Ontogeny of region-specific sex differences in androgen receptor messenger ribonucleic acid expression in the rat forebrain

Testosterone and its metabolites are the principal gonadal hormones responsible for sexual differentiation of the brain. However, the relative roles of the androgen receptor (AR) vs. the estrogen receptor in specific aspects of this process remain unclear due to the intracellular metabolism of testosterone to active androgenic and estrogenic compounds. In this study, we used an 35S-labeled riboprobe and in situ hybridization to analyze steady state, relative levels of AR messenger RNA (mRNA) expression in the developing bed nucleus of the stria terminalis, medial preoptic area, and lateral septum, as well as the ventromedial and arcuate nuclei of the hypothalamus. Each area was examined on embryonic day 20 and postnatal days 0, 4, 10, and 20 to produce a developmental profile of AR mRNA expression. AR mRNA hybridization was present on embryonic day 20 in all areas analyzed. In addition, AR mRNA expression increased throughout the perinatal period in all areas examined in both males and females. However, between postnatal days 4 and 10, sharp increases in AR mRNA expression in the principal portion of the bed nucleus of the stria terminalis and the medial preoptic area occurred in the male that were not paralleled in the female. Subsequently, males exhibited higher levels of AR mRNA than females in these areas by postnatal day 10. There was no sex difference in AR mRNA content in the lateral septum, ventromedial nucleus, or arcuate nucleus at any age. These results suggest that sex differences in AR mRNA expression during development may lead to an early sex difference in sensitivity to the potential masculinizing effects of androgen.