Sex Steroids and Afferent Input: Their Roles in Brain Sexual Differentiation

Sexually dimorphic patterns of sexual behavior and gonadotropin secretion in adult rodents arise as a result of the presence or absence of androgen (or androgen metabolites, primarily estrogen) during the perinatal period (13, 14, 16, 22-24, 44, 51, 70). Central nervous system (CNS) regions underlying control of these reproductive processes show sex differences in various morphological parameters such as size of nuclei in neuronal groups, number of neurons, neuronal volume, extent of dendritic extension and branching, number of dendritic spines, morphology and localization of synapses, and cell nuclear volume and structure (5, 14, 23, 70). Moreover, there are reports of sex differences in the pattern of connectivity between various limbic regions related to reproductive functions (15, 63). The importance of hormones in directing or organizing brain sexual dimorphism has been illustrated con­ vincingly by two types of experiments: (a) Castration of genetic (XY) males during perinatal life demasculinizes and feminizes the neural substrate for sexual behavior and feminizes the brain-pituitary axis destined to mediate adult patterns of gonadotropin secretion, and (b) injection of androgen into