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

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Abstract
1 To determine their contributions to the rapid kinetic properties of GABAA receptor (GABAR) currents, α1 and β3 subunit subtypes without or with δ or γ2L subtypes were transiently coexpressed in mouse L929 fibroblasts to produce α1β3, α1β3δ, or α1β3γ2L GABAR isoforms. 2 Brief (2–3 ms) applications of 1 mM GABA to outside-out membrane patches containing α1β3, α1β3δ, or α1β3γ2L isoforms elicited currents that activated rapidly with monophasic time courses and deactivated rapidly with biphasic time courses. α1β3γ2L currents exhibited a slower mean deactivation rate (76.1 ms) than α1β3 (34.1 ms) or α1β3δ currents (42.8 ms). 3 During 1 mM GABA applications, α1β3γ2L currents activated more rapidly (0.46 ms) than α1β3 currents (1.7 ms) or α1β3δ currents (2.4 ms). During 4000 ms GABA applications, α1β3 and α1β3γ2L currents desensitized with triphasic time courses to similar extents (α1β3, 94.6%; α1β3γ2L, 92.4%) and with similar mean rates (α1β3, 352 ms; α1β3γ2L, 462 ms). In contrast, α1β3δ currents desensitized only 55.6% with a biphasic time course and slower mean rate (1260 ms). 4 These experiments demonstrated that the α1β3 heterodimer formed a GABAR channel with rapid deactivation and rapid and nearly complete desensitization. Addition of the δ subunit did not alter the activation rate, but produced a receptor with slower and less complete desensitization. Addition of the γ2L 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 α1β3γ2L and α1β3δ 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.