μ-opioid receptor

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.

OPRM1
Identifiers
AliasesOPRM1, LMOR, M-OR-1, MOP, MOR, MOR1, OPRM, opioid receptor mu 1
External IDsOMIM: 600018 MGI: 97441 HomoloGene: 37368 GeneCards: OPRM1
Gene location (Human)
Chr.Chromosome 6 (human)[1]
Band6q25.2Start154,010,496 bp[1]
End154,246,867 bp[1]
RNA expression pattern
More reference expression data
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]

Structure

A cryo-electron microscopy structure of the µ-opioid receptor–Gi protein complex was published in 2018.[6]

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.[7][8][9]

μ1More is known about the μ1 opioid receptor than the other variants.
μ2TRIMU 5 is a selective agonist of the μ2 receptor.[10]
μ3The μ3 variant was first described in 2003.[11] It is responsive to opiate alkaloids but not opioid peptides.[12]

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.[13]

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.[14] 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.[15]

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.[16] The most important regulatory proteins for the MOR are the β-arrestins arrestin beta 1 and arrestin beta 2,[17][18][19] and the RGS proteins RGS4, RGS9-2, RGS14, and RGSZ2.[20][21]

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.[22][23][24] 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.[25][26]

Tolerance and overdoses

Fatal opioid overdose typically occurs due to bradypnea, hypoxemia, and decreased cardiac output (hypotension occurs due to vasodilation, & bradycardia further contributes to decreased cardiac output).[27][28][29] A potentiation effect occurs when opioids are combined with ethanol, benzodiazepines, or barbiturates, which results in an increased risk for overdose to occur.[27][28] Substantial tolerance to respiratory depression develops quickly, and tolerant individuals can withstand larger doses.[30] However, tolerance to respiratory depression is lost just as quickly during withdrawal.[30] Many overdoses occur in people who misuse their medication after being in withdrawal long enough to lose the tolerance to respiratory depression. Less commonly, massive overdoses have been known to cause circulatory collapse.

Opioid overdoses can be rapidly reversed through the use of opioid antagonists, naloxone being the most widely used example.[27]

See also

References
  1. GRCh38: Ensembl release 89: ENSG00000112038 - Ensembl, May 2017
  2. 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". Arch. Biochem. Biophys. 375 (1): 31–49. doi:10.1006/abbi.1999.1529. PMID 10683246.
  6. Koehl A, Hu H, Maeda S, et al. (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.
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  15. Liu XY, Liu ZC, Sun YG, Ross M, Kim S, Tsai FF, Li QF, Jeffry J, Kim JY, Loh HH, Chen ZF (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.
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