Narcolepsy in orexin knockout mice: molecular genetics of sleep regulation

Neurons containing the neuropeptide orexin (hypocretin) are located exclusively in the lateral hypothalamus and send axons to numerous regions throughout the central nervous system, including the major nuclei implicated in sleep regulation. Here, we report that, by behavioral and electroencephalographic criteria, orexin knockout mice exhibit a phenotype strikingly similar to human narcolepsy patients, as well as canarc-1 mutant dogs, the only known monogenic model of narcolepsy. Moreover, modafinil, an anti-narcoleptic drug with ill-defined mechanisms of action, activates orexin-containing neurons. We propose that orexin regulates sleep/wakefulness states, and that orexin knockout mice are a model of human narcolepsy, a disorder characterized primarily by rapid eye movement (REM) sleep dysregulation.

The sleep disorder canine narcolepsy is caused by a mutation in the hypocretin (orexin) receptor 2 gene

Narcolepsy is a disabling sleep disorder affecting humans and animals. It is characterized by daytime sleepiness, cataplexy, and striking transitions from wakefulness into rapid eye movement (REM) sleep. In this study, we used positional cloning to identify an autosomal recessive mutation responsible for this sleep disorder in a well-established canine model. We have determined that canine narcolepsy is caused by disruption of the hypocretin (orexin) receptor 2 gene (Hcrtr2). This result identifies hypocretins as major sleep-modulating neurotransmitters and opens novel potential therapeutic approaches for narcoleptic patients.

Identification and functional analysis of mutations in the hypocretin (orexin) genes of narcoleptic canines

Narcolepsy is a sleep disorder affecting animals and humans. Exon skipping mutations of the Hypocretin/Orexin-receptor-2 (Hcrtr2) gene were identified as the cause of narcolepsy in Dobermans and Labradors. Preprohypocretin (Hcrt) knockout mice have symptoms similar to human and canine narcolepsy. In this study, 11 sporadic cases of canine narcolepsy and two additional multiplex families were investigated for possible Hcrt and Hcrtr2 mutations. Sporadic cases have been shown to have more variable disease onset, increased disease severity, and undetectable Hypocretin-1 levels in cerebrospinal fluid. The canine Hcrt locus was isolated and characterized for this project. Only one novel mutation was identified in these two loci. This alteration results in a single amino acid substitution (E54K) in the N-terminal region of the Hcrtr2 receptor and autosomal recessive transmission in a Dachshund family. Functional analysis of previously-described exon-skipping mutations and of the E54K substitution were also performed using HEK-293 cell lines transfected with wild-type and mutated constructs. Results indicate a truncated Hcrtr2 protein, an absence of proper membrane localization, and undetectable binding and signal transduction for exon-skipping mutated constructs. In contrast, the E54K abnormality was associated with proper membrane localization, loss of ligand binding, and dramatically diminished calcium mobilization on activation of the receptor. These results are consistent with a loss of function for all three mutations. The absence of mutation in sporadic cases also indicates genetic heterogeneity in canine narcolepsy, as reported previously in humans.

Narcolepsy and immunity

Narcolepsy is a neurological disorder known to be associated with human leukocyte antigen (HLA)-DQB1*0602 in humans. In a canine model, the disorder is also genetically linked to a gene of high homology with the human mu-switch-like immunoglobulin (Ig) gene (current LOD score 13.6 at 0% recombination). Since association with HLA or other immune function polymorphic genes (T cell receptor of Ig, mainly) is a hallmark of most autoimmune diseases, it is proposed that autoimmunity may also play a role in the development of narcolepsy. Arguments for and against this hypothesis are reviewed. It is shown that both on the basis of the most recent molecular studies, and because of some of its clinical features, narcolepsy may be an autoimmune disorder. However, neither systemic nor central nervous system (CNS) evidence of any autoimmune abnormality have ever been found. To reconcile this discrepancy, it is suggested that the pathological immune process involved in narcolepsy could be difficult to detect because it is restricted to a very small region of the brain or targets a low abundance neuroeffector. Alternatively, it is possible that a more fundamental relationship is involved between sleep generation and immune regulation. The pathophysiology of narcolepsy may then involve new CNS-immune mechanisms that may shed new light on the sleep process itself.

Sleep studies on canine narcolepsy: pattern and cycle comparisons between affected and normal dogs

Two narcoleptic and 2 normal poodle or mixed poodle dogs were polygraphically monitored for 48 h with one narcoleptic and one normal monitored concurrently. Data were categorized by 15-sec epochs into wakefulness, light sleep, slow wave sleep, REM sleep, cataplexy (immobility preceded by wakefulness with partial or complete electromyographic quiescence and pronounced theta activity from subcortical leads), and atonia with no theta (15-30 sec periods like cataplexy but without theta). In narcoleptics we could see no gross differences between the polygraphic records of cataplexy and those of REM sleep; scoring distinctions between the two states depended on the antecedent state. Results indicated that narcoleptic dogs do not differ from normals with respect to percent of time spent in wakefulness (39.8% vs. 42.6%), light sleep (16.2% vs. 18.4%), or slow wave sleep (27.2% vs. 28.0%). Narcoleptic dogs spent slightly less time than normals in REM sleep (6.9% vs. 11.1%) and spend 9.1% and 0.8% of the recording time in cataplexy and atonia with no theta respectively. Normal dogs presented neither of these pathological states.

Genetic linkage of autosomal recessive canine narcolepsy with a mu immunoglobulin heavy-chain switch-like segment

Identification of genes determining narcolepsy susceptibility is important not only for understanding that disorder but also for possible clues to general sleep-control mechanisms. Studies in humans reveal at least one such gene related to the major histocompatibility complex and in dog an as-yet-unmapped single, autosomal recessive gene canarc-1. Gene markers for canarc-1 were therefore sought by DNA restriction fragment length polymorphisms in our colony of narcoleptic dogs. A human mu-switch immunoglobulin probe and the enzyme Hae III identified a gene cosegregating with canarc-1 in backcrossed animals (logarithm of odds scores: m = 24, Z max = 7.2 at theta = 0%). canarc-1 was also shown not to be tightly linked with the dog major histocompatibility complex (m = 40, Z less than -2 at theta less than 4.8%). These results represent the mapping of a non-major histocompatibility complex narcolepsy gene and strongly suggest involvement of the immune system in the pathophysiology of that disease.

Narcolepsy-cataplexy in a female dog

Narcolepsy in human beings is a life-long illness afflicting 100,000–150,000 Americans. No current treatment for this neurological disorder is satisfactory and a definitive approach to the disease requires an animal model of narcolepsy. This report summarizes data on a female toy poodle which has a canine form of narcolepsy-cataplexy. At approximately 4 mo of age, the dog presented cataplectic attacks. Physical examination disclosed no systemic factors to account for such atonic episodes. Observation indicated that attacks were frequently partial, involving only the neck musculature and hind legs. However, such attacks could develop into complete cataplexy, causing postural collapse and areflexia, although extrinsic eye muscles and the muscles of respiration were spared. Presentation of food, water, or a plaything were the most frequent elicitors of attacks. During 41 hr of EEG, EOG, and EMG monitoring in conjunction with behavioral observation, the dog exhibited normal polygraphic wakefulness, slow-wave sleep and REM sleep. Unambiguous sleep onset REM periods and cataplectic attacks were also observed. These pathological manifestations are analogous to those characteristic of human narcolepsy. The diagnosis of canine narcolepsy was further confirmed by two negative trials with neostigmine (ruling out myasthenia) and two positive trials with imipramine (cataplexy in human narcolepsy responds to imipramine treatment). The dog will be bred either with a littermate or a similarly afflicted male in an attempt to produce a population of afflicted dogs.

Genetic studies in the sleep disorder narcolepsy

Narcolepsy is a chronic neurologic disorder characterized by excessive daytime sleepiness and abnormal manifestations of REM sleep including cataplexy, sleep paralysis, and hypnagogic hallucinations. Narcolepsy is both a significant medical problem and a unique disease model for the study of sleep. Research in human narcolepsy has led to the identification of specific HLA alleles (DQB1*0602 and DQA1*0102) that predispose to the disorder. This has suggested the possibility that narcolepsy may be an autoimmune disorder, a hypothesis that has not been confirmed to date. Genetic factors other than HLA are also likely to be involved. In a canine model of narcolepsy, the disorder is transmitted as a non-MHC single autosomal recessive trait with full penetrance (canarc-1). A tightly linked marker for canarc-1 has been identified, and positional cloning studies are under way to isolate canarc-1 from a newly developed canine genomic BAC library. The molecular cloning of this gene may lead to a better understanding of sleep mechanisms, as has been the case for circadian rhythms following the cloning of frq, per, and Clock.

Effect of alpha 1-adrenoceptors blockade with prazosin in canine narcolepsy

The role of central alpha 1-adrenergic receptors in cataplexy was investigated in 4 narcoleptic poodles and 6 genetically narcoleptic Doberman pinschers. Treatment of narcoleptic dogs with prazosin, a selective alpha 1-adrenergic receptor blocker, exacerbated cataplexy in both narcoleptic dog breeds. Control and heterozygous Dobermans were not affected by the drug. Binding studies using [3H]prazosin revealed an increase in alpha 1-receptor binding apparently limited to the amygdala. The present study suggests that central alpha 1-adrenoceptors, whose role is still mostly unknown, play a fundamental role in controlling mechanisms involved in cataplexy and REM sleep.

Cholinergic mechanisms in canine narcolepsy--I. Modulation of cataplexy via local drug administration into the pontine reticular formation

Cataplexy in the narcoleptic canine has been shown to increase after systemic administration of cholinergic agonists. Furthermore, the number of cholinergic receptors in the pontine reticular formation of narcoleptic canines is significantly elevated. In the present study we have investigated the effects of cholinergic drugs administered directly into the pontine reticular formation on cataplexy, as defined by brief episodes of hypotonia induced by emotions, in narcoleptic canines. Carbachol and atropine were perfused through microdialysis probes implanted bilaterally in the pontine reticular formation of freely moving, narcoleptic and control Doberman pinschers. Cataplexy was quantified using the Food-Elicited Cataplexy Test, and analysed using recordings of electroencephalogram, electrooculogram and electromyogram. Cataplexy was characterized by a desynchronized electroencephalogram and a drop in electromyogram and electrooculogram activity. In narcoleptic canines, both unilateral and bilateral carbachol (10(-5) to 10(-3) M) produced a dose-dependent increase in cataplexy, which resulted in complete muscle tone suppression at the highest concentration. In control canines, neither bilateral nor unilateral carbachol (10(-5) to 10(-3) M) produced cataplexy, although bilateral carbachol, did produce muscle atonia at the highest dose (10(-3)). The increase in cataplexy after bilateral carbachol (10(-4) M) was rapidly reversed when the perfusion medium was switched to one containing atropine (10(-4) M). Bilateral atropine (10(-3) to 10(-2) M) alone did not produce any significant effects on cataplexy in narcoleptic canines; however, bilateral atropine (10(-2) M) did reduce the increase in cataplexy produced by systemic administration of physostigmine (0.05 mg/kg, i.v.). These findings demonstrate that cataplexy in narcoleptic canines can be stimulated by applying cholinergic agonists directly into the pontine reticular formation. The ability of atropine to inhibit locally and systemically stimulated cataplexy indicates that the pontine reticular formation is a critical component in cholinergic stimulation of cataplexy. Therefore, it is suggested that the pontine reticular formation plays a significant role in the cholinergic regulation of narcolepsy.

Familial occurrence of narcolepsy in miniature horses

In an investigation of 2 closely related Miniature Horses with a history of excessive sleepiness, depression and episodes of collapse, a diagnosis of narcolepsy was made on the basis of neurological examination and pharmacological testing. Further investigations included electroencephalographic examination (EEG), and analysis of protein content, cell count and monoamine metabolite concentrations of lumbosacral cerebrospinal fluid (CSF). There were no abnormalities noted in the EEGs, and no consistent changes in CSF neurotransmitter metabolites in the narcoleptic horses when compared with 3 normal, unrelated Miniature Horses and 2 related, clinically unaffected animals. The breeding background of the 2 affected horses was investigated and a limited survey of Miniature Horse breeders in North America was conducted. These investigations have shown that narcolepsy is a rare but distinct syndrome in the Miniature Horse, and that the cases described here appear to represent a familial occurrence of the disease.

Cholinergic mechanisms in canine narcolepsy--II. Acetylcholine release in the pontine reticular formation is enhanced during cataplexy

Cataplexy in the narcoleptic canine has been shown to increase after local administration of carbachol into the pontine reticular formation. Rapid eye movement sleep has also been shown to increase after local administration of carbachol in the pontine reticular formation, and furthermore, acetylcholine release in the pontine tegmentum was found to increase during rapid eye movement sleep in rats. Therefore, in the present study we have investigated acetylcholine release in the pontine reticular formation during cataplexy in narcoleptic canines. Extracellular acetylcholine levels were measured in the pontine reticular formation of freely moving narcoleptic and control Doberman pinschers using in vivo microdialysis probes. Cataplexy was induced by the Food-Elicited Cataplexy Test and monitored using recordings of electroencephalogram, electrooculogram and electromyogram. Basal levels of acetylcholine in the microdialysis perfusates were approximately 0.5 pmol/10 min in both control and narcoleptic canines. Local perfusion with tetrodotoxin (10(-5) M) or artificial cerebrospinal fluid without Ca2+ produced a decrease, while intravenous injections of physostigmine (0.05 mg/kg) produced an increase in acetylcholine levels, indicating that the levels of acetylcholine levels measured are derived from neuronal release. During cataplexy induced by the Food-Elicited Cataplexy Test, acetylcholine levels increased by approximately 50% after four consecutive tests in narcoleptic canines, but did not change after four consecutive tests in control canines. Motor activity and feeding behavior, similar to that occurring during a Food-Elicited Cataplexy Test, had no effect on acetylcholine levels in the narcoleptic canines.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurologic diseases

The responses of apparently healthy newborn foals to neurologic testing differ significantly from those of adult horses. These responses and the diagnostic techniques pertinent to neurologic problems are reviewed as a basis for evaluation of the compromised neonatal foal. The more frequently encountered neurologic diseases are discussed in a problem-oriented format. These clinical problems include behavioral abnormalities, convulsions, changes in consciousness, blindness, ataxia without loss of strength, ataxia with weakness and paralysis, and the floppy foal.

Narcolepsy without unique MHC class II antigen association: studies in the canine model

Human narcolepsy is almost exclusively associated with the major histocompatibility complex (MHC) class II antigen HLA-DR2 and is the strongest HLA-disease association described to date. Canine narcolepsy resembles the human disease in its behavioral manifestations and responses to therapeutic drugs. Therefore, mixed leukocyte culture (MLC) was used to study differences in the canine MHC class II (DLA-D) antigens present in narcoleptic dogs to determine whether an analogous, unique DLA-D antigen could be identified in canine narcolepsy. Results show at least five different DLA-D antigens appear in potential narcoleptic haplotypes among the 29 dogs studied. The data demonstrate that, unlike man, in dogs there is no unique D locus antigen associated with narcolepsy and further suggest that linkage disequilibrium with a specific MHC antigen is unlikely to be essential for the manifestation of canine narcolepsy. Because human narcolepsy is thought to be multigenic, the canine narcolepsy-MHC dissociation suggests that the dog model may help elucidate the non-MHC narcolepsy gene(s).

DLA-DQB1 alleles and bone marrow transplantation experiments in narcoleptic dogs

Human narcolepsy is a neurological disorder known to be tightly associated with HLA-DQB1*0602. A clinically similar disorder has been described in various dog breeds. The canine form of the disease is inherited as an autosomal recessive disorder in Labrador retrievers and Doberman pinschers (canarc-1) but occurs sporadically in other breeds, most typically dachshunds and poodles. In this study, we have examined if there is a relationship between the development of narcolepsy and specific dog leukocyte antigen (DLA)-DQB1 alleles. Ninety-nine dogs were typed for DLA-DQB1-31 with narcolepsy and 68 control animals. Recent studies have linked the development of autosomal recessive canine narcolepsy to a disruption of the hypocretin receptor 2 (Hcrtr2) gene on the same chromosome as the canine MHC region (CFA12), but not close to the DLA. Four Hcrtr2-positive families (two Doberman pinscher families, one Labrador retriever family, one dachshund family) were analyzed at the DLA-DQ level. No relationship was found between narcolepsy and DLA in Hcrtr2-mediated narcolepsy but loose genetic linkage was observed (Zmax=2.3 at theta=25%, m= 40). Bone marrow transplantation between two DLA identical affected (Hcrtr2-/-) and unaffected (Hcrtr2+/-) siblings was also performed and found not to be successful neither in transmitting narcolepsy nor in relieving the symptoms in Doberman pinschers. DLA-DQB1 was next studied in 11 dogs with sporadic (non-familial) narcolepsy and in unrelated control animals of the same and different breeds. The allelic and carrier frequencies of various DLA-DQB1 alleles were analyzed. There was no strong positive or negative correlation between the development of narcolepsy and specific DLA-DQB1 alleles. These results do not support the involvement of DLA-DQ in canine narcolepsy, whether of sporadic or familial origin.

Review of pathophysiology and clinical management of narcolepsy in dogs

Narcolepsy is a chronic sleep disorder that affects human beings and animals. Up to 17 breeds of dogs are affected sporadically, and familial forms occur in dobermanns, labrador retrievers and dachshunds. These dogs display characteristics strikingly similar to those of human narcolepsy, including cataplexy (a sudden loss of muscle tone in response to emotional stimulation) and a shorter sleep latency. It has recently been shown that the aetiology of both the familial form (receptor null mutation) and the sporadic form (loss of ligand production) of canine narcolepsy is associated with a deficit in hypocretin/orexin neurotransmission. Hypocretin deficiency can be detected by the measurement of hypocretin-1 in cerebrospinal fluid, and this could be used to diagnose hypocretin ligand deficient cases in clinical practice. Narcolepsy is neither progressive nor life-threatening, but the clinical signs persist throughout life, and lifelong treatment and care are required. This article reviews the recent progress in narcolepsy research in dogs, and describes the diagnosis and treatment of the disease.

Breeding history of the Stanford colony of narcoleptic dogs

Narcolepsy is a disabling sleep disorder of unknown aetiology. In humans, the disease is mostly sporadic, with a few familial cases having been reported. In 1973 a sporadic case of narcolepsy was reported in a poodle, and in 1975 familial cases of narcolepsy occurred in dobermanns. As with human narcoleptics, these narcoleptic dogs exhibited excessive daytime sleepiness and cataplexy. A colony of narcoleptic dogs was established at Stanford University in 1976 to study the pathophysiology of the disease. Between 1976 and 1995, a total of 669 animals of various breeds were born, of which 487 survived. Dobermanns accounted for 78 per cent of the total. The narcolepsy genotype in dobermanns had no significant influence on puppy mortality rate (numbers of stillborn and survival rate). The sex, maternal parity or the age of the sire or dam had no significant effect. The percentage of stillborn puppies increased from 6.1 per cent in outbred litters to 15.4 per cent in inbred litters (P = 0.10). Birth season also had a significant effect, and the highest survival rate (P = 0.02), and the lowest percentage of stillborn puppies (P = 0.09) occurred between April and June.

Pharmacokinetics of imipramine in narcoleptic horses

OBJECTIVE

To validate use of high-performance liquid chromatography (HPLC) in determining imipramine concentrations in equine serum and to determine pharmacokinetics of imipramine in narcoleptic horses.

ANIMALS

5 horses with adult-onset narcolepsy.

PROCEDURE

Blood samples were collected before (time 0) and 3, 5, 10, 15, 20, 30, and 45 minutes and 1, 2, 3, 4, 6, 8, 12, and 24 hours after IV administration of imipramine hydrochloride (2 or 4 mg/kg of body weight). Serum was analyzed, using HPLC, to determine imipramine concentration. The serum concentration-versus-time curve for each horse was analyzed separately to estimate pharmacokinetic values.

RESULTS

Adverse effects (muscle fasciculations, tachycardia, hyperresponsiveness to sound, and hemolysis) were detected in most horses when serum imipramine concentrations were high, and these effects were most severe in horses receiving 4 mg of imipramine/kg. Residual adverse effects were not apparent. Value (mean +/- SD) for area under the curve was 3.9 +/- 0.7 h X microg/ml, whereas volume of distribution was 584 +/- 161.7 ml/kg, total body clearance was 522 +/- 102 ml/kg/h, and mean residence time was 1.8 +/- 0.6 hours. One horse had signs of narcolepsy 6 and 12 hours after imipramine administration; corrresponding serum imipramine concentrations were less than the therapeutic range.

CONCLUSIONS AND CLINICAL RELEVANCE

Potentially serious adverse effects may be seen in horses administered doses of imipramine that exceed a dosage of 2 mg/kg. Total body clearance of imipramine in horses is slower than that in humans; thus, the interval between subsequent doses should be longer in horses.

Prostaglandin E2 and its methyl ester reduce cataplexy in canine narcolepsy

The effects of intravenous administration of prostaglandins (PGs) were investigated in genetically narcoleptic Doberman pinschers. The treatment of narcoleptic dogs with PGE2 and PGE2 methyl ester, but not PGD2 and PGD2 methyl ester, induced a dose-dependent reduction of canine cataplexy, a dissociated manifestation of rapid-eye-movement sleep. The effect was specific and not associated with any change in other behavior. Furthermore, the effect was long-lasting (up to 2 hr) and could not be explained by the acute cardiovascular changes seen after intravenous PG administration. PGE2 methyl ester, a lipophilic derivative of PGE2 with more central penetration than PGE2, was 4 times more potent than PGE2. These results indicate that PGE2 modifies cataplexy through a central effect and suggest that this prostaglandin may play a role in rapid-eye-movement sleep regulation.

Inheritance of von Willebrand's disease in a colony of Doberman Pinschers

OBJECTIVE

To determine the mode of inheritance of von Willebrand's disease (vWD) and perform linkage analysis between vWD and coat color or narcolepsy in a colony of Doberman Pinschers.

ANIMALS

159 Doberman Pinschers.

PROCEDURE

von Willebrand factor antigen (vWF:Ag) concentration was measured by use of ELISA, and results were used to classify dogs as having low (< 20%), intermediate (20 to 65%), or high (> 65%) vWF:Ag concentration, compared with results of analysis of standard pooled plasma. Buccal bleeding time was measured, and mode of inheritance of vWD was assessed by pedigree analysis.

RESULTS

von Willebrand's disease was transmitted as a single autosomal gene defect. Results suggested that 27.04% of dogs were homozygous for vWD, 62.26% were heterozygous, and 10.69% did not have the defect. Most homozygous and some heterozygous dogs had prolonged bleeding times. Dogs with diluted coat colors (blue and fawn) were significantly overrepresented in the homozygous group, compared with black and red dogs, but a significant link between vWD and coat color was not detected.

CONCLUSIONS AND CLINICAL RELEVANCE

von Willebrand's disease is transmitted as an autosomal dominant trait with variable penetrance; most dogs in this colony (89.3%) were carriers of vWD. Homozygosity for vWD is not likely to be lethal. Some heterozygous dogs have prolonged bleeding times. An association between diluted coat colors and vWD may exist.