SOME ASPECTS OF OOCYTE MATURATION IN CATFISH

Plasma levels of cortisol, testosterone and oestradiol increase in the gravid catfish and reach peak values at 15, 90 and 120min, respectively, following administration of ovine luteinizing hormone (LH) or partially-purified salmon gonadotropin (SG-G100). Of the three steroids, cortisol is of interrenal origin, whereas testosterone and oestradiol are contributed by the ovaries. Further, the ovarian tissue of the gravid catfish, under the influence of gonadotropin, can also synthesize pregnanolone (3α-hydroxy-5β-pregnan-20-one). These findings suggest that the injected gonadotropin stimulates the interrenal to produce cortisol and also promotes steroidogenesis in the ovary of the gravid catfish. Experiments were designed to evaluate the interactions between gonadotropin, cortisol and ovarian steroids during oocyte maturation. In vitro culture of catfish oocytes with cortisol acetate or LH alone and in various combinations revealed that not only is cortisol acetate a much more potent maturation-inducing agent than LH but that the two hormones act synergistically over a wide range of dosages. Prior exposure of oocytes to LH or pregnanolone for as short a duration as 90 min sensitized them to subsequent cortisol action on maturation, thereby indicating that possibly LH sensitizes oocytes through the formation of pregnanolone. Addition of testosterone or oestradiol to the culture medium simultaneously with cortisol, LH or pregnanolone markedly inhibited oocyte maturation. On the basis of our data a new hypothesis has been proposed to explain the hormonal regulation of oocyte maturation in the catfish. The gonadotropin injected to induce ovulation, or released spontaneously in the gravid catfish at the time of ovulation, acts at two sites, the ovarian tissue and the interrenal; the former produces pregnanolone, testosterone and oestradiol, while the latter produces cortisol. Pregnanolone sensitizes oocytes so that the maturation response to cortisol is enhanced. The significance of the increase in the plasma levels of testosterone and oestradiol is not properly understood. Further work will be necessary to elucidate this problem. © 1979.