Distribution of calcitonin gene-related peptide in vertebrate neuromuscular junctions: relationship to the acetylcholine receptor.

Abstract

Calcitonin gene-related peptide (CGRP), regarded by several authors to be involved in maintenance of the acetylcholine receptor, is present in the motor axons of various striated rat muscles. It is present, however, only in motor endplates of several selected striated muscles, where it is located in presynaptic axon terminals of neuromuscular junctions. No immunoreactivity could be seen within synaptic vesicles themselves. In the non-human primate Macaca fasciculata, neuromuscular junctions, including those in the diaphragm, display an intense CGRP reaction. The structure of the simian motor endplates is more elaborate than that of the rat. Amphibian motor nerve endings, both in tetanic and tonic muscles, display CGRP immunoreactivity. In tetanic muscles the CGRP reaction outlines "terminaisons en placque" (true motor end plates) and weakly reacting "terminaisons en grappe" (grape-like endings) in tonic muscles. On supramaximal stimulation of the motor nerve, CGRP is depleted from the affected neuromuscular junctions. Wallerian degeneration of the motor axon results in complete disappearance of CGRP. In most rat muscles in which motor endplates do not normally exhibit CGRP immunoreactivity, e.g., the diaphragm and buccinator muscles, the pre-terminal motor axons are CGRP-positive. After immobilization of such muscles by local bupivacaine injection to rats under brief chloral hydrate anesthesia, CGRP immunoreactivity of the neuromuscular junctions can be elicited because blockade of neuromuscular transmission results in accumulation of CGRP in the endplates. Even more striking is the appearance of CGRP immunoreactivity in normally non-reactive motor endplates during axon regeneration after an experimentally induced Wallerian degeneration of the motor axons. We conclude that CGRP is a regular, genotypically determined component of neuromuscular junctions, present either in a manifest or in a latent form. The latter can be elicited by various experimental approaches. The presence of CGRP in the motor endplate supports the theory that this peptide is instrumental in maintenance of the acetylcholine receptor.