Nitric oxide synthase-containing nerve elements in the pylorus of the cat

Combined nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry, nitric oxide synthase (NOS) and vasoactive intestinal polypeptide (VIP) immunocytochemistry were used to study the distribution of NOS- and VIP-containing nerve elements in the feline pylorus. A large number of stained multipolar neurons was found in the myenteric plexus. However, some NADPH-d and NOS positive neurons were also observed in the submucous plexus and in the internal part of muscular layer. A few stained perikarya were found in the tunica mucosa, in a very close situation to the blood vessels. A large number of thin varicose fibres, with intense reaction for all markers were seen around or in close contact with the unstained perikarya to the blood vessels and some of them around the pyloric glands. The density of NOS and NADPH-d positive nerve elements was much higher than that of VIP-immunoreactive (IR) nerve elements. Our results suggest that nitric oxide (NO) might act as a regulatory neurotransmitter of the pyloric sphincter, blood flow and secretion in this region.