Identification of vagal efferent fibers and putative target neurons in the enteric nervous system of the rat.

The stomach and small intestine receive an efferent innervation from the dorsal motor nucleus of the vagus (DMX). The current experiments were undertaken as a partial test of the hypothesis that the CNS innervates only a small number of command neurons in a restricted number of enteric ganglia. The anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) was injected into the DMX by iontophoresis, and 10-21 days later PHA-L was visualized in the bowel by immunofluorescence. Varicose vagal efferent fibers, labeled by PHA-L, were found in the myenteric plexus as far distally as the ileo-colic junction. PHA-L-labeled varicose axons were rare in comparison to nonlabeled fibers, entered a minority of myenteric ganglia, and contacted a small proportion of the neurons. Ganglia thus innervated by vagal efferent fibers were more numerous in the stomach than in the small intestine. Within the stomach, these ganglia were common in the antrum than in the corpus and none were found in the wall of the rumen. Innervated ganglia in the small intestine became progressively more sparse distally. No PHA-L-labeled axons were observed in the submucosal plexus, thus raising the possibility that vagal modulation of secretomotor responses involves an intermediate synapse in the myenteric plexus. Nonvaricose bundles of PHA-L-labeled fibers were also observed. These bundles appeared to utilize the connectives of the myenteric plexus as a pathway within which to descend within the bowel. Vagal efferent bundles were found to pass through the pyloric sphincter to enter the small intestine from the stomach; thus vagal fibers can reach the distal intestine by an intraenteric route that is not lesioned by crushing mesenteric nerves. The existence of this pathway affects the interpretation of experiments seeking to utilize such lesions to distinguish intrinsic from extrinsic neurites. Possible target neurons of the vagal efferent innervation were identified by simultaneously demonstrating the immunoreactivities of 5-hydroxytryptamine (5-HT), vasoactive intestinal polypeptide (VIP), enkephalin (ENK), galanin (GAL), and tyrosine hydroxylase (TH) along with that of PHA-L. Vagal terminals in the myenteric plexus appeared selectively to contact 5-HT- and, to a significantly lesser extent, VIP-, but not ENK- or GAL-immunoreactive neurons. Apparent vagal innervation of 5-HT-immunoreactive neurons was significantly more common in the duodenum, where a majority of the 5-HT-immunoreactive cells were encircled by varicose PHA-L-labeled axons, than in the stomach.(ABSTRACT TRUNCATED AT 400 WORDS)