NEURONAL AND NONNEURONAL AROMATASE IN PRIMARY CULTURES OF DEVELOPING ZEBRA FINCH TELENCEPHALON

Estrogenic metabolites of circulating androgens have important effects on the organization and activation of neural circuits controlling rep roductive behavior and physiology in males of many vertebrate species. Previous studies indicate that aromatase, the enzyme that converts an drogens to estrogens, is expressed most abundantly in neurons in limbi c brain regions. Songbirds are unique in that aromatase is expressed a t unusually high levels throughout the telencephalon of both males and females. We assume that estrogens formed in the telencephalon itself masculinize neural circuits controlling song, since the brain is a maj or source of circulating estrogens in adult males. However, the cellul ar localization of telencephalic aromatase in songbirds remains unknow n. We have established primary cultures from telencephalons of develop ing zebra finches and found aromatase activity (conversion of H-3-andr ostenedione or H-3-testosterone to H-3-estrone plus H-3-estradiol) at some of the highest levels reported for brain tissue of any species. B oth neurons and glia were identified in these cultures based on cell m orphology and labeling by specific immunohistochemical markers. Howeve r, when culture conditions were manipulated to reduce the incidence of either neurons or glia by varying the age of cultures or their platin g density, treating with the neurotoxin kainic acid, physically shakin g off loosely attached neurons, or preparing cultures in media that en couraged enrichment of neurons, high levels of aromatase persisted. Fu rthermore, Northern blot analysis of total RNA extracted from enriched neuronal or glial cultures indicated the presence of aromatase mRNA i n both cell preparations. In site hybridization with a zebra finch aro matase cDNA probe conjugated to digoxigenin showed the cultures contai ned darkly labeled neurons and lightly labeled non-neuronal cells, pre sumably astrocytes. We conclude that aromatase is expressed in both ne uronal and non-neuronal cells in these cultures, suggesting that both cell types may also express the enzyme in vivo. The presence of aromat ase outside of neurons suggests that glia may be targets of estrogen a ction or that glia may supply some estrogen to the estrogen-sensitive neural circuits in this species.