Almost everyone would agree that gonadal steroid hormones cause sex differences in the nervous system. Testosterone, acting via androgenic or estrogenic metabolites, alters the form and function of the developing brain. What is not so clear, however, is exactly where androgens and estrogens act to cause neural sex differences. Consider the dilemma: most brain regions are composed of multiple cell types, several of which may express androgen or estrogen receptors. Each region receives input from dozens (if not hundreds) of sites and in turn projects to dozens more; many of these afferent and efferent cell groups are also likely to express the relevant receptors. Thus, testosterone may cause a given sex difference by acting at the sexually dimorphic cells themselves, at afferents or target cells or even at neighboring glial cells. Given this complexity, the problem of identifying the crucial site(s) of hormone action becomes almost insurmountable.
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