AN ATLAS OF AROMATASE MESSENGER-RNA EXPRESSION IN THE ZEBRA FINCH BRAIN

Neural conversion of androgen to estrogen by aromatase is an important step in the development and expression of masculine behavior in mamma ls and birds. In contrast to the low telencephalic levels of aromatase in adult mammals and nonsongbirds, the zebra finch telencephalon poss esses high aromatase activity. This study maps, by in situ hybridizati on, cells that express aromatase mRNA in the adult zebra finch telence phalon, diencephalon, midbrain, and pens. High aromatase mRNA expressi on was observed in the caudal neostriatum, limbic archistriatum, and h ypothalamus. The hippocampus, parahippocampal area, and hyperstriatum accessorium contained cells expressing moderate amounts of aromatase m essage. Weakly labeled cells were found in the rostral neostriatum, lo bus parolfactorius, and mesencephalic reticular formation. These findi ngs are consistent with aromatase activity measurements of zebra finch tissue and document with anatomical precision both the widespread exp ression of aromatase mRNA in the brain and novel sites of brain aromat ase. This study identifies the caudal neostriatum as a major site of t elencephalic aromatase. A previous survey (Gahr et al., 1993: J. Comp. Neurol. 327:112-122) of several avian species found that the presence of estrogen receptors in parts of the caudal neostriatum is unique to songbirds, which are the only birds to possess the elaborated telence phalic song system. Together, these findings suggest that the heighten ed estrogen synthesis and estrogen sensitivity of the passerine caudal neostriatum may have some functional relation with the telencephalic circuits responsible for song. (C) 1995 Wiley-Liss, Inc.