Glutamate Sensitivity of Denervated Insect Muscle Fibres

Abstract

L-GLUTAMATE, rather than acetylcholine, is probably the transmitter at the excitatory synapses found on insect skeletal muscle fibres^1–8. By applying L-glutamate iontophoretically from a micropipette to localized areas on the surface of locust muscle fibres the glutamate sensitivity of the muscle membrane is found to decrease sharply with distance from a synaptic site^5,7,8. Within 10–35 microns of a synapse the glutamate sensitivity falls either to zero or to a low background level about 10,000 times less than that at the synaptic sites⁵. Insect skeletal muscle fibres are multiterminally innervated and the synapses are usually on the inner faces of the fibres. The areas of fibre membrane most sensitive to glutamate can therefore be most frequently located by positioning the glutamate electrode in a cleft between two fibres (Fig. 1b), Usually the outer, exposed surfaces of the fibres are devoid of synaptic membrane and are either insensitive to glutamate or sensitive to only very high concentrations of this amino-acid (Fig. 2). Here I report studies of denervated locust muscle fibres, which demonstrate that, following denervation, the glutamate receptors can be found over the entire surface of the muscle fibres.