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Asymmetry of brain aromatase activity: region- and sex-specific developmental patterns.

Developmental patterns of aromatase activity (AA) were characterized in individual forebrain regions of the rat at gestational day (GD) 22 and postnatal days (PN) 6 and 15. Aromatase activity was measured separately in homogenates of left and right preoptic area, anterior amygdaloid area, medial amygdaloid nucleus, anterior hypothalamic area and posterior hypothalamic area, by the tritiated water method with [1 beta-3H]-androstenedione as a substrate. Region- and sex-dependent asymmetries of AA with either left-to-right or right-to-left gradients were found. They change between GD22 and PN6 and PN15 according to region-specific patterns. Thus, AA of the male medial amygdaloid nucleus of the left side is higher at GD22, lower at PN6 and equal to the right side at PN15; in females, AA of the left side is lower than AA of the right side at GD22 and higher at PN6 and PN15. In preoptic area, a side difference (left side higher) was only detected in males. Asymmetries may result from differences in the expression of the enzyme by individual cell groups, or from differences in the number of cells per area expressing the enzyme. In either case, the stage-dependent patterns of asymmetry in AA would be expected to influence sex steroid-dependent differentiation processes in individual forebrain areas.