PROJECT SUMMARY/ABSTRACT Reproductive success and survival of progeny in organisms that undergo sexual reproduction are contingent on social interactions, and in humans of course these behaviors are also critical for mental well-being and success in our professional lives. There have been significant advances in understanding how the nervous system of diverse animals, ranging from worms to mammals, enables successful display of social interactions. In mice for example, central components of neural circuits underlying reproductive behavior, territorial aggression, and parental care have been clearly identified in both males and females using molecular genetic approaches. Recent studies have also provided insights into how specific chemosensory cues act on these neural circuits to guide ongoing social interactions. How social interactions and their outcomes are internally sensed to modulate downstream long term changes in the organism is less well understood. Mammalian females show dramatic changes following reproductive behavior as they transition from seeking mates to preparing for pregnancy, childbirth, and subsequent nursing. The neural circuits that sense and mediate these major transitions are poorly characterized. Our goal is to characterize these neural circuits mechanistically in order to understand how they sense changes in peripheral, visceral reproductive organs to guide subsequent behavioral transitions. In unpublished findings, we have identified a population of neurons in the female mouse brain that specifically senses successful culmination of reproductive behavior but not the social interactions that precede it. In Aim 1 of this project, we propose to image ongoing activity of these neurons to define how and when they sense this behavioral endpoint. In Aim 2, we will functionally characterize these neurons to understand their contribution to post-reproductive behavior related transitions in females. In Aim 3, we will characterize projection targets of, and presynaptic inputs to, these neurons in order to understand mechanistically how this neural circuit senses an internal event to mediate these transitions; in addition, we will use molecular genetic approaches to determine the identity of these neurons. If successful, our project will provide insights into neural circuit mechanisms that regulate flexibility in female mouse social behaviors centered around reproduction. Women also experience major transitions in behavior and physiology centered around reproduction, and our work has the potential to shed light on this important aspect of human biology in health as well as potentially the many disease conditions that can impact women during this process.