Divergence of volatile anesthetic effects in inhibitory neurotransmitter receptors

Background: The mechanism of volatile anesthetic (VA) action is unknown. Inhibitory 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), which is uniquely inhibited by VAs, suggested a chimeric receptor approach to identify portions of these proteins that may be necessary for anesthetic effects. Methods: A silent mutation resulting in the addition of a unique restriction enzyme recognition site was introduced in GABA receptor type A cu,, glycine cu,, and p subunit cDNAs, Chimeras mere constructed by rejoining restriction digest fragments and were expressed in Xenopus oocytes. Modulation of submaximal agonist-evoked peak currents by the VAs chloroform, enflurane, halothane, or isoflurane was measured using two-electrode voltage clamp. Results: Four chimeras mere constructed and designated glyrho, rhogly, alpha (2)rho, and rho alpha (2). Glyrho formed glycine-gated receptors with currents that were enhanced by chloroform or halothane but were inhibited by enflurane or isoflurane. Chimeras rhogly and rho alpha (2), each formed GABA-gated receptors with currents that mere inhibited by chloroform or halothane 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 grouping parallels the chemistry of these compounds. Our results support the involvement of multiple protein domains in the mechanism of VA modulation of GABA and glycine receptors.