1-Benzyl-N-[3-[spiroisobenzofuran-1(3H),4'-piperidin-1-yl]propyl]pyrrolidine-2-carboxamide (Compound 24) antagonizes NOP receptor-mediated potassium channel activation in rat periaqueductal gray slices

Non-peptide orphanin receptor antagonist activity in rat myocardial ischemia-induced cardiac arrhythmias

One of the causes of sudden cardiac death is arrhythmia after acute myocardial ischemia. After ischemia, endogenous orphanin (N/OFQ) plays a role in the development of arrhythmias. It is discussed in this paper how nonpeptide orphanin receptor (ORL1) antagonists such as J-113397, SB-612111 and compound-24 (C-24) affect arrhythmia in rats following acute myocardial ischemia and what the optimal concentrations for these antagonists are. The electrocardiogram of the rat was recorded as part of the experiment. The concentrations of tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β) in the myocardium were measured following euthanasia. Following the use of three antagonists, we found the lowest inflammatory factor concentrations and the smallest number of ischemic arrhythmia episodes. All of them had a small impact on cardiac function. LF/HF values were significantly reduced in all three antagonist groups, suggesting that they are involved in the regulation of sympathetic nerves. In conclusion, pretreatment with the three antagonist groups can effectively reduce the concentration of TNF-α and IL-1β, and the occurrence of arrhythmias after ischemia can also be significantly reduced. Inflammation and sympathetic activity may be related to the mechanism of action of antagonists.

Regulation of N-type calcium channels by nociceptin receptors and its possible role in neurological disorders

Activation of nociceptin opioid peptide receptors (NOP, a.k.a. opioid-like receptor-1, ORL-1) by the ligand nociceptin/orphanin FQ, leads to G protein-dependent regulation of Cav2.2 (N-type) voltage-gated calcium channels (VGCCs). This typically causes a reduction in calcium currents, triggering changes in presynaptic calcium levels and thus neurotransmission. Because of the widespread expression patterns of NOP and VGCCs across multiple brain regions, the dorsal horn of the spinal cord, and the dorsal root ganglia, this results in the alteration of numerous neurophysiological features. Here we review the regulation of N-type calcium channels by the NOP-nociceptin system in the context of neurological conditions such as anxiety, addiction, and pain.

BPR1M97, a dual mu opioid receptor/nociceptin-orphanin FQ peptide receptor agonist, produces potent antinociceptive effects with safer properties than morphine

There is unmet need to design an analgesic with fewer side effects for severe pain management. Although traditional opioids are the most effective painkillers, they are accompanied by severe adverse responses, such as respiratory depression, constipation symptoms, tolerance, withdrawal, and addiction. We indicated BPR1M97 as a dual mu opioid receptor (MOP)/nociceptin-orphanin FQ peptide (NOP) receptor full agonist and investigated the pharmacology of BPR1M97 in multiple animal models. In vitro studies on BPR1M97 were assessed using cyclic-adenosine monophosphate production, β-arrestin, internalization, and membrane potential assays. In vivo studies were characterized using the tail-flick, tail-clip, lung functional, heart functional, acetone drop, von Frey hair, charcoal meal, glass bead, locomotor activity, conditioned place preference (CPP) and naloxone precipitation tests. BPR1M97 elicited full agonist properties for all cell-based assays tested in MOP-expressing cells. However, it acted as a G protein-biased agonist for NOP. BPR1M97 initiated faster antinociceptive effects at 10 min after subcutaneous injection and elicited better analgesia in cancer-induced pain than morphine. Unlike morphine, BPR1M97 caused less respiratory, cardiovascular, and gastrointestinal dysfunction. In addition, BPR1M97 decreased global activity and induced less withdrawal jumping precipitated by naloxone. Thus, BPR1M97 could serve as a novel small molecule dual receptor agonist for antinociception with fewer side effects than morphine. This article is part of the Special Issue entitled 'New Vistas in Opioid Pharmacology'.

Nociceptin/Orphanin FQ Receptor Structure, Signaling, Ligands, Functions, and Interactions with Opioid Systems

The NOP receptor (nociceptin/orphanin FQ opioid peptide receptor) is the most recently discovered member of the opioid receptor family and, together with its endogenous ligand, N/OFQ, make up the fourth members of the opioid receptor and opioid peptide family. Because of its more recent discovery, an understanding of the cellular and behavioral actions induced by NOP receptor activation are less well developed than for the other members of the opioid receptor family. All of these factors are important because NOP receptor activation has a clear modulatory role on mu opioid receptor-mediated actions and thereby affects opioid analgesia, tolerance development, and reward. In addition to opioid modulatory actions, NOP receptor activation has important effects on motor function and other physiologic processes. This review discusses how NOP pharmacology intersects, contrasts, and interacts with the mu opioid receptor in terms of tertiary structure and mechanism of receptor activation; location of receptors in the central nervous system; mechanisms of desensitization and downregulation; cellular actions; intracellular signal transduction pathways; and behavioral actions with respect to analgesia, tolerance, dependence, and reward. This is followed by a discussion of the agonists and antagonists that have most contributed to our current knowledge. Because NOP receptors are highly expressed in brain and spinal cord and NOP receptor activation sometimes synergizes with mu receptor-mediated actions and sometimes opposes them, an understanding of NOP receptor pharmacology in the context of these interactions with the opioid receptors will be crucial to the development of novel therapeutics that engage the NOP receptor.

Tritium-labelled isovaleryl-RYYRIK-NH2 as potential antagonist probe for ORL1 nociceptin receptor

IsoVa-RYYRIK-NH2 is a highly specific antagonist ligand of the opioid receptor-like 1 (ORL1) receptor, an endogenous ligand of which is 17-mer peptide nociceptin. ORL1 antagonists have potential for clinical use as analgesic and antineuropathic drugs, and thus information on the receptor-binding characteristics of antagonists is very important for rational drug design. In the present study, we prepared tritium-labelled isova-RYYRIK-NH2 from its precursor with the 3-methylcrotonyl (CH3)2CCHCO group by a catalytic reduction using tritium gas. The resulting [(3)H]isoVa-RYYRIK-NH2 was evaluated in a saturation binding assay using the COS-7 cell membrane preparations of transiently expressed ORL1. It exhibited more than 90% specific binding with a dissociation constant of 1.21±0.03nM. From the mutual heterologous binding assays using [(3)H]isoVa-RYYRIK-NH2 and [(3)H]nociceptin, isoVa-RYYRIK-NH2 and nociceptin were found to share the receptor-binding site, but each also had a separate specific binding site of its own. They differentiated the two different binding states or conformations of ORL1, which might represent the agonist-active and antagonist-inactive conformations of ORL1. [(3)H]isoVa-RYYRIK-NH2 is thus a key tracer to uncover the amino acid residues important for receptor inactivation.

Translational approach to develop novel medications on alcohol addiction: focus on neuropeptides

Research on alcohol and drug dependence has shown that the development of addiction depends on a complex interplay of psychological factors, genetic or epigenetic predisposing factors, and neurobiological adaptations induced by drug consumption. A greater understanding of the mechanisms leading to alcohol abuse will allow researchers to identify genetic variation that corresponds to a specific biological vulnerability to addiction, thus defining robust endophenotypes that might help deconstruct these complex syndromes into more tractable components. To this end, it is critical to develop a translational framework that links alterations at the molecular level, to changes in neuronal function, and ultimately to changes at the behavioral and clinical levels. Translational phenotypes can be identified by the combination of animal and human studies designed to elucidate the neurofunctional, anatomical and pharmacological mechanisms underlying the etiology of alcohol addiction. The present article offers an overview of medication development in alcoholism with a focus on the critical aspect of translational research. Moreover, significant examples of promising targets from neuropeptidergic systems, namely nociceptin/orphanin FQ and neuropeptide S are given.

Capsaicin in the periaqueductal gray induces analgesia via metabotropic glutamate receptor-mediated endocannabinoid retrograde disinhibition

BACKGROUND AND PURPOSE

Capsaicin, an agonist of transient receptor potential vanilloid 1 (TRPV1) channels, is pro-nociceptive in the periphery but is anti-nociceptive when administered into the ventrolateral periaqueductal gray (vlPAG), a midbrain region for initiating descending pain inhibition. Here, we investigated how activation of TRPV1 channels in the vlPAG leads to anti-nociception.

EXPERIMENTAL APPROACH

We examined synaptic transmission and neuronal activity using whole-cell recordings in vlPAG slices in vitro and hot-plate nociceptive responses in rats after drug microinjection into the vlPAG in vivo.

KEY RESULTS

Capsaicin (1-10 µM) depressed evoked GABAergic inhibitory postsynaptic currents (eIPSCs) in vlPAG slices presynaptically, while increasing miniature excitatory PSC frequency. Capsaicin-induced eIPSC depression was antagonized by cannabinoid CB₁ and metabotropic glutamate (mGlu₅) receptor antagonists, and prevented by inhibiting diacylglycerol lipase (DAGL), which converts DAG into 2-arachidonoylglycerol (2-AG), an endocannabinoid. Capsaicin induced membrane depolarization in 2/3 neurons recorded but, overall, increased neuronal firings by increasing evoked postsynaptic potentials. Intra-vlPAG capsaicin reduced hot-plate responses in rats, effects blocked by CB₁ and mGlu receptor antagonists. Effects of capsaicin were antagonized by SB 366791, a TRPV1 channel antagonist.

CONCLUSIONS AND IMPLICATIONS

Capsaicin activated TRPV1s on glutamatergic terminals to release glutamate which activated postsynaptic mGlu₅ receptors, yielding 2-AG from DAG by DAGL hydrolysis. 2-AG induces retrograde inhibition (disinhibition) of GABA release via presynaptic CB₁ receptors. This disinhibition in the vlPAG leads to anti-nociception by activating the descending pain inhibitory pathway. This is a novel TRPV1 channel-mediated anti-nociceptive mechanism in the brain and a new interaction between vanilloid and endocannabinoid systems.

Functional heterogeneity of nociceptin/orphanin FQ receptors revealed by (+)-5a Compound and Ro 64-6198 in rat periaqueductal grey slices

The nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor is a non-opioid branch of the opioid receptor family implicated in several neurological and psychological disorders, such as pain, anxiety, depression, involuntary movement, addiction, seizure and dementia. Heterogeneity of NOP receptors has been proposed based on the findings of splicing variants and from binding and functional studies. We have previously reported that Ro 64-6198, a NOP receptor agonist, activated a subset, but not all, of N/OFQ-sensitive NOP receptors in midbrain ventrolateral periaqueductal grey (vlPAG). In this study, we found that a new NOP receptor ligand, (+)-5a Compound ((3aS, 6aR)-1-(cis-4-isopropylcyclohexyl)-5'-methyl-2'-phenylhexahydrospiro[piperidine-4,1'-pyrrolo[3, 4-c]pyrrole]), also activated a subset of NOP receptors in vlPAG neurons. (+)-5a Compound (0.1-30 μm) concentration-dependently activated G-protein-coupled inwardly-rectifying potassium (GIRK) channels mediated through the NOP receptors in about 35% of the recorded vlPAG neurons. (+)-5a Compound (EC50: 605 nm) was less potent (1/12) and efficacious (47%) than N/OFQ. In (+)-5a Compound-insensitive neurons, Ro 64-6198 was also ineffective, and vice versa, but N/OFQ activated GIRK channels through NOP receptors. In (+)-5a Compound-sensitive neurons, (+)-5a Compound precluded the effect of Ro 64-6198. Immunofluorecent and morphometric studies showed that most of the (+)-5a Compound-sensitive neurons were multipolar with intensive dendritic arborization and immunoreactive to glutamic acid decarboxylase-67. It is suggested that (+)-5a Compound activates a subset of NOP receptors, similar to the Ro 64-6198-sensitive subset, in the vlPAG neurons which are mostly GABAergic. These results further support the presence of functional heterogeneity of NOP receptors in the midbrain PAG.

Quantitative study of the antagonistic effect of (-)-cis-1-Methyl-7-[[4-(2,6-dichlorophenyl)piperidin-1-yl]methyl]-6,7,8,9-tetrahydro-5H-benzocyclohepten-5-ol (SB-612111) on nociceptin/orphanin FQ-mediated potassium channel activation in rat periaqueductal gray slices

Nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor, a non-opioid branch of the opioid receptor family, shows structural similarities to traditional opioid receptors but binds opioid with very poor affinity. This receptor has been implicated in many physiological functions including pain regulation. This study quantitatively investigated the effect of (-)-cis-1-Methyl-7-[[4-(2,6-dichlorophenyl)piperidin-1 -yl]methyl]-6,7,8,9-tetrahydro-5H-benzocyclohepten-5-ol (SB-612111), a novel non-peptide ligand of NOP receptor, on the native NOP receptors in the midbrain ventrolateral periaqueductal gray (vlPAG), a crucial region for pain regulation. SB-612111 concentration-dependently antagonized N/OFQ-induced G-protein coupled inwardly rectifying K(+) (GIRK) current in vlPAG neurons. The IC(50) value of SB-612111 estimated from dose-response curves is 87.7±1.2nM. SB-612111 had no intrinsic agonistic activity and did not affect the GIRK current induced by [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin, a mu-opioid receptor agonist, when tested at concentrations of up to 1μM. It is concluded that SB-612111 is a pure, potent and selective antagonist of NOP receptors that mediate GIRK channel activation in the vlPAG neurons.

Endogenous opiates and behavior: 2009

This paper is the 32nd consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2009 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Recently patented and promising ORL-1 ligands: where have we been and where are we going?

IMPORTANCE OF THE FIELD

The interactions of nociceptin/orphanin FQ (N/OFQ) and the opioid receptor-like receptor 1 (nociceptin opioid peptide--NOP) have been implicated in a variety of systems including cardiovascular, respiratory, immune, and the central and peripheral nervous systems.

AREAS COVERED IN THIS REVIEW

To elucidate the endogenous role of the N/OFQ-NOP system through the use of knockout and knockdown animal preparations, though most advances have been made using a host of synthetic agonists and antagonists. This review gives a brief history of the receptor-ligand discovery, the development of these agonists and antagonists within the last 10 years as published, and the therapeutic indications thereof focusing on pain.

WHAT THE READER WILL GAIN

The use of NOP ligands in pain has been controversial at best; however, there are indications that both agonists and antagonists have a place in the clinical setting for acute and chronic pain. NOP ligands have potential as novel therapeutics, interestingly, when incorporated into a rationally-designed multi-target agent.

TAKE HOME MESSAGE

The discovery of N/OFQ and NOP opened a new option for the treatment of pain with the potential for a decreased side effect profile. Numerous compounds have been designed to target this system, the most promising of which have mixed profiles.