Abstract
ABSTRACT The time of ovulation in immature rats primed with 3 IU PMSG on day 22 of age was between 1:00 and 4:00 a. m. on day 25 as determined by the presence of ova in the oviducts. When 0.5 mg progesterone was given at 10:00 a. m. on day 24, ovulation was between 1:00 and 2:00 a. m. on day 25. However, ovulation was advanced to 11:00 p. m. of day 24 when 0.5 or 2.0 mg progesterone was injected at 5:30 a. m. on day 24. In animals pretreated with 12 IU PMSG. ovulation occurred between 10:00 p. m. of day 24 and 2:00 a. m. of day 25 when the same steroid (2.0 mg) was given at 5:30 a. m. on day 24. Ovulation was inhibited with 4.5 mg phenobarbital injected at 10:30 a. m. or 11:00 a. m. on day 24 in progesterone-injected rats which had been pretreated with 3 or 12 IU PMSG, respectively, indicating that the critical period has been shifted forward by 3 hours. The highest plasma LH occurred at 3:00 and 8:00 p. m. on day 24 in rats primed with 3 IU PMSG alone, whereas the highest plasma LH in rats injected with 3 IU PMSG and progesterone occurred at 10:00 a. m. and 3:00 p. m. of day 24, as determined by a modified method of the ovarian ascorbic acid depletion (OAAD) assay. It is concluded that progesterone advanced the time of ovulation by shifting forward the critical period and causing an adequate release of ovulatory gonadotrophins between 10:00 a. m. and 3:00 p. m. on the day preceding ovulation.
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