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Opioid receptor

Opioid receptors are a group of G-protein coupled receptors with opioids as ligands. The endogenous opioids are dynorphins, enkephalins, endorphins, endomorphins and nociceptin/orphanin FQ. The opioid receptors are ~40% identical to somatostatin receptors (SSTRs).

Types of receptors

There are three major subtypes of opioid receptors: [1]

Receptor Subtypes Location [2] Function [2]
delta (δ)
OP1 (I)
δ1, δ2
kappa (κ)
OP2 (I)
κ1, κ2, κ3
  • Brain
    • hypothalamus
    • periaqueductal gray
    • claustrum
  • spinal cord
    • substantia gelatinosa
mu (μ)
OP3 (I)
μ1, μ2, μ3
  • brain
    • cortex (laminae III and IV)
    • thalamus
    • periaqueductal gray
  • spinal cord
    • substantia gelatinosa


  • respiratory depression
  • miosis
  • euphoria
  • reduced GI motility
  • physical dependence

(I). Name based on order of discovery

Sigma receptors (σ) were once considered to be opioid receptors, but are not usually currently classified as such.

The receptors were named using the first letter of the first ligand that was found to bind to them. Morphine was the first chemical shown to bind to mu receptors. The first letter of the drug morphine is `m', but in biochemistry there is a tendency to use Greek letters, thus turning the 'm' to μ. Similarly a drug known as ketocyclazocine was first shown to attach itself to kappa receptors.[3]

The opioid receptor types are ~70% identical with differences located at N and C termini. The μ receptor (the μ represents morphine) is perhaps the most important. It is thought that the G protein binds to the third intracellular loop of the opioid receptors. Both in mice and humans the genes for the various receptor subtypes are located on different chromosomes.

Separate subtypes have been identified in human tissue. Research has so far failed to identify the genetic evidence of the subtypes, and it is thought that they arise from post-translational modification of cloned receptor types.[4]

An additional opioid receptor has been identified and cloned based on homology with the cDNA. This receptor is known as the nociceptin receptor or ORL 1 receptor.

An IUPHAR (International Union of Pharmacology) subcommittee (2000)[5] has recommended that appropriate terminology for the 3 classical (μ, δ, κ) receptors, and the non-classical (nociceptin) receptor, should be MOP, DOP, KOP and NOP respectively.


  1. ^ Corbett AD, Henderson G, McKnight AT, Paterson SJ (2006).75 years of opioid research: the exciting but vain quest for the Holy Grail. Brit. J. Pharmacol.147, S153–S162
  2. ^ a b Unless else specified, then reference Opioid HDBK 5x8 FIN - ENDOGENOUS OPIOID SYSTEMS
  3. ^
  4. ^ Fries, DS (2002). Opioid Analgesics. In Williams DA, Lemke TL. Foye's Principles of Medicinal Chemistry (5 ed.). Philadelphia: Lippincott Williams & Wilkins. ISBN 0-683-30737-1.
  5. ^ Cox BM, Chavkin C, Christie MJ, Civelli O, Evans C, Hamon MD, et al (2000). Opioid receptors. In The IUPHAR Compendium of Receptor Characterization and Classification. ed. Girdlestone, D. London IUPHAR Media Ltd.pp 321-333

See also

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Opioid_receptor". A list of authors is available in Wikipedia.
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