Effects of isoflurane on γ-aminobutyric acid type A receptors activated by full and partial agonists

Background: Volatile anesthetics prolong inhibitory postsynaptic potentials in central neurons via an allosteric action on the gamma-aminobutyric acid type A (GABA(A)) receptor, an effect that may underlie the hypnotic actions of these agents. Inhaled anesthetics such as isoflurane act to enhance responses to submaximal concentrations of GABA, but it is not clear whether their effect is mediated by an increase in the binding of the agonist or by changes in receptor gating behavior. To address this question, the authors studied the effects of isoflurane on a mutant GABA(A) receptor with a gating defect that decreases receptor sensitivity by lowering agonist efficacy. They then compared the effects of clinically relevant concentrations of isoflurane on the actions of GABA and piperidine-4-sulfonic acid (P4S), a partial agonist at the GABA(A) receptor. Methods: The authors created a mutant of the GABA(A) receptor alpha(1) subunit (L277A) by site-directed mutagenesis. The mutant subunit was coexpressed with beta(2) and gamma(2s) subunits in HEK293 cells, and responses to GABA and P4S were recorded using the whole cell patch clamp technique. EC50 values were determined for the full agonist GABA and the partial agonist P4S. The authors also determined the relative efficacy (epsilon) of P4S. These measurements were then repeated in the presence of isoflurane. Results: The concentration-response curve for GABA was shifted to the right (EC50 = 278 muM) in the alpha(1)(L277A)beta(2)gamma(2S) mutant receptor, compared with the corresponding wild-type alpha(1)beta(2)gamma(2S) GABA(A) receptor (EC50 = 16 muM). P4S is a partial agonist at both receptors, with a dramatically decreased relative efficacy at the mutant receptor (epsilon = 0.24). When the mutant receptor was studied in the presence of isoflurane, the concentration-response curves for both GABA and P4S were shifted to the left (EC50 for GABA = 78 muM); the efficacy of P4S also increased significantly (epsilon = 0.40). Conclusion: By studying a mutant GABA(A) receptor with impaired gating, the authors were able to demonstrate clearly that isoflurane can increase the efficacy of a partial agonist, as well as increase agonist potency. These data suggest that the volatile anesthetic isoflurane exerts at least some of its effects on the GABA(A) receptor via alterations in gating rather than simply changing binding or unbinding of the agonist.