Ontogenetic expression and sex differences of aromatase and estrogen receptor-alpha/beta mRNA in the mouse hippocampus.

Estrogen plays an important role during brain development interfering with the maturation of distinct neural systems and, in particular, with the sexual differentiation of brain structures and function. Similar to other brain regions, estrogen is known to influence neuronal differentiation and plasticity in the hippocampus. The present study is concerned with the developmental expression of mRNAs for the estrogen-synthesizing enzyme aromatase and the two known nuclear estrogen receptors (alpha/beta) in the male and female mouse hippocampus. Using semiquantitative reverse transcriptase polymerase chain reaction (RT-PCR) analysis, we found that aromatase as well as estrogen receptors (alpha/beta) are already expressed prenatally in the hippocampus of both sexes. Aromatase expression increased during the first two postnatal weeks and decreased, thereafter, to lower levels in adults. Sex differences in aromatase expression were observed postnatally with higher levels in males. Estrogen receptor-alpha/beta mRNAs did not fluctuate obviously throughout pre- and postnatal development but revealed a distinct sex-specific pattern at the end of the first postnatal week. Again, higher expression was detected in males. These findings clearly demonstrate the capacity of estrogen formation and the presence of both estrogen receptor subtypes in the developing hippocampus. Sex differences in aromatase mRNA levels paralleled the sex-specific pattern of estrogen receptor expression. Thus, our data support the idea that the developing hippocampus is a target for estrogen action and estrogen receptor-mediated sexual differentiation.