A Possible Hypothalamic Site of Action of Progesterone in the Facilitation of Ovulation in the Rat

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Abstract
The effect of progesterone on the induction of ovulation in rats made persistently estrous by prepubertal androgen treatment or by hypothalamic lesions was studied. In the androgen-sterilized rat, spaced injections of various dosages of progesterone (0.5–2 mg) failed to induce ovulation even though treatment was continued for 30 days. Furthermore, daily doses of 0.25–2 mg did not induce ovulation although vaginal cycles were restored by the 0.5 mg dosage. In an attempt to determine if the site of action of progesterone in the facilitation of ovulation was in the anterior hypothalamus, lesions of various sizes were made in the medial preoptic area and/or the suprachiasmatic nuclei of normal rats. Three weeks after persistent estrus had become established the response of these animals to progesterone was studied. The following results were obtained: Rats with lesions of the suprachiasmatic nuclei exhibited persistent estrus and ovulated in response to progesterone. Destruction of the suprachiasmatic nuclei and the major portion of the medial preoptic area produced persistent estrus but progesterone did not induce ovulation. The smallest region which prevented progesteroneinduced ovulation when destroyed included a part of the suprachiasmatic nuclei and the periventricular portion of the medial preoptic area. That the median eminence, tuberal region and the adenohypophysis were still functional was demonstrated by the ovulation which followed electrical stimulation of these hypothalamic areas in rats with preoptic lesions. (Endocrinology75: 838, 1964)