Progestins and cortisol delay while estradiol-17β induces early parturition in the guppy, Poecilia reticulata

Abstract

Fertilization and gestation are intrafollicular in the guppy ( Poecilia reticulata), and ovulation occurs at the end of gestation prior to parturition. In this study, the effects in vivo of the ovarian steroids, progesterone, 17α,20β-dihydroxy-4-pregnen-3-one (17α,20β-P), cortisol and estradiol-17β, the antiprogestin RU 486, and aromatase inhibitor, 4-hydroxyandrost-4-ene-3,17-dione (4-HAD), on gestation and parturition were studied in the guppy. Progesterone (0.05 and 0.10 μg/ml of water), 17α,20β-P (0.01 μg/ml and greater), cortisol (0.10 μg/ml) and 4-HAD (0.10 μg/ml) all prolonged gestation presumably by inhibiting ovulation. 17α,20β-P was most effective in inhibiting ovulation and parturition for up to 36 days postpartum. This inhibition was reversed when fish were transferred to steroidfree water. Besides extending gestation, 17α,20β-P and 4-HAD also inhibited development of vitellogenic oocytes. Estradiol-17β (0.05 and 0.10 μg/ml) and RU 486 (10 μg/g body weight) both induced premature parturition presumably by accelerating onset of ovulation. These results, together with our previous observations on the steroid profile in the guppy, strongly suggest roles for estradiol-17β and cortisol in regulating ovulation and parturition.