GABA(A) receptors as molecular targets of general anesthetics: identification of binding sites provides clues to allosteric modulation.
PURPOSE: The purpose of this review is to summarize current knowledge of detailed biochemical evidence for the role of gamma-aminobutyric acid type A receptors (GABA(A)-Rs) in the mechanisms of general anesthesia. PRINCIPAL FINDINGS: With the knowledge that all general anesthetics positively modulate GABA(A)-R-mediated inhibitory transmission, site-directed mutagenesis comparing sequences of GABA(A)-R subunits of varying sensitivity led to identification of amino acid residues in the transmembrane domain that are critical for the drug actions in vitro. Using a photo incorporable analogue of the general anesthetic, R(+)etomidate, we identified two transmembrane amino acids that were affinity labelled in purified bovine brain GABA(A)-R. Homology protein structural modelling positions these two residues, alphaM1-11' and betaM3-4', close to each other in a single type of intersubunit etomidate binding pocket at the beta/alpha interface. This position would be appropriate for modulation of agonist channel gating. Overall, available information suggests that these two etomidate binding residues are allosterically coupled to sites of action of steroids, barbiturates, volatile agents, and propofol, but not alcohols. Residue alpha/betaM2-15' is probably not a binding site but allosterically coupled to action of volatile agents, alcohols, and intravenous agents, and alpha/betaM1-(-2') is coupled to action of intravenous agents. CONCLUSIONS: Establishment of a coherent and consistent structural model of the GABA(A)-R lends support to the conclusion that general anesthetics can modulate function by binding to appropriate domains on the protein. Genetic engineering of mice with mutation in some of these GABA(A)-R residues are insensitive to general anesthetics in vivo, suggesting that further analysis of these domains could lead to development of more potent and specific drugs.
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amino acids etomidate gamma-aminobutyric acid type A receptors GABA(A)-R GABA(A)-Rs GABA(A)-R GABA(A)-R GABA(A)-R bovine brain GABA(A)-R intersubunit etomidate binding pocket etomidate beta/alpha interface GABA(A) receptors steroids barbiturates gamma-aminobutyric acid propofol alcohols alcohols amino acid R(+)etomidate