Sexual differentiation of projections from the principal nucleus of the bed nuclei of the stria terminalis.

The principal nucleus of the bed nuclei of the stria terminalis (BSTp) is sexually dimorphic and participates in several aspects of reproduction. A detailed analysis of its projections revealed that the BSTp provides major inputs to forebrain regions that are sexually dimorphic and contain high densities of neurons that express receptors for sex steroid hormones in a pattern that is remarkably similar to that of the medial amygdaloid nucleus. The BSTp sends its strongest outputs to the periventricular zone of the hypothalamus and innervates structures thought to play important roles in regulating hormone secretion from the anterior pituitary, but it also provides strong inputs to the medial preoptic and ventromedial nuclei of the hypothalamus. The BSTp also sends a strong return projection to the medial nucleus of the amygdala. The projections of the BSTp appear to be more robust in males with striking sex differences observed in most, but not all, major terminal fields. Moreover, various terminal fields appeared to differ in their developmental sensitivity to manipulation of circulating levels of sex steroids during the neonatal period. Thus, the organization of projections from the BSTp suggests that it plays a particularly important role in regulating neuroendocrine function and that neurons in this nucleus may relay olfactory information to the hypothalamus differently in male and female rats. Furthermore, the differential action of sex steroids on the density of afferents from the BSTp in various regions indicates that these hormones exert a target-specific influence on the development of BSTp projections.