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

Targeting Nociceptin/Orphanin FQ receptor to rescue cognitive symptoms in a mouse neuroendocrine model of chronic stress

Chronic stress causes cognitive deficits, such as impairments in episodic-like hippocampus-dependent memory. Stress regulates an opioid-related neuropeptide named Nociceptin/Orphanin FQ (N/OFQ), the ligand of the G protein-coupled receptor NOP. Since this peptide has deleterious effects on memory, we hypothesized that the N/OFQ system could be a mediator of the negative effects of stress on memory. Chronic stress was mimicked by chronic exposure to corticosterone (CORT). The NOP receptor was either acutely blocked using selective antagonists, or knocked-down specifically in the hippocampus using genetic tools. Long-term memory was assessed in the object recognition (OR) and object location (OL) paradigms. Acute injection of NOP antagonists before learning had a negative impact on memory in naive mice whereas it restored memory performances in the chronic stress model. This rescue was associated with a normalization of neuronal cell activity in the CA3 part of the hippocampus. Chronic CORT induced an upregulation of the N/OFQ precursor in the hippocampus. Knock-down of the NOP receptor in the CA3/Dentate Gyrus region prevented memory deficits in the CORT model. These data demonstrate that blocking the N/OFQ system can be beneficial for long-term memory in a neuroendocrine model of chronic stress. We therefore suggest that NOP antagonists could be useful for the treatment of memory deficits in stress-related disorders.

Acute single non-sedative doses of NOP receptor agonists affect acquisition of object location memory but repeated high doses do not induce long-lasting deficits

The Nociceptin/Orphanin FQ (N/OFQ) system has been shown to modulate various aspects of long-term memory. It is therefore important to study the effects on memory impairment by nociceptin receptor (NOP) agonists under preclinical development. In the present study, we investigated the effect of systemic injection of two small molecule selective NOP agonists, AT-202 and AT-524, in the object location memory task in male and female mice. Since high doses of NOP agonists have been shown to induce sedation, we first determined the sedative doses for the two compounds and found them to be higher in female than in male mice. We then observed that sub-sedative doses of NOP agonists administered before learning, induced memory impairment during a test session performed 24 h later. Again, female mice were less sensitive to the amnesic effects than males. On the contrary, in male mice, NOP agonists did not produce amnesia when they were injected after learning, suggesting that they do not affect the consolidation of object location memory. Finally, repeated administration of high doses of NOP agonists over 7 days did not impair long-term spatial memory. Together, our data show for the first time that NOP receptor agonists impair the acquisition of object location memory with sex-dependent potency but do not affect memory consolidation, and that repeated stimulation of the receptor does not compromise long-term episodic-like spatial memory.

Endogenous and exogenous opioid effects on oligodendrocyte biology and developmental brain myelination

The elevated presence of opioid receptors and their ligands throughout the developing brain points to the existence of maturational functions of the endogenous opioid system that still remain poorly understood. The alarmingly increasing rates of opioid use and abuse underscore the urgent need for clear identification of those functions and the cellular bases and molecular mechanisms underlying their physiological roles under normal and pathological conditions. This review is focused on current knowledge on the direct and indirect regulatory roles that opioids may have on oligodendrocyte development and their generation of myelin, a complex insulating membrane that not only facilitates rapid impulse conduction but also participates in mechanisms of brain plasticity and adaptation. Information is examined in relation to the importance of endogenous opioid function, as well as direct and indirect effects of opioid analogues, which like methadone and buprenorphine are used in medication-assisted therapies for opioid addiction during pregnancy and pharmacotherapy in neonatal abstinence syndrome. Potential opioid effects are also discussed regarding late myelination of the brain prefrontal cortex in adolescents and young adults. Such knowledge is fundamental for the design of safer pharmacological interventions for opioid abuse, minimizing deleterious effects in the developing nervous system.

Fear expression is reduced after acute and repeated nociceptin/orphanin FQ (NOP) receptor antagonism in rats: therapeutic implications for traumatic stress exposure

RATIONALE

Evaluation of pharmacotherapies for acute stress disorder (ASD) or post-traumatic stress disorder (PTSD) is challenging due to robust heterogeneity of trauma histories and limited efficacy of any single candidate to reduce all stress-induced effects. Pursuing novel mechanisms, such as the nociceptin/orphanin FQ (NOP) system, may be a viable path for therapeutic development and of interest as it is involved in regulation of relevant behaviors and recently implicated in PTSD and ASD.

OBJECTIVES

First, we evaluated NOP receptor antagonism on general behavioral performance and again following a three-species predator exposure model (Experiment 1). Then, we evaluated effects of NOP antagonism on fear memory expression (Experiment 2).

METHODS

Adult, male rats underwent daily administration of NOP antagonists (J-113397 or SB-612,111; 0-20 mg/kg, i.p.) and testing in acoustic startle, elevated plus maze, tail-flick, and open field tests. Effects of acute NOP antagonism on behavioral performance following predator exposure were then assessed. Separately, rats underwent fear conditioning and were later administered SB-612,111 (0-3 mg/kg, i.p.) prior to fear memory expression tests.

RESULTS

J-113397 and SB-612,111 did not significantly alter most general behavioral performance measures alone, suggesting minimal off-target behavioral effects of NOP antagonism. J-113397 and SB-612,111 restored performance in measures of exploratory behavior (basic movements on the elevated plus maze and total distance in the open field) following predator exposure. Additionally, SB-612,111 significantly reduced freezing behavior relative to control groups across repeated fear memory expression tests, suggesting NOP antagonism may be useful in dampening fear responses. Other measures of general behavioral performance were not significantly altered following predator exposure.

CONCLUSIONS

NOP antagonists may be useful as pharmacotherapeutics for dampening fear responses to trauma reminders, and the present results provide supporting evidence for the implication of the NOP system in the neuropathophysiology of dysregulations in fear learning and memory processes observed in trauma- and stress-related disorders.

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.

Reproducible Genetic Risk Loci for Anxiety: Results From ∼200,000 Participants in the Million Veteran Program

OBJECTIVE

Anxiety disorders are common and often disabling. The goal of this study was to examine the genetic architecture of anxiety disorders and anxiety symptoms, which are also frequently comorbid with other mental disorders, such as major depressive disorder.

METHODS

Using one of the world's largest biobanks including genetic, environmental, and medical information, the Million Veteran Program, the authors performed a genome-wide association study (GWAS) of a continuous trait for anxiety (based on score on the Generalized Anxiety Disorder 2-item scale [GAD-2], N=199,611) as the primary analysis and self-report of physician diagnosis of anxiety disorder (N=224,330) as a secondary analysis.

RESULTS

The authors identified five genome-wide significant signals for European Americans and one for African Americans on GAD-2 score. The strongest were on chromosome 3 (rs4603973) near SATB1, a global regulator of gene expression, and on chromosome 6 (rs6557168) near ESR1, which encodes an estrogen receptor. The locus identified on chromosome 7 (rs56226325, MAF=0.17) near MAD1L1 was previously identified in GWASs of bipolar disorder and schizophrenia. The authors replicated these findings in the summary statistics of two major published GWASs for anxiety, and also found evidence of significant genetic correlation between the GAD-2 score results and previous GWASs for anxiety (r_g=0.75), depression (r_g=0.81), and neuroticism (r_g=0.75).

CONCLUSIONS

This is the largest GWAS of anxiety traits to date. The authors identified novel genome-wide significant associations near genes involved with global regulation of gene expression (SATB1) and the estrogen receptor alpha (ESR1). Additionally, the authors identified a locus (MAD1L1) that may have implications for genetic vulnerability across several psychiatric disorders. This work provides new insights into genetic risk mechanisms underpinning anxiety and related psychiatric disorders.

Endogenous opiates and behavior: 2017

This paper is the fortieth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2017 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

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.

Decreased Nociceptin Receptors Are Related to Resilience and Recovery in College Women Who Have Experienced Sexual Violence: Therapeutic Implications for Posttraumatic Stress Disorder

BACKGROUND

Posttraumatic stress disorder (PTSD) is a stress disorder that develops in only some individuals following a traumatic event. Data suggest that a substantial fraction of women recover after sexual violence. Thus, the investigation of stress and antistress neuropeptides in this sample has the potential to inform the neurochemistry of resilience following trauma. Nociceptin is an antistress neuropeptide in the brain that promotes resilience in animal models of PTSD.

METHODS

[¹¹C]NOP-1A positron emission tomography was used to measure the in vivo binding to nociceptin receptors in 18 college women who had experienced sexual violence irrespective of whether they met DSM-5 diagnostic criteria for PTSD. [¹¹C]NOP-1A data from 18 healthy control subjects were also included to provide a contrast with the sexual violence group. [¹¹C]NOP-1A total distribution volume (V_T) in the regions of interest were measured with kinetic analysis using the arterial input function. The relationships between regional V_T and Clinician-Administered PTSD Scale for DSM-5 total symptom and subscale severity were examined using correlational analyses.

RESULTS

No differences in [¹¹C]NOP-1A V_T were noted between the sexual violence and control groups. V_T in the midbrain and cerebellum were positively correlated with PTSD total symptom severity in the past month before positron emission tomography. Intrusion/re-experiencing and avoidance subscale symptoms drove this relationship. Stratification of subjects by a DSM-5 PTSD diagnosis and contrasting their V_T with that in control subjects showed no group differences.

CONCLUSIONS

Decreased midbrain and cerebellum nociceptin receptors are associated with less severe PTSD symptoms. Medications that target nociceptin should be explored to prevent and treat PTSD.

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.

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.

Therapeutic Approaches for NOP Receptor Antagonists in Neurobehavioral Disorders: Clinical Studies in Major Depressive Disorder and Alcohol Use Disorder with BTRX-246040 (LY2940094)

Conventional antidepressants increase the efflux of biogenic amine neurotransmitters (the monoamine hypothesis of depression) in the central nervous system (CNS) and are the principle drugs used to treat major depressive disorder (MDD). However, the lack of efficacy in some patients, the slow onset of action, and the side effect profiles of existing antidepressants necessitate the exploration of additional treatment options. The discovery of the nociceptin/orphanin FQ peptide NOP receptor (N/OFQ-NOP receptor) system and its characterization in preclinical biological and pharmacological stress-related conditions supports the potential antidepressant and anti-stress properties of a NOP receptor antagonist for the treatment of neurobehavioral disorders. BTRX-246040 (formerly LY2940094) was designed to test this hypothesis in the clinic. A small clinical proof of concept study demonstrated efficacy of BTRX-246040 in MDD patients. In this study, BTRX-246040 (40 mg, p.o.) significantly reduced negative bias as assessed by the facial recognition test within 1 week of treatment and decreased depression symptoms after 8 weeks. BTRX-246040 also reduced depression symptoms in a second trial with heavy alcohol drinkers. Given the comorbidity of MDD and alcohol use disorder, a compound with such effects in patients could be a valuable addition to the medications available. A proof of concept study showed efficacy of BTRX-246040 in reducing heavy drinking and increasing the probability of abstinence in individuals diagnosed with alcohol dependence. In addition, plasma levels of gamma-glutamyl transferase were decreased by BTRX-246040 compared to placebo control implying improvement in liver function. Collectively, the clinical data reviewed within this chapter suggest that BTRX-264040 functions to normalize dysfunction in reward circuits. The overall efficacy and safety of this compound with a novel mechanism of action are encouraging of further clinical development. BTRX-246040 is currently under development for MDD by BlackThorn Therapeutics.

Regulation of the Genes Encoding the ppN/OFQ and NOP Receptor

Over the years, the ability of N/OFQ-NOP receptor system in modulating several physiological functions, including the release of neurotransmitters, anxiety-like behavior responses, modulation of the reward circuitry, inflammatory signaling, nociception, and motor function, has been examined in several brain regions and at spinal level. This chapter collects information related to the genes encoding the ppN/OFQ and NOP receptor, their regulation, and relative transcriptional control mechanisms. Furthermore, genetic manipulations, polymorphisms, and epigenetic alterations associated with different pathological conditions are discussed. The evidence here collected indicates that the study of ppN/OFQ and NOP receptor gene expression may offer novel opportunities in the field of personalized therapies and highlights this system as a good "druggable target" for different pathological conditions.

HTLV-1 infection-induced motor dysfunction, memory impairment, depression, and brain tissues oxidative damage in female BALB/c mice

AIMS

The HTLV-1 infection is associated with a neuro-inflammatory disease. In the present study, the behavioral consequences and brain oxidative damages were evaluated in HTLV-1-infected BALB/c mice.

MATERIAL AND METHODS

20 female BALB/c mice were divided into two groups comprising control and HTLV-1-infected. The HTLV-1-infected group was inoculated with a 10⁶ MT-2 HTLV-1-infected cell line. Two months later, the behavioral tests were conducted. Finally, oxidative stress was assessed in the cortex and hippocampus tissues.

KEY FINDINGS

In the HTLV-1-infected group, running time and latency to fall, travel distance and time spent in the peripheral zone, total crossing number and total traveled distance in open field test, the latency of entrance into the dark compartment in the passive avoidance test, the new object exploration percentage, and discrimination ratio were significantly lower than in the control group. The immobility time, time spent in the dark compartment in passive avoidance test, and total exploration time significantly increased in the HTLV-1-infected group compared to the control group. In the cortical tissue of the HTLV-1 group, the malondialdehyde levels were elevated while the total thiol levels decreased in comparison to the control group. The activity of superoxide dismutase in the cortical and hippocampal tissues, and catalase activity in cortical tissue significantly decreased in the HTLV-1 group in comparison to the control group.

SIGNIFICANCE

The HTLV-1 infection seems to induce depression-like behavior, motor dysfunction, disruption in working and fear memory and also oxidative stress in the cortex and hippocampus.

The Nociceptin Receptor (NOP) Agonist AT-312 Blocks Acquisition of Morphine- and Cocaine-Induced Conditioned Place Preference in Mice

Treatment of drug addiction remains an unmet medical need due to the dearth of approved pharmacotherapies. There are no approved treatments for cocaine addiction, whereas the current opioid crisis has revealed the stark reality of the limited options to treat prescription and illicit opioid abuse. Preclinical studies in rodents and nonhuman primates have shown that orphanin FQ/nociceptin (N/OFQ), the endogenous ligand for the nociceptin opioid receptor (NOP) reduces the rewarding effects of several abused substances, including opioids, psychostimulants and alcohol. A few nonpeptide small-molecule NOP agonists have also shown efficacy in attenuating the rewarding effects of various abused drugs. We previously demonstrated that a high affinity small-molecule NOP agonist AT-312 selectively reduced the rewarding effects of ethanol in the conditioned place preference paradigm in mice. In the present study, we examined if AT-312 (3 mg/kg, i.p. or s.c. respectively), would alter the rewarding action of morphine (7.5 mg/kg, s.c.) or cocaine (15 mg/kg, i.p.). The effect of AT-312 on morphine- and cocaine-induced motor stimulation was also assessed on the conditioning days. The role of the NOP receptor in the effects of AT-312 was further confirmed by conducting the place conditioning experiments in NOP knockout mice and compared to their wild-type controls. Our results showed that AT-312 significantly reduced the acquisition of morphine and cocaine CPP in wild-type mice but not in mice lacking NOP receptors. AT-312 also suppressed morphine-induced and completely abolished cocaine-induced motor stimulation in NOP wild-type mice, but not in NOP knockout mice. These results show that small-molecule NOP receptor agonists have promising efficacy for attenuating the rewarding effects of morphine and cocaine, and may have potential as pharmacotherapy for opioid and psychostimulant addiction or for treating polydrug addiction.