CHANGES IN ARGININE VASOTOCIN CONTENT IN THE PITUITARY OF THE MEDAKA (ORYZIAS-LATIPES) DURING OSMOTIC-STRESS

Changes in the arginine vasotocin (AVT) neurons of the medaka, Oryzias latipes, during osmotic stress were studied by means of immunohistochemistry and radioimmunoassay. AVT neurons were identified by their immunoreactivity to anti-arginine vasopressin serum, which crossreacts with AVT. When freshwater (FW)-adapted fish were transferred to seawater (SW), the number of immunoreactive magnocellular neurons decreased, while the cell nuclear size increased. AVT content in the pituitary significantly decreased 2 hr or 1 day after transfer to SW, but returned to approximately the initial level 1 week after transfer. However, when SW-adapted fish were transferred to FW, the number of immunoreactive magnocellular neurons increased from 1 hr after readaptation to FW, but the cell nuclear size failed to show significant changes. AVT content in the pituitary significantly increased 1 or 2 hr after transfer. These results seem to indicate that AVT secretion in the medaka was temporarily accelerated by exposure to SW and inhibited immediately after transfer to FW. © 1991.