PROJECT SUMMARY (Project 3) MDMA, an amphetamine derivative known both as an ‘empathogen’ and the recreational drug ‘ecstasy’, may soon be approved for treating Post-Traumatic Stress Disorder. MDMA’s therapeutic efficacy is linked to its unique ability to foster feelings of social connection and trust. However, MDMA‘s well-known abuse potential, and associated cardiovascular and neuropsychiatric toxicity, present a major public health risk. The psychological and behavioral effects of MDMA in human subjects contrast strongly with the closely related psychostimulant, methamphetamine (MA). While MDMA and MA share chemical and pharmacological similarities, MA has an even higher abuse liability and, accordingly, a more devastating societal impact. We hypothesize that the unique prosocial properties of MDMA are mechanistically linked to its comparatively lower abuse potential. A deeper understanding of how MDMA’s unique prosocial effect mitigates abuse of amphetamine-class compounds may lead to entirely new therapeutic strategies for amphetamine use disorders. We exploit the contrasting behavioral effects of MDMA and MA to probe brain-wide patterns of neural activity corresponding to these drugs’ differential regulation of natural reward sensitivity as well as their shared abuse potential. We propose a rigorous, systematic screening process to identify and test novel neural circuits that differentiate these behavioral properties of MA and MDMA. Using simple, reproducible behavior assays that parallel human imaging experiments in Project 4, we identify, validate and characterize these novel circuits in three broad steps. First, we use a whole-brain imaging and mapping process to identify candidate brain regions where neural activity is differentially regulated by MDMA, MA, or a drug-by-behavioral context interaction. Second, we perform loss-of-function experiments to define the contribution of these identified regional ensembles to specific MA and MDMA-induced behaviors using a transgenic mouse line that allows for capture-and-control of drug-activated ensembles. Third, we focus on the drug-induced temporal structure of ensemble activity within those identified brain regions, collaborating across Projects to perform detailed recordings of cellular activity in vivo. Developing effective, scalable therapies for existing and emerging amphetamine-derivative abuse disorders requires approaches that build upon, and transcend, simple preclinical models of ligand-receptor interference. To our knowledge, no other research group has proposed or executed a circuit-based approach that leverages parallel mouse-human behavior and imaging modalities so directly.