In the brain, the synthesis of neurosteroids and receptor activation during gonadal sex differentiation in teleosts are poorly understood. In the present study, the protogynous orange-spotted grouper (Epinephelus coioides) was selected as a model fish, and we hypothesized that de novo synthesis of neural estrogen may play a role in the female grouper brain during gonadal sex differentiation. We investigated the temporal expression of the genes StAR, cyp19a1b and pcna and the sex steroid nuclear receptors for estrogen (ERα, ERβ1 and ERβ2), androgen (AR) and the plasma membrane-associated estrogen receptor (GPR30) in the brain during early developmental ages from 90 days after hatching (dah) to 180dah after gonadal sex differentiation. Our results revealed that mRNA for ERs and GPR30 but not AR was significantly increased at 110dah (a time close to gonadal sex differentiation) in the forebrain and midbrain and for cyp19a1b at 110dah in the forebrain. Brain aromatase activity and estradiol (E2) levels, but not testosterone (T), were increased in the forebrain at 110 and 120dah, respectively. Furthermore, exogenous E2 stimulated cyp19a1b transcripts in the forebrain and hypothalamus and immunoreactive (ir)Cyp19a1b (aromatase enzyme) in the forebrain. irCyp19a1b localized in the glial cells of the forebrain regions. Therefore, we identified a peak of functional aromatase activity and estrogen signaling in the early grouper brain during gonadal sex differentiation. Moreover, pcna transcripts (a marker for cell proliferation activity) were higher in the early brain at 110-150dah. Thus, a peak time of development in the brain is suggested to occur during gonadal sex differentiation in the grouper.
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