Localization of the Estradiol Receptor MRNA in the Forebrain of the Rainbow Trout

In situ hybridization was used to localize the cells that express the estradiol receptor gene (ER) in the forebrain (hypothalamus, preoptic area, telencephalon) of the rainbow trout (Oncorhynchus mykiss). Both sense and anti-sense [35S]UTP-labeled single-stranded RNA probes were generated from the estradiol binding domain of the ER cDNA. The sense probe was used to evaluate the background of the hybridization reaction. In the forebrain, specific signal appeared in three areas: the posterior hypothalamus, the preoptic area, and the ventral telencephalon. Our localization correlates with [3H]estradiol binding studies in other teleost species. In the pituitary, we observed a weak signal when compared to the signal observed in the forebrain (about ten grains/cell in the pituitary against 35 grains/cell in the posterior hypothalamus). A significant difference was also observed between the intensity of labeling per cell when different forebrain nuclei were compared. We provide here evidence for a tissue-specific regulation of the ER mRNA levels in the trout hypothalamo-pituitary axis.