Distribution and seasonal variations in levels of three native GnRHs in the brain and pituitary of perciform fish

Specific and sensitive radioimmunoassays (RIAs) were newly developed for tw o types of gonadotropin-releasing hormone (GnRH), namely, seabream (sb) GnR H and chicken (c) GnRH-II. We employed these two RIAs together with a previ ously reported RIA for salmon (s) GnRH to study the presence and regional d istribution of these three GnRHs in the brains and pituitaries of four perc iform fishes (red seabream, Pagrus major; black seabream, Acanthopagrus sch legeli; striped knifejaw, Oplegnathus fasciatus; and Nile tilapia, Oreochro mis niloticus), as well as clarify seasonal changes in levels of these GnRH s in the brain and pituitary of red seabream, All three GnRHs were found in brains of all fishes examined, with regional distributions in the brains o f the three GnRHs being rather similar, sbGnRH was abundant in telencephalo n and hypothalamus, cGnRH-II was concentrated from the middle to posterior part of the brain and distributed throughout the brain. sGnRH was concentra ted in the olfactory bulb and distributed all over the brain, as was cGnRH- II. The dominant form of GnRH in the pituitary was sbGnRH, with levels 500- to 2400-fold higher than those of sGnRH, while cGnRH-II was undetectable i n all four species. In the brain and pituitary of female red seabream, leve ls of both brain and pituitary sbGnRH increased from October (immature phas e) and reached a peak in April (spawning phase), reflecting the increase in gonadosomatic index and vitellogenesis, However, levels of sbGnRH remained high only in the pituitary of completely regressed fish in June. Levels of both sGnRH and cGnRH-II in the brain were higher in the regressed phase an d remained lower during the spawning phase. From these and previous results , it appears that sbGnRH is physiologically the most important form of GnRH in reproduction in red seabream and, probably, in other perciforms also.