Narcolepsy: clinical features, new pathophysiologic insights, and future perspectives

Hypocretin-1 CSF levels in anti-Ma2 associated encephalitis

Idiopathic narcolepsy is associated with deficient hypocretin transmission. Narcoleptic symptoms have recently been described in paraneoplastic encephalitis with anti-Ma2 antibodies. The authors measured CSF hypocretin-1 levels in six patients with anti-Ma2 encephalitis, and screened for anti-Ma antibodies in patients with idiopathic narcolepsy. Anti-Ma autoantibodies were not detected in patients with idiopathic narcolepsy. Four patients with anti-Ma2 encephalitis had excessive daytime sleepiness; hypocretin-1 was not detectable in their cerebrospinal fluid, suggesting an immune-mediated hypocretin dysfunction.

Is motor inhibition during laughter due to emotional or respiratory influences?

We compared the effects of laughter and several respiratory movements on spinal motor excitability to unravel their respective influences. We measured H-reflexes in 13 healthy volunteers during 10 different tasks (including laughter, simulated laughter, and various respiratory movements). We compared the percentage that remained of the initial H-reflex during each task with that during a neutral task. H-reflex percentage differed between the neutral task (79.4 +/- 16.1%), true laughter (43.7 +/- 17.9%), and simulated laughter (66.6 +/- 24.3%), and between the two latter tasks. Coughing also resulted in H-reflex suppression, but not as deeply as true laughter. During the other respiratory maneuvers, the H-reflex increased compared to the neutral task. Our finding that true laughter evoked more H-reflex depression than simulated laughter suggests that mirth on its own depresses the H-reflex. This mechanism may also be involved in the pathophysiology of cataplexy, the main symptom of narcolepsy.

Crystal structure of HLA-DQ0602 that protects against type 1 diabetes and confers strong susceptibility to narcolepsy

The MHC class II molecule DQ0602 confers strong susceptibility to narcolepsy but dominant protection against type 1 diabetes. The crystal structure of DQ0602 reveals the molecular features underlying these contrasting genetic properties. Structural comparisons to homologous DQ molecules with differential disease associations highlight a previously unrecognized interplay between the volume of the P6 pocket and the specificity of the P9 pocket, which implies that presentation of an expanded peptide repertoire is critical for dominant protection against type 1 diabetes. In narcolepsy, the volume of the P4 pocket appears central to the susceptibility, suggesting that the presentation of a specific peptide population plays a major role.

Migraine and idiopathic narcolepsy--a case-control study

In order to evaluate a possible association between migraine and idiopathic narcolepsy, we performed a multicentre case-control study on the comorbidity of narcolepsy and different headaches. In total, 96 patients with idiopathic narcolepsy were enrolled. The migraine frequency in the patients and in the control group was 21.9% and 19.8%, respectively (P = 0.722). The migraine features did not differ significantly between both groups. However, headache fulfilling the criteria for tension-type headache was significantly more often reported by narcolepsy patients than by the control group (60.3% vs. 40.7%, P= 0.006). We conclude that there is no association between migraine and narcolepsy but that patients with narcolepsy show more unspecific headache, probably due to sleep disturbances.

Glucose hypometabolism of hypothalamus and thalamus in narcolepsy

It has been hypothesized that hypothalamus is involved in narcolepsy. The relative difference between cerebral glucose metabolism of 24 narcoleptic patients and 24 normal controls was studied using 18F-fluorodeoxy glucose positron emission tomography. Patients with narcolepsy showed significantly reduced cerebral glucose metabolism in bilateral rectal and subcallosal gyri, the medial convexity of right superior frontal gyrus, bilateral precuneus, right inferior parietal lobule, and in left supramarginal gyrus (uncorrected p < 0.001). Bilateral posterior hypothalami and mediodorsal thalamic nuclei showed hypometabolism with significance at the level of corrected p < 0.05, with small volume correction. This study showed cerebral glucose hypometabolism of the hypothalamus-thalamus-orbitofrontal pathways in the narcoleptic brain.

The Xyrem?? Risk Management Program

Sodium oxybate, also known as gamma-hydroxybutyric acid (GHB), was discovered in 1960 and has been described both as a therapeutic agent with high medical value and, more recently, a substance of abuse. The naturally occurring form of this drug is found in various body tissues but has been studied most extensively in the CNS where its possible function as a neurotransmitter continues to be studied. Sodium oxybate has been approved in different countries for such varied uses as general anaesthesia, the treatment of alcohol withdrawal and addiction, and, most recently, cataplexy associated with narcolepsy. During the 1980s, easy access to GHB-containing products led to various unapproved uses, including weight loss, bodybuilding and the treatment of sleeplessness, sometimes with serious long-term effects. The availability of these unapproved and unregulated forms of the drug led to GHB and its analogues being popularised as substances of abuse and subsequent notoriety as agents used in drug-facilitated sexual assault, or 'date rape', eventually leading to the prohibition of GHB sales in the US. Legal efforts to control the sale and distribution of GHB and its analogues nearly prevented the clinical development of sodium oxybate for narcolepsy in the US. However, following extensive discussions with a variety of interested parties, a satisfactory solution was devised, including legislative action and the development of the Xyrem Risk Management Program. Amendments to the US Controlled Substances Act made GHB a schedule I drug, but also contained provisions that allow US FDA-approved products to be placed under schedule III. This unique, bifurcated schedule for sodium oxybate/GHB allowed the clinical development of sodium oxybate to proceed and, in July 2002, it was approved by the FDA as an orphan drug for the treatment of cataplexy in patients with narcolepsy as Xyrem(sodium oxybate) oral solution. To promote the safe use of sodium oxybate, as well as alleviate concerns over possible diversion and abuse following product approval, a proprietary restricted drug distribution system was created, called the Xyrem Success Program. Components of the programme include a centralised distribution and dispensing system, a physician and patient registry, compulsory educational materials for patients and physicians, a specially trained pharmacy staff, a method for tracking prescription shipments, and an initial post-marketing surveillance programme. The system has created a unique opportunity to provide both physician and patient education and ongoing patient counselling, promoting greater drug safety and enhanced patient compliance.

Neuropeptides in hypothalamic neuronal disorders

A few examples of hypothalamic, peptidergic disorders leading to clinical signs and symptoms are presented in this review. Increased activity of corticotropin-releasing hormone (CRH) neurons in the paraventricular nucleus (PVN) and decreased activity of the vasopressin neurons in the biological clock and of the thyroxine-releasing hormone (TRH) neurons in the PVN contribute to the signs and symptoms of depression. In men, the central nucleus of the bed nucleus of the stria terminalis (BSTc) is about twice as large and contains twice as many somatostatin neurons as in women. In transsexuals this sex difference is reversed, pointing to a role of this structure in gender. Luteinizing hormone-releasing hormone (LHRH) neurons are formed in the fetal olfactory placade and migrate along the terminal nerve fibers into the hypothalamus. In Kallmann's syndrome the migration process of the LHRH (gonadotropin-releasing hormone) neurons is aborted, which explains the joint occurrence of hypogonadotropic hypogonadism and anosmia in this syndrome. In postmenopausal women, the neurons of the infundibular nucleus hypertrophy and become hyperactive because of the disappearance of the estrogen feedback and contain hyperactive peptidergic neurons. Climacteric flushes may be caused by hyperactivity of the neurokinin-B or LHRH neurons in this nucleus. The hypocretin (orexin) neurons in the perifornical area are involved in sleep. In narcolepsy with cataplexy, a loss of these neurons, probably due to an autoimmune process, is found. Obese subjects with a mutation in the gene that encodes for leptin, the preproghrelin gene, or the alpha-melanocyte-stimulating hormone (alpha-MSH) gene have been described. Decreased numbers and activity of the oxytocin neurons in the PVN may be responsible for the absence of satiety in Prader-Willi syndrome. Moreover, a glucocorticoid receptor polymorphism is associated with obesitas and dysregulation of the hypothalamus-pituitary-adrenal axis. In contrast, two single nucleotide polymorphisms (SNPs) of the AGRP gene have been associated with anorexia nervosa.

Cataplexy: 'tonic immobility' rather than 'REM-sleep atonia'?

BACKGROUND

Cataplexy, a sudden loss of muscle tone in response to strong emotions, is the most specific symptom of narcolepsy. It is currently thought to be due to disturbed rapid eye movement (REM) sleep regulation, and portrayed as REM sleep atonia occurring at the wrong time. However, there are several arguments against including cataplexy in the 'state boundary control' hypothesis. It does not explain why cataplexy is triggered by emotions, and recent studies in narcoleptic dogs showed that REM sleep regulatory mechanisms were in fact intact in these animals.

METHODS

We review the literature on the REM sleep dissociation theory, discuss the merits and demerits of the theory, and propose an alternative hypothesis explaining cataplexy.

RESULTS

Cataplexy may represent an atavism (recurrence of an ancestral characteristic) of tonic immobility. Tonic immobility (TI) denotes a condition in which an animal is rendered immobile when faced with danger. Arguments in favor of the TI hypotheses are that it explains the emotional triggering. Furthermore, centers regulating narcolepsy and TI are both located in the lateral hypothalamic area. Finally, several drugs known for their ameliorating effect on cataplexy reduce the frequency and duration of TI in animals.

CONCLUSION

Cataplexy may be due to a mechanism different from the other clinical symptoms of narcolepsy.

THEGENETICS OFNARCOLEPSY

Human narcolepsy is a genetically complex disorder. Family studies indicate a 20-40 times increased risk of narcolepsy in first-degree relatives and twin studies suggest that nongenetic factors also play a role. The tight association between narcolepsy-cataplexy and the HLA allele DQB1*0602 suggests that narcolepsy has an autoimmune etiology. In recent years, extensive genetic studies in animals, using positional cloning in dogs and gene knockouts in mice, have identified abnormalities in hypothalamic hypocretin (orexin) neurotransmission as key to narcolepsy pathophysiology. Though most patients with narcolepsy-cataplexy are hypocretin deficient, mutations or polymorphisms in hypocretin-related genes are extremely rare. It is anticipated that susceptibility genes that are independent of HLA and impinge on the hypocretin neurotransmitter system are isolated in human narcolepsy.

Hypocretin deficiency in narcoleptic humans is associated with abdominal obesity

OBJECTIVE

To determine the prevalence of obesity among patients with narcolepsy, to estimate associated long-term health risks on the basis of waist circumference, and to distinguish the impact of hypocretin deficiency from that of increased daytime sleepiness (i.e., reduced physical activity) on these anthropometric measures.

RESEARCH METHODS AND PROCEDURES

A cross-sectional, case-control study was conducted. Patients with narcolepsy (n = 138) or idiopathic hypersomnia (IH) (n = 33) were included. Age-matched, healthy members of the Dutch population (Monitoring Project on Risk Factors for Chronic Diseases and Doetinchem Project; n = 10,526) were used as controls. BMI and waist circumference were determined.

RESULTS

Obesity (BMI > or = 30 kg/m(2)) and overweight (BMI 25 to 30 kg/m(2)) occurred more often among narcolepsy patients [prevalence: 33% (narcoleptics) vs. 12.5% (controls) and 43% (narcoleptics) vs. 36% (controls), respectively; both p < 0.05]. Narcoleptics had a larger waist circumference (mean difference 5 +/- 1.4 cm, p < 0.001). The BMI of patients with IH was significantly lower than that of narcolepsy patients (25.6 +/- 3.6 vs. 28.5 +/- 5.4 kg/m(2); p = 0.004).

DISCUSSION

Overweight and obesity occur frequently in patients with narcolepsy. Moreover, these patients have an increased waist circumference, indicating excess fat storage in abdominal depots. The fact that patients with IH had a lower BMI than narcoleptics supports the notion that excessive daytime sleepiness (i.e., inactivity) cannot account for excess body fat in narcoleptic patients.

From Club Drug to Orphan Drug: Sodium Oxybate (Xyrem) for the Treatment of Cataplexy

Narcolepsy, a rare disease with a prevalence of 0.05% in the general population, affects an estimated 140,000 patients in the United States. Patients have been able to lead fuller personal and professional lives since the Food and Drug Administration approved sodium oxybate (Xyrem) in 2002 for treatment of cataplexy in patients with narcolepsy. Previously, gamma-hydroxybutyrate (GHB), the active ingredient of sodium oxybate, had been a substance of abuse, most notoriously as a date-rape drug. Public Law 106-172, the date-rape prohibition act enacted in 2000, was modified to allow the drug to be legally administered for medical purposes. Because of the apprehension regarding the risk of possible drug diversion after the approval of sodium oxybate and concerns about safety, the Xyrem Risk Management Program was created. This program has been successful in satisfying the needs of patients and physicians while ensuring responsible distribution of the drug.

Effects of lateral hypothalamic lesion with the neurotoxin hypocretin-2-saporin on sleep in Long-Evans rats

Narcolepsy, a disabling neurological disorder characterized by excessive daytime sleepiness, sleep attacks, sleep fragmentation, cataplexy, sleep-onset rapid eye movement sleep periods and hypnagogic hallucinations was recently linked to a loss of neurons containing the neuropeptide hypocretin. There is considerable variability in the severity of symptoms between narcoleptic patients, which could be related to the extent of neuronal loss in the lateral hypothalamus. To investigate this possibility, we administered two concentrations (90 ng or 490 ng in a volume of 0.5 microl) of the neurotoxin hypocretin-2-saporin, unconjugated saporin or saline directly to the lateral hypothalamus and monitored sleep, the entrained and free-running rhythm of core body temperature and activity. Neurons stained for hypocretin or for the neuronal specific marker were counted in the perifornical area, dorsomedial and ventromedial nucleus of the hypothalamus. More neuronal nuclei (NeuN) cells were destroyed by the higher concentration of hypocretin-2-saporin (-55%) compared with the lower concentration (-34%) in the perifornical area, although both concentrations lesioned the hypocretin neurons almost equally well (high concentration=91%; low concentration=88%). The high concentration of hypocretin-2-saporin also lesioned neurons in the dorsomedial nucleus of the hypothalamus and ventromedial nucleus of the hypothalamus. Narcoleptic-like sleep behavior was produced by both concentrations of the hypocretin-2-saporin. The high concentration produced a larger increase in non-rapid eye movement sleep amounts during the normally active night cycle than low concentration. Neither concentration of hypocretin-2-saporin disrupted the phase or period of the core temperature or activity rhythms. The low concentration of unconjugated saporin did not significantly lesion hypocretin or neurons and did not alter sleep. The high concentration of unconjugated saporin produced some loss of neuronal nuclei-immunoreactive (NeuN-ir) neurons and hypocretin immunoreactive neurons, but only a transient increase in non-rapid eye movement sleep. These results led us to conclude that the extent of hypocretin neuronal loss together with an accompanying loss of cells in the lateral hypothalamus may explain the differences in severity of symptoms seen in human narcolepsy.

The abrupt cessation of therapeutically administered sodium oxybate (GHB) does not cause withdrawal symptoms

Sodium oxybate (gamma-hydroxybutyrate; GHB) has demonstrated efficacy for the treatment of narcolepsy. However, there are reports of withdrawal following chronic abuse of illicit GHB which involve escalating both doses and dosing frequency. The present trial afforded an opportunity to test the hypothesis that chronic daily therapeutic dosing of sodium oxybate in narcoleptics does not cause withdrawal following abrupt cessation. Fifty-five narcoleptic patients, taking sodium oxybate (dose range 3-9 gm/night) for 7-44 months (mean 21 months), were randomized into a 2-week double-blind period: 29 patients received placebo and 26 continued to receive sodium oxybate. During this 2-week trial period, the following symptoms were reported in patients receiving placebo (N): anxiety (2), dizziness (1), insomnia (1) and somnolence (1). While these symptoms may represent possible symptoms of mild GHB withdrawal, they are also highly consistent with the returning symptoms of narcolepsy. We conclude there is minimal evidence of withdrawal symptoms following abrupt cessation of chronic sodium oxybate dosing in the therapeutic range.

Screening for anti-ganglioside antibodies in hypocretin-deficient human narcolepsy

Narcolepsy is a sleep disorder caused by defective hypocretin (orexin) neurotransmission. It is thought to result from an autoimmune destruction of hypocretin producing neurons. Recently, low hypocretin levels were found in patients with Guillain-Barré syndrome, a post-infectious immune-mediated disorder in which a variety of circulating antibodies against neuronal gangliosides are found. We therefore considered gangliosides to be candidate antigens in narcolepsy as well, and screened for the presence of a panel of serum anti-ganglioside antibodies in a group of 28 well-characterized narcoleptic patients. We did not find a correlation between increased titers of anti-ganglioside antibodies and hypocretin-deficient narcolepsy. This study does not support the hypothesis that an autoimmune response is involved in narcolepsy. However, as an autoimmune attack may be selective and/or transient, future studies are needed to ultimately refute or confirm the autoimmune hypothesis.

Somatotropic axis in hypocretin-deficient narcoleptic humans: altered circadian distribution of GH-secretory events

Narcolepsy is a sleep disorder caused by impaired hypocretin (orexin) neurotransmission. Growth hormone (GH) secretion may be altered in narcolepsy for various reasons. Slow-wave sleep episodes, which are closely associated with GH-secretory events, are more randomly dispersed over 24 h in narcoleptics. Furthermore, hypocretins may inhibit pituitary GH release. We assessed the function of the somatotropic axis in narcolepsy by deconvolving 24-h (10-min sampling interval) plasma GH concentration profiles in seven hypocretin-deficient narcoleptic patients and in seven healthy controls matched for age, sex, and body weight. Both basal and pulsatile GH secretion rate and secretagogue-induced GH release were similar in patients and controls. However, narcoleptics secreted approximately 50% of their total production during the daytime, whereas controls secreted only 25% during the day. Also, the GH output pattern of narcoleptics was significantly less regular. We propose that hypocretin deficiency disrupts the circadian distribution of hypothalamic GH-releasing hormone release in narcoleptic patients to simultaneously cause daytime GH release and promote their propensity to fall asleep during the day.

HLA typing does not predict REM sleep behaviour disorder and hallucinations in Parkinson's disease

HLA-DR2 haplotype and DQ1 DNA alleles, characterizing 90 to 100% of all narcoleptic patients, were found to be equally distributed in 20 Parkinson's disease (PD) patients with early hallucinations, rapid eye movement (REM) sleep-related behaviour disturbances (RBD), and sleep onset in REM (SOREM), and in 20 PD patients without hallucinations, despite 10 to 15 years of treatment, and no RBD or SOREM.

Narcolepsy

Narcolepsy is a chronic disorder of EDS. All patients experience EDS. Other symptoms include cataplexy, sleep paralysis, hypnagogic hallucinations, and disrupted nocturnal sleep. Treatment, usually with stimulants and low-doses of antidepressant medications, can dramatically improve the patient's quality of life. Although only advanced practice nurses may be actively involved in the diagnosis and treatment of this disorder, all nurses can encourage their patients who complain of EDS to consult a specialist in sleep disorders medicine, provide emotional support after diagnosis, and educate patients and their families about narcolepsy and its treatment.

Measurement of hypocretin/orexin content in the mouse brain using an enzyme immunoassay: the effect of circadian time, age and genetic background

The hypocretins (1 and 2) have emerged as key regulators of sleep and wakefulness. We developed a high-throughput enzyme immunoassay (EIA) to measure total brain hypocretin levels from large numbers of mice. Hypocretin levels were not altered by circadian time or age. However, significant differences in one or both hypocretin peptides were observed between different mouse strains. We studied hypocretin levels in knockout and transgenic mouse models with obesity, circadian gene mutations or monoaminergic defects. Compared to controls, only histamine receptor knockouts had lower hypocretin levels. This was most pronounced in H1 receptor knockouts suggesting the existence of a positive feedback loop between hypocretin and histaminergic neurons.

Functions of the orexinergic/hypocretinergic system

Orexin A and orexin B are hypothalamic peptides that act on their targets via two G protein-coupled receptors (OX1 and OX2 receptors). In the central nervous system, the cell bodies producing orexins are localized in a narrow region within the lateral hypothalamus and project mainly to regions involved in feeding, sleep, and autonomic functions. Via putative pre- and postsynaptic effects, orexins increase synaptic activity in these regions. In isolated neurons and cells expressing recombinant receptors orexins cause Ca2+ elevation, which is mainly dependent on influx. The activity of orexinergic cells appears to be controlled by feeding- and sleep-related signals via a variety of neurotransmitters/hormones from the brain and other tissues. Orexins and orexin receptors are also found outside the central nervous system, particularly in organs involved in feeding and energy metabolism, e.g., gastrointestinal tract, pancreas, and adrenal gland. In the present review we focus on the physiological properties of the cells that secrete or respond to orexins.

The hypothalamus in episodic brain disorders

Episodic brain disorders (EBD) form an intriguing group of neurological diseases in which at least some of the symptoms occur in attacks. The hypothalamus integrates many brain functions, including endocrine and autonomic control, and governs various body rhythms. It seems a likely site in which the initiation of attacks of EBD can be modulated. Indeed, the hypothalamus has a crucial role in EBD such as narcolepsy and cluster headache. The same may be true for migraine and depression. Here we summarise the evidence supporting an important role for the hypothalamus in the initiation of disease episodes in various EBD. Study of the various pathophysiological concepts of EBD within the context of the hypothalamus may prove a fruitful example of cross-fertilisation between various research areas.

The role of hypocretins (orexins) in sleep regulation and narcolepsy

The hypocretins (orexins) are two novel neuropeptides (Hcrt-1 and Hcrt-2), derived from the same precursor gene, that are synthesized by neurons located exclusively in the lateral, posterior, and perifornical hypothalamus. Hypocretin-containing neurons have widespread projections throughout the CNS with particularly dense excitatory projections to monoaminergic centers such as the noradrenergic locus coeruleus, histaminergic tuberomammillary nucleus, serotoninergic raphe nucleus, and dopaminergic ventral tegmental area. The hypocretins were originally believed to be primarily important in the regulation of appetite; however, a major function emerging from research on these neuropeptides is the regulation of sleep and wakefulness. Deficiency in hypocretin neurotransmission results in the sleep disorder narcolepsy in mice, dogs, and humans. The hypocretins are also uniquely positioned to link sleep, appetite, and neuroendocrine control. The aim of this review is to describe and discuss the current knowledge regarding the hypocretin neurotransmitter system in narcolepsy and normal sleep.

Reduced cortical gray matter in narcolepsy: preliminary findings with voxel-based morphometry

There is a selective loss of hypocretin/orexin-containing hypothalamic neurons in patients with narcolepsy. The authors compared MRI-derived gray matter maps of 12 patients with narcolepsy with matched controls using voxel-based morphometry to ascertain whether there are other structural brain abnormalities. Patients with narcolepsy showed bilateral cortical gray matter reductions predominantly in inferior temporal and inferior frontal brain regions. Relative global gray matter loss was independent of disease duration or medication history. No significant subcortical gray matter alterations were noted.

Integration of human sleep-wake regulation and circadian rhythmicity

The human sleep-wake cycle is generated by a circadian process, originating from the suprachiasmatic nuclei, in interaction with a separate oscillatory process: the sleep homeostat. The sleep-wake cycle is normally timed to occur at a specific phase relative to the external cycle of light-dark exposure. It is also timed at a specific phase relative to internal circadian rhythms, such as the pineal melatonin rhythm, the circadian sleep-wake propensity rhythm, and the rhythm of responsiveness of the circadian pacemaker to light. Variations in these internal and external phase relationships, such as those that occur in blindness, aging, morning and evening, and advanced and delayed sleep-phase syndrome, lead to sleep disruptions and complaints. Changes in ocular circadian photoreception, interindividual variation in the near-24-h intrinsic period of the circadian pacemaker, and sleep homeostasis can contribute to variations in external and internal phase. Recent findings on the physiological and molecular-genetic correlates of circadian sleep disorders suggest that the timing of the sleep-wake cycle and circadian rhythms is closely integrated but is, in part, regulated differentially.