Functional magnetic resonance imaging reveals different neural substrates for the effects of orexin-1 and orexin-2 receptor antagonists

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.

The Role of Orexin Receptor Antagonists in Inhibiting Drug Addiction: A Review Article

The orexinergic system and its receptors are involved in many physiological processes. Their functions in energy homeostasis, arousal, cognition, stress processing, endocrine functions, and pain modulation have been investigated. Many studies have shown that the orexinergic system cooperates with the dopaminergic system in the addiction process. Emerging evidence suggests that the orexinergic system can be effective in the induction of drug dependence and tolerance. Therefore, several researches have been conducted on the effect of orexin receptor (OXR) antagonists on reducing tolerance and dependence caused by drug abuse. Due to the significant growth of the studies on the orexinergic system, the current literature was conducted to collect the findings of previous studies on orexin and its receptors in the induction of drug addiction. In addition, cellular and molecular mechanisms of the possible role of orexin in drug tolerance and dependence are discussed. The findings indicate that the administration of OXR antagonists reduces drug dependence. OXR blockers seem to counteract the addictive effects of drugs through multiple mechanisms, such as preventing neuronal adaptation. This review proposes the potential clinical use of OXR antagonists in the treatment of drug dependence.

Associations of sleep disorders with all-cause MCI/dementia and different types of dementia - clinical evidence, potential pathomechanisms and treatment options: A narrative review

Due to worldwide demographic change, the number of older persons in the population is increasing. Aging is accompanied by changes of sleep structure, deposition of beta-amyloid (Aß) and tau proteins and vascular changes and can turn into mild cognitive impairment (MCI) as well as dementia. Sleep disorders are discussed both as a risk factor for and as a consequence of MCI/dementia. Cross-sectional and longitudinal population-based as well as case-control studies revealed sleep disorders, especially sleep-disorderded breathing (SDB) and excessive or insufficient sleep durations, as risk factors for all-cause MCI/dementia. Regarding different dementia types, SDB was especially associated with vascular dementia while insomnia/insufficient sleep was related to an increased risk of Alzheimer's disease (AD). Scarce and still inconsistent evidence suggests that therapy of sleep disorders, especially continuous positive airway pressure (CPAP) in SDB, can improve cognition in patients with sleep disorders with and without comorbid dementia and delay onset of MCI/dementia in patients with sleep disorders without previous cognitive impairment. Regarding potential pathomechanisms via which sleep disorders lead to MCI/dementia, disturbed sleep, chronic sleep deficit and SDB can impair glymphatic clearance of beta-amyloid (Aß) and tau which lead to amyloid deposition and tau aggregation resulting in changes of brain structures responsible for cognition. Orexins are discussed to modulate sleep and Aß pathology. Their diurnal fluctuation is suppressed by sleep fragmentation and the expression suppressed at the point of hippocampal atrophy, contributing to the progression of dementia. Additionally, sleep disorders can lead to an increased vascular risk profile and vascular changes such as inflammation, endothelial dysfunction and atherosclerosis which can foster neurodegenerative pathology. There is ample evidence indicating that changes of sleep structure in aging persons can lead to dementia and also evidence that therapy of sleep disorder can improve cognition. Therefore, sleep disorders should be identified and treated early.

[Sleep and dementia]

Aging is associated with changes in sleep structure and cerebral deposition of amyloid beta and tau proteins. Sleep disturbances precede the onset of dementia by years. Comorbid sleep disorders, such as insomnia and sleep-disordered breathing, a family history of dementia and epigenetic factors can contribute to the development of dementia. This article explores the question of the interaction between sleep and dementia based on the existing literature. Alterations caused by slow wave sleep lead to changes in the glymphatic clearance of amyloid beta, tau proteins and other proteins. Transient and chronic sleep disorders cause disturbances in the brain areas responsible for cognition and behavior. Sleep-regulating brain areas are the first to be affected in the neurodegenerative process and accelerate the risk of dementia. Circadian age-related changes in amyloid beta and tau proteins affect the amount and depth of sleep and vice versa. Amyloid beta in cerebrospinal fluid shows an inverse correlation with sleep. Orexins modulate amyloid beta and sleep.

Modeling Brain Dysconnectivity in Rodents

Altered or atypical functional connectivity as measured with functional magnetic resonance imaging (fMRI) is a hallmark feature of brain connectopathy in psychiatric, developmental, and neurological disorders. However, the biological underpinnings and etiopathological significance of this phenomenon remain unclear. The recent development of MRI-based techniques for mapping brain function in rodents provides a powerful platform to uncover the determinants of functional (dys)connectivity, whether they are genetic mutations, environmental risk factors, or specific cellular and circuit dysfunctions. Here, we summarize the recent contribution of rodent fMRI toward a deeper understanding of network dysconnectivity in developmental and psychiatric disorders. We highlight substantial correspondences in the spatiotemporal organization of rodent and human fMRI networks, supporting the translational relevance of this approach. We then show how this research platform might help us comprehend the importance of connectional heterogeneity in complex brain disorders and causally relate multiscale pathogenic contributors to functional dysconnectivity patterns. Finally, we explore how perturbational techniques can be used to dissect the fundamental aspects of fMRI coupling and reveal the causal contribution of neuromodulatory systems to macroscale network activity, as well as its altered dynamics in brain diseases. These examples outline how rodent functional imaging is poised to advance our understanding of the bases and determinants of human functional dysconnectivity.

Involvement of orexin-2 receptors in the CA1 region of the hippocampus in the extinction and reinstatement of methamphetamine-induced conditioned place preference in the rats

The present study, regarding the orexin receptors having a pivotal role in reward-related psychostimulant use disorder (PUD), aimed to investigate the role of orexin-2 (OX2) receptors in the CA1 region of the hippocampus (HPC) in the extinction and reinstatement of methamphetamine (METH)-induced conditioned place preference (CPP). In the first set of investigations, to determine the role of OX2 receptors in the extinction of METH-induced CPP, rats were daily given (during the extinction) bilaterally intra-CA1 region different doses of TCS OX2 29 (1, 3, 10, and 30 nmol/0.5 μl 12% DMSO) as the selective OX2 receptor antagonist. Then, to demonstrate the role of OX2 receptors in the reinstatement of METH-induced CPP after the extinction was established, each rat bilaterally received TCS OX2 29 at the same doses in the CA1 region before injection of the sub-threshold (priming) dose of METH (0.25 mg/kg, sc) on the reinstatement day. The data revealed that the administration of TCS OX2 29 in the CA1 region reduces the mean extinction latency and suppresses the reinstatement of METH-seeking behavior in extinguished rats. Additionally, the potency of TCS OX2 29 to inhibit the reinstatement phase was higher compared to the potency of this drug to modulate the extinction phase of METH-induced CPP. Accordingly, it could be concluded that the blockade of the OX2 receptors in this area might be an essential application and potential therapeutics in treating METH use disorder.

Hypocretin/Orexin Receptor Pharmacology and Sleep Phases

The hypocretins/orexins are two excitatory neuropeptides, alternately called HCRT1 or orexin-A and HCRT2 or orexin-B, that are the endogenous ligands for two G-protein-coupled receptors, HCRTR1/OX1R and HCRTR2/OX2R. Shortly after the discovery of this system, degeneration of hypocretin/orexin-producing neurons was implicated in the etiology of the sleep disorder narcolepsy. The involvement of this system in a disorder characterized by the loss of control over arousal state boundaries also suggested its role as a critical component of endogenous sleep-wake regulatory circuitry. The broad projections of the hypocretin/orexin-producing neurons, along with differential expression of the two receptors in the projection fields of these neurons, suggest distinct roles for these receptors. While HCRTR1/OX1R is associated with regulation of motivation, reward, and autonomic functions, HCRTR2/OX2R is strongly linked to sleep-wake control. The association of hypocretin/orexin with these physiological processes has led to intense interest in the therapeutic potential of compounds targeting these receptors. Agonists and antagonists for the hypocretin/orexin receptors have shown potential for the treatment of disorders of excessive daytime somnolence and nocturnal hyperarousal, respectively, with the first antagonists approved by the US Food and Drug Administration (FDA) in 2014 and 2019 for the treatment of insomnia. These and related compounds have also been useful tools to advance hypocretin/orexin neurobiology.

Bidirectional gut-brain communication: A role for orexin-A

It is increasingly evident that bidirectional gut-brain signaling provides a communication pathway that uses neural, hormonal, and immunological routes to regulate homeostatic mechanisms such as hunger/satiety as well as emotions and inflammation. Hence, disruption of the gut-brain axis can cause numerous pathophysiologies, including obesity and intestinal inflammatory diseases. One chemical mediator in the gut-brain axis is orexin-A, given that hypothalamic orexin-A affects gastrointestinal motility and secretion, and peripheral orexin in the intestinal mucosa can modulate brain functions, making possible an orexinergic gut-brain network. It has been proposed that orexin-A acts on this axis to regulate nutritional processes, such as short-term intake, gastric acid secretion, and motor activity associated with the cephalic phase of feeding. Orexin-A has also been related to stress systems and stress responses via the hypothalamic-pituitary-adrenal axis. Recent studies on the relationship of orexin with immune system-brain communications in an animal model of colitis suggested an immunomodulatory role for orexin-A in signaling and responding to infection by reducing the production of pro-inflammatory cytokines (e.g., tumor necrosis factor α, interleukin-6, and monocyte chemoattractant protein-1). These studies suggested that orexin administration might be of potential therapeutic value in irritable bowel syndrome or chronic intestinal inflammatory diseases, in which gastrointestinal symptoms frequently coexist with behavioral disorders, including loss of appetite, anxiety, depression, and sleeping disorders. Interventions in the orexinergic system have been proposed as a therapeutic approach to these diseases and for the treatment of chemotherapeutic drug-related hyperalgesia and fatigue in cancer patients.

Substituted Azabicyclo[2.2.1]heptanes as Selective Orexin-1 Antagonists: Discovery of JNJ-54717793

The orexin system consists of two neuropeptides (orexin-A and orexin-B) that exert their mode of action on two receptors (orexin-1 and orexin-2). While the role of the orexin-2 receptor is established as an important modulator of sleep wake states, the role of the orexin-1 receptor is believed to play a role in addiction, panic, or anxiety. In this manuscript, we describe the optimization of a nonselective substituted azabicyclo[2.2.1]heptane dual orexin receptor antagonist (DORA) into orally bioavailable, brain penetrating, selective orexin-1 receptor (OX1R) antagonists. This resulted in the discovery of our first candidate for clinical development, JNJ-54717793.

From orexin receptor agonist YNT-185 to novel antagonists with drug-like properties for the treatment of insomnia

YNT-185 is the first known small molecule acting as orexin 2 receptor (OX₂R) agonist with implication to narcolepsy treatment, served as a template scaffold in generating a small set of seven compounds with predictive affinity to OX₂R. The design of the new small molecules was driven mostly by improving physicochemical properties of the parent drug YNT-185 in parallel with in silico studies, later suggesting their favorable binding modes within the active site of OX₂R. We obtained seven new potential OX₂R binders that were evaluated in vitro for their CNS availability, cytotoxicity, and behavior pattern on OX₂R. Out of them, 15 emerged as the most potent modulator of OX₂R, which, contrary to YNT-185, displayed inverse mode of action, i.e. antagonist profile. 15 was also submitted to an in vivo experiment revealing its ability to permeate through BBB into the brain with a short half-life.

Translational evaluation of novel selective orexin-1 receptor antagonist JNJ-61393215 in an experimental model for panic in rodents and humans

Orexin neurons originating in the perifornical and lateral hypothalamic area project to anxiety- and panic-associated neural circuitry, and are highly reactive to anxiogenic stimuli. Preclinical evidence suggests that the orexin system, and particularly the orexin-1 receptor (OX1R), may be involved in the pathophysiology of panic and anxiety. Selective OX1R antagonists thus may constitute a potential new treatment strategy for panic- and anxiety-related disorders. Here, we characterized a novel selective OX1R antagonist, JNJ-61393215, and determined its affinity and potency for human and rat OX1R in vitro. We also evaluated the safety, pharmacokinetic, and pharmacodynamic properties of JNJ-61393215 in first-in-human single- and multiple-ascending dose studies conducted. Finally, the potential anxiolytic effects of JNJ-61393215 were evaluated both in rats and in healthy men using 35% CO₂ inhalation challenge to induce panic symptoms. In the rat CO₂ model of panic anxiety, JNJ-61393215 demonstrated dose-dependent attenuation of CO₂-induced panic-like behavior without altering baseline locomotor or autonomic activity, and had minimal effect on spontaneous sleep. In phase-1 human studies, JNJ-61393215 at 90 mg demonstrated significant reduction (P < 0.02) in CO₂-induced fear and anxiety symptoms that were comparable to those obtained using alprazolam. The most frequently reported adverse events were somnolence and headache, and all events were mild in severity. These results support the safety, tolerability, and anxiolytic effects of JNJ-61393215, and validate CO₂ exposure as a translational cross-species experimental model to evaluate the therapeutic potential of novel anxiolytic drugs.

Cyclic tailor-made amino acids in the design of modern pharmaceuticals

Tailor-made AAs are indispensable components of modern medicinal chemistry and are becoming increasingly prominent in new drugs. In fact, about 30% of small-molecule pharmaceuticals contain residues of tailor-made AAs or structurally related diamines and amino-alcohols. Cyclic tailor-made AAs present a particular value to rational structural design by virtue of their local conformational constraints and are widely used in lead optimization programs. The present review article highlights 34 compounds, all of which are derived from cyclic AAs, representing recently-approved, small-molecule pharmaceuticals as well as promising drug candidates currently in various phases of clinical study. For each compound, the discussion includes the discovery, therapeutic profile and optimized synthesis, with a focus on the preparation of cyclic tailor-made AA as the principal structural feature. The present review article is intended to serve as a reference source for organic, medicinal and process chemists along with other professionals working in the fields of drug design and pharmaceutical discovery.

Tailor-Made Amino Acids and Fluorinated Motifs as Prominent Traits in Modern Pharmaceuticals

Structural analysis of modern pharmaceutical practices allows for the identification of two rapidly growing trends: the introduction of tailor-made amino acids and the exploitation of fluorinated motifs. Curiously, the former represents one of the most ubiquitous classes of naturally occurring compounds, whereas the latter is the most xenobiotic and comprised virtually entirely of man-made derivatives. Herein, 39 selected compounds, featuring both of these traits in the same molecule, are profiled. The total synthesis, source of the corresponding amino acids and fluorinated residues, and medicinal chemistry aspects and biological properties of the molecules are discussed.

Comparison of Orexin 1 and Orexin 2 Ligand Binding Modes Using X-ray Crystallography and Computational Analysis

The orexin system, which consists of the two G protein-coupled receptors OX1 and OX2, activated by the neuropeptides OX-A and OX-B, is firmly established as a key regulator of behavioral arousal, sleep, and wakefulness and has been an area of intense research effort over the past two decades. X-ray structures of the receptors in complex with 10 new antagonist ligands from diverse chemotypes are presented, which complement the existing structural information for the system and highlight the critical importance of lipophilic hotspots and water molecules for these peptidergic GPCR targets. Learnings from the structural information regarding the utility of pharmacophore models and how selectivity between OX1 and OX2 can be achieved are discussed.

The hypocretin (orexin) system: from a neural circuitry perspective

Hypocretin/orexin neurons are distributed restrictively in the hypothalamus, a brain region known to orchestrate diverse functions including sleep, reward processing, food intake, thermogenesis, and mood. Since the hypocretins/orexins were discovered more than two decades ago, extensive studies have accumulated concrete evidence showing the pivotal role of hypocretin/orexin in diverse neural modulation. New method of viral-mediated tracing system offers the possibility to map the monosynaptic inputs and detailed anatomical connectivity of Hcrt neurons. With the development of powerful research techniques including optogenetics, fiber-photometry, cell-type/pathway specific manipulation and neuronal activity monitoring, as well as single-cell RNA sequencing, the details of how hypocretinergic system execute functional modulation of various behaviors are coming to light. In this review, we focus on the function of neural pathways from hypocretin neurons to target brain regions. Anatomical and functional inputs to hypocretin neurons are also discussed. We further briefly summarize the development of pharmaceutical compounds targeting hypocretin signaling. This article is part of the special issue on Neuropeptides.

phMRI, neurochemical and behavioral responses to psychostimulants distinguishing genetically selected alcohol-preferring from genetically heterogenous rats

Alcoholism is often associated with other forms of drug abuse, suggesting that innate predisposing factors may confer vulnerability to addiction to diverse substances. However, the neurobiological bases of these factors remain unknown. Here, we have used a combination of imaging, neurochemistry and behavioral techniques to investigate responses to the psychostimulant amphetamine in Marchigian Sardinian (msP) alcohol-preferring rats, a model of vulnerability to alcoholism. Specifically, we employed pharmacological magnetic resonance imaging to investigate the neural circuits engaged by amphetamine challenge, and to relate functional reactivity to neurochemical and behavioral responses. Moreover, we studied self-administration of cocaine in the msP rats. We found stronger functional responses in the extended amygdala, alongside with increased release of dopamine in the nucleus accumbens shell and augmented vertical locomotor activity compared with controls. Wistar and msP rats did not differ in operant cocaine self-administration under short access (2 hours) conditions, but msP rats exhibited a higher propensity to escalate drug intake following long access (6 hours). Our findings suggest that neurobiological and genetic mechanisms that convey vulnerability to excessive alcohol drinking also facilitate the transition from psychostimulants use to abuse.

Orexin as a modulator of fear-related behavior: Hypothalamic control of noradrenaline circuit

Fear is an important physiological function for survival. It appears when animals or humans are confronted with an environmental threat. The amygdala has been shown to play a highly important role in emergence of fear. Hypothalamic orexin neurons are activated by fearful stimuli to evoke a 'defense reaction' with an increase in arousal level and sympathetic outflow to deal with the imminent danger. However, how this system contributes to the emergence of fear-related behavior is not well understood. Orexin neurons in the hypothalamus send excitatory innervations to noradrenergic neurons in the locus coeruleus (NA^LC) which express orexin receptor 1 (OX1R) and send projections to the lateral amygdala (LA). Inhibition of this di-synaptic orexin → NA^LC → LA pathway by pharmacological or opto/chemogenetic methods reduces cue-induced fear expression. Excitatory manipulation of this pathway induces freezing, a fear-related behavior that only occurs when the environment contains some elements suggestive of danger. Although, fear memory helps animals respond to a context or cue previously paired with an aversive stimulus, fear-related behavior is sometimes evoked even in a distinct context containing some similar elements, which is known as fear generalization. Our recent observation suggests that the orexin → NA^LC → LA pathway might contribute to this response. This review focuses on recent advances regarding the role of hypothalamic orexin neurons in behavioral fear expression. We also discuss the potential effectiveness of orexin receptor antagonists for treating excessive fear response or overgeneralization seen in anxiety disorder and post-traumatic stress disorder (PTSD).

Anti-stress neuropharmacological mechanisms and targets for addiction treatment: A translational framework

Stress-related substance use is a major challenge for treating substance use disorders. This selective review focuses on emerging pharmacotherapies with potential for reducing stress-potentiated seeking and consumption of nicotine, alcohol, marijuana, cocaine, and opioids (i.e., key phenotypes for the most commonly abused substances). I evaluate neuropharmacological mechanisms in experimental models of drug-maintenance and relapse, which translate more readily to individuals presenting for treatment (who have initiated and progressed). An affective/motivational systems model (three dimensions: valence, arousal, control) is mapped onto a systems biology of addiction approach for addressing this problem. Based on quality of evidence to date, promising first-tier neurochemical receptor targets include: noradrenergic (α1 and β antagonist, α2 agonist), kappa-opioid antagonist, nociceptin antagonist, orexin-1 antagonist, and endocannabinoid modulation (e.g., cannabidiol, FAAH inhibition); second-tier candidates may include corticotropin releasing factor-1 antagonists, serotonergic agents (e.g., 5-HT reuptake inhibitors, 5-HT3 antagonists), glutamatergic agents (e.g., mGluR2/3 agonist/positive allosteric modulator, mGluR5 antagonist/negative allosteric modulator), GABA-promoters (e.g., pregabalin, tiagabine), vasopressin 1b antagonist, NK-1 antagonist, and PPAR-γ agonist (e.g., pioglitazone). To address affective/motivational mechanisms of stress-related substance use, it may be advisable to combine agents with actions at complementary targets for greater efficacy but systematic studies are lacking except for interactions with the noradrenergic system. I note clinically-relevant factors that could mediate/moderate the efficacy of anti-stress therapeutics and identify research gaps that should be pursued. Finally, progress in developing anti-stress medications will depend on use of reliable CNS biomarkers to validate exposure-response relationships.

Increased training compensates for OX1R blockage-impairment of spatial memory and c-Fos expression in different cortical and subcortical areas

It has been suggested that the orexin system modulates learning and memory-related processes. However, the possible influence that training could have on the effect of the blockade of orexin-A selective receptor (OX1R) on a spatial memory task has not been explored. Therefore, the present study attempts to compare the effects of OX1R antagonist SB-334867 infusion on spatial memory in two different conditions in the Morris Water Maze (MWM). This experiment evaluated the animals' performance in weak training (2 trials per session) vs strong training (6 trials per session) protocols in a spatial version of the MWM. We found that in the 2-trial condition the post-training SB-334867 infusion had a negative effect on consolidation as well as on the retention and reversal learning of the task 72 h later. This effect was not apparent in the 6-trial condition. In addition, while the strong training groups showed a general increase in c-Fos expression in several brain areas of the hippocampal-thalamic-cortical circuit, SB-334867 administration had the opposite effect in areas that have been previously reported to have a high density of OX1R. Specifically, the SB-infused group in the 2-trial condition showed a decrease in c-Fos immunoreactivity in the dentate gyrus, granular retrosplenial and prelimbic cortices, and centrolateral thalamic nucleus. This was not observed for subjects in the 6-trial condition. The activation of these areas could constitute a neuroanatomical substrate involved in the compensatory mechanisms of training upon SB-334867 impairing effects on a MWM spatial task.

Role of medial hypothalamic orexin system in panic, phobia and hypertension

Orexin has been implicated in a number of physiological functions, including arousal, regulation of sleep, energy metabolism, appetitive behaviors, stress, anxiety, fear, panic, and cardiovascular control. In this review, we will highlight research focused on orexin system in the medial hypothalamic regions of perifornical (PeF) and dorsomedial hypothalamus (DMH), and describe the role of this hypothalamic neuropeptide in the behavioral expression of panic and consequent fear and avoidance responses, as well as sympathetic regulation and possible development of chronic hypertension. We will also outline recent data highlighting the clinical potential of single and dual orexin receptor antagonists for neuropsychiatric conditions including panic, phobia, and cardiovascular conditions, such as in hypertension.

Therapeutics development for addiction: Orexin-1 receptor antagonists

The orexin system includes the neuropeptides orexin A and B and the cognate receptors of orexin-1 (OX1) and -2 (OX2) and has been indicated in a number of important physiological processes. It is generally accepted that the OX1 receptor is mainly involved in motivation and reward and the OX2 receptor in the modulation of sleep/wake cycle and energy homeostasis. A variety of OX1 selective antagonists (1-SORAs) have been disclosed in the literature and some of them have been evaluated as potential therapeutics for addiction treatment. In this review we summarize all OX1 antagonists reported thus far based on their core structure. Several dual orexin receptor antagonists (DORAs) and OX2 selective antagonist (2-SORAs) have also been recently evaluated in reward and addiction models. While DORAs may seem pharmacologically advantageous for alcohol addiction given the recent findings on the OX2 receptor in reward and alcohol consumption, 1-SORAs are the better options for other drugs of addiction such as cocaine due to the absence of the sedative effects inherently associated with dual antagonists.

Cognitive profile of ketamine-dependent patients compared with methamphetamine-dependent patients and healthy controls

BACKGROUND

Ketamine has emerged as a major substance of abuse worldwide and has been listed with methamphetamine (METH) as two of the most widely available illicit substances in Taiwan. Only a few studies have examined the long-term consequences of chronic and heavy ketamine abuse. We compared the cognitive function of ketamine-dependent patients with that of METH-dependent patients and healthy controls.

METHODS

We recruited 165 participants (58 ketamine-dependent and 49 METH-dependent patients who sought treatment and 58 healthy controls) and evaluated them by using a cognitive test battery, the Brief Assessment of Cognition in Schizophrenia, with scores being estimated in reference to normative data in general population.

RESULTS

The ketamine-dependent patients had significantly poorer performance than did the controls in many cognitive tests, including verbal memory, motor speed, verbal fluency, and attention and processing speed, and the battery as a whole. METH-dependent patients exhibited poorer function in motor speed, verbal fluency, and attention and processing speed. The ketamine group performed poorer than did METH group in the domains of verbal memory, working memory, and attention and processing speed and the composite battery scores. A previous experience of ketamine-induced psychotomimetic symptoms, using higher doses of ketamine, and longer abstinence appeared to be associated with performance in some tests; however, the significance disappeared after multiple comparison correction.

CONCLUSIONS

The ketamine-dependent patients had impaired cognitive function, and METH-dependent patients exhibited intermediate performance between ketamine-dependent patients and healthy controls. Given the growing population of ketamine abusers, public education on the cognitive consequences should be provided.

Hypocretin as a Hub for Arousal and Motivation

The lateral hypothalamus is comprised of a heterogeneous mix of neurons that serve to integrate and regulate sleep, feeding, stress, energy balance, reward, and motivated behavior. Within these populations, the hypocretin/orexin neurons are among the most well studied. Here, we provide an overview on how these neurons act as a central hub integrating sensory and physiological information to tune arousal and motivated behavior accordingly. We give special attention to their role in sleep-wake states and conditions of hyper-arousal, as is the case with stress-induced anxiety. We further discuss their roles in feeding, drug-seeking, and sexual behavior, which are all dependent on the motivational state of the animal. We further emphasize the application of powerful techniques, such as optogenetics, chemogenetics, and fiber photometry, to delineate the role these neurons play in lateral hypothalamic functions.

Synthesis and Evaluation of Orexin-1 Receptor Antagonists with Improved Solubility and CNS Permeability

Orexins are hypothalamic neuropeptides playing important roles in many functions including the motivation of addictive behaviors. Blockade of the orexin-1 receptor has been suggested as a potential strategy for the treatment of drug addiction. We have previously reported OX₁ receptor antagonists based on the tetrahydroisoquinoline scaffold with excellent OX₁ potency and selectivity; however, these compounds had high lipophilicity (clogP > 5) and low to moderate solubility. In an effort to improve their properties, we have designed and synthesized a series of analogues where the 7-position substituents known to favor OX₁ potency and selectivity were retained, and groups of different nature were introduced at the 1-position where substitution was generally tolerated as demonstrated in previous studies. Compound 44 with lower lipophilicity (clogP = 3.07) displayed excellent OX₁ potency ( K_e = 5.7 nM) and selectivity (>1,760-fold over OX₂) in calcium mobilization assays. In preliminary ADME studies, 44 showed excellent kinetic solubility (>200 μM), good CNS permeability ( P_app = 14.7 × 10^-6 cm/sec in MDCK assay), and low drug efflux (efflux ratio = 3.3).

Orexin OX2 Receptor Antagonists as Sleep Aids

The discovery of the orexin system represents the single major progress in the sleep field of the last three to four decades. The two orexin peptides and their two receptors play a major role in arousal and sleep/wake cycles. Defects in the orexin system lead to narcolepsy with cataplexy in humans and dogs and can be experimentally reproduced in rodents. At least six orexin receptor antagonists have reached Phase II or Phase III clinical trials in insomnia, five of which are dual orexin receptor antagonists (DORAs) that target both OX1 and OX2 receptors (OX2Rs). All clinically tested DORAs induce and maintain sleep: suvorexant, recently registered in the USA and Japan for insomnia, represents the first hypnotic principle that acts in a completely different manner from the current standard medications. It is clear, however, that in the clinic, all DORAs promote sleep primarily by increasing rapid eye movement (REM) and are almost devoid of effects on slow-wave (SWS) sleep. At present, there is no consensus on whether the sole promotion of REM sleep has a negative impact in patients suffering from insomnia. However, sleep onset REM (SOREM), which has been documented with DORAs, is clearly an undesirable effect, especially for narcoleptic patients and also in fragile populations (e.g. elderly patients) where REM-associated loss of muscle tone may promote an elevated risk of falls. Debate thus remains as to the ideal orexin agent to achieve a balanced increase in REM and non-rapid eye movement (NREM) sleep. Here, we review the evidence that an OX2R antagonist should be at least equivalent, or perhaps superior, to a DORA for the treatment of insomnia. An OX2R antagonist may produce more balanced sleep than a DORA. Rodent sleep experiments show that the OX2R is the primary target of orexin receptor antagonists in sleep modulation. Furthermore, an OX2R antagonist should, in theory, have a lower narcoleptic/cataplexic potential. In the clinic, the situation remains equivocal, since OX2R antagonists are in early stages: MK-1064 has completed Phase I, and MIN202 is currently in clinical Phase II/III trials. However, data from insomnia patients have not yet been released. Promotional material suggests that balanced sleep is indeed induced by MIN-202, whereas in volunteers MK-1064 has been reported to act similarly to DORAs.

Orexin-1 receptor signalling in the prelimbic cortex and ventral tegmental area regulates cue-induced reinstatement of ethanol-seeking in iP rats

Orexins (hypocretins) are hypothalamic neuropeptides that innervate the entire neuraxis, including the prelimbic cortex and ventral tegmental area and have been implicated in ethanol-seeking behaviour. The present study aimed to use the orexin-1 (OX1 ) receptor antagonist SB-334867 to examine the role of prelimbic cortex and ventral tegmental area OX1 receptors in cue-induced reinstatement of ethanol-seeking. Ethanol-preferring rats (iP) rats were trained to self-administer ethanol (10 percent v/v, FR3) or sucrose (0.2-1 percent w/v, FR3) in the presence of reward-associated cues before being implanted with indwelling guide cannulae. Rats then underwent extinction training for 11 days. On test days, rats were given a microinjection of vehicle or SB-334867 (3 μg/side) and presented with reward-associated cues to precipitate reinstatement. Results show SB-334867 infused into the prelimbic cortex attenuated cue-induced reinstatement of ethanol-seeking, but not sucrose-seeking. OX1 antagonism in the ventral tegmental area also attenuated cue-induced reinstatement of ethanol-seeking. These findings suggest that OX1 receptors located in the prelimbic cortex and ventral tegmental area are part of a circuit driving cue-mediated ethanol-seeking behaviour.

Orexin A-mediated AKT signaling in the dentate gyrus contributes to the acquisition, expression and reinstatement of morphine-induced conditioned place preference

Accumulating evidence indicates that the hippocampal dentate gyrus (DG), a critical brain region contributing to learning and memory, is involved in the addiction and relapse to abused drugs. Emerging studies also suggest the role of orexin signaling in the rewarding behavior induced by repeated exposure to opiates. In the present study, we investigated the dynamic adaptation of orexin signaling in the DG and its functional significance in the acquisition, expression, maintenance of and relapse to rewarding behavior induced by morphine. Repeated place conditioning with morphine significantly increased the orexin A content released from the lateral hypothalamic area projecting neurons into the DG. Local infusions of orexin A into the DG sensitized the acquisition of and relapse to the conditioned place preference induced by morphine. The application of the orexin receptor type 1 (OXR1) antagonist SB334867 significantly abolished the acquisition, expression and maintenance of the conditioned place preference induced by repeated exposure to morphine. Furthermore, the significant increase of the phosphorylation of AKT in the DG was associated with preference for the morphine-paired chamber in rats, which was reversed by the local administration of an OXR1 antagonist. Thus, these findings suggested that the dynamic upregulation of orexin A signaling, via the AKT pathway in the DG, may promote the acquisition and maintenance of opioid-induced craving behaviors and may increase sensitivity to the rewarding effect of subsequent opioids.

Temperament and arousal systems: A new synthesis of differential psychology and functional neurochemistry

This paper critically reviews the unidimensional construct of General Arousal as utilised by models of temperament in differential psychology for example, to underlie 'Extraversion'. Evidence suggests that specialization within monoamine neurotransmitter systems contrasts with the attribution of a "general arousal" of the Ascending Reticular Activating System. Experimental findings show specialized roles of noradrenaline, dopamine, and serotonin systems in hypothetically mediating three complementary forms of arousal that are similar to three functional blocks described in classical models of behaviour within kinesiology, clinical neuropsychology, psychophysiology and temperament research. In spite of functional diversity of monoamine receptors, we suggest that their functionality can be classified using three universal aspects of actions related to expansion, to selection-integration and to maintenance of chosen behavioural alternatives. Monoamine systems also differentially regulate analytic vs. routine aspects of activities at cortical and striatal neural levels. A convergence between main temperament models in terms of traits related to described functional aspects of behavioural arousal also supports the idea of differentiation between these aspects analysed here in a functional perspective.

The highly selective orexin/hypocretin 1 receptor antagonist GSK1059865 potently reduces ethanol drinking in ethanol dependent mice

The orexin/hypocretin (ORX) system plays a major role in motivation for natural and drug rewards. In particular, a number of studies have shown that ORX signaling through the orexin 1 receptor (OX1R) regulates alcohol seeking and consumption. Despite the association between ORX signaling and motivation for alcohol, no study to date has investigated what role the ORX system plays in alcohol dependence, an understanding of which would have significant clinical relevance. This study was designed to evaluate the effect of the highly selective OX1R antagonist GSK1059865 on voluntary ethanol intake in ethanol-dependent and control non-dependent mice. Mice were subjected to a protocol in which they were evaluated for baseline ethanol intake and then exposed to intermittent ethanol or air exposure in inhalation chambers. Each cycle of chronic intermittent ethanol (CIE), or air, exposure was followed by a test of ethanol intake. Once the expected effect of increased voluntary ethanol intake was obtained in ethanol dependent mice, mice were tested for the effect of GSK1059865 on ethanol and sucrose intake. Treatment with GSK1059865 significantly decreased ethanol drinking in a dose-dependent manner in CIE-exposed mice. In contrast GSK1059865 decreased drinking in air-exposed mice only at the highest dose used. There was no effect of GSK1059865 on sucrose intake. Thus, ORX signaling through the OX1R, using a highly-selective antagonist, has a profound influence on high levels of alcohol drinking induced in a dependence paradigm, but limited or no influence on moderate alcohol drinking or sucrose drinking. These results indicate that the ORX system may be an important target system for treating disorders of compulsive reward seeking such as alcoholism and other addictions in which motivation is strongly elevated.

Hypocretin receptor 1 blockade preferentially reduces high effort responding for cocaine without promoting sleep

Recent evidence suggests that blockade of the hypocretin receptor 1 may act as a useful pharmacotherapy for cocaine abuse. Here we investigated the extent to which various doses of a hypocretin receptor 1 antagonist, SB-334867, affect cocaine self-administration at varying doses of cocaine and across a range of effort requirements, and tested if these SB-334867 doses produce sedative effects. First, we trained animals to self-administer one of three doses of cocaine on a progressive ratio schedule, and then tested the effects of three doses of SB-334867. Responding for cocaine was then analyzed to segregate features of relatively high and low effort requirements across the progressive ratio session. In another set of experiments, we tested potential sleep-promoting effects of the same doses of SB-334867. Our data indicate that blockade of hypocretin receptor 1 preferentially reduces high effort responding for cocaine at levels that do not promote sedation.

Effect of 1-substitution on tetrahydroisoquinolines as selective antagonists for the orexin-1 receptor

Selective blockade of the orexin-1 receptor (OX1) has been suggested as a potential approach to drug addiction therapy because of its role in modulating the brain's reward system. We have recently reported a series of tetrahydroisoquinoline-based OX1 selective antagonists. Aimed at elucidating structure-activity relationship requirements in other regions of the molecule and further enhancing OX1 potency and selectivity, we have designed and synthesized a series of analogues bearing a variety of substituents at the 1-position of the tetrahydroisoquinoline. The results show that an optimally substituted benzyl group is required for activity at the OX1 receptor. Several compounds with improved potency and/or selectivity have been identified. When combined with structural modifications that were previously found to improve selectivity, we have identified compound 73 (RTIOX-251) with an apparent dissociation constant (Ke) of 16.1 nM at the OX1 receptor and >620-fold selectivity over the OX2 receptor. In vivo, compound 73 was shown to block the development of locomotor sensitization to cocaine in rats.

A selective orexin-1 receptor antagonist attenuates stress-induced hyperarousal without hypnotic effects

Orexins (OXs) are peptides produced by perifornical (PeF) and lateral hypothalamic neurons that exert a prominent role in arousal-related processes, including stress. A critical role for the orexin-1 receptor (OX1R) in complex emotional behavior is emerging, such as overactivation of the OX1R pathway being associated with panic or anxiety states. Here we characterize a brain-penetrant, selective, and high-affinity OX1R antagonist, compound 56 [N-({3-[(3-ethoxy-6-methylpyridin-2-yl)carbonyl]-3-azabicyclo[4.1.0]hept-4-yl}methyl)-5-(trifluoromethyl)pyrimidin-2-amine]. Ex vivo receptor binding studies demonstrated that, after subcutaneous administration, compound 56 crossed the blood-brain barrier and occupied OX1Rs in the rat brain at lower doses than standard OX1R antagonists GSK-1059865 [5-bromo-N-({1-[(3-fluoro-2-methoxyphenyl)carbonyl]-5-methylpiperidin-2-yl}methyl)pyridin-2-amine], SB-334867 [1-(2-methyl-1,3-benzoxazol-6-yl)-3-(1,5-naphthyridin-4-yl)urea], and SB-408124 [1-(6,8-difluoro-2-methylquinolin-4-yl)-3-[4-(dimethylamino)phenyl]urea]. Although compound 56 did not alter spontaneous sleep in rats and in wild-type mice, its administration in orexin-2 receptor knockout mice selectively promoted rapid eye movement sleep, demonstrating target engagement and specific OX1R blockade. In a rat model of psychological stress induced by cage exchange, the OX1R antagonist prevented the prolongation of sleep onset without affecting sleep duration. In a rat model of panic vulnerability (involving disinhibition of the PeF OX region) to threatening internal state changes (i.e., intravenous sodium lactate infusion), compound 56 attenuated sodium lactate-induced panic-like behaviors and cardiovascular responses without altering baseline locomotor or autonomic activity. In conclusion, OX1R antagonism represents a novel therapeutic strategy for the treatment of various psychiatric disorders associated with stress or hyperarousal states.

Orexin/hypocretin role in reward: implications for opioid and other addictions

Addiction is a devastating disorder that affects 15.3 million people worldwide. While prevalent, few effective treatments exist. Orexin receptors have been proposed as a potential target for anti-craving medications. Orexins, also known as hypocretins, are neuropeptides produced in neurons of the lateral and dorsomedial hypothalamus and perifornical area, which project widely throughout the brain. The absence of orexins in rodents and humans leads to narcolepsy. However, orexins also have an established role in reward seeking. This review will discuss some of the original studies describing the roles of the orexins in reward seeking as well as specific works that were presented at the 2013 International Narcotics Research Conference. Orexin signalling can promote drug-induced plasticity of glutamatergic synapses onto dopamine neurons of the ventral tegmental area (VTA), a brain region implicated in motivated behaviour. Additional evidence suggests that orexin signalling can also promote drug seeking by initiating an endocannabinoid-mediated synaptic depression of GABAergic inputs to the VTA, and thereby disinhibiting dopaminergic neurons. Orexin neurons co-express the inhibitory opioid peptide dynorphin. It has been proposed that orexin in the VTA may not mediate reward per se, but rather occludes the 'anti-reward' effects of dynorphin. Finally, orexin signalling in the prefrontal cortex and the central amygdala is implicated in reinstatement of reward seeking. This review will highlight recent work describing the role of orexin signalling in cellular processes underlying addiction-related behaviours and propose novel hypotheses for the mechanisms by which orexin signalling may impart drug seeking.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Both Ox1r and Ox2r orexin receptors contribute to the cardiovascular and locomotor components of the novelty stress response in the rat

Orexin contributes to the expression of the cardiovascular and behavioural response of some forms of stress, including novelty stress. Thus, Almorexant, a dual receptor antagonist that blocks the two known orexin receptors, Ox1R and Ox2R, reduces these responses. However, it is not known if the reduction results from blockade of one receptor only or both. To answer this question, the selective Ox1R antagonist ACT335827 and the selective Ox2R antagonist EMPA were injected intragastrically (300 mg/kg) or intraperitoneally (30 and 100 mg/kg) either alone or as a cocktail and compared to Almorexant in rats exposed to novelty stress. Cardiovascular and locomotor responses were recorded by radio-telemetry. Triple immunolabelling was also conducted to establish the distribution of Ox1R and Ox2R in sympathetic preganglionic neurons and orexin neurons. Intraperitoneal injections of ACT335827 (100 mg/kg) reduced the pressor and tachycardic but not the locomotor response of novelty (by 32% and 48%, respectively). Intraperitoneal injections of EMPA (100 mg/kg) only reduced the pressor response (42%). However when given together, ACT335827 and EMPA reduced all 3 components (65%, 60% and 57% of the tachycardic, pressor and locomotor responses, respectively) as Almorexant (100 mg/kg) did (69%, 87% and 72%, respectively). Triple immunolabelling revealed that sympathetic preganglionic neurons were mainly Ox1R positive only while orexin neurons were both Ox1R and Ox2R positive. This study shows that orexin's contribution to the cardiovascular and locomotor components of the novelty stress response is not mediated by one receptor alone, but by both receptors and at different levels of the neuraxis.

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.

Discovery and development of orexin receptor antagonists as therapeutics for insomnia

Insomnia persistently affects the quality and quantity of sleep. Currently approved treatments for insomnia primarily target γ-aminobutyric acid-A (GABA-A) receptor signalling and include benzodiazepines and GABA-A receptor modulators. These drugs are used to address this sleep disorder, but have the potential for side effects such as tolerance and dependence, making them less attractive as maintenance therapy. Forward and reverse genetic approaches in animals have implicated orexin signalling (also referred to as hypocretin signalling) in the control of vigilance and sleep/wake states. Screening for orexin receptor antagonists using in vitro and in vivo methods in animals has identified compounds that block one or other of the orexin receptors (single or dual orexin receptor antagonists [SORAs and DORAs], respectively) in animals and humans. SORAs have primarily been used as probes to further elucidate the roles of the individual orexin receptors, while a number of DORAs have progressed to clinical development as pharmaceutical candidates for insomnia. The DORA almorexant demonstrated significant improvements in a number of clinically relevant sleep parameters in animal models and in patients with insomnia but its development was halted. SB-649868 and suvorexant have demonstrated efficacy and tolerability in Phase II and III trials respectively. Furthermore, suvorexant is currently under review by the Food and Drug Administration for the treatment of insomnia. Based on the publication of recent non-clinical and clinical data, orexin receptor antagonists potentially represent a targeted, effective and well-tolerated new class of medications for insomnia.

Orexin/hypocretin based pharmacotherapies for the treatment of addiction: DORA or SORA?

Addiction is a chronic relapsing disorder which presents a significant global health burden and unmet medical need. The orexin/hypocretin system is an attractive potential therapeutic target as demonstrated by the successful clinical trials of antagonist medications like Suvorexant for insomnia. It is composed of two neuropeptides, orexin-A and orexin-B and two excitatory and promiscuous G-protein coupled receptors, OX1 and OX2. Orexins are known to have a variety of functions, most notably in regulating arousal, appetite and reward. The orexins have been shown to have a role in mediating the effects of several drugs of abuse, such as cocaine, morphine and alcohol via projections to key brain regions such as the ventral tegmental area, nucleus accumbens and prefrontal cortex. However, it has not yet been demonstrated whether the dual orexin receptor antagonists (DORAs) under development for insomnia are ideal drugs for the treatment of addiction. The question of whether to use a DORA or single orexin receptor antagonist (SORA) for the treatment of addiction is a key question that will need to be answered in order to maximize the clinical utility of orexin receptor antagonists. This review will examine the role of the orexin/hypocretin system in addiction, orexin-based pharmacotherapies under development and factors affecting the selection of one or both orexin receptors as drug targets for the treatment of addiction.

Diurnal inhibition of NMDA-EPSCs at rat hippocampal mossy fibre synapses through orexin-2 receptors

Diurnal release of the orexin neuropeptides orexin-A (Ox-A, hypocretin-1) and orexin-B (Ox-B, hypocretin-2) stabilises arousal, regulates energy homeostasis and contributes to cognition and learning. However, whether cellular correlates of brain plasticity are regulated through orexins, and whether they do so in a time-of-day-dependent manner, has never been assessed. Immunohistochemically we found sparse but widespread innervation of hippocampal subfields through Ox-A- and Ox-B-containing fibres in young adult rats. The actions of Ox-A were studied on NMDA receptor (NMDAR)-mediated excitatory synaptic transmission in acute hippocampal slices prepared around the trough (Zeitgeber time (ZT) 4-8, corresponding to 4-8 h into the resting phase) and peak (ZT 23) of intracerebroventricular orexin levels. At ZT 4-8, exogenous Ox-A (100 nm in bath) inhibited NMDA receptor-mediated excitatory postsynaptic currents (NMDA-EPSCs) at mossy fibre (MF)-CA3 (to 55.6 ± 6.8% of control, P = 0.0003) and at Schaffer collateral-CA1 synapses (70.8 ± 6.3%, P = 0.013), whereas it remained ineffective at non-MF excitatory synapses in CA3. Ox-A actions were mediated postsynaptically and blocked by the orexin-2 receptor (OX2R) antagonist JNJ10397049 (1 μm), but not by orexin-1 receptor inhibition (SB334867, 1 μm) or by adrenergic and cholinergic antagonists. At ZT 23, inhibitory effects of exogenous Ox-A were absent (97.6 ± 2.9%, P = 0.42), but reinstated (87.2 ± 3.3%, P = 0.002) when endogenous orexin signalling was attenuated for 5 h through i.p. injections of almorexant (100 mg kg(-1)), a dual orexin receptor antagonist. In conclusion, endogenous orexins modulate hippocampal NMDAR function in a time-of-day-dependent manner, suggesting that they may influence cellular plasticity and consequent variations in memory performance across the sleep-wake cycle.

Dysfunctional dopaminergic neurotransmission in asocial BTBR mice

Autism spectrum disorders (ASD) are neurodevelopmental conditions characterized by pronounced social and communication deficits and stereotyped behaviours. Recent psychosocial and neuroimaging studies have highlighted reward-processing deficits and reduced dopamine (DA) mesolimbic circuit reactivity in ASD patients. However, the neurobiological and molecular determinants of these deficits remain undetermined. Mouse models recapitulating ASD-like phenotypes could help generate hypotheses about the origin and neurophysiological underpinnings of clinically relevant traits. Here we used functional magnetic resonance imaging (fMRI), behavioural and molecular readouts to probe dopamine neurotransmission responsivity in BTBR T(+) Itpr3(tf)/J mice (BTBR), an inbred mouse line widely used to model ASD-like symptoms owing to its robust social and communication deficits, and high level of repetitive stereotyped behaviours. C57BL/6J (B6) mice were used as normosocial reference comparators. DA reuptake inhibition with GBR 12909 produced significant striatal DA release in both strains, but failed to elicit fMRI activation in widespread forebrain areas of BTBR mice, including mesolimbic reward and striatal terminals. In addition, BTBR mice exhibited no appreciable motor responses to GBR 12909. DA D1 receptor-dependent behavioural and signalling responses were found to be unaltered in BTBR mice, whereas dramatic reductions in pre- and postsynaptic DA D2 and adenosine A2A receptor function was observed in these animals. Overall these results document profoundly compromised DA D2-mediated neurotransmission in BTBR mice, a finding that is likely to have a role in the distinctive social and behavioural deficits exhibited by these mice. Our results call for a deeper investigation of the role of dopaminergic dysfunction in mouse lines exhibiting ASD-like phenotypes, and possibly in ASD patient populations.

The hypocretin/orexin system: an increasingly important role in neuropsychiatry

Hypocretins, also named as orexins, are excitatory neuropeptides secreted by neurons specifically located in lateral hypothalamus and perifornical areas. Orexinergic fibers are extensively distributed in various brain regions and involved in a number of physiological functions, such as arousal, cognition, stress, appetite, and metabolism. Arousal is the most important function of orexin system as dysfunction of orexin signaling leads to narcolepsy. In addition to narcolepsy, orexin dysfunction is associated with serious neural disorders, including addiction, depression, and anxiety. However, some results linking orexin with these disorders are still contradictory, which may result from differences of detection methods or the precision of tools used in measurements; strategies targeted to orexin system (e.g., antagonists to orexin receptors, gene delivery, and cell transplantation) are promising new tools for treatment of neuropsychiatric disorders, though studies are still in a stage of preclinical or clinical research.