Orphanin FQ inhibits synaptic transmission and long-term potentiation in rat hippocampus

Selected β2-, β3- and β2,3-Amino Acid Heterocyclic Derivatives and Their Biological Perspective

Heterocyclic moieties, especially five and six-membered rings containing nitrogen, oxygen or sulfur atoms, are broadly distributed in nature. Among them, synthetic and natural alike are pharmacologically active compounds and have always been at the forefront of attention due to their pharmacological properties. Heterocycles can be divided into different groups based on the presence of characteristic structural motifs. The presence of β-amino acid and heterocyclic core in one compound is very interesting; additionally, it very often plays a vital role in their biological activity. Usually, such compounds are not considered to be chemicals containing a β-amino acid motif; however, considering them as this class of compounds may open new routes of their preparation and application as new drug precursors or even drugs. The possibility of their application as nonproteinogenic amino acid residues in peptide or peptide derivatives synthesis to prepare a new class of compounds is also promising. This review highlights the actual state of knowledge about β-amino acid moiety-containing heterocycles presenting antiviral, anti-inflammatory, antibacterial compounds, anaplastic lymphoma kinase (ALK) inhibitors, as well as agonist and antagonists of the receptors.

[The role of nociceptin in opioid regulation of brain functions]

This review discusses our current knowledge on the nociceptin/orphanin (N/OFQ) system regarding its role in regulation of brain functions. Nociceptin receptor (NOPr) was identified in 1994 [Bunzow et al., 1994; Mollereau et al., 1994]. In 1995 a 17 amino acid endogenous peptide was found to be the high-affinity ligand for the NOPr [Reinscheid et al., 1995]. N/OFQ has a broad spectrum of activity and can act as on opioid-like as well as an anti-opioid peptide. Considering high level of N/OFQ and NOPr mRNA expression in the limbic brain regions, the N/OFQ/NOP system is suggested to be involved in regulation of emotions, resward, pain sensitivity, stress responsibility, sexual behavior, aggression, drug abuse and addiction. However it is still not well understood whether an increased vulnerability to drugs of abuse may be associated with dysregulation of N/OFQ/NOP system. Current review further highlights a need for further research on N/OFQ/NOP system as it could have clinical utility for substance abuse, depression, and anxiety pharmacotherapy.

Hippocampal changes in mice lacking an active prohormone convertase 2

Prohormone convertase 2 (PC2) is essential for the biosynthesis of many neuropeptides, including several of them in hippocampus. In mouse brain, lacking an enzymatically active PC2 (PC2-null) causes accumulation of many neuropeptides in their precursor or intermediate forms. Little is known about how a PC2-null state may affect the function of the hippocampus. In this study, adult PC2-null mice and their wildtype (WT) littermates were subjected to three analyses to determine possible changes associated with PC2-null at physiological, behavioral, and molecular levels, respectively, under normal and stressed conditions. Electrophysiological recordings of hippocampal slices were performed to measure evoked field-excitatory postsynaptic potentials (EPSP), long-term potentiation (LTP), and paired-pulse facilitation (PPF). Morris water maze (MWM) testing was conducted to examine behavioral changes that are indicative of hippocampal integrity. Quantitative mass spectrometry analysis was used to determine changes in the hippocampal proteome in response to a focal cerebral ischemic insult. We found that there were no significant differences in the threshold of evoked EPSPs between PC2-null and WT animals. However, an increase in LTP in both triggering rate and amplitude was observed in PC2-null mice, suggesting that PC2 may be involved in regulating synaptic strength. The PPF, on the other hand, showed a decrease in PC2-null mice, suggesting a presynaptic mechanism. Consistent with changes in LTP, PC2-null mice displayed decreased latencies in finding the escape platform in the MWM test. Further, after distal focal cerebral ischemia, the hippocampal proteomes incurred changes in both WT and PC2-null mice, with a prominent change in proteins associated with neurotransmission, exocytosis, and transport processes seen in the PC2-null but not WT mice. Taken together, our results suggest that PC2 is involved in regulating hippocampal synaptic plasticity, learning, and memory behaviors, as well as the hippocampal response to stresses originating in other regions of the brain.

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.

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.

The Nociceptin/Orphanin FQ System and the Regulation of Memory

Nociceptin/orphanin FQ (N/OFQ) is an endogenous neuropeptide of 17 amino acids, related to opioid peptides but with its own receptor, distinct from conventional opioid receptors, the ORL1 or NOP receptor. The NOP receptor is a G protein-coupled receptor which activates Gi/o proteins and thus induces an inhibition of neuronal activity. The peptide and its receptor are widely expressed in the central nervous system with a high density of receptors in regions involved in learning and memory. This review describes the consequences of the pharmacological manipulation of the N/OFQ system by NOP receptor ligands on learning processes and on the consolidation of various types of long-term memory. We also discuss the role of endogenous N/OFQ release in the modulation of learning and memory. Finally we propose several putative neuronal mechanisms taking place at the level of the hippocampus and amygdala and possibly underlying the behavioral amnestic or promnesic effects of NOP ligands.

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.

The Cholinergic System as a Treatment Target for Opioid Use Disorder

Opioid overdoses recently became the leading cause of accidental death in the US, marking an increase in the severity of the opioid use disorder (OUD) epidemic that is impacting global health. Current treatment protocols for OUD are limited to opioid medications, including methadone, buprenorphine, and naltrexone. While these medications are effective in many cases, new treatments are required to more effectively address the rising societal and interpersonal costs associated with OUD. In this article, we review the opioid and cholinergic systems, and examine the potential of acetylcholine (ACh) as a treatment target for OUD. The cholinergic system includes enzymes that synthesize and degrade ACh and receptors that mediate the effects of ACh. ACh is involved in many central nervous system functions that are critical to the development and maintenance of OUD, such as reward and cognition. Medications that target the cholinergic system have been approved for the treatment of Alzheimer's disease, tobacco use disorder, and nausea. Clinical and preclinical studies suggest that medications such as cholinesterase inhibitors and scopolamine, which target components of the cholinergic system, show promise for the treatment of OUD and further investigations are warranted.

Synthesis, Biological Evaluation, and SAR Studies of 14β-phenylacetyl Substituted 17-cyclopropylmethyl-7, 8-dihydronoroxymorphinones Derivatives: Ligands With Mixed NOP and Opioid Receptor Profile

A series of 14β-acyl substituted 17-cyclopropylmethyl-7,8-dihydronoroxymorphinone compounds has been synthesized and evaluated for affinity and efficacy for mu (MOP), kappa (KOP), and delta (DOP) opioid receptors and nociceptin/orphanin FQ peptide (NOP) receptors. The majority of the new ligands displayed high binding affinities for the three opioid receptors, and moderate affinity for NOP receptors. The affinities for NOP receptors are of particular interest as most classical opioid ligands do not bind to NOP receptors. The predominant activity in the [35S]GTPγS assay was partial agonism at each receptor. The results are consistent with our prediction that an appropriate 14β side chain would access a binding site within the NOP receptor and result in substantially higher affinity than displayed by the parent compound naltrexone. Molecular modeling studies, utilizing the recently reported structure of the NOP receptor, are also consistent with this interpretation.

Synaptic plasticity modulation by circulating peptides and metaplasticity: Involvement in Alzheimer's disease

Synaptic plasticity is a cellular process involved in learning and memory whose alteration in its two main forms (Long Term Depression (LTD) and Long Term Potentiation (LTP)), is observed in most brain pathologies, including neurodegenerative disorders such as Alzheimer's disease (AD). In humans, AD is associated at the cellular level with neuropathological lesions composed of extracellular deposits of β-amyloid (Aβ) protein aggregates and intracellular neurofibrillary tangles, cellular loss, neuroinflammation and a general brain homeostasis dysregulation. Thus, a dramatic synaptic environment perturbation is observed in AD patients, involving changes in brain neuropeptides, cytokines, growth factors or chemokines concentration and diffusion. Studies performed in animal models demonstrate that these circulating peptides strongly affect synaptic functions and in particular synaptic plasticity. Besides this neuromodulatory action of circulating peptides, other synaptic plasticity regulation mechanisms such as metaplasticity are altered in AD animal models. Here, we will review new insights into the study of synaptic plasticity regulatory/modulatory mechanisms which could influence the process of synaptic plasticity in the context of AD with a particular attention to the role of metaplasticity and peptide dependent neuromodulation.

A systematic review of the role of the nociceptin receptor system in stress, cognition, and reward: relevance to schizophrenia

Schizophrenia is a debilitating neuropsychiatric illness that is characterized by positive, negative, and cognitive symptoms. Research over the past two decades suggests that the nociceptin receptor system may be involved in domains affected in schizophrenia, based on evidence aligning it with hallmark features of the disorder. First, aberrant glutamatergic and striatal dopaminergic function are associated with psychotic symptoms, and the nociceptin receptor system has been shown to regulate dopamine and glutamate transmission. Second, stress is a critical risk factor for first break and relapse in schizophrenia, and evidence suggests that the nociceptin receptor system is also directly involved in stress modulation. Third, cognitive deficits are prevalent in schizophrenia, and the nociceptin receptor system has significant impact on learning and working memory. Last, reward processing is disrupted in schizophrenia, and nociceptin signaling has been shown to regulate reward cue salience. These findings provide the foundation for the involvement of the nociceptin receptor system in the pathophysiology of schizophrenia and outline the need for future research into this system.

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.

■ 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

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

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

Nociceptin Opioid Receptor (NOP) as a Therapeutic Target: Progress in Translation from Preclinical Research to Clinical Utility

In the two decades since the discovery of the nociceptin opioid receptor (NOP) and its ligand, nociceptin/orphaninFQ (N/OFQ), steady progress has been achieved in understanding the pharmacology of this fourth opioid receptor/peptide system, aided by genetic and pharmacologic approaches. This research spawned an explosion of small-molecule NOP receptor ligands from discovery programs in major pharmaceutical companies. NOP agonists have been investigated for their efficacy in preclinical models of anxiety, cough, substance abuse, pain (spinal and peripheral), and urinary incontinence, whereas NOP antagonists have been investigated for treatment of pain, depression, and motor symptoms in Parkinson's disease. Translation of preclinical findings into the clinic is guided by PET and receptor occupancy studies, particularly for NOP antagonists. Recent progress in preclinical NOP research suggests that NOP agonists may have clinical utility for pain treatment and substance abuse pharmacotherapy. This review discusses the progress toward validating the NOP-N/OFQ system as a therapeutic target.

The Nociceptin Receptor as an Emerging Molecular Target for Cocaine Addiction

Cocaine addiction is a global public health and socioeconomic issue that requires pharmacological and cognitive therapies. Currently there are no FDA-approved medications to treat cocaine addiction. However, in preclinical studies, interventions ranging from herbal medicine to deep-brain stimulation have shown promise for the therapy of cocaine addiction. Recent developments in molecular biology, pharmacology, and medicinal chemistry have enabled scientists to identify novel molecular targets along the pathways involved in drug addiction. In 1994, a receptor that showed a great deal of homology to the traditional opioid receptors was characterized. However, endogenous and exogenous opioids failed to bind to this receptor, which led scientists to name it opioid receptor-like receptor, now referred to as the nociceptin receptor. The endogenous ligand of NOPr was identified a year later and named orphanin FQ/nociceptin. Nociceptin and NOPr are widely distributed throughout the CNS and are involved in many physiological responses, such as food intake, nociceptive processing, neurotransmitter release, etc. Furthermore, exogenous nociceptin has been shown to regulate the activity of mesolimbic dopaminergic neurons, glutamate, and opioid systems, and the stress circuit. Importantly, exogenous nociceptin has been shown to reduce the rewarding and addictive actions of a number of drugs of abuse, such as psychostimulants, alcohol, and opioids. This paper reviews the existing literature on the role of endogenous nociceptin in the rewarding and addictive actions of cocaine. The effect of exogenous nociceptin on these processes is also reviewed. Furthermore, the effects of novel small-molecule NOPr ligands on these actions of cocaine are discussed. Overall, a review of the literature suggests that NOPr could be an emerging target for cocaine addiction pharmacotherapy.

Nociceptin and the nociceptin receptor in learning and memory

There are many processes in which the neuropeptide nociceptin/orphanin FQ (N/OFQ or nociceptin) is involved in the brain. The role of nociceptin in learning and memory holds promise in modulating these processes in health and disease in the human brain. This review summarizes the body of research focused on N/OFQ and its specific receptor, the nociceptin receptor (NOP receptor), in learning and memory, and its potential mechanisms of action, in which acetylcholine, NMDA receptor, and noradrenaline may be critical. Finally, the association between NOP receptor and posttraumatic stress disorder (PTSD), a psychiatric disorder with altered fear learning, is examined as one of the potential outcomes resulting from pathological consequences of dysregulation of N/OFQ-NOP receptor in the brain.

Nociceptin/orphanin FQ decreases glutamate transmission and blocks ethanol-induced effects in the central amygdala of naive and ethanol-dependent rats

The central nucleus of the amygdala (CeA) mediates several addiction-related processes and nociceptin/orphanin FQ (nociceptin) regulates ethanol intake and anxiety-like behaviors. Glutamatergic synapses, in the CeA and throughout the brain, are very sensitive to ethanol and contribute to alcohol reinforcement, tolerance, and dependence. Previously, we reported that in the rat CeA, acute and chronic ethanol exposures significantly decrease glutamate transmission by both pre- and postsynaptic actions. In this study, using electrophysiological techniques in an in vitro CeA slice preparation, we investigated the effects of nociceptin on glutamatergic transmission and its interaction with acute ethanol in naive and ethanol-dependent rats. We found that nociceptin (100-1000 nM) diminished basal-evoked compound glutamatergic receptor-mediated excitatory postsynaptic potentials (EPSPs) and spontaneous and miniature EPSCs (s/mEPSCs) by mainly decreasing glutamate release in the CeA of naive rats. Notably, nociceptin blocked the inhibition induced by acute ethanol (44 mM) and ethanol blocked the nociceptin-induced inhibition of evoked EPSPs in CeA neurons of naive rats. In neurons from chronic ethanol-treated (ethanol-dependent) rats, the nociceptin-induced inhibition of evoked EPSP amplitude was not significantly different from that in naive rats. Application of [Nphe1]Nociceptin(1-13)NH2, a nociceptin receptor (NOP) antagonist, revealed tonic inhibitory activity of NOP on evoked CeA glutamatergic transmission only in ethanol-dependent rats. The antagonist also blocked nociceptin-induced decreases in glutamatergic responses, but did not affect ethanol-induced decreases in evoked EPSP amplitude. Taken together, these studies implicate a potential role for the nociceptin system in regulating glutamatergic transmission and a complex interaction with ethanol at CeA glutamatergic synapses.

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.

Ghrelin, neuropeptide Y, and other feeding-regulatory peptides active in the hippocampus: role in learning and memory

The hippocampus is a brain region of primary importance for neurogenesis, which occurs during early developmental states as well as during adulthood. Increases in neuronal proliferation and in neuronal death with age have been associated with drastic changes in memory and learning. Numerous neurotransmitters are involved in these processes, and some neuropeptides that mediate neurogenesis also modulate feeding behavior. Concomitantly, feeding peptides, which act primarily in the hypothalamus, are also present in the hippocampus. This review aims to ascertain the role of several important feeding peptides in cognitive functions, either through their local synthesis in the hippocampus or through their actions via specific receptors in the hippocampus. A link between neurogenesis and the orexigenic or anorexigenic properties of feeding peptides is discussed.

The role of NOP receptors in psychomotor stimulation and locomotor sensitization induced by cocaine and amphetamine in mice

We have previously shown that orphanin FQ (also known as nociceptin; OFQ/N) attenuates the motor stimulatory effect of cocaine and blocks locomotor sensitization induced by cocaine. Furthermore, we have shown that cocaine treatment altered the level of endogenous OFQ/N, raising the possibility that endogenous OFQ/N and its receptor (NOP) may be crucial in these actions of cocaine. Accordingly, in the present study, we sought to determine the role of NOP receptors in psychomotor stimulation and locomotor sensitization induced by cocaine or amphetamine. Mice lacking the NOP receptor and their wild-type littermates were habituated to motor activity chambers for 1h, injected with cocaine (0, 15 or 30 mg/kg) or amphetamine (0, 1 or 3mg/kg), and motor activity was recorded for 1h. For sensitization induced by these drugs, mice were treated with saline or the highest dose of each drug once daily for three consecutive days and tested on day 8. On this day, mice were habituated to the chambers for 1h, then received a challenge dose of cocaine (15 mg/kg) or amphetamine (1mg/kg), and motor activity was recorded for 1h. Cocaine and amphetamine each induced hyperlocomotion but the extent of this response was not different between NOP receptor null mice and their controls. Sensitization developed to the motor stimulatory action of each drug, but the magnitude of cocaine-induced sensitization was only higher in null mice compared to their controls. Together, the present results suggest that the endogenous OFQ/N/NOP receptor system may modulate the development of cocaine-induced locomotor sensitization.

Cellular mechanisms of nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor regulation and heterologous regulation by N/OFQ

The nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor is the fourth and most recently discovered member of the opioid receptor superfamily that also includes μ, δ, and κ opioid receptor subtypes (MOR, DOR, and KOR, respectively). The widespread anatomic distribution of the NOP receptor enables the modulation of several physiologic processes by its endogenous agonist, N/OFQ. Accordingly, the NOP receptor has gained a lot of attention as a potential target for the development of ligands with therapeutic use in several pathophysiological states. NOP receptor activation frequently results in effects opposing classic opioid receptor action; therefore, regulation of the NOP receptor and conditions affecting its modulatory tone are important to understand. Mounting evidence reveals a heterologous interaction of the NOP receptor with other G protein-coupled receptors, including MOR, DOR, and KOR, which may subsequently influence their function. Our focus in this review is to summarize and discuss the findings that delineate the cellular mechanisms of NOP receptor signaling and regulation and the regulation of other receptors by N/OFQ and the NOP receptor.

Deciphering intracellular localization and physiological role of nociceptin and nocistatin

Nociceptin and nocistatin are endogenous ligands of G protein coupled receptor family. Numerous techniques have been used to study the diverse parameters including, localization, distribution and ultrastructure of these peptides. The majority of the study parameters are based on their physiological roles in different organ systems. The present study presents an overview of the different methods used for the study of nociceptin, nocistatin and their receptors. Nociceptin has been implicated in many physiological functions including, nociception, locomotion, stressed-induced analgesia, learning and memory, neurotransmitter and hormone release, renal function, neuronal differentiation, sexual and reproductive behavior, uterine contraction, feeding, anxiety, gastrointestinal motility, cardiovascular function, micturition, cough, hypoxic-ischemic brain injury, diuresis and sodium balance, temperature regulation, vestibular function, and mucosal transport. It has been noted that the use of light and electron microscopy was less frequent, though it may be one of the most promising tools to study the intracellular localization of these neuropeptides. In addition, more studies on the level of circulating nociceptin and nocistatin are also necessary for investigating their clinical roles in health and disease. A variety of modern tools including physiological, light and electron microscopy (EM) are needed to decipher the extent of intracellular localization, tissue distribution and function of these peptides. The intracellular localization of nociceptin and nocistatin will require a high resolution transmission EM capable of identifying these peptides and other supporting molecules that co-localize with them. A tracing technique could also elucidate a possible migratory ability of nociceptin and nocistatin from one cellular compartment to the other.

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.

Identification of an orally active opioid receptor-like 1 (ORL1) receptor antagonist 4-{3-[(2R)-2,3-dihydroxypropyl]-2-oxo-2,3-dihydro-1H-benzimidazol-1-yl}-1-[(1S,3S,4R)-spiro[bicyclo[2.2.1]heptane-2,1'-cyclopropan]-3-ylmethyl]piperidine as clinical candidate

Our efforts to optimize prototype opioid receptor-like 1 (ORL1) antagonist 1 led to the discovery of 4-{3-[(2R)-2,3-dihydroxypropyl]-2-oxo-2,3-dihydro-1H-benzimidazol-1-yl}-1-[(1S,3S,4R)-spiro[bicyclo[2.2.1]heptane-2,1'-cyclopropan]-3-ylmethyl]piperidine 10. 10 showed potent ORL1 antagonistic activity, excellent selectivity over other opioid receptors, and in vivo efficacy after oral dosing. Currently clinical trials of 10 are underway.

Activation of nociceptin opioid peptide (NOP) receptor impairs contextual fear learning in mice through glutamatergic mechanisms

The present study investigated whether the selective nociceptin opioid peptide (NOP) receptor agonist, Ro64-6198, impairs acquisition of fear conditioning through glutamatergic mechanisms. Systemic administration of Ro64-6198 (0.3 and 1mg/kg) or the non-competitive NMDA receptor antagonist, MK-801 (0.03 and 0.1mg/kg) prior to conditioning severely impaired contextual but not cued fear learning in C57BL/6N mice. When administered together at sub-effective doses, Ro64-6198 (0.5mg/kg) and MK-801 (0.05mg/kg), synergistically impaired contextual fear learning, but left cued fear learning intact. We next used the immediate shock deficit paradigm (ISD) to examine the effects of Ro64-6198 and MK-801 on contextual memory formation in the absence of the foot-shock. As expected, naive mice that were shocked briefly after being placed in the training chamber displayed no contextual fear conditioning. This learning deficit was elevated by prior exposure of mice to the training context. Furthermore, administration of Ro64-6198 and MK-801, either separately at amnesic doses (1mg/kg and 0.1mg/kg, respectively) or concomitantly at sub-effective doses (0.5mg/kg and 0.05mg/kg, respectively) significantly reduced the facilitating effects of context preexposure. These findings demonstrate the existence of functional antagonism between NOP and NMDA receptors that predominantly contributes to modulation of conditioned fear learning which involves spatial-processing demands.

Alterations in the level of OFQ/N-IR in rat brain regions by cocaine

We have previously shown that administration of orphanin FQ/nociceptin (OFQ/N), the endogenous ligand of the opioid receptor-like (ORL-1) receptor, into the lateral ventricles or VTA blocked cocaine sensitization. In the present study, we determined the effect of acute and chronic cocaine treatment on the level of endogenous OFQ/N in rat brain regions. Male Sprague Dawley rats were tested for motor activity in response to saline or cocaine (20 mg/kg) injection once daily for three consecutive days. To determine the effect of single or repeated cocaine administration on the level of OFQ/N, rats were sacrificed 1 h following saline or cocaine injection either on day 1 or 3, respectively. Additional groups of rats were treated similarly with saline or cocaine on days 1-3 and sacrificed or tested for locomotor sensitization on day 8. Consistent with previous studies, repeated cocaine administration induced locomotor sensitization to a challenge dose of cocaine (7.5 mg/kg) given on day 8. Measurements of tissue content of OFQ/N-IR using radioimmunoassay indicated that the rat hypothalamus and striatum, respectively, contained the highest and lowest levels of the peptide among the brain regions tested. Acute cocaine decreased the level of OFQ-IR in the rat midbrain and to a lesser extent in the striatum. On the other hand, the level of OFQ/N was higher in rats treated with cocaine on days 1-3 and sacrificed on day 8. These findings suggest that endogenous OFQ/N may be involved in the actions of cocaine and possibly in cocaine-induced motor stimulation and locomotor sensitization.

The role of the opioid receptor-like (ORL1) receptor in motor stimulatory and rewarding actions of buprenorphine and morphine

We have previously shown that the ability of buprenorphine to activate the opioid receptor-like (ORL1) receptor compromises its antinociceptive effect. Furthermore, morphine has been shown to alter the level of orphanin FQ/nociceptin (OFQ/N), the endogenous ligand of the ORL1 receptor, raising the possibility that the endogenous OFQ/N/ORL1 receptor system may be involved in the actions of these opioids. Thus, using mice lacking the ORL1 receptor and their wild-type littermates, the present study assessed the role of the ORL1 receptor in psychomotor stimulant and rewarding actions of buprenorphine and morphine. Morphine (5, 10 mg/kg) dose-dependently increased motor activity and induced conditioned place preference. However, the magnitude of each response was comparable for the mutant mice and their wild-type littermates. In contrast, buprenorphine (1 mg/kg) induced greater motor stimulation in ORL1 receptor knockout mice as compared with their wild-type littermates. Further, single conditioning with buprenorphine (3 mg/kg) induced place preference in mutant mice but not in their wild-type littermates. The results of binding assay showed that buprenorphine concentration-dependently (0-1000 nM) displaced specific binding of [(3)H]-OFQ/N in brain membrane of wild-type mice. Together, the present results suggest that the ability of buprenorphine to interact with the ORL1 receptor modulates its acute motor stimulatory and rewarding effects.

Nociceptin receptor impairs recognition memory via interaction with NMDA receptor-dependent mitogen-activated protein kinase/extracellular signal-regulated kinase signaling in the hippocampus

Strong evidence suggests a role for nociceptin/orphanin FQ (N/OFQ) neuropeptide and its receptor (NOP) in cognition. However, the signaling mechanisms underlying N/OFQ modulation of memory are less understood. Here, we show that intracerebroventricular or intrahippocampal infusions of N/OFQ impair long-term memory formation in the mouse object recognition task. The synthetic NOP receptor 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), administered systemically, also produced amnesic effects that were blocked by coinfusion of the NOP receptor antagonist, [Nphe1,Arg14,Lys15]nociceptin-NH2 (UFP-101), into the dorsal hippocampus. In contrast, Ro64-6198 had no effect on short-term memory or recall performances. Immunoblotting analysis revealed a strong suppressive action of Ro64-6198 on learning-induced upregulation of hippocampal extracellular signal-regulated kinase (ERK) phosphorylation, which is crucial for long-term information storage. Accordingly, pharmacological inhibition of ERK activation after systemic injection of SL327 [alpha-[amino[(4-aminophenyl)thio]methylene]-2-(trifluoromethyl)benzene acetonitrile], a selective inhibitor of the upstream kinase MEK (mitogen-activated protein kinase kinase), abolished long-term recognition memory formation. The noncompetitive NMDA receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d]cyclohepten-5,10-imine maleate (MK-801), given systemically, also suppressed ERK activation and disrupted recognition memory. In contrast, no effect of MK-801 was observed on recall, as for Ro64-6198. When administered concurrently at subthreshold doses, Ro64-6198 and MK-801 synergistically suppressed hippocampal ERK activation and impaired long-term memory formation. Under resting conditions, neither Ro64-6198 nor MK-801 affected spontaneous ERK activity in the hippocampus at the amnesic doses whereas at higher doses, only MK-801 had a suppressive effect. We conclude that N/OFQ-NOP receptor system negatively regulates long-term recognition memory formation through hippocampal ERK signaling mechanisms. This modulation may in part take place by inhibiting glutamatergic function at the NMDA receptor.

Nociceptin/orphanin FQ reverses mecamylamine-induced learning and memory impairment as well as decrease in hippocampal acetylcholine release in the rat

Nociceptin/orphanin FQ is an endogenous neuropeptide that plays important roles in several physiological functions including pain, anxiety, locomotion, learning, and memory. We previously reported that low doses of nociceptin improved the scopolamine-induced impairment of learning and memory in the passive avoidance test and the spontaneous Y-maze alternation task in mice. In the present study, the effects of nociceptin on learning and memory impairment as well as the decrease in acetylcholine release induced by mecamylamine were investigated in rats. Mecamylamine (49 micromol/kg, s.c.), a nicotinic acetylcholine receptor antagonist, impaired learning and memory in the step-through type passive avoidance test and decreased acetylcholine release in the hippocampus, as determined by in vivo microdialysis. The administration of nociceptin (10 fmol/rat, i.c.v.) reversed the impairment of learning and memory and blocked the decrease in acetylcholine release induced by mecamylamine. This ameliorating effect on the mecamylamine-induced impairment of learning and memory was not blocked by [NPhe(1)]nociceptin(1-13)NH(2) (1 nmol/rat, i.c.v.), an opioid receptor-like 1 (NOP) receptor antagonist. These results suggest that nociceptin improves the impairment of learning and memory as well as decrease in acetylcholine release induced by mecamylamine, and that these effects may not be mediated by NOP receptors.

The orphanin FQ/nociceptin (OFQ/N) system

Orphanin FQ/nociceptin (OFQ/N) was the first novel neuropeptide discovered as the natural ligand of an orphan G protein-coupled receptor (GPCR). Orphan GPCRs are proteins classified as receptors on the basis of their sequence similarities to known GPCRs but that lack the ligands that activate them in vivo. One such orphan GPCR exhibited sequence similarities with the opioid receptors. OFQ/N was isolated as its natural ligand and shown to also share sequence similarities to the opioid peptides. This led to numerous studies attempting to find functional similarities and differences between the OFQ/N and opioid systems. This chapter will summarize our knowledge of the OFQ/N system and of its roles in the organism.

Presynaptic neuropeptide receptors

Presynaptic receptors for four families of neuropeptides will be discussed: opioids, neuropeptide Y, adrenocorticotropic hormone (ACTH), and orexins. Presynaptic receptors for the opioids (micro, delta, kappa, and ORL(1)) and neuropeptide Y (Y(2)) inhibit transmitter release from a variety of neurones, both in the peripheral and central nervous systems. These receptors, which were also identified in human tissue, are coupled to G(i/o) proteins and block voltage-dependent Ca(2+) channels, activate voltage-dependent K(+) channels, and/or interfere with the vesicle release machinery. Presynaptic receptors for ACTH (MC(2) receptors) have so far been identified almost exclusively in cardiovascular tissues from rabbits, where they facilitate noradrenaline release; they are coupled to G(s) protein and act via stimulation of adenylyl cyclase. Presynaptic receptors for orexins (most probably OX(2) receptors) have so far almost exclusively been identified in the rat and mouse brain, where they facilitate the release of glutamate and gamma-aminobutyric acid (GABA); they are most probably linked to G(q) and directly activate the vesicle release machinery or act via a transduction mechanism upstream of the release process. Agonists and antagonists at opioid receptors owe at least part of their therapeutic effects to actions on presynaptic receptors. Therapeutic drugs targeting neuropeptide Y and orexin receptors and presynaptic ACTH receptors so far are not available.

A new synthesis of the ORL-1 antagonist 1-[(3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidinyl]-3-ethyl-1,3-dihydro-2H-benzimidazol-2-one (J-113397) and activity in a calcium mobilization assay

A new chiral synthesis of the ORL-1 antagonist 1-[(3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidinyl]-3-ethyl-1,3-dihydro-2H-benzimidazol-2-one (2, J-113397) was developed. J-113397 has a K(e)=0.85nM in an ORL-1 calcium mobilization assay and is 89-, 887-, and 227-fold selective for the ORL-1 receptor relative to the mu, delta, and kappa opioid receptors.

Nocistatin attenuated the nociceptin induced c-Fos expression in the mouse hippocampus

Nocistatin and nociceptin/orphaninFQ (N/OFQ) are the two new peptides which may have roles in nociception, memory, anxiety, and other biological functions. Nocistatin acts as a functional antagonist to N/OFQ in several functions, but their neuro-anatomical sites of interaction are unknown. We investigated the effect of combined intracerebroventricular (i.c.v.) injection of nocistatin with N/OFQ, on N/OFQ induced c-Fos expression in the mouse hippocampus, using c-Fos immunohistochemistry. We found that co-injection of nocistatin with N/OFQ significantly attenuated N/OFQ induced c-Fos expression in the hippocampus.

Presynaptic inhibition of glutamate release by neuropeptides: use-dependent synaptic modification

Neuropeptides are signaling molecules that interact with G-protein coupled receptors located both pre- and postsynaptically. Presynaptically, these receptors are localized in axons and terminals away from presynaptic specializations. Neuropeptides are stored in dense core vesicles that are distinct from the clear synaptic vesicles containing classic neurotransmitters such as glutamate and GABA. Because they require a stronger Ca(2+) signal than synaptic vesicles, dense core vesicles do not release neuropeptides with single action potentials but rather require high-frequency trains. Thus, neuropeptides only modulate strongly stimulated synapses, providing negative or positive feedback. Many neuropeptides have been found to inhibit glutamate release from presynaptic terminals, and the major mechanism is likely direct interaction of betagamma G-protein subunits with presynaptic proteins such as SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor). The use of mouse genetic models and specific receptor antagonists are beginning to unravel the function of inhibitory neuropeptides. The opioid receptors kappa and mu, which are activated by endogenous opioid peptides such as dynorphin, enkephalin, and possibly the endomorphins, are important in modulating pain transmission. Dynorphin, nociceptin/orphanin FQ, and somatostatin and its related peptide cortistatin appear to play a role in modulation of learning and memory. Neuropeptide Y has important functions in ingestive behavior and also in entraining circadian rhythms. The existence of neuropeptides greatly expands the computational ability of the brain by providing additional levels of modulation.

The neuropeptide nociceptin is a synaptically released endogenous inhibitor of hippocampal long-term potentiation

Hippocampal long-term potentiation (LTP) of excitatory synaptic transmission has been regarded as a cellular model of learning and memory. Its induction is regulated by many functional molecules at synapses, including the neuropeptide nociceptin, which is identified as an endogenous ligand for the orphan opioid receptor. Mutant mice lacking the receptor exhibit enhanced LTP and hippocampus-dependent memory formation; however, the precise molecular and cellular mechanism is largely unknown. Here, we show that LTP in the hippocampal CA1 region is inhibited by nociceptin synaptically released from interneurons by tetanic stimulation. This endogenous nociceptin downregulates the excitability of pyramidal cells by the hyperpolarization induced by the activation of K(+) channels, which are the common target shared with GABA(B) receptors, although the mode of action is considerably different. Interestingly, the modulation of LTP by endogenous nociceptin is not observed when theta-burst stimulation is used instead of tetanic stimulation, suggesting that relatively longer high-frequency synaptic activation is required for the release of endogenous nociceptin. These results indicate that, in addition to GABA, nociceptin released from interneurons by their high-frequency activation is a novel endogenous neuromodulator that negatively regulates LTP induction in the hippocampus through direct modulation of pyramidal cells.

Enhanced hippocampal acetylcholine release in nociceptin-receptor knockout mice

Nociceptin (NOC), an endogenous ligand of the opioid receptor-like 1 receptor, is thought to be involved in learning and memory processes. Since acetylcholine (ACh) is involved in hippocampal function, and the hippocampus plays a critical role on the learning and memory function, hippocampal ACh release in NOC-receptor knockout mice was examined using an in vivo microdialysis method. The release of hippocampal ACh was largely increased in the knockout mice. Furthermore, in the knockout mice, an enhanced hippocampal theta rhythm, which is known to be linked to hippocampal memory function, was also observed. Immunohistochemically, in septum, co-existence of NOC receptor with cholinergic, but not with GABAergic neurons, was verified. The findings demonstrate that the NOC receptor is involved in hippocampal cholinergic function.