NOP Receptor/ORL1

NOP receptor/ORL1 is the most recently identified opioid receptor family member and responds to nociceptin/orphanin FQ as its endogenous agonist[1][2]. Mechanistically, N/OFQ activates an opioidlike G protein-coupled receptor and inhibits adenylate cyclase in ORL1-expressing cells, linking NOP receptor signaling to cellular transduction and nociceptive regulation[2][3]. In pain-related models, OFQ/N shows overlapping distribution in pain-processing areas with opioids but produces diverse nociceptive responses, including hyperalgesia after intracerebroventricular administration in mice[3][4]. Compared with MOP, KOP, and DOP opioid receptors, NOP/ORL1 is homologous to classical opioid receptors but remains insensitive to traditional opioids, supporting its distinction as a separate opioid receptor isoform[4]. Structural studies of human NOP bound to peptide-mimetic antagonist C-24 provide atomic-level information useful for ligand design[5]. For experimental applications, J-113397 functions as a potent and selective nonpeptidyl ORL1 antagonist, while Ro 64-6198 is a systemically active nonpeptide NOP agonist with high brain penetration and strong selectivity over classic opioid receptors[6][7].