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

The role of orexinergic system in the regulation of cataplexy

Loss of orexin/hypocretin causes serious sleep disorder; narcolepsy. Cataplexy is the most striking symptom of narcolepsy, characterized by abrupt muscle paralysis induced by emotional stimuli, and has been considered pathological activation of REM sleep atonia system. Clinical treatments for cataplexy/narcolepsy and early pharmacological studies in narcoleptic dogs tell us about the involvement of monoaminergic and cholinergic systems in the control of cataplexy/narcolepsy. Muscle atonia may be induced by activation of REM sleep-atonia generating system in the brainstem. Emotional stimuli may be processed in the limbic systems including the amygdala, nucleus accumbens, and medial prefrontal cortex. It is now considered that orexin/hypocretin prevents cataplexy by modulating the activity of different points of cataplexy-inducing circuit, including monoaminergic/cholinergic systems, muscle atonia-generating systems, and emotion-related systems. This review will describe the recent advances in understanding the neural mechanisms controlling cataplexy, with a focus on the involvement of orexin/hypocretin system, and will discuss future experimental strategies that will lead to further understanding and treatment of this disease.

Neurobiology of cataplexy

Cataplexy is the pathognomonic and the most striking symptom of narcolepsy. It has originally been, and still is now, widely considered as an abnormal manifestation of rapid eye movement (REM) sleep during wakefulness due to the typical muscle atonia. The neurocircuits of cataplexy, originally confined to the brainstem as those of REM sleep atonia, now include the hypothalamus, dorsal raphe (DR), amygdala and frontal cortex, and its neurochemistry originally focused on catecholamines and acetylcholine now extend to hypocretin (HCRT) and other neuromodulators. Here, we review the neuroanatomy and neurochemistry of cataplexy and propose that cataplexy is a distinct brain state that, despite similarities with REM sleep, involves cataplexy-specific features.

Discharge and Role of Acetylcholine Pontomesencephalic Neurons in Cortical Activity and Sleep-Wake States Examined by Optogenetics and Juxtacellular Recording in Mice

Acetylcholine (ACh) neurons in the pontomesencephalic tegmentum (PMT) are thought to play an important role in promoting cortical activation with waking (W) and paradoxical sleep [PS; or rapid eye movement (REM)], but have yet to be proven to do so by selective stimulation and simultaneous recording of identified ACh neurons. Here, we employed optogenetics combined with juxtacellular recording and labeling of neurons in transgenic (TG) mice expressing ChR2 in choline acetyltransferase (ChAT)-synthesizing neurons. We established in vitro then in vivo in anesthetized (A) and unanesthetized (UA), head-fixed mice that photostimulation elicited a spike with short latency in neurons which could be identified by immunohistochemical staining as ACh neurons within the laterodorsal (LDT)/sublaterodorsal (SubLDT) and pedunculopontine tegmental (PPT) nuclei. Continuous light pulse stimulation during sleep evoked tonic spiking by ACh neurons that elicited a shift from irregular slow wave activity to rhythmic θ and enhanced γ activity on the cortex without behavioral arousal. With θ frequency rhythmic light pulse stimulation, ACh neurons discharged in bursts that occurred in synchrony with evoked cortical θ. During natural sleep-wake states, they were virtually silent during slow wave sleep (SWS), discharged in bursts during PS and discharged tonically during W. Yet, their bursting during PS was not rhythmic or synchronized with cortical θ but associated with phasic whisker movements. We conclude that ACh PMT neurons promote θ and γ cortical activity during W and PS by their tonic or phasic discharge through release of ACh onto local neurons within the PMT and/or more distant targets in the hypothalamus and thalamus.

Orexin Neurons Respond Differentially to Auditory Cues Associated with Appetitive versus Aversive Outcomes

Orexin (Orx) neurons are known to be involved in the promotion and maintenance of waking because they discharge in association with cortical activation and muscle tone during waking and because, in their absence, waking with muscle tone cannot be maintained and narcolepsy with cataplexy ensues. Whether Orx neurons discharge during waking in association with particular conditions, notably with appetitive versus aversive stimuli or positive versus negative emotions, is debated and considered important in understanding their role in supporting particular waking behaviors. Here, we used the technique of juxtacellular recording and labeling in head-fixed rats to characterize the discharge of Orx neurons during the performance of an associative discrimination task with auditory cues for appetitive versus aversive outcomes. Of 57 active, recorded, and neurobiotin-labeled neurons in the lateral hypothalamus, 11 were immunohistochemically identified as Orx-positive (Orx(+)), whereas none were identified as melanin-concentrating hormone-positive. Orx(+) neurons discharged at significantly higher rates during the tone associated with sucrose than during the tone associated with quinine delivered upon licking. They also discharged at high rates after the tone associated with sucrose. Across periods and outcomes, their discharge was positively correlated with EEG gamma activity and EMG activity, which is indicative of cortical activation and behavioral arousal. These results suggest that Orx neurons discharge in a manner characteristic of reward neurons yet also characteristic of arousal neurons. Accordingly, the Orx neurons may respond to and participate in reward processes while modulating cortical activity and muscle tone to promote and maintain arousal along with learned adaptive behavioral responses.

SIGNIFICANCE STATEMENT

Orexin neurons play a critical role in promoting and maintaining a waking state because, in their absence, narcolepsy with cataplexy ensues. Known to discharge during waking and not during sleep, they have also been proposed to be selectively active during appetitive behaviors. Here, we recorded and labeled neurons in rats to determine the discharge of immunohistochemically identified orexin neurons during performance of an associative discrimination task. Orexin neurons responded differentially to auditory cues associated with appetitive sucrose versus aversive quinine, indicating that they behave like reward neurons. However, correlated discharge with cortical and muscle activity indicates that they also behave like arousal neurons and can thus promote cortical activation with behavioral arousal and muscle tone during adaptive waking behaviors.

Gray matter atrophy in narcolepsy: An activation likelihood estimation meta-analysis

The authors reviewed the literature on the use of voxel-based morphometry (VBM) in narcolepsy magnetic resonance imaging (MRI) studies via the use of a meta-analysis of neuroimaging to identify concordant and specific structural deficits in patients with narcolepsy as compared with healthy subjects. We used PubMed to retrieve articles published between January 2000 and March 2014. The authors included all VBM research on narcolepsy and compared the findings of the studies by using gray matter volume (GMV) or gray matter concentration (GMC) to index differences in gray matter. Stereotactic data were extracted from 8 VBM studies of 149 narcoleptic patients and 162 control subjects. We applied activation likelihood estimation (ALE) technique and found significant regional gray matter reduction in the bilateral hypothalamus, thalamus, globus pallidus, extending to nucleus accumbens (NAcc) and anterior cingulate cortex (ACC), left mid orbital and rectal gyri (BAs 10 and 11), right inferior frontal gyrus (BA 47), and the right superior temporal gyrus (BA 41) in patients with narcolepsy. The significant gray matter deficits in narcoleptic patients occurred in the bilateral hypothalamus and frontotemporal regions, which may be related to the emotional processing abnormalities and orexin/hypocretin pathway common among populations of patients with narcolepsy.

Role of the medial prefrontal cortex in cataplexy

Narcolepsy is characterized by chronic sleepiness and cataplexy, episodes of profound muscle weakness that are often triggered by strong, positive emotions. Narcolepsy with cataplexy is caused by a loss of orexin (also known as hypocretin) signaling, but almost nothing is known about the neural mechanisms through which positive emotions trigger cataplexy. Using orexin knock-out mice as a model of narcolepsy, we found that palatable foods, especially chocolate, markedly increased cataplexy and activated neurons in the medial prefrontal cortex (mPFC). Reversible suppression of mPFC activity using an engineered chloride channel substantially reduced cataplexy induced by chocolate but did not affect spontaneous cataplexy. In addition, neurons in the mPFC innervated parts of the amygdala and lateral hypothalamus that contain neurons active during cataplexy and that innervate brainstem regions known to regulate motor tone. These observations indicate that the mPFC is a critical site through which positive emotions trigger cataplexy.

Animal models of sleep disorders

Problems with sleep affect a large part of the general population, with more than half of all people in the United States reporting difficulties with sleep or insufficient sleep at various times and about 40 million affected chronically. Sleep is a complex physiologic process that is influenced by many internal and environmental factors, and problems with sleep are often related to specific personal circumstances or are based on subjective reports from the affected person. Although human subjects are used widely in the study of sleep and sleep disorders, the study of animals has been invaluable in developing our understanding about the physiology of sleep and the underlying mechanisms of sleep disorders. Historically, the use of animals for the study of sleep disorders has arguably been most fruitful for the condition of narcolepsy, in which studies of dogs and mice revealed previously unsuspected mechanisms for this condition. The current overview considers animal models that have been used to study 4 of the most common human sleep disorders-insomnia, narcolepsy, restless legs syndrome, and sleep apnea-and summarizes considerations relevant to the use of animals for the study of sleep and sleep disorders. Animal-based research has been vital to the elucidation of mechanisms that underlie sleep, its regulation, and its disorders and undoubtedly will remain crucial for discovering and validating sleep mechanisms and testing interventions for sleep disorders.

Control of sleep and wakefulness

This review summarizes the brain mechanisms controlling sleep and wakefulness. Wakefulness promoting systems cause low-voltage, fast activity in the electroencephalogram (EEG). Multiple interacting neurotransmitter systems in the brain stem, hypothalamus, and basal forebrain converge onto common effector systems in the thalamus and cortex. Sleep results from the inhibition of wake-promoting systems by homeostatic sleep factors such as adenosine and nitric oxide and GABAergic neurons in the preoptic area of the hypothalamus, resulting in large-amplitude, slow EEG oscillations. Local, activity-dependent factors modulate the amplitude and frequency of cortical slow oscillations. Non-rapid-eye-movement (NREM) sleep results in conservation of brain energy and facilitates memory consolidation through the modulation of synaptic weights. Rapid-eye-movement (REM) sleep results from the interaction of brain stem cholinergic, aminergic, and GABAergic neurons which control the activity of glutamatergic reticular formation neurons leading to REM sleep phenomena such as muscle atonia, REMs, dreaming, and cortical activation. Strong activation of limbic regions during REM sleep suggests a role in regulation of emotion. Genetic studies suggest that brain mechanisms controlling waking and NREM sleep are strongly conserved throughout evolution, underscoring their enormous importance for brain function. Sleep disruption interferes with the normal restorative functions of NREM and REM sleep, resulting in disruptions of breathing and cardiovascular function, changes in emotional reactivity, and cognitive impairments in attention, memory, and decision making.

Cholinergic modulation of narcoleptic attacks in double orexin receptor knockout mice

To investigate how cholinergic systems regulate aspects of the sleep disorder narcolepsy, we video-monitored mice lacking both orexin (hypocretin) receptors (double knockout; DKO mice) while pharmacologically altering cholinergic transmission. Spontaneous behavioral arrests in DKO mice were highly similar to those reported in orexin-deficient mice and were never observed in wild-type (WT) mice. A survival analysis revealed that arrest lifetimes were exponentially distributed indicating that random, Markovian processes determine arrest lifetime. Low doses (0.01, 0.03 mg/kg, IP), but not a high dose (0.08 mg/kg, IP) of the cholinesterase inhibitor physostigmine increased the number of arrests but did not alter arrest lifetimes. The muscarinic antagonist atropine (0.5 mg/kg, IP) decreased the number of arrests, also without altering arrest lifetimes. To determine if muscarinic transmission in pontine areas linked to REM sleep control also influences behavioral arrests, we microinjected neostigmine (50 nl, 62.5 µM) or neostigmine + atropine (62.5 µM and 111 µM respectively) into the nucleus pontis oralis and caudalis. Neostigmine increased the number of arrests in DKO mice without altering arrest lifetimes but did not provoke arrests in WT mice. Co-injection of atropine abolished this effect. Collectively, our findings establish that behavioral arrests in DKO mice are similar to those in orexin deficient mice and that arrests have exponentially distributed lifetimes. We also show, for the first time in a rodent narcolepsy model, that cholinergic systems can regulate arrest dynamics. Since perturbations of muscarinic transmission altered arrest frequency but not lifetime, our findings suggest cholinergic systems influence arrest initiation without influencing circuits that determine arrest duration.

Dopaminergic regulation of sleep and cataplexy in a murine model of narcolepsy

STUDY OBJECTIVES

To determine if the dopaminergic system modulates cataplexy, sleep attacks and sleep-wake behavior in narcoleptic mice.

DESIGN

Hypocretin/orexin knockout (i.e., narcoleptic) and wild-type mice were administered amphetamine and specific dopamine receptor modulators to determine their effects on sleep, cataplexy and sleep attacks.

PATIENTS OR PARTICIPANTS

Hypocretin knockout (n = 17) and wild-type mice (n = 21).

INTERVENTIONS

Cataplexy, sleep attacks and sleep-wake behavior were identified using electroencephalogram, electromyogram and videography. These behaviors were monitored for 4 hours after an i.p. injection of saline, amphetamine and specific dopamine receptor modulators (D1- and D2-like receptor modulators).

MEASUREMENTS AND RESULTS

Amphetamine (2 mg/kg), which increases brain dopamine levels, decreased sleep attacks and cataplexy by 61% and 67%, suggesting that dopamine transmission modulates such behaviors. Dopamine receptor modulation also had powerful effects on sleep attacks and cataplexy. Activation (SKF 38393; 20 mg/kg) and blockade (SCH 23390; 1 mg/kg) of D1-like receptors decreased and increased sleep attacks by 77% and 88%, without affecting cataplexy. Pharmacological activation of D2-like receptors (quinpirole; 0.5 mg/kg) increased cataplectic attacks by 172% and blockade of these receptors (eticlopride; 1 mg/kg) potently suppressed them by 97%. Manipulation of D2-like receptors did not affect sleep attacks.

CONCLUSIONS

We show that the dopaminergic system plays a role in regulating both cataplexy and sleep attacks in narcoleptic mice. We found that cataplexy is modulated by a D2-like receptor mechanism, whereas dopamine modulates sleep attacks by a D1-like receptor mechanism. These results support a role for the dopamine system in regulating sleep attacks and cataplexy in a murine model of narcolepsy.

Narcoleptic orexin receptor knockout mice express enhanced cholinergic properties in laterodorsal tegmental neurons

Pharmacological studies of narcoleptic canines indicate that exaggerated pontine cholinergic transmission promotes cataplexy. As disruption of orexin (hypocretin) signaling is a primary defect in narcolepsy with cataplexy, we investigated whether markers of cholinergic synaptic transmission might be altered in mice constitutively lacking orexin receptors (double receptor knockout; DKO). mRNA for Choline acetyltransferase (ChAT), vesicular acetylcholine transporter (VAChT) and the high-affinity choline transporter (CHT1) but not acetylcholinesterase (AChE) was significantly higher in samples from DKO than wild-type (WT) mice. This was region-specific; levels were elevated in samples from the laterodorsal tegmental nucleus (LDT) and the fifth motor nucleus (Mo5) but not in whole brainstem samples. Consistent with region-specific changes, we were unable to detect significant differences in Western blots for ChAT and CHT1 in isolates from brainstem, thalamus and cortex or in ChAT enzymatic activity in the pons. However, using ChAT immunocytochemistry, we found that while the number of cholinergic neurons in the LDT and Mo5 were not different, the intensity of somatic ChAT immunostaining was significantly greater in the LDT, but not Mo5, from DKO than from WT mice. We also found that ChAT activity was significantly reduced in cortical samples from DKO compared with WT mice. Collectively, these findings suggest that the orexins can regulate neurotransmitter expression and that the constitutive absence of orexin signaling results in an up-regulation of the machinery necessary for cholinergic neurotransmission in a mesopontine population of neurons that have been associated with both normal rapid eye movement sleep and cataplexy.

Potentiation of a functional autoantibody in narcolepsy by a cholinesterase inhibitor

We have recently reported the presence of an immunoglobulin G (IgG) autoantibody (Ab) in patients with narcolepsy with cataplexy that abolishes spontaneous colonic migrating motor complexes (CMMCs) and increases smooth muscle tension and atropine-sensitive phasic contractions in a physiological assay of an isolated colon. In this study, we used the cholinesterase inhibitor, neostigmine, to explore the mechanism of the narcoleptic IgG-mediated disruption of enteric motor function in four patients with narcolepsy with cataplexy and to identify a pharmacological mimic of the Ab. Neostigmine potentiated the narcoleptic IgG-mediated increase in smooth muscle resting tension and phasic smooth muscle contractions by an atropine-sensitive mechanism but exerted no effect on resting tension in the presence of control IgG. Decreased frequency of CMMCs mediated by IgG with anti-M3R activity was reversed by neostigmine. Therefore, a challenge with a cholinesterase inhibitor improves the specificity of the CMMC assay for narcoleptic IgG. Tetrodotoxin (TTX), a neuronal sodium channel blocker, also abolished CMMCs and increased resting tone, and a similar potentiation was observed with neostigmine; thus, TTX is a mimic of the functional effects of the narcoleptic IgG in this bioassay. These findings provide a link to pharmacological studies of canine narcolepsy and are consistent with a functional blockade of both excitatory and inhibitory motor neurons by the narcoleptic Ab, similar to the TTX mimic, presumably by binding to an autoantigenic target expressed in both populations of neurons.

Animal models of narcolepsy

Narcolepsy is a debilitating sleep disorder with excessive daytime sleepiness and cataplexy as its two major symptoms. Although this disease was first described about one century ago, an animal model was not available until the 1970s. With the establishment of the Stanford canine narcolepsy colony, researchers were able to conduct multiple neurochemical studies to explore the pathophysiology of this disease. It was concluded that there was an imbalance between monoaminergic and cholinergic systems in canine narcolepsy. In 1999, two independent studies revealed that orexin neurotransmission deficiency was pivotal to the development of narcolepsy with cataplexy. This scientific leap fueled the generation of several genetically engineered mouse and rat models of narcolepsy. To facilitate further research, it is imperative that researchers reach a consensus concerning the evaluation of narcoleptic behavioral and EEG phenomenology in these models.

A consensus definition of cataplexy in mouse models of narcolepsy

People with narcolepsy often have episodes of cataplexy, brief periods of muscle weakness triggered by strong emotions. Many researchers are now studying mouse models of narcolepsy, but definitions of cataplexy-like behavior in mice differ across labs. To establish a common language, the International Working Group on Rodent Models of Narcolepsy reviewed the literature on cataplexy in people with narcolepsy and in dog and mouse models of narcolepsy and then developed a consensus definition of murine cataplexy. The group concluded that murine cataplexy is an abrupt episode of nuchal atonia lasting at least 10 seconds. In addition, theta activity dominates the EEG during the episode, and video recordings document immobility. To distinguish a cataplexy episode from REM sleep after a brief awakening, at least 40 seconds of wakefulness must precede the episode. Bouts of cataplexy fitting this definition are common in mice with disrupted orexin/hypocretin signaling, but these events almost never occur in wild type mice. It remains unclear whether murine cataplexy is triggered by strong emotions or whether mice remain conscious during the episodes as in people with narcolepsy. This working definition provides helpful insights into murine cataplexy and should allow objective and accurate comparisons of cataplexy in future studies using mouse models of narcolepsy.

Modulation of cortical activation and behavioral arousal by cholinergic and orexinergic systems

Multiple neuronal systems contribute to the promotion and maintenance of the wake state, which is characterized by cortical activation and behavioral arousal. Using predominantly glutamate as a neurotransmitter, neurons within the reticular formation of the brainstem give rise to either ascending projections into the forebrain or descending projections into the spinal cord to promote through relays fast cortical activity or motor activity with postural muscle tone. Using acetylcholine, cholinergic neurons in the brainstem project to forebrain relays and others in the basal forebrain to the cortex, by which they stimulate fast gamma activity during waking and during rapid eye movement or paradoxical sleep (PS). Other neuromodulatory systems, such as noradrenergic locus coeruleus neurons, give rise to highly diffuse projections through brain and spinal cord and simultaneously stimulate cortical activation and behavioral arousal. Although such neuromodulatory systems were thought to be redundant, a recently discovered peptide called orexin (Orx) or hypocretin, contained in diffusely projecting neurons of the hypothalamus, was found to be essential for the maintenance of waking with muscle tone, since in its absence narcolepsy with cataplexy occurred. Orx neurons discharge during active waking and cease firing during sleep. Since cholinergic neurons discharge during waking and PS, they would stimulate cortical activation in association with muscle tone when orexinergic neurons are also active but would stimulate cortical activation with muscle atonia when orexinergic neurons are silent, as in natural PS, or absent, as in pathological narcolepsy with cataplexy.

Clinical and neurobiological aspects of narcolepsy

Narcolepsy is characterized by excessive daytime sleepiness (EDS), cataplexy and/or other dissociated manifestations of rapid eye movement (REM) sleep (hypnagogic hallucinations and sleep paralysis). Narcolepsy is currently treated with amphetamine-like central nervous system (CNS) stimulants (for EDS) and antidepressants (for cataplexy). Some other classes of compounds such as modafinil (a non-amphetamine wake-promoting compound for EDS) and gamma-hydroxybutyrate (GHB, a short-acting sedative for EDS/fragmented nighttime sleep and cataplexy) given at night are also employed. The major pathophysiology of human narcolepsy has been recently elucidated based on the discovery of narcolepsy genes in animals. Using forward (i.e., positional cloning in canine narcolepsy) and reverse (i.e., mouse gene knockout) genetics, the genes involved in the pathogenesis of narcolepsy (hypocretin/orexin ligand and its receptor) in animals have been identified. Hypocretins/orexins are novel hypothalamic neuropeptides also involved in various hypothalamic functions such as energy homeostasis and neuroendocrine functions. Mutations in hypocretin-related genes are rare in humans, but hypocretin-ligand deficiency is found in many narcolepsy-cataplexy cases. In this review, the clinical, pathophysiological and pharmacological aspects of narcolepsy are discussed.

Reduced expression of TAC1, PENK and SOCS2 in Hcrtr-2 mutated narcoleptic dog brain

Background

Narcolepsy causes dramatic behavioral alterations in both humans and dogs, with excessive sleepiness and cataplexy triggered by emotional stimuli. Deficiencies in the hypocretin system are well established as the origin of the condition; both from studies in humans who lack the hypocretin ligand (HCRT) and in dogs with a mutation in hypocretin receptor 2 (HCRTR2). However, little is known about molecular alterations downstream of the hypocretin signals.

Results

By using microarray technology we have screened the expression of 29760 genes in the brains of Doberman dogs with a heritable form of narcolepsy (homozygous for the canarc-1 [HCRTR-2-2] mutation), and their unaffected heterozygous siblings. We identified two neuropeptide precursor molecules, Tachykinin precursor 1 (TAC1) and Proenkephalin (PENK), that together with Suppressor of cytokine signaling 2 (SOCS2), showed reduced expression in narcoleptic brains. The difference was particularly pronounced in the amygdala, where mRNA levels of PENK were 6.2 fold lower in narcoleptic dogs than in heterozygous siblings, and TAC1 and SOCS2 showed 4.4 fold and 2.8 fold decrease in expression, respectively. The results obtained from microarray experiments were confirmed by real-time RT-PCR. Interestingly, it was previously shown that a single dose of amphetamine-like stimulants able to increase wakefulness in the dogs, also produce an increase in the expression of both TAC1 and PENK in mice.

Conclusion

These results suggest that TAC1, PENK and SOCS2 might be intimately connected with the excessive daytime sleepiness not only in dogs, but also in other species, possibly including humans.

Orexinergic projections to the cat midbrain mediate alternation of emotional behavioural states from locomotion to cataplexy

Orexinergic neurones in the perifornical lateral hypothalamus project to structures of the midbrain, including the substantia nigra and the mesopontine tegmentum. These areas contain the mesencephalic locomotor region (MLR), and the pedunculopontine and laterodorsal tegmental nuclei (PPN/LDT), which regulate atonia during rapid eye movement (REM) sleep. Deficiencies of the orexinergic system result in narcolepsy, suggesting that these projections are concerned with switching between locomotor movements and muscular atonia. The present study characterizes the role of these orexinergic projections to the midbrain. In decerebrate cats, injecting orexin-A (60 microm to 1.0 mm, 0.20-0.25 microl) into the MLR reduced the intensity of the electrical stimulation required to induce locomotion on a treadmill (4 cats) or even elicit locomotor movements without electrical stimulation (2 cats). On the other hand, when orexin was injected into either the PPN (8 cats) or the substantia nigra pars reticulata (SNr, 4 cats), an increased stimulus intensity at the PPN was required to induce muscle atonia. The effects of orexin on the PPN and the SNr were reversed by subsequently injecting bicuculline (5 mm, 0.20-0.25 microl), a GABA(A) receptor antagonist, into the PPN. These findings indicate that excitatory orexinergic drive could maintain a higher level of locomotor activity by increasing the excitability of neurones in the MLR, while enhancing GABAergic effects on presumably cholinergic PPN neurones, to suppress muscle atonia. We conclude that orexinergic projections from the hypothalamus to the midbrain play an important role in regulating motor behaviour and controlling postural muscle tone and locomotor movements when awake and during sleep. Furthermore, as the excitability is attenuated in the absence of orexin, signals to the midbrain may induce locomotor behaviour when the orexinergic system functions normally but elicit atonia or narcolepsy when the orexinergic function is disturbed.

[The neurophysiology of cataplexy]

Cataplexy and excessive daytime sleepiness are the leading symptoms of narcolepsy. Electrophysiological studies in humans do not show a clear association between cataplexy and rapid eye movement (REM) sleep. Even a decrement of the H reflex is not specific for cataplexy and may be caused by unspecific triggers such as coughing. Cholinomimetics, which may induce status cataplecticus, do not influence REM sleep, thus evidencing a REM-independent mechanism. Recent studies demonstrate a lack of the neuropeptide hypocretin in the CSF of narcoleptics. Hypocretin controls wakefulness and the motor and autonomous systems. In hypocretin-1 and -2 knockout mice, sudden stops of motor activity could be observed in emotional situations that were accompanied by sudden shifts from wakefulness to REM sleep and could be terminated by application of anticataplectic medication. The lack of hypocretin not only causes a noradrenergic-cholinergic imbalance in the midbrain but also influences motoneurons directly by juxtacellular hypocretin-containing membranes. Intravenous application of hypocretin in a dog with hypocretin deficiency in the CSF caused a dose-dependent decrease of cataplexies. An understanding of the neuronal mechanisms responsible for cataplexies is essential for the development of new anticataplectic medications.

Clinical aspects and pathophysiology of narcolepsy

Narcolepsy is a chronic debilitating sleep disorder first described in the late 19th century. It is characterized by two major symptoms, excessive daytime sleepiness and cataplexy, and two so-called auxiliary symptoms, hypnagogic hallucinations and sleep paralysis. The final diagnosis relies on polysomnography showing the presence of sleep onset rapid eye movement periods (SOREMPs) during the multiple sleep latency test. The presence of HLA DQA1*0102-DQB1*0602 is supportive of the diagnosis. The pathophysiology of the disorder is still unknown but an imbalance between monoamines and acetylcholine is generally accepted. Recent findings in narcoleptic dogs, a natural model of narcolepsy, and in knockout mice revealed that a mutation of type 2 hypocretin receptor plays a major role in the etiology of narcolepsy. Up to now, no mutation has been found in humans except a case of early onset and atypical narcolepsy. However, a marked reduction of hypocretin type 1 has been found in the cerebrospinal fluid (CSF) of a majority of patients and a global loss of hypocretins was noted in post-mortem brain tissue of narcoleptic subjects. Conversely, no hypocretin neuron degeneration has been observed in the genetic form of narcolepsy in dogs but no trace of hypocretin was seen in the brain or the CSF in cases of sporadic canine narcolepsy. This suggests that different hypocretinergic mechanisms are involved in sporadic and genetic forms of canine narcolepsy. Treatment has not evolved significantly over the last few years. However, new drugs, such as hypocretin agonists, are currently being developed.

SIGNIFICANCE

After the discovery of the type 2 hypocretin receptor mutation in canine narcolepsy and the finding of a CSF hypocretin-1 deficiency in human narcolepsy, the major stream of research has involved the hypocretinergic system. However, other lines of research deserve to be pursued simultaneously, in view of comprehensive advancements in the understanding of narcolepsy.

Pharmacogenomics in the treatment of narcolepsy

Narcolepsy is a neurological disorder characterized by excessive daytime sleepiness and cataplexy. Available treatments of narcolepsy include stimulants and antidepressants but the recent discovery of orexin/hypocretin deficiency in narcolepsy opens up new perspectives. Narcolepsy is a complex disorder involving genetic, immune and environmental factors. Although only a strong association is found with the HLA DQB1*0602 gene, other genetic susceptibility factors might be involved. Among these, the functional polymorphism of the catechol-O-methyltransferase (COMT) gene is critically involved in the severity of narcolepsy and in the response to the stimulant modafinil. Other pharmacogenetic targets include the orexinergic, noradrenergic and possibly the serotonergic pathways.

Direct and indirect excitation of laterodorsal tegmental neurons by Hypocretin/Orexin peptides: implications for wakefulness and narcolepsy

Compelling evidence links the recently discovered hypothalamic peptides Hypocretin/Orexin (Hcrt/Orx) to rapid eye movement sleep (REM) control and the sleep disorder narcolepsy, yet how they influence sleep-related systems is not well understood. We investigated the action of Hcrt/Orx on mesopontine cholinergic (MPCh) neurons of the laterodorsal tegmental nucleus (LDT), a target group whose function is altered in canine narcolepsy and appears pivotal for normal REM and wakefulness. Extracellular recordings from mouse brainstem slices revealed that Hcrt/Orx evoked prolonged firing of LDT neurons. Whole-cell recordings revealed that Hcrt/Orx had actions on both presynaptic neurons and at postsynaptic sites. Hcrt/Orx produced an increase in frequency and amplitude of spontaneous EPSCs without equivalent effect on IPSCs, by triggering action potentials and enhancing spike-evoked synaptic transmission in glutamatergic afferents. Postsynaptically, Hcrt/Orx produced an inward current and an increase in membrane current noise, which were accompanied by a conductance increase. These persisted in TTX, ionotropic glutamate receptor antagonists, and low extracellular calcium. Both presynaptic and postsynaptic actions were specific because they were not mimicked by an Hcrt/Orx fragment, and both actions were observed for cholinergic and noncholinergic LDT neurons. Finally, extracellular recordings during postsynaptic potential blockade demonstrated that postsynaptic actions of Hcrt/Orx alone could evoke prolonged firing. In the context of other recent work, our findings suggest that Hcrt/Orx neurons may coordinate the activity of the entire reticular activating system during waking. Moreover, these findings address specific hypotheses regarding the cellular mechanisms underlying REM disregulation in narcolepsy.

Rat sleep and eye movement density as biological markers of demyelinating disease

Myelin mutants provide an opportunity to study neurophysiological and behavioral effects of demyelination. The taiep rats are myelin mutants with progressive demyelination of the central nervous system (CNS), resulting in five neurological symptoms: tremor, ataxia, immobility, epilepsy, and paralysis. The demyelination affects the brainstem, an important area in the control of sleep. This study compared eye movement density (EMD) in taiep vs. normal control rats during paradoxical sleep (PS). It was hypothesized that taiep rats would have significantly reduced EMD during PS in comparison to normal controls due to their demyelinating disease. In addition, demyelination of brainstem structures would suggest possible changes in sleep-wake structure. Hence, we compared sleep-wake stages in taiep vs. normal, control rats. The results confirmed significantly reduced EMD during PS in taiep rats compared to normal rats during the 12-h (light) recording period. In addition, analysis of EMD values across the 12-h light period revealed significant differences in EMD values as a function of time of day in the taeip rats only. Comparison of waking and sleep values across the 12-h light phase revealed an "immobility episode" in three taiep rats, which was not present in normal controls. In addition, PS percentage was significantly lower and low-voltage sleep was significantly higher in taiep rats. These results suggest that EMD, immobility episodes, and sleep architecture may be useful as measurable biological events in the study of demyelinating disease. The results were discussed in terms of possible mechanisms underlying these differences, as well as possible implications for future studies.

Narcolepsy: genetic predisposition and neuropharmacological mechanisms. REVIEW ARTICLE

Narcolepsy is a disabling sleep disorder characterized by excessive daytime somnolence (EDS), cataplexy and REM sleep-related abnormalities. It is a frequently-occurring but under-diagnosed condition that affects 0.02 to 0.18% of the general population in various countries. Although most cases occur sporadically, familial clustering may be observed; the risk of a first-degree relative of a narcoleptic developing narcolepsy is 10-40 times higher than in the general population. The disorder is tightly associated with the specific human leukocyte antigen (HLA) allele, DQB1*0602 [most often in combination with HLA-DR2 (DRB1*15)]. Genetic transmission is, however, likely to be polygenic in most cases, and genetic factors other than HLA-DQ are also likely to be implicated. In addition, environmental factors are involved in disease predisposition; most monozygotic twins pairs reported in the literature are discordant for narcolepsy. Narcolepsy was reported to exist in canines in the early 1970s. Both sporadic and familial cases are also observed in this animal species. A highly-penetrant single autosomal recessive gene, canarc-1, is involved in the transmission of narcolepsy in Doberman pinschers and Labrador retrievers. Positional cloning of this gene is in progress, and a human homologue of this gene, or a gene with a functional relationship to canarc-1, might be involved in some human cases. Human narcolepsy is currently treated with central nervous system (CNS) stimulants for EDS and antidepressants for cataplexy and abnormal REM sleep. These treatments are purely symptomatic and induce numerous side effects. These compounds disturb nocturnal sleep in many patients, and tolerance may develop as a result of continuous treatment. The canine model is an invaluable resource for studying the pharmacological and physiological control of EDS and cataplexy. Experiments using canine narcolepsy have demonstrated that increased cholinergic and decreased monoaminergic transmission are likely to be at the basis of the pathophysiology of the disorder. Pharmacological studies have shown that blockade of norepinephrine uptake mediates the anticataplectic effect of currently prescribed antidepressants, while blockade of dopamine uptake and/or stimulation of dopamine release mediates the awake-promoting effect of CNS stimulants. Studies in canine narcolepsy also suggest that mechanisms and brain sites for triggering cataplexy are not identical to those regulating REM sleep. It may thus be possible to develop new pharmacological compounds that specifically target abnormal symptoms in narcolepsy, but do not disturb physiological sleep/wake cycles. (See also postscript remarks).

Neuronal activity in the cholinoceptive basal forebrain of freely moving narcoleptic dobermans

Cholinergic stimulation in the basal forebrain (BF) triggers cataplexy in canine narcolepsy. Extracellular single unit recordings in the BF were carried out in freely moving narcoleptic dogs to study the neuronal mechanisms mediating cataplexy induction in the BF. Among the 64 recorded neurons, 12 were wake-active, three were slow wave sleep (SWS)-active, 17 were wake-/REM-active, 11 were REM sleep-active, three were cataplexy-active, and the other 18 were state-independent. Systemic administration of physostigmine, a cholinesterase inhibitor, induces status cataplecticus, decreases SWS and increases acetylcholine levels in the BF. Firing of most of the state-dependent neurons in the BF was significantly modified by physostigmine. Some of these neurons may thus mediate sleep stage changes or the effect on cataplexy observed after cholinergic stimulation in the BF.

Neuropharmacological characterization of basal forebrain cholinergic stimulated cataplexy in narcoleptic canines

Basal forebrain (BF) cholinergic regulation of cataplexy was investigated in narcoleptic canines. Specific cholinergic agonists and antagonists, and excitatory or inhibitory amino acid neurotransmitter receptor agonists, were perfused through microdialysis probes implanted bilaterally in the BF of narcoleptic canines. Cataplexy was monitored using the food-elicited cataplexy test (FECT) and recordings of electroencephalogram, electrooculogram, and electromyogram. In narcoleptic canines, carbachol and oxotremorine (10(-5)-10(-3) M), but not McN-A-343 or nicotine (10(-4)-10(-3) M), produced a dose-dependent increase in cataplexy. In addition, N-methyl-d-aspartate (10(-4)-10(-3) M) and kainic acid (10(-5)-10(-4) M) did not have any effects, while muscimol (10(-3) M) produced a weak (P < 0.10) increase in cataplexy. In control canines, carbachol (10(-5)-10(-3) M), but not oxotremorine (10(-4)-10(-3) M), produced muscle atonia after the highest concentration in one of three animals. Carbachol (10(-3) M)-induced cataplexy in narcoleptic canines was blocked by equimolar perfusion with the muscarinic antagonists atropine, gallamine, and 4-DAMP but not pirenzepine. These findings indicate that carbachol-stimulated cataplexy in the BF of narcoleptic canines is mediated by M2, and perhaps M3, muscarinic receptors. The release of acetylcholine in the BF was also examined during FECT and non-FECT behavioral stimulation in narcoleptic and control canines. A significant increase in acetylcholine release was found in both narcoleptic and control BF during FECT stimulation. In contrast, simple motor activity and feeding, approximating that which occurs during an FECT, did not affect acetylcholine release in the BF of narcoleptic canines. These findings indicate that BF acetylcholine release is enhanced during learned emotion/reward associated behaviors in canines.

Neurobiology of Narcolepsy

Narcolepsy is characterized by excessive daytime sleepiness and abnormal rapid eye movement sleep. It affects about 0.05% of the Caucasian population. Human narcolepsy involves the interaction of environmental factors with a specific immunogenetic background. It is tightly associated with a major histocompatibility complex allele, human leukocyte antigen (HLA) DQB1*0602. Genetic factors other than HLA are also involved. In contrast, narcolepsy in Dobermans is transmitted as a single autosomal recessive trait. This canine narcolepsy gene is unlinked to the major histocompatibility complex class II but co-segregates with a DNA segment with high homology to the human immunoglobulin μ-switch sequence, further suggesting immunopathology in narcolepsy. However, attempts to demonstrate that narcolepsy is an autoimmune disease have been unsuccessful. Narcolepsy is treated with antidepressants for rapid eye movement sleep-related symptoms and with amphetamine-like stimulants for sleepiness. Pharmacological studies using narcoleptic canines indicate that monoaminergic and cholinergic systems are involved in the pathophysiology of narcolepsy. Dopaminergic uptake mechanisms and D2(3) autoreceptors are involved in the control of alertness, whereas adrenergic uptake mechanisms, α-1 and α-2/dopaminergic D2(3) receptors, are involved in the control of cataplexy, suggesting that amphetamine-like stimulants act via the dopaminergic system and that antidepressants exhibit their anticataplectic effects via the adrenergic system. Local drug perfusion studies indicate that D2(3) agonists in the ventral tegmental area induce cataplexy and sleepiness in narcoleptic dogs but not in control dogs. Furthermore, perfusion of M2 agonists in the pontine reticular formation and the basal forebrain induces cataplexy in narcoleptic dogs. Extracellular single-unit and acetylcholine measurement studies suggest that basal forebrain cholinoceptive sites mediate the emotional trigger for cataplexy. Although narcolepsy does not seem to be a classical autoimmune disease, concomitant increases in microglial HLA class II expression with the development of the disease occur in canine narcolepsy. A neuroimmune-related process at an early age is thus likely to contribute to the neurochemical imbalance seen in narcolepsy. NEUROSCIENTIST 4:133–143, 1998

Effects of thyrotropin-releasing hormone and its analogs on daytime sleepiness and cataplexy in canine narcolepsy

The therapeutic potential of thyrotropin-releasing hormone (TRH) and TRH analogs in narcolepsy, a sleep disorder characterized by abnormal rapid eye movement (REM) sleep and daytime sleepiness, was examined using the canine model. The effects of TRH and the biologically stable TRH analogs CG3703, CG3509, and TA0910 on daytime sleep and cataplexy, a symptom of abnormal REM sleep, were assessed using polysomnographic recordings and the food elicited cataplexy test (FECT), respectively. CG3703 (100 and 400 microg/kg, i.v.) and TA0910 (100 and 400 microg/kg, i.v.) significantly increased wakefulness and decreased sleep in narcoleptic canines, whereas TRH (400 and 1600 microg/kg, i.v.) had no significant effect. TRH (25-1600 microg/kg, i.v.) and all three TRH analogs, CG3703 (6. 25-400 microg/kg, i.v., and 0.25-16 mg/kg, p.o.), CG3509 (25-1600 microg/kg, i.v.), and TA0910 (25-1600 microg/kg, i.v.), significantly reduced cataplexy in canine narcolepsy. These compounds did not produce any significant side effects during behavioral assays, nor did they alter free T3 and T4 levels in serum even when used at doses that completely suppressed cataplexy. Although more work is needed to establish the mode of action of TRH analogs on alertness and REM sleep-related symptoms, our results suggest a possible therapeutic application for TRH analogs in human sleep disorders.

Effects of thyrotropin-releasing hormone and its analogs on daytime sleepiness and cataplexy in canine narcolepsy

The therapeutic potential of thyrotropin-releasing hormone (TRH) and TRH analogs in narcolepsy, a sleep disorder characterized by abnormal rapid eye movement (REM) sleep and daytime sleepiness, was examined using the canine model. The effects of TRH and the biologically stable TRH analogs CG3703, CG3509, and TA0910 on daytime sleep and cataplexy, a symptom of abnormal REM sleep, were assessed using polysomnographic recordings and the food elicited cataplexy test (FECT), respectively. CG3703 (100 and 400 microg/kg, i.v.) and TA0910 (100 and 400 microg/kg, i.v.) significantly increased wakefulness and decreased sleep in narcoleptic canines, whereas TRH (400 and 1600 microg/kg, i.v.) had no significant effect. TRH (25-1600 microg/kg, i.v.) and all three TRH analogs, CG3703 (6. 25-400 microg/kg, i.v., and 0.25-16 mg/kg, p.o.), CG3509 (25-1600 microg/kg, i.v.), and TA0910 (25-1600 microg/kg, i.v.), significantly reduced cataplexy in canine narcolepsy. These compounds did not produce any significant side effects during behavioral assays, nor did they alter free T3 and T4 levels in serum even when used at doses that completely suppressed cataplexy. Although more work is needed to establish the mode of action of TRH analogs on alertness and REM sleep-related symptoms, our results suggest a possible therapeutic application for TRH analogs in human sleep disorders.

Pharmacological aspects of human and canine narcolepsy

Narcolepsy-cataplexy is a disabling neurological disorder that affects 1/2000 individuals. The main clinical features of narcolepsy, excessive daytime sleepiness and symptoms of abnormal REM sleep (cataplexy, sleep paralysis, hypnagogic hallucinations) are currently treated using amphetamine-like compounds or modafinil and antidepressants. Pharmacological research in the area is facilitated greatly by the existence of a canine model of the disorder. The mode of action of these compounds involves presynaptic activation of adrenergic transmission for the anticataplectic effects of antidepressant compounds and presynaptic activation of dopaminergic transmission for the EEG arousal effects of amphetamine-like stimulants. The mode of action of modafmil is still uncertain, and other neurochemical systems may offer interesting avenues for therapeutic development. Pharmacological and physiological studies using the canine model have identified primary neurochemical and neuroanatomical systems that underlie the expression of abnormal REM sleep and excessive sleepiness in narcolepsy. These involve mostly the pontine and basal forebrain cholinergic, the pontine adrenergic and the mesolimbic and mesocortical dopaminergic systems. These studies confirm a continuing need for basic research in both human and canine narcolepsy, and new treatments that act directly at the level of the primary defect in narcolepsy might be forthcoming.

Mesopontine organization of cholinergic and catecholaminergic cell groups in the normal and narcoleptic dog

Canine narcolepsy is a unique experimental model of a human sleep disorder characterized by excessive daytime sleepiness and cataplexy. There is a consensus recognition of an imbalance between cholinergic and catecholaminergic systems in narcolepsy although the underlying mechanisms remain poorly understood. Possible substrates could be an abnormal organization, numbers and/or ratio of cholinergic to catecholaminergic cells in the brain of narcoleptic dogs. Therefore, we sought to characterize the corresponding neuronal populations in normal and narcoleptic dogs (Doberman Pinscher) by using choline acetyltransferase (ChAT), nicotinamide adenosine dinucleotide phosphate (NADPH)-diaphorase, tyrosine hydroxylase (TH), and dopamine beta-hydroxylase (DBH). Cholinergic cell groups were found in an area extending from the central to the gigantocellular tegmental field and the periventricular gray corresponding to the pedunculopontine tegmental nucleus (PPT), the laterodorsal tegmental nucleus (LDT), and the parabrachial nucleus. An almost perfect co-localization of ChAT and NADPH-diaphorase was also observed. Catecholaminergic cell groups detected included the ventral tegmental area, the substantia nigra, and the locus coeruleus nucleus (LC). The anatomical distribution of catecholaminergic neurons was unusual in the dog in two important aspects: i) TH- and/or DBH-immunoreactive neurons of the LC were found almost exclusively in the reticular formation and not within the periventricular gray, ii) very few, if any TH-positive neurons were found in the central gray and dorsal raphe. Quantitative analysis did not reveal any significant differences in the organization and the number of cells identified in the LDT, PPT, and LC of normal and narcoleptic dogs. Moreover, the cholinergic to catecholaminergic ratio was found identical in the two groups. In conclusion, the present results do not support the hypothesis that the neurochemical imbalance in narcolepsy could result from abnormal organization, numbers, or ratio of the corresponding neuronal populations.

Major histocompatibility class II molecules in the CNS: increased microglial expression at the onset of narcolepsy in canine model

Human narcolepsy is a neurological disorder known to be closely associated with HLA-DR2 and DQB1*0602. Because most autoimmune diseases are HLA-associated, a similar mechanism has been proposed for narcolepsy. However, neither systemic nor CNS evidence of an autoimmune abnormality has ever been reported. In this study, major histocompatibility (MHC) class I and class II expression was studied in the CNS of human and canine narcoleptics using immunohistochemistry and Northern analysis. Results indicated that canine narcolepsy is associated with a significant increase of MHC class II expression by the microglia. Moreover, the highest values were found between 3 and 8 months of age, strikingly concomitant to the development of narcolepsy in the canine model. In humans, class II expression was not found significantly different between control and narcoleptic subjects. This result could be explained by the old age of the subjects (69.86 +/- 5.31 and 68.36 +/- 4.74 years in narcoleptics and controls, respectively), because class II expression is significantly correlated with age in both humans and dogs. For the first time, this study demonstrated that the expression of MHC class II molecules in the CNS is age-dependent and that a consistent increase of their expression by the microglia might be critically involved in the development of narcolepsy.

Local administration of dopaminergic drugs into the ventral tegmental area modulates cataplexy in the narcoleptic canine

Cataplexy in the narcoleptic canine may be modulated by systemic administration of monoaminergic compounds. In the present study, we have investigated the effects of monoaminergic drugs on cataplexy in narcoleptic canines when perfused locally via microdialysis probes in the amygdala, globus pallidus/putamen, basal forebrain, pontine reticular formation and ventral tegmental area of narcoleptic and control Doberman pinchers. Cataplexy was quantified using the Food-Elicited Cataplexy Test and analyzed by electroencephalogram, electroculogram and electromyogram. Local perfusion with the monoaminergic agonist quinpirole, 7-OH-DPAT and BHT-920, into the ventral tegmental area produced a dose-dependent increase in cataplexy without significantly reducing basal muscle tone. Perfusion with the antagonist raclopride in the same structure produced a moderate reduction in cataplexy. Local perfusion with quinpirole, 7-OH-DPAT and BHT-920 into the globus pallidus/putamen also produced an increase, while raclopride produced a decrease, in cataplexy in narcoleptic canines. In control animals, none of the above drugs produced cataplexy or muscle atonia when perfused into either the ventral tegmental area or the globus pallidus/putamen. Other monoaminergic drugs tested in these two brain areas; prazosin, yohimbine, amphetamine, SKF 38393 and SCH 23390 had no effects on cataplexy. Local perfusion with each of the above listed drugs had no effect on cataplexy in any of the other brain regions examined. These findings show that cataplexy may be regulated by D2/D3 dopaminergic receptors in the ventral tegmental area and perhaps the globus pallidus/ putamen. It is suggested that neurons in the mesolimbic dopamine system of narcoleptics are hypersensitive to dopaminergic autoreceptor agonists.

Major histocompatibility class II molecules in the CNS: increased microglial expression at the onset of narcolepsy in canine model

Human narcolepsy is a neurological disorder known to be closely associated with HLA-DR2 and DQB1*0602. Because most autoimmune diseases are HLA-associated, a similar mechanism has been proposed for narcolepsy. However, neither systemic nor CNS evidence of an autoimmune abnormality has ever been reported. In this study, major histocompatibility (MHC) class I and class II expression was studied in the CNS of human and canine narcoleptics using immunohistochemistry and Northern analysis. Results indicated that canine narcolepsy is associated with a significant increase of MHC class II expression by the microglia. Moreover, the highest values were found between 3 and 8 months of age, strikingly concomitant to the development of narcolepsy in the canine model. In humans, class II expression was not found significantly different between control and narcoleptic subjects. This result could be explained by the old age of the subjects (69.86 +/- 5.31 and 68.36 +/- 4.74 years in narcoleptics and controls, respectively), because class II expression is significantly correlated with age in both humans and dogs. For the first time, this study demonstrated that the expression of MHC class II molecules in the CNS is age-dependent and that a consistent increase of their expression by the microglia might be critically involved in the development of narcolepsy.

Altered distribution of cholinergic cells in the narcoleptic dog

Narcolepsy is characterized by excessive sleepiness and episodes of cataplexy brought on by emotional excitation. Cataplexy and sleep paralysis have been hypothesized to be produced by the triggering during waking of brain stem cholinergic mechanisms normally acting to induce atonia in REM sleep. We hypothesized that narcoleptics have an abnormal number of LDT and/or PPN cholinergic neurons. A comparison was made of cholinergic cell numbers in the brain stems of normal and narcoleptic canines. Cholinergic neurons were identified by NADPH-diaphorase histochemistry. We found increased numbers of cholinergic neurons at the R6-R7 level of the LDT and PPN in narcoleptic canines. This abnormality can explain alterations in cholinergic receptor number, acetylcholine release, and the occurrence of cataplexy and sleep paralysis that characterize narcolepsy.

Cholinergic regulation of cataplexy in canine narcolepsy in the pontine reticular formation is mediated by M2 muscarinic receptors

Both rapid eye movement sleep and cataplexy in the narcoleptic canine have been shown to increase after both systemic and local administration of cholinergic agonists in the pontine reticular formation. Furthermore, binding studies indicate an increase in the number of M2 muscarinic receptors in the pontine reticular formation of narcoleptic canines. In the present study we have investigated the receptor subtypes involved in mediating the cholinergic stimulation of cataplexy, as defined by brief periods of hypotonia induced by emotions, within the pontine reticular formation of narcoleptic canines. Specific cholinergic and monoaminergic agonists and antagonists, and excitatory or inhibitory amino-acid neurotransmitter receptor agonists, were perfused through microdialysis probes implanted bilaterally in the pontine reticular formation of narcoleptic canines, and cataplexy was monitored using the Food-Elicited Cataplexy Test and recordings of electroencephalogram, electrooculogram and electromyogram. In narcoleptic canines, bilateral perfusion with oxotremorine (M2 muscarinic) (10(-5)-10(-3) M) in the pontine reticular formation produced a dose-dependent increase in cataplexy, which reached complete muscle atonia (status cataplecticus) during the highest concentration. In control canines bilateral perfusion with oxotremorine (10(-5)-10(-3) M) did not produce any cataplectic attacks, but did produce muscle atonia after the highest concentration. Bilateral perfusion with either McN-A-343 (M1 muscarinic) or nicotine (both 10(-5)-10(-3) M) did not have any effect on cataplexy in either narcoleptic or control canines. The increase in cataplexy in narcoleptic canines produced by local perfusion with carbachol (10(-4) M) followed by equimolar perfusion with a muscarinic antagonist was rapidly reversed by atropine (muscarinic) and gallamine (M2 muscarinic), partially reversed by 4-DAMP (M3/M1 muscarinic) and completely unaffected by pirenzepine (M1 muscarinic). Bilateral perfusion with excitatory, glutamatergic receptor agonists N-methyl-D-aspartate, AMPA (both at 10(-4)-10(-3) M) and kainic acid (10(-5)-10(-4) M) did not have any effect on cataplexy, whereas bilateral perfusion with the inhibitory GABAergic receptor agonist muscimol (10(-4)-10(-3) M) produced a moderate increase in cataplexy in the narcoleptic canines. Bilateral perfusion with numerous monoaminergic compounds, BHT-920 (alpha-2 agonist), yohimbine (alpha-2 antagonist), propranolol (beta antagonist) and prazosin (alpha-1 antagonist), did not have any effect on cataplexy. These findings demonstrate that cholinergic regulation of cataplexy in the narcoleptic canine at the level of the pontine reticular formation is mediated by M2, and possibly M3, muscarinic receptors. The effects of muscimol indicate that the stimulation of cataplexy might be elicited by local neuronal inhibition.

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)