GABAA receptor subunit γ2 and δ subtypes confer unique kinetic properties on recombinant GABAA receptor currents in mouse fibroblasts

1. To determine their contributions to the rapid kinetic properties of GABA(A) receptor (GABAR) currents, alpha 1 and beta 3 subunit subtypes without or with delta or gamma 2L subtypes were transiently coexpressed in mouse L929 fibroblasts to produce alpha 1 beta 3, alpha 1 beta 3 delta, or alpha 1 beta 3 gamma 2L GABAR isoforms. 2. Brief (2-3 ms) applications of 1 mM GABA to outside-out membrane patches containing alpha 1 beta 3, alpha 1 beta 3 delta, or alpha 1 beta 3 gamma 2L isoforms elicited currents that activated rapidly with monophasic time courses and deactivated rapidly with biphasic time courses. alpha 1 beta 3 gamma 2L currents exhibited a slower mean deactivation rate (76.1 ms) than alpha 1 beta 3 (341. ms) or alpha 1 beta 3 delta currents (42.8 ms). 3. During 1 mM GABA applications, alpha 1 beta 3 gamma 2L currents activated more rapidly (0.46 ms) than alpha 1 beta 3 currents (1.7 ms) or alpha 1 beta 3 delta currents (2.4 ms). During 4000 ms GABA applications, alpha 1 beta 3 and alpha 1 beta 3 gamma 2L currents desensitized with triphasic time courses to similar extents (alpha 1 beta 3, 94.6%; alpha 1 beta 3 gamma 2L, 92.4%) and with similar mean rates (alpha 1 beta 3, 352 ms; alpha 1 beta 3 gamma 2L, 462 ms). In contrast, alpha 1 beta 3 delta currents desensitized only 55.6% with a biphasic time course and slower mean rate (1260 ms). 4. These experiments demonstrated that the alpha 1 beta 3 heterodimer formed a GABAR channel with rapid deactivation and rapid and nearly complete desensitization. Addition of the delta subunit did not alter the activation rate, but produced a receptor with slower and less complete desensitization. Addition of the gamma 2L subtype increased activation rate, prolonged deactivation and changed the pattern of rapid desensitization. 5. Rapid kinetic and steady-state single-channel data were used to construct kinetic models that predicted the behaviour of the alpha 1 beta 3 gamma 2L and alpha 1 beta 3 delta currents. These models represent a reconciliation of macroscopic and steady-state single-channel data; for GABARs and provide a framework for systematically assessing the functional significance of different GABAR isoforms.