Project abstract Social connectedness is crucially important to human health, and is increasingly recognized as such, especially in these days of “social distancing”. While strong social relationships may confer a survivorship advantage with large effect sizes similar to those for health behaviors like diet and exercise1, disruption and loss of social relationships carry significant health risks and mortality, including increased risk of mental illness, cardiovascular disease, stroke, etc.2–4. However, neurobiological mechanisms for positive and negative aspects of social connectedness have been under-studied. The κ opioid system, which is systematically activated during stress (including social stress), and oxytocin (OT), a hormone strongly implicated in social connectedness (and in particular, their interactions), have not been fully explored in the contexts of positive and negative effects on social connectedness. In the current proposal, we will investigate biological mechanisms for positive and negative mechanisms of social connectedness (social buffering and separation), particularly in the κ opioid and oxytocin (OT) systems. By using a unique non-human primate model, the titi monkey (Plecturocebus cupreus), we will be able to address mechanism and causality in a way difficult or impossible to do in humans. Titi monkeys are a socially monogamous New World primate. They exhibit all the signs of adult pair-bonding both in the wild5–7 and in the laboratory8–10, including a preference for the familiar partner11; distress upon separation from the partner specifically12; and the ability of the partner to buffer the stress response12; they are thus the perfect primate model in which to study the neurobiology of pair bonding in all its aspects, including social buffering and separation13. This socially monogamous mating system allows us to study adult attachment in adult males as well as females, which is not possible in other common laboratory models like rats, mice, and rhesus monkeys. We have shown the separation response is modulated by opioids and that the OT system is also responsive to separation. We will use C11 PET imaging and pharmacological treatments during social buffering, short term separation, and longer-term separation to test the hypothesis that the κ opioid receptor and OT interact such that activation of the κ system results in inhibition of OT in the nucleus accumbens during social separation and loss; chronically, social separation and loss will result in down-regulation of κ opioid receptors and stimulation of OT in the hypothalamus; while inhibition of κ receptors, and downstream effects on OT, are implicated in the positive, anti-stress components of social bonds. Ultimately, this project will improve our understanding of the neurobiological basis of social connectedness in both its positive and negative aspects, providing for mechanistic connections to human health.