Recent advances towards the discovery of ORL-1 receptor agonists and antagonists

Since their discovery a decade ago, remarkable progress has been made toward understanding the biological function and significance of the opioid receptor-like-1 (ORL-1) receptor and its endogenous peptide ligand, nociceptin. The human nociceptin receptor, herein referred to as ORL-1, but also known as OP4 (the fourth member of opioid peptide receptor family) or nociceptin/orphanin FQ peptide (NOP) receptor, was first identified as an orphan opioid receptor with close homology to the classical μ-, κ-, and δ-opioid receptors. ORL-1 does not bind endogenous ligands of the other opioid receptors with high affinity, but instead prefers the 17 amino acid peptide nociceptin. The obvious homologies of ORL-1 to opioid receptors, and its ligand nociceptin to opioid peptide ligands, led to a period of intense investigation that resulted in a number of significant reports describing the biology of the receptor and ligand. The emerging pharmacological evidence from these reports suggests that ORL-1 agonists may be clinically useful for treatment of stress, anxiety, substance abuse (opioid and alcohol), anorexia, cachexia, cough, asthma, and possibly neuropathic pain/allodynia. The peripheral effects of nociceptin suggest that agonists may have utility in the treatment of gastrointestinal motility disorders, water retention, and hypertension. ORL-1 antagonists may be useful in enhancing cognitive function and treating locomotor disorders such as Parkinsonism. In addition to research into the fundamental biology of ORL-1 and nociceptin, noteworthy advances have been made in the discovery of new peptide and non-peptide agonists and antagonists of the ORL-1 receptor leading to a better understanding of its involvement in a variety of biological processes. This review highlights the rationale for the development of ORL-1 ligands and recent progress made by different research groups towards the development of peptidic and non-peptidic ORL-1 agonists or antagonists over the last four years. To add perspective on the commercial potential of this research area, the development status of advanced new molecules is addressed together with any pharmacological characterisation of these entities.     

Nociceptin/orphanin FQ up‐regulates P2X3 receptors in primary cultures of neonatal rat trigeminal ganglion neurons

Recent evidence indicates a nociceptive role of the nociceptin/orphanin FQ–opioid receptor‐like receptor (N/OFQ‐ORL1) system in craniofacial pain; however, the mechanisms of such an effect remain unclear. We investigated whether the action of N/OFQ involves the modulation of P2X₃, a pain‐transducing ionotropic receptor. Double‐labeled immunohistochemical staining was used to determine the co‐localization of ORL1 and P2X₃ receptors in the trigeminal ganglia (TG) of neonatal Sprague‐Dawley rats. The effect of N/OFQ (at a concentration ranging from 1 pM to 10 nM) on the expression of P2X₃ receptors was detected by RT‐PCR, western blotting, and immunocytochemical staining. We found that ORL1 receptors were co‐localized with P2X₃ receptors and that the application of N/OFQ could up‐regulate, in a concentration‐dependent manner, the expression of P2X₃ receptor mRNA and P2X₃ receptor protein in TG neurons. Immunocytochemical staining also revealed enhanced P2X₃ immunoreactivity beneath the neuronal membrane and an increased percentage of P2X₃‐positive neurons after treatment with N/OFQ. Those effects were completely blocked by a specific ORL1 antagonist, UFP‐101. Our results suggest that the activation of ORL1 receptors by N/OFQ can potentiate P2X₃ receptors in primary cultures of neonatal trigeminal neurons, which may be a mechanism for the nociceptive role of N/OFQ in the modulation of craniofacial pain. Our findings may also have implications in treating craniofacial pain.     

The endogenous nociceptin/orphanin FQ‐NOP receptor system as a potential therapeutic target for intestinal disorders

In 1995, by reverse pharmacology approach, used here for the first time in the history of pharmacology, nociceptin/orphanin FQ (N/OFQ) has been discovered as the endogenous ligand of a preidentified receptor named opioid receptor like 1. Subsequent studies showed that N/OFQ and its receptor (N/OFQergic system) are widely distributed in central and peripheral nervous systems as well as in peripheral organs of human and animals, and represent a system that is involved in a very large range of biological functions such as pain perception, intestinal motility and secretion, immune modulation, stress. From the time of its discovery to now, a high number of NOP agonists and antagonists have been synthesized and tested in various pathologies. Nevertheless, none of the molecules targeting N/OFQergic system have currently succeeded in going through clinical trials concerning gut pathologies, indicating that further studies are required. The work from Dr. Fichna et al., published in the present issue of Neurogastroenterology and Motility, adds another brick in the wall of understanding the role of N/OFQergic system in IBS‐D pathology by the pharmacological evaluation of a new NOP receptor agonist, SCH 221510, in animal models of intestinal alterations (diarrhea and visceral hyperalgesia). Interestingly, authors report clinical data confirming the involvement of N/OFQergic system in IBS‐D patients and, consequently, suggest this system as a valuable therapeutic target for IBS‐D pathology. This minireview aims to give a brief summary of experimental and clinical studies focusing on the N/OFQergic system as pharmacological target for the therapeutic treatment of intestinal pathologies such as IBS and IBD.     

孤啡肽对大鼠顶叶皮层神经元延迟整流钾电流的抑制作用

目的研究孤啡肽(orphanin FQ/nociceptin,OFQ/N)对大鼠顶叶皮层神经元延迟整流钾电流(IK)的影响,初步探讨其干扰学习记忆过程的离子机制。方法采用全细胞膜片钳技术,观察OFQ/N对急性分离的大鼠顶叶皮层神经元IK的作用。结果①OFQ/N明显抑制IK,并呈剂量依赖性(P<0.05)。②0.1μmol.L-1OFQ/N使IK的电流-电压(I-V)曲线降低(n=8,P<0.01)。③0.1μmol.L-1OFQ/N使IK激活曲线的半数激活电压(V1/2)和斜率因子(k)分别由给药前的(-43.4±6.1)mV和(11.5±1.1)mV变为给药后的(-19.1±4.6)mV和(17.3±3.2)mV(n=8,P<0.01)。④0.1μmol.L-1OFQ/N使IK失活曲线的半数失活电压(V1/2)和斜率因子(k)分别由给药前的(-68.8±2.6)mV和(16.5±1.7)mV变为给药后的(-76.8±2.8)mV(n=5,P<0.01)和(15.7±3.1)mV(n=5,P>0.05)。结论OFQ/N可抑制大鼠顶叶皮层神经元IK,使IK激活曲线右移,失活曲线左移。     

Role of nitric oxide in mediating vasodilator responses to opioid peptides in the rat

1. Endomorphins 1 and 2, endogenous ligands for the mu-opioid receptor, and nociceptin (orphanin FQ; OFQ), an endogenous ligand for the ORL1 receptor, have vasodilator activity in the vascular bed of the hindquarters of the rat. In the present study, the role of nitric oxide (NO), vasodilator prostaglandins and the opening of KATP channels in mediating vasodilator responses to these novel agonists was investigated in the rat. 2. Under constant-flow conditions, injections of endomorphins 1 and 2, PL017 ([N-MePhe3,D-Pro4]-morphiceptin), nociceptin and Tyr-D-Ala-Gly-MePhe-Gly(ol)-enkephalin (DAMGO) produced dose-dependent decreases in hindquarters perfusion pressure. Vasodilator responses to endomorphin 1 and 2, acetylcholine and adrenomedullin, were attenuated by the NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) at a time when vasodilator responses to nociceptin, adrenomedullin and the NO donor diethylamine/NO were not altered. 3. Vasodilator responses to endomorphins 1 and 2, nociceptin, PL017 and DAMGO were not altered after administration of sodium meclofenamate at a time when vasodilator responses to arachidonic acid were reduced significantly or after administration of U-37883A at a time when vasodilator responses to levcromakalim were reduced significantly. 4. The results of these studies indicate that vasodilator responses to endomorphins 1 and 2, PL017 and DAMGO are mediated, in large part, by the release of NO, whereas vasodilator responses to nociceptin are mediated by an L-NAME-insensitive mechanism. Moreover, these results demonstrate that the vasodilator responses to these peptides are not due to the release of vasodilator prostaglandins or the opening of KATP channels in the hindquarters vascular bed of the rat.     

孤啡肽和阿片类配体与阿片孤儿受体相互作用的机制研究(英文)

目的:研究孤啡肽和阿片类配体与阿片孤儿受体相 互作用的分子机制。方法:用分子动力学方法计算孤啡肽的最低能构象;通过分子对接程序将孤啡肽、阿片类配体对接到阿片孤儿受体的结合口袋中;通过结合能的计算研究配体对受体的亲和力与它们的结合能之间的关系。结果:孤啡肽(1-4)残基位于结合口袋的底部,孤啡肽(5-7)残基位于结合口袋的顶部,孤啡肽(8-17)残基与孤儿受体的第二膜外环区结合;阿片类配体和孤儿受体的结合方式与孤啡肽的情况类似,区别在于孤儿受体参与配体结合的残基种类和数量不同,因而亲和力不同;配体-受体的结合能与配体的亲和力之间有很好的相关性;预测了洛芬太尼四个异构体与阿片孤儿受体的亲和力。结论:该研究能够解释许多实验事实,有助于进一步理解阿片受体与配体相互作用的分子机制并设计新的分子生物学实验。     

Circulating nociceptin levels during the cluster headache period

The trigeminal innervation of the dura and its vessels has a prominent role in the mechanism of cluster headache. Nociceptin, an opioid neuropeptide, is the endogenous ligand of the OP-4 receptor, with both algesic and analgesic properties depending on the site of action. Nociceptin and its receptor are expressed by trigeminal ganglion cells where they co-localize with calcitonin gene-related peptide, a marker peptide of the trigeminovascular neurones. Nociceptin inhibits neurogenic dural vasodilatation, a phenomenon related to trigeminovascular activation. To explore its possible involvement in cluster headache, we studied circulating levels of nociceptin when attack-free during the cluster period, and also after the termination of the cluster period, using radioimmunoassay. In 14 cluster headache patients nociceptin levels during the cluster period were significantly lower than in age-, and sex-matched controls (4.91 +/- 1.96 vs. 9.58 +/- 2.57 pg/ml, P < 0.01). After the termination of the cluster period nociceptin levels (8.60 +/- 1.47 pg/ml) were not statistically different from controls. Nociceptin levels did not correlate with age, length of disease or episode length. Lower nociceptin levels during the cluster period may result in a defective regulation of trigeminal activity that might not protect sufficiently against the attacks.     

Possible mechanism underlying the antiarrhythmic effect of peptides nociceptin and DALDA on the heart

During aconitine-induced arrhythmias the antiarrhythmic effect of DALDA (Tyr-D-Arg-Phe-Lys-NH2) and nociceptin (orphanin FQ) administered intravenously depended on activation of nitric oxide synthase. KATP channels were not involved in the realization of this effect. Endogenous prostanoids played a minor role in the antiarrhythmic effect of nociceptin and did not contribute to the protective influence of DALDA. The antiarrhythmic effect of orphanin FQ administered intravenously did not depend on functional activity of the autonomic nervous system. However, the effect of orphanin FQ after intracerebroventricular infusion was determined by changes in the state of this system.     

Nociceptin (1-13)NH2 inhibits stimulated calcitonin-gene-related-peptide release from primary cultures of rat trigeminal ganglia neurones

In this work we have developed and characterized primary cultures of neonatal rat trigeminal ganglia neurones; calcitonin-gene-related-peptide (CGRP) released from cells was taken as a marker of neuronal function. A significant and consistent increase in CGRP secretion was elicited by non-specific (56 mm KCl or veratridine) or specific (capsaicin) depolarizing stimuli. This paradigm was subsequently used to investigate the effects of nociceptin, an opioid-like peptide involved in central and peripheral control of nociception. We found that the nociceptin analogue nociceptin (1-13)NH2 (NOC) did not affect baseline CGRP release, but it reduced in a concentration-dependent manner CGRP release induced by all tested stimuli. NOC-induced reduction was statistically significant from 0.01 nm onward and achieved maximal effects at 10 nm. Such effects of NOC were seemingly mediated by the activation of specific ORL1 receptors, as a well-known nociceptin antagonist, N(Phe1)nociceptin (1-13)NH2, was able to completely revert NOC inhibition of capsaicin-stimulated CGRP release.