Neuroanatomical specificity of sex differences in expression of aromatase mRNA in the quail brain

In birds and mammals, aromatase activity in the preoptic-hypothalamic region (HPOA) is usually higher in males than in females. It is, however, not known whether the enzymatic sex difference reflects the differential activation of aromatase transcription or some other control mechanism. Although sex differences in aromatase activity are clearly documented in the HPOA of Japanese quail (Coturnix japonica), only minimal or even no differences at all were observed in the number of aromatase-immunoreactive (ARO-ir) cells in the medial preoptic nucleus (POM) and in the medial part of the bed nucleus striae terminalis (BSTM). We investigated by in situ hybridization the distribution and possible sex differences in aromatase mRNA expression in the brain of sexually active adult quail. The distribution of aromatase mRNA matched very closely the results of previous immunocytochemical studies with the densest signal being observed in the POM, BSTM and in the mediobasal hypothalamus (MBH). Additional weaker signals were detected in the rostral forebrain, arcopallium and mesencephalic regions. No sex difference in the optical density of the hybridization signal could be found in the POM and MBH but the area covered by mRNA was larger in males than in females, indicating a higher overall expression in males. In contrast, in the BSTM, similar areas were covered by the aromatase expression in both sexes but the density of the signal was higher in females than in males. The physiological control of aromatase is thus neuroanatomically specific and with regard to sex differences, these controls are at least partially different if one compares the level of transcription, translation and activity of the enzyme. These results also indirectly suggest that the sex difference in aromatase enzyme activity that is present in the quail HPOA largely results from differentiated controls of enzymatic activity rather than differences in enzyme concentration.