Ryanodine‐ and CaMKII‐dependent release of endogenous CGRP induces an increase in acetylcholine quantal size in neuromuscular junctions of mice

OBJECTIVE: The aim of this study was to identify the mechanism responsible for an increase in miniature endplate potentials (MEPPs) amplitude, induced by ryanodine as an agonist of ryanodine receptors in mouse motor nerve terminals. METHODS: Using intracellular microelectrode recordings of MEPPs and evoked endplate potentials (EPPs), the changes in spontaneous and evoked acetylcholine release in motor synapses of mouse diaphragm neuromuscular preparations were studied. RESULTS: Ryanodine (0.1 μM) increased both the amplitudes of MEPPs and EPPs to a similar extent (up to 130% compared to control). The ryanodine effect was prevented by blockage of receptors of calcitonin gene‐related peptide (CGRP) by a truncated peptide CGRP (8‐37). Endogenous CGRP is stored in large dense‐core vesicles in motor nerve terminals and may be released as a co‐transmitter. The ryanodine‐induced increase in MEPPs amplitude may be fully prevented by inhibition of vesicular acetylcholine transporter by vesamicol or by blocking the activity of protein kinase A with H‐89, suggesting that endogenous CGRP is released in response to the activation of ryanodine receptors. Activation of CGRP receptors can, in turn, upregulate the loading of acetylcholine into synaptic vesicles, which will increase the quantal size. This new feature of endogenous CGRP activity looks similar to recently described action of exogenous CGRP in motor synapses of mice. The ryanodine effect was prevented by inhibitors of Ca/Calmodulin‐dependent kinase II (CaMKII) KN‐62 or KN‐93. Inhibition of CaMKII did not prevent the increase in MEPPs amplitude, which was caused by exogenous CGRP. CONCLUSIONS: We propose that the activity of presynaptic CaMKII is necessary for the ryanodine‐stimulated release of endogenous CGRP from motor nerve terminals, but CaMKII does not participate in signaling downstream the activation of CGRP‐receptors followed by quantal size increase.