Divergence of volatile anesthetic effects in inhibitory neurotransmitter receptors

Background: The mechanism of volatile anesthetic (VA) action is unknown. In hibitory receptors for the neurotransmitters gamma -aminobutyric acid (GABA ) or glycine are typically positively modulated by VAs and may be important targets for their action. The existence of a GABA receptor subtype (p), wh ich is uniquely inhibited by VAs, suggested a chimeric receptor approach to identify portions of these proteins that may be necessary for anesthetic e ffects. Methods: A silent mutation resulting in the addition of a unique restrictio n enzyme recognition site was introduced in GABA receptor type A cu,, glyci ne cu,, and p subunit cDNAs, Chimeras mere constructed by rejoining restric tion digest fragments and were expressed in Xenopus oocytes. Modulation of submaximal agonist-evoked peak currents by the VAs chloroform, enflurane, h alothane, or isoflurane was measured using two-electrode voltage clamp. Results: Four chimeras mere constructed and designated glyrho, rhogly, alph a (2)rho, and rho alpha (2). Glyrho formed glycine-gated receptors with cur rents that were enhanced by chloroform or halothane but were inhibited by e nflurane or isoflurane. Chimeras rhogly and rho alpha (2), each formed GABA -gated receptors with currents that mere inhibited by chloroform or halotha ne but enhanced by enflurane or isoflurane. Conclusions: These data show, for the first time, functional divergence of VA action on a single protein target, The VAs in this study fall into, two distinct groups with respect to their effects on these receptors. This grou ping parallels the chemistry of these compounds. Our results support the in volvement of multiple protein domains in the mechanism of VA modulation of GABA and glycine receptors.