Comparative distribution of estrogen receptor alpha and beta immunoreactivities in the forebrain and the midbrain of the female guinea pig.

Estrogen plays an important role in regulating gonadotropin secretion and reproductive behavior. The estrogen receptor alpha (ERalpha) was believed to be the only receptor which mediated the actions of the hormone until the identification of a novel ER called ERbeta. In the present study, the map of ERalpha immunoreactive (IR) neurons was compared with the distribution pattern of ERbeta-IR neurons in the forebrain and midbrain of ovariectomized guinea pigs using immunocytochemistry. The immunoreactivities appeared to be mainly nuclear in their subcellular distribution. Both ERalpha- and ERbeta-like immunoreactivities were highly expressed in the bed nucleus of the stria terminalis and the ventrolateral hypothalamic nucleus but were found to be differentially expressed in discrete subregions of the amygdaloid complex. A large number of intensely labeled ERalpha cells were observed throughout the rostrocaudal extent of the preoptic region, whereas only a few ERbeta-IR neurons were found in the periventricular preoptic nucleus bordering the third ventricle or scattered in the medial preoptic area. In contrast, only ERalpha-immunoreactivity was seen in the septum, and in the magnocellular supraoptic, paraventricular, arcuate, and premammillary nuclei. In the midbrain, neurons containing ERalpha were observed throughout the rostrocaudal extent of the gray matter, whereas ERbeta was only detected within the dorsal raphe nucleus. These observations provide evidence of a distinct neuroanatomical pattern for the two subtypes of the ER which may have different roles in regulating behavior and the neuroendocrine mechanisms of reproduction. Species similarities and differences in the distributions of ERalpha and ERbeta immunoreactivities are discussed.