Desensitization and binding properties determine distinct α1β2γ2 and α3β2γ2 GABAA receptor-channel kinetic behavior

GABAA receptor subtypes comprising the al and alpha 3 subunits change with development and have a specific anatomical localization in the adult brain. These receptor subtypes have been previously demonstrated to greatly differ in deactivation kinetics but the underlying gating mechanisms have not been fully elucidated. Therefore, we expressed rat alpha 1 beta 2 gamma 2 and alpha 3 beta 2 gamma 2 receptors in human embryonic kidney 293 cells and recorded current responses to ultrafast GABA applications at macroscopic and single-channel levels. We found that the slow deactivation of alpha 3 beta 2 gamma 2-mediatecl currents is associated with a relatively small rate and extent of apparent desensitization. In contrast, responses mediated by alpha 1 beta 2 gamma 2 receptors had faster deactivation and stronger desensitization. alpha 3 beta 2 gamma 2 receptors had faster recovery in the paired-pulse agonist applications than alpha 1 beta 2 gamma 2 channels. The onset of currents mediated by alpha 3 beta 2 gamma 2 receptors was slower than that of alpha 1 beta 2 gamma 2 for a wide range of GABA concentrations. Single-channel analysis did not reveal differences in the opening/closing kinetics of alpha 1 beta 2 gamma 2 and alpha 3 beta 2 gamma 2 channels but burst durations were longer in alpha 3 beta 2 gamma 2 receptors. Simulation with a previously reported kinetic model was used to explore the differences in respective rate constants. Reproduction of major kinetic differences required a smaller desensitization rate as well as smaller binding and unbinding rates in alpha 3 beta 2 gamma 2 compared with alpha 1 beta 2 gamma 2 receptors. Our work describes the mechanisms underlying the kinetic differences between two major GABAA receptor subtypes and provides a framework to interpret data from native GABA receptors.