Hormonal control of the development and regulation of tyrosine hydroxylase expression within a sexually dimorphic population of dopaminergic cells in the hypothalamus.

In situ hybridization histochemistry was used to examine the development and regulation of tyrosine hydroxylase (TH) mRNA within the sexually dimorphic population of dopaminergic cells in the anteroventral periventricular nucleus (AVPv) of the hypothalamus. The AVPv contains over 3 times as many TH mRNA-containing cells in female rats, compared with males. This sexual dimorphism appears to be dependent on perinatal levels of gonadal steroids since orchidectomy of newborn males increased, and treatment of newborn females with testosterone decreased, the number of TH mRNA-containing cells detected within the AVPv. In addition, circulating gonadal steroids appear to downregulate TH expression within these cells in both adult male and female rats. In adult male animals, gonadectomy increased the number of TH mRNA cells in the AVPv within 7 days. Similarly, estradiol treatment prevented the increase in the number of TH mRNA-containing cells within the AVPv seen in ovariectomized female rats. No sexual differences were detected in the number of TH mRNA-containing cells within the suprachiasmatic preoptic nucleus, located just ventral to the AVPv. These findings indicate that perinatal gonadal steroids influence the number of cells within the AVPv that express TH in detectable amounts by determining the number of cells that are capable of producing sufficient quantities of TH message, as opposed to sex-specific alterations in the post-translational mechanisms. In the adult, circulating gonadal steroids appear to downregulate TH expression within these cells suggesting that testosterone and/or estrogen may exert a sustained influence on the biosynthetic activity of this sexually dimorphic population of dopaminergic cells.