RESUMO
Delayed effects of exposure to small amounts of estrogenic compounds during the critical period of brain sexual differentiation were investigated by subcutaneous treatment of female Sprague-Dawley rats with 0 (vehicle control), 0.08, 0.4, or 2 µg/kg of 17α-ethynylestradiol (EE) on postnatal day (PND) 1. The treatment did not affect growth and development of the treated animals, and the timings of vaginal opening were similar between the EE-treated and control groups. The animals were periodically examined for the estrous cycle from postnatal week (PNW) 8-9 to PNW 32-33. Patterns of the estrous cycle were similar among the groups until PNW 17. None of the control animals showed persistent estrus until PNW 33. The animals treated with 0.4 µg/kg or more EE showed persistent estrus from PNW 20. The alteration was reflected in the number of days judged as proestrus or estrus, and was found to gradually increase in the EE-treated groups. At necropsy on PNW 32-33, ovulation was not confirmed in most EE-treated animals, even on the day of estrus. In addition, sporadic milk accumulations were observed in the mammary gland of the EE-treated animals. Histological evaluation revealed cystic follicle formation in the EE-treated ovaries and also revealed hyperplasia of mammary glands. Furthermore, ovaries from the animals showing persistent estrus lacked corpus luteum, indicating long-term anovulation. These results clearly show that single exposure to EE during the critical period of brain sexual differentiation can exert effects on reproductive functions at a later period in rats.