Identification of small molecule ligands for GPR83 that modulate morphine antinociception and reward

The opioid crisis showcases a need for novel therapeutic avenues to treat drug abuse. GPR83, a recently deorphanized G protein-coupled receptor shown to blunt morphine reward learning and regulate pain, is activated by the neuropeptide PEN and peptides derived from cholecystokinin. To identify small molecule ligands, we generated a homology model based on the crystal structure of related receptors and used virtual screening on a library of 7 million compounds. The top 50 hits were screened in a cell-based assay that identified 2 selective GPR83 agonists, CPD1 and CPD27, and 1 antagonist, CPD25. The model was validated by site-directed mutagenesis of GPR83 residues predicted to interact with these ligands; these mutations disrupted ligand binding to GPR83. The molecular pharmacological properties of these compounds were characterized, and their GPR83 specificity validated using knockdown cells generated using GPR83 short hair pin RNA. Peripheral antagonist (CPD25) administration to wild-type and GPR83 knockout mice blocked morphine conditioned place preference only in the wild-type mice supporting that CPD25-mediated blockade is through GPR83 antagonism. Interestingly, morphine antinociception was blunted by the GPR83 agonist (CPD1) and enhanced by CPD25 with a medium to large effect size estimate, demonstrating a role for GPR83 in regulating morphine analgesia. Taken together, we identified and validated small molecule modulators of GPR83 that could be used to probe its role in neuropsychiatric disorders. Our in vivo studies highlight GPR83 as a target that could be used to limit the addictive effects of opioids in the treatment of pain. SIGNIFICANCE STATEMENT: There is a need to identify targets that limit opioid abuse potential while maintaining the pain-relieving effects. This study identifies small molecule GPR83 ligands that block opioid reward learning while enhancing their pain-relieving effect.