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

Studies in avian species have often been useful in elucidating basic concepts relevant to the regulation of reproductive behaviors by sex steroid hormones. Once a link between a steroid hormone and a behavioral response has been established, one can use the localization of steroid hormone receptors in the brain to facilitate the identification of neural circuits that control behavior. The recent identification of a second type of estrogen receptor called estrogen receptor β or ERβ 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 ERα (the name given to the ER that was previously described) is important for reproduction while ERβ is more important for non-reproductive functions. In this paper we apply this hypothesis more generally by examining possible functions of ERβ in avian species. We have initiated studies of the ERβ in the brain of two avian species, the Japanese quail (Coturnix japonica) and the European starling (Sturnus vulgaris). ERβ was cloned in both species and the mRNA for this receptor type was localized in the brain employing in situ hybridization histochemistry methods. In both species ERβ was found to be diffusely present in telencephalic areas consistent with a role for this receptor subtype in cognitive functions. However, ERβ mRNA was also found in many brain areas that are traditionally thought to be important in the regulation of reproductive functions such as the preoptic region, the bed nucleus of the stria terminalis and the nucleus taeniae. Of the two receptor types, only mRNA for ERα was observed in the telencephalic vocal control nucleus HVc of male starlings. Steroid receptors in this nucleus are thought to be an example of an evolutionary specialization that has evolved to coordinate the production of courtship vocalizations with other aspects of reproduction. The lack of ERβ mRNA expression in HVc is consistent with the hypothesis that ERα is preferentially involved in reproductive behaviors while ERα is involved in the steroid regulation of other neural functions. However, the widespread occurrence of ERα in other nuclei involved in reproductive function suggests that one must be cautious about the general applicability of the above hypothesis until more is known about ERβ function in these other nuclei.