Steroid sensitive sites in the avian brain: Does the distribution of the estrogen receptor alpha and beta types provide insight into their function?

Studies in avian species have often been useful in elucidating basic concep ts relevant to the regulation of reproductive behaviors by sex steroid horm ones. Once a link between a steroid hormone and a behavioral response has b een established, one can use the localization of steroid hormone receptors in the brain to facilitate the identification of neural circuits that contr ol behavior. The recent identification of a second type of estrogen recepto r called estrogen receptor beta or ER beta has raised new issues about the action of steroid hormones in the brain. A hypothesis has been proposed by Kuiper et al. [1998] based on studies in mammalian species suggesting that ER alpha (the name given to the ER that was previously described) is import ant for reproduction while ER beta is more important for non-reproductive f unctions. In this paper we apply this hypothesis more generally by examinin g possible functions of ER beta in avian species. We have initiated studies of the ERP in the brain of two avian species, the Japanese quail (Coturnix japonica) and the European starling (Sturnus vulgaris). ER beta was cloned in both species and the mRNA for this receptor type was localized in the b rain employing in situ hybridization histochemistry methods. In both specie s ER beta was found to be diffusely present in telencephalic areas consiste nt with a role for this receptor subtype in cognitive functions. However, E R beta mRNA was also found in many brain areas that are traditionally thoug ht to be important in the regulation of reproductive functions such as the preoptic region, the bed nucleus of the stria terminalis and the nucleus ta eniae. Of the two receptor types, only mRNA for ER alpha was observed in th e telencephalic vocal control nucleus HVc of male starlings. Steroid recept ors in this nucleus are thought to be an example of an evolutionary special ization that has evolved to coordinate the production of courtship vocaliza tions with other aspects of reproduction. The lack of ER beta mRNA expressi on in HVc is consistent with the hypothesis that ER alpha is preferentially involved in reproductive behaviors while ER beta is involved in the steroi d regulation of other neural functions. However, the widespread occurrence of ER beta in other nuclei involved in reproductive function suggests that one must be cautious about the general applicability of the above hypothesi s until more is known about ER beta function in these other nuclei.