CSF Hypocretin Levels are Normal in Huntington's Disease Patients

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.

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.

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

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

Circadian alterations in patients with neurodegenerative diseases: Neuropathological basis of underlying network mechanisms

Circadian organization of physiology and behavior is an important biological process that allows organisms to anticipate and prepare for daily changes and demands. Disruptions in this system precipitates a wide range of health issues. In patients with neurodegenerative diseases, alterations of circadian rhythms are among the most common and debilitating symptoms. Although a growing awareness of these symptoms has occurred during the last decade, their underlying neuropathophysiological circuitry remains poorly understood and consequently no effective therapeutic strategies are available to alleviate these health issues. Recent studies have examined the neuropathological status of the different neural components of the circuitry governing the generation of circadian rhythms in neurodegenerative diseases. In this review, we will dissect the potential contribution of dysfunctions in the different nodes of this circuitry to circadian alterations in patients with neurodegenerative diseases. A deeper understanding of these mechanisms will provide not only a better understanding of disease neuro-pathophysiology, but also hold the promise for developing effective and mechanisms-based therapies.

The Role of Hypothalamic Pathology for Non-Motor Features of Huntington's Disease

 Huntington’s disease (HD) is a fatal genetic neurodegenerative disorder. It has mainly been considered a movement disorder with cognitive symptoms and these features have been associated with pathology of the striatum and cerebral cortex. Importantly, individuals with the mutant huntingtin gene suffer from a spectrum of non-motor features often decades before the motor disorder manifests. These symptoms and signs include a range of psychiatric symptoms, sleep problems and metabolic changes with weight loss particularly in later stages. A higher body mass index at diagnosis is associated with slower disease progression. The common psychiatric symptom of apathy progresses with the disease. The fact that non-motor features are present early in the disease and that they show an association to disease progression suggest that unravelling the underlying neurobiological mechanisms may uncover novel targets for early disease intervention and better symptomatic treatment. The hypothalamus and the limbic system are important brain regions that regulate emotion, social cognition, sleep and metabolism. A number of studies using neuroimaging, postmortem human tissue and genetic manipulation in animal models of the disease has collectively shown that the hypothalamus and the limbic system are affected in HD. These findings include the loss of neuropeptide-expressing neurons such as orexin (hypocretin), oxytocin, vasopressin, somatostatin and VIP, and increased levels of SIRT1 in distinct nuclei of the hypothalamus. This review provides a summary of the results obtained so far and highlights the potential importance of these changes for the understanding of non-motor features in HD.

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.

A Critical Evaluation of Wet Biomarkers for Huntington's Disease: Current Status and Ways Forward

There is an unmet clinical need for objective biomarkers to monitor disease progression and treatment response in Huntington's disease (HD). The aim of this review is, therefore, to provide practical advice for biomarker discovery and to summarise studies on biofluid markers for HD. A PubMed search was performed to review literature with regard to candidate saliva, urine, blood and cerebrospinal fluid biomarkers for HD. Information has been organised into tables to allow a pragmatic approach to the discussion of the evidence and generation of practical recommendations for future studies. Many of the markers published converge on metabolic and inflammatory pathways, although changes in other analytes representing antioxidant and growth factor pathways have also been found. The most promising markers reflect neuronal and glial degeneration, particularly neurofilament light chain. International collaboration to standardise assays and study protocols, as well as to recruit sufficiently large cohorts, will facilitate future biomarker discovery and development.

Fluid and imaging biomarkers for Huntington's disease

Huntington's disease is a chronic progressive neurodegenerative condition for which there is no disease-modifying treatment. The known genetic cause of Huntington's disease makes it possible to identify individuals destined to develop the disease and instigate treatments before the onset of symptoms. Multiple trials are already underway that target the cause of HD, yet clinical measures are often insensitive to change over typical clinical trial duration. Robust biomarkers of drug target engagement, disease severity and progression are required to evaluate the efficacy of treatments and concerted efforts are underway to achieve this. Biofluid biomarkers have potential advantages of direct quantification of biological processes at the molecular level, whilst imaging biomarkers can quantify related changes at a structural level in the brain. The most robust biofluid and imaging biomarkers can offer complementary information, providing a more comprehensive evaluation of disease stage and progression to inform clinical trial design and endpoints.

Biofluid Biomarkers in Huntington's Disease

Huntington's disease (HD) is a chronic progressive neurodegenerative condition where new markers of disease progression are needed. So far no disease-modifying interventions have been found, and few interventions have been proven to alleviate symptoms. This may be partially explained by the lack of reliable indicators of disease severity, progression, and phenotype.Biofluid biomarkers may bring advantages in addition to clinical measures, such as reliability, reproducibility, price, accuracy, and direct quantification of pathobiological processes at the molecular level; and in addition to empowering clinical trials, they have the potential to generate useful hypotheses for new drug development.In this chapter we review biofluid biomarker reports in HD, emphasizing those we feel are likely to be closest to clinical applicability.

Hypersomnia in Neurodegenerative Diseases

Hypersomnia is a common complaint in many patients with neurodegenerative diseases and a major cause of decreased quality of life. This article discusses the prevalence and factors associated with hypersomnia in patients with a variety of neurodegenerative diseases affecting the central nervous system, including tauopathies, synucleinopathies, and other conditions. Common nocturnal sleep problems that may result in daytime hypersomnia are delineated. A clinical approach to hypersomnia in patients with neurodegenerative diseases, recommended diagnostic testing, and available treatment options are also discussed.

Chronic sleep disturbance and neural injury: links to neurodegenerative disease

Sleep-wake disruption is frequently observed and often one of the earliest reported symptoms of many neurodegenerative disorders. This provides insight into the underlying pathophysiology of these disorders, as sleep-wake abnormalities are often accompanied by neurodegenerative or neurotransmitter changes. However, in addition to being a symptom of the underlying neurodegenerative condition, there is also emerging evidence that sleep disturbance itself may contribute to the development and facilitate the progression of several of these disorders. Due to its impact both as an early symptom and as a potential factor contributing to ongoing neurodegeneration, the sleep-wake cycle is an ideal target for further study for potential interventions not only to lessen the burden of these diseases but also to slow their progression. In this review, we will highlight the sleep phenotypes associated with some of the major neurodegenerative disorders, focusing on the circadian disruption associated with Alzheimer's disease, the rapid eye movement behavior disorder and sleep fragmentation associated with Parkinson's disease, and the insomnia and circadian dysregulation associated with Huntington's disease.

Mouse Models of Huntington's Disease

In this review, we explore the similarities and differences in the behavioural neurobiology found in the mouse models of Huntington's disease (HD) and the human disease state. The review is organised with a comparative focus on the functional domains of motor control, cognition and behavioural disturbance (akin to psychiatric disturbance in people) and how our knowledge of the underlying physiological changes that are manifest in the HD mouse lines correspond to those seen in the HD clinical population. The review is framed in terms of functional circuitry and neurotransmitter systems and how abnormalities in these systems impact on the behavioural readouts across the mouse lines and how these may correspond to the deficits observed in people. In addition, interpretational issues associated with the data from animal studies are discussed.

The hypocretin/orexin system: an increasingly important role in neuropsychiatry

Hypocretins, also named as orexins, are excitatory neuropeptides secreted by neurons specifically located in lateral hypothalamus and perifornical areas. Orexinergic fibers are extensively distributed in various brain regions and involved in a number of physiological functions, such as arousal, cognition, stress, appetite, and metabolism. Arousal is the most important function of orexin system as dysfunction of orexin signaling leads to narcolepsy. In addition to narcolepsy, orexin dysfunction is associated with serious neural disorders, including addiction, depression, and anxiety. However, some results linking orexin with these disorders are still contradictory, which may result from differences of detection methods or the precision of tools used in measurements; strategies targeted to orexin system (e.g., antagonists to orexin receptors, gene delivery, and cell transplantation) are promising new tools for treatment of neuropsychiatric disorders, though studies are still in a stage of preclinical or clinical research.

Sleep disturbance in mental health problems and neurodegenerative disease

Sleep has been described as being of the brain, by the brain, and for the brain. This fundamental neurobiological behavior is controlled by homeostatic and circadian (24-hour) processes and is vital for normal brain function. This review will outline the normal sleep-wake cycle, the changes that occur during aging, and the specific patterns of sleep disturbance that occur in association with both mental health disorders and neurodegenerative disorders. The role of primary sleep disorders such as insomnia, obstructive sleep apnea, and REM sleep behavior disorder as potential causes or risk factors for particular mental health or neurodegenerative problems will also be discussed.

Circadian and sleep disorder in Huntington's disease

Huntington's disease is a progressive neurological disorder that starts insidiously with motor, cognitive or psychiatric disturbance, and progresses through a distressing range of symptoms to end with a devastating loss of function, both motor and executive. There is a growing awareness that, in addition to cognitive and psychiatric symptoms, there are other important non-motor symptoms in HD, including sleep and circadian abnormalities. It is not clear if sleep-wake changes are caused directly by HD gene-related pathology, or if they are simply a consequence of having a neurodegenerative disease. From a patient point of view, the answer is irrelevant, since sleep and circadian disturbances are deleterious to good daily living, even in neurologically normal people. The assumption should be that, at the very least, sleep and/or circadian disturbance in HD patients will contribute to their symptoms. At worst, they may contribute to the progressive decline in HD. Here I review the state of our understanding of sleep and circadian abnormalities in HD. I also outline a set of simple rules that can be followed to improve the chances of a good night's sleep, since preventing any 'preventable' symptoms is the a logical first step in treating disease. The long-term impact of sleep disruption in HD is unknown. There have been no large-scale systematic studies of in sleep in HD. Furthermore, there has never been a study of the efficacy of pharmaceuticals that are typically used to treat sleep deficits in HD patients. Thus treatment of sleep disturbance in HD is necessarily empirical. A better understanding of the relationship between sleep/circadian abnormalities and HD pathology is needed, if treatment of this aspect of HD is to be optimized.

Development of biomarkers for Huntington's disease

Huntington's disease is an autosomal dominant, progressive neurodegenerative disorder, for which there is no disease-modifying treatment. By use of predictive genetic testing, it is possible to identify individuals who carry the gene defect before the onset of symptoms, providing a window of opportunity for intervention aimed at preventing or delaying disease onset. However, without robust and practical measures of disease progression (ie, biomarkers), the efficacy of therapeutic interventions in this premanifest Huntington's disease population cannot be readily assessed. Current progress in the development of biomarkers might enable evaluation of disease progression in individuals at the premanifest stage of the disease; these biomarkers could be useful in defining endpoints in clinical trials in this population. Clinical, cognitive, neuroimaging, and biochemical biomarkers are being investigated for their potential in clinical use and their value in the development of future treatments for patients with Huntington's disease.

Paradoxical function of orexin/hypocretin circuits in a mouse model of Huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder involving progressive motor disturbances, cognitive decline, and desynchronized sleep-wake rhythms. Recent studies revealed that restoring normal sleep-wake cycles can improve cognitive function in HD mice, suggesting that some sleep/wake systems remain operational and thus represent potential therapeutic targets for HD. Hypothalamic neurons expressing orexins/hypocretins (orexin neurons) are fundamental orchestrators of arousal in mammals, but it is unclear whether orexin circuits operate normally in HD. Here we analyzed the electrophysiology, histology, and gene expression of orexin circuits in brain slices from R6/2 mice, a transgenic model of HD with a progressive neurological phenotype. We report that in R6/2 mice, the size of an electrically distinct subpopulation of orexin neurons is reduced, as is the number of orexin-immunopositive cells in some hypothalamic regions. R6/2 orexin cells display altered glutamatergic inputs, and have an abnormal circadian profile of activity, despite normal circadian rhythmicity of the suprachiasmatic nucleus (SCN), the "master clock" of the brain. Nevertheless, even at advanced stages of HD, intrinsic firing properties of orexin cells remain normal and suppressible by serotonin, noradrenaline, and glucose. Furthermore, histaminergic neurons (key cells required for the propagation of orexin-induced arousal) also display normal responses to orexin. Together, these data suggest that the orexin system remains functional and modifiable in HD mice, although its circadian activity profile is disrupted and no longer follows that of the SCN.

Hypocretin and melanin-concentrating hormone in patients with Huntington disease

To evaluate whether hypocretin-1 (orexin-A) and melanin-concentrating hormone (MCH) neurotransmission are affected in patients with Huntington disease (HD), we immunohistochemically stained hypocretin and MCH neurons and estimated their total numbers in the lateral hypothalamus of both HD patients and matched controls. In addition, hypocretin-1 levels were determined in prefrontal cortical tissue and post-mortem ventricular cerebrospinal fluid (CSF) using a radioimmunoassay. The total number of hypocretin-1 neurons was significantly reduced by 30% in HD brains (P = 0.015), while the total number of MCH neurons was not significantly altered (P = 0.100). Levels of hypocretin-1 were 33% lower in the prefrontal cortex of the HD patients (P = 0.025), but ventricular CSF levels were similar to the control values (P = 0.306). Neuronal intranuclear and cytoplasmic inclusions of mutant huntingtin were present in all HD hypothalami, although with a variable distribution across different hypothalamic structures. We found a specific reduction in hypocretin signaling in patients with HD as MCH cell number was not significantly affected. It remains to be shown whether the moderate decrease in hypocretin neurotransmission could contribute to clinical symptoms. As the number of MCH-expressing neurons was not affected, alterations in MCH signaling are unlikely to have clinical effects in HD patients.

Hypothalamic dysfunction and neuroendocrine and metabolic alterations in Huntington's disease: clinical consequences and therapeutic implications

Huntington's disease (HD) is a hereditary neurodegenerative disorder characterized by cognitive, psychiatric, behavioural and motor disturbances. Although the course of HD is also frequently complicated by unintended weight loss, sleep disturbances and autonomic nervous system dysfunction, the aetiology of these signs and symptoms remains largely unknown. In recent years, many novel findings from both animal and human studies have emerged that indicate considerable hypothalamic, endocrine and metabolic alterations in HD. However, a comprehensive overview of these findings is lacking and their precise clinical significance is far from clear. Therefore, in this review we attempt to put these recent developments in the field into perspective by integrating them with previous findings in a comprehensible manner, and by discussing their clinical relevance, with a special focus on body weight, sleep and autonomic functions in HD, which will also allow for the identification of future lines of research in this area.

Hypothalamic-endocrine aspects in Huntington's disease

Huntington's disease (HD) is a hereditary and fatal disorder caused by an expanded CAG triplet repeat in the HD gene, resulting in a mutant form of the protein huntingtin. Wild-type and mutant huntingtin are expressed in most tissues of the body but the normal function of huntingtin is not fully known. In HD, the neuropathology is characterized by intranuclear and cytoplasmic inclusions of huntingtin aggregates, and cell death primarily in striatum and cerebral cortex. However, hypothalamic atrophy occurs at early stages of HD with loss of orexin- and somatostatin-containing cell populations. Several symptoms of HD such as sleep disturbances, alterations in circadian rhythm, and weight loss may be due to hypothalamic dysfunction. Endocrine changes including increased cortisol levels, reduced testosterone levels and increased prevalence of diabetes are found in HD patients. In HD mice, alterations in the hypothalamic-pituitary-adrenal axis occurs as well as pancreatic beta-cell and adipocyte dysfunction. Increasing evidence points towards important pathology of the hypothalamus and the endocrine system in HD. As many neuroendocrine factors are secreted into the cerebrospinal fluid, blood and urine, it is possible that their levels may reflect the disease state in the central nervous system. Investigating neuroendocrine changes in HD opens up the possibility of finding biomarkers to evaluate future therapies for HD, as well as of identifying novel targets for therapeutic interventions.