A new naturally occurring GABA(A) receptor subunit partnership with high sensitivity to ethanol

The main source of ambient GABA responsible for tonic inhibition in the mouse hippocampus

The extracellular space of the brain contains gamma-aminobutyric acid (GABA) that activates extrasynaptic GABA(A) receptors mediating tonic inhibition. The source of this GABA is uncertain: it could be overspill of vesicular release, non-vesicular leakage, reverse transport, dying cells or glia. Using a novel approach, we simultaneously measured phasic and tonic inhibitory currents and assessed their correlation. Enhancing or diminishing vesicular GABA release in hippocampal neurons caused highly correlated changes in the two inhibitions. During high-frequency phasic inhibitory bursts, tonic current was also enhanced as shown by simulating the summation of IPSCs and by recordings in knockout mice devoid of tonic inhibitory current. When vesicular release was reduced by blocking action potentials or the vesicular GABA transporter, phasic and tonic currents decreased in a correlated fashion. Our results are consistent with most of hippocampal tonic inhibitory current being mediated by GABA released from the very vesicles responsible for activating phasic inhibition.

Tonic for what ails us? high-affinity GABAA receptors and alcohol

Ethanol interactions with gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter in the brain, play key roles in acute intoxication. However, the exact mechanisms of these ethanol interactions have been the subject of considerable confusion and controversy. Many studies suggest that ethanol potentiates the function of the type A GABA receptor (GABAA-R). However, these findings have not been consistently replicated in experiments that directly examined the effects of ethanol on GABAA-R-mediated ion current. Differences in ethanol sensitivity of different GABAA-R subtypes have been invoked as a potential explanation for the inconsistent findings, and recent work suggests that GABAA-Rs that contain the delta subunit and/or mediate tonic extrasynaptic GABA responses may be especially ethanol sensitive. However, considerable disagreement has arisen over these findings. This special issue of Alcohol contains articles from eight research groups that are examining this issue. The authors present their work, their views on the present state of this area of alcohol research, and their ideas about how to proceed with future studies that may help to address the present confusion and controversy. This editorial provides an introduction to this line of research and the current findings and controversies.

Ethanol acts directly on extrasynaptic subtypes of GABAA receptors to increase tonic inhibition

Based on the similarity of ethanol intoxication to the behavioral effects of drugs known to target gamma-aminobutyric acid type A (GABAA) receptors (GABARs), it has been suspected for decades that ethanol facilitates the activity of GABA. Even so, it has been surprisingly difficult to identify molecular targets of ethanol. Research conducted over the past several years suggests that a subclass of GABARs (those containing delta subunits) responds in a relevant concentration range to ethanol. Although delta subunit-containing GABARs are not ubiquitously expressed at inhibitory synapses like their gamma subunit-containing, synaptic counterparts, they are found in many neurons in extrasynaptic locations. Here, they give rise to a tonic form of inhibition that can potently suppress neuronal excitability. Studies have shown that both recombinant and native delta subunit-containing GABARs (1) are modulated by behaviorally relevant (i.e., low millimolar) concentrations of ethanol, (2) directly bind ethanol over the same concentration range, (3) show altered function upon single amino substitutions linked to changes in behavioral responsiveness to ethanol, and (4) are a site of action of Ro15-4513, a competitive antagonist of ethanol binding and a drug which prevents many of the behavioral aspects of ethanol intoxication. Despite such comprehensive evidence, however, the field is not free from controversy. This review evaluates published data for and against a central role of delta subunit-containing GABARs in ethanol actions and suggests future directions that might help settle points of controversy.

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

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

A new meaning for "Gin & Tonic": tonic inhibition as the target for ethanol action in the brain

Gamma-aminobutyric acid (GABA) is the main chemical inhibitory neurotransmitter in the brain. In the central nervous system, it acts on two distinct types of receptor: an ion channel, that is, an "ionotropic" receptor permeable to Cl- and HCO3- (GABAA receptors [GABAARs]) and a G-protein coupled "metabotropic" receptor that is linked to various effector mechanisms (GABAB receptors). This review will summarize novel developments in the physiology and pharmacology of GABAARs, specifically those found outside synapses. The focus will be on a particular combination of GABAAR subunits responsible for mediating tonic inhibition and sensitive to concentrations of ethanol legally considered to be sobriety impairing. Since the same receptors are also a preferred target for the metabolites of steroid hormones synthesized in the brain (neurosteroids), the ethanol-sensitive tonic inhibition may be a common pathway for interactions between the effects of alcohol and those of ovarian and stress-related neurosteroids.

Ethanol effects on GABA-gated current in a model of increased alpha4betadelta GABAA receptor expression depend on time course and preexposure to low concentrations of the drug

Several recent studies have suggested that alphabetadelta subtypes of gamma-aminobutyric acid type A (GABAA) receptors (delta-GABAR) are a target for low dose ethanol (<30 mM). However, there are also conflicting reports suggesting that only high doses of the drug (100 mM) modulate these receptors. In addition, the studies which have demonstrated a clear effect of low dose ethanol on delta-GABAR find different effective concentrations for this effect. Here, we test the hypothesis that the apparent disparity in effective concentration is due to time-course effects when low (1-3 mM) dose ethanol is preapplied. To this end, we tested ethanol effects on native GABAR in CA1 hippocampus in a model of increased alpha4betadelta GABAR expression following 48h administration of the GABA-modulatory steroid THP (3alpha-OH-5beta-pregnan-20-one) to adult, female rats. GABA(EC20)-gated current was recorded with whole-cell patch clamp procedures from acutely isolated pyramidal cells. We assessed ethanol's effect on GABA-gated current using either (1) 2-5 min application of ethanol in increasing concentrations (0.1-30 mM) or (2) coadministration of ethanol with GABA. Two minute application of 1-3 mM ethanol produced optimal potentiation of GABA-gated current following steroid treatment, with higher concentrations less effective. In contrast, 30 mM ethanol produced optimal effects when ethanol was not preapplied. However, following preapplication of 1mM ethanol, 30 mM ethanol decreased the peak GABA-gated current. These findings suggest that ethanol may act at multiple interacting sites to affect GABAR efficacy and desensitization. These data also suggest that ethanol effects on GABA-gated current are affected by the time course of exposure and previous exposure to low concentrations of the drug.

Neurosteroid modulation of synaptic and extrasynaptic GABA(A) receptors

Certain naturally occurring pregnane steroids act in a nongenomic manner to potently and selectively enhance the interaction of the inhibitory neurotransmitter GABA with the GABA(A) receptor. Consequently such steroids exhibit anxiolytic, anticonvulsant, analgesic, sedative, hypnotic, and anesthetic properties. In both physiological and pathophysiological scenarios, the pregnane steroids may function as endocrine messengers (e.g., produced in the periphery and cross the blood-brain barrier) to influence behaviour. However, additionally "neurosteroids" can be synthesised in the brain and spinal cord to act in a paracrine or autocrine manner and thereby locally influence neuronal activity. Given the ubiquitous expression of the GABA(A) receptor throughout the mammalian central nervous system (CNS), physiological, pathophysiological, or drug-induced pertubations of neurosteroid levels may be expected to produce widespread changes in brain excitability. However, the neurosteroid/GABA(A) receptor interaction is brain region and indeed neuron specific. The molecular basis of this specificity will be reviewed here, including (1) the importance of the subunit composition of the GABA(A) receptor; (2) how protein phosphorylation may dynamically influence the sensitivity of GABA(A) receptors to neurosteroids; (3) the impact of local steroid metabolism; and (4) the emergence of extrasynaptic GABA(A) receptors as a neurosteroid target.

Mechanisms of neurosteroid interactions with GABA(A) receptors

Neuroactive steroids have some of their most potent actions by augmenting the function of GABA(A) receptors. Endogenous steroid actions on GABA(A) receptors may underlie important effects on mood and behavior. Exogenous neuroactive steroids have potential as anesthetics, anticonvulsants, and neuroprotectants. We have taken multiple approaches to understand more completely the interaction of neuroactive steroids with GABA(A) receptors. We have developed many novel steroid analogues in this effort. Recent work has resulted in synthesis of new enantiomer analogue pairs, novel ligands that probe various properties of the steroid pharmacophore, fluorescent neuroactive steroid analogues, and photoaffinity labels. Using these tools, combined with receptor binding and electrophysiological assays, we have begun to untangle the complexity of steroid actions at this important class of ligand-gated ion channel.

Presynaptic inhibition of excitatory afferents to hilar mossy cells

The hippocampus contains one very strong recurrent excitatory network formed by associational connections between CA3 pyramidal cells and another that depends largely on a disynaptic excitatory pathway between dentate granule cells. The recurrent excitatory network in CA3 has long been considered a possible location of autoassociative memory storage, whereas changes in the level and arrangement of recurrent excitation between granule cells are strongly implicated in epileptogenesis. Hilar mossy cells are likely to receive collateral input from CA3 pyramidal cells and they are key intermediaries (by mossy fiber inputs) in the recurrent excitatory network between granule cells. The current study uses minimal stimulation techniques in an in vitro preparation of the rat dentate gyrus to examine presynaptic modulation of both mossy fiber and non-mossy fiber inputs to hilar mossy cells. We report that both mossy fiber and non-mossy fiber inputs to hilar mossy cells express presynaptic gamma-aminobutyric acid type B (GABA(B)) receptors that are subject to tonic inhibition by ambient GABA. We further find that only non-mossy fiber inputs express presynaptic muscarinic acetylcholine receptors, but that bath application of cholinergic agonists produces action potential-dependent increases in ambient GABA that can indirectly inhibit mossy fiber inputs. Finally, we demonstrate that mossy cells express high-affinity postsynaptic GABA(A) receptors that are also capable of detecting changes in ambient GABA produced by cholinergic agonists. Our results are among the first to directly characterize these important collateral inputs to hilar mossy cells and may help facilitate informed comparison between primary and collateral projections in two major excitatory pathways.

Interneurons of the dentate gyrus: an overview of cell types, terminal fields and neurochemical identity

Interneurons of the dentate gyrus are a diverse group of neurons that use GABA as their primary neurotransmitter. Morphological studies of these neurons have been challenging since no single neuroanatomical method provides a complete view of these interneurons. However, through the integration of findings obtained from multiple methods, an interesting picture of this complex group of neurons is emerging, and this review focuses on studies in rats and mice. In situ hybridization of mRNAs for the two isoforms of the GABA synthesizing enzyme, glutamate decarboxylase (GAD65 and GAD67), demonstrates the abundance of GABA neurons in the dentate gyrus and their high concentration in the hilus and along the base of the granule cell layer. Likewise, immunohistochemical studies, particularly of GAD65, demonstrate the rich fields of GABA terminals not only around the somata of granule cells but also in the dendritic regions of the molecular layer. This broad group of GABA neurons and their terminals can be subdivided according to their morphological characteristics, including the distribution of their axonal plexus, and their neurochemical identity. Intracellular labeling of single interneurons has been instrumental in demonstrating the extensiveness of their axonal plexus and the relatively specific spatial distribution of their axonal fields. These findings have led to the broad classification of interneurons into those that terminate primarily at perisomatic regions and those that innervate the dendrites of granule cells. The interneurons also can be classified according to their neuropeptide and calcium-binding protein content. These and other molecules contribute to the rich diversity of dentate interneurons and may provide opportunities for selectively regulating specific groups of GABA neurons in the dentate gyrus in order to enhance their function or protect vulnerable neurons from damage.