PROJECT SUMMARY (Project 1, Young) Evidence from animal studies suggests that oxytocin (OT) plays an important role in facilitating social behavior. It is hypothesized that the influence of OT on social cognition is facilitated by increasing the salience and rewarding value of social stimuli. Human studies, inspired by the work in animal models, have investigated the effects of intranasal (IN) administration of OT on a wide range of human social behaviors, including psychiatric outcomes such as social functioning in individuals with autism spectrum disorder (ASD). The clinical efficacy of IN-OT is however questionable because of limited blood-brain-barrier penetration and diffusion within the brain. Stimulating endogenous OT release would circumvent these issues. Studies investigating the effects of targeting the melanocortin system have shown that melanocortin 4 receptors (MC4Rs) interact with several neurochemical systems known to modulate social behavior including OT. Work in our laboratory has shown that peripheral injection of melanotan II (MTII), a brain penetrant small-molecule selective MC4R agonist, facilitates partner preference formation in monogamous prairie voles. This effect is blocked by central infusion of an OT receptor (OXTR) antagonist (OTA). Further, MTII selectively activates hypothalamic OT neurons and potentiates OT release in the nucleus accumbens (NAc). MTII does not have a direct effect on OT release at projection terminals but is thought to result in local release of OT in the paraventricular nucleus of the hypothalamus (PVN) and thereby priming OT neurons in this region to be more responsive to stimuli. We have shown that MTII alone does not increase OT release outside of the PVN, but when combined with a hypertonic osmotic challenge, a potent stimulus for terminal OT release, results in approximately a 2-fold increase in OT release in the NAc. In line with this finding, preliminary data from our laboratory suggests that MTII combined with social exposure, also known to stimulate OT release, results in robust brain activation. This effect is absent under non-social conditions and blocked by central injection of an OTA. The aim of this project is to further investigate the mechanisms through which MTII affects brain circuitry by studying how this compound influences activation across multiple nodes in a brain network involved in social salience processing. Our hypothesis is that MTII paired with social interaction will result in increased activation of nodes in the network and coordinated activity across nodes. We further hypothesize that the effect of MTII on brain activity is mediated through the OT system and will therefore be attenuated by administration of an OTA, as well as in animals carrying alleles associated with low brain OXTR density. Finally, we will directly investigate if the effect of MTII is mediated through OXTRs in the PVN, through autoregulation on OT neurons, by using a viral vector mediated CRISPR-Cas9 approach to site specifically delete OXTRs in OT neurons. These experiments will give important insights into the neural mechanisms of a translationally relevant approach to manipulate the OT circuitry and will lay the ground for further non-invasive stimulation of the OT system.