ISOLATION AND CHARACTERIZATION OF A ZEBRA FINCH AROMATASE CDNA - IN-SITU HYBRIDIZATION REVEALS HIGH AROMATASE EXPRESSION IN BRAIN

The brain regions controlling song are much larger in male than in fem ale zebra finches. This sex difference is thought to arise development ally when the male's brain is exposed to higher levels of estrogen. Th e synthesis of estrogen from androgen is catalyzed by aromatase, a key enzyme implicated in song development in the zebra finch. To study th e role of estrogen synthesis in the organization of brain regions resp onsible for song; a cDNA encoding aromatase was isolated from a zebra finch ovarian cDNA library. The 3188 bp cDNA contains a 1527 bp open r eading frame with 5' and 3' untranslated regions of 116 bp and 1545 bp , respectively. The deduced polypeptide is 509 amino acids in length, and is highly homologous to aromatases reported for chicken (92%), hum an (72%), mouse (70%), rat (69%) and trout (53%). Northern blot analys is revealed 5.4 kb, 4.8 kb and 3.2 kb aromatase mRNAs in brain and ova ry. In situ hybridization histochemistry revealed the expression of ar omatase mRNA in ovarian thecal cells of some, but not all, follicles, suggesting that aromatase gene expression is regulated during follicul ar maturation. In the adult brain, the distribution of aromatase mRNA was surprisingly widespread, and included the preoptic area, hypothala mus, hippocampus and neostriatum. By contrast, little aromatase mRNA e xpression was noted in the song nuclei (HVC, RA, area X). This study, the first description of aromatase mRNA expression by in situ hybridiz ation in the brain of any species, identifies a surprisingly large num ber of cells that express aromatase mRNA in the zebra finch telencepha lon. This pattern may be unique feature of all songbirds.