Inhibitory synaptic channels activated by gamma-aminobutyric acid (GABA) in crayfish muscle.

Small crayfish muscle fibres were voltage clamped and synaptic current elicited by superfused GABA solutions was measured. Analysis of the fluctuations of synaptic current and of relaxations of the current after voltage steps yielded analogous results. The current has two components. The first component is characterized by the opening of synaptic channels with a single channel conductance gamma = 9 pS and an average open time tau = 5 ms, measured at 23 degrees C and - 100 mV. tau depends on the membrane potential, tau E = tau 0 x eE/epsilon, and epsilon was about +100 mV in the average. The channel open time agrees with the time constant of decay of the inhibitory postsynaptic current (IPSC) elicited by a nerve stimulus. The current is carried by chloride ions. The second current component is much slower, the average channel open time was tau s = 33 ms at 23 degrees C and -60 mV. The open time tau s of the slow component also was shortened on hyperpolarization. The reversal potential for the current component was more positive than -50 mV. This slow component also seems to be a synaptic one.