Sexually dimorphic expression of oxytocin binding sites in forebrain and spinal cord of the rat

The central actions of oxytocin on reproduction-related functions and behaviors are strongly steroid-dependent and gender specific. This study characterizes sexual differences in the oxytocin binding site expression in forebrain and spinal cord of the rat. Using film autoradiography, we quantified the density of oxytocin binding sites in the ventromedial hypothalamic nucleus, the medial and central nuclei of the amygdala, the medial bed nucleus of the stria terminalis and the spinal cord dorsal horns both in adult male and female rats, and during development. In addition, neonatal castrated males and intact neonatal females treated with a single injection of testosterone (1 mg) were examined. Data showed a sexual dimorphism in the expression of oxytocin binding sites in the spinal cord dorsal horns and in restricted areas of the forebrain that are sensitive to gonadal steroids such as the ventromedial hypothalamic nucleus, but not in gonadal steroid insensitive sites such as the central nucleus of the amygdala. Adult males had higher oxytocin binding site densities in the ventromedial hypothalamic nucleus and dorsal horns than females. In the forebrain, but not in the dorsal horn, this sexual difference required a perinatal exposure to testosterone. Neonatal castration only abolished the sexual difference in the ventromedial hypothalamic nucleus of adults, but not in the dorsal horn. Furthermore, females that received a single injection of testosterone 1 day after birth showed significant increases in the density of oxytocin binding sites in the ventromedial hypothalamic nucleus, medial nucleus of the amygdala and medial bed nucleus of the stria terminalis. In addition, the findings suggest that the sexual difference in the ventromedial hypothalamic nucleus also requires gonadal hormones in adulthood. Our data support the hypothesis that sexually dimorphic oxytocin binding sites may contribute to the regulatory central actions of oxytocin in gender specific functions and behaviors such as nociception and reproduction.