Article

Low-dose alcohol actions on α4β3δ GABAA receptors are reversed by the behavioral alcohol antagonist Ro15-4513

Department of Molecular and Medical Pharmacology, University of California, Los Angeles, CA 90095-1735, USA.
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 06/2006; 103(22):8540-5. DOI: 10.1073/pnas.0600194103
Source: PubMed

ABSTRACT

Although it is now more than two decades since it was first reported that the imidazobenzodiazepine Ro15-4513 reverses behavioral alcohol effects, the molecular target(s) of Ro15-4513 and the mechanism of alcohol antagonism remain elusive. Here, we show that Ro15-4513 blocks the alcohol enhancement on recombinant "extrasynaptic" alpha4/6beta3delta GABA(A) receptors at doses that do not reduce the GABA-induced Cl(-) current. At low ethanol concentrations (< or =30 mM), the Ro15-4513 antagonism is complete. However, at higher ethanol concentrations (> or =100 mM), there is a Ro15-4513-insensitive ethanol enhancement that is abolished in receptors containing a point mutation in the second transmembrane region of the beta3 subunit (beta3N265M). Therefore, alpha4/6beta3delta GABA receptors have two distinct alcohol modulation sites: (i) a low-dose ethanol site present in alpha4/6beta3delta receptors that is antagonized by the behavioral alcohol antagonist Ro15-4513 and (ii) a site activated at high (anesthetic) alcohol doses, defined by mutations in membrane-spanning regions. Receptors composed of alpha4beta3N265Mdelta subunits that lack the high-dose alcohol site show a saturable ethanol dose-response curve with a half-maximal enhancement at 16 mM, close to the legal blood alcohol driving limit in most U.S. states (17.4 mM). Like in behavioral experiments, the alcohol antagonist effect of Ro15-4513 on recombinant alpha4beta3delta receptors is blocked by flumazenil and beta-carboline-ethyl ester (beta-CCE). Our findings suggest that ethanol/Ro15-4513-sensitive GABA(A) receptors are important mediators of behavioral alcohol effects.

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    • "Hence there still remain some conflicting data about the effects of EtOH on native neuronal GABA A receptors. Previous work suggests that GABA A receptors containing the subunit and/or mediating tonic extrasynaptic GABA responses (tonic current) may be especially sensitive to EtOH (Wallner et al., 2006). Differences in EtOH sensitivity of various GABA A receptor subunit compositions have been invoked as a potential explanation for the inconsistent findings . "
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    • "Presumably, this Ro15-4513 sensitive tonic current that was ablated was normally composed of α4βγ2-containing GABA A -Rs since they are markedly potentiated by Ro15-4513 whereas α4δ-containing receptors are not (Wallner et al., 2006b). Even though α4δ receptors are not potentiated by Ro15-4513, they still bind this compound with high affinity (Hanchar et al., 2006) and it has been proposed that this binding to α4δ receptors antagonizes the effects of ethanol (Hanchar et al., 2006; Wallner et al., 2006b). However, our previously reported results that Ro15-4513 antagonism of ethanol-induced LORR was normal in δ subunit KO mice (Mihalek et al., 2001) argue against this. "
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