Three members of the family of opiod receptors are known to date that all belong to the class of GPCR-A4 (together with Somatostatin receptors). The family name originates from the active hallucinating component of Papaver somniferum (opium), whereas the first assignment of names of each member was based on the most potent opiate used to study the three subtypes: mu (morphine, OP3), kappa (ketocyclazocine, OP2), and sigma (SKF 10047). The later discovery of another subtype, named delta (named after the species vas deferens used for this study, OP1), and the finding that the sigma receptor was actually a non-opioid receptor resulted in the currently know classification of mu, kappa, and delta receptor subtypes (OP1-OP3). A fourth opioid receptor subtype (Nociceptin, OP4) has been identified as a result of cloning techniques. This receptor shows a significant degree of homology in the cDNA coding for this and the other subtypes. Opiate receptors are abundantly present in the brain, and present in the spinal cord and digestive tract. Besides the fact that these receptors are well known for their key interactions with opiates mediating hallucinating and analgesic effects, they do interact with endogenous ligands (endorphins) as well. Activation of opioid receptors by endogenous and exogenous ligands results in a multitude of effects, which include analgesia, respiratory depression, euphoria, feeding, the release of hormones, inhibition of gastrointestinal transit, and effects on anxiety
 The effects of morphine- and nalorphine-like drugs in the nondependent and morphine dependent chronic spinal dog. Martin, W.R., Eades, C.G., Thompson, J.A., Huppler, R.E., Gilbert, P.E. J. Pharmacol. Exp. Ther. 1976, 197, 517-532.
 Endogenous opioid peptides: multiple agonists and receptors. N Lord JA, Waterfield AA, Hughes J, Kosterlitz HW. Nature. 1977, 267, 495–499
 Psychotomimetic sigma-opiates andPCP. Mannalack, D.T., Beart, P.M., Gundlach, A.L. Trends Pharmacol. Sci. 1986, 7, 448-451.
 InternationalUnion of Pharmacology. XII. Classification of opioid receptors. Dhawan BN, Cesselin F, Raghubir R, Reisine T, Bradley PB, Portoghese PS, Hamon M. Pharmacol. Rev. 1996, 48, 567–92.
 ORL1, a novel member of the opioid receptor family. Cloning, functional expression and localization. Mollereau C, Parmentier M, Mailleux P, Butour JL, Moisand C, Chalon P, Caput D, Vassart G, Meunier JC. FEBS Lett. 1994, 341, 33–38.