Expression and localization of cytochrome P450 17α-hydroxylase/c17,20-lyase in the avian brain

Steroids synthesized de novo from cholesterol in the brain are generally called neurosteroids. We have recently demonstrated, using biochemical and molecular biological methods, that certain structures in the quail brain possess cytochrome P450 side-chain cleavage enzyme (P450scc) and 3β-hydroxysteroid dehydrogenaseΔ⁵-Δ⁴-isomerase (3β-HSD) and produce pregnenolone, pregnenolone sulfate and progesterone. To clarify the biosynthetic pathway of neurosteroids in the avian brain, therefore, we examined the expression of messenger RNA (mRNA) encoding for the enzyme cytochrome P450 17α-hydroxylase/c17,20-lyase (P450_17α,lyase), which converts pregnenolone to dehydroepiandrosterone via 17α-hydroxypregnenolone or progesterone to androstenedione via 17α-hydroxyprogesterone. RT-PCR analysis followed by Southern hybridization indicated the expression of P450_17α,lyase mRNA in the brain of sexually mature birds without a clear-cut sex difference. Employing biochemical techniques combined with HPLC analysis, the conversion of progesterone to 17α-hydroxyprogesterone was also found in brain slices of the mature male. P450_17α,lyase mRNA was detected in various brain regions, but there was a clear regional difference in the expression. The expressions of P450_17α,lyase mRNA in the diencephalon and mesencephalon were significantly higher than those in the cerebrum and cerebellum, unlike 3β-HSD mRNA, which showed no regional difference in the expression. In situ hybridization revealed the cellular localization of P450_17α,lyase mRNA. The cells expressing P450_17α,lyase mRNA were detected several diencephalic and mesencephalic regions, such as the preoptic area, the anterior hypothalamus, the dorsolateral thalamus, the optic tectum and the ventral midbrain. The expression was also localized in the septum, the hyperstriatum accessorium, and the ventral portions of the archistriatum in the telencephalon. Cerebellar Purkinje cells also expressed P450_17α,lyase mRNA. These results suggest that the avian brain possesses P450_17α,lyase as well as P450scc and 3β-HSD in both sexes. The expression of P450_17α,lyase in the avian brain may be region-dependent.