Sexual receptivity in aged female rats : Behavioral evidence for increased sensitivity to estrogen

Lordosis behavior and behavioral patterns indicative of sexual receptivity or nonreceptivity were evaluated in 3- and 25-month-old female rats. Intact, 25-month-old females revealing a pattern of prolonged vaginal cornification (PVC) were highly receptive, whereas 25-month-old pseudopregnant (PL) females were nonreceptive. These measures did not differ as a result of previous sexual experience (i.e., sexually naive vs retired breeders). Following ovariectomy, sexual behavior persisted significantly longer in 25-month-old PVC females than in 3-month-old females ovariectomized during vaginal proestrus. No difference was observed in the rate at which the vaginal smears of 25-month-old PVC and young ovariectomized females became leukocytic. When tested 5, 19, and 20 days after ovariectomy, sexual behavior was absent in all females. Following treatment with estradiol benzoate (0.5 or 1.0 μg/kg), the latency for the reappearance of sexual behavior in 25-month-old females was significantly shorter than that in similarly treated 3-month-old females. This was the case regardless of prior ovarian condition (PVC or PL) and breeding experience. In addition, the mean lordosis quotient (LQ) of 25-month-old females receiving either dose of EB was significantly greater than that of similarly treated 3-month-old females on at least 5 days of a 10-day treatment period. These results are discussed in terms of possible age-related changes in central and peripheral estrogen metabolism.