Point mutation in the first transmembrane region of the β2 subunit of the γ-aminobutyric acid type A receptor alters desensitization kinetics of γ-aminobutyric acid- and anesthetic-induced channel gating

A conserved glycine residue in the first transmembrane (TM1) domain of the β2 subunit has been identified to be involved with desensitization induced by γ-aminobutyric acid (GABA) and anesthetics. Recombinant GABA_A receptors expressed in Sf9 cells were recorded using semi-fast agonist application. Upon direct activation by GABA or anesthetics, the main effect of the TM1 point mutation on the β2 subunit (G219F) was to slow the time constant (τ) of desensitization. At GABA concentrations eliciting maximum currents, the corresponding median τ values were 0.87 s (25-75% interval (0.76; 1.04 s)), 0.93 s (0.76; 1.23 s), and 1.36 s (1.17; 1.57 s) for α1β2γ2, α1(G223F)β2γ2, and α1β2(G219F)γ2, respectively. The τ value for the β2-mutant receptor was significantly longer than α1β2γ2 (p < 0.01) and α1(G223F)β2γ2 (p < 0.05). For pentobarbital-induced currents (500 μM), the corresponding median τ values were 1.36 s (0.81; 1.41 s), 1.47 s (1.31; 2.38 s), and 2.82 s (2.21; 5.56 s) for α1β2γ2, α1(G223F)β2γ2, and α1β2(G219F)γ2, respectively. The τ value for the β2-mutant receptor was significantly longer than that for α1β2γ2 (p < 0.01). The present findings suggest that this TM1 glycine residue is critical for the rate at which desensitization occurs and that both GABA and intravenous anesthetics implement an analogous pathway for generating desensitization.