Expression of Cytochrome P450 Side‐Chain Cleavage Enzyme and 3β‐Hydroxysteroid Dehydrogenase in the Rat Central Nervous System: A Study by Polymerase Chain Reaction and In Situ Hybridization

: In examining steroid synthesis in the CNS, expression of the mRNAs encoding for cytochrome P450 side‐chain cleavage enzyme (P450SCC) and 3β‐hydroxysteroid dehydrogenase/Δ5‐Δ4 isomerase (3β‐HSD) has been studied in the rat brain. P450SCC transforms cholesterol into pregnenolone and 3β‐HSD transforms pregnenolone into progesterone. PCR was used to amplify cDNA sequences from total RNA extracts. Classical steroidogenic tissues, like adrenal and testis, as well as the non‐steroidogenic tissue lung have been used as controls. The expression of P450SCC and 3β‐HSD have been demonstrated by PCR in cortex, cerebellum, and spinal cord. In addition, primary cultures of rat cerebellar glial cells and rat cerebellar granule cells were found to express P450SCC and 3β‐HSD at comparable levels. Furthermore, three of the four known isoenzymes of 3β‐HSD were identified, as determined using selective PCR primers coupled with discriminative restriction enzymes and sequencing analysis of the amplified brain products. Using RNA probes, in situ hybridization indicated that P450SCC and 3β‐HSD are expressed throughout the brain at a low level and mainly in white matter. Enrichment of glial cell cultures in oligodendrocytes, however, does not increase the relative abundance of P450SCC and 3β‐HSD mRNA detected by PCR. This discrepancy suggests that the developmental state of cultured cells and their intercellular environment may be critical for regulating the expression of these enzymes. These findings support the proposal that the brain apparently has the capacity to synthesize progesterone from cholesterol, through pregnenolone, but that the expression of these enzymes appears to be quite low. Furthermore, the identification of these messages in cerebellar granule cell cultures implies that certain neurons, in addition to glial cells, may express these steroidogenic enzymes.