Gamma-aminobutyric acid receptors on cultured cockroach brain neurones

Abstract

Gamma-aminobutyric acid (GABA) at 10-4 moll-1 inhibited spontaneous activity and produced conductance changes in 60% of cultured cockroach neurones tested. The reversal potential for the GABA-evoked response was between -65 mV and -75 mV. Under whole-cell voltage-clamp conditions, with 114 mmoll-1 potassium chloride in the electrode, the reversal potential had a similar value to that predicted for a chloride current. The response was blocked by 10-5 moll-1 picrotoxin but was not affected by 10-5 moll-1 bicuculline. In the whole-cell voltage-clamp conditions, 50 μmoll-1 GABA evoked an inward current that was accompanied by an increase in current noise. Fluctuation analysis of the noise gave a mean channel opening time of 11.8 ms for GABA and 6.5 ms for muscimol. The single-channel conductance was 18.6 pS for GABA and 15.2 pS for muscimol. When 50 μmoll-1 GABA was applied in the presence of the benzodiazepine, flunitrazepam, there was an increase in both the evoked current and the accompanying current noise. Analysis of this noise gave values of 14.3 ms for the mean channel opening time and 18.3 pS for the single-channel conductance. The variance of the noise was increased by approximately 60% in the presence of flunitrazepam, suggesting that this drug potentiates the GABA responses of cockroach neurones by increasing the frequency of channel events.