Estrogen but not progesterone facilitates the lordosis reaction to cervicovaginal stimulation of ovariectomized rats.

Ovariectomized rats were treated sequentially with 1, 10 or 100 micrograms/kg of estradiol benzoate and 8 mg/kg of progesterone (or vehicles) and were tested for lordosis reactivity. A male was used to apply cutaneous stimulation only, or cutaneous and cervicovaginal stimulation, in females with masked or unmasked vaginal orifices, respectively. Graded cervicovaginal stimulation alone was obtained with a manually operated penile dummy. Progesterone caused a significant increase in the lordosis quotient to male stimulation after the doses of 10 and 100 micrograms/kg of estrogen but not after 1 microgram/kg of the hormone. This effect of progesterone was unaffected when the vaginal orifice was masked to avoid penile intromission and the resulting cervicovaginal stimulation. Lordosis reactions to artificially graded cervicovaginal stimulation following progesterone only or vehicle only were both infrequent and weak. The three estrogen doses employed increased markedly the reactivity to cervicovaginal stimulation, both when the percent frequency of lordosis and when lordosis intensity ratings were considered. In contraposition with the effect on male stimulation, progesterone did not increase the reactivity to cervicovaginal stimulation, as measured by the percent incidence of lordosis. This was also the case when lordosis intensity scores were considered, with the sole exception of a significant difference when 1 microgram/kg of estradiol and the strongest stimulation were used. Our results suggest that the sensitivity of the afferent pathways to elicit the lordosis reaction are under different hormonal modulations: cutaneous afferents seem to be facilitated by estrogen and progesterone, while cervicovaginal afferents would be facilitated only by estrogen.