μ Opioid Receptor/MOR

µ Opioid Receptor/MOR mediates opioid analgesia as a heptahelical G protein-coupled receptor (GPCR) and suppresses cellular activity through predominantly Gᵢ/ₒ-coupled signaling^[1][2]. Mechanistically, MOR activation inhibits the adenylyl cyclase/cAMP pathway, activates inwardly rectifying potassium channels, and inhibits voltage-dependent calcium channels, linking receptor signaling to reduced pain transmission^[2]. In pain models, MOR signaling is regulated by receptor phosphorylation, β-arrestin2 recruitment, desensitization, internalization, and agonist-dependent endocytosis^[3][4]. In disease models, altered MOR-mediated G-protein activation appears in schizophrenia-relevant rat brain regions and HIV-1 Tat-exposed mouse forebrain regions^[5][6]. Compared with related isoforms, OPRM1 alternative splicing generates full-length 7TM, truncated 6TM, and single-transmembrane variants, and these isoforms show different trafficking, G-protein activation, β-arrestin2 recruitment, and biased signaling^[7][8]. The 6TM MOR1K isoform differs from MOR1 because morphine stimulation increases Ca²⁺ and nitric oxide release and couples to stimulatory Gαs, whereas MOR1 couples to inhibitory Gαᵢ/ₒ^[9]. For experimental applications, DAMGO, morphine, fentanyl, methadone, and receptor inhibitors support studies of MOR signaling, endocytosis, analgesia, tolerance, and disease-associated receptor function^[3][4][10].