PREOVULATORY CHANGES IN THE LEVELS OF 3 GONADOTROPIN-RELEASING HORMONE-ENCODING MESSENGER RIBONUCLEIC-ACIDS (MESSENGER-RNAS), GONADOTROPIN BETA-SUBUNIT MESSENGER-RNAS, PLASMA GONADOTROPIN, AND STEROIDS IN THE FEMALE GILTHEAD SEABREAM, SPARUS-AURATA

Gilthead seabream females undergo daily cycles of final oocyte maturat ion (FOM), ovulation, and spawning throughout their spawning season. F OM consists of lipid droplet and yolk granule coalescence, germinal ve sicle (GV) migration, and GV breakdown. Plasma maturational gonadotrop in (CtH-II) levels fluctuate throughout the day, reaching a peak at 8 h before spawning, when the GV is at the periphery of the oocyte. The preovulatory GtH-II surge is accompanied by an increase in the plasma levels of 17 alpha,20 beta-dihydroxy-4-pregnen-3-one and estradiol, wh ile testosterone and 17 alpha,20 beta,21-trihydroxy-4-pregnen-3-one le vels remain unchanged. Concurrent with the preovulatory GtH-II surge, there is an increase in pituitary GtH-II beta subunit mRNA levels foll owed by an increase in GtH-I beta mRNA levels. Gilthead seabream brain contains three different forms of GnRH: salmon (s)GnRH, seabream (sb) GnRH, and chicken (c)GnRH-II. All three GnRH-encoding mRNAs fluctuate throughout the day, reaching highest levels 8 h before spawning, concu rrent with the preovulatory GtH-II surge. On the basis of these correl ations and of the anatomical organization of the three GnRH systems, i t is hypothesized that in the daily-spawning gilthead seabream females , preovulatory GtH-II secretion, and probably synthesis, are induced b y a surge of sbGnRH secretion. The involvement of the other two GnRH f orms, sGnRH and cGnRH-II, in the control of ovulation and spawning is presumed, on the basis of the elevation of their mRNA levels at the ti me of the preovulatory GtH-II secretion and spawning.