Elsevier

Alcohol

Volume 41, Issue 3, May 2007, Pages 211-221
Alcohol

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

https://doi.org/10.1016/j.alcohol.2007.04.011Get rights and content

Abstract

Based on the similarity of ethanol intoxication to the behavioral effects of drugs known to target γ-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 δ subunits) responds in a relevant concentration range to ethanol. Although δ subunit-containing GABARs are not ubiquitously expressed at inhibitory synapses like their γ 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 δ 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 δ subunit-containing GABARs in ethanol actions and suggests future directions that might help settle points of controversy.

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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