Effect of long-term castration and long-term androgen treatment on sexually dimorphic estrogen-inducible progesterone receptor mRNA levels in the ventromedial hypothalamus of whiptail lizards.

In whiptail lizards, as in laboratory rodents, females will respond to exogenous estrogen by increasing progesterone receptor (PR) or PR mRNA in the ventromedial hypothalamus (VMH) while males show an attenuated response to the same treatment. In rodents, neonatal hormone manipulations affect the adult expression of this trait; however, few investigators have examined the effects of hormone treatment in adulthood. Therefore the current study was carried out to determine whether observed sex differences in the estrogen response in adulthood may be modified by steroid hormone manipulation. We castrated male whiptail lizards for 1 week (short term) or 6 weeks (long term). We also gonadectomized female whiptails and implanted them with either a Silastic capsule containing testosterone or an empty capsule. At the end of that time all implants were removed and the animals were injected with either estradiol benzoate (EB) or steroid suspension vehicle and their brains were assayed for PR mRNA expression using in situ hybridization. The results demonstrate that in male whiptail lizards, long-term castration increases sensitivity to estradiol as measured by induction of PR mRNA in the VMH; EB-injected long-term castrated males were not different from EB-injected females. However, long-term androgenization did not attenuate the estrogen response in females. This suggests that attenuation of the estrogen response in males requires activation by testicular secretions, but that females cannot be made to show a male phenotype via testosterone administration.