PROJECT SUMMARY/ABSTRACT There has been a significant rise in opioid use disorder (OUD) in the United States over the past decade, making it imperative to gain a better understanding of the behavioral characteristics underlying OUD vulnerability. Current rodent models focus on how one or few traits interact in a linear manner to predict substance use disorder (SUD), however, OUD consists of several symptoms that interact with one another across the addiction process and can vary between individuals to affect OUD vulnerability or resiliency. I contributed toward a rat model that captures this behavioral complexity using male and female heterogeneous stock rats in an effort to better model human OUD. Bayesian stochastic block model (SBM) network-based clustering analysis is used to separate rats into resilient and vulnerable subpopulations. Using this model, we are able to assess the neurobiological mechanisms contributing toward OUD vulnerability and resiliency, the latter of which is not well understood. These opposing phenotypes are likely mediated by different cellular and circuitry adaptations, and will be the focus of this proposal. The K99 aims assess how functional and morphological neuroplasticity differences in nucleus accumbens core (NAcc) D1/D2 medium-spiny neuron (MSN) contribute to OUD resiliency and vulnerability. I will first use whole-cell patch-clamp electrophysiology to characterize changes in AMPA/NMDA ratios in vulnerable and resilient subpopulations following cued reinstatement (Aim 1). I will then inject an intracellular label into the recorded cell and examine differences in D1/D2-MSN dendritic spine morphology (Aim 2). This approach will allow for the tracking of NAcc neuroplastic adaptations within the same cell across the two phenotypes. The R00 portion of the proposal will assess how pathway specific regulation of dorsolateral ventral pallidum (dlVP) projections contribute to OUD resiliency and vulnerability. Comparable to the NAcc, the dlVP, the main functional output of the NAcc, shows cell-specific functional regulation of drug seeking and refraining behavior, making it an ideal structure to evaluate circuit heterogeneity in individual variation in OUD propensity. Using confocal microscopy and viral tracers, I will assess functional connectivity from the dlVP to the subthalamic nucleus (STN), a region known to enhance seeking and likely vulnerability, and to the lateral habenula (LHb), known to mediate aversion and hypothesized to promote resiliency (Aim 3a). I will then employ chemogenetic technology to selectively isolate and manipulate these pathways in OUD resilient and vulnerable rats. (Aim 3b). Lastly, using whole-cell patch-clamp electrophysiology, I will evaluate input adaptations in the STN and LHb following dlVP stimulation within the two phenotypes following cued reinstatement (Aim 4). Experiments in this proposal employ a novel rat model capturing individual variation in OUD propensity similar to what is o...