Beta-arrestin 2 rather than G protein efficacy determines the anxiolytic-versus antidepressant-like effects of nociceptin/orphanin FQ receptor ligands

Recommended Opioid Receptor Tool Compounds: Comparative In Vitro for Receptor Selectivity Profiles and In Vivo for Pharmacological Antinociceptive Profiles

Opioid agonist ligands bind opioid receptors and stimulate downstream signaling cascades for various biological processes including pain and reward. Historically, before cloning the receptors, muscle contraction assays using isolated organ tissues were used followed by radiolabel ligand binding assays on native tissues. Upon cloning of the opioid G protein-coupled receptors (GPCRs), cell assays using transfected opioid receptor DNA plasmids became the standard practice including 35S-GTPγS functional and cAMP based assays. A number of research laboratories have studied key "tool" reference opioid receptor ligands for decades and used them as control reference compounds. Some, but not all, of these commonly used tool compounds have been characterized and compared side by side in parallel assays for selectivity profiles at the different human opioid receptors isoforms. Herein, we performed the standard FLIPR calcium mobilization assay using HEK293 cells engineered to stably express the GαΔ6qi4myr in parallel, at human MOR, KOR, DOR, and NOP opioid receptors. The following tool compounds: morphine, fentanyl, oxycodone, DAMGO, DPDPE, U69593, deltorphin II, and nociceptin, were examined herein. These included the substance use disorder (SUD) compounds morphine, fentanyl, and oxycodone. Additionally, the antagonist tool compounds naloxone, NTI, norBNI, and β-FNA were assayed in parallel at the human MOR, KOR, DOR, and NOP opioid receptors. Furthermore, the agonist tool compounds were tested in the same in vivo tail-flick antinociception assays via intrathecal injection for ED50 potencies. These data provide both in vitro comparative pharmacology as a reference for cellular activities and in vivo antinociception profiles for these tool compounds.

Induction of orofacial pain potentiates fibromyalgia symptoms in mice: Relevance of nociceptin system

AIMS

Fibromyalgia patients might experience temporomandibular disorder (TMD) as a comorbidity. However, the connection between these two syndromes is not fully understood. Nociceptin (N/OFQ) and NOP receptors are implicated in both conditions, but their relevance in the comorbidity needs investigation. This study featured a comorbidity model of fibromyalgia plus TMD in mice, attempting to evaluate the significance of the N/OFQ-NOP receptor in this paradigm.

MATERIALS AND METHODS

Female CF-1 mice were submitted to the fibromyalgia model induced by three daily consecutive injections of reserpine (0.25 mg/kg) and received an intra-masseter injection of complete Freund's adjuvant (CFA; 10 μl; diluted 1:1) on day four.

KEY FINDINGS

There was a rise in nocifensive and depression-like behaviors in the comorbidity group, as evaluated by the Grimace scores and the tail suspension test (TST). This group displayed anxiogenic-like effects in the hole board and the elevated plus maze tests. The comorbidity group showed an increment of c-Fos immunopositivity in the ipsilateral side of CFA injection, in the trigeminal ganglion (TG) and thalamus. The administration of N/OFQ (1 nmol/kg, i.p.) boosted the Grimace scores in the comorbidity group, with no effect for the NOP receptor antagonist UFP-101 (1 nmol/kg, i.p.). Either NOP ligand failed to alter depression or anxiety behavioral changes. Alternatively, pregabalin (30 mg/kg; i.p.) reduced the nociceptive responses and the number of head dips in the hole board.

SIGNIFICANCE

Data reveal new evidence suggesting that inducing TMD with CFA may worsen fibromyalgia symptoms in reserpine-treated mice, an effect partially regulated by systemic N/OFQ.

Effects of Stress Exposure to Pain Perception in Pre-Clinical Studies: Focus on the Nociceptin/Orphanin FQ-NOP Receptor System

Exposure to physical and psychological stress modulates pain transmission in a dual manner. Stress-induced analgesia (SIA) refers to the reduction in pain sensitivity that can occur in response to acute stress. On the contrary, chronic stress exposure may lead to a phenomenon named stress-induced hyperalgesia (SIH). SIH is a clinically relevant phenomenon since it has been well documented that physical and psychological stress exacerbates pain in patients with several chronic pain syndromes, including migraine. The availability of animal models of SIA and SIH is of high importance for understanding the biological mechanisms leading to these phenomena and for the identification of pharmacological targets useful to alleviate the burden of stress-exacerbated chronic pain. Among these targets, the nociceptin/orphanin FQ (N/OFQ)-N/OFQ peptide (NOP) receptor system has been identified as a key modulator of both pain transmission and stress susceptibility. This review describes first the experimental approaches to induce SIA and SIH in rodents. The second part of the manuscript summarizes the scientific evidence that suggests the N/OFQ-NOP receptor system as a player in the stress-pain interaction and candidates NOP antagonists as useful drugs to mitigate the detrimental effects of stress exposure on pain perception.

Activation of NOP receptor increases vulnerability to stress: role of glucocorticoids and CRF signaling

RATIONALE

Recently, we demonstrated that the activation of the nociceptin/orphanin FQ (N/OFQ) receptor (NOP) signaling facilitates depressive-like behaviors. Additionally, literature findings support the ability of the N/OFQ-NOP system to modulate the hypothalamic-pituitary-adrenal (HPA) axis.

OBJECTIVES

Considering that dysfunctional HPA axis is strictly related to stress-induced psychopathologies, we aimed to study the role of the HPA axis in the pro-depressant effects of NOP agonists.

METHODS

Mice were treated prior to stress with the NOP agonist Ro 65-6570, and immobility time in the forced swimming task and corticosterone levels were measured. Additionally, the role of endogenous glucocorticoids and CRF was investigated using the glucocorticoid receptor antagonist mifepristone and the CRF1 antagonist antalarmin in the mediation of the effects of Ro 65-6570.

RESULTS

The NOP agonist in a dose-dependent manner further increased the immobility of mice in the second swimming session compared to vehicle. By contrast, under the same conditions, the administration of the NOP antagonist SB-612111 before stress reduced immobility, while the antidepressant nortriptyline was inactive. Concerning in-serum corticosterone in mice treated with vehicle, nortriptyline, or SB-612111, a significant decrease was observed after re-exposition to stress, but no differences were detected in Ro 65-6570-treated mice. Administration of mifepristone or antalarmin blocked the Ro 65-6570-induced increase in the immobility time in the second swimming session.

CONCLUSIONS

Present findings suggest that NOP agonists increase vulnerability to depression by hyperactivating the HPA axis and then increasing stress circulating hormones and CRF1 receptor signaling.

The Nociceptin/Orphanin FQ peptide receptor antagonist, SB-612111, improves cerebral blood flow in a rat model of traumatic brain injury

Traumatic brain injury (TBI) affects more than 2.5 million people in the U.S. each year and is the leading cause of death and disability in children and adults ages 1 to 44. Approximately 90% of TBI cases are classified as mild but may still lead to acute detrimental effects such as impaired cerebral blood flow (CBF) that result in prolonged impacts on brain function and quality of life in up to 15% of patients. We previously reported that nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor antagonism reversed mild blast TBI-induced vestibulomotor deficits and prevented hypoxia. To explore mechanisms by which the NOP receptor-N/OFQ pathway modulates hypoxia and other TBI sequelae, the ability of the NOP antagonist, SB-612111 (SB), to reverse TBI-induced CBF and associated injury marker changes were tested in this study. Male Wistar rats randomly received sham craniotomy or craniotomy + TBI via controlled cortical impact. Injury severity was assessed after 1 h (modified neurological severity score (mNSS). Changes in CBF were assessed 2 h post-injury above the exposed cortex using laser speckle contrast imaging in response to the direct application of increasing concentrations of vehicle or SB (1, 10, and 100 µM) to the brain surface. TBI increased mNSS scores compared to baseline and confirmed mild TBI (mTBI) severity. CBF was significantly impaired on the ipsilateral side of the brain following mTBI, compared to contralateral side and to sham rats. SB dose-dependently improved CBF on the ipsilateral side after mTBI compared to SB effects on the respective ipsilateral side of sham rats but had no effect on contralateral CBF or in uninjured rats. N/OFQ levels increased in the cerebral spinal fluid (CSF) following mTBI, which correlated with the percent decrease in ipsilateral CBF. TBI also activated ERK and cofilin within 3 h post-TBI; ERK activation correlated with increased CSF N/OFQ. In conclusion, this study reveals a significant contribution of the N/OFQ-NOP receptor system to TBI-induced dysregulation of cerebral vasculature and suggests that the NOP receptor should be considered as a potential therapeutic target for TBI.

A complex relation between levels of adult hippocampal neurogenesis and expression of the immature neuron marker doublecortin

We investigated the mechanisms underlying the effects of the antidepressant fluoxetine on behavior and adult hippocampal neurogenesis (AHN). After confirming our earlier report that the signaling molecule β-arrestin-2 (β-Arr2) is required for the antidepressant-like effects of fluoxetine, we found that the effects of fluoxetine on proliferation of neural progenitors and survival of adult-born granule cells are absent in the β-Arr2 knockout (KO) mice. To our surprise, fluoxetine induced a dramatic upregulation of the number of doublecortin (DCX)-expressing cells in the β-Arr2 KO mice, indicating that this marker can be increased even though AHN is not. We discovered two other conditions where a complex relationship occurs between the number of DCX-expressing cells compared to levels of AHN: a chronic antidepressant model where DCX is upregulated and an inflammation model where DCX is downregulated. We concluded that assessing the number of DCX-expressing cells alone to quantify levels of AHN can be complex and that caution should be applied when label retention techniques are unavailable.

Biased Agonism: Lessons from Studies of Opioid Receptor Agonists

In ligand bias different agonist drugs are thought to produce distinct signaling outputs when activating the same receptor. If these signaling outputs mediate therapeutic versus adverse drug effects, then agonists that selectively activate the therapeutic signaling pathway would be extremely beneficial. It has long been thought that μ-opioid receptor agonists that selectively activate G protein- over β-arrestin-dependent signaling pathways would produce effective analgesia without the adverse effects such as respiratory depression. However, more recent data indicate that most of the therapeutic and adverse effects of agonist-induced activation of the μ-opioid receptor are actually mediated by the G protein-dependent signaling pathway, and that a number of drugs described as G protein biased in fact may not be biased, but instead may be low-intrinsic-efficacy agonists. In this review we discuss the current state of the field of bias at the μ-opioid receptor and other opioid receptor subtypes.

Anxiety and Depression: What Do We Know of Neuropeptides?

In modern society, there has been a rising trend of depression and anxiety. This trend heavily impacts the population's mental health and thus contributes significantly to morbidity and, in the worst case, to suicides. Modern medicine, with many antidepressants and anxiolytics at hand, is still unable to achieve remission in many patients. The pathophysiology of depression and anxiety is still only marginally understood, which encouraged researchers to focus on neuropeptides, as they are a vast group of signaling molecules in the nervous system. Neuropeptides are involved in the regulation of many physiological functions. Some act as neuromodulators and are often co-released with neurotransmitters that allow for reciprocal communication between the brain and the body. Most studied in the past were the antidepressant and anxiolytic effects of oxytocin, vasopressin or neuropeptide Y and S, or Substance P. However, in recent years, more and more novel neuropeptides have been added to the list, with implications for the research and development of new targets, diagnostic elements, and even therapies to treat anxiety and depressive disorders. In this review, we take a close look at all currently studied neuropeptides, their related pathways, their roles in stress adaptation, and the etiology of anxiety and depression in humans and animal models. We will focus on the latest research and information regarding these associated neuropeptides and thus picture their potential uses in the future.

Cardiovascular and renal effects of novel nonpeptide nociceptin opioid peptide receptor agonists

BACKGROUND AND PURPOSE

Partial agonists of the nociceptin opioid peptide (NOP) receptor have potential therapeutic use as antihypertensive and water diuretics (aquaretics). To date, peptide NOP receptor ligands have failed to progress in clinical trials due to poor pharmacokinetics and adverse effects. Nonpeptide, small-molecule NOP receptor ligands may be more suitable as therapeutic agents. This study investigated the cardiovascular and renal responses produced by the novel nonpeptide NOP agonists AT-403, AT-090, AT-127, and AT-039.

EXPERIMENTAL APPROACH

Changes in mean arterial pressure (MAP), heart rate (HR), renal excretory function and occurrence of sedation and hyperphagia were determined before and after i.v. bolus injection or infusion of the NOP agonists in conscious Sprague-Dawley rats. Additional studies involving (i) measurement of renal sympathetic nerve activity (RSNA) and (ii) renal denervation were conducted to investigate the role of the renal nerves in the cardiorenal responses to AT-039.

KEY RESULTS

Bolus i.v. injection of AT-403, AT-090, AT-127 and AT-039 produced significant decreases in MAP and HR and a sodium-sparing diuresis. AT-403, AT-090, and AT-127, but not AT-039, induced sedation and hyperphagia at all doses tested. Infusion i.v. of AT-039 produced hypotension and aquaresis without adverse central nervous system effects or change in HR, responses that were also observed in renal denervated rats.

CONCLUSIONS AND IMPLICATIONS

Nonpeptide NOP agonists decrease blood pressure and produce aquaresis in conscious rodents. Due to lack of sedation and hyperphagia, AT-039 represents a novel NOP agonist that may be useful for treatment of hypertension and/or volume overload/hyponatraemic states.

In silico analysis of energy interactions between nociceptin/orfanin FQ receptor and two antagonists with potential antidepressive action

This study addresses the binding energies of NOPR-ligand complexes and presents the main amino acid residues involved in the interaction between these complexes.

Role of Nociceptin/Orphanin FQ-NOP Receptor System in the Regulation of Stress-Related Disorders

Nociceptin/orphanin FQ (N/OFQ) is a 17-residue neuropeptide that binds the nociceptin opioid-like receptor (NOP). N/OFQ exhibits nucleotidic and aminoacidics sequence homology with the precursors of other opioid neuropeptides but it does not activate either MOP, KOP or DOP receptors. Furthermore, opioid neuropeptides do not activate the NOP receptor. Generally, activation of N/OFQ system exerts anti-opioids effects, for instance toward opioid-induced reward and analgesia. The NOP receptor is widely expressed throughout the brain, whereas N/OFQ localization is confined to brain nuclei that are involved in stress response such as amygdala, BNST and hypothalamus. Decades of studies have delineated the biological role of this system demonstrating its involvement in significant physiological processes such as pain, learning and memory, anxiety, depression, feeding, drug and alcohol dependence. This review discusses the role of this peptidergic system in the modulation of stress and stress-associated psychiatric disorders in particular drug addiction, mood, anxiety and food-related associated-disorders. Emerging preclinical evidence suggests that both NOP agonists and antagonists may represent a effective therapeutic approaches for substances use disorder. Moreover, the current literature suggests that NOP antagonists can be useful to treat depression and feeding-related diseases, such as obesity and binge eating behavior, whereas the activation of NOP receptor by agonists could be a promising tool for anxiety.

Antipanic-like effect of esketamine and buprenorphine in rats exposed to acute hypoxia

The antidepressant effect of ketamine has been widely acknowledged and the use of one of its enantiomers, S-ketamine (esketamine), has recently been approved for the clinical management of treatment-resistant depression. As with ketamine, the non-selective opioid receptor-interacting drug buprenorphine is reported to have antidepressant and anxiolytic properties in humans and rodents. Given the fact that antidepressant drugs are also first line treatment for panic disorder, it is surprising that the potential panicolytic effect of these compounds has been scarcely (ketamine), or not yet (buprenorphine) investigated. We here evaluated the effects of ketamine (the racemic mixture), esketamine, and buprenorphine in male Wistar rats submitted to a panicogenic challenge: acute exposure to hypoxia (7% O₂). We observed that esketamine (20 mg/kg), but not ketamine, decreased the number of escape attempts made during hypoxia, and this effect could be observed even 7 days after the drug administration. A panicolytic-like effect was also observed with MK801, which like esketamine, antagonizes NMDA glutamate receptors. Buprenorphine (0.3 mg/kg) also impaired hypoxia-induced escape, an effect blocked by the non-selective opioid receptor antagonist naloxone, indicating an interaction with classical ligand sites, such as µ and kappa receptors, but not with nociception/orphanin FQ receptors. Altogether, the results suggest that esketamine and buprenorphine cause rapid-onset panicolytic-like effects, and may be alternatives for treating panic disorder, particularly in patients who are refractory to standard pharmacological treatment.

Differential In Vitro Pharmacological Profiles of Structurally Diverse Nociceptin Receptor Agonists in Activating G Protein and Beta-Arrestin Signaling at the Human Nociceptin Opioid Receptor

Agonists at the nociceptin opioid peptide receptor (NOP) are under investigation as therapeutics for nonaddicting analgesia, opioid use disorder, Parkinson's disease, and other indications. NOP full and partial agonists have both been of interest, particularly since NOP partial agonists show a reduced propensity for behavioral disruption than NOP full agonists. Here, we investigated the in vitro pharmacological properties of chemically diverse NOP receptor agonists in assays measuring functional activation of the NOP receptor such as guanosine 5'-O-[gamma-thio]triphosphate (GTPγS) binding, cAMP inhibition, G protein-coupled inwardly rectifying potassium (GIRK) channel activation, phosphorylation, β-arrestin recruitment and receptor internalization. When normalized to the efficacy of the natural agonist nociceptin/orphanin FQ (N/OFQ), we found that different functional assays that measure intrinsic activity produce inconsistent levels of agonist efficacy, particularly for ligands that were partial agonists. Agonist efficacy obtained in the GTPγS assay tended to be lower than that in the cAMP and GIRK assays. These structurally diverse NOP agonists also showed differential receptor phosphorylation profiles at the phosphosites we examined and induced varying levels of receptor internalization. Interestingly, although the rank order for β-arrestin recruitment by these NOP agonists was consistent with their ability to induce receptor internalization, their phosphorylation signatures at the time point we investigated were not indicative of the levels of β-arrestin recruitment or internalization induced by these agonists. It is possible that other phosphorylation sites, yet to be identified, drive the recruitment of NOP receptor ensembles and subsequent receptor trafficking by some nonpeptide NOP agonists. These findings potentially help understand NOP agonist pharmacology in the context of ligand-activated receptor trafficking. SIGNIFICANCE STATEMENT: Chemically diverse agonist ligands at the nociceptin opioid receptor G protein-coupled receptor showed differential efficacy for activating downstream events after receptor binding, in a suite of functional assays measuring guanosine 5'-O-[gamma-thio]triphosphate binding, cAMP inhibition, G protein-coupled inwardly rectifying protein channel activation, β-arrestin recruitment, receptor internalization and receptor phosphorylation. These analyses provide a context for understanding nociceptin opioid peptide receptor (NOP) agonist pharmacology driven by ligand-induced differential NOP receptor signaling.

Functional Selectivity Does Not Predict Antinociceptive/Locomotor Impairing Potencies of NOP Receptor Agonists

Nociceptin/orphanin FQ controls several functions, including pain transmission, via stimulation of the N/OFQ peptide (NOP) receptor. Here we tested the hypothesis that NOP biased agonism may be instrumental for identifying innovative analgesics. In vitro experiments were performed with the dynamic mass redistribution label free assay and the NOP non-peptide agonists Ro 65-6570, AT-403 and MCOPPB. In vivo studies were performed in wild type and β-arrestin 2 knockout mice using the formalin, rotarod and locomotor activity tests. In vitro all compounds mimicked the effects of N/OFQ behaving as potent NOP full agonists. In vivo Ro 65-6570 demonstrated a slightly higher therapeutic index (antinociceptive vs. motor impairment effects) in knockout mice. However, all NOP agonists displayed very similar therapeutic index in normal mice despite significant differences in G protein biased agonism. In conclusion the different ability of inducing G protein vs. β-arrestin 2 recruitment of a NOP agonist cannot be applied to predict its antinociceptive vs. motor impairment properties.

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.

Negative relationship between brain α1A-AR neurotransmission and βArr2 levels in anxious adolescent rats subjected to early life stress

Early-life stress is correlated with the development of anxiety-related behavior in adolescence, but underlying mechanisms remain poorly known. The α_1A-adrenergic receptor (AR) is linked to mood regulation and its function is assumed to be regulated by β-arrestins (βArrs) via desensitization and downregulation. Here, we investigated correlation between changes in α_1A-AR and βArr2 levels in the prefrontal cortex (PFC) and hippocampus of adolescent and adult male rats subjected to maternal separation (MS) and their relationship with anxiety-like behavior in adolescence. MS was performed 3 h per day from postnatal days 2-11 and anxiety-like behavior was evaluated in the elevated plus-maze and open field tests. The protein levels were examined using western blot assay. MS decreased α_1A-AR expression and increased βArr2 expression in both brain regions of adolescent rats, while induced reverse changes in adulthood. MS adolescent rats demonstrated higher anxiety-type behavior and lower activity in behavioral tests than controls. Decreased α_1A-AR levels in MS adolescence strongly correlated with reduced time spent in the open field central area, consistent with increased anxiety-like behavior. An anxiety-like phenotype was mimicked by acute and chronic treatment of developing rats with prazosin, an α_1A-AR antagonist, suggesting α_1A-AR downregulation may facilitate anxiety behavior in MS adolescent rats. Together, our results indicate a negative correlation between α_1A-AR neurotransmission and βArr2 levels in both adults and anxious-adolescent rats and suggest that increased βArr2 levels may contribute to posttranslational regulation of α_1A-AR and modulation of anxiety-like behavior in adolescent rats. This may provide a path to develop more effective anxiolytic treatments.

Biased Agonism at Nociceptin/Orphanin FQ Receptors: A Structure Activity Study on N/OFQ(1-13)-NH2

Nociceptin/orphanin FQ (N/OFQ) controls different biological functions via selective stimulation of the N/OFQ peptide (NOP) receptor. The pleiotropic actions of N/OFQ may limit the development of NOP ligands as innovative drugs in different therapeutic areas. The pharmacological concept of functional selectivity (aka biased agonism) might be useful for amplifying beneficial actions and/or counteracting side effects. Thus, molecules with large bias factors toward G protein or β arrestin are required for investigating the translational value of NOP biased modulation. Herein, the biased behavior of a heterogeneous library of NOP-targeting peptide derivatives was evaluated in vitro with the aim to provide possible insights into the structural determinants that govern the selective activation of G protein versus β-arrestin. Our results demonstrate that lipidation of N/OFQ(1-13)-NH2 is a useful strategy for obtaining G protein biased agonists for the NOP receptor.

Blockade of nociceptin/orphanin FQ signaling facilitates an active copying strategy due to acute and repeated stressful stimuli in mice

The role of stress in the etiology of depression has been largely reported. In this line, exogenous glucocorticoids are employed to mimic the influence of stress on the development of depression. The N/OFQ-NOP receptor system has been implicated in the modulation of stress and emotional behaviors. In fact, the blockade of NOP receptors induces antidepressant effects and increases resilience to acute stress. This study investigated the effects of the NOP receptor blockade on dexamethasone-treated mice exposed to acute and prolonged swimming stress. Swiss and NOP(+/+) and NOP(−/−) mice were treated with dexamethasone, and the protective effects of the NOP antagonist SB-612111 (10 mg/kg, ip) or imipramine (20 mg/kg, ip) were investigated in three swimming sessions. The re-exposure to swim stress increased immobility time in Swiss and NOP(+/+), but not in NOP(−/−) mice. Acute and repeated dexamethasone administration induced a further increase in the immobility time, and facilitated body weight loss in Swiss mice. Single administration of SB-612111, but not imipramine, prevented swimming stress- and dexamethasone-induced increase in the immobility time. Repeated administrations of SB-612111 prevented the deleterious effects of 5 days of dexamethasone treatment. Imipramine also partially prevented the effects of repeated glucocorticoid administration on the immobility time, but did not affect the body weight loss. NOP(−/−) mice were more resistant than NOP(+/+) mice to inescapable swimming stress, but not dexamethasone-induced increase in the immobility time and body weight loss. In conclusion, the blockade of the NOP receptor facilitates an active stress copying response and attenuates body weight loss due to repeated stress.

Biased Opioid Ligands

Achieving effective pain management is one of the major challenges associated with modern day medicine. Opioids, such as morphine, have been the reference treatment for moderate to severe acute pain not excluding chronic pain modalities. Opioids act through the opioid receptors, the family of G-protein coupled receptors (GPCRs) that mediate pain relief through both the central and peripheral nervous systems. Four types of opioid receptors have been described, including the μ-opioid receptor (MOR), κ-opioid receptor (KOR), δ-opioid receptor (DOR), and the nociceptin opioid peptide receptor (NOP receptor). Despite the proven success of opioids in treating pain, there are still some inherent limitations. All clinically approved MOR analgesics are associated with adverse effects, which include tolerance, dependence, addiction, constipation, and respiratory depression. On the other hand, KOR selective analgesics have found limited clinical utility because they cause sedation, anxiety, dysphoria, and hallucinations. DOR agonists have also been investigated but they have a tendency to cause convulsions. Ligands targeting NOP receptor have been reported in the preclinical literature to be useful as spinal analgesics and as entities against substance abuse disorders while mixed MOR/NOP receptor agonists are useful as analgesics. Ultimately, the goal of opioid-related drug development has always been to design and synthesize derivatives that are equally or more potent than morphine but most importantly are devoid of the dangerous residual side effects and abuse potential. One proposed strategy is to take advantage of biased agonism, in which distinct downstream pathways can be activated by different molecules working through the exact same receptor. It has been proposed that ligands not recruiting β-arrestin 2 or showing a preference for activating a specific G-protein mediated signal transduction pathway will function as safer analgesic across all opioid subtypes. This review will focus on the design and the pharmacological outcomes of biased ligands at the opioid receptors, aiming at achieving functional selectivity.

Effects of non-peptide nociceptin/orphanin FQ receptor ligands on methylphenidate-induced hyperactivity in mice: Implications for bipolar disorders

Bipolar disorder is a psychiatric pathology characterized by biphasic mood episodes of mania or hypomania and depression. The pharmacotherapy of bipolar disorder has significant adverse effects impairing treatment adherence and patient quality of life. The N/OFQ-NOP receptor system has been widely implicated with mood disorders. Clinical and preclinical findings suggest antidepressants actions for NOP antagonists. More recently, the administration of NOP agonists has shown to promote depressant states. The present study aimed to investigate the effects of non-peptide NOP ligands in methylphenidate-induced manic-like behavior in mice. The NOP agonist Ro 65-6570 (0.01-1 mg/kg, ip), at the higher dose, did not affect spontaneous locomotion per se, but prevented the methylphenidate (10 mg/kg, sc)-induced hyperlocomotion. The NOP partial agonist AT-090 (0.001-0.03 mg/kg, ip) and the NOP antagonist SB-612111 (1-10 mg/kg, ip) did not significantly affect the psychostimulant-induced hyperactivity. Experiments performed with mice lacking the NOP receptor (NOP(-/-)) demonstrated that the treatment with methylphenidate induced similar hyperlocomotion in NOP(-/-) and NOP(+/+) mice. In conclusion, these findings suggest a potential role for NOP agonists in the prevention of manic states, especially by counteracting the hyperactivity symptom of bipolar patients. However, more studies are necessary in order to evaluate these compounds in other features of bipolar disorder.

Blockade of NOP receptor modulates anxiety-related behaviors in mice exposed to inescapable stress

RATIONALE

Depression and anxiety frequently co-occur, and this has important clinical implications. Previous studies showed that activation of the nociceptin/orphanin FQ receptor (NOP) elicits anxiolytic effects, while its blockade promotes consistent antidepressant actions. NOP antagonists are effective in reversing footshock-induced depressive-like behaviors, but their effects on stress-induced anxiety are still unclear.

OBJECTIVE

This study aimed to investigate the effects of the NOP antagonist SB-612111 on footshock stress-induced anxiety behaviors.

METHODS

Male Swiss mice were exposed to inescapable electric footshock stress, and behavioral phenotype was screened based on the ability to escape from footshock (i.e., helpless or non-helpless). Animals were then treated with diazepam (1 mg/kg) and SB-612111 (0.1-10 mg/kg), and their behavior was assessed in the elevated plus-maze (EPM) and open field test.

RESULTS

When compared with non-stressed mice, helpless, but not non-helpless, animals displayed significant reductions in the time spent in and entries into open arms in the EPM. Diazepam significantly increased open arms exploration in helpless, non-helpless, and non-stressed mice. However, treatment with the NOP antagonist SB-612111 was inactive in naive mice, while it reversed anxiogenic-related behaviors in helpless mice and increased anxiety states in non-helpless mice. No effects on locomotion were observed.

CONCLUSION

Helpless mice displayed increased anxiety compared to non-stressed and non-helpless animals, thus supporting use of this approach as an animal model to investigate anxiety/depression comorbidity. Additionally, SB-612111 modulated anxiety-like behaviors in male mice depending on individual stress susceptibility. Ultimately, NOP antagonists could be useful for treating anxiety in depressed patients.

Managing Parkinson's disease: moving ON with NOP

The opioid-like neuropeptide nociceptin/orphanin FQ (N/OFQ) and its receptor (NOP receptor) contribute to Parkinson's disease (PD) and motor complications associated with levodopa therapy. The N/OFQ-NOP receptor system is expressed in cortical and subcortical motor areas and, notably, in dopaminergic neurons of the substantia nigra compacta. Dopamine depletion, as in rodent models of PD results in up-regulation of N/OFQ transmission in the substantia nigra and down-regulation of N/OFQ transmission in the striatum. Consistent with this, NOP receptor antagonists relieve motor deficits in PD models by reinstating the physiological balance between excitatory and inhibitory inputs impinging on nigro-thalamic GABAergic neurons. NOP receptor antagonists also counteract the degeneration of nigrostriatal dopaminergic neurons, possibly by attenuating the excitotoxicity or modulating the immune response. Conversely, NOP receptor agonists attenuate levodopa-induced dyskinesia by attenuating the hyperactivation of striatal D1 receptor signalling in neurons of the direct striatonigral pathway. The N/OFQ-NOP receptor system might represent a novel target in the therapy of PD.

Limbic circuit connectivity and the stress response: New insights into the mammalian nociceptin peptide system

Considerable progress has been made in understanding the neurobehavioral effects of nociceptin peptide (N/OFQ) and its NOP receptor. Recent interest has focused on its role in stress and cognition, with consideration of therapeutic potential in regard to anxiety and mood disorders. Research has interrogated the mechanisms of action of N/OFQ peptide in the context of stress-related behavior. We are interested in the endogenous role of N/OFQ and NOP receptor in terms of adaptation to chronic stress. Our research has highlighted the importance of associated limbic regions including the bed nucleus, extended amygdala, in addition to thalamic reticular nucleus as important sites for long-term adaptations in endogenous N/OFQ function in chronic stress. Such research raises interest in appreciation of extended limbic connections and novel pathways which allow us to reevaluate current understanding of stress neurocircuitry. Examination of endogenous N/OFQ-NOP receptor modulation of monoaminergic and amino acid transmitter systems in this extended limbic architecture will facilitate deeper understanding of the tonic control of behavior. Application of in vivo experimental approaches to models of abnormal neurodevelopment and heightened stress vulnerability in adulthood will enable the role of N/OFQ in complex neuropsychiatric disorders including schizophrenia and post-traumatic stress disorder to be defined.

Targeting opioid dysregulation in depression for the development of novel therapeutics

Since the serendipitous discovery of the first class of modern antidepressants in the 1950's, all pharmacotherapies approved by the Food and Drug Administration for major depressive disorder (MDD) have shared a common mechanism of action, increased monoaminergic neurotransmission. Despite the widespread availability of antidepressants, as many as 50% of depressed patients are resistant to these conventional therapies. The significant length of time required to produce meaningful symptom relief with these medications, 4-6 weeks, indicates that other mechanisms are likely involved in the pathophysiology of depression which may yield more viable targets for drug development. For decades, no viable candidate target with a different mechanism of action to that of conventional therapies proved successful in clinical studies. Now several exciting avenues for drug development are under intense investigation. One of these emerging targets is modulation of endogenous opioid tone. This review will evaluate preclinical and clinical evidence pertaining to opioid dysregulation in depression, focusing on the role of the endogenous ligands endorphin, enkephalin, dynorphin, and nociceptin/orphanin FQ (N/OFQ) and their respective receptors, mu (MOR), delta (DOR), kappa (KOR), and the N/OFQ receptor (NOP) in mediating behaviors relevant to depression and anxiety. Finally, putative opioid based antidepressants that are under investigation in clinical trials, ALKS5461, JNJ-67953964 (formerly LY2456302 and CERC-501) and BTRX-246040 (formerly LY-2940094) will be discussed. This review will illustrate the potential therapeutic value of targeting opioid dysregulation in developing novel therapies for MDD.

NOP-Targeted Nonpeptide Ligands

The development of nonpeptide systemically active small-molecule NOP-targeted ligands has contributed tremendously to validating the NOP receptor as a promising target for therapeutics. Although a NOP-targeted compound is not yet approved for clinical use, a few NOP ligands are in clinical trials for various indications. Both successful and failed human clinical trials with NOP ligands provide opportunities for rational development of new and improved NOP-targeted compounds. A few years after the discovery of the NOP receptor in 1994, and its de-orphanization upon discovery of the endogenous peptide nociceptin/orphanin FQ (N/OFQ) in 1995, there was a significant effort in the pharmaceutical industry to discover nonpeptide NOP ligands from hits obtained from high-throughput screening campaigns of compound libraries. Depending on the therapeutic indication to be pursued, NOP agonists and antagonists were discovered, and some were optimized as clinical candidates. Advances such as G protein-coupled receptor (GPCR) structure elucidation, functional selectivity in ligand-driven GPCR activation, and multi-targeted ligands provide new scope for the rational design of novel NOP ligands fine-tuned for successful clinical translation. This article reviews the field of nonpeptide NOP ligand drug design in the context of these exciting developments and highlights new optimized nonpeptide NOP ligands possessing interesting functional profiles, which are particularly attractive for several unmet clinical applications involving NOP receptor pharmacomodulation.

Nociceptin/orphanin FQ receptor agonists increase aggressiveness in the mouse resident-intruder test

Aggressive behaviors can be considered symptoms of bipolar disorder, schizophrenia, post-traumatic stress, intermittent explosive, and personality disorders. Nociceptin/orphanin FQ (N/OFQ) is a peptide acting as endogenous ligand of the NOP receptor. Preclinical and clinical findings suggest the NOP receptor as an innovative target for the treatment of psychopathologies, such as anxiety, depression, and drug abuse. This study investigated the effects of NOP ligands and the behavioral phenotype of mice lacking the NOP receptor in an animal model of aggressiveness, the resident-intruder test. Mood stabilizers, such as valproate, lithium, and carbamazepine reduced aggressive behaviors of resident mice, while diazepam was inactive. In contrast, para-chlorophenylalanine (PCPA), an inhibitor of 5-HT synthesis, increased aggressiveness in mice. Similar to PCPA, the treatment with the NOP agonists Ro 65-6570 and AT-090 also increased aggressive behaviors. The systemic administration of the NOP antagonist SB-612111 did not modify the behavior of resident mice, but it prevented the aggressive behavior of Ro 65-6570. NOP receptor knockout mice did not display any behavioral difference compared to wild-type animals in the resident-intruder test. None of the treatments affected non-agonistic behaviors and spontaneous locomotion. In conclusion, NOP receptor agonists increased aggressiveness, while the pharmacological and genetic blockade of NOP receptor signaling did not modify agonistic behaviors. Ultimately, the aggressive profile of action of NOP agonists should be taken into account in the development of innovative psychiatric drugs targeting the NOP receptor.

NOP Receptor Signaling Cascades

The nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor is a G protein-coupled receptor with wide distribution throughout the peripheral and central nervous system. Similar to other opioid receptors, NOP receptors couple to intracellular second messengers and regulatory proteins to affect biological systems. In this chapter, we review the current literature for NOP signaling cascades including their role as classic GPCRs, the investigation of their kinase and arrestin signaling pathways, and the importance of examining biased signaling to critically evaluate the therapeutic potential of novel NOP agonists.

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.

NOP agonists prevent the antidepressant-like effects of nortriptyline and fluoxetine but not R-ketamine

RATIONALE

Nociceptin/orphanin FQ (N/OFQ) is the endogenous ligand of a Gi protein-coupled receptor named NOP. Both N/OFQ and NOP receptor are widely expressed in brain areas involved in the control of emotional processes. Clinical and preclinical studies support antidepressant effects due to the blockade of NOP receptor signaling. By contrast, NOP receptor activation did not evoke any change in behavioral despair tests.

OBJECTIVES

The present study aimed to investigate the effects of the co-administration of NOP agonists and classic antidepressant drugs in the forced swimming test (FST) and learned helplessness model (LH) in mice.

METHODS

Male Swiss mice were co-administered with NOP agonists (N/OFQ and Ro 65-6570) and antidepressants (nortriptyline, fluoxetine, and R-ketamine) or SB-612111 (NOP antagonist) and the behavioral effects were assessed in the FST and LH tests.

RESULTS

Fluoxetine, nortriptyline, R-ketamine and the NOP antagonist SB-612111 displayed antidepressant-like effects in the FST. The administration of the NOP agonists N/OFQ and Ro 65-6570 did not induce any behavioral change. However, co-administration of NOP agonists blocked the antidepressant effects of SB-612111, fluoxetine, and nortriptyline, but not R-ketamine in the FST. Similarly, in the LH, the systemic injection of SB-612111, nortriptyline, and R-ketamine reversed helplessness. The co-administration of Ro 65-6570 blocked the antidepressant-like effects of SB-612111 and nortriptyline, but not R-ketamine.

CONCLUSIONS

NOP receptor activation inhibits the acute antidepressant effects of nortriptyline and fluoxetine, but not R-ketamine. The present findings contribute to further understand the role played by the N/OFQ-NOP receptor system in regulating mood states.

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.

Endogenous Opiates and Behavior: 2016

This paper is the thirty-ninth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2016 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, and the roles of these opioid peptides and receptors in pain and analgesia, stress and social status, tolerance and dependence, learning and memory, eating and drinking, drug abuse and alcohol, sexual activity and hormones, pregnancy, development and endocrinology, mental illness and mood, seizures and neurologic disorders, electrical-related activity and neurophysiology, general activity and locomotion, gastrointestinal, renal and hepatic functions, cardiovascular responses, respiration and thermoregulation, and immunological responses.

NOP Ligands for the Treatment of Anxiety and Mood Disorders

Many studies point toward the nociceptin/orphanin FQ (N/OFQ) and the N/OFQ peptide receptor (NOP) as targets for the development of innovative drugs for treating anxiety- and mood-related disorders. Evidence supports the view that the activation of NOP receptors with agonists elicits anxiolytic-like effects, while its blockade with NOP antagonists promotes antidepressant-like actions in rodents. Genetic studies showed that NOP receptor knockout mice display an antidepressant-like phenotype, and NOP antagonists are inactive in these animals. In contrast, the genetic blockade of NOP receptor signaling generally displays an increase of anxiety states in the elevated plus-maze test. In this chapter we summarized the most relevant findings of NOP receptor ligands in the modulation of anxiety and mood disorders, and the putative mechanisms of action are discussed.

Activation of nociceptin/orphanin FQ receptors inhibits contextual fear memory reconsolidation

Several neuropeptidergic systems act as modulators of cognitive performances. Among them, nociceptin, an opioid-like peptide also known as orphanin FQ (N/OFQ), has recently gained attention. Stimulation of its receptor, the N/OFQ opioid receptor (NOP), which is expressed in brain regions involved in emotion, memory and stress response, has inhibitory effects on the acquisition and/or consolidation of spatial and emotional memory in rodents. Recently, N/OFQ was also proposed to be linked to the pathogenesis of Post-Traumatic Stress Disorder in humans. However, until now the effect of the activation of the N/OFQ-NOP system on already consolidated memory, such as during retrieval and reconsolidation phases, has never been explored. In the present study, we investigated the consequences of systemic injection of NOP agonists or i.c.v. injection of the N/OFQ peptide on the retrieval and the reconsolidation of contextual fear memory in mice. We demonstrate that the activation of the N/OFQ system impairs the reconsolidation of context-dependent but not cue-dependent aversive memories. We also show that this amnestic effect is associated with decreased c-Fos expression in the hippocampus and amygdala. Our data thus provide the first evidence that the NOP receptor could be targeted during the reconsolidation process to weaken maladaptive memories. The N/OFQ-NOP system might constitute in the future an interesting pharmacological target for interfering with so-called "pathological memories", in particular those involving maladaptive contextual memories.

Antinociceptive action of NOP and opioid receptor agonists in the mouse orofacial formalin test

Nociceptin/orphanin FQ (N/OFQ) modulates several biological functions, including pain transmission via selective activation of a specific receptor named NOP. The aim of this study was the investigation of the antinociceptive properties of NOP agonists and their interaction with opioids in the trigeminal territory. The orofacial formalin (OFF) test in mice was used to investigate the antinociceptive potential associated to the activation of NOP and opioid receptors. Mice subjected to OFF test displayed the typical biphasic nociceptive response and sensitivity to opioid and NSAID drugs. Mice knockout for the NOP gene displayed a robust pronociceptive phenotype. The NOP selective agonist Ro 65-6570 (0.1-1mgkg^-1) and morphine (0.1-10mgkg^-1) elicited dose dependent antinociceptive effects in the OFF with the alkaloid showing larger effects; the isobologram analysis of their actions demonstrated an additive type of interaction. The mixed NOP/opioid receptor agonist cebranopadol elicited potent (0.01-0.1mgkg^-1) and robust antinociceptive effects. In the investigated dose range, all drugs did not modify the motor performance of the mice in the rotarod test. Collectively the results of this study demonstrated that selective NOP agonists and particularly mixed NOP/opioid agonists are worthy of development as innovative drugs to treat painful conditions of the trigeminal territory.

In vitro pharmacological characterization of a novel unbiased NOP receptor-selective nonpeptide agonist AT-403

Nociceptin/orphanin FQ (N/OFQ) regulates several biological functions via selective activation of the N/OFQ receptor (NOP), a member of the opioid receptor family. We recently identified a new high affinity and highly selective NOP agonist AT-403. In this study, we characterized the functional profile of AT-403 and compared it to other known nonpeptide NOP agonists Ro 65-6570, Ro 2q, SCH-221510, MCOPPB, AT-202 and SCH-486757, using the following assays: GTPγ[³⁵ S] stimulated binding, calcium mobilization assay in cells-expressing human NOP or classical opioid receptors and chimeric G proteins, bioluminescence resonance energy transfer (BRET) based assay for studying NOP receptor interaction with G protein and arrestin, and the electrically stimulated mouse vas deferens bioassay. All compounds behaved as NOP full agonists consistently showing the following rank order of potency MCOPPB > AT-403 > Ro 65-6570 = Ro 2q > SCH-221510 >  AT-202 > SCH-486757. AT-403 and MCOPPB displayed the highest NOP selectivity both at human and murine receptors. Interestingly, while all the other nonpeptide NOP agonists displayed bias toward G protein-mediated signaling in the BRET assay, AT-403, similar to the natural ligand N/OFQ, behaved as an unbiased agonist, activating G-protein-mediated function as well as arrestin recruitment. AT-403 may be a useful nonpeptide tool compound to study the pharmacology of NOP activation in disease states.

Effects of [Nphe1, Arg14, Lys15] N/OFQ-NH2 (UFP-101), a potent NOP receptor antagonist, on molecular, cellular and behavioural alterations associated with chronic mild stress

The present study investigated the effect of [Nphe¹] Arg¹⁴, Lys¹⁵-N/OFQ-NH₂ (UFP-101), a selective NOP receptor antagonist, in chronic mild stress (CMS) in male Wistar rats. NOP receptor antagonists were reported to elicit antidepressant-like effects in rodents. Our aim was to investigate UFP-101 effects on CMS-induced anhedonia and impairment of hippocampal neurogenesis. UFP-101 (10 nmol/rat intracerebroventricularly) did not influence sucrose intake in non-stressed animals, but reinstated basal sucrose consumption in stressed animals from the second week of treatment. UFP-101 also reversed stress effects in forced swimming test and in open field. Fluoxetine (10 mg/kg intraperitoneally) produced similar effects. Moreover, we investigated whether UFP-101 could affect CMS-induced impairment in hippocampal cell proliferation and neurogenesis, and in fibroblast growth factor (FGF-2) expression. Our data confirm that CMS reduced neural stem cell proliferation and neurogenesis in adult rat hippocampus. Chronic UFP-101 treatment did not affect the reduced proliferation (bromodeoxyuridine-positive cells) observed in stressed animals. However, UFP-101 increased the number of doublecortin-positive cells, restoring neurogenesis. Finally, UFP-101 significantly increased FGF-2 expression, reduced by CMS. These findings support the view that blockade of NOP receptors produces antidepressant-like effects in CMS associated with positive effects on neurogenesis and FGF-2 expression. Therefore, NOP receptors may represent a target for innovative antidepressant drugs.

Pharmacological studies on the NOP and opioid receptor agonist PWT2-[Dmt1]N/OFQ(1-13)

An innovative chemical strategy named peptide welding technology (PWT) has been developed for the facile synthesis of tetrabranched peptides. [Dmt¹]N/OFQ(1-13)-NH₂ acts as a universal agonist for nociceptin/orphanin FQ (N/OFQ) and classical opioid receptors. The present study investigated the pharmacological profile of the PWT derivative of [Dmt¹]N/OFQ(1-13)NH₂ (PWT2-[Dmt¹]) in several assays in vitro and in vivo after spinal administration in monkeys subjected to the tail withdrawal assay. PWT2-[Dmt¹] mimicked the effects of [Dmt¹]N/OFQ(1-13)-NH₂ displaying full agonist activity, similar affinity/potency and selectivity at human recombinant N/OFQ (NOP) and opioid receptors in receptor binding, stimulation of [³⁵S]GTPγS binding, calcium mobilization in cells expressing chimeric G proteins, and BRET studies for measuring receptor/G-protein and receptor/β-arrestin 2 interaction. In vivo in monkeys PWT2-[Dmt¹] elicited dose-dependent and robust antinociceptive effects being more potent and longer lasting than [Dmt¹]N/OFQ(1-13)-NH₂. The analgesic action of PWT2-[Dmt¹] was sensitive to the NOP receptor antagonist J-113397, but not naltrexone. Thus, the present study demonstrated that the tetrabranched derivative of [Dmt¹]N/OFQ(1-13)-NH₂ obtained with the PWT technology maintains the in vitro pharmacological profile of the parent peptide but displays higher potency and longer lasting action in vivo.

In vitro functional characterization of novel nociceptin/orphanin FQ receptor agonists in recombinant and native preparations

Nociceptin/Orphanin FQ (N/OFQ) regulates several biological functions via selective activation of the N/OFQ receptor (NOP). In this study novel nonpeptide NOP ligands were characterized in vitro in receptor binding and [³⁵S]GTPγS stimulated binding in membranes of cells expressing human NOP and classical opioid receptors, calcium mobilization assay in cells coexpressing the receptors and chimeric G proteins, bioluminescence resonance energy transfer (BRET) based assay for studying NOP receptor interaction with G protein and arrestin, the electrically stimulated mouse vas deferens and the mouse colon bioassays. The action of the AT compounds were compared with standard NOP agonists (N/OFQ and Ro 65-6570) and the NOP selective antagonist SB-612111. AT compounds displayed high NOP affinity and behaved as NOP agonists in all the functional assays consistently showing the following rank order of potency AT-127≥AT-090≥AT-035>AT-004= AT-001. AT compounds behaved as NOP full agonists in the calcium mobilization and mouse colon assays and as partial agonists in the [³⁵S]GTPγS and BRET assays. Interestingly AT-090 and AT-127, contrary to standard nonpeptide agonists that display G protein biased agonism, behaved as an unbiased agonists. AT-090 and AT-127 displayed higher NOP selectivity than Ro 65-6570 at native mouse receptors. AT-090 and AT-127 might be useful pharmacological tools for investigating the therapeutic potential of NOP partial agonists.

The nociceptin/orphanin FQ receptor agonist SR-8993 as a candidate therapeutic for alcohol use disorders: validation in rat models

RATIONALE

Alcoholism is a complex disorder in which diverse pathophysiological processes contribute to initiation and progression, resulting in a high degree of heterogeneity among patients. Few pharmacotherapies are presently available, and patient responses to these are variable. The nociceptin/orphanin FQ (NOP) receptor has been suggested to play a role both in alcohol reward and in negatively reinforced alcohol seeking. Previous studies have shown that NOP-receptor activation reduces alcohol intake in genetically selected alcohol-preferring as well as alcohol-dependent rats. NOP activation also blocks stress- and cue-induced reinstatement of alcohol-seeking behavior.

OBJECTIVES

Here, we aimed to examine a novel, potent, and brain-penetrant small-molecule NOP-receptor agonist, SR-8993, in animal models of alcohol- as well as anxiety-related behavior using male Wistar rats.

RESULTS

SR-8993 was mildly anxiolytic when given to naïve animals and potently reversed acute alcohol withdrawal-induced ("hangover") anxiety. SR-8993 reduced both home-cage limited access drinking, operant responding for alcohol, and escalation induced through prolonged intermittent access to alcohol. SR-8993 further attenuated stress- as well as cue-induced relapse to alcohol seeking. For the effective dose (1.0 mg/kg), non-specific effects such as sedation may be limited, since a range of control behaviors were unaffected, and this dose did not interact with alcohol elimination.

CONCLUSION

These findings provide further support for NOP-receptor agonism as a promising candidate treatment for alcoholism and establish SR-8993 or related molecules as suitable for further development as therapeutics.