Agonist potency differentiates G protein activation and Ca2+ signalling by the orexin receptor type 1

Imaging flow cytometry of tumoroids: A new method for studying GPCR expression

Fluorescence confocal microscopy is commonly used to analyze the regulation membrane proteins expression such as G protein-coupled receptors (GPCRs). With this approach, the internal movement of GPCRs within the cell can be observed with a high degree of resolution. However, these microscopy techniques led to complex and time-consuming analysis and did not allow a large population of events to be sampled. A recent approach termed imaging flow cytometry (IFC), which combines flow cytometry and fluorescence microscopy, had two main advantages to study the regulation of GPCRs expression such as orexins receptors (OXRs): the ability (1) to analyze large numbers of cells and; (2) to visualize cell integrity and fluorescent markers localization. Here, we compare these two technologies using the orexin A (OxA) ligand coupled to rhodamine (OxA-rho) to investigate anti-tumoral OX1R expression in human digestive cancers. IFC has been adapted for cancer epithelial adherent cells and also to 3D cell culture tumoroids which partially mimic tumoral structures. In the absence of specific antibody, expression of OX1R is examined in the presence of OxA-rho. 2D-culture of colon cancer cells HT-29 exhibits a maximum level of OX1R internalization induced by OxA with 19% ± 3% colocalizing to early endosomes. In 3D-culture of HT-29 cells, internalization of OX1R/OxA-rho reached its maximum at 60 min, with 30.7% ± 6.4% of OX1R colocalizing with early endosomes. This is the first application of IFC to the analysis of the expression of a native GPCR, OX1R, in both 2D and 3D cultures of adherent cancer cells.

Orexin Signaling: A Complex, Multifaceted Process

The orexin system comprises two G protein-coupled receptors, OX1 and OX2 receptors (OX1R and OX2R, respectively), along with two endogenous agonists cleaved from a common precursor (prepro-orexin), orexin-A (OX-A) and orexin-B (OX-B). For the receptors, a complex array of signaling behaviors has been reported. In particular, it becomes obvious that orexin receptor coupling is very diverse and can be tissue-, cell- and context-dependent. Here, the early signal transduction interactions of the orexin receptors will be discussed in depth, with particular emphasis on the direct G protein interactions of each receptor. In doing so, it is evident that ligands, additional receptor-protein interactions and cellular environment all play important roles in the G protein coupling profiles of the orexin receptors. This has potential implications for our understanding of the orexin system's function in vivo in both central and peripheral environments, as well as the development of novel agonists, antagonists and possibly allosteric modulators targeting the orexin system.

Disruption of 5-hydroxytryptamine 1A receptor and orexin receptor 1 heterodimer formation affects novel G protein-dependent signaling pathways and has antidepressant effects in vivo

G protein-coupled receptor (GPCR) heterodimers are new targets for the treatment of depression. Increasing evidence supports the importance of serotonergic and orexin-producing neurons in numerous physiological processes, possibly via a crucial interaction between 5-hydroxytryptamine 1A receptor (5-HT1AR) and orexin receptor 1 (OX1R). However, little is known about the function of 5-HT1AR/OX1R heterodimers. It is unclear how the transmembrane domains (TMs) of the dimer affect its function and whether its modulation mediates antidepressant-like effects. Here, we examined the mechanism of 5-HT1AR/OX1R dimerization and downstream G protein-dependent signaling. We found that 5-HT1AR and OX1R form constitutive heterodimers that induce novel G protein-dependent signaling, and that this heterodimerization does not affect recruitment of β-arrestins to the complex. In addition, we found that the structural interface of the active 5-HT1AR/OX1R dimer transforms from TM4/TM5 in the basal state to TM6 in the active conformation. We also used mutation analyses to identify key residues at the interface (5-HT1AR R151^4.40, 5-HT1AR Y198^5.41, and OX1R L230^5.54). Injection of chronic unpredictable mild stress (CUMS) rats with TM4/TM5 peptides improved their depression-like emotional status and decreased the number of endogenous 5-HT1AR/OX1R heterodimers in the rat brain. These antidepressant effects may be mediated by upregulation of BDNF levels and enhanced phosphorylation and activation of CREB in the hippocampus and medial prefrontal cortex. This study provides evidence that 5-HT1AR/OX1R heterodimers are involved in the pathological process of depression. Peptides including TMs of the 5-HT1AR/OX1R heterodimer interface are candidates for the development of compounds with fast-acting antidepressant-like effects.

Hippocampal orexin receptors: Localization and function

Orexin (hypocretin) is secreted from the perifornical/lateral hypothalamus and is well known for sleep regulation. Orexin has two, orexin A and B, transcripts and two receptors, type 1 and 2 (OX1R and OX2R), located in the plasma membrane of neurons in different brain areas, including the hippocampus involved in learning, memory, seizures, and epilepsy, as physiologic and pathologic phenomena. OX1R is expressed in the dentate gyrus and CA1 and the OX2R in the CA3 areas. Orexin enhances learning and memory as well as reward, stress, seizures, and epilepsy, partly through OX1Rs, while either aggravating or alleviating those phenomena via OX2Rs. OX1Rs activation induces long-term changes of synaptic responses in the hippocampus, an age and concentration-dependent manner. Briefly, we will review the localization and functions of hippocampal orexin receptors, their role in learning, memory, stress, reward, seizures, epilepsy, and hippocampal synaptic plasticity.

Orexin receptor blockers: A tool for lowering alcohol intake and alcohol addictive behavior in the light of preclinical studies

Alcohol use disorder (AUD) is a severe and globally widespread neurological and psychiatric problem. The treatment with currently used drugs often does not bring the expected effect. New optimization methods or directions in pharmacotherapy are still being sought. The group of bioactive ligands, targeted at neuropeptides called orexins (OXs) and their receptors (OXRs), affects a number of functions including ingestion, sleep-wake regulation, as well as the brain reward system which is the basis of addiction.

The purpose of this paper is to systematize the knowledge in the field of preclinical behavioral studies on rodents (rats and mice) in several models of alcohol consumption using the OXRs antagonists.

The results of the experiments indicated a potential efficacy of particular OXRs antagonists in the AUD treatment, especially those selectively blocking the OX1R. Among them, SB-334867 in the lowest effective dose of 3 mg/kg i.p. was most studied, as shown in the model of two-bottle choice using C57BL/6 mice. Moreover, this compound did not affect the reduction of cognitive functions. GSK1059865 was also involved in the selective reduction of ethanol intake, and simultaneously did not alter the consumption of sugar solution. The other group of selective OX2R antagonists, such as TCS-OX2-29 and LSN2424100, was less efficient.

In summary, the OX1R antagonists proved to have the potential in AUD therapy, not only through the reduction of ethanol consumption but also in the treatment of coexisting behavioral and physiological disorders, such as insomnia and anxiety.

Orexin receptors in GtoPdb v.2021.3

Orexin receptors (nomenclature as agreed by the NC-IUPHAR Subcommittee on Orexin receptors [42]) are activated by the endogenous polypeptides orexin-A and orexin-B (also known as hypocretin-1 and -2; 33 and 28 aa) derived from a common precursor, preproorexin or orexin precursor, by proteolytic cleavage and some typical peptide modifications [109]. Currently the only orexin receptor ligands in clinical use are suvorexant and lemborexant, which are used as hypnotics. Orexin receptor crystal structures have been solved [134, 133, 54, 117, 46].

Orexin system is expressed in avian liver and regulates hepatic lipogenesis via ERK1/2 activation

Orexins are originally characterized as orexigenic hypothalamic neuropeptides in mammals. Subsequent studies found orexin to be expressed and perform pleiotropic functions in multiple tissues in mammals. In avian (non-mammalian) species, however, orexin seemed to not affect feeding behavior and its physiological roles are poorly understood. Here, we provide evidence that orexin and its related receptors are expressed in chicken hepatocytes. Double immunofluorescence staining showed that orexin is localized in the ER, Golgi, and in the lysosomes in LMH cells. Brefeldin A treatment reduced orexin levels in the culture media, but increased it in the cell lysates. Administration of recombinant orexins upregulated the expression of orexin system in the liver of 9-day old chicks, but did not affect feed intake. Recombinant orexins increased fatty acid synthase (FASN) protein levels in chicken liver, activated acetyl-CoA carboxylase (ACCα), and increased FASN, ATP citrate lyase(ACLY), and malic enzyme (ME) protein expression in LMH cells. Blockade ERK1/2 activation by PD98059 attenuated these stimulating effects of orexin on lipogenic factors. Overexpression of ERK1/2 increased the expression of lipogenic genes, and orexin treatment induced the phosphorylated levels of ERK1/2^{Thr202/Tyr204}, but not that of p38 ^{Thr180/Tyr182} or JNK1/2 ^{Thr183/Tyr185} in chicken liver and LMH cells. Taken together, this is the first report evidencing that orexin is expressed and secreted from chicken hepatocytes, and that orexin induced hepatic lipogenesis via activation of ERK1/2 signaling pathway.

A novel phosphorylation site on orexin receptor 1 regulating orexinA-induced GRK2-biased signaling

Drug discovery efforts targeting G protein-coupled receptors (GPCRs) have succeeded in developing multiple medications for treating various human diseases including cancer, metabolic disorders, and inflammatory disorders. These medications are broadly classified as either agonists or antagonists that respectively promote or inhibit receptor activation by endogenous stimuli. However, there has been a growing appreciation that GPCR biased signaling between G protein- and β-arrestin-dependent signaling in particular is a promising method for improving drug efficacy and therapy. Orexin receptor 1 (OX1R), a member of the GPCRs, is an important drug target in the central nervous system. In this study, we identified a novel regulatory phosphorylation site (Ser-262) on OX1R that abolished its capability to interact with GRK2, but did not affect its interaction with G proteins, GRK5, or β-arrestin1/2 activation, indicating that Ser-262 is a key amino acid for OX1R internalization that contributes to induction of GRK2-dependent biased signaling via orexin A. Our findings could potentially lead to the development of new drug targets for the prevention and treatment of insomnia, narcolepsy, and substance abuse, with fewer side effects than existing therapies.

Orexins and stress

The neuropeptides orexins are important in regulating the neurobiological systems that respond to stressful stimuli. Furthermore, orexins are known to play a role many of the phenotypes associated with stress-related mental illness such as changes in cognition, sleep-wake states, and appetite. Interestingly, orexins are altered in stress-related psychiatric disorders such as Major Depressive Disorder and Anxiety Disorders. Thus, orexins may be a potential target for treatment of these disorders. In this review, we will focus on what is known about the role of orexins in acute and repeated stress, in stress-induced phenotypes relevant to psychiatric illness in preclinical models, and in stress-related psychiatric illness in humans. We will also briefly discuss how orexins may contribute to sex differences in the stress response and subsequent phenotypes relevant to mental health, as many stress-related psychiatric disorders are twice as prevalent in women.

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.

Orexin Receptor Multimerization versus Functional Interactions: Neuropharmacological Implications for Opioid and Cannabinoid Signalling and Pharmacogenetics

Orexins/hypocretins are neuropeptides formed by proteolytic cleavage of a precursor peptide, which are produced by neurons found in the lateral hypothalamus. The G protein-coupled receptors (GPCRs) for these ligands, the OX₁ and OX₂ orexin receptors, are more widely expressed throughout the central nervous system. The orexin/hypocretin system has been implicated in many pathways, and its dysregulation is under investigation in a number of diseases. Disorders in which orexinergic mechanisms are being investigated include narcolepsy, idiopathic sleep disorders, cluster headache and migraine. Human narcolepsy has been associated with orexin deficiency; however, it has only rarely been attributed to mutations in the gene encoding the precursor peptide. While gene variations within the canine OX₂ gene hcrtr2 have been directly linked with narcolepsy, the majority of human orexin receptor variants are weakly associated with diseases (the idiopathic sleep disorders, cluster headache and polydipsia-hyponatremia in schizophrenia) or are of potential pharmacogenetic significance. Evidence for functional interactions and/or heterodimerization between wild-type and variant orexin receptors and opioid and cannabinoid receptors is discussed in the context of its relevance to depression and epilepsy.

Multiple excitatory actions of orexins upon thalamo-cortical neurons in dorsal lateral geniculate nucleus - implications for vision modulation by arousal

The orexinergic system of the lateral hypothalamus plays a crucial role in maintaining wakefulness and mediating arousal in a circadian time-dependent manner. Due to the extensive connections of orexinergic neurons, both orexins (OXA and OXB) exert mainly excitatory effects upon remote brain areas, including the thalamus. The dorsal lateral geniculate nucleus (DLG) is a relay thalamic centre for the visual system. Its thalamo-cortical (TC) neurons convey photic information from the retina to the primary visual cortex. The present study shows that orexins are powerful modulators of neuronal activity in the DLG. OXA directly depolarised the majority of neurons tested, acting predominately on postsynaptic OX2 receptors. Moreover, OXA was found to increase excitability and enhance neuronal responses to both glutamate and γ-aminobutyric acid (GABA). Mechanistic studies showed the involvement of voltage-gated calcium currents and GIRK channels in the observed depolarisations. Immunohistochemical staining showed sparse orexinergic innervation of the DLG during the light phase, with increased density at night. We hypothesise that the depolarising effects of orexins upon DLG neurons may facilitate signal transmission through the visual thalamo-cortical pathway during behavioural arousal. Thus, the action of orexin on DLG TC neurons may underlie the circadian/behavioural modulation of vision.

Orexin/Hypocretin System: Role in Food and Drug Overconsumption

The neuropeptide orexin/hypocretin (OX), while largely transcribed within the hypothalamus, is released throughout the brain to affect complex behaviors. Primarily through the hypothalamus itself, OX homeostatically regulates adaptive behaviors needed for survival, including food intake, sleep-wake regulation, mating, and maternal behavior. However, through extrahypothalamic limbic brain regions, OX promotes seeking and intake of rewarding substances of abuse, like palatable food, alcohol, nicotine, and cocaine. This neuropeptide, in turn, is stimulated by the intake of or early life exposure to these substances, forming a nonhomeostatic, positive feedback loop. The specific OX receptor involved in these behaviors, whether adaptive behavior or substance seeking and intake, is dependent on the particular brain region that contributes to them. Thus, we propose that, while the primary function of OX is to maintain arousal for the performance of adaptive behaviors, this neuropeptide system is readily co-opted by rewarding substances that involve positive feedback, ultimately promoting their abuse.

Aβ and Inflammatory Stimulus Activate Diverse Signaling Pathways in Monocytic Cells: Implications in Retaining Phagocytosis in Aβ-Laden Environment

Background: Accumulation of amyloid β (Aβ) is one of the main hallmarks of Alzheimer’s disease (AD). The enhancement of Aβ clearance may provide therapeutic means to restrict AD pathology. The cellular responses to different forms of Aβ in monocytic cells are poorly known. We aimed to study whether different forms of Aβ induce inflammatory responses in monocytic phagocytes and how Aβ may affect monocytic cell survival and function to retain phagocytosis in Aβ-laden environment.

Methods: Monocytic cells were differentiated from bone marrow hematopoietic stem cells (HSC) in the presence of macrophage-colony stimulating factor. Monocytic cells were stimulated with synthetic Aβ42 and intracellular calcium responses were recorded with calcium imaging. The formation of reactive oxygen species (ROS), secretion of cytokines and cell viability were also assessed. Finally, monocytic cells were introduced to native Aβ deposits ex vivo and the cellular responses in terms of cell viability, pro-inflammatory activation and phagocytosis were determined. The ability of monocytic cells to phagocytose Aβ plaques was determined after intrahippocampal transplantation in vivo.

Results: Freshly solubilized Aβ induced calcium oscillations, which persisted after removal of the stimulus. After few hours of aggregation, Aβ was not able to induce oscillations in monocytic cells. Instead, lipopolysaccharide (LPS) induced calcium responses divergent from Aβ-induced response. Furthermore, while LPS induced massive production of pro-inflammatory cytokines, neither synthetic Aβ species nor native Aβ deposits were able to induce pro-inflammatory activation of monocytic cells, contrary to primary microglia. Finally, monocytic cells retained their viability in the presence of Aβ and exhibited phagocytic activity towards native fibrillar Aβ deposits and congophilic Aβ plaques.

Conclusion: Monocytic cells carry diverse cellular responses to Aβ and inflammatory stimulus LPS. Even though Aβ species cause specific responses in calcium signaling, they completely lack the ability to induce pro-inflammatory phenotype of monocytic cells. Monocytes retain their viability and function in Aβ-laden brain.

Histamine may contribute to vortioxetine's procognitive effects; possibly through an orexigenic mechanism

Vortioxetine is a novel multimodal antidepressant that acts as a serotonin (5-HT)3, 5-HT7, and 5-HT1D receptor antagonist; 5-HT1B receptor partial agonist; 5-HT1A receptor agonist; and 5-HT transporter inhibitor in vitro. In preclinical and clinical studies vortioxetine demonstrates positive effects on cognitive dysfunction. Vortioxetine's effect on cognitive function likely involves the modulation of several neurotransmitter systems. Acute and chronic administration of vortioxetine resulted in changes in histamine concentrations in microdialysates collected from the rat prefrontal cortex and ventral hippocampus. Based on these results and a literature review of the current understanding of the interaction between the histaminergic and serotonergic systems and the role of histamine on cognitive function, we hypothesize that vortioxetine through an activation of the orexinergic system stimulates the tuberomammilary nucleus and enhances histaminergic neurotransmission, which contributes to vortioxetine's positive effects on cognitive function.

G-protein-dependency of orexin/hypocretin receptor signalling in recombinant Chinese hamster ovary cells

Multiple signalling pathways for orexin receptors have been discovered, and most thoroughly mapped in Chinese hamster ovary K1 (CHO-K1) cells. It is also known that orexin receptors can couple to the G-protein families Gi, Gs and Gq. However, the connection between the G-proteins and the downstream signals is only vaguely established, and we now set out to resolve this for human orexin receptors expressed in CHO-K1 cells. Adenylyl cyclase (AC), phospholipase A2, C and D, and diacylglycerol lipase activities were assessed by precursor radiolabelling and chromatographic separation, and calcium by fluorescent methods. Pertussis toxin, cholera toxin and the cyclic depsipeptide, UBO-QIC a.k.a. FR900359, were used to assess the involvement of Gi-, Gs- and Gq-family G-proteins, respectively. Calcium elevations as well as activation of the phospholipases and diacylglycerol lipase were dependent on Gq, as they were fully blocked by UBO-QIC. The low-potency AC activation fully depended on Gs. Surprisingly, the assumed Gi-dependent inhibition of AC was (fully or partially) inhibited by UBO-QIC, in opposition to the previous findings of no sensitivity of Gi proteins to UBO-QIC. Orexin receptor signalling is indeed mostly Gq-driven in CHO-K1 cells, even with respect to the less clearly mapped cascades such as phospholipase A2 and C and calcium influx, underlining the importance of Gq even under physiological conditions. AC regulation warrants more studies.

Hypothalamic neuropeptide signaling in alcohol addiction

The hypothalamus is now known to regulate alcohol intake in addition to its established role in food intake, in part through neuromodulatory neurochemicals termed neuropeptides. Certain orexigenic neuropeptides act in the hypothalamus to promote alcohol drinking, although they affect different aspects of the drinking response. These neuropeptides, which include galanin, the endogenous opioid enkephalin, and orexin/hypocretin, appear to stimulate alcohol intake not only through mechanisms that promote food intake but also by enhancing reward and reinforcement from alcohol. Moreover, these neuropeptides participate in a positive feedback relationship with alcohol, whereby they are upregulated by alcohol intake to promote even further consumption. They contrast with other orexigenic neuropeptides, such as melanin-concentrating hormone and neuropeptide Y, which promote alcohol intake under limited circumstances, are not consistently stimulated by alcohol, and do not enhance reward. They also contrast with neuropeptides that can be anorexigenic, including the endogenous opioid dynorphin, corticotropin-releasing factor, and melanocortins, which act in the hypothalamus to inhibit alcohol drinking as well as reward and therefore counter the ingestive drive promoted by orexigenic neuropeptides. Thus, while multiple hypothalamic neuropeptides may work together to regulate different aspects of the alcohol drinking response, excessive signaling from orexigenic neuropeptides or inadequate signaling from anorexigenic neuropeptides can therefore allow alcohol drinking to become dysregulated.

Effect of Orexin-A on Cortisol Secretion in H295R Cells via p70S6K/4EBP1 Signaling Pathway

Orexin-A is a neuropeptide that orchestrates diverse central and peripheral processes. It is now clear that orexin system plays a central role in the regulation of endocrine, paracrine, and neurocrine. It is involved in the regulation of growth hormone, adrenocorticotropic hormone, thyroid, mineralocorticoid, and cortisol secretion. These hormones may also serve as a kind of signal linking energy balance regulation, reproduction, stress response, and cardiovascular regulation. Many studies have demonstrated the ability of orexin-A to regulate adrenocortical cells through the MAPK (mitogen-activated protein kinases) pathway. The aim of our study is to investigate the effect of orexin-A on cortisol secretion via the protein 70 ribosomal protein S6 kinase-1 (p70S6K) and eukaryotic translation initiation factor 4E binding proteins (4EBP1) signaling pathway in adrenocortical cells. We reported the first evidence that orexin-A stimulated p70S6K and 4EBP1 in human H295R adrenocortical cells in a concentration and time-dependent manner. 10(-6) M orexin-A treatment for 1 hour was the most potent. Our results also indicated that p70S6K and 4EBP1 kinases participated in controlling cortisol secretion via OX1 receptor in H295R cells, which implied important role of p70S6K and 4EBP1 kinases in regulating adrenal function induced by orexin-A.

Orexin/hypocretin receptor signalling cascades

Orexin (hypocretin) peptides and their two known G-protein-coupled receptors play essential roles in sleep-wake control and powerfully influence other systems regulating appetite/metabolism, stress and reward. Consequently, drugs that influence signalling by these receptors may provide novel therapeutic opportunities for treating sleep disorders, obesity and addiction. It is therefore critical to understand how these receptors operate, the nature of the signalling cascades they engage and their physiological targets. In this review, we evaluate what is currently known about orexin receptor signalling cascades, while a sister review (Leonard & Kukkonen, this issue) focuses on tissue-specific responses. The evidence suggests that orexin receptor signalling is multifaceted and is substantially more diverse than originally thought. Indeed, orexin receptors are able to couple to members of at least three G-protein families and possibly other proteins, through which they regulate non-selective cation channels, phospholipases, adenylyl cyclase, and protein and lipid kinases. In the central nervous system, orexin receptors produce neuroexcitation by postsynaptic depolarization via activation of non-selective cation channels, inhibition of K⁺ channels and activation of Na⁺/Ca²⁺ exchange, but they also can stimulate the release of neurotransmitters by presynaptic actions and modulate synaptic plasticity. Ca²⁺ signalling is also prominently influenced by these receptors, both via the classical phospholipase C-Ca²⁺ release pathway and via Ca²⁺ influx, mediated by several pathways. Upon longer-lasting stimulation, plastic effects are observed in some cell types, while others, especially cancer cells, are stimulated to die. Thus, orexin receptor signals appear highly tunable, depending on the milieu in which they are operating.

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.

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.

OX1 and OX2 orexin/hypocretin receptor pharmacogenetics

Orexin/hypocretin peptide mutations are rare in humans. Even though human narcolepsy is associated with orexin deficiency, this is only extremely rarely due to mutations in the gene coding prepro-orexin, the precursor for both orexin peptides. In contrast, coding and non-coding variants of the OX₁ and OX₂ orexin receptors have been identified in many human populations; sometimes, these have been associated with disease phenotype, although most confer a relatively low risk. In most cases, these studies have been based on a candidate gene hypothesis that predicts the involvement of orexins in the relevant pathophysiological processes. In the current review, the known human OX₁/HCRTR1 and OX₂/HCRTR2 genetic variants/polymorphisms as well as studies concerning their involvement in disorders such as narcolepsy, excessive daytime sleepiness, cluster headache, polydipsia-hyponatremia in schizophrenia, and affective disorders are discussed. In most cases, the functional cellular or pharmacological correlates of orexin variants have not been investigated—with the exception of the possible impact of an amino acid 10 Pro/Ser variant of OX₂ on orexin potency—leaving conclusions on the nature of the receptor variant effects speculative. Nevertheless, we present perspectives that could shape the basis for further studies. The pharmacology and other properties of the orexin receptor variants are discussed in the context of GPCR signaling. Since orexinergic therapeutics are emerging, the impact of receptor variants on the affinity or potency of ligands deserves consideration. This perspective (pharmacogenetics) is also discussed in the review.

Orexin/hypocretin receptor signalling: a functional perspective

Multiple homeostatic systems are regulated by orexin (hypocretin) peptides and their two known GPCRs. Activation of orexin receptors promotes waking and is essential for expression of normal sleep and waking behaviour, with the sleep disorder narcolepsy resulting from the absence of orexin signalling. Orexin receptors also influence systems regulating appetite/metabolism, stress and reward, and are found in several peripheral tissues. Nevertheless, much remains unknown about the signalling pathways and targets engaged by native receptors. In this review, we integrate knowledge about the orexin receptor signalling capabilities obtained from studies in expression systems and various native cell types (as presented in Kukkonen and Leonard, this issue of British Journal of Pharmacology) with knowledge of orexin signalling in different tissues. The tissues reviewed include the CNS, the gastrointestinal tract, the pituitary gland, pancreas, adrenal gland, adipose tissue and the male reproductive system. We also summarize the findings in different native and recombinant cell lines, especially focusing on the different cascades in CHO cells, which is the most investigated cell line. This reveals that while a substantial gap exists between what is known about orexin receptor signalling and effectors in recombinant systems and native systems, mounting evidence suggests that orexin receptor signalling is more diverse than originally thought. Moreover, rather than being restricted to orexin receptor 'overexpressing' cells, this signalling diversity may be utilized by native receptors in a site-specific manner.

Orexins protect neuronal cell cultures against hypoxic stress: an involvement of Akt signaling

Orexins A and B are peptides produced mainly by hypothalamic neurons that project to numerous brain structures. We have previously demonstrated that rat cortical neurons express both types of orexin receptors, and their activation by orexins initiates different intracellular signals. The present study aimed to determine the effect of orexins on the Akt kinase activation in the rat neuronal cultures and the significance of that response in neurons subjected to hypoxic stress. We report the first evidence that orexins A and B stimulated Akt in cortical neurons in a concentration- and time-dependent manner. Orexin B more potently than orexin A increased Akt phosphorylation, but the maximal effect of both peptides on the kinase activation was very similar. Next, cultured cortical neurons were challenged with cobalt chloride, an inducer of reactive oxygen species and hypoxia-mediated signaling pathways. Under conditions of chemical hypoxia, orexins potently increased neuronal viability and protected cortical neurons against oxidative stress. Our results also indicate that Akt kinase plays an important role in the pro-survival effects of orexins in neurons, which implies a possible mechanism of the orexin-induced neuroprotection.

Enhanced early expression of membrane receptors with the Rous sarcoma virus promoter in baculovirus-infected insect cells

The effect of the Rous sarcoma virus (RSV) long terminal repeat enhancer/promoter on expression levels of complementary DNAs (cDNAs) encoding seven transmembrane receptors was studied using the baculovirus expression vector system. Expression of the human α(2B)-adrenoceptor (AR) cDNA under the control of the polyhedrin (POL) promoter produced up to 7.6 pmol/mg protein at 28 H post infection (p.i.) in Sf9 cells. The addition of the RSV promoter increased the expression to 11.6 pmol/mg protein. Dramatic increases in expression levels at early times were also obtained with the α(2A)-AR, the M₁ and M₄ muscarinic receptors, and the orexin OX₁ receptor. Analysis of the time-dependent expression revealed that expression driven by the RSV promoter reaches almost maximum 24 H p.i. and that this promoter is superior to the often used POL promoter at early times p.i. when functional studies need to be performed. Functional enhancement of signaling as a result of early expression is demonstrated with the α(2B)-AR and the OX₁ receptor. Finally, enhanced green fluorescent protein fluorescence in living cells was used to monitor expression by various viral promoters. The results verified the early transcriptional activity of the RSV promoter, whereas the cytomegalovirus promoter was found to be poorly active in Sf9 cells.

Viral-mediated gene delivery for cell-based assays in drug discovery

BACKGROUND

Adenovirus, retrovirus and lentivirus-based vectors, originally engineered and optimized for in vivo and ex vivo gene therapy, have become increasingly useful for viral-mediated gene delivery to support in vitro cell-based assays. Viral vectors underpin functional genomics screening of cDNA, shRNA and aptamer libraries, are used for a variety of target validation studies and importantly, for high-throughput cell-based drug discovery and compound profiling assays. The baculovirus/insect cell expression system had gained prevalence as a tool for recombinant protein production when it was observed that recombinant baculovirus vectors too could serve as efficient gene delivery vehicles for a wide range of mammalian cells. Although the use of baculovirus vectors in vivo has lagged behind retroviral, adenoviral and lentiviral vectors, they have gained prominence for development of in vitro cell-based assays due to the ease of generation, broad host range and excellent biosafety profile. There is an increasing emphasis on cell-based assays in high-throughput automated drug discovery laboratories and a variety of commercially available viral-vectors can be used for supporting these assays.

OBJECTIVE

We compare and contrast the current viral-mediated gene delivery vector systems and highlight their suitability for cell-based drug discovery assays.

CONCLUSION

Viral-mediated gene delivery is increasingly being used in support of genome scale target validation studies and cell-based assay development for specific drug target genes such as ion channels, G protein-coupled receptors and intracellular enzymes. The choice of a delivery system over another for a particular application is largely dictated by the cell types and cell lines in use, virus cellular tropism, assay throughput, safety requirements and ease/cost of reagent generation.

Autocrine endocannabinoid signaling through CB1 receptors potentiates OX1 orexin receptor signaling

It has been proposed that OX(1) orexin receptors and CB(1) cannabinoid receptors can form heteromeric complexes, which affect the trafficking of OX(1) receptors and potentiate OX(1) receptor signaling to extracellular signal-regulated kinase (ERK). We have recently shown that OX(1) receptor activity releases high levels of the endocannabinoid 2-arachidonoyl glycerol (2-AG), suggesting an alternative route for OX(1)-CB(1) receptor interaction in signaling, for instance, in retrograde synaptic transmission. In the current study, we set out to investigate this possibility utilizing recombinant Chinese hamster ovary K1 cells. 2-AG released from OX(1) receptor-expressing cells acted as a potent paracrine messenger stimulating ERK activity in neighboring CB(1) receptor-expressing cells. When OX(1) and CB(1) receptors were expressed in the same cells, OX(1) stimulation-induced ERK phosphorylation and activity were strongly potentiated. The potentiation but not the OX(1) response as such was fully abolished by specific inhibition of CB(1) receptors or the enzyme responsible for 2-AG generation, diacylglycerol lipase (DAGL). Although the results do not exclude the previously proposed OX(1)-CB(1) heteromerization, they nevertheless unequivocally identify DAGL-dependent 2-AG generation as the pivotal determinant of the OX(1)-CB(1) synergism and thus suggest a functional rather than a molecular interaction of OX(1) and CB(1) receptors.

Physiology of the orexinergic/hypocretinergic system: a revisit in 2012

The neuropeptides orexins and their G protein-coupled receptors, OX(1) and OX(2), were discovered in 1998, and since then, their role has been investigated in many functions mediated by the central nervous system, including sleep and wakefulness, appetite/metabolism, stress response, reward/addiction, and analgesia. Orexins also have peripheral actions of less clear physiological significance still. Cellular responses to the orexin receptor activity are highly diverse. The receptors couple to at least three families of heterotrimeric G proteins and other proteins that ultimately regulate entities such as phospholipases and kinases, which impact on neuronal excitation, synaptic plasticity, and cell death. This article is a 10-year update of my previous review on the physiology of the orexinergic/hypocretinergic system. I seek to provide a comprehensive update of orexin physiology that spans from the molecular players in orexin receptor signaling to the systemic responses yet emphasizing the cellular physiological aspects of this system.

Brain orexin promotes obesity resistance

Resistance to obesity is becoming an exception rather than the norm, and understanding mechanisms that lead some to remain lean in spite of an obesigenic environment is critical if we are to find new ways to reverse this trend. Levels of energy intake and physical activity both contribute to body weight management, but it is challenging for most to adopt major long-term changes in either factor. Physical activity outside of formal exercise, also referred to as activity of daily living, and in stricter form, spontaneous physical activity (SPA), may be an attractive modifiable variable for obesity prevention. In this review, we discuss individual variability in SPA and NEAT (nonexercise thermogenesis, or the energy expended by SPA) and its relationship to obesity resistance. The hypothalamic neuropeptide orexin (hypocretin) may play a key role in regulating SPA and NEAT. We discuss how elevated orexin signaling capacity, in the context of a brain network modulating SPA, may play a major role in defining individual variability in SPA and NEAT. Greater activation of this SPA network leads to a lower propensity for fat mass gain and therefore may be an attractive target for obesity prevention and therapy.

Role of orexin receptors in obesity: from cellular to behavioral evidence

The orexin peptides and their two receptors are involved in multiple physiological processes, including energy homeostasis, arousal, stress and reward. Higher signaling of the orexin peptides at the orexin receptors (OXR) protects against obesity, but it is less clear how their activation in different brain regions contributes to this behavioral output. This review summarizes the evidence available for a role of central OXR in energy homeostasis and their contribution to obesity. A detailed analysis of anatomical, cellular and behavioral evidence shows that modulation of energy homeostasis by the OXR is largely dependent upon anatomical and cellular context. It also shows that obesity resistance provided by activation of the OXR is distributed across multiple brain sites with site-specific actions. We suggest that understanding the role of the OXR in the development of obesity requires considering both specific mechanisms within brain regions and interactions of orexinergic input between multiple sites.

Activation of orexin/hypocretin type 1 receptors stimulates cAMP synthesis in primary cultures of rat astrocytes

The effects of orexins, which are also named hypocretins, on cAMP formation were examined in primary cultures of rat astrocytes. Orexin A, an agonist of OX₁ and OX₂ receptors, stimulated cAMP production with an EC₅₀ value of 0.68 μM and potentiated the forskolin-induced increase in the nucleotide synthesis. [Ala¹¹-D-Leu¹⁵]orexin B, an agonist of OX₂ receptors, was inactive. The effects of orexin A were antagonized by SB 408124, a selective blocker of OX₁ receptors, but were not affected by TCS OX2 29, a selective antagonist of OX₃ receptors. We hypothesized that the activation of OX₁ receptors stimulated cAMP synthesis in primary rat astrocyte cultures.

Orexins/hypocretins acting at Gi protein-coupled OX 2 receptors inhibit cyclic AMP synthesis in the primary neuronal cultures

Orexins A and B are newly discovered neuropeptides with pleiotropic activity. They signal through two G protein-coupled receptors: OX₁ and OX₂. In this study, we examined the expression of orexin receptors and effects of the receptors’ activation on cyclic AMP formation in the primary neuronal cell cultures from rat cerebral cortex. Both types of orexin receptors were expressed in rat cortical neurons; the level of OX₂R was markedly higher compared to OX₁R. Orexin A (an agonist of OX₁R and OX₂R) and [Ala¹¹-D-Leu¹⁵]orexin B (a selective agonist of OX₂R) did not affect basal cyclic AMP formation in the primary neuronal cell cultures. Both peptides (0.001–1 μM) inhibited, in a concentration-dependent manner and IC₅₀ values in low nanomolar range, the increase in the nucleotide production evoked by forskolin (1 μM; a direct activator of adenylyl cyclase), pituitary adenylate cyclase-activating polypeptide (PACAP27; 0.1 μM), and vasoactive intestinal peptide (VIP; 3 μM). Effects of orexin A on forskolin-, PACAP27-, and VIP-stimulated cyclic AMP synthesis were blocked by TCS OX2 29 (a selective antagonist of OX₂R), and unaffected by SB 408124 (a selective antagonist of OX₁R). Pretreatment of neuronal cell cultures with pertussis toxin (PTX) abolished the inhibitory action of orexin A on forskolin- and PACAP-stimulated cyclic AMP accumulation. It is suggested that in cultured rat cortical neurons orexins, acting at OX₂ receptors coupled to PTX-sensitive G_i protein, inhibit cyclic AMP synthesis.

Knockdown of orexin type 1 receptor in rat locus coeruleus increases REM sleep during the dark period

The locus coeruleus (LC) regulates sleep/wakefulness and is densely innervated by orexinergic neurons in the lateral hypothalamus. Here we used small interfering RNAs (siRNAs) to test the role of LC orexin type 1 receptor (OxR1) in sleep–wake control. In sleep studies, bilateral OxR1 siRNA injections led to an increase of time spent in rapid eye movement (REM) sleep, which was selective for the dark (active) period, peaked at approximately 30% of control during the second dark period after injection and then disappeared after 4 days. Cataplexy-like episodes were not observed. The percentage time spent in wakefulness and non-REM (NREM) sleep and the power spectral profile of NREM and REM sleep were unaffected. Control animals, injected with scrambled siRNA, had no sleep changes after injection. Quantification of the knockdown revealed that unilateral microinjection of siRNAs targeting OxR1 into the rat LC on two consecutive days induced a 45.5% reduction of OxR1 mRNA in the LC 2 days following the injections when compared with the contralateral side receiving injections of control (scrambled) siRNAs. This reduction disappeared 4 days after injection. Similarly, unilateral injection of OxR1 siRNA into the LC revealed a marked (33.5%) reduction of OxR1 staining 2 days following injections. In contrast, both the mRNA level and immunohistochemical staining for tyrosine hydroxylase were unaffected. The results indicate that a modest knockdown of OxR1 is sufficient to induce observable sleep changes. Moreover, orexin neurons, by acting on OxR1 in the LC, play a role in the diurnal gating of REM sleep.

Orexins/hypocretins and orexin receptors in apoptosis: a mini-review

An unexpected and fascinating aspect of the neuropeptides orexins has recently emerged when it was shown that orexins acting at orexin receptors OX1R or OX2R induce dramatic apoptosis resulting in massive reduction in cell growth in various cancer cell lines. This mini-review will provide the reader with recent findings related to the proapoptotic actions of orexins and the entirely novel mechanism whereby the seven membrane-spanning G-protein-coupled receptor (GPCR) OX1R triggers apoptosis. Recent data show that orexins induce tyrosine phosphorylation of the tyrosine-based motifs - immunoreceptor tyrosine-based inhibitory motif and immunoreceptor tyrosine-based switch motif - in OX1R. These phosphorylations result in the recruitment and activation of the phosphotyrosine phosphatase SHP-2 and subsequent cytochrome c-mediated mitochondrial apoptosis. Finally, this mini-review will also speculate on: (1) the potential importance of tyrosine-based motifs in the large family of GPCRs; (2) the interest of orexin receptors as therapeutic targets in cancer therapy; (3) the possible role of orexin receptor-mediated apoptosis in physiology and pathophysiology in the brain (neurodevelopment, neurodegenerative diseases) and in the periphery.

Discovery of a functional immunoreceptor tyrosine-based switch motif in a 7-transmembrane-spanning receptor: role in the orexin receptor OX1R-driven apoptosis

The orexin neuropeptides promote robust apoptosis in cancer cells. We have recently shown that the 7-transmembrane-spanning orexin receptor OX1R mediates apoptosis through an original mechanism. OX1R is equipped with a tyrosine-based inhibitory motif ITIM, which is tyrosine-phosphorylated on receptor activation, allowing the recruitment and activation of the tyrosine phosphatase SHP-2, leading to apoptosis. We show here that another motif, immunoreceptor tyrosine-based switch motif (ITSM), is present in OX1R and is mandatory for OX1R-mediated apoptosis. This conclusion is based on the following observations: 1) a canonical ITSM sequence is present in the first intracellular loop of OX1R; 2) mutation of Y(83) to F within ITSM abolished OX1R-mediated apoptosis but did not alter orexin-induced inositol phosphate formation or calcium transient via coupling of OX1R to G(q) protein; 3) mutation of Y(83) to F further abolished orexin-induced tyrosine phosphorylation in ITSM and subsequent recruitment of SHP-2 by the receptor. Finally, we developed a structural model of OX1R showing that the spatial localization of phosphotyrosines in ITSM and ITIM in OX1R is compatible with their interaction with the two SH2 domains of SHP-2. These data represent the first evidence for a functional role of an ITSM in a 7-transmembrane-spanning receptor.

Orexin A-induced extracellular calcium influx in prefrontal cortex neurons involves L-type calcium channels

Orexins, novel excitatory neuropeptides from the lateral hypothalamus, have been strongly implicated in the regulation of sleep and wakefulness. In this study, we explored the effects and mechanisms of orexin A on intracellular free Ca2+ concentration ([Ca2+]i) of freshly dissociated neurons from layers V and VI in prefrontal cortex (PFC). Changes in [Ca2+]i were measured with fluo-4/AM using confocal laser scanning microscopy. The results revealed that application of orexin A (0.1-1 microM) induced increase of [Ca2+]i in a dose-dependent manner. This elevation of [Ca2+]i was completely blocked by pretreatment with selective orexin receptor 1 antagonist SB 334867. While depletion of intracellular Ca2+ stores by the endoplasmic reticulum inhibitor thapsigargin (2 pM), [Ca2+]i in PFC neurons showed no increase in response to orexin A. Under extracellular Ca2+-free condition, orexin A failed to induce any changes of Ca2+ fluorescence intensity in these acutely dissociated cells. Our data further demonstrated that the orexin A-induced increase of [Ca2+]i was completely abolished by the inhibition of intracellular protein kinase C or phospholipase C activities using specific inhibitors, BIS II (1 microM) and D609 (10 microM), respectively. Selective blockade of L-type Ca2+ channels by nifedipine (5 microM) significantly suppressed the elevation of [Ca2+]i induced by orexin A. Therefore, these findings suggest that exposure to orexin A could induce increase of [Ca2+]i in neurons from deep layers of PFC, which depends on extracellular Ca2+ influx via L-type Ca2+ channels through activation of intracellular PLC-PKC signaling pathway by binding orexin receptor 1.

Involvement of TRPC3 channels in calcium oscillations mediated by OX(1) orexin receptors

Oscillations of intracellular Ca2+ provide a novel mechanism for sustained activation of cellular processes. Receptor-activated oscillations are mainly thought to occur through rhythmic IP3-dependent store discharge. However, as shown here in HEK293 cells 1 nM orexin-A (Ox-A) acting at OX1 receptors (OX1R) triggered oscillatory Ca2+ responses, requiring external Ca2+. These responses were attenuated by interference with TRPC3 channel (but not TRPC1/4) function using dominant negative constructs, elevated Mg2+ (a blocker of many TRP channels) or inhibition of phospholipase A2. These treatments did not affect Ca2+ oscillations elicited by high concentrations of Ox-A (100 nM) in the absence of external Ca2+. OX1R are thus able to activate TRPC(3)-channel-dependent oscillatory responses independently of store discharge.

Orexins stimulate steroidogenic acute regulatory protein expression through multiple signaling pathways in human adrenal H295R cells

Orexins mediate a variety of physiological processes, including feeding behavior, the circadian pathway, and cortisol secretion. Steroidogenesis is regulated by a variety of neuropeptides, and one of the key rate-limiting steps is cholesterol transport across the mitochondrial membrane by the steroidogenic acute regulatory protein (StAR). StAR expression can be regulated through several different signaling pathways. Despite the clear link between orexins and steroid production, the actions of the orexin family of hormones on steroid biosynthesis are not fully understood. We present data showing that 100 nm of both orexins A and B for 4 or 24 h significantly up-regulates StAR, in H295R pluripotent adrenocortical cells. We present the dose-dependent and time-dependent characteristics of StAR up-regulation at the protein level, showing significant increases after 4 h at a relatively low agonist concentration (1 nm). We have provided a key analysis of the precise G protein-coupled signaling pathways required for the up-regulation of StAR in response to orexins A and B. This has involved dominant-negative G protein analysis, and the direct inhibition of the protein kinase A, protein kinase C, ERK1/2, and p38 pathways. This shows a fundamental role for multiple G protein-coupled and MAPK-mediated signaling pathways leading to StAR expression. Antagonist analysis also showed that orexin effects on StAR were primarily, but not exclusively, acting through the orexin receptor type 1. This is the first study linking orexin action on StAR expression and comprehensively describes the signaling pathways involved in regulating the complexity of hormone biosynthesis.

Regulation of OX1 orexin/hypocretin receptor-coupling to phospholipase C by Ca2+ influx

BACKGROUND AND PURPOSE

Orexin (OX) receptors induce Ca2+ elevations via both receptor-operated Ca2+ channels (ROCs) and the "conventional" phospholipase C (PLC)-Ca2+ release-store-operated Ca2+ channel (SOC) pathways. In this study we assessed the ability of these different Ca2+ influx pathways to amplify OX1 receptor signalling to PLC in response to stimulation with the physiological ligand orexin-A.

EXPERIMENTAL APPROACH

PLC activity was assessed in CHO cells stably expressing human OX1 receptors.

KEY RESULTS

Inhibition of total Ca2+ influx by reduction of the extracellular [Ca2+] to 1 microM effectively inhibited the receptor-stimulated PLC activity at low orexin-A concentrations (by 93% at 1 nM), and this effect was gradually reduced by higher orexin-A concentrations. A similar but weaker inhibitory effect (84% at 1 nM) was obtained on depolarization to approximately 0 mV, which disrupts most of the driving force for Ca2+ entry. The inhibitor of the OX1 receptor-activated ROCs, tetraethylammonium chloride (TEA), was somewhat less effective than the reduction in extracellular [Ca2+] at inhibiting PLC activation, probably because it only partially blocks ROCs. The partial inhibitor of both ROCs and SOCs, Mg2+, and the SOC inhibitors, dextromethorphan, SKF-96365 (1-[beta-(3-(4-methoxyphenyl)propoxy)-4-methoxyphenethyl]-1H-imidazole HCL) and 2-APB (2-aminoethoxydiphenyl borate), inhibited PLC activity at low concentrations of orexin-A, but were not as effective as TEA.

CONCLUSIONS AND IMPLICATIONS

Both ROCs and SOCs markedly amplify the OX(1) receptor-induced PLC response, but ROCs are more central for this response. These data indicate the crucial role of ROCs in orexin receptor signalling.