A Single M1 Residue in the β2 Subunit Alters Channel Gating of GABAA Receptor in Anesthetic Modulation and Direct Activation

Abstract

General anesthetics allosterically modulate the activity of neuronal γ-aminobutyric acid, type A (GABAA), receptors. Previous mutational studies from our laboratory and others have shown that the regions in transmembrane domain 1 (M1) and pre-M1 of α and β subunits in GABA receptors are essential for positive modulation of GABA binding and function by the intravenous (IV) general anesthetics. Mutation of β2Gly-219 to Phe corresponded in ρ nearly eliminated the modulatory effect of IV anesthetics in α1/β2/γ2S combination. However, the general anesthetics retained the ability to directly open the channel of mutant G219F, and the apparent affinity for GABA was increased, and desensitization rate was reduced. In this study, we made additional single mutations such as 219 Ser, Cys, Ile, Asp, Arg, Tyr, and Trp. The larger side chains of the replacement residues produced the greatest reduction in enhancement of GABA currents by IV anesthetics at clinical concentrations (Trp > Tyr = Phe > Arg > Asp > Ile > Cys > Ser = wild type). Compared with a 2-3-fold response in wild type, pentobarbital and propofol enhanced less than 0.5-fold; etomidate and alphaxalone modulation was reduced from more than 4- to 1-fold in G219F, G219Y, and G219W. A linear correlation was observed between the volume of the residue at position 219 and the loss of modulation. An identical correlation was found for the effect of modulation on left-shift in the GABA EC50 value; furthermore, the same rank order of residues, related to size, was found for reduction in the maximal direct channel-gating by pentobarbital (1 mM) and etomidate (100 μM) and for increased apparent affinity for direct gating by the IV anesthetics.