Activation of lordosis in ovariectomized guinea pigs by free and esterified forms of estrone, estradiol-17β and estriol

The threshold doses of estradiol-17β, estrone, estriol, estradiol-17β-3-benzoate and estrone-3-benzoate required to activate lordosis in ovariectomized adult guinea pigs were determined by injecting these steroids in combination with progesterone. The doses of the 3 free steroids which activated lordosis in about 50% of the animals ranged from 20 to 50 μg/animal. In contrast, threshold doses of estradiol benzoate and estrone benzoate were only 0.4 and 1.7 μg/animal, respectively. The data indicate that (1) conversion to estradiol-17β is not an absolute requirement for activation of lordosis and (2) esterified forms of estradiol and estrone are far more potent in inducing lordosis than the corresponding free forms of these steroids. The increased potency of the esterified estrogens may be attributable to their prolonged action. Testosterone propionate also induced lordosis in 6/18 ovariectomized guinea pigs when given in a dose of 500 μg/animal for 7 days.