Nonneuronal localization for steroid converting enzyme: 3 alpha-hydroxysteroid oxidoreductase in olfactory tubercle of rat brain.

3 alpha-Hydroxysteroid oxidoreductase (EC 1.1.1.50) was localized in the rat brain by cryostat sectioning, microassay, and neurochemical lesions. Single 16-microns sections were cut, homogenized, and assayed. In the olfactory tubercle 3 alpha-hydroxysteroid oxidoreductase activity is high in the piaglial layer at the surface, 20-fold lower at a depth of 50 microns, and 50-fold lower at a depth of 200 microns. A similar pattern of activity was seen in the olfactory bulb, the interpeduncular nucleus, the frontal pole of the cortex, and the frontoparietal cortex. When kainic acid, a toxin that destroys neurons but leaves glia and axons of passage intact, was injected into the olfactory tubercle, 3 alpha-hydroxysteroid oxidoreductase activity was undiminished whereas glutamic acid decarboxylase activity was reduced by 80%. This laminar distribution and insensitivity to kainic acid are consistent with a nonneuronal localization. The high concentration of astrocytes in the piaglial layer, where 3 alpha-hydroxysteroid oxidoreductase activity is highest, lead us to suggest that this enzyme is localized to astrocytes. The presence of particular enzymes in some brain regions and not in others determines which products are synthesized and which are inactivated in those regions. Thus, the location of 3 alpha-hydroxysteroid oxidoreductase and other steroid converting enzymes can affect the activity of neuronal circuits and the behaviours regulated by those circuits.