μ Opioid receptors

μ-Opioid receptors, also referred to as MOP, MOR, OP3 and mu opioid receptors, are members of the opioid family of G-protein-coupled receptors that have high affinity for enkephalins and beta-endorphin, but low affinity for dynorphins.   Three splice variants of μ-opioid receptors exist, μ1, μ2 and μ3, with the μ3 variant being responsive to opiate alkaloids but not opioid peptides.  The prototypical μ-opioid receptor agonist is morphine, the primary psychoactive alkaloid in opium. 

μ Opioid receptors are distributed throughout the neocortex, thalamus, nucleus accumbens, hippocampus, and amygdala  and in the peripheral nervous system.  μ Opioid receptors are involved in respiration, cardiovascular functions, feeding, learning and memory, intestinal transit, locomotor activity, thermoregulation, hormone secretion and immune functions.  Most importantly for human medicine, activation of the μ-opioid receptor by agonists such as morphine causes analgesia and sedation, with u opioid receptor agonists being among the most powerful analgesic medications known, but also among the most addictive.

Further reading

Pasternak and Pan (2013) Mu opioids and their receptors: evolution of a concept. Pharmacol Rev. 65(4) 1257 PMID: 24076545

Valentino and Volkow (2018) Untangling the complexity of opioid receptor function. Neuropsychopharmacol 43 2514 PMID: 30250308

Borsodi et al (2019). Opioid receptors (version 2019.4) in the IUPHAR/BPS Guide to Pharmacology Database. IUPHAR/BPS Guide to Pharmacology 2019(4)  https://doi.org/10.2218/gtopdb/F50/2019.4