Mu-opioid receptor, Kappa-opioid receptor, and Delta-opioid receptor (Mu (MOR), Kappa (KOR), and Delta (DOR))

The mu-opioid receptor (MOR), kappa-opioid receptor (KOR), and delta-opioid receptor (DOR) are three principal subtypes of opioid receptors, classified as G protein-coupled receptors and widely expressed in the central and peripheral nervous systems. They mediate the pharmacological effects of both endogenous opioid peptides (like endorphins, enkephalins, and dynorphins) and exogenous drugs (e.g., morphine, fentanyl, salvinorin A). MOR is best known for mediating potent analgesic effects as well as euphoria and addiction risk; KOR modulates pain and negative emotional states (dysphoria, aversion); DOR contributes to analgesia, emotional regulation, and tends to have lower adverse effects. All three are major drug targets in pain management, anesthesia, and neuropsychiatric diseases, although therapeutic use is challenged by tolerance, dependence, and side effects. Recent approaches seek to target receptor heterodimers or exploit biased signaling to separate desired effects (analgesia) from unwanted ones (addiction, sedation, dysphoria).

Other names

MORμ-opioid receptorKORκ-opioid receptorDORδ-opioid receptoropioid receptorsclassical opioid receptors

Agonists: Activate inhibitory G-proteins (Gi/o) leading to decreased neuronal excitability and antinociception. Antagonists: Block receptor signaling and reverse opioid effects (e.g., naloxone). Biased agonism: Selective activation of receptor pathways (G protein vs β-arrestin), aimed at pain relief with fewer side effects. Receptor internalization/trafficking modulation. Heterodimer targeting: Modulate distinct receptor pharmacology.