Steroids induce hypothalamic progestin receptors and facilitate female sexual behavior in neonatal rats.

This report provides the first evidence that two activational actions of estrogen can occur within the first week of life in the rat. Priming 4-day-old rats with exogenous estradiol benzoate facilitates lordosis and ear wiggling-like behavior, and induces cytosol progestin receptors in the medial basal hypothalamus and preoptic region (Hyp-PoA) 44 h later. Moreover, unprimed male infants show more intense lordosis and higher concentrations of cytosol progestin receptors in Hyp-PoA than infant females. Neonatal castration of male infants decreases their concentration of cytosol progestin receptors in Hyp-PoA to the levels normally seen in infant females. In addition, priming 6-day-old infants with progesterone alone facilitates lordosis in male but not female infants. Together, these data demonstrate that in the male infant, endogenous steroids of testicular origin have activational as well as organizational actions.