Sensitization of sexual behavior in ovariectomized rats by chronic estradiol treatment

Abstract

The ovariectomized (OVX) rat treated with estradiol benzoate (EB) is used to elucidate neuroendocrine mechanisms of sexual behavior. Chronic behavioral and pharmacological manipulations can be confounded by rising baselines, since females are behaviorally more sensitive to repeated EB injections. The literature lacks a systematic examination of chronic effects of EB administered alone to the sexually experienced OVX rat. Long–Evans rats were repeatedly treated (8 tests) with s.c. injections of 2, 5, or 10μg EB at different time intervals (4 or 8days). Female sexual behaviors as well as receipt of mounts, intromissions and ejaculations from the male were observed in the unilevel 4-hole pacing chamber. The effects of adrenalectomy (ADX) and strain (Long–Evans vs. Wistar) were also assessed. Long–Evans OVX rats treated with 5μg EB every 8days showed persistently low levels of sexual behavior. Sensitization was most robust following 10μg EB at 4-day intervals. Very few sexual behaviors were ever induced by 2μg EB. ADX did not affect the development of behavioral sensitization by 10μg EB. Therefore, to achieve a low steady state of sexual behaviors in sexually experienced Long-Evans OVX rats 5μg of EB administered every 8days is optimal, whereas a persistently high level of sexual behaviors is induced with 10μg EB administered every 4days. OVX Wistar rats are behaviorally more sensitive to EB. Behavioral sensitization to EB may serve as a mechanism to optimize reproductive success.