POSSIBLE ROLES FOR PROSTAGLANDINS-E2 AND F2-ALPHA IN SEASONAL-CHANGESIN OVARIAN STEROIDOGENESIS IN THE FROG (RANA-ESCULENTA)

Concentrations of PGE2, PGF2alpha, androgens and oestradiol in plasma, and ovary weights were measured in the female frog, Rana esculenta, d uring the annual breeding cycle. Experiments were carried out in vivo to study the effects of PGE2 and PGF2alpha on plasma sex steroids duri ng the following stages: pre-reproduction (April), reproduction (May), post-reproduction (June) and recovery (October). Experiments were per formed in vitro during these stages to evaluate the effects of these t wo prostaglandins on the secretion of ovarian steroids. Concentrations of PGE2 were low in plasma during winter hibernation, the reproductio n and post-reproduction stages, whereas they were high during the pre- reproduction and recovery stages. PGE2 treatment in vivo increased and rogen secretion in April, whereas PGF2alpha treatment increased oestra diol secretion in June and October. In experiments in vitro, PGE2 incr eased androgen secretion and decreased oestradiol secretion from ovari es collected in April, whereas PGF2alpha increased oestradiol secretio n from ovaries collected in October. These results suggest that a seas onal increase in plasma PGE2 may inhibit breeding activity, probably b y stimulating ovarian androgen secretion, whereas, as previously repor ted, a seasonal increase in plasma PGF2alpha may inhibit breeding, by stimulating ovarian oestradiol secretion.