Abstract
We have studied the effect of water temperature on brain-pituitary-gonadal axis in male honmoroko, with particular focus on gonadotropin-releasing hormone (GnRH) contents in the brain and pituitary. Mature fish in the spawning season were exposed to long photoperiod (15L9D) combined with high (30°C) or low (15°C) water temperature for a period of 30 days. Salmon GnRH (sGnRH) contents in the brain and pituitary increased and maintained high levels at 15°C coinciding with the high levels of plasma testosterone and an increase in GSI. No marked changes in sGnRH content were observed at 30°C while GSI values and plasma testosterone levels decreased. These results indicate that the activities of GnRH neurons were associated with the gonadal activities. They may not, however, be involved in gonadal regression caused by high water temperature.
