The Brain of Teleost Fish, a Source, and a Target of Sexual Steroids

The composition of cytoplasmic messenger ribonucleoproteins (mRNPs) is determined by their nuclear and cytoplasmic histories and reflects past functions and future fates. The protein components of selected mRNP complexes promote their assembly into microscopically visible cytoplasmic RNA granules, including stress granules, processing bodies and germ cell (or polar) granules. We propose that RNA granules can be both a cause and a consequence of altered mRNA translation, decay or editing. In this capacity, RNA granules serve as key modulators of post-transcriptional and epigenetic gene expression.

Over the past decade, it has become clear that the brain is a steroidogenic organ. 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 ion-gated neurotransmitter receptors. This paper summarizes what is known about the biosynthesis of neurosteroids, the enzymes mediating these reactions, their localization during development and in the adult, and their function and mechanisms of action in the developing and adult central and peripheral nervous systems. The expression of the steroidogenic enzymes is developmentally regulated, with some enzymes being expressed only during development, while others are expressed during development and in the adult. These enzymes are expressed in both neurons and glia, suggesting that these two cell types must work in concert to produce the appropriate active neurosteroid. The functions attributed to specific neurosteroids include modulation of GABA(A) and NMDA function, modulation of sigma receptor function, regulation of myelinization, neuroprotection, and growth of axons and dendrites. Neurosteroids have also been shown to modulate expression of particular subunits of GABA(A) and NMDA receptors, providing additional sites at which these compounds can regulate neural function. The pharmacological properties of specific neurosteroids are described, and potential uses of neurosteroids in specific neuropathologies and during normal aging in humans are also discussed. Copyright 2000 Academic Press.