Selective deafferentation of convergent inputs to trigeminal subnucleus caudalis: effects on calcitonin gene-related...

Electrophysiological studies (Sessle, 1987, 1991) suggest that trigeminal deafferenting injuries can cause an "unmasking" of existing but normally suppressed convergent inputs to the spinal trigeminal nucleus, including many that arise from the cervical spinal cord. However, the spatial arrangement of this projection has not been examined, particularly with reference to nociceptive components that might become involved in pathological changes leading to chronic pain. Therefore, the purpose of this study was to apply selective interruptions of the trigeminal and/or cervical primary afferent inputs to the spinal trigeminal subnucleus caudalis (Vc) in the cat, followed by (1) demonstration and quantification of axonal degeneration in the spinal trigeminal tract to determine the extent of trigeminal-cervical primary afferent overlap; and (2) an analysis of lesion-induced alterations in the distribution of calcitonin gene-related peptide immunoreactivity (CGRP-IR) in laminae I and II of Vc, since recent evidence strongly suggests that CGRP is involved in pathophysiological elevations of central nervous system neuronal excitability. Degenerating fibers were found throughout the spinal tract following a trigeminal rhizotomy or tractotomy, with the largest numbers adjacent to the rostral two-thirds of Vc, but with a significant number extending caudally to at least the level of C2. CGRP-IR was reduced or eliminated from the rostral one-third and periobex region of Vc, except for a dorsomedial zone that was minimally affected. Retention of CGRP-IR was greater at more caudal levels. Following a combined trigmeninal and cervical tractotomy, fiber degeneration was massive throughout the spinal tract, yet a population of small myelinated fibers persisted at 60 days after surgery. Concomitantly, CGRP-IR was profoundly reduced throughout Vc, except for a small dorsomedial zone of retention, which became more extensive caudally. A cervical tractotomy resulted in moderate numbers of degenerating fibers adjacent to the caudal one-third of Vc, and this number declined rostrally; however, degenerating fibers could be seen at the level of the obex. CGRP-IR was reduced in the dorsomedial and ventrolateral zones of Vc, particularly in its caudal one-third. Electron-microscopic analysis revealed a population of CGRP-IR boutons, most of which were of the simple axodendritic type with asymmetrical contacts. A few examples of axoaxonic contacts were observed. Loss of labeled boutons observed with the electron microscope was consistent with light-microscopic quantitative results. Those boutons that were retained were variable in size and displayed simple axodendritic contacts.(ABSTRACT TRUNCATED AT 400 WORDS)