Low CSF hypocretin-1/orexin-A associated with hypersomnia secondary to hypothalamic lesion in a case of multiple sclerosis

Recent insights into the pathophysiology of narcolepsy type 1

Narcolepsy type 1 (NT1) is a sleep-wake disorder in which people typically experience excessive daytime sleepiness, cataplexy and other sleep-wake disturbances impairing daily life activities. NT1 symptoms are due to hypocretin deficiency. The cause for the observed hypocretin deficiency remains unclear, even though the most likely hypothesis is that this is due to an auto-immune process. The search for autoantibodies and autoreactive T-cells has not yet produced conclusive evidence for or against the auto-immune hypothesis. Other mechanisms, such as reduced corticotrophin-releasing hormone production in the paraventricular nucleus have recently been suggested. There is no reversive treatment, and the therapeutic approach is symptomatic. Early diagnosis and appropriate NT1 treatment is essential, especially in children to prevent impaired cognitive, emotional and social development. Hypocretin receptor agonists have been designed to replace the attenuated hypocretin signalling. Pre-clinical and clinical trials have shown encouraging initial results. A better understanding of NT1 pathophysiology may contribute to faster diagnosis or treatments, which may cure or prevent it.

Potential immunological triggers for narcolepsy and idiopathic hypersomnia: Real-world insights on infections and influenza vaccinations

OBJECTIVE

It is hypothesized that narcolepsy type 1 (NT1) develops in genetically susceptible people who encounter environmental triggers leading to immune-mediated hypocretin-1 deficiency. The pathophysiologies of narcolepsy type 2 (NT2) and idiopathic hypersomnia (IH) remain unknown. The main aim of this study was to collect all reported immunological events before onset of a central disorder of hypersomnolence.

METHODS

Medical records of 290 people with NT1, and 115 with NT2 or IH were retrospectively reviewed to extract infection and influenza vaccination history. Prevalence, distribution of immunological events, and time until hypersomnolence onset were compared between NT1 and the combined group of NT2 and IH.

RESULTS

Immunological events were frequently reported before hypersomnolence disorder onset across groups. Flu and H1N1 influenza vaccination were more common in NT1, and Epstein-Barr virus and other respiratory and non-respiratory infections in NT2 and IH. Distributions of events were comparable between NT2 and IH. Rapid symptom onset within one month of infection was frequent across groups, especially after flu infection in NT1. Hypersomnolence disorder progression after an immunological event was reported in ten individuals.

CONCLUSIONS

Our findings suggest a variety of immunological triggers potentially related to NT1, including H1N1 influenza infection or vaccination, infection with other flu types, and other respiratory and non-respiratory infections. Frequent reports of immunological events (other than those reported in NT1) immediately prior to the development of NT2 and IH support the specificity of triggers for NT1, and open important new research avenues into possible underlying immunological mechanisms in NT2 and IH.

Targeting the orexin/hypocretin system for the treatment of neuropsychiatric and neurodegenerative diseases: from animal to clinical studies

Orexins (also known as hypocretins) are neuropeptides located exclusively in hypothalamic neurons that have extensive projections throughout the central nervous system and bind two different G protein-coupled receptors (OX1R and OX2R). Since its discovery in 1998, the orexin system has gained the interest of the scientific community as a potential therapeutic target for the treatment of different pathological conditions. Considering previous basic science research, a dual orexin receptor antagonist, suvorexant, was the first orexin agent to be approved by the US Food and Drug Administration to treat insomnia. In this review, we discuss and update the main preclinical and human studies involving the orexin system with several psychiatric and neurodegenerative diseases. This system constitutes a nice example of how basic scientific research driven by curiosity can be the best route to the generation of new and powerful pharmacological treatments.

Orexinergic System in Neurodegenerative Diseases

Orexinergic system consisting of orexins and orexin receptors plays an essential role in regulating sleep–wake states, whereas sleep disruption is a common symptom of a number of neurodegenerative diseases. Emerging evidence reveals that the orexinergic system is disturbed in various neurodegenerative diseases, including Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease (HD), and multiple sclerosis (MS), whereas the dysregulation of orexins and/or orexin receptors contributes to the pathogenesis of these diseases. In this review, we summarized advanced knowledge of the orexinergic system and its role in sleep, and reviewed the dysregulation of the orexinergic system and its role in the pathogenesis of AD, PD, HD, and MS. Moreover, the therapeutic potential of targeting the orexinergic system for the treatment of these diseases was discussed.

Focus on the Complex Interconnection between Cancer, Narcolepsy and Other Neurodegenerative Diseases: A Possible Case of Orexin-Dependent Inverse Comorbidity

Simple Summary

This narrative review first describes from several points of view the complex interrelationship between cancer and neurodegeneration, with special attention to the mechanisms that might underlie an inverse relationship between them. In particular, the mechanisms that might induce an imbalance between cell apoptotic and proliferative stimuli are discussed. Second, the review summarizes findings on orexins and their involvement in narcolepsy, neurodegenerative diseases, and cancer, starting from epidemiological data then addressing laboratory findings, animal models, and human clinical observational and interventional investigations. Important research efforts are warranted on these topics, as they might lead to novel therapeutic approaches to both neurodegenerative diseases and cancer.

Abstract

Conditions such as Alzheimer’s (AD) and Parkinson’s diseases (PD) are less prevalent in cancer survivors and, overall, cancer is less prevalent in subjects with these neurodegenerative disorders. This seems to suggest that a propensity towards one type of disease may decrease the risk of the other. In addition to epidemiologic data, there is also evidence of a complex biological interconnection, with genes, proteins, and pathways often showing opposite dysregulation in cancer and neurodegenerative diseases. In this narrative review, we focus on the possible role played by orexin signaling, which is altered in patients with narcolepsy type 1 and in those with AD and PD, and which has been linked to β-amyloid brain levels and inflammation in mouse models and to cancer in cell lines. Taken together, these lines of evidence depict a possible case of inverse comorbidity between cancer and neurodegenerative disorders, with a role played by orexins. These considerations suggest a therapeutic potential of orexin modulation in diverse pathologies such as narcolepsy, neurodegenerative disorders, and cancer.

Hormones as adaptive control systems in juvenile fish

Growth is an important theme in biology. Physiologists often relate growth rates to hormonal control of essential processes. Ecologists often study growth as a function of gradients or combinations of environmental factors. Fewer studies have investigated the combined effects of environmental and hormonal control on growth. Here, we present an evolutionary optimization model of fish growth that combines internal regulation of growth by hormone levels with the external influence of food availability and predation risk. The model finds a dynamic hormone profile that optimizes fish growth and survival up to 30 cm, and we use the probability of reaching this milestone as a proxy for fitness. The complex web of interrelated hormones and other signalling molecules is simplified to three functions represented by growth hormone, thyroid hormone and orexin. By studying a range from poor to rich environments, we find that the level of food availability in the environment results in different evolutionarily optimal strategies of hormone levels. With more food available, higher levels of hormones are optimal, resulting in higher food intake, standard metabolism and growth. By using this fitness-based approach we also find a consequence of evolutionary optimization of survival on optimal hormone use. Where foraging is risky, the thyroid hormone can be used strategically to increase metabolic potential and the chance of escaping from predators. By comparing model results to empirical observations, many mechanisms can be recognized, for instance a change in pace-of-life due to resource availability, and reduced emphasis on reserves in more stable environments.This article has an associated First Person interview with the first author of the paper.

Sleep disturbance and cognitive decline in multiple sclerosis patients with isolated optic neuritis as the first demyelinating event

PURPOSE

Multiple sclerosis (MS) patients whose first demyelinating event is optic neuritis have been claimed to display a milder disease course and reduced physical disability. Our aim was to investigate the impact of the clinical features of the first clinical episode on cognitive disability and sleep dysfunction in MS.

METHODS

A total of 26 (10 with optic neuritis as the first clinical event) MS patients were recruited. A comprehensive sleep study was performed, and a panel of tests were administered to examine cognitive and motor performance. Serum levels of sleep-related mediators orexin-A and melatonin were measured by enzyme-linked immunosorbent assay. Subjective sleep quality was evaluated by Pittsburgh sleep quality test, and daytime excessive sleepiness was tested by Epworth sleepiness scale.

RESULTS

MS patients with the first clinical episode of optic neuritis and patients with at least one optic neuritis attack exhibited increased daytime sleepiness, higher sleep efficiency and NREM duration and lower total wake time. Patients with a history of optic neuritis obtained more favorable scores in neuropsychological tests measuring executive functions and complex attention as compared to those who had never experienced optic neuritis. Melatonin and orexin-A levels were lower in patients with optic neuritis onset. The higher no. of optic neuritis attacks was associated with reduced wake time and higher symbol digit modalities test scores.

CONCLUSIONS

Having a history of optic neuritis is associated with improved sleep quality and executive functions but increased daytime sleepiness. Reduction of orexin-A and melatonin levels might be one of the underlying mechanisms.

Systemic administration of orexin A ameliorates established experimental autoimmune encephalomyelitis by diminishing neuroinflammation

BACKGROUND

Orexins (hypocretins, Hcrt) A and B are GPCR-binding hypothalamic neuropeptides known to regulate sleep/wake states and feeding behavior. A few studies have shown that orexin A exhibits anti-inflammatory and neuroprotective properties, suggesting that it might provide therapeutic effects in inflammatory and neurodegenerative diseases like multiple sclerosis (MS). In MS, encephalitogenic Th1 and Th17 cells trigger an inflammatory response in the CNS destroying the myelin sheath. Here, we investigated the effects of peripheral orexin A administration to mice undergoing experimental autoimmune encephalomyelitis (EAE), a widely used model of MS.

METHODS

Mice were subcutaneously immunized with myelin oligodendrocyte glycoprotein peptide (MOG)_35-55 in CFA. Mice were treated intraperitoneally for five consecutive days with either PBS or 300 μg of orexin A starting at a moderate EAE score. Molecular, cellular, and histological analysis were performed by real-time PCR, ELISA, flow cytometry, and immunofluorescence.

RESULTS

Orexin A strongly ameliorated ongoing EAE, limiting the infiltration of pathogenic CD4⁺ T lymphocytes, and diminishing chemokine (MCP-1/CCL2 and IP-10/CXCL10) and cytokine (IFN-γ (Th1), IL-17 (Th17), TNF-α, IL-10, and TGF-β) expressions in the CNS. Moreover, orexin A treatment was neuroprotective, decreasing demyelination, astrogliosis, and microglial activation. Despite its strong local therapeutic effects, orexin A did not impair peripheral draining lymph node cell proliferation and Th1/Th17 cytokine production in response to MOG_35-55 in vitro.

CONCLUSIONS

Peripherally-administered orexin A ameliorated EAE by reducing CNS neuroinflammation. These results suggest that orexins may represent new therapeutic candidates that should be further investigated for MS treatment.

Neuroprotective and antihyperalgesic effects of orexin-A in rats with painful diabetic neuropathy

AIM OF STUDY

Diabetes mellitus is related to the development of neuronal tissue injury in different peripheral and central nervous system regions. A common complication of diabetes is painful diabetic peripheral neuropathy (PDN). We have studied the neuroprotective and anti-nociceptive properties of neuropeptide orexin-A in an animal experimental model of diabetic neuropathy.

METHODS

All experiments were carried out on male Wistar rats (220-250 g). Diabetes was induced by a single intraperitoneal injection of 55 mg/kg (i.p.) streptozotocin (STZ). Orexin-A was chronically administrated into the implanted intrathecal catheter (0.6, 2.5 and 5 nM/L, daily, 4 weeks). The tail-flick and rotarod treadmill tests were used to evaluate the nociceptive threshold and motor coordination of these diabetic rats, respectively. Cleaved caspase-3, Bax, Bcl2 and the Bax/Bcl-2 ratio, as the biochemical indicators of apoptosis, were investigated in the dorsal half of the lumbar spinal cord tissue by western blotting method.

RESULTS

Treatment of the diabetic rats with orexin-A (5 nM/L) significantly attenuated the hyperalgesia and motor deficit in diabetic animals. Furthermore, orexin-A (5 nM/L) administration suppressed pro-apoptotic cleaved caspase-3 and Bax proteins. Also, orexin-A (5 nM/L) reduced the expression of Bax/Bcl-2 ratio in spinal cord dorsal half of rats with PDN.

CONCLUSIONS

Altogether our data suggest that the orexin-A has anti-hyperalgesic and neuroprotective effects in rats with PDN. Cellular mechanisms underlying the observed effects may, at least partially, be related to reducing the neuronal apoptosis.

Is there a link between inflammation and fatigue in multiple sclerosis?

Purpose

Among autoimmune diseases of the central nervous system stands multiple sclerosis (MS), which is characterized by demyelination, synaptopathy, and neurodegeneration. MS fatigue can affect up to 90% of patients and be very disabling, with a drastic impact on their quality of life. To date, the evaluation of MS fatigue has relied mainly on subjective scales, and actual therapeutic interventions are challenged by modest efficacy and numerous undesirable effects. Therefore, finding biomarkers of MS fatigue might help in optimizing evaluation and treatment strategies. The main objective here was to assess the relationship between MS fatigue and inflammatory or other immunomediated markers.

Methods

Research was conducted according to PRISMA guidelines. Computerized databases (ie, PubMed/Medline and Scopus) were consulted till February 2018 aiming to identify articles that addressed inflammation and MS fatigue. Studies in English and French published at any time were considered.

Results

A total of 27 studies matched the research criteria. Inconsistency existed regarding the relationship between fatigue and the orexin A system, hypothalamus–pituitary–adrenal axis, and cerebrospinal fluid inflammatory markers. As for peripheral markers, although there was scarcity in the available data, serum proinflammatory cytokines (ie, IL6, TNFα, and IFNγ) seem to be associated with MS fatigue. Finally, no link was found between MS fatigue and T-cell populations (ie, CD3⁺CD4⁺ T lymphocytes, regulatory T cells) or other peripheral markers of inflammation (ie, CRP, erythrocyte-sedimentation rate, soluble ICAM1).

Conclusion

Future large-scale studies would benefit from comparing the relationship between fatigue and immune measures in patients with different disease phenotypes with and without disease-modifying drugs. With the subjective nature of fatigue scales, finding objective biomarkers for fatigue would be of great help.

Demyelination in Multiple Sclerosis: Reprogramming Energy Metabolism and Potential PPARγ Agonist Treatment Approaches

Demyelination in multiple sclerosis (MS) cells is the site of several energy metabolic abnormalities driven by dysregulation between the opposed interplay of peroxisome proliferator-activated receptor γ (PPARγ) and WNT/β-catenin pathways. We focus our review on the opposing interactions observed in demyelinating processes in MS between the canonical WNT/β-catenin pathway and PPARγ and their reprogramming energy metabolism implications. Demyelination in MS is associated with chronic inflammation, which is itself associated with the release of cytokines by CD4⁺ Th17 cells, and downregulation of PPARγ expression leading to the upregulation of the WNT/β-catenin pathway. Upregulation of WNT/β-catenin signaling induces activation of glycolytic enzymes that modify their energy metabolic behavior. Then, in MS cells, a large portion of cytosolic pyruvate is converted into lactate. This phenomenon is called the Warburg effect, despite the availability of oxygen. The Warburg effect is the shift of an energy transfer production from mitochondrial oxidative phosphorylation to aerobic glycolysis. Lactate production is correlated with increased WNT/β-catenin signaling and demyelinating processes by inducing dysfunction of CD4⁺ T cells leading to axonal and neuronal damage. In MS, downregulation of PPARγ decreases insulin sensitivity and increases neuroinflammation. PPARγ agonists inhibit Th17 differentiation in CD4⁺ T cells and then diminish release of cytokines. In MS, abnormalities in the regulation of circadian rhythms stimulate the WNT pathway to initiate the demyelination process. Moreover, PPARγ contributes to the regulation of some key circadian genes. Thus, PPARγ agonists interfere with reprogramming energy metabolism by directly inhibiting the WNT/β-catenin pathway and circadian rhythms and could appear as promising treatments in MS due to these interactions.

Hypothalamic Dysfunction and Multiple Sclerosis: Implications for Fatigue and Weight Dysregulation

Signs and symptoms of multiple sclerosis are usually attributed to demyelinating lesions in the spinal cord or cerebral cortex. The hypothalamus is a region that is often overlooked yet controls many important homeostatic functions, including those that are perturbed in multiple sclerosis. In this review we discuss how hypothalamic dysfunction may contribute to signs and symptoms in people with multiple sclerosis. While dysfunction of the hypothalamic-pituitary-adrenal axis is common in multiple sclerosis, the effects and mechanisms of this dysfunction are not well understood. We discuss three hypothalamic mechanisms of fatigue in multiple sclerosis: (1) general hypothalamic-pituitary-adrenal axis hyperactivity, (2) disordered orexin neurotransmission, (3) abnormal cortisol secretion. We then review potential mechanisms of weight dysregulation caused by hypothalamic dysfunction. Lastly, we propose future studies and therapeutics to better understand and treat hypothalamic dysfunction in multiple sclerosis. Hypothalamic dysfunction appears to be common in multiple sclerosis, yet current studies are underpowered and contradictory. Future studies should contain larger sample sizes and standardize hormone and neuropeptide measurements.

Autonomic dysfunction in multiple sclerosis

Multiple sclerosis (MS) is the leading cause of neurological disability in young adults. Since the pathophysiology of MS is characterized by dissemination in space, as well as in time, the autonomic nervous system is inevitably damaged in the course of the disease in many patients and the proportion of affected patients increases with disease duration. Autonomic dysfunction (AD) in MS is explained by lesions in regions responsible for autonomic regulation such as nuclei in the periventricular region of fourth ventricle in the brainstem as well as medullar lesions. Reports about frequency of AD in MS patients vary notably between groups. Nevertheless its impact on quality of life is substantial but, unfortunately, often overlooked. The aim of this article is to present a concise review of various symptoms and signs of autonomic system dysfunction in MS.

Protective effect of orexin-A on 6-hydroxydopamine-induced neurotoxicity in SH-SY5Y human dopaminergic neuroblastoma cells

Parkinson's disease (PD) is a progressive neurodegenerative disease characterized by progressive and selective death of midbrain dopaminergic neurons. Pharmacologic treatment of PD can be divided into symptomatic and neuroprotective therapies. Orexin-A (hypocretin-1) is a hypothalamic peptide that exerts its biological effects by stimulation of two specific, membrane-bound orexin receptors. Recent studies have shown that orexin-A has a protective role during neuronal damage. Here, we investigated the effects of orexin-A on 6-OHDA-induced neurotoxicity in human neuroblastoma SH-SY5Y cell line as an in vitro model of Parkinson's disease. Cell damage was induced by 150μM 6-OHDA and the cells viability was examined by MTT assay. Intracellular reactive oxygen species (ROS) was determined by fluorescence spectrophotometry method. Immunoblotting and DNA analysis were also employed to determine the levels of biochemical markers of apoptosis in the cells. The data showed that 6-OHDA could decrease the viability of the cells. In addition, intracellular ROS, activated caspase 3, Bax/Bcl-2 ratio, cytochrome c as well as DNA fragmentation were significantly increased in 6-OHDA-treated cells. Pretreatment of cells with orexin-A (80pM) elicited protective effect and reduced biochemical markers of cell death. The results suggest that orexin-A has protective effects against 6-OHDA-induced neurotoxicity and its protective effects are accompanied by its antioxidant and anti-apoptotic properties and contribute to our knowledge of the pharmacology of orexin-A.

Clinical assessment and management of sleep disorders in multiple sclerosis: a literature review

The major sleep disorders are common in multiple sclerosis (MS) and are associated with significant morbidity. Despite this, the rate of recognition and management of these conditions are low. All types of sleep disorders are seen in patients with MS: insomnia, circadian rhythm sleep disorders, sleep-related movement disorders, sleep-related breathing disorders, hypersomnia (narcolepsy), and parasomnia (REM sleep behavior disorder; RBD). This literature review covers the prevalence, clinical features, and treatment of sleep disorders in MS. Based on clinical experience, the spectrum of symptoms associated with MS, and the current knowledge of MS pathophysiology, we have also enclosed proposed strategies for clinical assessment and investigation of sleep disorders in MS patients.

Fatigue in multiple sclerosis - a brief review

Fatigue is the most common and debilitating symptom in multiple sclerosis (MS) and is believed to be distinctly different from fatigue seen in other chronic conditions. It can affect a patient's mood, sleep and have a detrimental effect on their quality of life. In the recent years much literature has emerged in an attempt to elucidate the potential causes and treatment of this common symptom. This review article aims to examine the most recent theories on the pathophysiology of fatigue in MS as well as its association with sleep and depression. We describe the pharmacological and non-pharmacological approaches to its treatment and propose a multidisciplinary, patient enabled and individualised manner to the management of fatigue in MS.

Orexins: tissue localization, functions, and its relation to insulin secretion and diabetes mellitus

Orexins play a role in many biological functions include sleep, feeding, and energy balance. They also regulate circadian rhythms and the way that we feel pain. Orexins have been identified in a variety of tissues including the cerebrospinal fluid, blood, hypothalamus, spinal cord, sensory ganglion, enteric nervous system, pituitary, adrenal, salivary and lacrimal glands, testis, vestibular gland, and skin. Orexins play a role in a variety of biological functions including arousal, sleeping, food and fluid intake, pain, memory, perception of odor, and sexual activity. Orexins have also been implicated in the regulation of glucose metabolism. The expression of orexin is induced by hypoglycemia, low food, pregnancy, and hemodialysis. In contrast, factors that inhibit the expression of orexins include obstructive sleep apnea, aging, depression, obesity, traumatic brain injury, and inflammatory molecules such as liposaccharide. In conclusion, orexins are widely distributed and involved in a large variety of biological activities.

[Hypothalamic involvement in multiple sclerosis]

Hypothalamic involvement is a rare condition in patients with multiple sclerosis (MS). We report two patients with a long history of MS who presented with severe acute hypothermia with associated thrombocytopenia and elevated transaminase levels. Several cases of hypothermia or hyperthermia in patients with MS have been reported in the literature. They could be linked with hypothalamic lesions, in particular in the pre-optic area. However, other anatomical locations seem to be involved in thermoregulation and can be affected by MS. Besides, some cases of syndrome of inappropriate antidiuretic hormone secretion have been reported in patients with MS. Finally, some sleep disorders, particularly hypersomnia or narcolepsy, could be related to hypothalamic lesions, through the fall in hypocretin-1 in the cerebrospinal fluid. Hypocretin-1 is a neuropeptide that is secreted by some hypothalamic cells. It plays a role in the sleep-awake rhythm. We report one patient with narcolepsy and cataplexy before the first symptoms of MS appeared. Hypothalamic signs are rare in MS. However, several series of autopsies have shown a high frequency of demyelinating lesions in the hypothalamic area. Among these lesions, the proportion of active lesions seems elevated. Yet only few of them have a clinical or biological translation such as thermoregulation dysfunction, sleep disorders or natremia abnormalities. Thus, it seems unlikely that inflammatory hypothalamic lesions alone, even when bilateral, could be the explanation of these signs. A sufficient number of inflammatory demyelinating lesions, which we can observe in patients with a long history of MS and an already severe disability, is probably necessary to develop such a rare symptomatology. Hypothalamic signs might be a factor of poor prognosis for the disease course and progression of the disability.

Orexin A (hypocretin-1) levels are not reduced while cocaine/amphetamine regulated transcript levels are increased in the cerebrospinal fluid of patients with multiple sclerosis: no correlation with fatigue and sleepiness

BACKGROUND

Fatigue and sleep disturbance are common features of multiple sclerosis (MS). Our objectives were to determine cerebrospinal fluid levels of orexin A (hypocretin-1), a hypothalamic peptide involved in sleep, in patients with MS, and correlate them with fatigue, sleepiness, and levels of cocaine and amphetamine regulated transcript (CART) another neuropeptide regulating metabolism with wider nervous system distribution.

METHODS

Consecutive patients with MS (n=34), other inflammatory (n=24) or non-inflammatory (n=42) neurological diseases, undergoing lumbar puncture were investigated. Orexin and CART were measured by RIA by investigators unaware of the patients' diagnosis.

RESULTS

Orexin A was slightly decreased in the cerebrospinal fluid of patients with inflammatory disease. There was no evidence of orexin A deficiency in MS, although there was a non-significant trend toward a decrease compared to non-inflammatory neurological diseases (p=0.06). CART levels were increased in MS compared to the non-inflammatory disease group (p=0.03). There were no significant correlations between CSF levels of orexin A and CART, fatigue, and hypersomnolence.

CONCLUSIONS

Cerebrospinal fluid orexin A is decreased in CNS inflammatory diseases other than MS, where it shows a trend toward reduction, but does not correlate significantly with CART or with measures of fatigue and hypersomnolence.

Hypocretin/orexin and narcolepsy: new basic and clinical insights

Narcolepsy is a chronic sleep disorder, characterized by excessive daytime sleepiness (EDS), cataplexy, sleep paralysis and hypnagogic hallucinations. Both sporadic (95%) and familial (5%) forms of narcolepsy exist in humans. The major pathophysiology of human narcolepsy has been recently discovered based on the discovery of narcolepsy genes in animals; the genes involved in the pathology of the hypocretin/orexin ligand and its receptor. Mutations in hypocretin-related genes are rare in humans, but hypocretin ligand deficiency is found in a large majority of narcolepsy with cataplexy. Hypocretin ligand deficiency in human narcolepsy is probably due to the post-natal cell death of hypocretin neurones. Although a close association between human leucocyte antigen (HLA) and human narcolepsy with cataplexy suggests an involvement of autoimmune mechanisms, this has not yet been proved. Hypocretin deficiency is also found in symptomatic cases of narcolepsy and EDS with various neurological conditions, including immune-mediated neurological disorders, such as Guillain-Barre syndrome, MA2-positive paraneoplastic syndrome and neuromyelitis optica (NMO)-related disorder. The findings in symptomatic narcoleptic cases may have significant clinical relevance to the understanding of the mechanisms of hypocretin cell death and choice of treatment option. The discoveries in human cases lead to the establishment of the new diagnostic test of narcolepsy (i.e. low cerebrospinal fluid hypocretin-1 levels for 'narcolepsy with cataplexy' and 'narcolepsy due to medical condition'). As a large majority of human narcolepsy patients are ligand deficient, hypocretin replacement therapy may be a promising new therapeutic option, and animal experiments using gene therapy and cell transplantations are in progress.

Clinical and neurobiological aspects of narcolepsy

Narcolepsy is characterized by excessive daytime sleepiness (EDS), cataplexy and/or other dissociated manifestations of rapid eye movement (REM) sleep (hypnagogic hallucinations and sleep paralysis). Narcolepsy is currently treated with amphetamine-like central nervous system (CNS) stimulants (for EDS) and antidepressants (for cataplexy). Some other classes of compounds such as modafinil (a non-amphetamine wake-promoting compound for EDS) and gamma-hydroxybutyrate (GHB, a short-acting sedative for EDS/fragmented nighttime sleep and cataplexy) given at night are also employed. The major pathophysiology of human narcolepsy has been recently elucidated based on the discovery of narcolepsy genes in animals. Using forward (i.e., positional cloning in canine narcolepsy) and reverse (i.e., mouse gene knockout) genetics, the genes involved in the pathogenesis of narcolepsy (hypocretin/orexin ligand and its receptor) in animals have been identified. Hypocretins/orexins are novel hypothalamic neuropeptides also involved in various hypothalamic functions such as energy homeostasis and neuroendocrine functions. Mutations in hypocretin-related genes are rare in humans, but hypocretin-ligand deficiency is found in many narcolepsy-cataplexy cases. In this review, the clinical, pathophysiological and pharmacological aspects of narcolepsy are discussed.

Hypocretins (orexins) and sleep–wake disorders

Since their discovery in 1998, the hypocretins (orexins)-peptides that are produced by a group of neurons situated in the posterolateral hypothalamus--have been shown to excite many CNS areas including many neuronal systems that regulate sleep and wakefulness. Animal studies indicate that hypocretins play a part in the regulation of various functions including arousal, muscle tone, locomotion, regulation of feeding behaviour, and neuroendocrine and autonomic functions. A link between hypocretin deficiency and narcoleptic symptoms was first shown in canine and rodent models of narcolepsy. Hypocretin deficiency, as shown by low or absent concentrations in CSF, was subsequently found in 90% of patients with sporadic narcolepsy-cataplexy, and less commonly in familial narcolepsy. In most other sleep-wake and neurological disorders, hypocretin concentrations are normal. Low concentrations were also found in hypothalamic disorders, acute traumatic brain injury, and a few other disorders. The exact function of the hypocretin system in sleep-wake regulation and its pathophysiological role in hypocretin-deficient and non-deficient narcolepsy as well as in non-narcoleptic, hypocretin-deficiency syndromes remain unclear.

Hypocretins (orexins): clinical impact of the discovery of a neurotransmitter

Hypothalamic excitatory hypocretin (orexin) neurons have been discovered in 1998 and found to have widespread projections to basal forebrain, monoaminergic and cholinergic brainstem, and spinal cord regions. The hypocretin system is influenced both neuronally (e.g. suprachiasmatic nucleus, GABAergic, cholinergic and aminergic brainstem nuclei) as well as metabolically (e.g. glucose, ghrelin, and leptin). Physiologically the hypocretin system has been implicated in the regulation of behaviours that are associated with wakefulness, locomotion, and feeding. A role in REM sleep, neuroendocrine, autonomic and metabolic functions has also been suggested. Pathophysiologically a deficient hypocretin neurotransmission has been found in human narcolepsy and (engineered) animal models of the disorder. Different mechanisms are involved including (1) degeneration of hypocretin neurons (mice), (2) hypocretin ligand deficiency (humans, mice, dogs), (3) hypocretin receptor deficiency (mice, dogs). Reports of low hypocretin-1 cerebrospinal fluid levels in neurologic conditions (e.g. Guillain-Barré syndrome, traumatic brain injury, hypothalamic lesions) with and without sleep-wake disturbances and, on the other hand, observations of normal levels in about 11% of narcoleptics raise questions about the exact nature and pathophysiological base of the link between hypocretin deficiency and clinical manifestations in human narcolepsy.

Symptomatic narcolepsy, cataplexy and hypersomnia, and their implications in the hypothalamic hypocretin/orexin system

Human narcolepsy is a chronic sleep disorder affecting 1:2000 individuals. The disease is characterized by excessive daytime sleepiness, cataplexy and other abnormal manifestations of REM sleep, such as sleep paralysis and hypnagogic hallucinations. Recently, it was discovered that the pathophysiology of (idiopathic) narcolepsy-cataplexy is linked to hypocretin ligand deficiency in the brain and cerebrospinal fluid (CSF), as well as the positivity of the human leukocyte antigen (HLA) DR2/DQ6 (DQB1*0602). The symptoms of narcolepsy can also occur during the course of other neurological conditions (i.e. symptomatic narcolepsy). We define symptomatic narcolepsy as those cases that meet the International Sleep Disorders Narcolepsy Criteria, and which are also associated with a significant underlying neurological disorder that accounts for excessive daytime sleepiness (EDS) and temporal associations. To date, we have counted 116 symptomatic cases of narcolepsy reported in literature. As, several authors previously reported, inherited disorders (n=38), tumors (n=33), and head trauma (n=19) are the three most frequent causes for symptomatic narcolepsy. Of the 116 cases, 10 are associated with multiple sclerosis, one case of acute disseminated encephalomyelitis, and relatively rare cases were reported with vascular disorders (n=6), encephalitis (n=4) and degeneration (n=1), and hererodegenerative disorder (three cases in a family). EDS without cataplexy or any REM sleep abnormalities is also often associated with these neurological conditions, and defined as symptomatic cases of EDS. Although it is difficult to rule out the comorbidity of idiopathic narcolepsy in some cases, review of the literature reveals numerous unquestionable cases of symptomatic narcolepsy. These include cases with HLA negative and/or late onset, and cases in which the occurrences of the narcoleptic symptoms are parallel with the rise and fall of the causative disease. A review of these cases (especially those with brain tumors), illustrates a clear picture that the hypothalamus is most often involved. Several cases of symptomatic cataplexy (without EDS) were also reported and in contrast, these cases appear to be often associated with non-hypothalamic structures. CSF hypocretin-1 measurement were also carried out in a limited number of symptomatic cases of narcolepsy/EDS, including narcolepsy/EDS associated with tumors (n=5), head trauma (n=3), vascular disorders (n=5), encephalopathies (n=3), degeneration (n=30), demyelinating disorder (n=7), genetic/congenital disorders (n=11) and others (n=2). Reduced CSF hypocretin-1 levels were seen in most symptomatic narcolepsy cases of EDS with various etiologies and EDS in these cases is sometimes reversible with an improvement of the causative neurological disorder and an improvement of the hypocretin status. It is also noted that some symptomatic EDS cases (with Parkinson diseases and the thalamic infarction) appeared, but they are not linked with hypocretin ligand deficiency. In contrast to idiopathic narcolepsy cases, an occurrence of cataplexy is not tightly associated with hypocretin ligand deficiency in symptomatic cases. Since CSF hypocretin measures are still experimental, cases with sleep abnormalities/cataplexy are habitually selected for CSF hypocretin measures. Therefore, it is still not known whether all or a large majority of cases with low CSF hypocretin-1 levels with CNS interventions, exhibit EDS/cataplexy. It appears that further studies of the involvement of the hypocretin system in symptomatic narcolepsy and EDS are helpful to understand the pathophysiological mechanisms for the occurrence of EDS and cataplexy.