Loss Of Orexin Neurons Research Articles

TAK-861, a potent, orally available orexin receptor 2-selective agonist, produces wakefulness in monkeys and improves narcolepsy-like phenotypes in mouse models

Narcolepsy type 1 (NT1) is associated with severe loss of orexin neurons and characterized by symptoms including excessive daytime sleepiness and cataplexy. Current medications indicated for NT1 often show limited efficacy, not addressing the full spectrum of symptoms, demonstrating a need for novel drugs. We discovered a parenteral orexin receptor 2 (OX2R) agonist, danavorexton, and an orally available OX2R agonist, TAK-994; both improving NT1 phenotypes in mouse models and individuals with NT1. However, danavorexton has limited oral availability and TAK-994 has a risk of off-target liver toxicity. To avoid off-target-based adverse events, a highly potent molecule with low effective dose is preferred. Here, we show that a novel OX2R-selective agonist, TAK-861 [N-{(2S,3R)-4,4-Difluoro-1-(2-hydroxy-2-methylpropanoyl)-2-[(2,3′,5′-trifluoro[1,1′-biphenyl]-3-yl)methyl]pyrrolidin-3-yl}ethanesulfonamide], activates OX2R with a half-maximal effective concentration of 2.5 nM and promotes wakefulness at 1 mg/kg in mice and monkeys, suggesting ~ tenfold higher potency and lower effective dosage than TAK-994. Similar to TAK-994, TAK-861 substantially ameliorates wakefulness fragmentation and cataplexy-like episodes in orexin/ataxin-3 and orexin-tTA;TetO DTA mice (NT1 mouse models). Compared with modafinil, TAK-861 induces highly correlated brain-wide neuronal activation in orexin-tTA;TetO DTA mice, suggesting efficient wake-promoting effects. Thus, TAK-861 has potential as an effective treatment for individuals with hypersomnia disorders including narcolepsy, potentially with a favorable safety profile.

Cerebrospinal fluid proteomics in recent-onset Narcolepsy type 1 reveals activation of the complement system.

Narcolepsy type 1 (NT1) is a rare, chronic and disabling neurological disease causing excessive daytime sleepiness and cataplexy. NT1 is characterized pathologically by an almost complete loss of neurons producing the orexin neuropeptides in the lateral hypothalamus. Genetic and environmental factors strongly suggest the involvement of the immune system in the loss of orexin neurons. The cerebrospinal fluid (CSF), secreted locally and surrounding the central nervous system (CNS), represents an accessible window into CNS pathological processes. To gain insight into the biological and molecular changes in NT1 patients, we performed a comparative proteomics analysis of the CSF from 21 recent-onset NT1 patients and from two control groups: group 1 with somatoform disorders, and group 2 patients with hypersomnia other than NT1, to control for any potential effect of sleep disturbances on CSF composition. To achieve an optimal proteomic coverage analysis, the twelve most abundant CSF proteins were depleted, and samples were analyzed by nano-flow liquid chromatography tandem mass spectrometry (nano-LC-MS/MS) using the latest generation of hybrid Orbitrap mass spectrometer. Our study allowed the identification and quantification of up to 1943 proteins, providing a remarkably deep analysis of the CSF proteome. Interestingly, gene set enrichment analysis indicated that the complement and coagulation systems were enriched and significantly activated in NT1 patients in both cohorts analyzed. Notably, the lectin and alternative complement pathway as well as the downstream lytic membrane attack complex were congruently increased in NT1. Our data suggest that the complement dysregulation in NT1 patients can contribute to immunopathology either by directly promoting tissue damage or as part of local inflammatory responses. We therefore reveal an altered composition of the CSF proteome in NT1 patients, which points to an ongoing inflammatory process contributed, at least in part, by the complement system.

TAK-994, a Novel Orally Available Brain-Penetrant Orexin 2 Receptor-Selective Agonist, Suppresses Fragmentation of Wakefulness and Cataplexy-Like Episodes in Mouse Models of Narcolepsy.

Loss of orexin neurons is associated with narcolepsy type 1 (NT1), which is characterized by multiple symptoms including excessive daytime sleepiness and cataplexy. Orexin 2 receptor (OX2R) knockout (KO) mice, but not orexin 1 receptor (OX1R) KO mice, show narcolepsy-like phenotypes, thus OX2R agonists are potentially promising for treating NT1. In fact, in early proof-of-concept studies, intravenous infusion of danavorexton, an OX2R-selective agonist, significantly increased wakefulness in individuals with NT1. However, danavorexton has limited oral availability. Here, we report pharmacological characteristics of a novel OX2R agonist, TAK-994 [N-{(2S,3S)-1-(2-hydroxy-2-methylpropanoyl)-2-[(2,3',5'-trifluorobiphenyl-3-yl)methyl]pyrrolidin-3-yl}methanesulfonamide sesquihydrate]. TAK-994 activated recombinant human OX2R (EC50 value of 19 nM) with > 700-fold selectivity against OX1R and activated OX2R-downstream signaling similar to those by orexin peptides in vitro. Oral administration of TAK-994 promoted wakefulness in normal mice but not in OX2R KO mice. TAK-994 also ameliorated narcolepsy-like symptoms in two mouse models of narcolepsy: orexin/ataxin-3 mice and orexin-tTA;TetO diphtheria toxin A mice. The wake-promoting effects of TAK-994 in orexin/ataxin-3 mice were maintained after chronic dosing for 14 days. These data suggest that overall in vitro and in vivo properties, except oral availability, are very similar between TAK-994 and danavorexton. Preclinical characteristics of TAK-994 shown here, together with upcoming clinical study results, can improve our understanding for orally available OX2R agonists as new therapeutic drugs for NT1 and other hypersomnia disorders. SIGNIFICANCE STATEMENT: Narcolepsy type 1 (NT1) is caused by a loss of orexin neurons, and thus an orexin 2 receptor (OX2R) agonist is considered to address the underlying pathophysiology of NT1. Oral administration of TAK-994, a novel OX2R agonist, promoted wakefulness in normal mice, but not in OX2R knockout mice, and ameliorated fragmentation of wakefulness and cataplexy-like episodes in mouse models of narcolepsy. These findings indicate that TAK-994 is an orally available brain-penetrant OX2R-selective agonist with potential to improve narcolepsy-like symptoms.

TCM syndrome differentiation and treatment of narcolepsy based on neurobiological mechanism: A review

Narcolepsy is a relatively rare brain disorder caused by the selective loss of orexin neurons. Narcolepsy is divided into Narcolepsy Type 1 (NT1) and Narcolepsis Type 2 (NT2). The pathogenesis of NT1 has been well established due to the severe loss of orexin neurons, while NT2 is still poorly understood, and little is known about its underlying neurobiological mechanisms. human leukocyte antigen alleles have been found to strongly influence the development of narcolepsy, with more than 90% of NT1 patients carrying the human leukocyte antigen II allele DQB1*06:02. In addition to the genetic evidence for the DQBI*06:02 allele, some other evidence suggests that a T cell-mediated immune mechanism destroys the orexin neurons of NT1, with CD4 + T cells being key. For this disease, traditional Chinese medicine (TCM) therapy has its own characteristics and advantages, especially the combination of acupuncture and medicine in the treatment of this disease in TCM, which has made considerable and gratifying progress. The purpose of this review is to introduce the frontier progress of neurobiology of narcolepsy, and to explore the syndrome differentiation and treatment of narcolepsy with the combined use of TCM and Western medicine combined with TCM.

Danavorexton, a selective orexin 2 receptor agonist, provides a symptomatic improvement in a narcolepsy mouse model

Narcolepsy type 1 (NT1), caused by loss of orexin neurons, is a neurological disorder characterized by excessive daytime sleepiness, cataplexy, disrupted nighttime sleep, hypnagogic/hypnopompic hallucinations and sleep paralysis, as well as a high risk of obesity. Danavorexton (TAK-925) is a novel brain-penetrant orexin 2 receptor (OX2R)-selective agonist currently being evaluated in clinical trials for the treatment of hypersomnia disorders including NT1. Thus, detailed characterization of danavorexton is critical for validating therapeutic potential of OX2R-selective agonists. Here, we report preclinical characteristics of danavorexton as a therapeutic drug for NT1. Danavorexton showed rapid association/dissociation kinetics to OX2R. The activation mode of endogenous OX2R by danavorexton and orexin peptide was very similar in an electrophysiological analysis. In orexin/ataxin-3 mice, a mouse model of NT1, danavorexton promoted wakefulness, and ameliorated fragmentation of wakefulness during the active phase after both acute and repeated administration, suggesting a low risk of receptor desensitization. Electroencephalogram (EEG) power spectral analysis revealed that danavorexton, but not modafinil, normalized dysregulated EEG power spectrum in orexin/ataxin-3 mice during the active phase. Finally, repeated administration of danavorexton significantly suppressed the body weight gain in orexin/ataxin-3 mice. Danavorexton may have the potential to treat multiple symptoms of NT1. These preclinical findings, together with upcoming clinical observations of danavorexton, could improve our understanding of the pathophysiology of NT1 and therapeutic potential of OX2R agonists.

Brain Circuits Underlying Narcolepsy.

Narcolepsy is a sleep disorder manifesting symptoms such as excessive daytime sleepiness and often cataplexy, a sudden and involuntary loss of muscle activity during wakefulness. The underlying neuropathological basis of narcolepsy is the loss of orexin neurons from the lateral hypothalamus. To date numerous animal models of narcolepsy have been produced in the laboratory, being invaluable tools for delineating the brain circuits of narcolepsy. This review will examine the evidence regarding the function of the orexin system, and how loss of this wake-promoting system manifests in excessive daytime sleepiness. This review will also outline the brain circuits controlling cataplexy, focusing on the contribution of orexin signaling loss in narcolepsy. Although our understanding of the brain circuits of narcolepsy has made great progress in recent years, much remains to be understood.

Histamine in murine narcolepsy: What do genetic and immune models tell us?

An increased number of histaminergic neurons, identified by labeling histidine‐decarboxylase (HDC) its synthesis enzyme, was unexpectedly found in patients with narcolepsy type 1 (NT1). In quest for enlightenment, we evaluate whether an increase in HDC cell number and expression level would be detected in mouse models of the disease, in order to provide proof of concepts reveling possible mechanisms of compensation for the loss of orexin neurons, and/or of induced expression as a consequence of local neuroinflammation, a state that likely accompanies NT1. To further explore the compensatory hypothesis, we also study the noradrenergic wake‐promoting system. Immunohistochemistry for HDC, orexin, and melanin‐concentrating hormone (MCH) was used to count neurons. Quantitative‐PCR of HDC, orexin, MCH, and tyrosine‐hydroxylase was performed to evaluate levels of mRNA expression in the hypothalamus or the dorsal pons. Both quantifications were achieved in genetic and neuroinflammatory models of narcolepsy with major orexin impairment, namely the orexin‐deficient (Orex‐KO) and orexin‐hemagglutinin (Orex‐HA) mice respectively. The number of HDC neurons and mRNA expression level were unchanged in Orex‐KO mice compared to controls. Similarly, we found no change in tyrosine‐hydroxylase mRNA expression in the dorsal pons between groups. Further, despite the presence of protracted local neuroinflammation as witnessed by the presence of reactive microglia, we found no change in the number of neurons nor the expression of HDC in Orex‐HA mice compared to controls. Importantly, no correlation was found in all conditions between HDC and orexin. Our findings indicate that, in mice, the expression of histamine and noradrenalin, two wake‐promoting systems, are not modulated by orexin level whether the lack of orexin is constitutive or induced at adult age, showing thus no compensation. They also show no recruitment of histamine by local neuroinflammation. Further studies will be needed to further define the role of histamine in the pathophysiology of NT1.

006 The effect of sodium oxybate on cataplexy in orexin knockout mice

Abstract Introduction Narcolepsy is a neurological disorder that is characterized by the loss of orexin neurons in the lateral hypothalamus. Cataplexy is a symptom of narcolepsy and is identified by a sudden loss of muscle tone during wakefulness. Cataplexy abruptly interrupts day-to-day activities, and makes activities like driving dangerous and potentially fatal. It is hypothesized that cataplexy occurs due to the intrusion of REM sleep muscle atonia during wakefulness. It has been demonstrated that a GABAergic mechanism is responsible for silencing the REM sleep atonia circuit and preventing REM sleep muscle atonia from occurring during wakefulness. Sodium oxybate (SXB), a low affinity agonist of GABAB receptors, is currently an approved treatment for narcolepsy; however, its mechanism of action in the brain remains unknown. Here, we investigate the hypothesis that SXB prevents cataplexy through a GABAB mediated mechanism by potentially suppressing the REM sleep atonia circuit. Methods We intraperitoneally (IP) injected orexin-/- mice with one of five treatments; either Lactated Ringers Solution, SXB at 50, 100, or 200 mg/kg, or Phaclophen at 10 mg/kg followed by SXB at 100 mg/kg. Cataplexy was assessed by video recordings. We gathered and analyzed data at three distinct chronological time points (i.e, at baseline, after three consecutive weeks of daily dosing with a treatment, and after one week of no treatment). These experiments were designed and conducted to determine whether; 1) SXB reduces cataplexy, 2) the cessation of SXB administration has an effect on cataplexy, 3) SXB is mediated through a GABAB mechanism. Results We first confirmed that mice used were indeed orexin-/- using immunohistochemical analysis to show that no orexin-a expressing neurons were located in the lateral hypothalamus. Then, we determined that: 1) SXB reduces cataplexy compared to time matched controls (unpaired t-test, p=0.0027, n=18); 2) When we stopped administering SXB IP injections, cataplexy levels increased towards control levels (paired t-test, p=0.0024, n=18); 3) it remains unclear if SXB’s effect is solely mediated through a GABAB mechanism. Conclusion Our findings demonstrate that SXB effectively reduces cataplexy in orexin-/- mice. Support (if any) This research was funded by Jazz Pharmaceuticals

001 Exploring the Orexin-tTA/TetO-DTA Mouse as a Model for Pediatric Narcolepsy

Abstract Introduction Narcolepsy is a sleep disorder caused by selective death of the orexin neurons that often begins in childhood. Orexin neuron loss disinhibits REM sleep during the active period and produces cataplexy, an abnormal behavioral state between REM sleep and wakefulness. Cataplexy is often more severe when narcolepsy develops in children compared to adults, but the mechanisms underlying this difference remain unknown. Methods We used orexin-tTA/TetO-DTA mice to model narcolepsy at different ages. When doxycycline is removed from the diet, the orexin neurons of these mice express diphtheria toxin A and die within 2–3 weeks. We removed doxycycline at 4 weeks (young-onset) or 14 weeks (adult-onset) of age in male and female mice. We implanted EEG and EMG electrodes for sleep recordings one week later and then recorded EEG/EMG/video for 24h at 3 and 13 weeks after removal of doxycycline. Age-matched controls had access to doxycycline diet for the entire experiment. Results Three weeks after doxycycline removal, both young-onset and adult-onset mice developed cataplexy and the sleep-wake fragmentation characteristic of narcolepsy. Age of orexin cell loss did not significantly affect cataplexy severity, however, female mice had more cataplexy than male mice overall. Both young- and adult-onset mice showed a 99% loss of orexin neurons at 3 weeks. Conclusion Considered together, our results suggest that the orexin-tTA/TetO-DTA mouse model of narcolepsy does not capture the severe cataplexy that is often seen in the human pediatric population. Support (if any):

Pitolisant in an Adolescent with Prader-Willi Syndrome.

This case report evaluates the potential benefit of pitolisant in a 15-year-old female with Prader-Willi syndrome, obsessive-compulsive disorder, autism spectrum disorder, and mild intellectual disability. Due to its action on the H3 receptor, it enhances central activity of histaminergic neurons resulting in increased alertness, irrespective of the loss of orexin neurons seen in narcolepsy. Additionally, it is thought to modulate various other neurotransmitter systems including acetylcholine, norepinephrine, and dopamine. Pitolisant has the potential to improve many symptoms in patients with Prader-Willi syndrome and it appears to be well tolerated with minimal side effects observed. Therefore, the use of pitolisant should be considered in patients with Prader-Willi syndrome who fail a psychostimulant trial.

Loss of the metabolism and sleep regulating neuronal populations expressing orexin and oxytocin in the hypothalamus in amyotrophic lateral sclerosis.

To determine the underlying cellular changes and clinical correlates associated with pathology of the hypothalamus in amyotrophic lateral sclerosis (ALS), as hypothalamic atrophy occurs in the preclinical phase of the disease. The hypothalamus was pathologically examined in nine patients with amyotrophic lateral sclerosis in comparison to eight healthy control subjects. The severity of regional atrophy (paraventricular nucleus: PVN, fornix and total hypothalamus) and peptidergic neuronal loss (oxytocin, vasopressin, cocaine- and amphetamine-regulating transcript: CART, and orexin) was correlated with changes in eating behaviour, sleep function, cognition, behaviour and disease progression. Tar DNA-binding protein 43 (TDP-43) inclusions were present in the hypothalamus of all patients with amyotrophic lateral sclerosis. When compared to controls, there was atrophy of the hypothalamus (average 21% atrophy, p=0.004), PVN (average 30% atrophy p=0.014) and a loss of paraventricular oxytocin-producing neurons (average 49% loss p=0.02) and lateral hypothalamic orexin-producing neurons (average 37% loss, significance p=0.02). Factor analysis identified strong relationships between abnormal eating behaviour, hypothalamic atrophy and loss of orexin-producing neurons. With increasing disease progression, abnormal sleep behaviour and cognition associated with atrophy of the fornix. Substantial loss of hypothalamic oxytocin-producing neurons occurs in ALS, with regional atrophy and the loss of orexin neurons relating to abnormal eating behaviour in ALS. Oxytocin- and orexin neurons display TDP43 inclusions. Our study points to significant pathology in the hypothalamus that may play a key role in metabolic and pathogenic changes in ALS.

Orexin gene expression is downregulated in alcohol dependent rats during acute alcohol withdrawal

Alcohol use disorders (AUD) are chronic relapsing brain disorder characterized by compulsive and heavy alcohol consumption. During acute withdrawal, patients with AUD display excessive daytime sleepiness, a condition linked to serious life-threatening complications, however, the mechanism is not known. Orexin and melanin-concentrating hormone (MCH) are the two hypothalamic neuropeptides that regulate many behaviors including sleep-wakefulness, and alcohol consumption, reinforcement, and reinstatement. Importantly, loss of orexin neurons causes narcolepsy, a severe sleep disorder with excessive daytime sleepiness. Does acute alcohol withdrawal reduce orexin gene expression? To investigate this, male Sprague-Dawley rats were divided in two groups: Rats were either administered with alcohol, mixed with infant formula (alcohol group) or control mixture containing water and infant formula (Controls) by gastric intubation every 8 h for 4 days using Majchrowicz’s chronic binge drinking protocol. The doses of alcohol were adjusted depending on degree of intoxication, exhibited by animals, prior to each dose. The animals were euthanized after 12 h of last alcohol/water administration. During withdrawal, the hypothalamus was rapidly dissected out, and the expressions of orexin and MCH genes were examined by Real-time PCR. There was a significant reduction in orexin gene expression in rats subjected to alcohol withdrawal as compared to controls. No such change was observed in the MCH gene expression. These results suggest that downregulation of orexin gene expression may be a possible mechanism responsible for excessive daytime sleepiness associated with alcohol withdrawal in patients with AUD.

The neurobiological basis of narcolepsy.

Narcolepsy is the most common neurological cause of chronic sleepiness. The discovery about 20 years ago that narcolepsy is caused by selective loss of the neurons producing orexins (also known as hypocretins) sparked great advances in the field. Here, we review the current understanding of how orexin neurons regulate sleep-wake behaviour and the consequences of the loss of orexin neurons. We also summarize the developing evidence that narcolepsy is an autoimmune disorder that may be caused by a T cell-mediated attack on the orexin neurons and explain how these new perspectives can inform better therapeutic approaches.

Cellular mechanisms involved in induction of selective degeneration of orexin neurons in the hypothalamus

Narcolepsy is a kind of sleep disorder featured by selective loss of orexin neurons in the lateral hypothalamus. Several lines of evidence, including association with specific HLA haplotypes, gene polymorphism in T cell receptor and detection of autoantibodies in a subpopulation of patients, suggest that autoimmune responses play an important role in the pathogenesis of this disorder. Potential relationship with influenza virus infection has also been a matter of interest. However, these events may not be able to explain all cases of narcolepsy. Based on the structural features of orexin, in addition to the findings on the characteristics of orexin neurons obtained from studies in organotypic hypothalamic slice cultures, we proposed novel mechanisms potentially involved in selective degeneration of orexin neurons. Increase in local production of nitric oxide induced by several life style-related conditions such as shortage of sleep and intake of high fat diet leads to inactivation of protein disulfide isomerase. Consequently, abnormal aggregates of orexin and/or its precursor that possess two intra-molecular disulfide bonds accumulate within orexin neurons. In addition to the increase in endoplasmic reticulum stress, accumulation of orexin as abnormal aggregates leads to increased excitability of orexin neurons by shutdown of feedback inhibition resulting from deficits in orexin release. These mechanisms may provide a clue to understand the pathogenic mechanisms of various neurological and psychiatric disorders accompanied by a decrease of orexin.

Cystamine-mediated inhibition of protein disulfide isomerase triggers aggregation of misfolded orexin-A in the Golgi apparatus and prevents extracellular secretion of orexin-A

Orexins (orexin-A and orexin-B) are neuropeptides that are reduced in narcolepsy, a sleep disorder that is characterized by excessive daytime sleepiness, sudden sleep attacks and cataplexy. However, it remains unclear how orexins in the brain and orexin neurons are reduced in narcolepsy. Orexin-A has two closely located intramolecular disulfide bonds and is prone to misfolding due to the formation of incorrect disulfide bonds. Protein disulfide isomerase (PDI) possesses disulfide interchange activity. PDI can modify misfolded orexin-A to its native form by rearrangement of two disulfide bonds. We have previously demonstrated that sleep deprivation and a high fat diet increase nitric oxide in the brain. This increase triggers S-nitrosation and inactivation of PDI, leading to aggregation of orexin-A and reduction of orexin neurons. However, the relationship between PDI inactivation and loss of orexin neurons has not yet been fully elucidated. In the present study, we used a PDI inhibitor, cystamine, to elucidate the precise molecular mechanism by which PDI inhibition reduces the number of orexin neurons. In rat hypothalamic slice cultures, cystamine induced selective depletion of orexin-A, but not orexin-B and melanin-concentrating hormone. Moreover, cystamine triggered aggregation of orexin-A, but not orexin-B in the Golgi apparatus of hypothalamic slice cultures and in vivo mouse brains. However, cystamine did not induce endoplasmic reticulum (ER) stress, and an ER stress inducer did not trigger aggregation of orexin-A in slice cultures. Finally, we demonstrated that cystamine significantly decreased extracellular secretion of orexin-A in AD293 cells overexpressing prepro-orexin. These findings suggest that cystamine-induced PDI inhibition induces selective depletion, aggregation in the Golgi apparatus and impaired secretion of orexin-A. These effects may represent an initial step in the pathogenesis of narcolepsy.

Role of the Orexin/Hypocretin System in Stress-Related Psychiatric Disorders.

Orexins (hypocretins) are critically involved in coordinating appropriate physiological and behavioral responses to aversive and threatening stimuli. Acute stressors engage orexin neurons via direct projections from stress-sensitive brain regions. Orexin neurons, in turn, facilitate adaptive behavior via reciprocal connections as well as via direct projections to the hypophysiotropic neurons that coordinate the hypothalamic-pituitary-adrenal (HPA) axis response to stress. Consequently, hyperactivity of the orexin system is associated with increased motivated arousal and anxiety, and is emerging as a key feature of panic disorder. Accordingly, there has been significant interest in the therapeutic potential of pharmacological agents that antagonize orexin signaling at their receptors for the treatment of anxiety disorders. In contrast, disorders characterized by inappropriately low levels of motivated arousal, such as depression, generally appear to be associated with hypoactivity of the orexin system. This includes narcolepsy with cataplexy, a disorder characterized by the progressive loss of orexin neurons and increased rates of moderate/severe depression symptomology. Here, we provide a comprehensive overview of both clinical and preclinical evidence highlighting the role of orexin signaling in stress reactivity, as well as how perturbations to this system can result in dysregulated behavioral phenotypes.

Increased anxiety but normal fear and safety learning in orexin-deficient mice

The loss of orexin neurons in humans leads to the disease narcolepsy, characterized by daytime sleepiness and cataplexy. Recent data suggest that orexin is also involved in emotional processing. The goal of the present study was to evaluate fear and safety learning as well as unconditioned fear (anxiety) in orexin-deficient animals. Orexin-deficient mice are an established animal model used to investigate the neuropathology and potential treatments for narcolepsy. Here, we present novel data showing that orexin-deficient mice express increased anxiety in the open field, light-dark box test and carnivore odor-induced avoidance, but are normal in fear and safety learning. These findings suggest an important role of orexin in brain areas involved in anxiety.

Erratum to: Progressive changes of orexin system in a rat model of 6-hydroxydopamine-induced Parkinson’s disease

[This corrects the article DOI: 10.1007/s12264-010-0410-9.].

Histamine Transmission Modulates the Phenotype of Murine Narcolepsy Caused by Orexin Neuron Deficiency.

Narcolepsy type 1 is associated with loss of orexin neurons, sleep-wake derangements, cataplexy, and a wide spectrum of alterations in other physiological functions, including energy balance, cardiovascular, and respiratory control. It is unclear which narcolepsy signs are directly related to the lack of orexin neurons or are instead modulated by dysfunction of other neurotransmitter systems physiologically controlled by orexin neurons, such as the histamine system. To address this question, we tested whether some of narcolepsy signs would be detected in mice lacking histamine signaling (HDC-KO). Moreover, we studied double-mutant mice lacking both histamine signaling and orexin neurons (DM) to evaluate whether the absence of histamine signaling would modulate narcolepsy symptoms produced by orexin deficiency. Mice were instrumented with electrodes for recording the electroencephalogram and electromyogram and a telemetric arterial pressure transducer. Sleep attacks fragmenting wakefulness, cataplexy, excess rapid-eye-movement sleep (R) during the activity period, and enhanced increase of arterial pressure during R, which are hallmarks of narcolepsy in mice, did not occur in HDC-KO, whereas they were observed in DM mice. Thus, these narcolepsy signs are neither caused nor abrogated by the absence of histamine. Conversely, the lack of histamine produced obesity in HDC-KO and to a greater extent also in DM. Moreover, the regularity of breath duration during R was significantly increased in either HDC-KO or DM relative to that in congenic wild-type mice. Defects of histamine transmission may thus modulate the metabolic and respiratory phenotype of murine narcolepsy.

Neurons containing orexin or melanin concentrating hormone reciprocally regulate wake and sleep.

Neurons containing orexin (hypocretin), or melanin concentrating hormone (MCH) are intermingled with each other in the perifornical and lateral hypothalamus. Each is a separate and distinct neuronal population, but they project to similar target areas in the brain. Orexin has been implicated in regulating arousal since loss of orexin neurons is associated with the sleep disorder narcolepsy. Microinjections of orexin into the brain or optogenetic stimulation of orexin neurons increase waking. Orexin neurons are active in waking and quiescent in sleep, which is consistent with their role in promoting waking. On the other hand, the MCH neurons are quiet in waking but active in sleep, suggesting that they could initiate sleep. Recently, for the first time the MCH neurons were stimulated optogenetically and it increased sleep. Indeed, optogenetic activation of MCH neurons induced sleep in both mice and rats at a circadian time when they should be awake, indicating the powerful effect that MCH neurons have in suppressing the wake-promoting effect of not only orexin but also of all of the other arousal neurotransmitters. Gamma-Aminobutyric acid (GABA) is coexpressed with MCH in the MCH neurons, although MCH is also inhibitory. The inhibitory tone of the MCH neurons is opposite to the excitatory tone of the orexin neurons. We hypothesize that strength in activity of each determines wake vs. sleep.