Influence of peripheral targets on the expression of calcitonin gene-related peptide immunoreactivity in rat cranial motoneurones.

Calcitonin gene-related peptide-like immunoreactivity (CGRP-ir) is displayed by motoneurons that innervate striated muscle but is absent from preganglionic parasympathetic motoneurons. One hypothesis to explain this is that CGRP gene expression in motoneurons is, in part, dependent on influences from the innervated organ. To test this hypothesis, we cross-anastomosed the right hypoglossal and cervical vagal nerves of rats so that the vagal motoneurons grew to innervate the musculature of the tongue. Following a recovery period of 17 to 52 weeks, the distribution of CGRP-ir in the dorsal motor vagal nucleus was determined in both cross-anastomosed animals and self-anastomosed control animals. Successful reinnervation of the tongue musculature by vagal motoneurons was demonstrated by showing that electrical stimulation of the central vagus/peripheral hypoglossal nerve produced a twitch of the tongue muscles. Motoneurones of the dorsal motor vagal nucleus, which now innervated the tongue were found to express CGRP-ir, which was evident from the double labeling of neurons with both horseradish peroxidase and CGRP-ir. Motoneurones of the dorsal motor vagal nucleus contralateral to the cross-anastomosis remained CGRP negative. Similarly, motoneurons of the dorsal motor vagal nucleus in control animals where the vagus nerve was self-anastomosed remained CGRP negative, showing that an induction of CGRP expression is not a result of nerve section itself. We suggest that a signal from the striated muscle transported retrogradely via the motor axon regulates expression of CGRP-ir in motoneurons.