Neurosecretory changes in the hypothalamico-hypophysial system of the rainbow trout (Salmo gairdneri)

Variations in concentration, if any, of pharmacologically active principles of the hypothalamico-neurohypophysial system of the rainbow trout Salmo gairdneri during early periods of transfer to sea water were investigated. The concentrations of oxytocic and antidiuretic principles in the brains and pituitaries of handled fish, fish transferred to sea water, and fish transferred to fresh water, were measured employing the isolated virgin guinea pig uterus for assaying oxytocic activity, and the ethanol anaesthetized water loaded rat for assaying antidiuretic activity. Handling the fish resulted in an increase of oxytocic and antidiuretic activity of pituitary extracts. Transfer of experimental fish to a sea water environment resulted in a transitory increase of oxytocic activity of extracts of the pituitary and hypothalamus for the first and second hours with a subsequent return to control levels. After transfer of the fish to a sea water environment the antidiuretic activity of pituitary extracts was observed to decrease during the first and third hour, with a return to control levels at the sixth hour. This evidence suggests that active principles which are known to play active roles in water balance in animals higher, phylogenetically, than fish, are liberated from the hypothalamico-neurohypophysial system of Salmo gairdneri in response to a hypertonic environment.Science, Faculty ofZoology, Department ofGraduat