Structure of aromatase mRNA in the rat brain

In order to elucidate the mechanism of the region-specific expression of rat brain aromatase, we investigated the multiple promoter systems in the brain. Total RNA extracted from the amygdala (AMY) was subjected to a reverse transcription-polymerase chain reaction (RT-PCR) with the primers for mouse brain-specific exon I (exon I-f). The nucleotide sequence of the RT-PCR product had 89.4% homology to the corresponding region of exon I-f of the human aromatase cDNA. It was first demonstrated that aromatase mRNA with exon I-f was expressed in the rat brain. This type message was expressed in the hypothalamus-preoptic area (HPOA) but not in the cerebral cortex (CC). Furthermore, total RNAs from the HPOA, AMY and CC of the rat brain were analysed by RT-PCR with the primers for rat ovarian type message-specific exon II. Significant levels of the products were generated in the HPOA and AMY with no signals in the CC. Cortical aromatase mRNA seemed to be unique in that the tissue had no exon I-f and ovarian type mRNAs. We thus analysed the 5'-region of aromatase mRNA in the rat CC by 5'-rapid amplification of cDNA ends (RACE) with antisense primers for exon V. The upstream sequence from the 5'-end of exon IV of the aromatase RACE clones was completely different from that of the aromatase mRNA which encoded the full-translated region. These results indicated that the aromatase gene in the rat brain contained at least three promoters: hypothalamus type (exon I-f), ovarian type and cortical type, and that the region-specific expression of the brain aromatase seemed to be regulated, at least in part, by differential promoter usage.