Opioid receptors are a group of inhibitory G protein-coupled receptors with opioids as ligands. The endogenous opioids are dynorphins, enkephalins, endorphins, endomorphins and nociceptin. The opioid receptors are ~40% identical to somatostatin receptors (SSTRs). Opioid receptors are distributed widely in the brain, in the spinal cord, on peripheral neurons, and digestive tract.
Hyperalgesia is an abnormally increased sensitivity to pain, which may be caused by damage to nociceptors or peripheral nerves and can cause hypersensitivity to stimulus. Prostaglandins E and F are largely responsible for sensitizing the nociceptors. Temporary increased sensitivity to pain also occurs as part of sickness behavior, the evolved response to infection.
An enkephalin is a pentapeptide involved in regulating nociception in the body. The enkephalins are termed endogenous ligands, as they are internally derived and bind to the body's opioid receptors. Discovered in 1975, two forms of enkephalin have been found, one containing leucine ("leu"), and the other containing methionine ("met"). Both are products of the proenkephalin gene.
β-Endorphin (beta-endorphin) is an endogenous opioid neuropeptide and peptide hormone that is produced in certain neurons within the central nervous system and peripheral nervous system. It is one of three endorphins that are produced in humans, the others of which include α-endorphin and γ-endorphin.
Opioid peptides or opiate peptides are peptides that bind to opioid receptors in the brain; opiates and opioids mimic the effect of these peptides. Such peptides may be produced by the body itself, for example endorphins. The effects of these peptides vary, but they all resemble those of opiates. Brain opioid peptide systems are known to play an important role in motivation, emotion, attachment behaviour, the response to stress and pain, control of food intake, and the rewarding effects of alcohol and nicotine.
Endomorphins are considered to be natural opioid neuropeptides central to pain relief. The two known endomorphins, endomorphin-1 and endomorphin-2, are tetrapeptides, consisting of Tyr-Pro-Trp-Phe and Tyr-Pro-Phe-Phe amino acid sequences respectively. These sequences fold into tertiary structures with high specificity and affinity for the μ-opioid receptor, binding it exclusively and strongly. Bound μ-opioid receptors typically induce inhibitory effects on neuronal activity. Endomorphin-like immunoreactivity exists within the central and peripheral nervous systems, where endomorphin-1 appears to be concentrated in the brain and upper brainstem, and endomorphin-2 in the spinal cord and lower brainstem. Because endomorphins activate the μ-opioid receptor, which is the target receptor of morphine and its derivatives, endomorphins possess significant potential as analgesics with reduced side effects and risk of addiction.
Opiorphin is an endogenous chemical compound first isolated from human saliva. Initial research with mice shows the compound has a painkilling effect greater than that of morphine. It works by stopping the normal breakup of enkephalins, natural pain-killing opioids in the spinal cord. It is a relatively simple molecule consisting of a five-amino acid polypeptide, Gln-Arg-Phe-Ser-Arg (QRFSR).
The nociceptin opioid peptide receptor (NOP), also known as the nociceptin/orphanin FQ (N/OFQ) receptor or kappa-type 3 opioid receptor, is a protein that in humans is encoded by the OPRL1 gene. The nociceptin receptor is a member of the opioid subfamily of G protein-coupled receptors whose natural ligand is the 17 amino acid neuropeptide known as nociceptin (N/OFQ). This receptor is involved in the regulation of numerous brain activities, particularly instinctive and emotional behaviors. Antagonists targeting NOP are under investigation for their role as treatments for depression and Parkinson's disease, whereas NOP agonists have been shown to act as powerful, non-addictive painkillers in non-human primates.
The neuropeptide FF receptors are members of the G-protein coupled receptor superfamily of integral membrane proteins which bind the pain modulatory neuropeptides AF and FF. The Neuropeptide FF receptor family is a member of the G protein-coupled receptor superfamily containing two subtypes, NPFF1 and NPFF2, which exhibit a high affinity for Neuropeptide FF (NPFF) peptides. NPFF1 is broadly distributed in the central nervous system with the highest levels found in the limbic system and the hypothalamus. NPFF2 is present in high density, particularly in mammals in the superficial layers of the spinal cord where it is involved in nociception and modulation of opioid functions. These receptors participate to the modulation of opioid receptor function in the brain and spinal cord, and can either reduce or increase opioid receptor function depending which tissue they are released in, reflecting a complex role for neuropeptide FF in pain responses.
JTC-801 is an opioid analgesic drug used in scientific research.
RB-101 is a drug that acts as an enkephalinase inhibitor, which is used in scientific research.
J-113,397 is an opioid drug which was the first compound found to be a highly selective antagonist for the nociceptin receptor, also known as the ORL-1 receptor. It is several hundred times selective for the ORL-1 receptor over other opioid receptors, and its effects in animals include preventing the development of tolerance to morphine, the prevention of hyperalgesia induced by intracerebroventricular administration of nociceptin, as well as the stimulation of dopamine release in the striatum, which increases the rewarding effects of cocaine, but may have clinical application in the treatment of Parkinson's disease.
SB-612,111 is an opioid receptor ligand which is a potent and selective antagonist for the nociceptin receptor (ORL-1), several times more potent than the older drug J-113,397. It does not have analgesic effects in its own right, but prevents the development of hyperalgesia, and also shows antidepressant effects in animal studies.
Ro64-6198 is an opioid drug used in scientific research. It acts as a potent and selective agonist for the nociceptin receptor, also known as the ORL-1 receptor, with over 100x selectivity over the other opioid receptors. It produces anxiolytic effects in animal studies equivalent to those of benzodiazepine drugs, but has no anticonvulsant effects and does not produce any overt effects on behaviour. However it does impair short-term memory, and counteracts stress-induced anorexia. It also has antitussive effects, and reduces the rewarding and analgesic effects of morphine, although it did not prevent the development of dependence. It has been shown to reduce alcohol self-administration in animals and suppressed relapses in animal models of alcoholism, and ORL-1 agonists may have application in the treatment of alcoholism.
(+)-Naloxone (dextro-naloxone) is a drug which is the opposite enantiomer of the opioid antagonist drug (−)-naloxone. Unlike (−)-naloxone, (+)-naloxone has no significant affinity for opioid receptors, but instead has been discovered to act as a selective antagonist of Toll-like receptor 4. This receptor is involved in immune system responses, and activation of TLR4 induces glial activation and release of inflammatory mediators such as TNF-α and Interleukin-1.
MCOPPB is a drug which acts as a potent and selective agonist for the nociceptin receptor, with a pKi of 10.07 and much weaker activity at other opioid receptors. It has only moderate affinity for the mu opioid receptor, weak affinity for the kappa opioid receptor and negligible binding at the delta opioid receptor. In animal studies, MCOPPB produces potent anxiolytic effects, with no inhibition of memory or motor function, and only slight sedative side effects which do not appear until much higher doses than the effective anxiolytic dose range.
Cebranopadol is an opioid analgesic of the benzenoid class which is currently under development internationally by Grünenthal, a German pharmaceutical company, and its partner Depomed, a pharmaceutical company in the United States, for the treatment of a variety of different acute and chronic pain states. As of November 2014, it is in phase III clinical trials.
Olivier Civelli is a molecular biologist, a researcher in the field of neuropharmacology and an educator. He is the Eric L. and Lila D. Nelson Professor of Neuropharmacology at University of California, Irvine. He is also a Professor in the Department of Developmental and Cell Biology at University of California, Irvine. He is most known for his work in advancing understanding of neurotransmission and his impact on drug discovery.
SR-16435 is a drug which acts as a potent partial agonist at both the μ-opioid receptor and nociceptin receptor. In animal studies it was found to be a potent analgesic, with results suggestive of reduced development of tolerance and increased activity against neuropathic pain compared to classic μ-selective agonists.
Ro65-6570 is an opioid drug. It has a potential use in preventing the addiction to other opioids.
