Colocalization of NADPH-diaphorase staining and VIP immunoreactivity in neurons in opossum internal anal sphincter.

Nitric oxide and vasoactive intestinal polypeptide (VIP) are important inhibitory neurotransmitters mediating relaxation of the internal anal sphincter. The location and coexistence of these two neurotransmitters in the internal anal sphincter has not been examined. We performed a double-labeling study to examine the coexistence of nitric oxide synthase and VIP in the opossum internal anal sphincter using the NADPH-diaphorase technique which is a histochemical stain for nitric oxide synthase. In perfusion-fixed, frozen-sectioned tissue, VIP-immunoreactive neurons were labeled using immunofluorescence histochemistry. After photographing the VIP-immunoreactive neurons, nitric oxide synthase was labeled using the NADPH-diaphorase technique. Ganglia containing neuronal cell bodies were present in the myenteric plexus for the entire extent of the internal anal sphincter. VIP-immunoreactive and NADPH-diaphorase-positive neurons were present in ganglia in the myenteric as well as the submucosal plexuses. Most of the VIP-immunoreactive neurons were also NADPH-diaphorase positive. VIP and nitric oxide synthase are present and frequently coexist in neurons in the internal anal sphincter of the opossum. These neurons may be an important source of inhibitory innervation mediating the rectoanal reflex-induced relaxation of the sphincter. The demonstration of the coexistence of these two neurotransmitters will be of fundamental importance in unraveling their relationship and interaction in the internal anal sphincter as well as other systems.