Expression and localization of cytochrome P450 17 alpha-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 3beta-hydroxysteroid dehydrogenase/Delta(5)-Delta(4)-isomerase (3beta-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 17alpha-hydroxylase/c17,20-lyase (P450(17alpha,lyase)), which converts pregnenolone to dehydroepiandrosterone via 17alpha-hydroxypregnenolone or progesterone to androstenedione via 17alpha-hydroxyprogesterone. RT-PCR analysis followed by Southern hybridization indicated the expression of P450(17alpha,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 17alpha-hydroxyprogesterone was also found in brain slices of the mature male. P450(17alpha,lyase) mRNA was detected in various brain regions, but there was a clear regional difference in the expression. The expressions of P450(17alpha,lyase) mRNA in the diencephalon and mesencephalon were significantly higher than those in the cerebrum and cerebellum, unlike 3beta-HSD mRNA, which showed no regional difference in the expression. In situ hybridization revealed the cellular localization of P450(17alpha,lyase) mRNA. The cells expressing P450(17alpha,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(17alpha,lyase) mRNA. These results suggest that the avian brain possesses P450(17alpha,lyase) as well as P450scc and 3beta-HSD in both sexes. The expression of P450(17alpha,lyase) in the avian brain may be region-dependent.