Μ-opioid receptor

OPRM1
Identifiers
AliasesOPRM1, LMOR, M-OR-1, MOP, MOR, MOR1, OPRM, opioid receptor mu 1
External IDsOMIM: 600018 MGI: 97441 HomoloGene: 37368 GeneCards: OPRM1
Orthologs
SpeciesHumanMouse
Entrez

4988

18390

Ensembl

ENSG00000112038

ENSMUSG00000000766

UniProt

P35372

P42866

RefSeq (mRNA)
RefSeq (protein)
Location (UCSC)Chr 6: 154.01 – 154.25 MbChr 10: 6.76 – 7.04 Mb
PubMed search[3][4]
Wikidata
View/Edit HumanView/Edit Mouse
Active and inactive μ-opioid receptors[5]

The μ-opioid receptors (MOR) are a class of opioid receptors with a high affinity for enkephalins and beta-endorphin, but a low affinity for dynorphins. They are also referred to as μ(mu)-opioid peptide (MOP) receptors. The prototypical μ-opioid receptor agonist is morphine, the primary psychoactive alkaloid in opium. It is an inhibitory G-protein coupled receptor that activates the Gi alpha subunit, inhibiting adenylate cyclase activity, lowering cAMP levels.

Structure

The structure of the μ-opioid receptor has been determined with the antagonist β-FNA,[6] the agonist BU72,[7] and in a complex with DAMGO and Gi protein.[8]

Splice variants

Three variants of the μ-opioid receptor are well characterized, though RT-PCR has identified up to 10 total splice variants in humans.[9][10][11]

μ1More is known about the μ1 opioid receptor than the other variants.
μ2TRIMU 5 is a selective agonist of the μ2 receptor.[12]
μ3The μ3 variant was first described in 2003.[13] It is responsive to opiate alkaloids but not opioid peptides.[14]

Location

They can exist either presynaptically or postsynaptically depending upon cell types.

The μ-opioid receptors exist mostly presynaptically in the periaqueductal gray region, and in the superficial dorsal horn of the spinal cord (specifically the substantia gelatinosa of Rolando). Other areas where they have been located include the external plexiform layer of the olfactory bulb, the nucleus accumbens, in several layers of the cerebral cortex, and in some of the nuclei of the amygdala, as well as the nucleus of the solitary tract.

Some MORs are also found in the intestinal tract. Activation of these receptors inhibits peristaltic action which causes constipation, a major side effect of μ agonists.[15]

Activation

MOR can mediate acute changes in neuronal excitability via suppression of presynaptic release of GABA. Activation of the MOR leads to different effects on dendritic spines depending upon the agonist, and may be an example of functional selectivity at the μ-receptor.[16] The physiological and pathological roles of these two distinct mechanisms remain to be clarified. Perhaps, both might be involved in opioid addiction and opioid-induced deficits in cognition.

Activation of the μ-opioid receptor by an agonist such as morphine causes analgesia, sedation, slightly reduced blood pressure, itching, nausea, euphoria, decreased respiration, miosis (constricted pupils), and decreased bowel motility often leading to constipation. Some of these effects, such as analgesia, sedation, euphoria, itching and decreased respiration, tend to lessen with continued use as tolerance develops. Miosis and reduced bowel motility tend to persist; little tolerance develops to these effects.

The canonical MOR1 isoform is responsible for morphine-induced analgesia, whereas the alternatively spliced MOR1D isoform (through heterodimerization with the gastrin-releasing peptide receptor) is required for morphine-induced itching.[17]

Deactivation

As with other G protein-coupled receptors, signalling by the μ-opioid receptor is terminated through several different mechanisms, which are upregulated with chronic use, leading to rapid tachyphylaxis.[18] The most important regulatory proteins for the MOR are the β-arrestins arrestin beta 1 and arrestin beta 2,[19][20][21] and the RGS proteins RGS4, RGS9-2, RGS14, and RGSZ2.[22][23]

Long-term or high-dose use of opioids may also lead to additional mechanisms of tolerance becoming involved. This includes downregulation of MOR gene expression, so the number of receptors presented on the cell surface is actually reduced, as opposed to the more short-term desensitisation induced by β-arrestins or RGS proteins.[24][25][26] Another long-term adaptation to opioid use can be upregulation of glutamate and other pathways in the brain which can exert an opioid-opposing effect, so reduce the effects of opioid drugs by altering downstream pathways, regardless of MOR activation.[27][28]

Tolerance and overdoses

Fatal opioid overdose typically occurs due to bradypnea, hypoxemia, and decreased cardiac output (hypotension occurs due to vasodilation, and bradycardia further contributes to decreased cardiac output).[29][30][31] A potentiation effect occurs when opioids are combined with ethanol, benzodiazepines, barbiturates, or other central depressants which can result in rapid loss of consciousness and an increased risk of fatal overdose.[29][30]

Substantial tolerance to respiratory depression develops quickly, and tolerant individuals can withstand larger doses.[32] However, tolerance to respiratory depression is quickly lost during withdrawal and may be completely reversed within a week. Many overdoses occur in people who return to their previous dose after having lost their tolerance following cessation of opioids. This puts addicts who receive medical treatment for opioid addiction at great risk of overdose when they are released, as they may be particularly vulnerable to relapse.

Less commonly, massive overdoses have been known to cause circulatory collapse from vasodilation and bradycardia.[33]

Opioid overdoses can be rapidly reversed through the use of opioid antagonists, naloxone being the most widely used example.[29] Opioid antagonists work by binding competitively to µ-opioid receptors and displacing opioid agonists. Additional doses of naloxone may be necessary and supportive care should be given to prevent hypoxic brain injury by monitoring vital signs.

Tramadol and tapentadol carry additional risks associated with their dual effects as SNRIs and can cause serotonin syndrome and seizures. Despite these risks, there is evidence to suggest that these drugs have a lower risk of respiratory depression compared to morphine.[34]

See also

References

  1. 1 2 3 GRCh38: Ensembl release 89: ENSG00000112038 - Ensembl, May 2017
  2. 1 2 3 GRCm38: Ensembl release 89: ENSMUSG00000000766 - Ensembl, May 2017
  3. ↑ "Human PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
  4. ↑ "Mouse PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
  5. ↑ Zhorov BS, Ananthanarayanan VS (March 2000). "Homology models of mu-opioid receptor with organic and inorganic cations at conserved aspartates in the second and third transmembrane domains". Archives of Biochemistry and Biophysics. 375 (1): 31–49. doi:10.1006/abbi.1999.1529. PMID 10683246.
  6. ↑ Manglik A, Kruse AC, Kobilka TS, Thian FS, Mathiesen JM, Sunahara RK, et al. (March 2012). "Crystal structure of the µ-opioid receptor bound to a morphinan antagonist". Nature. 485 (7398): 321–326. doi:10.1038/nature10954. PMC 3523197. PMID 22437502.
  7. ↑ Huang W, Manglik A, Venkatakrishnan AJ, Laeremans T, Feinberg EN, Sanborn AL, et al. (August 2015). "Structural insights into µ-opioid receptor activation". Nature. 524 (7565): 315–321. doi:10.1038/nature14886. PMC 4639397. PMID 26245379.
  8. ↑ Koehl A, Hu H, Maeda S, Zhang Y, Qu Q, Paggi JM, et al. (June 2018). "Structure of the µ-opioid receptor-Gi protein complex". Nature. 558 (7711): 547–552. Bibcode:2018Natur.558..547K. doi:10.1038/s41586-018-0219-7. PMC 6317904. PMID 29899455.
  9. ↑ Dortch-Carnes J, Russell K (January 2007). "Morphine-stimulated nitric oxide release in rabbit aqueous humor". Experimental Eye Research. 84 (1): 185–190. doi:10.1016/j.exer.2006.09.014. PMC 1766947. PMID 17094965.
  10. ↑ Pan L, Xu J, Yu R, Xu MM, Pan YX, Pasternak GW (2005). "Identification and characterization of six new alternatively spliced variants of the human mu opioid receptor gene, Oprm". Neuroscience. 133 (1): 209–220. doi:10.1016/j.neuroscience.2004.12.033. PMID 15893644. S2CID 22410194.
  11. ↑ Xu J, Lu Z, Narayan A, Le Rouzic VP, Xu M, Hunkele A, et al. (April 2017). "Alternatively spliced mu opioid receptor C termini impact the diverse actions of morphine". The Journal of Clinical Investigation. 127 (4): 1561–1573. doi:10.1172/JCI88760. PMC 5373896. PMID 28319053.
  12. ↑ Eisenberg RM (April 1994). "TRIMU-5, a mu 2-opioid receptor agonist, stimulates the hypothalamo-pituitary-adrenal axis". Pharmacology, Biochemistry, and Behavior. 47 (4): 943–946. doi:10.1016/0091-3057(94)90300-X. PMID 8029266. S2CID 54354971.
  13. ↑ Cadet P, Mantione KJ, Stefano GB (May 2003). "Molecular identification and functional expression of mu 3, a novel alternatively spliced variant of the human mu opiate receptor gene". Journal of Immunology. 170 (10): 5118–5123. doi:10.4049/jimmunol.170.10.5118. PMID 12734358.
  14. ↑ Stefano GB (June 2004). "Endogenous morphine: a role in wellness medicine". Medical Science Monitor. 10 (6): ED5. PMID 15173675.
  15. ↑ Chen W, Chung HH, Cheng JT (March 2012). "Opiate-induced constipation related to activation of small intestine opioid μ2-receptors". World Journal of Gastroenterology. 18 (12): 1391–1396. doi:10.3748/wjg.v18.i12.1391. PMC 3319967. PMID 22493554.
  16. ↑ Liao D, Lin H, Law PY, Loh HH (February 2005). "Mu-opioid receptors modulate the stability of dendritic spines". Proceedings of the National Academy of Sciences of the United States of America. 102 (5): 1725–1730. Bibcode:2005PNAS..102.1725L. doi:10.1073/pnas.0406797102. JSTOR 3374498. PMC 545084. PMID 15659552.
  17. ↑ Liu XY, Liu ZC, Sun YG, Ross M, Kim S, Tsai FF, et al. (October 2011). "Unidirectional cross-activation of GRPR by MOR1D uncouples itch and analgesia induced by opioids". Cell. 147 (2): 447–458. doi:10.1016/j.cell.2011.08.043. PMC 3197217. PMID 22000021. Lay summary – Washington University in St. Louis Press Release. {{cite journal}}: Cite uses deprecated parameter |lay-url= (help)
  18. ↑ Martini L, Whistler JL (October 2007). "The role of mu opioid receptor desensitization and endocytosis in morphine tolerance and dependence". Current Opinion in Neurobiology. 17 (5): 556–564. doi:10.1016/j.conb.2007.10.004. PMID 18068348. S2CID 29491629.
  19. ↑ Zuo Z (September 2005). "The role of opioid receptor internalization and beta-arrestins in the development of opioid tolerance". Anesthesia and Analgesia. 101 (3): 728–734. doi:10.1213/01.ANE.0000160588.32007.AD. PMID 16115983.
  20. ↑ Marie N, Aguila B, Allouche S (November 2006). "Tracking the opioid receptors on the way of desensitization". Cellular Signalling. 18 (11): 1815–1833. doi:10.1016/j.cellsig.2006.03.015. PMID 16750901.
  21. ↑ DuPen A, Shen D, Ersek M (September 2007). "Mechanisms of opioid-induced tolerance and hyperalgesia". Pain Management Nursing. 8 (3): 113–121. doi:10.1016/j.pmn.2007.02.004. PMID 17723928.
  22. ↑ Garzón J, Rodríguez-Muñoz M, Sánchez-Blázquez P (May 2005). "Morphine alters the selective association between mu-opioid receptors and specific RGS proteins in mouse periaqueductal gray matter". Neuropharmacology. 48 (6): 853–868. doi:10.1016/j.neuropharm.2005.01.004. PMID 15829256. S2CID 23797166.
  23. ↑ Hooks SB, Martemyanov K, Zachariou V (January 2008). "A role of RGS proteins in drug addiction". Biochemical Pharmacology. 75 (1): 76–84. doi:10.1016/j.bcp.2007.07.045. PMID 17880927.
  24. ↑ Sirohi S, Dighe SV, Walker EA, Yoburn BC (November 2008). "The analgesic efficacy of fentanyl: relationship to tolerance and mu-opioid receptor regulation". Pharmacology, Biochemistry, and Behavior. 91 (1): 115–120. doi:10.1016/j.pbb.2008.06.019. PMC 2597555. PMID 18640146.
  25. ↑ Lopez-Gimenez JF, Vilaró MT, Milligan G (November 2008). "Morphine desensitization, internalization, and down-regulation of the mu opioid receptor is facilitated by serotonin 5-hydroxytryptamine2A receptor coactivation". Molecular Pharmacology. 74 (5): 1278–1291. doi:10.1124/mol.108.048272. PMID 18703670. S2CID 6310244.
  26. ↑ Kraus J (June 2009). "Regulation of mu-opioid receptors by cytokines". Frontiers in Bioscience. 1: 164–170. doi:10.2741/e16. PMID 19482692.
  27. ↑ García-Fuster MJ, Ramos-Miguel A, Rivero G, La Harpe R, Meana JJ, García-Sevilla JA (November 2008). "Regulation of the extrinsic and intrinsic apoptotic pathways in the prefrontal cortex of short- and long-term human opiate abusers". Neuroscience. 157 (1): 105–119. doi:10.1016/j.neuroscience.2008.09.002. PMID 18834930. S2CID 9022097.
  28. ↑ Ueda H, Ueda M (June 2009). "Mechanisms underlying morphine analgesic tolerance and dependence". Frontiers in Bioscience. 14: 5260–5272. doi:10.2741/3596. PMID 19482614.
  29. 1 2 3 Blok (2017). "Opioid toxicity" (PDF). Clinical Key. Elsevier.
  30. 1 2 Hughes CG, McGrane S, Pandharipande PP (2012). "Sedation in the intensive care setting". Clinical Pharmacology. 4 (53): 53–63. doi:10.2147/CPAA.S26582. PMC 3508653. PMID 23204873.
  31. ↑ Shanazari AA, Aslani Z, Ramshini E, Alaei H (2011). "Acute and chronic effects of morphine on cardiovascular system and the baroreflexes sensitivity during severe increase in blood pressure in rats". ARYA Atherosclerosis. 7 (3): 111–117. doi:10.1016/0277-9536(88)90399-1. PMC 3347855. PMID 22577457.
  32. ↑ Algera MH, Olofsen E, Moss L, Dobbins RL, Niesters M, van Velzen M, et al. (March 2021). "Tolerance to Opioid-Induced Respiratory Depression in Chronic High-Dose Opioid Users: A Model-Based Comparison With Opioid-Naïve Individuals". Clinical Pharmacology and Therapeutics. 109 (3): 637–645. doi:10.1002/cpt.2027. PMC 7983936. PMID 32865832.
  33. ↑ Krantz MJ, Palmer RB, Haigney MC (January 2021). "Cardiovascular Complications of Opioid Use: JACC State-of-the-Art Review". Journal of the American College of Cardiology. 77 (2): 205–223. doi:10.1016/j.jacc.2020.11.002. PMID 33446314. S2CID 231613932.
  34. ↑ Houmes RJ, Voets MA, Verkaaik A, Erdmann W, Lachmann B (April 1992). "Efficacy and safety of tramadol versus morphine for moderate and severe postoperative pain with special regard to respiratory depression". Anesthesia and Analgesia. 74 (4): 510–514. doi:10.1213/00000539-199204000-00007. PMID 1554117. S2CID 24530179.
  • "Opioid Receptors: μ". IUPHAR Database of Receptors and Ion Channels. International Union of Basic and Clinical Pharmacology.
  • mu+Opioid+Receptor at the US National Library of Medicine Medical Subject Headings (MeSH)
  • Human OPRM1 genome location and OPRM1 gene details page in the UCSC Genome Browser.
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