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Preovulatory changes in the levels of three
gonadotropin-releasing hormone-encoding messenger ribonucleic acids
(mRNAs), gonadotropin beta-subunit mRNAs, plasma gonadotropin, and
steroids in the female gilthead seabream, Sparus
aurata.
Gothilf Y, Meiri I, Elizur A, Zohar
Y.
Center of Marine Biotechnology, University of Maryland,
Baltimore 21202, USA.
Gilthead seabream females undergo daily
cycles of final oocyte maturation (FOM), ovulation, and spawning
throughout their spawning season. FOM consists of lipid droplet and yolk
granule coalescence, germinal vesicle (GV) migration, and GV breakdown.
Plasma maturational gonadotropin (GtH-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
17alpha,20beta-dihydroxy-4-pregnen-3-one and estradiol, while
testosterone and 17alpha,20beta,21-trihydroxy-4-pregnen-3-one levels
remain unchanged. Concurrent with the preovulatory GtH-II surge, there
is an increase in pituitary GtH-II beta subunit mRNA levels followed by
an increase in GtH-Ibeta 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, concurrent with
the preovulatory GtH-II surge. On the basis of these correlations and of
the anatomical organization of the three GnRH systems, it is
hypothesized that in the daily-spawning gilthead seabream females,
preovulatory GtH-II secretion, and probably synthesis, are induced by a
surge of sbGnRH secretion. The involvement of the other two GnRH forms,
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 time
of the preovulatory GtH-II secretion and spawning.
PMID: 9369182
[PubMed - indexed for MEDLINE]
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