Bidirectional modulatory effect of orphanin FQ on morphine-induced analgesia: antagonism in brain and potentiation in spinal cord of the rat

Synthesis, Biological Activity and Molecular Docking of Chimeric Peptides Targeting Opioid and NOP Receptors

Recently, mixed opioid/NOP agonists came to the spotlight for their favorable functional profiles and promising outcomes in clinical trials as novel analgesics. This study reports on two novel chimeric peptides incorporating the fragment Tyr-c[D-Lys-Phe-Phe]Asp-NH₂ (RP-170), a cyclic peptide with high affinity for µ and κ opioid receptors (or MOP and KOP, respectively), conjugated with the peptide Ac-RYYRIK-NH₂, a known ligand of the nociceptin/orphanin FQ receptor (NOP), yielding RP-170-RYYRIK-NH₂ (KW-495) and RP-170-Gly₃-RYYRIK-NH₂ (KW-496). In vitro, the chimeric KW-496 gained affinity for KOP, hence becoming a dual KOP/MOP agonist, while KW-495 behaved as a mixed MOP/NOP agonist with low nM affinity. Hence, KW-495 was selected for further in vivo experiments. Intrathecal administration of this peptide in mice elicited antinociceptive effects in the hot-plate test; this action was sensitive to both the universal opioid receptor antagonist naloxone and the selective NOP antagonist SB-612111. The rotarod test revealed that KW-495 administration did not alter the mice motor coordination performance. Computational studies have been conducted on the two chimeras to investigate the structural determinants at the basis of the experimental activities, including any role of the Gly₃ spacer.

Therapeutic potential of nociceptin/orphanin FQ peptide (NOP) receptor modulators for treatment of traumatic brain injury, traumatic stress, and their co-morbidities

The nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor is a member of the opioid receptor superfamily with N/OFQ as its endogenous agonist. Wide expression of the NOP receptor and N/OFQ, both centrally and peripherally, and their ability to modulate several biological functions has led to development of NOP receptor modulators by pharmaceutical companies as therapeutics, based upon their efficacy in preclinical models of pain, anxiety, depression, Parkinson's disease, and substance abuse. Both posttraumatic stress disorder (PTSD) and traumatic brain injury (TBI) are debilitating conditions that significantly affect the quality of life of millions of people around the world. PTSD is often a consequence of TBI, and, especially for those deployed to, working and/or living in a war zone or are first responders, they are comorbid. PTSD and TBI share common symptoms, and negatively influence outcomes as comorbidities of the other. Unfortunately, a lack of effective therapies or therapeutic agents limits the long term quality of life for either TBI or PTSD patients. Ours, and other groups, demonstrated that PTSD and TBI preclinical models elicit changes in the N/OFQ-NOP receptor system, and that administration of NOP receptor ligands alleviated some of the neurobiological and behavioral changes induced by brain injury and/or traumatic stress exposure. Here we review the past and most recent progress on understanding the role of the N/OFQ-NOP receptor system in PTSD and TBI neurological and behavioral sequelae. There is still more to understand about this neuropeptide system in both PTSD and TBI, but current findings warrant further examination of the potential utility of NOP modulators as therapeutics for these disorders and their co-morbidities. We advocate the development of standards for common data elements (CDE) reporting for preclinical PTSD studies, similar to current preclinical TBI CDEs. That would provide for more standardized data collection and reporting to improve reproducibility, interpretation and data sharing across studies.

Interactive Mechanisms of Supraspinal Sites of Opioid Analgesic Action: A Festschrift to Dr. Gavril W. Pasternak

Almost a half century of research has elaborated the discoveries of the central mechanisms governing the analgesic responses of opiates, including their receptors, endogenous peptides, genes and their putative spinal and supraspinal sites of action. One of the central tenets of "gate-control theories of pain" was the activation of descending supraspinal sites by opiate drugs and opioid peptides thereby controlling further noxious input. This review in the Special Issue dedicated to the research of Dr. Gavril Pasternak indicates his contributions to the understanding of supraspinal mediation of opioid analgesic action within the context of the large body of work over this period. This review will examine (a) the relevant supraspinal sites mediating opioid analgesia, (b) the opioid receptor subtypes and opioid peptides involved, (c) supraspinal site analgesic interactions and their underlying neurophysiology, (d) molecular (particularly AS) tools identifying opioid receptor actions, and (e) relevant physiological variables affecting site-specific opioid analgesia. This review will build on classic initial studies, specify the contributions that Gavril Pasternak and his colleagues did in this specific area, and follow through with studies up to the present.

Oxycodone in the Opioid Epidemic: High 'Liking', 'Wanting', and Abuse Liability

It is estimated that nearly a third of people who abuse drugs started with prescription opioid medicines. Approximately, 11.5 million Americans used prescription drugs recreationally in 2016, and in 2018, 46,802 Americans died as the result of an opioid overdose, including prescription opioids, heroin, and illicitly manufactured fentanyl (National Institutes on Drug Abuse (2020) Opioid Overdose Crisis. https://www.drugabuse.gov/drugs-abuse/opioids/opioid-overdose-crisis . Accessed 06 June 2020). Yet physicians will continue to prescribe oral opioids for moderate-to-severe pain in the absence of alternative therapeutics, underscoring the importance in understanding how drug choice can influence detrimental outcomes. One of the opioid prescription medications that led to this crisis is oxycodone, where misuse of this drug has been rampant. Being one of the most highly prescribed opioid medications for treating moderate-to-severe pain as reflected in the skyrocketed increase in retail sales of 866% between 1997 and 2007, oxycodone was initially suggested to be less addictive than morphine. The false-claimed non-addictive formulation of oxycodone, OxyContin, further contributed to the opioid crisis. Abuse was often carried out by crushing the pills for immediate burst release, typically by nasal insufflation, or by liquefying the pills for intravenous injection. Here, we review oxycodone pharmacology and abuse liability as well as present the hypothesis that oxycodone may exhibit a unique pharmacology that contributes to its high likability and abuse susceptibility. We will discuss various mechanisms that likely contribute to the high abuse rate of oxycodone including clinical drug likability, pharmacokinetics, pharmacodynamics, differences in its actions within mesolimbic reward circuity compared to other opioids, and the possibility of differential molecular and cellular receptor interactions that contribute to its selective effects. We will also discuss marketing strategies and drug difference that likely contributes to the oxycodone opioid use disorders and addiction.

The NOP Receptor System in Neurological and Psychiatric Disorders: Discrepancies, Peculiarities and Clinical Progress in Developing Targeted Therapies

The nociceptin opioid peptide (NOP) receptor and its endogenous ligand nociceptin/orphanin FQ (N/OFQ) are the fourth members of the opioid receptor and opioid peptide families. Although they have considerable sequence homology to the other family members, they are not considered opioid per se because they do not have pharmacological profiles similar to the other family members. The number of NOP receptors in the brain is higher than the other family members, and NOP receptors can be found throughout the brain. Because of the widespread distribution of NOP receptors, N/OFQ and other peptide and small molecule agonists and antagonists have extensive CNS activities. Originally thought to be anti-opioid, NOP receptor agonists block some opioid activities, potentiate others, and modulate other activities not affected by traditional opiates. Because the effect of receptor activation can be dependent upon site of administration, state of the animal, and other variables, the study of NOP receptors has been fraught with contradictions and inconsistencies. In this article, the actions and controversies pertaining to NOP receptor activation and inhibition are discussed with respect to CNS disorders including pain (acute, chronic, and migraine), drug abuse, anxiety and depression. In addition, progress towards clinical use of NOP receptor-directed compounds is discussed.

Potential of Nociceptin/Orphanin FQ Peptide Analogs for Drug Development

Nociceptin receptor (NOP) belongs to the family of opioid receptors but was discovered and characterized much later than the so called classical opioid receptors, μ, δ and κ (or MOP, DOP and KOP, resp.). Nociceptin/orphanin FQ (N/OFQ) is the endogenous ligand of this receptor and it controls numerous important functions in the central nervous system and in the periphery, so its analogs may be developed as innovative drugs for the treatment of a variety of conditions and pathological states. Availability of potent and selective ligands with high affinity to NOP receptor is essential to fully understand the role of NOP-N/OFQ system in the body, which in turn may lead to designing novel therapeutics. Here, we have focused on reviewing the structure of potent peptide-based agonists, antagonists, biased analogs and bivalent ligands that target NOP receptor.

Molecular Biology of Opioid Analgesia and Its Clinical Considerations

Understanding the molecular biology of opioid analgesia is essential for its proper implementation and mechanistic approach to its modulation in order to maximize analgesia and minimize undesired effects. By appreciating the molecular mechanisms intrinsic to opioid analgesia, one can manipulate a molecular target to augment or diminish a specific effect using adjuvant drugs, select an appropriate opioid for opioid rotation or define a molecular target for new opioid drug development. In this review, we present the cellular and molecular mechanisms of opioid analgesia and that of the associated phenomena of tolerance, dependence, and hyperalgesia. The specific mechanisms highlighted are those that presently can be clinically addressed.

Crosstalk between Opioid and Anti-Opioid Systems: An Overview and Its Possible Therapeutic Significance

Opioid peptides and receptors are broadly expressed throughout peripheral and central nervous systems and have been the subject of intense long-term investigations. Such studies indicate that some endogenous neuropeptides, called anti-opioids, participate in a homeostatic system that tends to reduce the effects of endogenous and exogenous opioids. Anti-opioid properties have been attributed to various peptides, including melanocyte inhibiting factor (MIF)-related peptides, cholecystokinin (CCK), nociceptin/orphanin FQ (N/OFQ), and neuropeptide FF (NPFF). These peptides counteract some of the acute effects of opioids, and therefore, they are involved in the development of opioid tolerance and addiction. In this work, the anti-opioid profile of endogenous peptides was described, mainly taking into account their inhibitory influence on opioid-induced effects. However, the anti-opioid peptides demonstrated complex properties and could show opioid-like as well as anti-opioid effects. The aim of this review is to detail the phenomenon of crosstalk taking place between opioid and anti-opioid systems at the in vivo pharmacological level and to propose a cellular and molecular basis for these interactions. A better knowledge of these mechanisms has potential therapeutic interest for the control of opioid functions, notably for alleviating pain and/or for the treatment of opioid abuse.

NOP receptor agonist attenuates nitroglycerin-induced migraine-like symptoms in mice

Migraine is an extraordinarily prevalent and disabling headache disorder that affects one billion people worldwide. Throbbing pain is one of several migraine symptoms including sensitivity to light (photophobia), sometimes to sounds, smell and touch. The basic mechanisms underlying migraine remain inadequately understood, and current treatments (with triptans being the primary standard of care) are not well tolerated by some patients. NOP (Nociceptin OPioid) receptors, the fourth member of the opioid receptor family, are expressed in the brain and periphery with particularly high expression known to be in trigeminal ganglia (TG). The aim of our study was to further explore the involvement of the NOP receptor system in migraine. To this end, we used immunohistochemistry to examine NOP receptor distribution in TG and trigeminal nucleus caudalus (TNC) in mice, including colocalization with specific cellular markers, and used nitroglycerin (NTG) models of migraine to assess the influence of the selective NOP receptor agonist, Ro 64-6198, on NTG-induced pain (sensitivity of paw and head using von Frey filaments) and photophobia in mice. Our immunohistochemical studies with NOP-eGFP knock-in mice indicate that NOP receptors are on the majority of neurons in the TG and are also very highly expressed in the TNC. In addition, Ro 64-6198 can dose dependently block NTG-induced paw and head allodynia, an effect that is blocked by the NOP antagonist, SB-612111. Moreover, Ro 64-6198, can decrease NTG-induced light sensitivity in mice. These results suggest that NOP receptor agonists should be futher explored as treatment for migraine symptoms. This article is part of the special issue on Neuropeptides.

NOP-Related Mechanisms in Pain and Analgesia

Since the discovery of the NOP receptor and N/OFQ as the endogenous ligand, evidence has appeared demonstrating the involvement of this receptor system in pain. This was not surprising for members of the opioid receptor and peptide families, particularly since both the receptor and N/OFQ are highly expressed in brain regions involved in pain, spinal cord, and dorsal root ganglia. What has been surprising is the complicated picture that has emerged from 25 years of research. The original finding that N/OFQ decreased tail flick and hotplate latency, when administered i.c.v., led to the hypothesis that NOP receptor antagonists could have analgesic activity without abuse liability. However, as data accumulated, it became clear that not only the potency but the activity per se was different when N/OFQ or small molecule NOP agonists were administered in the brain versus the spinal cord and it also depended upon the pain assay used. When administered systemically, NOP receptor agonists are generally ineffective in attenuating heat pain but are antinociceptive in an acute inflammatory pain model. Most antagonists administered systemically have no antinociceptive activity of their own, even though selective peptide NOP antagonists have potent antinociceptive activity when administered i.c.v. Chronic pain models provide different results as well, as small molecule NOP receptor agonists have potent anti-allodynic and anti-hyperalgesic activity after systemic administration. A considerable number of electrophysiological and anatomical experiments, in particular with NOP-eGFP mice, have been conducted in an attempt to explain the complicated profile resulting from NOP receptor modulation, to examine receptor plasticity, and to elucidate mechanisms by which selective NOP agonists, bifunctional NOP/mu agonists, or NOP receptor antagonists modulate acute and chronic pain.

Electrophysiological Actions of N/OFQ

Whilst the nociceptin/orphanin FQ (N/OFQ) receptor (NOP) has similar intracellular coupling mechanisms to opioid receptors, it has distinct modulatory effects on physiological functions such as pain. These actions range from agonistic to antagonistic interactions with classical opioids within the spinal cord and brain, respectively. Understanding the electrophysiological actions of N/OFQ has been crucial in ascertaining the mechanisms by which these agonistic and antagonistic interactions occur. These similarities and differences between N/OFQ and opioids are due to the relative location of NOP versus opioid receptors on specific neuronal elements within these CNS regions. These mechanisms result in varied cellular actions including postsynaptic modulation of ion channels and presynaptic regulation of neurotransmitter release.

Cebranopadol: A Novel First-in-Class Potent Analgesic Acting via NOP and Opioid Receptors

Cebranopadol is a novel first-in-class analgesic with highly potent agonistic activity at nociceptin/orphanin FQ peptide (NOP) and opioid receptors. It is highly potent and efficacious across a broad range of preclinical pain models. Its side effect profile is better compared to typical opioids. Mechanistic studies have shown that cebranopadol's activity at NOP receptors contributes to its anti-hyperalgesic effects while ameliorating some of its opioid-type side effects, including respiratory depression and abuse potential. Phase II of clinical development has been completed, demonstrating efficacy and good tolerability in acute and chronic pain conditions.This article focusses on reviewing data on the preclinical in vitro and in vivo pharmacology, safety, and tolerability, as well as clinical trials with cebranopadol.

Role of mu-opioid agonist efficacy on antinociceptive interactions between mu agonists and the nociceptin opioid peptide agonist Ro 64-6198 in rhesus monkeys

Mu-opioid receptor agonists are clinically effective analgesics, but also produce undesirable effects that limit their clinical utility. The nociceptin opioid peptide (NOP) receptor system also modulates nociception, and NOP agonists might be useful adjuncts to enhance the analgesic effects or attenuate the undesirable effects of mu-opioid agonists. The present study determined behavioral interactions between the NOP agonist (-)-Ro 64-6198 and mu-opioid ligands that vary in mu-opioid receptor efficacy (17-cyclopropylmethyl-3,14β-dihyroxy-4,5α-epoxy-6α-[(3 ́-isoquinolyl)acetamindo]morphinan (NAQ) < buprenorphine < nalbuphine < morphine = oxycodone < methadone) in male rhesus monkeys. For comparison, Ro 64-6198 interactions were also examined with the kappa-opioid receptor agonist nalfurafine. Each opioid ligand was examined alone and following fixed-dose Ro 64-6198 pretreatments in assays of thermal nociception (n = 3-4) and schedule-controlled responding (n = 3). Ro 64-6198 alone failed to produce significant antinociception up to doses (0.32 mg/kg, IM) that significantly decreased rates of responding. All opioid ligands, except NAQ and nalfurafine, produced dose- and thermal intensity-dependent antinociception. Ro 64-6198 enhanced the antinociceptive potency of buprenorphine, nalbuphine, methadone, and nalfurafine. Ro 64-6198 enhancement of nalbuphine antinociception was NOP antagonist SB-612111 reversible and occurred under a narrow range of dose and time conditions. All opioid ligands, except NAQ and buprenorphine, produced dose-dependent decreases in rates of responding. Ro 64-6198 did not significantly alter mu-opioid ligand rate-decreasing effects. Although these results suggest that NOP agonists may selectively enhance the antinociceptive vs. rate-suppressant effects of some mu-opioid agonists, this small enhancement occurred under a narrow range of conditions dampening enthusiasm for NOP agonists as candidate "opioid-sparing" adjuncts.

Nociceptin/orphanin FQ receptor ligands and translational challenges: focus on cebranopadol as an innovative analgesic

Opioids are characterised as classical (mu, delta, and kappa) along with the non-classical nociceptin/orphanin FQ (N/OFQ) receptor or NOP. Targeting NOP has therapeutic indications in control of the cardiovascular and respiratory systems and micturition, and a profile as an antidepressant. For all of these indications, there are translational human data. Opioids such as morphine and fentanyl (activating the mu receptor) are the mainstay of pain treatment in the perioperative period, despite a challenging side-effect profile. Opioids in general have poor efficacy in neuropathic pain. Moreover, longer term use is associated with tolerance. There is good evidence interactions between opioid receptors, and receptor co-activation can reduce side-effects without compromising analgesia; this is particularly true for mu and NOP co-activation. Recent pharmaceutical development has produced a mixed opioid/NOP agonist, cebranopadol. This new chemical entity is effective in animal models of nociceptive and neuropathic pain with greater efficacy in the latter. In animal models, there is little evidence for respiratory depression, and tolerance (compared with morphine) only develops after long treatment periods. There is now early phase clinical development in diabetic neuropathy, cancer pain, and low back pain where cebranopadol displays significant efficacy. In 1996, N/OFQ was formally identified with an innovative analgesic profile. Approximately 20 yr later, cebranopadol as a clinical ligand is advancing through the human trials process.

ORL1 Activation Mediates a Novel ORL1 Receptor Agonist SCH221510 Analgesia in Neuropathic Pain in Rats

Opioid receptor like 1 (ORL₁) receptor activation displayed an anti-nociceptive effect at spinal level for acute and neuropathic pain. SCH221510, an orally active non-peptide ORL₁ agonist, was reported to be effective in treating neuropathic pain. The present study used ORL₁ antagonist and siRNA to investigate that ORL₁ activation mediates intrathecal SCH221510 analgesia in neuropathic pain induced by chronic constrictive injury (CCI) to rat sciatic nerve. Paw withdrawal latency and 50% mechanical threshold were measured for thermal and mechanical hypersensitivity in rats. CCI significantly decreased paw withdrawal latency and mechanical threshold. SCH221510 (3, 10, 30 μg) or ORL₁ antagonist ([Nphe¹]nociceptin(1-13)NH₂, 10 μg) was intrathecally injected to test the behavioral effects on neuropathic pain. Intrathecal siRNA was started on 1 day before CCI surgery and maintained for 7 days. L4-L5 spinal cord ORL₁ mRNA and protein were measured by real-time PCR and Western blot. The effect of intrathecal siRNA on SCH2210510 was tested in CCI rats on day 7. Intrathecal SCH221510 dose-dependently reduced thermal and mechanical hypersensitivity induced by CCI. [Nphe¹]nociceptin(1-13)NH₂ blocked SCH221510 analgesia in CCI rats. Intrathecal siRNA blocked ORL₁ mRNA and protein increase induced by CCI. Intrathecal ORL₁ siRNA did not change thermal and mechanical hypersensitivity induced by nerve injury. Intrathecal siRNA blocked SCH221510 analgesia in neuropathic pain at spinal level. Conclusively, ORL₁ activation mediates SCH221510 analgesia in neuropathic pain at spinal level. The results warrant a potential clinically applicable drug in treating neuropathic pain.

Analysis of the distribution of spinal NOP receptors in a chronic pain model using NOP-eGFP knock-in mice

BACKGROUND AND PURPOSE

The nociceptin/orphanin FQ opioid peptide (NOP) receptor system plays a significant role in the regulation of pain. This system functions differently in the spinal cord and brain. The mechanism by which the NOP receptor agonists regulate pain transmission in these regions is not clearly understood. Here, we investigate the peripheral and spinal NOP receptor distribution and antinociceptive effects of intrathecal nociceptin/orphanin FQ (N/OFQ) in chronic neuropathic pain.

EXPERIMENTAL APPROACH

We used immunohistochemistry to determine changes in NOP receptor distribution triggered by spinal nerve ligation (SNL) using NOP-eGFP knock-in mice. Antinociceptive effects of intrathecal N/OFQ on SNL-mediated allodynia and heat/cold hyperalgesia were assessed in wild-type mice.

KEY RESULTS

NOP-eGFP immunoreactivity was decreased by SNL in the spinal laminae I and II outer, regions that mediate noxious heat stimuli. In contrast, immunoreactivity of NOP-eGFP was unchanged in the ventral border of lamina II inner, which is an important region for the development of allodynia. NOP-eGFP expression was also decreased in a large number of primary afferents in the L4 dorsal root ganglion (DRG) of SNL mice. However, SNL mice showed increased sensitivity, compared to sham animals to the effects of i.t administered N/OFQ with respect to mechanical as well as thermal stimuli.

CONCLUSIONS AND IMPLICATIONS

Our findings suggest that the spinal NOP receptor system attenuates injury-induced hyperalgesia by direct inhibition of the projection neurons in the spinal cord that send nociceptive signals to the brain and not by inhibiting presynaptic terminals of DRG neurons in the superficial lamina.

Evaluation of cebranopadol, a dually acting nociceptin/orphanin FQ and opioid receptor agonist in mouse models of acute, tonic, and chemotherapy-induced neuropathic pain

BACKGROUND

Cebranopadol (a.k.a. GRT-6005) is a dually acting nociceptin/orphanin FQ and opioid receptor agonist that has been recently developed in Phase 2 clinical trials for painful diabetic neuropathy or cancer pain. It also showed analgesic properties in various rat models of pain and had a better safety profile as compared to equi-analgesic doses of morphine. Since antinociceptive properties of cebranopadol have been studied mainly in rat models, in the present study, we assessed analgesic activity of subcutaneous cebranopadol (10 mg/kg) in various mouse pain models.

METHODS

We used models of acute, tonic, and chronic pain induced by thermal and chemical stimuli, with a particular emphasis on pharmacoresistant chronic neuropathic pain evoked by oxaliplatin in which cebranopadol was used alone or in combination with simvastatin.

KEY RESULTS

As shown in the hot plate test, the analgesic activity of cebranopadol developed more slowly as compared to morphine (90-120 min vs. 60 min). Cebranopadol displayed a significant antinociceptive activity in acute pain models, i.e., the hot plate, writhing, and capsaicin tests. It attenuated nocifensive responses in both phases of the formalin test and reduced cold allodynia in oxaliplatin-induced neuropathic pain model. Its efficacy was similar to that of morphine. Used in combination and administered simultaneously, 4 or 6 h after simvastatin, cebranopadol did not potentiate antiallodynic activity of this cholesterol-lowering drug. Cebranopadol did not induce any motor deficits in the rotarod test.

CONCLUSION

Cebranopadol may have significant potential for the treatment of various pain types, including inflammatory and chemotherapy-induced neuropathic pain.

Involvement of the N/OFQ-NOP system in rat morphine antinociceptive tolerance: Are astrocytes the crossroad?

The development of tolerance to the antinociceptive effect is a main problem associated with the repeated administration of opioids. The progressively higher doses required to relieve pain reduce safety and exacerbate the side effects of classical opioid receptor agonists like morphine. Nociceptin/orphanin FQ (N/OFQ) and its NOP receptor constitute the fourth endogenous opioid system that is involved in the control of broad spectrum of biological functions, including pain transmission. Aim of this work was to evaluate the relevance of the N/OFQ-NOP system in morphine antinociceptive action and in the development of morphine tolerance in the rat. Continuous spinal intrathecal infusion of morphine (1-3 nmol/h) evoked analgesic effects for 5 days in wild type animals. The same doses infused in NOP(-/-) rats showed a lower analgesic efficacy, while the onset of tolerance was delayed to day 9. N/OFQ (1-3 nmol/h), continuously infused in NOP(+/+) animals, showed an analgesic profile similar to morphine. Immunohistochemical analysis of the dorsal horn of the spinal cord of morphine tolerant NOP(+/+) rats showed an increased number of Iba1- and GFAP-positive cells (microglia and astrocytes, respectively). Interestingly, microglia but not astrocyte activation was observed in NOP(-/-) morphine tolerant rat. A selective activation of astrocytes was observed in the dorsal horn of wild type N/OFQ tolerant rats. The antinociceptive effect of morphine partially depends by the N/OFQ-NOP system that participates in the development of morphine tolerance. In particular, NOP receptors are involved in morphine-induced astrocyte activation, and N/OFQ per se increases astrocyte density.

Respiratory Effects of the Nociceptin/Orphanin FQ Peptide and Opioid Receptor Agonist, Cebranopadol, in Healthy Human Volunteers

Background

Cebranopadol is a novel strong analgesic that coactivates the nociceptin/orphanin FQ receptor and classical opioid receptors. There are indications that activation of the nociceptin/orphanin FQ receptor is related to ceiling in respiratory depression. In this phase 1 clinical trial, we performed a pharmacokinetic-pharmacodynamic study to quantify cebranopadol’s respiratory effects.

Methods

Twelve healthy male volunteers received 600 μg oral cebranopadol as a single dose. The following main endpoints were obtained at regular time intervals for 10 to 11 h after drug intake: ventilation at an elevated clamped end-tidal pressure of carbon dioxide, pain threshold and tolerance to a transcutaneous electrical stimulus train, and plasma cebranopadol concentrations. The data were analyzed using sigmoid Emax (respiration) and power (antinociception) models.

Results

Cebranopadol displayed typical opioid-like effects including miosis, analgesia, and respiratory depression. The blood-effect-site equilibration half-life for respiratory depression and analgesia was 1.2 ± 0.4 h (median ± standard error of the estimate) and 8.1 ± 2.5 h, respectively. The effect-site concentration causing 50% respiratory depression was 62 ± 4 pg/ml; the effect-site concentration causing 25% increase in currents to obtain pain threshold and tolerance was 97 ± 29 pg/ml. The model estimate for minimum ventilation was greater than zero at 4.9 ± 0.7 l/min (95% CI, 3.5 to 6.6 l/min).

Conclusions

At the dose tested, cebranopadol produced respiratory depression with an estimate for minimum ventilation greater than 0 l/min. This is a major advantage over full μ-opioid receptor agonists that will produce apnea at high concentrations. Further clinical studies are needed to assess whether such behavior persists at higher doses.

Bifunctional peptide-based opioid agonist/nociceptin antagonist ligand for dual treatment of nociceptive and neuropathic pain

Drugs able to treat both nociceptive and neuropathic pain effectively without major side effects are lacking. We developed a bifunctional peptide-based hybrid (KGNOP1) that structurally combines a mu-opioid receptor agonist (KGOP1) with antinociceptive activity and a weak nociceptin receptor antagonist (KGNOP3) with anti-neuropathic pain activity. We investigated KGNOP1-related behavioral effects after intravenous administration in rats by assessing thermal nociception, cold hyperalgesia in a model of neuropathic pain induced by chronic constriction injury of the sciatic nerve, and plethysmography parameters including inspiratory time (TI) and minute ventilation (VM) in comparison to the well-known opioid analgesics, tramadol and morphine. Time-course and dose-dependent effects were investigated for all behavioral parameters to determine the effective doses 50% (ED50). Pain-related effects on cold hyperalgesia were markedly increased by KGNOP1 as compared to KGNOP3 and tramadol (ED50: 0.0004, 0.32, and 12.1 μmol/kg, respectively), whereas effects on thermal nociception were significantly higher with KGNOP1 as compared to morphine (ED50: 0.41 and 14.7 μmol/kg, respectively). KGNOP1 and KGOP1 produced a larger increase in TI and deleterious decrease in VM in comparison to morphine and tramadol (ED50(TI): 0.63, 0.52, 12.2, and 50.9 μmol/kg; ED50(VM): 0.57, 0.66, 10.6, and 50.0 μmol/kg, respectively). Interestingly, the calculated ratios of anti-neuropathic pain/antinociceptive to respiratory effects revealed that KGNOP1 was safer than tramadol (ED50 ratio: 5.44 × 10 vs 0.24) and morphine (ED50 ratio: 0.72 vs 1.39). We conclude that KGNOP1 is able to treat both experimental neuropathic and nociceptive pain, more efficiently and safely than tramadol and morphine, respectively, and thus should be a candidate for future clinical developments.

Pharmacological characterization of cebranopadol a novel analgesic acting as mixed nociceptin/orphanin FQ and opioid receptor agonist

The aim of the study was to investigate the in vitro and in vivo pharmacological profile of cebranopadol, a novel agonist for opioid and nociceptin/orphanin FQ (N/OFQ) receptors (NOP). In vitro cebranopadol was assayed in calcium mobilization studies in cells coexpressing NOP or opioid receptors and chimeric G‐proteins and in a bioluminescence resonance energy transfer (BRET) assay for studying receptor interaction with G‐protein and β‐arrestin 2. The mouse tail withdrawal and formalin tests were used for investigating cebranopadol antinociceptive properties. In calcium mobilization studies cebranopadol showed the following rank order of potency NOP = mu > kappa ≥ delta. In BRET studies, cebranopadol promoted NOP and mu receptors interaction with G‐protein with similar high potency and efficacy. However, cebranopadol did not stimulated NOP–β‐arrestin 2 interactions and displayed reduced potency at mu/β‐arrestin 2. In vivo, cebranopadol exhibits highly potent and extremely long‐lasting antinociceptive effects. The effects of cebranopadol in the tail withdrawal assay were sensitive to both SB‐612111 and naloxone. Collectively the present results confirm and extend previous finding demonstrating that cebranopadol, by acting as mixed NOP/opioid receptor agonist, elicits robust analgesic effects in different pain models.

■ REVIEW : ORL-1: An Awkward Child of the Opioid Receptor Family

The cloning of the δ-opioid receptor allowed for the rapid cloning of the two other classically defined opioid receptors, the μ- and κ-opioid receptors. However, several groups cloned a fourth receptor (ORL-1, for opioid receptor-like) that had high homology to the opioid receptors but did not bind any known endogenous opioid peptides (i.e., endorphins) or exogenous opiates. Recently, two independent groups isolated a 17- amino-acid peptide that is an endogenous ligand for ORL-1; one group named it orphanin FQ (OFQ), the other named it nociceptin (N). It was reported that intracerebroventricular administration of this heptadeca peptide (OFQ/N) in mice induced an increased responsiveness to painful stimuli, an effect in striking contrast to the analgesia that is a hallmark of classical opiate drugs. Further research has revealed that OFQ/N has complex effects on pain perception: OFQ/N has been touted as having analgesic, hyperalgesic, and anti opioid properties. In addition to discussing these disparate findings, this review highlights the structural and pharmacological parallels between ORL-1 and opioid receptors as well as their respective endogenous ligands. NEUROSCIENTIST 4:172-184, 1998

Characterisation of the Novel Mixed Mu-NOP Peptide Ligand Dermorphin-N/OFQ (DeNo)

INTRODUCTION

Opioid receptors are currently classified as Mu (μ), Delta (δ), Kappa (κ) plus the opioid related nociceptin/orphanin FQ (N/OFQ) peptide receptor (NOP). Despite compelling evidence for interactions and benefits of targeting more than one receptor type in producing analgesia, clinical ligands are Mu agonists. In this study we have designed a Mu-NOP agonist named DeNo. The Mu agonist component is provided by dermorphin, a peptide isolated from the skin of Phyllomedusa frogs and the NOP component by the endogenous agonist N/OFQ.

METHODS

We have assessed receptor binding profile of DeNo and compared with dermorphin and N/OFQ. In a series of functional screens we have assessed the ability to (i) increase Ca2+ in cells coexpressing recombinant receptors and a the chimeric protein Gαqi5, (ii) stimulate the binding of GTPγ[35S], (iii) inhibit cAMP formation, (iv) activate MAPKinase, (v) stimulate receptor-G protein and arrestin interaction using BRET, (vi) electrically stimulated guinea pig ileum (gpI) assay and (vii) ability to produce analgesia via the intrathecal route in rats.

RESULTS

DeNo bound to Mu (pKi; 9.55) and NOP (pKi; 10.22) and with reasonable selectivity. This translated to increased Ca2+ in Gαqi5 expressing cells (pEC50 Mu 7.17; NOP 9.69), increased binding of GTPγ[35S] (pEC50 Mu 7.70; NOP 9.50) and receptor-G protein interaction in BRET (pEC50 Mu 8.01; NOP 9.02). cAMP formation was inhibited and arrestin was activated (pEC50 Mu 6.36; NOP 8.19). For MAPK DeNo activated p38 and ERK1/2 at Mu but only ERK1/2 at NOP. In the gpI DeNO inhibited electrically-evoked contractions (pEC50 8.63) that was sensitive to both Mu and NOP antagonists. DeNo was antinociceptive in rats.

CONCLUSION

Collectively these data validate the strategy used to create a novel bivalent Mu-NOP peptide agonist by combining dermorphin (Mu) and N/OFQ (NOP). This molecule behaves essentially as the parent compounds in vitro. In the antonocicoeptive assays employed in this study DeNo displays only weak antinociceptive properties.

Bifunctional Peptide-Based Opioid Agonist-Nociceptin Antagonist Ligands for Dual Treatment of Acute and Neuropathic Pain

Herein, the opioid pharmacophore H-Dmt-d-Arg-Aba-β-Ala-NH2 (7) was linked to peptide ligands for the nociceptin receptor. Combination of 7 and NOP ligands (e.g., H-Arg-Tyr-Tyr-Arg-Ile-Lys-NH2) led to binding affinities in the low nanomolar domain. In vitro, the hybrids behaved as agonists at the opioid receptors and antagonists at the nociceptin receptor. Intravenous administration of hybrid 13a (H-Dmt-d-Arg-Aba-β-Ala-Arg-Tyr-Tyr-Arg-Ile-Lys-NH2) to mice resulted in potent and long lasting antinociception in the tail-flick test, indicating that 13a was able to permeate the BBB. This was further supported by a cell-based BBB model. All hybrids alleviated allodynia and hyperalgesia in neuropathic pain models. Especially with respect to hyperalgesia, they showed to be more effective than the parent compounds. Hybrid 13a did not result in significant respiratory depression, in contrast to an equipotent analgesic dose of morphine. These hybrids hence represent a promising avenue toward analgesics for the dual treatment of acute and neuropathic pain.

Central N/OFQ-NOP Receptor System in Pain Modulation

Two decades have passed since the peptide, nociceptin/orphanin FQ (N/OFQ), and its cognate (NOP) receptor were discovered. Although NOP receptor activation causes a similar pattern of intracellular actions as mu-opioid (MOP) receptors, NOP receptor-mediated pain modulation in rodents are more complicated than MOP receptor activation. This review highlights the functional evidence of spinal, supraspinal, and systemic actions of NOP receptor agonists for regulating pain. In rodents, effects of the N/OFQ-NOP receptor system in spinal and supraspinal sites for modulating pain are bidirectional depending on the doses, assays, and pain modalities. The net effect of systemically administered NOP receptor agonists may depend on relative contribution of spinal and supraspinal actions of the N/OFQ-NOP receptor signaling in rodents under different pain states. In stark contrast, NOP receptor agonists produce only antinociception and antihypersensitivity in spinal and supraspinal regions of nonhuman primates regardless of doses and assays. More importantly, NOP receptor agonists and a few bifunctional NOP/MOP receptor agonists do not exhibit reinforcing effects (abuse liability), respiratory depression, itch pruritus, nor do they delay the gastrointestinal transit function (constipation) in nonhuman primates. Depending upon their intrinsic efficacies for activating NOP and MOP receptors, bifunctional NOP/MOP receptor agonists warrant additional investigation in primates regarding their side effect profiles. Nevertheless, NOP receptor-related agonists display a much wider therapeutic window as compared to that of MOP receptor agonists in primates. Both selective NOP receptor agonists and bifunctional NOP/MOP receptor agonists hold great potential as effective and safe analgesics without typical opioid-associated side effects in humans.

Cholesterol-rich lipid rafts are involved in neuropeptide FF anti-nociceptin/orphanin FQ effect

The participation of a signaling platform to the anti-nociceptin/orphanin FQ (N/OFQ) effect of neuropeptide FF (NPFF) receptors was investigated in both acutely dissociated neurons and SH-SY5Y human neuroblastoma cells. The NPFF anti-N/OFQ, not anti-μ opioid effect, on the Ca²⁺ transient triggered by depolarization was reversed by methyl-β-cyclodextrin which depletes cholesterol from cell membranes. While the inactive α-cyclodextrin had no effect. By using [³⁵ S]GTPγS binding assay, a significant 20% decrease in the activity of nociceptin/orphanin FQ peptide receptors induced by the NPFF analog 1DMe was observed in detergent-resistant membranes, but not in total membranes of SH-SY5Y cells. Moreover, siRNA knock-down of G-protein-coupled receptor kinase 2 indicated that G-protein-coupled receptor kinase 2, but not protein kinase C, acted as the mediator in the NPFF anti-N/OFQ process. These data indicate that cholesterol-rich lipid rafts play an important role in the anti-N/OFQ effect of NPFF receptors. We proposed the participation of a signaling platform to the anti-Nociceptin/Orphanin FQ (N/OFQ) effect of Neuropeptide FF (NPFF) receptors both in mouse neurons and SH-SY5Y cells, with GRK2 protein acting as the mediator in this process. These findings should provide a more precise way to understand the anti-opioid effect of NPFF. NOP, Nociceptin/Orphanin FQ peptide.

Knock-In Mice with NOP-eGFP Receptors Identify Receptor Cellular and Regional Localization

The nociceptin/orphanin FQ (NOP) receptor, the fourth member of the opioid receptor family, is involved in many processes common to the opioid receptors including pain and drug abuse. To better characterize receptor location and trafficking, knock-in mice were created by inserting the gene encoding enhanced green fluorescent protein (eGFP) into the NOP receptor gene (Oprl1) and producing mice expressing a functional NOP-eGFP C-terminal fusion in place of the native NOP receptor. The NOP-eGFP receptor was present in brain of homozygous knock-in animals in concentrations somewhat higher than in wild-type mice and was functional when tested for stimulation of [(35)S]GTPγS binding in vitro and in patch-clamp electrophysiology in dorsal root ganglia (DRG) neurons and hippocampal slices. Inhibition of morphine analgesia was equivalent when tested in knock-in and wild-type mice. Imaging revealed detailed neuroanatomy in brain, spinal cord, and DRG and was generally consistent with in vitro autoradiographic imaging of receptor location. Multicolor immunohistochemistry identified cells coexpressing various spinal cord and DRG cellular markers, as well as coexpression with μ-opioid receptors in DRG and brain regions. Both in tissue slices and primary cultures, the NOP-eGFP receptors appear throughout the cell body and in processes. These knock-in mice have NOP receptors that function both in vitro and in vivo and appear to be an exceptional tool to study receptor neuroanatomy and correlate with NOP receptor function.

SIGNIFICANCE STATEMENT

The NOP receptor, the fourth member of the opioid receptor family, is involved in pain, drug abuse, and a number of other CNS processes. The regional and cellular distribution has been difficult to determine due to lack of validated antibodies for immunohistochemical analysis. To provide a new tool for the investigation of receptor localization, we have produced knock-in mice with a fluorescent-tagged NOP receptor in place of the native NOP receptor. These knock-in mice have NOP receptors that function both in vitro and in vivo and have provided a detailed characterization of NOP receptors in brain, spinal cord, and DRG neurons. They appear to be an exceptional tool to study receptor neuroanatomy and correlate with NOP receptor function.

Intrathecal administration of nociceptin/orphanin FQ receptor agonists in rats: A strategy to relieve chemotherapy-induced neuropathic hypersensitivity

Oxaliplatin and paclitaxel are considered central components in the treatment of colorectal and breast cancer, respectively. The development of neuropathy during chronic treatment represents the major dose-limiting side effect that leads to discontinuation or interruption of therapies. The management of neuropathy is a challenge to individuate innovative therapeutic strategies based on new targets and correct routes of administration. We evaluated the hypersensitivity reliever effect of different opioid receptor agonists in rat models of oxaliplatin and paclitaxel-induced neuropathy. Compounds were spinally infused by intrathecal catheter. In oxaliplatin-treated rats, 0.3 nmol morphine induced the reversion of the mechanical hypersensitivity (Paw-pressure test), nociceptin/orphanin FQ (N/OFQ; 0.3-3 nmol) significantly increased the pain threshold without reaching the values of the control animals. The N/OFQ peptide (NOP) receptor full agonist UFP-112 reverted pain threshold alterations at lower dosage (0.1 nmol) vs morphine and N/OFQ, the partial agonist UFP-113 (0.1-1 nmol) was similar to N/OFQ. The higher efficacy of morphine vs N/OFQ was highlighted also in paclitaxel-treated rats. The mechanical hypersensitivity was fully reverted by 0.1 nmol UFP-112 and UFP-113. In conclusion, intrathecal μ opioid peptide (MOP) and NOP receptor agonists relieved chemotherapy-induced neuropathic pain. The synthetic peptides showed valuable potency and efficacy suggesting the NOP system as an exploitable target.

Supraspinal actions of nociceptin/orphanin FQ, morphine and substance P in regulating pain and itch in non-human primates

BACKGROUND AND PURPOSE

Nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor agonists display a promising analgesic profile in preclinical studies. However, supraspinal N/OFQ produced hyperalgesia in rodents and such effects have not been addressed in primates. Thus, the aim of this study was to investigate the effects of centrally administered ligands on regulating pain and itch in non-human primates. In particular, nociceptive thresholds affected by intracisternal N/OFQ were compared with those of morphine and substance P, known to provide analgesia and mediate hyperalgesia, respectively, in humans.

EXPERIMENTAL APPROACH

Intrathecal catheters were installed to allow intracisternal and lumbar intrathecal administration in awake and unanaesthetized rhesus monkeys. Nociceptive responses were measured using the warm water tail-withdrawal assay. Itch scratching responses were scored from videotapes recording behavioural activities of monkeys in their home cages. Antagonist studies were conducted to validate the receptor mechanisms underlying intracisternally elicited behavioural responses.

KEY RESULTS

Intracisternal morphine (100 nmol) elicited more head scratches than those after intrathecal morphine. Distinct dermatomal scratching locations between the two routes suggest a corresponding activation of supraspinal and spinal μ receptors. Unlike intracisternal substance P, which induced hyperalgesia, intracisternal N/OFQ (100 nmol) produced antinociceptive effects mediated by NOP receptors. Neither peptide increased scratching responses.

CONCLUSIONS AND IMPLICATIONS

Taken together, these results demonstrated differential actions of ligands in the primate supraspinal region in regulating pain and itch. This study not only improves scientific understanding of the N/OFQ-NOP receptor system in pain processing but also supports the therapeutic potential of NOP-related ligands as analgesics.

Functional plasticity of the N/OFQ-NOP receptor system determines analgesic properties of NOP receptor agonists

Despite high sequence similarity between NOP (nociceptin/orphanin FQ opioid peptide) and opioid receptors, marked differences in endogenous ligand selectivity, signal transduction, phosphorylation, desensitization, internalization and trafficking have been identified; underscoring the evolutionary difference between NOP and opioid receptors. Activation of NOP receptors affects nociceptive transmission in a site-specific manner, with antinociceptive effects prevailing after peripheral and spinal activation, and pronociceptive effects after supraspinal activation in rodents. The net effect of systemically administered NOP receptor agonists on nociception is proposed to depend on the relative contribution of peripheral, spinal and supraspinal activation, and this may depend on experimental conditions. Functional expression and regulation of NOP receptors at peripheral and central sites of the nociceptive pathway exhibits a high degree of plasticity under conditions of neuropathic and inflammatory pain. In rodents, systemically administered NOP receptor agonists exerted antihypersensitive effects in models of neuropathic and inflammatory pain. However, they were largely ineffective in acute pain while concomitantly evoking severe motor side effects. In contrast, systemic administration of NOP receptor agonists to non-human primates (NHPs) exerted potent and efficacious antinociception in the absence of motor and sedative side effects. The reason for this species difference with respect to antinociceptive efficacy and tolerability is not clear. Moreover, co-activation of NOP and μ-opioid peptide (MOP) receptors synergistically produced antinociception in NHPs. Hence, both selective NOP receptor as well as NOP/MOP receptor agonists may hold potential for clinical use as analgesics effective in conditions of acute and chronic pain.

Beneficial effects of co-treatment with dextromethorphan on prenatally methadone-exposed offspring

Background

Heroin use among young women of reproductive age has drawn much attention around the world. Although methadone is widely used in maintenance therapy for heroin/morphine addiction, the long-term effects of prenatal exposure to methadone and preventative therapy remain unclear. For revealing this question, female pregnant Sprague–Dawley rats were sub-grouped to receive (1) vehicle, (2) methadone 5 mg/kg at embryonic day 3 (E3) and then 7 mg/kg from E4 to E20, (3) dextromethorphan (DM) 3 mg/kg, and (4) methadone + DM (the rats received methadone followed by DM treatment), subcutaneously, twice a day from E3 to E20. The body weight, natural withdrawal, pain sensitivity, ED50, conditioned place preference and water maze were conducted at different postnatal stages (P1 to P79) of offspring. The quantitative real-time RT-PCR and electrophysiology were also used to measure the gene expression of opioid receptors in the spinal cord and changes of LTP/LTD in the hippocampus, separately.

Results

Prenatal exposure to methadone or DM did not affect survival rate, body weight, water maze and LTP or LTD of offspring. However, prenatal methadone significantly increased the withdrawal symptoms, pain sensitivity, addiction liability and decreased the mRNA expression of pain related opioid receptors. Co-administration of DM with methadone in the maternal rats effectively prevented these abnormalities of offspring induced by methadone.

Conclusions

Our study clearly showed that co-administration of dextromethorphan with methadone in the maternal rats prevented the adverse effects induced by prenatal methadone exposure. It implies that dextromethorphan may have a potential to be used in combination with methadone for maintenance treatment in pregnant heroin-addicted women to prevent the adverse effects induced by methadone on offspring.

Electronic supplementary material

The online version of this article (doi:10.1186/s12929-015-0126-2) contains supplementary material, which is available to authorized users.

Pharmacological attenuation of chronic alcoholic pancreatitis induced hypersensitivity in rats

AIM

To characterize an alcohol and high fat diet induced chronic pancreatitis rat model that mimics poor human dietary choices.

METHODS

Experimental rats were fed a modified Lieber-DeCarli alcohol (6%) and high-fat (65%) diet (AHF) for 10 wk while control animals received a regular rodent chow diet. Weekly behavioral tests determined mechanical and heat sensitivity. In week 10 a fasting glucose tolerance test was performed, measuring blood glucose levels before and after a 2 g/kg bodyweight intraperitoneal (i.p.) injection of glucose. Post mortem histological analysis was performed by staining pancreas and liver tissue sections with hematoxylin and eosin. Pancreas sections were also stained with Sirius red and fast green to quantify collagen content. Insulin-expressing cells were identified immunohistochemically in separate sections. Tissue staining density was quantified using Image J software. After mechanical and heat sensitivity became stable (weeks 6-10) in the AHF-fed animals, three different drugs were tested for their efficacy in attenuating pancreatitis associated hypersensitivity: a Group II metabotropic glutamate receptor specific agonist (2R,4R)-4-Aminopyrrolidine-2,4-dicarboxylate (APDC, 3 mg/kg, ip; Tocris, Bristol, United Kingdom), nociceptin (20, 60, 200 nmol/kg, ip; Tocris), and morphine sulfate (3 mg/kg, μ-opioid receptor agonist; Baxter Healthcare, Deerfield, IL, United States).

RESULTS

Histological analysis of pancreas and liver determined that unlike control rats, AHF fed animals had pancreatic fibrosis, acinar and beta cell atrophy, with steatosis in both organs. Fat vacuolization was significantly increased in AHF fed rats (6.4% ± 1.1% in controls vs 23.8% ± 4.2%, P < 0.05). Rats fed the AHF diet had reduced fasting glucose tolerance in week 10 when peak blood glucose levels reached significantly higher concentrations than controls (127.4 ± 9.2 mg/dL in controls vs 161.0 ± 8.6 mg/dL, P < 0.05). This concurred with a 3.5 fold higher incidence of single and small 2-10 cell insulin-positive cell clusters (P < 0.05). Insulin expressing islet of Langerhans cells appeared hypertrophied while islet number and area measurements were not different from controls. Weekly behavioral tests determined that mechanical and heat sensitivities were significantly increased by 4 wk on AHF diet compared to controls. Hypersensitivity was attenuated with efficacy similar to morphine with single dose treatment of either metabotropic glutamate receptor 2/3 agonist APDC, or nociceptin, the endogenous ligand for opioid-receptor-like 1 receptor.

CONCLUSION

The AHF diet induces a chronic alcoholic pancreatitis in rats with measurable features resembling clinical patients with chronic pancreatitis and type 3c diabetes mellitus.

Endogenous nociceptin system involvement in stress responses and anxiety behavior

The mechanisms underpinning stress-related behavior and dysfunctional events leading to the expression of neuropsychiatric disorders remain incompletely understood. Novel candidates involved in the neuromodulation of stress, mediated both peripherally and centrally, provide opportunities for improved understanding of the neurobiological basis of stress disorders and may represent targets for novel therapeutic development. This chapter provides an overview of the mechanisms by which the opioid-related peptide, nociceptin, regulates the neuroendocrine stress response and stress-related behavior. In our research, we have employed nociceptin receptor antagonists to investigate endogenous nociceptin function in tonic control over stress-induced activity of the hypothalamo-pituitary-adrenal axis. Nociceptin demonstrates a wide range of functions, including modulation of psychological and inflammatory stress responses, modulation of neurotransmitter release, immune homeostasis, in addition to anxiety and cognitive behaviors. Greater appreciation of the complexity of limbic-hypothalamic neuronal networks, together with attention toward gender differences and the roles of steroid hormones, provides an opportunity for deeper understanding of the importance of the nociceptin system in the context of the neurobiology of stress and behavior.

Nociceptin/orphanin FQ peptide receptor antagonist JTC-801 reverses pain and anxiety symptoms in a rat model of post-traumatic stress disorder

BACKGROUND AND PURPOSE

Single-prolonged stress (SPS), a rat model of post-traumatic stress disorder (PTSD), also induces long-lasting hyperalgesia associated with hypocortisolism and elevated nociceptin/orphanin FQ (N/OFQ) levels in serum and CSF. Here, we determined the effect of JTC-801 (N-(4-amino-2-methylquinolin-6-yl)-2-(4-ethylphenoxymethyl) benzamide monohydrochloride), a nociceptin/orphanin FQ peptide (NOP) receptor antagonist, on symptoms of pain and anxiety in rats after SPS exposure, and examined N/OFQ-NOP receptor system changes.

EXPERIMENTAL APPROACH

Male Sprague Dawley rats received JTC-801 (6 mg kg(-1) i.p., once daily) during days 7-21 of SPS. The ability of JTC-801 to inhibit N/OFQ-stimulated [(35) S]-GTPγS binding was confirmed in rat brain membranes. Anxiety-like behaviour and pain sensitivity were monitored by changes in elevated plus maze performance and withdrawal responses to thermal and mechanical stimuli. Serum corticosterone and N/OFQ content in CSF, serum and brain tissues were determined by radioimmunoassay; NOP receptor protein and gene expression in amygdala, hippocampus and periaqueductal grey (PAG) were examined by immunoblotting and real-time PCR respectively.

KEY RESULTS

JTC-801 treatment reversed SPS-induced mechanical allodynia, thermal hyperalgesia, anxiety-like behaviour and hypocortisolism. Elevated N/OFQ levels in serum, CSF, PAG and hippocampus at day 21 of SPS were blocked by JTC-801; daily JTC-801 treatment also reversed NOP receptor protein and mRNA up-regulation in amygdala and PAG.

CONCLUSION AND IMPLICATIONS

JTC-801 reversed SPS-induced anxiety- and pain-like behaviours, and NOP receptor system up-regulation. These findings suggest that N/OFQ plays an important role in hyperalgesia and allodynia maintenance after SPS. NOP receptor antagonists may provide effective treatment for co-morbid PTSD and pain.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Stimulation of δ opioid receptor and blockade of nociceptin/orphanin FQ receptor synergistically attenuate parkinsonism

δ opioid peptide (DOP) receptors are considered a therapeutic target in Parkinson's disease, although the use of DOP agonists may be limited by side effects, including convulsions. To circumvent this issue, we evaluated whether blockade of nociceptin/orphanin FQ (N/OFQ) tone potentiated the antiparkinsonian effects of DOP agonists, thus allowing for reduction of their dosage. Systemic administration of the N/OFQ receptor (NOP) antagonist J-113397 [(3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H benzimidazol-2-one] and the DOP receptor agonist SNC-80 [(+)-4-[(αR)-α-(2S,5R)-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxy-benzyl]-N-N-diethylbenzamide] revealed synergistic attenuation of motor deficits in 6-hydroxydopamine hemilesioned rats and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated mice. In this model, repeated administration of the combination produced reproducible antiparkinsonian effects and was not associated with rescued striatal dopamine terminals. Microdialysis studies revealed that either systemic administration or local intranigral perfusion of J-113397 and SNC-80 led to the enhancement of nigral GABA, reduction of nigral Glu, and reduction of thalamic GABA levels, consistent with the view that NOP receptor blockade and DOP receptor stimulation caused synergistic overinhibition of nigro-thalamic GABA neurons. Whole-cell recording of GABA neurons in nigral slices confirmed that NOP receptor blockade enhanced the DOP receptor-induced effect on IPSCs via presynaptic mechanisms. Finally, SNC-80 more potently stimulated stepping activity in mice lacking the NOP receptor than wild-type controls, confirming the in vivo occurrence of an NOP-DOP receptor interaction. We conclude that endogenous N/OFQ functionally opposes DOP transmission in substantia nigra reticulata and that NOP receptor antagonists might be used in combination with DOP receptor agonists to reduce their dosage while maintaining their full therapeutic efficacy.

Calcium-permeable ion channels in pain signaling

The detection and processing of painful stimuli in afferent sensory neurons is critically dependent on a wide range of different types of voltage- and ligand-gated ion channels, including sodium, calcium, and TRP channels, to name a few. The functions of these channels include the detection of mechanical and chemical insults, the generation of action potentials and regulation of neuronal firing patterns, the initiation of neurotransmitter release at dorsal horn synapses, and the ensuing activation of spinal cord neurons that project to pain centers in the brain. Long-term changes in ion channel expression and function are thought to contribute to chronic pain states. Many of the channels involved in the afferent pain pathway are permeable to calcium ions, suggesting a role in cell signaling beyond the mere generation of electrical activity. In this article, we provide a broad overview of different calcium-permeable ion channels in the afferent pain pathway and their role in pain pathophysiology.

NOP receptor mediates anti-analgesia induced by agonist-antagonist opioids

Clinical studies have shown that agonist-antagonist opioid analgesics that produce their analgesic effect via action on the kappa-opioid receptor, produce a delayed-onset anti-analgesia in men but not women, an effect blocked by co-administration of a low dose of naloxone. We now report the same time-dependent anti-analgesia and its underlying mechanism in an animal model. Using the Randall-Selitto paw-withdrawal assay in male rats, we found that nalbuphine, pentazocine, and butorphanol each produced analgesia during the first hour followed by anti-analgesia starting at ∼90min after administration in males but not females, closely mimicking its clinical effects. As observed in humans, co-administration of nalbuphine with naloxone in a dose ratio of 12.5:1 blocked anti-analgesia but not analgesia. Administration of the highly selective kappa-opioid receptor agonist U69593 produced analgesia without subsequent anti-analgesia, and confirmed by the failure of the selective kappa antagonist nor-binaltorphimine to block nalbuphine-induced anti-analgesia, indicating that anti-analgesia is not mediated by kappa-opioid receptors. We therefore tested the role of other receptors in nalbuphine anti-analgesia. Nociceptin/orphanin FQ (NOP) and sigma-1 and sigma-2 receptors were chosen on the basis of their known anti-analgesic effects and receptor binding studies. The selective NOP receptor antagonists, JTC801, and J-113397, but not the sigma receptor antagonist, BD 1047, antagonized nalbuphine anti-analgesia. Furthermore, the NOP receptor agonist NNC 63-0532 produced anti-analgesia with the same delay in onset observed with the three agonist-antagonists, but without producing preceding analgesia and this anti-analgesia was also blocked by naloxone. These results strongly support the suggestion that clinically used agonist-antagonists act at the NOP receptor to produce anti-analgesia.

Neuropeptide orphanin FQ inhibits dendritic morphogenesis through activation of RhoA

Brain-derived neurotrophic factor (BDNF) plays a facilitatory role in neuronal development and promotion of differentiation. Mechanisms that oppose BDNF's stimulatory effects create balance and regulate dendritic growth. However, these mechanisms have not been studied. We have focused our studies on the BDNF-induced neuropeptide OrphaninFQ/ Nociceptin (OFQ); while BDNF is known to enhance synaptic activity, OFQ has opposite effects on activity, learning, and memory. We have now examined whether OFQ provides a balance to the stimulatory effects of BDNF on neuronal differentiation in the hippocampus. Golgi staining in OFQ knockout (KO) mice revealed an increase in primary dendrite length as well as spine density, suggesting that endogenous OFQ inhibits dendritic morphology. We have also used cultured hippocampal neurons to demonstrate that exogenous OFQ has an inhibitory effect on dendritic growth and that the neuropeptide alters the response to BDNF when pre-administered. To determine if BDNF and OFQ act in a feedback loop, we inhibited the actions of the BDNF and OFQ receptors, TrkB and NOP using ANA-12 and NOP KO mice respectively but our data suggest that the two factors do not act in a negative feedback loop. We found that the inhibition of dendritic morphology induced by OFQ is via enhanced RhoA activity. Finally, we have evidence that RhoA activation is required for the inhibitory effects of OFQ on dendritic morphology. Our results reveal basic mechanisms by which neurons not only regulate the formation of proper dendritic growth during development but also control plasticity in the mature nervous system.

Designing bifunctional NOP receptor-mu opioid receptor ligands from NOP receptor-selective scaffolds. Part I

The nociceptin receptor (NOP) and its endogenous agonist, nociceptin/orphanin FQ (N/OFQ), members of the opioid receptor and peptide families respectively, modulate the pharmacological effects of classical opioids, particularly opioid-induced reward and nociception. We hypothesized that compounds containing both NOP and opioid receptor activity in a single molecule may have useful pharmacological profiles as non-addicting analgesics or as drug abuse medications. We report here our forays into the structure-activity relationships for discovering 'bifunctional' NOP-mu opioid receptor (MOP) ligands, starting from our NOP-selective scaffolds. This initial SAR suggests pharmacophoric elements that may be modified to modulate/increase opioid affinity, while maintaining high affinity for the NOP receptor, to result in potent bifunctional small-molecule NOP/MOP ligands.

Effects of spinally administered bifunctional nociceptin/orphanin FQ peptide receptor/μ-opioid receptor ligands in mouse models of neuropathic and inflammatory pain

Nociceptin/orphanin FQ peptide receptor (NOP) agonists produce antinociceptive effects in animal models after spinal administration and potentiate μ-opioid receptor (MOP)-mediated antinociception. This study determined the antinociceptive effects of spinally administered bifunctional NOP/MOP ligands and the antinociceptive functions of spinal NOP and MOP receptors in mice. Antinociceptive effects of bifunctional NOP/MOP ligands BU08028 [(2S)-2-[(5R,6R,7R,14S)-N-cyclopropylmethyl-4,5-epoxy-6,14-ethano-3-hydroxy-6-methoxymorphinan-7-yl]-3,3-dimethylpentan-2-ol] and SR16435 [1-(1-(2,3,3α,4,5,6-hexahydro-1H-phenalen-1-yl)piperidin-4-yl)-indolin-2-one] were pharmacologically compared with the putative bifunctional ligand buprenorphine, selective NOP agonist SCH221510 [3-endo-8-[bis(2-methylphenyl)methyl]-3-phenyl-8-azabicyclo[3.2.1]octan-3-ol] and selective MOP agonist morphine in neuropathic and inflammatory pain models. Additionally, the degree of tolerance development to the antiallodynic effects of SR16435 and buprenorphine were determined after repeated intrathecal administration. Our data indicated that BU08028 and SR16435 were more potent than morphine and SCH221510 in attenuating nerve injury-induced tactile allodynia and inflammation-induced thermal hyperalgesia. Coadministration of receptor-selective antagonists further revealed that both NOP and MOP in the spinal cord mediated the antiallodynic effects of BU08028 and SR16435, but intrathecal buprenorphine-induced antiallodynic effects were primarily mediated by MOP. Repeated intrathecal administration of SR16435 resulted in reduced and slower development of tolerance to its antiallodynic effects compared with buprenorphine. In conclusion, both NOP and MOP receptors in the spinal cord independently drive antinociception in mice. Spinally administered bifunctional NOP/MOP ligands not only can effectively attenuate neuropathic and inflammatory pain, but also have higher antinociceptive potency with reduced tolerance development to analgesia. Such ligands therefore display a promising profile as spinal analgesics.

Effects of co-administration of intrathecal nociceptin/orphanin FQ and opioid antagonists on formalin-induced pain in rats

PURPOSE

Nociceptin/orphanin FQ (N/OFQ) as an endogeneous hexadecapeptide is known to exert antinociceptive effects spinally. The aims of this study were to demonstrate the antinociceptive effects of i.t. N/OFQ and to investigate the possible interaction between N/OFQ and endogenous opioid systems using selective opioid receptor antagonists in rat formalin tests.

MATERIALS AND METHODS

I.t. N/OFQ was injected in different doses (1-10 nmol) via a lumbar catheter prior to a 50 μL injection of 5% formalin into the right hindpaw of rats. Flinching responses were measured from 0-10 min (phase I, an initial acute state) and 11-60 min (phase II, a prolonged tonic state). To observe which opioid receptors are involved in the anti-nociceptive effect of i.t. N/OFQ in the rat-formalin tests, naltrindole (5-20 nmol), β-funaltrexamine (1-10 nmol), and norbinaltorphimine (10 nmol), selective δ-, μ- and κ-opioid receptor antagonists, respectively, were administered intrathecally 5 min after i.t. N/OFQ.

RESULTS

I.t. N/OFQ attenuated the formalin-induced flinching responses in a dose-dependent manner in both phases I and II. I.t. administration of naltrindole and β-funaltrexamine dose-dependently reversed the N/OFQ-induced attenuation of flinching responses in both phases; however, norbinaltorphimine did not.

CONCLUSION

I.t. N/OFQ exerted an antinociceptive effect in both phases of the rat-formalin test through the nociceptin opioid peptide receptor. In addition, the results suggested that δ- and μ-opioid receptors, but not κ-opioid receptors, are involved in the antinociceptive effects of N/OFQ in the spinal cord of rats.

Differential control of opioid antinociception to thermal stimuli in a knock-in mouse expressing regulator of G-protein signaling-insensitive Gαo protein

Regulator of G-protein signaling (RGS) proteins classically function as negative modulators of G-protein-coupled receptor signaling. In vitro, RGS proteins have been shown to inhibit signaling by agonists at the μ-opioid receptor, including morphine. The goal of the present study was to evaluate the contribution of endogenous RGS proteins to the antinociceptive effects of morphine and other opioid agonists. To do this, a knock-in mouse that expresses an RGS-insensitive (RGSi) mutant Gαo protein, Gαo(G184S) (Gαo RGSi), was evaluated for morphine or methadone antinociception in response to noxious thermal stimuli. Mice expressing Gαo RGSi subunits exhibited a naltrexone-sensitive enhancement of baseline latency in both the hot-plate and warm-water tail-withdrawal tests. In the hot-plate test, a measure of supraspinal nociception, morphine antinociception was increased, and this was associated with an increased ability of opioids to inhibit presynaptic GABA neurotransmission in the periaqueductal gray. In contrast, antinociception produced by either morphine or methadone was reduced in the tail-withdrawal test, a measure of spinal nociception. In whole-brain and spinal cord homogenates from mice expressing Gαo RGSi subunits, there was a small loss of Gαo expression and an accompanying decrease in basal G-protein activity. Our results strongly support a role for RGS proteins as negative regulators of opioid supraspinal antinociception and also reveal a potential novel function of RGS proteins as positive regulators of opioid spinal antinociceptive pathways.

Increased nociceptive sensitivity and nociceptin/orphanin FQ levels in a rat model of PTSD

BACKGROUND

Clinical studies indicate that post-traumatic stress disorder (PTSD) frequently shares co-morbidity with chronic pain. Although in animals acute stress-induced antinociception is well documented, the effect of PTSD-like stress on nociceptive sensitivity is unclear. Though a few studies measured nociceptive responses at a single time point, no studies have examined changes in nociceptive sensitivity over time following exposure to PTSD-like stress. Nociceptin/orphanin FQ (N/OFQ), an endogenous ligand for the N/OFQ peptide (NOP) receptor, modulates various biological functions in the central nervous system that are affected by PTSD, including nociceptive sensitivity, stress and anxiety, learning and memory.

RESULTS

The present study examined thermal and mechanical nociceptive sensitivity in male Sprague Dawley rats between 7 and 28 days after single-prolonged stress (SPS), an established animal model for PTSD. Rat paw withdrawal thresholds (PWT) to von Frey and paw withdrawal latencies (PWL) to radiant heat stimuli, respectively, dramatically decreased as early as 7 days after initiation of SPS and lasted the length of the study, 28 days. In addition, N/OFQ levels increased in cerebrospinal fluid (CSF; on days 9, 14 and 28) and serum (day 28), while levels of circulating corticosterone (CORT) decreased 28 days after initiation of SPS. SPS exposure induced anxiety-like behavior and enhanced inhibition of the hypothalamo-pituitary-adrenal (HPA) axis, as previously reported for this model.

CONCLUSIONS

Our results demonstrate that SPS induces the development of persistent mechanical allodynia and thermal hyperalgesia that is accompanied by increased N/OFQ content in the CSF, and eventually, in serum. These findings suggest a link between N/OFQ and the development of hyperalgesia and allodynia in a rat model of PTSD.

Sex differences in the Nociceptin/Orphanin FQ system in rat spinal cord following chronic morphine treatment

Nociceptin/Orphanin FQ (N/OFQ) appears to contribute to the development of morphine tolerance, as blockade of its actions will block or reverse the process. To better understand the contribution of N/OFQ to the development of morphine tolerance, this study examined the effect of chronic morphine treatment on levels of N/OFQ and levels and activity of the N/OFQ peptide (NOP) receptor in spinal cord (SC) from male and female rats. Both male and female Wistar rats showed less responsiveness to morphine after subcutaneous injection of escalating doses of morphine (10, 20, 40, 60 and 80 mg/kg, respectively) twice daily for five consecutive days. Male rats were more tolerant to the antinociceptive actions of morphine than females. The N/OFQ content of SC extracts was higher in females than in males, regardless of treatment; following chronic morphine treatment the difference in N/OFQ levels between males and females was more pronounced. N/OFQ content in cerebrospinal fluid (CSF) was reduced 40% in male and 16% in female rats with chronic morphine exposure, but increased in periaqueductal grey of both sexes. Chronic morphine treatment increased NOP receptor levels 173% in males and 137% in females, while decreasing affinity in both. Chronic morphine increased the efficacy of N/OFQ-stimulated [³⁵S]GTPγS binding to SC membranes from male rats, consistent with increased receptor levels. Taken together, these findings demonstrate sex differences in N/OFQ-NOP receptor expression and NOP receptor activity following chronic morphine treatment. They also suggest interplay between endogenous N/OFQ and chronic morphine treatment that results in nociceptive modulation.

Orphanin-FQ/nociceptin: Lack of antinociceptive, hyperalgesic or allodynic effects in acute thermal or mechanical tests following intracerebroventricular or intrathecal administration to mice or rats

A recent review calls attention to the discrepant results resulting from studies that have examined the nociceptive or antinociceptive properties of orphanin-FQ/nociceptin (Phe-Gly-Gly-Phe-Thr-Gly-Ala-Arg-Lys-Ser-Ala-ArgLys-Leu-Ala-Asn-Gln; OFQ/N), the heptadecapeptide isolated from rat (nociceptin) and pig (orphanin FQ) brain that binds with high affinity to the opioid 'orphan' receptor (a seven transmembrane protein with sequence homology to opioid receptors), but exhibits only low affinity binding with conventional opioid ligands. Some of the discrepancy might result from differences in species, test, route of administration or time-course. We undertook a comprehensive examination of the effects of spinal (i.t.) or supraspinal (i.c.v.) administration of OFQ/N in mice and rats. Mice treated with OFQ/N either i.t. or i.c.v. demonstrated no significant nociceptive effect in the hot plate, warm-water or radiant heat tail-flick tests (except for the highest and most sedative dose of 10 nmol i.c.v. in the mouse warm-water tail-flick test). Pretreatment with the opioid antagonist naloxone or with peptidase inhibitors did not enhance the nociceptive effects of OFQ/N peptide in the warm-water tail-flick test. The motor activity in mice administered OFQ/N i.c.v. decreased significantly compared to controls. Rats administered i.c.v. or i.t. OFQ/N displayed no significant difference from vehicle-treated animals in similar noxious stimulus tests and OFQ/N-treated rats did not exhibit allodynia in a paw-withdrawal test. Overall, OFQ/N was ineffective in significantly altering response to noxious stimuli, regardless of whether the peptide was given at supraspinal or spinal sites in mice or in rats. In addition, i.c.v. or i.t. application of antisense or mismatch ODN to the orphan receptor did not modify tail-flick latency in either mice or rats, arguing against a tonic nociceptive tone mediated via the OFQ/N receptor.

Differential effects of nociceptin/orphanin FQ (NOP) receptor agonists in acute versus chronic pain: studies with bifunctional NOP/μ receptor agonists in the sciatic nerve ligation chronic pain model in mice

1-(1-Cyclooctylpiperidin-4-yl)-indolin-2-one (SR14150) and 1-(1-(2,3,3a,4,5,6-hexahydro-1H-phenalen-1-yl)piperidinl-4-yl)-indolin-2-one (SR16835) are moderately selective nociceptin/orphanin FQ (NOP) receptor agonists. In the [(35)S]guanosine 5'-O-(3-thiotriphosphate) assay in vitro, SR14150 is a partial agonist at both the NOP and μ-opioid receptors, whereas SR16835 is a full agonist at the NOP receptor and has low efficacy at μ receptors. These compounds were tested for antinociceptive and antiallodynic activity, using mice in chronic pain, subsequent to spinal nerve ligation (SNL) surgery. When administered subcutaneously to mice after SNL surgery, SR14150 but not SR16835 increased tail-flick latency, which was blocked by the opioid antagonist naloxone, but not by the NOP receptor antagonist (-)-cis-1-methyl-7-[[4-(2,6-dichlorophenyl)piperidin-1-yl]methyl]-6,7,8,9-tetrahydro-5H-benzocyclohepten-5-ol (SB-612111). In contrast, both SR14150 and SR16835 had antiallodynic activity when mechanical allodynia was measured with von Frey monofilaments. This effect was completely blocked by SB-612111 but not by naloxone. On the other hand, morphine antinociception and antiallodynia were both blocked by naloxone and potentiated by SB-612111. These results indicate that, in mice, circuitry mediating antinociceptive activity in acute and chronic pain states is different. It is possible that during a chronic pain state, an up-regulated NOP system in the spinal cord leads to NOP receptor-mediated antiallodynia, which is blocked by NOP antagonists. However, supraspinal up-regulation could lead to an attenuation of morphine antinociception and antiallodynia, which can be alleviated by an NOP receptor antagonist. Thus, although neither NOP agonists nor antagonists are effective as analgesics in acute pain, they may have efficacy as analgesics, either alone or in combination with morphine, for treatment of chronic pain.