Propofol and Other Intravenous Anesthetics Have Sites of Action on the γ-Aminobutyric Acid Type A Receptor Distinct from That for Isoflurane

Abstract

Both volatile and intravenous general anesthetics allosterically enhance γ-aminobutyric acid (GABA)-evoked chloride currents at the GABA type A (GABA_A) receptor. Recent work has revealed that two specific amino acid residues within transmembrane domain (TM)2 and TM3 are necessary for positive modulation of GABA_A and glycine receptors by the volatile anesthetic enflurane. We now report that mutation of these residues within either GABA_A α2 (S270 or A291) or β1 (S265 or M286) subunits resulted in receptors that retain normal or near-normal gating by GABA but are insensitive to clinically relevant concentrations of another inhaled anesthetic, isoflurane. To determine whether receptor modulation by intravenous general anesthetics also was affected by these point mutations, we examined the effects of propofol, etomidate, the barbiturate methohexital, and the steroid alphaxalone on wild-type and mutant GABA_A receptors expressed in human embryonic kidney 293 cells. In most cases, these mutations had little or no effect on the actions of these intravenous anesthetics. However, a point mutation in the β1 subunit (M286W) abolished potentiation of GABA by propofol but did not alter direct activation of the receptor by high concentrations of propofol. These data indicate that the receptor structural requirements for positive modulation by volatile and intravenous general anesthetics may be quite distinct.