GABA(A) receptor subunit gamma 2 and delta subtypes confer unique kinetic properties on recombinant GABA(A) 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 L9 29 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 2 L isoforms elicited currents that activated rapidly with monophasic time co urses and deactivated rapidly with biphasic time courses. alpha 1 beta 3 ga mma 2L currents exhibited a slower mean deactivation rate (76.1 ms) than al pha 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 activate d more rapidly (0.46 ms) than alpha 1 beta 3 currents (1.7 ms) or alpha 1 b eta 3 delta currents (2.4 ms). During 4000 ms GABA applications, alpha 1 be ta 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 forme d a GABAR channel with rapid deactivation and rapid and nearly complete des ensitization. Addition of the delta subunit did not alter the activation ra te, but produced a receptor with slower and less complete desensitization. Addition of the gamma 2L subtype increased activation rate, prolonged deact ivation and changed the pattern of rapid desensitization. 5. Rapid kinetic and steady-state single-channel data were used to construc t kinetic models that predicted the behaviour of the alpha 1 beta 3 gamma 2 L and alpha 1 beta 3 delta currents. These models represent a reconciliatio n of macroscopic and steady-state single-channel data; for GABARs and provi de a framework for systematically assessing the functional significance of different GABAR isoforms.