Projections of enteric peptide-containing neurons in the rat

In order to understand better the neuronal circuitry involved in the regulation of gut functions, we have studied the distribution and projections of those enteric neurons in the rat intestines that contain vasoactive intestinal peptide (VIP), neuropeptide Y (NPY), galanin, substance P (SP), calcitonin gene-related peptide (CGRP), gastrin releasing peptide (GRP), somatostatin and enkephalin. The origin of the peptide-containing nerve fibers was examined by immunocytochemistry after extrinsic denervation. Most of them were found to be intramural in origin, each population displaying its own characteristic topographic distribution. The projections of each neuronal population were studied immunocytochemically by examining the subsequent axonal degeneration after local severing of nervous pathways. This study revealed that myenteric neurons issue predominantly descending projections to other myenteric ganglia and to the muscle layers. Submucous neurons project to other submucous ganglia and to the mucosa and submucosa. Most of these neurons issue both ascending and descending projections. The projection distances varied considerably between the different neuronal populations, the majority being in the range of 4-10 mm. Myenteric GRP and galanin neurons in the small intestine issued the longest projections, 20 and 15 mm, respectively. The shortest projections were those issued from myenteric VIP/NPY neurons and submucosal galanin and GRP neurons in the small intestine and from submucosal VIP neurons in the large intestine (2 mm in length). On the whole our results on the projections of enteric neurons in the rat agree with observations in the guinea pig and dog. However, there are species differences that remain to be explained.