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 sexu al differentiation of brain structures and function. Similar to other brain regions, estrogen is known to influence neuronal differentiation and plast icity in the hippocampus. The present study is concerned with the developme ntal expression of mRNAs for the estrogen-synthesizing enzyme aromatase and the two known nuclear estrogen receptors (alpha/beta) in the male and fema le mouse hippocampus. Using semiquantitative reverse transcriptase polymera se chain reaction (RT-PCR) analysis, we found that aromatase as well as est rogen receptors (alpha/beta) are already expressed prenatally in the hippoc ampus of both sexes. Aromatase expression increased during the first two po stnatal weeks and decreased, thereafter, to lower levels in adults. Sex dif ferences in aromatase expression were observed postnatally with higher leve ls in males. Estrogen receptor-alpha/beta mRNAs did not fluctuate obviously throughout pre- and postnatal development but revealed a distinct sex-spec ific pattern at the end of the first postnatal week. Again, higher expressi on was detected in males. These findings clearly demonstrate the capacity o f estrogen formation and the presence of both estrogen receptor subtypes in the developing hippocampus. Sex differences in aromatase mRNA levels paral leled the sex-specific pattern of estrogen receptor expression. Thus, our d ata support the idea that the developing hippocampus is a target for estrog en action and estrogen receptor-mediated sexual differentiation.