Urotensin II-immunoreactive neurons in the caudal neurosecretory system of freshwater and seawater fish.

Antiserum generated against synthetic urotensin II of the goby, Gillichthys mirabilis, was used to localize urotensin II in the caudal neurosecretory system in six species of freshwater teleosts: Cyprinus carpio, Carassius auratus, Oreochromis mossambicus, Oreochromis niloticus, Salmo gairdneri and Plecoglossus altivelis, and six species of seawater teleosts: Acanthogobius flavimanus, Pagrus major, Parapristipoma trilineatum, Trachurus japonicus, Seriola dumerili and Seriola quinqueradiata. In the carp, urotensin II-immunoreactive perikarya were classified into three groups according to their size and shape. Small cells were located in the spinal cord dorsal to the urophysis, medium-sized cells immediately anterior to the urophysis, and large cells anterior to the medium-sized cells. In each group, a small number of nonreactive cells was found. Urotensin II-immunoreactive nerve fibers extended toward the urophysis and terminated around the blood vessels. Other species of teleosts showed a similar immunoreaction to that observed in the carp. The immunoreaction of the urophysis was stronger in seawater fish than freshwater fish. Urotensin II-immunoreactive elements could not be detected in the brains of the carp, goldfish and goby.