EXPERIMNTAL STUDIES ON REBOUND PHENOMENON OF OVARIAN FUNCTION IN RAT

Abstract

The rebound phenomenon of ovarian function has widely been recognized since Matsumoto and Igarashi reported on clinical findings adout it.The author made investigation whether the same phenomenon is also found in rats.(1) When adult female rats were consecutively given a large dose of estrogen (estradiol benzoate subcutaneously, 500r once every 3 days for 30 days or 100r once daily for 20 days), the ovary was atrophied, and histologically corpora lutea was remarkably decreased or lost, indicating evident inhibitory effect. At the same time, gonadotrophic potency of the anterior hypophysis, as measured by mouse uterus weight method, showed evident decline, and PAS-positive cells in the hypophysis were also decreased.(2) When the administration of estrogen was abruptly suspended, ovarian weight was gradually restored, and after 70 days, it, as well as its histological picture, went above the normal level of controls. In this way, the rebouud phenomenon of ovarian function could be demonstrated also in rats.(3) In accordance with these findings, gonadotrophic potency of the anterior hypophysis also increased beyond the level before the estrogen administration. It was consequently elucidated that the rebound phenomenon of ovarian function is due to that of the anterior hypohysis.(4) When a large dose of estrogen was consecutively given (estradiol benzoate, 500r once every 3 days for 30 days, subcutaneously), the function of the pituitary-ovarian system was inhibited. And when a small inhibitory dose of estrogen was further given (10r once every 3 days for 40 days, subcutaneously), the inhibitory effect was no more produced. In this way, Hohlweg's so-called desensitization (“Desensibilisierung”) was demonstrated in the ovary of the rat. These two-desensitization and rebound phenomenon-were compared each other under the same conditions, and the effect of the latter was found more prominent.(5) Within 50 days after the administration of a large dose of estrogen (estradiol benzoate 100r daily for20 days), the duration of pseudo-pregnancy was evidently prolonged from the preadministration average of 13.8±0.5 days to 17.4±1.3 days. This is considered to be due to the same mechanism with that of prolongation of high phase of basal bodytemperature, which is observed in association with the rebound phenomenon of ovarian function in women, and is considered to be induced by activated function of corpora lutea in the rebound phenomenon.(6) After consecutive administration of a large dose of progesterone, slight reduction of ovarian weight and inhibitory effect on vaginal smear were observed, but neither evident inhibitory effect on the hypophysis, nor the rebound phenomenon after the suspension of administration. At70 days after the administration, however, gonadotrophic potency of the hypophysis was increased. This is a problem which must await future study.