mureceptors

Mureceptors, typically called mu-opioid receptors (MORs), are a class of G protein-coupled receptors within the opioid receptor family that mediate the effects of endogenous and exogenous opioids. The receptor is encoded by the OPRM1 gene in humans. MORs are widely distributed in the central nervous system, including the periaqueductal gray, dorsal horn of the spinal cord, thalamus, limbic regions, and brainstem, as well as in peripheral nerves.

Endogenous ligands such as beta-endorphin and enkephalins, and exogenous ligands such as morphine, heroin, and fentanyl, bind MORs with high affinity. Activation couples to Gi/o proteins, leading to inhibition of adenylyl cyclase, reduced cAMP levels, opening of G protein–gated inwardly rectifying potassium (GIRK) channels, and closure of voltage-gated calcium channels. This results in hyperpolarization of neurons and decreased neurotransmitter release, producing analgesia and mood effects, as well as respiratory depression, sedation, and euphoria.

Physiological roles include modulation of pain signaling in the spinal cord and brain, reward and reinforcement, respiration, gastrointestinal motility, and stress responses. Clinically, mu-opioid receptor agonists are used for analgesia but carry risks of tolerance, dependence, respiratory depression, constipation, and pinpoint pupils. Antagonists such as naloxone or naltrexone block MORs and are used to treat overdose or to precipitate withdrawal in dependent individuals. Research on MORs also informs understanding of addiction, pain management, and the development of biased agonists with potentially improved safety profiles.