Calcitonin gene-related peptide prevents disuse-induced sprouting of rat motor nerve terminals

Calcitonin gene-related peptide (CGRP) coexists with acetylcholine (ACh) in motor nerve terminals. Externally applied CGRP has been shown to increase the synthesis of ACh receptors in cultured myotubes by a mechanism independent of muscle activity. Thus, CGRP is suggested to be a neurotrophic factor that may regulate the expression of several long-term events occurring at the neuromuscular junction. We have examined the effect of CGRP on the sprouting of motor nerve terminals induced by chronic block of nerve-muscle activity in adult rats. Daily treatment with CGRP suppressed the disuse-induced terminal sprouting in a dose-dependent manner, whereas the morphology of motor nerve terminals in active muscles was unaffected by CGRP. CGRP may be a possible candidate for an antisprouting agent which has been postulated to exist in nerve terminals. The disuse-induced outgrowth of terminal sprouts was accompanied by an increase in the mean quantum content of end-plate potentials, as well as in the frequency of spontaneous miniature end-plate potentials. This increased transmitter release was still maintained at the junctions in which disuse-induced terminal sprouting had been suppressed by CGRP. It is suggested that the formation of terminal sprouts per se is not responsible for the plastic change of transmitter release induced by prolonged disuse of the neuromuscular junction.