ArticleEthanol acts directly on extrasynaptic subtypes of GABAA receptors to increase tonic inhibition
Introduction
Determining the precise molecular target of a psychoactive drug is notoriously difficult and for few drugs has this been more challenging than for ethanol. Considering that ethanol is the most widely used drug in society and that ethanol abuse is by far the most common form of substance abuse, understanding where and how this compound acts in the brain is one of the key challenges of neuroscience.
It was first recognized decades ago that ethanol shares a pharmacological profile with drugs known to act on γ-aminobutyric acid type A (GABAA) receptors (GABARs), however, experimental evidence that ethanol exerts effects on specific molecules involved in GABAergic signaling has been limited and controversial. Indeed, since the first published account suggesting that a specific GABAR subunit (γ2L) is required for ethanol sensitivity (Wafford et al., 1991), findings in this field have been difficult for other labs to replicate and there is currently no consensus regarding specific molecules at which ethanol might act in exerting its effects on behavior.
There are several reasons why progress in identifying targets has been slow and at times contentious. First, the actions of ethanol in the brain occur at unusually high concentrations. Behavioral signs of intoxication appear only when millimolar levels of ethanol are present in blood, implying a weak receptor–drug interaction. Such low affinity renders traditional biochemical approaches, so useful in defining molecular sites of action, impractical. Second, most candidate targets for ethanol are ion channels which are complex molecules. Native GABARs (whose role in ethanol effects is the subject of this review) exist as heteropentamers composed of combinations selected from 19 homologous subunits. While stoichiometry and general rules for biosynthesis of the most common isoforms of native GABARs are becoming clearer (2 α subunits, 2 β subunits, and either a γ or δ subunit are thought to be required in the pentamer), many uncertainties remain. Third, even if the precise molecular makeup of a native GABAR was established, reconstitution of native GABARs with the desired subunit composition and spatial arrangement of subunits is problematic. Standard recombinant expression methods may not assure that multiple subunits will assemble with one another in their native spatial confirmations. Of course, issues such as accessory subunits, posttranslational modifications, and regulation by second messengers complicate matters further.
This review begins by summarizing positive evidence that δ subunit-containing GABARs are key targets for low (<30 mM) concentrations of ethanol. Much of these data have been obtained from complimentary studies of recombinant and native GABARs. Published objections to these findings are then discussed and, where possible, attempts are made to reconcile the negative findings with the supportive evidence. The review concludes with several suggestions for future experiments that might help clarify outstanding issues.
Section snippets
In recombinant studies, low concentrations of ethanol enhance the function of extrasynaptic GABAR subtypes
A number of studies have shown that recombinant GABARs composed of specific combinations of subunits are special in that they respond to ethanol within a dosage range corresponding to blood ethanol concentrations achieved during moderate social consumption (Hanchar et al., 2005, Sundstrom-Poromaa et al., 2002, Wallner et al., 2003). The most common isoforms of GABARs contain γ subunits, and these receptors respond to ethanol at concentrations greater than 100 mM if at all (Wallner et al., 2006b
Objections to the hypothesis that δ subunit-containing GABARs contribute to low-dose ethanol actions in brain
Despite the comprehensive, cross-confirmatory evidence reviewed above, four papers have reported unsuccessful attempts to extend these results (Borghese et al., 2006, Casagrande et al., 2007, Valenzuela et al., 2005, Yamashita et al., 2006). Upon simple inspection, these failures seem impossible to reconcile with the extensive evidence implicating δ subunit-containing GABARs in ethanol actions. We remain puzzled by these reports as we have repeated all of the key experiments (many are
Suggestions for future studies
While a review is rarely able to settle all outstanding issues, especially with regard to a complicated and controversial topic of this sort, such an effort does hold the possibility of clarifying key issues and suggesting critical future experiments. On the bright side, there is detailed, substantial, and accumulating evidence that extrasynaptic GABARs are responsive to low doses of ethanol. In addition, the negative findings reviewed here, although confusing, have come from experiments that
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