Exposure to Estrogen Mimicking the Level of Late Pregnancy Suppresses Estrus Subsequently Induced by Estrogen at the Level of the Follicular Phase in Ovariectomized Shiba Goats

A high-estrogen environment during late pregnancy is suspected to cause postpartum silent ovulation, and progesterone (P(4)) is suggested to recover estrus. However, few attempts have been undertaken to elucidate the influence of these steroids on estrus by analyzing hormonal profiles. We investigated estrus and luteinizing hormone (LH) surges in ovariectomized goats (n=6) assigned to three treatments in a cross-over design. In groups 1 and 2, 200 µg/kg body weight/day estradiol benzoate (Dose-200 E(2)B) was administered for 14 days concurrent with P(4) for 11 days, while in the control, saline solution and P(4) were administered likewise. Ten days after the final administration of Dose-200 E(2)B, group 2 was treated with P(4) for 8 days, and all groups were treated with 2 µg/kg body weight E(2)B (Dose-2 E(2)B) 20 days after the final administration of Dose-200 E(2)B (or saline solution). The proportion of cases expressing estrus after the administration of Dose-2 E(2)B was smaller (P<0.01) in group 1 than in the control (1/6, 3/6 and 6/6; groups 1 and 2 and the control, respectively). The proportions of cases generating LH surges did not differ (P>0.1) among the groups (5/6, 5/6 and 6/6; groups 1 and 2 and the control, respectively), but the peak concentrations in groups 1 and 2 (26.2 ± 14.7 and 11.3 ± 6.7 ng/ml) were lower (P<0.01) than those in the control (67.8 ± 19.4 ng/ml). These results demonstrated that elevation of plasma estrogen mimicking late pregnancy inhibits the subsequent estrus induced by estrogen simulating the follicular phase.