THE BIPHASIC EFFECT OF PROGESTERONE ON OVULATION IN THE RAT

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