Dominant gating governing transient GABA_A receptor activity: A first latency and P_o/o analysis

Steady-state, single-channel gating of GABA_A receptors (GABARs) is complex. Simpler gating may dominate when triggered by rapid GABA transients present during fast inhibitory synaptic transmission and is critical to understanding the time course of fast IPSCs. We studied the single-channel activity of expressed α1β1γ2 GABARs in outside-out patches from human embryonic kidney 293 cells triggered by rapidly applied GABA (10-2000 μM) pulses (2-300 msec). Activation was analyzed with the time to first channel opening after GABA presentation, or first latency (FL). FL distributions are monoexponential at low GABA concentrations and biexponential above 30 μM GABA. The fast rate increases supralinearly to a plateau of ∼ 1100 sec^-1, the apparent activation rate. The slow rate and amplitude are insensitive to GABA concentration. The results argue that doubly liganded receptors can rapidly desensitize before opening. Gating after the first opening was quantified with analysis of open probability conditioned on the first opening (P_o/o). P_o/o functions are biexponential, dominated by a fast component, and insensitive to GABA concentration. This suggests that open channels convert primarily to fast but also to slow desensitized states. Furthermore, dual modes of fast desensitization may influence IPSC amplitude and thereby synaptic efficacy. The findings provided for the construction of a mathematical gating model that accounts for FL and P_o/o functions. In addition, the model predicts the time course of macroscopic current responses thought to mimic IPSCs. The results provide new insights into dominant gating that is likely operational during fast GABAergic synaptic transmission.