Structural sex differences in the brain: influence of gonadal steroids and behavioral correlates.

INTRODUCTION Considerable debate exists in the contemporary scientific literature regarding the origin and existence of sex differences in the behavior of women and men (1). Recently, evidence was found for the existence of DNA loci that may relate to sexual orientation in man (2,3). A number of other studies [for reviews see (4-6)] have attempted to establish a biological basis for gender diffrences in humans. However these studies are often inconclusive, due in part to the tremendous variability and role of learning. Sexually dimorphic behaviors, particularly those important in reproduction are very evident when considering animal models. Historically, there was debate concerning the possibility that both sexes were capable of showing maleand female-typical behavior, with testicular steroids eliciting male-typical behavior and ovarian steroids stimulating femaletypical behavior. Further, gonadectomy during the prepuberal period resulted in a loss of mating behavior, which was restored by replacing the appropriate hormone or some of its relevant metabolites (7,8). More recent investigations have demonstrated that sex differences in behavior rely on neonatal exposure to gonadal steroids and the later presence of appropriate gonadal steroids in the adult for full expression of sexual behavior (9-14). Conversely, alterations that occur in neuroendocrine systems during aging, particularly relative to the effects of loss of estrogen on the function of the female hypothalamic neuroendocrine systems also provide new insights into how steroid modulate neuronal circuitries. However, it has to be pointed out that a