Isolation and structure of the endogenous agonist of opioid receptor-like ORL1 receptor

Kappa-opioid receptor antagonism in the nucleus accumbens shell distinguishes escalated alcohol consumption and negative affective-like behavior from physiological withdrawal in alcohol-dependence

Alcohol use disorder (AUD) is a chronic relapsing disease that is deleterious at individual, familial, and societal levels. Although AUD is one of the highest preventable causes of death in the USA, therapies for the treatment of AUD are not sufficient given the heterogeneity of the disorder and the limited number of approved medications. To provide better pharmacological strategies, it is important to understand the neurological underpinnings of AUD. Evidence implicates the endogenous dynorphin (DYN)/κ-opioid receptor (KOR) system recruitment in dysphoric and negative emotional states in AUD to promote maladaptive behavioral regulation. The nucleus accumbens shell (AcbSh), mediating motivational and emotional processes that is a component of the mesolimbic dopamine system and the extended amygdala, is an important site related to alcohol's reinforcing actions (both positive and negative) and neuroadaptations in the AcbSh DYN/KOR system have been documented to induce maladaptive symptoms in AUD. We have previously shown that in other nodes of the extended amygdala, site-specific KOR antagonism can distinguish different symptoms of alcohol dependence and withdrawal. In the current study, we examined the role of the KOR signaling in the AcbSh of male Wistar rats in operant alcohol self-administration, measures of negative affective-like behavior, and physiological symptoms during acute alcohol withdrawal in alcohol-dependence. To induce alcohol dependence, rats were exposed to chronic intermittent ethanol vapor for 14 h/day for three months, during which stable escalation of alcohol self-administration was achieved and pharmacological AcbSh KOR antagonism ensued. The results showed that AcbSh KOR antagonism significantly reduced escalated alcohol intake and negative affective-like states but did not alter somatic symptoms of withdrawal. Understanding the relative contribution of these different drivers is important to understand and inform therapeutic efficacy approaches in alcohol dependence and further emphasis the importance of the KOR/DYN system as a target for AUD therapeutics.

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

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

Opioids and the kidney: two sides of the same coin

Renal dysfunction, including acute renal failure (ARF) and chronic kidney disease (CKD), continues to present significant health challenges, with renal ischemia-reperfusion injury (IRI) being a pivotal factor in their development and progression. This condition, notably impacting kidney transplantation outcomes, underscores the urgent need for innovative therapeutic interventions. The role of opioid agonists in this context, however, remains a subject of considerable debate. Current reviews tend to offer limited perspectives, focusing predominantly on either the protective or detrimental effects of opioids in isolation. Our review addresses this gap through a thorough and comprehensive evaluation of the existing literature, providing a balanced examination of the dualistic nature of opioids' influence on renal health. We delve into both the nephroprotective and nephrotoxic aspects of opioids, dissecting the complex interactions and paradoxical effects that embody the "two sides of the same coin" phenomenon. This comprehensive analysis is vital for understanding the intricate roles of opioids in renal pathophysiology, potentially informing the development of novel therapeutic strategies for preventing or treating hypoxic kidney injury.

Safety, Tolerability, and Pharmacokinetics of Single- and Multiple-Ascending Doses of Sunobinop in Healthy Participants

Sunobinop is an investigational, potent, selective partial agonist at the nociceptin/orphanin FQ peptide receptor in vitro. Three phase 1 studies were conducted to evaluate the safety, tolerability, and pharmacokinetics (PK) of escalating single- and multiple-dose administration of sunobinop in healthy participants. Study 1 was a randomized, double-blind, placebo-controlled, single-ascending dose study. Study 2 was a randomized, double-blind, placebo-controlled, multiple-ascending dose study. Study 3 was a randomized, open-label, single-dose, 4-way crossover study of oral and sublingual sunobinop comparing morning (AM) and bedtime (PM) administration. Seventy participants were included. Systemic exposure (peak plasma concentration [Cmax], area under the plasma concentration-time curve from time 0 to the time of last quantifiable concentration [AUC0-t], and area under the plasma concentration-time curve from time 0 extrapolated to infinity [AUCinf]) of sunobinop was characterized by dose proportionality from 0.6 to 2 mg and increased less than proportionally from 3 to 30 mg. The PKs of sunobinop were similar, regardless of AM or PM administration, for both the oral and sublingual formulations. The majority of absorbed sunobinop was excreted unchanged in the urine within 8 hours of dosing, thereby showing rapid elimination with no appreciable accumulation following 14 consecutive days of once-daily dosing and suggesting exclusive renal elimination. Most treatment-emergent adverse events (TEAEs) were mild in severity; 1 severe TEAE occurred and all TEAEs resolved by the end of the studies. Sunobinop was generally well-tolerated and safe across the range of doses evaluated and presents a clinical profile suitable for continued development.

Development of a genetically encoded sensor for probing endogenous nociceptin opioid peptide release

Nociceptin/orphanin-FQ (N/OFQ) is a recently appreciated critical opioid peptide with key regulatory functions in several central behavioral processes including motivation, stress, feeding, and sleep. The functional relevance of N/OFQ action in the mammalian brain remains unclear due to a lack of high-resolution approaches to detect this neuropeptide with appropriate spatial and temporal resolution. Here we develop and characterize NOPLight, a genetically encoded sensor that sensitively reports changes in endogenous N/OFQ release. We characterized the affinity, pharmacological profile, spectral properties, kinetics, ligand selectivity, and potential interaction with intracellular signal transducers of NOPLight in vitro. Its functionality was established in acute brain slices by exogeneous N/OFQ application and chemogenetic induction of endogenous N/OFQ release from PNOC neurons. In vivo studies with fibre photometry enabled direct recording of NOPLight binding to exogenous N/OFQ receptor ligands, as well as detection of endogenous N/OFQ release within the paranigral ventral tegmental area (pnVTA) during natural behaviors and chemogenetic activation of PNOC neurons. In summary, we show here that NOPLight can be used to detect N/OFQ opioid peptide signal dynamics in tissue and freely behaving animals.

Drug repurposing opportunities for chronic kidney disease

The development of targeted drugs for the early prevention and management of chronic kidney disease (CKD) is of great importance. However, the success rates and cost-effectiveness of traditional drug development approaches are extremely low. Utilizing large sample genome-wide association study data for drug repurposing has shown promise in many diseases but has not yet been explored in CKD. Herein, we investigated actionable druggable targets to improve renal function using large-scale Mendelian randomization and colocalization analyses. We combined two population-scale independent genetic datasets and validated findings with cell-type-dependent eQTL data of kidney tubular and glomerular samples. We ultimately prioritized two drug targets, opioid receptor-like 1 and F12, with potential genetic support for restoring renal function and subsequent treatment of CKD. Our findings explore the potential pathological mechanisms of CKD, bridge the gap between the molecular mechanisms of pathogenesis and clinical intervention, and provide new strategies in future clinical trials of CKD.

Role of Spinal Cholecystokinin Octapeptide, Nociceptin/Orphanin FQ, and Hemokinin-1 in Diabetic Allodynia

A complication of diabetes is neuropathic pain, which is difficult to control with medication. We have confirmed that neuropathic pain due to mechanical allodynia in diabetic mice is mediated by a characteristic neuropeptide in the spinal cord. We evaluated the strength of mechanical allodynia in mice using von Frey filaments. When mice were intravenously injected with streptozotocin, mechanical allodynia appeared 3 days later. Antibodies of representative neuropeptides were intrathecally (i.t.) administered to allodynia-induced mice 7 days after the intravenous administration of streptozotocin, and allodynia was reduced by anti-cholecystokinin octapeptide antibodies, anti-nociceptin/orphanin FQ antibodies, and anti-hemokinin-1 antibodies. In contrast, i.t.-administered anti-substance P antibodies, anti-somatostatin antibodies, and anti-angiotensin II antibodies did not affect streptozotocin-induced diabetic allodynia mice. Mechanical allodynia was attenuated by the i.t. administration of CCK-B receptor antagonists and ORL-1 receptor antagonists. The mRNA level of CCK-B receptors in streptozotocin-induced diabetic allodynia mice increased in the spinal cord, but not in the dorsal root ganglion. These results indicate that diabetic allodynia is caused by cholecystokinin octapeptide, nociceptin/orphanin FQ, and hemokinin-1 released from primary afferent neurons in the spinal cord that transmit pain to the brain via the spinal dorsal horn.

Endogenous opioids in the olfactory tubercle and their roles in olfaction and quality of life

Olfactory dysfunctions decrease daily quality of life (QOL) in part by reducing the pleasure of eating. Olfaction plays an essential role in flavor sensation and palatability. The decreased QOL due to olfactory dysfunction is speculated to result from abnormal neural activities in the olfactory and limbic areas of the brain, as well as peripheral odorant receptor dysfunctions. However, the specific underlying neurobiological mechanisms remain unclear. As the olfactory tubercle (OT) is one of the brain's regions with high expression of endogenous opioids, we hypothesize that the mechanism underlying the decrease in QOL due to olfactory dysfunction involves the reduction of neural activity in the OT and subsequent endogenous opioid release in specialized subregions. In this review, we provide an overview and recent updates on the OT, the endogenous opioid system, and the pleasure systems in the brain and then discuss our hypothesis. To facilitate the effective treatment of olfactory dysfunctions and decreased QOL, elucidation of the neurobiological mechanisms underlying the pleasure of eating through flavor sensation is crucial.

Development of a genetically encoded sensor for probing endogenous nociceptin opioid peptide release

Nociceptin/orphanin-FQ (N/OFQ) is a recently appreciated critical opioid peptide with key regulatory functions in several central behavioral processes including motivation, stress, feeding, and sleep. The functional relevance of N/OFQ action in the mammalian brain remains unclear due to a lack of high-resolution approaches to detect this neuropeptide with appropriate spatial and temporal resolution. Here we develop and characterize NOPLight, a genetically encoded sensor that sensitively reports changes in endogenous N/OFQ release. We characterized the affinity, pharmacological profile, spectral properties, kinetics, ligand selectivity, and potential interaction with intracellular signal transducers of NOPLight in vitro. Its functionality was established in acute brain slices by exogeneous N/OFQ application and chemogenetic induction of endogenous N/OFQ release from PNOC neurons. In vivo studies with fibre photometry enabled direct recording of NOPLight binding to exogenous N/OFQ receptor ligands, as well as detection of endogenous N/OFQ release within the paranigral ventral tegmental area (pnVTA) during natural behaviors and chemogenetic activation of PNOC neurons. In summary, we show here that NOPLight can be used to detect N/OFQ opioid peptide signal dynamics in tissue and freely behaving animals.

Developmental and adult striatal patterning of nociceptin ligand marks striosomal population with direct dopamine projections

Circuit influences on the midbrain dopamine system are crucial to adaptive behavior and cognition. Recent developments in the study of neuropeptide systems have enabled high-resolution investigations of the intersection of neuromodulatory signals with basal ganglia circuitry, identifying the nociceptin/orphanin FQ (N/OFQ) endogenous opioid peptide system as a prospective regulator of striatal dopamine signaling. Using a prepronociceptin-Cre reporter mouse line, we characterized highly selective striosomal patterning of Pnoc mRNA expression in mouse dorsal striatum, reflecting early developmental expression of Pnoc . In the ventral striatum, Pnoc expression was was clustered across the nucleus accumbens core and medial shell, including in adult striatum. We found that Pnoc tdTomato reporter cells largely comprise a population of dopamine receptor D1 ( Drd1 ) expressing medium spiny projection neurons localized in dorsal striosomes, known to be unique among striatal projections neurons for their direct innervation of midbrain dopamine neurons. These findings provide new understanding of the intersection of the N/OFQ system among basal ganglia circuits with particular implications for developmental regulation or wiring of striatal-nigral circuits.

Pharmacological blockage of NOP receptors decreases ventral tegmental area dopamine neuronal activity through GABAB receptor-mediated mechanism

The Nociceptin/Orphanin FQ (N/OFQ) peptide and its receptor NOP are highly expressed within several regions of the mesolimbic system, including the ventral tegmental area (VTA). Evidence indicates that the N/OFQ-NOP receptor system is involved in reward processing and historically it has been proposed that activation of NOP receptors attenuates the motivation for substances of abuse. However, recent findings demonstrated that drug self-administration and relapse to drug-seeking are also attenuated after administration of NOP receptor antagonists. Here, to shed light on the mechanisms through which NOP receptor blockers modulate these processes, we utilized ex vivo patch-clamp recordings to investigate the effect of the selective NOP receptor antagonist LY2817412 on VTA dopaminergic (DA) function in male rats. Results showed that, similar to the endogenous NOP receptor agonist N/OFQ, LY2817412 reduced the spontaneous basal firing discharge of VTA DA neurons. Consistently, we found that NOP receptors are expressed both in VTA DA and GABA cells and that LY2817412 slice perfusion increased GABA release onto VTA DA cells. Finally, in the attempt to dissect the role of postsynaptic and presynaptic NOP receptors, we tested the effect of N/OFQ and LY2817412 in the presence of GABA receptors blockers. Results showed that the effect of LY2817412 was abolished following pretreatment with GABABR, but not GABAAR, blockers. Conversely, inhibition of DA neuronal activity by N/OFQ was unaffected by blockade of GABA receptors. Altogether, these results suggest that both NOP receptor agonists and antagonists can decrease VTA DA neuronal activity, but through distinct mechanisms of action. The effect of NOP receptor antagonists occurs through a GABABR-mediated mechanism while NOP receptor agonists seem to act via a direct effect on VTA DA neurons.

Analgesic Peptides: From Natural Diversity to Rational Design

Pain affects one-third of the global population and is a significant public health issue. The use of opioid drugs, which are the strongest painkillers, is associated with several side effects, such as tolerance, addiction, overdose, and even death. An increasing demand for novel, safer analgesic agents is a driving force for exploring natural sources of bioactive peptides with antinociceptive activity. Since the G protein-coupled receptors (GPCRs) play a crucial role in pain modulation, the discovery of new peptide ligands for GPCRs is a significant challenge for novel drug development. The aim of this review is to present peptides of human and animal origin with antinociceptive potential and to show the possibilities of their modification, as well as the design of novel structures. The study presents the current knowledge on structure-activity relationship in the design of peptide-based biomimetic compounds, the modification strategies directed at increasing the antinociceptive activity, and improvement of metabolic stability and pharmacodynamic profile. The procedures employed in prolonged drug delivery of emerging compounds are also discussed. The work summarizes the conditions leading to the development of potential morphine replacements.

Pd(II)-Catalyzed β-C(sp3)-H Alkynylation of Alanine in Di- and Tripeptides with Asn as an Endogenous Directing Group

The introduction of alkyne moieties into peptides remains in demand as it represents a promising approach for further structural diversification of peptides. Herein, we describe the Pd(II)-catalyzed C(sp3)-H alkynylation of Ala-Asn-embedded di- and tripeptides using Asn as the endogenous lead group. In addition, a key building block for the glycopeptide Tyc4PG-14 and Tyc4PG-15 was produced by our methodology.

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.

Interactions Between Endogenous Opioids and the Immune System

The endogenous opioid system, which consists of opioid receptors and their ligands, is widely expressed in the nervous system and also found in the immune system. As a part of the body's defense machinery, the immune system is heavily regulated by endogenous opioid peptides. Many types of immune cells, including macrophages, dendritic cells, neutrophils, and lymphocytes are influenced by endogenous opioids, which affect cell activation, differentiation, proliferation, apoptosis, phagocytosis, and cytokine production. Additionally, immune cells also synthesize and secrete endogenous opioid peptides and participate peripheral analgesia. This chapter is structured into two sections. Part one focuses on immunoregulatory functions of central endogenous opioids; and part two describes how opioid peptide-containing immune cells participate in local analgesia.

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.

Discovery and structure-activity relationships (SAR) of a novel class of 2-substituted N-piperidinyl indole-based nociceptin opioid receptor ligands

The development of SAR around substituted N-piperidinyl indole-based nociceptin opioid receptor (NOP) ligands led to the discovery of a novel series of 2-substituted N-piperidinyl indoles that provide both selective NOP full agonists and bifunctional NOP full agonists-μ opioid (MOP) receptor partial agonists. 2-substituted N-piperidinyl indoles have improved potency at the NOP receptor and are NOP full agonists, compared to our previously reported 3-substituted N-piperidinyl indoles that are selective NOP partial agonists. SAR in this series of 2-substituted N-piperidinyl indoles shows that 2-substitution versus 3-substitution on the indole moiety affects their intrinsic activity and opioid receptor selectivity. Molecular docking of these 2-substituted N-piperidinyl indoles in an active-state NOP homology model and MOP receptor structures provides a rationale for the differences observed in the binding, functional profiles and selectivity of 2-substituted versus 3-substituted N-piperidinyl indoles.

The neurobiology of social play behaviour: Past, present and future

Social play behaviour is a highly energetic and rewarding activity that is of great importance for the development of brain and behaviour. Social play is abundant during the juvenile and early adolescent phases of life, and it occurs in most mammalian species, as well as in certain birds and reptiles. To date, the majority of research into the neural mechanisms of social play behaviour has been performed in male rats. In the present review we summarize studies on the neurobiology of social play behaviour in rats, including work on pharmacological and genetic models for autism spectrum disorders, early life manipulations and environmental factors that influence play in rats. We describe several recent developments that expand the field, and highlight outstanding questions that may guide future studies.

A Prospective Observational Study on the Role of Immunohistochemical Expression of Orphanin in Laryngeal Squamous Cell Carcinoma Recurrence

To date, histological biomarkers expressed by laryngeal cancer are poorly known. The identification of biomarkers associated with laryngeal squamous cell carcinoma (SCC), would help explain the tumorogenesis and prevent the possible recurrence of the lesion after treatment. For this reason, the aim of this study is to investigate, for the first time, the Orphanin expression in 48 human specimens of laryngeal SCC and evaluate its possible correlation with patients prognosis. We analyzed pathological specimens from 48 patients with laryngeal SCC to detect the presence of Orphanin by using an immunohistochemistry test. We compared the findings with healthy tissue acquired from patients who underwent surgery for mesenchymal benign tumours of the larynx. The specimens were stained with anti-Orphanin monoclonal antibodies. Results were processed through a computerised image analysis system to determine a scale of staining intensity. All the tumoural specimens examined showed a significant immunoreaction for Orphanin when compared with healthy tissues (p < 0.05) but with a different immune reactivity related to clinical-pathological features. A high Orphanin expression was not significantly related to Histological Grading (HG), TNM, and stage (p > 0.05). In the multivariate analysis, the Orphanin expression was significantly related only to the malignant recurrence (p < 0.05). Our study suggests that Orphanin could have a role in tumorigenesis by increasing the recurrence of cancer; therefore, it should be further explored as a possible biomarker for laryngeal cancer.

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.

Avian opioid peptides: evolutionary considerations, functional roles and a challenge to address critical questions

The present review considers the putative hormonal opioid peptides in birds. In birds and all other vertebrates, there are four opioid related genes encoding a series of peptides. These genes are, respectively, proenkephalin (PENK), prodynorphin (PDYN), pronociceptin (PNOC) and proopiomelanocortin (POMC). Proenkephalin (PENK) encodes Met- and Leu-enkephalin together with peptides containing met enkephalin motifs in birds, mammals and reptiles. Proopiomelanocortin (POMC) encodes β endorphin together with adrenocorticotropic hormone (ACTH), and melanocyte stimulating hormone (MSH). Prodynorphin (PDYN) encoding dynorphins A and B with α- and β-neoendorphins together intermediate polypeptides across the vertebrates. Pronociceptin (PNOC) encodes nociceptin together with possibly putative avian nocistatin and a non-opioid peptide derived from the C terminal of pronociceptin. There is a high degree of identity in the sequences of enkephalin peptides, dynorphin-A and B and nociceptin in birds and, to a less extent, across vertebrates. The opioid peptides exert effects related to pain together with other biological actions such as growth/development acting via a series of opioid receptors. What is unclear, particularly in birds, is the biological roles and interactions (additivity, antagonistic and synergistic) for the individual opioid peptides, the processing of the prohormones in different tissues and the physiological relevance of the different peptides and, particularly, of the circulating forms.

Nociceptin Receptor-Related Agonists as Safe and Non-addictive Analgesics

As clinical use of currently available opioid analgesics is often impeded by dose-limiting adverse effects, such as abuse liability and respiratory depression, new approaches have been pursued to develop safe, effective, and non-addictive pain medications. After the identification of the nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor more than 25 years ago, NOP receptor-related agonists have emerged as a promising target for developing novel and effective opioids that modulate the analgesic and addictive properties of mu-opioid peptide (MOP) receptor agonists. In this review, we highlight the effects of the NOP receptor-related agonists compared with those of MOP receptor agonists in experimental rodent and more translational non-human primate (NHP) models and the development status of key NOP receptor-related agonists as potential safe and non-addictive analgesics. Several lines of evidence demonstrated that peptidic and non-peptidic NOP receptor agonists produce potent analgesic effects by intrathecal delivery in NHPs. Moreover, mixed NOP/MOP receptor partial agonists (e.g., BU08028, BU10038, and AT-121) display potent analgesic effects when administered intrathecally or systemically, without eliciting adverse effects, such as respiratory depression, itch behavior, and signs of abuse liability. More importantly, cebranopadol, a mixed NOP/opioid receptor agonist with full efficacy at NOP and MOP receptors, produces robust analgesic efficacy with reduced adverse effects, conferring promising outcomes in clinical studies. A balanced coactivation of NOP and MOP receptors is a strategy that warrants further exploration and refinement for the development of novel analgesics with a safer and effective profile.

Chemical tools for the opioids

The opioids are potent and widely used pain management medicines despite also possessing severe liabilities that have fueled the opioid crisis. The pharmacological properties of the opioids primarily derive from agonism or antagonism of the opioid receptors, but additional effects may arise from specific compounds, opioid receptors, or independent targets. The study of the opioids, their receptors, and the development of remediation strategies has benefitted from derivatization of the opioids as chemical tools. While these studies have primarily focused on the opioids in the context of the opioid receptors, these chemical tools may also play a role in delineating mechanisms that are independent of the opioid receptors. In this review, we describe recent advances in the development and applications of opioid derivatives as chemical tools and highlight opportunities for the future.

Nociceptin/Orphanin FQ Opioid Peptide-Receptor Expression in the Endometriosis-Associated Nerve Fibers-Possible Treatment Option?

Endometriosis (EM) is a chronic inflammatory disease affecting millions of women worldwide. Chronic pelvic pain is one of the main problems of this condition, leading to quality-of-life impairment. Currently, available treatment options are not able to treat these women accurately. A better understanding of the pain mechanisms would be beneficial to integrate additional therapeutic management strategies, especially specific analgesic options. To understand pain in more detail, nociceptin/orphanin FQ peptide (NOP) receptor expression was analyzed in EM-associated nerve fibers (NFs) for the first time. Laparoscopically excised peritoneal samples from 94 symptomatic women (73 with EM and 21 controls) were immunohistochemically stained for NOP, protein gene product 9.5 (PGP9.5), substance P (SP), calcitonin gene-related peptide (CGRP), tyrosine hydroxylase (TH), and vasoactive intestinal peptide (VIP). Peritoneal NFs of EM patients and healthy controls were positive for NOP and often colocalized with SP-, CGRP-, TH-, and VIP-positive nerve fibers, suggesting that NOP is expressed in sensory and autonomic nerve fibers. In addition, NOP expression was increased in EM associate NF. Our findings highlight the potential of NOP agonists, particularly in chronic EM-associated pain syndromes and deserve further study, as the efficacy of NOP-selective agonists in clinical trials.

Recapitulating phenotypes of alcohol dependence via overexpression of Oprk1 in the ventral tegmental area of non-dependent TH::Cre rats

The dynorphin (DYN)/kappa-opioid receptor (KOR) system is involved in dysphoria and negative emotional states. Dysregulation of KOR function promotes maladaptive behavioral regulation during withdrawal associated with alcohol dependence. Mesolimbic dopaminergic (DA) projections from the ventral tegmental area (VTA) innervate the extended amygdala circuitry and presynaptic KORs attenuate DA in these regions leading to an excessive alcohol consumption and negative affective-like behavior, whereas mesocortical KOR-regulated DA projections have been implicated in executive function and decision-making. Thus, the neuroadaptations occurring in DYN/KOR systems are important aspects to consider for the development of personalized therapeutic solutions. Herein, we study the contribution of the VTA DA neuron Oprk1 (KOR gene) in excessive alcohol consumption, negative emotional state, and executive function. To do so, Oprk1 mRNA expression and KOR function were characterized to confirm alcohol dependence-induced dysregulation in the VTA. Then, a transgenic Cre-Lox rat model (male and female TH::Cre rats) was used to allow for conditional and inducible overexpression of Oprk1 in VTA DA neurons. The effect of this overexpression was evaluated on operant alcohol self-administration, negative emotional states, and executive function. We found that VTA Oprk1 overexpression recapitulates some phenotypes of alcohol dependence including escalated alcohol self-administration and depressive-like behavior. However, working memory performance was not impacted following VTA Oprk1 overexpression in TH::Cre rats. This supports the hypothesis that dysregulated KOR signaling within the mesolimbic DA system is an important contributor to symptoms of alcohol dependence and shows that understanding Oprk1-mediated contributions to alcohol use disorder (AUD) should be an important future goal.

Opioids, sleep, analgesia and respiratory depression: Their convergence on Mu (μ)-opioid receptors in the parabrachial area

Opioids provide analgesia, as well as modulate sleep and respiration, all by possibly acting on the μ-opioid receptors (MOR). MOR's are ubiquitously present throughout the brain, posing a challenge for understanding the precise anatomical substrates that mediate opioid induced respiratory depression (OIRD) that ultimately kills most users. Sleep is a major modulator not only of pain perception, but also for changing the efficacy of opioids as analgesics. Therefore, sleep disturbances are major risk factors for developing opioid overuse, withdrawal, poor treatment response for pain, and addiction relapse. Despite challenges to resolve the neural substrates of respiratory malfunctions during opioid overdose, two main areas, the pre-Bötzinger complex (preBötC) in the medulla and the parabrachial (PB) complex have been implicated in regulating respiratory depression. More recent studies suggest that it is mediation by the PB that causes OIRD. The PB also act as a major node in the upper brain stem that not only receives input from the chemosensory areas in medulla, but also receives nociceptive information from spinal cord. We have previously shown that the PB neurons play an important role in mediating arousal from sleep in response to hypercapnia by its projections to the forebrain arousal centers, and it may also act as a major relay for the pain stimuli. However, due to heterogeneity of cells in the PB, their precise roles in regulating, sleep, analgesia, and respiratory depression, needs addressing. This review sheds light on interactions between sleep and pain, along with dissecting the elements that adversely affects respiration.

Activation of the nociceptin/orphanin-FQ receptor promotes NREM sleep and EEG slow wave activity

Sleep/wake control involves several neurotransmitter and neuromodulatory systems yet the coordination of the behavioral and physiological processes underlying sleep is incompletely understood. Previous studies have suggested that activation of the Nociceptin/orphanin FQ (N/OFQ) receptor (NOPR) reduces locomotor activity and produces a sedation-like effect in rodents. In the present study, we systematically evaluated the efficacy of two NOPR agonists, Ro64-6198 and SR16835, on sleep/wake in rats, mice, and Cynomolgus macaques. We found a profound, dose-related increase in non-Rapid Eye Movement (NREM) sleep and electroencephalogram (EEG) slow wave activity (SWA) and suppression of Rapid Eye Movement sleep (REM) sleep in all three species. At the highest dose tested in rats, the increase in NREM sleep and EEG SWA was accompanied by a prolonged inhibition of REM sleep, hypothermia, and reduced locomotor activity. However, even at the highest dose tested, rats were immediately arousable upon sensory stimulation, suggesting sleep rather than an anesthetic state. NOPR agonism also resulted in increased expression of c-Fos in the anterodorsal preoptic and parastrial nuclei, two GABAergic nuclei that are highly interconnected with brain regions involved in physiological regulation. These results suggest that the N/OFQ-NOPR system may have a previously unrecognized role in sleep/wake control and potential promise as a therapeutic target for the treatment of insomnia.

The Downregulation of Opioid Receptors and Neuropathic Pain

Neuropathic pain (NP) refers to pain caused by primary or secondary damage or dysfunction of the peripheral or central nervous system, which seriously affects the physical and mental health of 7-10% of the general population. The etiology and pathogenesis of NP are complex; as such, NP has been a hot topic in clinical medicine and basic research for a long time, with researchers aiming to find a cure by studying it. Opioids are the most commonly used painkillers in clinical practice but are regarded as third-line drugs for NP in various guidelines due to the low efficacy caused by the imbalance of opioid receptor internalization and their possible side effects. Therefore, this literature review aims to evaluate the role of the downregulation of opioid receptors in the development of NP from the perspective of dorsal root ganglion, spinal cord, and supraspinal regions. We also discuss the reasons for the poor efficacy of opioids, given the commonness of opioid tolerance caused by NP and/or repeated opioid treatments, an angle that has received little attention to date; in-depth understanding might provide a new method for the treatment of NP.

The life and times of endogenous opioid peptides: Updated understanding of synthesis, spatiotemporal dynamics, and the clinical impact in alcohol use disorder

The opioid G-protein coupled receptors (GPCRs) strongly modulate many of the central nervous system structures that contribute to neurological and psychiatric disorders including pain, major depressive disorder, and substance use disorders. To better treat these and related diseases, it is essential to understand the signaling of their endogenous ligands. In this review, we focus on what is known and unknown about the regulation of the over two dozen endogenous peptides with high affinity for one or more of the opioid receptors. We briefly describe which peptides are produced, with a particular focus on the recently proposed possible synthesis pathways for the endomorphins. Next, we describe examples of endogenous opioid peptide expression organization in several neural circuits and how they appear to be released from specific neural compartments that vary across brain regions. We discuss current knowledge regarding the strength of neural activity required to drive endogenous opioid peptide release, clues about how far peptides diffuse from release sites, and their extracellular lifetime after release. Finally, as a translational example, we discuss the mechanisms of action of naltrexone (NTX), which is used clinically to treat alcohol use disorder. NTX is a synthetic morphine analog that non-specifically antagonizes the action of most endogenous opioid peptides developed in the 1960s and FDA approved in the 1980s. We review recent studies clarifying the precise endogenous activity that NTX prevents. Together, the works described here highlight the challenges and opportunities the complex opioid system presents as a therapeutic target.

Experiences and reflections about behavioral pain assays in laboratory animals

Pharmacological assays based on the measurement of nociceptive responses in laboratory animals are a fundamental tool to assess analgesic strategies. During our experience with this type of experiments, we have been repeatedly challenged by different concerns related to their interpretation or relevance. Although these subjects are frequently discussed in our lab, they do not usually find a place in research articles with original data, in which the focus on results seems mandatory. In the present manuscript we try to discuss as central issues some of these aspects that often cross transversally our research. We have gathered them in five topics inspired by the results obtained in our laboratory. The two initial sections are devoted to the influence of the behavioral method used to assess nociception on the results achieved, as well as to the possibility that data may be more easily accepted when obtained with standard methods than with alternative ones. The third topic is related to the difficulties encountered when working with a molecule that may evoke dual effects, acting as pronociceptive or antinociceptive depending on the dose. The fourth point deals with the situation in which a particular hyperalgesic reaction is related to several molecules but the single inhibition of only one of them can completely prevent it. Finally, the last issue is addressed to comment the impact in the progress of pain research of experiments performed in animal models of pathological settings.

Fixed Dose Versus Loose Dose: Analgesic Combinations

Combinations of drugs may be fixed (two or more entities in a single product) or loose (two or more agents taken together but as individual agents) to help address multimechanistic pain. The use of opioids plus nonopioids can result in lower opioid consumption without sacrificing analgesic benefits. Drug combinations may offer additive or synergistic benefits. A variety of fixed-dose combination products are available on the market such as diclofenac plus thiocolchicoside, acetaminophen and caffeine, acetaminophen and opioid, ibuprofen and acetaminophen, tramadol and acetaminophen, and others. Fixed-dose combination products offer predictable pharmacokinetics and pharmacodynamics, known adverse events, and can reduce the pill burden. However, they are limited to certain drug combinations and doses; loose dosing allows prescribers the versatility to meet individual patient requirements as well as the ability to titrate as needed. Not all drug combinations offer synergistic benefits, which depend on the drugs and their doses. Certain drugs offer dual mechanisms of action in a single molecule, such as tapentadol, and these may further be used in combination with other analgesics. New technology allows for co-crystal productions of analgesic agents which may further improve drug characteristics, such as bioavailability. Combination analgesics are important additions to the analgesic armamentarium and may offer important benefits at lower doses than monotherapy.

A nociceptive neuronal ensemble in the dorsomedial prefrontal cortex underlies pain chronicity

Pain chronicity involves unpleasant experience in both somatosensory and affective aspects, accompanied with the prefrontal cortex (PFC) neuroplastic alterations. However, whether specific PFC neuronal ensembles underlie pain chronicity remains elusive. Here we identify a nociceptive neuronal ensemble in the dorsomedial prefrontal cortex (dmPFC), which shows prominent reactivity to nociceptive stimuli. We observed that this ensemble shows distinct molecular characteristics and is densely connected to pain-related regions including basolateral amygdala (BLA) and lateral parabrachial nuclei (LPB). Prolonged chemogenetic activation of this nociceptive neuronal ensemble, but not a randomly transfected subset of dmPFC neurons, induces chronic pain-like behaviors in normal mice. By contrast, silencing the nociceptive dmPFC neurons relieves both pain hypersensitivity and anxiety in mice with chronic inflammatory pain. These results suggest the presence of specific dmPFC neuronal ensembles in processing nociceptive information and regulating pain chronicity.

Regulation of N-type calcium channels by nociceptin receptors and its possible role in neurological disorders

Activation of nociceptin opioid peptide receptors (NOP, a.k.a. opioid-like receptor-1, ORL-1) by the ligand nociceptin/orphanin FQ, leads to G protein-dependent regulation of Cav2.2 (N-type) voltage-gated calcium channels (VGCCs). This typically causes a reduction in calcium currents, triggering changes in presynaptic calcium levels and thus neurotransmission. Because of the widespread expression patterns of NOP and VGCCs across multiple brain regions, the dorsal horn of the spinal cord, and the dorsal root ganglia, this results in the alteration of numerous neurophysiological features. Here we review the regulation of N-type calcium channels by the NOP-nociceptin system in the context of neurological conditions such as anxiety, addiction, and pain.

A Crosstalk between the Cannabinoid Receptors and Nociceptin Receptors in Colitis-Clinical Implications

Inflammatory bowel diseases (IBD) refer to a group of gastrointestinal (GI) disorders with complex pathogenesis characterized by chronic intestinal inflammation with a variety of symptoms. Cannabinoid and nociceptin opioid receptors (NOPs) and their ligands are widely distributed in the GI tract. The nociceptin opioid receptor is a newly discovered member of the opioid receptor family with unique characteristics. Both cannabinoid and NOP systems exhibit antinociceptive and anti-inflammatory activity and contribute to maintaining proper motility, secretion and absorption in the GI tract. Furthermore, they influence high and low voltage calcium channels, which play a crucial role in the processing of pain, and share at least two kinases mediating their action. Among them there is NF-κB, a key factor in the regulation of inflammatory processes. Therefore, based on functional similarities between cannabinoid and nociceptin receptors and the anti-inflammatory effects exerted by their ligands, there is a high likelihood that there is an interaction between cannabinoid receptors 1 and 2 and the nociceptin receptor in colitis. In this review, we discuss potential overlaps between these two systems on a molecular and functional level in intestinal inflammation to create the basis for novel treatments of IBD.

Synthesis, Biological Activity and Molecular Docking of Chimeric Peptides Targeting Opioid and NOP Receptors

Recently, mixed opioid/NOP agonists came to the spotlight for their favorable functional profiles and promising outcomes in clinical trials as novel analgesics. This study reports on two novel chimeric peptides incorporating the fragment Tyr-c[D-Lys-Phe-Phe]Asp-NH2 (RP-170), a cyclic peptide with high affinity for µ and κ opioid receptors (or MOP and KOP, respectively), conjugated with the peptide Ac-RYYRIK-NH2, a known ligand of the nociceptin/orphanin FQ receptor (NOP), yielding RP-170-RYYRIK-NH2 (KW-495) and RP-170-Gly3-RYYRIK-NH2 (KW-496). In vitro, the chimeric KW-496 gained affinity for KOP, hence becoming a dual KOP/MOP agonist, while KW-495 behaved as a mixed MOP/NOP agonist with low nM affinity. Hence, KW-495 was selected for further in vivo experiments. Intrathecal administration of this peptide in mice elicited antinociceptive effects in the hot-plate test; this action was sensitive to both the universal opioid receptor antagonist naloxone and the selective NOP antagonist SB-612111. The rotarod test revealed that KW-495 administration did not alter the mice motor coordination performance. Computational studies have been conducted on the two chimeras to investigate the structural determinants at the basis of the experimental activities, including any role of the Gly3 spacer.

Endogenous opioid systems alterations in pain and opioid use disorder

Decades of research advances have established a central role for endogenous opioid systems in regulating reward processing, mood, motivation, learning and memory, gastrointestinal function, and pain relief. Endogenous opioid systems are present ubiquitously throughout the central and peripheral nervous system. They are composed of four families, namely the μ (MOPR), κ (KOPR), δ (DOPR), and nociceptin/orphanin FQ (NOPR) opioid receptors systems. These receptors signal through the action of their endogenous opioid peptides β-endorphins, dynorphins, enkephalins, and nociceptins, respectfully, to maintain homeostasis under normal physiological states. Due to their prominent role in pain regulation, exogenous opioids-primarily targeting the MOPR, have been historically used in medicine as analgesics, but their ability to produce euphoric effects also present high risks for abuse. The ability of pain and opioid use to perturb endogenous opioid system function, particularly within the central nervous system, may increase the likelihood of developing opioid use disorder (OUD). Today, the opioid crisis represents a major social, economic, and public health concern. In this review, we summarize the current state of the literature on the function, expression, pharmacology, and regulation of endogenous opioid systems in pain. Additionally, we discuss the adaptations in the endogenous opioid systems upon use of exogenous opioids which contribute to the development of OUD. Finally, we describe the intricate relationship between pain, endogenous opioid systems, and the proclivity for opioid misuse, as well as potential advances in generating safer and more efficient pain therapies.

Involvement of the opioidergic and nociceptinergic systems in the analgesic effects of novel nociceptin analogues after acute and chronic immobilization stress

Stress is known to exert an influence on neuroendocrine, autonomic, hormonal functioning. Various stress models have been reported to induce analgesia. This is a phenomenon, referred to as stress-induced analgesia (SIA). Nociceptin/Orphanin FQ(N/OFQ) is a heptadecapeptide that has been found to play a direct role on pain perception.

This study aimed to investigate the effects of novel nociceptin analogues on nociception after acute and chronic immobilization stress (CIS) and the involvement of the opioid and nociceptinergic systems in analgesic effects. Analgesic effects were examined by paw-pressure (PP) and hot-plate (HP) tests.

Our data showed that acute immobilization stress induced hypoalgesia. The analgesic effect was more pronounced in pain caused by a mechanical stimulus than by a thermal one. CIS attenuated the hyperalgesic effect of naloxone and JTC-801 for mechanical and thermal stimulation. The effects of the opioid system are more pronounced in acute immobilization stress, while the nociceptin mechanisms predominate after chronic stress.

Antagonism of N/OFQ attenuates externalization of β1-adrenergic receptor and ventricular arrhythmias in acute myocardial ischemia rat model

Nociceptin/orphanin FQ (N/OFQ) and adrenergic activations play roles in promoting cardiac arrhythmia in acute myocardial ischemia but whether N/OFQ and β1-adrenergic activities interact and how they interact in the arrhythmogenesis are still unknown. We designed this study to investigate the potential interaction of N/OFQ and β1-adrenergic activities and the underlying mechanism in arrhythmogenesis in acute myocardial ischemia. Ventricular arrhythmia was evaluated in anaesthetized rats following permanent coronary artery occlusion (CAO), in presence and absence of UFP-101 (a selective antagonist of N/OFQ receptor). The changes of β1-adrenergic receptor (β1-AR) in plasma membrane of cardiomyocytes were quantitatively evaluated and the relations with the alterations of phosphorylated Raf kinase inhibitor protein (p-RKIP) and phosphorylated connexin 43 (p-Cx43) were investigated. The ventricular arrhythmia was 59% less in the animals pre-treated with UFP-101 than the placebo-treated control (difference of means = -2.41; 95% confidence interval (CI) -2.84 to -1.99; P < 0.001). Meanwhile, p-RKIP and membrane β1-AR in the myocardium were downregulated by 59% and 24%, respectively (p-RKIP: difference of means = -6.91; 95% CI -8.38 to -5.45; P < 0.001; membrane β1-AR difference of means = -27.06; 95% CI -29.89 to -24.23; P < 0.001). Artificial upregulation of RKIP by didymin significant increased β1-AR in plasma membrane of the cardiomyocytes in the animals prone to ventricular arrhythmia. The findings may suggest that activation of N/OFQ receptor in acute myocardial ischemia induces upregulation of p-RKIP, externalization of β1-adrenergic receptor and downregulation of p-Cx43 in the cardiomyocytes, which promotes ventricular arrhythmia.

Cebranopadol as a Novel Promising Agent for the Treatment of Pain

Opioids are used to treat pain, but despite their effectiveness, they possess several side effects such as respiratory depression, tolerance and physical dependence. Cebranopadol has been evaluated as a solution to this problem. The compound acts on the mu opioid receptor and the nociceptin/orphanin receptor and these receptors co-activation can reduce opioid side-effects without compromising analgesia. In the present review, we have compiled information on the effects of cebranopadol, its pharmacokinetics, and clinical trials involving cebranopadol, to further explore its promise in pain management.

Current and Future Therapeutic Options in Pain Management: Multi-mechanistic Opioids Involving Both MOR and NOP Receptor Activation

Opioids are widely used in chronic pain management, despite major concerns about their risk of adverse events, particularly abuse, misuse, and respiratory depression from overdose. Multi-mechanistic opioids, such as tapentadol and buprenorphine, have been widely studied as a valid alternative to traditional opioids for their safer profile. Special interest was focused on the role of the nociceptin opioid peptide (NOP) receptor in terms of analgesia and improved tolerability. Nociceptin opioid peptide receptor agonists were shown to reinforce the antinociceptive effect of mu opioid receptor (MOR) agonists and modulate some of their adverse effects. Therefore, multi-mechanistic opioids involving both MOR and NOP receptor activation became a major field of pharmaceutical and clinical investigations. Buprenorphine was re-discovered in a new perspective, as an atypical analgesic and as a substitution therapy for opioid use disorders; and buprenorphine derivatives have been tested in animal models of nociceptive and neuropathic pain. Similarly, cebranopadol, a full MOR/NOP receptor agonist, has been clinically evaluated for its potent analgesic efficacy and better tolerability profile, compared with traditional opioids. This review overviews pharmacological mechanisms of the NOP receptor system, including its role in pain management and in the development of opioid tolerance. Clinical data on buprenorphine suggest its role as a safer alternative to traditional opioids, particularly in patients with non-cancer pain; while data on cebranopadol still require phase III study results to approve its introduction on the market. Other bifunctional MOR/NOP receptor ligands, such as BU08028, BU10038, and AT-121, are currently under pharmacological investigations and could represent promising analgesic agents for the future.

Challenges and new opportunities for detecting endogenous opioid peptides in reward

The endogenous opioid peptide system, comprised of enkephalins, endorphins, dynorphins, and nociceptin, is a highly complex neurobiological system. Opioid peptides are derived from four precursor molecules and undergo several processing events yielding over 20 unique opioid peptides. This diversity together with low in vivo concentration and complex processing and release dynamics has challenged research into each peptide's unique function. Despite the subsequent challenges in detecting and quantifying opioid peptides in vivo, researchers have pioneered several techniques to directly or indirectly assay the roles of opioid peptides during behavioral manipulations. In this review, we describe the limitations of the traditional techniques used to study the role of endogenous opioid peptides in food and drug reward and bring focus to the wealth of new techniques to measure endogenous opioid peptides in reward processing.

Endogenous Opioids and Their Role in Stem Cell Biology and Tissue Rescue

Opioids are considered the oldest drugs known by humans and have been used for sedation and pain relief for several centuries. Nowadays, endogenous opioid peptides are divided into four families: enkephalins, dynorphins, endorphins, and nociceptin/orphanin FQ. They exert their action through the opioid receptors (ORs), transmembrane proteins belonging to the super-family of G-protein-coupled receptors, and are expressed throughout the body; the receptors are the δ opioid receptor (DOR), μ opioid receptor (MOR), κ opioid receptor (KOR), and nociceptin/orphanin FQ receptor (NOP). Endogenous opioids are mainly studied in the central nervous system (CNS), but their role has been investigated in other organs, both in physiological and in pathological conditions. Here, we revise their role in stem cell (SC) biology, since these cells are a subject of great scientific interest due to their peculiar features and their involvement in cell-based therapies in regenerative medicine. In particular, we focus on endogenous opioids’ ability to modulate SC proliferation, stress response (to oxidative stress, starvation, or damage following ischemia–reperfusion), and differentiation towards different lineages, such as neurogenesis, vasculogenesis, and cardiogenesis.

New Insights on Metabolic and Genetic Basis of Migraine: Novel Impact on Management and Therapeutical Approach

Migraine is a hereditary disease, usually one-sided, sometimes bilateral. It is characterized by moderate to severe pain, which worsens with physical activity and may be associated with nausea and vomiting, may be accompanied by photophobia and phonophobia. The disorder can occur at any time of the day and can last from 4 to 72 h, with and without aura. The pathogenic mechanism is unclear, but extensive preclinical and clinical studies are ongoing. According to electrophysiology and imaging studies, many brain areas are involved, such as cerebral cortex, thalamus, hypothalamus, and brainstem. The activation of the trigeminovascular system has a key role in the headache phase. There also appears to be a genetic basis behind the development of migraine. Numerous alterations have been identified, and in addition to the genetic cause, there is also a close association with the surrounding environment, as if on the one hand, the genetic alterations may be responsible for the onset of migraine, on the other, the environmental factors seem to be more strongly associated with exacerbations. This review is an analysis of neurophysiological mechanisms, neuropeptide activity, and genetic alterations that play a fundamental role in choosing the best therapeutic strategy. To date, the goal is to create a therapy that is as personalized as possible, and for this reason, steps forward have been made in the pharmacological field in order to identify new therapeutic strategies for both acute treatment and prophylaxis.

Therapeutic potential of nociceptin/orphanin FQ peptide (NOP) receptor modulators for treatment of traumatic brain injury, traumatic stress, and their co-morbidities

The nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor is a member of the opioid receptor superfamily with N/OFQ as its endogenous agonist. Wide expression of the NOP receptor and N/OFQ, both centrally and peripherally, and their ability to modulate several biological functions has led to development of NOP receptor modulators by pharmaceutical companies as therapeutics, based upon their efficacy in preclinical models of pain, anxiety, depression, Parkinson's disease, and substance abuse. Both posttraumatic stress disorder (PTSD) and traumatic brain injury (TBI) are debilitating conditions that significantly affect the quality of life of millions of people around the world. PTSD is often a consequence of TBI, and, especially for those deployed to, working and/or living in a war zone or are first responders, they are comorbid. PTSD and TBI share common symptoms, and negatively influence outcomes as comorbidities of the other. Unfortunately, a lack of effective therapies or therapeutic agents limits the long term quality of life for either TBI or PTSD patients. Ours, and other groups, demonstrated that PTSD and TBI preclinical models elicit changes in the N/OFQ-NOP receptor system, and that administration of NOP receptor ligands alleviated some of the neurobiological and behavioral changes induced by brain injury and/or traumatic stress exposure. Here we review the past and most recent progress on understanding the role of the N/OFQ-NOP receptor system in PTSD and TBI neurological and behavioral sequelae. There is still more to understand about this neuropeptide system in both PTSD and TBI, but current findings warrant further examination of the potential utility of NOP modulators as therapeutics for these disorders and their co-morbidities. We advocate the development of standards for common data elements (CDE) reporting for preclinical PTSD studies, similar to current preclinical TBI CDEs. That would provide for more standardized data collection and reporting to improve reproducibility, interpretation and data sharing across studies.

Contributions of the International Narcotics Research Conference to Opioid Research Over the Past 50 years

The International Narcotics Research Conference (INRC), founded in 1969, has been a successful forum for research into the actions of opiates, with an annual conference since 1971. Every year, scientists from around the world have congregated to present the latest data on novel opiates, opiate receptors and endogenous ligands, mechanisms of analgesic activity and unwanted side effects, etc. All the important discoveries in the opiate field were discussed, often first, at the annual INRC meeting. With an apology to important events and participants not discussed, this review presents a short history of INRC with a discussion of groundbreaking discoveries in the opiate field and the researchers who presented from the first meeting up to the present.

Lack of effect of the nociceptin opioid peptide agonist Ro 64-6198 on pain-depressed behavior and heroin choice in rats

RATIONALE AND OBJECTIVE

One objective of the National Institutes of Health Helping to End Addiction Long-term (HEAL) initiative is to accelerate research on safer and more effective medications for both pain and opioid use disorder. Ligands that activate the nociceptin opioid peptide receptor (NOP) constitute one class of candidate drugs for both applications. The present preclinical study determined the effectiveness of the NOP agonist Ro 64-6198 to produce antinociception in a pain-depressed behavior procedure and attenuate opioid self-administration in a heroin-vs-food choice procedure.

METHODS

In Experiment 1, Adult Sprague-Dawley rats were equipped with microelectrodes and trained to respond for electrical brain stimulation in an intracranial self-stimulation (ICSS) procedure. The potency, time course, and receptor mechanism of effects produced by R0 64-6198 alone (0.32-3.2 mg/kg) on ICSS were examined, followed by evaluation of 0.32-1.0 mg/kg Ro 64-6198 effectiveness to block lactic acid-induced depression of ICSS. In Experiment 2, rats self-administered heroin under a heroin-vs-food choice procedure during a regimen of repeated, daily intraperitoneal administration of vehicle or Ro 64-6198 (1-3.2 mg/kg/day).

RESULTS

Ro 64-6198 produced dose- and time-dependent ICSS depression that was blocked by the selective NOP antagonist SB612111 but not by naltrexone. Ro 64-6198 failed to block acid-induced depression of ICSS. Repeated Ro 64-6198 pretreatment also failed to attenuate heroin-vs-food choice up to doses that significantly decreased operant behavior.

CONCLUSIONS

These results do not support the utility of Ro 64-6198 as a stand-alone medication for either acute pain or opioid use disorder.

MOP and NOP receptor interaction: Studies with a dual expression system and bivalent peptide ligands

Opioids targeting mu;μ (MOP) receptors produce analgesia in the peri-operative period and palliative care. They also produce side effects including respiratory depression, tolerance/dependence and addiction. The N/OFQ opioid receptor (NOP) also produces analgesia but is devoid of the major MOP side effects. Evidence exists for MOP-NOP interaction and mixed MOP-NOP ligands produce analgesia with reduced side effects. We have generated a HEKMOP/NOP human expression system and used bivalent MOP-NOP and fluorescent ligands to (i) probe for receptor interaction and (ii) consequences of that interaction. We used HEKMOP/NOP cells and two bivalent ligands; Dermorphin-N/OFQ (MOP agonist-NOP agonist; DeNO) and Dermorphin-UFP101 (MOP agonist-NOP antagonist; De101). We have determined receptor binding profiles, GTPγ[35S] binding, cAMP formation and ERK1/2 activation. We have also probed MOP and NOP receptor interactions in HEK cells and hippocampal neurones using the novel MOP fluorescent ligand, DermorphinATTO488 and the NOP fluorescent ligand N/OFQATTO594. In HEKMOP/NOP MOP ligands displaced NOP binding and NOP ligands displaced MOP binding. Using fluorescent probes in HEKMOP/NOP cells we demonstrated MOP-NOP probe overlap and a FRET signal indicating co-localisation. MOP-NOP were also co-localised in hippocampal tissue. In GTPγ[35S] and cAMP assays NOP stimulation shifted the response to MOP rightwards. At ERK1/2 the response to bivalent ligands generally peaked later. We provide evidence for MOP-NOP interaction in recombinant and native tissue. NOP activation reduces responsiveness of MOP activation; this was shown with conventional and bivalent ligands.