Tryptophan Mutations at Azi-Etomidate Photo-Incorporation Sites on α1 or β2 Subunits Enhance GABAA Receptor Gating and Reduce Etomidate Modulation

The potent general anesthetic etomidate produces its effects by enhancing GABA(A) receptor activation. Its photolabel analog [H-3]azi-etomidate labels residues within transmembrane domains on alpha and beta subunits: alpha Met236 and beta Met286. We hypothesized that these methionines contribute to etomidate sites formed at alpha-beta subunit interfaces and that increasing side-chain bulk and hydrophobicity at either locus would mimic etomidate binding and block etomidate effects. Channel activity was electrophysiologically quantified in alpha(1)beta(2)gamma(2L) receptors with alpha(1)M236W or beta(2)M286W mutations, in both the absence and the presence of etomidate. Measurements included spontaneous activation, GABA EC50, etomidate agonist potentiation, etomidate direct activation, and rapid macrocurrent kinetics. Both alpha(1)M236W and beta(2)M286W mutations induced spontaneous channel opening, lowered GABA EC50, increased maximal GABA efficacy, and slowed current deactivation, mimicking effects of etomidate on alpha(1)beta(2)gamma(2L) channels. These changes were larger with alpha(1)M236W than with beta(2)M286W. Etomidate (3.2 mu M) reduced GABA EC50 much less in alpha(1)M236W beta(2)gamma(2L) receptors (2-fold) than in wild type (23-fold). However, etomidate was more potent and efficacious in directly activating alpha(1)M236W beta(2)gamma(2L) compared with wild type. In alpha(1)beta(2)M286W gamma(2L) receptors, etomidate induced neither agonist-potentiation nor direct channel activation. These results support the hypothesis that alpha(1)Met236 and beta(2)Met286 are within etomidate sites that allosterically link to channel gating. Although alpha(1)M236W produced the larger impact on channel gating, beta(2)M286W produced more profound changes in etomidate sensitivity, suggesting a dominant role in drug binding. Furthermore, quantitative mechanistic analysis demonstrated that wild-type and mutant results are consistent with the presence of only one class of etomidate sites mediating both agonist potentiation and direct activation.