The Anesthetic-Like Effects of Diverse Compounds on Wild-Type and Mutant γ-Aminobutyric Acid Type A and Glycine Receptors

Abstract

No theory of inhaled anesthetic action requires volatility of the anesthetic to accomplish the biophysical interaction of anesthetic with biological target. The identification of mutations that attenuate the effect of inhaled anesthetics on various receptors raises the possibility that nonvolatile compounds with anesthetic effects can be identified with the aid of these receptors. In previous studies, we identified compounds that were either charged or had an exceptionally low vapor pressure and which modulated anesthetic-sensitive receptors in a manner similar to inhaled anesthetics. We tested whether these, and another charged compound, shared a common mechanism with volatile anesthetics, by comparing their effect on wild-type gamma-aminobutyric acid type A (GABA(A)) or glycine receptors and mutant receptors that were engineered to be relatively resistant to inhaled anesthetics. The effect of beta-hydroxybutyric acid, ammonium chloride, diethylhexyl phthalate, and GABA were tested on homomeric alpha1 and mutant alpha1 (S267I) glycine receptors. The effect of sodium dodecyl sulfate and glycine were tested on alpha1 b2 gamma2s and mutant alpha1(S270I) beta2 gamma2s GABA(A) receptors. Receptors were expressed in Xenopus laevis oocytes and studied using two-electrode voltage clamping. For both GABA(A) and glycine receptors, isoflurane and ethanol were used as positive controls and propofol as a negative control (i.e., unaffected by the mutation). Beta-hydroxybutyric acid, ammonium chloride, diethylhexyl phthalate, and GABA all enhanced glycine receptor function. This effect was reduced by the S267I mutations. Sodium dodecyl sulfate and glycine enhanced GABA(A) receptor function, and the S270I mutation attenuated this effect. These findings support the hypothesis that the compounds studied modulate GABA(A) or glycine receptors by a mechanism similar to that of isoflurane and ethanol. Comparing the effect of drugs on anesthetic-sensitive wild-type receptors with relatively less sensitive mutant receptors may help identify compounds with anesthetic effects.