A review of the role of orexin system in pain modulation

The involvement of orexin-1 receptors in modulation of feeding and anxiety-like behavior in rats with complete Freund's adjuvant-induced temporomandibular joint disorder

Orexin-A (OXA), a neuropeptide produced in the hypothalamus, is recognized for its role in modulating orofacial nociception and regulating feeding behaviors, as well as its impact on psychophysiological responses. This study investigated the role of orexin-1 receptors (OX1R) in modulating nociceptive behaviors induced by noxious stimulation of the temporomandibular joint (TMJ) and the associated changes in mood and feeding behaviors in rats with complete Freund's adjuvant (CFA)-induced temporomandibular disorders (TMDs). Bilateral cannulation of the lateral ventricles was performed in rats. To induce nociception, CFA was injected unilaterally into the left TMJ of the rats. Nociceptive behaviors were assessed using the hot plate and tail flick tests, while anxiety-like behavior and food intake were evaluated using an elevated plus maze (EPM) and a food preference device, respectively. The results demonstrated a significant increase in nociceptive scores and anxiety-like behaviors, along with reductions in water and food consumption following CFA injection. However, post-treatment with OXA at concentrations of 50 and 100 pM/rat significantly decreased thermal nociceptive scores, alleviated anxiety-like behavior, and increased water and food intake. These beneficial effects were reversed when OXA was co-administered with SB-334867 (40 nM/rat), an OX1R antagonist. Collectively, our findings suggest that OX1R signaling plays a role in the modulation of anxiety-like behavior and abnormalities in food intake in CFA-treated rats. Understanding the involvement of OXA and its receptors in CFA-induced TMJ nociception and behavioral changes may pave the way for potential therapeutic interventions targeting OX1R signaling in the management of TMD-associated symptoms.

Orexinergic projections to substantia innominata mediate arousal and analgesia

Understanding neural circuits involved in anesthesia is crucial for improving its safety and efficacy. Hypothalamic orexin neurons (LHA OX ), projecting broadly, are essential in regulating arousal and pain. However, the precise targets remain unclear. Here we investigated the orexin projections to the substantia innominata (SI). Combining optogenetics, fiber photometry, and EEG/EMG allowed us to manipulate orexin activities, while simultaneously recording local ligand release and global cortical activities during anesthesia. Brain slice electrophysiology revealed the synaptic connections in the SI, while RNAscope was employed to examine the distribution of orexin receptors and downstream neuronal types. Presynaptic vesicles were identified in the orexin terminals in the SI, where 49.16% of cells expressed OX2R and 6.8% expressed OX1R. Orexin release in the SI was reversibly suppressed by isoflurane. Optogenetic activation of the LHA OX →SI circuit significantly increased orexin release and promoted arousal from various anesthesia stages, including reanimation during 0.75% isoflurane (p < 0.0001), prolongation of 3% isoflurane induction (p = 0.0033), and acceleration of emergence from 2% isoflurane (p < 0.0001). Furthermore, activating this circuit induced analgesia to both thermal (p = 0.0074) and inflammatory (p = 0.0127) pain. Patch-clamp recordings revealed that optogenetic activation of orexin terminals in the SI elicited excitatory postsynaptic currents, which were blocked by the OX2R antagonist. SI contains more GABAergic (28.17%) and glutamatergic (11.96%) neurons than cholinergic neurons (4.13%), all of which expressed OX2R. Thus, LHA OX neurons innervate SI neurons to regulate both arousal and pain predominantly through OX2R.

Restraint stress-induced antinociceptive effects in acute pain: Involvement of orexinergic system in the nucleus accumbens

The complicated relevance between stress and pain has been identified. Neurotransmitters and neuropeptides of various brain areas play a role in this communication. Pain inhibitory response is known as stress-induced analgesia (SIA). The studies demonstrated that the nucleus accumbens (NAc) is critical in modulating pain. As a neuropeptide, orexin is crucially involved in initiating behavioral and physiological responses to threatening and unfeeling stimuli. However, the role of the orexin receptors of the NAc area after exposure to restraint stress (RS) as acute physical stress in the modulation of acute pain is unclear. One hundered twenty adult male albino Wistar rats (230-250 g) were used. Animals were unilaterally implanted with cannulae above the NAc. The SB334867 and TCS OX2 29 were used as antagonists for OX1r and OX2r, respectively. Different doses of the antagonists (1, 3, 10, and 30 nmol/0.5 µl DMSO) were microinjected intra-NAc five minutes before exposure to RS (3 hours). Then, the tail-flick test as a model of acute pain was performed, and the nociceptive threshold (Tail-flick latency; TFL) was measured in 60-minute time set intervals. According to this study's findings, the antinociceptive effects of RS in the tail-flick test were blocked during intra-NAc administration of SB334867 or TCS OX2 29. The RS as acute stress increased TFL and deceased pain-like behavior responses. The 50 % effective dose values of the OX1r and OX2r antagonists were 12.82 and 21.64 nmol, respectively. The result demonstrated contribution of the OX1r into the NAc was more remarkable than that of the OX2r on antinociceptive responses induced by the RS. Besides, in the absence of RS, the TFL was attenuated. The current study's data indicated that OX1r and OX2r into the NAc induced pain modulation responses during RS in acute pain. In conclusion, the findings revealed the involvement of intra-NAc orexin receptors in improving SIA.

Pain-Insomnia-Depression Syndrome: Triangular Relationships, Pathobiological Correlations, Current Treatment Modalities, and Future Direction

Pain-insomnia-depression syndrome (PIDS) is a complex triad of chronic pain, insomnia, and depression that has profound effects on an individual's quality of life and mental health. The pathobiological context of PIDS involves complex neurobiological and physiological mechanisms, including alterations in neurotransmitter systems and impaired pain processing pathways. The first-line therapeutic approaches for the treatment of chronic pain, depression, and insomnia are a combination of pharmacological and non-pharmacological therapies. In cases where patients do not respond adequately to these treatments, additional interventions such as deep brain stimulation (DBS) may be required. Despite advances in understanding and treatment, there are still gaps in knowledge that need to be addressed. To improve our understanding, future research should focus on conducting longitudinal studies to uncover temporal associations, identify biomarkers and genetic markers associated with PIDS, examine the influence of psychosocial factors on treatment responses, and develop innovative interventions that address the complex nature of PIDS. The aim of this study is to provide a comprehensive overview of these components and to discuss their underlying pathobiological relationships.

Orexin receptor 2 agonist activates diaphragm and genioglossus muscle through stimulating inspiratory neurons in the pre-Bötzinger complex, and phrenic and hypoglossal motoneurons in rodents

Orexin-mediated stimulation of orexin receptors 1/2 (OX[1/2]R) may stimulate the diaphragm and genioglossus muscle via activation of inspiratory neurons in the pre-Bötzinger complex, which are critical for the generation of inspiratory rhythm, and phrenic and hypoglossal motoneurons. Herein, we assessed the effects of OX2R-selective agonists TAK-925 (danavorexton) and OX-201 on respiratory function. In in vitro electrophysiologic analyses using rat medullary slices, danavorexton and OX-201 showed tendency and significant effect, respectively, in increasing the frequency of inspiratory synaptic currents of inspiratory neurons in the pre-Bötzinger complex. In rat medullary slices, both danavorexton and OX-201 significantly increased the frequency of inspiratory synaptic currents of hypoglossal motoneurons. Danavorexton and OX-201 also showed significant effect and tendency, respectively, in increasing the frequency of burst activity recorded from the cervical (C3-C5) ventral root, which contains axons of phrenic motoneurons, in in vitro electrophysiologic analyses from rat isolated brainstem-spinal cord preparations. Electromyogram recordings revealed that intravenous administration of OX-201 increased burst frequency of the diaphragm and burst amplitude of the genioglossus muscle in isoflurane- and urethane-anesthetized rats, respectively. In whole-body plethysmography analyses, oral administration of OX-201 increased respiratory activity in free-moving mice. Overall, these results suggest that OX2R-selective agonists enhance respiratory function via activation of the diaphragm and genioglossus muscle through stimulation of inspiratory neurons in the pre-Bötzinger complex, and phrenic and hypoglossal motoneurons. OX2R-selective agonists could be promising drugs for various conditions with respiratory dysfunction.

Orexins and primary headaches: an overview of the neurobiology and clinical impact

INTRODUCTION

Primary headaches, including migraines and cluster headaches, are highly prevalent disorders that significantly impact quality of life. Several factors suggest a key role for the hypothalamus, including neuroimaging studies, attack periodicity, and the presence of altered homeostatic regulation. The orexins are two neuropeptides synthesized almost exclusively in the lateral hypothalamus with widespread projections across the central nervous system. They are involved in an array of functions including homeostatic regulation and nociception, suggesting a potential role in primary headaches.

AREAS COVERED

This review summarizes current knowledge of the neurobiology of orexins, their involvement in sleep-wake regulation, nociception, and functions relevant to the associated symptomology of headache disorders. Preclinical reports of the antinociceptive effects of orexin-A in preclinical models are discussed, as well as clinical evidence for the potential involvement of the orexinergic system in headache.

EXPERT OPINION

Several lines of evidence support the targeted modulation of orexinergic signaling in primary headaches. Critically, orexins A and B, acting differentially via the orexin 1 and 2 receptors, respectively, demonstrate differential effects on trigeminal pain processing, indicating why dual-receptor antagonists failed to show clinical efficacy. The authors propose that orexin 1 receptor agonists or positive allosteric modulators should be the focus of future research.

The Role of Orexin Receptor Antagonists in Inhibiting Drug Addiction: A Review Article

The orexinergic system and its receptors are involved in many physiological processes. Their functions in energy homeostasis, arousal, cognition, stress processing, endocrine functions, and pain modulation have been investigated. Many studies have shown that the orexinergic system cooperates with the dopaminergic system in the addiction process. Emerging evidence suggests that the orexinergic system can be effective in the induction of drug dependence and tolerance. Therefore, several researches have been conducted on the effect of orexin receptor (OXR) antagonists on reducing tolerance and dependence caused by drug abuse. Due to the significant growth of the studies on the orexinergic system, the current literature was conducted to collect the findings of previous studies on orexin and its receptors in the induction of drug addiction. In addition, cellular and molecular mechanisms of the possible role of orexin in drug tolerance and dependence are discussed. The findings indicate that the administration of OXR antagonists reduces drug dependence. OXR blockers seem to counteract the addictive effects of drugs through multiple mechanisms, such as preventing neuronal adaptation. This review proposes the potential clinical use of OXR antagonists in the treatment of drug dependence.

Contribution of the intra-hippocampal orexin system in the regulation of restraint stress response to pain-related behaviors in the formalin test

Stress-induced antinociception (SIA) is due to the activation of several neural pathways and neurotransmitters that often suppress pain perception. Studies have shown that the orexin neuropeptide system is essential in pain modulation. Therefore, this study aimed to investigate the role of orexinergic receptors in the hippocampal CA1 region in modulating SIA response during the formalin test as an animal model of inflammatory pain. The orexin-1 receptor (OX1r) antagonist, SB334867, at 1, 3, 10, and 30 nmol or TCS OX2 29 as an orexin-2 receptor (OX2r) antagonist at the same doses were microinjected into the CA1 region in rats. Five minutes later, rats were exposed to restraint stress (RS) for 3 h, and pain-related behaviors were monitored in 5-min blocks for the 60-min test period in the formalin test. Results showed that applying RS for 3 h reduced pain responses in the early and late phases of the formalin test. The main findings showed that intra-CA1 injection of orexin receptor antagonists reduced the antinociception caused by stress in both phases of the formalin test. In addition, the contribution of OX2r in mediating the antinociceptive effect of stress was more prominent than that of OX1r in the early phase of the formalin test. However, in the late phase, both receptors worked similarly. Accordingly, the orexin system and its two receptors in the CA1 region of the hippocampus regulate SIA response to this animal model of pain in formalin test.

Intra-accumbal orexinergic system contributes to the stress-induced antinociceptive behaviors in the animal model of acute pain in rats

Stress and pain are interleaved at numerous levels - influencing each other. Stress can increase the nociception threshold in animals, long-known as stress-induced analgesia (SIA). Orexin is known as a neuropeptide that modulates pain. The effect of stress on the mesolimbic system in the modulation of pain is known. The role of the intra-accumbal orexin receptors in the modulation of acute pain by forced swim stress (FSS) is unclear. In this study, 117 adult male albino Wistar rats (270-300 g) were used. The animals were unilaterally implanted with cannulae above the NAc. The antagonist of the orexin-1 receptor (OX1r), SB334867, and antagonist of the orexin-2 receptor (OX2r), TCS OX2 29, were microinjected into the NAc in different doses (1, 3, 10, and 30 nmol/0.5 µl DMSO) before exposure to FSS for a 6-min period. The tail-flick test was carried out as an assay nociception of acute pain, and the nociceptive threshold [tail-flick latency (TFL)] was measured for 60-minute. The findings demonstrated that exposure to acute stress could remarkably increase the TFLs and antinociceptive responses. Moreover, intra-accumbal microinjection of SB334867 or TCS OX2 29 blocked the antinociceptive effect of stress in the tail-flick test. The contribution of orexin receptors was almost equally modulating SIA. The present study's findings suggest that OX1r and OX2r within the NAc modulate stress-induced antinociceptive responses. The intra-accumbal microinjection of orexin receptors antagonists declares inducing antinociceptive responses by FSS in acute pain. Proposedly, intra-accumbla orexinergic receptors have a role in the development of SIA.

An orexin-receptor-2-mediated heart-brain axis in cardiac pain

Orexin is a neuropeptide released from hypothalamus regulating feeding, sleeping, arousal, and cardiovascular activity. Past research has demonstrated that orexin receptor 2 (OX2R) agonist infusion in the brain results in sympathoexcitatory responses. Here, we found that epicardial administration of OX2R agonism leads to opposite responses. We proved that OX2R is expressed mainly in DRG neurons and transported to sensory nerve endings innervating the heart. In a capsaicin-induced cardiac sympathetic afferent reflex (CSAR) model, we recorded the calcium influx in DRG neurons, measured heart rate variability, and examined the PVN c-Fos activity to prove that epicardial OX2R agonism administration could attenuate capsaicin-induced CSAR. We further showed that OX2R agonism could partially rescue acute myocardial infarction by reducing sympathetic overactivation. Our data indicate that epicardial application of OX2R agonist exerts a cardioprotective effect by attenuating CSAR. This OX2R-mediated heart-brain axis may provide therapeutic targets for acute cardiovascular diseases.

Orexin neurons play contrasting roles in itch and pain neural processing via projecting to the periaqueductal gray

Pain and itch are recognized as antagonistically regulated sensations; pain suppresses itch, whilst pain inhibition enhances itch. The neural mechanisms at the central nervous system (CNS) underlying these pain-itch interactions still need to be explored. Here, we revealed the contrasting role of orexin-producing neurons (ORX neurons) in the lateral hypothalamus (LH), which suppresses pain while enhancing itch neural processing, by applying optogenetics to the acute pruritus and pain model. We also revealed that the circuit of ORX neurons from LH to periaqueductal gray regions served in the contrasting modulation of itch and pain processing using optogenetic terminal inhibition techniques. Additionally, by using an atopic dermatitis model, we confirmed the involvement of ORX neurons in regulating chronic itch processing, which could lead to a novel therapeutic target for persistent pruritus in clinical settings. Our findings provide new insight into the mechanism of antagonistic regulation between pain and itch in the CNS.

Effects of the orexin receptor 2 agonist danavorexton on emergence from general anaesthesia and opioid-induced sedation, respiratory depression, and analgesia in rats and monkeys

BACKGROUND

Delayed emergence from general anaesthesia, opioid-induced sedation, and opioid-induced respiratory depression is associated with perioperative complications. We characterised the preclinical effects of the orexin receptor 2 (OX2R)-selective agonist danavorexton (TAK-925) on emergence from anaesthesia and reversal of fentanyl-induced sedation, respiratory depression, and analgesia.

METHODS

Emergence from isoflurane- or propofol-induced anaesthesia and fentanyl-induced sedation were investigated by righting reflex, rotarod, and electroencephalography in rats or monkeys. Fentanyl-induced respiratory depression was assessed by arterial blood gas analysis and whole-body plethysmography in rats and monkeys. Analgesia was evaluated using formalin- and skin incision-induced pain models in rats.

RESULTS

Danavorexton shortened emergence from isoflurane- or propofol-induced anaesthesia and from fentanyl-induced sedation at 1 (P=0.005), 3 (P=0.006), and 3 mg kg-1 s.c. (P=0.022), respectively, by righting reflex in rats. Danavorexton (10 mg kg-1 s.c.) accelerated recovery from isoflurane-, propofol- and fentanyl-induced motor impairment in separate rotarod tests in rats (P=0.008, P=0.007, P=0.017, respectively), and reversed anaesthesia and fentanyl-induced delta-power increases. Danavorexton shortened emergence (return of righting reflex) from isoflurane- or propofol-induced anaesthesia at 1 (P=0.002) and 3 mg kg-1 (P=0.004), respectively, in cynomolgus monkeys. Danavorexton (10 mg kg-1 s.c.) reversed fentanyl-induced increase in Pco2 (P=0.006), and decrease in Po2 (P=0.015) and pH (P<0.001) in rats, and at 3 mg kg-1 s.c. reversed fentanyl-induced increase in Pco2 (P=0.007), and decrease in Po2 (P=0.013) and SO2 (P=0.036) in monkeys. Danavorexton increased minute volume and tidal volume in fentanyl-treated animals. Danavorexton at ≤10 mg kg-1 s.c. did not compromise fentanyl analgesia in rat formalin- and skin incision-induced pain models.

CONCLUSIONS

Danavorexton promoted recovery from anaesthesia and fentanyl-induced sedation, and antagonised fentanyl-induced respiratory depression without compromising fentanyl analgesia.

The role of orexin in Parkinson's disease

Emerging evidence has implicated the orexin system in non-motor pathogenesis of Parkinson's disease. It has also been suggested the orexin system is involved in the modulation of motor control, further implicating the orexin system in Parkinson's disease. Parkinson's disease is the second most common neurodegenerative disease with millions of people suffering worldwide with motor and non-motor symptoms, significantly affecting their quality of life. Treatments are based solely on symptomatic management and no cure currently exists. The orexin system has the potential to be a treatment target in Parkinson's disease, particularly in the non-motor stage. In this review, the most current evidence on the orexin system in Parkinson's disease and its potential role in motor and non-motor symptoms of the disease is summarized. This review begins with a brief overview of Parkinson's disease, animal models of the disease, and the orexin system. This leads into discussion of the possible roles of orexin neurons in Parkinson's disease and levels of orexin in the cerebral spinal fluid and plasma in Parkinson's disease and animal models of the disease. The role of orexin is then discussed in relation to symptoms of the disease including motor control, sleep, cognitive impairment, psychological behaviors, and the gastrointestinal system. The neuroprotective effects of orexin are also summarized in preclinical models of the disease.

The Orexin/Hypocretin System, the Peptidergic Regulator of Vigilance, Orchestrates Adaptation to Stress

The orexin/hypocretin neuropeptide family has emerged as a focal point of neuroscientific research following the discovery that this family plays a crucial role in a variety of physiological and behavioral processes. These neuropeptides serve as powerful neuromodulators, intricately shaping autonomic, endocrine, and behavioral responses across species. Notably, they serve as master regulators of vigilance and stress responses; however, their roles in food intake, metabolism, and thermoregulation appear complementary and warrant further investigation. This narrative review provides a journey through the evolution of our understanding of the orexin system, from its initial discovery to the promising progress made in developing orexin derivatives. It goes beyond conventional boundaries, striving to synthesize the multifaceted activities of orexins. Special emphasis is placed on domains such as stress response, fear, anxiety, and learning, in which the authors have contributed to the literature with original publications. This paper also overviews the advancement of orexin pharmacology, which has already yielded some promising successes, particularly in the treatment of sleep disorders.

Analgesic effect of linalool odor on oral ulcerative mucositis-induced pain in rats

Oral ulcerative mucositis (OUM) induces severe pain, leading to a low quality of life. Linalool odor exposure has recently been reported to suppress inflammatory pain in the hind paws. However, the analgesic effect of linalool odor on orofacial pain remains unclear. In this study, we examined the mechanism underlying the analgesic effect of linalool odor on oral pain caused by OUM using nocifensive behavioral and immunohistochemical analyses in rats. OUM was developed by treating the labial fornix region of the inferior incisors with acetic acid. Linalool at 1% was exposed for 5 min at 30 min before nocifensive behavioral measurements. OUM induced spontaneous pain and mechanical allodynia, which were suppressed by the linalool odor. Mechanical allodynia in the hind paw following the injection of complete Freund's adjuvant was also suppressed by linalool odor. Application of lidocaine to the olfactory bulb attenuated the inhibition of spontaneous pain and hyperactivation of trigeminal spinal nucleus caudalis neurons in OUM model rats. Linalool odor exposure-induced neuronal activation in the locus coeruleus (LC) of OUM model rats was decreased by lidocaine application to the olfactory bulb. The decrease in neuronal activation in the LC was attenuated by the administration of orexin 1 receptor (OX-1) antagonist to the LC. These results suggest that linalool odor stimulation through the olfactory pathway activates LC neurons via OX-1 signaling, leading to the suppression of OUM-induced oral pain.

The orexin story and orexin receptor antagonists for the treatment of insomnia

Insomnia is present in up to one third of the adult population worldwide, and it can present independently or with other medical conditions such as mental, metabolic, or cardiovascular diseases, which highlights the importance of treating this multifaceted disorder. Insomnia is associated with an abnormal state of hyperarousal (increased somatic, cognitive, and cortical activation) and orexin has been identified as a key promotor of arousal and vigilance. The current standards of care for the treatment of insomnia recommend non-pharmacological interventions (cognitive behavioural therapy) as first-line treatment and, if behavioural interventions are not effective or available, pharmacotherapy. In contrast to most sleep medications used for decades (benzodiazepines and 'Z-drugs'), the new orexin receptor antagonists do not modulate the activity of γ-aminobutyric acid receptors, the main inhibitory mechanism of the central nervous system. Instead, they temporarily block the orexin pathway, causing a different pattern of effects, e.g., less morning or next-day effects, motor dyscoordination, and cognitive impairment. The pharmacokinetic/pharmacodynamic properties of these drugs are the basis of the different characteristics explained in the package inserts, including the recommended starting dose. Orexin receptor antagonists seem to be devoid of any dependence and tolerance-inducing effects, rendering them a viable option for longer-term treatment. Safety studies did not show exacerbation of existing respiratory problems, but more real-world safety and pharmacovigilance experience is needed. This review provides an overview of the orexin history, the mechanism of action, the relation to insomnia, and key features of available drugs mediating orexin signalling.

Antinociceptive effect of intermittent fasting via the orexin pathway on formalin-induced acute pain in mice

It has been suggested that stress responses induced by fasting have analgesic effects on nociception by elevating the levels of stress-related hormones, while there is limited understanding of pain control mechanisms. Here, we investigated whether acute or intermittent fasting alleviates formalin-induced pain in mice and whether spinal orexin A (OXA) plays a role in this process. 6, 12, or 24 h acute fasting (AF) and 12 or 24 h intermittent fasting (IF) decreased the second phase of pain after intraplantar formalin administration. There was no difference in walking time in the rota-rod test and distance traveld in the open field test in all groups. Plasma corticosterone level and immobility time in the forced swim test were increased after 12 h AF, but not after 12 h IF. 12 h AF and IF increased not only the activation of OXA neurons in the lateral hypothalamus but also the expression of OXA in the lateral hypothalamus and spinal cord. Blockade of spinal orexin 1 receptor with SB334867 restored formalin-induced pain and spinal c-Fos immunoreactivity that were decreased after 12 h IF. These results suggest that 12 h IF produces antinociceptive effects on formalin-induced pain not by corticosterone elevation but by OXA-mediated pathway.

Age-dependent down-regulation of orexin receptors in trigeminal nucleus caudalis correlated with attenuation of orexinergic analgesia in rats

Aging is related to a variety of physiological organ changes, including central and peripheral nervous systems. It has been reported that the orexin signaling has a potential analgesic effect in different models of pain, especially inflammatory pulpal pain. However, the age-induced alteration in dental pain perception and orexin analgesia has not yet been fully elucidated. Here, we tested that how aging may change the effect of orexin-A on nociceptive behaviors in a rat dental pulp pain model. The expression levels of orexin receptors and the nociceptive neuropeptides substance P (SP) and calcitonin-related gene peptide (CGRP) were also assessed in the trigeminal nucleus caudalis (TNC) of young and aged rats. Dental pulp pain was induced by intradental application of capsaicin (100 μg). The immunofluorescence technique was used to evaluate the expression levels. The results show less efficiency of orexin-A to ameliorate pain perception in aged rats as compared to young rats. In addition, a significant decrease in the number of orexin 1 and 2 receptors was observed in the TNC of aged as compared to young rats. Dental pain-induced SP and CGRP overexpression was also significantly inhibited by orexin-A injection into the TNC of young animals. In contrast, orexin-A could not produce such effects in the aged animals. In conclusion, the older age-related reduction of the antinociceptive effect of orexin may be due to the downregulation of its receptors and inability of orexin signaling to inhibit the expression of nociceptive neuropeptides such as SP and CGRP in aged rats.

Similar functional roles of the Orexin-1 and Orexin-2 receptors within the dentate gyrus area of the hippocampus in the stress-induced antinociceptive responses in the acute pain model in the rat

Studies establish that the brain's Orexin system is involved in pain modulation. Orexin-1 and orexin-2 receptors (OX1 and OX2r, respectively) are essential in responsiveness to stressful stimuli. Some evidence indicates that the hippocampus's dentate gyrus (DG) potentially modulates pain and stress. The present study examined the involvement of OX1 and OX2 receptors within the DG in response to acute pain after exposure to forced swim stress (FSS). Five to seven days post-stereotaxic surgery, the baseline tail-flick latency (TFL) was taken from the animal, then rats unilaterally received through an implanted cannula either different doses of OX1r antagonist (SB334867; 1, 3, 10, and 30 nmol), OX2r antagonist (TCS OX2 29; 1, 3, 10 and 30 nmol), or vehicle (0.5 μl solution of 12% DMSO). After 5 min, rats were exposed to the FSS for six minutes. Subsequently, the tail-flick test was conducted, and the TFLs were measured at the 60-min time set intervals. Results indicated that FSS produces antinociceptive responses in the tail-flick test. Two-way ANOVA analysis showed that Microinjection of OX1r and OX2r antagonists into the DG region of the brain reduced FSS-induced analgesia in the tail-flick test. The decrement effects of these two antagonists were almost the same. Additionally, results showed that the role of both receptors was the same in modulating stress-induced analgesia (SIA). These findings show that the orexin system in the hippocampal DG region might be partially involved in the SIA in acute pain.

Brain nuclei and neural circuits in neuropathic pain and brain modulation mechanisms of acupuncture: a review on animal-based experimental research

Neuropathic pain (NP) is known to be associated with abnormal changes in specific brain regions, but the complex neural network behind it is vast and complex and lacks a systematic summary. With the help of various animal models of NP, a literature search on NP brain regions and circuits revealed that the related brain nuclei included the periaqueductal gray (PAG), lateral habenula (LHb), medial prefrontal cortex (mPFC), and anterior cingulate cortex (ACC); the related brain circuits included the PAG-LHb and mPFC-ACC. Moreover, acupuncture and injurious information can affect different brain regions and influence brain functions via multiple aspects to play an analgesic role and improve synaptic plasticity by regulating the morphology and structure of brain synapses and the expression of synapse-related proteins; maintain the balance of excitatory and inhibitory neurons by regulating the secretion of glutamate, γ-aminobutyric acid, 5-hydroxytryptamine, and other neurotransmitters and receptors in the brain tissues; inhibit the overactivation of glial cells and reduce the release of pro-inflammatory mediators such as interleukins to reduce neuroinflammation in brain regions; maintain homeostasis of glucose metabolism and regulate the metabolic connections in the brain; and play a role in analgesia through the mediation of signaling pathways and signal transduction molecules. These factors help to deepen the understanding of NP brain circuits and the brain mechanisms of acupuncture analgesia.

Effects of light therapy on sleep/wakefulness, daily rhythms, and the central orexin system in a diurnal rodent model of seasonal affective disorder

BACKGROUND

Bright light therapy (BLT) is the first-line treatment for seasonal affective disorder. However, the neural mechanisms underlying BLT are unclear. To begin filling this gap, the present study examined the impact of BLT on sleep/wakefulness, daily rhythms, and the wakefulness-promoting orexin/hypocretin system in a diurnal rodent, Nile grass rats (Arvicanthis niloticus).

METHODS

Male and female grass rats were housed under a 12:12 h light/dark cycle with dim light (50 lx) during the day. The experimental group received daily 1-h early morning BLT (full-spectrum white light, 10,000 lx), while the control group received narrowband red light for 4 weeks. Sleep/wakefulness and in-cage locomotor activity were monitored, followed by examination of hypothalamic prepro-orexin and orexin receptors OX1R and OX2R expression in corticolimbic brain regions.

RESULTS

The BLT group had higher wakefulness during light treatment, better nighttime sleep quality, and improved daily rhythm entrainment compared to controls. The impact of BLT on the orexin system was sex- and brain region-specific, with males showing higher OX1R and OX2R in the CA1, while females showed higher prepro-orexin but lower OX1R and OX2R in the BLA, compared to same-sex controls.

LIMITATIONS

The present study focused on the orexin system in a limited number of brain regions at a single time point. Sex wasn't a statistical factor, as male and female cohorts were run independently.

CONCLUSIONS

The diurnal grass rats show similar behavioral responses to BLT as humans, thus could be a good model for further elucidating the neural mechanisms underlying the therapeutic effects of BLT.

Emerging experimental drugs in clinical trials for migraine: observations and key talking points

INTRODUCTION

There have been significant advances in the treatment of migraine. In response to the clinical success of monoclonal antibodies targeting calcitonin gene-related peptide, there is interest in the clinical trial outcomes of alternative emerging drugs that act on novel targets associated with migraine pathophysiology. As approximately 50% of patients do not respond to CGRP therapies, there is significant value in future drug innovation. Emerging drugs in clinical trials for the treatment of migraine aim to fill this need.

AREAS COVERED

The emerging drugs that will be discussed in this review include zavegepant, lasmiditan, delta opioid receptor agonists, neuronal nitric oxide synthase inhibitors, monoclonal antibodies targeting pituitary adenylate cyclase-activating polypeptide (PACAP) and its receptor, dual orexin receptor antagonists, metabotropic glutamate receptor 5 antagonists, and inducers of ketosis.

EXPERT OPINION

When considering the preclinical and clinical research related to the emerging drug classes discussed in this review, most therapies are derived from highly supported targets of migraine pathogenesis. Although the individual drugs discussed in this review may be of dubious clinical value, the importance of the therapeutic targets on which they act cannot be understated. Future research is necessary to appropriately target the pathways elucidated by preclinical studies.

Orexin A, an amphipathic α-helical neuropeptide involved in pleiotropic functions in the nervous and immune systems: Synthetic approach and biophysical studies of the membrane-bound state

This research reports on the membrane interactions of orexin A (OXA), an α-helical and amphipathic neuropeptide that contains 33 residues and two disulfide bonds in the N-terminal region. OXA, which activates the orexins 1 and 2 receptors in neural and immune cell membranes, has essential pleiotropic physiological effects, including at the levels of arousal, sleep/wakefulness, energy balance, neuroprotection, lipid signaling, the inflammatory response, and pain. As a result, the orexin system has become a prominent target to treat diseases such as sleep disorders, drug addiction, and inflammation. While the high-resolution structure of OXA has been investigated in water and bound to micelles, there is a lack of information about its conformation bound to phospholipid membranes and its receptors. NMR is a powerful method to investigate peptide structures in a membrane environment. To facilitate the NMR structural studies of OXA exposed to membranes, we present a novel synthetic scheme, leading to the production of isotopically-labeled material at high purity. A receptor activation assay shows that the ¹⁵N-labeled peptide is biologically active. Biophysical studies are performed using surface plasmon resonance, circular dichroism, and NMR to investigate the interactions of OXA with phospholipid bilayers. The results demonstrate a strong interaction between the peptide and phospholipids, an increase in α-helical content upon membrane binding, and an in-plane orientation of the C-terminal region critical to function. This new knowledge about structure-activity relationships in OXA could inspire the design of novel therapeutics that leverage the anti-inflammatory and neuro-protective functions of OXA, and therefore could help address neuroinflammation, a major issue associated with neurological disorders such as Alzheimer's disease.

Targeting Orexin Receptors for the Treatment of Insomnia: From Physiological Mechanisms to Current Clinical Evidence and Recommendations

After a detailed description of orexins and their roles in sleep and other medical disorders, we discuss here the current clinical evidence on the effects of dual (DORAs) or selective (SORAs) orexin receptor antagonists on insomnia with the aim to provide recommendations for their further assessment in a context of personalized and precision medicine. In the last decade, many trials have been conducted with orexin receptor antagonists, which represent an innovative and valid therapeutic option based on the multiple mechanisms of action of orexins on different biological circuits, both centrally and peripherally, and their role in a wide range of medical conditions which are often associated with insomnia. A very interesting aspect of this new category of drugs is that they have limited abuse liability and their discontinuation does not seem associated with significant rebound effects. Further studies on the efficacy of DORAs are required, especially on children and adolescents and in particular conditions, such as menopause. Which DORA is most suitable for each patient, based on comorbidities and/or concomitant treatments, should be the focus of further careful research. On the contrary, studies on SORAs, some of which seem to be appropriate also in insomnia in patients with psychiatric diseases, are still at an early stage and, therefore, do not allow to draw definite conclusions.

Migraine and obesity: what is the real direction of their association?

INTRODUCTION

In recent decades, studies have addressed the issue of how migraine and obesity are related and have suggested obesity as a risk factor for migraine headache. However, the exact direction of this relationship remains under debate. In this review, the authors summarize the evidence that have suggested migraine as a risk factor for obesity and overweightness.

AREAS COVERED

This article reviews the results of the previous research published on PubMed and Scopus databases (from 2000 to 2020) concerning the association between migraine and obesity to determine the actual direction of their association. Special attention has been given to the common mechanistic pathways involved in the pathophysiology of migraine and obesity.

EXPERT OPINION

The majority of research conducted thus far has considered obesity as a risk factor for migraine. However, because of the cross-sectional design of available research, we cannot be certain of the proposed direction of this association. There is evidence supporting the hypothesis that obesity can serve as a consequence of migraine through the effects of neuropeptides, inflammatory mediators, adipokines, gut microbiota and modifications in eating behavior and lifestyle. However, the real direction of the relationship between migraine and obesity should be further investigated in large prospective studies.

Blockade of orexin receptor type-1 by SB-334867 and activation of orexin receptor type-2 attenuate morphine tolerance in rats

Purpose

The interaction of orexinergic neurons with the opioidergic system and their effects on morphine analgesia and tolerance have not been fully elucidated. The purpose of the study was to evaluate the effects of the orexin-1 and orexin-2 receptor (OX1R and OX2R) agonist and antagonist on morphine analgesia and tolerance in rats.

Material and methods

A total of 90 Wistar albino male rats weighing 180–220 g were used in the experiments. To induce morphine tolerance, rats were injected with a single dose of morphine (50 mg kg−1, s.c.) for 3 days. Morphine tolerance was assessed on day 4 in randomly selected rats by analgesia tests. In order to evaluate morphine tolerance situation, orexin-A, SB-334867, orexin-B and TCS OX2 29 were administered together with morphine for 3 days. The analgesic effects of orexin-A (10 μg kg−1), OXR1 antagonist SB-334867 (10 mg kg−1), OXR2 agonist orexin-B (15 μg kg−1), OXR2 antagonist TCS OX2 29 (0.5 mg kg−1) and morphine (5 mg kg−1) were measured at 15 or 30-min intervals by tail-flick and hot-plate antinociceptive tests.

Results

The results suggested that the combination of orexin-1 receptor antagonist SB-334867 and orexin-B with morphine significantly increased the analgesic effect compared to morphine-tolerant rats. In addition, administration of orexin-A and -B alone showed significant analgesic effects compared to the saline group. However, co-administration of orexin-A and -B with morphine did not increase the analgesic efficacy of morphine.

Conclusions

The results of this study demonstrated that co-administration of SB-334867 and orexin-B with morphine attenuated morphine tolerance. Further studies are needed to elucidate the details of the interaction between orexin receptors and the opioidergic system.

Antagonism of the orexin receptors in the ventral tegmental area diminished the stress-induced analgesia in persistent inflammatory pain

As a part of descending pain inhibitory system, orexin (OXs) in the ventral tegmental area (VTA) are implicated in nociceptive responses. The current study aimed to evaluate the role of OX receptors (OXRs) in the VTA in stress-induced analgesia in persistent inflammatory pain. Ninety-nine adult male Wistar rats underwent forced swim stress (FSS) following intra-VTA infusion of various doses of SB334867 or TCS OX2 29 (1, 3, 10, and 30 nmol/0.3 μL) as an OX1R or OX2R antagonist, respectively. The nociceptive threshold was evaluated using the formalin test as an animal model of persistent inflammatory pain. Current results demonstrated FSS as acute stress produced analgesic responses in the persistent inflammatory pain. Moreover, either OX1R or OX2R antagonist infusion in the VTA hindered the FSS-induced analgesia in both early and late phases. The inhibitory effect of SB334768 in the FSS-induced analgesia was stronger than TCS OX2 29 in both early and late phases of the formalin test. Neither SB334768 nor TCS OX2 29 alone affects pain-related behaviors in formalin tests. Intra-VTA microinjection of each treatment could not modify locomotion in rats. The findings suggest that OX1R and OX2R in the VTA are implicated in FSS-induced analgesia mechanisms.

Orexin/Hypocretin System Dysfunction in ESSENCE (Early Symptomatic Syndromes Eliciting Neurodevelopmental Clinical Examinations)

Early Symptomatic Syndromes Eliciting Neurodevelopmental Clinical Examinations (ESSENCE) is an umbrella term covering a wide range of neurodevelopmental difficulties and disorders. Thus, ESSENCE includes attention-deficit/hyperactivity disorder (ADHD), autism spectrum disorder (ASD), and other neurodevelopmental disorders (NDDs) and difficulties, with a variety of symptoms in cognitive, motor, sensory, social, arousal, regulatory, emotional, and behavioral developmental domains, frequently co-occurring and likely having partly common neurobiological substrates. The ESSENCE concept is a clinical paradigm that promotes organizing NDDs in everyday clinical practice according to their coexistence, symptom dimensions overlapping, and treatment possibilities. Despite increased knowledge regarding NDDs, the neurobiological mechanisms that underlie them and other ESSENCE-related problems, are not well understood. With its wide range of neural circuits and interactions with numerous neurotransmitters, the orexin/hypocretin system (Orx-S) is possibly associated with a variety of neurocognitive, psychobiological, neuroendocrine, and physiological functions and behaviors. Dysfunction of Orx-S has been implicated in various psychiatric and neurological disorders. This article provides an overview of Orx-S dysfunctions' possible involvement in the development, presentation, and maintenance of ESSENCE. We provide a focused review of current research evidence linking orexin neuropeptides with specific clinical NDDs symptoms, mostly in ADHD and ASD, within the Research Domain Criteria (RDoC) framework. We propose that Orx-S dysfunction might have an important role in some of these neurodevelopmental symptom domains, such as arousal, wakefulness, sleep, motor and sensory processing, mood and emotional regulation, fear processing, reward, feeding, attention, executive functions, and sociability. Our perspective is presented from a clinical point of view. Further, more thorough systematic reviews are needed as well as planning of extensive new research into the Orx-S's role in ESSENCE, especially considering RDoC elements.

An interaction between basolateral amygdala orexinergic and endocannabinoid systems in inducing anti-nociception in the rat formalin test

The amygdala has emerged as the main brain center for the emotional affective dimension of pain and pain modulation. In the amygdala, orexin and cannabinoid receptors are expressed in relatively high concentrations. To investigate the possible interaction between the amygdala orexin and cannabinoid systems on the modulation of inflammatory pain, we conducted formalin, rotarod, and plethysmometer tests, as well as analyzing mRNA expression of orexin and cannabinoid receptors in male rats. The basolateral amygdala (BLA) was unilaterally implanted by a guide cannula. Our results showed that, compared to saline and DMSO/saline, intra-BLA microinjection of orexin-A (50 and 100 µM) decreased flinch response in the early phase, but not in the late phase of the formalin test. However, these injections had no significant effect on the mRNA expression level of BLA, orexin receptor type-1 (Orx₁), and cannabinoid receptor type-1 (Cb1). Moreover, intra-BLA administration of Orx₁ receptor antagonist (SB-334867; 50 nM) and Cb1 receptor antagonist (AM251; 250 and 500 nM) decreased flinch response only in the early phase of the formalin test as compared to the DMSO group. Although the intra-BLA infusion of orexin-A alone and along with SB-334867 or AM251 decreased flinch response in the early phase of the formalin test, intra-BLA co-microinjection of SB-334867/AM251/OrxA increased flinch response in both early and late phases of the formalin test when compared to the DMSO/OrxA group. Interestingly, in the SB-334867/AM251/OrxA group, the Cb1 receptor was upregulated in all groups in comparison to Orx₁ receptors. Our results revealed an interaction between BLA, orexin-A, and Cb1 receptors in inducing anti-nociception in the formalin test.

The Orexin System: A Potential Player in the Pathophysiology of Absence Epilepsy

BACKGROUND

Absence epilepsy is characterized by the presence of spike-and-wave discharges (SWDs) at the EEG generated within the cortico-thalamo-cortical circuit. The molecular mechanisms involved in the pathophysiology of absence epilepsy are only partially known. WAG/Rij rats older than 2-3 months develop spontaneous SWDs, and they are sensitive to anti- absence medications. Hence, WAG/Rij rats are extensively used as a model for absence epilepsy with predictive validity.

OBJECTIVE

The aim of the study was to examine the possibility that the orexin system, which supports the wake status in experimental animals and humans, plays a role in the pathophysiology of absence seizures.

METHODS

The perspective grounds its method from recent literature along with measurements of orexin receptor type-1 (OX1) protein levels in the thalamus and somatosensory cortex of WAG/Rij rats and non-epileptic Wistar control rats at two ages (25 days and 6-7 months). OX1 protein levels were measured by immunoblotting.

RESULTS

The analysis of the current literature suggests that the orexin system might be involved in the pathophysiology of absence epilepsy and might be targeted by therapeutic intervention. Experimental data are in line with this hypothesis, showing that OX1 protein levels were reduced in the thalamus and somatosensory cortex of symptomatic WAG/Rij rats (6-7 months of age) with respect to non-epileptic controls, whereas these differences were not seen in pre-symptomatic, 25 days-old WAG/Rij rats.

CONCLUSION

This perspective might pave the way for future studies on the involvement of the orexinergic system in the pathophysiology of SWDs associated with absence epilepsy and its comorbidities.

Role of orexinergic receptors within the ventral tegmental area in the development of morphine sensitization induced by forced swim stress in the rat

The ventral tegmental area (VTA) has been suggested as part of a common system for reward, stress, and morphine sensitization. Repeated exposure to stress enhances sensitivity to drugs such as morphine. The role of orexin receptor type 1 (OX1R) and type 2 (OX2R) within the VTA in cross-sensitization of morphine with stress was assessed in this study. Various doses of OX1R antagonist (SB334867) and OX2R antagonist (TCS OX2 29) were microinjected into the VTA of 134 adult male albino Wistar rats through cannulae, which had been bilaterally implanted above this region. Five min after microinjection, animals were forced to swim for 6 min, and 10 min after forced swim stress (FSS) termination, a low dose of morphine (i.e., ineffective dose for sensitization) was subcutaneously injected (1 mg/kg; sc). This procedure was repeated for three consecutive days as a sensitization period followed by a 5-day drug/stress-free period. On the 9th day, sensitivity to morphine was examined by measuring antinociceptive responses to the ineffective dose of morphine via tail-flick test. The obtained findings revealed that while concurrent administration of FSS and an ineffective dose of morphine (1 mg/kg; sc) for three consecutive days induced sensitivity to morphine, intra-VTA administration of OX1R- and OX2R antagonists, dose-dependently blocked this sensitization. These results suggested that both orexin receptors located in the VTA have a considerable role in morphine sensitization induced by concurrent administration of FSS and a low dose of morphine. So, there is a contribution of the orexin system partly to stress-induced sensitization to morphine.

Orexin receptor type 2 agonism inhibits thermogenesis in brown adipose tissue by attenuating afferent innervation

Orexin signaling has been associated with energy expenditure and brown adipose tissue (BAT) function. However, conflicting data exist in the field about how orexin signaling regulates BAT thermogenesis. In this study, we show that a specific orexin receptor type 2 (OX2R) agonist [Ala11, D-Leu15]-OxB (OB-Ala) inhibited intrascapular brown adipose tissue (iBAT) thermogenesis by reducing sympathetic output to iBAT. This effect is mediated by OX2Rs located on afferent nerve endings innervating iBAT instead of brown adipocyte itself. Microinjection of OB-Ala into iBAT inhibited iBAT thermogenesis in mice upon cold exposure and neuronal activity in the paraventricular nucleus. Findings suggest that OB-Ala could inhibit iBAT thermogenesis by attenuating sensory input thereby inhibiting the sympathetic-sensory iBAT feedback loop. Our study uncovers a novel primary action site of orexin in the regulation of energy balance.

Reviewing the role of the orexinergic system and stressors in modulating mood and reward-related behaviors

In this review study, we aimed to introduce the orexinergic system as an important signaling pathway involved in a variety of cognitive functions such as memory, motivation, and reward-related behaviors. This study focused on the role of orexinergic system in modulating reward-related behavior, with or without the presence of stressors. Cross-talk between the reward system and orexinergic signaling was also investigated, especially orexinergic signaling in the ventral tegmental area (VTA), the nucleus accumbens (NAc), and the hippocampus. Furthermore, we discussed the role of the orexinergic system in modulating mood states and mental illnesses such as depression, anxiety, panic, and posttraumatic stress disorder (PTSD). Here, we narrowed down our focus on the orexinergic signaling in three brain regions: the VTA, NAc, and the hippocampus (CA1 region and dentate gyrus) for their prominent role in reward-related behaviors and memory. It was concluded that the orexinergic system is critically involved in reward-related behavior and significantly alters stress responses and stress-related psychiatric and mood disorders.

Circadian rhythms and pain

The goal of this review is to provide a perspective on the nature and importance of the relationship between the circadian and pain systems. We provide: 1) An overview of the circadian and pain systems, 2) a review of direct and correlative evidence that demonstrates diurnal and circadian rhythms within the pain system; 3) a perspective highlighting the need to consider the role of a proposed feedback loop of circadian rhythm disruption and maladaptive pain; 4) a perspective on the nature of the relationship between circadian rhythms and pain. In summary, we propose that there is no single locus responsible for producing the circadian rhythms of the pain system. Instead, circadian rhythms of pain are a complex result of the distributed rhythms present throughout the pain system, especially those of the descending pain modulatory system, and the rhythms of the systems with which it interacts, including the opioid, endocrine, and immune systems.

Hypothalamic structural integrity and temporal complexity of cortical information processing at rest in migraine without aura patients between attacks

The hypothalamus has been attributed an important role during the premonitory phase of a migraine attack. Less is known about the role played by the hypothalamus in the interictal period and its relationship with the putative neurocognitive networks previously identified in the pathophysiology of migraine. Our aim was to test whether the hypothalamic microstructure would be altered during the interictal period and whether this co-existed with aberrant connectivity at cortical level. We collected multimodal MRI data from 20 untreated patients with migraine without aura between attacks (MO) and 20 healthy controls (HC) and studied fractional anisotropy, mean (MD), radial (RD), and axial diffusivity of the hypothalamus ROI as a whole from diffusion tensor imaging (DTI). Moreover, we performed an exploratory analysis of the same DTI metrics separately for the anterior and posterior hypothalamic ROIs bilaterally. From resting-state functional MRI, we estimated the Higuchi's fractal dimension (FD), an index of temporal complexity sensible to describe non-periodic patterns characterizing BOLD signature. Finally, we correlated neuroimaging findings with migraine clinical features. In comparison to HC, MO had significantly higher MD, AD, and RD values within the hypothalamus. These findings were confirmed also in the exploratory analysis on the sub-regions of the hypothalamus bilaterally, with the addition of lower FA values on the posterior ROIs. Patients showed higher FD values within the salience network (SN) and the cerebellum, and lower FD values within the primary visual (PV) network compared to HC. We found a positive correlation between cerebellar and SN FD values and severity of migraine. Our findings of hypothalamic abnormalities between migraine attacks may form part of the neuroanatomical substrate that predisposes the onset of the prodromal phase and, therefore, the initiation of an attack. The peculiar fractal dimensionality we found in PV, SN, and cerebellum may be interpreted as an expression of abnormal efficiency demand of brain networks devoted to the integration of sensory, emotional, and cognitive information related to the severity of migraine.

Exploring the Role of Orexinergic Neurons in Parkinson's Disease

Parkinson's disease (PD) is a neurodegenerative disease affecting about 2% of the population. A neuropeptide, orexin, is linked with sleep abnormalities in the parkinsonian patient. This study aimed to review the changes in the orexinergic system in parkinsonian subjects and the effects of orexin. A number of search techniques were used and presumed during the search, including cloud databank searches of PubMed and Medline using title words, keywords, and MeSH terms. PD is characterised by motor dysfunctions (postural instability, rigidity, tremor) and cognitive disorders, sleep-wake abnormalities grouped under non-motor disorders. The Orexinergic system found in the hypothalamus is linked with autonomic function, neuroprotection, learning and memory, and the sleep-wake cycle. Prepro-orexin, a precursor peptide (130 amino acids), gives rise to orexins (Orx-A and Orx-B). Serum orexin level measurement is vital for evaluating several neurological disorders (Alzheimer's disease, Huntington's disease, and PD). Orexinergic neurons are activated by hypoglycemia and ghrelin, while they are restrained by food consumption and leptin. Orexinergic system dysfunctioning was found to be linked with non-motor symptoms (sleep abnormalities) in PD. Orexinergic neuron's behaviour may be either inhibitory or excitatory depending on the environment in which they are present. As well, orexin antagonists are found to improve the abnormal sleep pattern. Since the orexinergic system plays a role in several psychological and neurological disorders, therefore, these disorders can be managed by targeting this system.

Role of hippocampal orexin receptors in antinociception elicited by chemical stimulation of the lateral hypothalamus in the tail-flick test

The lateral hypothalamus (LH) orexinergic neurons project to numerous brain regions implicated in pain perception, including the CA1 part of the hippocampal formation. Moreover, the roles of orexin receptors (OXRs) in the CA1 in anti-analgesic consequences of the LH chemical stimulation by carbachol, muscarinic receptor agonist, in acute pain have not been clarified. The current research showed OXRs antagonist administration's effect in the CA1 on analgesia elicited by the LH chemical stimulation in a tail-flick test as an acute model of pain. The control groups, including vehicle-control groups, were given intra-LH administration of saline (0.5 μL), following intra-CA1 infusion of DMSO (12 %; 0.5 μL), and carbachol-control groups were treated with carbachol (250 nM/0.5 μL saline) into the LH following DMSO in the CA1. Treated groups received SB334867 (1, 3, 10, and 30 nM/0.5 μL DMSO) or TCS OX2 29 (0.1, 1, 10, and 20 nM/0.5 μL DMSO) as OX1R or OX2R antagonist, respectively, in the CA1 prior intra-LH administration of carbachol. After all injections, all rats underwent the tail-flick test over a 60-min time. Infusion of SB334867 or TCS OX2 29 in the CA1 impaired the analgesic consequences following chemical stimulation of the LH in acute pain. Meanwhile suppressive impact of the OX1R or OX2R antagonist on the analgesic impact of LH chemical stimulation was approximately identical. The current investigation provided a new document about the critical involvement of hippocampal orexinergic system in the modulatory role of the LH-CA1 path in pain perception.

Gene polymorphism association studies in cluster headache: A field synopsis and systematic meta-analyses

BACKGROUND

A plethora of studies have attempted to identify genetic determinants of disease susceptibility and treatment response of patients with cluster headache (CH), but results are often conflicting, and no comprehensive overview with a quantitative summary of the evidence in this field is available.

METHODS

A systematic search of relevant publications was performed without any language restrictions on PubMed, Web of Knowledge, Cochrane Library, and OpenGrey, up to December 2020. A standardized data extraction form was used to collect relevant data from each included study. Meta-analyses were conducted for gene polymorphisms investigated in at least two studies and the Bayesian false discovery probability (BFDP) test was applied to the pooled odds ratios (ORs) to assess the credibility of the observed associations.

RESULTS

Among the 27 articles identified by the systematic review, 17 studies evaluating 12 single nucleotide polymorphisms (SNPs) were included in the quantitative data analysis. The pooled results showed no significant association with CH risk of 10 SNPs, including five SNPs of HCRTR2 (rs2653349, rs2653342, rs3122156, rs10498801, and rs3800539), two SNPs of ADH4 (rs1800759 and rs1126671), CLOCK rs1801260, and two SNPs (rs1006417 and ADCYAP1R1 rs12668955) previously identified by a genome-wide association study (GWAS). Conversely, the pooled results revealed the association of the HCRTR2 rs9357855 A allele with a higher risk of CH (A vs. G, OR: 1.33, 95% CI: 1.04-1.72, p = 0.026), and of GNB3 rs5443 with a higher response rate of patients with CH to triptan drugs (CT+TT vs. CC, OR: 1.96, 95% CI: 1.04-3.72, p = 0.038). However, assuming a prior probability of 0.001, the respective BFDP values being higher than 0.8 (BFDP_rs9357855  = 0.998; BFDP_rs5443  = 0.998) revealed lack of noteworthy results.

CONCLUSIONS

Well-designed GWASs and large replication studies are still needed to identify reliable genetic variants of disease susceptibility and treatment response of patients with CH.

Activation of Hypocretin Neurons in Endometriosis

Endometriosis is a gynecological disease affecting 6-10% of women of reproductive age. In addition to gynecologic symptoms, endometriosis is associated with various systemic effects, including inflammation, altered body weight, and behavioral changes. Previous murine studies demonstrate that endometriosis is causally inked to increased pain sensitization, behavioral changes, and low body mass index (BMI). One possible cellular target that may mediate some of these findings is the hypocretin/orexin neurons. This neuronal system plays a role in regulating wakefulness/sleep cycles, pain perception, and appetite. We hypothesize that endometriosis alters activity level of the hypocretin/orexin (Hcrt) neuronal system. Mice underwent endometriosis induction surgeries (endo) or sham surgeries (sham) for the development of the experimental model. Immunocytochemistry was performed on harvested samples from the lateral hypothalamus, and activation levels of Hcrt cells were examined by quantifying the expression of phosphorylation of cAMP-responsive element binding protein (CREB) in these cells after an acute stress in sham and endo mice. Mice with endometriosis had greater Hcrt neurons activation than sham mice. Mice with endometriosis fed with high fat diet showed a lower fat/body weight and fat/lean tissue ratio compared to mice without endometriosis. There was no significant difference in food intake between sham and endometriosis mice. These results demonstrate that endometriosis is associated with low body mass and increased hypocretin/orexin activity, which could be implicated in the behavioral changes and to differences in body composition.

The orexin/hypocretin system in neuropsychiatric disorders: Relation to signs and symptoms

Hypocretin-1 and 2 (or orexin A and B) are neuropeptides exclusively produced by a group of neurons in the lateral and dorsomedial hypothalamus that project throughout the brain. In accordance with this, the two different hypocretin receptors are also found throughout the brain. The hypocretin system is mainly involved in sleep-wake regulation, but also in reward mechanisms, food intake and metabolism, autonomic regulation including thermoregulation, and pain. The disorder most strongly linked to the hypocretin system is the primary sleep disorder narcolepsy type 1 caused by a lack of hypocretin signaling, which is most likely due to an autoimmune process targeting the hypocretin-producing neurons. However, the hypocretin system may also be affected, but to a lesser extent and less specifically, in various other neurological disorders. Examples are neurodegenerative diseases such as Alzheimer's, Huntington's and Parkinson's disease, immune-mediated disorders such as multiple sclerosis, neuromyelitis optica, and anti-Ma2 encephalitis, and genetic disorders such as type 1 diabetus mellitus and Prader-Willi Syndrome. A partial hypocretin deficiency may contribute to the sleep features of these disorders.

The role of dentate gyrus dopaminergic receptors in the lateral hypothalamic-induced antinociception during persistent inflammatory pain in male rats

The lateral hypothalamus (LH) is one of the key brain areas involved in pain modulation. Also, the dentate gyrus (DG) of the hippocampus expresses various receptors, including dopaminergic receptors. Dopaminergic receptors play a key role in pain transmission and modulation within the brain. The present study aimed to investigate the involvement of DG dopaminergic receptors in the LH-induced antinociception during the presence of inflammatory pain. Male Wistar rats were used in this study. Cannulae were unilaterally implanted in their skull for microinjections into the LH and DG. The LH was chemically stimulated by carbachol injection (250 nM/0.5 μl saline). In separate groups, different doses (0.25, 1, and 4 μg/0.5 μl vehicle) of the D1- and D2-like dopamine receptor antagonists (SCH23390 and Sulpiride, respectively) were microinjected into the DG, 5 min prior to intra-LH injection of carbachol. Five min after the second injection, formalin test as a persistent inflammatory pain model in animals was done in all rats. The results revealed that carbachol could induce antinociception following formalin injection into rat's hind paw. The 4 μg dose of both antagonists significantly reduced the LH stimulation-induced antinociception in both phases of formalin pain responses. Although the 1 μg dose of sulpiride significantly reduced antinociception during both phases, 1 μg SCH23390 could only reduce this antinociception during the late phase. These findings demonstrate the involvement of DG dopaminergic receptors in the LH-induced antinociception. The results also suggest that the effectiveness of DG dopaminergic receptors is more pronounced during the late phase of formalin-induced pain responses.

Impact of daytime light intensity on the central orexin (hypocretin) system of a diurnal rodent (Arvicanthis niloticus)

The neuropeptide orexin/hypocretin is implicated in sleep and arousal, energy expenditure, reward, affective state and cognition. Our previous work using diurnal Nile grass rats (Arvicanthis niloticus) found that orexin mediates the effects of environmental light, particularly daytime light intensity, on affective and cognitive behaviours. The present study further investigated how daytime light intensity affects the central orexin system in male and female grass rats. Subjects were housed for 4 weeks in 12:12 hr dim light:dark (50 lux, dimLD) or in 12:12 hr bright light:dark cycle (1000 lux, brightLD). Day/night fluctuations in some orexin measures were also assessed. Despite similar hypothalamic prepro-orexin mRNA expression across all conditions, there were significantly more orexin-immunoreactive neurons, larger somata, greater optical density or higher orexin A content at night (ZT14) than during the day (ZT2), and/or in animals housed in brightLD compared to dimLD. Grass rats in brightLD also had higher cisternal CSF levels of orexin A. Furthermore, orexin receptor OX1R and OX2R proteins in the medial prefrontal cortex were higher in brightLD than dimLD males, but lower in brightLD than dimLD females. In the CA1 and dorsal raphe nucleus, females had higher OX1R than males without any significant effects of light condition, and OX2R levels were unaffected by sex or light. These results reveal that daytime light intensity alters the central orexin system of both male and female diurnal grass rats, sometimes sex-specifically, and provides insight into the mechanisms underlying how daytime light intensity impacts orexin-regulated functions.

Dopaminergic receptors in the ventral tegmental area modulated the lateral hypothalamic stimulation-induced antinociception in an animal model of tonic pain

The role of the ventral tegmental area (VTA) and the lateral hypothalamus (LH) in the modulation of formalin-induced nociception is well documented individually. The present study aimed to investigate the role of dopamine receptors of the VTA in the modulation of the LH stimulation-induced antinociception during both phases of the formalin test as an animal model of tonic pain. In this study, male Wistar rats were unilaterally implanted with two guide cannulae in the VTA and LH. In two separate groups, animals received different doses (0.25, 1, and 4 μg/rat) of D1- or D2-like dopamine receptor antagonists (SCH-23,390 or Sulpiride, respectively) into the VTA before intra-LH injection of carbachol (22.83 ng/rat) following formalin injection (50 μL; s.c.) into their contralateral hind paws. The blockade of these two receptors reduced intra-LH carbachol-induced antinociception during both phases of the formalin test. This reduction during the late phase of the formalin test was more than that of the early phase. The results indicated that LH stimulation-induced antinociception was mediated by D1- and D2-like dopamine receptors in the VTA, and so, the neural pathway projecting from the LH to the VTA contributes to the modulation of formalin-induced nociception in the rats.

The Endocannabinoid System Contributes to Electroacupuncture Analgesia

The extensive involvement of the endocannabinoid system (ECS) in vital physiological and cognitive processes of the human body has inspired many investigations into the role of the ECS and drugs, and therapies that target this system and its receptors. Activation of cannabinoid receptors 1 and 2 (CB₁ and CB₂) by cannabinoid treatments, including synthetic cannabinoids, alleviates behavioral responses to inflammatory and neuropathic pain. An increasing body of scientific evidence details how electroacupuncture (EA) treatments achieve effective analgesia and reduce inflammation by modulating cannabinoid signaling, without the adverse effects resulting from synthetic cannabinoid administration. CB₁ receptors in the ventrolateral area of the periaqueductal gray are critically important for the mechanisms of the EA antinociceptive effect, while peripheral CB₂ receptors are related to the anti-inflammatory effects of EA. This review explores the evidence detailing the endocannabinoid mechanisms involved in EA antinociception.

Effect of suvorexant on morphine tolerance and dependence in mice: Role of NMDA, AMPA, ERK and CREB proteins

The major problems of morphine use in the clinic are its tolerance and dependence. This study aimed to investigate the effect of suvorexant, a dual orexin receptor antagonist, on morphine-induced dependence and tolerance in mice and evaluate the level of NMDA, AMPA, ERK, p-ERK, CREB and p-CREB proteins in the brain. Tolerance and dependence were induced by repeated injection of morphine in mice (three times a day for 3 days, 50, 50, and 75 mg/kg /day). To evaluate the effects of the drugs on morphine-induced tolerance and dependence, suvorexant (30, 60 and 90 mg/kg), clonidine (positive control, 0.1 mg/kg) and saline were injected intraperitoneally 30 min before each injection of morphine. Tolerance and locomotor activity were assessed by tail-flick and open-field tests, respectively. The effect of suvorexant on the naloxone (5 mg/kg, ip)-induced morphine withdrawal, was also evaluated. Finally, the expression of proteins in the brain of mice was measured by western blot. Administration of suvorexant with morphine significantly reduced morphine-induced tolerance. Also, suvorexant attenuated the naloxone-precipitated opioid withdrawal. Suvorexant decreased morphine-enhanced levels of CREB and p-ERK proteins but did not affect the expression of NMDA and AMPA proteins compared to the morphine group. Suvorexant reduced morphine-induced tolerance and dependence through the inhibition of orexin receptors as well as changes in CREB and p-ERK protein levels in the brain.

Multitargeting the sleep-pain interaction with pharmacological approaches: A narrative review with suggestions on new avenues of investigation

The multimorbidity formed by sleep disturbances and pain conditions is highly prevalent and has a significant impact in global health and in the socioeconomic system. Although different approaches have been directed toward its management, evidence regarding an optimal treatment is lacking, and pharmacological options are often preferred. Health professionals (e.g., pain and sleep clinicians) tend to focus on their respective expertise, targeting a single symptom with a single drug. This may increase polypharmacy and the risk of drug interactions, adverse events, and mortality. Hence, the use of medications that can directly or indirectly improve sleep, pain, and other possible accompanying conditions without exacerbating them becomes especially relevant. The objectives of this comprehensive review are to: a) describe the beneficial or deleterious effects that some commonly used medications to manage pain have on sleep and sleep disorders; and b) describe the beneficial or deleterious effects that frequently prescribed medications for sleep may have on pain. Moreover, medications targeting some specific sleep-pain interactions will be suggested and future directions for improving sleep and alleviating pain of these patients will be provided with clinical and research perspectives.

Inhibition of Orexin/Hypocretin Neurons Ameliorates Elevated Physical Activity and Energy Expenditure in the A53T Mouse Model of Parkinson's Disease

Aside from the classical motor symptoms, Parkinson's disease also has various non-classical symptoms. Interestingly, orexin neurons, involved in the regulation of exploratory locomotion, spontaneous physical activity, and energy expenditure, are affected in Parkinson's. In this study, we hypothesized that Parkinson's-disease-associated pathology affects orexin neurons and therefore impairs functions they regulate. To test this, we used a transgenic animal model of Parkinson's, the A53T mouse. We measured body composition, exploratory locomotion, spontaneous physical activity, and energy expenditure. Further, we assessed alpha-synuclein accumulation, inflammation, and astrogliosis. Finally, we hypothesized that chemogenetic inhibition of orexin neurons would ameliorate observed impairments in the A53T mice. We showed that aging in A53T mice was accompanied by reductions in fat mass and increases in exploratory locomotion, spontaneous physical activity, and energy expenditure. We detected the presence of alpha-synuclein accumulations in orexin neurons, increased astrogliosis, and microglial activation. Moreover, loss of inhibitory pre-synaptic terminals and a reduced number of orexin cells were observed in A53T mice. As hypothesized, this chemogenetic intervention mitigated the behavioral disturbances induced by Parkinson's disease pathology. This study implicates the involvement of orexin in early Parkinson's-disease-associated impairment of hypothalamic-regulated physiological functions and highlights the importance of orexin neurons in Parkinson's disease symptomology.