Studies on locust neuromuscular physiology in relation to glutamic acid.

It is well known that application of L-glutamic acid to insect excitatory neuromuscular synapses leads to the production of depolarizing postsynaptic potentials, and this, with other evidence, has led to the belief that L-glutamic acid may be the chemical transmitter at these synapses (Pitman, 1971). It is also known that the haemolymph that bathes insect muscles contains L-glutamate at concentrations which would block neuromuscular transmission in experimental preparations, and it has been uncertain how the neuromuscular system of the living insect functions under such conditions. The work described here was undertaken to learn more about the action of glutamate on insect excitatory neuromuscular systems, and to try to explain how they function in the presence of haemolymph glutamate. Observations were made predominantly upon the mechanical responses of the locust retractor unguis muscle. Compared with the use of microelectrodes this approach told little about cellular mechanisms, but it did provide valuable information which could not be obtained with the other methods. For example, it revealed that certain experimental conditions could cause abnormal muscle contractions. The experimental results were sometimes difficult to interpret, partly because the physiological salines differed so much in chemical composition from locust haemolymph that it was difficult to extrapolate from the experimental situation to the living insect, and partly because the experimental conditions could generate abnormal responses. The study was extended to investigate some of these additional problems, and this gave us a better understanding of the physiology of locust muscle under natural and experimental conditions. We consider also that our study has thrown some light on the mechanisms which protect locust neuromuscular synapses from haemolymph glutamate.