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 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 important for reproduction while ER beta is more important for non-reproductive functions. In this paper we apply this hypothesis more generally by examining 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 brain employing in situ hybridization histochemistry methods. In both species ER beta was found to be diffusely present in telencephalic areas consistent with a role for this receptor subtype in cognitive functions. However, ER beta 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 ER alpha 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 ER beta mRNA expression in HVc is consistent with the hypothesis that ER alpha is preferentially involved in reproductive behaviors while ER beta is involved in the steroid 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 hypothesis until more is known about ER beta function in these other nuclei.