Pituitary levels of three forms of GnRH in the male European sea bass (Dicentrarchus labrax, L.) during sex differentiation and first spawning season

In the present study, levels of three GnRH forms [seabream GnRH (sbGnRH), c hicken GnRH-II (cGnRH-II), and salmon GnRH (sGnRH)] were analyzed in the pi tuitary of male sea bass during sex differentiation and the first spawning season. Plasma levels of gonadotropin (GTH-2), testosterone (T), and 11-ket otestosterone (11-KT) were determined during the same periods. All GnRH for ms were present in the pituitary. sbGnRH levels were 9-fold higher than cGn RH-II and 17-fold higher than sGnRH levels. The highest GnRHs levels were d etected in November 1995, when fish were 9 months old and when the gonads s tarted to differentiate. Levels of the three forms decreased and remained l ow during the first spawning season, with the exception of sbGnRH, which sh owed a significant increase in November 1996. Plasma GTH-2 levels were lowe st in November 1995, later increasing 2.5 times during the next months. Dur ing the first spawning season, plasma GTH-2 levels peaked in December 1996, 1 month after the peak of sbGnRH. During sex differentiation, plasma T lev els were high in November 1995 but decreased over the next months, while le vels of 11-KT remained low and unchanged. During the first spawning season, both steroids peaked in January 1997. These results suggest a possible rol e for all three GnRH forms in achieving gonadal differentiation, while sbGn RH may be the most relevant form in the regulation of the first spawning se ason in male sea bass. Moreover, GTH-2 and 11-KT may play important roles i n gonadal maturation, since plasma GTH-2 and 11-KT levels were high through out the period of spermiation. (C) 2000 Academic Press.