Short- and long-term influences of calcitonin gene-related peptide on the synthesis of acetylcholinesterase in mammalian myotubes

1 The present study analyses the short- (15 min-2 h) and long-term (24-48 h ) influences of calcitonin gene-related peptide (CGRP) on acetylcholinester ase (AChE) expression in the rat cultured skeletal muscle and the signal tr ansduction events underlying CGRP actions. 2 To assess the effect of CGRP on AChE synthesis, myotubes were pre-exposed to the irreversible AChE inhibitor diisopropyl fluorophosphate (DFP) and t reated with CGRP or forskolin, an adenylyl cyclase (AC) activator. Treatmen t of myotubes with 1 - 100 nM CGRP for 2 h increased by up to 42% the synth esis of catalytically active AChE with a parallel increase in the intracell ular cyclic AMP. 3 The stimulation of AChE synthesis induced by CGRP was mimicked by direct activation of AC with 3 - 30 muM forskolin. In contrast, pre-treatment of c ultures with 100 nM CGRP for 20 h reduced by 37% the subsequent synthesis o f AChE, resulting in a 15% decrease in total AChE activity after 48 h CGRP treatment. 4 Moreover, 24 h treatment of myotubes with 100 nM CGRP reduced by 54% the accumulation of cyclic AMP induced by a subsequent CGRP treatment. 5 These findings indicate that, in skeletal muscle cells, CGRP modulates th e AChE expression in a time-dependent manner, initially stimulating the enz yme synthesis through a cyclic AMP-dependent mechanism. The decreased AChE synthesis observed after long-term CGRP treatment suggests that CGRP signal ling system is subject to desensitization or down-regulation, that might fu nction as an important adaptative mechanism of the muscle fibre in response to long-term changes in neuromuscular transmission.