PROJECT SUMMARY/ABSTRACT Opioid use disorder (OUD) is a public health crisis in America. Even among those in treatment, long-term adherence to medication is low, and relapse is the norm. A core trigger for relapse is the experience of stress, often involving psychosocial or interpersonal challenges. Preclinical models of OUD show that stress impacts neural, endocrine, and behavioral functioning, increasing one’s vulnerability for initiating and sustaining the addictive behavior. Here, we aim to translate these preclinical findings to human OUD patients on medication- assisted treatment (primary drug: heroin; stabilized on the same medication dose for 1-6 months). We will employ two broad approaches for examining psychosocial stress in patients, one utilizing chronic markers (i.e., trait-like behavioral and imaging phenotypes) and one utilizing acute markers (i.e., state-like reactivity to a laboratory induction paradigm which tests the adaptive capacity of the stress system). These approaches are fully complementary (orthogonal), in that the chronic markers depend on between-person differences while the acute markers depend on within-person fluctuations from each participant’s baseline. The chronic psychosocial stress markers, to be acquired on Day 1 of the study, include: (1) social-cognitive functioning (emotion recognition task), (2) fMRI activation during a decision-making task that involves exposure to threatening and drug-related images, and (3) gray matter volume assessed with structural MRI and voxel-based morphometry. The acute psychosocial stress markers will be acquired on Day 2 of the study (two weeks after Day 1), during which participants undergo an experimental stress induction via personalized imagery. The acute stress markers include: experimentally- induced changes in craving, salivary cortisol (marker of HPA functioning), and salivary alpha amylase (marker of sympathetic activation). For each of these two approaches, we will test for hypothesized differences between OUD participants and matched healthy controls (HC), and then within OUD we will test for hypothesized correlations of the stress variables with scores from the Cumulative Adversity Interview (CAI), a well-validated measure of lifetime cumulative psychosocial stress. Finally, OUD participants will be followed for 8 months to track relapse status and drug use, which we hypothesize will be prospectively predicted by our multimodal stress markers. With this design, we will address our study aims of uncovering whether there are abnormalities in the neural, endocrine, and behavioral correlates of stress in OUD versus HC; whether such abnormalities in OUD are exacerbated by cumulative adversity; and whether stress and its biological mechanisms predict OUD relapse trajectories. Our study will advance basic knowledge of stress biology in OUD, a research area that to date has been profoundly understudied compared with other addictions. Such knowledge may inform new therapeuti...