Activity and expression of steroidogenic enzymes in the brain of adult zebrafish.

The brain of adult teleost fish exhibits several unique and interesting features, notably an intense neurogenic activity linked to persistence of radial glial cells acting as neural progenitors, and a high aromatase activity supported by strong expression of the cyp19a1b gene. Strikingly, cyp19a1b expression is restricted to radial glial cells, suggesting that estrogens are able to modulate their activity. This raises the question of the origin, central or peripheral, of C19 androgens available for aromatization. This study aimed to investigate the activity and expression of other main steroidogenic enzymes in the brain of adult zebrafish. We demonstrate by high-performance liquid chromatography that the zebrafish brain has the ability to convert [³H]-pregnenolone into a variety of radiolabeled steroids such as 17OH-pregnenolone, dehydroepiandrosterone, androstenedione, testosterone, dihydro-testosterone, estrone, estradiol, progesterone, and dihydro- and tetrahydro-progesterone. Next, we show by in situ hybridization that messengers for key steroidogenic enzymes, such as Cyp11a1 (P450(SCC)), 3β-Hsd, Cyp17 and Cyp19a1b, are widely expressed in the forebrain where they exhibit an overall similar pattern. By combining aromatase B immunohistochemistry with in situ hybridization, we show that cyp11a1, 3β-hsd and cyp17 messengers are found in part in aromatase B-positive radial processes, suggesting mRNA export. This set of results provides the first demonstration that the brain of fish can produce true neurosteroids, possibly in radial glial cells. Given that radial glial cells are brain stem cells during the entire lifespan of fish, it is suggested that at least some of these neurosteroids are implicated in the persisting neurogenic process.