Localization of NADPH-diaphorase and neuronal NO-synthase in the digestive tract of the Masu salmon, Oncorhynchus masou (Osteichthyes: Salmonidae)

The morphology, localization, and distribution of nitrergic neurons in the digestive tract of juvenile and adult Masu salmon, Oncorhynchus masou, were studied using histochemical and immunohistochemical methods. We revealed general and age-related peculiarities in the distribution and number of NO-ergic enteric neurons. These neurons were detected in juveniles only in the esophagus and the stomach, while in the adult fish, the neurons were present throughout the gastrointestinal tract. The total number of labeled cells was considerably lower in the proximal intestine than in the esophagus and the stomach, but was some-what greater in the distal intestine. Both in juveniles and adults, neurons and nerve fibers were located within the branches of the vagus nerve, in the myenteric nervous plexus and the circular muscle, and, more rarely, in the submucous nervous plexus and in the longitudinal muscle. Unipolar and bipolar neurons and a large number of undifferentiated and subdifferentiated cells were found, with the latter being more numerous in juveniles. Among multipolar cells, the neurons of Dogiel’s morphological types I and II occurred and they were more abundant in adult fish. Moreover, positive nerve fibers were present in the muscle wall of numerous blood vessels and the swim bladder ducts of juvenile and adult fish.