The hypocretins (orexins) mediate the "phasic" components of REM sleep: A new hypothesis

Treatment of Narcolepsy Type 1 With Orexin: A Systematic Review

Narcolepsy is a rare, chronic neurological disorder characterized by excessive daytime sleepiness (EDS). Narcolepsy type 1 is probably caused by an autoimmune-mediated loss of orexin-producing neurons. Type 2 patients retain the physiological functioning of orexigenic neurons. The basis for treating narcolepsy type 1 with orexin-A is that if narcolepsy develops because of a loss of orexigenic neurons, then administering orexin should be able to eliminate, reduce, or prevent the impact of this loss. The aim of this review was to capture and analyze studies to elucidate the efficacy of orexin-A in the treatment of narcolepsy type 1 in humans. The search strategy included the following descriptors: "narcolepsy," "orexin," and "treatment," with filters for randomized clinical trials (RCT) and human studies. A total of 70 publications were retrieved from the databases. Duplicate records were removed before screening (n = 13), and 54 were then excluded for the following reasons: off-topic (n = 18), reviews (n = 14), use of a different intervention other than orexin (n = 14), non-human studies (n = 4), out-of-population selection criteria (n = 2), and case report (n = 2). Thus, the studies included in the review were three. Treatment of narcolepsy with orexin decreases the number of wake-REM (rapid eye movement) transitions and total time of REM sleep, although it does not increase wake time. The failure of orexin to alleviate daytime sleepiness suggests that orexin deficiency is not the only factor involved in the pathophysiology of type I narcolepsy.

Orexin-mediated motivated arousal and reward seeking

The neuromodulator orexin has been identified as a key factor for motivated arousal including recent evidence that sleep deprivation-induced enhancement of reward behavior is modulated by orexin. While orexin is not necessary for either reward or arousal behavior, orexin neurons' broad projections, ability to sense the internal state of the animal, and high plasticity of signaling in response to natural rewards and drugs of abuse may underlie heightened drug seeking, particularly in a subset of highly motivated reward seekers. As such, orexin receptor antagonists have gained deserved attention for putative use in addiction treatments. Ongoing and future clinical trials are expected to identify individuals most likely to benefit from orexin receptor antagonist treatment to promote abstinence, such as those with concurrent sleep disorders or high craving, while attention to methodological considerations will aid interpretation of the numerous preclinical studies investigating disparate aspects of the role of orexin in reward and arousal.

Inactivation of hypocretin receptor-2 signaling in dopaminergic neurons induces hyperarousal and enhanced cognition but impaired inhibitory control

Hypocretin/Orexin (HCRT/OX) and dopamine (DA) are both key effectors of salience processing, reward and stress-related behaviors and motivational states, yet their respective roles and interactions are poorly delineated. We inactivated HCRT-to-DA connectivity by genetic disruption of Hypocretin receptor-1 (Hcrtr1), Hypocretin receptor-2 (Hcrtr2), or both receptors (Hcrtr1&2) in DA neurons and analyzed the consequences on vigilance states, brain oscillations and cognitive performance in freely behaving mice. Unexpectedly, loss of Hcrtr2, but not Hcrtr1 or Hcrtr1&2, induced a dramatic increase in theta (7-11 Hz) electroencephalographic (EEG) activity in both wakefulness and rapid-eye-movement sleep (REMS). DAHcrtr2-deficient mice spent more time in an active (or theta activity-enriched) substate of wakefulness, and exhibited prolonged REMS. Additionally, both wake and REMS displayed enhanced theta-gamma phase-amplitude coupling. The baseline waking EEG of DAHcrtr2-deficient mice exhibited diminished infra-theta, but increased theta power, two hallmarks of EEG hyperarousal, that were however uncoupled from locomotor activity. Upon exposure to novel, either rewarding or stress-inducing environments, DAHcrtr2-deficient mice featured more pronounced waking theta and fast-gamma (52-80 Hz) EEG activity surges compared to littermate controls, further suggesting increased alertness. Cognitive performance was evaluated in an operant conditioning paradigm, which revealed that DAHcrtr2-ablated mice manifest faster task acquisition and higher choice accuracy under increasingly demanding task contingencies. However, the mice concurrently displayed maladaptive patterns of reward-seeking, with behavioral indices of enhanced impulsivity and compulsivity. None of the EEG changes observed in DAHcrtr2-deficient mice were seen in DAHcrtr1-ablated mice, which tended to show opposite EEG phenotypes. Our findings establish a clear genetically-defined link between monosynaptic HCRT-to-DA neurotransmission and theta oscillations, with a differential and novel role of HCRTR2 in theta-gamma cross-frequency coupling, attentional processes, and executive functions, relevant to disorders including narcolepsy, attention-deficit/hyperactivity disorder, and Parkinson's disease.

Sleep, Narcolepsy, and Sodium Oxybate

Sodium oxybate (SO) has been in use for many decades to treat narcolepsy with cataplexy. It functions as a weak GABAB agonist but also as an energy source for the brain as a result of its metabolism to succinate and as a powerful antioxidant because of its capacity to induce the formation of NADPH. Its actions at thalamic GABAB receptors can induce slow-wave activity, while its actions at GABAB receptors on monoaminergic neurons can induce or delay REM sleep. By altering the balance between monoaminergic and cholinergic neuronal activity, SO uniquely can induce and prevent cataplexy. The formation of NADPH may enhance sleep’s restorative process by accelerating the removal of the reactive oxygen species (ROS), which accumulate during wakefulness. SO improves alertness in normal subjects and in patients with narcolepsy. SO may allay severe psychological stress - an inflammatory state triggered by increased levels of ROS and characterized by cholinergic supersensitivity and monoaminergic deficiency. SO may be able to eliminate the inflammatory state and correct the cholinergic/ monoaminergic imbalance.

Orexin signaling modulates synchronized excitation in the sublaterodorsal tegmental nucleus to stabilize REM sleep

The relationship between orexin/hypocretin and rapid eye movement (REM) sleep remains elusive. Here, we find that a proportion of orexin neurons project to the sublaterodorsal tegmental nucleus (SLD) and exhibit REM sleep-related activation. In SLD, orexin directly excites orexin receptor-positive neurons (occupying ~3/4 of total-population) and increases gap junction conductance among neurons. Their interaction spreads the orexin-elicited partial-excitation to activate SLD network globally. Besides, the activated SLD network exhibits increased probability of synchronized firings. This synchronized excitation promotes the correspondence between SLD and its downstream target to enhance SLD output. Using optogenetics and fiber-photometry, we consequently find that orexin-enhanced SLD output prolongs REM sleep episodes through consolidating brain state activation/muscle tone inhibition. After chemogenetic silencing of SLD orexin signaling, a ~17% reduction of REM sleep amounts and disruptions of REM sleep muscle atonia are observed. These findings reveal a stabilization role of orexin in REM sleep.

Orexin signaling is provided by diffusely distributed fibers and involved in different brain circuits that orchestrate sleep and wakefulness states. Here, the authors show that a proportion of orexin neurons project to the sublaterodorsal tegmental nucleus and exhibit rapid eye movement (REM) sleep-related actions.

Hypocretin (orexin) immunoreactivity in the feline midbrain: Relevance for the generation of wakefulness

Hypocretins (Hcrt) 1 and 2 are two neuropeptides synthesized from neurons that are located in the perifornical area of the lateral hypothalamus. These neurons project diffusely throughout the central nervous system, and have been implicated in the generation and maintenance of wakefulness, as well as in critical physiological processes that occur during this behavioral state, such as motivation. The hypocretinergic projections towards the feline midbrain have not been studied before. Therefore, the aim of the present study was to analyze their relationship to the midbrain neurons, that are critically involved in the control of sleep and wakefulness. With this purpose, we examined the distribution of Hcrt1-positive fibers in the midbrain and pontomesencephalic area of the domestic cat (Felis catus), and their relationship with catecholaminergic and cholinergic neurons by means of single and double immunohistochemistry. Hcrtergic axons with distinctive varicosities and buttons were heterogeneously distributed, exhibiting different densities in distinct regions of the midbrain. High Hcrtergic fiber densities were observed in the periaqueductal gray, interpeduncular nucleus, locus coeruleus and cholinergic mesopontine regions. In addition, we studied in detail the Hcrtergic projection towards the dopaminergic nuclei of the midbrain. While very few Hcrt + fibers were observed in the substantia nigra pars compacta, the highest density of Hcrtergic fibers was found in the dopaminergic ventral periaqueductal gray area (also called A10dc area); appositions between Hcrtergic terminals and dopaminergic somata and dendrites were observed within this area. Because this dopaminergic area has been involved in the control of wakefulness, the present anatomical data provides relevant support about the role of the Hcrtergic system in the generation of this behavioral state.

Stress and Systemic Inflammation: Yin-Yang Dynamics in Health and Diseases

Studies in psychoneuroimmunology (PNI) would provide better insights into the "whole mind-body system." Systems biology models of the complex adaptive systems (CASs), such as a conceptual framework of "Yin-Yang dynamics," may be helpful for identifying systems-based biomarkers and targets for more effective prevention and treatment. The disturbances in the Yin-Yang dynamical balance may result in stress, inflammation, and various disorders including insomnia, Alzheimer's disease, obesity, diabetes, cardiovascular diseases, skin disorders, and cancer. At the molecular and cellular levels, the imbalances in the cytokine pathways, mitochondria networks, redox systems, and various signaling pathways may contribute to systemic inflammation. In the nervous system, Yin and Yang may represent the dynamical associations between the progressive and regressive processes in aging and neurodegenerative diseases. In response to the damages to the heart, the Yin-Yang dynamical balance between proinflammatory and anti-inflammatory cytokine networks is crucial. The studies of cancer have revealed the importance of the Yin-Yang dynamics in the tumoricidal and tumorigenic activities of the immune system. Stress-induced neuroimmune imbalances are also essential in chronic skin disorders including atopic dermatitis and psoriasis. With the integrative framework, the restoration of the Yin-Yang dynamics can become the objective of dynamical systems medicine.

Neural Control of the Upper Airway: Respiratory and State-Dependent Mechanisms

Upper airway muscles subserve many essential for survival orofacial behaviors, including their important role as accessory respiratory muscles. In the face of certain predisposition of craniofacial anatomy, both tonic and phasic inspiratory activation of upper airway muscles is necessary to protect the upper airway against collapse. This protective action is adequate during wakefulness, but fails during sleep which results in recurrent episodes of hypopneas and apneas, a condition known as the obstructive sleep apnea syndrome (OSA). Although OSA is almost exclusively a human disorder, animal models help unveil the basic principles governing the impact of sleep on breathing and upper airway muscle activity. This article discusses the neuroanatomy, neurochemistry, and neurophysiology of the different neuronal systems whose activity changes with sleep-wake states, such as the noradrenergic, serotonergic, cholinergic, orexinergic, histaminergic, GABAergic and glycinergic, and their impact on central respiratory neurons and upper airway motoneurons. Observations of the interactions between sleep-wake states and upper airway muscles in healthy humans and OSA patients are related to findings from animal models with normal upper airway, and various animal models of OSA, including the chronic-intermittent hypoxia model. Using a framework of upper airway motoneurons being under concurrent influence of central respiratory, reflex and state-dependent inputs, different neurotransmitters, and neuropeptides are considered as either causing a sleep-dependent withdrawal of excitation from motoneurons or mediating an active, sleep-related inhibition of motoneurons. Information about the neurochemistry of state-dependent control of upper airway muscles accumulated to date reveals fundamental principles and may help understand and treat OSA. © 2016 American Physiological Society. Compr Physiol 6:1801-1850, 2016.

Hypoxia and hypercapnia inhibit hypothalamic orexin neurons in rats

Evidence of impaired function of orexin neurons has been found in individuals with cardiorespiratory disorders, such as obstructive sleep apnea (OSA) and sudden infant death syndrome (SIDS), but the mechanisms responsible are unknown. Individuals with OSA and SIDS experience repetitive breathing cessations and/or rebreathing of expired air, resulting in hypoxia/hypercapnia (H/H). In this study, we examined the responses of fluorescently identified rat orexin neurons in the lateral hypothalamus to acute H/H to test if and how these neurons alter their activity and function during this challenge. Experiments were conducted in an in vitro slice preparation using voltage-clamp and current-clamp configurations. H/H (10 min) induced hyperpolarization, accompanied by rapid depression, and finally, cessation of firing activity in orexin neurons. Hypoxia alone had similar but less potent effects. H/H did not alter the frequency of inhibitory glycinergic postsynaptic currents. The frequency of GABAergic currents was diminished but only at 8-10 min of H/H. In contrast, the frequency of excitatory glutamatergic postsynaptic events was diminished as early as 2-4 min of H/H. In the presence of glutamatergic receptor blockers, the inhibitory effects of H/H on the firing activity and membrane potential of orexin neurons persisted but to a lesser extent. In conclusion, both direct alteration of postsynaptic membrane properties and diminished glutamatergic neurotransmission likely contribute to the inhibition of orexin neurons by H/H. These mechanisms could be responsible for the decreased function of orexin in individuals at risk for OSA and SIDS.

Hypocretinergic system in the medial preoptic area promotes maternal behavior in lactating rats

Hypocretin-1 and 2 (HCRT-1 and HCRT-2, respectively) are neuropeptides synthesized by neurons located in the postero-lateral hypothalamus, whose projections are widely distributed throughout the brain. The hypocretinergic (HCRTergic) system has been associated with the generation and maintenance of wakefulness, as well as with the promotion of motivated behaviors. In lactating rats, intra-cerebroventricular HCRT-1 administration stimulates maternal behavior, whilst lactation per se increases the expression of HCRT type 1 receptor (HCRT-R1). Due to the fact that HCRTergic receptors are expressed in the medial preoptic area (mPOA), a region critically involved in maternal behavior, we hypothesize that HCRT-1 promotes maternal behavior acting on this region. In order to evaluate this hypothesis, we assessed the maternal behavior of lactating rats following microinjections of HCRT-1 (10 or 100μM) and the selective HCRT-R1 antagonist SB-334867 (250μM) into the mPOA, during the first and second postpartum weeks. While intra-mPOA microinjections of HCRT-1 (100μM) increased corporal pup licking during the second postpartum week, the blockade of HCRT-R1 significantly decreased active components of maternal behavior, such as retrievals, corporal and ano-genital lickings, and increased the time spent in nursing postures in both postpartum periods. We conclude that HCRTergic system in the mPOA may stimulate maternal behavior, suggesting that endogenous HCRT-1 is necessary for the natural display of this behavior.

Subchronical treatment with Fluoxetine modifies the activity of the MCHergic and hypocretinergic systems. Evidences from peptide CSF concentration and gene expression

In the postero-lateral hypothalamus are located two neuronal systems that utilize the neuropeptides melanin-concentrating hormone (MCH) and hypocretins (also called orexins) as neuromodulators. These systems have reciprocal connections between them, and project throughout the central nervous system. MCH has been involved in the generation of sleep, mainly REM sleep, while hypocretins have a critical role in the generation of wakefulness. MCHergic activity is also involved in the pathophysiology of major depressive disorder (MD). In this regards, intracerebral administration of MCH promotes pro-depressive behaviors (i.e., immobility in the forced swimming test) and REM sleep hypersomnia, which is an important trait of depression. Furthermore, the antagonism of the MCHR-1 receptor has a reliable antidepressant effect, suggesting that MCH is a pro-depressive factor. Hypocretins have been also involved in mood regulation; however, their role in depression is still on debate. Taking these data into account, we explored whether systemic subchronical treatment with Fluoxetine (FLX), a serotonergic antidepressant, modifies the concentration of MCH in the cerebrospinal fluid (CSF), as well as the preproMCH mRNA expression. We also evaluated the hypocretinergic system by quantifying the hypocretin-levels in the CSF and the preprohypocretin mRNA expression. Compared to control, FLX increased the levels of preprohypocretin mRNA without affecting the hypocretin-1 CSF levels. On the contrary, FLX significantly decreased the MCH CSF concentration without affecting the preproMCH gene expression. This result is in agreement with the fact that MCH serum level diminishes during the antidepressant treatment in MD, and supports the hypothesis that an increase in the MCHergic activity could have pro-depressive consequences.

Melanin-Concentrating Hormone (MCH): Role in REM Sleep and Depression

The melanin-concentrating hormone (MCH) is a peptidergic neuromodulator synthesized by neurons of the lateral sector of the posterior hypothalamus and zona incerta. MCHergic neurons project throughout the central nervous system, including areas such as the dorsal (DR) and median (MR) raphe nuclei, which are involved in the control of sleep and mood. Major Depression (MD) is a prevalent psychiatric disease diagnosed on the basis of symptomatic criteria such as sadness or melancholia, guilt, irritability, and anhedonia. A short REM sleep latency (i.e., the interval between sleep onset and the first REM sleep period), as well as an increase in the duration of REM sleep and the density of rapid-eye movements during this state, are considered important biological markers of depression. The fact that the greatest firing rate of MCHergic neurons occurs during REM sleep and that optogenetic stimulation of these neurons induces sleep, tends to indicate that MCH plays a critical role in the generation and maintenance of sleep, especially REM sleep. In addition, the acute microinjection of MCH into the DR promotes REM sleep, while immunoneutralization of this peptide within the DR decreases the time spent in this state. Moreover, microinjections of MCH into either the DR or MR promote a depressive-like behavior. In the DR, this effect is prevented by the systemic administration of antidepressant drugs (either fluoxetine or nortriptyline) and blocked by the intra-DR microinjection of a specific MCH receptor antagonist. Using electrophysiological and microdialysis techniques we demonstrated also that MCH decreases the activity of serotonergic DR neurons. Therefore, there are substantive experimental data suggesting that the MCHergic system plays a role in the control of REM sleep and, in addition, in the pathophysiology of depression. Consequently, in the present report, we summarize and evaluate the current data and hypotheses related to the role of MCH in REM sleep and MD.