PROJECT SUMMARY Opioid addiction is a chronic, relapsing disorder, characterized by bouts of compulsive drug intake, protracted withdrawal states, and a high vulnerability to relapse. Opioid addiction in the United States is a national public health emergency, and is responsible for the deaths of more than half a million individuals since 2000. Contributing to the dire state of this crisis is the lack of pharmacotherapeutics for the long-term treatment of opioid addiction, as currently available treatments focus on either reversing overdoses or delaying the onset of withdrawal symptoms. Similar to other addictive behaviors, opioid addiction is believed to arise in part due to aberrations within the cortico-basal ganglia circuit, a network involved in associative learning, decision-making, and motivation. Central to this circuit is the nucleus accumbens (NAc), a heterogeneous structure comprised of two interspersed populations of GABAergic medium spiny neurons (MSNs): direct pathway MSNs (dMSNs) and indirect pathway MSNs (iMSNs). dMSNs express the excitatory dopamine D1 receptor and project directly to the ventral tegmental area (VTA), whereas iMSNs express the inhibitory dopamine D2 receptor and project indirectly to the VTA via the ventral pallidum (VP). Both dMSNs and iMSNs receive extensive glutamatergic input from cortical and subcortical nuclei as well as dopaminergic modulation from the midbrain, but dMSNs and iMSNs have opposing control over behavioral output: stimulation of dMSNs facilitates behavior and serves as a ?go? signal, whereas stimulation of iMSNs suppresses behavior and serves as a ?stop? signal. Disruptions in the balance of signaling between dMSNs and iMSNs (excessive ?go? signal or inadequate ?stop? signal) is thought to underlie the development of compulsive behaviors, such as those associated with drug addiction. Previous research using cell type-specific modulation of dMSNs and iMSNs has supported this hypothesis, though this research has relied primarily on the use of psychostimulants, so the applicability of such findings to opioid addiction is uncertain. Moreover, the bulk of research into the neurobiology of drug addiction fails to take into account the individual variability associated with the development and progression of addiction, due to either methodological or statistical design. As only a small percentage of individuals who use opioids become addicted, it is essential to investigate the neurobiological mechanisms underlying addiction vulnerability and resilience if effective treatments are to be developed. Thus, the research proposed herein will employ novel viral-mediated gene transfer approaches for the real-time monitoring (via in vivo calcium imaging) and transient manipulation (via chemogenetics) of dMSNs and iMSNs during a model of opioid addiction to test the hypothesis that compulsive drug use disrupts the balance of striatal signaling to drive pathological opioid craving and consumption in a subset of individuals. These findings will provide much-needed insight into the neurobiological mechanisms underlying opioid addiction and will have the potential to inform the development of future, targeted pharmacotherapeutics for the long-term treatment of opioid addiction.