Sex differences in male-typical copulatory behaviors in response to androgen and estrogen treatment in rats.

The present experiment was performed to test the hypothesis that gender differences in the capacity for brain estrogen synthesis could constitute a sexually dimorphic mechanism that limits the activational effects of testosterone (T) in females, and enhances them in males. We determined the effects of treatments with equivalent levels of either T or estradiol (E2) on olfactory behavior and mounting in age-matched heterosexually naive gonadectomized male and female rats that were genitally ansthetized with lidocaine paste in order to minimize the contribution of sexually dimorphic somatosensory inputs to the expression of copulatory behavior. We found that T stimulated mounting to a greater extent in males than in females, but had equivalent effects on mount latency and genital investigation in the two sexes. On the other hand, E2 stimulated equivalent levels of mounting in males and females and reduced mount latency to a similar extent in males and females. However, E2 had a pronounced effect on the levels of genital investigation in males but not in females. Serum steroid levels and the levels of nuclear steroid receptor occupation in the brain were not different between males and females, suggesting that the behavioral differences between males and females cannot be attributed to differences in peripheral steroid metabolism or brain uptake. The results obtained corroborate previous studies suggesting that female rats normally undergo considerable male-typical behavioral masculinization during fetal development. However, such male-typical features of normal development in female rats do not extend to the regulation of preoptic aromatase activity or to the capacity of females to display olfactory behaviors in response to adult E2 exposure, functions which are sexually dimorphic even in the rat. The present results support the view that gender differences in the capacity for brain estrogen synthesis contribute to the sexually dimorphic display of T-stimulated male-typical sexual motivation and copulatory behavior in rats.