PROPOFOL AND OTHER INTRAVENOUS ANESTHETICS HAVE SITES OF ACTION ON THE GAMMA-AMINOBUTYRIC-ACID TYPE-A RECEPTOR DISTINCT FROM THAT FOR ISOFLURANE

Both volatile and intravenous general anesthetics allosterically enhan ce gamma-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 nec essary for positive modulation of GABA(A) and glycine receptors by the volatile anesthetic enflurane. We now report that mutation of these r esidues within either GABA(A) alpha 2 (S270 or A291) or beta 1 (S265 o r M286) subunits resulted in receptors that retain normal or near-norm al gating by GABA but are insensitive to clinically relevant concentra tions of another inhaled anesthetic, isoflurane. To determine whether receptor modulation by intravenous general anesthetics also was affect ed by these point mutations, we examined the effects of propofol, etom idate, the barbiturate methohexital, and the steroid alphaxalone on wi ld-type and mutant GABA(A) receptors expressed in human embryonic kidn ey 293 cells. In most cases, these mutations had little or no effect o n the actions of these intravenous anesthetics. However, a point mutat ion in the beta 1 subunit (M286W) abolished potentiation of GABA by pr opofol but did not alter direct activation of the receptor by high con centrations of propofol. These data indicate that the receptor structu ral requirements for positive modulation by volatile and intravenous g eneral anesthetics may be quite distinct.