Roles of the orexin system in central motor control

Orexin Neurons to Sublaterodorsal Tegmental Nucleus Pathway Prevents Sleep Onset REM Sleep-Like Behavior by Relieving the REM Sleep Pressure

Proper timing of vigilance states serves fundamental brain functions. Although disturbance of sleep onset rapid eye movement (SOREM) sleep is frequently reported after orexin deficiency, their causal relationship still remains elusive. Here, we further study a specific subgroup of orexin neurons with convergent projection to the REM sleep promoting sublaterodorsal tegmental nucleus (OXSLD neurons). Intriguingly, although OXSLD and other projection-labeled orexin neurons exhibit similar activity dynamics during REM sleep, only the activation level of OXSLD neurons exhibits a significant positive correlation with the post-inter-REM sleep interval duration, revealing an essential role for the orexin-sublaterodorsal tegmental nucleus (SLD) neural pathway in relieving REM sleep pressure. Monosynaptic tracing reveals that multiple inputs may help shape this REM sleep-related dynamics of OXSLD neurons. Genetic ablation further shows that the homeostatic architecture of sleep/wakefulness cycles, especially avoidance of SOREM sleep-like transition, is dependent on this activity. A positive correlation between the SOREM sleep occurrence probability and depression states of narcoleptic patients further demonstrates the possible significance of the orexin-SLD pathway on REM sleep homeostasis.

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.

Clinical usefulness of dual orexin receptor antagonism beyond insomnia: Neurological and psychiatric comorbidities

Orexin is a neurotransmitter produced by a small group of hypothalamic neurons. Besides its well-known role in the regulation of the sleep-wake cycle, the orexin system was shown to be relevant in several physiological functions including cognition, mood and emotion modulation, and energy homeostasis. Indeed, the implication of orexin neurotransmission in neurological and psychiatric diseases has been hypothesized via a direct effect exerted by the projections of orexin neurons to several brain areas, and via an indirect effect through orexin-mediated modulation of sleep and wake. Along with the growing evidence concerning the use of dual orexin receptor antagonists (DORAs) in the treatment of insomnia, studies assessing their efficacy in insomnia comorbid with psychiatric and neurological diseases have been set in order to investigate the potential impact of DORAs on both sleep-related symptoms and disease-specific manifestations. This narrative review aimed at summarizing the current evidence on the use of DORAs in neurological and psychiatric conditions comorbid with insomnia, also discussing the possible implication of modulating the orexin system for improving the burden of symptoms and the pathological mechanisms of these disorders. Target searches were performed on PubMed/MEDLINE and Scopus databases and ongoing studies registered on Clinicaltrials.gov were reviewed. Despite some contradictory findings, preclinical studies seemingly support the possible beneficial role of orexin antagonism in the management of the most common neurological and psychiatric diseases with sleep-related comorbidities. However, clinical research is still limited and further studies are needed for corroborating these promising preliminary results.

Treatment of Sleep, Motor and Sensory Symptoms with the Orexin Antagonist Suvorexant in Adults with Idiopathic Restless Legs Syndrome: A Randomized Double-Blind Crossover Proof-of-Concept Study

BACKGROUND AND OBJECTIVES

Current treatment guidelines for restless legs syndrome (RLS) recommend treatment be initiated with non-dopaminergic drugs. Given the potential role of orexins in the pathophysiology of RLS, we performed a pilot, proof-of-concept study to investigate the therapeutic effects of suvorexant, a dual orexin receptor antagonist (DORA), on sleep and sensory/motor symptoms in individuals with idiopathic RLS.

METHODS

This was a randomized, double-blind, crossover and placebo-controlled study. Inclusion criteria were diagnosis with idiopathic RLS, an International RLS Study Group Severity Rating Scale (IRLS) score > 15, and the absence of significant RLS symptoms before 9 pm. Following washout from any previous central nervous system (CNS)-active drugs, patients were randomized to receive either suvorexant or placebo for two consecutive 2-week treatment periods. Treatment was administered at 9 pm at a fixed dose of 10 mg/day during the first week, and 20 mg during the second week. Primary and coprimary endpoints were wake after sleep onset (WASO) and total sleep time (TST), respectively, while IRLS rating scale score, multiple suggested immobilization tests (m-SIT), and periodic limb movements (PLMs) were secondary endpoints. RLS severity was measured weekly using the IRLS and Clinical Global Improvement (CGI) scales. m-SIT were also performed between 8 pm and midnight at the end of each treatment phase and were followed by a sleep study.

RESULTS

A total of 41 participants were randomized, 40 of whom completed the study. Compared with placebo, treatment with suvorexant significantly improved RLS symptoms (according to IRLS total score, CGI, and the m-SIT), PLM during sleep, and PLM with arousal. Improvement of RLS symptoms was greater in those who had not been exposed to dopaminergic agents in the past. Sleep architecture also improved with significant changes in TST, WASO, sleep onset latency, sleep efficiency, non rapid-eye movement stage 1 (N1) %, non rapid-eye movement stage 2 (N2) %, and rapid eye movement (REM) %. Suvorexant was well tolerated in RLS, with few and mild adverse events.

CONCLUSIONS

Our results provide the first proof of evidence of the therapeutic efficacy of DORAs in improving sleep and sensory and motor symptoms in RLS. Given orexin's role in pain and sensory processing, potential mechanisms of action are discussed.

CLASSIFICATION OF EVIDENCE

The study provides class II evidence supporting the therapeutic efficacy of suvorexant in patients with RLS with sleep disturbance.

TRIAL REGISTRATION

EudraCT#: 2017-004580-12.

Eat, seek, rest? An orexin/hypocretin perspective

Seeking and ingesting nutrients is an essential cycle of life in all species. In classical neuropsychology these two behaviours are viewed as fundamentally distinct from each other, and known as appetitive and consummatory, respectively. Appetitive behaviour is highly flexible and diverse, but typically involves increased locomotion and spatial exploration. Consummatory behaviour, in contrast, typically requires reduced locomotion. Another long-standing concept is "rest and digest", a hypolocomotive response to calorie intake, thought to facilitate digestion and storage of energy after eating. Here, we note that the classical seek➔ingest➔rest behavioural sequence is not evolutionarily advantageous for all ingested nutrients. Our limited stomach capacity should be invested wisely, rather than spent on the first available nutrient. This is because nutrients are not simply calories: some nutrients are more essential for survival than others. Thus, a key choice that needs to be made soon after ingestion: to eat more and rest, or to terminate eating and search for better food. We offer a perspective on recent work suggesting how nutrient-specific neural responses shape this choice. Specifically, the hypothalamic hypocretin/orexin neurons (HONs) - cells that promote hyperlocomotive explorative behaviours - are rapidly and differentially modulated by different ingested macronutrients. Dietary non-essential (but not essential) amino acids activate HONs, while glucose depresses HONs. This nutrient-specific HON modulation engages distinct reflex arcs, seek➔ingest➔seek and seek➔ingest➔rest, respectively. We propose that these nutri-neural reflexes evolved to facilitate optimal nutrition despite the limitations of our body.

Orexin recruits non-selective cationic conductance and endocannabinoid to dynamically modulate firing of caudal pontine reticular nuclear neurones

The neuropeptide orexin is involved in motor circuit function. However, its modulation on neuronal activities of motor structures, integrating orexin's diverse downstream molecular cascades, remains elusive. By combining whole-cell patch-clamp recordings and neuropharmacological methods, we revealed that both non-selective cationic conductance (NSCC) and endocannabinoids (eCBs) are recruited by orexin signalling on reticulospinal neurones in the caudal pontine reticular nucleus (PnC). The orexin-NSCC cascade provides a depolarizing force that proportionally enhances the firing-responsive gain of these neurones. Meanwhile, the orexin-eCB cascade selectively attenuates excitatory synaptic strength in these neurones by activating presynaptic cannabinoid receptor type 1. This cascade restrains the firing response of the PnC reticulospinal neurones to excitatory inputs. Intriguingly, non-linear or linear interactions between orexin postsynaptic excitation and presynaptic inhibition can influence the firing responses of PnC reticulospinal neurones in different directions. When presynaptic inhibition is in the lead, non-linear interactions can prominently downregulate or even gate the firing response. Conversely, linear interactions occur to promote the firing response, and these linear interactions can be considered a proportional reduction in the contribution of depolarization to firing by presynaptic inhibition. Through the dynamic employment of these interactions, adaptive modulation may be achieved by orexin to restrain or even gate the firing output of the PnC to weak/irrelevant input signals and facilitate those to salient signals. KEY POINTS: This study investigated the effects of orexin on the firing activity of PnC reticulospinal neurones, a key element of central motor control. We found that orexin recruited both the non-selective cationic conductances (NSCCs) and endocannabinoid (eCB)-cannabinoid receptor type 1 (CB1R) system to pontine reticular nucleus (PnC) reticulospinal neurones. The orexin-NSCC cascade exerts a postsynaptic excitation that enhances the firing response, whereas the orexin-eCB-CB1R cascade selectively attenuates excitatory synaptic strength that restrains the firing response. The postsynaptic and presynaptic actions of orexins occur in an overlapping time window and interact to dynamically modulate firings in PnC reticulospinal neurones. Non-linear interactions occur when presynaptic inhibition of orexin is in the lead, and these interactions can prominently downregulate or even gate firing responses in PnC reticulospinal neurones. Linear interactions occur when postsynaptic excitation of orexin is in the lead, and these interactions can promote the firing response. These linear interactions can be considered a proportional reduction of the contribution of depolarization to firing by presynaptic inhibition.

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.

Neuro-consequences of the spaceflight environment

As human space exploration advances to establish a permanent presence beyond the Low Earth Orbit (LEO) with NASA's Artemis mission, researchers are striving to understand and address the health challenges of living and working in the spaceflight environment. Exposure to ionizing radiation, microgravity, isolation and other spaceflight hazards pose significant risks to astronauts. Determining neurobiological and neurobehavioral responses, understanding physiological responses under Central Nervous System (CNS) control, and identifying putative mechanisms to inform countermeasure development are critically important to ensuring brain and behavioral health of crew on long duration missions. Here we provide a detailed and comprehensive review of the effects of spaceflight and of ground-based spaceflight analogs, including simulated weightlessness, social isolation, and ionizing radiation on humans and animals. Further, we discuss dietary and non-dietary countermeasures including artificial gravity and antioxidants, among others. Significant future work is needed to ensure that neural, sensorimotor, cognitive and other physiological functions are maintained during extended deep space missions to avoid potentially catastrophic health and safety outcomes.

Orexin-A potentiates glycine currents by activating OX1R and IP3/Ca2+/PKC signaling pathways in spinal cord ventral horn neurons

Orexin-A/B modulates multiple physical functions by activating their receptors (OX₁R and OX₂R), but its effects in the spinal cord motor control remain unknown. Using acute separation (by digestive enzyme) of cells and patch-clamp recordings, we aimed to investigate the effect and mechanisms of orexin-A on the glycine receptors in the spinal cord ventral horn neurons. Orexin-A potentiated the glycine currents by activating OX₁R. In Ca²⁺-free extracellular solution, orexin-A still increased the glycine currents. While, the orexin-A-induced potentiation was blocked when Ca²⁺ was chelated by internal infusion of BAPTA, and the orexin-A effect was abolished by the IP₃ receptor antagonists heparin and Xe-C. The PKC inhibitor Bis-IV nullified the orexin-A effect. In addition, orexin-A did not cause a further enhancement of the glycine currents after bath application of the PKC activator PMA. In conclusion, after OX₁R is activated, a distinct IP₃/Ca²⁺-dependent PKC signaling pathway, is likely responsible for the orexin-A potentiation on glycine currents in the spinal cord ventral horn neurons.

Orexin enhances firing activities in the gigantocellular reticular nucleus through the activation of non-selective cationic conductance

Orexin/hypocretin has been implicated in central motor control. The gigantocellular reticular nucleus (Gi), a key element of the brainstem motor inhibitory system, also receives orexinergic innervations. However, the modulations of orexin on the neuronal activities and the underlying cellular mechanisms in Gi neurons remain unknown. Here, through whole-cell patch-clamp recordings, we first observed that orexin increased the firing frequency in Gi neurons. Interestingly, a postsynaptic depolarization elicited by orexin was observed in the presence of tetrodotoxin, without altering the input resistance of Gi neurons at around -60 mV. Moreover, through comparing the current-frequency curves constructed by identical current injections from equal membrane potentials, we found that orexin also increased the repetitive firing ability of Gi neurons. This action appeared to be caused by the shortening of inter-spike intervals, without altering the waveform of individual action potentials. We finally revealed that activation of the non-selective cationic conductance contributed to the orexin-elicited excitation in Gi neurons. Together, these results suggest that orexin may facilitate Gi-mediated motor functions through enhancing the neuronal activities of Gi neurons.

Orexin signaling modulates synchronized excitation in the sublaterodorsal tegmental nucleus to stabilize REM sleep

The relationship between orexin/hypocretin and rapid eye movement (REM) sleep remains elusive. Here, we find that a proportion of orexin neurons project to the sublaterodorsal tegmental nucleus (SLD) and exhibit REM sleep-related activation. In SLD, orexin directly excites orexin receptor-positive neurons (occupying ~3/4 of total-population) and increases gap junction conductance among neurons. Their interaction spreads the orexin-elicited partial-excitation to activate SLD network globally. Besides, the activated SLD network exhibits increased probability of synchronized firings. This synchronized excitation promotes the correspondence between SLD and its downstream target to enhance SLD output. Using optogenetics and fiber-photometry, we consequently find that orexin-enhanced SLD output prolongs REM sleep episodes through consolidating brain state activation/muscle tone inhibition. After chemogenetic silencing of SLD orexin signaling, a ~17% reduction of REM sleep amounts and disruptions of REM sleep muscle atonia are observed. These findings reveal a stabilization role of orexin in REM sleep.

Orexin signaling is provided by diffusely distributed fibers and involved in different brain circuits that orchestrate sleep and wakefulness states. Here, the authors show that a proportion of orexin neurons project to the sublaterodorsal tegmental nucleus and exhibit rapid eye movement (REM) sleep-related actions.

Orexins role in neurodegenerative diseases: From pathogenesis to treatment

Orexin is a neurotransmitter that mainly regulates sleep/wake cycle. In addition to its sleep cycle regulatory role, it is involved in regulation of attention, energy homeostasis, neurogenesis and cognition. Several evidences has shown the involvement of orexin in narcolepsy, but there are also growing evidences that shows the disturbance in orexin system in neurodegenerative diseases including Alzheimer's, Parkinson's, Epilepsy, Huntington's diseases and Amyotrophic lateral sclerosis. Pathogenesis and clinical symptoms of these disorders can be partly attributed from orexin system imbalance. However, there are controversial reports on the exact relationship between orexin and these neurodegenerative diseases. Therefore, the aim of this review is to summarize the current evidences regarding the role of orexin in these neurodegenerative diseases.

Role of spontaneous and sensory orexin network dynamics in rapid locomotion initiation

Appropriate motor control is critical for normal life, and requires hypothalamic hypocretin/orexin neurons (HONs). HONs are slowly regulated by nutrients, but also display rapid (subsecond) activity fluctuations in vivo. The necessity of these activity bursts for sensorimotor control and their roles in specific phases of movement are unknown. Here we show that temporally-restricted optosilencing of spontaneous or sensory-evoked HON bursts disrupts locomotion initiation, but does not affect ongoing locomotion. Conversely, HON optostimulation initiates locomotion with subsecond delays in a frequency-dependent manner. Using 2-photon volumetric imaging of activity of >300 HONs during sensory stimulation and self-initiated locomotion, we identify several locomotion-related HON subtypes, which distinctly predict the probability of imminent locomotion initiation, display distinct sensory responses, and are differentially modulated by food deprivation. By causally linking HON bursts to locomotion initiation, these findings reveal the sensorimotor importance of rapid spontaneous and evoked fluctuations in HON ensemble activity.

The globus pallidus as a target for neuropeptides and endocannabinoids participating in central activities

The globus pallidus in the basal ganglia plays an important role in movement regulation. Neuropeptides and endocannabinoids are neuronal signalling molecules that influence the functions of the whole brain. Endocannabinoids, enkephalin, substance P, neurotensin, orexin, somatostatin and pituitary adenylate cyclase-activating polypeptides are richly concentrated in the globus pallidus. Neuropeptides and endocannabinoids exert excitatory or inhibitory effects in the globus pallidus mainly by modulating GABAergic, glutamatergic and dopaminergic neurotransmission, as well as many ionic mechanisms. Pallidal neuropeptides and endocannabinoids are associated with the pathophysiology of a number of neurological disorders, such as Parkinson's disease, Huntington's disease, schizophrenia, and depression. The levels of neuropeptides and endocannabinoids and their receptors in the globus pallidus change in neurological diseases. It has been demonstrated that spontaneous firing activity of globus pallidus neurons is closely related to the manifestations of Parkinson's disease. Therefore, the neuropeptides and endocannabinoids in the globus pallidus may function as potential targets for treatment in some neurological diseases. In this review, we highlight the morphology and function of neuropeptides and endocannabinoids in the globus pallidus and their involvement in neurological diseases.

Orexins alleviate motor deficits via increasing firing activity of pallidal neurons in a mouse model of Parkinson's disease

Orexin is a peptide neurotransmitter released in the globus pallidus. Morphological evidence reveals that both orexin 1 receptor (OX₁R) and orexin 2 receptor (OX₂R) exist in the globus pallidus. Here we showed that bilateral microinjection of both orexin-A and orexin-B into the globus pallidus alleviated motor deficits in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced parkinsonian mice. Further in vivo extracellular single-unit recording revealed that the basal spontaneous firing rate of the globus pallidus neurons in MPTP parkinsonian mice was slower than that of normal mice. Application of orexin-A or orexin-B significantly increased the spontaneous firing rate of pallidal neurons. The influx of Ca²⁺ through the L-type Ca²⁺ channel is the major mechanism involved in orexin-induced excitation in the globus pallidus. Orexin-A-induced increase in firing rate of pallidal neurons in MPTP parkinsonian mice was stronger than that of normal mice. Orexin-A exerted both electrophysiological and behavioral effects mainly via OX₁R, and orexin-B exerted the effects via OX₂R. Endogenous orexins modulated the excitability of globus pallidus neurons mainly through OX₁R. The present behavioral and electrophysiological results suggest that orexins ameliorate parkinsonian motor deficits through increasing the spontaneous firing of globus pallidus neurons.

Medial Prefrontal Cortex-Pontine Nuclei Projections Modulate Suboptimal Cue-Induced Associative Motor Learning

Diverse and powerful mechanisms have evolved to enable organisms to modulate learning and memory under a variety of survival conditions. Cumulative evidence has shown that the prefrontal cortex (PFC) is closely involved in many higher-order cognitive functions. However, when and how the medial PFC (mPFC) modulates associative motor learning remains largely unknown. Here, we show that delay eyeblink conditioning (DEC) with the weak conditioned stimulus (wCS) but not the strong CS (sCS) elicited a significant increase in the levels of c-Fos expression in caudal mPFC. Both optogenetic inhibition and activation of the bilateral caudal mPFC, or its axon terminals at the pontine nucleus (PN) contralateral to the training eye, significantly impaired the acquisition, recent and remote retrieval of DEC with the wCS but not the sCS. However, direct optogenetic activation of the contralateral PN had no significant effect on the acquisition, recent and remote retrieval of DEC. These results are of great importance in understanding the elusive role of the mPFC and its projection to PN in subserving the associative motor learning under suboptimal learning cue.

Behavior of mice aboard the International Space Station

Interest in space habitation has grown dramatically with planning underway for the first human transit to Mars. Despite a robust history of domestic and international spaceflight research, understanding behavioral adaptation to the space environment for extended durations is scant. Here we report the first detailed behavioral analysis of mice flown in the NASA Rodent Habitat on the International Space Station (ISS). Following 4-day transit from Earth to ISS, video images were acquired on orbit from 16- and 32-week-old female mice. Spaceflown mice engaged in a full range of species-typical behaviors. Physical activity was greater in younger flight mice as compared to identically-housed ground controls, and followed the circadian cycle. Within 7-10 days after launch, younger (but not older), mice began to exhibit distinctive circling or 'race-tracking' behavior that evolved into coordinated group activity. Organized group circling behavior unique to spaceflight may represent stereotyped motor behavior, rewarding effects of physical exercise, or vestibular sensation produced via self-motion. Affording mice the opportunity to grab and run in the RH resembles physical activities that the crew participate in routinely. Our approach yields a useful analog for better understanding human responses to spaceflight, providing the opportunity to assess how physical movement influences responses to microgravity.

Functional characterization of an orexin neuropeptide in amphioxus reveals an ancient origin of orexin/orexin receptor system in chordate

Amphioxus belongs to the subphylum cephalochordata, an extant representative of the most basal chordates, whose regulation of endocrine system remains ambiguous. Here we clearly demonstrated the existence of a functional orexin neuropeptide in amphioxus, which is able to interact with orexin receptor, activate both PKC and PKA pathways, decrease leptin expression, and stimulate lipogenesis. We also showed the transcription level of amphioxus orexin was affected by fasting or temperature, indicating a role of this gene in the regulation of energy balance. In addition, the expression of the amphioxus orexin was detected at cerebral vesicle, which has been proposed to be a homolog of the vertebrate brain. These data collectively suggest that a functional orexin neuropeptide has already emerged in amphioxus, which provide insights into the evolutionary origin of orexin in chordate and the functional homology between the cerebral vesicle and vertebrate brain.

Fluctuation of primary motor cortex excitability during cataplexy in narcolepsy

Objective

Cataplexy is a complicated and dynamic process in narcolepsy type 1 (NT1) patients. This study aimed to clarify the distinct stages during a cataplectic attack and identify the changes of the primary motor cortex (PMC) excitability during these stages.

Methods

Thirty-five patients with NT1 and 29 healthy controls were recruited to this study. Cataplectic stages were distinguished from a cataplectic attack by video-polysomnogram monitoring. Transcranial magnetic stimulation motor-evoked potential (TMS-MEP) was performed to measure the excitability of PMC during quiet wakefulness, laughter without cataplexy, and each cataplectic stage.

Results

Based on the video and electromyogram observations, a typical cataplectic attack (CA) process is divided into four stages: triggering (CA1), resisting (CA2), atonic (CA3), and recovering stages (CA4). Compared with healthy controls, NT1 patients showed significantly decreased intracortical facilitation during quiet wakefulness. During the laughter stage, both patients and controls showed increased MEP amplitude compared with quiet wakefulness. The MEP amplitude significantly increased even higher in CA1 and 2, and then dramatically decreased in CA3 accompanied with prolonged MEP latency compared with the laughter stage and quiet wakefulness. The MEP amplitude and latency gradually recovered during CA4.

Interpretation

This study identifies four stages during cataplectic attack and reveals the existence of a resisting stage that might change the process of cataplexy. The fluctuation of MEP amplitude and MEP latency shows a potential participation of PMC and motor control pathway during cataplexy, and the increased MEP amplitude during CA1 and 2 strongly implies a compensatory mechanism in motor control that may resist or avoid cataplectic attack.

Orexin-A Exerts Neuroprotective Effects via OX1R in Parkinson's Disease

Parkinson's disease (PD) is a common neurodegenerative disorder characterized by progressive and selective death of dopaminergic neurons. Orexin-A is involved in many biological effects of the body. It has been reported that orexin-A has protective effects in cellular models of PD. However, little is known about the protective effects of orexin-A in animal parkinsonian models and the cellular mechanism has not yet been fully clarified. The aim of this study was to evaluate the effects of orexin-A in MPTP mice model of PD as well as the possible neuroprotective mechanisms of orexin-A on dopaminergic neurons. The results from animal experiments demonstrated that orexin-A attenuated the loss of dopaminergic neurons and the decrease of tyrosine hydroxylase (TH) expression in the substantia nigra, normalized the striatal dopaminergic fibers, and prevented the depletion of dopamine and its metabolites in the striatum. MPTP-treated mice showed cognitive impairments accompanied with significant motor deficiency. Orexin-A improved MPTP-induced impairments in both motor activity and spatial memory. Importantly, orexin-A increased the protein level of brain-derived neurotrophic factor (BDNF) in dopaminergic neurons of the substantia nigra. Furthermore, the protective effects of orexin-A on MPTP parkinsonian mice could be blocked by orexinergic receptor 1 (OX1R) antagonist, SB334867. In another set of experiments with SH-SY5Y dopaminergic cells, orexin-A significantly induced the expression of BDNF in a dose and time-dependent manner. The upregulation of BDNF is mainly concerned with PI3K and PKC signaling pathways via OX1R. The present study demonstrated that orexin-A exerted neuroprotective effects on MPTP parkinsonian mice, which may imply orexin-A as a potential therapeutic target for PD.

Orexin-A Exerts Neuroprotective Effects via OX1R in Parkinson's Disease

Parkinson’s disease (PD) is a common neurodegenerative disorder characterized by progressive and selective death of dopaminergic neurons. Orexin-A is involved in many biological effects of the body. It has been reported that orexin-A has protective effects in cellular models of PD. However, little is known about the protective effects of orexin-A in animal parkinsonian models and the cellular mechanism has not yet been fully clarified. The aim of this study was to evaluate the effects of orexin-A in MPTP mice model of PD as well as the possible neuroprotective mechanisms of orexin-A on dopaminergic neurons. The results from animal experiments demonstrated that orexin-A attenuated the loss of dopaminergic neurons and the decrease of tyrosine hydroxylase (TH) expression in the substantia nigra, normalized the striatal dopaminergic fibers, and prevented the depletion of dopamine and its metabolites in the striatum. MPTP-treated mice showed cognitive impairments accompanied with significant motor deficiency. Orexin-A improved MPTP-induced impairments in both motor activity and spatial memory. Importantly, orexin-A increased the protein level of brain-derived neurotrophic factor (BDNF) in dopaminergic neurons of the substantia nigra. Furthermore, the protective effects of orexin-A on MPTP parkinsonian mice could be blocked by orexinergic receptor 1 (OX1R) antagonist, SB334867. In another set of experiments with SH-SY5Y dopaminergic cells, orexin-A significantly induced the expression of BDNF in a dose and time-dependent manner. The upregulation of BDNF is mainly concerned with PI3K and PKC signaling pathways via OX1R. The present study demonstrated that orexin-A exerted neuroprotective effects on MPTP parkinsonian mice, which may imply orexin-A as a potential therapeutic target for PD.

Recent advances in understanding the roles of hypocretin/orexin in arousal, affect, and motivation

The hypocretins (Hcrts) are two alternatively spliced neuropeptides (Hcrt1/Ox-A and Hcrt2/Ox-B) that are synthesized exclusively in the hypothalamus. Data collected in the 20 years since their discovery have supported the view that the Hcrts play a broad role in the control of arousal with a particularly important role in the maintenance of wakefulness and sleep-to-wake transitions. While this latter point has received an overwhelming amount of research attention, a growing literature has begun to broaden our understanding of the many diverse roles that the Hcrts play in physiology and behavior. Here, we review recent advances in the neurobiology of Hcrt in three sections. We begin by surveying findings on Hcrt function within normal sleep/wake states as well as situations of aberrant sleep (that is, narcolepsy). In the second section, we discuss research establishing a role for Hcrt in mood and affect (that is, anxiety, stress, and motivation). Finally, in the third section, we briefly discuss future directions for the field and place an emphasis on analytical modeling of Hcrt neural activity. We hope that the data discussed here provide a broad overview of recent progress in the field and make clear the diversity of roles played by these neuromodulators.

The Subthalamic Neurons are Activated by Both Orexin-A and Orexin-B

The subthalamic nucleus is an important nucleus in the indirect pathway of the basal ganglia circuit and therefore is involved in motor control under both normal and pathological conditions. Morphological studies reveal that the subthalamic nucleus receives relatively dense orexinergic projections originating from the hypothalamus. Both orexin-1 (OX₁) and orexin-2 (OX₂) receptors are expressed in the subthalamic nucleus. To explore the functions of orexinergic system in the subthalamic nucleus, extracellular electrophysiological recordings and behavioral tests were performed in the present study. Exogenous application of orexin-A significantly increased the spontaneous firing rate from 5.70 ± 0.66 Hz to 9.87 ± 1.18 Hz in 64.00% subthalamic neurons recorded. OX₁ receptors are involved in orexin-A-induced excitation. Application of orexin-B increased the firing rate from 7.47 ± 0.92 Hz to 11.85 ± 1.39 Hz in 80.95% subthalamic neurons recorded, entirely through OX₂ receptors. Both OX₁ and OX₂ receptor antagonists decreased the firing rate in 43.75% and 62.50% subthalamic neurons recorded respectively, suggesting the involvement of endogenous orexinergic system in the control of spontaneous firing activity. Further elevated body swing test revealed that microinjection of orexins and the receptor antagonists into the subthalamic nucleus induced contralateral-biased swing and ipsilateral-biased swing, respectively. Taken together, the present study suggests that orexins play important roles in the subthalamic nucleus which may provide further evidence for the involvement of subthalamic orexinergic tone in Parkinson's disease.

SIGNIFICANCE

Previous morphological studies indicate that the subthalamic nucleus receives orexinergic innervation and expresses both OX₁ and OX₂ receptors. Using in vivo multibarrel electrophysiological recordings, the present study revealed that exogenous application of orexin-A and orexin-B increased the spontaneous firing rate of the subthalamic neurons through OX₁ and OX₂ receptors. Endogenous orexinergic system was involved in the control of spontaneous firing of the subthalamic neurons. Further behavioral test revealed that intrasubthalamic application of orexins and the receptor antagonists induced biased swing behavior. The present study may provide further evidence for the involvement of subthalamic orexinergic tone in Parkinson's disease.

Reversible inactivation of the lateral hypothalamus reversed high reward choices in cost-benefit decision-making in rats

The Lateral hypothalamus (LH) is an important component of the networks underlying the control of feeding and other motivated behaviors. Cost-benefit decision-making is mediated largely by the prefrontal cortex (PFC) which strongly innervates the LH. Therefore, in the current study, we conducted a series of experiments to elucidate the role of the perifornical area of the lateral hypothalamus (PeF-LH) in effort and/or delay-based decision-making. We trained different groups of rats in a delay-based and/or an effort-based form of cost-benefit T-maze decision- making task in which they could either choose to pay the cost to obtain a high reward in one arm or could obtain a low reward in the other arm with no cost. During test days, the rats received local injections of either vehicle or lidocaine4% (0.5 μl/side), in the PeF-LH. In an effort-based decision task, PeF-LH inactivation led to decrease in high reward choice. Similarly, in a delay-based decision task animals' preference changed to a low but immediately available reward. This was not caused by a spatial memory or motor deficit. PeF-LH inactivation modified decision behavior. The results imply that PeF-LH is important for allowing the animal to pay a cost to acquire greater rewards.

Calcium, mitochondrial dysfunction and slowing the progression of Parkinson's disease

Parkinson's disease is characterized by progressively distributed Lewy pathology and neurodegeneration. The motor symptoms of clinical Parkinson's disease (cPD) are unequivocally linked to the degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNc). Several features of these neurons appear to make them selectively vulnerable to factors thought to cause cPD, like aging, genetic mutations and environmental toxins. Among these features, Ca²⁺ entry through Cav1 channels is particularly amenable to pharmacotherapy in early stage cPD patients. This review outlines the linkage between these channels, mitochondrial oxidant stress and cPD pathogenesis. It also summarizes considerations that went into the design and execution of the ongoing Phase 3 clinical trial with an inhibitor of these channels - isradipine.

The spectrum of REM sleep-related episodes in children with type 1 narcolepsy

Type 1 narcolepsy is a central hypersomnia due to the loss of hypocretin-producing neurons and characterized by cataplexy, excessive daytime sleepiness, sleep paralysis, hypnagogic hallucinations and disturbed nocturnal sleep. In children, close to the disease onset, type 1 narcolepsy has peculiar clinical features with severe cataplexy and a complex admixture of movement disorders occurring while awake. Motor dyscontrol during sleep has never been systematically investigated. Suspecting that abnormal motor control might affect also sleep, we systematically analysed motor events recorded by means of video polysomnography in 40 children with type 1 narcolepsy (20 females; mean age 11.8 ± 2.6 years) and compared these data with those recorded in 22 age- and sex-matched healthy controls. Motor events were classified as elementary movements, if brief and non-purposeful and complex behaviours, if simulating purposeful behaviours. Complex behaviours occurring during REM sleep were further classified as 'classically-defined' and 'pantomime-like' REM sleep behaviour disorder episodes, based on their duration and on their pattern (i.e. brief and vivid-energetic in the first case, longer and with subcontinuous gesturing mimicking daily life activity in the second case). Elementary movements emerging either from non-REM or REM sleep were present in both groups, even if those emerging from REM sleep were more numerous in the group of patients. Conversely, complex behaviours could be detected only in children with type 1 narcolepsy and were observed in 13 patients, with six having 'classically-defined' REM sleep behaviour disorder episodes and seven having 'pantomime-like' REM sleep behaviour disorder episodes. Complex behaviours during REM sleep tended to recur in a stereotyped fashion for several times during the night, up to be almost continuous. Patients displaying a more severe motor dyscontrol during REM sleep had also more severe motor disorder during daytime (i.e. status cataplecticus) and more complaints of disrupted nocturnal sleep and of excessive daytime sleepiness. The neurophysiological hallmark of this severe motor dyscontrol during REM sleep was a decreased atonia index. The present study reports for the first time the occurrence of a severe and peculiar motor disorder during REM sleep in paediatric type 1 narcolepsy and confirms the presence of a severe motor dyscontrol in these patients, emerging not only from wakefulness (i.e. status cataplecticus), but also from sleep (i.e. complex behaviours during REM sleep). This is probably related to the acute imbalance of the hypocretinergic system, which physiologically acts by promoting movements during wakefulness and suppressing them during sleep.

Orexin exerts excitatory effects on reticulospinal neurons in the rat gigantocellular reticular nucleus through the activation of postsynaptic orexin-1 and orexin-2 receptors

Previous studies have revealed that orexin may actively participate in central motor control. The gigantocellular reticular nucleus (Gi) is a key element of the brainstem motor inhibitory system. The descending orexinergic projections also reach Gi region, and microinjection of orexin into Gi causes robust muscle tone inhibition. However, the modulation effects of orexin on Gi neurons remain unclear. In the present study, using whole-cell patch-clamp recordings, we initially observed that orexin elicited an inward current in Gi neurons at a holding potential of -70mV in a concentration-dependent manner. By combining electrophysiology with neuropharmacological methods, we further determined that the orexin-induced inward current was directly mediated by the activation of postsynaptic orexin-1 and orexin-2 receptors. Moreover, orexin did not affect the frequency and amplitude of miniature excitatory and inhibitory postsynaptic currents in Gi neurons, which suggests that orexin had no effects on neurotransmission to these neurons. Therefore, the direct excitatory effect of orexin on an inhibitory motor structure, the Gi, was reported in the present study. This modulation may be integrated into the role of orexin in central motor control.

Selective neuronal vulnerability in Parkinson disease

Intracellular α-synuclein (α-syn)-rich protein aggregates called Lewy pathology (LP) and neuronal death are commonly found in the brains of patients with clinical Parkinson disease (cPD). It is widely believed that LP appears early in the disease and spreads in synaptically coupled brain networks, driving neuronal dysfunction and death. However, post-mortem analysis of human brains and connectome-mapping studies show that the pattern of LP in cPD is not consistent with this simple model, arguing that, if LP propagates in cPD, it must be gated by cell- or region-autonomous mechanisms. Moreover, the correlation between LP and neuronal death is weak. In this Review, we briefly discuss the evidence for and against the spreading LP model, as well as evidence that cell-autonomous factors govern both α-syn pathology and neuronal death.

Calcium and Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disease in the world. Its causes are poorly understood and there is no proven therapeutic strategy for slowing disease progression. The core motor symptoms of PD are caused by the death of dopaminergic neurons in the substantia nigra pars compacta (SNc). In these neurons, Ca²⁺entry through plasma membrane Cav1 channels drives a sustained feed-forward stimulation of mitochondrial oxidative phosphorylation. Although this design helps prevent bioenergetic failure when activity needs to be sustained, it leads to basal mitochondrial oxidant stress. Over decades, this basal oxidant stress could compromise mitochondrial function and increase mitophagy, resulting in increased vulnerability to other proteostatic stressors, like elevated alpha synuclein expression. Because this feedforward mechanism is no longer demanded by our lifestyle, it could be dispensed with. Indeed, use of dihydropyridines - negative allosteric modulators of Cav1 Ca²⁺ channels - comes with little or no effect on brain function but is associated with decreased risk and progression of PD. An ongoing, NIH sponsored, Phase 3 clinical trial in North America is testing the ability of one member of the dihydropyridine class (isradipine) to slow PD progression in early stage patients. The review summarizes the rationale for the trial and outlines some unanswered questions.

Evidence for a Role of Orexin/Hypocretin System in Vestibular Lesion-Induced Locomotor Abnormalities in Rats

Vestibular damage can induce locomotor abnormalities in both animals and humans. Rodents with bilateral vestibular loss showed vestibular deficits syndrome such as circling, opisthotonus as well as locomotor and exploratory hyperactivity. Previous studies have investigated the changes in the dopamine system after vestibular loss, but the results are inconsistent and inconclusive. Numerous evidences indicate that the orexin system is implicated in central motor control. We hypothesized that orexin may be potentially involved in vestibular loss-induced motor disorders. In this study, we examined the effects of arsanilate- or 3,3′-iminodipropionitrile (IDPN)-induced vestibular lesion (AVL or IVL) on the orexin-A (OXA) labeling in rat hypothalamus using immunohistochemistry. The vestibular lesion-induced locomotor abnormalities were recorded and verified using a histamine H4 receptor antagonist JNJ7777120 (20 mg/kg, i.p.). The effects of the orexin receptor type 1 antagonist SB334867 (16 μg, i.c.v.) on these behavior responses were also investigated. At 72 h post-AVL and IVL, animals exhibited vestibular deficit syndrome and locomotor hyperactivity in the home cages. These responses were significantly alleviated by JNJ7777120 which also eliminated AVL-induced increases in exploratory behavior in an open field. The numbers of OXA-labeled neurons in the hypothalamus were significantly increased in the AVL animals at 72 h post-AVL and in the IVL animals at 24, 48, and 72 h post-IVL. SB334867 significantly attenuated the vestibular deficit syndrome and locomotor hyperactivity at 72 h post-AVL and IVL. It also decreased exploratory behavior in the AVL animals. These results suggested that the alteration of OXA expression might contribute to locomotor abnormalities after acute vestibular lesion. The orexin receptors might be the potential therapeutic targets for vestibular disorders.

Orexin-A increases the activity of globus pallidus neurons in both normal and parkinsonian rats

Orexin is a member of neuropeptides which was first identified in the hypothalamus. The globus pallidus is a key structure in the basal ganglia, which is involved in both normal motor function and movement disorders. Morphological studies have shown the expression of both OX1 and OX2 receptors in the globus pallidus. Employing single unit extracellular recordings and behavioural tests, the direct in vivo electrophysiological and behavioural effects of orexin-A in the globus pallidus were studied. Micro-pressure administration of orexin-A significantly increased the spontaneous firing rate of pallidal neurons. Correlation analysis revealed a negative correlation between orexin-A induced excitation and the basal firing rate. Furthermore, application of the specific OX1 receptor antagonist, SB-334867, decreased the firing rate of pallidal neurons, suggesting that endogenous orexinergic systems modulate the firing activity of pallidal neurons. Orexin-A increased the excitability of pallidal neurons through both OX1 and OX2 receptors. In 6-hydroxydopamine parkinsonian rats, orexin-A-induced increase in firing rate of pallidal neurons was stronger than that in normal rats. Immunostaining revealed positive OX1 receptor expression in the globus pallidus of both normal and parkinsonian rats. Finally, postural test showed that unilateral microinjection of orexin-A led to contralateral deflection in the presence of systemic haloperidol administration. Further elevated body swing test revealed that pallidal orexin-A and SB-334867 induced contralateral-biased swing and ipsilateral-biased swing respectively. Based on the electrophysiological and behavioural findings of orexin-A in the globus pallidus, the present findings may provide a rationale for the pathogenesis and treatment of Parkinson's disease.

Orexin-A affects gastric distention sensitive neurons in the hippocampus and gastric motility and regulation by the perifornical area in rats

Orexin-A is mainly produced in the lateral hypothalamus (LHA) and the perifornical area (PeF). Here, we aim to elucidate the effects of orexin-A in the hippocampus (Hi) on gastric distention (GD)-sensitive neurons and gastric motility, and potential regulation mechanisms by the PeF. Retrograde tracing and fluorescent-immunohistochemical staining were used to determine orexin-A neuronal projections. Single unit discharges in the Hi were recorded extracellularly and gastric motility in conscious rats was monitored during administration of orexin-A to the Hi or electrical stimulation of the PeF. Orexin-A administration to the Hi excited most of the GD-excitatory (GD-E) neurons and GD-inhibitory (GD-I) neurons, and increased gastric motility in a dose-dependent manner. All of effects induced by orexin-A could be partly blocked by pretreatment with orexin-A antagonist, SB-334867. Electrical stimulation of the PeF excited the majority of the orexin-A-responsive GD neurons in the Hi and promoted gastric motility. Additionally, pretreatment with SB-334867 in the Hi increased the firing rate of GDI and GDE neurons following electrical stimulation of the PeF. These findings suggest that orexin-A could regulate activities of GD-sensitive neurons and gastric motility. Furthermore, the PeF may be involved in this regulatory pathway.

Contribution of Orexin to the Neurogenic Hypertension in BPH/2J Mice

BPH/2J mice are a genetic model of hypertension associated with an overactive sympathetic nervous system. Orexin is a neuropeptide which influences sympathetic activity and blood pressure. Orexin precursor mRNA expression is greater in hypothalamic tissue of BPH/2J compared with normotensive BPN/3J mice. To determine whether enhanced orexinergic signaling contributes to the hypertension, BPH/2J and BPN/3J mice were preimplanted with radiotelemetry probes to compare blood pressure 1 hour before and 5 hours after administration of almorexant, an orexin receptor antagonist. Mid frequency mean arterial pressure power and the depressor response to ganglion blockade were also used as indicators of sympathetic nervous system activity. Administration of almorexant at 100 (IP) and 300 mg/kg (oral) in BPH/2J mice during the dark-active period (2 hours after lights off) markedly reduced blood pressure (-16.1 ± 1.6 and -11.0 ± 1.1 mm Hg, respectively;P<0.001 compared with vehicle). However, when almorexant (100 mg/kg, IP) was administered during the light-inactive period (5 hours before lights off) no reduction from baseline was observed (P=0.64). The same dose of almorexant in BPN/3J mice had no effect on blood pressure during the dark (P=0.79) or light periods (P=0.24). Almorexant attenuated the depressor response to ganglion blockade (P=0.018) and reduced the mid frequency mean arterial pressure power in BPH/2J mice (P<0.001), but not BPN/3J mice (P=0.70). Immunohistochemical labeling revealed that BPH/2J mice have 29% more orexin neurons than BPN/3J mice which are preferentially located in the lateral hypothalamus. The results suggest that enhanced orexinergic signaling contributes to sympathetic overactivity and hypertension during the dark period in BPH/2J mice.

Theta synchronization between medial prefrontal cortex and cerebellum is associated with adaptive performance of associative learning behavior

Associative learning is thought to require coordinated activities among distributed brain regions. For example, to direct behavior appropriately, the medial prefrontal cortex (mPFC) must encode and maintain sensory information and then interact with the cerebellum during trace eyeblink conditioning (TEBC), a commonly-used associative learning model. However, the mechanisms by which these two distant areas interact remain elusive. By simultaneously recording local field potential (LFP) signals from the mPFC and the cerebellum in guinea pigs undergoing TEBC, we found that theta-frequency (5.0-12.0 Hz) oscillations in the mPFC and the cerebellum became strongly synchronized following presentation of auditory conditioned stimulus. Intriguingly, the conditioned eyeblink response (CR) with adaptive timing occurred preferentially in the trials where mPFC-cerebellum theta coherence was stronger. Moreover, both the mPFC-cerebellum theta coherence and the adaptive CR performance were impaired after the disruption of endogenous orexins in the cerebellum. Finally, association of the mPFC -cerebellum theta coherence with adaptive CR performance was time-limited occurring in the early stage of associative learning. These findings suggest that the mPFC and the cerebellum may act together to contribute to the adaptive performance of associative learning behavior by means of theta synchronization.