Estrogen receptor alpha, but not beta, is expressed in the interneurons of the hippocampus in prepubertal rats: an in situ hybridization study.

Estradiol is involved in the differentiation and plasticity of hippocampal neurons. In the CA1 region, estrogen treatment increases dendritic spines and synapse density on pyramidal cells. In the adult hippocampus, immunoreactivity for estrogen receptor alpha (ERalpha) has been reported in inhibitory interneurons, but neither in the pyramidal neurons nor in granule cells. Estrogens also mediate aspects of sexual differentiation of the hippocampus. To examine the possibility that an alteration in expression of the two types of estrogen receptors (ERalpha and ERbeta) in the hippocampus underlies different roles of estrogen and/or ERs during development and in adult life, we applied non-isotopic, digoxigenin (dig)-labeled, in situ hybridization histochemistry (ISHH) for the both ER forms and examined the distribution pattern of their messages in serial, frontal sections over the postnatal period and in the adult. ERalpha mRNA expression was found scattered throughout the hippocampus especially in the hilar region of the dentate gyrus, and in the strata radiatum and pyramidale in the cornus ammonis at postnatal days (PND) 14, 21 and 35. In the hilus of the dorsal hippocampus, the density of ERalpha-labelled cells was greater in the rostro-medial aspect, while less in the lateral and the caudal region. In the ventral hippocampus the signals for ERalpha mRNA were also found in relatively high density in the hilus. No significant sex difference in distribution and intensity of the ERalpha mRNA positive cells were detected. The hippocampal distribution of ERalpha mRNA expression at PND 14 remained the same on PND 21 and 35 and in adulthood. As reported for adults, ERalpha mRNA signals appear to be in interneurons of the hippocampus but neither in the pyramidal cells nor in the dentate granular cells based on their size and location. In contrast to the result of ERalpha, no clear signals for ERbeta mRNA were detected in the hippocampus across all ages examined, whereas they were clearly detected in the hypothalamus.