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DONALD C. PAUP, Effect of Progesterone and Nembutal in Altering Estrous Running and Vaginal Periodicity in the Female Rat, Endocrinology, Volume 92, Issue 5, 1 May 1973, Pages 1530–1535, https://doi.org/10.1210/endo-92-5-1530
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Hormonal regulation of activity wheel running and vaginal cycles was studied in female rats using agents and methods known to influence estrous receptivity, ovulation, and ovarian hormone levels. Injection of 0.5 mg/100 g bw progesterone on proestrus did not alter running or vaginal periodicity, whereas, when given on diestrus. it delayed both by one day. Progesterone dosages of 2 mg/100 g bw and 4 mg/100 g bw injected on diestrus increased mean delay of vaginal and activity estrus from approximately 1 to 2 and 2.5 days, respectively, and on metestrus, from 0 to 1 and 1.5 days, respectively. Peak running occurred on days of vaginal cornification. Induced delays in estrous activity were present only during the cycle in which progesterone was administered. Advancement of 1 day in estrous activity was obtained by injecting 0.5 mg/100 g bw progesterone on the day before proestrus in 5–day cyclic rats. Nembutal (3.3 mg/100 g bw) was found to delay both peak running and vaginal cornification by 1 day when injected prior to the “critical period” on the afternoon of proestrus. The results are congruent with expectations based upon effects of progesterone and Nembutal on estrous receptivity and ovulation. A model is suggested in which progesterone delays estrous cyclicity through modification of estrogen secretion or utilization (Endocrinology92: 1530, 1973)