Pre- and postnatal ontogeny of aromatase cytochrome P450 messenger ribonucleic acid expression in the male rat brain studied by in situ hybridization.

A central step in the development and sexual differentiation of the brain is the intraneuronal conversion of testosterone to estrogen. This conversion is catalyzed by an enzyme complex comprised of cytochrome P450 aromatase and the ubiquitous NADPH-dependent cytochrome P450 reductase. Brain aromatase activity is developmentally regulated and expressed only in specific hypothalamic and limbic structures. To determine whether developmental change in aromatase activity are the result of differential regulation of aromatase gene expression, we designed two specific oligonucleotides complementary to a 5'-region or to the heme-binding region of the rat aromatase complementary DNA. Relative amounts of messenger RNA (mRNA) encoding aromatase cytochrome P450 were examined on cryostat sections from male fetuses and offspring of time-pregnant Long-Evans rats using the in situ hybridization technique. On gestational day (GD) 16 (GD 1 = 24 h after mating), aromatase-specific mRNA was detected in the preoptic/hypothalamic area. On GD18 and GD20, aromatase mRNA expression was more abundant and more widespread. High levels of mRNA were detected in the medical preoptic nucleus, the sexually dimorphic nucleus of the preoptic area, the bed nucleus of stria terminalis, and the medial amygdala. Lower levels were found in the periventricular preoptic nucleus and the ventromedial hypothalamic nucleus. On postnatal day (PN) 2 and throughout further development (PN6, PN15, and adult), aromatase mRNA decreased to much lower levels in the preoptic area, but remained high in the bed nucleus of stria terminalis and amygdala. An intermediate situation, i.e. transient expression with a peak around birth, was observed for aromatase mRNA levels in the ventromedial hypothalamic nucleus. Interestingly, during early postnatal development, specific aromatase mRNA was also detected in the hippocampal formation as well as in the subfornical organ. In summary, these results demonstrate that the low quantities of aromatase transcripts in the developing rat brain can be detected by means of in situ hybridization. Depending on the particular region examined, aromatase expression appears to be subjected to different developmental programs. As the expression of aromatase mRNA parallels the results obtained from activity assays, it is likely that developmental changes in aromatase activity are the result of regulation of aromatase gene expression.