Analysis of calcitonin gene-related peptide (CGRP)-containing nerve fibres in the rat spinal cord using light and electron microscopy

Calcitonin gene-related peptide (CGRP) content in areas surrounding the central canals of cervical, thoracic, lumbar and sacral spinal cords of rats were investigated by light microscopy, conventional transmission electron microscopy (CTEM) (100-200 kV) and high-voltage transmission electron microscopy (HVTEM) (1000 kV) using immunocytochemistry. Tissues were examined using either the peroxidase-antiperoxidase technique or an immuno-cryoultramicrotomy technique. Light microscopically, more intense CGRP localization was observed ventral to the central canals in the lumbar and sacral cord compared with the dorsal area in the same regions. HVTEM revealed that high levels of CGRP labelling were found adjacent to the basal side of ependymal cells ventral to the central canal. Analysis using CTEM operated at 200 kV demonstrated that the CGRP immunoreactivity was present within unmyelinated nerve fibres in the vicinity of the basal side of ependymal cells ventral to the central canal. Immuno-cryoultramicrotomy revealed that immuno-gold particles indicating CGRP labelling were localized on vesicle-like electron-dense bodies in unmyelinated and some fine caliber myelinated nerve fibres. Existence of CGRP as a primary afferent marker in the area surrounding the central canal suggest that there may be an intimate relationship between the modulation of nociceptive information and the area surrounding the central canal in rat spinal cord.