Distribution of substance P-like immunoreactivity in the enteric neurons of the large colon of normal and amitraz-treated ponies: an immunocytochemical study.

The distribution of the putative motor excitatory neurotransmitter, substance P, was studied immunocytochemically in the left dorsal colon of four normal control ponies and three ponies with amitraz-induced impaction colic. Substance P-like immunoreactivity in the control ponies was observed in nerve fibres in all layers of the bowel wall and in the nerve cell bodies of the enteric ganglia. The substance P-like immunoreactivity was clearly more intense in the cell bodies of submucosal ganglia than in those of the myenteric ganglia. The internodal nerve strands of the myenteric plexus were very rich in substance P-like immunoreactivity and within the ganglia they formed dense varicose networks around the neuronal cell bodies. Nerve bundles rich in substance P-like immunoreactivity diverged inward from the myenteric plexus to contribute an abundance of varicose immunoreactive fibres to the circular muscle of the tunica muscularis. Nerve fascicles with substance P-like immunoreactivity were sparse in the longitudinal muscle except in the thickened taenial band. In the submucosa many of the nerve fibres with substance P-like immunoreactivity appeared to arise from ganglionic cell bodies. Immunoreactive fibres commonly condensed around arterial vessels in the submucosa. Fine immunoreactive nerve fascicles from the submucosal plexus also projected internally to supply the muscularis mucosae and form periglandular arrays in the lamina propria. The distribution of substance P-like immunoreactivity in the normal equine colon differed in some respects from patterns observed in large intestines of other mammals. When the colons of normal and amitraz-treated ponies were compared no differences were discerned in the distribution or intensity of substance P-like reactivity.