In situ generation of nitric oxide by myenteric neurons but not by mononuclear cells of the human colon

1. Production of nitric oxide (NO) is implicated in the pathogenesis of inf lammatory bo bowel disease, However, the cells responsible for the producti on of NO in situ in the human colon remain unknown, 2. Surgical samples from 12 patients with ulcerative colitis, eight patient s with Crohn's disease and 10 controls were studied. Possible generation of NO was visualized by reduced nicotinamide adenine dinucleotide phosphate ( NADPH) diaphorase activity in human colon. Immunohistological staining for various NO synthase (NOS) isoforms (endothelial, neuronal and inducible), n itrotyrosine and interleukin-2 was also performed. 3. Reduced NADPH diaphorase activity was not found in lamina propria mononu clear cells, but was found in colonic epithelium, endothelium and myenteric neurons and their processes, 4. The NADPH-diaphorase activity positive processes were significantly less common in colon from patients with Crohn's disease compared with control c olon. 5. Endothelial NOS was constitutively expressed an colonic endothelium, 6. Neuronal NOS was constitutively expressed on myenteric neurons. 7. Expression of inducible NOS (iNOS) was increased in the epithelium and e ndothelium of the colon of patients with ulcerative colitis. 8. No correlation was found between expression of iNOS and NADPH diaphorase activity. 9. Nitrotyrosine was expressed by lamina propria leucocytes, but not by epi thelium, 10. Interleukin-2 was expressed on both leucocytes and myenteric neurons. 11. Colonic epithelium, endothelium and myenteric neurons synthesize NO. My enteric neurons were principally responsible for NO production and NO may a ct as a neurotransmitter in the enteric nervous system.