Changes in brainstem calcitonin gene-related peptide after VIIth and VIIIth cranial nerve lesions in guinea pig

Abstract

The present study investigated the effect of seventh and eighth cranial nerve lesions on the prominence of calcitonin gene-related peptide in the hypoglossal (XII), facial (VII), abducens (VI), and oculomotor (III) cranial nerve nuclei. Guinea pigs were anesthetized and subjected to unilateral cochlear removal, vestibular end organ ablation, and seventh nerve transection. After a survival period ranging from 4 h to 5 days, each animal was anesthetized and perfused intracardially. Frozen sections were collected through the brainstem and stained immunohistochemically for calcitonin gene-related peptide using a polyclonal antibody with the Vectastain ABC kit and protocol. Positive cells were counted in each nucleus bilaterally and analyzed for side to side differences. Nuclei XII and III showed no significant difference in the numbers of cells staining positively for calcitonin gene-related peptide between the ipsilateral and the contralateral sides to the lesion. However, nuclei VII and VI showed elevated numbers ipsilateral to the lesion on some days, but not all. For VII, there was no significant difference before 24 h, but there were significant differences 1–5 days after the lesion. Similarly, in VI, there was no difference before 24 h, but differences were significant beginning with day 1 and continuing through day 3, and finally disappearing by day 4. Changes in the numbers of CGRP positive cells in VII measurable 24 h after the lesion and continuing for at least 5 days afterward indicate a central nervous system retrograde response to peripheral motor nerve injury. However, since no peripheral damage occurred to any structure other than those related to VII and VIII, increased numbers of calcitonin gene-related peptide positive cells in VI indicates the presence of a separate mediating mechanism. We believe this increase may be due, not to the direct loss of a peripheral nerve as in the case of VII, but instead, to an indirect motor stimulation of the eye muscles (indicated by nystagmus) that accompanies unilateral vestibular damage. Thus, while the central CGRP response in VII is activated by a retrograde neuronal mechanism, the central calcitonin gene-related peptide response in VI is activated by an anterorade transsynaptic neuronal mechanism.