The role of orexin in motivated behaviours

HYPOTHesizing about central comBAT against obesity

The hypothalamus is a brain region in charge of many vital functions. Among them, BAT thermogenesis represents an essential physiological function to maintain body temperature. In the metabolic context, it has now been established that energy expenditure attributed to BAT function can contribute to the energy balance in a substantial extent. Thus, therapeutic interest in this regard has increased in the last years and some studies have shown that BAT function in humans can make a real contribution to improve diabetes and obesity-associated diseases. Nevertheless, how the hypothalamus controls BAT activity is still not fully understood. Despite the fact that much has been known about the mechanisms that regulate BAT activity in recent years, and that the central regulation of thermogenesis offers a very promising target, many questions remain still unsolved. Among them, the possible human application of knowledge obtained from rodent studies, and drug administration strategies able to specifically target the hypothalamus. Here, we review the current knowledge of homeostatic regulation of BAT, including the molecular insights of brown adipocytes, its central control, and its implication in the development of obesity.

Synthesis and characterization of a novel 18F-labeled 2,5-diarylnicotinamide derivative targeting orexin 2 receptor

Orexin 2 receptor (OX2R) is thought to play an important role in the arousal-promoting function, but its distribution and function in the pathophysiology of orexin-mediated disorders remains to be fully elucidated. In the present study, we synthesized and characterized a novel 18F-labeled 2,5-diarylnicotinamide (DAN) derivative as a potential positron emission tomography (PET) probe for in vivo imaging of OX2R. In in vitro binding experiments, [18F]DAN-1 selectively bound to OX2R. In a biodistribution study using normal mice, [18F]DAN-1 displayed moderate brain uptake (2.10% ID per g at 10 min post-injection). In addition, the radioactivity in the mouse brain at 30 min post-injection was significantly decreased by co-injection with nonradioactive DAN-1, but high nonspecific binding was observed. These results suggested that further structural modifications of [18F]DAN-1 are needed to use it for imaging OX2R in the brain.

A Preliminary Description of the Sleep-Related Neural Systems in the Brain of the Blue Wildebeest, Connochaetes taurinus

The current study provides a detailed qualitative description of the organization of the cholinergic, catecholaminergic, serotonergic, orexinergic, and GABAergic sleep-related systems in the brain of the blue wildebeest (Connocheates taurinus), along with a quantitative analysis of the pontine cholinergic and noradrenergic neurons, and the hypothalamic orexinergic neurons. The aim of this study was to compare the nuclear organization of these systems to other mammalian species and specifically that reported for other Cetartiodactyla. In the brain of the blue wildebeest, from the basal forebrain to the pons, the nuclear organization of the cholinergic, catecholaminergic, serotonergic, and orexinergic systems, for the most part, showed a corresponding nuclear organization to that reported in other mammals and more specifically the Cetartiodactyla. Furthermore, the description and distribution of the GABAergic system, which was examined through immunostaining for the calcium binding proteins calbindin, calretinin, and parvalbumin, was also similar to that seen in other mammals. These findings indicate that sleep in the blue wildebeest is likely to show typically mammalian features in terms of the global brain activity of the generally recognized sleep states of mammals, but Cetartiodactyl-specific features of the orexinergic system may act to lower overall daily total sleep time in relation to similar sized non-Cetartiodactyl mammals. Anat Rec, 2019. © 2019 American Association for Anatomy Anat Rec, 303:1977-1997, 2020. © 2019 American Association for Anatomy.

Effects of hunger on emotional arousal responses and attention/memory biases

In this study, we examined how emotional arousal interacts with hunger states and the processing of food stimuli. In general, arousal enhances the processing of high-priority information at the expense of lower priority information (Mather & Sutherland, 2011). Because food has been a biologically relevant stimulus in primates throughout evolution, detecting it in the environment and remembering its location has high priority. In our study, inducing arousal enhanced attention to subsequent food stimuli. In addition, we manipulated whether participants were hungry or sated to examine how hunger states would influence emotional processing. Previous research reveals that being hungry is associated with increases in norepinephrine, a key neurotransmitter involved in the arousal response. We found that, when sated, participants showed greater pupil dilation to emotional than neutral stimuli. In contrast, when hungry, pupil dilation responses were as strong to neutral as to emotional stimuli. Thus, when hungry, participants were less effective at differentiating the intensity of arousal responses to emotional versus neutral stimuli because of high arousal responses to neutral stimuli. Memory for food stimuli was enhanced compared with memory for nonfood stimuli for all participants but especially for hungry participants. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Differential Roles of Each Orexin Receptor Signaling in Obesity

Orexins are hypothalamic neuropeptides that regulate feeding, energy expenditure, and sleep. Although orexin-deficient mice are susceptible to obesity, little is known about the roles of the orexin receptors in long-term energy metabolism. Here, we performed the metabolic characterization of orexin receptor-deficient mice. Ox1r-deficient mice were resistant to diet-induced obesity, and their food intake was similar between chow and high-fat food. Ox2r-deficient mice exhibited less energy expenditure than wild-type mice when fed a high-fat diet. Neither Ox1r-deficient nor Ox2r-deficient mice showed body weight gain similar to orexin-deficient mice. Although the presence of a running wheel suppressed diet-induced obesity in wild-type mice, the effect was weaker in orexin neuron-ablated mice. Finally, we did not detect abnormalities in brown adipose tissues of orexin-deficient mice. Thus, each orexin receptor signaling has a unique role in energy metabolism, and orexin neurons are involved in the interactive effect of diet and exercise on body weight gain.

•

Food intakes of Ox1r-deficient mice are similar between chow and high-fat food

•

Ox2r-deficient mice exhibit less energy expenditure when fed a high-fat diet

•

Orexin neurons are involved in the interactive effect of diet and exercise

•

Orexin-deficient mice have normal brown adipose tissue

Involvement of orexin receptors within the hippocampal dentate gyrus in morphine-induced reinstatement in food-deprived rats

The orexinergic system is found to cooperate in mediating stress-induced drug relapse. The orexinergic terminals innervate neurons of the hippocampal dentate gyrus (DG) which is a key structure in the maintenance and reinstatement of drug addiction. However, the specific contribution of intra-DG orexin receptors to stress-induced reinstatement has not been completely known. In the current study, the effects of intra-DG administration of SB334867, an orexin-1 receptor (OX1R) antagonist, and TCS OX2 29, an orexin-2 receptor (OX2R) antagonist, were investigated on the reinstatement induced by a sub-threshold dose of morphine and food deprivation (FD) stress. Adult male rats received different doses of SB334867 or TCS OX2 29 (3, 10, and 30 nM/0.5 μl DMSO 12%) bilaterally into the DG in separate groups, following the acquisition and extinction of morphine-induced conditioned place preference (CPP). Then, the reinstatement was evaluated by the 24 h FD stress and/or a sub-threshold dose of morphine (0.5 mg/kg, s.c.). CPP scores and locomotor activities were recorded during the test. The findings indicated that pre-treatment with the highest dose of SB334867 (30 nM) and two higher doses of TCS OX2 29 (10 and 30 nM) blocked the sub-threshold dose and FD stress-induced reinstatement of morphine. The effect of TCS OX2 29 on reduction of reinstatement was more pronounced than that of SB334867. It suggests a role for the orexin receptors, especially OX2R within the DG region in the stress-induced reinstatement of morphine-seeking behaviours in extinguished rats.

Fatigue, Sleep, and Autoimmune and Related Disorders

Profound and debilitating fatigue is the most common complaint reported among individuals with autoimmune disease, such as systemic lupus erythematosus, multiple sclerosis, type 1 diabetes, celiac disease, chronic fatigue syndrome, and rheumatoid arthritis. Fatigue is multi-faceted and broadly defined, which makes understanding the cause of its manifestations especially difficult in conditions with diverse pathology including autoimmune diseases. In general, fatigue is defined by debilitating periods of exhaustion that interfere with normal activities. The severity and duration of fatigue episodes vary, but fatigue can cause difficulty for even simple tasks like climbing stairs or crossing the room. The exact mechanisms of fatigue are not well-understood, perhaps due to its broad definition. Nevertheless, physiological processes known to play a role in fatigue include oxygen/nutrient supply, metabolism, mood, motivation, and sleepiness-all which are affected by inflammation. Additionally, an important contributing element to fatigue is the central nervous system-a region impacted either directly or indirectly in numerous autoimmune and related disorders. This review describes how inflammation and the central nervous system contribute to fatigue and suggests potential mechanisms involved in fatigue that are likely exhibited in autoimmune and related diseases.

Essential structure of orexin 1 receptor antagonist YNT-707, Part IV: The role of D-ring in 4,5-epoxymorphinan on the orexin 1 receptor antagonistic activity

The orexin 1 receptor (OX₁R) antagonists carrying a morphinan skeleton such as YNT-707 (2) and YNT-1310 (3) showed potent and extremely high selective antagonistic activity against OX₁R. In the course of our study of the essential structure of YNT-707 for high binding affinity against OX₁R, we prepared derivatives of 2 without the D- and 4,5-epoxy rings to clarify the roles of these structural determinants toward OX₁R antagonistic activity. The D- and 4,5-epoxy rings played important roles for the active orientation of the 17-sulfonamide and 6-amide side chains. Finally, we identified the simple structure required for selective OX₁R antagonistic activity in the complex morphinan skeleton, which is expected to be a useful scaffold for further design of OX₁R ligands.

Neurophysiological and Behavioral Effects of Anti-Orexinergic Treatments in a Mouse Model of Huntington's Disease

Huntington's disease (HD) is associated with sleep and circadian disturbances in addition to hallmark motor and cognitive impairments. Electrophysiological studies on HD mouse models have revealed an aberrant oscillatory activity at the beta frequency, during sleep, that is associated with HD pathology. Moreover, HD animal models display an abnormal sleep-wake cycle and sleep fragmentation. In this study, we investigated a potential involvement of the orexinergic system dysfunctioning in sleep-wake and circadian disturbances and abnormal network (i.e., beta) activity in the R6/1 mouse model. We found that the age at which orexin activity starts to deviate from normal activity pattern coincides with that of sleep disturbances as well as the beta activity. We also found that acute administration of Suvorexant, an orexin 1 and orexin 2 receptor antagonist, was sufficient to decrease the beta power significantly and to improve sleep in R6/1 mice. In addition, a 5-day treatment paradigm alleviated cognitive deficits and induced a gain of body weight in female HD mice. These results suggest that restoring normal activity of the orexinergic system could be an efficient therapeutic solution for sleep and behavioral disturbances in HD.

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.

The Postpartum Marital Satisfaction, Maternal Serum Concentration of Orexin-A and Mode of Delivery

Aims: We aimed to assess the association of postpartum maternal serum concentration of orexin-A with postpartum marital satisfaction considering the effect of mode of delivery as an influential factor. Methods: This cohort study conducted among third trimester pregnant women, who met our eligibility criteria. Postpartum maternal and cord serum concentration of orexin-A were measured and their association with postpartum marital satisfaction were assessed considering the impact of mode of delivery. Results: There was a statistically significant positive association between postpartum maternal and cord serum levels of orexin-A (r = 0.79, p < 0.001) and postpartum marital satisfaction among women with cesarean section (r = 0.31, p < 0.01). The maternal orexin-A level of women delivered with cesarean section who had post-partum marital dissatisfaction was significantly lower than those one with marital satisfaction (84.13 ± 95.88 vs. 153.08 ± 95.88 pg/ml, p = 0.04). Logistic regression model showed that the type of delivery was not related to marital satisfaction (OR = 1.7, 95%CI: 0.6, 4.8), p = 0.280). Conclusions: The postpartum maternal serum orexin-A level was associated with marital satisfaction in women delivered through cesarean section.

Reduced Plasma Orexin-A Concentrations are Associated with Cognitive Deficits in Anorexia Nervosa

Orexins/hypocretins are neuropeptides implicated in numerous processes, including food intake and cognition. The role of these peptides in the psychopathology of anorexia nervosa (AN) remains poorly understood. The aim of the current study was to evaluate the associations between plasma orexin-A (OXA) concentrations and neuropsychological functioning in adult women with AN, and a matched control group. Fasting plasma OXA concentrations were taken in 51 females with AN and in 51 matched healthy controls. Set-shifting was assessed using the Wisconsin Card Sorting Test (WCST), whereas decision making was measured using the Iowa Gambling Task (IGT). The AN group exhibited lower plasma OXA levels than the HC group. Lower mean scores were obtained on the IGT in AN patients. WCST perseverative errors were significantly higher in the AN group compared to HC. In both the AN and HC group, OXA levels were negatively correlated with WCST non-perseverative errors. Reduced plasma OXA concentrations were found to be associated with set-shifting impairments in AN. Taking into consideration the function of orexins in promoting arousal and cognitive flexibility, future studies should explore whether orexin partly underpins the cognitive impairments found in AN.

Interleukin-1-related activity and hypocretin-1 in cerebrospinal fluid contribute to fatigue in primary Sjögren's syndrome

BACKGROUND

Fatigue is a common and sometimes debilitating phenomenon in primary Sjögren's syndrome (pSS) and other chronic inflammatory diseases. We aimed to investigate how IL-1 β-related molecules and the neuropeptide hypocretin-1 (Hcrt1), a regulator of wakefulness, influence fatigue.

METHODS

Hcrt1 was measured by radioimmunoassay (RIA) in cerebrospinal fluid (CSF) from 49 patients with pSS. Interleukin-1 receptor antagonist (IL-1Ra), IL-1 receptor type 2 (IL-1RII), IL-6, and S100B protein were measured by enzyme-linked immunosorbent assay (ELISA). Fatigue was rated by the fatigue visual analog scale (fVAS).

RESULTS

Simple univariate regression and multiple regression analyses with fatigue as a dependent variable revealed that depression, pain, and the biochemical variable IL-1Ra had a significant association with fatigue. In PCA, two significant components were revealed. The first component (PC1) was dominated by variables related to IL-1β activity (IL-1Ra, IL-1RII, and S100B). PC2 showed a negative association between IL-6 and Hcrt1. fVAS was then introduced as an additional variable. This new model demonstrated that fatigue had a higher association with the IL-1β-related PC1 than to PC2. Additionally, a third component (PC3) became significant between low Hcrt1 concentrations and fVAS scores.

CONCLUSIONS

The main findings of this study indicate a functional network in which several IL-1β-related molecules in CSF influence fatigue in addition to the classical clinical factors of depression and pain. The neuropeptide Hcrt1 seems to participate in fatigue generation, but likely not through the IL-1 pathway.

Eating Rewards the Gears of the Clock

Eating behavior is regulated by metabolic and hedonic brain networks, which interact with each other to balance the physiological regulation of hunger and satiety. The daily balance of this regulation is controlled by the central circadian clock. Importantly, metabolic and reward properties of food impact the functioning of circadian clocks, altering the oscillatory activity of the molecular clockwork and circadian rhythms. However, when feeding (metabolic or reward) is timed, the whole circadian system is entrained. Furthermore, besides synchronizing the clock, the timing of both metabolic and reward eating might be crucial for health, to improve circadian physiology, as well as to treat metabolic (e.g., diabetes, obesity) and neurological diseases (e.g., mental, neurodegenerative).

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.

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.

Orexin 1 receptors in the anterior cingulate and orbitofrontal cortex regulate cost and benefit decision-making

Orexin neurons are discretely localized within the lateral hypothalamus and have widespread projections into all areas of the brain. In addition, several lines of evidence specify that orexins may also participate in the regulation of a variety of affective and cognitive processes. The Orexin-1 receptor (OX1r) is distributed extensively throughout the prefrontal cortex (PFC). Delay-based decision- making is mediated largely by the orbitofrontal cortex (OFC) while effort- based decision-making is controlled by the anterior cingulated cortex (ACC). Hence, in the present study, a series of experiments were conducted to clarify the role of OX1r in the mPFC (ACC and/or OFC) in cost and benefit decision-making. The rats were trained in a delay and/or effort-based form of cost-benefit T-maze decision-making task. Two goal arms were different in the amount of accessible reward and cost. Before surgery, all animals were selecting the high reward arm and pay the cost on almost every trial. During the test days, the rats received local injections of either DMSO 20% /0.5 μl, as a vehicle, or SB334867 (3, 30 and 300 nM/0.5 μl), as a selective OX1r antagonist, within the ACC and/or OFC. The results of this study showed that the bilateral microinjection of SB334867 into ACC and/or OFC changed the preference to a low reward arm with no cost, indicating the role of OX1 receptors in cost and benefit decision- making. From these results, it can be implied that OX1 receptors in the mPFC play a crucial role for allowing the animal to evaluate and pay the cost to acquire greater rewards.

Age-related central regulation of orexin and NPY in the short-lived African killifish Nothobranchius furzeri

Orexin A (OXA) and neuropeptide Y (NPY) are two hypothalamic neuropeptides involved in the regulation of feeding behavior and food intake in all vertebrates. Accumulating evidences document that they undergo age-related modifications, with consequences on metabolism, sleep/wake disorders and progression of neurodegenerations. The present study addressed the age related changes in expression and distribution of orexin A (its precursor is also known as hypocretin-HCRT) and NPY, and their regulation by food intake in the short-lived vertebrate model Nothobranchius furzeri. Our experiments, conducted on male specimens, show that: (a) HCRT and OXA and NPY mRNA and protein are localized in neurons of diencephalon and optic tectum, as well as in numerous fibers projecting through the entire neuroaxis, and are colocalized in specific nuclei; (b) in course of aging, HCRT and NPY expressing neurons are localized also in telencephalon and rhombencephalon; (c) HCRT expressing neurons increased slightly in the diencephalic area of old animals and in fasted animals, whereas NPY increased sharply; (d) central HCRT levels are not regulated neither in course of aging nor by food intake; and (e) central NPY levels are augmented in course of aging, and regulated by food intake only in young. These findings represent a great novelty in the study of central orexinergic and NPY-ergic systems in vertebrates', demonstrating an uncommon and unprecedented described regulation of these two orexigenic neuropeptides.

Orexin in the anxiety spectrum: association of a HCRTR1 polymorphism with panic disorder/agoraphobia, CBT treatment response and fear-related intermediate phenotypes

Preclinical studies point to a pivotal role of the orexin 1 (OX₁) receptor in arousal and fear learning and therefore suggest the HCRTR1 gene as a prime candidate in panic disorder (PD) with/without agoraphobia (AG), PD/AG treatment response, and PD/AG-related intermediate phenotypes. Here, a multilevel approach was applied to test the non-synonymous HCRTR1 C/T Ile408Val gene variant (rs2271933) for association with PD/AG in two independent case-control samples (total n = 613 cases, 1839 healthy subjects), as an outcome predictor of a six-weeks exposure-based cognitive behavioral therapy (CBT) in PD/AG patients (n = 189), as well as with respect to agoraphobic cognitions (ACQ) (n = 483 patients, n = 2382 healthy subjects), fMRI alerting network activation in healthy subjects (n = 94), and a behavioral avoidance task in PD/AG pre- and post-CBT (n = 271). The HCRTR1 rs2271933 T allele was associated with PD/AG in both samples independently, and in their meta-analysis (p = 4.2 × 10^-7), particularly in the female subsample (p = 9.8 × 10^-9). T allele carriers displayed a significantly poorer CBT outcome (e.g., Hamilton anxiety rating scale: p = 7.5 × 10^-4). The T allele count was linked to higher ACQ sores in PD/AG and healthy subjects, decreased inferior frontal gyrus and increased locus coeruleus activation in the alerting network. Finally, the T allele count was associated with increased pre-CBT exposure avoidance and autonomic arousal as well as decreased post-CBT improvement. In sum, the present results provide converging evidence for an involvement of HCRTR1 gene variation in the etiology of PD/AG and PD/AG-related traits as well as treatment response to CBT, supporting future therapeutic approaches targeting the orexin-related arousal system.

Lateral Hypothalamic Mc3R-Expressing Neurons Modulate Locomotor Activity, Energy Expenditure, and Adiposity in Male Mice

The central melanocortin system plays a crucial role in the control of energy balance. Although the decreased energy expenditure and increased adiposity of melanocortin-3 receptor (Mc3R)-null mice suggest the importance of Mc3R-regulated neurons in energy homeostasis, the roles for specific subsets of Mc3R neurons in energy balance have yet to be determined. Because the lateral hypothalamic area (LHA) contributes to the control of energy expenditure and feeding, we generated Mc3rcre mice to determine the roles of LHA Mc3R (Mc3RLHA) neurons in energy homeostasis. We found that Mc3RLHA neurons overlap extensively with LHA neuron markers that contribute to the control of energy balance (neurotensin, galanin, and leptin receptor) and project to brain areas involved in the control of feeding, locomotion, and energy expenditure, consistent with potential roles for Mc3RLHA neurons in these processes. Indeed, selective chemogenetic activation of Mc3RLHA neurons increased locomotor activity and augmented refeeding after a fast. Although the ablation of Mc3RLHA neurons did not alter food intake, mice lacking Mc3RLHA neurons displayed decreased energy expenditure and locomotor activity, along with increased body mass and adiposity. Thus, Mc3R neurons lie within LHA neurocircuitry that modulates locomotor activity and energy expenditure and contribute to energy balance control.

Orexin prevents depressive-like behavior by promoting stress resilience

Hypothalamic neuropeptide orexin has been implicated in the pathophysiology of psychiatric disorders and accumulating clinical evidence indicates a potential link between orexin and depression. However, the exact role of orexin in depression, particularly the underlying neural substrates and mechanisms, remains unknown. In this study, we reveal a direct projection from the hypothalamic orexinergic neurons to the ventral pallidum (VP), a structure that receives an increasing attention for its critical position in rewarding processing, stress responses, and depression. We find that orexin directly excites GABAergic VP neurons and prevents depressive-like behaviors in rats. Two orexin receptors, OX1R and OX2R, and their downstream Na+-Ca2+ exchanger and L-type Ca2+ channel co-mediate the effect of orexin. Furthermore, pharmacological blockade or genetic knockdown of orexin receptors in VP increases depressive-like behaviors in forced swim test and sucrose preference test. Intriguingly, blockage of orexinergic inputs in VP has no impact on social proximity in social interaction test between novel partners, but remarkably strengthens social avoidance under an acute psychosocial stress triggered by social rank. Notably, a significantly increased orexin level in VP is accompanied by an increase in serum corticosterone in animals exposed to acute stresses, including forced swimming, food/water deprivation and social rank stress, rather than non-stress situations. These results suggest that endogenous orexinergic modulation on VP is especially critical for protecting against depressive reactions to stressful events. The findings define an indispensable role for the central orexinergic system in preventing depression by promoting stress resilience.

Orexins/Hypocretins: Key Regulators of Energy Homeostasis

Originally described to be involved in feeding regulation, orexins/hypocretins are now also considered as major regulatory actors of numerous biological processes, such as pain, sleep, cardiovascular function, neuroendocrine regulation, and energy expenditure. Therefore, they constitute one of the most pleiotropic families of hypothalamic neuropeptides. Although their orexigenic effect is well documented, orexins/hypocretins also exert central effects on energy expenditure, notably on the brown adipose tissue (BAT) thermogenesis. A better comprehension of the underlying mechanisms and potential interactions with other hypothalamic molecular pathways involved in the modulation of food intake and thermogenesis, such as AMP-activated protein kinase (AMPK) and endoplasmic reticulum (ER) stress, is essential to determine the exact implication and pathophysiological relevance of orexins/hypocretins on the control of energy balance. Here, we will review the actions of orexins on energy balance, with special focus on feeding and brown fat function.

Orexin/hypocretin receptor modulation of anxiolytic and antidepressive responses during social stress and decision-making: Potential for therapy

Hypothalmic orexin/hypocretin (Orx) neurons in the lateral and dorsomedial perifornical region (LH-DMH/PeF) innervate broadly throughout the brain, and receive similar inputs. This wide distribution, as well as two Orx peptides (Orx_A and Orx_B) and two Orx receptors (Orx₁ and Orx₂) allow for functionally related but distinctive behavioral outcomes, that include arousal, sleep-wake regulation, food seeking, metabolism, feeding, reward, addiction, and learning. These are all motivational functions, and tie the orexin systems to anxiety and depression as well. We present evidence, that for affective behavior, Orx₁ and Orx₂ receptors appear to have opposing functions. The majority of research on anxiety- and depression-related outcomes has focused on Orx₁ receptors, which appear to have primarily anxiogenic and pro-depressive actions. Although there is significant research suggesting contrary findings, the primary potential for pharmacotherapies linked to the Orx₁ receptor is via antagonists to block anxious and depressive behavior. Dual orexin receptor antagonists have been approved for treatment of sleep disorders, and are likely candidates for adaptation for affect disorder treatments. However, we present evidence here that demonstrates the Orx₂ receptors are anxiolytic and antidepressive. Using a new experimental pre-clinical model of anxious and depressive behavior stimulated by social stress and decision-making that produces two stable behavioral phenotypes, Escape/Resilient and Stay/Susceptible, we tested the effects of intracerebroventricular injections of Orx₂ agonist and antagonist drugs. Over ten behavioral measures, we have demonstrated that Orx₂ agonists promote resilience, as well as anxiolytic and antidepressive behavior. In contrast, Orx₂ antagonists or knockdown kindle anxious and pro-depressive behavior plus increase susceptibility. The results suggest that the Orx₂ receptor may be a useful target for pharmacotherapies to treat anxiety and depression.

Serotonergic Input to Orexin Neurons Plays a Role in Maintaining Wakefulness and REM Sleep Architecture

Neurons expressing neuropeptide orexins (hypocretins) in the lateral hypothalamus (LH) and serotonergic neurons in the dorsal raphe nucleus (DR) both play important roles in the regulation of sleep/wakefulness states, and show similar firing patterns across sleep/wakefulness states. Orexin neurons send excitatory projections to serotonergic neurons in the DR, which express both subtypes of orexin receptors (Mieda et al., 2011), while serotonin (5-HT) potently inhibits orexin neurons through activation of 5HT_1A receptors (5HT1ARs). In this study, we examined the physiological importance of serotonergic inhibitory regulation of orexin neurons by studying the phenotypes of mice lacking the 5HT1A receptor gene (Htr1a) specifically in orexin neurons (ox5HT1ARKO mice). ox5HT1ARKO mice exhibited longer NREM sleep time along with decreased wakefulness time in the later phase of the dark period. We also found that restraint stress induced a larger impact on REM sleep architecture in ox5HT1ARKO mice than in controls, with a larger delayed increase in REM sleep amount as compared with that in controls, indicating abnormality of REM sleep homeostasis in the mutants. These results suggest that 5HT1ARs in orexin neurons are essential in the regulation of sleep/wakefulness states, and that serotonergic regulation of orexin neurons plays a crucial role in the appropriate control of orexinergic tone to maintain normal sleep/wake architecture.

Improvement of glycemic control by treatment for insomnia with suvorexant in type 2 diabetes mellitus

Introduction

Acute and chronic insomnia can exacerbate type 2 diabetes mellitus (T2DM). We investigated suvorexant (an anti-insomnia drug that targets the orexin system) effects on sleep architecture and glucose metabolism in T2DM patients with insomnia.

Materials and methods

This 7 day open-label, single-arm, intervention trial included 18 subjects with T2DM and insomnia. After 1 day acclimatization, daily glucose levels, sleep architecture, and autonomic nervous function were evaluated by continuous glucose monitoring (CGM), single-channel electroencephalography, and accelerometry, respectively.

Results

Suvorexant treatment for 3 days significantly increased total sleep time and sleep efficiency, with partial suppression of sympathetic nerve activity. CGM-measured 24 h mean glucose level decreased significantly from 157.7 ± 22.9 to 152.3 ± 17.8 mg/dL, especially in the early glucose surge after the midnight nadir (from 28.3 ± 15.0 to 18.2 ± 9.9 mg/dL), and until supper with a significant improvement in homeostasis model assessment of insulin resistance from 4.0 ± 2.8 to 2.9 ± 1.6, respectively.

Conclusions

Suvorexant treatment for insomnia of subjects with T2DM significantly improved CGM-measured daily glycemic control, which was associated with changes in sympathomimetic tone and/or improved insulin sensitivity. The amelioration of insomnia may therefore be a target for improving glycemic control in T2DM patients with insomnia.

Serotonergic Input to Orexin Neurons Plays a Role in Maintaining Wakefulness and REM Sleep Architecture

Neurons expressing neuropeptide orexins (hypocretins) in the lateral hypothalamus (LH) and serotonergic neurons in the dorsal raphe nucleus (DR) both play important roles in the regulation of sleep/wakefulness states, and show similar firing patterns across sleep/wakefulness states. Orexin neurons send excitatory projections to serotonergic neurons in the DR, which express both subtypes of orexin receptors (Mieda et al., 2011), while serotonin (5-HT) potently inhibits orexin neurons through activation of 5HT_1A receptors (5HT1ARs). In this study, we examined the physiological importance of serotonergic inhibitory regulation of orexin neurons by studying the phenotypes of mice lacking the 5HT1A receptor gene (Htr1a) specifically in orexin neurons (ox5HT1ARKO mice). ox5HT1ARKO mice exhibited longer NREM sleep time along with decreased wakefulness time in the later phase of the dark period. We also found that restraint stress induced a larger impact on REM sleep architecture in ox5HT1ARKO mice than in controls, with a larger delayed increase in REM sleep amount as compared with that in controls, indicating abnormality of REM sleep homeostasis in the mutants. These results suggest that 5HT1ARs in orexin neurons are essential in the regulation of sleep/wakefulness states, and that serotonergic regulation of orexin neurons plays a crucial role in the appropriate control of orexinergic tone to maintain normal sleep/wake architecture.

To eat or to sleep: That is a lateral hypothalamic question

The lateral hypothalamus (LH) is a functionally and anatomically complex brain region that is involved in the regulation of many behavioral and physiological processes including feeding, arousal, energy balance, stress, reward and motivated behaviors, pain perception, body temperature regulation, digestive functions and blood pressure. Despite noteworthy experimental efforts over the past decades, the circuit, cellular and synaptic bases by which these different processes are regulated by the LH remains incompletely understood. This knowledge gap links in large part to the high cellular heterogeneity of the LH. Fortunately, the rapid evolution of newer genetic and electrophysiological tools is now permitting the selective manipulation, typically genetically-driven, of discrete LH cell populations. This, in turn, permits not only assignment of function to discrete cell groups, but also reveals that considerable synergistic and antagonistic interactions exist between key LH cell populations that regulate feeding and arousal. For example, we now know that while LH melanin-concentrating hormone (MCH) and orexin/hypocretin neurons both function as sensors of the internal metabolic environment, their roles regulating sleep and arousal are actually opposing. Additional studies have uncovered similarly important roles for subpopulations of LH GABAergic cells in the regulation of both feeding and arousal. Herein we review the role of LH MCH, orexin/hypocretin and GABAergic cell populations in the regulation of energy homeostasis (including feeding) and sleep-wake and discuss how these three cell populations, and their subpopulations, may interact to optimize and coordinate metabolism, sleep and arousal. This article is part of the Special Issue entitled 'Hypothalamic Control of Homeostasis'.

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

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

Orexinergic Modulation of Spinal Motor Activity in the Neonatal Mouse Spinal Cord

The role of orexin during development, and especially in terms of spinal cord function, is not well understood. It is for this reason that we focused on the network actions of orexin during the first week of development. We found that orexinergic fibers were present in the lumbar spinal cord of postnatal day 0 (P0) to P3 mice. The fibers were expressed mainly in the dorsal horn, but occasional fibers were observed in the ventral horn. Both orexin (OX) A and OXB increased the motoneurons (MNs) tonic neurogram discharge. However, only OXA was found to significantly increase spontaneous bursting activity and the frequency of fictive locomotor bursts. We show that OXA is able to act directly on MNs. To test the contribution of the recurrent MN collaterals, we blocked the nicotinic cholinergic drive and observed that OXA retained its ability to increase fictive locomotor activity. Additionally, we recorded neurograms from ventral lateral funiculi, where OXA had no effect on population discharge. These effects were also confirmed by recording from descending commissural interneurons via patch recordings. The loci of the effects of OXA were further investigated in a dorsal horn-removed preparation where OXA also shows an increase in the discharge from ventral root neurograms but no increase in the frequency of spontaneous or fictive locomotion burst activity. In summary, multiple lines of evidence from our work demonstrate the robust effects of orexins on spinal cord networks and MNs at the time of birth.

Enhanced food-related responses in the ventral medial prefrontal cortex in narcolepsy type 1

Narcolepsy type 1 is a chronic sleep disorder caused by a deficiency of the orexin (hypocretin) neuropeptides. In addition to sleep regulation, orexin is important for motivated control processes. Weight gain and obesity are common in narcolepsy. However, the neurocognitive processes associated with food-related control and overeating in narcolepsy are unknown. We explored the neural correlates of general and food-related attentional control in narcolepsy-type-1 patients (n = 23) and healthy BMI-matched controls (n = 20). We measured attentional bias to food words with a Food Stroop task and general executive control with a Classic Stroop task during fMRI. Moreover, using multiple linear regression, we assessed the relative contribution of neural responses during Food Stroop and Classic Stroop to spontaneous snack intake. Relative to healthy controls, narcolepsy patients showed enhanced ventral medial prefrontal cortex responses and connectivity with motor cortex during the Food Stroop task, but attenuated dorsal medial prefrontal cortex responses during the Classic Stroop task. Moreover, the former activity but not the latter, was a significant predictor of spontaneous snack intake. These findings demonstrate that narcolepsy, characterized by orexin deficiency, is associated with decreased dorsal medial prefrontal cortex responses during general executive control and enhanced ventral medial prefrontal cortex responses during food-driven attention.

Neuron-related blood inflammatory markers as an objective evaluation tool for major depressive disorder: An exploratory pilot case-control study

BACKGROUND

Neuroinflammation is suggested to be a crucial factor in the pathophysiology of major depressive disorder (MDD). Analysis of neuron-derived exosomes (NDE) in peripheral blood has recently been highlighted to reveal the pathophysiology of brain diseases without using brain biopsy. Currently, human NDE studies require a considerable amount of peripheral blood to measure multiple substances inside exosomes. Previously, NDE-based clinical studies focusing on MDD have not been reported.

METHODS

As an exploratory pilot case-control study between healthy controls (HC) and drug-free MDD patients (each; N = 34), we searched for NDE-related blood biomarkers with a small amount of peripheral blood using a novel sandwich immunoassay between anti-neuron antibody and antibodies against CD81 (an exosome marker) and against other proteins related to neuroinflammation and synaptic functions.

RESULTS

Most neuron-related blood biomarkers had moderately to strongly positive correlation with CD81 (NDE), thus we normalized the above biomarkers by CD81 (quantity of each biomarker/CD81) to predict NDE-related blood substances. Interleukin 34 (IL34)/CD81 levels were significantly higher in MDD group compared to HC group. Synaptophysin (SYP), SYP/CD81, and tumor necrosis factor receptor 1 (TNFR1)/CD81 were positively correlated with severities of depression and/or various sub-symptoms.

LIMITATIONS

We did not actually extract NDE from peripheral blood.

CONCLUSIONS

Using a small amount of peripheral blood, we have successfully detected possible NDE-related blood biomarkers. This is the first study to suggest that not only SYP and TNFR1 but also IL34 are important blood biomarkers for patients with MDD. Further studies are warranted to evaluate the present study.

Reactive and predictive homeostasis: Roles of orexin/hypocretin neurons

Homeostasis is the maintenance of a healthy physiological equilibrium in a changing world. Reactive (feedback, counter-regulatory) and predictive (feedforward, anticipatory) homeostatic control strategies are both important for survival. For example, in energy homeostasis, the pancreas reacts to ingested glucose by releasing insulin, whereas the brain prepares the body for ingestion through anticipatory salivation based on food-associated cues. Reactive control is largely innate, whereas predictive control is often acquired or modified through associative learning, though some important predictive control strategies are innate, e.g. avoidance of fox scent in mice that never met a fox. Traditionally, the hypothalamus has been viewed as a reactive controller, sensing deviations from homeostasis to elicit counter-regulatory responses, while "higher" areas such as the cortex have been viewed as predictive controllers. However, experimental evidence argues against such neuroanatomical segregation: for example, receptors for internal homeostatic indicators are found throughout the brain, while key interoceptive hypothalamic cells also rapidly sense external cues. Here a model is proposed where the brain-wide-projecting, non-neuroendocrine, neurons of the hypothalamus, exemplified by orexin/hypocretin neurons, function as "brain government" systems that convert integrated internal and external information into reactive and predictive autonomic, cognitive, and behavioural adaptations that ensure homeostasis. Like regions of a country without a government, individual brain regions can function normally without hypothalamic guidance, but these functions are uncoordinated, producing mismatch between supply and demand of arousal, and derailing decision-making as seen in orexin-deficient narcolepsy. This article is part of the Special Issue entitled 'Hypothalamic Control of Homeostasis'.

Optical probing of orexin/hypocretin receptor antagonists

Study Objectives

The present study investigated the function of Hypocretin (Hcrt or Orexin/OX) receptor antagonists in sleep modulation and memory function with optical methods in transgenic mice.

Methods

We used Hcrt-IRES-Cre knock-in mice and AAV vectors expressing channelrhodopsin-2 (ChR2) to render Hcrt neurons sensitive to blue light stimulation. We optogenetically stimulated Hcrt neurons and measured latencies to wakefulness in the presence or absence of OX1/2R antagonists and Zolpidem. We also examined endogenous Hcrt neuronal activity with fiber photometry. Changes in memory after optogenetic sleep disruption were evaluated by the novel object recognition test (NOR) and compared for groups treated with vehicle, OX1/2R antagonists, or Zolpidem. We also analyzed electroencephalogram (EEG) power spectra of wakefulness, rapid eye movement (REM) sleep, and non-REM (NREM) sleep following the injections of vehicle, OX1/2R antagonists, and Zolpidem in young adult mice.

Results

Acute optogenetic stimulation of Hcrt neurons at different frequencies resulted in wakefulness. Treatment with dual OX1/2R antagonists (DORAs) DORA12 and MK6096, as well as selective OX2R antagonist MK1064 and Zolpidem, but not selective OX1R antagonist 1SORA1, significantly reduced the bout length of optogenetic stimulation-evoked wakefulness episode. Fiber photometry recordings of GCaMP6f signals showed that Hcrt neurons are active during wakefulness, even in the presence of OXR antagonists. Treatment with dual OX1/2R antagonists improved memory function despite optogenetic sleep fragmentation caused impaired memory function in a NOR test.

Conclusions

Our results show DORAs and selective OX2R antagonists stabilize sleep and improve sleep-dependent cognitive processes even when challenged by optogenetic stimulation mimicking highly arousing stimuli.

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.

Daily Fluctuation of Orexin Neuron Activity and Wiring: The Challenge of "Chronoconnectivity"

In the heterogeneous hub represented by the lateral hypothalamus, neurons containing the orexin/hypocretin peptides play a key role in vigilance state transitions and wakefulness stability, energy homeostasis, and other functions relevant for motivated behaviors. Orexin neurons, which project widely to the neuraxis, are innervated by multiple extra- and intra-hypothalamic sources. A key property of the adaptive capacity of orexin neurons is represented by daily variations of activity, which is highest in the period of the animal’s activity and wakefulness. These sets of data are here reviewed. They concern the discharge profile during the sleep/wake cycle, spontaneous Fos induction, peptide synthesis and release reflected by immunostaining intensity and peptide levels in the cerebrospinal fluid as well as postsynaptic effects. At the synaptic level, adaptive capacity of orexin neurons subserved by remodeling of excitatory and inhibitory inputs has been shown in response to changes in the nutritional status and prolonged wakefulness. The present review wishes to highlight that synaptic plasticity in the wiring of orexin neurons also occurs in unperturbed conditions and could account for diurnal variations of orexin neuron activity. Data in zebrafish larvae have shown rhythmic changes in the density of inhibitory innervation of orexin dendrites in relation to vigilance states. Recent findings in mice have indicated a diurnal reorganization of the excitatory/inhibitory balance in the perisomatic innervation of orexin neurons. Taken together these sets of data point to “chronoconnectivity,” i.e., a synaptic rearrangement of inputs to orexin neurons over the course of the day in relation to sleep and wake states. This opens questions on the underlying circadian and homeostatic regulation and on the involved players at synaptic level, which could implicate dual transmitters, cytoskeletal rearrangements, hormonal regulation, as well as surrounding glial cells and extracellular matrix. Furthermore, the question arises of a “chronoconnectivity” in the wiring of other neuronal cell groups of the sleep-wake-regulatory network, many of which are characterized by variations of their firing rate during vigilance states.

The role of the orexin system in stress response

Orexins are neuropeptides that are exclusively produced by hypothalamic neurons, which project throughout the entire brain. Orexin, also known as hypocretins, were initially identified to play a fundamental role in food intake, arousal and the regulation of sleep and wakefulness. Recent studies identified orexins to be critical for diverse physiological processes including motivation, reward, attention, emotional regulation, stress and anxiety. Here, I review recent findings that indicate orexin has an important role in acute and chronic stress. I also summarize the recent optogenetic and chemogenetic studies that have advanced our understanding of the orexin system. I will conclude by discussing clinical studies that implicate orexins in mental health disorders. This article is part of the Special Issue entitled 'Hypothalamic Control of Homeostasis'.

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'.

Central Orexin A Affects Reproductive Axis by Modulation of Hypothalamic Kisspeptin/Neurokinin B/Dynorphin Secreting Neurons in the Male Wistar Rats

It is an established fact that orexin plays an important role in regulating the reproductive axis and the secretions of gonadotropin-releasing hormone (GnRH)/luteinizing hormone (LH). However, its precise cellular and molecular mechanisms are not fully recognized. Accordingly, the aim of the present study is to find out whether the central injection of orexin A (OXA) and its antagonists, SB-334867 (as orexin receptor antagonist 1; OX1RA) and JNJ-10397049 (as orexin receptor antagonist 2; OX2RA), either alone or in combination, can leave any impact on the reproductive axis (either hormonal or behavioral) in the male Wistar rats. Furthermore, in order to see whether OXA signals can be relayed through the pathway of kisspeptin/neurokinin B/dynorphin (known as KNDy neurons, a neural network which works upstream of GnRH neurons) or not, the relative gene expression of these neuropeptides were measured. Overall, the data from radioimmunoassay revealed that OXA significantly decreases the mean serum level of LH and testosterone and, in a similar vein, its antagonists neutralize this impact. Moreover, data from real-time quantitative PCR indicated that OXA has significantly reduced the hypothalamic expression of Gnrh. In this line, the gene expressions of Kisspeptin and Neurokinin b decreased. However, OXA antagonists neutralize this impact. Also, the expression of Dynorphin gene was upregulated by the following application of the OXA. The results of this study are related to the impact of orexin on the reproductive axis. It is recommended that KNDy neurons as the interneural pathway relay the information of orexin to the GnRH neurons.

How orexin signals bias action: Hypothalamic and accumbal circuits

Survival-maximizing, well-timed actions are a key responsibility of the brain. Hypothalamic neurons containing neurotransmitters orexins/hypocretins are important players in this process. Individuals without orexin neurons display inappropriately-timed transitions between arousal states, and other behavioural abnormalities including increased risk-taking. Deciphering neural circuits through which orexin neurons control brain states and behavior thus illuminates brain mechanisms of context-appropriate actions. This review outlines and puts into broader context recent examples of orexin circuit analyses in the lateral hypothalamus (LH) and the nucleus accumbens (NAc), two brain regions clasically implicated in context-appropriate actions. In the LH, orexin neurons excite GAD65-expressing neurons. The LH(GAD65) neuron excitation induces elevated locomotor activity, while inhibition of LH(GAD65) neuron natural activity depresses voluntary locomotion. The orexin → LH(GAD65) circuit may therefore assist in creating the drive to run. In the NAc shell region, orexin axons excite D2 neurons (dopamine-inhibited neurons expressing dopamine type-2 receptor). NAc(D2) cell activation increases risk-avoidance behaviors, while NAc(D2) cell inhibition reduces risk-avoidance. The excitatory orexin → NAc(D2) circuit may thus assist in reducing risk-taking, and oppose the inhibitory VTA(dopamine) → NAc(D2) circuit during computation of risk appetite. Neural computation in these local and long-range orexin circuits may thus assist in generating risk-avoiding locomotor responses to stressors known to activate orexin neurons, such as body energy depletion or potential external threats. A model is proposed where orexin-opposing, inhibitory inputs acting on the orexin target neurons may context-specifically channel orexin-induced brain excitation towards particular sets of actions.

Role of the orexin 2 receptor in palatable-food consumption-associated cardiovascular reactivity in spontaneously hypertensive rats

Hypertensive subjects often exhibit exaggerated cardiovascular reactivity. An overactive orexin system underlies the pathophysiology of hypertension. We examined orexin's roles in eating-associated cardiovascular reactivity in spontaneously hypertensive rats (SHRs) and Wistar-Kyoto (WKY) rats. Results showed eating regular chow or palatable food (sucrose agar) was accompanied by elevated arterial pressure and heart rate. In both SHRs and WKY rats, the cardiovascular responses associated with sucrose-agar consumption were greater than that with regular-chow consumption. Additionally, SHRs exhibited greater cardiovascular responses than WKY rats did to regular-chow and palatable food consumption. Central orexin 2 receptor (OX2R) blockade attenuated sucrose-agar consumption-associated cardiovascular response only in SHRs. In both SHRs and WKY rats, OX2R blockade did not affect regular-chow consumption-associated cardiovascular responses. Greater numbers of c-Fos-positive cells in the rostral ventrolateral medulla (RVLM) and of c-Fos-positive orexin neurons in the dorsomedial hypothalamus (DMH) were detected in sucrose agar-treated SHRs, compared to regular chow-treated SHRs and to sucrose agar-treated WKY rats. Central OX2R blockade reduced the number of c-Fos-positive cells in the RVLM only in sucrose agar-treated SHRs. We concluded that in SHRs, orexin neurons in the DMH might be overactive during eating palatable food and may further elicit exaggerated cardiovascular responses via an OX2R-RVLM pathway.