Low dose acute alcohol effects on GABAA receptor subtypes

GABAA receptors (GABAARs) 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 GABAAR subtypes, nonsynaptic GABAARs have recently emerged as important regulators of neuronal excitability. While high doses (≥ 100 mM) of ethanol have been reported to enhance activity of most GABAAR subtypes, most abundant synaptic GABAARs are essentially insensitive to ethanol concentrations that occur during social ethanol consumption (< 30 mM). However, extrasynaptic δ and β3 subunit-containing GABAARs, associated in the brain with α4 or 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 GABAARs bind [3H]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 GABAAR subtypes.