Molecular determinants for the action of general anesthetics at recombinant α₂β₃γ2 γ-aminobutyric acid_A receptors

General anesthetics modulate the activity of ligand-gated ion channels including the GABA_A receptor. Mutational studies mainly on the benzodiazepine-insensitive α₂β_1(M286w) and α₆β_3(N289M)γ₂ GABA_A receptors revealed that a serine in transmembrane domain 2 and a methionine in transmembrane domain 3 are essential for the action of most general anesthetics. We investigated whether these residues would similarly be relevant for their action at the benzodiazepine-sensitive GABA receptor subtype, α₂β₃γ₂. We found that not only the N265M but also the M286W mutation nearly abolished the modulatory effect of etomidate. However, the anti-convulsant loreclezole, a structural homologue of etomidate, was inactive on the N265M mutant, but displayed normal modulatory activity on the M286W mutant. Both mutations did not affect the modulatory action of the neurosteroid alphaxalone. The direct action of alphaxalone, however, was dramatically increased in the M286W mutant to about twice the maximal GABA current but not significantly affected in the N265M mutant. These data demonstrate that the structural requirements for modulatory and direct actions of various general anesthetics are distinct. The molecular switches induced by these mutations can be exploited to identify the molecular determinants for the action of general anesthetics.