Approved medications and available treatments for opioid use disorder (OUD) do not adequately address long- term changes in cravings, motivation, and mood; thus, while tragic, it is not surprising that between 60-90% of individuals with OUD experience relapse, even following inpatient rehabilitation. Currently available treatments for OUD target the µ opioid receptor (MOR) – the primary target responsible for opioids’ reinforcing properties. Changes in MOR expression and signaling contribute to dysphoria and gastrointestinal disruptions that plague dependent individuals during acute opioid withdrawal (Christie, 2008), and MOR agonist medications are available to sate aberrant MOR function to attenuate withdrawal symptoms (Blanco and Volkow, 2019). While these medications are critical tools for promoting public health, they have a limited ability to ameliorate persistent opioid cravings that can last long into abstinence (Volkow and Blanco, 2020). To combat this dire problem with breakthrough treatments, we desperately need to identify new molecular targets that can be harnessed to remediate long-term opioid craving and vulnerability to relapse. Prefrontal cortex (PFC) somatostatin-expressing inhibitory neurons (SST-INs) regulate the top-down control of motivated and affective behaviors. Because SST-INs arise from genetically-distinct progenitors and display specialized signaling pathways relative to other cell types, precise efforts to identify molecules selectively expressed by SST-INs are likely to yield druggable targets with minimized risk of adverse effects. Pharmacologically modulating SST-INs therefore represents a promising avenue towards developing breakthrough treatments for OUD. To that end, the goal of this proposal is to identify and validate G protein- coupled receptors (GPCRs) expressed by SST-INs that could be targeted in OUD. GPCR signaling pathways regulate nearly every aspect of cellular physiology, including synaptic plasticity, membrane physiology, protein translation, and gene expression. Through their profound abilities to regulate cell biology and brain function, GPCRs transmit the effects of hormones, growth factors, cell adhesion molecules, and nearly every major neurotransmitter. In this proposal, we will take an unbiased transcriptomics approach to identify GPCR transcripts expressed in SST-INs whose expression changes following long-access fentanyl self- administration. We will then use electrophysiology, behavioral techniques, pharmacology, and genetic engineering to define how candidate GPCRs regulate PFC activity and to begin determining whether such receptors comprise promising targets for OUD medications.