Vasoactive intestinal polypeptide and substance P in primary afferent pathways to the sacral spinal cord of the cat

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Abstract
An analysis of vasoactive intestinal polypeptide immunoreactivity (VIP‐IR) and substance P‐IR in the cat spinal cord has revealed marked differences in the distribution of the two peptides. While substance P‐IR was located at all levels of the cord, VIP‐IR was most prominent in the sacral segments in Lissauer's tract and lamina I on the lateral edge of the dorsal horn. VIP‐IR was also present in the sacral cord in (1) laminae V, VII, and X, (2) a thin band on the medial side of the dorsal horn, (3) the dorsal commissure, (4) the lateral band of the sacral parasympathetic ucleus, and (5) in a few animals in Onuf's nucleus. In other segments of the spinal cord VIP‐IR was much less prominent but was present in Lissauer's tract and laminae I, II, and X. Substance P‐IR was more uniformly distributed at all segmental levels in laminae I–III, V, VII, and X and in the dorsal commissure. In ventrolateral lamina I of the sacral spinal cord both VIP‐IR and substance P‐IR exhibited a distinctive periodic pattern in te rostrocaudal axis. The peptides were associated with bundles of dorsoventrally oriented axons and varicosities spaced at approximately 210‐μm intervals center to center along the length of the spinal cord. The bundles in lamina I continued into lamina V where they further divided into smaller bundles that extended medially through laminae V and VII. The most prominent bundles of VIP axons passed ventrally from lateral laminae V and VII to enter lamina X and the ventral part of te dorsal gray commissure. On the other hand the maority of substance P axons in lamina V turned dorsally to join wth axons on the medial side of the dorsal horn and to pass into the dorsal part of the dorsal gray commissure. Rostrocaudal VIP axons were present not only in Lissauer's tract but also in dorsolateral lamina I, in the lateral funiculus and in the ependymal cell layer of the central canal. Following unilateral transectionofthe sacral dorsal roots (2 weeks–22 months) the density of VIP axons and terminals was markedly reduced in ipsilateral Lissauer's trat and lateral laminae I and V; however, no change was detected in lamina X. Sacral deafferentation reduced substance P‐IR in the dorsal gray commissure and in lateral laminae I and V. It is concluded that VIP‐IR and substance P‐IR are present at all levels of the cat spinal cord, but that VIP‐IR is most prominent in sacral afferent pathways. The distribution of VIP‐IR in spinal cord was very similar to the central projections of sacral visceral afferents. These findings are consistent with data from other experiments indicating that VIP may be a transmitter in visceral afferent pathways.