Nitric oxide synthase in neurons of the gastrointestinal tract of an avian species, Coturnix coturnix.

The distribution of reduced nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase staining and its colocalisation with nitric oxide synthase immunoreactivity was examined in neurons of the quail gastrointestinal tract. Immunoreactivity and enzyme activity were found in identical populations of neurons. Nitric oxide synthase activity was present in 30-40% of myenteric nerve cells in each region. Numerous reactive cells were also in the submucous plexus and many positive nerve fibres innervated the muscle. In the small intestine, some of the fibres innervating the circular muscle formed a deep muscular plexus and in the large intestine there was a submuscular plexus. Nerve fibres formed baskets around nerve cells of myenteric and submucous ganglia. Intramural arterioles were innervated, most prominently in the oesophagus and stomach. Very few fibres innervated the mucosa, except for some oesophageal glands and the core of villi in the cloaca. It is concluded that nitric oxide is probably involved in transmission to muscle of the avian intestine, as it is in mammals, and that it is probably also produced at neuroneuronal and neurovascular junctions.