Expression and activity of 3β-hydroxysteroid dehydrogenase/Δ⁵-Δ⁴- isomerase in different regions of the avian brain

Recently, we have demonstrated, using biochemical and immunochemical methods, that the quail brain possesses the cholesterol side-chain cleavage enzyme (cytochrome P450scc) and produces pregnenolone and its sulfate ester. To clarify progesterone biosynthesis in the avian brain, therefore, we examined the expression of messenger RNA (mRNA) encoding for the enzyme 3β- hydroxysteroid dehydrogenase/Δ⁵-Δ⁴-isomerase (3β-HSD) and its enzymatic activity using the quail. RT-PCR analysis together with Southern hybridization indicated the expression of 3β-HSD mRNA in the brain of sexually mature birds but with no clear-cut sex difference. Employing biochemical techniques combined with HPLC analysis, the conversion of pregnenolone to progesterone was found in brain slices of mature males. Progesterone biosynthesis was increased in a time dependent manner and completely abolished by trilostane, a specific inhibitor of 3β-HSD. The enzymatic activity of 3β-HSD was greatest in the cerebrum and lowest in the mesencephalon. A specific RIA indicated that progesterone concentrations in the different brain regions closely followed the level of 3β-HSD activity. High levels of progesterone concentration were observed in the diencephalon and cerebrum with lowest values in the mesencephalon. Progesterone levels in the brain regions were significantly higher than those in the plasma. These results suggest that the avian brain possesses not only cytochrome P450scc but also 3β-HSD and produces progesterone. It is also indicated that progesterone biosynthesis in the avian brain may be region-dependent.