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Neurosecretory cells in the caudal spinal cord of fishes were first discovered in elasmobranches. However, most of the subsequent work on the caudal neurosecretory system has emphasized its morphology and function in teleosts. Two major peptides, urotensins I and II (UI and UII), have been isolated from this system in teleosts and their amino acid sequences have been determined. We have used immunohistochemical techniques to confirm and expand previous morphological and pharmacological findings regarding the phylogenetic occurrence and the anatomical organization of the caudal neurosecretory system and to localize putative neurohormonal products within its structural elements. UII-immunoreactive neuronal structures were found in the holocephalan, Hydrolagus collei; the elasmobranches, Squalus acanthias, Dasyatis sabina, and Raja binoculata; the dipnoan, Protopterus annectens; the brachiopterygian, Erpetoichthys calabaricus; the chondrostean, Polyodon spathula; and the holosteans Lepisosteus platyrhinchus and Amia calva. UI/corticotropin-releasing-factor immunoreactive elements were detected in the caudal spinal cords of Petromyzon marinus, Raja binoculata, Polydon spathula, Lepisosteus platyrhinchus, L. osseus, L. platostomus, and Amia calva. In addition, in several of these species, immunohistochemical techniques have provided evidence for serotoninergic input to the caudal neurosecretory system.

Urotensin I (UI), one of the biologically active peptides isolated from the caudal spinal cord and urophysis of fish, exhibits a strong sequence homology with mammalian corticotropin-releasing factor (CRF). We have applied an antiserum to ovine CRF that cross-reacts completely with UI to sections taken from the caudal spinal cord and urophysis of the channel catfish (Ictalurus punctatus). Using the indirect immunofluorescence technique, the presence of striking Ul-like immunoreactivity was observed within discrete neuronal structures. UI immunoreactivity was found within large-diameter neuronal perikarya in spinal cord segments immediately rostral to the urophysis, in fascicles of nerve fibers as the urophysis emerges, and in a dense plexus of nerve fibers and terminals which abut capillary loops within the urophysis. The localization of UI immunoreactivity within discrete neurosecretory neurons of this system will make it possible to more fully determine the nature of the regulatory mechanisms controlling its secretion.

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