VAGAL SENSORS IN THE RAT DUODENAL MUCOSA - DISTRIBUTION AND STRUCTUREAS REVEALED BY IN-VIVO DII-TRACING

Results from functional studies point to the importance of chemorecept ive endings in the duodenum innervated by vagal afferents in the regul ation of gastrointestinal functions such as gastric emptying and acid secretion, as well as in the process of satiation. In order to visuali ze the vagal sensory innervation of this gut segment, vagal afferents were selectively labeled in vivo by injecting the lipophilic carbocyan ine dye DiI into either the left or the right nodose ganglion of young adult rats. Thick cryostat sections or whole-mounted peels of muscula ris externa or submucosa of formalin-fixed tissue were analyzed with c onventional and/or confocal microscopy. In the mucosa, many DiI-labele d vagal afferent fibers were found with terminal arborizations mainly between the crypts and the villous lamina propria. In both areas, vaga l terminal branches came in close contact with the basal lamina, but d id not appear to penetrate it so as to make direct contact with epithe lial cells. Labeled vagal afferent fibers in the villous and cryptic l amina propria were found to be in intimate anatomical contact with fib rocyte-like cells that may belong to the class of interstitial cells o f Cajal, and with small granular cells that might be granulocytes or h istiocytes. Although our analysis was not quantitative, and considerin g that labeling was unilateral and not complete, it appears that the o verall density of vagal afferent mucosal innervation was variable; man y villi showed no evidence for innervation while other areas had quite dense networks of arborizing terminal fibers in several neighboring v illi. Analysis of separate whole-mounted muscularis externa and submuc osa peels revealed the presence of large bundles of Labeled afferent f ibers running within the myenteric plexus along the mesenteric attachm ent primarily in an aboral direction, with individual fibers turning t owards the antimesenteric pole, and either penetrating into the submuc osa or forming the characteristic intraganglionic laminar endings (IGL Es). Although the possibility of individual fibers issuing collaterals to myenteric IGLEs and at the same time to mucosal terminals was not demonstrated, it cannot be ruled out. These anatomical findings are di scussed in the context of absorptive mechanisms for the different macr onutrients and the implication of enteroendocrine cells such as CCK-co ntaining cells that may function as intestinal ''taste cells''.