Preferential immunohistochemical localization of vasoactive intestinal polypeptide (VIP) in the sacral spinal cord of the cat: Light and electron microscopic observations

In the present study we have employed immunoperoxidase techniques to investigate the distribution of vasoactive intestinal polypeptide (VIP)-like immunoreactivity in the spinal cord and sensory ganglia of the cat. The spinal distribution of VIP-containing neuronal processes was also compared with that of substance P (SP), somatostatin (SOM), and cholecystokinin-8 (CCK) at lumbar, sacral, and coccygeal levels. At sacral levels, VIP was found to be contained in small and medium-sized primary sensory neurons and in dorsal rootlets. Deafferentation, by either ganglionectomy or dorsal rhizotomy, resulted in a nearly complete loss of VIP immunoreactivity in the spinal cord. The spinal distribution of VIP fibers and terminals was most dense and extensive in sacral segments. Forming a thin shell around the dorsal horn, collaterals, apparently originating from Lissauer's tract, projected either medially or laterally through lamina I. Laterally, many VIP axons terminated in lateral laminae V to VII. Others projected further through the neck of the dorsal horn to medial lamina V and the gray matter near the central canal. Medially, VIP axons descended through lamina I to expand into terminal fields in the posterior commissure and medial lamina V. At the ultrastructural level, VIP-like immunoradioactivity was found in dense core and smaller clear round vesicles. Synaptic connections were infrequently observed but, when encountered, were of the simple axodendritic type. The spinal distribution of VIP-containing fibers was remarkably similar to that reported for pelvic nerve and visceral afferents, both in termination patterns within the spinal gray matter and in localization to the sacral cord. The density of SP-, SOM-, and CCK-containing fibers and terminals was constant at all levels examined (L4 to Co4). In marked contrast, the distribution of VIP fibers, much like that of pelvic nerve afferents, was mostly confined to sacral segments. Thus, although SP, SOM, and CCK may be contained within a population of sacral visceral afferents, they must be common to afferent systems in other segments as well. VIP, however, appears to be preferentially contained within pelvic visceral afferent fibers confined mostly to sacral segments.