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First published online March 2, 2006
Journal of Experimental Biology 209, 1147-1156 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02094

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Hydration of rainbow trout oocyte during meiotic maturation and in vitro regulation by 17,20ß-dihydroxy-4-pregnen-3-one and cortisol

Sylvain Milla, Bernard Jalabert, Helene Rime, Patrick Prunet and Julien Bobe^*

Institut National de la Recherche Agronomique, INRA-SCRIBE, IFR 140, Campus de Beaulieu, 35000 Rennes, France

^* Author for correspondence (e-mail: Julien.Bobe{at}rennes.inra.fr)

Accepted 12 January 2006

Although oocytes of many teleost fish, especially marine species, are subjected to a hydration process during meiotic maturation, which leads to an important volume increase, no noticeable hydration of the preovulatory oocyte has ever been reported in rainbow trout (Oncorhynchus mykiss). In the present study, oocyte water content and dry mass were monitored using consecutive samples taken in vivo from the same female rainbow trout, from 4–5 days prior to ovulation to up to 7 days post-ovulation. In addition, yolk protein electrophoretic patterns were compared between oocytes sampled prior to germinal vesicle breakdown (GVBD) and unfertilized eggs. Furthermore, the effect of the maturation-inducing steroid (17,20ß-dihydroxy-4-pregnen-3-one, 17,20ß-P), cortisol and 11-deoxycorticosterone (DOC) on oocyte dry and wet masses, as well as GVBD occurrence was assessed in vitro. Finally, mRNA expression profiles of glucocorticoid and mineralocorticoid receptors as well as 11ß-hydroxysteroid dehydrogenase (11ß-HSD) were monitored in the periovulatory ovary by real-time PCR. Both in vivo and in vitro data showed, for the first time in rainbow trout, that a significant oocyte hydration occurs during oocyte maturation. In addition, an intra-oocyte dry matter increase was reported in vivo during the periovulatory period. However, yolk protein migration patterns were similar in preGVBD oocytes and unfertilized eggs, suggesting that no or little yolk proteolysis occurs during oocyte maturation. We also showed that oocyte hydration can be induced in vitro by 17,20ß-P and cortisol but not by DOC. In contrast, GVBD was only observed after 17,20ß-P stimulation. Finally, real-time PCR analysis showed an up-regulation of 11ß-HSD and glucocorticoid receptor 2 transcripts in the ovary at the time of oocyte maturation. Together, these results suggest that cortisol could participate in the control of oocyte hydration and possibly in other periovulatory ovarian functions.

Key words: trout, Oncorhynchus mykiss, oocyte, meiotic maturation, hydration, cortisol