The Actions of Sevoflurane and Desflurane on the &ggr;-Aminobutyric Acid Receptor Type A: Effects of TM2 Mutations in the &agr; and &bgr; Subunits

Background Previous studies have shown that specific amino acid residues in the putative second transmembrane segment (TM2) of the &ggr;-aminobutyric acid receptor type A (GABAA) receptor play a critical role in the enhancement of GABAA receptor function by halothane, enflurane, and isoflurane. However, very little is known about the actions of sevoflurane and desflurane on recombinant GABAA receptors. The aim of this study was to examine the effects of sevoflurane and desflurane on potentiation of GABA-induced responses in the wild-type GABAA receptor and in receptors mutated in TM2 of the &agr;1, &agr;2, or &bgr;2 subunits. Methods GABAA receptor &agr;1 or &agr;2, &bgr;2 or &bgr;3, and &ggr;2s subunit cDNAs were expressed for pharmacologic study by transfection of human embryonic kidney 293 cells and assayed using the whole cell voltage clamp technique. Concentration-response curves and EC50 values for agonist were determined in the wild-type &agr;1&bgr;2&ggr;2s and &agr;2&bgr;3&ggr;2s receptors, and in receptors harboring mutations in TM2, such as &agr;1(S270W)&bgr;2&ggr;2s, &agr;1&bgr;2(N265W)&ggr;2s, and &agr;2(S270I)&bgr;3&ggr;2s. The actions of clinically relevant concentration of volatile anesthetics (isoflurane, sevoflurane, and desflurane) on GABA activated Cl− currents were compared in the wild-type and mutant GABAA receptors. Results Both sevoflurane and desflurane potentiated submaximal GABA currents in the wild-type GABAA &agr;1&bgr;2&ggr;2s receptor and &agr;2&bgr;3&ggr;2s receptor. Substitution of Ser270 in TM2 of the &agr; subunit by a larger amino acid, tryptophan (W) or isoleucine (I), as in &agr;1(S270W)&bgr;2&ggr;2s and &agr;2(S270I)&bgr;3&ggr;2s, completely abolished the potentiation of GABA-induced currents by these anesthetic agents. In contrast, mutation of Asn265 in TM2 of the &bgr; subunit to tryptophan (W) did not prevent potentiation of GABA-induced responses. The actions of sevoflurane and desflurane in the wild-type receptor and in mutated receptors were qualitatively and quantitatively similar to those observed for isoflurane. Conclusions Positions Ser270 of the GABAA &agr;1 and &agr;2 subunits, but not Asn265 in the TM2 of the &bgr;2 subunit, are critical for regulation of the GABAA receptor by sevoflurane and desflurane, as well as isoflurane, consistent with the idea that these three volatile anesthetics share a common site of actions on the &agr; subunit of the GABAA receptor.

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