Nitric oxide synthase in the enteric nervous system of the rainbow trout, Salmo gairdneri

The distribution of nitric oxide synthase (NOS) was determined in the gastrointestinal tract of the rainbow trout, Salmo gairdneri. NOS immunoreactivity and the NADPH diaphorase histochemical reaction were co-localized in nerve cells in the myenteric ganglia. However, only about 60% of NADPH diaphorase-stained nerve cells in the vagus nerve trunks were immunoreactive for NOS. Reactive myenteric nerve cells were found throughout the gastrointestinal tract, comprising about 10-15% of all nerve cells. Reactive nerve cells and fibres appeared in the myenteric ganglia and nerve trunks. The circular muscle was innervated by reactive nerve fibres throughout the gastrointestinal tract. Some NOS-containing cell bodies were also in this layer. The submucous plexus contained reactive nerve fibres in each region of the gut; in the large intestine a few reactive nerve cell bodies were also seen in this plexus. The muscle in the mucosal folds of the large intestine was densely innervated. The observations suggest that nitric oxide is an enteric transmitter in teleost fish, as it is in mammals.