RUN-DOWN OF THE GABA(A) RESPONSE UNDER EXPERIMENTAL-ISCHEMIA IN ACUTELY DISSOCIATED CA1 PYRAMIDAL NEURONS OF THE RAT

1. The effect of experimental ischaemia on the response to gamma-amino butyric acid (GABA) was assessed in acutely dissociated CA1 pyramidal neurones of rats, using the patch-clamp technique. 2. Rapid applicatio n of 3 x 10(-5) M GABA induced a bicuculline-sensitive inward Cl- curr ent (I-GABA) at a holding potential (V-h) of -44 mV. The peak amplitud e of I-GABA showed a time-dependent decrease (run-down) when it was re corded with the conventional whole-cell mode without internal ATP. The run-down was not observed when the intracellular ATP concentration ([ ATP](i)) was maintained by the nystatin-perforated recording with an i ntracellular Naf concentration ([Na+](i)) of 0 mM. 3. When [Na+](i) wa s increased to more than 30 mM, the I-GABA run-down was observed even with the nystatin-perforated recording. 4. The I-GABA run-down observe d at 60 mM [Na+](i) with tile nystatin method was further enhanced und er experimental ischaemia without changes in the reversal potential of I-GABA. The enhanced run-down was suppressed by application of the Na +,K+-ATPase inhibitors, ouabain and SPAI-1. 5. I-GABA run-down during ischaemia was also accompanied by an outward holding current and a con comitant increase in intracellular free Ca2+ concentration ([Ca2+](i)) in 48.5% of the neurones. The outward current was a Ca2+-activated K current, which was blocked by 3 x 10(-7) M charybdotoxin. 6. In the i nside-out mode of the single-channel analysis: GABA activated three su bconductance states with conductances of 33.4, 22.7 and 15.2 pS. Reduc tion of ATP concentration from 2 to 0 mM on tile intracellular side su ppressed the channel activities, while an increase in Ca2+ concentrati on from 0.7 x 10(-9) to 1.1 x 10(-6) M had no effect. 7. These results suggest that ischaemia induces the run-down of the postsynaptic GABA response at the GABA(A) receptor level, and that this run-down is trig gered by a decrease in [ATP](i).