Opioid receptor

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

1 Types of receptors
2 References
3 See also
4 External links

[edit] Types of receptors

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

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

Sigma receptors (σ) were once considered to be opiod 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 so they converted the 'm' to μ. Similarly a drug known as ketocyclazocine was first shown to attach itself to kappa receptors.^[2]

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.^[3]

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.

[edit] References

^ 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
^ http://opioids.com/narcotic-drugs/chapter-3.html
^ 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.