Distribution of ORL-1 receptor binding and receptor-activated G-proteins in rat forebrain and their experimental localization in anterior cingulate cortex

Therapeutic potentials of NOP and MOP receptor coactivation for the treatment of pain and opioid abuse

Following the identification of the nociceptin/orphanin FQ (N/OFQ) peptide (NOP) as an endogenous ligand for the NOP receptor, ample evidence has revealed unique functional profiles of the N/OFQ-NOP receptor system. NOP receptors are expressed in key neural substrates involved in pain and reward modulation. In nonhuman primates (NHPs), NOP receptor activation effectively exerts antinociception and anti-hypersensitivity at the spinal and supraspinal levels. Moreover, NOP receptor activation inhibits dopaminergic transmission and synergistically enhances mu-opioid peptide (MOP) receptor-mediated analgesia. In this article, we have discussed the functional profiles of ligands with dual NOP and MOP receptor agonist activities and highlight their optimal functional efficacy for pain relief and drug abuse treatment. Through coactivation of NOP and MOP receptors, bifunctional NOP/MOP receptor "partial" agonists (e.g., AT-121, BU08028, and BU10038) reveal a wider therapeutic window with fewer side effects. These newly developed ligands potently induce antinociception without MOP receptor agonist-associated side effects such as abuse potential, respiratory depression, itching sensation, and physical dependence. In addition, in both rodent and NHP models, bifunctional NOP/MOP receptor agonists can attenuate reward processing and/or the reinforcing effects of opioids and other abused drugs. While a mixed NOP/opioid receptor "full" agonist cebranopadol is undergoing clinical trials, bifunctional NOP/MOP "partial" agonists exhibit promising therapeutic profiles in translational NHP models for the treatment of pain and opioid abuse. This class of drugs demonstrates the therapeutic advantage of NOP and MOP receptor coactivation, indicating a greater potential for future development.

Roles of K+ and cation channels in ORL-1 receptor-mediated depression of neuronal excitability and epileptic activities in the medial entorhinal cortex

Nociceptin (NOP) is an endogenous opioid-like peptide that selectively activates the opioid receptor-like (ORL-1) receptors. The entorhinal cortex (EC) is closely related to temporal lobe epilepsy and expresses high densities of ORL-1 receptors. However, the functions of NOP in the EC, especially in modulating the epileptiform activity in the EC, have not been determined. We demonstrated that activation of ORL-1 receptors remarkably inhibited the epileptiform activity in entorhinal slices induced by application of picrotoxin or by deprivation of extracellular Mg2+. NOP-mediated depression of epileptiform activity was independent of synaptic transmission in the EC, but mediated by inhibition of neuronal excitability in the EC. NOP hyperpolarized entorhinal neurons via activation of K+ channels and inhibition of cation channels. Whereas application of Ba2+ at 300 μM which is effective for the inward rectifier K+ (Kir) channels slightly inhibited NOP-induced hyperpolarization, the current-voltage (I-V) curve of the net currents induced by NOP was linear without showing inward rectification. However, a role of NOP-induced inhibition of cation channels was revealed after inhibition of Kir channels by Ba2+. Furthermore, NOP-mediated augmentation of membrane currents was differently affected by application of the blockers selective for distinct subfamilies of Kir channels. Whereas SCH23390 or ML133 blocked NOP-induced augmentation of membrane currents at negative potentials, application of tertiapin-Q exerted no actions on NOP-induced alteration of membrane currents. Our results demonstrated a novel cellular and molecular mechanism whereby activation of ORL-1 receptors depresses epilepsy.

Agonist-selective NOP receptor phosphorylation correlates in vitro and in vivo and reveals differential post-activation signaling by chemically diverse agonists

Agonists of the nociceptin/orphanin FQ opioid peptide (NOP) receptor, a member of the opioid receptor family, are under active investigation as novel analgesics, but their modes of signaling are less well characterized than those of other members of the opioid receptor family. Therefore, we investigated whether different NOP receptor ligands showed differential signaling or functional selectivity at the NOP receptor. Using newly developed phosphosite-specific antibodies to the NOP receptor, we found that agonist-induced NOP receptor phosphorylation occurred primarily at four carboxyl-terminal serine (Ser) and threonine (Thr) residues, namely, Ser³⁴⁶, Ser³⁵¹, Thr³⁶², and Ser³⁶³, and proceeded with a temporal hierarchy, with Ser³⁴⁶ as the first site of phosphorylation. G protein-coupled receptor kinases 2 and 3 (GRK2/3) cooperated during agonist-induced phosphorylation, which, in turn, facilitated NOP receptor desensitization and internalization. A comparison of structurally distinct NOP receptor agonists revealed dissociation in functional efficacies between G protein-dependent signaling and receptor phosphorylation. Furthermore, in NOP-eGFP and NOP-eYFP mice, NOP receptor agonists induced multisite phosphorylation and internalization in a dose-dependent and agonist-selective manner that could be blocked by specific antagonists. Our study provides new tools to study ligand-activated NOP receptor signaling in vitro and in vivo. Differential agonist-selective NOP receptor phosphorylation by chemically diverse NOP receptor agonists suggests that differential signaling by NOP receptor agonists may play a role in NOP receptor ligand pharmacology.

NOP-Related Mechanisms in Substance Use Disorders

Nociceptin/orphanin FQ (N/OFQ) is a 17 amino acid peptide that was deorphanized in 1995 and has been widely studied since. The role of the N/OFQ system in drug abuse has attracted researchers' attention since its initial discovery. The first two scientific papers describing the effect of intracranial injection of N/OFQ appeared 20 years ago and reported efficacy of the peptide in attenuating alcohol intake, whereas heroin self-administration was insensitive. Since then more than 100 scientific articles investigating the role of the N/OFQ and N/OFQ receptor (NOP) system in drug abuse have been published. The present article provides an historical overview of the advances in the field with focus on three major elements. First, the most robust data supportive of the efficacy of NOP agonists in treating drug abuse come from studies in the field of alcohol research, followed by psychostimulant and opioid research. In contrast, activation of NOP appears to facilitate nicotine consumption. Second, emerging data challenge the assumption that activation of NOP is the most appropriate strategy to attenuate consumption of substances of abuse. This assumption is based first on the observation that animals carrying an overexpression of NOP system components are more prone to consume substances of abuse, whereas NOP knockout rats are less motivated to self-administer heroin, alcohol, and cocaine. Third, administration of NOP antagonists also reduces alcohol consumption. In addition, NOP blockade reduces nicotine self-administration. Hypothetical mechanisms explaining this apparent paradox are discussed. Finally, we focus on the possibility that co-activation of NOP and mu opioid (MOP) receptors is an alternative strategy, readily testable in the clinic, to reduce the consumption of psychostimulants, opiates, and, possibly, alcohol.

Nociceptin and the NOP receptor in aversive learning in mice

The endogenous neuropeptide nociceptin (N/OFQ), which mediates its actions via the nociceptin receptor (NOP), is implicated in multiple behavioural and physiological functions. This study examined the effects of the NOP agonists N/OFQ and the synthetic agonist Ro 64-6198, the antagonists NNN and NalBzoH, as well as deletion of the Pronociceptin gene on emotional memory in mice. The animals were tested in the passive avoidance (PA) task, dependent on hippocampal and amygdala functions. N/OFQ injected intraventricularly (i.c.v.) prior to training produced a biphasic effect on PA retention; facilitation at a low dose and impairment at higher doses. Ro 64-6198 also displayed a biphasic effect with memory facilitation at lower doses and impairment at a high dose. None of the agonists influenced PA training latencies. NNN did not significantly modulate retention in the PA task but antagonized the inhibitory effects of N/OFQ. NalBzoH facilitated memory retention in a dose-dependent manner and blocked the impairing effects of N/OFQ. However, neither NNN nor NalBzoH blocked the memory-impairing effects of Ro 64-6198. Finally, the Pnoc knockout mice exhibited enhanced PA retention latencies compared to the wild type mice. The biphasic effect of the natural ligand and Ro 64-6198 and the failure of the antagonists to block the action of Ro 64-6198 indicate complexity in ligand-receptor interaction. These results indicate that brain nociceptin and its NOP has a subtle role in regulation of mechanisms of relevance for treatment of disorders with processing disturbances of aversive events e.g. Alzheimer's disease, anxiety, depression and PTSD.

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.

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.

The nociceptin system and hippocampal cognition in mice: a pharmacological and genetic analysis

This study examines the effects of NOP agonists nociceptin/orphanin FQ (N/OFQ) and Ro 64-6198, NOP antagonists [Nphe(1)]N/OFQ(1-13)-NH(2) Nphe(1) and naloxone benzoylhydrazone (NalBzoH) on spatial memory in NMRI mice and pronociceptin (proNC) knockout (KO) mice using the water maze task. N/OFQ, administered i.c.v. (1, 5 and 10 nmol/mouse) and into hippocampal CA3 (1 nmol/mouse, bilaterally), impaired acquisition and retention in the maze. Impairments were blocked by pre-treatment with Nphe(1) (10 nmol, i.c.v.). Ro 64-6198 (0.1-0.3-1 mg/kg i.p.) also dose-dependently impaired learning. However, pre-treatment with NalBzoH (1 mg/kg, s.c.) failed to modify the effects of Ro 64-6198. Nphe(1) (10 nmol/mouse i.c.v.) and NalBzoH (1 mg/kg, s.c.) by themselves failed to affect maze performance, despite a tendency for enhanced performance. Prepro N/OFQ knockout (ppN/OFQ -/-) showed evidence of improved learning, evident at retention trials and in reversal training. ppN/OFQ -/- mice were not impaired by N/OFQ (10 nmol i.c.v.) in the task, suggesting that changes in postsynaptic NOP receptors may occur in such KO mice. It is concluded that N/OFQ and NOP receptors have an important role in hippocampus-dependent spatial learning and memory, probably by modulation of glutamatergic functions.

Discovery of 1-[1-(1-methylcyclooctyl)-4-piperidinyl]-2-[(3R)-3-piperidinyl]-1H-benzimidazole: integrated drug-design and structure-activity relationships for orally potent, metabolically stable and potential-risk reduced novel non-peptide nociceptin/orphanin FQ receptor agonist as antianxiety drug

Anxiety disorders, caused by continuous or acute stress or fear, have been highly prevailing psychiatric disorders. For the acute treatment of the disorders, benzodiazepines have been widely used despite having liabilities that limit their utility. Alternatively, endogenous nociceptin/orphanin FQ and nociceptin/orphanin FQ peptide receptor (or opioid-receptor-like-1 receptor) have important roles in the integration of emotional components, e.g. anxiolytic activity is the key behavioral action of nociceptin/orphanin FQ in brain. In our preceding study, various structurally novel 1,2-disubstituted benzimidazole derivatives were designed and synthesized as highly potent nociceptin/orphanin FQ peptide receptor selective full agonists in vitro with high or moderate nociceptin/orphanin FQ peptide receptor occupancy in the mice brain per os based on appropriate physicochemical properties for the oral brain activity [Hayashi et al. (2009) J Med Chem;52:610-625]. In the present study, drug design and structure-activity relationships for Vogel anticonflict activities in mice per os, metabolic stabilities in human liver microsome, CYP2D6 inhibitions, serum protein bindings, and human ether-a-go-go related gene binding affinities of novel nociceptin/orphanin FQ peptide receptor agonists were investigated. Through the series of coherent drug discovery studies, the strongest nociceptin/orphanin FQ peptide receptor agonist, 1-[1-(1-methylcyclooctyl)-4-piperidinyl]-2-[(3R)-3-piperidinyl]-1H-benzimidazole was designed and identified as a new-class orally potent anxiolytic with little side-effects, as significant findings.

Novel non-peptide nociceptin/orphanin FQ receptor agonist, 1-[1-(1-Methylcyclooctyl)-4-piperidinyl]-2-[(3R)-3-piperidinyl]-1H-benzimidazole: design, synthesis, and structure-activity relationship of oral receptor occupancy in the brain for orally potent antianxiety drug

An endogenous heptadecapeptide, nociceptin/orphanin FQ (N/OFQ), and a G-protein-coupled receptor, N/OFQ peptide (NOP) receptor [or opioid-receptor-like-1 (ORL1) receptor], have been described in terms of its structure, distribution, and pharmacology. Thus, the N/OFQ and NOP receptor are located in the central nervous systems in humans, primates, and rodents, and are involved in the integration of the emotional components in the brain; e.g., N/OFQ displays anxiolytic activity in the brain. For identifying orally potent anxiolytic, drug-design studies were performed with a series of 1,2-disubstituted benzimidazole derivatives, which resulted in the identification of various chemotypes of highly potent NOP selective full agonists in vitro with high or moderate NOP receptor occupancy in the mice brain per os such as 1-[1-(1-methylcyclooctyl)-4-piperidinyl]-2-[(3R)-3-piperidinyl]-1H-benzimidazole 1 (MCOPPB), the most potent novel non-peptide NOP full agonist in vitro and an orally potent anxiolytic in the mice.

Central and peripheral role of the nociceptin/orphaninFQ system on normal and disturbed colonic motor function and faecal pellet output in the rat

In this study, seeking further information on the role of the nociceptin/orphanin FQ (N/OFQ)-ergic system in normal and disturbed colonic motor function in rats, we compared the colonic effects of UFP-112, a novel highly potent agonist, with those of N/OFQ. When injected intracerebroventricularly (i.c.v.) and intraperitoneally (i.p.), UFP-112 and N/OFQ increased bead expulsion time in a statistically significant and dose-related manner and reduced the percentage of rats with castor oil-induced diarrhoea. UFP-112 showed greater efficacy, higher potency and longer-lasting inhibitory effects than N/OFQ, and pretreatment with UFP-101, a selective antagonist, blocked the N/OFQ analogue-induced responses in both tests. When injected i.c.v., UFP-112 and N/OFQ inhibited corticotrophin releasing factor- and restrain stress-stimulated faecal pellet excretion significantly and in a dose-related manner. Conversely, when injected peripherally both peptides significantly inhibited colonic propulsive motility but did so in a non-dose-related manner. In conclusion, these findings indicate that, in the rat, the central and peripheral N/OFQ systems have an inhibitory role in modulating distal colonic propulsive motility under physiological and pathological conditions. UFP-112 therefore promises to be a useful pharmacological tool for investigating the role of the N/OFQ system in motor functions in the distal colonic tract under physiological and pathological conditions.

Distribution of nociceptin and Ro64-6198 activated [35S]-GTPgammaS binding in the rat brain

The peptide, nociceptin, was discovered as the endogenous ligand for the opioid-like receptor, ORL1. Since its discovery, this peptide has been shown to modulate the perception of pain, modulate feeding and produce behavioral effects in rodent models of mood disorders. Recently, the non-peptide agonist {(1S,3aS)-8-(2,3,3a,4,5,6-hexahydro-1H-phenalen-1-yl)-1-phenyl-1,3,8-triaza-spiro[4,5]decan-4-one} (Ro64-6198) of the ORL1 receptor has been reported in the literature. In the present study, we compared the distribution and potency of Ro64-6198 with nociceptin for their ability to stimulate [(35)S]-GTPgammaS binding to sections of rat brain. In initial studies, Ro64-6198 inhibited (125)I-nociceptin binding to the hORL1 receptors with a K(i) of 1.75 nM compared with 0.20 nM for nociceptin. To assess agonist potency in a whole cell assay, a cell line expressing the hORL1 receptor and G(alpha15) was created and used for calcium mobilization studies. In this assay system, Ro64-6198 increased intracellular calcium with an EC(50) of 52nM compared with 24 nM for nociceptin. Having verified the agonist properties of Ro64-6198, we then assessed the potency and distribution of ORL1 receptor activation in rat brain sections. In dose-response studies, Ro64-6198 increased [(35)S]-GTPgammaS binding to a variety of brain regions with EC(50) values ranging from 84.9 to 2,143 nM depending on the brain regions evaluated. These potencies were similar to that seen for nociceptin, but substantially lower than values established using [(125)I] nociceptin binding to the cloned human ORL1 receptor. In general, the brain distribution of agonist stimulated [(35)S]-GTPgammaS binding was similar when either Ro64-6198 or nociceptin were used. Using these techniques, we have demonstrated, for the first time that Ro64-6198 activates [(35)S]-GTPgammaS binding to rat brain sections and this compound stimulates a similar population of receptors as nociceptin.

Increased opioid receptor binding and G protein coupling in the accumbens and ventral tegmental area of postnatal day 2 rats

In some regions of the developing rat brain such as the nucleus accumbens (Acb), mu opioid (MOP) receptor specific binding in the perinatal period exceeds that in the adult. To investigate the significance of these developmental changes, MOP and nociceptin/orphanin FQ (NOP) receptor binding and G protein coupling as determined by GTPgammaS binding experiments were examined in mesolimbic regions of postnatal day 2 (P2) pups and compared to those of their dams. Acb of the P2 pup exhibited 2-fold greater MOP receptor specific binding than that of the dam. In the ventral tegmental area (VTA), NOP specific binding was about 2-fold higher in the P2 pup. A correlation was found between MOP and NOP binding and their coupling to G protein on dam and P2 pup brain sections. However, the magnitude of increases in MOP and NOP receptor G protein coupling to G protein in P2 pups exceeded the 2-fold differences in binding between pups and dams. Furthermore, the amplitude of the MOP receptor G protein coupling in female P2 Acb was greater than increases in male P2 pup Acb. Differences in MOP and NOP binding and G protein coupling in other mesolimbic regions between P2 pups and dams were rarely observed. The data indicate that greater binding and G protein coupling of MOP and NOP receptors occur in discrete, mesolimbic regions of P2 pups when compared to their dams. It may be of significance that these brain regions, Acb and VTA, are undergoing maturation on P2.

Opioid receptor-like 1 (NOP) receptors in the rat dorsal raphe nucleus: evidence for localization on serotoninergic neurons and functional adaptation after 5,7-dihydroxytryptamine lesion

A high density of opioid receptor-like 1 (ORL1) receptor (also referred to as NOP receptor) is found in limbic areas and in regions containing monoamines, which are implicated in emotional activity and physiopathology of depression and anxiety. We aimed at defining precisely the localization of ORL1 receptors in dorsal raphe nucleus, by means of a lesion strategy and autoradiographic studies. In control rats, [3H]nociceptin and nociceptin-stimulated [35S]GTPgammaS bindings were found to be correlated in several brain regions. We performed in rats a selective destruction of serotoninergic neurons by surgical stereotaxic injection of 5,7-dihydroxytryptamine (5,7-DHT) in dorsal raphe nucleus. This led to a marked decrease in serotonin contents in striata and frontal cortices (about -60%) and in autoradiographic [3H]citalopram binding in posterior regions. In dorsal raphe nucleus, [3H]nociceptin binding was decreased to the same extent as [3H]citalopram binding, whereas it was unchanged in the other regions studied. Nevertheless, in the dorsal raphe, nociceptin-stimulated [35S]GTPgammaS binding was decreased to a lesser extent than [3H]nociceptin binding in 5,7-DHT-lesioned rats. The ratio between nociceptin-stimulated [35S]GTPgammaS binding and [3H]nociceptin binding was significantly increased in 5,7-DHT-lesioned rats compared with controls in this region. These data demonstrate 1) that ORL1 receptors are located on serotoninergic neurons in the dorsal raphe nucleus and 2) that, after a lesion, the functionality of remaining ORL1 receptors appears to be up-regulated, which could correspond to a compensatory mechanism.