Distribution, morphology and projections of nitrergic and non-nitrergic submucosal neurons in the pig small intestine

Sequential nitric oxide synthase immunohistochemistry and nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) histochemistry in pig small intestinal wholemounts revealed a complete colocalisation of the two nitrergic markers in submucous neurons. The external submucous plexus (ESP) contained nitrergic neurons throughout. In the internal submucous plexus (ISP) we found a moderate number of nitrergic neurons in the duodenum, while they were rare in the jejunum and nearly absent in the ileum. Combined NADPHd histochemistry and silver impregnation showed morphological ESP type III and VI neurons to be NADPHd positive whereas ESP type II, IV and V neurons were NADPHd negative. Axons of ESP type III, IV and VI neurons were often observed to enter interconnecting strands directed abluminally. ESP type II neurons projected mainly to the ISP. In special silver-impregnated wholemounts containing both external muscle layers and the abluminal part of the submucous layer, i.e. the myenteric plexus and the ESP, the great majority of impregnated axons within the interconnecting strands were observed to run between both plexuses and did not enter the circular muscle layer. We conclude that ESP type III and VI neurons are nitrergic while ESP type II, IV and V neurons are non-nitrergic. Furthermore, we assume that ESP type III, IV and VI neurons may represent a submucosal input to the myenteric plexus.