Relative ‘trophic’ influences of excitatory and inhibitory innervation of locust skeletal muscle fibres

Abstract

DENERVATION of locust skeletal muscle, where L-glutamate is thought to be the excitatory neurotransmitter¹, induces a pattern of physiological and pharmacological changes similar to that following denervation of vertebrate skeletal muscle^2,3. The degeneration of nerve terminals on locust muscle which follows axotomy⁴ is accompanied by a significant increase in the sensitivity of the extrajunctional membrane to L-glutamate^5–8, as well as a fall in resting membrane potential^8,9 and input conductance⁸ of the denervated fibres. Locust skeletal muscle fibres are multiterminally and sometimes polyneuronally innervated. In addition to excitatory inputs some fibres receive endings from inhibitory axons in which transmission is probably mediated by γ-aminobutyric acid (GABA)¹⁰. This dual nervous control at the peripheral level allows examination of the separate ‘trophic’ influences of excitatory and inhibitory inputs to a muscle fibre. In this report we demonstrate that in at least one locust muscle, the metathoracic retractor unguis (RU) of Schistocerca gregaria which contains fibres receiving both excitatory and inhibitory inputs, denervation-induced changes can occur in the presence of functionally intact inhibitory innervation.