SEX-SPECIFIC AROMATIZATION OF TESTOSTERONE IN MOUSE HYPOTHALAMIC NEURONS

Conversion of androgens to oestrogens by neural aromatase during brain development appears to be a prerequisite for sexual differentiation o f the mammalian central nervous system. In order to investigate the pr e- and perinatal patterns of testosterone (T) aromatization in the mal e and female mouse brain, aromatase activity (AA) was measured in hypo thalamic and cerebral homogenates of embryonic day (ED) 17 fetuses and neonates using an in vitro (H2O)-H-3 product formation microassay. In addition, AA was examined in gender-specific neuronal cell cultures p repared from ED 15 mouse cerebral hemisphere and hypthalamus at 3 and 6 days in vitro (DIV), and this was compared with enzyme activities in homogenates. The aromatase has also been evaluated in glial-enriched cultures from ED 20 mouse hypothalamus and cortex as well as in ED 15 cultures treated with the neurotoxin kainic acid in order to localize AA to neurons and/or glial cells. Significant sex differencees in AA w ere observed in hypothalamic tissue homogenates as early as ED 17, bec oming even more distinct in neonates, AA being always higher in males compared to females. Similar AA was also found in cells from both sexe s from cultured ED 15 hypothalamus after 3 DIV. However, significantly higher AA was observed after 6 DIV in ED 15 male hypothalamic culture s compared to female. ED 20 glial-enriched hypothalamic culutures (pur ity >95%) from both brain regions exhibited very low AA after 6 DIV, a nd no sex differences were found. After treatment of ED 15 hypothalami c cultures with kainic acid, a 70-80% decrease in AA was observed in b oth sexes compared to non-treated cultures and sex differences were no longer present. Kinetic studies performed on aromatase in male and fe male hypothalamic cells from ED 15 cultures after 6 DIV, from ED 17 fe tuses, and from neonatal homogenates with T as substrate indicate simi lar T binding affinities in cultures as in embryonic and neonatal homo genates from both genders, with an apparent K-m of similar to 40 nM. I n cortex, from either ED 15 cultures, ED 20 cultures, ED 17 fetuses or neonates, AA was low and sex differences were not observed. Taken tog ether, our results demonstrate: (1) sex differences in hypothalamic AA during pre- and perinatal development in the mouse with a higher capa city for oestrogen formation in male than in female; (2) no sex differ ences in AA from cortical tissues; (3) no sexual or developmental diff erences in hypothalamic aromatase T binding affinity; (4) aromatase is neuronal rather than astroglial, and (5) sex differences in AA are co nsequentially restricted to neurons.