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:

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 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 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.