Quisqualate action on the crayfish neuromuscular junction

Actions of quisqualic acid on the crayfish neuromuscular junction were examined and compared with those of glutamate. The glutamate and quisqualate currents were induced in the voltage-clamped muscle. When their peak amplitudes were plotted against the current intensity of iontophoretically applied drugs, the hyperbola resulted. The amplitude of the maximum response to quisqualate was similar to that to glutamate. When the amount of quisqualate which was simultaneously applied with glutamate was changed, the amplitudes of the maximum response of glutamate plus quisqualate did not vary much at the top of the dose-response curves. Bath application of quisqualate in concentrations above 10(-6) M produced a large depolarization of the muscle fiber, but this response was maintained after it reached a maximum, unlike glutamate. On the other hand, the amplitude of excitatory junctional potential (EJPs) was gradually decreased. A quantum analysis of the extracellular EJP demonstrated that this decrease in EJP amplitude produced by quisqualic acid was due to the postsynaptic event. This decrease in EJP amplitude could not be prevent by exposure of the muscle fiber to concanavalin A, which completely blocked development of desensitization of the glutamate receptor. Based on these results, it is doubtful that the decrease in EJP amplitude produced by quisqualic acid is due to desensitization of the neuroreceptor.