Narcolepsia en ingles

Narcolepsy: Clinical Features, New Pathophysiologic Insights,and Future Perspectives

*Sebastiaan Overeem, Emmanuel Mignot, *J. Gert van Dijk, and *Gert Jan Lammers*Department of Neurology and Clinical Neurophysiology, Leiden University Medical Center, The Netherlands; and the Center

for Narcolepsy, Department of Psychiatry, Stanford University School of Medicine, California, USA.

Summary: Narcolepsy is characterized by excessive daytime sleepiness and abnormalmanifestations of rapid eye movement sleep such as cataplexy. The authors review theclinical features of narcolepsy, including epidemiology, symptoms, diagnosis, andtreatment, in detail. Recent findings show that a loss of hypocretin-producing neuronslies at the root of the signs and symptoms of narcolepsy. The authors review the currentstate of knowledge on hypocretin anatomy, physiology, and function with specialemphasis on the research regarding the hypocretin deficiency in narcolepsy, whichmay also explain associated features of the disorder, such as obesity. Lastly, theydiscuss some future perspectives for research into the pathophysiology of sleep/wakedisorders, and the potential impact of the established hypocretin deficiency on thediagnosis and treatment of narcolepsy. Key Words: NarcolepsyCataplexyHypo-cretinOrexinREM sleepObesity.

Narcolepsy is currently defined as a disorder ofunknown etiology, which is characterized by excessivesleepiness that typically is associated with cataplexy(sudden bilateral loss of muscle tone provoked by emo-tion) and other [rapid eye movement] REM sleep phe-nomena such as sleep paralysis and hypnagogic halluci-nations (International Classification of Sleep Disorders,1997). This definition does not reveal that it puzzledneurologists and psychiatrists for more than a century.The fascination probably results from the combinationof, on the one hand, bizarre, and on the other hand,universally recognizable experiences. The impact ondaily life is profound, although standard physical andmental investigations produce normal results. The vari-

ous clinical symptoms are in part paroxysmal. Theycover consciousness, perception, and motor control, andare modulated by situational circumstances andemotions.

Recent discoveries challenge some aspects of the def-inition, in particular concerning the etiology. This reviewstarts with an overview of the clinical aspects of narco-lepsy, followed by an update on the pathophysiology,with emphasis on the role of the hypocretins. Lastly wediscuss the implications of impaired hypocretin transmis-sion for the diagnosis and treatment of narcolepsy.

THE BURDEN OF NARCOLEPSY

Psychosocial problems are a direct consequence of thesymptomatology. They are frequent, incapacitating, andoccur in all cultures (Broughton et al., 1981; Kales et al.,1982). The impact of narcolepsy is often more severethan that of other chronic diseases such as epilepsy(Broughton et al., 1984). Controlled questionnaire stud-ies demonstrate that most patients have educational andoccupational problems, even when treated properly. Nar-

The work by S.O. at Stanford University was supported by grantsfrom the Hersenstichting Nederland and Stichting De Drie Lichten.Parts of the authors research were supported by National Institutes ofHealth grants NS23724, NS33797, and HL59601 (E.M.).

Address correspondence and reprint requests to Dr. Gert Jan Lam-mers, Department of Neurology and Clinical Neurophysiology, LeidenUniversity Medical Center, PO Box 9600, 2300 RC Leiden, TheNetherlands.

Journal of Clinical Neurophysiology18(2):78105, Lippincott Williams & Wilkins, Inc., Philadelphia 2001 American Clinical Neurophysiology Society

78

colepsy has deleterious effects on performance, promo-tion, earning capacity, and interpersonal relations, in-cluding marriage and sexual function. There is anincreased risk of job loss, accidents, and additional in-surance payments (Broughton et al., 1983, 1984; Gos-wami, 1998). Driving is prohibited in several countries.Child and adolescent narcoleptics report embarrassment,academic decline, and loss of self-esteem (Broughton etal., 1994). Although the complaints may be very dis-abling, the core of the personality remains intact.

HISTORICAL REVIEW

In 1877, Westphal (1877) described the first unequiv-ocal case of the disease later called narcolepsy in hispaper entitled Eigenartige mit Einschlafen verbundeneAnflle. The patient presented reported attacks of mus-cle weakness with preserved consciousness, sleep at-tacks, and disturbed nocturnal sleep. There were evenearlier reports of patients, who in retrospect may havehad narcolepsy, but their descriptions do not excludeother forms of hypersomnia (Caff, 1862; Furukawa,1987; Graves, 1851; Lennox, 1939; Schindler, 1829).

Gelineau (1880) coined the term narcolepsy (inFrench, narcolepsie) in 1880 to describe the combinationof sleep attacks and attacks of astasiaa combinationhe considered to be a specific disease entity. A debateensued, continuing for many decades afterward: Wasnarcolepsy a (single) disease (entity), a morbus suigeneris, to be distinguished from epilepsy and hysteria?If so, were attacks of muscle weakness (cataplexy) es-sential for the diagnosis? As a consequence, many in-vestigators used the word narcolepsy inadvertently forvarious kinds of hypersomnia-related symptoms, leadingto a confusion of terms.

In 1902, Loewenfeld (1902) was the first who firmlyadvocated to structure the debate and to reserve the termnarcolepsy for the combination of hypersomnia and at-tacks of muscle weakness. He agreed with many con-temporary authors that isolated sleep attacks could be anonspecific phenomenon, indicative of many other dis-eases. However, it took more than 20 years before thisview became more generally accepted. Henneberg(1916) introduced the term cataplexy for attacks of mus-cle weakness as late as in 1916. Redlich (1925) intro-duced another term, affektive Tonusverlust, around thesame time, that has been used in German until now.However, in the Anglo-Saxon literature, which has dom-inated the literature on narcolepsy from the second halfof the 1920s, cataplexy has been used to describe suchattacks (Adie, 1926).

Yoss and Daley (1957) introduced the classic tetrad of

narcolepsy in 1957, adding hypnagogic hallucinationsand sleep paralysis. Both phenomena had been describedearlier, independent of, as well as associated with, nar-colepsy (Daniels, 1934; Fischer, 1878). Remarkably andparadoxically, the specificity of hypnagogic hallucina-tions and sleep paralysis for the diagnosis of narcolepsyhas been questioned again in recent years (Aldrich et al.,1997). In 1975, during the first International Conferenceon Narcolepsy, in France, disturbed nocturnal sleep wasadded as the fifth symptom, resulting in a pentad ratherthan a tetrad (Guilleminault, 1976).

In 1960, Vogel (1960) discovered sleep-onset REMperiods in the EEG of patients with narcolepsy. Heinterpreted this finding as evidence that narcolepsy was apsychogenic disorder: The early appearance of dream-ing in the sleep of a narcoleptic patient, in contrast to theabsence of dreaming in the early phase of normal sleep,supports the proposition that the narcoleptic patientmakes use of his sleep for the projection of fantasy whichis gratified in a dream in a way unacceptable duringwaking life (p. 428). Despite this nowadays remarkableif not astonishing interpretation, his discovery paved theway for the notion that cataplexy, hypnagogic hallucina-tions, and sleep paralysis were all partial expressions ofREM sleep (Rechtschaffen et al., 1963; Takahashi andJimbo, 1963). Objective tests to diagnose narcolepsycould now be developed, finally resulting in the MultipleSleep Latency Test (MSLT), which assesses sleep ten-dency as well as abnormal manifestations of REM sleep(Carskadon and Dement, 1977; Carskadon et al., 1986).

The detection of narcolepsy in dogs in the 1970s(Knecht et al., 1973; Mitler et al., 1974), and theirsuccessful breeding in several cases (Foutz et al., 1979),marked a new era in narcolepsy research, eventuallyleading to the discovery that impaired hypocretin trans-mission results in narcolepsy in dogs and humans in1999 (Lin et al., 1999; Nishino et al., 2000).

EPIDEMIOLOGY

Narcolepsy has an estimated prevalence of 20 to 60per 100,000 in Western countries (Aldrich, 1992; De-ment et al., 1973; Hublin et al., 1994c; Mignot, 1998;Roth, 1980). The highest prevalence is reported in Japan(Honda, 1979; Mignot, 1998) and the lowest in Israel(Lavie and Peled, 1987). These differences are probablythe result of different diagnostic criteria and differencesin experimental design.

Men and women are affected equally. Age at onset liesbetween 15 to 30 years in the majority of patients.Narcolepsy may start before puberty; in approximately6% of patients it starts before the age of 10 years. Onset

79NARCOLEPSY PERSPECTIVES

J Clin Neurophysiol, Vol. 18, No. 2, 2001

after the age of 55 is very uncommon (Broughton andMullington, 1994; Parkes, 1985; Roth, 1980). Life ex-pectancy is normal.

Using clinical criteria, largely corresponding to thegenerally used criteria of the international classificationof sleep disorders, only 10% to 15% of patients experi-ence the full tetrad (Parkes, 1985; Roth, 1980; Yoss andDaly, 1957). Approximately 70% of patients have cata-plexy, 30% have hypnagogic hallucinations, and 25%have sleep paralysis (Parkes, 1985; Yoss and Daly,1957). Excessive daytime sleepiness (EDS) alone or incombination with hypnagogic hallucinations and/or sleepparalysis is the presenting symptom in approximately90% of patients (Guilleminault et al., 1974; Roth, 1980).Cataplexy is the first symptom in approximately 5% to8% of patients (Guilleminault et al., 1974; Guilleminault,1976). The pattern does not appear to be different inprepubertal children (Guilleminault et al., 1998). Symp-toms usually develop gradually. It may take several yearsfor each subsequent symptom to occur. If cataplexy is thesecond symptom to occur, it appears on average 6 yearsafter the onset of EDS (Passouant and Billiard, 1976).Once the symptoms are present, there are usually onlyminor fluctuations in severity. Over several decades,cataplexy and sleep paralysis may diminish and almostdisappear in some patients. In the case of cataplexy thisis probably not simply the result of changes in lifestyleand avoidance of provocative situations. It is exceptionalfor new symptoms to appear 10 years or more after theonset of the disease. However, we witnessed the devel-opment of EDS in a patient who had had isolated cata-plexy for more than 40 years (van Dijk et al., 1991).

CLINICAL SYMPTOMS

Excessive Daytime Sleepiness

EDS is the principal symptom of narcolepsy. Clini-cally, it is expressed in two different ways, the first beinga continuous subjective feeling of sleepiness, and thesecond being the presence of irresistible sleep attacks.Most patients experience both aspects of sleepiness, buteither one may predominate in a single patient. Oncesleepiness has occurred, it will be present daily, never todisappear again. The severity is usually stable over theyears, with mild fluctuations over periods of weeks ormonths. Sleepiness must be distinguished from fatigue,which is a much less specific complaint, related to a greatmany other disorders. Sleepiness usually fluctuates dur-ing the day, with a stereotyped, individual pattern in themajority of patients. It is often relieved temporarily after

a sleep attack. However, no single amount of sleep willeliminate sleepiness for more than several hours.

EDS in narcolepsy is characterized by the tendency tofall asleep much more than by an increased amount ofsleep. Even though patients may fall asleep at every hourof the day, the total amount of sleep over a 24-hourperiod is increased only slightly, if at all. Although theymay be labeled as attacks, naps seldom start with anabrupt switch from alertness to sleep; patients go througha drowsy state instead. Blurred or double vision andptosis may be associated with this drowsiness (Chee,1968). Patients seem to experience a continuous waxingand waning of attention, modulated by situational cir-cumstances. Provocative circumstances resemble thoseof healthy sleep-deprived individuals. Examples are pas-sive activities and boring situations, such as watchingtelevision or being a passenger in a car. In more severecases, reading a newspaper is impossible. Rarely, sleepattacks may occur under highly unusual circumstances(e.g., while riding a bicycle or walking).

The duration of sleep attacks is usually less than 20minutes, but a longer duration does not preclude thediagnosis. Patients can be awakened during an attack.

Motor and, to a lesser extent, mental activity seems tobe a way to combat waning attention, because this mayprevent temporarily the occurrence of sleep as well as thesubjective feeling of sleepiness. For this reason patientsmay force themselves to be active to reduce or even(subjectively) to eliminate their complaints. Some evenfear weekends, or days with inevitable passivity such asholidays. However, all patients will have difficultiesstaying awake when seated in a dark, quiet room, regard-less of whether they just have slept, have taken stimu-lants or other medication, or whether they feel sleepybefore sitting down.

Cataplexy

Cataplexy (kataplhssv 5 to strike down) is char-acterized by a bilateral loss of muscle tone of suddenonset with preserved consciousness (Fig. 1). All striatedmuscles may be involved, but eye movements and res-piration are not compromised, although there may be afeeling of shortness of breath. The muscle weakness canbe partial or complete, and may involve some or, lessfrequently, all muscles. Two regions of the body arepredominantly involved in partial attacks: the knees,which give way; and the face and neck, with sagging ofthe jaw, inclination of the head, and some weakness ofthe muscles subserving speech. Speech may be slurred orthe patient may be unable to talk at all (Daniels, 1934).

80 S. OVEREEM ET AL.


Movements such as grimacing and jerks are observed,the latter particularly in the face.

Note that partial attacks can be so subtle that they areoften only recognized by experienced observers, such aspartners or specialized physicians. Patients themselvesmay even fail to interpret this phenomenon as abnormal.Although the attacks start abruptly, they usually takeseveral seconds to reach their maximum. Injury is un-common, even in more severe attacks, because mostpeople are able to find support or to sit down at the onsetof an attack. The extent of muscle weakness determineswhether a patient falls during an attack.

A consistent individual pattern of involved muscles isusually seen, although both partial and complete attacksmay occur in the same patient. The frequency variesbetween more than 10 per day to less than one per month,and seems to be rather stable over periods of 10 days(Gelb et al., 1994)

Neurologic examination during an attack shows, be-sides atonia, a loss of tendon reflexes and sometimes

extensor plantar responses (Adie, 1926; Guilleminaultand Gelb, 1995; Wilson, 1928).

Cataplexy lasts from several seconds to, rarely,several minutes. A longer duration is probably theresult of consecutive attacks, or a transition into(REM) sleep.

Attacks are provoked by emotions, the most typicalone being laughter (AnicLabat et al., 1999). Otherfrequent triggers include the following: when feelingamused, attempts at repartee, and, less often, unexpectedmeetings with acquaintances or anger. It is an intriguingquestion why the threshold for an attack seems to be thelowest during laughter among all various emotions. Anapparently universally occurring reduction in muscletone induced by laughter may be part of the explanation(Lammers et al., 2000; Overeem et al., 1999). Both theactual emotion as well as the anticipation of somethingspecial or funny may elicit an attack. Examples of thisanticipation are telling a joke but being unable to finishit because an attack starts just before the patient gets to

FIG. 1. Photographs showing two complete cataplectic attacks in a 51-year-old narcoleptic patient. This patient is severely affected, with multiplecomplete attacks per day when untreated. Note that the muscle paralysis is not complete immediately. In the upper series the patient first falls to hisknees, before collapsing completely. In the lower series the patient is able to stretch out his hands to break the fall. Both series span a time courseof approximately 2 seconds. The patient gave written consent to publish these photographs.



the punch line, or failure to hit the ball perfectly duringtennis because the excitement induces an attack. A spo-radic, not always obviously emotional, trigger is aninfirmity in response to a call for sudden action. Thismay impair the ability to participate in sports, or to catcha bus or train when it is almost leaving. Each patientexperiences an individual sensitivity for the varioustypes of emotions that will provoke an attack. Attackswithout any detectable trigger occur only rarely.

Although emotions are very important triggers, thecircumstances and state of the patient are also impor-tant. Attacks characteristically cannot be provoked dur-ing medical consultation or in laboratory situations. Thisis underscored by the fact that the first description of anobserved cataplectic attack was published in 1921, morethan 40 years after the discovery of the disease and afterthe introduction of the term cataplexy (Somer, 1921). Itseems that a certain intimacy lowers the threshold forattacks to occur in company. An illustration of thisphenomenon concerns the example of the patient with aninability to score in tennis: This only happened whileplaying with his partner or a close friend. Other circum-stances that frequently lower the attack threshold are astrong feeling of sleepiness and sleep deprivation. Aminority of patients also describe periods of hours ordays during which the attack threshold is lower, soattacks may occur without or with less obvious triggers.A specific sensation in the head, or sensations of warmthor nervousness at the beginning of or just before anattack have also been reported (Gelb et al., 1994). Inter-estingly, some patients still experience such sensationsduring treatment, when they no longer have a loss ofmuscle tone. Most patients learn tricks after severalyears to prevent attacks or to abort the attacks. Examplesof such tricks are leaning against a wall, or the voluntarycontraction of muscles that are not (yet) involved. Nar-coleptics with severe cataplexy tend to avoid (potential)emotional situations, causing them to withdraw in partfrom social life.

A so-called status cataplecticus has been describedespecially after acute withdrawal of medication (tricy-clics and mazindol) (Hishikawa and Shimizu, 1995;Parkes, 1985; Passouant et al., 1970) and in patientsinitiating prazosin treatment for concomitant hyperten-sion (Aldrich and Rogers, 1989; Guilleminault et al.,1988).

Less than 0.5% of the patients have unilateral cata-plexy. Structural brain lesions seem to be relativelycommon in such cases (Lascelles et al., 1976; Wilson,1928; Yoss and Daly, 1957).

Hypnagogic Hallucinations

Hypnagogic hallucinations are vivid dreamlike expe-riences that occur during the transition between wake-fulness and sleep. If similar experiences occur on awak-ening, the term hypnopompic hallucination is used.During the attack, these images are usually felt to be real.Many subjects have to be convinced by partners, or haveto check for themselves, that their experiences are notreal. In time, most patients get used to their occurrence,reducing their impact. In the majority of patients thecontent of the hypnagogic hallucinations is bizarre andfrightening; they are only rarely pleasant. Some patientsfeel they are able to manipulate the content; for example,by concentrating on pleasant experiences or changingbody position. Although visual imagery is predominant,auditory and tactile components are frequently present,but smell and taste feature only rarely. Constant orchanging colored forms are reported as well. A substan-tial proportion of patients additionally have out-of-bodyexperiences at sleep onset. In many patients these latterphenomena diminish after treatment along with the vi-sual experiences. In some narcoleptic patients, hypnago-gic hallucinations may be so intense, that they can beconsidered wrongly to be psychotic, leading to a diag-nosis of schizophrenia (Douglass et al., 1991; Howland,1997; Jackson and Bachman, 1996).

The duration of the attacks is almost always less than10 minutes. The frequency varies between less than oncea month to more than once a day. During the attack, sleepparalysis may occur in addition to the hallucinations. Inour opinion, it is the timing of dreaming at sleep onset,rather than the content, that is characteristic for hypna-gogic hallucinations in narcolepsy. The timing is alsoimportant for differentiation from unusual vivid dreamsthat are also considered to have a high prevalence innarcolepsy.

Many patients do not report hypnagogic hallucinationsspontaneously, because they are embarrassed to mentionsuch expressions of madness. They only admit havinghad these experiences if asked explicitly.

Hypnagogic hallucinations can occur on their own,without any of the other features of narcolepsy, butattack frequency is usually lower in those cases than innarcolepsy.

Sleep Paralysis

Sleep paralysis is the inability to move during theonset of sleep or on awakening, while patients are sub-jectively awake and conscious. It is therefore almostidentical to attacks of complete cataplexy, apart from theabsence of an emotional trigger and a longer duration (as



long as 10 minutes). Patients are unable to perform eventhe smallest movements, such as opening their eyes orlifting a finger. They may feel they have to struggle tomove. In most patients attacks are infrequent, occurringas much as once or twice a week. In addition to itsduration, sleep paralysis has other features in commonwith hypnagogic hallucinations, such as the timing at theonset of sleep or awakening and a high prevalence insubjects without narcolepsy. A prevalence of more than30% (5% to 62%) in the general population has beenreported, with familial clustering and an association withsleep deprivation (Dahlitz and Parkes, 1993; Ness,1978).

People are usually frightened during an attack, partic-ularly the first time. The anxiety is intensified by hyp-nagogic hallucinations that often accompany the paraly-sis (Hishikawa, 1976; Parkes, 1985). Sensory stimulationmay end the paralysis, but others consider such anattempt to be even more unpleasant than the attack itself.

Disturbed Nocturnal Sleep

Disturbed nocturnal sleep is considered to be the fifthcomponent of the tetrad. It may cause a great deal ofdiscomfort. Although patients typically have a shortsleep-onset latency, subsequent frequent awakenings arecommon. Most awakenings are brief, but some peoplestay awake for hours. A clear relationship between theseverity of the disturbed nocturnal sleep and the severityof daytime sleepiness has never been established(Broughton and Broughton, 1994; Lamphere et al.,1989).

Other sleep-related disorders such as periodic limbmovements disorder (PLMD), sleep apnea, REM sleepbehavior disorder, increased body movements, andsleeptalking may all contribute to the disturbed nocturnalsleep in narcolepsy. Some authors even state that thenocturnal sleep disturbances are exclusively the result ofsuch concomitant, in part age-related, disorders (Wittiget al., 1983), but others question their importance (Boivinet al., 1993b). Although many authors presume that thesephenomena occur more frequently in narcolepsy than inthe general population, well-designed studies on thissubject are lacking (Parkes, 1985; Schenck and Mahow-ald, 1992; van den Hoed et al., 1981; Wittig et al., 1983).

Other Symptoms

Several symptoms are less important for the diagnosisof narcolepsy, but are frequently reported and may, atleast in part, share pathophysiologic mechanisms withthe classic five symptoms.

Automatic behavior occurs in as many as 40% of

patients, and is characterized by semipurposeful activityduring a state of drowsiness that often precedes a sleepattack. The duration is usually minutes, with a variationof seconds to more than an hour. Partial or completeamnesia for these activities is the rule (Guilleminault etal., 1975b). The episodes typically involve the continu-ation of an activity that does not require extensive skill.If skill is needed, frequent mistakes are made. Examplesare meaningless written language, handwriting that be-comes illegible, or finding oneself in a place withoutknowing why or how one has arrived. During conversa-tions patients may utter a burst of words, sometimeswithout any meaning, and without any relation to whathas just been said (Guilleminault et al., 1975a).

Memory disturbances are reported by as many as 50%of patients, particularly regarding recent events (Aguirreet al., 1985; Broughton et al., 1981; Rogers and Rosen-berg, 1990; Schulz et al., 1997). Objective dysfunction istypically absent during standard memory testing. Thisdiscrepancy is often explained by a sleepiness-inducedlack of attention in normal everyday life, which does notoccur during the high-attention demands in the labora-tory situation. Other explanations are the use of invalidmemory tests, which do not represent everyday memoryfunctions. Alternatively, patients may have an inaccurateperception of their own memory, similar to the memorycomplaints in depression (Hood and Bruck, 1997).

Associated Features

Tiredness or fatigue is not a symptom of narcolepsy,but a nonspecific complaint, with a high prevalence.Confusion with sleepiness must be eliminated. If presentas a major complaint, a coexistent depression must beconsidered.

Sleep apnea is considered to be associated with nar-colepsy (Chokroverty, 1986; Parkes, 1985). However,controlled studies in large, unselected groups using cleardefinitions for sleep apnea and narcolepsy are not avail-able, so it remains unclear to what extent sleep apneaoccurs more often than chance would predict. PLMD isoften seen in narcolepsy, but probably does not contrib-ute remarkably to nocturnal sleep disruption or to day-time sleepiness (Boivin et al., 1993a; Hong et al., 2000).REM sleep behavior disorder has a prevalence of ap-proximately 7% in narcolepsy. Treatment with stimu-lants or tricyclics may induce or exacerbate its occur-rence (Schenck and Mahowald, 1992).

As many as 30% of narcoleptic patients fulfill thecriteria for depression (Broughton et al., 1981). Becausesome of the clinical features of depression and narco-lepsy may overlap, and sleep-onset REM periods are also



seen in depressed people, it is sometimes difficult toseparate the two (Hudson et al., 1992). Moreover, thepharmacologic treatment (tricyclic antidepressants) ofboth diseases overlaps. These notions led most investi-gators to believe that depression shares some of thepathophysiologic mechanisms with narcolepsy, and theseauthors do not consider depression merely a reaction tothe burden of narcolepsy (Reynolds et al., 1983).

Sexual dysfunction (diminished libido, impotence) arereported frequently (Broughton et al., 1994; Roy, 1976).Whether the prevalence in untreated patients is higherthan in other chronic diseases is not known. Treatment inparticular with tricyclic antidepressants may be an im-portant cause, particularly in men (Broughton et al.,1994; Roth, 1980).

Narcoleptic patients are frequently obese. Their bodymass index (BMI) is approximately 10% to 20% higherthan that of matched healthy control subjects (Bell, 1976;Daniels, 1934; Honda et al., 1986; Lammers et al., 1996;Schuld et al., 2000b). Inactivity resulting from sleep hasbeen postulated as an explanation. However, there is noconvincing evidence that narcoleptic patients are lessactive than healthy control subjects during the 24 hoursof a day (Middelkoop et al., 1995). An association witha tendency to crave sweet foods has been suggested aswell (Bell, 1976). The use of medication and/or coexist-ing depression, which were not exclusion criteria in thesestudies, could be an alternative explanation (Lammers etal., 1996). However, a recent retrospective study didinclude medicated (n 5 17) as well as nonmedicatedpatients (n 5 18), and reported a higher BMI in bothpatient groups compared with the normal population(Schuld et al., 2000b). Additionally, in a small study wefound that patients without medication and depressionhad a higher BMI compared with matched control sub-jects (28.9 6 3.6 kg/m2 vs. 23.8 6 1.6 kg/m2; meandifference, 5.2 kg/m2; 95% confidence interval, 3.07.3kg/m2) (Lammers et al., unpublished observation). Inter-estingly, patients were consuming less calories and inparticular less carbohydrates than control subjects in thatstudy (Lammers et al., 1996). These latter data suggestthat narcolepsy itself is associated with a higher BMI,perhaps as a result of a lower metabolic rate.

Hypothyroidism is often diagnosed in narcoleptics.Again, there are no systematic studies that show a higherprevalence in narcoleptics, nor have we found any re-ports that treatment of presumed hypothyroidism allevi-ates sleepiness. The diagnosis is probably made too oftenfrom erroneous interpretation of the complaints; for ex-ample, a selective attention to the coexistent complaint oftiredness (Yoss and Daly, 1957). Alternatively, if narco-

leptics indeed have a lower metabolic rate, thyroid func-tion may be (subclinically) involved.

DIAGNOSIS

The diagnosis is based primarily on the clinical symp-toms, obtained during a diagnostic interview. Unfortu-nately, taking a clear history may be hampered if patientshave read patient information forms before consultation.Therefore, the importance of specific questioning mustbe underlined. Ever since the first description, more thana century ago, there have been controversies about diag-nostic criteria. Currently, the criteria in the InternationalClassification of Sleep Disorders (ICSD), which includesclinical criteria and in case of doubt ancillary investiga-tions, are widely accepted (ICSD, 1997):A. The patient has a complaint of excessive sleepiness

or sudden muscle weakness.B. Recurrent daytime naps or lapses into sleep occur

almost daily for at least 3 months.C. Sudden bilateral loss of postural muscle tone occurs

in association with intense emotion (cataplexy).D. Associated features include

1. Sleep paralysis2. Hypnagogic hallucinations3. Automatic behaviors4. Disrupted major sleep episode

E. Polysomnography demonstrates one or more of thefollowing:1. Sleep latency less than 10 minutes2. REM sleep latency less than 20 minutes3. An MSLT that demonstrates a mean sleep latency

of less than 5 minutes4. Two or more sleep-onset REM periods

F. Human leukocyte antigen (HLA) typing demon-strates DQB1*0602 or DR2 positivity.

G. No medical or mental disorder accounts for thesymptoms.

H. Other sleep disorders (e.g., PLMD or central sleepapnea syndrome) may be present but are not theprimary cause of the symptoms.Minimal Criteria: B plus C, or A plus D plus E plusG.

There is mounting concern, however, that these crite-ria may be too broad (Aldrich, 1996; Guilleminault et al.,1994; Moscovitch et al., 1993; Rogers et al., 1997).

The diagnosis of narcolepsy is universally accepted tobe certain in patients with EDS and cataplexy. Contro-versies arise in patients without cataplexy. According tothe ICSD criteria, ancillary investigationsin particular,



the finding of sleep-onset REM periods during theMSLTare mandatory for the diagnosis of narcolepsyin these patients. But can these sleep-onset REM periodsbe used as a gold standard? There are two argumentsagainst this concept. The first is the relatively low sen-sitivity and specificity of sleep-onset REM periods inpatients who do have EDS in combination with cataplexy(Aldrich, 1996; Aldrich et al., 1997; Honda, 1988;Moscovitch et al., 1993). The second derives from asystematic study of EDS patients without cataplexy whohad more characteristics in common with other hyper-somnias (such as idiopathic hypersomnia, insufficientsleep syndrome, and mild sleep-disordered breathing)than with patients with cataplexy (Aldrich, 1996).

However, despite all doubts, the ICSD criteria are stillapplicable in todays practice and are a sufficient guidefor (pharmacotherapeutic) treatment.

ANCILLARY INVESTIGATIONS

Polysomnography

Nocturnal polysomnography is used mainly to elimi-nate other causes of EDS, such as sleep apnea andPLMD. In narcolepsy the most specific findings are ashort REM latency (,20 minutes) in 40% to 50% ofpatients, and a short sleep latency (,10 minutes) innearly all patients (Broughton et al., 1988b; Montplaisiret al., 1978). Furthermore, a low sleep efficiency withfrequent awakenings, sleep fragmentation (in particularduring REM sleep), and an increased percentage of stageI sleep are found (Broughton and Mamelak, 1980; Lam-mers et al., 1993; Montplaisir and Godbout, 1986a).

Twenty-four hour ambulatory polysomnography maybe performed to get an impression of the frequency andduration of sleep attacks. Typically, total sleep time isminimally increased (or not at all) over the 24-hourperiod. However, this technique cannot be used to diag-nose narcolepsy because of a lack of (validated) diag-nostic criteria.

The MSLT, introduced in 1977, is considered to be thestandard objective test for the assessment of sleepinessand the diagnosis of narcolepsy (Carskadon and Dement,1977; Mitler et al., 1979). The MSLT measures the sleeplatency in each of four or five 20-minute periods dividedover the day, during which subjects are asked to lie downon a bed in a dark and quiet room and try to fall asleep.Besides measuring sleep onset, special attention is paidto the occurrence of REM sleep. Definite narcolepsy isdefined by a mean sleep latency (stage I) of 5 minutes orless and at least two sleep-onset REM periods during fivenaps (see also the ICSD criteria). Some authors advocate

a mean sleep latency of less than 8 minutes (Moscovitchet al., 1993). Influences from sleep deprivation, drug useor its discontinuation, and others should be considered asdescribed in the guidelines (American Sleep DisordersAssociation, 1992; Carskadon et al., 1986).

Recent studies question both the sensitivity and spec-ificity of the MSLT in the diagnosis of narcolepsy(Aldrich, 1996; Bishop et al., 1996; Guilleminault et al.,1994; Moscovitch et al., 1993; Rogers et al., 1997). Forexample, patients without any sleep complaint may ful-fill the MSLT criteria, whereas only 70% of patients withclear cataplexy show these abnormalities (Aldrich, 1996;Bishop et al., 1996).

The role of the MSLT in the assessment of the severityof sleepiness in narcolepsy has also been questioned.After the introduction of the MSLT, it was generallyaccepted (without any study to validate this assumption)that the sleep latency in narcoleptics correlates with theseverity of sleepiness, as is the case in sleep-deprivedhealthy volunteers. However, this turned out to be amisconception. Clinical severity, the amount of sleepduring ambulatory recordings, as well as subjective feel-ings of sleepiness are in fact correlated poorly with sleeplatencies during the MSLT (Broughton et al., 1988a).Moreover, treatment effects are minimally or not at alldetected by the MSLT, even when the clinical effect isimpressive (Boivin and Montplaisir, 1991; Guilleminaultet al., 1986; Hartse et al., 1982; Lammers and van Dijk,1992; Lammers et al., 1991b; Mitler et al., 1993b). Forthis reason, other tests have been advocated. Polysom-nographic ambulatory 24-hour recordings (Genton et al.,1995), and in particular the Maintenance of WakefulnessTest (MWT), are the most widely accepted. The MWT isa variant of the MSLT, resembling the natural situationmore closely. During this test, sleep latency is recordedwhile patients are seated in a chair and are asked to stayawake rather than being instructed to attempt to fallasleep (Hartse et al., 1982; Mitler et al., 1982, 1998).

DIFFERENTIAL DIAGNOSIS

The combination of excessive daytime sleepiness andcataplexy is pathognomonic for narcolepsy. For the sep-arate symptoms of narcolepsy, several differential diag-noses are possible:

A. Excessive daytime sleepiness1. Intrinsic sleep disorders

a. Sleep apneab. PLMDc. Idiopathic hypersomniad. Recurrent hypersomnia



2. Extrinsic sleep disordersa. Drug intoxication/withdrawalb. Insufficient sleep syndrome

3. Circadian rhythm disorders4. Other

a. Organic neurologic diseases such as (thalamic)infarction, tumor, (metabolic) encephalopathy,encephalitis, myasthenia gravis

b. Depressionc. Fatigue (syndromes)d. Malingering

B. Cataplexy1. Isolated cataplexy

a. Niemann Pick disease type C (Challamel et al.,1994; Kandt et al., 1982).

b. PraderWilly syndrome (Helbing et al., 1993;Vgontzas et al., 1996).

c. Norrie disease (Koch et al., 1999; Vossler etal., 1996).

d. Secondary to diencephalic tumors (DCruz etal., 1994).

e. Familial cataplexy (Hartse et al., 1988).2. Noncataplectic attacks

a. Syncope (including orthostatic hypotension)b. Startle syndromesc. Drop attacksd. Atonic/gelastic seizurese. Psychogenicf. Malingering

C. Hypnagogic hallucinations1. Isolated, sporadic hypnagogic hallucinations with

or without sleep paralysis2. Hypnagogic hallucinations associated with other

sleep disorders (Aldrich, 1996; Ohayon et al.,1996).

3. Sleep terrors4. Nightmares5. Psychiatric diseases, especially schizophrenia

(Douglass et al., 1991; Jackson and Bachman,1996).

6. Peduncular hallucinosis (Manford and Ander-mann, 1998).

7. Metabolic encephalopathy (Manford and Ander-mann, 1998; Ohayon et al., 1996).

8. Seizures (Manford and Andermann, 1998).9. Psychoactive substance abuse (Khat, hashish)

(Granek et al., 1988; Ohayon et al., 1996).D. Sleep paralysis

1. Isolated, sporadic sleep paralysis with or withouthypnagogic hallucinations

2. Familial sleep paralysis (Dahlitz and Parkes,1993; Roth et al., 1968).

3. Sleep paralysis associated with other sleep disor-ders (Aldrich, 1996; Ohayon et al., 1996).

4. Sleep drunkenness

E. Automatic behavior1. Automatic behavior associated with other sleep

disorders (Aldrich, 1996).2. Psychiatric diseases3. Metabolic encephalopathy4. Partial complex seizures5. Extreme absent-mindedness

One has to bear in mind that the presence of anotherintrinsic sleep disorder or depression does not precludethe diagnosis of narcolepsy, especially if cataplexy ispresent. Important factors in distinguishing cataplexyfrom other, resembling attacks are the preserved con-sciousness during cataplexy and the emotional trigger. Itis important to note part of the normal population expe-riences some feelings of muscle weakness with strongemotions (AnicLabat et al., 1999).

TREATMENT

Nonpharmacologic

Patients should live a regular life, go to bed at thesame hour each night, and get up at the same time eachmorning. Scheduled naps or short naps just before activ-ities demanding a high degree of attention alleviatesleepiness in most patients. The optimal frequency, du-ration, and timing of these naps has to be established(Godbout and Montplaisir, 1986; Mullington andBroughton, 1993; Roehrs et al., 1986; Rogers and Al-drich, 1993).

Because narcoleptic patients are probably more sensi-tive to the sleep-inducing properties of carbohydrates,they should not eat large carbohydrate-rich meals (Brucket al., 1994). Alcohol consumption should preferably beavoided for similar reasons. There is no other lifestyleadvice or other nonpharmacologic therapies with provenefficacy (Garma and Marchand, 1994).

Pharmacologic

Traditionally pharmacologic treatment is divided intotreatment of EDS and treatment of REM dissociationphenomena. This strict distinction is artificial, but mostdrugs alleviate predominantly one of the two complaints.Treatment should be directed at the most disabling symp-tom first, starting with low doses. Treatment schedules



and doses should be tailored for the individual patientand should be adjusted to the times of the day when thecomplaints are the most serious. Combination therapy isoften necessary.

Treatment of EDS

Stimulants remain the mainstay of the treatment ofEDS (American Sleep Disorders Association, 1994).They include dextroamphetamine (560 mg), metham-phetamine (1050 mg), pemoline (20115 mg), methyl-phenidate (1060 mg), mazindol (16 mg), and, inearlier days, ephedrine. They enhance the release andinhibit the reuptake of catecholamines and, to a lesserextent, serotonin in the central nervous system (CNS)and the periphery (Mitler et al., 1994). They are alsoweak inhibitors of monoamine oxidase (MAO).

On choosing a stimulant, several considerationsshould be kept in mind:

1. Ideally, drug efficacy should be assessed with agenerally accepted, objective test to quantify sleep-iness, but there is no such test. Comparison ofeffectiveness is therefore difficult and inherentlyinvalid, although some attempts have been made(Mitler and Hajdukovic, 1991).

2. Individual differences in effectiveness, side effects,and tolerability appear to be large. Therefore, ef-fectiveness of a certain stimulant as assessed ingroups is of only relative importance when makinga choice for an individual.

3. The onset of action of a drug and its duration mayin certain cases be of more importance than theexpected effectiveness.

4. The treatment goal is of major importance in thejudgment of the effectiveness of pharmacologictreatment. Some aim for normal vigilance duringthe entire day by using very high doses to reachnormal results on objective tests, and accept anumber of side effects (Mitler et al., 1993a). Otherssearch for the optimum between resolution of com-plaints and side effects (Parkes and Dahlitz, 1993).

5. Mood-raising effects may occur (Zwicker et al.,1995).

With these considerations in mind, substantial improve-ments have been reported in 65% to 85% of patients (Mitleret al., 1994). Although stimulants may reduce daytimesleepiness considerably, they cannot abolish sleepiness al-together. Side effects and tolerance are major drawbacks tothe use of stimulants. The most important dose-related sideeffects are irritability, agitation, headache, and peripheralsympathetic stimulation. Tolerance occurs in approximately

one-third of patients and leads to high doses (Mitler et al.,1994; Parkes and Dahlitz, 1993). Some patients tend toincrease their dose because they prefer high alertness at theexpense of an overactive mind and body. However, addic-tion does not seem to be a problem in narcoleptics (Parkesand Dahlitz, 1993). Induction or aggravation of hyperten-sion seems not to be a notable problem in clinical practicewhen using doses in the normal therapeutic range (Wallinand Mahowald, 1998). Induction of psychosis and halluci-nations is rare (Mitler et al., 1994). Nocturnal sleep may bedisturbed in patients who use high doses or who takestimulants in the evening.

Long-acting agents seem to be tolerated better than theshort-acting ones. These can be used to good effect whentargeted at social events or difficult periods during theday. For this reason, combinations of stimulants may bethe most effective.

Modafinil is usually grouped with the stimulants, but ithas a different mode of action. An a1 agonistic actionwas presumed initially, but questioned later. Its exactmechanism has not yet been elucidated, but a role fordopaminergic uptake inhibition was suggested (Mignotet al., 1994c). The efficacy has been studied in large,randomized, placebo-controlled studies and is probablycomparable with that of the stimulants, although directcomparisons are lacking (Broughton et al., 1997; USModafinil Multicenter Study Group, 1998, 2000). Theadvantage of modafinil over the stimulants lies in thelower frequency and severity of side effects (Billiard etal., 1994a; Broughton et al., 1997; Fry, 1998; Laffont etal., 1994; US Modafinil Multicenter Study Group, 1998).In particular patients feel less irritable and agitated (Bas-tuji and Jouvet, 1988). A growing number of investiga-tors are suggesting that modafinil should become thedrug of choice (Broughton et al., 1997; Fry, 1998;Nishino and Mignot, 1997; US Modafinil MulticenterStudy Group, 2000).

Several other drugs also improve daytime sleepiness:selegiline (MAO B inhibitor; dose, 1040 mg; combinedwith a low-tyramine diet) (Hublin et al., 1994b), brofaro-mine (MAO A inhibitor) (Hohagen et al., 1993) and,probably less potent, L-dopa (Boivin and Montplaisir,1991). Long-term experience is limited, with the last twodrugs in particular. Codeine (30120 mg) was reportedto reduce sleepiness in an open study, but is less potentthan the previously mentioned substances (Fry et al.,1986).

Gamma-hydroxybutyrate (GHB), a putative neuro-transmitter that acts as a hypnotic, is given two to threetimes at night (as discussed later) and has a slight effecton daytime sleepiness together with a dose-sparing effecton stimulants (Broughton and Mamelak, 1980; Lammers



et al., 1993; Scrima et al., 1989). Ritanserin, (a selectiveserotonin type 2 inhibitor) had an effect on sleepiness,but is no longer being marketed (Lammers et al., 1991a).Prolintane, used mainly in Finland, has not been studiedin drug trials (Hublin, 1996). Data on the effectiveness ofL-tyrosine (Elwes et al., 1989; Mouret et al., 1988) andprotriptyline (Henry et al., 1988; Mitler et al., 1986) areconflicting, but not very convincing.

Ephedrine has been used before the introduction of theother stimulants, but was not tested in drug trials(Daniels, 1934; Mitler et al., 1994). The newer stimu-lants displaced ephedrine because of their milder sideeffects and lesser tendency to induce tolerance. Caffeinemay alleviate sleepiness, but only weakly: The alertingeffect of six cups of strong coffee is comparable with thatof 5 mg of dexamphetamine (Mitler et al., 1994; Parkesand Dahlitz, 1993). Mazindol has been withdrawn fromthe market in most countries because of reports of severeside effects (pulmonary hypertension and valvular insuf-ficiency) when used with related appetite suppressantsubstances, in particular fenfluramines (Rich et al., 2000;Ryan et al., 1999). However, there are patients whoexperience a better therapeutic response on mazindolcompared with any other drug. In these patients, closelymonitored treatment seems warranted.

Treatment of REM Dissociation Phenomena

Most studies concerning the treatment of the REMdissociation phenomena have focused on cataplexy.Generally, amelioration of cataplexy is associatedwith a reduction of hypnagogic hallucinations andsleep paralysis. Tricyclic antidepressants are the mosteffective treatment. The different substances all inhibitthe reuptake of norepinephrine and serotonin, and arepotent REM sleep inhibitors. The most commonlyused ones are imipramine (10 100 mg), protriptyline(2.5 40 mg), and clomipramine (10 150 mg) (Guil-leminault et al., 1976; Hishikawa et al., 1966; Mour-tazaev, 1982). Most authors consider clomipramine tobe the treatment of choice (Parkes, 1994). Some pa-tients experience improvement of EDS when treatedwith protriptyline or clomipramine. Tolerance mayoccur. As is the case with the stimulants, side effectsand, to a lesser extent, tolerance are major drawbacks.Side effects are largely the result of anticholinergicproperties. The most frequently reported ones are a drymouth, increased sweating, sexual dysfunction (impo-tence; delayed orgasm, erection; and ejaculation dys-function), weight gain, tachycardia, constipation,blurred vision, and urinary retention. Consequently,many patients refrain from these medications or re-

duce the dose. Tricyclics should never be stoppedabruptly because of the risk of severe aggravation ofcataplexy, which may even lead to status cataplecticus(Hishikawa and Shimizu, 1995; Parkes, 1985; Passou-ant et al., 1970).

Many alternative treatments to the tricyclics have beenstudied, especially selective serotonin reuptake inhibitorssuch as fluoxetine, zimelidine, femoxitine, fluvoxamine,and paroxetine in a relatively higher dose than the tricy-clics (Langdon et al., 1986; Nishino and Mignot, 1997;Schachter and Parkes, 1980; Schrader et al., 1986). Allthese substances appear to have anticataplectic propertiesand less (disabling) side effects compared with the tri-cyclics. These substances seem to act mainly via lessselective desmethyl metabolites, which are potent adren-ergic uptake inhibitors (Nishino et al., 1993). However,with the possible exception of zimelidine, clomipramineis more potent (Langdon et al., 1986; Montplaisir andGodbout, 1986b). Therefore, we still consider (low-dose)tricyclic antidepressants to be the first-choice treatmentfor cataplexy.

Other alternatives are viloxazine (a selective norad-renergic reuptake inhibitor) (Guilleminault et al., 1986),mazindol (Iijima et al., 1986; Parkes and Schachter,1979), and GHB, given orally at night divided into twodoses of 30 mg/kg each (Lammers et al., 1993; Mamelaket al., 1986; Scrima et al., 1989). Although the effective-ness of GHB is proved, the short duration of action(making a second dose at night necessary) and theundetermined long-term safety currently limits its use.However, compared with the tricyclics, reported sideeffects are less frequent and less severe (Mamelak et al.,1986). GHB has been submitted for approval by the Foodand Drug Administration as a first-line drug for thetreatment of cataplexy (personal communication, OrphanMedical, Minnesota, USA, November, 2000).

Selegiline and brofaromine are reported to be effective(Hohagen et al., 1993; Hublin et al., 1994a). Experiencewith these substances in the treatment of REM sleepdissociation phenomena is limited.

Several drugs may be expected, theoretically, to ag-gravate cataplexy. However, in the literature there areonly convincing reports of prazosin, an a1 antagonistused to treat hypertension (Aldrich and Naylor, 1989;Guilleminault et al., 1988).

As mentioned earlier, mazindol, selegiline, and prob-ably brofaromine may have a combined impact on sleep-iness as well as on the REM dissociation phenomena.Therefore, they can be used as monotherapy in patientswith EDS and REM dissociation phenomena, particu-larly in milder forms.



Treatment of Disturbed Nocturnal Sleep

Disturbed nocturnal sleep can be a major complaint ofpatients. Unfortunately, treatment options and their as-sociated efficacy are limited. Improvement has beendescribed during GHB treatment (Lammers et al., 1993;Mamelak et al., 1986; Scharf et al., 1985; Scrima et al.,1989, 1990). Short-term beneficial effects of benzodiaz-epines have been described as well (Thorpy et al., 1992).Although nocturnal sleep may be improved with thesedrugs, improvement of EDS is not the rule (Broughton etal., 1994; Lammers et al., 1991a).

THEORY OF STATE BOUNDARY CONTROL

All symptoms of narcolepsy can be explained basedon two principles, which have been described together asloss of state boundary control (Broughton et al., 1986).The first is the impossibility to sustain any given sleep/wake state for a length of time: When awake, patients fallasleep quickly, and when asleep, they awaken quickly.The second is that various phenomena that normallyoccur together in a sleep stage can occur dissociated (i.e.,on their own and out of their context). Several narcolep-tic symptoms are now discussed according to thisconcept.

Cataplexy, sleep paralysis, and hypnagogic hallucina-tions are clinical manifestations of dissociated REMsleep. Normal REM sleep is characterized by REMs,muscle atonia, dream experiences, diminished memoryfunction, and a specific EEG pattern. The classical non-EDS manifestations of narcolepsy can be recognizedeasily as dissociation of the normally joined REM fea-tures. Cataplexy and sleep paralysis represent intrusionsof REM sleep atonia in the waking state and, similarly,hypnagogic hallucinations represent the intrusion ofdream imagery into the waking state and/or drowsiness.Concomitant REM sleep behavior disorder and evenmemory dysfunction can be explained by this dissocia-tion theory as well. Some clinical observations remainunexplained: Why do some patients have only cataplexyand others only sleep paralysis, and why does sleepparalysis occur frequently without any other signs ofREM sleep disturbance?

Non-REM (NREM) sleep dissociation phenomena areless easy detected or delineated clinically. Examples maybe night terrors and sleepwalking, which have a pre-sumed higher prevalence in narcolepsy (Mahowald andSchenck, 1991, 1992). The existence of NREM dissoci-ation phenomena is supported by electrophysiologic ob-servations (Vogel, 1976).

Ambiguous sleep, a unique sleep stage in narcolep-

tics, combines features of REM sleep and NREM sleep,and completes the spectrum of dissociation of sleepstages (de BarrosFerreira and Lairy, 1976; Montplaisir,1976). For wake states, comparable disturbances havebeen suggested as well (Volk et al., 1984).

The absence of a clear correlation between the dis-turbed nocturnal sleep (and the improvement of noctur-nal sleep after treatment) on the one hand, and EDS onthe other is remarkable and unexplained (Broughton etal., 1994; Lammers et al., 1991a). The only correlationappears to be between REM period disruption and phaseshifts at night, and the frequency of daytime cataplexy(Lammers et al., 1993; Montplaisir and Godbout, 1986a).

PHYSIOLOGY OF REM SLEEPDISSOCIATION PHENOMENA

Cataplexy and sleep paralysis are presumed to repre-sent the atonia of REM sleep. REM sleep atonia isinduced by activity of the cholinoceptive neurons in thepontine reticular formation, the nucleus reticularis pontis(Hishikawa and Shimizu, 1995). These neurons descendthrough the medulla and inhibit motoneurons throughactivation of inhibitory glycinergic interneurons (Hish-ikawa and Shimizu, 1995). Connections between thelimbic system and pontine nuclei presumably explainhow emotional events can trigger cataplexy, althoughthese tracts have not yet been identified. The mecha-nisms or tracts by which sleep paralysis is elicited are notelucidated.

Polysomnographic studies revealed a link betweenhypnagogic hallucinations and sleep-onset REM periods,but not exclusively (Benbadis et al., 1995; Chetrit et al.,1994; Hishikawa et al., 1978). This may be explained bypolysomnographic dissociation phenomena or ambigu-ous sleep. Although hypnagogic hallucinations are gen-erally considered to be related to REM sleep, they maybe dissociation phenomena of NREM sleep as well(Benbadis et al., 1995; Cavallero et al., 1992). Theintensified experience of the dream content of hypna-gogic hallucinations may be explained by the experienceof dreams in a more vigilant state than normal physio-logic REM sleep.

NEUROCHEMICAL AND NEUROANATOMICASPECTS

Knowledge about human narcolepsy on a neurochem-ical and neuroanatomic level is sparse and scattered. Theavailable information is derived from patients with along history of drug treatment, and may have beeninfluenced by this (Aldrich et al., 1994; Khan et al.,



1994; Kish et al., 1992; Rinne et al., 1996; Staedt et al.,1996; Strittmatter et al., 1996). Reports of symptomaticnarcolepsy point to a important role of the diencephalonand upper brainstem (Aldrich and Naylor, 1989; Autretet al., 1994; Bonduelle and Degos, 1976; Erlich andItabashi, 1986; Stahl et al., 1980) A huge amount ofknowledge has been gathered from studies of narcolepticdogs at Stanford University. Resemblance with humannarcolepsy is remarkable, including juvenile onset, ex-cessive sleepiness, fragmented sleep, sleep-onset REMperiods, and cataplectic attacks triggered by emotion(Nishino and Mignot, 1997; Siegel et al., 1999). A recentextensive review summarizes the knowledge obtainedfrom canine narcolepsy (Nishino and Mignot, 1997).Here, the neurochemical and neuroanatomic model basedon these findings is described briefly.

EDS is considered to be a consequence of hypoactivityof dopaminergic transmission. Alleviation is achievedmainly by presynaptic modulation of dopamine.

Cataplexy is considered to be a consequence of re-duced monoaminergic (especially dopaminergic and/oradrenergic) tone in conjunction with cholinergic (M2muscarine receptor) hypersensitivity. Involved receptorsare the presynaptic a2/D2 (and/or D3) autoreceptor andpostsynaptic a1 adrenergic receptor. Alleviation isreached by blockade of a2/D2 or D3 and stimulation ofa1(b) receptors. Local injection studies suggest thatinhibitory dopaminergic autoreceptors are of primaryimportance in causing cataplexy (Reid et al., 1996).

It remains an enigma why anticholinergic drugs thatare effective in dogs are not effective in human narco-lepsy, considering the neurochemical hypotheses. Pro-posed explanations are the occurrence of dose-limitingside effects and rapid development of tolerance. Future,more selective M2 antagonists may resolve this issue.

On an anatomic level, cholinergic neurons in the basalforebrain and pontine reticular formation, and dopami-nergic neurons in the ventral tegmental area seem to beinvolved (Tafti et al., 1997). The finding that dopami-nergic neurons in the ventral tegmentum mediate theexperience of pleasure is of particular interest in thiscontext (Mirenowicz and Schultz, 1996). The site of thepotentially involved adrenergic neurons is still unknown.

GENETIC ASPECTS

In approximately 1% of cases, narcolepsy occurs infamilies. Although some early reports may have mis-taken other sleep disorders for narcolepsy, numerousfamilial cases have been reported. Genuine multiplex(i.e., more affected generations) families are very rare. Ifnarcolepsy runs in families, it typically shows an auto-

somal dominant mode of inheritance (Billiard et al.,1994b; Guilleminault et al., 1989; Nevsimalova et al.,1997).

The majority of patients (99%) experience nonfamilial(sporadic) narcolepsy, but genetic factors are still impor-tant in those cases. Prevalence studies have shown thatthe risk for a first-degree relative of a patient withnarcolepsy is 1% to 2% (for a review see Mignot [1998]).This risk, although small, is still 30 to 40 times higherthan the estimated prevalence in the general population.These data indicate that there are genetic factors thatpredispose to the development of narcolepsy.

Although genetic factors play a role in sporadic nar-colepsy, they are neither necessary nor sufficient to causenarcolepsy. Twin studies showed that only 25% to 31%of monozygotic twins are concordant for the disease (forreferences, see Mignot [1998]). Narcolepsy, therefore,has a complex etiology in the majority of cases, withgenetic factors forming a susceptibility background onwhich other (environmental) factors act.

HLA ASSOCIATION

One of the most important predisposing genetic fac-tors is a specific HLA subtype. The HLA complex is acollection of genes on chromosome 6. The HLA DR andDQ genes are located very close to each other, andencode the a and b chains of the corresponding HLAprotein (which are imported in the presentation of anti-gens to the rest of the immune system). These genes arehighly polymorphic (i.e., have subtle differences be-tween people), and these variants can be determined byeither serologic techniques, or at the deoxyribonucleicacid (DNA) level (Fig. 2).

In the early 1980s, Honda et al. (1983) reported that inJapan, all patients carried a specific HLA class II allelenamely, HLA DR2. In whites, this association exists aswell, with more than 85% of patients being positive forDR2 (Billiard and Cadilhac, 1985; Langdon et al., 1984;Rogers et al., 1997). For diagnostic purposes, it is im-portant to know that a DNA-defined subtype of HLADQnamely, HLA DQB1*0602is an even more spe-cific marker. This is especially important in AfricanAmericans, because DR2 is a poor marker for narcolepsyin that population (Mignot et al., 1994b; Rogers et al.,1997) (see Fig. 2).

There has been discussion whether HLA DQB1*0602is a narcolepsy susceptibility gene itself. It was conceiv-able that the actual gene involved was only located veryclose to the HLA DQ region, and therefore transmittedtogether. However, the exact genetic region responsiblefor the susceptibility to develop narcolepsy has been



pinpointed and completely sequenced. No additionalgenes have been found in the region, making HLADQB1*0602 itself the gene involved (Ellis et al., 1998;Mignot et al., 1997). Additional evidence is given by thefact that people homozygous for HLA DQB1*0602 havean even higher relative risk to acquire narcolepsy thanpeople heterozygous for the locus (Pelin et al., 1998).

One has to bear in mind that 12% to 35% of thegeneral population carry the same HLA subtype men-tioned earlier. In addition, there have been a few reportsof patients with unquestionable narcolepsy being nega-tive for HLA DQB1*0602. Interestingly, more than one-third of patients in multiplex families are negative forthis subtype (Mignot, 1996; Singh et al., 1990). There-fore, other genes must be involved in these patients. Thepossible contribution of hypocretin genes in the humandisorder are discussed later in this review.

IMMUNOLOGIC ASPECTS

Almost all diseases associated with a specific HLAallele are autoimmune in nature. In narcolepsy, extensivestudies have been performed to look for general markersof (auto)immune activation (e.g., oligoclonal bands inthe cerebrospinal fluid [CSF], serum immunoglobulinlevels, lymphocyte subset ratios) (Fredrikson et al., 1990;HinzeSelch et al., 1998; Matsuki et al., 1985). To date,no evidence for an autoimmune process has been estab-lished. There are several possible explanations for this. Itis conceivable that an unknown, fundamental interactionbetween the immune system and sleep regulation exists.There is increasing evidence for immune-related mech-

anisms specific for the CNS (Perry et al., 1993). Such anovel immuneCNS interaction is likely to remain un-detected based on our current, limited knowledge. It isalso possible that a classic immune destruction of a partof the brain is involved. This would most likely be atransient process, and if the area involved is small, itcould easily be missed (see Mignot et al. [1995]). Thepossibility of autoimmune cell destruction in a small partof the brain has become the most likely, now that weknow that the hypocretin system plays a central role inthe development of narcolepsy.

THE DISCOVERY OF THE HYPOCRETINS

The hypocretins were isolated for the first time in1996. Gautvik et al. (1996) isolated 38 messenger ribo-nucleic acids (mRNAs) that were expressed selectivelyin the hypothalamus, using a rat complementary DNAlibrary enhanced for hypothalamus-specific clones.Among a number of earlier characterized hypothalamicpeptides (e.g., vasopressin and oxytocin), one novelmolecule restricted to the lateral hypothalamus wasfound. This molecule, at that time designated as cloneH35, turned out to be a prohormone, predicted to pro-duce two peptides (de Lecea et al., 1998). Because oftheir hypothalamic origin and a weak sequence homol-ogy to the incretin hormone family, these peptides werenamed hypocretin-1 and -2 (hcrt-1 and -2) (see Sutcliffeand de Lecea [2000]). The N-terminal of hcrt-1 remainedunknown at that time, so it could only be guessed to be38 amino acids long (de Lecea et al., 1998). Hcrt-2 wasdetermined to be a 28-amino acid peptide, based on

FIG. 2. Schematic overview of the location of the human leukocyte antigen (HLA) complex on chromosome 6, including the DQ/DR region withthe haplotypes typically associated with narcolepsy. The HLA gene complex spans more than 4,000 kilobases, but the HLA DR and DQ genes arelocated very close to each other. Three polymorphic genes in this regionnamely, DQB1, DQA1, and DRB1are mainly responsible for the allelicvariation in this region. DQB1*0602 (boxed), a subtype on the deoxyribonucleic acid level, is the most specific marker for narcolepsy across all ethnicgroups. It is almost always associated with DQA1*0102. The DRB1 subtype that is typically associated with DQB1*0602 differs between ethnicgroups. In whites and Asians it is mainly DRB1*1501; in AfricanAmericans it is typically DRB1*1503 (both DRB1*1501 and DRB1*1503 aresubtypes of DR2). p; short arm; q, long arm; kb, kilobases.



flanking dipeptide cleavage sites and the predicted likelyconversion of the C-terminal glycine to an amide (Fig.3).

Only 6 weeks after the description of the hypocretins,Sakurai et al. (1998a) independently described the samepeptides. Orphan receptors are receptors, cloned on thebasis of conserved structure with known receptors, butwithout a known biologic ligand. Sakurai et al. (1998a)used a technique in which cell lines, stably transfectedwith orphan G-protein-coupled receptors, were chal-lenged with purified hypothalamic extracts. Two hypo-thalamic peptides were isolated that activated one of theorphan receptors named. One of these peptides was 33amino acids in length, with an N-terminal pyroglutamylresidue and an amidated C-terminus. Four cysteine res-idues in the peptide formed two sets of intrachain disul-fide bonds. The other was a 28-amino acid peptide, withan amidated C-terminal (see Fig. 3).

Historically the hypothalamus is implicated in thecontrol of food intake and energy homeostasis. This ledSakurai et al. (1998a) to test the effect on food intake ofrats when the two peptides were injected intracerebrov-entricularly (ICV). It turned out that both peptides stim-ulated food intake in rats. These molecules were there-fore designated orexin-A and -B (orejh 5 appetite). Inthe same paper the cloning of two receptors for thepeptides was reported, with orexin receptor 1 displayinga preferential affinity for orexin-A, and orexin receptor 2binding both orexins with equal affinity.

The solution structure of hcrt-2 has been determinedby two-dimensional nuclear magnetic resonance imaging(Lee et al., 1999). Based on their amino acid structure,and their location in the CNS, the hypocretins werethought to act as neurotransmitters. In their initial paper,de Lecea et al.(1998) already showed that hcrt-2 inducedan increase in the firing rate of hypothalamic neurons in

vitro. Patch-clamp experiments revealed that hypocretinsincreased the axonal release of both g-aminobutyric acidand glutamate (van den Pol et al., 1998). Excitatorycellular actions of the hypocretins were mediated by acalcium-dependent pathway (Lund et al., 2000; van denPol et al., 1998).

There has been some confusion about the relationbetween the hypocretins and the orexins. In a comment,Sakurai et al. (1998b) acknowledged that the hypocretinsand orexins were identical. Unfortunately, a minireviewaccompanying the original paper described them to bedifferent (Flier and MaratosFlier, 1998). To make mat-ters even worse, one study compared nonamidated hypo-cretins (biologically the nonactive form) with orexins, onthe basis of which a difference in agonistic activity waspostulated (Smart et al., 2000). In recent reviews, thehypocretins and orexins are still described as distinctentities (Williams et al., 2000). Part of this confusion liesin the fact that in the original description, hcrt-1 was notcompletely determined. However, it is now clear that thehypocretin gene is identical to the orexin gene, and thehypocretins and orexins are the same peptides (Kilduffand Peyron, 2000).

The existence of two different names for the samemolecules is troublesome. We use the name hypocretinin this review for a number of reasons. First, hypocretinwas the first name given to the molecules. Second, theorexigenic effect of the peptides is now thought to be notas strong as originally believed (see Hypocretins andFood Intake). In addition, several other functions of thehypocretin/orexin system have been established by now.The name hypocretin does not imply a specific function.Lastly, in the major genetic databases, the term hypocre-tin is used.

LOCALIZATION AND PROJECTIONS OFHYPOCRETIN NEURONS

Several groups have examined the localization ofhypocretin neurons, using various techniques such asin situ hybridization (de Lecea et al., 1998; Sakurai etal., 1998a) and immunohistochemistry (Chen et al.,1999; Date et al., 1999; Nambu et al., 1999; Peyron etal., 1998). Hypocretin-containing neurons are local-ized in a subregion of the dorsolateral hypothalamus,centered around the perifornical nucleus. In the rat,estimates of the number of hypocretin-containing neu-rons range from 1,128 to 4,092, depending on theantiserum and/or calculation method used (Harrison etal., 1999; Kilduff and Peyron, 2000; Peyron et al.,1998). In the human brain, this number was estimatedto be 15,000 to 20,000 (Peyron et al., 2000), but other

FIG. 3. Diagram of preprohypocretin. GKR and GRR depict dibasicresidues that are potential cleavage sites for prohormone convertases.The derived amino acid sequences for hypocretin-1 (hcrt-1) and hcrt-2are shown as well. The C-terminal ends of both peptides are amidated.Note the two intrachain disulfide bridges in hcrt-1.



investigators, using different techniques, counted50,000 to 80,000 hypocretin-producing cells(Thannickal et al., 2000).

In contrast to the small area in which the hypocretin-containing neurons lie, these cells project widelythroughout the brain (Chen et al., 1999; Date et al., 1999;Nambu et al., 1999; Peyron et al., 1998) (Fig. 4). Pro-jection sites include the cerebral cortex, basal forebrainstructures such as the diagonal band of Brocca, theamygdala, and brainstem areas including the reticularformation, raphe nuclei, and the locus ceruleus (LC). Thespinal cord is innervated by hypocretin fibers as well(van den Pol, 1999).

In the hypothalamus, hypocretin neurons lie in thesame area as cells producing melanin-concentrating hor-mone (MCH; another orexigenic molecule), but in con-trast with initial ideas, these peptides are not produced bythe same cells (Broberger et al., 1998; Elias et al., 1998).Prolactin does seem to colocalize with hypocretin in therat hypothalamus (Risold et al., 1999). Because of theestablished coexpression of prolactin with dynorphin Band bradykinin (Griffond et al., 1993, 1994), these lattermolecules may be used as a marker for hypocretinneurons (Kilduff and Peyron, 2000).

In accordance with the widespread innervation, hypo-cretin receptors are found throughout the brain. Interest-ingly, the distribution of hypocretin receptor 1 (hcrtr-1)in the brain differs greatly from that of hcrtr-2 (Trivedi etal., 1998). hcrtr-1 is localized mainly in the ventromedialhypothalamic nucleus, dorsal raphe, and LC. In contrast,hcrtr-2 is expressed more abundantly in the paraventricu-lar nucleus and the nucleus accumbens.

HYPOCRETINS AND FOOD INTAKE

In their original paper, Sakurai et al. (1998a) hadimplicated hypocretins in the regulation of food intake.Centrally administered hcrt-1 and -2 stimulated appetitein rats, and preprohypocretin mRNA was upregulated onfasting (Sakurai et al., 1998a). Local injection of hcrt-1in several hypothalamic areas, such as the dorsomedialnucleus, induced feeding behavior (Dube et al., 1999).Administration of antibodies against hcrt-1 suppressedfeeding in rats (Yamada et al., 2000). Food consumptionin rats was also reduced by a selective hcrtr-1 antagonist(Haynes et al., 2000). A growing body of evidence placesthe hypocretin system in the complex network of feedingregulating systems that we know to date. For example,neurons that produce the appetite-inducing peptides neu-ropeptide Y (NPY) and agouti-related protein, project tohypocretin neurons (Yamanaka et al., 2000). In addition,hypocretin-induced feeding can be blocked with NPYreceptor antagonists (Dube et al., 2000; Jain et al., 2000).Leptin is another molecule essential for the regulation offood intake and body weight. Leptin-dependent systemsinteract closely with hypocretin neurons: There are datasuggesting that leptin receptor-bearing neurons are in-nervated by hypocretin neurons. Hypocretin neuronsshow leptin receptor immunoreactivity and expressSTAT3, a transcription factor that is activated by leptin(Funahashi et al., 2000; Hakansson et al., 1999; Horvathet al., 1999).

The effects of hypocretins on food consumption havealso been disputed (Edwards et al., 1999; Ida et al., 1999;Lubkin and StrickerKrongrad, 1998; Moriguchi et al.,1999). Hypocretin administration does not alter 24-hour

FIG. 4. Projections of hypocretin neu-rons in the rat brain. Hypocretin neuronsproject widely throughout the brain. Pro-jection fields can be divided in four maingroups: an intrahypothalamic part, an as-cending pathway, a medial pathway to anumber of thalamic nuclei, and a de-scending pathway. Projections are re-traced from the original figure, to facili-tate black-and-white printing. Reprintedwith permission from the Society forNeuroscience. Source: Peyron C, TigheDK, Den Pol AN, et al. Neurons contain-ing hypocretin (orexin) project to multi-ple neuronal systems. J Neurosci1998;18:10013.



total food consumption (Ida et al., 1999). Moreover, theappetite-inducing activity of the peptides is much lesscompared with, for example, NPY (Edwards et al.,1999), and is sometimes even absent (Ida et al., 2000).Some investigators were unable to detect upregulation ofhcrt-1 in the rat hypothalamus after fasting (Taheri et al.,1999). Prolonged administration of hcrt-1 did not haveany effect on body weight in rats (Yamanaka et al.,1999). Recently, it was suggested that hcrt-1 modulatesfood intake not by a postingestive mechanism of satiety,but as a rebound to initial behavioral suppression (Rodg-ers et al., 2000).

Most likely, the hypocretin system is involved in theoverall control of energy metabolism rather than in theregulation of food intake per se. A number of otherstudies support this view. First, hypocretin neurons areglucose sensitive: Preprohypocretin levels and Fos-likeimmunoreactivity in hypocretin neurons increase in re-sponse to hypoglycemia (Griffond et al., 1999; Morigu-chi et al., 1999). Furthermore, there is direct evidencethat ICV administration of hcrt-1 increases the metabolicrate in mice (Lubkin and StrickerKrongrad, 1998).

OTHER FUNCTIONS OF THE HYPOCRETINSYSTEM

In a number of animal studies, centrally administeredhypocretins evoked a general arousal, together with ste-reotypic motor activities (Hagan et al., 1999; Piper et al.,2000). hcrt-1 increased the neuronal firing rate in the LC,and when administered in rats, induced arousal (Hagan etal., 1999). Stereotypic behaviors such as burrowing andgrooming in response to hypocretins have been described(Hagan et al., 1999; Ida et al., 1999). There is evidencethat hcrt-1-induced hyperlocomotion, and stereotypy wasmediated by the dopaminergic system, based on theblocking of these hcrt-1-induced behaviors by dopamineD1 or D2 antagonists (Nakamura et al., 2000).

Both hcrt-1 and hcrt-2 increased blood pressure andheart rate in rats when injected ICV (Chen et al., 2000;Samson et al., 1999; Shirasaka et al., 1999). Theseeffects suggest a role for the hypocretins in autonomicfunction. Hypocretin-containing neurons seem to be in-volved in the activation of medullary sympathetic out-flow (Dun et al., 2000).

Hypocretin neurotransmission seems to play a role inneuroendocrine regulation as well. Prolactin and soma-totropin levels were decreased after hcrt-1 treatment inrats (Hagan et al., 1999; Russell et al., 2000). Thehypothalamopituitaryadrenal axis seems to be activatedby hcrt-1 (Kuru et al., 2000; Malendowicz et al., 1999),most likely at a central level (Jaszberenyi et al., 2000).

There is some evidence of effects of hypocretins on thereproductive system; for example, through modulation ofpituitary luteinizing hormone secretion (Pu et al., 1998;Tamura et al., 1999). Hypocretins stimulate gastric acidsecretion and were found in the enteric nervous system,so there appear to be some peripheral gastrointestinaleffects of the hypocretin system (Kirchgessner and Liu,1999; Takahashi et al., 1999). Most of these functionsneed to be established further, however.

HYPOCRETIN NEUROTRANSMISSION ANDNARCOLEPSY

Canine Narcolepsy

The most intensely studied animal model for narco-lepsy is the canine model. A large body of physiologic,pharmacologic, and clinical evidence shows striking sim-ilarities between human and canine narcolepsy, includingsleepiness, cataplexy, and sleep-onset REM periods(Baker, 1985; Foutz et al., 1979; Nishino and Mignot,1997). One important difference is that in humans, nar-colepsy rarely runs in families (Mignot, 1998) whereas inselected dogs, breeding made it possible to establishautosomal recessive transmission with full penetrance inLabrador retrievers and Doberman pinschers (Baker,1985; Foutz et al., 1979).

Using the dog model, a cloning effort was initiated inthe early 1990s to identify the canine narcolepsy gene,designated canarc-1. Initially, linkage analysis in a num-ber of backcrosses generated a m switchlike marker(Cederberg et al., 1998; Mignot et al., 1991). Additionalcloning and sequencing studies did not show an adjacentimmunoglobulin constant region, suggesting that themarker was not a genuine immunoglobulin switch seg-ment, but only a cross-reacting sequence of unknownsignificance (Mignot et al., 1994a).

The project was hampered by several restrictionsposed by the available canine genomic libraries. Toovercome these problems, it was next decided to build alarge insert canine genomic library, to facilitate chromo-some walking from the m switchlike marker (Lin et al.,1999). The project accelerated after the identification ofan insert containing a sequence with a high homology tothe human gene Myo6 (located on chromosome 6). Amethod based on conserved synteny between the canineand human genome was then used to identify the sus-ceptibility region (Lin et al., 1999). In short, humanexpressed sequence tags, known to be located closely toMyo6, were used to select clones from the dog library.Polymorphic markers in these clones were used to typethe canine crosses, in addition to unrelated narcoleptic



dogs. Finally a critical region was established, containingonly one previously described gene, namely hcrtr-2.

In Dobermans, hcrtr-2 complementary DNAs weremarkedly shorter in narcoleptic dogs than in controls,suggesting a deletion in the transcripts. A so-calledshort, interspersed nucleotide element was found tocause aberrant splicing of the receptor mRNA, result-ing in a truncated receptor (Lin et al., 1999) (fordetails, see Fig. 5). The short, interspersed nucleotideelement insertion was not observed in the Labradorretrievers. In narcoleptic Labrador retrievers, a singlenucleotide change results in a splicing defect as well(for details, see Fig. 5). Both the Doberman andLabrador mutations result in a receptor lacking theintracellular end, and therefore an absence of function.A milder, nontruncating mutation in the hcrtr-2 genehas been identified in familial narcolepsy in Dachs-hunds (Hungs et al., 2000). Functional studies areunderway to determine the biologic sequelae of thismutation. None of the mutations were found in a largenumber of control animals tested. In a number ofother, unrelated narcoleptic dogs, no mutations inhcrtr-2 were found. This means genetic heterogeneityin the genesis of the canine disorder, similar to thehuman condition (discussed later).

The disrupted function of hcrtr-2 in narcoleptic Do-bermans does not alter the number, morphology, anddistribution of hypocretin neurons (Okura et al., 2000).hcrt-1 levels in the CSF of these dogs are not differentfrom control animals, suggesting intact hcrtr-1-mediatedtransmission (Ripley et al., 2000).

Taken together, these findings show convincingly that

mutations in the hcrtr-2 gene are responsible for theautosomal recessive form of canine narcolepsy.

Murine Narcolepsy

Only two weeks after the publication of the caninenarcolepsy gene, Yanagisiwas group reported on thephenotype of preprohypocretin knockout mice (Chemelliet al., 1999). Searching for abnormalities in food intakein this mouse model, lacking both hcrt-1 and -2, theywere surprised not to find any obvious changes. Whilestudying the locomotor activity of the knockout mice toassess energy expenditure, Chemelli et al. (1999) ob-served periods of reduced activity at night, the periodwhen mice are normally most active. With the inventiveuse of infrared video recording, it became obvious thathomozygous mice had brief periods of what was calledbehavioral arrest. At first, the possibility of a seizuredisorder was considered. Twenty-four-hour EEG/electro-myographic recordings excluded this possibility. Insteadit was found that the hypocretin knockouts had severalsleep abnormalities, including an increased but frag-mented amount of sleep during the dark period, and adecreased REM latency as well as sleep-onset REMperiods. These findings are remarkably similar to humanand canine narcolepsy. This led the authors to the justi-fied proposal that hypocretin knockout mice have narco-lepsy, and that the episodes of behavioral arrest were infact cataplectic attacks (Chemelli et al., 1999).

It is obvious that the availability of a mouse modelwill exert a tremendous impact on the research into thepathophysiology of narcolepsy. Tools have been devel-

FIG. 5. Schematic representation of the hypocretin-2 (hcrtr-2) locus in dogs. Hcrtr-2 is encoded by seven exons. ATG and TGA are start and stopcodons respectively. Exon sequence is shown in italics. Mutations in both Dobermans and Labradors are boxed. In narcoleptic Dobermans, afterdetermining exonintron boundaries, and sequencing of the boundary preceding the deletion, an insertion of a canine short, interspersed nucleotideelement (SINE) was found near the 5' splice site of the fourth exon (Lin et al., 1999). The SINE element causes aberrant splicing and results in atruncated receptor because of a premature stop codon at position 932 of the encoded ribonucleic acid. In narcoleptic Labradors a G-to-A transitionin the 5' splice junction consensus sequence (exon 6intron 6, shown in gray with the critical A underlined) was found to result in deletion of exon6, resulting in a truncated protein as well (Lin et al., 1999).



oped to monitor EEG and electromyograms over longperiods of time, without restricting movements of themice (Sinton et al., 2000). When hypocretin knockoutsare held on a restricted feeding schedule, the resultingarousal makes it possible to elicit a relatively stablenumber of cataplectic attacks in a certain amount of time,thereby facilitating pharmacologic studies (Willie et al.,2000). The phenotype of hcrtr-2 knockout mice has beendescribed recently (Chemelli et al., 2000). These knock-outs have a sleep/wake pattern very similar to but some-what milder than the peptide knockouts. These resultsmay implicate a role for hcrtr-1 in the pathogenesis ofnarcolepsy as well.

Human Narcolepsy

Shortly after the implication of the hypocretin systemin the pathogenesis of narcolepsy in animals, we wereable to publish the first evidence of hypocretin involve-ment in the human disorder (Nishino et al., 2000). Wemeasured hcrt-1 levels in the CSF of nine narcolepticpatients and eight control subjects. All patients wereHLA DQB1*0602 positive, had typical MSLT results,and had cataplexy. In control subjects, hcrt-1 concentra-tions displayed very little variation; levels were all be-tween 250 and 285 pg/mL (Nishino et al., 2000). Thisnarrow range suggests a high level of regulation ofnormal hcrt-1 production. Seven of nine patients studiedhad no detectable hcrt-1 in their CSF (detection limit, 40pg/mL) (Nishino et al., 2000). Two patients, clinicallyindistinguishable from the others, had detectable levels,with one displaying a value more than twice than normal.These last two patients may have downstream defects intheir hypocretin system (for example, a receptor dysfunc-tion) or defects in intracellular signal transmission. In

hcrtr-2-mutated dogs, hypocretin levels are normal in-deed (Ripley et al., 2000). The high levels in one patientmay then reflect secondary ligand upregulation, to aug-ment diminished downstream signaling.

These results showed that a hypocretin deficiency isinvolved in the human condition as well. Extended stud-ies are underway to determine hcrt-1 concentrations inatypical patients, and in patients with a family history ofnarcolepsy.

Parallel to our studies in sporadic patients, we initiateda mutation screening of the hypocretin genes in humanpatients (Peyron et al., 2000). After determination of theintronexon boundaries of the human hcrtr-1 and hcrtr-2genes (the genomic sequence of hcrt had been publishedpreviously by Sakurai et al. (1999)), primers were devel-oped to sequence all the exons, together with 50 basepairs of flanking intron on each side, to allow detectionof splice site mutations. A total of 74 patients wereincluded in the study, including patients with and withouta family history of narcolepsy, and patients with andwithout the HLA DQB1*0602 antigen. Although 14polymorphisms were detected in the three genes, none ofthem were associated with or linked to narcolepsy. Ad-ditional studies are warranted to determine the possibleeffect of these polymorphisms on disease susceptibility,severity, and, for example, response to treatment.

Only one patient was found to be heterozygous for aG-to-T transversion in the hcrt signal peptide (Fig. 6A).The disease phenotype of this patient is remarkablysevere. Cataplexy started to occur at the age of 6 months,reaching five to 20 attacks per day when untreated. Fromthis young age, sleep attacks started to occur as well,with sleep-onset REM periods detected for the fist timeat the age of 2.5 years. hcrt-1 was undetectable in this

FIG. 6. (A, B) Signal peptide mutation in a human patient. One human patient was found to be heterozygous for a G-to-T transversion in thehypocretin signal peptide sequence. In A, sequence chromatograms show both the wild-type (lower half) and the mutated sequences (upper half).In B, immunofluorescence pictures of representative cells, transfected with a wild-type hypocretin construct (left) and a construct carrying themutation (right) are shown. The normal distribution of peptide (diffuse in the cell body, with a high concentration at the axon terminal) is markedlychanged for the mutant peptide. Additional colocalization studies (Peyron et al., 2000) showed that the mutant peptide accumulates in the smoothendoplasmic reticulum. Immunofluorescence pictures were inverted to facilitate black-and-white printing.



patients CSF. To examine the cellular phenotype of themutation, wild-type and mutant alleles were transfectedinto Neuro-2A cells, and the distribution of the producedpeptides was studied by immunofluorescence. In short,the mutant allele showed a remarkably different distri-bution in the cells, with accumulation of what wasdetermined to be smooth endoplasmic reticulum (Fig.6B). This accumulation is thought to be lethal, based onsimilar mutations in a n

Narcolepsia en ingles

Documents

Transcript of Narcolepsia en ingles