Distribution of Calcitonin Gene-related Peptide mRNA and Immunoreactivity in the Rat Central and Peripheral Vestibular System

The distribution of calcitonin gene-related peptide (CGRP) mRNA and immunoreactivity (CGRPi) was examined in sections of brainstem and decalcified temporal bones to study the peptidergic innervation patterns in both the central and peripheral vestibular system. In the brainstem, neuron somata associated with the vestibular system and containing CGRP mRNA and CGRPi were found in both the dorsolateral and medial regions of the group E vestibular efferents as well as scattered throughout the caudal pontine reticular nucleus (CPR). The only labelled neuron somata containing CGRP mRNA and CGRPi in the temporal bone were found in the gustatory geniculate ganglion. Coarse CGRPi axons were traced ventrally from the dorsolateral and medial group E somata and dorsally from the CPR somata. CGRPi axons from the medial group E and CPR crossed the facial nerve ventral to the facial genu before entering the eighth nerve root. Immunoreactive axons projected from medial group E cells, beneath the ependyma of the fourth ventricle, to the contralateral brainstem. Unmyelinated CGRPi fibers passed among the primary afferent fibers in Scarpa's ganglion and continued through the subepithelial regions of the vestibular end-organs. Within the neurosensory epithelia of the maculae and cristae, the CGRPi axons ramified to produce numerous CGRPi terminals. An efferent neuromodulatory role of CGRP in the vestibular system is inferred from the wide distribution of CGRP mRNA and immunoreactivity found within efferent vestibular neurons.