Development of steroid-receptor systems in guinea pig brain. II. Cytoplasmic progestin receptors.

The development of the cytoplasmic progestin receptor (CPR) system in male and female guinea pigs was examined to determine if insensitivity to the lordosis promoting activity of estrogen-progestin treatment in adult males and neonatal guinea pigs is correlated with alterations in this receptor system. Gonadectomized neonatal (4-6 days old) and adult (50-65 days old) guinea pigs of both sexes were injected with estradiol benzoate (EB) and killed 40 h later. CPRs were measured in the hypothalamus (HYPO), preoptic area (POA) and cortex ( CORT ). Neonatal brain contained both high (Kd congruent to 0.1 nM) and low (Kd greater than 10 nM) affinity binders for [3H]R5020 similar to those found in adult brain. In the absence of EB-priming the concentration of CPR in HYPO, POA and CORT was lower in neonatal animals than adults. In both males and females, neither a low (1.6 micrograms) nor a high (10 micrograms) dose of EB resulted in a concentration of estrogen inducible CPR in neonatal HYPO greater than that found in adult HYPO after 1.6 micrograms EB. Even after treatment with 50 micrograms EB the concentration of CPR in neonatal female HYPO was not different from adult female HYPO after 1.6 micrograms EB. In neonatal POA, 10 micrograms EB resulted in a concentration of CPR greater than that found in adult POA after 1.6 micrograms EB. Finally, there was a sex difference in the concentration of estrogen inducible CPR in HYPO (female greater than male) but not POA of adult guinea pigs. No sex difference in CPR concentration was found in any neonatal brain area.