Chronic exposures to low levels of estradiol and their effects on the ovaries and reproductive hormones: Comparison with aging.

Aging in female rats is characterized by a state called "constant estrous" in which rats are unable to ovulate, have polycystic ovaries and moderately elevated estrogen levels. We hypothesized that chronic exposure of young animals to low levels of E2 can produce reproductive changes similar to that seen in aging animals. Adult female rats were sham-implanted (control) or implanted with slow-release E2 (20 ng/day) pellets for 30, 60, or 90 days. Old constant estrous (OCE) rats were used for comparison. Estrous cyclicity was monitored periodically. At the end of treatment, animals were sacrificed, trunk blood was collected for hormone measurements and ovaries for immunohistochemistry. Young animals became acyclic with increasing duration of E2 exposure while OCE rats were in a state of acyclicity. Ovaries became increasingly more cystic with E2 exposure, and were comparable to OCE rats; however, there was a marked reduction in interstitial tissue with exogenous E2 treatment. Exogenous E2 also decreased Mullerian inhibiting substance expression, increased infiltration of macrophages without much impact on apoptosis in the ovaries. Serum testosterone levels decreased in E2-treated young animals, while it increased significantly in OCE rats. There was a marked reduction in LH but not FSH levels with E2 exposure in both young and old animals. These results indicate that even very low doses of E2 are capable of inducing aging-like changes in young animals.