Regulation of aromatase gene expression in the adult rat brain

Brain aromatase plays an important role in the regulation of adult reproductive behavior in male rodents. This report focuses on recent experiments from our laboratory that examined the distribution and regulation of aromatase mRNA in the rat brain. Aromatase mRNA was measured by a highly sensitive ribonuclease protection assay using a 32P-labeled antisense RNA probe that was complimentary to the 5' coding region of rat aromatase mRNA. This probe protects two RNA fragments in rat brain tissue: a 430-nt length fragment and a shorter 300-nt fragment. The presence of the 300-nt RNA fragment is not associated with enzyme activity in the rat brain and appears to represent an alternative brain-specific aromatase transcript whose function, if any, is unknown. In contrast, the 430-nt RNA fragment represents mRNA, which is thought to encode functional aromatase enzyme because its levels are correlated with aromatase activity concentrations in preoptic area, hypothalamus, amygdala, and ovary. Aromatase activity and mRNA levels in the preoptic area and hypothalamus decreased by 7 days after castration and were maintained at intact levels by treatment with testosterone and dihyhdrotestosterone, but not with estradiol. In contrast, neither aromatase activity nor mRNA levels in the amygdala are affected by castration or hormone replacement. In addition, sex differences in the regulation of aromatase mRNA were apparent in both the preoptic area and hypothalamus. These results demonstrate that androgens regulate the transcription or stability of aromatase mRNA in specific brain areas. Moreover, they suggest that gender differences in androgen responsiveness play an important role in regulating gene expression in the adult rat brain.