Closing of membrane channels effected by gamma-amino-butyric acid (GABA) in crayfish muscle.

In a small crayfish muscle fibers current elicited by superfusion of GABA was studied. In the majority of the fibers the GABA induced synaptic current relaxed after voltage steps in agreement with the known voltage and time dependence of the inhibitory chloride channels. In about 20% of the preparations, however, at low GABA concentrations of 5-50 mumol/l an anomalous i--response occurs, namely a reduced total membrane conductance in response to GABA. This i--response can be described by a closing of membrane channels, in which the average closed time increases on hyperpolarization. Equivalent conductance reductions are also observed in measurements of the power spectral density of the current noise. In addition to the noise spectrum of the inhibitory chloride current (Dudel et al. 1980), a spectrum with a higher value of the corner frequency appeared, which seems to represent the i- component. In contrast to the inhibitory chloride current, the i--response is not blocked by picrotoxin. The receptors and channels which mediate the i--response thus seem to be different from those of the inhibitory iCl. Several lines of evidence indicate that the i--response is due to the closing of potassium channels. The i--response counteracts the inhibitory effect of GABA at low GABA concentrations and can distort the dose-response curve.