Increased urotensin I and II immunoreactivity in the urophysis of Gillichthys mirabilis transferred to low salinity water

Abstract

The function of the fish caudal neurosecretory system is uncertain, but a role in osmoregulation has been suggested by many investigators. Our objective was to determine if acclimation to water of different salinity has an effect on immunoreactive patterns and staining intensities of the two caudal neuropeptides, urotensins I (UI) and II (UII), in Gillichthys mirabilis. Five fish, originally maintained in seawater, were transferred to deionized fresh water (FW), and five were transferred to new seawater (SW). After 24 hr spinal cords were removed and fixed, FW and SW spinal cords were paired in blocks to receive identical treatment, and cryostat sections were double immunostained for both peptides using a double sequential immunofluorescence procedure. FW spinal cords exhibited increased staining intensities for both UI and UII in their urophyses (the neurohemal organ) compared to the SW spinal cords. The magnitude of intensity difference appeared greater for UI than for UII. In addition, the FW urophyses had more loci displaying intense, perivascular UI immunoreactivity than the SW urophyses. Thus, it appears that environmental salinity has an effect on the urophysial content of UI and UII in this euryhaline fish. The increased immunoreactivity in FW fish could reflect increased synthesis and storage, decreased release of the stored peptides, or decreased peptide degradation.