In the rat an abrupt discharge of testicular testosterone in the newborn male figures prominently in the development of mechanisms controlling gonadotropin secretion, sexual behavior, and also promotes the functional differentiation of the accessory sex glands. In this study we detail the temporal characteristics of this surge in the rat, and we provide comparative data documenting a similar surge-like appearance of testosterone in neonatal male mice, recently foaled male horses, and newborn human infants. Although the physiological and behavioral significance of this phenomenon for species other than the rat remains to be determined, the apparently ubiquitous appearance of the neonatal testosterone surge suggests that it may be of special significance in the sexual differentiation of many mammalian species.

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.

Changes in serum and pituitary LH and FSH concentrations have been measured in the newborn male rat before, during, and up to 24 h after birth. A sudden and transient increase of serum and pituitary gonadotropins is observed at birth, which is followed by a rapid increase of absolute and relative testicular weights between 2--12 h (P less than 0.0001) and by a transient increase of serum testosterone between 0 h in utero (810 +/- 26 pg/ml) and 2 h (2820 +/- 318 pg/ml; P less than 0.0001). Similarly, premature newborn rats obtained by cesarian delivery on day 20 of gestation also exhibited an increase in testicular weight between 0--6 h and an increase in serum testosterone levels between 0 h (730 +/- 170 pg/ml) and 2 h (3400 +/- 300 pg/ml; P less than 0.001) with only a slight increase in serum LH. These results show that the hypophyseo-testicular axis of the rats is stimulated at the moment of birth. The factors responsible for this stimulation are discussed. This transient testicular crisis occurring at birth could affect the process of masculinization of the central nervous system of the rat.