Narcolepsy is a disabling neurological disorder that was first recognized 118 years ago by Gelineau, J. B. (De la narcolepsie, Gazette des Hopitaux Paris (1880) 53: 626-628). For a review of narcolepsy, see generally Chokroverty, S. (ed.), Sleep Disorders Medicine: Basic Science, Technical Considerations, and Clinical Aspects, 2.sub.2 nd edition, Butterworth Heinemann, Boston, Mass. U.S.A. 1999; Aldrich, M., Sleep Medicine, Oxford University Press, New York, N.Y. U.S.A. 1999; Vgnotzas, A. N. et al., Annu. Rev. Med. (1999) 50:387-400; and Guillenminault, C., Narcolepsy Syndrome in Principles and Practice of Sleep Medicine, 2.sup.nd edition (Kryger, M. H., et al. (eds.), (W. B. Saunders Philadelphia, Pa. U.S.A. 1989), pages 338-246). The symptoms of narcolepsy include excessive daytime sleepiness (EDS), hypnagogic and hypnopompic hallucinations (hallucinations during transitions into and out of sleep, respectively), cataplexy (sudden and reversible loss of muscle tone), sleep paralysis (an inability to move at sleep onset or awakening) and REM sleep at sleep onset (Guilleminault, C. 1989). In narcoleptics, sleep occurs at inappropriate times and in dangerous and embarrassing situations. Although total sleep time is near normal, nighttime sleep is disrupted by frequent awakenings (Mitler, M. et al., Psych Clin. N. Amer. (1987) 10:593-606). Cataplexy, a temporary, partial or complete paralysis due to a sudden loss of muscle tone, with unimpaired consciousness, is typically triggered by sudden strong emotions, such as those accompanying laughter, anger and embarrassment. In some patients, status cataplecticus, or periods of repetitive loss of muscle tone, occurs and can last for hours or days.
Narcolepsy has also been reported to occur in other animals and has been most intensively studied in canines (Foutz, A. S., et al., (1979) Sleep 1:413-421; Nishino, S. and Mignot, E. (1997) Prog. Neurobiol. 52:27-78; Cederberg, R., et al., (1998) Vet. Rec. 142, 31-36). Canine narcolepsy in Doberman pinschers and Labrador Retrievers is transmitted as an apparently single gene autosomal recessive trait with full penetrance, canarc-1 (Foutz, A. S., et al., (1979) Sleep 1:413-421; Baker, T.L. and Dement, W. C. (1985), Canine narcolepsy-cataplexy syndrome: evidence for an inherited monoaminergic-cholinergic imbalance in Brain Mechanisms of Sleep, D. J. McGinty, R. Drucker-Colin, A. Morrison, and P. L. Parmeggiani, eds. (New York: Raven Press), pages 199-233). A large number of physiological and pharmacological studies have demonstrated a close similarity between human and canine narcolepsy (Baker, T. L. and Dement, W. C. (1985) and Nishino, S. and Mignot, E. (1997)). These animals have all the major symptoms defining narcolepsy in humans, including episodes of cataplexy. Canine narcoleptics also exhibit excessive daytime sleepiness and interrupted sleep periods (Kaitin, K. I. et al., Electroenceph. Clin. Neurophysiol. (1986) 64:447-454). Cholinergic antagonists block cataplexy in both canine and human narcoleptics (Delashaw et al., (1979) Exp. Neurology 66:745-757). .alpha.1 blockers (such as prazosin) exacerbate cataplexy in dogs and humans and can produce status cataplecticus in both species (Mignot et al., (1988) Brain Res. 444:184-188; Guilleminault et al., (1988) The Lancet 2: 511). Drugs used to treat cataplexy and excessive sleepiness in humans are also effective in narcoleptics dogs (Baker and Dement, 1985). Narcolepsy usually does not develop until adolescence in humans, but it can be seen as early as three or as late as 45 years of age or older (Yoss and Daly, (1960) Pediatrics 25:1025-1033; Billiard, (1985) Ann. Clin.Res 17:220-226). The appearance of cataplexy, as a proxy variable for the onset of narcolepsy/cataplexy, in canine narcolepsy, develops between 4 and 24 weeks of age.
Approximately 250,000 Americans have narcolepsy (Aldrich, M. S., New Eng. J. Med. (1990) 323:389-394). Although familial cases of narcolepsy have been reported, most human occurrences are sporadic, and the disorder is generally believed to be multigenic and environmentally influenced (Honda, Y., and Matsuki, K., Genetic Aspects of Narcolepsy in Handbook of Sleep Disorders, M. Thorpy (ed.) (Marcel Dekker, Inc., New York, N.Y. 1990), pages 217-234). One predisposing genetic factor is a specific HLA-DQ allele, HLA-DQB1*0602 (Matsuki, K., et al., (1992) Lancet 339:1052. Mignot, E., et al., (1994) Sleep 17:S60-S67; Mignot, E. (1998) Neurology 50:S16-S22). Approximately 95% of narcoleptics have this HLA haplotype, compared to only 30% of the general population (Aldrich, M. S., New Eng. J Med. (1990) 323:389-394). An autoimmune mechanism has been reported in some HLA-associated diseases such as juvenile diabetes, celiac disease, systemic lupus erythematosus and rheumatoid arthritis (Sinha, A. et al., Science (1990) 248:1380-1388); however, all attempts to date to test the autoimmune hypothesis for narcolepsy have failed (Mignot, E., et al., Adv. Neuroimmunol. (1995) 5:23-37).
It has recently been reported that narcolepsy is linked to dysfunction of the newly discovered hypocretin (Hcrt) (orexin) peptide system. This report was based on a deletion in the transcripts of the hypocretin receptor 2 (Hcrtr2) gene in narcoleptic Dobermans and Labradors (Lin, L. et. al., Cell (1999) 97:365-376). Chemelli et al. created Hcrt knockout mice which have abnormalities of sleep control resembling aspects of narcolepsy (Chemelli, R. M. et al., Cell (1999) 98:437-451), as well.
Narcolepsy requires long-term management of symptoms (Fry, J., Neurology (1998) 50(2 Suppl 1):S8-15). Interventions can be nonpharmacologic, such as lifestyle changes, and pharmacologic, for relief of daytime sleepiness, cataplexy, sleep paralysis, hypnagogic hallucinations, and/or hypnopompic hallucinations. Pharmacologic treatment of narcolepsy has depended on the use of central nervous system (CNS) stimulants to increase wakefulness or to reduce the number and severity of cataplectic attacks or hypnagogic hallucinations. CNS stimulants can be effective in relieving the sleepiness of narcolepsy; however, extremely high doses are necessary to restore alertness to normal levels (Mitler, M. et al., Sleep (1993) 16:306-317). Such doses can have very dangerous side effects. Because of these side effects, most narcoleptics use stimulants only when absolutely needed or continuously use low-level doses not capable of restoring normal levels of alertness. Periodic "drug holidays" can sometimes be employed to maintain the effectiveness of stimulants (Mitler, M. S. Sleep (1994) 17:S103-S106). Frequent naps can be effective in permitting periods of waking alertness (Aldrich, M. S., Neurology (1992) 42(S6):34-43). Cataplexy can sometimes be treated successfully with tricyclic antidepressants or selective serotonin reuptake inhibitors (SSRI's), among other medications. Both tricyclic antidepressant drugs and SSRI's all appear to act by producing metabolites that activate noradrenergic receptors (Nishino, S. et al., Sleep (1993) 16:706-712; Mignot, E. et al., Psychopharmacology (1993) 113:76-82). Even with these treatments, accidents due to sleepiness and cataplexy are common and professional and educational attainment are significantly reduced in narcoleptics (Broughton, W. A. and Broughton, R. J., Sleep (1994) 17:S45-S49).