Calcitonin gene-related peptide lengthens acetylcholine receptor channel open time in developing muscle

The effect of calcitonin gene-related peptide (CGRP) on nicotinic acetylcholine receptor (AChR) function was examined in developing Xenopus myotomal muscle. Short term exposure to CGRP was found to prolong the open time of AChR channels in intermediate stage muscle. Of the two predominant conductance classes of AChRs expressed in Xenopus muscle, only the low conductance channels exhibited a lengthened open time in response to CGRP. High conductance channels were not affected, nor were low conductance channels at an earlier developmental stage. The lengthening of open time was reflected by an increase in the duration of synaptic currents at intermediate stages in response to CGRP. Application of 8 bromo-cAMP also prolonged synaptic currents, suggesting that the CGRP effects were mediated by cAMP. CGRP is known to be present in motor nerve terminals of Xenopus, and its release at developing synapses could potentiate the effects of ACh on muscle membrane.