Structure and function of human prepro-orexin gene

Neuropeptides Modulate Feeding via the Dopamine Reward Pathway

Dopamine (DA) is a catecholamine neurotransmitter widely distributed in the central nervous system. It participates in various physiological functions, such as feeding, anxiety, fear, sleeping and arousal. The regulation of feeding is exceptionally complex, involving energy homeostasis and reward motivation. The reward system comprises the ventral tegmental area (VTA), nucleus accumbens (NAc), hypothalamus, and limbic system. This paper illustrates the detailed mechanisms of eight typical orexigenic and anorexic neuropeptides that regulate food intake through the reward system. According to recent literature, neuropeptides released from the hypothalamus and other brain regions regulate reward feeding predominantly through dopaminergic neurons projecting from the VTA to the NAc. In addition, their effect on the dopaminergic system is mediated by the prefrontal cortex, paraventricular thalamus, laterodorsal tegmental area, amygdala, and complex neural circuits. Research on neuropeptides involved in reward feeding can help identify more targets to treat diseases with metabolic disorders, such as obesity.

Targeting the orexin/hypocretin system for the treatment of neuropsychiatric and neurodegenerative diseases: from animal to clinical studies

Orexins (also known as hypocretins) are neuropeptides located exclusively in hypothalamic neurons that have extensive projections throughout the central nervous system and bind two different G protein-coupled receptors (OX1R and OX2R). Since its discovery in 1998, the orexin system has gained the interest of the scientific community as a potential therapeutic target for the treatment of different pathological conditions. Considering previous basic science research, a dual orexin receptor antagonist, suvorexant, was the first orexin agent to be approved by the US Food and Drug Administration to treat insomnia. In this review, we discuss and update the main preclinical and human studies involving the orexin system with several psychiatric and neurodegenerative diseases. This system constitutes a nice example of how basic scientific research driven by curiosity can be the best route to the generation of new and powerful pharmacological treatments.

A molecular network map of orexin-orexin receptor signaling system

Orexins are excitatory neuropeptides, which are predominantly associated with feeding behavior, sleep-wake cycle and energy homeostasis. The orexinergic system comprises of HCRTR1 and HCRTR2, G-protein-coupled receptors of rhodopsin family and the endogenous ligands processed from HCRT pro-hormone, Orexin A and Orexin B. These neuropeptides are biosynthesized by the orexin neurons present in the lateral hypothalamus area, with dense projections to other brain regions. The orexin-receptor signaling is implicated in various metabolic as well as neurological disorders, making it a promising target for pharmacological interventions. However, there is limited information available on the collective representation of the signal transduction pathways pertaining to the orexin-orexin receptor signaling system. Here, we depict a compendium of the Orexin A/B stimulated reactions in the form of a basic signaling pathway map. This map catalogs the reactions into five categories: molecular association, activation/inhibition, catalysis, transport, and gene regulation. A total of 318 downstream molecules were annotated adhering to the guidelines of NetPath curation. This pathway map can be utilized for further assessment of signaling events associated with orexin-mediated physiological functions and is freely available on WikiPathways, an open-source pathway database ( https://www.wikipathways.org/index.php/Pathway:WP5094 ).

Multiomics integration reveals the effect of Orexin A on glioblastoma

Objectives: This study involved a multi-omics analysis of glioblastoma (GBM) samples to elaborate the potential mechanism of drug treatment. Methods: The GBM cells treated with or without orexin A were acquired from sequencing analysis. Differentially expressed genes/proteins/metabolites (DEGs/ DEPs/ DEMs) were screened. Next, combination analyses were conducted to investigate the common pathways and correlations between the two groups. Lastly, transcriptome-proteome-metabolome association analysis was carried out to determine the common pathways, and the genes in these pathways were analyzed through Kaplan-Meier (K-M) survival analysis in public databases. Cell and animal experiments were performed to investigate the anti-glioma activity of orexin A. Results: A total of 1,527 DEGs, 52 DEPs, and 153 DEMs were found. Moreover, the combination analyses revealed that 6, 4, and 1 common pathways were present in the transcriptome-proteome, proteome-metabolome, and transcriptome-metabolome, respectively. Certain correlations were observed between the two data sets. Finally, 11 common pathways were discovered in association analysis, and 138 common genes were screened out in these common pathways. Six genes showed significant differences in terms of survival in both TCGA and CGGA. In addition, orexin A inhibited the proliferation, migration, and invasion of glioma in vitro and in vivo. Conclusion: Eleven common KEGG pathways with six common genes were found among different omics participations, revealing the underlying mechanisms in different omics and providing theoretical basis and reference for multi-omics research on drug treatment.

Physiological Role of Orexin/Hypocretin in the Human Body in Motivated Behavior: A Comprehensive Review

Neurohormones are neurosecretory materials released by neurosecretory cells that serve both as neuromodulators in the brain and spinal cord and as circulating regulatory hormones. They serve a wide range of functions, including homeostasis, development, and modulation of neuronal and muscle activity. In the hypothalamus, neurohormones called hypocretins are created that were discovered in the late nineties. Orexin receptors (OXRs) have been shown to enhance synaptic signaling in the central nervous system at the cellular level. The orexins improve stimulated neural activity in the hippocampus, which, in turn, aids with spatial memory, learning, and mood. They present themselves as mediators for the hypothalamic functions. They have been shown to regulate sleep-wake cycles, arousal mechanisms, addiction, sympathetic nerve activity (SNA), blood pressure, and thermogenesis. Its role in storing brown adipose tissue has implications for thermal homeostasis. The significant role of orexins is seen in tumorigenesis when orexin A (OrxA) and orexin B (OrxB) induce apoptosis in fast-growing tumor cells. Orexin-null subjects show clinical narcolepsy, indicating that orexins were responsible for keeping them awake. Orexin microinjections in mice brains stimulated increased physical activity, thus possibly countering diet-induced obesity. Physical activity significantly increased plasma orexin-A levels, which facilitated the process of energy homeostasis. The amount of adrenocorticotropic hormone (ACTH) increases in stress conditions, which further facilitates the release of the stress hormone cortisol. No increase in the ACTH hormone is seen in stressed mice administered with orexin receptor 2 (OX2R) antagonists thus showing orexin's role in stress reaction. As a result of linking hypocretin/orexin to various physiological procedures, increased research into the medicinal potential of drugs targeting these receptors is emerging. We summed up in this review the recent advances in our understanding of how orexin and its receptor system play an essential role in clinical and pathological functions. This research summarizes a new area for research in human medicine, providing the possibility of controlling a vast array of physiological functions through intra-cerebroventricular injections of a single neuropeptide.

Functional roles of orexin in obstructive sleep apnea: From clinical observation to mechanistic insights

Obstructive sleep apnea is the most common sleep-related breathing disorder. Repetitive episodes of the obstructive respiratory events lead to arousal, sleep fragmentation, and excessive daytime sleepiness. Orexin, also known as hypocretin, is one of the most important neurotransmitters responsible for sleep and arousal regulation. Deficiency of orexin has been shown to be involved in the pathogenesis of narcolepsy, which shares cardinal symptoms of sleep apnea and excessive daytime sleep with obstructive sleep apnea. However, the relationship between orexin and obstructive sleep apnea is not well defined. In this review, we summarize the current evidence, from in vitro, in vivo, and clinical data, regarding the association between orexin and obstructive sleep apnea. The effects of orexin on sleep apnea, as well as how the consequences of obstructive sleep apnea affect the orexin system function are also discussed. Additionally, the contrary findings are also included and discussed.

Avian Orexin: Feed Intake Regulator or Something Else?

Originally named for its expression in the posterior hypothalamus in rats and after the Greek word for “appetite”, hypocretin, or orexin, as it is known today, gained notoriety as a neuropeptide regulating feeding behavior, energy homeostasis, and sleep. Orexin has been proven to be involved in both central and peripheral control of neuroendocrine functions, energy balance, and metabolism. Since its discovery, its ability to increase appetite as well as regulate feeding behavior has been widely explored in mammalian food production animals such as cattle, pigs, and sheep. It is also linked to neurological disorders, leading to its intensive investigation in humans regarding narcolepsy, depression, and Alzheimer’s disease. However, in non-mammalian species, research is limited. In the case of avian species, orexin has been shown to have no central effect on feed-intake, however it was found to be involved in muscle energy metabolism and hepatic lipogenesis. This review provides current knowledge and summarizes orexin’s physiological roles in livestock and pinpoints the present lacuna to facilitate further investigations.

Dual orexin and MCH neuron-ablated mice display severe sleep attacks and cataplexy

Orexin/hypocretin-producing and melanin-concentrating hormone-producing (MCH) neurons are co-extensive in the hypothalamus and project throughout the brain to regulate sleep/wakefulness. Ablation of orexin neurons decreases wakefulness and results in a narcolepsy-like phenotype, whereas ablation of MCH neurons increases wakefulness. Since it is unclear how orexin and MCH neurons interact to regulate sleep/wakefulness, we generated transgenic mice in which both orexin and MCH neurons could be ablated. Double-ablated mice exhibited increased wakefulness and decreased both rapid eye movement (REM) and non-REM (NREM) sleep. Double-ablated mice showed severe cataplexy compared with orexin neuron-ablated mice, suggesting that MCH neurons normally suppress cataplexy. Double-ablated mice also showed frequent sleep attacks with elevated spectral power in the delta and theta range, a unique state that we call 'delta-theta sleep'. Together, these results indicate a functional interaction between orexin and MCH neurons in vivo that suggests the synergistic involvement of these neuronal populations in the sleep/wakefulness cycle.

Role of spontaneous and sensory orexin network dynamics in rapid locomotion initiation

Appropriate motor control is critical for normal life, and requires hypothalamic hypocretin/orexin neurons (HONs). HONs are slowly regulated by nutrients, but also display rapid (subsecond) activity fluctuations in vivo. The necessity of these activity bursts for sensorimotor control and their roles in specific phases of movement are unknown. Here we show that temporally-restricted optosilencing of spontaneous or sensory-evoked HON bursts disrupts locomotion initiation, but does not affect ongoing locomotion. Conversely, HON optostimulation initiates locomotion with subsecond delays in a frequency-dependent manner. Using 2-photon volumetric imaging of activity of >300 HONs during sensory stimulation and self-initiated locomotion, we identify several locomotion-related HON subtypes, which distinctly predict the probability of imminent locomotion initiation, display distinct sensory responses, and are differentially modulated by food deprivation. By causally linking HON bursts to locomotion initiation, these findings reveal the sensorimotor importance of rapid spontaneous and evoked fluctuations in HON ensemble activity.

Hypocretin/orexin modulates body weight and the metabolism of glucose and insulin

The hypocretin/orexin (Hcrt/orexin) unit affects the functions of the nervous, cardiovascular, gastrointestinal, and reproductive systems. Hcrt/orexin ligands and receptors have been localized to different parts of the central and peripheral nervous systems, cerebrospinal fluid and blood, exocrine (pancreas, salivary, lacrimal) as well as endocrine (pancreatic islets, pituitary, adrenal) glands. Several factors including stress, glucagon-like peptide-1 agonists, glutamate, nicotine, glucose, and hypoglycaemia stimulate the expression of Hcrt/orexin system, but it is inhibited by ageing, bone morphogenetic protein, hypoxia/hypercapnia, melanocortin receptor accessory protein 2, and glucagon. Literature reports show that Hcrt/orexin can significantly increase insulin secretion from normal and diabetic rat pancreata. Hcrt/orexin decreases blood glucose concentration and reduces insulin resistance partly via increased tissue expression of glucose transporter type 4. It reduces obesity by increasing browning of fat cells and energy expenditure. Taken together, Hcrt/orexin modulates obesity and the metabolism of glucose and insulin. The Hcrt/orexin system may thus be a target in the development of new therapies for the treatment of diabetes mellitus.

Transgenic Archaerhodopsin-3 Expression in Hypocretin/Orexin Neurons Engenders Cellular Dysfunction and Features of Type 2 Narcolepsy

Narcolepsy, characterized by excessive daytime sleepiness, is associated with dysfunction of the hypothalamic hypocretin/orexin (Hcrt) system, either due to extensive loss of Hcrt cells (Type 1, NT1) or hypothesized Hcrt signaling impairment (Type 2, NT2). Accordingly, efforts to recapitulate narcolepsy-like symptoms in mice have involved ablating these cells or interrupting Hcrt signaling. Here, we describe orexin/Arch mice, in which a modified archaerhodopsin-3 gene was inserted downstream of the prepro-orexin promoter, resulting in expression of the yellow light-sensitive Arch-3 proton pump specifically within Hcrt neurons. Histological examination along with ex vivo and in vivo electrophysiological recordings of male and female orexin/Arch mice demonstrated silencing of Hcrt neurons when these cells were photoilluminated. However, high expression of the Arch transgene affected cellular and physiological parameters independent of photoillumination. The excitability of Hcrt neurons was reduced, and both circadian and metabolic parameters were perturbed in a subset of orexin/Arch mice that exhibited high levels of Arch expression. Orexin/Arch mice also had increased REM sleep under baseline conditions but did not exhibit cataplexy, a sudden loss of muscle tone during wakefulness characteristic of NT1. These aberrations resembled some aspects of mouse models with Hcrt neuron ablation, yet the number of Hcrt neurons in orexin/Arch mice was not reduced. Thus, orexin/Arch mice may be useful to investigate Hcrt system dysfunction when these neurons are intact, as is thought to occur in narcolepsy without cataplexy (NT2). These results also demonstrate the utility of extended phenotypic screening of transgenic models when specific neural circuits have been manipulated.SIGNIFICANCE STATEMENT Optogenetics has become an invaluable tool for functional dissection of neural circuitry. While opsin expression is often achieved by viral injection, stably integrated transgenes offer some practical advantages. Here, we demonstrate successful transgenic expression of an inhibitory opsin in hypocretin/orexin neurons, which are thought to promote or maintain wakefulness. Both brief and prolonged illumination resulted in inhibition of these neurons and induced sleep. However, even in the absence of illumination, these cells exhibited altered electrical characteristics, particularly when transgene expression was high. These aberrant properties affected metabolism and sleep, resulting in a phenotype reminiscent of the narcolepsy Type 2, a sleep disorder for which no good animal model currently exists.

Dissociating orexin-dependent and -independent functions of orexin neurons using novel Orexin-Flp knock-in mice

Uninterrupted arousal is important for survival during threatening situations. Activation of orexin/hypocretin neurons is implicated in sustained arousal. However, orexin neurons produce and release orexin as well as several co-transmitters including dynorphin and glutamate. To disambiguate orexin-dependent and -independent physiological functions of orexin neurons, we generated a novel Orexin-flippase (Flp) knock-in mouse line. Crossing with Flp-reporter or Cre-expressing mice showed gene expression exclusively in orexin neurons. Histological studies confirmed that orexin was knock-out in homozygous mice. Orexin neurons without orexin showed altered electrophysiological properties, as well as received decreased glutamatergic inputs. Selective chemogenetic activation revealed that both orexin and co-transmitters functioned to increase wakefulness, however, orexin was indispensable to promote sustained arousal. Surprisingly, such activation increased the total time spent in cataplexy. Taken together, orexin is essential to maintain basic membrane properties and input-output computation of orexin neurons, as well as to exert awake-sustaining aptitude of orexin neurons.

Serotonergic neurons in the dorsal raphe nucleus mediate the arousal-promoting effect of orexin during isoflurane anesthesia in male rats

Previous studies have demonstrated that the activation of orexinergic neurons facilitates the recovery of animals from general anesthesia. Moreover, serotonergic neurons that receive projections from orexin neurons have also been shown to participate in sleep-wakefulness regulation. In the present study, we aimed to explore whether orexinergic neurons facilitate emergence from isoflurane anesthesia in rats by activating serotonergic neurons. Orexin A (30 or 100 pmol), orexin B (30 or 100 pmol), and their respective antagonists SB-334867 and TCS-OX2-29 (5 or 20 μg) were microinjected into the dorsal raphe nucleus (DRN) of rats, and their effects on induction and emergence times were analyzed. Electroencephalogram (EEG) changes were also recorded and analyzed to illuminate the effect of orexin injection into the DRN on cortical excitability under isoflurane anesthesia. Activation of serotonergic neurons was detected via immunohistochemical analysis of c-Fos expression following orexin administration. Our results indicated that injection of neither orexins nor orexin antagonists into the rat DRN exerted an impact on induction time, whereas orexin-A injection (100 pmol) enhanced arousal when compared with the saline group. In contrast, administration of orexin receptor type 1 antagonist SB-334867 (20 μg) prolonged emergence time from isoflurane anesthesia. Microinjection of orexin-A induced an arousal pattern on EEG, and decreased the burst suppression ratio under isoflurane anesthesia. Isoflurane anesthesia inhibited the activity of serotonergic neurons, as shown by decrease in the number of c-Fos-immunoreactive serotonergic neurons when compared with the sham group. This inhibitory effect was partially reversed by administration of orexin-A. Taken together, our findings suggest that orexinergic signals facilitate emergence from isoflurane anesthesia, at least partially, by reversing the effects of isoflurane on serotonergic neurons of the DRN.

Functional characterization of an orexin neuropeptide in amphioxus reveals an ancient origin of orexin/orexin receptor system in chordate

Amphioxus belongs to the subphylum cephalochordata, an extant representative of the most basal chordates, whose regulation of endocrine system remains ambiguous. Here we clearly demonstrated the existence of a functional orexin neuropeptide in amphioxus, which is able to interact with orexin receptor, activate both PKC and PKA pathways, decrease leptin expression, and stimulate lipogenesis. We also showed the transcription level of amphioxus orexin was affected by fasting or temperature, indicating a role of this gene in the regulation of energy balance. In addition, the expression of the amphioxus orexin was detected at cerebral vesicle, which has been proposed to be a homolog of the vertebrate brain. These data collectively suggest that a functional orexin neuropeptide has already emerged in amphioxus, which provide insights into the evolutionary origin of orexin in chordate and the functional homology between the cerebral vesicle and vertebrate brain.

Involvement of PLAGL1/ZAC1 in hypocretin/orexin transcription

The hypocretin/orexin neuropeptide system coordinates the regulation of various physiological processes. Our previous study reported that a reduction in the expression of pleomorphic adenoma gene-like 1 (Plagl1), which encodes a C2H2 zinc-finger transcription factor, occurs in hypocretin neuron-ablated transgenic mice, suggesting that PLAGL1 is co-expressed in hypocretin neurons and regulates hypocretin transcription. The present study examined whether canonical prepro-hypocretin transcription is functionally modulated by PLAGL1. Double immunostaining indicated that the majority of hypocretin neurons were positive for PLAGL1 immunore-activity in the nucleus. Notably, PLAGL1 immunoreactivity in hypocretin neurons was altered in response to several conditions affecting hypocretin function. An uneven localization of PLAGL1 was detected in the nuclei of hypocretin neurons following sleep deprivation. Chromatin immunoprecipitation revealed that endogenous PLAGL1 may bind to a putative PLAGL1-binding site in the proximal region of the hypocretin gene, in the murine hypothalamus. In addition, electroporation of the PLAGL1 expression vector into the fetal hypothalamus promoted hypothalamic hypocretin transcription. These results suggested that PLAGL1 may regulate hypothalamic hypocretin transcription.

Expression of Dopamine Receptors in the Lateral Hypothalamic Nucleus and Their Potential Regulation of Gastric Motility in Rats With Lesions of Bilateral Substantia Nigra

Most Parkinson’s Disease (PD) patients experience gastrointestinal (GI) dysfunction especially the gastroparesis, but its underlying mechanism is not clear. We have previously demonstrated that the neurons in the substantia nigra (SN) project to the lateral hypothalamic nucleus (LH) and the dorsal motor nucleus of vagus (DMV) receives the neural projection from LH by the means of anterograde and retrograde neural tracing technology. Orexin A (OXA) is predominately expressed in the LH. It has been reported that OXA can alter the gastric motility through the orexin receptor 1 (OX1R) in DMV. We speculated that this SN-LH-DMV pathway could modulate the motility of stomach because of the important role of LH and DMV in the regulation of gastric motility. However, the distribution and expression of dopamine receptors (DR) in the LH is unknown. In the present study, using a double-labeling immunofluorescence technique combined with confocal microscopy, we significantly extend our understanding of the SN-LH-DMV pathway by showing that (1) a considerable quantity of dopamine receptor 1 and 2 (D1 and D2) was expressed in the LH as well as the OX1R was expressed in the DMV; (2) Nearly all of the D1-immuoreactve (IR) neurons were also OXA-positive while only a few neurons express both D2 and OXA in the LH, and the DR-positive neurons were surrounded by the dopaminergic neural fibers; In the DMV, OX1R were colocalized with choline acetyltransferase (ChAT)-labeled motor neurons; (3) When the gastroparesis was induced by the destruction of dopaminergic neurons in the SN, the decreased expression of D1 and OXA was observed in the LH as well as the reduced OX1R and ChAT expression in the DMV. These findings suggest that SN might regulate the function of OXA-positive neurons via D1 receptor, which then affect the motor neurons in the DMV through OX1R. If the SN is damaged the vagal pathway would be affected, which may lead to gastric dysfunction. The present study raises the possibility that the SN-LH-DMV pathway can regulate the movement of stomach.

Single-cell transcriptomic analysis of the lateral hypothalamic area reveals molecularly distinct populations of inhibitory and excitatory neurons

The lateral hypothalamic area (LHA) coordinates an array of fundamental behaviors, including sleeping, waking, feeding, stress and motivated behavior. The wide spectrum of functions ascribed to the LHA may be explained by a heterogeneous population of neurons, the full diversity of which is poorly understood. We employed a droplet-based single-cell RNA-sequencing approach to develop a comprehensive census of molecularly distinct cell types in the mouse LHA. Neuronal populations were classified based on fast neurotransmitter phenotype and expression of neuropeptides, transcription factors and synaptic proteins, among other gene categories. We define 15 distinct populations of glutamatergic neurons and 15 of GABAergic neurons, including known and novel cell types. We further characterize a novel population of somatostatin-expressing neurons through anatomical and behavioral approaches, identifying a role for these neurons in specific forms of innate locomotor behavior. This study lays the groundwork for better understanding the circuit-level underpinnings of LHA function.

Activation of lateral hypothalamic group III metabotropic glutamate receptors suppresses cocaine-seeking following abstinence and normalizes drug-associated increases in excitatory drive to orexin/hypocretin cells

The perifornical/lateral hypothalamic area (LHA) orexin (hypocretin) system is involved in drug-seeking behavior elicited by drug-associated stimuli. Cocaine exposure is associated with presynaptic plasticity at LHA orexin cells such that excitatory input to orexin cells is enhanced acutely and into withdrawal. These changes may augment orexin cell reactivity to drug-related cues during abstinence and contribute to relapse-like behavior. Studies in hypothalamic slices from drug-naïve animals indicate that agonism of group III metabotropic glutamate receptors (mGluRs) reduces presynaptic glutamate release onto orexin cells. Therefore, we examined the group III mGluR system as a potential target to reduce orexin cell excitability in-vivo, including in animals with cocaine experience. First, we verified that group III mGluRs regulate orexin cell activity in behaving animals by showing that intra-LHA infusions of the selective agonist L-(+)-2-Amino-4-phosphonobutyric acid (L-AP4) reduces c-fos expression in orexin cells following 24 h food deprivation. Next, we extended these findings to show that intra-LHA L-AP4 infusions reduced discriminative stimulus-driven cocaine-seeking following withdrawal. Importantly, L-AP4 had no effect on lever pressing for sucrose pellets or general motoric behavior. Finally, using whole-cell patch-clamp recordings from identified orexin cells in orexin-GFP transgenic mice, we show enhanced presynaptic drive to orexin cells following 14d withdrawal and that this plasticity can be normalized by L-AP4. Together, these data indicate that activation of group III mGluRs in LHA reduces orexin cell activity in vivo and may be an effective strategy to suppress cocaine-seeking behavior following withdrawal. These effects are likely mediated, at least in part, by normalization of presynaptic plasticity at orexin cells that occurs as a result of cocaine exposure. This article is part of the Special Issue entitled 'Hypothalamic Control of Homeostasis'.

Orexin/hypocretin and dysregulated eating: Promotion of foraging behavior

At its discovery, orexin/hypocretin (OX) was hypothesized to promote food intake. Subsequently, with the identification of the participation of OX in numerous other phenomena, including arousal and drug seeking, this neuropeptide was proposed to be involved in highly motivated behaviors. The present review develops the hypothesis that the primary evolutionary function of OX is to promote foraging behavior, seeking for food under conditions of limited availability. Thus, it will first describe published literature on OX and homeostatic food intake, which shows that OX neurons are activated by conditions of food deprivation and in turn stimulate food intake. Next, it will present literature on excessive and binge-like food intake, which demonstrates that OX stimulates both intake and willingness to work for palatable food. Importantly, studies show that binge-like eating can be inhibited by OX antagonists at doses far lower than those required to suppress homeostatic intake (3 mg/kg vs. 30 mg/kg), suggesting that an OX-based pharmacotherapy, at the right dose, could specifically control dysregulated eating. Finally, the review will discuss the role of OX in foraging behavior, citing literature which shows that OX neurons, which are activated during the anticipation of food reward, can promote a number of phenomena involved in successful foraging, including food-anticipatory locomotor behavior, olfactory sensitivity, visual attention, spatial memory, and mastication. Thus, OX may promote homeostatic eating, as well as binge eating of palatable food, due to its ability to stimulate and coordinate the activities involved in foraging behavior.

Pyrrole: An insight into recent pharmacological advances with structure activity relationship

Pyrrole is a heterocyclic ring template with multiple pharmacophores that provides a way for the generation of library of enormous lead molecules. Owing to its vast pharmacological profile, pyrrole and its analogues have drawn much attention of the researchers/chemists round the globe to be explored exhaustively for the benefit of mankind. This review focusses on recent advancements; pertaining to pyrrole scaffold, discussing various aspects of structure activity relationship and its bioactivities.

Orexins, Sleep, and Blood Pressure

Purpose of Review

The aim of this review was to summarize collected data on the role of orexin and orexin neurons in the control of sleep and blood pressure.

Recent Findings

Although orexins (hypocretins) have been known for only 20 years, an impressive amount of data is now available regarding their physiological role. Hypothalamic orexin neurons are responsible for the control of food intake and energy expenditure, motivation, circadian rhythm of sleep and wake, memory, cognitive functions, and the cardiovascular system. Multiple studies show that orexinergic stimulation results in increased blood pressure and heart rate and that this effect may be efficiently attenuated by orexinergic antagonism. Increased activity of orexinergic neurons is also observed in animal models of hypertension.

Summary

Pharmacological intervention in the orexinergic system is now one of the therapeutic possibilities in insomnia. Although the role of orexin in the control of blood pressure is well described, we are still lacking clinical evidence that this is a possibility for a new approach in the treatment of cardiovascular diseases.

A Review of Single-Nucleotide Polymorphisms in Orexigenic Neuropeptides Targeting G Protein-Coupled Receptors

Many physiological pathways are involved in appetite, food intake, and the maintenance of energy homeostasis. In particular, neuropeptides within the central nervous system have been demonstrated to be critical signaling molecules for modulating appetite. Both anorexigenic (appetite-decreasing) and orexigenic (appetite-stimulating) neuropeptides have been described. The biological effects of these neuropeptides can be observed following central administration in animal models. This review focuses on single nucleotide polymorphisms (SNPs) in six orexigenic neuropeptides: agouti-related protein (AGRP), galanin, melanin concentrating hormone (MCH), neuropeptide Y (NPY), orexin A, and orexin B. Following a brief summary of the neuropeptides and their orexigenic activities, reports associating SNPs within the orexigenic neuropeptides to energy homeostasis, food intake, obesity, and BMI in humans are reviewed. Additionally, the NIH tool Variation Viewer was utilized to identify missense SNPs within the mature, biologically active neuropeptide sequences. For SNPs found through Variation Viewer, a concise discussion on relevant pharmacological structure-activity relationship studies for select SNPs is included. This review is meant to update reported orexigenic neuropeptide SNPs and demonstrate the potential utility of genomic sequence databases for finding SNPs that may result in altered receptor signaling for neuropeptide pathways associated with appetite.

Narcolepsy with cataplexy and hyperthyroidism sudden appeared after H1N1 vaccination

Narcolepsy type 1 (NT1) is a chronic sleep disorder, characterized by excessive daytime sleepiness, cataplexy and fragmented nocturnal sleep. It is caused by a hypocretin deficiency due to a significant reduction of the neurons producing it. In the last years, it has been postulated that an autoimmune mechanism would be responsible for the destruction of these neurons in those genetically predisposed patients. The increased incidence of narcolepsy after the pandemic H1N1 influenza vaccination campaign in 2009-2010 is known. We present below the case of an adult patient who, 10 days after receiving H1N1 vaccination, suffers a traffic accident after falling asleep. Subsequent studies revealed hyperthyroidism due to Graves disease. In spite of the treatment, the patient persisted with daily and disabling daytime sleepiness, sleep attacks and episodes of generalized muscle atony with preservation of consciousness. A nocturnal polysomnography and multiple sleep latency test (MSLT) were performed with a diagnosis of NT1. The particularity of this case is the presentation of 2 autoimmune diseases triggered by an H1N1 vaccine without adjuvant, so far there is only evidence of NT1 associated with vaccines with adjuvant and viral infection. The association of both entities has made us reflect on the autoimmune mechanism, reinforcing the theory of its role in the onset of the disease.

The neuropeptide genes SST, TAC1, HCRT, NPY, and GAL are powerful epigenetic biomarkers in head and neck cancer: a site-specific analysis

Background

Staging and pathological grading systems are convenient but imperfect predictors of recurrence in head and neck squamous cell carcinoma (HNSCC). Identifying biomarkers for HNSCC that will progress and cause death is a critical research area, particularly if the biomarker can be linked to selection of patients. Therefore, to identify potential alternative prognostic markers, we investigated the methylation status of five neuropeptide gene promoters. The promoter methylation status was determined by quantitative methylation-specific PCR in 230 cases of HNSCC; 58 hypopharynx, 45 larynx, 56 oropharynx, and 71 oral cavity tumor samples were studied.

Results

The somatostatin (SST), tachykinin precursor 1 (TAC1), hypocretin neuropeptide precursor (HCRT), neuropeptide Y (NPY), and galanin (GAL) promoters were methylated in 84.3, 63.5, 32.6, 28.3, and 20.0%, respectively, of the samples. The mean number of methylated genes per sample was 2.29 (range, 0–5). Disease-free survival was lower in patients with 3–5 methylated genes than in those with 0–2 methylated genes (log-rank test, P = 0.007). In multivariate Cox proportional hazards analysis, TAC1 and GAL promoter methylation independently predicted recurrence (odds ratios 1.620, 95% confidence interval [CI] 1.018–2.578, P = 0.042, and odds ratios 1.692, 95% CI 1.063–2.694, P = 0.027, respectively). In patients with oral cancer, TAC1 methylation showed the best correlation with poor survival (odds ratio 4.427, 95% CI 1.634–12.00, P = 0.003). Similar findings were observed for HCRT and GAL in patients with laryngeal cancer and oropharyngeal cancer, respectively.

Conclusion

In this study, we demonstrated the methylation status of the neuropeptide-encoding genes SST, TAC1, HCRT, NPY, and GAL and its relationship with recurrence and survival in HNSCC. These methylation changes may serve as potential molecular markers for defining the risk and prognosis of HNSCC.

Electronic supplementary material

The online version of this article (10.1186/s13148-018-0485-0) contains supplementary material, which is available to authorized users.

DEC2 modulates orexin expression and regulates sleep

Adequate sleep is essential for physical and mental health. We previously identified a missense mutation in the human DEC2 gene (BHLHE41) leading to the familial natural short sleep behavioral trait. DEC2 is a transcription factor regulating the circadian clock in mammals, although its role in sleep regulation has been unclear. Here we report that prepro-orexin, also known as hypocretin (Hcrt), gene expression is increased in the mouse model expressing the mutant hDEC2 transgene (hDEC2-P384R). Prepro-orexin encodes a precursor protein of a neuropeptide producing orexin A and B (hcrt1 and hcrt2), which is enriched in the hypothalamus and regulates maintenance of arousal. In cell culture, DEC2 suppressed prepro-orexin promoter-luc (ore-luc) expression through cis-acting E-box elements. The mutant DEC2 has less repressor activity than WT-DEC2, resulting in increased orexin expression. DEC2-binding affinity for the prepro-orexin gene promoter is decreased by the P384R mutation, likely due to weakened interaction with other transcription factors. In vivo, the decreased immobility time of the mutant transgenic mice is attenuated by an orexin receptor antagonist. Our results suggested that DEC2 regulates sleep/wake duration, at least in part, by modulating the neuropeptide hormone orexin.

Yokukansan (Kampo medicinal formula) prevents the development of morphine tolerance by inhibiting the secretion of orexin A

Background

Yokukansan (YKS), a traditional herbal (Kampo) medicine consisting of seven herbs, is effective in the treatment of pain disorders, such as headache, postherpetic neuralgia, fibromyalgia, and trigeminal neuralgia, and we have previously shown it to be effective against morphine analgesic tolerance in rats. It has been reported that orexin receptor antagonists prevent the development of morphine tolerance and that YKS inhibits the secretion of orexin A in the hypothalamus. This study examined whether the inhibition of the secretion of orexin A by YKS is one mechanism underlying its effect against morphine analgesic tolerance.

Methods

Male Wistar rats were administered a subcutaneous injection of morphine hydrochloride (10 mg/kg/day) for 5 days. One group was preadministered YKS, starting 3 days before the morphine. The withdrawal latency following thermal stimulation was measured daily using a hot plate test. On day 5, the levels of orexin A in the plasma and the midbrain were measured, and the appearance of activated astrocytes in the midbrain was examined by immunofluorescence staining.

Results

The preadministration of YKS prevented the development of morphine tolerance. The repeated administration of morphine significantly increased the plasma and midbrain levels of orexin A and the activation of astrocytes. These increases were significantly inhibited by the preadministration of YKS.

Conclusion

These results suggest that the preadministration of YKS attenuated the development of antinociceptive morphine tolerance and that the inhibition of orexin A secretion may be one mechanism underlying this phenomenon.

Involvement of orexin neurons in fasting- and central adenosine-induced hypothermia

We examined whether orexin neurons might play a protective role against fasting- and adenosine-induced hypothermia. We first measured body temperature (BT) in orexin neuron-ablated (ORX-AB) mice and wild-type (WT) controls during 24 hours of fasting. As expected, the magnitude of BT drop and the length of time suffering from hypothermia were greater in ORX-AB mice than in WT mice. Orexin neurons were active just before onset of hypothermia and during the recovery period as revealed by calcium imaging in vivo using G-CaMP. We next examined adenosine-induced hypothermia via an intracerebroventricular administration of an adenosine A1 receptor agonist, N6-cyclohexyladenosine (CHA), which induced hypothermia in both ORX-AB and WT mice. The dose of CHA required to initiate a hypothermic response in ORX-AB mice was more than 10 times larger than the dose for WT mice. Once hypothermia was established, the recovery was seemingly slower in ORX-AB mice. Activation of orexin neurons during the recovery phase was confirmed by immunohistochemistry for c-Fos. We propose that orexin neurons play dual roles (enhancer in the induction phase and compensator during the recovery phase) in adenosine-induced hypothermia and a protective/compensatory role in fasting-induced hypothermia.

Acute Heat Stress Alters the Expression of Orexin System in Quail Muscle

Accumulating evidences indicate that the hypothalamic neuropeptide orexins or hypocretins are involved in stress-induced responses in mammals. Recently, we found that orexin is expressed and secreted from avian muscle cells, however its regulation is still unknown. In this study, we investigated the effect of heat and oxidative stress, the most challenging stressors in poultry production, on the expression of orexin system in quail muscle tissues and myoblast cell lines. Four week-old genetically selected susceptible and resistant Japanese quail (Coturnix coturnix Japonica) lines were exposed to acute heat stress (HS, 37°C for 1.5 h) or maintained at thermoneutral conditions (24°C). Quail myoblast (QM7) cell line was exposed to heat stress (45°C) for 0.5, 1, 2, or 4 h. The control cells were maintained at 37°C. The cells were also treated with several doses of hydrogen peroxide (H₂O₂, 10-200 μM) or 4-Hydroxynonenal (4-HNE, 10-30 μM) as oxidative stress. Untreated cells were used as controls. Acute HS significantly induced the expression of HSP70 and down-regulated orexin system in both quail muscle tissue and QM7 cells. Similarly, H₂O₂ but not 4-HNE treatment significantly increased HSP70 protein levels and dysregulated the expression of orexin and its related receptors in a dose-dependent manner in QM7 cells. Transient overexpression of HSP70 down-regulated the expression of orexin system in QM7 cells. Taken together, these data indicate that orexin may be a key player in stress response in avian muscle by demonstrating that heat and oxidative stress alter the expression of orexin system in quail muscle. This effect might be mediated through HSP70. Unraveling the upstream regulators and downstream effectors of orexin in avian muscle merits further in depth investigations.

Modular organization of a hypocretin gene minimal promoter

Orexins or hypocretins are neurotransmitters produced by a small population of neurons in the lateral hypothalamus. This family of peptides modulates sleep-wake cycle, arousal and feeding behaviors; however, the mechanisms regulating their expression remain to be fully elucidated. There is an interest in defining the key molecular elements in orexin regulation, as these may serve to identify targets for generating novel therapies for sleep disorders, obesity and addiction. Our previous studies showed that the expression of orexin was decreased in mice carrying null-mutations of the transcription factor early B-cell factor 2 (ebf2) and that the promoter region of the prepro-orexin (Hcrt) gene contained two putative ebf-binding sites, termed olf-1 sites. In the present study, a minimal promoter region of the murine Hcrt gene was identified, which was able to drive the expression of a luciferase reporter gene in the human 293 cell line. Deletion of the olf1-site proximal to the transcription start site of the Hcrt gene increased reporter gene expression, whereas deletion of the distal olf1-like site decreased its expression. The lentiviral transduction of murine transcription factor ebf2 cDNA into 293 cells increased the gene expression driven by this minimal Hcrt-gene promoter and an electrophoretic mobility shift assays demonstrated that the distal olf1-like sequence was a binding site for ebf2.

CD4+ T-Cell Reactivity to Orexin/Hypocretin in Patients With Narcolepsy Type 1

Introduction

Narcolepsy type 1 is accompanied by a selective loss of orexin/hypocretin (hcrt) neurons in the lateral hypothalamus caused by yet unknown mechanisms. Epidemiologic and genetic associations strongly suggest an immune-mediated pathogenesis of the disease.

Methods

We compared specific T-cell reactivity to orexin/hcrt peptides in peripheral blood mononuclear cells of narcolepsy type 1 patients to healthy controls by a carboxyfluorescein succinimidyl ester proliferation assay. Orexin/hcrt-specific T-cell reactivity was also determined by cytokine (interferon gamma and granulocyte-macrophage colony-stimulating factor) analysis. Individuals were considered as responders if the cell division index of CD3+CD4+ T cells and both stimulation indices of cytokine secretion exceeded the cutoff 3. Additionally, T-cell reactivity to orexin/hcrt had to be confirmed by showing reactivity to single peptides present in different peptide pools.

Results

Using these criteria, 3/15 patients (20%) and 0/13 controls (0%) showed orexin/hcrt-specific CD4+ T-cell proliferation (p = .2262). The heterogeneous reactivity pattern did not allow the identification of a preferential target epitope.

Conclusions

A significant role of orexin/hcrt-specific T cells in narcolepsy type 1 patients could not be confirmed in this study. Further studies are needed to assess the exact role of CD4+ T cells and possible target antigens in narcolepsy type 1 patients.

Neurochemical Heterogeneity Among Lateral Hypothalamic Hypocretin/Orexin and Melanin-Concentrating Hormone Neurons Identified Through Single-Cell Gene Expression Analysis

The lateral hypothalamic area (LHA) lies at the intersection of multiple neural and humoral systems and orchestrates fundamental aspects of behavior. Two neuronal cell types found in the LHA are defined by their expression of hypocretin/orexin (Hcrt/Ox) and melanin-concentrating hormone (MCH) and are both important regulators of arousal, feeding, and metabolism. Conflicting evidence suggests that these cell populations have a more complex signaling repertoire than previously appreciated, particularly in regard to their coexpression of other neuropeptides and the machinery for the synthesis and release of GABA and glutamate. Here, we undertook a single-cell expression profiling approach to decipher the neurochemical phenotype, and heterogeneity therein, of Hcrt/Ox and MCH neurons. In transgenic mouse lines, we used single-cell quantitative polymerase chain reaction (qPCR) to quantify the expression of 48 key genes, which include neuropeptides, fast neurotransmitter components, and other key markers, which revealed unexpected neurochemical diversity. We found that single MCH and Hcrt/Ox neurons express transcripts for multiple neuropeptides and markers of both excitatory and inhibitory fast neurotransmission. Virtually all MCH and approximately half of the Hcrt/Ox neurons sampled express both the machinery for glutamate release and GABA synthesis in the absence of a vesicular GABA release pathway. Furthermore, we found that this profile is characteristic of a subpopulation of LHA glutamatergic neurons but contrasts with a broad population of LHA GABAergic neurons. Identifying the neurochemical diversity of Hcrt/Ox and MCH neurons will further our understanding of how these populations modulate postsynaptic excitability through multiple signaling mechanisms and coordinate diverse behavioral outputs.

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.

Anatomical and electrophysiological development of the hypothalamic orexin neurons from embryos to neonates

The amount, quality, and diurnal pattern of sleep change greatly during development. Developmental changes of sleep/wake architecture are in a close relationship to brain development. The fragmentation of wake episodes is one of the salient features in the neonatal period, which is also observed in mature animals and human individuals lacking neuropeptide orexin/hypocretin signaling. This raises the possibility that developmental changes of lateral hypothalamic orexin neurons are relevant to the development of sleep/wake architecture. However, little information is available on morphological and physiological features of developing orexin neurons. To address the cellular basis for maturation of the sleep/wake regulatory system, we investigated the functional development of orexin neurons in the lateral hypothalamus. The anatomical development as well as the changes in the electrophysiological characteristics of orexin neurons was examined from embryonic to postnatal stages in orexin-EGFP mice. Prepro-orexin promoter activity was detectable at embryonic day (E) 12.0, followed by expression of orexin A after E14.0. The number of orexin neurons and their membrane capacitance reached similar levels to adults by postnatal day (P) 7, while their membrane potentials, firing rates, and action potential waveforms were developed by P21. The hyperpolarizing effect of serotonin, which is a major inhibitory signal for adult orexin neurons, was detected after E18.0 and matured at P1. These results suggest that the expression of orexin peptides precedes the maturation of electrophysiological activity of orexin neurons. The function of orexin neurons gradually matures by 3 weeks after birth, coinciding with maturation of sleep/wake architecture.

Hypocretin/orexin loss changes the hypothalamic immune response

Hypocretin, also known as orexin, maintains the vigilance state and regulates various physiological processes, such as arousal, sleep, food intake, energy expenditure, and reward. Previously, we found that when wild-type mice and hypocretin/ataxin-3 littermates (which are depleted of hypothalamic hypocretin-expressing neurons postnatally) were administered lipopolysaccharide (LPS), the two genotypes exhibited significant differences in their sleep/wake cycle, including differences in the degree of increase in sleep periods and in recovery from sickness behaviour. In the present study, we examined changes in the hypothalamic vigilance system and in the hypothalamic expression of inflammatory factors in response to LPS in hypocretin/ataxin-3 mice. Peripheral immune challenge with LPS affected the hypothalamic immune response and vigilance states. This response was altered by the loss of hypocretin. Hypocretin expression was inhibited after LPS injection in both hypocretin/ataxin-3 mice and their wild-type littermates, but expression was completely abolished only in hypocretin/ataxin-3 mice. Increases in the number of histidine decarboxylase (HDC)-positive cells and in Hdc mRNA expression were found in hypocretin/ataxin-3 mice, and this increase was suppressed by LPS. Hypocretin loss did not impact the change in expression of hypothalamic inflammatory factors in response to LPS, except for interferon gamma and colony stimulating factor 3. The number of c-Fos-positive/HDC-positive cells in hypocretin/ataxin-3 mice administered LPS injections was elevated, even during the rest period, in all areas, suggesting that there is an increase in the activity of histaminergic neurons in hypocretin/ataxin-3 mice following LPS injection. Taken together, our results suggest a novel role for hypocretin in the hypothalamic response to peripheral immune challenge. Our findings contribute to the understanding of the pathophysiology of narcolepsy.

The integrative role of orexin/hypocretin neurons in nociceptive perception and analgesic regulation

The level of wakefulness is one of the major factors affecting nociception and pain. Stress-induced analgesia supports an animal’s survival via prompt defensive responses against predators or competitors. Previous studies have shown the pharmacological effects of orexin peptides on analgesia. However, orexin neurons contain not only orexin but also other co-transmitters such as dynorphin, neurotensin and glutamate. Thus, the physiological importance of orexin neuronal activity in nociception is unknown. Here we show that adult-stage selective ablation of orexin neurons enhances pain-related behaviors, while pharmacogenetic activation of orexin neurons induces analgesia. Additionally, we found correlative activation of orexin neurons during nociception using fiber photometry recordings of orexin neurons in conscious animals. These findings suggest an integrative role for orexin neurons in nociceptive perception and pain regulation.

Progressive Loss of the Orexin Neurons Reveals Dual Effects on Wakefulness

STUDY OBJECTIVES

Narcolepsy is caused by loss of the orexin (also known as hypocretin) neurons. In addition to the orexin peptides, these neurons release additional neurotransmitters, which may produce complex effects on sleep/wake behavior. Currently, it remains unknown whether the orexin neurons promote the initiation as well as the maintenance of wakefulness, and whether the orexin neurons influence initiation or maintenance of sleep. To determine the effects of the orexin neurons on the dynamics of sleep/wake behavior, we analyzed sleep/wake architecture in a novel mouse model of acute orexin neuron loss.

METHODS

We used survival analysis and other statistical methods to analyze sleep/wake architecture in orexin-tTA ; TetO diphtheria toxin A mice at different stages of orexin neuron degeneration.

RESULTS

Progressive loss of the orexin neurons dramatically reduced survival of long wake bouts, but it also improved survival of brief wake bouts. In addition, with loss of the orexin neurons, mice were more likely to wake during the first 30 sec of nonrapid eye movement sleep and then less likely to return to sleep during the first 60 sec of wakefulness.

CONCLUSIONS

These findings help explain the sleepiness and fragmented sleep that are characteristic of narcolepsy. Orexin neuron loss impairs survival of long wake bouts resulting in poor maintenance of wakefulness, but this neuronal loss also fragments sleep by increasing the risk of awakening at the beginning of sleep and then reducing the likelihood of quickly returning to sleep.

Leptin acts via lateral hypothalamic area neurotensin neurons to inhibit orexin neurons by multiple GABA-independent mechanisms

The adipocyte-derived hormone leptin modulates neural systems appropriately for the status of body energy stores. Leptin inhibits lateral hypothalamic area (LHA) orexin (OX; also known as hypocretin)-producing neurons, which control feeding, activity, and energy expenditure, among other parameters. Our previous results suggest that GABAergic LHA leptin receptor (LepRb)-containing and neurotensin (Nts)-containing (LepRb(Nts)) neurons lie in close apposition with OX neurons and control Ox mRNA expression. Here, we show that, similar to leptin, activation of LHA Nts neurons by the excitatory hM3Dq DREADD (designer receptor exclusively activated by designer drugs) hyperpolarizes membrane potential and suppresses action potential firing in OX neurons in mouse hypothalamic slices. Furthermore, ablation of LepRb from Nts neurons abrogated the leptin-mediated inhibition, demonstrating that LepRb(Nts) neurons mediate the inhibition of OX neurons by leptin. Leptin did not significantly enhance GABAA-mediated inhibitory synaptic transmission, and GABA receptor antagonists did not block leptin-mediated inhibition of OX neuron activity. Rather, leptin diminished the frequency of spontaneous EPSCs onto OX neurons. Furthermore, leptin indirectly activated an ATP-sensitive potassium (K(ATP)) channel in OX neurons, which was required for the hyperpolarization of OX neurons by leptin. Although Nts did not alter OX activity, galanin, which is coexpressed in LepRb(Nts) neurons, inhibited OX neurons, whereas the galanin receptor antagonist M40 (galanin-(1-12)-Pro3-(Ala-Leu)2-Ala amide) prevented the leptin-induced hyperpolarization of OX cells. These findings demonstrate that leptin indirectly inhibits OX neurons by acting on LHA LepRb(Nts) neurons to mediate two distinct GABA-independent mechanisms of inhibition: the presynaptic inhibition of excitatory neurotransmission and the opening of K(ATP) channels.

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.

Concurrent and robust regulation of feeding behaviors and metabolism by orexin neurons

Orexin neurons in the hypothalamus regulate energy homeostasis by coordinating various physiological responses. Past studies have shown the role of the orexin peptide itself; however, orexin neurons contain not only orexin but also other neurotransmitters such as glutamate and dynorphin. In this study, we examined the physiological role of orexin neurons in feeding behavior and metabolism by pharmacogenetic activation and chronic ablation. We generated novel orexin-Cre mice and utilized Cre-dependent adeno-associated virus vectors to express Gq-coupled modified GPCR, hM3Dq or diphtheria toxin fragment A in orexin neurons. By intraperitoneal injection of clozapine-N oxide in orexin-Cre mice expressing hM3Dq in orexin neurons, we could selectively manipulate the activity of orexin neurons. Pharmacogenetic stimulation of orexin neurons simultaneously increased locomotive activity, food intake, water intake and the respiratory exchange ratio (RER). Elevation of blood glucose levels and RER persisted even after locomotion and feeding behaviors returned to basal levels. Accordantly, 83% ablation of orexin neurons resulted in decreased food and water intake, while 70% ablation had almost no effect on these parameters. Our results indicate that orexin neurons play an integral role in regulation of both feeding behavior and metabolism. This regulation is so robust that greater than 80% of orexin neurons were ablated before significant changes in feeding behavior emerged.

Conditional ablation of orexin/hypocretin neurons: a new mouse model for the study of narcolepsy and orexin system function

The sleep disorder narcolepsy results from loss of hypothalamic orexin/hypocretin neurons. Although narcolepsy onset is usually postpubertal, current mouse models involve loss of either orexin peptides or orexin neurons from birth. To create a model of orexin/hypocretin deficiency with closer fidelity to human narcolepsy, diphtheria toxin A (DTA) was expressed in orexin neurons under control of the Tet-off system. Upon doxycycline removal from the diet of postpubertal orexin-tTA;TetO DTA mice, orexin neurodegeneration was rapid, with 80% cell loss within 7 d, and resulted in disrupted sleep architecture. Cataplexy, the pathognomic symptom of narcolepsy, occurred by 14 d when ∼5% of the orexin neurons remained. Cataplexy frequency increased for at least 11 weeks after doxycycline. Temporary doxycycline removal followed by reintroduction after several days enabled partial lesion of orexin neurons. DTA-induced orexin neurodegeneration caused a body weight increase without a change in food consumption, mimicking metabolic aspects of human narcolepsy. Because the orexin/hypocretin system has been implicated in the control of metabolism and addiction as well as sleep/wake regulation, orexin-tTA; TetO DTA mice are a novel model in which to study these functions, for pharmacological studies of cataplexy, and to study network reorganization as orexin input is lost.

Orexin-A promotes cell migration in cultured rat astrocytes via Ca2+-dependent PKCα and ERK1/2 signals

Orexin-A is an important neuropeptide involved in the regulation of feeding, arousal, energy consuming, and reward seeking in the body. The effects of orexin-A have widely studied in neurons but not in astrocytes. Here, we report that OX1R and OX2R are expressed in cultured rat astrocytes. Orexin-A stimulated the phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2), and then induced the migration of astrocytes via its receptor OX1R but not OX2R. Orexin-A-induced ERK1/2 phosphorylation and astrocytes migration are Ca2+-dependent, since they could be inhibited by either chelating the extracellular Ca2+ or blocking the pathway of store-operated calcium entry (SOCE). Furthermore, both non-selective protein kinase C (PKC) inhibitor and PKCα selective inhibitor, but not PKCδ inhibitor, prevented the increase in ERK1/2 phosphorylation and the migration of astrocytes, indicating that the Ca2+-dependent PKCα acts as the downstream of the OX1R activation and mediates the orexin-A-induced increase in ERK1/2 phosphorylation and cell migration. In conclusion, these results suggest that orexin-A can stimulate ERK1/2 phosphorylation and then facilitate the migration of astrocytes via PLC-PKCα signal pathway, providing new knowledge about the functions of the OX1R in astrocytes.

GABAergic neurons in the preoptic area send direct inhibitory projections to orexin neurons

Populations of neurons in the hypothalamic preoptic area (POA) fire rapidly during sleep, exhibiting sleep/waking state-dependent firing patterns that are the reciprocal of those observed in the arousal system. The majority of these preoptic "sleep-active" neurons contain the inhibitory neurotransmitter GABA. On the other hand, a population of neurons in the lateral hypothalamic area (LHA) contains orexins, which play an important role in the maintenance of wakefulness, and exhibit an excitatory influence on arousal-related neurons. It is important to know the anatomical and functional interactions between the POA sleep-active neurons and orexin neurons, both of which play important, but opposite roles in regulation of sleep/wakefulness states. In this study, we confirmed that specific pharmacogenetic stimulation of GABAergic neurons in the POA leads to an increase in the amount of non-rapid eye movement (NREM) sleep. We next examined direct connectivity between POA GABAergic neurons and orexin neurons using channelrhodopsin 2 (ChR2) as an anterograde tracer as well as an optogenetic tool. We expressed ChR2-eYFP selectively in GABAergic neurons in the POA by AAV-mediated gene transfer, and examined the projection sites of ChR2-eYFP-expressing axons, and the effect of optogenetic stimulation of ChR2-eYFP on the activity of orexin neurons. We found that these neurons send widespread projections to wakefulness-related areas in the hypothalamus and brain stem, including the LHA where these fibers make close appositions to orexin neurons. Optogenetic stimulation of these fibers resulted in rapid inhibition of orexin neurons. These observations suggest direct connectivity between POA GABAergic neurons and orexin neurons.

Lack of Association Between Variants within the AHSG , HCRT and NPY2R Genes and Anthropometrical Parameters in Czech Post-Monica Study

The aim of this study was to examine single nucleotide polymorphisms (SNPs) of candidate genes α 2-Heremans-Schmid glycoprotein ( AHSG , rs4917), Hypocretin ( HCRT , rs760282) and Neuropetide Y2 receptor (NPY2R , rs 1047214), which are known to have a potential effect on body mass index (BMI) and other indicators of obesity. A population study was performed in 2007/2008 on 2559 adults (1191 males and 1368 females) from the Czech post-MONICA project. The SNPs were examined using the polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method. We did not find any significant association between the examined SNPs and BMI across the whole population. A significantly lower triglyceride level was found in the AHSG gene CC homozygotes compared to T allele carriers in the entire population ( p = 0.009). In conclusion, we are not able to confirm the hypothesis that polymorphisms within the AHSG , HCRT and NPY2R genes are major genetic determinants of BMI and plasma lipids in the Czech-Slavonic population.

Optogenetic examination identifies a context-specific role for orexins/hypocretins in anxiety-related behavior

Maladaptation to stress is associated with psychopathology. However, our understanding of the underlying neural circuitry involved in adaptations to stress is limited. Previous work from our lab indicated the paraventricular hypothalamic neuropeptides orexins/hypocretins regulate behavioral and neuroendocrine responses to stress. To further elucidate the role of orexins in adaptation to stress, we employed optogenetic techniques to specifically examine the effects of orexin cell activation on behavior in the social interaction test and in the home cage as well as orexin receptor 1 internalization and ERK phosphorylation in brain regions receiving orexin inputs. In the social interaction test, optogenetic stimulation of orexin neurons decreased time spent in the interaction zone while increasing the frequency of entries into the interaction zone. In addition, optogenetic stimulation of orexin neurons increased the total distance traveled in the social interaction arena but had no effect on their home cage behavior. Together, these results suggest that orexin release increases anxiety in the social interaction test while increasing the salience of novel but not familiar environmental stimuli. Consistent with activation of orexin neurons, optogenetic stimulation increased orexin receptor1 internalization and ERK phosphorylation in the paraventricular thalamus (PVT) and locus coeruleus (LC), two regions heavily innervated by orexin neurons. Together these results show for the first time that elevation of orexin activity, possibly in the PVT and LC, is associated with increased anxiety, activity, and arousal in a context-specific manner.

Influence of inhibitory serotonergic inputs to orexin/hypocretin neurons on the diurnal rhythm of sleep and wakefulness

STUDY OBJECTIVE

Serotonergic (5HT) neurons of the dorsal raphe nuclei receive excitatory input from hypothalamic orexin (hypocretin) neurons and reciprocally inhibit orexin neurons through the 5HT1A receptor. However, the physiological significance of this negative feedback circuit for sleep/wakefulness regulation is little understood.

DESIGN

5HT1A receptor expression level was specifically and reversibly controlled in the orexin neurons using the Tet-off system. The responsiveness of orexin neurons to 5HT in vitro and the sleep/wakefulness patterns were compared between 5HT1A-overexpressing and control mice.

MEASUREMENTS AND RESULTS

When the 5HT1A receptor was overexpressed in orexin neurons of Orexin-EGFP; orexin-tTA; TetO Htr1a mice, 5HT-induced inhibition of orexin neurons was prolonged. In the absence of doxycycline, Orexin-tTA; TetO Htr1a mice exhibited severe fragmentation of sleep/wakefulness during the first half of the dark period-the time of maximal activity in nocturnal rodents-without affecting sleep/wakefulness during the light period when sleep time is maximal. However, when the 5HT1A receptor in orexin neurons was reduced to basal expression levels in the presence of doxycycline, sleep/wakefulness patterns in Orexin-tTA; TetO Htr1a mice during the early active period were indistinguishable from those of littermate TetO Htr1a mice. These results strongly suggest that enhancement of inhibitory serotonergic input to orexin neurons caused fragmentation of wakefulness. In contrast, sleep/wakefulness architecture in the light period was unaffected by 5HT1A receptor overexpression in the orexin neurons.

CONCLUSION

Inhibitory serotonergic input likely functions as negative feedback to orexin neurons in the early dark period and helps stabilize wakefulness bouts, thereby contributing to the diurnal rhythm of sleep and wakefulness.