DISTRIBUTION OF NITRIC-OXIDE SYNTHASE CONTAINING NEURONS IN THE RECTAL MYENTERIC PLEXUS AND ANAL-CANAL - MORPHOLOGIC EVIDENCE THAT NITRIC-OXIDE MEDIATES THE RECTOANAL INHIBITORY REFLEX

PURPOSE: Following the demonstration that a novel neurotransmitter, ni tric oxide (NO), is released during neurogenic relaxation of the inter nal anal sphincter in vitro, it has been suggested that NO could media te the rectoanal inhibitory reflex in vivo. The aim of this study was to establish whether the distribution of NO-producing nerves in the an orectum is consistent with this proposed role. METHODS: NO is synthesi zed in neurons which contain the enzyme nitric oxide synthase and thei r presence in the anorectum was determined in tissue obtained from nin e abdominoperineal and three anterior resection specimens in patients undergoing surgery for rectal carcinoma. Cryostat sections were staine dfor nitric oxide synthase immunoreactivity, pan-neuronal/axonal immun oreactivity, and NADPH diaphorase activity. RESULTS: Nitric oxide synt hase immunoreactivity is present in a subpopulation of neurons in rect al myenteric ganglia which also contain NADPH diaphorase activity. Use of the latter histochemical technique enabled the structure and distr ibution of nitric oxide synthase containing neurons to be determined i n whole-mount preparations. Individual neurons have Dogiel type 1 morp hology and are present throughout the rectal myenteric plexus. In the distal rectum, positively stained axons enter shunt fascicles which de scend into the anal canal, where they ramify into and throughout the i nternal anal sphincter. Within the sphincter, positively stained nerve s lie in close proximity to smooth muscle cells. CONCLUSION: These res ults are consistent with the hypothesis that NO is the neurotransmitte r that mediates the rectonal inhibitory reflex.