α1 subunit levels, although PKA type II regulatory subunits RIIα and RIIβ were increased at 10 and 60 min when PKC isozymes are also known to be elevated. To determine if PKA activation was responsible for the ethanol-induced elevation of GABA_A-R α1 subunits, the PKA antagonist H89 was administered to rats prior to ethanol exposure. H89 administration prevented ethanol-induced increases in GABA_A-R α1 subunit expression. Moreover, increasing PKA activity intracerebroventricularly with Sp-cAMP prior to a hypnotic dose of ethanol increased ethanol-induced loss of righting reflex (LORR) duration. This effect appears to be mediated in part by GABA_A-R as increasing PKA activity also increased the duration of muscimol-induced LORR. Overall, these data suggest that PKA mediates ethanol-induced GABA_A-R expression and contributes to behavioral effects of ethanol involving GABA_A-R.

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.

Glycine receptor internalization by protein kinases activation

Although glycine-induced currents in the central nervous system have been proven to be modulated by protein kinases A (PKA) and C (PKC), the mechanism is not well understood. In order to better comprehend the mechanism involved in this phenomenon, we tested the PKA and PKC activation effect on the specific [(3) H]glycine and [(3) H]strychnine binding to postsynaptic glycine receptor (GlyR) in intact rat retina. The specific binding constituted about 20% of the total radioligand binding. Kinetic analysis of the specific binding exhibited a sigmoidal behavior with three glycine and two strychnine binding sites and affinities of 212 nM for [(3) H]glycine and 50 nM for [(3) H]strychnine. Specific radioligand binding was decreased (60-85%) by PKA and PKC activation, an effect that was blocked by specific kinases inhibitors, as well as by cytochalasin D. GlyR expressed in the plasma membrane decreased about 50% in response to kinases activation, which was consistent with an increase of the receptor in the microsomal fraction when PKA was activated. Moreover, immunoprecipitation studies indicated that these kinases lead to a time-dependent receptor phosphorylation. Our results suggest that in retina, GlyR is cross-regulated by G protein-coupled receptors, activating PKA and PKC.

Plasticity of GABAA receptors after ethanol pre-exposure in cultured hippocampal neurons

Alcohol use causes many physiological changes in brain with behavioral sequelae. We previously observed (J Neurosci 27:12367-12377, 2007) plastic changes in hippocampal slice recordings paralleling behavioral changes in rats treated with a single intoxicating dose of ethanol (EtOH). Here, we were able to reproduce in primary cultured hippocampal neurons many of the effects of in vivo EtOH exposure on GABA(A) receptors (GABA(A)Rs). Cells grown 11 to 15 days in vitro demonstrated GABA(A)R δ subunit expression and sensitivity to enhancement by short-term exposure to EtOH (60 mM) of GABA(A)R-mediated tonic current (I(tonic)) using whole-cell patch-clamp techniques. EtOH gave virtually no enhancement of mIPSCs. Cells pre-exposed to EtOH (60 mM) for 30 min showed, 1 h after EtOH withdrawal, a 50% decrease in basal I(tonic) magnitude and tolerance to short-term EtOH enhancement of I(tonic), followed by reduced basal mIPSC area at 4 h. At 24 h, we saw considerable recovery in mIPSC area and significant potentiation by short-term EtOH; in addition, GABA(A)R currents exhibited reduced enhancement by benzodiazepines. These changes paralleled significant decreases in cell-surface expression of normally extrasynaptic δ and α4 GABA(A)R subunits as early as 20 min after EtOH exposure and reduced α5-containing GABA(A)Rs at 1 h, followed by a larger reduction of normally synaptic α1 subunit at 4 h, and then by increases in α4γ2-containing cell-surface receptors by 24 h. Measuring internalization of biotinylated GABA(A)Rs, we showed for the first time that the EtOH-induced loss of I(tonic) and cell-surface δ/α4 20 min after withdrawal results from increased receptor endocytosis rather than decreased exocytosis.

PKCγ is required for ethanol-induced increases in GABA(A) receptor α4 subunit expression in cultured cerebral cortical neurons

Ethanol exposure produces alterations in GABA(A) receptor function and expression associated with CNS hyperexcitability, but the mechanisms of these effects are unknown. Ethanol is known to increase both GABA(A) receptor α4 subunits and protein kinase C (PKC) isozymes in vivo and in vitro. Here, we investigated ethanol regulation of GABA(A) receptor α4 subunit expression in cultured cortical neurons to delineate the role of PKC. Cultured neurons were prepared from rat pups on postnatal day 0-1 and tested after 18 days. GABA(A) receptor α4 subunit surface expression was assessed using P2 fractionation and surface biotinylation following ethanol exposure for 4 h. Miniature inhibitory post-synaptic currents were measured using whole cell patch clamp recordings. Ethanol increased GABA(A) receptor α4 subunit expression in both the P2 and biotinylated fractions, while reducing the decay time constant in miniature inhibitory post-synaptic currents, with no effect on γ2 or δ subunits. PKC activation mimicked ethanol effects, while the PKC inhibitor calphostin C prevented ethanol-induced increases in GABA(A) receptor α4 subunit expression. PKCγ siRNA knockdown prevented ethanol-induced increases in GABA(A) receptor α4 subunit expression, but inhibition of the PKCβ isoform with PKCβ pseudosubstrate had no effect. We conclude that PKCγ regulates ethanol-induced alterations in α4-containing GABA(A) receptors.