Opioid receptors

In addition, opioids can be categorised according to the type of opioid receptor at which they produce their effects. Classically, there are considered to be three opioid receptors. These receptors are all G-protein-coupled receptors, and were originally named mu (after morphine, its most commonly recognised exogenous ligand), delta (after vas deferens, the tissue within which it was first isolated) and kappa (after the first ligand to act at this receptor, ketocyclazocine). In 1996 the International Union of Pharmacology (IUPHAR) renamed the receptors OP1 (the delta receptor), OP2 (the kappa receptor) and OP3 (the mu receptor). In 2000 this nomenclature was again changed to DOP, KOP and MOP (Table 2).3,4 Now, however, owing to the extensive literature previously using the Greek nomenclature for opioid receptors (δ, κ and µ) IUPHAR recommends using this classification and the DOP, KOP and MOP classification of 2000. Some authorities describe the existence of multiple subtypes of the three classical opioid receptors, but this is not a belief held by all researchers within the field.5 The classical opioid receptors are distributed widely within the central nervous system and, to a lesser extent, throughout the periphery, occupying sites within the vas deferens, knee joint, gastrointestinal tract, heart and immune system, amongst others.6

Soon after the discovery of the opioid receptors, a series of endogenous ligands active at the receptors were discovered in brain extracts. Three pro-hormone precursors provide the parent compounds from which these endogenous ligands are derived. Proenkephalin is cleaved to form met-enkephalin and leu-enkephalin, which bind to the DOP receptor. Dynorphin A and B are derived from prodynorphin and are agonists at the KOP receptor. Pro-opiomelancortin (POMC) is the parent compound for β-endorphin, an agonist at the MOP receptor, though it is capable of displaying agonist activity at all three classical opioid receptors.2,3,7 Two further endogenous peptides act as agonists at the MOP receptor, endomorphin 1 and 2, but no precursor has yet been identified (Table 3). There is significant cross-talk between the endogenous agonists and the three classical receptors.

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In 1994 a fourth G-protein-coupled endogenous opioid like receptor was found, and was subsequently named the nociceptin (NOP) receptor. Quickly after this discovery came the isolation from brain extracts of its endogenous ligand, nociceptin/orphanin FQ (N/OFQ). This endogenous ligand is similarly derived from a precursor compound, in this instance from the polypeptide precursor pre-pro-nociceptin. Though the N/OFQ/NOP system does not bind naloxone, nor are its effects reversed by naloxone, it is a G-protein-coupled receptor system that shares a marked similarity to the known amino acid sequences of the classical opioid receptors.3,8 At a cellular level, N/OFQ acts to produce similar actions to those described for the classical opioid receptors above. For these reasons it has been classified as the fourth opioid receptor; however, owing to its lack of response to the classical opioid antagonist (naloxone) some pharmacologists have questioned the wisdom of this classification. IUPHAR considers the NOP receptor to be a non-opioid branch of the opioid receptor family.4

In clinical practice the stimulation of the differing opioid receptors produces a range of effects, which are often dependent upon the location of the receptor, along with analgesia. Agonists binding to MOP receptors may cause analgesia, but also sedation, respiratory depression, bradycardia, nausea and vomiting and a reduction in gastric motility. Activation of DOP receptors can cause spinal and supraspinal analgesia and reduce gastric motility, while KOP receptor stimulation may produce spinal analgesia, diuresis and dysphoria. Spinally, N/OFQ has been shown to produce analgesia and hyperalgesia, dependent upon the administered concentration, and allodynia. Supraspinally, when administered intracerebrovascularly it is thought to produce a pro-nociceptive anti-analgesic effect, owing to an inhibition of endogenous opioid tone.3,9

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