Sequential Changes in Urotensin Immunoreactivity Patterns in the Trout, Oncorhynchus mykiss, Caudal Neurosecretory System in Response to Seawater Challenge

Document Type


Publication Date


Publication Title

Zoological Science




We have been investigating the possible relationship between the teleost caudal neurosecretory system and osmoregulation, by comparing immunostaining intensities of the caudal neuropeptides, urotensins I (UI) and II (UII), in fish sequentially following transfer to different water salinities. Freshwater trout (Oncorhynchus mykiss) were transferred from fresh water (FW) to new FW and from FW to 100% seawater (SW). After 2, 10 and 48 hr posterior spinal cords were removed, fixed and double sequentially immunostained. The 2 hr SW urophyses exhibited more UII and less UI intensity than FW ones. Perikarya anterior to the SW urophyses had less UII and more UI intensity. The 10 hr SW spinal cords showed lower intensity of UI and UII in urophyses and higher intensity of both in anterior perikarya than FW spinal cords. The 48 hr spinal cords did not show any difference in intensity for either UI or UII. We conclude that UI and UII are differentially regulated, that urophysial UI release and perikaryal synthesis are stimulated 2 and 10 hr following transfer to seawater, and that there is an initial inhibition followed within 10 hr by a stimulation of urophysial UII release and perikaryal synthesis following transfer to seawater. By 48 hr the caudal neurosecretory response to SW challenge appears to have subsided, and we hypothesize that the caudal system's role in osmoregulation may be only acute (i.e. within 48 hr following a challenge).