Neurosteroid biosynthesis in the quail brain: A review

The brain traditionally has been considered to be a target site of peripheral steroid hormones. In contrast to this classical concept, new findings over the past decade have shown that the brain itself also has the capability of forming steroids de novo, the so-called "neurosteroids". De novo neurosteroidogenesis in the brain from cholesterol is a conserved property of vertebrates. Our studies using the quail, as an excellent animal model, have demonstrated that the avian brain possesses cytochrome P450 side-chain cleavage enzyme (P450scc), 3β-hydroxysteroid dehydrogenase/Δ⁵- Δ⁴-isomerase (3β-HSD), cytochrome P450 17α-hydroxylase/c17,20-lyase (P450_17α,lyase), 17β-HSD, etc., and produces pregnenolone, progesterone, 3β, 5β-tetrahydroprogesterone, androstenedione, testosterone and estradiol from cholesterol. However, the biosynthetic pathway of neurosteroids in the avian brain from cholesterol may be still incomplete, because we recently found that the quail brain actively produces 7α-hydroxypregnenolone, a previously undescribed avian neurosteroid. This paper summarize the advances made in our understanding of biosynthesis of neurosteroids in the avian brain.