Distribution and morphological features of nitrergic neurons in the porcine large intestine.

The distribution of nitric oxide synthase (NOS), an enzyme involved in the synthesis of the presumed non-adrenergic noncholinergic inhibitory neurotransmitter nitric oxide (NO), was demonstrated in the enteric nervous system of the porcine caecum, colon and rectum. Techniques used were NOS-immunocytochemistry and nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd)-histochemistry. Throughout the entire large intestine, NOS-immunoreactive (IR) and NADPHd-positive neurons were abundant in the myenteric and outer submucous plexus. In the inner submucous plexus, only a small number of positive neurons were found in the caecum and colon, while a moderate number was observed in the rectum. The nitrergic neurons in the porcine enteric nerve plexuses were of a range of sizes and shapes, with a small proportion showing immunostaining for vasoactive intestinal polypeptide. Varicose and non-varicose NOS-IR and NADPHd-positive nerve fibres were present in the ganglia and connecting strands of all three plexuses. Nerve fibres were also numerous in the circular muscle layer, scarce in the longitudinal muscle coat and negligible in the mucosal region. The abundance of NOS/NADPHd in the intrinsic innervation of the caecum, colon and rectum of the pig implicates NO as an important neuronal messenger in these regions of the gastrointestinal tract.