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Structures Immunoreactive with Porcine NPY in the Caudal Neurosecretory System of Several Fishes and Cyclostomes

Abstract Using a double immunostaining technique, we examined the relationships between neuropeptide Y (NPY)- and urotensin I (UI)- and/or urotensin II (UII)-positive structures in the caudal spinal cord of various species of fishes, particularly in the teleost Oncorhynchus masou and in elasmobranchs Scyliorhinus torazame, Triakis scyllium, and Raja kenojei. Primitive actinopterygians, i.e., Acipenser transmontanus, Polypterus senegalus, and Lepisosteus productus, and cyclostomes Lethenteron japonica and Eptatretus atami were also studied from the viewpoint of comparative endocrinology. In the teleost and elasmobranchs, NPY-positive fibers were demonstrated to be often in contact with UI- and/or UII-positive cells and fibers, suggesting a control of the caudal neurosecretory neurons by NPY or a related substance. Similar association was seen in the primitive actinopterygians. As to the cyclostomes, immunoreactivities for NPY and UII were found in the lamprey, but none of the immunoreactivity for these peptides was seen in the hagfish. In the lamprey, NPY-positive varicose fibers abutting on the UII-positive cells were also demonstrated.