Minimal Age for Induction of Ovulation with Progesterone in Rats: Evidence for Neural Control

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Abstract
Injection of progesterone at 10:00 AM on the second day after PMS increased the percentage of rats which ovulated in response to a single injection of 3.75-7.5 IU of PMS given at 16, 18, 20 or 22 days of age. While this effect of progesterone was observed when as much as 15.0 IU of PMS was given to 20-or 22-day-old rats, the maximal dose of PMS at which it occurred in younger rats was 7.5 IU. Evidence obtained from the intravenous injection of 2.0 IU of HCG at 18 days of age (PMS given when 16 days old) suggests that follicular sensitivity to ovulating hormone is lower when high doses of PMS are used than when low doses are given. Neither progesterone nor HCG produced ovulation in rats given PMS at 14 days of age. Phenobarbital prevented progesterone-induced ovulation if given at 1:30 PM on the second day after PMS, but did not if given at 5:30 PM. Such data suggest that ovulation induced by progesterone was caused by a neurally controlled release of pituitary ovulating hormone. If this interpretation is correct, the data offer evidence that neural control of pituitary ovulating hormone release is established at as early an age as the follicles can be “ripened” to such a degree that they will ovulate in response to a release of pituitary ovulating hormone. (Endocrinology74: 793, 1964)