Recently, we demonstrated that cytochrome P450 side-chain cleavage enzyme (P450scc) occurs in the rat cerebellar Purkinje cell after differentiation and remains during neonatal development and into adulthood. 3β-Hydroxysteroid dehydrogenase/Δ5-Δ4-isomerase (3βHSD) is also an essential enzyme for progesterone biosynthesis not only in peripheral steroidogenic glands but also in the nervous system. In the present study, therefore, the expression of 3βHSD in the rat cerebellum was investigated during neonatal development and in the adult. RT-PCR analysis showed that the expression of 3βHSD messenger RNA (mRNA) in the cerebellum was higher at 7- 14 days of age than at other times. Biochemical studies together with HPLC analysis revealed that cerebellar slices at 10 days of age converted pregnenolone to progesterone, suggesting enzymatic activity of 3βHSD. This conversion was significantly reduced by trilostane, a specific inhibitor of 3βHSD. A specific RIA indicated that progesterone concentrations in the cerebellum were higher at 3 and 10 days of age than at 60 days of age. The progesterone level in the cerebellum was significantly higher than that in plasma at 10 days of age. In contrast, the concentrations in both cerebellum and plasma at 3 and 60 days of age were similar. In the present study, the site of 3βHSD mRNA expression in the cerebellum was further examined in neonatal and adult rats using in situ hybridization. The cerebellar expression of 3βHSD mRNA was obscure at 3 days of age, whereas intense expression occurred in Purkinje cells and external granule cells throughout the cerebellum at 10 days of age. 3βHSD mRNA was also expressed in Purkinje cells and granule cells at 60 days of age, but a restricted expression was observed along the cerebellar meninges. These results suggest that the steroidogenic enzyme 3βHSD as well as P450scc are expressed at least in the cerebellar Purkinje cell. The expression of 3βHSD, however, may increase for a limited period around 10 days of age, unlike P450scc.
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