Immunohistochemical localization and biological activity of 3β-hydroxysteroid dehydrogenase and 5α-reductase in the brain of the frog, Rana esculenta, during development

The occurrence of several enzymes responsible for the biosynthesis of neurosteroids in the brain of adult frogs is now firmly established but the expression of these enzymes during ontogenesis has not yet been investigated. In the present report, we describe the immunohistochemical distribution and biological activity of 3β-hydroxysteroid dehydrogenase (3β-HSD) and 5α-reductase (5α-R) in the brain of the European green frog, Rana esculenta, during larval development. The spatio-temporal distribution of 3β-HSD and 5α-R immunoreactivities in the tadpole brain was generally different, although these two enzymes were occasionally detected in the same areas such as the olfactory bulbs and cerebellum. Identification of neurons based on their morphological aspect as well as labeling of astrocytes with an antiserum against glial fibrillary acidic protein (GFAP) revealed that, in the tadpole brain, 3β-HSD- and 5α-R-immunoreactive materials were contained in both neurons and glial cells. Incubation of tadpole brain explants with [3H]-pregnenolone resulted in the formation of several tritiated steroids including progesterone, 17-hydroxyprogesterone, androstenedione, 5α-dihydroprogesterone and 5α-dihydrotestosterone. The present study provides the first immunocytochemical mapping of two key steroidogenic enzymes in the developing frog brain. The data also indicate that neurosteroid biosynthesis occurs in the brain of tadpoles, as previously shown for adult amphibians, birds and mammals. The transient expression of steroidogenic enzymes in several regions of the tadpole brain suggests that, in amphibians, neurosteroids may be implicated in neurotrophic activities during larval development.