The activity and expression of aromatase in songbirds

Songbirds have emerged as extremely important animals for investigating sex steroid hormone effects on the central nervous system. The masculinizing effects of exogenous estrogen on the neural circuits controlling song in female zebra finches are well documented. There is evidence that estrogens are necessary for the full activation of singing behavior in several species. These kinds of studies have led us and others to investigate the mechanisms whereby estrogens are made available to the brains of songbirds during development and adulthood. In this article, I review results of some of these studies examining the estrogen synthetic enzyme aromatase and its expression and activity in brain and in other tissues of songbirds. I will discuss some results and thoughts we have about the interactions of aromatase with the two remaining androgen-metabolizing enzymes in the avian brain, 5alpha-reductase, the enzyme that converts T into the active androgen 5alpha-dihydrotestosterone (DHT); and 5beta-reductase, the enzyme that converts T into the inactive 5beta-DHT. Finally, I will consider some ideas raised by these studies concerning potential sources of the androgen substrate for brain aromatization as well as some possible new functions that aromatase might be playing in the songbird telencephalon.