THE BIPHASIC EFFECT OF PROGESTERONE ON OVULATION IN THE RAT

Abstract

ABSTRACT Injection of 1 or 10 mg progesterone at the 6th–11th hour on the morning of pro-oestrus in 4- and 5-day cyclic rats results in neurohumoral induction of ovulation, which cannot be blocked by pentobarbitone administered at 13.45 h on the same day. Data on hypophysectomy suggest that premature release of ovulation-inducing gonadotrophins occurs after such progesterone injections. After timely pentobarbitone administration, however, induction of premature ovulation is inhibited. Injection of progesterone about 8 hours earlier (on the day before prooestrus at the 22nd hour) inhibits the induction of ovulation which would normally occur during the afternoon at pro-oestrus. Administration of progesterone at the 14th hour on day 3 of the 5-day cycle, advances ovulation by 24 hours, whereas earlier injection of the hormone at the 6th hour inhibits ovulation in most cases. These data suggest a biphasic effect of progesterone on induction of ovulation in the rat, with time characteristics similar to those observed earlier in the rabbit.