GABA concentration sets the conductance of delayed GABA(A) channels in outside-out patches from rat hippocampal neurons

GABA, channels were activated by GABA in outside-out patches from rat cultu red hippocampal neurons. They were blocked by bicuculline and potentiated b y diazepam. In 109 of 190 outside-out patches, no channels were active befo re exposure to GABA (silent patches). The other 81 patches showed spontaneo us channel activity. In patches containing spontaneous channel activity, ra pid application of GABA rapidly activated channels. In 93 of the silent pat ches, channels could be activated by GABA but only after a delay that was s ometimes as long as 10 minutes. The maximum channel conductance of the chan nels activated after a delay increased with GABA concentration from less th an 10 pS (0.5 muM GABA) to more than 100 pS (10 mM GABA). Fitting the data with a Hill-type equation gave an EC50 value of 33 muM and a Hill coefficie nt of 0.6. The channels showed outward rectification and were chloride sele ctive. In the presence of 1 muM diazepam, the GABA EC50 decreased to 0.2 mu M but the maximum conductance was unchanged. Diazepam decreased the average latency for channel opening. Bicuculline. a GABA antagonist, caused a conc entration-dependent decrease in channel conductance. In channels activated with 100 muM GABA the bicuculline EC50 was 19 muM. The effect of GABA on ch annel conductance shows that the role of the ligand in GABA, receptor chann el function is more complex than previously thought.