Biologically active estrogen receptor-beta: evidence from in vivo autoradiographic studies with estrogen receptor...

Estrogen receptor-1 (ER beta) messenger RNA (mRNA) has been detected in the brain of wild-type and estrogen receptor-alpha knockout (ER alphaKO) mice. The present study used in vivo autoradiography to evaluate the binding of 125I-estrogen, a compound with a similar affinity for both ERs to ascertain whether ER beta mRNA is translated into biologically active receptor. Mice were injected with 125I-estrogen, and sections were mounted on slides and opposed to emulsion. After exposure, labeled cells were seen in ER alphaKO brain regions where ER beta is expressed (preoptic and paraventricular nuclei of the hypothalamus; bed nucleus of the stria terminalis; amygdala; entorhinal cortex; and dorsal raphe). Competition studies with 17beta-estradiol eliminated binding in the ER alphaKO brain, whereas 16alphaIE2, an ER alpha selective agonist and dihydrotestosterone had no effect. In contrast, competition studies with 16alphaIE2 in wild-type mice eliminated 125I-estrogen binding to ER alpha and resulted in a pattern of residual binding comparable to that seen in the ER alphaKO brain. The results demonstrate that residual estrogen binding sites are present in regions of the ER alphaKO brain where ER beta is expressed, brain regions that were also seen after eliminating binding to ER alpha in wild-type mice. These data provide the first evidence that ER beta mRNA is translated into a biologically active protein in the rodent brain.