Human neuronal nicotinic acetylcholine receptors expressed in Xenopus oocytes predict efficacy of halogenated compounds that disobey the Meyer- Overton rule

Abstract

Background: According to the Meyer-Overton rule, anesthetic potency of a substance can be predicted by its lipid solubility, but a group of halogenated volatile compounds predicted to induce anesthesia does not obey this rule. Thus, these compounds are useful tools for studies of molecular targets of anesthetics. Human neuronal nicotinic acetylcholine receptor (hnAChR) subunits have been recently cloned, which allowed the authors to assess whether these receptors could differentiate among volatile anesthetic and nonimmobilizer compounds. This study provides the first data regarding anesthetic sensitivity of hnAChRs. Methods: α2β4, α3β4, and α4β6 hnAChRs were expressed in Xenopus oocytes, and effects of volatile anesthetics isoflurane and F3 (1-chloro-1,2,2-trifluorocyclobutane, 1A) and nonimmobilizers F6 (1,2-dichlorohexafluorocyclobutane, 2N) and F8 (2,3- dichlorooctafluorobutane) on the peak acetylcholine-gated currents were studied using the two-electrode voltage-clamp technique. Results: Isoflurane and F3 inhibited all the hnAChRs tested in a concentration-dependent manner. Isoflurane at a concentration corresponding to 1 minimum alveolar concentration (MAC) inhibited 83, 69, and 71% of ACh-induced currents in α2β4, α3β4, and α4β6 hnAChRs, respectively, and 1 MAC of F3 inhibited 64, 44, and 61% of currents gated in those receptors. F6 (8-34μM) did not cause any changes in currents gated by any of the receptors tested. F8 (4-18μM) did not alter the currents gated in either α3β4 or α4β2 receptors, but caused a small potentiation of α2β4 hnAChRs without a concentration-response relation. Conclusion: The in vivo potency and effectiveness of volatile anesthetic and nonimmobilizer compounds were consistent with their actions on hnAChRs expressed in a recombinant expression system, suggesting a potential participation of these receptors in the mechanisms of anesthesia.