Bicuculline, pentobarbital and diazepam modulate spontaneous GABA(A) channels in rat hippocampal neurons

1 Spontaneously opening, chloride-selective channels that showed outward re ctification were recorded in ripped-off patches from rat cultured hippocamp al neurons and in cell-attached patches from rat hippocampal CAI pyramidal neurons in slices. 2 In both preparations, channels had multiple conductance states and the mo st common single-channel conductance varied. In the outside-out patches it ranged from 12 to 70 pS (Vp = 40 mV) whereas in the cell-attached patches i t ranged from 56 to 85 pS (-Vp = 80 mV). 3 Application of GABA to a patch showing spontaneous channel activity evoke d a rapid, synchronous activation of channels. During prolonged exposure to either 5 or 100 muM GABA, the open probability of channels decreased. Appl ication of GABA appeared to have no immediate effect on single-channel cond uctance. 4 Exposure of the patches to 100 muM bicuculline caused a gradual decrease on the single-channel conductance of the spontaneous channels. The time for complete inhibition to take place was slower in the outside-out than in th e cell-attached patches. 5 Application of 100 muM pentobarbital or 1 muM diazepam caused 2-4 fold in crease in the maximum channel conductance of low conductance (<40 pS) spont aneously active channels. 6 The observation of spontaneously opening GABA(A) channels in cell-attache d patches on neurons in slices suggests that they may have a role in neuron s in vivo and could be an important site of action for some drugs such as b enzodiazepines, barbiturates and general anaesthetics.