Overlap and co-expression of estrogen synthetic and responsive neurons in the songbird brain-a double-label immunocytochemical study

The songbird telencephalon exhibits the capacity to both synthesize and respond to estrogen. Several telencephalic loci in addition to those in the diencephalon express aromatase (estrogen synthase) and estrogen receptors (ER). Little is known about the interactions between cells that contain aromatase and those that contain ER, particularly at the level of protein expression. Consequently, we do not know if locally synthesized estrogens affect ER via autocrine and/or paracrine mechanisms. Here we have mapped the distributions, identified areas of overlap, and measured the degree of co-expression of aromatase and ERα in the zebra finch (Taeniopygia guttata). First, alternate sections were stained with antibodies against either aromatase or ERα, revealing the distributions and therefore, the overlap between these proteins. Subsequently, using double-label light microscopy we have measured the number of aromatase soma, ERα soma, and co-expressing soma in areas of overlap in adult males and females. In the preoptic area about 10% of aromatase-positive soma co-express ERα. In the bed nucleus of the stria terminalis, ventromedial nucleus, nucleus taeniae, and the caudomedial nidopallium, although cells containing either protein were easily detectable, the level of co-expression was minimal. The degree of co-expression and the number of aromatase-positive soma did not differ between sexes. However, the number of ERα cells was higher in the female preoptic area relative to that in the male. Conversely, ERα is more abundant in the male bed nucleus of the stria terminalis relative to the female. We conclude that while local aromatization in the preoptic area may modulate ERα-containing neurons via autocrine pathways, paracrine mechanisms may predominate in other areas of the songbird brain.