An analysis of the inhibitory post‐synaptic current in the voltage‐clamped crayfish muscle.

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Abstract
Inhibitory post-synaptic currents (IPSC) were recorded from the feed-back current through a wire electrode inserted longitudinally into the opener muscle fiber of the claw in the crayfish (Cambarus clarkii). IPSC rose to its peak in about 3-4 ms and decayed exponentially. The decay time constant at -100 mV was 9.4 ms. The decay time constant decreased as the membrane was hyperpolarized and increased during depolarization. The time constant (.tau.) depends on voltage (V) according to the relation .tau. = a exp (AV), with a = 18.6 ms and A = 0.0065 mV-1. Voltage dependence was opposite in direction to that seen at frog end-plates, but in the same direction as that of EPSC [excitatory postsynaptic current] in crayfish muscle. At lower temperatures, the rise and fall times of IPSC were prolonged. Q10 for the decay time constant was 2.4 from 22.6-12.5.degree. C. When pH was decreased from 7.2 to 5.5, the decay time constant increased by about 50% with little change in the voltage dependence of the time course. When Cl- in the solution was changed to iodide, the decay time constant was increased by a factor of 3, while voltage dependence of the time course was not changed. In Br- solution, the decay time constant increased by about 50%. Peak amplitudes of IPSC were approximately linear as the membrane was depolarized, but they leveled off as the membrane was hyperpolarized beyond the reversal potential. The non-linear I-V [inhibition-voltage] relation did not result from inadequate voltage clamping nor from a change in the inside concentration of Cl-. After equilibration with iodide solution the I-V relation was approximately linear. The decay time constant was increased after repetitive nerve stimulation. This prolongation became more pronounced at lower temperatures. The kinetic process of the transmitter action is discussed. The rate limiting process for IPSC is probably the binding and unbinding of the transmitter to the receptor.