Channel properties determine the transient activation kinetics of recombinant GABA(A) receptors

To understand the mechanisms underlying activation and deactivation of GABA , receptor (GABAR) channels, we studied the properties of an identified GAB AR isoform under conditions similar to those present at central GABAergic s ynapses. Recombinant (alpha 5 beta 3 gamma 2L GABARs were expressed in L929 fibroblasts and studied using patch-clamp recording techniques. Brief appl ication of a high GABA concentration to outside-our membrane patches elicit ed transient currents that resembled those reported for miniature inhibitor y postsynaptic currents (mIPSCs), as well as native GABAR currents recorded under similar conditions. Characteristic of these currents was a rapid act ivation phase followed by a prolonged biphasic deactivation phase that far outlasted GABA application. Single-channel recordings revealed unique patte rns of channel activity with two channel conductance states of 22 and 16 pS , The prolonged deactivation phase appeared to be sustained by entry into a nd reopening from long-lasting closures or desensitized states. Agonist aff inity determined the time course of deactivation, indicating that occupied receptors drove the channel activity underlying the decay of current. The t ime course of deactivation was also longer at depolarized membrane potentia ls. The similarities between transient activation kinetics of recombinant a lpha 5 beta 3 gamma 2L GABARs to activation of synaptic GABARs (rapid activ ation and prolonged, voltage-dependent deactivation) suggest that intrinsic channel properties determine much of the response patterns of native GABAR s. (C) 1999 Published by Elsevier Science B.V. All rights reserved.