STIMULATORY AND INHIBITORY EFFECTS OF PROGESTERONE ON THE RELEASE OF PITUITARY OVULATING HORMONE IN THE RABBIT1

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Abstract
A study of the effects of progesterone on ovulation in the rabbit reveals that the steroid at first facilitates and subsequently inhibits the release of pituitary ovulating hormone. Forty estrous or estrogen-primed female rabbits were treated with 2 ing. progesterone in oil subcutaneously and subjected within 1–4 hours to glass-rod stimulation of the vagina; 11 of them ovulated as compared with only 2⁄30 controls. Mating or intravenous copper acetate, within 4 hours after injection of progesterone, stimulated ovulation in all of 10 estrous rabbits. A few progesterone-treated females ovulated “spontaneously.” The facilitation appeared to depend on synergism of progesterone with estrogen; 10 anestrous animals failed to respond to progesterone and vaginal stimulation by ovulating. Twenty-four hours after treatment with progesterone, 21 initially estrous or estrogen-primed rabbits were allowed to mate; only 14 would permit copulation and 10 of these failed to ovulate. None of 10 estrogen-progesterone-treated females ovulated after receiving artificial vaginal stimulation under these conditions. Furthermore, of 17 estrogen-treated rabbits injected with copper acetate 24 hours after progesterone only 9 revealed ruptured follicles. Although these results confirm the classical inhibitory effect of progesterone, the present work emphasizes that preceding this phase there is a period of faciliation of the ovulatory response.