The interaction of the general anesthetic etomidate with the γ-aminobutyric acid type A receptor is influenced by a single amino acid

Abstract

The γ-aminobutyric acid type A (GABA A ) receptor is a transmitter-gated ion channel mediating the majority of fast inhibitory synaptic transmission within the brain. The receptor is a pentameric assembly of subunits drawn from multiple classes (α 1–6 , β 1–3 , γ 1–3 , δ 1 , and ɛ 1 ). Positive allosteric modulation of GABA A receptor activity by general anesthetics represents one logical mechanism for central nervous system depression. The ability of the intravenous general anesthetic etomidate to modulate and activate GABA A receptors is uniquely dependent upon the β subunit subtype present within the receptor. Receptors containing β 2 - or β 3 -, but not β 1 subunits, are highly sensitive to the agent. Here, chimeric β 1 /β 2 subunits coexpressed in Xenopus laevis oocytes with human α 6 and γ 2 subunits identified a region distal to the extracellular N-terminal domain as a determinant of the selectivity of etomidate. The mutation of an amino acid (Asn-289) present within the channel domain of the β 3 subunit to Ser (the homologous residue in β 1 ), strongly suppressed the GABA-modulatory and GABA-mimetic effects of etomidate. The replacement of the β 1 subunit Ser-290 by Asn produced the converse effect. When applied intracellularly to mouse L(tk−) cells stably expressing the α 6 β 3 γ 2 subunit combination, etomidate was inert. Hence, the effects of a clinically utilized general anesthetic upon a physiologically relevant target protein are dramatically influenced by a single amino acid. Together with the lack of effect of intracellular etomidate, the data argue against a unitary, lipid-based theory of anesthesia.