Immunohistochemical localization of 3β‐hydroxysteroid dehydrogenase and 5α‐reductase in the brain of the African lungfish Protopterus annectens

The localization of the enzymes responsible for the biosynthesis of neurosteroids in the brain of dipnoans has not yet been determined. In the present study, we investigated the immunohistochemical distribution of 3β‐hydroxysteroid dehydrogenase (3β‐HSD) and 5α‐reductase (5α‐R) in the brain and pituitary of the African lungfish Protopterus annectens by using antibodies raised against type I human 3β‐HSD and type I human 5α‐R. The 3β‐HSD and 5α‐R immunoreactivities were detected in cell bodies and fibers located in the same areas of the lungfish brain, namely, in the pallium, thalamus, hypothalamus, tectum, and periaqueductal gray. Identification of astrocytes, oligodendrocytes, and neurons with antisera against glial fibrillary acidic protein, galactocerebroside and neurofilaments revealed that, in the lungfish brain, 3β‐HSD immunolabeling is expressed exclusively by neurons, whereas the 5α‐R‐immunoreactive material is contained in both neurons and glial cells. In the pituitary gland, 3β‐HSD‐ and 5α‐R‐like immunoreactivity was localized in both the pars distalis and the pars intermedia. The present study provides the first immunocytochemical mapping of two key steroidogenic enzymes in the brain and pituitary of a lungfish. These data strongly suggest that neurosteroid biosynthesis occurs in the brain of fishes, as previously shown for amphibians, birds, and mammals. J. Comp. Neurol. 438:123–135, 2001. © 2001 Wiley‐Liss, Inc.