Localization of nitric oxide synthase in enteric neurons of the porcine and human ileocaecal junction.

Recent studies, using pharmacological or indirect morphological techniques, suggest that the non-adrenergic non-cholinergic (NANC) control of the ileocaecal junction (ICJ) is largely regulated by nitric oxide (NO). In this study, NO synthase (NOS) has been localized immunocytochemically and enzyme histochemically, using NADPH-diaphorase (NADPH-d), in enteric neurons of the myenteric and submucous plexuses of the ICJ of man and pig. The myenteric plexus, as well as the outer submucous plexus of both the porcine and the human ICJ, harboured NOS-containing neurons, which varied widely in size and shape, but which all displayed a multidendritic, uniaxonal appearance. Compared to the myenteric plexus, significantly fewer NOS-containing neurons were encountered in the outer submucous plexus. Neurofilament immunohistochemistry following NADPH-d application made it possible to distinguish a variety of cells that stained for both markers. Some of the larger neurons were of the Dogiel type-I morphology, whereas others showed a type III or a type VI-like morphology. A large number of NOS-immunoreactive nerve fibers were detected in the enlarged circular muscle of the ICJ and in the adjacent ileum. No NOS staining was detected in the smooth muscle cells of the outer circular or longitudinal muscle layer. The latter finding, together with the abundance of NOS positive nerve fibers in the smooth muscle layer, suggest a neuronal origin for NO as an important inhibitory neurotransmitter in the ICJ.