DISTINCT LOCI MEDIATE THE DIRECT AND INDIRECT ACTIONS OF THE ANESTHETIC ETOMIDATE AT GABA(A) RECEPTORS

Most general anesthetics produce two distinct actions at GABA(A) recep tors. Thus, these drugs augment GABA-gated chloride currents (referred to as an indirect action) and, at higher concentrations, elicit chlor ide currents in the absence of GABA (referred to as a direct action). Because a beta subunit appears to be required for the direct action of intravenous anesthetics in recombinant GABA(A) receptors, site-direct ed mutagenesis of the beta 3 subunit was performed to identify amino a cid residues that are critical for this action. In HEK293 cells expres sing a prototypical GABA(A) receptor composed of alpha 1 beta 3 gamma 2 subunits, mutation of amino acid 290 from Asn to Ser dramatically re duced both etomidate-induced chloride currents and its ability to stim ulate [H-3]flunitrazepam binding. By contrast, the ability of etomidat e to augment GABA-gated chloride currents and GABA-enhanced [H-3]fluni trazepam binding was retained. The demonstration that the direct, but not the indirect, actions of etomidate are dependent an beta 3(Asn(290 )) indicates that the dual actions of this intravenous anesthetic at G ABA(A) receptors are mediated via distinct loci.