GABA and the GABA A Receptor

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<ul><li><p>VOL. 21, NO. 2, 1997 127</p><p>campus. Journal of Neurochemistry 62:16351638, 1994.</p><p>TRUJILLO, K.A., AND AKIL, H. Excitatory amino acids and drugs of abuse: Arole for N-methyl-D-aspartate receptors in drug tolerance, sensitization andphysical dependence. Drug and Alcohol Dependence 38:139154, 1995.</p><p>TSAI, G.; GASTFRIEND, D.R.; AND COYLE, J.T. The glutamatergic basis ofhuman alcoholism. American Journal of Psychiatry 152:332340, 1995.</p><p>WOODWARD, J.J., AND GONZALES, R.A. Ethanol inhibition of N-methyl-D-aspartate-stimulated endogenous dopamine release from rat striatal slices:Reversal by glycine. Journal of Neurochemistry 54:712715, 1990.</p><p>GABA AND THE GABAA RECEPTOR</p><p>S. John Mihic, Ph.D., and R. Adron Harris, Ph.D.</p><p>The neurotransmitter gamma-aminobutyric acid(GABA) inhibits the activity of signal-receiving neuronsby interacting with the GABAA receptor on these cells.The GABAA receptor is a channel-forming protein thatallows the passage of chloride ions into the cells.Excessive GABAA activation may play a role in mediat-ing the sedative effects of alcohol and other sedatingand anesthetic agents. For example, alcohol enhancesthe GABAA-mediated chloride flow into cells and maythereby enhance neuronal inhibition. Alcohols effectson the GABAA-receptor function likely involve othermolecules (e.g., other neurotransmitters and proteinsthat add phosphate groups to the receptor [i.e., pro-tein kinases]). Several experimental approaches alsohave suggested that changes in GABAA-receptor func-tion contribute to the tolerance to and dependence onalcohol. Finally, individual differences in the GABAsystem may play a role in determining a persons sus-ceptibility to developing alcohol dependence. KEYWORDS: GABA; GABA receptors; neurotransmission;brain; sedative hypnotics; receptor proteins; chloridechannel; ion; protein kinases; AOD dependence; AODtolerance; AOD intoxication; AOD use susceptibility;animal model; literature review</p><p>Nerve cells, or neurons, in the brain communicatethrough chemical messengers called neurotransmit-ters. These molecules are released by the signal-emitting neuron and bind to specific proteins (i.e.,receptors) on the signal-receiving neuron. (For more infor-mation on signal transmission within and among nervecells, see the article The Principles of Nerve CellCommunication, pp. 107-108.) Two main types of neuro-transmitters and neurotransmitter receptorsexcitatoryand inhibitorydetermine the response of the signal-receiving neuron. Excitatory neurotransmitters and theirreceptors increase the neurons intrinsic electrical activityand excitability, whereas inhibitory neurotransmitters and</p><p>their receptors reduce neuronal excitability. For optimalfunctioning, the brain must balance the excitatory andinhibitory influences: Excessive excitation can lead toseizures, whereas excessive neuronal inhibition can resultin incoordination, sedation, and anesthesia.</p><p>Gamma-aminobutyric acid (GABA) is the primary in-hibitory neurotransmitter in the central nervous system.Because alcohol intoxication is accompanied by the incoor-dination and sedation indicative of neuronal inhibition, re-searchers have investigated alcohols effects on GABA andits receptors. This article summarizes findings that alcoholsignificantly alters GABA-mediated neurotransmission andpresents some evidence that the primary GABA receptor(called the GABAA receptor) may play a crucial role in thedevelopment of tolerance to and dependence on alcohol aswell as contribute to the predisposition to alcoholism.</p><p>THE GABAA RECEPTOR</p><p>GABAA receptors are large proteins1 embedded in the cell</p><p>membranes of neurons (see figure). Each receptor consistsof five protein molecules, or subunits, that assemble so thata channel is formed at the center of the complex. WhenGABA molecules or GABA-like compounds bind to thereceptor and activate it, this channel temporarily opens andallows the passage of negatively charged molecules (i.e.,ions), such as chloride ions (Cl-), to pass from the cellsexterior to its interior. This ion flow decreases the cellsexcitability. The cumulative neuronal inhibition caused byGABAs binding to many neurons results in sedation andintoxication (Whiting et al. 1995). In laboratory animals,these effects manifest themselves as loss of the rightingreflexthat is, the animals can not get up when placed ontheir backs. Compounds that enhance the GABAA recep-tors activity cause increased neuronal inhibition. In con-trast, compounds that reduce GABAA receptor activityresult in the excitation of the signal-receiving neurons.</p><p>The subunits that constitute the GABAA receptor eachconsist of a large extracellular region located on the out-side of the cell membrane, four segments spanning the cellmembrane, and several intracellular regions that are ex-posed to the neurons interior. Whereas the extracellularprotein region is responsible for GABA binding, the intra-</p><p>NEUROTRANSMITTER REVIEW</p><p>S. JOHN MIHIC, PH.D., is an assistant professor in theDepartment of Physiology and Pharmacology, BowmanGrey School of Medicine, Winston-Salem, North Carolina.</p><p>R. ADRON HARRIS, PH.D., is a professor in the Departmentof Pharmacology and director of the Alcohol ResearchCenter, University of Colorado Health Sciences Center,and a research career scientist at the Denver VeteransAdministration Medical Center, Denver, Colorado.</p><p>1For a definition of this and other technical terms used in this article, seecentral glossary, pp. 177179.</p></li><li><p>128 ALCOHOL HEALTH &amp; RESEARCH WORLD</p><p>cellular regions can be modified by the addition of phos-phate groups (i.e., can become phosphorylated). As de-scribed later in this article, this phosphorylation, which isperformed by enzymes such as protein kinase C (PKC) andoccurs at specific sites of the GABA receptor subunits,regulates the receptors functioning.</p><p>Many different GABAA receptor subunits have beenidentified. These fall into three groups: , , and sub-units. Each of these groups contains several different sub-units (e.g., 1 and 2). The exact subunit composition ofmost GABAA receptors is not known. Most likely, eachreceptor consists of two subunits, one subunit, and two subunits (see figure). Each subunit type only interactswith specific molecules. Thus, the and subunits can interact with GABA,whereas the and subunits containthe binding site for benzodiazepines(see below). Different subunits withineach of the three groups also differ intheir pharmacological properties (e.g.,the sensitivity to alcohol). Consequently,the specific subunit composition ofeach GABAA receptor molecule determines that receptorsoverall characteristics. GABAA receptors in differentneurons or brain regions or at various developmentalstages therefore can differ in their pharmacological proper-ties (McKernan and Whiting 1996).</p><p>GABAA receptors are found throughout the brain. Thiswide distribution likely is responsible for the plethora ofbehaviors (e.g., sedation, relief of anxiety, and motor in-coordination) produced by agents that activate these receptors, such as alcohol.</p><p>THE GABAA RECEPTORS ROLE IN ALCOHOLINTOXICATION</p><p>Numerous clinically useful sedating medications (e.g.,benzodiazepines, such as Valium, and barbiturates, suchas phenobarbital) and anesthetic agents (e.g., halothane)exert their effects at least in part by enhancing GABAsinfluence on GABAA receptors. Thus, these agents tilt thebalance of excitatory and inhibitory influences in the braintoward inhibition, thereby causing the incoordination,sedation, and even anesthesia that accompany their use.Because alcohol produces similar effects, it also likelypromotes neuronal inhibition through the GABAA receptor(Tabakoff and Hoffman 1996).</p><p>Using several different approaches, researchers haveattempted to determine which of alcohols behavioral ef-fects are mediated by changes in GABAA receptor func-tion. One strategy has been to administer alcohol togetherwith other compounds that interact with the GABAA recep-tor and then determine whether alcohol enhances or im-pedes the effects of these compounds. For example,injections of GABA or GABA-like compounds into thebrains of rats increased alcohols incoordinating and hyp-notic effects (Deitrich et al. 1989). Similarly, rats that were</p><p>treated with a compound that inhibits GABA degradationexhibited increased alcohol-induced incoordination(Deitrich et al. 1989). Finally, a compound called Ro 15-4513, which inhibits GABAA receptor function, has beenshown to prevent some of alcohols behavioral effects. Forexample, Ro 15-4513 reduced the severity of alcoholshypnotic effects and decreased alcohol consumption inanimals (Mihic and Harris 1996). Such studies, however,provide only indirect evidence of alcohols actions andtherefore must be interpreted with caution.</p><p>More direct evidence of alcohols interaction with theGABAA receptor derives from neurochemical analyses andfrom studies in mouse and rat strains bred to differ in their</p><p>sensitivities to some of alcohols be-havioral effects. Neurochemical stud-ies have analyzed alcohols effects onGABA-mediated Cl- uptake into brainmicrosacsmembranes isolatedfrom brain cells that form sealedbagsand spinal-cord neurons grownin tissue culture. Many of these studiesfound that alcohol increased Cl- up-</p><p>take, suggesting that alcohol could enhance GABA-medi-ated inhibition of neurons (Mihic and Harris 1996). </p><p>Researchers also have investigated alcohols effects onGABAA receptor function in mouse and rat strains specifi-cally bred to differ in their susceptibilities to alcohol-induced incoordination or loss of righting reflex. Forexample, so-called long-sleep (LS) mice exhibit a longerduration of the loss of righting reflex after an acute alcoholinjection than do short-sleep (SS) mice. Studies in thesemice found that alcohol enhanced GABA-mediated Cl-</p><p>uptake into brain microsacs obtained from LS mice but notinto microsacs obtained from SS mice (Mihic and Harris1996). These findings suggest that a biochemical differ-ence in alcohols effects on the GABAA receptor mayunderlie the behavioral differences observed between thetwo strains.</p><p>Alcohols effects on GABAA receptor function likelyinvolve the actions of other cellular proteins, such as thePKC enzymes that phosphorylate the GABAA receptor atspecific sites. In one experiment, for example, mice lack-ing a certain PKC subtype in the brain displayed reducedsensitivity to alcohol on several behavioral tests. More-over, alcohol no longer enhanced the GABA-induced flowof Cl- into brain microsacs prepared from these PKCknock-out mice (Mihic and Harris 1996). This observa-tion further strengthens the hypothesis that alcohol-inducedenhancement of GABAA receptor activity not only in-volves proteins other than the receptor proteins but alsorequires protein phosphorylation.</p><p>Other studies have used electrophysiological techniques toassess alcohols effects on GABAA receptor function. Thesestudies have employed different experimental systems: (1)neurons that are still in an intact brain, (2) neurons in thinslices of isolated brain tissue, (3) isolated brain cells that havebeen grown in tissue culture, and (4) nonneuronal cells that</p><p>NEUROTRANSMITTER REVIEW</p><p>Some alcoholics mayexhibit abnormal</p><p>GABA metabolism</p></li><li><p>VOL. 21, NO. 2, 1997 129</p><p>normally do not produce GABAA receptors but which can beinduced artificially to manufacture receptors composed ofspecific subunits. Like the experiments described previously,these electrophysiological analyses indicate that the mecha-nisms underlying alcohol-induced enhancement of GABA-mediated signal transmission are complex and may involveneurotransmitter receptors other than the GABAA receptor.For example, one study found that alcohol enhanced theactivity of the GABAA receptor on certain cells in the cere-bellum of rats only in the presence of the neurotransmitternorepinephrine, which acts through another receptor, the -adrenergic receptor (Freund and Palmer 1997). These find-ings suggest that alcohol-dependent enhancement of GABAactivity in the cerebellum requires the activation of the -adrenergic receptor. This receptor is located on the same cellsin the cerebellum as the GABAA receptor. Both receptorsalso interact in the absence of alcohol, but this interactionmay be enhanced in the presence of alcohol.</p><p>At least three plausible mechanisms could explain theinteractions among the -adrenergic receptor, the GABAAreceptor, and alcohol, as follows:</p><p> Norepinephrine could increase the GABAA receptorssensitivity to alcohol.</p><p> Alcohol could interact with the -adrenergic receptor,thereby increasing that receptors ability to modulateGABAA receptor function.</p><p> Alcohol may further increase -adrenergic enhancementof GABAA receptor function by inhibiting the removalof norepinephrine from the synapses.</p><p>-adrenergic signal transmission results in increased pro-tein phosphorylation. Thus, whatever the exact mechanismmay be, the association between the activities of theGABAA and -adrenergic receptors supports the conclu-sions from the C1--flow analyses described above thatalcohols effect on the GABAA receptor may require acti-vation of phosphorylating proteins, such as PKC (for adetailed discussion, see Weiner et al. 1997).</p><p>The link between protein phosphorylation and the sensi-tivity to alcohol of the GABAA receptor also has been con-firmed in studies analyzing alcohols effects on GABAAreceptors with known subunit composition. For example,the 2 subunit of the GABAA receptor exists in two formsa short variant (2S) and a long variant (2L)which differin size by eight amino acids. Analyses in cultured cellsfound that receptors containing the 2L subunit showedalcohol-induced enhancement of their activity, whereasreceptors containing the 2S subunit generally were insensi-tive to intoxicating alcohol concentrations (Wafford andWhiting 1992; Whitten et al. 1996; Harris et al. 1997). Theadditional eight amino acids present in 2L contain a sitethat can be phosphorylated by PKC, indicating that phos-phorylation is a prerequisite for the GABAA receptorssensitivity to alcohol. However, these experiments only can</p><p>be performed in cultured cells or other artificial systems,not in intact brains. Therefore, one cannot conclude un-equivocally from these studies whether the GABAA recep-tors sensitivity to alcohol in an intact organism isdetermined by differences in receptor subunits, phosphory-lating enzymes, or other unknown factors (see Weiner et al.1997; Harris et al. 1997; Mihic and Harris 1996).</p><p>THE GABAA RECEPTOR AND ALCOHOL TOLERANCEAND DEPENDENCE</p><p>After continuous alcohol consumption, both humansand laboratory animals develop tolerance to alcoholseffectsthat is, they require larger amounts of alcoholto achieve the same effects. Moreover, continuousalcohol consumption leads to the development of de-</p><p>NEUROTRANSMITTER REVIEW</p><p>PP P</p><p>? ?</p><p>GABA</p><p>Schematic representation of the gamma-aminobutyric acid(GABAA) receptor. The functional receptor consists of fiveproteins, or subunitsmost likely two subunits, one subunit,and two subunits. (Question marks indi...</p></li></ul>