THE INTERACTION OF THE GENERAL ANESTHETIC ETOMIDATE WITH THE GAMMA-AMINOBUTYRIC-ACID TYPE-A RECEPTOR IS INFLUENCED BY A SINGLE AMINO-ACID

The gamma-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 o f subunits drain from multiple classes (alpha(1-6), beta(1-3), gamma(1 -3), delta(1), and epsilon(1)), Positive allosteric modulation of GABA (A) receptor activity by general anesthetics represents one logical me chanism for central nervous system depression, The ability of the intr avenous general anesthetic etomidate to modulate and activate GABA(A) receptors is uniquely dependent upon the beta subunit subtype present within the receptor, Receptors containing beta(2)- or beta(3)-, but no t beta(1) subunits, are highly sensitive to the agent, Here, chimeric beta(1)/beta(2) subunits coexpressed in Xenopus laevis oocytes with hu man alpha(6) and gamma(2) subunits identified a region distal to the e xtracellular 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 beta(3) subunit to Ser (the homologous residue i n beta(1)), strongly suppressed the GABA-modulatory and GABA mimetic e ffects of etomidate, The replacement of the beta(1) subunit Ser-290 by Asn produced the converse effect, When applied intracellularly to mou se L(tk-) cells stably expressing the alpha(6) beta(3) gamma(2) subuni t combination, etomidate was inert. Hence, the effects of a clinically utilized general anesthetic upon a physiologically relevant target pr otein are dramatically influenced by a single amino acid. Together wit h the lack of effect of intracellular etomidate, the data argue agains t a unitary, lipid-based theory of anesthesia.