NEUROPEPTIDE DISTRIBUTIONS IN THE COLON, CECUM, AND JEJUNUM OF THE HORSE

The pelvic flexure portion of the equine large colon is the proposed l ocation of a pacemaker mechanism. This study was conducted to ascertai n whether the distribution of certain putative neurotransmitters diffe rs at the pelvic flexure compared to other sampling sites. Tissue samp les were collected from the intestinal tracts of six horses. Serial se ctions from these samples were reacted with primary antisera specific for substance P, vasoactive intestinal polypeptide (VIP), methionine-E nkephalin, and calcitonin gene-related peptide (CGRP). The regional di stribution of immunoreactive neuronal elements was uniform for each of the neuropeptides except VIP. Although neurons exhibiting VIP-like im munoreactivity were abundant throughout the colon, they were somewhat more plentiful near the apex of the pelvic flexure and the left dorsal colon. These neurons may participate in the initiation and propagatio n of the propulsive/retropulsive contraction waves, which emanate from this location and are believed to lend a sphincter-like capacity to t he pelvic flexure. The submucosal plexus was replete with neurons with intense substance P and VIP-like reactivity. Reactive fibers left sub mucosal ganglia to project to the intestinal mucosa, reflecting a poss ible secretogogic role for these neurons. This role may be especially important for the horse as a hindgut fermenter. There were abundant me thionine-Enkephalin and substance P-like reactive varicosities through out the myenteric plexus, many of which established a pericellular ple xus of varicose fibers. The abundance of these varicosities, which may correlate with a high degree of neuronal integration, did not vary re gionally. These data may enhance our understanding of both normal colo nic peristalsis and motility disorders caused by a depletion of these neuropeptides.