Increased and decreased muscle tone with orexin (hypocretin) microinjections in the locus coeruleus and pontine inhibitory area

Central mechanisms of muscle tone regulation: implications for pain and performance

Muscle tone represents a foundational property of the motor system with the potential to impact musculoskeletal pain and motor performance. Muscle tone is involuntary, dynamically adaptive, interconnected across the body, sensitive to postural demands, and distinct from voluntary control. Research has historically focused on pathological tone, peripheral regulation, and contributions from passive tissues, without consideration of the neural regulation of active tone and its consequences, particularly for neurologically healthy individuals. Indeed, simplistic models based on the stretch reflex, which neglect the central regulation of tone, are still perpetuated today. Recent advances regarding tone are dispersed across different literatures, including animal physiology, pain science, motor control, neurology, and child development. This paper brings together diverse areas of research to construct a conceptual model of the neuroscience underlying active muscle tone. It highlights how multiple tonic drive networks tune the excitability of complex spinal feedback circuits in concert with various sources of sensory feedback and in relation to postural demands, gravity, and arousal levels. The paper also reveals how tonic muscle activity and excitability are disrupted in people with musculoskeletal pain and how tone disorders can lead to marked pain and motor impairment. The paper presents evidence that integrative somatic methods address the central regulation of tone and discusses potential mechanisms and implications for tone rehabilitation to improve pain and performance.

The protective effects of orexin B in neuropathic pain by suppressing inflammatory response

Chronic pain induced by pathological insults to the sensorimotor system is a typical form of neuropathic pain (NP), and the underlying mechanism is complex. Currently, there are no successful therapeutic interventions for NP. Orexin B is a neuropeptide with a wide range of biological functions. However, the pharmacological function of orexin B in chronic neuropathic pain has been less studied. Here, we aim to examine the neuroprotective effects of orexin B in chronic constriction injury (CCI)- induced NP. Firstly, we found that orexin type 2 receptor (OX2R) but not orexin type 1 receptor (OX1R) was reduced in the spinal cord (SC) of CCI-treated rats. Mechanical withdrawal threshold and thermal withdrawal latency assays display that administration of orexin B clearly ameliorated CCI-evoked neuropathic pain dose-dependently. Notably, orexin B treatment also effectively prevented microglia activation by reducing the levels of IBA1. Additionally, orexin B was also found to suppress the inflammatory response in the SC tissue by reducing the levels of IL-6, TNF-α, iNOS, and COX-2 as well as the production of NO and PGE2 in CCI-treated rats. Furthermore, orexin B administration attenuated oxidative stress (OS) by increasing the activity of SOD and the levels of GSH. Mechanically, orexin B prevented activation of JNK/NF-κB signaling in the SC of CCI-treated rats. Based on these findings, we conclude that orexin B might have a promising role in ameliorating CCI-evoked neuropathic pain through the inhibition of microglial activation and inflammatory response.

Major alteration of motor control during rapid eye movements sleep in mice models of sleep disorders

Alteration of motor control during rapid eye movements (REM) sleep has been extensively described in sleep disorders, in particular in isolated REM sleep behavior disorder (iRBD) and narcolepsy type 1 (NT1). NT1 is caused by the loss of orexin/hypocretin (ORX) neurons. Unlike in iRBD, the RBD comorbid symptoms of NT1 are not associated with alpha-synucleinopathies. To determine whether the chronic absence of ORX neuropeptides is sufficient to induce RBD symptoms, we analyzed during REM sleep the EMG signal of the prepro-hypocretin knockout mice (ORX-/-), a recognized mouse model of NT1. Then, we evaluated the severity of motor alterations by comparing the EMG data of ORX-/- mice to those of mice with a targeted suppression of the sublaterodorsal glutamatergic neurotransmission, a recognized rodent model of iRBD. We found a significant alteration of tonic and phasic components of EMG during REM sleep in ORX-/- mice, with more phasic events and more REM sleep episodes without atonia compared to the control wild-type mice. However, these phasic events were fewer, shorter, and less complex in ORX-/- mice compared to the RBD-like ORX-/- mice. We thus show that ORX deficiency, as seen in NT1, is sufficient to impair muscle atonia during REM sleep with a moderate severity of alteration as compared to isolated RBD mice. As described in NT1 patients, we report a major interindividual variability in the severity and frequency of RBD symptoms in ORX-deficient mice.

The role of orexin in Parkinson's disease

Emerging evidence has implicated the orexin system in non-motor pathogenesis of Parkinson's disease. It has also been suggested the orexin system is involved in the modulation of motor control, further implicating the orexin system in Parkinson's disease. Parkinson's disease is the second most common neurodegenerative disease with millions of people suffering worldwide with motor and non-motor symptoms, significantly affecting their quality of life. Treatments are based solely on symptomatic management and no cure currently exists. The orexin system has the potential to be a treatment target in Parkinson's disease, particularly in the non-motor stage. In this review, the most current evidence on the orexin system in Parkinson's disease and its potential role in motor and non-motor symptoms of the disease is summarized. This review begins with a brief overview of Parkinson's disease, animal models of the disease, and the orexin system. This leads into discussion of the possible roles of orexin neurons in Parkinson's disease and levels of orexin in the cerebral spinal fluid and plasma in Parkinson's disease and animal models of the disease. The role of orexin is then discussed in relation to symptoms of the disease including motor control, sleep, cognitive impairment, psychological behaviors, and the gastrointestinal system. The neuroprotective effects of orexin are also summarized in preclinical models of the disease.

Central effects of trigeminal electrical stimulation

This is a review of the literature on the main neuromodulation techniques, focusing on the possibility of introducing sensory threshold ULFTENS into them. Electro neuromodulation techniques have been in use for many years as promising methods of therapy for cognitive and emotional disorders. One of the most widely used forms of stimulation for orofacial pain is transcutaneous trigeminal stimulation on three levels: supraorbital area, dorsal surface of the tongue, and anterior skin area of the tragus. The purpose of this review is to trigger interest on using dental ULFTENS as an additional trigeminal neurostimulation and neuromodulation technique in the context of TMD. In particular, we point out the possibility of using ULFTENS at a lower activation level than that required to trigger a muscle contraction that is capable of triggering effects at the level of the autonomic nervous system, with extreme ease of execution and few side effects.

Deficiency of orexin signaling during sleep is involved in abnormal REM sleep architecture in narcolepsy

Narcolepsy is a sleep disorder caused by deficiency of orexin signaling. However, the neural mechanisms by which deficient orexin signaling causes the abnormal rapid eye movement (REM) sleep characteristics of narcolepsy, such as cataplexy and frequent transitions to REM states, are not fully understood. Here, we determined the activity dynamics of orexin neurons during sleep that suppress the abnormal REM sleep architecture of narcolepsy. Orexin neurons were highly active during wakefulness, showed intermittent synchronous activity during non-REM (NREM) sleep, were quiescent prior to the transition from NREM to REM sleep, and a small subpopulation of these cells was active during REM sleep. Orexin neurons that lacked orexin peptides were less active during REM sleep and were mostly silent during cataplexy. Optogenetic inhibition of orexin neurons established that the activity dynamics of these cells during NREM sleep regulate NREM-REM sleep transitions. Inhibition of orexin neurons during REM sleep increased subsequent REM sleep in "orexin intact" mice and subsequent cataplexy in mice lacking orexin peptides, indicating that the activity of a subpopulation of orexin neurons during the preceding REM sleep suppresses subsequent REM sleep and cataplexy. Thus, these results identify how deficient orexin signaling during sleep results in the abnormal REM sleep architecture characteristic of narcolepsy.

Noradrenergic neuromodulation in ageing and disease

The locus coeruleus (LC) is a small brainstem structure located in the lower pons and is the main source of noradrenaline (NA) in the brain. Via its phasic and tonic firing, it modulates cognition and autonomic functions and is involved in the brain's immune response. The extent of degeneration to the LC in healthy ageing remains unclear, however, noradrenergic dysfunction may contribute to the pathogenesis of Alzheimer's (AD) and Parkinson's disease (PD). Despite their differences in progression at later disease stages, the early involvement of the LC may lead to comparable behavioural symptoms such as preclinical sleep problems and neuropsychiatric symptoms as a result of AD and PD pathology. In this review, we draw attention to the mechanisms that underlie LC degeneration in ageing, AD and PD. We aim to motivate future research to investigate how early degeneration of the noradrenergic system may play a pivotal role in the pathogenesis of AD and PD which may also be relevant to other neurodegenerative diseases.

Hypothalamic Control of Forelimb Motor Adaptation

The ability to perform skilled arm movements is central to everyday life, as limb impairments in common neurologic disorders such as stroke demonstrate. Skilled arm movements require adaptation of motor commands based on discrepancies between desired and actual movements, called sensory errors. Studies in humans show that this involves predictive and reactive movement adaptations to the errors, and also requires a general motivation to move. How these distinct aspects map onto defined neural signals remains unclear, because of a shortage of equivalent studies in experimental animal models that permit neural-level insights. Therefore, we adapted robotic technology used in human studies to mice, enabling insights into the neural underpinnings of motivational, reactive, and predictive aspects of motor adaptation. Here, we show that forelimb motor adaptation is regulated by neurons previously implicated in motivation and arousal, but not in forelimb motor control: the hypothalamic orexin/hypocretin neurons (HONs). By studying goal-oriented mouse-robot interactions in male mice, we found distinct HON signals occur during forelimb movements and motor adaptation. Temporally-delimited optosilencing of these movement-associated HON signals impaired sensory error-based motor adaptation. Unexpectedly, optosilencing affected neither task reward or execution rates, nor motor performance in tasks that did not require adaptation, indicating that the temporally-defined HON signals studied here were distinct from signals governing general task engagement or sensorimotor control. Collectively, these results reveal a hypothalamic neural substrate regulating forelimb motor adaptation.SIGNIFICANCE STATEMENT The ability to perform skilled, adaptable movements is a fundamental part of daily life, and is impaired in common neurologic diseases such as stroke. Maintaining motor adaptation is thus of great interest, but the necessary brain components remain incompletely identified. We found that impaired motor adaptation results from disruption of cells not previously implicated in this pathology: hypothalamic orexin/hypocretin neurons (HONs). We show that temporally confined HON signals are associated with skilled movements. Without these newly-identified signals, a resistance to movement that is normally rapidly overcome leads to prolonged movement impairment. These results identify natural brain signals that enable rapid and effective motor adaptation.

Orexin-B exerts excitatory effects on nigral dopaminergic neurons and alleviates motor disorders in MPTP parkinsonian mice

The study aimed to investigate the effects of orexin-B in Parkinson's disease. The present study showed that orexin-B exerted marked excitatory effects via orexin-2 receptor on the nigral dopaminergic neurons in MPTP parkinsonian mice, while blocking orexin-2 receptor decreased the firing rate of dopaminergic neurons significantly. Furthermore, intracerebroventricular application of orexin-B relieved the degeneration of dopaminergic neurons, increased the general spontaneous activity and alleviated motor coordination in MPTP parkinsonian mice. The present study suggests that orexin-B could exert protective effects on dopaminergic neurons and improve motor disorders in parkinsonian mice. Such protective effects of orexin-B on Parkinson's disease may be partially attributed to the excitatory effects on the nigral dopaminergic neurons.

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.

Firing activity of locus coeruleus noradrenergic neurons decreases in necdin-deficient mice, an animal model of Prader-Willi syndrome

BACKGROUND

Prader-Willi syndrome (PWS) is a neurodevelopmental disorder characterized by multiple respiratory, cognitive, endocrine, and behavioral symptoms, such as central apnea, intellectual disabilities, exaggerated stress responses, and temper tantrums. The locus coeruleus noradrenergic system (LC-NE) modulates a diverse range of behaviors, including arousal, learning, pain modulation, and stress-induced negative affective states, which are possibly correlated with the pathogenesis of PWS phenotypes. Therefore, we evaluated the LC-NE neuronal activity of necdin-deficient mice, an animal model of PWS.

METHODS

Heterozygous necdin-deficient mice (B6.Cg-Ndntm1ky) were bred from wild-type (WT) females to generate WT (+m/+p) and heterozygotes (+m/-p) animals, which were examined of LC-NE neuronal activity, developmental reflexes, and plethysmography.

RESULTS

On slice electrophysiology, LC-NE neurons of Ndntm1ky mice with necdin deficiency showed significantly decreased spontaneous activities and impaired excitability, which was mediated by enhanced A-type voltage-dependent potassium currents. Ndntm1ky mice also exhibited the neonatal phenotypes of PWS, such as hypotonia and blunt respiratory responses to hypercapnia.

CONCLUSIONS

LC-NE neuronal firing activity decreased in necdin-deficient mice, suggesting that LC, the primary source of norepinephrine in the central nervous system, is possibly involved in PWS pathogenesis.

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.

REM sleep behavior disorder in narcolepsy: A secondary form or an intrinsic feature?

Disrupted nighttime sleep is one of the pentad of symptoms defining Narcolepsy. REM sleep behavior disorder (RBD) largely contributes to night sleep disruption and narcolepsy is the most common cause of secondary RBD. However, RBD linked to narcolepsy (N-RBD) has been insufficiently characterized, leaving unsolved a number of issues. Indeed, it is still debated whether N-RBD is an intrinsic feature of narcolepsy, as indubitable for cataplexy, and therefore strictly linked to the cerebrospinal fluid hypocretin-1 (CSF hcrt-1) deficiency, or an associated feature, with a still unclear pathophysiology. The current review aims at rendering a comprehensive state-of-the-art of N-RBD, highlighting the open and unsettled topics. RBD reportedly affects 30-60% of patients with Narcolepsy type 1 (NT1), but it may be seen also in Narcolepsy type 2 (NT2). When compared to idiopathic/isolated RBD (iRBD), N-RBD has been reported to be characterized by less energetic and quieter episode, which however occur with the same probability in the first and the second part of the night and sometime even subcontinuously. N-RBD patients are generally younger than those with iRBD. N-RBD has been putatively linked to wake-sleep instability due to CSF hcrt-1 deficiency, but this latter by itself cannot explain completely the phenomenon as N-RBD has not been universally linked to low CSF hcrt-1 levels and it may be observed also in NT2. Therefore, other factors may probably play a role and further studies are needed to clarify this issue. In addition, therapeutic options have been poorly investigated.

Molecular mechanism of tumour necrosis factor alpha regulates hypocretin (orexin) expression, sleep and behaviour

Hypocretin 1 and hypocretin 2 (orexin A and B) regulate sleep, wakefulness and emotion. Tumour necrosis factor alpha (TNF-α) is an important neuroinflammation mediator. Here, we examined the effects of TNF-α treatment on hypocretin expression in vivo and behaviour in mice. TNF-α decreased hypocretin 1 and hypocretin 2 expression in a dose-dependent manner in cultured hypothalamic neurons. TNF-α decreased mRNA stability of prepro-hypocretin, the single precursor of hypocretin 1 and hypocretin 2. Mice challenged with TNF-α demonstrated decreased expression of prepro-hypocretin, hypocretin 1 and hypocretin 2 in hypothalamus. In response to TNF-α, prepro-hypocretin mRNA decay was increased in hypothalamus. TNF-α neutralizing antibody restored the expression of prepro-hypocretin, hypocretin 1 and hypocretin 2 in vivo in TNF-α challenged mice, supporting hypocretin system can be impaired by increased TNF-α through decreasing hypocretin expression. Repeated TNF-α challenge induced muscle activity during rapid eye movement sleep and sleep fragmentation, but decreased learning, cognition and memory in mice. TNF-α neutralizing antibody blocked the effects of TNF-α; in contrast, hypocretin receptor antagonist enhanced the effects of TNF-α. The data support that TNF-α is involved in the regulation of hypocretin expression, sleep and cognition. The findings shed some lights on the role of neuroinflammation in neurodegenerative diseases including Alzheimer's disease and Parkinson's disease.

Locus Coeruleus Phasic, But Not Tonic, Activation Initiates Global Remapping in a Familiar Environment

Locus coeruleus (LC) neurons, the source of hippocampal norepinephrine (NE), are activated by novelty and changes in environmental contingencies. Based on the role of monoamines in reconfiguring invertebrate networks, and data from mammalian systems, a network reset hypothesis for the effects of LC activation has been proposed. We used the cellular compartmental analysis of temporal FISH technique based on the cellular distribution of immediate early genes to examine the effect of LC activation and inactivation, on regional hippocampal maps in male rats, when LC activity was manipulated just before placement in a second familiar (A/A) and/or novel environment (A/B). We found that bilateral phasic, but not tonic, activation of LC reset hippocampal maps in the A/A condition, whereas silencing the LC with clonidine before placement in the A/B condition blocked map reset and a familiar map emerged in the dentate gyrus, proximal and distal CA1, and CA3c. However, CA3a and CA3b encoded the novel environment. These results support a role for phasic LC responses in generating novel hippocampal sequences during memory encoding and, potentially, memory updating. The silencing experiments suggest that novel environments may not be recognized as different by dentate gyrus and CA1 without LC input. The functional distinction between phasic and tonic LC activity argues that these parameters are critical for determining network changes. These data are consistent with the hippocampus activating internal network representations to encode novel experiential episodes and suggest LC input is critical for this role.SIGNIFICANCE STATEMENT Burst activation of the broadly projecting novelty signaling system of the locus coeruleus initiates new network representations throughout the hippocampus despite unchanged external environments. Tonic activation does not alter network representations in the same condition. This suggests differences in the temporal parameters of neuromodulator network activation are critical for neuromodulator function. Silencing this novelty signaling system prevented the appearance of new network representations in a novel environment. Instead, familiar representations were expressed in a subset of hippocampal areas, with another subset encoding the novel environment. This "being in two places at once" argues for independent functional regions within the hippocampus. These experiments strengthen the view that internal states are major determinants of the brain's construction of environmental representations.

Orexin supplementation in narcolepsy treatment: A review

Narcolepsy is a rare, chronic neurological disease characterized by excessive daytime sleepiness, cataplexy, vivid hallucinations, and sleep paralysis. Narcolepsy occurs in approximately 1 of 3000 people, affecting mainly adolescents aged 15 to 30 years. Recently, people with narcolepsy were shown to exhibit extensive orexin/hypocretin neuronal loss. The orexin system regulates sleep/wake control via complex interactions with monoaminergic, cholinergic and GABA-ergic neuronal systems. Currently, no cure for narcolepsy exists, but some symptoms can be controlled with medication (eg, stimulants, antidepressants, etc). Orexin supplementation represents a more sophisticated way to treat narcolepsy because it addresses the underlying cause of the disease and not just the symptoms. Research on orexin supplementation in the treatment of sleep disorders has strongly increased over the past two decades. This review focuses on a brief description of narcolepsy, the mechanisms by which the orexin system regulates sleep/wake cycles, and finally, possible therapeutic options based on orexin supplementation in animal models and patients with narcolepsy.

The hypocretin/orexin system as a target for excessive motivation in alcohol use disorders

The hypocretin/orexin (ORX) system has been repeatedly demonstrated to regulate motivation for drugs of abuse, including alcohol. In particular, ORX seems to be critically involved in highly motivated behaviors, as is observed in high-seeking individuals in a population, in the seeking of highly palatable substances, and in models of dependence. It seems logical that this system could be considered as a potential target for treatment for addiction, particularly alcohol addiction, as ORX pharmacological manipulations significantly reduce drinking. However, the ORX system also plays a role in a wide range of other behaviors, emotions, and physiological functions and is disrupted in a number of non-dependence-associated disorders. It is therefore important to consider how the ORX system might be optimally targeted for potential treatment for alcohol use disorders either in combination with or separate from its role in other functions or diseases. This review will focus on the role of ORX in alcohol-associated behaviors and whether and how this system could be targeted to treat alcohol use disorders while avoiding impacts on other ORX-relevant functions. A brief overview of the ORX system will be followed by a discussion of some of the factors that makes it particularly intriguing as a target for alcohol addiction treatment, a consideration of some potential challenges associated with targeting this system and, finally, some future directions to optimize new treatments.

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.

GABA Cells in the Central Nucleus of the Amygdala Promote Cataplexy

Cataplexy is a hallmark of narcolepsy characterized by the sudden uncontrollable onset of muscle weakness or paralysis during wakefulness. It can occur spontaneously, but is typically triggered by positive emotions such as laughter. Although cataplexy was identified >130 years ago, its neural mechanism remains unclear. Here, we show that a newly identified GABA circuit within the central nucleus of the amygdala (CeA) promotes cataplexy. We used behavioral, electrophysiological, immunohistochemical, and chemogenetic strategies to target and manipulate CeA activity selectively in narcoleptic (orexin^-/-) mice to determine its functional role in controlling cataplexy. First, we show that chemogenetic activation of the entire CeA produces a marked increase in cataplexy attacks. Then, we show that GABA cells within the CeA are responsible for mediating this effect. To manipulate GABA cells specifically, we developed a new mouse line that enables genetic targeting of GABA cells in orexin^-/- mice. We found that chemogenetic activation of GABA CeA cells triggered a 253% increase in the number of cataplexy attacks without affecting their duration, suggesting that GABA cells play a functional role in initiating but not maintaining cataplexy. We show that GABA cell activation only promotes cataplexy attacks associated with emotionally rewarding stimuli, not those occurring spontaneously. However, we found that chemogenetic inhibition of GABA CeA cells does not prevent cataplexy, suggesting these cells are not required for initiating cataplexy attacks. Our results indicate that the CeA promotes cataplexy onset and that emotionally rewarding stimuli may trigger cataplexy by activating GABA cells in the CeA.SIGNIFICANCE STATEMENT Although cataplexy has been closely linked to positive emotions for >130 years, the neural circuitry that underlies this relationship is poorly understood. Recent work suggests that the amygdala, a brain area important for processing emotion, may be part of this circuit. This study provides the first functional evidence to implicate GABA cells in the amygdala as regulators of cataplexy triggered by positive emotions and identifies the amygdala as the brain region important more for gating the entrance into rather than the exit from cataplexy. We also generated a new mouse model for studying GABA neurons in narcoleptic mice, which could serve as a useful tool for studying the neurobiological underpinnings of narcolepsy.

Orexin signaling in rostral lateral hypothalamus and nucleus accumbens shell in the control of spontaneous physical activity in high- and low-activity rats

Spontaneous physical activity (SPA) describes activity outside of formal exercise and shows large interindividual variability. The hypothalamic orexin/hypocretin peptides are key regulators of SPA. Orexins drive SPA within multiple brain sites, including rostral lateral hypothalamus (LH) and nucleus accumbens shell (NAcSh). Rats with high basal SPA (high activity, HA) show higher orexin mRNA expression and SPA after injection of orexin-A in rostral LH compared with low-activity (LA) rats. Here, we explored the contribution of orexin signaling in rostral LH and NAcSh to the HA/LA phenotype. We found that HA rats have higher sensitivity to SPA after injection of orexin-A in rostral LH, but not in NAcSh. HA and LA rats showed similar levels of orexin receptor expression in rostral LH, and activation of orexin-producing neurons after orexin-A injection in rostral LH. Also, in HA and LA rats, the coinjection of orexin-A in rostral LH and NAcSh failed to further increase SPA beyond the effects of orexin-A in rostral LH. Pretreatment with muscimol, a GABA_A receptor agonist, in NAcSh potentiated SPA produced by orexin-A injection in rostral LH in HA but not in LA rats. Our results suggest that a feedback loop from orexin-responsive neurons in rostral LH to orexin neurons and a the NAcSh-orexin neuron-rostral LH circuit regulate SPA. Overall, our data suggest that differences in orexin sensitivity in rostral LH and its modulation by GABA afferents from NAcSh contribute to individual SPA differences.

The hypocretins/orexins: integrators of multiple physiological functions

The hypocretins (Hcrts), also known as orexins, are two peptides derived from a single precursor produced in the posterior lateral hypothalamus. Over the past decade, the orexin system has been associated with numerous physiological functions, including sleep/arousal, energy homeostasis, endocrine, visceral functions and pathological states, such as narcolepsy and drug abuse. Here, we review the discovery of Hcrt/orexins and their receptors and propose a hypothesis as to how the orexin system orchestrates these multifaceted physiological functions.

Influence of hypercapnia on the synthesis of neuropeptides and their receptors in murine brain

BACKGROUND AND OBJECTIVE

Sleep disorders are a complicated and major public health concern affecting millions of individuals. Obstructive sleep apnoea (OSA) is a common but still under-recognized disease which can cause intermittent nocturnal hypercapnia. Neuropeptides play critical roles in neurotransmission, acting as transmitters or modulators. Results from recent studies have implicated several neuropeptides in sleep and breathing regulation, including orexin, neuropeptides Y and galanin. Therefore, the present study aimed to evaluate the influence of hypercapnia on these neuropeptides and their receptors in order to assess their potential role in the pathogenesis of OSA.

METHODS

Fifteen C57BL/6J mice were randomly divided into three groups and exposed to moderate hypercapnia (5% CO(2) with balanced room air), or severe hypercapnia (10% CO(2) with balanced room air) or room air for 3 h (9:00-12:00 h), respectively. Immediately following exposure the brainstem and hypothalamus were excised for real-time reverse transcription polymerase chain reaction and western blot analyses.

RESULTS

In the hypothalamus gene expression including galanin, orexin and neuropeptide Y receptor 1 (NPYR1) was downregulated by hypercapnia. However, protein and mRNA levels of orexin-A receptor were upregulated by severe hypercapnia. In the brainstem only NPYR1 mRNA expression was decreased in moderate hypercapnia compared with that in severe hypercapnia.

CONCLUSIONS

These findings suggest that hypercapnia can affect these neuropeptides and their receptors, especially the orexin and orexin-A receptor. The potential relationships between these peptides and OSA are worthy of further investigation.

Ventricular orexin-A (hypocretin-1) levels correlate with rapid-eye-movement sleep without atonia in Parkinson's disease

OBJECTIVE

Patients with Parkinson's disease frequently complain of sleep disturbances and loss of muscle atonia during rapid-eye-movement (REM) sleep is not rare. The orexin-A (hypocretin-1) hypothalamic system plays a central role in controlling REM sleep. Loss of orexin neurons results in narcolepsy-cataplexy, a condition characterized by diurnal sleepiness and REM sleep without atonia. Alterations in the orexin-A system have been also documented in Parkinson's disease, but whether these alterations have clinical consequences remains unknown.

METHODS

Here, we measured orexin-A levels in ventricular cerebrospinal fluid from eight patients with Parkinson's disease (four males and four females) who underwent ventriculography during deep brain-stimulation surgery and performed full-night polysomnography before surgery.

RESULTS

Our results showed a positive correlation between orexin-A levels and REM sleep without muscle atonia.

CONCLUSION

Our results suggest that high levels of orexin-A in Parkinson's disease may be associated with loss of REM muscle atonia.

Parkinson's disease and sleep/wake disturbances

Parkinson's disease (PD) has traditionally been characterized by its cardinal motor symptoms of bradykinesia, rigidity, resting tremor, and postural instability. However, PD is increasingly being recognized as a multidimensional disease associated with myriad nonmotor symptoms including autonomic dysfunction, mood disorders, cognitive impairment, pain, gastrointestinal disturbance, impaired olfaction, psychosis, and sleep disorders. Sleep disturbances, which include sleep fragmentation, daytime somnolence, sleep-disordered breathing, restless legs syndrome (RLS), nightmares, and rapid eye movement (REM) sleep behavior disorder (RBD), are estimated to occur in 60% to 98% of patients with PD. For years nonmotor symptoms received little attention from clinicians and researchers, but now these symptoms are known to be significant predictors of morbidity in determining quality of life, costs of disease, and rates of institutionalization. A discussion of the clinical aspects, pathophysiology, evaluation techniques, and treatment options for the sleep disorders that are encountered with PD is presented.

Effect of orexin-B-saporin-induced lesions of the lateral hypothalamus on performance on a progressive ratio schedule

It has been suggested that a sub-population of orexinergic neurones whose somata lie in the lateral hypothalamic area (LHA) play an important role in regulating the reinforcing value of both food and drugs. This experiment examined the effect of disruption of orexinergic mechanisms in the LHA on performance on the progressive ratio schedule of reinforcement, in which the response requirement increases progressively for successive reinforcers. The data were analysed using a mathematical model which yields a quantitative index of reinforcer value and dissociates effects of interventions on motor and motivational processes. Rats were trained under a progressive ratio schedule using food-pellet reinforcement. They received bilateral injections of conjugated orexin-B-saporin (OxSap) into the LHA or sham lesions. Training continued for a further 40 sessions after surgery. Equations were fitted to the response rate data from each rat, and the parameters of the model were derived for successive blocks of 10 sessions. The OxSap lesion reduced the number of orexin-containing neurones in the LHA by approximately 50% compared with the sham-lesioned group. The parameter expressing the incentive value of the reinforcer was not significantly altered by the lesion. However, the parameter related to the maximum response rate was significantly affected, suggesting that motor capacity was diminished in the OxSap-lesioned group. The results indicate that OxSap lesions of the LHA disrupted food-reinforced responding on the progressive ratio schedule. It is suggested that this disruption was brought about by a change in non-motivational (motor) processes.

Energy expenditure: role of orexin

The orexins/hypocretins are endogenous, modulatory and multifunctional neuropeptides with prominent influence on several physiological processes. The influence of orexins on energy expenditure is highlighted with focus on orexin action on individual components of energy expenditure. As orexin stabilizes and maintains normal states of arousal and the sleep/wake cycle, we also highlight orexin mediation of sleep and how sleep interacts with energy expenditure.

Neuropeptides controlling energy balance: orexins and neuromedins

In this chapter, we review the feeding and energy expenditure effects of orexin (also known as hypocretin) and neuromedin. Orexins are multifunctional neuropeptides that affect energy balance by participating in regulation of appetite, arousal, and spontaneous physical activity. Central orexin signaling for all functions originates in the lateral hypothalamus-perifornical area and is likely functionally differentiated based on site of action and on interacting neural influences. The effect of orexin on feeding is likely related to arousal in some ways but is nonetheless a separate neural process that depends on interactions with other feeding-related neuropeptides. In a pattern distinct from other neuropeptides, orexin stimulates both feeding and energy expenditure. Orexin increases in energy expenditure are mainly by increasing spontaneous physical activity, and this energy expenditure effect is more potent than the effect on feeding. Global orexin manipulations, such as in transgenic models, produce energy balance changes consistent with a dominant energy expenditure effect of orexin. Neuromedins are gut-brain peptides that reduce appetite. There are gut sources of neuromedin, but likely the key appetite-related neuromedin-producing neurons are in the hypothalamus and parallel other key anorectic neuropeptide expression in the arcuate to paraventricular hypothalamic projection. As with other hypothalamic feeding-related peptides, hindbrain sites are likely also important sources and targets of neuromedin anorectic action. Neuromedin increases physical activity in addition to reducing appetite, thus producing a consistent negative energy balance effect. Together with the other various neuropeptides, neurotransmitters, neuromodulators, and neurohormones, neuromedin and orexin act in the appetite network to produce changes in food intake and energy expenditure, which ultimately influences the regulation of body weight.

Narcolepsy with hypocretin/orexin deficiency, infections and autoimmunity of the brain

The loss of hypothalamic hypocretin/orexin (hcrt) producing neurons causes narcolepsy with cataplexy. An autoimmune basis for the disease has long been suspected and recent results have greatly strengthened this hypothesis. Narcolepsy with hcrt deficiency is now known to be associated with a Human Leukocyte Antigen (HLA) and T-cell receptor (TCR) polymorphisms, suggesting that an autoimmune process targets a single peptide unique to hcrt-cells via specific HLA-peptide-TCR interactions. Recent data have shown a robust seasonality of disease onset in children and associations with Streptococcus Pyogenes, and influenza A H1N1-infection and H1N1-vaccination, pointing towards processes such as molecular mimicry or bystander activation as crucial for disease development. We speculate that upper airway infections may be common precipitants of a whole host of CNS autoimmune complications including narcolepsy.

The injection of hypocretin-1 into the nucleus pontis oralis induces either active sleep or wakefulness depending on the behavioral state when it is administered

STUDY OBJECTIVES

We previously reported that the microinjection of hypocretin (orexin) into the nucleus pontis oralis (NPO) induces a behavioral state that is comparable to naturally occurring active (rapid eye movement) sleep. However, other laboratories have found that wakefulness occurs following injections of hypocretin into the NPO. The present study tested the hypothesis that the discrepancy in behavioral state responses to hypocretin injections is due to the fact that hypocretin was not administered during the same states of sleep or wakefulness.

DESIGN

Adult cats were implanted with electrodes to record sleep and waking states. Hypocretin-1 (0.25 microL, 500microM) was microinjected into the NPO while the animals were awake or in quiet (non-rapid eye movement) sleep.

MEASUREMENTS AND RESULTS

When hyprocretin-1 was microinjected into the NPO during quiet sleep, active sleep occurred with a short latency. In addition, there was a significant increase in the time spent in active sleep and in the number of episodes of this state. On the other hand, the injection of hyprocretin-1 during wakefulness resulted not only in a significant increase in wakefulness, but also in a decrease in the percentage and frequency of episodes of active sleep.

CONCLUSIONS

The present data demonstrate that the behavioral state of the animal dictates whether active sleep or wakefulness is induced following the injection of hypocretin. Therefore, we suggest that hypocretin-1 enhances ongoing states of wakefulness and their accompanying patterns of physiologic activity and that hypocretin-1 is also capable of promoting active sleep and the changes in various processes that occur during this state.

Functional neuroanatomy of the noradrenergic locus coeruleus: its roles in the regulation of arousal and autonomic function part I: principles of functional organisation

The locus coeruleus (LC) is the major noradrenergic nucleus of the brain, giving rise to fibres innervating extensive areas throughout the neuraxis. Recent advances in neuroscience have resulted in the unravelling of the neuronal circuits controlling a number of physiological functions in which the LC plays a central role. Two such functions are the regulation of arousal and autonomic activity, which are inseparably linked largely via the involvement of the LC. The LC is a major wakefulness-promoting nucleus, resulting from dense excitatory projections to the majority of the cerebral cortex, cholinergic neurones of the basal forebrain, cortically-projecting neurones of the thalamus, serotoninergic neurones of the dorsal raphe and cholinergic neurones of the pedunculopontine and laterodorsal tegmental nucleus, and substantial inhibitory projections to sleep-promoting GABAergic neurones of the basal forebrain and ventrolateral preoptic area. Activation of the LC thus results in the enhancement of alertness through the innervation of these varied nuclei. The importance of the LC in controlling autonomic function results from both direct projections to the spinal cord and projections to autonomic nuclei including the dorsal motor nucleus of the vagus, the nucleus ambiguus, the rostroventrolateral medulla, the Edinger-Westphal nucleus, the caudal raphe, the salivatory nuclei, the paraventricular nucleus, and the amygdala. LC activation produces an increase in sympathetic activity and a decrease in parasympathetic activity via these projections. Alterations in LC activity therefore result in complex patterns of neuronal activity throughout the brain, observed as changes in measures of arousal and autonomic function.

Hypocretin/orexin and energy expenditure

The hypocretins or orexins are endogenous neuropeptides synthesized in discrete lateral, perifornical and dorsal hypothalamic neurones. These multi-functional neuropeptides modulate energy homeostasis, arousal, stress, reward, reproduction and cardiovascular function. This review summarizes the role of hypocretins in modulating non-sleep-related energy expenditure with specific focus on the augmentation of whole body energy expenditure as well as hypocretin-induced physical activity and sympathetic outflow. We compare the efficacy of hypocretin-1 and 2 on energy expenditure and evaluate whether the literature implicates hypocretin signalling though the hypocretin-1 and -2 receptor as having shared and or functionally specific physiological effects. Thus far data suggest that hypocretin-1 has a more robust stimulatory effect relative to hypocretin-2. Furthermore, hypocretin-1 receptor predominantly mediates behaviours known to influence energy expenditure. Further studies on the hypocretin-2 receptor are needed.

State-dependent control of lumbar motoneurons by the hypocretinergic system

Neurons in the lateral hypothalamus (LH) that synthesize hypocretins (Hcrt-1 and Hcrt-2) are active during wakefulness and excite lumbar motoneurons. Because hypocretinergic cells also discharge during phasic periods of rapid eye movement (REM) sleep, we sought to examine their action on the activity of motoneurons during this state. Accordingly, cat lumbar motoneurons were intracellularly recorded, under alpha-chloralose anesthesia, prior to (control) and during the carbachol-induced REM sleep-like atonia (REMc). During control conditions, LH stimulation induced excitatory postsynaptic potentials (composite EPSP) in motoneurons. In contrast, during REMc, identical LH stimulation induced inhibitory PSPs in motoneurons. We then tested the effects of LH stimulation on motoneuron responses following the stimulation of the nucleus reticularis gigantocellularis (NRGc) which is part of a brainstem-spinal cord system that controls motoneuron excitability in a state-dependent manner. LH stimulation facilitated NRGc stimulation-induced composite EPSP during control conditions whereas it enhanced NRGc stimulation-induced IPSPs during REMc. These intriguing data indicate that the LH exerts a state-dependent control of motor activity. As a first step to understand these results, we examined whether hypocretinergic synaptic mechanisms in the spinal cord were state dependent. We found that the juxtacellular application of Hcrt-1 induced motoneuron excitation during control conditions whereas motoneuron inhibition was enhanced during REMc. These data indicate that the hypocretinergic system acts on motoneurons in a state-dependent manner via spinal synaptic mechanisms. Thus, deficits in Hcrt-1 may cause the coexistence of incongruous motor signs in cataplectic patients, such as motor suppression during wakefulness and movement disorders during REM sleep.

Hypocretin/orexin in arousal and stress

Multiple lines of evidence indicate that hypocretin/orexin (HCRT) participates in the regulation of arousal and arousal-related process. For example, HCRT axons and receptors are found within a variety of arousal-related systems. Moreover, when administered centrally, HCRT exerts robust wake-promoting actions. Finally, a dysregulation of HCRT neurotransmission is associated with the sleep/arousal disorder, narcolepsy. Combined, these observations suggested that HCRT might be a key transmitter system in the regulation of waking. Nonetheless, subsequent evidence indicates that HCRT may not play a prominent role in the initiation of normal waking. Instead HCRT may participate in a variety of processes such as consolidation of waking and/or coupling metabolic state with behavioral state. Additionally, substantial evidence suggests a potential involvement of HCRT in high-arousal conditions, including stress. Thus, HCRT neurotransmission is closely linked to high-arousal conditions, including stress, and HCRT administration exerts a variety of stress-like physiological and behavioral effects that are superimposed on HCRT-induced increases in arousal. Combined, this evidence suggests the hypothesis that HCRT may participate in behavioral responding under high-arousal aversive conditions. Importantly, these actions of HCRT may not be limited to stress. Like stress, appetitive conditions are associated with elevated arousal levels and a stress-like activation of various physiological systems. These and other observations suggest that HCRT may, at least in part, exert affectively neutral actions that are important under high-arousal conditions associated with elevated motivation and/or need for action.

Dorsomedial pontine neurons with descending projections to the medullary reticular formation express orexin-1 and adrenergic alpha2A receptor mRNA

Neurons located in the dorsomedial pontine rapid eye movement (REM) sleep-triggering region send axons to the medial medullary reticular formation (mMRF). This pathway is believed to be important for the generation of REM sleep motor atonia, but other than that they are glutamatergic little is known about neurochemical signatures of these pontine neurons important for REM sleep. We used single-cell reverse transcription and polymerase chain reaction (RT-PCR) to determine whether dorsomedial pontine cells with projections to the mMRF express mRNA for selected membrane receptors that mediate modulatory influences on REM sleep. Fluorescein (FITC)-labeled latex microspheres were microinjected into the mMRF of 26-34-day-old rats under pentobarbital anesthesia. After 5-6 days, rats were sacrificed, pontine slices were obtained and neurons were dissociated from 400 to 600 microm micropunches extracted from dorsomedial pontine reticular formation. We found that 32 out of 51 FITC-labeled cells tested (63+/-7% (SE)) contained the orexin type 1 receptor (ORX1r) mRNA, 27 out of 73 (37+/-6%) contained the adrenergic alpha(2A) receptor (alpha(2A)r) RNA, and 6 out of 31 (19+/-7%) contained both mRNAs. The percentage of cells positive for the ORX1r mRNA was significantly lower (p<0.04) for the dorsomedial pontine cells that were not retrogradely labeled from the mMRF (32+/-11%), whereas alpha(2A)r mRNA was present in a similar percentage of FITC-labeled and unlabeled neurons. Our data suggest that ORX and adrenergic pathways converge on a subpopulation of cells of the pontine REM sleep-triggering region that have descending projections to the medullary region important for the motor control during REM sleep.

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.

Noradrenaline triggers muscle tone by amplifying glutamate-driven excitation of somatic motoneurones in anaesthetized rats

Postural muscle tone is potently suppressed during sleep and cataplexy. Since brainstem noradrenergic cell discharge activity is tightly coupled with state-dependent changes in muscle activity, it is assumed that noradrenergic drive on to somatic motoneurones modulates basal muscle tone. However, it has never been determined whether noradrenergic neurotransmission acts to directly regulate motoneurone activity or whether it functions to modulate prevailing synaptic activity. This is an important distinction because noradrenaline regulates cell excitability by both directly depolarizing neurones and by indirectly potentiating glutamate-mediated excitation. We used reverse-microdialysis, electrophysiology, neuro-pharmacological and histological techniques in anaesthetized rats to determine whether strengthening noradrenergic drive (via exogenous noradrenaline application) on to trigeminal motoneurones affects masseter muscle tone by increasing spontaneous motoneurone activity or whether it acts to amplify prevailing glutamate-driven excitation. Although noradrenaline is hypothesized to modulate motor activity, we found that direct stimulation of trigeminal motoneurones by alpha(1)-adrenoceptor activation had no direct effect on basal masseter tone. However, when glutamate-driven excitation was increased at the trigeminal motor pool by either endogenous glutamate release (induced by the monosynaptic masseteric reflex) or exogenous AMPA application, noradrenaline triggered a potent increase in basal masseter tone. The stimulatory effects of noradrenaline were unmasked and rapidly switched on only in the presence of glutamatergic transmission. Blockade of AMPA receptors abolished this excitatory effect, indicating that noradrenergic drive requires ongoing glutamatergic activity. Our data indicate that exogenous noradrenergic drive does not directly affect spontaneous motoneurone discharge activity in anaesthetized rats; rather, it triggers postural muscle tone by amplifying prevailing glutamate-driven excitation.

Systemic and nasal delivery of orexin-A (Hypocretin-1) reduces the effects of sleep deprivation on cognitive performance in nonhuman primates

Hypocretin-1 (orexin-A) was administered to sleep-deprived (30-36 h) rhesus monkeys immediately preceding testing on a multi-image delayed match-to-sample (DMS) short-term memory task. The DMS task used multiple delays and stimulus images and effectively measures cognitive defects produced by sleep deprivation (Porrino et al., 2005). Two methods of administration of orexin-A were tested, intravenous injections (2.5-10.0 microg/kg, i.v.) and a novel method developed for nasal delivery via an atomizer spray mist to the nostrils (dose estimated 1.0 microg/kg). Results showed that orexin-A delivered via the intravenous and nasal routes significantly improved performance in sleep-deprived monkeys; however, the nasal delivery method was significantly more effective than the highest dose (10 microg/kg) of intravenous orexin-A tested. The improvement in performance by orexin-A was specific to trials classified as high versus low cognitive load as determined by performance difficulty under normal testing conditions. Except for the maximum intravenous dose (10 microg/kg), neither delivery method affected task performance in alert non-sleep-deprived animals. The improved performance in sleep-deprived animals was accompanied by orexin-A related alterations in local cerebral glucose metabolism (CMRglc) in specific brain regions shown previously to be engaged by the task and impaired by sleep deprivation (Porrino et al., 2005). Consistent with the differential effects on performance, nasal delivered orexin-A produced a more pronounced reversal of sleep deprivation induced changes in brain metabolic activity (CMRglc) than intravenous orexin-A. These findings provide strong evidence for the effectiveness of intranasal orexin-A in alleviating cognitive deficits produced by loss of sleep.

Central neural and endocrine mechanisms of non-exercise activity thermogenesis and their potential impact on obesity

The rise in obesity is associated with a decline in the amount of physical activity in which people engage. The energy expended through everyday non-exercise activity, called non-exercise activity thermogenesis (NEAT), has a considerable potential impact on energy balance and weight gain. Comparatively little attention has been paid to the central mechanisms of energy expenditure and how decreases in NEAT might contribute to obesity. In this review, we first examine the sensory and endocrine mechanisms through which energy availability and energy balance are detected that may influence NEAT. Second, we describe the neural pathways that integrate these signals. Lastly, we consider the effector mechanisms that modulate NEAT through the alteration of activity levels as well as through changes in the energy efficiency of movement. Systems that regulate NEAT according to energy balance may be linked to neural circuits that modulate sleep, addiction and the stress response. The neural and endocrine systems that control NEAT are potential targets for the treatment of obesity.

Possible use of repeated cold stress for reducing fatigue in chronic fatigue syndrome: a hypothesis

BACKGROUND

Physiological fatigue can be defined as a reduction in the force output and/or energy-generating capacity of skeletal muscle after exertion, which may manifest itself as an inability to continue exercise or usual activities at the same intensity. A typical example of a fatigue-related disorder is chronic fatigue syndrome (CFS), a disabling condition of unknown etiology and with uncertain therapeutic options. Recent advances in elucidating pathophysiology of this disorder revealed hypofunction of the hypothalamic-pituitary-adrenal axis and that fatigue in CFS patients appears to be associated with reduced motor neurotransmission in the central nervous system (CNS) and to a smaller extent with increased fatigability of skeletal muscle. There is also some limited evidence that CFS patients may have excessive serotonergic activity in the brain and low opioid tone.

PRESENTATION OF THE HYPOTHESIS

This work hypothesizes that repeated cold stress may reduce fatigue in CFS because brief exposure to cold may transiently reverse some physiological changes associated with this illness. For example, exposure to cold can activate components of the reticular activating system such as raphe nuclei and locus ceruleus, which can result in activation of behavior and increased capacity of the CNS to recruit motoneurons. Cold stress has also been shown to reduce the level of serotonin in most regions of the brain (except brainstem), which would be consistent with reduced fatigue according to animal models of exercise-related fatigue. Finally, exposure to cold increases metabolic rate and transiently activates the hypothalamic-pituitary-adrenal axis as evidenced by a temporary increase in the plasma levels of adrenocorticotropic hormone, beta-endorphin and a modest increase in cortisol. The increased opioid tone and high metabolic rate could diminish fatigue by reducing muscle pain and accelerating recovery of fatigued muscle, respectively.

TESTING THE HYPOTHESIS

To test the hypothesis, a treatment is proposed that consists of adapted cold showers (20 degrees Celsius, 3 minutes, preceded by a 5-minute gradual adaptation to make the procedure more comfortable) used twice daily.

IMPLICATIONS OF THE HYPOTHESIS

If testing supports the proposed hypothesis, this could advance our understanding of the mechanisms of fatigue in CFS.

Neuropeptides as possible targets in sleep disorders

Insomnia and hypersomnia are frequent sleep disorders, and they are most often treated pharmacologically with hypnotics and wake-promoting compounds. These compounds act on classical neurotransmitter systems, such as benzodiazepines on GABA-A receptors, and amfetamine-like stimulants on monoaminergic terminals to modulate neurotransmission. In addition, acetylcholine, amino acids, lipids and proteins (cytokines) and peptides, are known to significantly modulate sleep and are, therefore, possibly involved in the pathophysiology of some sleep disorders. Due to the recent developments of molecular biological techniques, many neuropeptides have been newly identified, and some are found to significantly modulate sleep. It was also discovered that the impairment of the hypocretin/orexin neurotransmission (a recently isolated hypothalamic neuropeptide system) is the major pathophysiology of narcolepsy, and hypocretin replacement therapy is anticipated to treat the disease in humans. In this article, the authors briefly review the history of neuropeptide research, followed by the sleep modulatory effects of various neuropeptides. Finally, general strategies for the pharmacological therapeutics targeting the peptidergic systems for sleep disorders are discussed.

Promotion of sleep by targeting the orexin system in rats, dogs and humans

Orexins are hypothalamic peptides that play an important role in maintaining wakefulness in mammals. Permanent deficit in orexinergic function is a pathophysiological hallmark of rodent, canine and human narcolepsy. Here we report that in rats, dogs and humans, somnolence is induced by pharmacological blockade of both orexin OX(1) and OX(2) receptors. When administered orally during the active period of the circadian cycle, a dual antagonist increased, in rats, electrophysiological indices of both non-REM and, particularly, REM sleep, in contrast to GABA(A) receptor modulators; in dogs, it caused somnolence and increased surrogate markers of REM sleep; and in humans, it caused subjective and objective electrophysiological signs of sleep. No signs of cataplexy were observed, in contrast to the rodent, dog or human narcolepsy syndromes. These results open new perspectives for investigating the role of endogenous orexins in sleep-wake regulation.

Gabrb3 gene deficient mice exhibit increased risk assessment behavior, hypotonia and expansion of the plexus of locus coeruleus dendrites

Gabrb3 gene deficient (gabrb3(-/-)) mice, control littermates (gabrb3(+/+)) and their progenitor strains C57Bl/6J and 129/SvJ were assessed for changes in the morphology of the main noradrenergic nuclei, the locus coeruleus (LC) and LC-associated behaviors including anxiety and muscle tone. While the area defined by the cell bodies of the LC was found not to differ between gabrb3(-/-) mice and controls, the pericoerulear dendritic zone of the LC was found to be significantly enlarged in gabrb3(-/-) mice. Relative to controls, gabrb3(-/-) mice were also found to be hypotonic, as was indicated by poor performance on the wire hanging task. Gabrb3(-/-) mice also exhibited a significant increase in stretch-attend posturing, a form of risk assessment behavior associated with anxiety. However, in the plus maze, a commonly used behavioral test for assessing anxiety, no significant difference was observed between gabrb3(-/-) and control mice. Lastly, relative to controls, gabrb3(-/-) mice exhibited significantly less marble burying behavior, a method commonly used to assess obsessive-compulsive behavior. However, the poor marble burying performance of the gabrb3(-/-) mice could be associated with the hypotonic condition exhibited by these mice. In conclusion, the results of this study indicate that the gabrb3 gene contributes to LC noradrenergic dendrite development with the disruption of this gene in mice resulting in an enlarged plexus of LC dendrites with a concurrent reduction in muscle tone and marble burying behavior, an increase in risk assessment behavior but no change in the plus maze parameters that are commonly used for assessing anxiety.

Electrophysiological characterization of neurons in the dorsolateral pontine rapid-eye-movement sleep induction zone of the rat: Intrinsic membrane properties and responses to carbachol and orexins

Pharmacological, lesion and single-unit recording techniques in several animal species have identified a region of the pontine reticular formation (subcoeruleus, SubC) just ventral to the locus coeruleus as critically involved in the generation of rapid-eye-movement (REM) sleep. However, the intrinsic membrane properties and responses of SubC neurons to neurotransmitters important in REM sleep control, such as acetylcholine and orexins/hypocretins, have not previously been examined in any animal species and thus were targeted in this study. We obtained whole-cell patch-clamp recordings from visually identified SubC neurons in rat brain slices in vitro. Two groups of large neurons (mean diameter 30 and 27 mum) were tentatively identified as cholinergic (rostral SubC) and noradrenergic (caudal SubC) neurons. SubC reticular neurons (non-cholinergic, non-noradrenergic) showed a medium-sized depolarizing sag during hyperpolarizing current pulses and often had a rebound depolarization (low-threshold spike, LTS). During depolarizing current pulses they exhibited little adaptation and fired maximally at 30-90 Hz. Those SubC reticular neurons excited by carbachol (n=27) fired spontaneously at 6 Hz, often exhibited a moderately sized LTS, and varied widely in size (17-42 mum). Carbachol-inhibited SubC reticular neurons were medium-sized (15-25 mum) and constituted two groups. The larger group (n=22) was silent at rest and possessed a prominent LTS and associated one to four action potentials. The second, smaller group (n=8) had a delayed return to baseline at the offset of hyperpolarizing pulses. Orexins excited both carbachol excited and carbachol inhibited SubC reticular neurons. SubC reticular neurons had intrinsic membrane properties and responses to carbachol similar to those described for other reticular neurons but a larger number of carbachol inhibited neurons were found (>50%), the majority of which demonstrated a prominent LTS and may correspond to pontine-geniculate-occipital burst neurons. Some or all carbachol-excited neurons are presumably REM-on neurons.

Behavioral and sleep/wake characteristics of mice lacking norepinephrine and hypocretin

We investigated the interaction between norepinephrine (NE) and orexin/hypocretin (Hcrt) in the control of sleep behavior and narcoleptic symptoms by creating mice that were deficient in both neurotransmitters. Mice with a targeted disruption of the dopamine beta-hydroxylase (Dbh) gene (deficient in NE and epinephrine) or the Hcrt gene were bred to generate double knockouts (DKOs), each single KO (Dbh-KO and Hcrt-KO), and control mice. The duration of wake, non-rapid eye movement (NREM) and REM sleep were monitored by electroencephalogram (EEG)/electromyogram (EMG) recording over a 24-h period, and the occurrence of behavioral arrests was monitored by video/EEG recording for 4 h. Overall, there was very little interaction between the two genes; for most parameters that were measured, the DKO mice resembled either Dbh-KO or Hcrt-KO mice. REM sleep was increased in both DKO and Hcrt-KO mice at night relative to the other groups, but DKO mice had significantly more REM sleep during the day than the other three groups. Sleep latency in response to saline or amphetamine injections was reduced in Dbh-KO and DKO mice relative to other groups. Behavioral arrests, that are frequent in Hcrt-KO mice, were not exacerbated in DKO mice.

Cataplexy associated with narcolepsy: epidemiology, pathophysiology and management

Although narcolepsy was first described over 100 years ago, most of what is known about the pathological changes in the CNS that are responsible for this unusual disease has been learned during the past few years. It is now known that narcolepsy is caused by the loss of a relatively few neurons that are responsible for producing the neuropeptide hypocretin in the CNS. The onset of narcolepsy typically occurs in early adulthood and may consist of a variety of symptoms; however, cataplexy (an abrupt, bilateral loss of skeletal muscle tone) is most specific to narcolepsy. TCAs were found to be beneficial for the treatment of cataplexy over 40 years ago and, more recently, the SSRIs have been used to treat the condition. The recent availability of sodium oxybate (the first drug to receive regulatory approval for the treatment of cataplexy) represents a significant advance in the treatment of narcolepsy, as it is highly efficacious for the treatment of cataplexy and shows promise for the treatment of excessive sleepiness and for improving sleep quality in patients with narcolepsy.