The ontogeny of cytosol and nuclear progestin receptors in male rat brain and its male-female differences.

The fetal and postnatal development of the progestin receptor systems in the intact male rat brain was investigated by means of the in vitro cytosol binding and the nuclear exchange assay using [3H]R5020 [( 17 alpha-methyl-3H]17 alpha, 21-dimethyl-19-nor-4, 9-pregnadiene-3,20-dione). The cortical cytosol receptors, first detectable at day 0, rapidly increased at day 7, reaching a maximum at day 10, then gradually declined thereafter. The receptors in the HPOA appeared clearly at day 1, increased during the first 10 days, then remained constant at days 14-21. The postnatal developmental patterns of cytosol brain progestin receptors in males were essentially similar to those in females, but there were some differences between both sexes. The male HPOA at days 10-14 contained more receptors than the female one. Nuclear progestin binding was low in the neonatal male brain at days 1-3. Despite the low level of serum progesterone, the cortical nuclear binding suddenly increased at days 7-10, then remained high at days 14-21. A similar, though less pronounced, pattern was seen in the HPOA. The male pattern of nuclear binding, thus, essentially resembled the female one. However, lower binding in the cortex and, possibly, HPOA was found in males than in females at days 10-21. After progesterone injection postnatal male rats accumulated a lower concentration of progestin receptors in the cortex and, possibly, HPOA than similarly-treated females. It is concluded from these results that progestin receptors in male rat brain appear immediately after birth and develop differentially in the cortex and HPOA. The sudden onset of increased nuclear translocation of endogenous progestin receptor complexes may occur in the brain at around days 7-10. There is a marked sex difference in the nuclear progestin receptor system in the postnatal brain, particularly the cortex. Moreover, the postnatal male brain has lower capacities of nuclear receptor translocation than does the female one. The progestin receptor system in the cortex and, possibly, HPOA of rats in the early postnatal life might be involved with some processes in the mechanism of sexual differentiation of the brain.