Loop G in the GABAA receptor α1 subunit influences gating efficacy.

KEY POINTS: The functional importance of residues in loop G of the GABAA receptor has not been investigated. D43 and T47 in the α1 subunit are of particular significance as their structural modification inhibits activation by GABA. While the T47C substitution had no significant effect, non-conservative substitution of either residue (D43C or T47R) reduced the apparent potency of GABA. Propofol potentiated maximal GABA-evoked currents mediated by α1(D43C)ß2γ2 and α1(T47R)ß2γ2 receptors. Non-stationary variance analysis revealed a reduction in maximal GABA-evoked Popen , suggesting impaired agonist efficacy. Further analysis of α1(T47R)ß2γ2 receptors revealed that the efficacy of the partial agonist THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridine-3-ol) relative to GABA was impaired. GABA-, THIP- and propofol-evoked currents mediated by α1(T47R)ß2γ2 receptors deactivated faster than those mediated by α1ß2γ2 receptors, indicating that the mutation impairs agonist-evoked gating. Spontaneous gating caused by the ß2(L285R) mutation was also reduced in α1(T47R)ß2(L285R)γ2 compared to α1ß2(L285R)γ2 receptors, confirming that α1(T47R) impairs gating independently of agonist activation. ABSTRACT: The modification of cysteine residues (substituted for D43 and T47) by 2-aminoethyl methanethiosulfonate in the GABAA α1 subunit loop G is known to impair activation of α1ß2γ2 receptors by GABA and propofol. While the T47C substitution had no significant effect, non-conservative substitution of either residue (D43C or T47R) reduced the apparent potency of GABA. Propofol (1 µm), which potentiates sub-maximal but not maximal GABA-evoked currents mediated by α1ß2γ2 receptors, also potentiated maximal currents mediated by α1(D43C)ß2γ2 and α1(T47R)ß2γ2 receptors. Furthermore, the peak open probabilities of α1(D43C)ß2γ2 and α1(T47R)ß2γ2 receptors were reduced. The kinetics of macroscopic currents mediated by α1(D43C)ß2γ2 and α1(T47R)ß2γ2 receptors were characterised by slower desensitisation and faster deactivation. Similar changes in macroscopic current kinetics, together with a slower activation rate, were observed with the loop D α1(F64C) substitution, known to impair both efficacy and agonist binding, and when the partial agonist THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridine-3-ol) was used to activate WT or α1(T47R)ß2γ2 receptors. Propofol-evoked currents mediated by α1(T47R)ß2γ2 and α1(F64C)ß2γ2 receptors also exhibited faster deactivation than their WT counterparts, revealing that these substitutions impair gating through a mechanism independent of orthosteric binding. Spontaneous gating caused by the introduction of the ß2(L285R) mutation was also reduced in α1(T47R)ß2(L285R)γ2 compared to α1ß2(L285R)γ2 receptors, confirming that α1(T47R) impairs gating independently of activation by any agonist. These findings implicate movement of the GABAA receptor α1 subunit's ß1 strand during agonist-dependent and spontaneous gating. Immobilisation of the ß1 strand may provide a mechanism for the inhibition of gating by inverse agonists such as bicuculline.