The wake-promoting hypocretin/orexin neurons change their response to noradrenaline after sleep deprivation

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.

Hypothalamic orexinergic neuron changes during the hibernation of the Syrian hamster

Hibernation in small mammals is a highly regulated process with periods of torpor involving drops in body temperature and metabolic rate, as well as a general decrease in neural activity, all of which proceed alongside complex brain adaptive changes that appear to protect the brain from extreme hypoxia and low temperatures. All these changes are rapidly reversed, with no apparent brain damage occurring, during the short periods of arousal, interspersed during torpor—characterized by transitory and partial rewarming and activity, including sleep activation, and feeding in some species. The orexins are neuropeptides synthesized in hypothalamic neurons that project to multiple brain regions and are known to participate in the regulation of a variety of processes including feeding behavior, the sleep-wake cycle, and autonomic functions such as brown adipose tissue thermogenesis. Using multiple immunohistochemical techniques and quantitative analysis, we have characterized the orexinergic system in the brain of the Syrian hamster—a facultative hibernator. Our results revealed that orexinergic neurons in this species consisted of a neuronal population restricted to the lateral hypothalamic area, whereas orexinergic fibers distribute throughout the rostrocaudal extent of the brain, particularly innervating catecholaminergic and serotonergic neuronal populations. We characterized the changes of orexinergic cells in the different phases of hibernation based on the intensity of immunostaining for the neuronal activity marker C-Fos and orexin A (OXA). During torpor, we found an increase in C-Fos immunostaining intensity in orexinergic neurons, accompanied by a decrease in OXA immunostaining. These changes were accompanied by a volume reduction and a fragmentation of the Golgi apparatus (GA) as well as a decrease in the colocalization of OXA and the GA marker GM-130. Importantly, during arousal, C-Fos and OXA expression in orexinergic neurons was highest and the structural appearance and the volume of the GA along with the colocalization of OXA/GM-130 reverted to euthermic levels. We discuss the involvement of orexinergic cells in the regulation of mammalian hibernation and, in particular, the possibility that the high activation of orexinergic cells during the arousal stage guides the rewarming as well as the feeding and sleep behaviors characteristic of this phase.

Neurobiology of cataplexy

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

A circuit perspective on narcolepsy

The sleep disorder narcolepsy is associated with symptoms related to either boundary state control that include excessive daytime sleepiness and sleep fragmentation, or rapid eye movement (REM) sleep features including cataplexy, sleep paralysis, hallucinations, and sleep-onset REM sleep events (SOREMs). Although the loss of Hypocretin/Orexin (Hcrt/Ox) peptides or their receptors have been associated with the disease, here we propose a circuit perspective of the pathophysiological mechanisms of these narcolepsy symptoms that encompasses brain regions, neuronal circuits, cell types, and transmitters beyond the Hcrt/Ox system. We further discuss future experimental strategies to investigate brain-wide mechanisms of narcolepsy that will be essential for a better understanding and treatment of the disease.

Arousal and sleep circuits

The principal neurons of the arousal and sleep circuits are comprised by glutamate and GABA neurons, which are distributed within the reticular core of the brain and, through local and distant projections and interactions, regulate cortical activity and behavior across wake-sleep states. These are in turn modulated by the neuromodulatory systems that are comprised by acetylcholine, noradrenaline, dopamine, serotonin, histamine, orexin (hypocretin), and melanin-concentrating hormone (MCH) neurons. Glutamate and GABA neurons are heterogeneous in their profiles of discharge, forming distinct functional cell types by selective or maximal discharge during (1) waking and paradoxical (REM) sleep, (2) during slow wave sleep, (3) during waking, or (4) during paradoxical (REM) sleep. The neuromodulatory systems are each homogeneous in their profile of discharge, the majority discharging maximally during waking and paradoxical sleep or during waking. Only MCH neurons discharge maximally during sleep. They each exert their modulatory influence upon other neurons through excitatory and inhibitory receptors thus effecting a concerted differential change in the functionally different cell groups. Both arousal and sleep circuit neurons are homeostatically regulated as a function of their activity in part through changes in receptors. The major pharmacological agents used for the treatment of wake and sleep disorders act upon GABA and neuromodulatory transmission.

Pharmacosynthetic Deconstruction of Sleep-Wake Circuits in the Brain

Over the past decade, basic sleep research investigating the circuitry controlling sleep and wakefulness has been boosted by pharmacosynthetic approaches, including chemogenetic techniques using designed receptors exclusively activated by designer drugs (DREADD). DREADD offers a series of tools that selectively control neuronal activity as a way to probe causal relationship between neuronal sub-populations and the regulation of the sleep-wake cycle. Following the path opened by optogenetics, DREADD tools applied to discrete neuronal sub-populations in numerous brain areas quickly made their contribution to the discovery and the expansion of our understanding of critical brain structures involved in a wide variety of behaviors and in the control of vigilance state architecture.

Adenosine has two faces: Regionally dichotomous adenosine tone in a model of epilepsy with comorbid sleep disorders

OBJECTIVE

Adenosine participates in maintaining the excitatory/inhibitory balance in neuronal circuits. Studies indicate that adenosine levels in the cortex and hippocampus increase and exert sleep pressure in sleep-deprived and control animals, whereas in epilepsy reduced adenosine tone promotes hyperexcitability. To date, the role of adenosine in pathological conditions that result in both seizures and sleep disorders is unknown. Here, we determined adenosine tone in sleep and seizure regulating brain regions of K_v1.1 knockout (KO) mice, a model of temporal epilepsy with comorbid sleep disorders.

METHODS

1) Reverse phase-high performance liquid chromatography (RP-HPLC) was performed on brain tissue to determine levels of adenosine and adenine nucleotides. 2) Multi-electrode array extracellular electrophysiology was used to determine adenosine tone in the hippocampal CA1 region and the lateral hypothalamus (LH).

RESULTS

RP-HPLC indicated a non-significant decrease in adenosine (~50%, p = 0.23) in whole brain homogenates of KO mice. Regional examination of relative levels of adenine nucleotides indicated decreased ATP and increased AMP in the cortex and hippocampus and increased adenosine in cortical tissue. Using electrophysiological and pharmacological techniques, estimated adenosine levels were ~35% lower in the KO hippocampal CA1 region, and 1-2 fold higher in the KO LH. Moreover, the increased adenosine in KO LH contributed to lower spontaneous firing rates of putative wake-promoting orexin/hypocretin neurons.

INTERPRETATION

This is the first study to demonstrate a direct correlation of regionally distinct dichotomous adenosine levels in a single model with both epilepsy and comorbid sleep disorders. The weaker inhibitory tone in the dorsal hippocampus is consistent with lower seizure threshold, whereas increased adenosine in the LH is consistent with chronic partial sleep deprivation. This work furthers our understanding of how adenosine may contribute to pathological conditions that underlie sleep disorders within the epileptic brain.

Dose-dependent and opposite effects of orexin A on prepulse inhibition response in sleep-deprived and non-sleep-deprived rats

Orexin is a novel neurotransmitter released from lateral hypothalamus, that is a crucial modulator in sleep/wakefulness system. Recent studies also suggest its possible role in the neurodevelopmental disorders, such as schizophrenia. Our study consists of two experiments, where we investigate the effect of orexin A (OXA), one of two isoforms of orexin that can pass blood brain barrier, on the prepulse inhibition of acoustic startle reflex. The first experiment tested the effect of OXA on PPI response of non-sleep-deprived rats via intraperitoneal injection 30min before testing. Our results show that 40μg/kg OXA attenuates PPI% at 78dB and 86dB prepulse intensities. The second experiment utilized 72-h REM sleep deprivation, as a model for sleep-deprivation-induced impairment of PPI response. Here, we tested the effect of OXA on PPI% of sleep-deprived rats via intraperitoneal injection at the last 30min of sleep deprivation, testing for PPI immediately afterwards. Our results showed that (1) sleep deprivation attenuates the PPI% at 74dB, 78dB and 86dB prepulse intensities and (2) 10μg/kg OXA completely restores the impaired PPI% at 78dB only, where the highest PPI% impairment was observed. These results suggest that orexin A modulates PPI response in rats in a dose-dependent manner, oppositely for non-sleep-deprived and sleep-deprived rats, and a more detailed investigation for the etiology of this effect should follow.

Partial Sleep Deprivation Reduces the Efficacy of Orexin-A to Stimulate Physical Activity and Energy Expenditure

OBJECTIVE

Sufficient sleep is required for weight maintenance. Sleep deprivation due to noise exposure stimulates weight gain by increasing hyperphagia and reducing energy expenditure (EE). Yet the mechanistic basis underlying the weight gain response is unclear. Orexin-A promotes arousal and negative energy balance, and orexin terminals project to the ventrolateral preoptic area (VLPO), which is involved in sleep-to-wake transitions. To determine whether sleep deprivation reduces orexin function in VLPO and to test the hypothesis that sleep deprivation would attenuate the orexin-A-stimulated increase in arousal, physical activity (PA), and EE.

METHODS

Electroencephalogram, electromyogram, distance traveled, and EE were determined in male Sprague-Dawley rats following orexin-A injections into VLPO both before and after acute (12-h) and chronic (8 h/d, 9 d) sleep deprivation by noise exposure.

RESULTS

Orexin-A in the VLPO significantly increased arousal, PA, total EE, and PA-related EE and reduced sleep and respiratory quotient before sleep deprivation. In contrast to after acute sleep deprivation in which orexin-A failed to stimulate EE during PA only, orexin-A failed to significantly increase arousal, PA, fat oxidation, total EE, and PA-related EE after chronic sleep deprivation.

CONCLUSIONS

Sleep deprivation may reduce sensitivity to endogenous stimuli that enhance EE due to PA and thus stimulate weight gain.

Hypocretin (orexin) is critical in sustaining theta/gamma-rich waking behaviors that drive sleep need

Hcrt gene inactivation in mice leads to behavioral state instability, abnormal transitions to paradoxical sleep, and cataplexy, hallmarks of narcolepsy. Sleep homeostasis is, however, considered unimpaired in patients and narcoleptic mice. We find that whereas Hcrt^ko/ko mice respond to 6-h sleep deprivation (SD) with a slow-wave sleep (SWS) EEG δ (1.0 to 4.0 Hz) power rebound like WT littermates, spontaneous waking fails to induce a δ power reflecting prior waking duration. This correlates with impaired θ (6.0 to 9.5 Hz) and fast-γ (55 to 80 Hz) activity in prior waking. We algorithmically identify a theta-dominated wakefulness (TDW) substate underlying motivated behaviors and typically preceding cataplexy in Hcrt^ko/ko mice. Hcrt^ko/ko mice fully implement TDW when waking is enforced, but spontaneous TDW episode duration is greatly reduced. A reformulation of the classic sleep homeostasis model, where homeostatic pressure rises exclusively in TDW rather than all waking, predicts δ power dynamics both in Hcrt^ko/ko and WT mouse baseline and recovery SWS. The low homeostatic impact of Hcrt^ko/ko mouse spontaneous waking correlates with decreased cortical expression of neuronal activity-related genes (notably Bdnf, Egr1/Zif268, and Per2). Thus, spontaneous TDW stability relies on Hcrt to sustain θ/fast-γ network activity and associated plasticity, whereas other arousal circuits sustain TDW during SD. We propose that TDW identifies a discrete global brain activity mode that is regulated by context-dependent neuromodulators and acts as a major driver of sleep homeostasis. Hcrt loss in Hcrt^ko/ko mice causes impaired TDW maintenance in baseline wake and blunted δ power in SWS, reproducing, respectively, narcolepsy excessive daytime sleepiness and poor sleep quality.

Orexin/Hypocretin and Organizing Principles for a Diversity of Wake-Promoting Neurons in the Brain

An enigmatic feature of behavioural state control is the rich diversity of wake-promoting neural systems. This diversity has been rationalized as 'robustness via redundancy', wherein wakefulness control is not critically dependent on one type of neuron or molecule. Studies of the brain orexin/hypocretin system challenge this view by demonstrating that wakefulness control fails upon loss of this neurotransmitter system. Since orexin neurons signal arousal need, and excite other wake-promoting neurons, their actions illuminate nonredundant principles of arousal control. Here, we suggest such principles by reviewing the orexin system from a collective viewpoint of biology, physics and engineering. Orexin peptides excite other arousal-promoting neurons (noradrenaline, histamine, serotonin, acetylcholine neurons), either by activating mixed-cation conductances or by inhibiting potassium conductances. Ohm's law predicts that these opposite conductance changes will produce opposite effects on sensitivity of neuronal excitability to current inputs, thus enabling orexin to differentially control input-output gain of its target networks. Orexin neurons also produce other transmitters, including glutamate. When orexin cells fire, glutamate-mediated downstream excitation displays temporal decay, but orexin-mediated excitation escalates, as if orexin transmission enabled arousal controllers to compute a time integral of arousal need. Since the anatomical and functional architecture of the orexin system contains negative feedback loops (e.g. orexin ➔ histamine ➔ noradrenaline/serotonin-orexin), such computations may stabilize wakefulness via integral feedback, a basic engineering strategy for set point control in uncertain environments. Such dynamic behavioural control requires several distinct wake-promoting modules, which perform nonredundant transformations of arousal signals and are connected in feedback loops.

Monoaminergic control of brain states and sensory processing: Existing knowledge and recent insights obtained with optogenetics

Monoamines are key neuromodulators involved in a variety of physiological and pathological brain functions. Classical studies using physiological and pharmacological tools have revealed several essential aspects of monoaminergic involvement in regulating the sleep-wake cycle and influencing sensory responses but many features have remained elusive due to technical limitations. The application of optogenetic tools led to the ability of monitoring and controlling neuronal populations with unprecedented temporal precision and neurochemical specificity. Here, we focus on recent advances in revealing the roles of some monoamines in brain state control and sensory information processing. We summarize the central position of monoamines in integrating sensory processing across sleep-wake states with an emphasis on research conducted using optogenetic techniques. Finally, we discuss the limitations and perspectives of new integrated experimental approaches in understanding the modulatory mechanisms of monoaminergic systems in the mammalian brain.

Optogenetic evidence for inhibitory signaling from orexin to MCH neurons via local microcircuits

The lateral hypothalamus (LH) is a key regulator of multiple vital behaviors. The firing of brain-wide-projecting LH neurons releases neuropeptides promoting wakefulness (orexin/hypocretin; OH), or sleep (melanin-concentrating hormone; MCH). OH neurons, which coexpress glutamate and dynorphin, have been proposed to excite their neighbors, including MCH neurons, suggesting that LH may sometimes coengage its antagonistic outputs. However, it remains unclear if, when, and how OH actions promote temporal separation of the sleep and wake signals, a process that fails in narcolepsy caused by OH loss. To explore this directly, we paired optogenetic stimulation of OH cells (at rates that promoted awakening in vivo) with electrical monitoring of MCH cells in mouse brain slices. Membrane potential recordings showed that OH cell firing inhibited action potential firing in most MCH neurons, an effect that required GABAA but not dynorphin receptors. Membrane current analysis showed that OH cell firing increased the frequency of fast GABAergic currents in MCH cells, an effect blocked by antagonists of OH but not dynorphin or glutamate receptors, and mimicked by bath-applied OH peptide. In turn, neural network imaging with a calcium indicator genetically targeted to MCH neurons showed that excitation by bath-applied OH peptides occurs in a minority of MCH cells. Collectively, our data provide functional microcircuit evidence that intra-LH feedforward loops may facilitate appropriate switching between sleep and wake signals, potentially preventing sleep disorders.

Wake-active neurons across aging and neurodegeneration: a potential role for sleep disturbances in promoting disease

Sleep/wake disturbance is a feature of almost all common age-related neurodegenerative diseases. Although the reason for this is unknown, it is likely that this inability to maintain sleep and wake states is in large part due to declines in the number and function of wake-active neurons, populations of cells that fire only during waking and are silent during sleep. Consistent with this, many of the brain regions that are most susceptible to neurodegeneration are those that are necessary for wake maintenance and alertness. In the present review, these wake-active populations are systematically assessed in terms of their observed pathology across aging and several neurodegenerative diseases, with implications for future research relating sleep and wake disturbances to aging and age-related neurodegeneration.

A unifying computational framework for stability and flexibility of arousal

Arousal and consciousness flexibly adjust to salient cues, but remain stable despite noise and disturbance. Diverse, highly interconnected neural networks govern the underlying transitions of behavioral state; these networks are robust but very complex. Frameworks from systems engineering provide powerful tools for understanding functional logic behind component complexity. From a general systems viewpoint, a minimum of three communicating control modules may enable flexibility and stability to coexist. Comparators would subtract current arousal from desired arousal, producing an error signal. Regulators would compute control signals from this error. Generators would convert control signals into arousal, which is fed back to comparators, to make the system noise-proof through self-correction. Can specific neurons correspond to these control elements? To explore this, here we consider the brain-wide orexin/hypocretin network, which is experimentally established to be vital for flexible and stable arousal. We discuss whether orexin neurons may act as comparators, and their target neurons as regulators and generators. Experiments are proposed for testing such predictions, based on computational simulations showing that comparators, regulators, and generators have distinct temporal signatures of activity. If some regulators integrate orexin-communicated errors, robust arousal control may be achieved via integral feedback (a basic engineering strategy for tracking a set-point despite noise). An integral feedback view also suggests functional roles for specific molecular aspects, such as differing life-spans of orexin peptides. The proposed framework offers a unifying logic for molecular, cellular, and network details of arousal systems, and provides insight into behavioral state transitions, complex behavior, and bases for disease.

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.

A physiologically based model of orexinergic stabilization of sleep and wake

The orexinergic neurons of the lateral hypothalamus (Orx) are essential for regulating sleep-wake dynamics, and their loss causes narcolepsy, a disorder characterized by severe instability of sleep and wake states. However, the mechanisms through which Orx stabilize sleep and wake are not well understood. In this work, an explanation of the stabilizing effects of Orx is presented using a quantitative model of important physiological connections between Orx and the sleep-wake switch. In addition to Orx and the sleep-wake switch, which is composed of mutually inhibitory wake-active monoaminergic neurons in brainstem and hypothalamus (MA) and the sleep-active ventrolateral preoptic neurons of the hypothalamus (VLPO), the model also includes the circadian and homeostatic sleep drives. It is shown that Orx stabilizes prolonged waking episodes via its excitatory input to MA and by relaying a circadian input to MA, thus sustaining MA firing activity during the circadian day. During sleep, both Orx and MA are inhibited by the VLPO, and the subsequent reduction in Orx input to the MA indirectly stabilizes sustained sleep episodes. Simulating a loss of Orx, the model produces dynamics resembling narcolepsy, including frequent transitions between states, reduced waking arousal levels, and a normal daily amount of total sleep. The model predicts a change in sleep timing with differences in orexin levels, with higher orexin levels delaying the normal sleep episode, suggesting that individual differences in Orx signaling may contribute to chronotype. Dynamics resembling sleep inertia also emerge from the model as a gradual sleep-to-wake transition on a timescale that varies with that of Orx dynamics. The quantitative, physiologically based model developed in this work thus provides a new explanation of how Orx stabilizes prolonged episodes of sleep and wake, and makes a range of experimentally testable predictions, including a role for Orx in chronotype and sleep inertia.

Hypocretin (orexin) regulation of sleep-to-wake transitions

The hypocretin (Hcrt), also known as orexin, peptides are essential for arousal stability. Here we discuss background information about the interaction of Hcrt with other neuromodulators, including norepinephrine and acetylcholine probed with optogenetics. We conclude that Hcrt neurons integrate metabolic, circadian and limbic inputs and convey this information to a network of neuromodulators, each of which has a different role on the dynamic of sleep-to-wake transitions. This model may prove useful to predict the effects of orexin receptor antagonists in sleep disorders and other conditions.

Optogenetic control of hypocretin (orexin) neurons and arousal circuits

In 1998, our group discovered a cDNA that encoded the precursor of two putative neuropeptides that we called hypocretins for their hypothalamic expression and their similarity to the secretin family of neuropeptides. In the last 16 years, numerous studies have placed the hypocretin system as an integrator of homeostatic functions with a crucial, non-redundant function as arousal stabilizer. We recently applied optogenetic methods to interrogate the role of individual neuronal circuits in sleep-to-wake transitions. The neuronal connections between the hypocretin system and the locus coeruleus (LC) seem to be crucial in establishing the appropriate dynamic of spontaneous awakenings.

Sleep and synaptic homeostasis

In the last decades a substantial knowledge about sleep mechanisms has been accumulated. However, the function of sleep still remains elusive. The difficulty with unraveling sleep's function may arise from the lack of understanding of how the multitude of processes associated with waking and sleep-from gene expression and single neuron activity to the whole brain dynamics and behavior-functionally and mechanistically relate to each other. Therefore, novel conceptual frameworks, which integrate and take into account the variety of phenomena occurring during waking and sleep at different levels, will likely lead to advances in our understanding of the function of sleep, above and beyond what merely descriptive or correlative approaches can provide. One such framework, the synaptic homeostasis hypothesis, focuses on wake- and sleep-dependent changes in synaptic strength. The core claim of this hypothesis is that learning and experience during wakefulness are associated with a net increase in synaptic strength. In turn, the proposed function of sleep is to provide synaptic renormalization, which has important implications with respect to energy needs, intracranial space, metabolic supplies, and, importantly, enables further plastic changes. In this article we review the empirical evidence for this hypothesis, which was obtained at several levels-from gene expression and cellular excitability to structural synaptic modifications and behavioral outcomes. We conclude that although the mechanisms behind the proposed role of sleep in synaptic homeostasis are undoubtedly complex, this conceptual framework offers a unique opportunity to provide mechanistic and functional explanation for many previously disparate observations, and define future research strategies.

A unified survival theory of the functioning of the hypocretinergic system

This article advances the theory that the hypocretinergic (orexinergic) system initiates, coordinates, and maintains survival behaviors and survival-related processes (i.e., the Unified Survival Theory of the Functioning of the Hypocretinergic System or "Unified Hypocretinergic Survival Theory"). A priori presumptive support for the Unified Hypocretinergic Survival Theory emanates from the fact that neurons that contain hypocretin are located in the key executive central nervous system (CNS) site, the lateral hypothalamus, that for decades has been well-documented to govern core survival behaviors such as fight, flight, and food consumption. In addition, the hypocretinergic system exhibits the requisite morphological and electrophysiological capabilities to control survival behaviors and related processes. Complementary behavioral data demonstrate that all facets of "survival" are coordinated by the hypocretinergic system and that hypocretinergic directives are not promulgated except during survival behaviors. Importantly, it has been shown that survival behaviors are selectively impacted when the hypocretinergic system is impaired or rendered nonfunctional, whereas other behaviors are relatively unaffected. The Unified Hypocretinergic Survival Theory resolves the disparate, perplexing, and often paradoxical-appearing results of previous studies; it also provides a foundation for future hypothesis-driven basic science and clinical explorations of the hypocretinergic system.

The lateral hypothalamic area controls paradoxical (REM) sleep by means of descending projections to brainstem GABAergic neurons

It has recently been shown that the ventrolateral part of the periaqueductal gray (VLPAG) and the adjacent dorsal deep mesencephalic nucleus (dDpMe) contain GABAergic neurons gating paradoxical sleep (PS) onset by means of their projection to the glutamatergic PS-on neurons of the sublaterodorsal tegmental nucleus (SLD). To determine the mechanisms responsible for the cessation of activity of these GABAergic PS-off neurons at the onset and during PS, we combined the immunostaining of c-FOS, a marker of neuronal activation, with cholera toxin b subunit (CTb) retrograde tracing from the VLPAG/dDpMe in three groups of rats (control, PS deprived, and PS hypersomniac). We found that the lateral hypothalamic area (LH) is the only brain structure containing a very large number of neurons activated during PS hypersomnia and projecting to the VLPAG/dDpMe. We further demonstrated that 44% of these neurons express the neuropeptide melanin concentrating hormone (MCH). We then showed that bilateral injections in the LH of two inhibitory compounds, clonidine (an α-2 adrenergic agonist) and muscimol (a GABAa agonist) induce an inhibition of PS. Furthermore, after muscimol injections in the LH, the VLPAG/dDpMe contained a large number of activated neurons, mostly GABAergic, and projecting to the SLD. Altogether, our results indicate for the first time that the activation of a population of LH neurons, in part MCH containing, is necessary for PS to occur. Furthermore, our results strongly suggest that these neurons trigger PS by means of their inhibitory projection to the PS-off GABAergic neurons located in the VLPAG/dDpMe.

Sleep-wake sensitive mechanisms of adenosine release in the basal forebrain of rodents: an in vitro study

Adenosine acting in the basal forebrain is a key mediator of sleep homeostasis. Extracellular adenosine concentrations increase during wakefulness, especially during prolonged wakefulness and lead to increased sleep pressure and subsequent rebound sleep. The release of endogenous adenosine during the sleep-wake cycle has mainly been studied in vivo with microdialysis techniques. The biochemical changes that accompany sleep-wake status may be preserved in vitro. We have therefore used adenosine-sensitive biosensors in slices of the basal forebrain (BFB) to study both depolarization-evoked adenosine release and the steady state adenosine tone in rats, mice and hamsters. Adenosine release was evoked by high K⁺, AMPA, NMDA and mGlu receptor agonists, but not by other transmitters associated with wakefulness such as orexin, histamine or neurotensin. Evoked and basal adenosine release in the BFB in vitro exhibited three key features: the magnitude of each varied systematically with the diurnal time at which the animal was sacrificed; sleep deprivation prior to sacrifice greatly increased both evoked adenosine release and the basal tone; and the enhancement of evoked adenosine release and basal tone resulting from sleep deprivation was reversed by the inducible nitric oxide synthase (iNOS) inhibitor, 1400 W. These data indicate that characteristics of adenosine release recorded in the BFB in vitro reflect those that have been linked in vivo to the homeostatic control of sleep. Our results provide methodologically independent support for a key role for induction of iNOS as a trigger for enhanced adenosine release following sleep deprivation and suggest that this induction may constitute a biochemical memory of this state.

Control of sleep and wakefulness

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

Shining light on wakefulness and arousal

Alterations in arousal states are associated with multiple neuropsychiatric disorders, including generalized anxiety disorders, addiction, schizophrenia, and depression. Therefore, elucidating the neurobiological mechanisms controlling the boundaries between arousal, hyperarousal, and hypoarousal is a crucial endeavor in biological psychiatry. Substantial research over several decades has identified distinct arousal-promoting neural populations in the brain; however, how these nuclei act individually and collectively to promote and maintain wakefulness and various arousal states is unknown. We have recently applied optogenetic technology to the repertoire of techniques used to study arousal. Here, we discuss the recent results of these experiments and propose future use of this approach as a way to understand the complex dynamics of neural circuits controlling arousal and arousal-related behaviors.

The orexin-1 receptor antagonist SB-334867 decreases sympathetic responses to a moderate dose of methamphetamine and stress

We recently discovered that inhibiting neurons in the dorsomedial hypothalamus (DMH) attenuated hyperthermia, tachycardia, hypertension, and hyperactivity evoked by the substituted amphetamine 3, 4-methylenedioxymethamphetamine (MDMA). Neurons that synthesize orexin are also found in the region of the DMH. As orexin and its receptors are involved in the regulation of heart rate and temperature, they would seem to be logical candidates as mediators of the effects evoked by amphetamines. The goal of this study was to determine if blockade of orexin-1 receptors in conscious rats would suppress cardiovascular and thermogenic responses evoked by a range of methamphetamine (METH) doses. Male Sprague-Dawley rats (n=6 per group) were implanted with telemetric transmitters measuring body temperature, heart rate, and mean arterial pressure. Animals were randomized to receive pretreatment with either the orexin-1 receptor antagonist SB-334867 (10mg/kg) or an equal volume of vehicle. Thirty minutes later animals were given intraperitoneal (i.p.) injections of either saline, a low (1mg/kg), moderate (5mg/kg) or high (10mg/kg) dose of METH. Pretreatment with SB-334867 significantly attenuated increases in body temperature and mean arterial pressure evoked by the moderate but not the low or high dose of METH. Furthermore, animals treated with SB-334867, compared to vehicle, had lower temperature and heart rate increases after the stress of an i.p. injection. In conclusion, temperature and cardiovascular responses to a moderate dose of METH and to stress appear to involve orexin-1 receptors. The failure to affect a low and a high dose of METH suggests a complex pharmacology dependent on dose. A better understanding of this may lead to the knowledge of how monoamines influence the orexin system and vice versa.

Local network regulation of orexin neurons in the lateral hypothalamus

Obesity and inadequate sleep are among the most common causes of health problems in modern society. Thus, the discovery that orexin (hypocretin) neurons play a pivotal role in sleep/wake regulation, energy balance, and consummatory behaviors has sparked immense interest in understanding the regulatory mechanisms of these neurons. The local network consisting of neurons and astrocytes within the lateral hypothalamus and perifornical area (LH/PFA), where orexin neurons reside, shapes the output of orexin neurons and the LH/PFA. Orexin neurons not only send projections to remote brain areas but also contribute to the local network where they release multiple neurotransmitters to modulate its activity. These neurotransmitters have opposing actions, whose balance is determined by the amount released and postsynaptic receptor desensitization. Modulation and negative feedback regulation of excitatory glutamatergic inputs as well as release of astrocyte-derived factors, such as lactate and ATP, can also affect the excitability of orexin neurons. Furthermore, distinct populations of LH/PFA neurons express neurotransmitters with known electrophysiological actions on orexin neurons, such as melanin-concentrating hormone, corticotropin-releasing factor, thyrotropin-releasing hormone, neurotensin, and GABA. These LH/PFA-specific mechanisms may be important for fine tuning the firing activity of orexin neurons to maintain optimal levels of prolonged output to sustain wakefulness and stimulate consummatory behaviors. Building on these exciting findings should shed further light onto the cellular mechanisms of energy balance and sleep-wake regulation.

Paradoxical function of orexin/hypocretin circuits in a mouse model of Huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder involving progressive motor disturbances, cognitive decline, and desynchronized sleep-wake rhythms. Recent studies revealed that restoring normal sleep-wake cycles can improve cognitive function in HD mice, suggesting that some sleep/wake systems remain operational and thus represent potential therapeutic targets for HD. Hypothalamic neurons expressing orexins/hypocretins (orexin neurons) are fundamental orchestrators of arousal in mammals, but it is unclear whether orexin circuits operate normally in HD. Here we analyzed the electrophysiology, histology, and gene expression of orexin circuits in brain slices from R6/2 mice, a transgenic model of HD with a progressive neurological phenotype. We report that in R6/2 mice, the size of an electrically distinct subpopulation of orexin neurons is reduced, as is the number of orexin-immunopositive cells in some hypothalamic regions. R6/2 orexin cells display altered glutamatergic inputs, and have an abnormal circadian profile of activity, despite normal circadian rhythmicity of the suprachiasmatic nucleus (SCN), the "master clock" of the brain. Nevertheless, even at advanced stages of HD, intrinsic firing properties of orexin cells remain normal and suppressible by serotonin, noradrenaline, and glucose. Furthermore, histaminergic neurons (key cells required for the propagation of orexin-induced arousal) also display normal responses to orexin. Together, these data suggest that the orexin system remains functional and modifiable in HD mice, although its circadian activity profile is disrupted and no longer follows that of the SCN.

Sleep-deprivation regulates α-2 adrenergic responses of rat hypocretin/orexin neurons

We recently demonstrated, in rat brain slices, that the usual excitation by noradrenaline (NA) of hypocretin/orexin (hcrt/orx) neurons was changed to an inhibition following sleep deprivation (SD). Here we describe that in control condition (CC), i.e. following 2 hours of natural sleep in the morning, the α(2)-adrenergic receptor (α(2)-AR) agonist, clonidine, had no effect on hcrt/orx neurons, whereas following 2 hours of SD (SDC), it hyperpolarized the neurons by activating G-protein-gated inwardly rectifying potassium (GIRK) channels. Since concentrations of clonidine up to a thousand times (100 µM) higher than those effective in SDC (100 nM), were completely ineffective in CC, a change in the availability of G-proteins is unlikely to explain the difference between the two conditions. To test whether the absence of effect of clonidine in CC could be due to a down-regulation of GIRK channels, we applied baclofen, a GABA(B) agonist known to also activate GIRK channels, and found that it hyperpolarized hcrt/orx neurons in that condition. Moreover, baclofen occluded the response to clonidine in SDC, indicating that absence of effect of clonidine in CC could not be attributed to down-regulation of GIRK channels. We finally tested whether α(2)-ARs were still available at the membrane in CC and found that clonidine could reduce calcium currents, indicating that α(2)-ARs associated with calcium channels remain available in that condition. Taken together, these results suggest that a pool of α(2)-ARs associated with GIRK channels is normally down-regulated (or desensitized) in hcrt/orx neurons to only become available for their inhibition following sleep deprivation.

Orexins/hypocretins and aminergic systems

Orexin/hypocretin neurones in the posterior hypothalamus are mutually connected with noradrenergic, serotonergic, dopaminergic, histaminergic, and cholinergic neurone systems. They activate these targets by direct post-synaptic and indirect pre-synaptic mechanisms and in turn receive inhibitory feedback and excitatory feed forward control. With respect to behavioural state control, orexin/hypocretin neurones are conducting the orchestra of biogenic amines. This review highlights the role of these players in the control of energy administration, sleep-wake architecture, cortical activation, plasticity, and memory functions in health and disease.

A decade of hypocretins: past, present and future of the neurobiology of arousal

In 1998, two groups independently identified the hypocretins, also known as orexins, as two hypothalamic peptides derived from the same precursor expressed in a few thousand neurones restricted to the perifornical area. A decade later, an amazing set of discoveries has demonstrated a key role for this neurotransmitter system in arousal and beyond. Here I review some of the experiments that led to these discoveries and the implications in the neurobiology of the hypothalamus and our understanding of brain arousal.

Role of polysialylated neural cell adhesion molecule in rapid eye movement sleep regulation in rats

Recent evidence suggests that synaptic plasticity occurs during homeostatic processes, including sleep-wakefulness regulation, although the underlying mechanisms are not well understood. Polysialylated neural cell adhesion molecule (PSA NCAM) is a transmembrane protein that has been implicated in various forms of plasticity. To investigate whether PSA NCAM is involved in the neuronal plasticity associated with spontaneous sleep-wakefulness regulation and sleep homeostasis, four studies were conducted using rats. First, we showed that PSA NCAM immunoreactivity is present in close proximity to key neurons in several nuclei of the sleep-wakefulness system, including the tuberomammillary hypothalamic nucleus, dorsal raphe nucleus, and locus coeruleus. Second, using western blot analysis and densitometric image analysis of immunoreactivity, we found that 6 h of sleep deprivation changed neither the levels nor the general location of PSA NCAM in the sleep-wakefulness system. Finally, we injected endoneuraminidase (Endo N) intracerebroventricularly to examine the effects of polysialic acid removal on sleep-wakefulness states and electroencephalogram (EEG) slow waves at both baseline and during recovery from 6 h of sleep deprivation. Endo N-treated rats showed a small but significant decrease in baseline rapid eye movement (REM) sleep selectively in the late light phase, and a facilitated REM sleep rebound after sleep deprivation, as compared with saline-injected controls. Non-REM sleep and wakefulness were unaffected by Endo N. These results suggest that PSA NCAM is not particularly involved in the regulation of wakefulness or non-REM sleep, but plays a role in the diurnal pattern of REM sleep as well as in some aspects of REM sleep homeostasis.

NR2A at CA1 synapses is obligatory for the susceptibility of hippocampal plasticity to sleep loss

A loss in the necessary amount of sleep alters expression of genes and proteins implicated in brain plasticity, but key proteins that render neuronal circuits sensitive to sleep disturbance are unknown. We show that mild (4-6 h) sleep deprivation (SD) selectively augmented the number of NR2A subunits of NMDA receptors on postsynaptic densities of adult mouse CA1 synapses. The greater synaptic NR2A content facilitated induction of CA3-CA1 long-term depression in the theta frequency stimulation range and augmented the synaptic modification threshold. NR2A-knock-out mice maintained behavioral response to SD, including compensatory increase in post-deprivation resting time, but hippocampal synaptic plasticity was insensitive to sleep loss. After SD, the balance between synaptically activated and slowly recruited NMDA receptor pools during temporal summation was disrupted. Together, these results indicate that NR2A is obligatory for the consequences of sleep loss on hippocampal synaptic plasticity. These findings could advance pharmacological strategies aiming to sustain hippocampal function during sleep restriction.

Orexin/hypocretin: a neuropeptide at the interface of sleep, energy homeostasis, and reward system

Recent studies have implicated the orexin system as a critical regulator of sleep/wake states as well as feeding behavior and reward processes. Orexin deficiency results in narcolepsy in humans, dogs, and rodents, suggesting that the orexin system is particularly important for maintenance of wakefulness. In addition, orexin deficiency also cause abnormalities in energy homeostasis and reward systems. Orexin activates waking active monoaminergic and cholinergic neurons in the hypothalamus and brainstem regions to maintain a long, consolidated waking period. Orexin neurons receive abundant input from the limbic system. Orexin neurons also have reciprocal links with the hypothalamic arcuate nucleus, which regulates feeding. Moreover, the responsiveness of orexin neurons to peripheral metabolic cues, such as leptin and glucose, suggest that these neurons have important role as a link between the energy homeostasis and vigilance states. Orexin neurons also have a link with the dopaminergic reward system in the ventral tegmental nucleus. These findings suggest that the orexin system interacts with systems that regulate emotion, reward, and energy homeostasis to maintain proper vigilance states. Therefore, this system may be a potentially important therapeutic target for treatment of sleep disorder, obesity, emotional stress, and addiction.

Sleep function: current questions and new approaches

The mammalian brain oscillates through three distinct global activity states: wakefulness, non-rapid eye movement (NREM) sleep and REM sleep. The regulation and function of these 'vigilance' or 'behavioural' states can be investigated over a broad range of temporal and spatial scales and at different levels of functional organization, i.e. from gene expression to memory, in single neurons, cortical columns or the whole brain and organism. We summarize some basic questions that have arisen from recent approaches in the quest for the functions of sleep. Whereas traditionally sleep was viewed to be regulated through top-down control mechanisms, recent approaches have emphasized that sleep is emerging locally and regulated in a use-dependent (homeostatic) manner. Traditional markers of sleep homeostasis, such as the electroencephalogram slow-wave activity, have been linked to changes in connectivity and plasticity in local neuronal networks. Thus waking experience-induced local network changes may be sensed by the sleep homeostatic process and used to mediate sleep-dependent events, benefiting network stabilization and memory consolidation. Although many questions remain unanswered, the available data suggest that sleep function will best be understood by an analysis which integrates sleep's many functional levels with its local homeostatic regulation.

Delayed orexin signaling consolidates wakefulness and sleep: physiology and modeling

Orexin-producing neurons are clearly essential for the regulation of wakefulness and sleep because loss of these cells produces narcolepsy. However, little is understood about how these neurons dynamically interact with other wake- and sleep-regulatory nuclei to control behavioral states. Using survival analysis of wake bouts in wild-type and orexin knockout mice, we found that orexins are necessary for the maintenance of long bouts of wakefulness, but orexin deficiency has little impact on wake bouts <1 min. Since orexin neurons often begin firing several seconds before the onset of waking, this suggests a surprisingly delayed onset (>1 min) of functional effects. This delay has important implications for understanding the control of wakefulness and sleep because increasing evidence suggests that different mechanisms are involved in the production of brief and sustained wake bouts. We incorporated these findings into a mathematical model of the mouse sleep/wake network. Orexins excite monoaminergic neurons and we hypothesize that orexins increase the monoaminergic inhibition of sleep-promoting neurons in the ventrolateral preoptic nucleus. We modeled orexin effects as a time-dependent increase in the strength of inhibition from wake- to sleep-promoting populations and the resulting simulated behavior accurately reflects the fragmented sleep/wake behavior of narcolepsy and leads to several predictions. By integrating neurophysiology of the sleep/wake network with emergent properties of behavioral data, this model provides a novel framework for investigating network dynamics and mechanisms associated with normal and pathologic sleep/wake behavior.

Orexin neuronal circuitry: role in the regulation of sleep and wakefulness

Orexin A and orexin B were initially identified as endogenous ligands for two orphan G protein-coupled receptors [104]. They were initially recognized as regulators of feeding behavior in view of their exclusive production in the lateral hypothalamic area (LHA), a region known as the feeding center, and their pharmacological activity [104,30,49,107]. Subsequently, the finding that orexin deficiency causes narcolepsy in humans and animals suggested that these hypothalamic neuropeptides play a critical role in regulating sleep/wake cycle [22,46,71,95,117]. These peptides activate waking-active monoaminergic and cholinergic neurons in the hypothalamus/brain stem regions to maintain a long, consolidated awake period. Recent studies on efferent and afferent systems of orexin neurons, and phenotypic characterization of genetically modified mice in the orexin system further suggested roles of orexin in the coordination of emotion, energy homeostasis, reward system, and arousal [3,80,106,137]. A link between the limbic system and orexin neurons might be important for increasing vigilance during emotional stimuli. Orexin neurons are also regulated by peripheral metabolic cues, including ghrelin, leptin, and glucose, suggesting that they might have important roles as a link between energy homeostasis and vigilance states [137]. Recent research has also implicated orexins in reward systems and the mechanisms of drug addiction [13,48,91]. These observations suggest that orexin neurons sense the outer and inner environment of the body, and maintain proper wakefulness of animals for survival. This review discusses the mechanism by which orexins maintain sleep/wakefulness states, and how this mechanism relates to other systems that regulate emotion, reward, and energy homeostasis.

Effects of rapid eye movement sleep deprivation on hypocretin neurons in the hypothalamus of a rat model of depression

Hypocretin (Hcrt, also known as orexin) is a hypothalamic neuropeptide linked to narcolepsy, a disorder diagnosed by the appearance of rapid eye-movement sleep (REMS)-state characteristics during waking. Major targets of Hcrt-containing fibers include the locus coeruleus and the raphe nucleus, areas with important roles in regulation of mood and sleep. A relationship between REMS and mood is suggested by studies demonstrating that REMS-deprivation (REMSD) ameliorates depressive symptoms in humans. Additional support is found in animal studies where antidepressants and REMSD have similar effects on monoamiergic systems thought to be involved in major depression. Recently, we have reported that Wistar-Kyoto (WKY) rats, an animal model of depression, have reduced number and size of hypothalamic cells expressing Hcrt-immunoractivity compared to the parent, Wistar (WIS) strain, suggesting the possibility that the depressive-like attributes of the WKY rat may be determined by this relative reduction in Hcrt cells [Allard, J.S., Tizabi, Y., Shaffery, J.P., Trouth, C.O., Manaye, K., 2004. Stereological analysis of the hypothalamic hypocretin/orexin neurons in an animal model of depression. Neuropeptides 38, 311-315]. In this study, we sought to test the hypothesis that REMSD would result in a greater increase in the number and/or size of hypothalamic, Hcrt-immunoreactive (Hcrt-ir) neurons in WKY, compared to WIS rats. The effect of REMSD, using the multiple-small-platforms-over-water (SPRD) method, on size and number of Hcrt-ir cells were compared within and across strains of rats that experienced multiple-large-platforms-over-water (LPC) as well as to those in a normal, home-cage-control (CC) setting. In accord with previous findings, the number of Hcrt-ir cells was larger in all three WIS groups compared to the respective WKY groups. REMSD produced a 20% increase (p<0.02) in the number of hypothalamic Hcrt-ir neurons in WKY rats compared to cage control WKY (WKY-CC) animals. However, an unexpected higher increase in number of Hcrt-ir cells was also observed in the WKY-LPC group compared to both WKY-CC (31%, p<0.001) and WKY-SPRD (20%, p<0.002) rats. A similar, smaller, but non-significant, pattern of change was noted in WIS-LPC group. Overall the data indicate a differential response to environmental manipulations where WKY rats appear to be more reactive than WIS rats. Moreover, the findings do not support direct antidepressant-like activity for REMSD on hypothalamic Hcrt neurons in WKY rats.

Orexin and psychiatric symptoms in suicide attempters

BACKGROUND

The orexins (hypocretins) are recently discovered hypothalamic peptides that are involved in the regulation of sleep, appetite and state of arousal. In the present study, we investigated the relationship between cerebrospinal fluid (CSF) orexin and specific psychiatric symptoms in suicidal patients.

METHODS

A total of 101 patients were enrolled in the study shortly after a suicide attempt. All patients underwent a lumbar puncture after a wash-out period during which they did not receive any antipsychotic or antidepressive medication. Structured interviews were performed using the Comprehensive Psychopathological Rating Scale (CPRS). CSF-orexin-A was measured and correlated with ratings of psychiatric symptoms.

RESULTS

There were significant and negative correlations between CSF-orexin and the symptoms lassitude (difficulty to initiate activities) and slowness of movement, as well as the ratings of global illness (p<0.005 for all three items, Spearman's rho).

LIMITATIONS

Correlation analysis is an indirect method of investigation and does not demonstrate causal relationships.

CONCLUSION

Low CSF-orexin levels are related to pronounced symptoms of inertia and reduced motor activity in suicidal patients. Interestingly, the lower the orexin levels, the higher were ratings of overall illness, as observed by a specialist in psychiatry. Our results suggest that reduced orexin levels are involved in the etiology of specific psychiatric symptoms.

The hypothalamic peptidergic system, hypocretin/orexin and vigilance control

Using forward and reverse genetics, the genes (hypocretin/orexin ligand and its receptor) involved in the pathogenesis of the sleep disorder, narcolepsy, in animals, have been identified. Mutations in hypocretin related-genes are extremely rare in humans, but hypocretin-ligand deficiency is found in most narcolepsy-cataplexy cases. Hypocretin deficiency in humans can be clinically detected by CSF hypocretin-1 measures, and undetectably low CSF hypocretin-1 is now included in the revised international diagnostic criteria of narcolepsy. Since hypocretin-ligand deficiency is the major pathophysiology in human narcolepsy, hypocretin replacements (using hypocretin agonists or gene therapy) are promising future therapeutic options. New insights into the roles of hypocretin system on sleep physiology have also rapidly increased. Hypocretins are involved in various fundamental hypothalamic functions such as feeding, energy homeostasis and neuroendocrine regulation. Hypocretin neurons project to most ascending arousal systems (including monoaminergic and cholinergic systems), and generally exhibit excitatory inputs. Together with the recent finding of the sleep promoting system in the hypothalamus (especially in the GABA/galanin ventrolateral preoptic area which exhibits inhibitory inputs to these ascending systems), the hypothalamus is now recognized as the most important brain site for the sleep switch, and other peptidergic systems may also participate in this regulation. Meanwhile, narcolepsy now appears to be a more complex condition than previously thought. The pathophysiology of the disease is involved in the abnormalities of sleep and various hypothalamic functions due to hypocretin deficiency, such as the changes in energy homeostasis, stress reactions and rewarding. Narcolepsy is therefore, an important model to study the link between sleep regulation and other fundamental hypothalamic functions.

The neural circuit of orexin (hypocretin): maintaining sleep and wakefulness

Sleep and wakefulness are regulated to occur at appropriate times that are in accordance with our internal and external environments. Avoiding danger and finding food, which are life-essential activities that are regulated by emotion, reward and energy balance, require vigilance and therefore, by definition, wakefulness. The orexin (hypocretin) system regulates sleep and wakefulness through interactions with systems that regulate emotion, reward and energy homeostasis.

Narcolepsy and the hypocretin system--where motion meets emotion

Narcolepsy is a neurological disorder that is characterized by excessive daytime sleepiness and cataplexy--a loss of muscle tone generally triggered by certain strong emotions with sudden onset. The underlying cause of most cases of human narcolepsy is a loss of neurons that produce hypocretin (Hcrt, also known as orexin). These cells normally serve to drive and synchronize the activity of monoaminergic and cholinergic cells. Sleepiness results from the reduced activity of monoaminergic, cholinergic and other cells that are normally activated by Hcrt neurons, as well as from the loss of Hcrt itself. Cataplexy is caused by an episodic loss of activity in noradrenergic cells that support muscle tone, and a linked activation of a medial medullary cell population that suppresses muscle tone. Current treatments for narcolepsy include stimulants to combat sleepiness and antidepressants to reduce cataplexy. Sodium oxybate produces both reductions in cataplexy and improved waking alertness. Future treatments are likely to include Hcrt or Hcrt agonists to reverse the underlying neurochemical deficit.

Orexin neurons are directly and indirectly regulated by catecholamines in a complex manner

We reported elsewhere that orexin neurons are directly hyperpolarized by noradrenaline (NA) and dopamine. In the present study, we show that NA, dopamine, and adrenaline all directly hyperpolarized orexin neurons. This response was inhibited by the alpha2 adrenergic receptor (alpha2-AR) antagonist, idazoxan or BRL44408, and was mimicked by the alpha2-AR-selective agonist, UK14304. A low concentration of Ba2+ inhibited NA-induced hyperpolarization, which suggests that activation of G protein coupled inward rectifier potassium channels is involved in the response. In the presence of a high concentration of idazoxan, NA induced depolarization or inward current. This response was inhibited by alpha1-AR antagonist, prazosin, which suggests the existence of alpha1-ARs on the orexin neurons along with alpha2-AR. We also examined the effects of NA on glutamatergic and GABAergic synaptic transmission. NA application dramatically increased the frequency and amplitude of spontaneous inhibitory synaptic currents (sIPSCs) and inhibited excitatory synaptic currents (sEPSCs) in orexin neurons; however, NA decreased the frequency of miniature EPSCs (mEPSCs) and IPSCs and the amplitude of evoked EPSCs and IPSCs through the alpha2-AR, because the NA response on mPSCs was inhibited by idazoxan. These results suggest that the NA-induced increase in sIPSC frequency and amplitude is mediated via alpha1-ARs on the somata of GABAergic neurons that innervate the orexin neurons. Calcium imaging using orexin/YC2.1 transgenic mouse brain revealed that NA-induced inhibition of orexin neurons is not altered by sleep deprivation or circadian time in mice. The evidence presented here revealed that orexin neurons are regulated by catecholamines in a complex manner.

CSF prostaglandin D synthase is reduced in excessive daytime sleepiness

Lipocalin-type prostaglandin D synthase (L-PGDS) is a brain enzyme, which produces prostaglandin D(2), a substance with endogenous somnogenic effects. Using a standardized protocol for immunonephelometric determination of cerebrospinal fluid (CSF) L-PGDS levels, we show that CSF L-PGDS levels are significantly lower in 34 patients with excessive daytime sleepiness when compared with levels in 22 healthy controls. Thus, L-PGDS may represent the first neurochemical measure of excessive daytime sleepiness.

Metabolic state signalling through central hypocretin/orexin neurons

Hypothalamic neurons that produce the peptide transmitters hypocretins/orexins have attracted much recent attention. They provide direct and predominantly excitatory inputs to all major brain areas except the cerebellum, with the net effect of stimulating wakefulness and arousal. These inputs are essential for generating sustained wakefulness in mammals, and defects in hypocretin signalling result in narcolepsy. In addition, new roles for hypocretins/orexins are emerging in reward-seeking, learning, and memory. Recent studies also indicate that hypocretin/orexin neurons can alter their intrinsic electrical activity according to ambient fluctuations in the levels of nutrients and appetite-regulating hormones. These intriguing electrical responses are perhaps the strongest candidates to date for the elusive neural correlates of after-meal sleepiness and hunger-induced wakefulness. Hypocretin/orexin neurons may thus directly translate rises and falls in body energy levels into different states of consciousness.

Cholecystokinin activates orexin/hypocretin neurons through the cholecystokinin A receptor

Orexin A and B are neuropeptides implicated in the regulation of sleep/wakefulness and energy homeostasis. The regulatory mechanism of the activity of orexin neurons is not precisely understood. Using transgenic mice in which orexin neurons specifically express yellow cameleon 2.1, we screened for factors that affect the activity of orexin neurons (a total of 21 peptides and six other factors were examined) and found that a sulfated octapeptide form of cholecystokinin (CCK-8S), neurotensin, oxytocin, and vasopressin activate orexin neurons. The mechanisms that underlie CCK-8S-induced activation of orexin neurons were studied by both calcium imaging and slice patch-clamp recording. CCK-8S induced inward current in the orexin neurons. The CCKA receptor antagonist lorglumide inhibited CCK-8S-induced activation of orexin neurons, whereas the CCKB receptor agonists CCK-4 (a tetrapeptide form of cholecystokinin) and nonsulfated CCK-8 had little effect. The CCK-8S-induced increase in intracellular calcium concentration was eliminated by removing extracellular calcium but not by an addition of thapsigargin. Nifedipine, omega-conotoxin, omega-agatoxin, 4-ethylphenylamino-1,2-dimethyl-6-methylaminopyrimidinium chloride, and SNX-482 had little effect, but La3+, Gd3+, and 2-aminoethoxydiphenylborate inhibited CCK-8S-induced calcium influx. Additionally, the CCK-8S-induced inward current was dramatically enhanced in the calcium-free solution and was inhibited by the cation channel blocker SKF96365, suggesting an involvement of extracellular calcium-sensitive cation channels. CCK-8S did not induce an increase in intracellular calcium concentration when membrane potential was clamped at -60 mV, suggesting that the calcium increase is induced by depolarization. The evidence presented here expands our understanding of the regulation of orexin neurons and the physiological role of CCK in the CNS.