Sexual dimorphism of brain aromatase activity in medaka: Induction of a female phenotype by estradiol

In this study we identified sex-dependent dimorphism of brain aromatase in the teleost medaka and examined its regulation by sex steriods. We first in vestigated differential distribution of brain aromatase activity in sexuall y mature male and female medaka in serial coronal sections of the brain and identified the hypothalamic nuclei contained in each section using the bra in atlas of medaka. In the brain of male medaka, high levels of activity ar e localized in sections containing the preoptic (POA) and suprachiasmatic n uclei (SC) (63-75 fmol/hr) and low levels in the nuclei periventricular dor salis (HD), ventralis (HV), and caudalis (Hc), nuclei diffusus of lobulus i nferiores (NDIL), and nuclei tuberi anteriores (TA) and posteriores (TP) (< 25 fmol/hr). In the brain of female medaka high aromatase activity is loca lized in sections containing the HD, I-IV, Hc, NDIL, TA, and TP (85-80 fmol /hr) and highly variable levels in the POA and SC (23-70 fmol/hr). The conc entration and time dependency of the exposure of male medaka to estradiol o n the total brain aromatase activity and morphologic sex characteristics we re determined next. Estradiol increased the activity of brain aromatase in a concentration-dependent manner at 2.5 and 25 <mu>g/L, but the increase wa s lower at higher concentrations of the hormone. The effect was time depend ent, gradually increasing up to the fifth day of exposure, after which it r eached a plateau. Estradiol induction of brain aromatase analyzed using Lin eweaver-Burke plots of saturation assays revealed a non-first-order reactio n. The results indicate that a positive Feedback mechanism regulates brain aromatase and imply that the sexual dimorphic distribution of aromatase may be highly sensitive to physiologic cues and environmental perturbations in fish.