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2 and 
subtypes confer unique kinetic properties on recombinant GABAA receptor currents in mouse fibroblasts
To determine their contributions to the rapid kinetic properties of GABAA receptor (GABAR) currents, 
1 and 
3 subunit subtypes without or with or 2L subtypes were transiently coexpressed in mouse L929 fibroblasts to produce 13, 13, or 132L GABAR isoforms.
Brief (2-3 ms) applications of 1 mM GABA to outside-out membrane patches containing 13, 13, or 132L isoforms elicited currents that activated rapidly with monophasic time courses and deactivated rapidly with biphasic time courses. 132L currents exhibited a slower mean deactivation rate (76·1 ms) than 13 (34·1 ms) or 13 currents (42·8 ms).
During 1 mM GABA applications, 132L currents activated more rapidly (0·46 ms) than 13 currents (1·7 ms) or 13 currents (2·4 ms). During 4000 ms GABA applications, 13 and 132L currents desensitized with triphasic time courses to similar extents (13, 94·6 %; 132L, 92·4 %) and with similar mean rates (13, 352 ms; 132L, 462 ms). In contrast, 13 currents desensitized only 55·6 % with a biphasic time course and slower mean rate (1260 ms).
These experiments demonstrated that the 13 heterodimer formed a GABAR channel with rapid deactivation and rapid and nearly complete desensitization. Addition of the subunit did not alter the activation rate, but produced a receptor with slower and less complete desensitization. Addition of the 2L subtype increased activation rate, prolonged deactivation and changed the pattern of rapid desensitization.
Rapid kinetic and steady-state single-channel data were used to construct kinetic models that predicted the behaviour of the 132L and 13 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.
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