Oral Orexin Receptor 2 Agonist in Narcolepsy Type 1.

BACKGROUND: Narcolepsy type 1 is caused by severe loss or lack of brain orexin neuropeptides. METHODS: We conducted a phase 2, randomized, placebo-controlled trial of TAK-994, an oral orexin receptor 2-selective agonist, in patients with narcolepsy type 1. Patients with confirmed narcolepsy type 1 according to clinical criteria were randomly assigned to receive twice-daily oral TAK-994 (30 mg, 90 mg, or 180 mg) or placebo. The primary end point was the mean change from baseline to week 8 in average sleep latency (the time it takes to fall asleep) on the Maintenance of Wakefulness Test (range, 0 to 40 minutes; normal ability to stay awake, ≥20 minutes). Secondary end points included the change in the Epworth Sleepiness Scale (ESS) score (range, 0 to 24, with higher scores indicating greater daytime sleepiness; normal, <10) and the weekly cataplexy rate. RESULTS: Of the 73 patients, 17 received TAK-994 at a dose of 30 mg twice daily, 20 received 90 mg twice daily, 19 received 180 mg twice daily, and 17 received placebo. The phase 2 trial and an extension trial were terminated early owing to hepatic adverse events. Primary end-point data were available for 41 patients (56%); the main reason for missing data was early trial termination. Least-squares mean changes to week 8 in average sleep latency on the MWT were 23.9 minutes in the 30-mg group, 27.4 minutes in the 90-mg group, 32.6 minutes in the 180-mg group, and -2.5 minutes in the placebo group (difference vs. placebo, 26.4 minutes in the 30-mg group, 29.9 minutes in the 90-mg group, and 35.0 minutes the 180-mg group; P<0.001 for all comparisons). Least-squares mean changes to week 8 in the ESS score were -12.2 in the 30-mg group, -13.5 in the 90-mg group, -15.1 in the 180-mg group, and -2.1 in the placebo group (difference vs. placebo, -10.1 in the 30-mg group, -11.4 in the 90-mg group, and -13.0 in the 180-mg group). Weekly incidences of cataplexy at week 8 were 0.27 in the 30-mg group, 1.14 in the 90-mg group, 0.88 in the 180-mg group, and 5.83 in the placebo group (rate ratio vs. placebo, 0.05 in the 30-mg group, 0.20 in the 90-mg group, and 0.15 in the 180-mg group). A total of 44 of 56 patients (79%) receiving TAK-994 had adverse events, most commonly urinary urgency or frequency. Clinically important elevations in liver-enzyme levels occurred in 5 patients, and drug-induced liver injury meeting Hy's law criteria occurred in 3 patients. CONCLUSIONS: In a phase 2 trial involving patients with narcolepsy type 1, an orexin receptor 2 agonist resulted in greater improvements on measures of sleepiness and cataplexy than placebo over a period of 8 weeks but was associated with hepatotoxic effects. (Funded by Takeda Development Center Americas; TAK-994-1501 and TAK-994-1504 ClinicalTrials.gov numbers, NCT04096560 and NCT04820842.).

Sleep modulates haematopoiesis and protects against atherosclerosis.

Sleep is integral to life1. Although insufficient or disrupted sleep increases the risk of multiple pathological conditions, including cardiovascular disease2, we know little about the cellular and molecular mechanisms by which sleep maintains cardiovascular health. Here we report that sleep regulates haematopoiesis and protects against atherosclerosis in mice. We show that mice subjected to sleep fragmentation produce more Ly-6Chigh monocytes, develop larger atherosclerotic lesions and produce less hypocretin-a stimulatory and wake-promoting neuropeptide-in the lateral hypothalamus. Hypocretin controls myelopoiesis by restricting the production of CSF1 by hypocretin-receptor-expressing pre-neutrophils in the bone marrow. Whereas hypocretin-null and haematopoietic hypocretin-receptor-null mice develop monocytosis and accelerated atherosclerosis, sleep-fragmented mice with either haematopoietic CSF1 deficiency or hypocretin supplementation have reduced numbers of circulating monocytes and smaller atherosclerotic lesions. Together, these results identify a neuro-immune axis that links sleep to haematopoiesis and atherosclerosis.

The neurobiological basis of narcolepsy.

Narcolepsy is the most common neurological cause of chronic sleepiness. The discovery about 20 years ago that narcolepsy is caused by selective loss of the neurons producing orexins (also known as hypocretins) sparked great advances in the field. Here, we review the current understanding of how orexin neurons regulate sleep-wake behaviour and the consequences of the loss of orexin neurons. We also summarize the developing evidence that narcolepsy is an autoimmune disorder that may be caused by a T cell-mediated attack on the orexin neurons and explain how these new perspectives can inform better therapeutic approaches.

Hyperarousal features in the sleep architecture of individuals with and without insomnia.

Sleep architecture encodes relevant information on the structure of sleep and has been used to assess hyperarousal in insomnia. This study investigated whether polysomnography-derived sleep architecture displays signs of hyperarousal in individuals with insomnia compared with individuals without insomnia. Data from Phase 3 clinical trials, private clinics and a cohort study were analysed. A comprehensive set of sleep architecture features previously associated with hyperarousal were retrospectively analysed focusing on sleep-wake transition probabilities, electroencephalographic spectra and sleep spindles, and enriched with a novel machine learning algorithm called the Wake Electroencephalographic Similarity Index. This analysis included 1710 individuals with insomnia and 1455 individuals without insomnia. Results indicate that individuals with insomnia had a higher likelihood of waking from all sleep stages, and showed increased relative alpha during Wake and N1 sleep and increased theta power during Wake when compared with individuals without insomnia. Relative delta power was decreased and Wake Electroencephalographic Similarity Index scores were elevated across all sleep stages except N3, suggesting more wake-like activity during these stages in individuals with insomnia. Additionally, sleep spindle density was decreased, and spindle dispersion was increased in individuals with insomnia. These findings suggest that insomnia is characterized by a dysfunction in sleep quality with a continuous hyperarousal, evidenced by changes in sleep-wake architecture.

Harriet Tubman's Hypersomnia: Insights from Historical and Medical Perspectives.

Harriet Tubman, a hero of the abolitionist movement and early civil rights advocate, suffered a head injury in childhood and subsequently developed sleep attacks associated with visions that were extensively documented in historical accounts. Her contemporaries perceived these visions together with unpredictable and unavoidable urges to sleep as manifestations of her deep faith, rather than as symptoms of an illness. While religious perspectives remain crucial to understanding Tubman's sleep-related experiences, some may consider them insufficient in view of modern medical advances. We propose the parallel explanation that her sleep attacks, usually attributed to temporal lobe epilepsy, actually represent a hypersomnia that is most consistent with the modern diagnosis of post-traumatic narcolepsy. Using historical analysis as well as current understandings of sleep medicine, we aim to shed light on this under-recognized aspect of Tubman's life. In addition, this case study allows us to review the potential long-term effects of severe traumatic brain injuries; consider a differential for excessive daytime sleepiness and hypnagogic hallucinations; and familiarize readers with the pathophysiology, diagnosis, and treatment of narcolepsy. Whether her symptoms are viewed through the lens of the past or measured against current biomedical standards, Tubman demonstrated an inspiring ability to persevere despite intrusive sleep episodes and to realize her dreams for the betterment of others.

Critical Dynamics and Coupling in Bursts of Cortical Rhythms Indicate Non-Homeostatic Mechanism for Sleep-Stage Transitions and Dual Role of VLPO Neurons in Both Sleep and Wake.

Origin and functions of intermittent transitions among sleep stages, including brief awakenings and arousals, constitute a challenge to the current homeostatic framework for sleep regulation, focusing on factors modulating sleep over large time scales. Here we propose that the complex micro-architecture characterizing sleep on scales of seconds and minutes results from intrinsic non-equilibrium critical dynamics. We investigate θ- and δ-wave dynamics in control rats and in rats where the sleep-promoting ventrolateral preoptic nucleus (VLPO) is lesioned (male Sprague-Dawley rats). We demonstrate that bursts in θ and δ cortical rhythms exhibit complex temporal organization, with long-range correlations and robust duality of power-law (θ-bursts, active phase) and exponential-like (δ-bursts, quiescent phase) duration distributions, features typical of non-equilibrium systems self-organizing at criticality. We show that such non-equilibrium behavior relates to anti-correlated coupling between θ- and δ-bursts, persists across a range of time scales, and is independent of the dominant physiologic state; indications of a basic principle in sleep regulation. Further, we find that VLPO lesions lead to a modulation of cortical dynamics resulting in altered dynamical parameters of θ- and δ-bursts and significant reduction in θ-δ coupling. Our empirical findings and model simulations demonstrate that θ-δ coupling is essential for the emerging non-equilibrium critical dynamics observed across the sleep-wake cycle, and indicate that VLPO neurons may have dual role for both sleep and arousal/brief wake activation. The uncovered critical behavior in sleep- and wake-related cortical rhythms indicates a mechanism essential for the micro-architecture of spontaneous sleep-stage and arousal transitions within a novel, non-homeostatic paradigm of sleep regulation.SIGNIFICANCE STATEMENT We show that the complex micro-architecture of sleep-stage/arousal transitions arises from intrinsic non-equilibrium critical dynamics, connecting the temporal organization of dominant cortical rhythms with empirical observations across scales. We link such behavior to sleep-promoting neuronal population, and demonstrate that VLPO lesion (model of insomnia) alters dynamical features of θ and δ rhythms, and leads to significant reduction in θ-δ coupling. This indicates that VLPO neurons may have dual role for both sleep and arousal/brief wake control. The reported empirical findings and modeling simulations constitute first evidences of a neurophysiological fingerprint of self-organization and criticality in sleep- and wake-related cortical rhythms; a mechanism essential for spontaneous sleep-stage and arousal transitions that lays the bases for a novel, non-homeostatic paradigm of sleep regulation.

Reassessing the Role of Histaminergic Tuberomammillary Neurons in Arousal Control.

The histaminergic neurons of the tuberomammillary nucleus (TMNHDC) of the posterior hypothalamus have long been implicated in promoting arousal. More recently, a role for GABAergic signaling by the TMNHDC neurons in arousal control has been proposed. Here, we investigated the effects of selective chronic disruption of GABA synthesis (via genetic deletion of the GABA synthesis enzyme, glutamic acid decarboxylase 67) or GABAergic transmission (via genetic deletion of the vesicular GABA transporter (VGAT)) in the TMNHDC neurons on sleep-wake in male mice. We also examined the effects of acute chemogenetic activation and optogenetic inhibition of TMNHDC neurons upon arousal in male mice. Unexpectedly, we found that neither disruption of GABA synthesis nor GABAergic transmission altered hourly sleep-wake quantities, perhaps because very few TMNHDC neurons coexpressed VGAT. Acute chemogenetic activation of TMNHDC neurons did not increase arousal levels above baseline but did enhance vigilance when the mice were exposed to a behavioral cage change challenge. Similarly, acute optogenetic inhibition had little effect upon baseline levels of arousal. In conclusion, we could not identify a role for GABA release by TMNHDC neurons in arousal control. Further, if TMNHDC neurons do release GABA, the mechanism by which they do so remains unclear. Our findings support the view that TMNHDC neurons may be important for enhancing arousal under certain conditions, such as exposure to a novel environment, but play only a minor role in behavioral and EEG arousal under baseline conditions.SIGNIFICANCE STATEMENT The histaminergic neurons of the tuberomammillary nucleus of the hypothalamus (TMNHDC) have long been thought to promote arousal. Additionally, TMNHDC neurons may counter-regulate the wake-promoting effects of histamine through co-release of the inhibitory neurotransmitter, GABA. Here, we show that impairing GABA signaling from TMNHDC neurons does not impact sleep-wake amounts and that few TMNHDC neurons contain the vesicular GABA transporter, which is presumably required to release GABA. We further show that acute activation or inhibition of TMNHDC neurons has limited effects upon baseline arousal levels and that activation enhances vigilance during a behavioral challenge. Counter to general belief, our findings support the view that TMNHDC neurons are neither necessary nor sufficient for the initiation and maintenance of arousal under baseline conditions.

Complementary roles of gasotransmitters CO and H2S in sleep apnea.

Sleep apnea, which is the periodic cessation of breathing during sleep, is a major health problem affecting over 10 million people in the United States and is associated with several sequelae, including hypertension and stroke. Clinical studies suggest that abnormal carotid body (CB) activity may be a driver of sleep apnea. Because gaseous molecules are important determinants of CB activity, aberrations in their signaling could lead to sleep apnea. Here, we report that mice deficient in heme oxygenase-2 (HO-2), which generates the gaseous molecule carbon monoxide (CO), exhibit sleep apnea characterized by high apnea and hypopnea indices during rapid eye movement (REM) sleep. Similar high apnea and hypopnea indices were also noted in prehypertensive spontaneously hypertensive (SH) rats, which are known to exhibit CB hyperactivity. We identified the gaseous molecule hydrogen sulfide (H2S) as the major effector molecule driving apneas. Genetic ablation of the H2S-synthesizing enzyme cystathionine-γ-lyase (CSE) normalized breathing in HO-2-/- mice. Pharmacologic inhibition of CSE with l-propargyl glycine prevented apneas in both HO-2-/- mice and SH rats. These observations demonstrate that dysregulated CO and H2S signaling in the CB leads to apneas and suggest that CSE inhibition may be a useful therapeutic intervention for preventing CB-driven sleep apnea.

Cholinergic, Glutamatergic, and GABAergic Neurons of the Pedunculopontine Tegmental Nucleus Have Distinct Effects on Sleep/Wake Behavior in Mice.

The pedunculopontine tegmental (PPT) nucleus has long been implicated in the regulation of cortical activity and behavioral states, including rapid eye-movement (REM) sleep. For example, electrical stimulation of the PPT region during sleep leads to rapid awakening, whereas lesions of the PPT in cats reduce REM sleep. Though these effects have been linked with the activity of cholinergic PPT neurons, the PPT also includes intermingled glutamatergic and GABAergic cell populations, and the precise roles of cholinergic, glutamatergic, and GABAergic PPT cell groups in regulating cortical activity and behavioral state remain unknown. Using a chemogenetic approach in three Cre-driver mouse lines, we found that selective activation of glutamatergic PPT neurons induced prolonged cortical activation and behavioral wakefulness, whereas inhibition reduced wakefulness and increased non-REM (NREM) sleep. Activation of cholinergic PPT neurons suppressed lower-frequency electroencephalogram rhythms during NREM sleep. Last, activation of GABAergic PPT neurons slightly reduced REM sleep. These findings reveal that glutamatergic, cholinergic, and GABAergic PPT neurons differentially influence cortical activity and sleep/wake states. SIGNIFICANCE STATEMENT: More than 40 million Americans suffer from chronic sleep disruption, and the development of effective treatments requires a more detailed understanding of the neuronal mechanisms controlling sleep and arousal. The pedunculopontine tegmental (PPT) nucleus has long been considered a key site for regulating wakefulness and REM sleep. This is mainly because of the cholinergic neurons contained in the PPT nucleus. However, the PPT nucleus also contains glutamatergic and GABAergic neurons that likely contribute to the regulation of cortical activity and sleep-wake states. The chemogenetic experiments in the present study reveal that cholinergic, glutamatergic, and GABAergic PPT neurons each have distinct effects on sleep/wake behavior, improving our understanding of how the PPT nucleus regulates cortical activity and behavioral states.

GABAergic Neurons of the Central Amygdala Promote Cataplexy.

Narcolepsy is characterized by chronic sleepiness and cataplexy-sudden muscle paralysis triggered by strong, positive emotions. This condition is caused by a lack of orexin (hypocretin) signaling, but little is known about the neural mechanisms that mediate cataplexy. The amygdala regulates responses to rewarding stimuli and contains neurons active during cataplexy. In addition, lesions of the amygdala reduce cataplexy. Because GABAergic neurons of the central nucleus of the amygdala (CeA) target brainstem regions known to regulate muscle tone, we hypothesized that these cells promote emotion-triggered cataplexy. We injected adeno-associated viral vectors coding for Cre-dependent DREADDs or a control vector into the CeA of orexin knock-out mice crossed with vGAT-Cre mice, resulting in selective expression of the excitatory hM3 receptor or the inhibitory hM4 receptor in GABAergic neurons of the CeA. We measured sleep/wake behavior and cataplexy after injection of saline or the hM3/hM4 ligand clozapine-N-oxide (CNO) under baseline conditions and under conditions that should elicit positive emotions. In mice expressing hM3, CNO approximately doubled the amount of cataplexy in the first 3 h after dosing under baseline conditions. Rewarding stimuli (chocolate or running wheels) also increased cataplexy, but CNO produced no further increase. In mice expressing hM4, CNO reduced cataplexy in the presence of chocolate or running wheels. These results demonstrate that GABAergic neurons of the CeA are sufficient and necessary for the production of cataplexy in mice, and they likely are a key part of the mechanism through which positive emotions trigger cataplexy.SIGNIFICANCE STATEMENT Cataplexy is one of the major symptoms of narcolepsy, but little is known about how strong, positive emotions trigger these episodes of muscle paralysis. Prior research shows that amygdala neurons are active during cataplexy and cataplexy is reduced by lesions of the amygdala. We found that cataplexy is substantially increased by selective activation of GABAergic neurons in the central nucleus of the amygdala (CeA). We also demonstrate that inhibition of these neurons reduces reward-promoted cataplexy. These results build upon prior work to establish the CeA as a crucial element in the neural mechanisms of cataplexy. These results demonstrate the importance of the CeA in regulating responses to rewarding stimuli, shedding light on the broader neurobiology of emotions and motor control.

Melanin-concentrating hormone neurons contribute to dysregulation of rapid eye movement sleep in narcolepsy.

The lateral hypothalamus contains neurons producing orexins that promote wakefulness and suppress REM sleep as well as neurons producing melanin-concentrating hormone (MCH) that likely promote REM sleep. Narcolepsy with cataplexy is caused by selective loss of the orexin neurons, and the MCH neurons appear unaffected. As the orexin and MCH systems exert opposing effects on REM sleep, we hypothesized that imbalance in this REM sleep-regulating system due to activity in the MCH neurons may contribute to the striking REM sleep dysfunction characteristic of narcolepsy. To test this hypothesis, we chemogenetically activated the MCH neurons and pharmacologically blocked MCH signaling in a murine model of narcolepsy and studied the effects on sleep-wake behavior and cataplexy. To chemoactivate MCH neurons, we injected an adeno-associated viral vector containing the hM3Dq stimulatory DREADD into the lateral hypothalamus of orexin null mice that also express Cre recombinase in the MCH neurons (MCH-Cre::OX-KO mice) and into control MCH-Cre mice with normal orexin expression. In both lines of mice, activation of MCH neurons by clozapine-N-oxide (CNO) increased rapid eye movement (REM) sleep without altering other states. In mice lacking orexins, activation of the MCH neurons also increased abnormal intrusions of REM sleep manifest as cataplexy and short latency transitions into REM sleep (SLREM). Conversely, a MCH receptor 1 antagonist, SNAP 94847, almost completely eliminated SLREM and cataplexy in OX-KO mice. These findings affirm that MCH neurons promote REM sleep under normal circumstances, and their activity in mice lacking orexins likely triggers abnormal intrusions of REM sleep into non-REM sleep and wake, resulting in the SLREM and cataplexy characteristic of narcolepsy.

Genetic identity of thermosensory relay neurons in the lateral parabrachial nucleus.

The parabrachial nucleus is important for thermoregulation because it relays skin temperature information from the spinal cord to the hypothalamus. Prior work in rats localized thermosensory relay neurons to its lateral subdivision (LPB), but the genetic and neurochemical identity of these neurons remains unknown. To determine the identity of LPB thermosensory neurons, we exposed mice to a warm (36°C) or cool (4°C) ambient temperature. Each condition activated neurons in distinct LPB subregions that receive input from the spinal cord. Most c-Fos+ neurons in these LPB subregions expressed the transcription factor marker FoxP2. Consistent with prior evidence that LPB thermosensory relay neurons are glutamatergic, all FoxP2+ neurons in these subregions colocalized with green fluorescent protein (GFP) in reporter mice for Vglut2, but not for Vgat. Prodynorphin (Pdyn)-expressing neurons were identified using a GFP reporter mouse and formed a caudal subset of LPB FoxP2+ neurons, primarily in the dorsal lateral subnucleus (PBdL). Warm exposure activated many FoxP2+ neurons within PBdL. Half of the c-Fos+ neurons in PBdL were Pdyn+, and most of these project into the preoptic area. Cool exposure activated a separate FoxP2+ cluster of neurons in the far-rostral LPB, which we named the rostral-to-external lateral subnucleus (PBreL). These findings improve our understanding of LPB organization and reveal that Pdyn-IRES-Cre mice provide genetic access to warm-activated, FoxP2+ glutamatergic neurons in PBdL, many of which project to the hypothalamus.

Damage to histaminergic tuberomammillary neurons and other hypothalamic neurons with traumatic brain injury.

The need for increased sleep after traumatic brain injury is a common and disabling complaint, yet its etiology is unknown. Previous studies have demonstrated diffuse damage to various hypothalamic systems, but the integrity of the histaminergic tuberomammillary nucleus, a major arousal-promoting system located in the posterior hypothalamus, has never been examined in head trauma patients. Here, we demonstrate that severe head trauma is associated with a marked loss (41%) of histaminergic neurons. Reduced histamine signaling may contribute to increased sleep need, and therapies that enhance histaminergic tone may improve arousal after head trauma or other conditions.

Optogenetic-mediated release of histamine reveals distal and autoregulatory mechanisms for controlling arousal.

Histaminergic neurons in the tuberomammillary nucleus (TMN) are an important component of the ascending arousal system and may form part of a "flip-flop switch" hypothesized to regulate sleep and wakefulness. Anatomical studies have shown that the wake-active TMN and sleep-active ventrolateral preoptic nucleus (VLPO) are reciprocally connected, suggesting that each region can inhibit its counterpart when active. In this study, we determined how histamine affects the two branches of this circuit. We selectively expressed channelrhodopsin-2 (ChR2) in TMN neurons and used patch-clamp recordings in mouse brain slices to examine the effects of photo-evoked histamine release in the ventrolateral TMN and VLPO. Photostimulation decreased inhibitory GABAergic inputs to the ventrolateral TMN neurons but produced a membrane hyperpolarization and increased inhibitory synaptic input to the VLPO neurons. We found that in VLPO the response to histamine was indirect, most likely via a GABAergic interneuron. Our experiments demonstrate that release of histamine from TMN neurons can disinhibit the TMN and suppresses the activity of sleep-active VLPO neurons to promote TMN neuronal firing. This further supports the sleep-wake "flip-flop switch" hypothesis and a role for histamine in stabilizing the switch to favor wake states.

The roles of prostaglandin E2 and D2 in lipopolysaccharide-mediated changes in sleep.

When living organisms become sick as a result of a bacterial infection, a suite of brain-mediated responses occur, including fever, anorexia and sleepiness. Systemic administration of lipopolysaccharide (LPS), a common constituent of bacterial cell walls, increases body temperature and non-rapid eye movement (NREM) sleep in animals and induces the production of pro-inflammatory prostaglandins (PGs). PGE2 is the principal mediator of fever, and both PGE2 and PGD2 regulate sleep-wake behavior. The extent to which PGE2 and PGD2 are involved in the effect of LPS on NREM sleep remains to be clarified. Therefore, we examined LPS-induced changes in body temperature and NREM sleep in mice with nervous system-specific knockouts (KO) for the PGE2 receptors type EP3 or EP4, in mice with total body KO of microsomal PGE synthase-1 or the PGD2 receptor type DP, and in mice treated with the cyclooxygenase (COX) inhibitor meloxicam. We observed that LPS-induced NREM sleep was slightly attenuated in mice lacking EP4 receptors in the nervous system, but was not affected in any of the other KO mice or in mice pretreated with the COX inhibitor. These results suggest that the effect of LPS on NREM sleep is partially dependent on PGs and is likely mediated mainly by other pro-inflammatory substances. In addition, our data show that the main effect of LPS on body temperature is hypothermia in the absence of nervous system EP3 receptors or in the presence of a COX inhibitor.

Amygdala lesions reduce cataplexy in orexin knock-out mice.

Narcolepsy is characterized by excessive sleepiness and cataplexy, sudden episodes of muscle weakness during waking that are thought to be an intrusion of rapid eye movement sleep muscle atonia into wakefulness. One of the most striking aspects of cataplexy is that it is often triggered by strong, generally positive emotions, but little is known about the neural pathways through which positive emotions trigger muscle atonia. We hypothesized that the amygdala is functionally important for cataplexy because the amygdala has a role in processing emotional stimuli and it contains neurons that are active during cataplexy. Using anterograde and retrograde tracing in mice, we found that GABAergic neurons in the central nucleus of the amygdala heavily innervate neurons that maintain waking muscle tone such as those in the ventrolateral periaqueductal gray, lateral pontine tegmentum, locus ceruleus, and dorsal raphe. We then found that bilateral, excitotoxic lesions of the amygdala markedly reduced cataplexy in orexin knock-out mice, a model of narcolepsy. These lesions did not alter basic sleep-wake behavior but substantially reduced the triggering of cataplexy. Lesions also reduced the cataplexy events triggered by conditions associated with high arousal and positive emotions (i.e., wheel running and chocolate). These observations demonstrate that the amygdala is a functionally important part of the circuitry underlying cataplexy and suggest that increased amygdala activity in response to emotional stimuli could directly trigger cataplexy by inhibiting brainstem regions that suppress muscle atonia.

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.

Orexin receptors: pharmacology and therapeutic opportunities.

Orexin-A and -B (also known as hypocretin-1 and -2) are neuropeptides produced in the lateral hypothalamus that promote many aspects of arousal through the OX1 and OX2 receptors. In fact, they are necessary for normal wakefulness, as loss of the orexin-producing neurons causes narcolepsy in humans and rodents. This has generated considerable interest in developing small-molecule orexin receptor antagonists as a novel therapy for the treatment of insomnia. Orexin antagonists, especially those that block OX2 or both OX1 and OX2 receptors, clearly promote sleep in animals, and clinical results are encouraging: Several compounds are in Phase III trials. As the orexin system mainly promotes arousal, these new compounds will likely improve insomnia without incurring many of the side effects encountered with current medications.

Emerging therapeutics in sleep.

The development of new therapeutics for sleep disorders is increasingly dependent upon understanding the basic brain circuitry that underlies sleep-wake regulation, and how it may be pharmacologically manipulated. In this review, we consider the pathophysiological basis of major sleep disorders that often are seen by neurologists, including excessive daytime sleepiness, insomnia, narcolepsy, rapid eye movement sleep behavior disorder, and restless legs syndrome, as well as circadian disorders, and we review the current and potential future therapeutic approaches.