PROJECT SUMMARY 3,4-Methylenedioxymethamphetamine (MDMA) a Phase 3 clinical trials as an adjunct to psychotherapy for Post-Traumatic Stress Disorder (PTSD). Published data show that MDMA therapy has a rapid onset and a ability to foster feelings of social connection, empathy and trust. However, MDMA itself may not be an ideal therapeutic, as it has a well-known potential for abuse and is associated with cardiovascular and neuro- psychiatric toxicity. Despite these and other limitations, the apparent efficacy of MDMA suggests that directly enhancing sociability and social reward sensitivity are feasible, potentially powerful therapeutic strategies. In mouse models we can use MDMA as a unique probe to understand evolutionarily conserved social behaviors with potential therapeutic relevance. Conventional approaches to understanding the mechanism of MDMA and other psychiatric drugs, focusing on high affinity receptor interactions and select brain areas, have had limited success at developing novel therapeutics for psychiatric disease. My lab has developed a way to define with few assumptions about pharmacology or brain areas involved. This method, in mice, maps brain-wide activity evoked du for MDMA-linked behaviors. Combining social behavioral testing and imaging could be used to screen novel therapeutics for MDMA-like profiles and provides testable hypotheses for human imaging studies with MDMA. MDMA releases supraphysiological levels of serotonin (5-HT) and dopamine (DA) among other neuromodulators and evokes acute social preference, social reward learning, and nonsocial drug reward in humans and mice. Mechanistically similar drugs that primarily release 5-HT (d-fenfluramine, FEN) or DA (d- methamphetamine, METH) recapitulate selective components of the total MDMA effect, but neither induces social reward learning. Here, we propose to take advantage of the overlapping yet distinct behavioral and unique prosocial effects. First, we compare brain-wide Fos expression maps between groups of mice under drug/environmental conditions that on brain-wide neural activity -like behavioral effects. Second, we test whether activity in identified regions is required for expression of four MDMA-evoked behaviors: acute social preference, drug craving, social reward craving and social operant conditioning. My preliminary data demonstrates proof-of-concept: we have discovered that the dorsal endopiriform nucleus/ ventral claustrum (DEn/VC) has an obligate role in MDMA-evoked acute social preference. Third, we detail the anatomy and connectivity of the DEn/VC, and test whether its activity can suffice to drive prosocial behaviors.