Projections of nitric oxide synthase- and peptide-containing neurons in the small and large intestines of the toad (Bufo marinus)

Abstract

The projections of galanin (GAL)- and vasoactive intestinal peptide (VIP)-immunoreactive (IR) and nitric oxide synthase (NOS)-containing neurons in the small and large intestines of the amphibian Bufo marinus were investigated by their reactions to surgical interruption (myotomy). In the small intestine, myotomy caused accumulation of GAL- and VIP-IR and of NADPH diaphorase reaction product (revealing NOS) in cut axons on the oral side of the operation site. On the anal side there was loss of GAL-IR axons from the circular muscle and myenteric plexus and long, anally directed processes could be traced from GAL-IR nerve cell bodies. There was no significant loss of VIP-IR or NADPH diaphorase from nerve fibres in the myenteric plexus or circular muscle layer, although anally-directed axons could be traced from nerve cell bodies on the anal side of the operation sites. In the large intestine, myotomy caused accumulation of VIP-IR and of NADPH diaphorase reaction product in cut axons on the oral side of the operation site. Anal to the cut, although there was no significant loss of these fibres from the muscle or myenteric plexus, anally directed axons could be traced from nerve cell bodies. GAL-IR fibres in the large intestine are of two types: a few contain GAL-IR alone and are thought to arise from enteric neurons; many contain both GAL- and SOM-IR and are thought to arise from nerve cell bodies in the hindgut. Myotomy caused an accumulation of GAL/SOM-IR material in fibres on the anal side of the cut and a substantial decrease in the number of fibres on the oral side. There was no detectable effect of myotomy on the GAL-IR fibres, although an abnormally high density of GAL-IR nerve cell bodies was found oral to the cut. These results indicate that VIP-IR and NOS-containing enteric neurons project in an oral to anal direction in the toad small and large intestines. Some of the neurons have short anal projections to the circular muscle. GAL-IR enteric neurons have similar projections in the small intestine, but their projections could not be determined in the large intestine. GAL/SOM-IR axons in the large intestine project from anal to oral. Myotomy in the large intestine appears to induce an increased or de novo expression of GAL-IR in enteric neurons oral to the cut.