Project Summary This project aims to understand the function of sex difference in the neural control of social behavior by focusing on the sexually dimorphic vasopressin system, where males have much higher vasopressin expression than females. Experiments proposed during the fellowship period will identify whether the sex difference in vasopressin innervation contributes to sex differences in the control of social behavior and communication. I will gain exceptional training in powerful genetic approaches that allow us to target vasopressin cells and the vasopressin gene specifically. The first aim tests whether excitation or inhibition of the sexually dimorphic vasopressin cells in the bed nucleus of the stria terminalis (BNST) affects social behavior differently in males and females. The second aim tests whether vasopressin is the key neuropeptide mediating sex-different effects on social communication observed in a previous study that ablated BNST vasopressin cells. This project will significantly enhance our mechanistic understanding of how the brain controls social behavior differently in males and females and may explain why many behavioral disorders show striking sex differences in morbidity. Although recently major advances have been made in understanding the neural basis of social behavior in adulthood, how such behavior is controlled differently in males and females is, by and large, unknown. This training grant will address these issues.