Nitrergic innervation of the gut in health and disease.

Abstract

At the outset of this study, anatomical and physiological findings indicated that nitric oxide (NO) was a putative inhibitory transmitter of at least a subpopulation of enteric non-adrenergic, non-cholinergic (NANC) myenteric motor neurons. Biochemical and molecular analyses confirmed that NO synthase, under appropriate conditions of pH and fixation, could be histochemically identified using an NADPH-dependent diaphorase technique. In this work, this histochemical technique was applied to whole mounts and tissue sections of the guinea-pig, rat and human intestine in order to provide a complete report on the distribution of sites of NO synthase activity in these species. NO synthase reactive neural elements were found to occur within the myenteric plexus as well as the submucosa throughout all regions of the intestine. The potential for blood vessels of the gut to synthesize NO was examined using NADPH diaphorase histochemistry and endothelial cell immunohistochemistry in laminar preparations and cryosectioned tissue of the guinea-pig, rat and human intestine. This aspect of the study provided the first evidence to indicate that (1) both the endothelial and vascular smooth muscle cells of the microvessels irrigating the intestinal wall of these species possess NO synthesis potential; (2) high endothelial venules in both the perifollicular area of Peyer's patches and extralymphoid regions of the submucosa and lamina propria also display NO synthase activity and (3) NO synthase activity is consistently and predominantly localized to discrete endothelial cell subcellular patches. The functional significance of the nitrergic innervation in the human intestine was addressed by examining whether this innervation could be distinguished on the basis of existing peptidergic neurons, such as those utilizing neuropeptide Y (NPY). In addition to NO and NPY exerting similar biological actions in the mammalian intestine including modulation of food intake, blood flow, motility, and secretion, these substances coexist in submucosal secretomotor neurons of the rodent intestine. This study examined the relative disposition of elements displaying NPY immunoreactivity and NADPH diaphorase activity in the nerve networks of the infant human colon. Neural elements containing NPY immunoreactivity and NADPH diaphorase activity were identified in the external muscle layers, myenteric plexus, and all nerve layers of the submucosa, including Henle's plexus, the Intermediate nerve layer, and Meissner's plexus. Perivascular NPY-immunoreactive nerve fibers did not contain NO synthase activity. This study also sought to determine whether some populations of nitrergic neurons and GABAergic interneurons represent the same subset of neurons in the human colon by assessing the colocalization of NADPH diaphorase activity in GABAergic nerve cells. (Abstract shortened by UMI.)