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: 
|Greek letter name||Name based on order of discovery||Subtypes|
|delta Opioid receptor (δ)||OP1||δ1, δ2|
|kappa Opioid receptor (κ)||OP2||κ1, κ2, κ3|
|mu Opioid receptor (μ)||OP3||μ1, μ2, μ3|
|Nociceptin Opioid receptor||OP4||ORL1|
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.
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.
An IUPHAR subcommittee has suggested that appropriate terminology for the 3 typical (μ, δ, κ) and the atypical (nociceptin) receptors, should be MOR, DOR, KOR and NOR respectively.
There are four major subtypes of opioid receptors:
|κ1, κ2, κ3|
|μ1, μ2, μ3||μ1:
(I). Name based on order of discovery
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- Corbett AD, Henderson G, McKnight AT, Paterson SJ (2006). "75 years of opioid research: the exciting but vain quest for the Holy Grail". Br. J. Pharmacol. 147 Suppl 1 (Suppl 1): S153–62. doi:10.1038/sj.bjp.0706435. PMC 1760732. PMID 16402099.
- Stein C, Schäfer M, Machelska H (August 2003). "Attacking pain at its source: new perspectives on opioids". Nat. Med. 9 (8): 1003–8. doi:10.1038/nm908. PMID 12894165.
- Fine PG, Portenoy RK (2004). "Chapter 2: The Endogenous Opioid System" (PDF). A Clinical Guide to Opioid Analgesia. McGraw Hill.
- IUPHAR GPCR Database - Opioid Receptors
- Opioid+Receptors at the US National Library of Medicine Medical Subject Headings (MeSH)
- Calculated spatial position of mu-opioid receptor in the lipid bilayer, inactive state with antagonist and active state with agonist