Biosynthesis and action of neurosteroids.

Over the past decade, it has become clear that the brain, like the gonad, adrenal and placenta, is a steroidogenic organ. However, unlike classic steroidogenic tissues, the synthesis of steroids in the nervous system requires the coordinate expression and regulation of the genes encoding the steroidogenic enzymes in several different cell types (neurons and glia) at different locations in the nervous system, and at distances from the cell bodies. The steroids synthesized by the brain and nervous system, given the name neurosteroids, have a wide variety of diverse functions. In general, they mediate their actions, not through classic steroid hormone nuclear receptors, but through other mechanisms such as through ion gated neurotransmitter receptors, or through direct or indirect modulation of other neurotransmitter receptors. We have briefly summarized the biochemistry of the enzymes involved in the biosynthesis of neurosteroids, their localization during development and in the adult, and the regulation of their expression, highlighting both similarities and differences between expression in the brain and in classic steroidogenic tissues.