ONTOGENY OF AROMATASE MESSENGER-RIBONUCLEIC-ACID AND AROMATASE-ACTIVITY IN THE RAT MIDBRAIN

Estrogen formation catalyzed by neural aromatase is crucial for the se xual differentiation of the brain. Ontogenic expression of aromatase m RNA and aromatase activity were studied in male and female rat midbrai ns. Aromatase mRNA was transiently expressed in both sexes showing max imum levels on postnatal day (P)2 and being absent on P20 and in adult s. Developmental expression of aromatase mRNA preceded that of aromata se activity. These data demonstrate that the capacity for estrogen for mation is present during a distinct phase of midbrain development. Our findings suggest an active role for estrogens in the differentiation of midbrain neurons.