EFFECTS OF PARTIAL VERSUS PURE ANTIESTROGENS ON OVULATION AND THE PITUITARY-OVARIAN AXIS IN THE RAT

The present study was undertaken to compare the effects of the pure an tiestrogen ZM 182780 (ZM) and the partial agonist tamoxifen (TAM) on o vulation and peripheral hormone levels in the rat. Adult female rats w ere treated with ZM (5 mg/kg/d) or TAM (5 mg/kg/d) and sacrificed at v arying times during the estrous cycle. ZM and TAM were able to inhibit ovulation to nearly the same extent (60% and 70%, respectively). ZM i nduced a persistent diestrus whereas TAM led to a mixed picture in the vaginal smear. Both antiestrogens suppressed the preovulatory surge o f LH, FSH and progesterone but had no effect on basal FSH values. In c ontrast to TAM, ZM caused an increase in basal levels of LH. Moreover, basal and preovulatory levels of estradiol and androgens were increas ed by ZM. By contrast, TAM decreased basal and preovulatory values of LH, estradiol, and androgens. These data suggest that ZM inhibits not only the positive but also negative feedback effect of estrogens and T AM seems to inhibit only the positive feedback. Moreover, it is concei vable that the suppressed preovulatory progesterone surge induced by Z M and TAM could be responsible for the antiovulatory effect. (C) 1998 Elsevier Science Ltd. All rights reserved.