Sex differences in estrogen-induced progesterone and estrogen receptor mRNA in the ventromedial hypothalamus of hatchling whiptail lizards.

The ventromedial hypothalamus (VMH) is an important neural locus for the control of female-typical sexual behavior in vertebrates, and exogenous estrogen stimulates a strong increase in progesterone receptor (PR) in the VMH of adult females. Estrogen also regulates its own receptor (ER), though the direction of the response varies from species to species. In rodents and whiptail lizards, males either lack estrogen regulation of PR and ER mRNA in the VMH or display a greatly attenuated response. We examined hatchlings of two closely related species of whiptail lizards, one of which is parthenogenetic. Though normally all female, the parthenogens can be made to develop as gonadal males by treating with aromatase inhibitor early in development. Thus, we were able to ask whether the brain sex of these 'created male' parthenogens corresponded to their gonadal sex or their genetic sex. We injected 1- and 30-day-old animals of both species and sexes with estradiol benzoate (EB) and assayed for PR and ER mRNA using in situ hybridization. All animals given EB responded with a strong increase in PR mRNA in the VMH. However, females of the sexual species had higher EB-induced PR mRNA levels than did conspecific males; there was no sex difference between the normal parthenogens and the created males of the parthenogenetic species. EB also stimulated an increase in ER mRNA in the VMH, but the pattern of response was more complex. Normal parthenogens did not increase ER mRNA in response to EB in either age group, in contrast to the strong response of 1-day-old males and females of the sexual species and 30-day-old created males. The results indicate that hatchling whiptails show striking species and sexual differences in the regulation of sex steroid receptor mRNAs in an area of the brain important for adult sexual behavior. This variation may play a role in the development of species and sexual differences in the adult neuroendocrine phenotype.