It is well established that testosterone from testicular origin plays a critical role in the activation of male sexual behavior in most, if not all, vertebrate species. These effects take place to a large extent in the preoptic area although other brain sites are obviously also implicated. In its target areas, testosterone is actively metabolized either into estrogenic and androgenic steroids that have specific behavioral effects or into inactive metabolites. These transformations either amplify the behavioral activity of testosterone or, alternatively, metabolism to an inactive compound dissipates any biological effect. Androgens and estrogens then bind to nuclear receptors that modulate the transcription of specific genes. This process is controlled by a variety of co-activators and co-repressors that, respectively, enhance or inhibit these transcriptional processes. In addition, recent work has shown that the production of estrogens by brain aromatase can be modulated within minutes by changes in neural activity and that these rapid changes in neuroestrogen production impact sexual behavior, in particular sexual motivation within the same time frame. Estrogens thus affect specific aspects of male sexual behavior in two different time frames via two types of mechanisms that are completely different. Multiple questions remain open concerning the cellular brain mechanisms that mediate testosterone action on male sexual behavior.
Jacques Balthazart and Gregory F. Ball
Paul E. Micevych and Melinda A. Mittelman-Smith
In the last two decades of the 20th century, key findings in the field of estrogen signaling completely changed our understanding of hormones: first, steroidogenesis was demonstrated in the CNS; second, a vast majority of cells in the nervous system were shown to have estrogen receptors; third, a second nuclear estrogen receptor (ERß) was cloned; and finally, “nuclear” receptors were shown to be present and functional in the cell membrane. Shortly thereafter, even more membrane estrogen receptors were discovered. Steroids (estrogens, in particular) began to be considered as neurotransmitters and their receptors were tethered to G protein-coupled receptor signaling cascades. In some parts of the brain, levels of steroids appeared to be independent of those found in the circulation and yet, circulating steroids had profound actions on the brain physiology. In this review, we discuss the interaction of peripheral and central estrogen action in the context of female reproduction—one of the best-studied aspects of steroid action. In addition to reviewing the evidence for steroidogenesis in the hypothalamus, we review membrane-localized nuclear receptors coupling to G protein-signaling cascades and the downstream physiological consequences for reproduction. We will also introduce newer work that demonstrates cell signaling for a common splice variant of estrogen receptor-α (ERα), and membrane action of neuroprogesterone in regulating estrogen positive feedback.