The Effects of Chronic Administration of Testosterone Propionate with or without Estradiol on the Sexual Behavior and Plasma Steroid Levels of Aged Female Rats

Low sexual desire concomitant with feelings of distress is reported in naturally and surgically menopausal women. A combination of estradiol (E2) and testosterone (T) restores sexual desire and interest in these women. The central mechanisms by which E2 and T act to restore desire are poorly understood. Here we examined the effect of chronic treatment with testosterone propionate (TP) administered by a sc SILASTIC brand capsule in aged ovary-intact female rats. Females were first treated with TP alone, followed by a second phase when TP was administered in combination with estradiol benzoate (EB; 10 μg) by sc injection 48 h prior to testing (EB+TP). Each phase consisted of 5 test days at 4-d intervals. Appetitive and consummatory female sexual behaviors were observed in bilevel chambers, and plasma E2 and T concentrations were measured with ELISA. Sexual solicitations and hops and darts were facilitated by the highest TP dose, and the lordosis quotient was increased by the two highest TP doses when administered alone, coinciding with an increase in plasma T, but those behavioral effects were not maintained across time. The lordosis quotient was inversely related to the TP dose in the EB+TP phase. These results suggest that the administration of TP by sc capsules to aged female rats facilitates appetitive and consummatory sexual behaviors; however, chronic treatment appears to be inhibitory. This is the first study to assess sexual behavior after SILASTIC brand implants of TP in the aged female rat. Additional research is needed to elucidate the mechanisms underlying the effects of T on female sexual function.