Low dose acute alcohol effects on GABAA receptor subtypes

Abstract

GABA A receptors (GABA A Rs) are the main inhibitory neurotransmitter receptors and have long been implicated in mediating at least part of the acute actions of ethanol. For example, ethanol and GABAergic drugs including barbiturates and benzodiazepines share many pharmacological properties. Besides the prototypical synaptic GABA A R subtypes, nonsynaptic GABA A Rs have recently emerged as important regulators of neuronal excitability. While high doses (≥100 mM) of ethanol have been reported to enhance activity of most GABA A R subtypes, most abundant synaptic GABA A Rs are essentially insensitive to ethanol concentrations that occur during social ethanol consumption (<30 mM). However, extrasynaptic δ and β3 subunit-containing GABA A Rs, associated in the brain with α4or α6 subunits, are sensitive to low millimolar ethanol concentrations, as produced by drinking half a glass of wine. Additionally, we found that a mutation in the cerebellar α6 subunit (α6R100Q), initially reported in rats selectively bred for increased alcohol sensitivity, is sufficient to produce increased alcohol-induced motor impairment and further increases of alcohol sensitivity in recombinant α6β3δ receptors. Furthermore, the behavioral alcohol antagonist Ro15-4513 blocks the low dose alcohol enhancement on α4/6/β3δ receptors, without reducing GABA-induced currents. In binding assays α4β3δ GABA A Rs bind [ 3 H] Ro15-4513 with high affinity, and this binding is inhibited, in an apparently competitive fashion, by low ethanol concentrations, as well as analogs of Ro15-4513 that are active to antagonize ethanol or Ro15-4513's block of ethanol. We conclude that most low to moderate dose alcohol effects are mediated by alcohol actions on alcohol/Ro15-4513 binding sites on GABA A R subtypes.

Studies of ethanol actions on recombinant δ-containing γ-aminobutyric acid type A receptors yield contradictory results

“…We were not able to do that comparison, because the authors did not publish their maximal currents. However, the EC 20 GABA tracings shown in their papers (Wallner et al, 2003;Wallner et al, 2006) were approximately of the same magnitude of ours (Figure 1 and 2C), and the EC 20 GABA responses in our experiments involving ethanol modulation ranged from 300 to 1800 nA (mean ± SEM= 1060 ± 100 nA), and there was no correlation between the magnitude of the GABA response and the ethanol effect. Therefore, the differences observed in ethanol sensitivity were not related to the magnitude of the current itself, and we are not sure if the maximal currents could be so divergent.…”

“…Therefore, even though there was a shift to the right for α 4 β 3 δ compared with Wallner et al (2003), we feel that the overall results were consistent with the literature. Point c: the desensitization rate for α 4 β 3 δ was higher in our preparation than in Wallner et al (2003Wallner et al ( ,2006, but α 4 β 3 δ always presented less desensitization than α 4 β 3 γ 2L (Figure 1). The δ-knockout mice could provide interesting insight into the situation.…”

“…Point a: in Wallner et al (2006), the authors made a reference to our maximal GABA responses being five times higher than the currents they obtained. We were not able to do that comparison, because the authors did not publish their maximal currents.…”

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

“…Recent experiments in various expression systems including oocytes, the stable L(tk -) cell linescell line expressing δ subunit-containing GABA A Rs (Borghese et al, 2005) and CHO cells (Yamashita et al, 2006) ethanol failed to enhance currents evoked by GABA, but the reason for the discrepancies between the results in various expression systems remains unknown. The reader should consult recent review articles summarizing the findings in expression systems (Hanchar et al, 2004;Wallner et al, 2006), in genetically altered mice (Boehm et al, 2006), and in brain slice preparations (Weiner and Valenzuela, 2006) from which it is clear that certain GABA A R combinations, including those containing δ subunits, may be responsible for specific actions of ethanol on the brain. In addition to the role of δ subunits, point mutations in the α1 subunit have also been shown to affect ethanol sensitivity, but only at higher (80 mM) ethanol concentrations .…”

GABAA receptor subtypes: the “one glass of wine” receptors

“…This finding generated considerable excitement when subsequently verified in large numbers of studies and in many independent laboratories (Kolata, 1986;Suzdak et al, 1986b;Lister and Nutt, 1987;Syapin et al, 1987;Hellevuo and Korpi, 1988;Lister and Nutt, 1988;Ticku and Kulkarni, 1988;Dar, 1992Dar, , 1995, but see (Hellevuo and Korpi, 1988) Given the surprising effectiveness of Ro15-4513 (and the complete ineffectiveness of the close structural analog flumazenil) in reversing pharmacologically relevant EtOH effects, it may not be surprising that there were skeptics that argued it must be the weak GABA A R blocking activity (partial inverse agonism) of Ro15-4513 that must be responsible for the alcohol antagonism (Lister and Nutt, 1987;Britton et al, 1988). However, almost all other negative modulators of GABA receptors (a.k.a., inverse agonists) do not act as alcohol antagonists (for recent review see (Wallner et al, 2006a)). Two exceptions of inverse agonists that do show alcohol antagonist actions are the Ro15-4513 structural analogs, RY080 and RY024, that also have been reported to show behavioral alcohol antagonism (McKay et al, 2004;Cook et al, 2005); these compounds also bind with high affinity to the [ 3 H] Ro15-4513/alcohol site on α4β3δ receptors .…”