LIGHT-MICROSCOPIC LOCALIZATION OF CALCITONIN-GENE-RELATED PEPTIDE IN THE NORMAL FELINE TRIGEMINAL SYSTEM AND FOLLOWING RETROGASSERIAN RHIZOTOMY

Calcitonin gene-related peptide (CGRP) is a neuropeptide that has been implicated in the transmission and modulation of primary afferent noc iceptive stimuli. In this study, we describe the light microscopic dis tribution of CGRP immunoreactivity (IR) within the feline trigeminal g anglion and trigeminal nucleus of normal adult subjects and in subject s 10 and 30 days following complete retrogasserian rhizotomy. Within t he trigeminal ganglion of normal subjects, cell bodies and fibers show ed CGRP-IR, whereas immunoreactive fibers were rare in the central roo t region. Within the normal spinal trigeminal and main sensory nuclei, CGRP-IR was seen to form a reproducible pattern that varied between t he different nuclei. Following rhizotomy, most, but not all, of the CG RP-IR was lost from the spinal trigeminal and main sensory nuclei, exc ept in regions where the upper cervical roots and cranial nerves VII, IX, and X project into the trigeminal nucleus. The pattern seen at 10 days contained more CGRP-IR than that seen at 30 days and suggests tha t degenerating fibers still show CGRP-IR. In contrast to the decrease seen in the nuclei after rhizotomy, examination of the central root th at was still attached to the trigeminal ganglion showed an increase in CGRP-IR within fibers, some of which ended in growth conelike enlarge ments. Rhizotomy induced a dramatic increase in CGRP-IR within trigemi nal motoneurons and their fibers, which was strongest 10 days after rh izotomy and weaker at 30 days, which was still stronger than normal. T hese results indicate that the majority of CGRP-IR found in the trigem inal nucleus originates from trigeminal primary afferents and that an upregulation of CGRP-IR occurs in trigeminal motoneurons and in regene rating fibers in the part of the central root that was still attached to the ganglion. In addition, the persistence of CGRP-IR fibers in the trigeminal nucleus provides one possible explanation for the preserva tion of pain in humans following trigeminal rhizotomy. (C) 1996 Wiley- Liss, Inc.