CGRP in Brain Stem Motoneurons

Immunohistochemistry and in sit14 hybridization have shown that calcitonin gene-related peptide (CGRP) is present in various groups of cranial and spinal motone~rons , ' -~ yet it is not contained in all motoneurons. It is totally absent from motor nuclei comprising exclusively preganglionic parasympathetic motoneurons, such as the dorsal motor vagal nucleus (DMVN); and in the nucleus ambiguus, some vagal motoneurons contain CGRP-immunoreactivity (-IR), but others do In a recent study on the nucleus ambiguus of the cat we found that CGRP-IR was absent from vagal preganglionic motoneurons projecting to the thorax and abdomen but was present in vagal motoneurons projecting to striated muscle of the larynx and pharynx.' Our hypothesis to explain the differential distribution of CGRP in motoneurons is that CGRP gene expression is in part dependent on influences from the innervated target, possibly via a retrogradely transported signal. This is in accord with evidence showing that the appearance of CGRP-IR in rnotoneurons coincides with the development of functional contacts between motor axons and striated m ~ s c l e , ~ and that peripheral nerve section leads to a transient increase in motoneuron CGRP-IR.' It is possible that motoneuronal expression of CGRP IS "switched on" directly by a signal from striated muscle fibers. To investigate this, we cross-anastomosed the hypoglossal and cervical vagal nerves in adult rats (n = 9). so that they grew to innervate each other's target tissues, and then, once reinnervation was established (18-24 weeks), we investigated the CGRP-IR of the two groups of motoneurons. The two nerves were chosen because: ( I ) virtually all hypoglossal motoneurons contain CGRP-IR, but motoneurons in the nearby DMVN do ( 2 ) the nerves are anatomically adjacent in the neck and can be accessed via a small midline incision; (3) no afferent fibers from the target organs connect monosynaptically with the motoneurons of these nerves; therefore, CGRP expression cannot depend on signals from the target conveyed via the activity of muscle afferents. It was first necessary to ascertain if the two nerves had grown to provide functional innervation of their foreign targets. Innervation of the tongue by vagal motoneurons in all operated animals was demonstrated by recording muscular contraction on nerve stimulation. That these motor axons now originate from motoneurons in the DMVN was confirmed by injecting a neuronal tracer, horseradish peroxidase (HRP), into the tongue. Very few HRP-labeled motoneurons were found in the hypoglossal nucleus on the operated side, but numerous HRP-labeled cells were now present in the DMVN on this side (FIG. Ic). Successful innervation