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

The interplay between Kynurenine and Nociceptin/Orphanin FQ pathways can be related to depressive-like phenotype

AIMS

This study aims to evaluate whether systemic administration of a nociceptin/orphanin FQ peptide (NOP) receptor agonist and exposure to the social defeat protocol are associated with upregulation of Kynurenine pathway. Additionally, we aim to investigate the potential association between NOP receptor-induced Kynurenine upregulation and markers of oxidative stress, neuroinflammation, and depressive-like behaviors.

METHODS

Behavioral tests were applied to evaluate depressive behavior in male Swiss mice submitted to social defeat protocol and treated with Ro 65-6570 (1.5 or 2 mg/kg; IP). Biochemical analyses were performed in the animals' brains and blood using spectrophotometry and ELISA. The software SPSS v.29 and R programming language were used to analyze the data.

RESULTS

High concentrations of Kynurenine were detected in the hippocampus, prefrontal cortex, and serum of animals treated with Ro 65-6570 and submitted to the social defeat protocol. In the prefrontal cortex, high doses of the agonist combined with the defeat protocol were associated with the Kynurenine upregulation. In the hippocampus, administration of the NOP agonist alone was associated with Kynurenine upregulation. Kynurenine levels were negatively correlated with the gain of weight, distance traveled, and interaction time of animals, as well as with IL-10 concentrations in the hippocampus, while Kynurenine levels were positively correlated with the immobility time of animals, IL-6 and carbonylated protein levels, as well as malondialdehyde in serum.

CONCLUSION

Neuroinflammatory and oxidative effects related to the NOP receptor activation may contribute to the Kynurenine upregulation and this indirect interaction between Nociceptin and Kynurenine pathways can be related to sub-chronic depressive phenotype.

Receptor-mediated Gi-3 activation in mammalian and human brain membranes: Reestablishment method and its application to nociceptin/orphanin FQ opioid peptide (NOP) receptor/Gi-3 interaction

Functional activation of heterotrimeric guanine nucleotide-binding proteins (G-proteins) via G-protein-coupled receptors (GPCRs) has been extensively explored using guanosine-5'-O-(3-[35S]thio)triphosphate ([35S]GTPγS) binding assay. However, the conventional method is primarily applicable to Gi/o family without discrimination among G-protein subtypes. Therefore, this study aims to reestablish a novel method termed "[35S]GTPγS binding/immunoprecipitation assay" by identifying a most suitable anti-Gαi-3 antibody instead of the previously utilized, now withdrawn antibody. In the initial screening of commercially available anti-Gαi-3 antibodies, two were identified and one was selected for further investigations based on efficacy with adenosine-the most potent agonist in our previous research. After optimizing experimental conditions with rat and postmortem human brain membranes, the stimulatory effects of various agonists were evaluated. Some agonists, including nociceptin, exhibited sufficient stimulatory effects for further pharmacological characterization. Nociceptin increased [35S]GTPγS binding to Gαi-3 in a concentration-dependent manner, response that was insensitive to naloxone but potently inhibited using (±)-J-113397. The method described in this study provides a valuable strategy for determining the intrinsic efficacy of ligands at various GPCRs. This includes nociceptin/orphanin FQ opioid peptide (NOP) receptor selectively coupled to Gαi-3, providing insights into the pharmacological concept of "functional selectivity."

Nociceptin/OrphaninFQ Receptor Modulates the Maturation of Adult-Born Neurons in the Mouse Dentate Gyrus Under Physiological Conditions and in a Chronic Stress Model

Neurogenesis persists in the adult dentate gyrus (DG) of the hippocampus, playing a critical role in memory and stress adaptation. Dysregulation of this process is implicated in cognitive deficits and depressive behaviors induced by chronic stress, while classical antidepressants are known to enhance neurogenesis. The Nociceptin/Orphanin FQ (N/OFQ) system, comprising N/OFQ and its NOP receptor, modulates memory and the stress response, yet its role in adult neurogenesis remains underexplored. Here, we investigated the impact of N/OFQ signaling on neurogenesis in the mouse DG using genetic and pharmacological approaches under basal and chronic stress conditions. In constitutive NOP receptor knockout (KO) mice, adult neurogenesis was only mildly altered, with subtle changes in neuronal maturation. However, spine density in 4-week-old adult-born DG neurons increased following conditional NOP Receptor KO in the DG. The increase was specific to stubby and thin spines, while mature mushroom spine density decreased. When NOP KO was restricted to newly born neurons, no significant differences were observed in spine density suggesting that the absence of NOP receptors in mature DG neurons influences the local environment to regulate spinogenesis in adult-born neurons indirectly. Finally, chronic corticosterone exposure impaired spinogenesis in immature neurons, and this was mitigated by systemic administration of a NOP antagonist. Our findings suggest that N/OFQ signaling indirectly regulates the maturation and connectivity of adult-born neurons through modulation of local and distal inputs. This regulation may contribute to the antidepressant and pro-cognitive effects of NOP receptor antagonists.

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

AIMS

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

MATERIALS AND METHODS

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

KEY FINDINGS

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

SIGNIFICANCE

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

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.

Naturally Inspired Molecules for Neuropathic Pain Inhibition-Effect of Mirogabalin and Cebranopadol on Mechanical and Thermal Nociceptive Threshold in Mice

BACKGROUND

Neuropathic pain is drug-resistant to available analgesics and therefore novel treatment options for this debilitating clinical condition are urgently needed. Recently, two drug candidates, namely mirogabalin and cebranopadol have become a subject of interest because of their potential utility as analgesics for chronic pain treatment. However, they have not been investigated thoroughly in some types of neuropathic pain, both in humans and experimental animals.

METHODS

This study used the von Frey test, the hot plate test and the two-plate thermal place preference test supported by image analysis and machine learning to assess the effect of intraperitoneal mirogabalin and subcutaneous cebranopadol on mechanical and thermal nociceptive threshold in mouse models of neuropathic pain induced by streptozotocin, paclitaxel and oxaliplatin.

RESULTS

Mirogabalin and cebranopadol effectively attenuated tactile allodynia in models of neuropathic pain induced by streptozotocin and paclitaxel. Cebranopadol was more effective than mirogabalin in this respect. Both drugs also elevated the heat nociceptive threshold in mice. In the oxaliplatin model, cebranopadol and mirogabalin reduced cold-exacerbated pain.

CONCLUSIONS

Since mirogabalin and cebranopadol are effective in animal models of neuropathic pain, they seem to be promising novel therapies for various types of neuropathic pain in patients, in particular those who are resistant to available analgesics.

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.

IUPHAR Review - Bivalent and bifunctional opioid receptor ligands as novel analgesics

Though efficacious in managing chronic, severe pain, opioid analgesics are accompanied by significant adverse effects including constipation, tolerance, dependence, and respiratory depression. The life-threatening risks associated with µ opioid receptor agonist-based analgesics challenges their use in clinic. A rational approach to combatting these adverse effects is to develop agents that incorporate activity at a second pharmacologic target in addition to µ opioid receptor activation. The promise of such bivalent or bifunctional ligands is the development of an analgesic with an improved side effect profile. In this review, we highlight ongoing efforts in the development of bivalent and bifunctional analgesics that combine µ agonism with efficacy at κ and δ opioid receptors, the nociceptin opioid peptide (NOP) receptor, σ receptors, and cannabinoid receptors. Several examples of bifunctional analgesics in preclinical and clinical development are highlighted, as are strategies being employed toward the rational design of novel agents.

Involvement of the Opioid Peptide Family in Cancer Progression

Peptides mediate cancer progression favoring the mitogenesis, migration, and invasion of tumor cells, promoting metastasis and anti-apoptotic mechanisms, and facilitating angiogenesis/lymphangiogenesis. Tumor cells overexpress peptide receptors, crucial targets for developing specific treatments against cancer cells using peptide receptor antagonists and promoting apoptosis in tumor cells. Opioids exert an antitumoral effect, whereas others promote tumor growth and metastasis. This review updates the findings regarding the involvement of opioid peptides (enkephalins, endorphins, and dynorphins) in cancer development. Anticancer therapeutic strategies targeting the opioid peptidergic system and the main research lines to be developed regarding the topic reviewed are suggested. There is much to investigate about opioid peptides and cancer: basic information is scarce, incomplete, or absent in many tumors. This knowledge is crucial since promising anticancer strategies could be developed alone or in combination therapies with chemotherapy/radiotherapy.

Neuromodulation in Parkinson's disease targeting opioid and cannabinoid receptors, understanding the role of NLRP3 pathway: a novel therapeutic approach

Parkinson's disease (PD) is a prevalent neurodegenerative disorder characterized by the progressive loss of dopaminergic neurons in the substantia nigra pars compacta, resulting in motor and non-motor symptoms. Although levodopa is the primary medication for PD, its long-term use is associated with complications such as dyskinesia and drug resistance, necessitating novel therapeutic approaches. Recent research has highlighted the potential of targeting opioid and cannabinoid receptors as innovative strategies for PD treatment. Modulating opioid transmission, particularly through activating µ (MOR) and δ (DOR) receptors while inhibiting κ (KOR) receptors, shows promise in preventing motor complications and reducing L-DOPA-induced dyskinesia. Opioids also possess neuroprotective properties and play a role in neuroprotection and seizure control. Similar to this, endocannabinoid signalling via CB1 and CB2 receptors influences the basal ganglia and may contribute to PD pathophysiology, making it a potential therapeutic target. In addition to opioid and cannabinoid receptor targeting, the NLRP3 pathway, implicated in neuroinflammation and neurodegeneration, emerges as another potential therapeutic avenue for PD. Recent studies suggest that targeting this pathway holds promise as a therapeutic strategy for PD management. This comprehensive review focuses on neuromodulation and novel therapeutic approaches for PD, specifically highlighting the targeting of opioid and cannabinoid receptors and the NLRP3 pathway. A better understanding of these mechanisms has the potential to enhance the quality of life for PD patients.

Antidepressant-like effect of tofisopam in mice: A behavioural, molecular docking and MD simulation study

BACKGROUND

Depression is a disease that affects millions of people worldwide, and the discovery and development of effective and safe antidepressant drugs is one of the important topics of psychopharmacology.

OBJECTIVES

In this study, it was aimed to investigate the antidepressant-like activity potential of tofisopam, an anxiolytic drug with 2,3-benzodiazepine structure, and to elucidate the pharmacological mechanisms mediating this effect.

METHODS

The antidepressant-like activity of tofisopam was investigated using tail suspension and modified forced swimming tests. Possible interactions of tofisopam with µ- and δ-opioid receptor subtypes were clarified by pharmacological antagonism, molecular docking and molecular dynamics simulation studies.

RESULTS

Tofisopam (50 and 100 mg/kg) significantly shortened the immobility time of mice in both the tail suspension and the modified forced swimming tests. The drug, at the same doses, prolonged the duration of swimming and climbing behaviours measured in modified forced swimming tests. A dosage of 25 mg/kg was ineffective. Mechanistic studies showed that the pretreatment with p-chlorophenylalanine methyl ester (serotonin synthesis inhibitor; 4 consecutive days, 100 mg/kg), α-methyl-para-tyrosine methyl ester (catecholamine synthesis inhibitor; 100 mg/kg), naloxonazine (selective µ-opioid receptor blocker, 7 mg/kg) and naltrindole (a selective δ-opioid receptor blocker, 0.99 mg/kg) abolished the anti-immobility effect induced by the 50 mg/kg dose of tofisopam in the tail suspension tests. Our in silico studies supported the behavioural findings that the antidepressant-like effect of tofisopam is mediated by μ- and δ-opioid receptors.

CONCLUSION

This study is the first to show that tofisopam has antidepressant-like activity mediated by the serotonergic, catecholaminergic and opioidergic systems.

PPL-138 (BU10038): A bifunctional NOP/mu partial agonist that reduces cocaine self-administration in rats

The search for new and effective treatments for cocaine use disorder (CUD) is a priority. We determined whether PPL-138 (BU10038), a compound with partial agonist activity at both nociceptin opioid peptide (NOP) and mu-opioid receptors, reduces cocaine consumption, reinstatement, and whether the compound itself produces reinforcing effects in rats. Using an intermittent access (IntA) cocaine self-administration procedure, we found that PPL-138 (0.1 and 0.3 mg/kg) effectively decreased the total number of cocaine infusions and burst-like cocaine intake in both male and female rats. Responses for food in an IntA model of food self-administration were not altered for either sex, although locomotor activity was increased in female but not male rats. Blockade of NOP receptors with the selective antagonist J-113397 (5 mg/kg) did not prevent the PPL-138-induced suppression of cocaine self-administration, whereas blockade of mu-opioid receptors by naltrexone (1 mg/kg) reversed such effect. Consistently, treatment with morphine (1, 3, and 10 mg/kg) dose-dependently reduced IntA cocaine self-administration measures. PPL-138 also reduced reinstatement of cocaine seeking at all doses examined. Although an initial treatment with PPL-138 (2.5, 10, and 40 μg/kg/infusion) appeared rewarding, the compound did not maintain self-administration behavior. Animals treated with PPL-138 showed initial suppression of cocaine self-administration, which was eliminated following repeated daily dosing. However, suppression of cocaine self-administration was retained when subsequent PPL-138 treatments were administered 48 h apart. These findings demonstrate that the approach of combining partial NOP/mu-opioid activation successfully reduces cocaine use, but properties of PPL-138 seem to depend on the timing of drug administration.