LOCALIZATION OF NITRIC-OXIDE SYNTHASE IN CANINE ILEOCOLONIC AND PYLORIC SPHINCTERS

The distribution of neurons containing NADPH-diaphorase (NADPH-d) acti vity and nitric oxide synthase-like immunoreactivity (NOS-LI) in the c anine pyloric and ileocolonic sphincters was studied. Cells within the myenteric and submucosal ganglia were positive for NADPH-d. These cel ls generally had the morphology of Dogiel type-I enteric neurons, howe ver, there was some diversity in the morphology of NADPH-d-positive ne urons in the myenteric plexus of the pylorus. Intramuscular ganglia we re observed in both sphincters, and NADPH-d was found in a sub-populat ion of neurons within these ganglia. Dual staining with an antiserum r aised against nitric oxide synthase (NOS) demonstrated that almost all cells with NOS-LI were also NADPH-d positive. Varicose fibers within ganglia and within the circular and longitudinal muscle layers also po ssed NOS-LI and NADPH-d activity. Dual staining with anti-VIP antibodi es showed that some of the NADPH-d-positive cells in the myenteric and submucosal ganglia also contained VIP-LI, but all VIP-LI-positive cel ls did not express NADPH-d activity. These data are consistent with re cent physiological studies suggesting that nitric oxide serves as an i nhibitory neurotransmitter in the pyloric and ileocolonic sphincters. The data also suggest that VIP is expressed in a sub-population of NAD PH-d-positive neurons and may therefore act as a co-transmitter in ent eric inhibitory neurotransmission to these specialized muscular region s.