Gonadal Steroid Concentrations in Serum and Hypothalamus of the Rat at Birth: Aromatization of Testosterone to 17β-Estradiol*

Endogenous concentrations of testosterone (T), 5 alpha-dihydrotestosterone, progesterone (P), and estradiol-17 beta (E2) were determined with specific RIAs in serum and hypothalami of male and female rats before, during, and up to 24 h after birth. In the male, a dramatic and transient increase in T concentration was observed in the serum and the hypothalamus between 0 h in utero and 2 h after delivery. At all times studied, T levels were undetectable in the female. We failed to detect any significant 5 alpha-dihydrotestosterone levels in the serum and the hypothalamus of both sexes. Serum E2 levels decreased between the 21-day fetal stage and 24 h postpartum. However, in males, hypothalamic E2 dramatically increased between 0 h in utero and 1 h after delivery and decreased between 2 and 24 h. This surge was absent in females and males gonadectomized at 0 h, suggesting that this surge is linked to the presence of the testes. E2 was undetectable in the cerebral cortex. P presented the same pattern of declining levels in the male and the female, and no sex difference was noted for the mean concentrations in the serum or the hypothalamus. The fall in P levels and the sudden increase in hypothalamic T and E2 levels could be determinant factors in the initiation of central nervous system sexual differentiation in the rat. The fact that in the male rat, hypothalamic E2 increases during the time when testicular secretions defeminize the brain strengthens the view that E2 mediates some of the effects of T.