Mu opioid receptor 1 (MOR)

The mu opioid receptor, encoded by the OPRM1 gene, is a class A G protein-coupled receptor that serves as the principal target for endogenous opioids such as beta‑endorphin and enkephalins. It mediates most clinically relevant effects of both natural opiates like morphine/heroin and synthetic opioids including fentanyl, oxycodone, methadone, buprenorphine, among others. Activation leads primarily to inhibition of neuronal activity through Gi/o-protein signaling—resulting in analgesia but also euphoria/reward effects that underlie its high abuse potential. The mu opioid receptor plays a central role in pain regulation but is also implicated in substance dependence/addiction pathways via modulation of dopaminergic reward circuits. Variations in its gene sequence or epigenetic regulation can influence individual responses to both therapeutic opioids and antagonists used in addiction treatment. Major safety concerns include respiratory depression leading to overdose death; tolerance; physical dependence; withdrawal syndrome; genetic variability affecting drug response; as well as increased risk for developing chronic postsurgical pain when expression is epigenetically repressed.

Other names

Mu receptorMOPOP3MOPrOpioid receptor, mu 1MOR1OPRM1 (gene symbol)μ-opioid receptor

Agonists: * Bind to the mu opioid receptor and activate Gi/o proteins. * Inhibit adenylate cyclase activity → decrease cAMP. * Open potassium channels/hyperpolarize neurons → reduce neuronal excitability. * Close voltage-gated calcium channels → inhibit neurotransmitter release. Antagonists: * Block binding site for endogenous/exogenous opioids; reverse or prevent opioid effects. Partial agonists/mixed action agents may have both agonist and antagonist properties depending on context.