Distribution and localization of 3β-hydroxysteroid dehydrogenase (3β-HSD) in the brain and its regions of the catfish Heteropneustes fossilis.

In vertebrates, steroids are synthesized de novo in the central and peripheral nervous system, independent of peripheral steroidogenic glands, such as the adrenal, gonads and placenta. 3β-Hydroxysteroid dehydrogenase/Δ5-Δ4-isomerase (3β-HSD) is a key steroidogenic enzyme in vertebrate gonads, placenta and adrenal. It mediates the oxidation and isomerization reactions of progesterone from pregnenolone, 17-hydroxyprogesterone from 17-hydroxypregnenolone and androstenedione from dehydroepiandrosterone. In the present study, we examined the expression of 3β-HSD cDNA by real time-PCR and localization of the mRNA by in situ hybridization in the brain and its regions during the different phases of the reproductive cycle of the catfish Heteropneustes fossilis. Further, 3β-HSD activity was assayed biochemically to show seasonal variations. We showed significant seasonal and sexual dimorphic changes in the levels of transcript abundance in the whole brain and its regions. In whole brain, level was the highest in post-spawning phase and lowest in spawning phase in males. In females, there was a progressive increase through resting phase to pre-spawning phase, a decline in the spawning phase and increase in the post-spawning phase. In the preparatory phase, the highest transcript level was seen in medulla oblongata and the lowest in pituitary in males. In females, the level was the highest in the hypothalamus and lowest in olfactory bulb and pituitary. However, in the pre-spawning phase, in males it was the highest in telencephalon and hypothalamus and lowest in pituitary. In females, the highest transcript level was in olfactory bulb and lowest in pituitary. 3β-HSD enzyme activity showed significant seasonal variation in the brain, the highest in the resting phase and lowest in the preparatory and spawning phases. In situ hybridization showed the presence of 3β-HSD transcript was especially high in the cerebellum region. The presence of 3β-HSD in the brain may indicate steroidogenesis in the catfish brain.