Genetic ablation of orexin neurons in mice results in narcolepsy, hypophagia, and obesity

Detection of reduced orexin-A/hypocretin-1 and its fragments by orexin-A "gold-standard" radioimmunoassay

BACKGROUND

Orexin-A/hypocretin-1 level is determined in the cerebrospinal fluid samples as a part of clinical narcolepsy diagnostics utilizing a specific commercial radioimmunoassay (RIA); this assay is also widely used in research of many other conditions. The specificity of RIAs is in general variable, and little has been firmly disclosed about the specificity of this RIA assay. Thus, the validity of many research results obtained using the kit is unclear. At least metabolites of orexin-A have been proposed as potential interfering substances.

METHODS

Since this issue has not been systematically assessed, we decided to investigate it using synthetic variants of orexin-A and -B (intact peptides and peptide fragments as well as reduced orexin-A).

RESULTS

Our synthetic orexin-A bound correspondingly to the orexin-A standard included in the kit while orexin-B did not bind even at 10000-fold higher concentrations. Reduction of the disulfide bridges in orexin-A (giving orexin-A-SS) decreased its binding 25-fold. C-terminal truncation of orexin-A-SS was well tolerated - some of the fragments actually bound better than orexin-A-SS - while N-terminal truncation was not allowed.

CONCLUSIONS

The results demonstrate that the RIA kit is fairly selective for intact orexin-A among the peptides tested. However, this does not as such prove that it measures intact orexin-A in the physiological samples, and further studies including identification of physiological orexin-A metabolites are thus required. We also suggest that the redox milieu of cerebrospinal fluid - that has been suggested to vary in different diseases - may have an impact on what is measured with the kit.

Changes of food-cue processing in major depressive disorder patients with decreased appetite: An event-related potential study

Appetite decrease is a common symptom in major depressive disorder (MDD). However, published research discussing the cognitive process to food stimuli in MDD patients with decreased appetite is lacking, as are objective indicators to assess their degree of appetitive loss. The current study evaluated the disparities in food-related cognition between healthy controls and MDD patients, explored the brain regions contributing to these changes, and evaluated the potential of event-related potentials for assessing appetite loss severity. A total of 149 subjects (healthy controls, n = 50; MDD patients with decreased appetite, n = 52; MDD patients without appetite change, n = 47) were included in this study. We used the Dimensional Anhedonia Rating Scale (DARS) to measure the degree of appetite decrease, and assessed their alterations in food-related cognition with the late positive potential (LPP). The standardized low-resolution brain electromagnetic tomography (sLORETA) method was used to explore the source activity of the LPP. We found the two groups of MDD patients did not differ in the disease severity, while those with appetite decrease got the lowest DARS food/drink score. And MDD patients with decreased appetite allocated fewer attentional resources to food stimuli with significantly lower LPP amplitude evoked by food in this group. Within depressed patients, LPP source activations were reduced in lingual gyrus, cuneus, inferior and middle occipital lobe, and inferior occipital gyrus in appetite-decreased patients, indicating altered occipital activity may be associated with attentional processing in MDD patients with decreased appetite. And correlation analysis revealed a moderate, positive correlation between LPP amplitude and DARS food/drink score. This study demonstrates the cognitive differences between MDD patients with appetite decrease and without appetite change, and provides a potential biomarker for evaluating the degree of appetite loss in MDD patients experiencing decreased appetite.

TDP-43 overexpression in the hypothalamus drives neuropathology, dysregulates metabolism and impairs behavior in mice

TAR DNA-binding protein 43 (TDP-43) pathology is linked to the neurodegenerative disorders amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD) and Huntington disease (HD). Dysregulation of metabolism and emotion is shared across these disorders and may be caused by hypothalamic pathology. Inclusions with TDP-43 are present in the hypothalamus in clinical ALS, as well as selective loss of hypothalamic neurons expressing the metabolism and emotion regulating neuropeptides hypocretin (orexin), melanin-concentrating hormone (MCH) and oxytocin. We aimed to investigate whether there is a casual link between the effects of TDP-43 in the hypothalamus and the development of neuropathology, as well as changes in metabolism and behavior. We generated an adeno-associated viral (AAV) vector expressing human TDP-43 under the neuronal-specific synapsin promoter, which was injected bilaterally into the hypothalamus of wild-type FVB/N mice. TDP-43 overexpression resulted in hypothalamic pathology in a dose-dependent fashion replicating clinical pathology with hypothalamic atrophy and loss of hypocretin-, MCH- and oxytocin-expressing neurons. Nuclear and cytoplasmic inclusions of TDP-43 were found in the hypothalamus. Mice overexpressing TDP-43 in the hypothalamus developed metabolic dysregulation with hyperglycaemia independent of food intake. Additionally, mice overexpressing TDP-43 in the hypothalamus exhibited reduced motor activity and nesting ability, suggesting the development of an apathy-like phenotype. Taken together, AAV-vector mediated TDP-43 overexpression in the hypothalamus leads to neuropathology with the development of metabolic dysfunction and apathy-like behavior. These results indicate that TDP-43 can exert direct pathological effects in the hypothalamus, which may contribute to the development of the non-motor phenotype in TDP-43 proteinopathies.

Discovery of a New Class of Orexin 2 Receptor Agonists as a Potential Treatment for Narcolepsy

The orexinergic system, projecting from the lateral hypothalamus, operates through two receptors, orexin receptor type-1 (OX1) and orexin receptor type-2 (OX2), stabilizing wakefulness, mainly via OX2. Narcolepsy Type 1 (NT1) is characterized by excessive sleepiness and cataplexy, and is linked to a loss of orexin-producing neurons. Current therapies manage the symptoms but do not address the underlying cause of the disease. For example, psychostimulants (e.g., modafinil) reduce excessive daytime sleepiness (EDS) and sodium oxybate (gammaaminobutyric acid receptor agonist) reduces both EDS and cataplexy. Despite decades of research, no small-molecule OX2 agonist has reached the market. This study presents the discovery of two new brain-penetrant, orally bioavailable OX2 agonists with a phenylglycine-like scaffold. These compounds stabilized wakefulness and reduced cataplexy in a mouse model of NT1. In healthy dogs, they increased time in wakefulness. These results highlight their potential as treatment for narcolepsy and other types of hypersomnolence.

An orexin agonist promotes wakefulness and inhibits cataplexy through distinct brain regions

Narcolepsy type 1, caused by selective loss of the orexin-producing neurons, is characterized by poor maintenance of wakefulness and cataplexy. Clinical trials show that orexin receptor 2 (OX2R) agonists substantially improve narcolepsy symptoms, but the key brain regions through which OX2R signaling produces these benefits are only partially understood. To address this question, we produced recombinant mice expressing the human diphtheria toxin receptor driven by the endogenous orexin promoter (orexinDTR mice). After injection with diphtheria toxin, orexinDTR mice had severe and selective loss of the orexin neurons, leading to narcolepsy symptoms, including poor maintenance of wakefulness and cataplexy; these symptoms were substantially improved by an OX2R-selective agonist OX-201. We then crossed orexinDTR mice with OX2R transcription-disrupted (TD) mice to produce a new model lacking orexin neurons and OX2R. We focally restored OX2R expression in specific brain regions of OX2R TD::orexinDTR mice and assessed whether OX-201 improves specific aspects of narcolepsy. In mice expressing OX2R only in the tuberomammillary nucleus (TMN) or basal forebrain (BF) regions, OX-201 improved maintenance of wakefulness but did not suppress cataplexy. In contrast, in mice expressing OX2R in the ventrolateral periaqueductal gray and lateral pontine tegmentum (vlPAG/LPT), OX-201 suppressed cataplexy without improving maintenance of wakefulness. These results suggest that OX2R signaling in the TMN and BF regions can stabilize wakefulness and OX2R signaling in the vlPAG/LPT region can suppress cataplexy, providing key insights into how orexins regulate wakefulness and muscle tone and how OX2R agonists improve the symptoms of narcolepsy. VIDEO ABSTRACT.

Regional cerebral blood perfusion impairment in type 1 narcolepsy patients: An arterial spin labeling study

OBJECTIVE

To investigate the pathophysiological characteristics of cerebral blood flow (CBF) in patients with narcolepsy type 1 (NT1) via the arterial spin labeling (ASL) technique.

METHODS

Thirty patients with diagnostic NT1 (PTs) and 34 age- and sex-matched healthy controls (HCs) were recruited for this study. Basic information was collected, and clinical evaluation and neuroimaging, including ASL and T1-3DBRAVO, was performed. The z-normalized CBF (zCBF) and spatial coefficient of variation (sCoV) were calculated, and the changes in NT1 were compared via analysis of covariate (ANCOVA). Furthermore, spearman's correlation analysis between impaired regional perfusion and clinical features was performed. Age, sex, and normalized grey matter volume were included as covariates.

RESULTS

Compared with that of HCs, the zCBF of PTs significantly differed in regions of fronto-temporal-occipital cortex, right insula and posterior insula, and left rostral/dorsal anterior cingulate gyrus (ACG) (P < 0.006). Moreover, the sCoV was significantly altered in the frontotemporal cortex, rostral ACG, right hippocampus, and posterior insula (P < 0.003). In PTs, positive correlations were identified between the zCBF of the right superior/middle frontal gyrus (SFG/MFG) and mean sleep latency, and between the zCBF of the left SFG of the frontal pole and sleep hallucination severity. Moreover, the sCoV of the right MFG/hippocampus were positively associated with Rapid Eye Movement efficiency and negatively associated with Hamilton Anxiety Scale score, respectively.

CONCLUSION

PTs exhibited abnormal regional perfusion in the frontal-temporal-occipital cortex and limbic system regions, which may serve as patient-specific imaging markers. Alterations in perfusion may lead to the clinical manifestations of underlying psychological and sleep disorders in PTs.

Measuring energy expenditure in narcolepsy using doubly labeled water and respiration chamber calorimetry

STUDY OBJECTIVES

Hypocretin deficiency causes type 1 narcolepsy, a condition characterized by excessive daytime sleepiness, cataplexy, and fragmented nocturnal sleep. Two-thirds of people with narcolepsy are also overweight, of which half are obese. The pathophysiology behind weight gain in people with narcolepsy remains unknown. We assessed a possible decrease in energy expenditure as a cause for overweight in narcolepsy using respiration chamber calorimetry and doubly labeled water.

METHODS

Ten males with type I narcolepsy and nine matched (for age, sex, and BMI) healthy controls were enrolled. Participants stayed in a respiration chamber for 24 hours. They subsequently received doubly labeled water and wore an accelerometer for 2 weeks to assess energy expenditure and physical activity under daily living conditions. Total daily energy expenditure, resting energy expenditure (REE), overnight metabolic rate, physical activity level, and activity-induced energy expenditure were measured.

RESULTS

No significant differences were found in REE, mean 24-hour respiration chamber energy expenditure, overnight metabolic rate, and activity-induced energy expenditure when comparing people with narcolepsy type 1 to controls. Physical activity was also comparable between groups.

CONCLUSIONS

Energy expenditure in narcolepsy type 1 is similar to matched controls, suggesting comparable metabolism and physical activity rates. It remains possible that metabolic changes are most pronounced around disease onset. In addition, patients had to discontinue their medication which may have influenced the results. Still, our findings suggest that other factors may also play a role in weight gain in narcolepsy, such as differences in dietary behavior.

Orexin receptors: possible therapeutic targets for psychiatric disorders

RATIONALE

Orexins, comprising orexin-A and orexin-B, are neuropeptides with extensive projections throughout the central nervous system. They are implicated in a variety of physiological processes through their receptors, orexin type 1 (OX1) and orexin type 2 (OX2) receptors. Among the physiological functions of orexins, their role in sleep/wake regulation has garnered significant attention. Consequently, three orexin receptor antagonists that block both OX1 and OX2 receptors (dual orexin receptor antagonist; DORA) are available on the market for the treatment of insomnia. Additionally, another DORA, vornorexant, has been submitted for approval.

OBJECTIVE

Beyond sleep disorders, the orexin system is deeply implicated in the pathophysiology of several psychiatric disorders, including depression, anxiety, and substance use disorders.

RESULTS

Accumulating evidence indicates that orexin receptor antagonists improve behavioral abnormalities that mimic certain psychiatric disorders in animal models and are effective in treating these disorders or their symptoms in humans. Moreover, orexin receptor antagonists are expected not only to alleviate core symptoms of psychiatric disorders but also to improve sleep disturbances, which are often comorbid with these conditions.

CONCLUSION

Drug discovery and development targeting orexin receptors should provide novel therapeutic options for the treatment of psychiatric disorders.

The mechanism of different orexin/hypocretin neuronal projections in wakefulness and sleep

Since the discovery of orexin/hypocretin, numerous studies have accumulated evidence demonstrating its key role in various aspects of neuromodulation, including addiction, motivation, and arousal. This paper focuses on the projection of orexin neurons to specific target brain regions through distinct neural pathways to regulate sleep and arousal. We provide a detailed discussion of the projection mechanisms of orexin neurons to downstream neurons, particularly emphasizing their activation of monoaminergic and cholinergic neurons associated with arousal. Additionally, we briefly explore the immune response and inflammatory factors linked to the loss of orexin neurons. Our findings underscore the significance of understanding specific neural projections in the generation and maintenance of arousal, which could guide advancements in neuroscience and lead to new therapeutic opportunities for treating insomnia or narcolepsy.

The Lehigh Valley Health Network Narcolepsy Cohort: clinical and polysomnographic analysis of 304 cases

STUDY OBJECTIVE

We aimed to characterize clinical features, comorbidities, and polysomnographic characteristics of a large cohort of patients with narcolepsy.

METHODS

We undertook a retrospective chart and polysomnographic review of all patients with a diagnosis of narcolepsy type 1 (NT1) or narcolepsy type 2 (NT2) seen within the Lehigh Valley Health Network between 2000 and 2022.

RESULTS

We found 304 cases with a diagnosis of narcolepsy (52 NT1, 252 NT2), based on International Classification of Sleep Disorders, third edition criteria. Compared to NT2, patients with NT1 had younger diagnosis age (24.5 vs 27.4 years, P = .03), shorter diagnostic gap (3.0 vs 4.6 years, P = .002), more frequent sleep paralysis (55.8% vs 19.4%, P < .0001) and hypnagogic hallucinations (46.2% vs 25.4%, P = .003), and higher Epworth Sleepiness Scale scores (17.8 vs 16.7, P = .02). The most common comorbid sleep disorders were breathing disorders (17.4%) and insomnia (15.5%). Migraine was the most common neurological disorder. Depression was more common in NT2 than NT1 (12 [23.1%] vs 94 [37.3%], P = .05). On the Multiple Sleep Latency Test, patients with NT1 had more sleep onset rapid eye movement periods than patients with NT2 (≥ 3 sleep onset rapid eye movement periods in 59.2% vs 26.9%, P < .0001). Only in NT2, hypnagogic hallucinations and higher Epworth Sleepiness Scale scores were associated with higher numbers of sleep onset rapid eye movement periods (P = .0277 and P = .0179, respectively).

CONCLUSION

This is one of the largest monocentric studies to date of patients with narcolepsy and confirms the frequent comorbidities of narcolepsy. Specific clinical characteristics and comorbidities may help differentiate NT1 from NT2.

CITATION

Jiang RY, Rochart R, Chu I, Duka S, Vendrame M. The Lehigh Valley Health Network Narcolepsy Cohort: clinical and polysomnographic analysis of 304 cases. J Clin Sleep Med. 2025;21(3):479-491.

The Orexin System and Its Impact on the Autonomic Nervous and Cardiometabolic System in Post-Acute Sequelae of COVID-19

Orexins (OXs) are critical for regulating circadian rhythms, arousal, appetite, energy metabolism, and electrolyte balance, affecting both the autonomic nervous system (ANS) and the cardiovascular system (CVS). Disruption of the OX system can result in symptoms similar to those observed in post-acute sequelae of COVID-19 (PASC). This review emphasizes the adverse effects of OX dysregulation on autonomic and cardiometabolic functions in patients with PASC. Additionally, we highlight the potential of anti-OX therapies to provide neuroprotective, anti-inflammatory, and immunoregulatory benefits, offering hope for alleviating some of the debilitating symptoms associated with PASC.

Genetic associations between orexin genes and phenotypes related to behavioral regulation in humans, including substance use

The hypothalamic neuropeptide system of orexin (hypocretin) neurons provides projections throughout the neuraxis and has been linked to sleep regulation, feeding and motivation for salient rewards including drugs of abuse. However, relatively little has been done to examine genes associated with orexin signaling and specific behavioral phenotypes in humans. Here, we tested for association of twenty-seven genes involved in orexin signaling with behavioral phenotypes in humans. We tested the full gene set, functional subsets, and individual genes involved in orexin signaling. Our primary phenotype of interest was Externalizing, a composite factor comprised of behaviors and disorders associated with reward-seeking, motivation, and behavioral regulation. We also tested for association with additional phenotypes that have been related to orexin regulation in model organism studies, including alcohol consumption, problematic alcohol use, daytime sleepiness, insomnia, cigarettes per day, smoking initiation, and body mass index. The composite set of 27 genes corresponding to orexin function was highly associated with Externalizing, as well as with alcohol consumption, insomnia, cigarettes per day, smoking initiation and BMI. In addition, all gene subsets (except the OXR2/HCRTR2 subset) were associated with Externalizing. BMI was significantly associated with all gene subsets. The "validated factors for PPOX/HCRT" and "PPOX/HCRT upregulation" gene subsets also were associated with alcohol consumption. Individually, 8 genes showed a strong association with Externalizing, 12 with BMI, 7 with smoking initiation, 3 with alcohol consumption, and 2 with problematic alcohol use, after correction for multiple testing. This study indicates that orexin genes are associated with multiple behaviors and disorders related to self-regulation in humans. This is consistent with prior work in animals that implicated orexin signaling in motivational activation induced by salient stimuli, and supports the hypothesis that orexin signaling is an important potential therapeutic target for numerous behavioral disorders.

Utility of complexity analysis in electroencephalography and electromyography for automated classification of sleep-wake states in mice

We explore an innovative approach to sleep stage analysis by incorporating complexity features into sleep scoring methods for mice. Traditional sleep scoring relies on the power spectral features of electroencephalogram (EEG) and the electromyogram (EMG) amplitude. We introduced a novel methodology for sleep stage classification based on two types of complexity analysis, namely multiscale entropy and detrended fluctuation analysis. Our analysis revealed significant variances in these complexities, not only within the specific theta and delta bands but across a wide frequency spectrum. Based on these findings, we developed a sleep stage scoring model, termed Sleep Analyzer Complex (SAC), a convolutional neural network model that integrates these complexity features with conventional EEG spectrum and EMG amplitude analysis. This integrated model significantly enhances the accuracy of sleep stage identification, achieving an accuracy of 97.4-98.1% for novel wild-type mice, on par with the agreement level among human scorers (97.3-97.8%). The efficacy of SAC was validated through tests conducted on wild-type mice, and it demonstrated remarkable success in identifying sleep architecture abnormalities in narcoleptic mice as well. This approach not only facilitates automated scoring of sleep/wakefulness states but also holds the potential to uncover detailed physiological insights, thereby advancing EEG-based sleep research.

Bibliometric and visual study of narcolepsy from 2000 to 2023

BACKGROUND

More studies explored the prevalence, causes, associated conditions, and therapeutic strategies of narcolepsy. With an increasing focus on understanding narcolepsy's prevalence, associated conditions, and therapeutic strategies, there's a notable absence of bibliometric analyses summarizing trends in research and identifying emerging areas of focus within this field.

AIM

To conduct a bibliometric analysis to investigate the current status and frontiers of narcolepsy.

METHODS

The documents related to narcolepsy are obtained from the Web of Science Core Collection database (WoSCC) from January 1, 2000, to December 31, 2023, and VOS viewer 1.6.16, and the WoSCC's literature analysis wire were used to conduct the bibliometric analysis.

RESULTS

A total of 4672 publications related to narcolepsy were included, and 16182 authors across 4397 institutions and 96 countries/regions contributed to these documents in 1131 different journals. The most productive author, institution, country and journal were Yves Dauvilliers, Stanford University, United States, and Sleep Medicine, respectively. The first high-cited document was published in Nature in 2005 by Saper et al, and this research underscores the role of certain neurons in ensuring the stability of sleep-wake transitions, offering insights into narcolepsy's pathophysiology.

CONCLUSION

In conclusion, the main research hotspots and frontiers in the field of narcolepsy are the diagnosis of narcolepsy, pathological mechanism of narcolepsy and the treatment of narcolepsy. More studies are needed to explore effective strategies for the diagnosis and treatment of narcolepsy.

Harnessing Simple Animal Models to Decode Sleep Mysteries

Whether it involves human subjects or non-human animals, basic, translational, or clinical sleep research poses significant ethical challenges for researchers and ethical committees alike. Sleep research greatly benefits from using diverse animal models, each offering unique insights into sleep control mechanisms. The fruit fly (Drosophila melanogaster) is a superior genetic model due to its quick generation period, large progenies, and rich genetic tools. Its well-characterized genome and ability to respond to hypnotics and stimulants make it an effective tool for studying sleep genetics and physiological foundations. The nematode (Caenorhabditis elegans) has a simpler neural organization and transparent body, allowing researchers to explore molecular underpinnings of sleep control. Vertebrate models, like zebrafish (Danio rerio), provide insights into circadian rhythm regulation, memory consolidation, and drug effects on sleep. Invertebrate models, like California sea hare (Aplysia californica) and Upside-down jellyfish (Cassiopea xamachana), have simpler nervous systems and behave similarly to humans, allowing for the examination of sleep principles without logistical and ethical challenges. Combining vertebrate and invertebrate animal models offers a comprehensive approach to studying sleep, improving our understanding of sleep regulation and potentially leading to new drug discovery processes for sleep disorders and related illnesses.

Can Neuromodulation Improve Sleep and Psychiatric Symptoms?

PURPOSE OF REVIEW

In this review, we evaluate recent studies that employ neuromodulation, in the form of non-invasive brain stimulation, to improve sleep in both healthy participants, and patients with psychiatric disorders. We review studies using transcranial electrical stimulation, transcranial magnetic stimulation, and closed-loop auditory stimulation, and consider both subjective and objective measures of sleep improvement.

RECENT FINDINGS

Neuromodulation can alter neuronal activity underlying sleep. However, few studies utilizing neuromodulation report improvements in objective measures of sleep. Enhancements in subjective measures of sleep quality are replicable, however, many studies conducted in this field suffer from methodological limitations, and the placebo effect is robust. Currently, evidence that neuromodulation can effectively enhance sleep is lacking. For the field to advance, methodological issues must be resolved, and the full range of objective measures of sleep architecture, alongside subjective measures of sleep quality, must be reported. Additionally, validation of effective modulation of neuronal activity should be done with neuroimaging.

Influence of sleep on physiological systems in atherosclerosis

Sleep is a fundamental requirement of life and is integral to health. Deviation from optimal sleep associates with numerous diseases including those of the cardiovascular system. Studies, spanning animal models to humans, show that insufficient, disrupted or inconsistent sleep contribute to poor cardiovascular health by disrupting body systems. Fundamental experiments have begun to uncover the molecular and cellular links between sleep and heart health while large-scale human studies have associated sleep with cardiovascular outcomes in diverse populations. Here, we review preclinical and clinical findings that demonstrate how sleep influences the autonomic nervous, metabolic and immune systems to affect atherosclerotic cardiovascular disease.

Contribution of hypothalamic orexin (hypocretin) circuits to pathologies of motivation

The orexin (also known as hypocretin) system, consisting of neuropeptides orexin-A and orexin-B, was discovered over 25 years ago and was immediately identified as a central regulator of sleep and wakefulness. These peptides interact with two G-protein coupled receptors, orexin 1 (OX1) and orexin 2 (OX2) receptors which are capable of coupling to all heterotrimeric G-protein subfamilies, but primarily transduce increases in calcium signalling. Orexin neurons are regulated by a variety of transmitter systems and environmental stimuli that signal reward availability, including food and drug related cues. Orexin neurons are also activated by anticipation, stress, cues predicting motivationally relevant information, including those predicting drugs of abuse, and engage neuromodulatory systems, including dopamine neurons of the ventral tegmental area (VTA) to respond to these signals. As such, orexin neurons have been characterized as motivational activators that coordinate a range of functions, including feeding and arousal, that allow the individual to respond to motivationally relevant information, critical for survival. This review focuses on the role of orexins in appetitive motivation and highlights a role for these neuropeptides in pathologies characterized by inappropriately high levels of motivated arousal (overeating, anxiety and substance use disorders) versus those in which motivation is impaired (depression).

Regulation of wakefulness by neurotensin neurons in the lateral hypothalamus

The lateral hypothalamic region (LH) has been identified as a key region for arousal regulation, yet the specific cell types and underlying mechanisms are not fully understood. While neurons expressing orexins (OX) are considered the primary wake-promoting population in the LH, their loss does not reduce daily wake levels, suggesting the presence of additional wake-promoting populations. In this regard, we recently discovered that a non-OX cell group in the LH, marked by the expression of neurotensin (Nts), could powerfully drive wakefulness. Activation of these NtsLH neurons elicits rapid arousal from non-rapid eye movement (NREM) sleep and produces uninterrupted wakefulness for several hours in mice. However, it remains unknown if these neurons are necessary for spontaneous wakefulness and what their precise role is in the initiation and maintenance of this state. To address these questions, we first examined the activity dynamics of the NtsLH population across sleep-wake behavior using fiber photometry. We find that NtsLH neurons are more active during wakefulness, and their activity increases concurrently with, but does not precede, wake-onset. We then selectively destroyed the NtsLH neurons using a diphtheria-toxin-based conditional ablation method, which significantly reduced wake amounts and mean duration of wake bouts and increased the EEG delta power during wakefulness. These findings demonstrate a crucial role for NtsLH neurons in maintaining normal arousal levels, and their loss may be associated with chronic sleepiness in mice.

Neurobehavioral meaning of pupil size

Pupil size is a widely used metric of brain state. It is one of the few signals originating from the brain that can be readily monitored with low-cost devices in basic science, clinical, and home settings. It is, therefore, important to investigate and generate well-defined theories related to specific interpretations of this metric. What exactly does it tell us about the brain? Pupils constrict in response to light and dilate during darkness, but the brain also controls pupil size irrespective of luminosity. Pupil size fluctuations resulting from ongoing "brain states" are used as a metric of arousal, but what is pupil-linked arousal and how should it be interpreted in neural, cognitive, and computational terms? Here, we discuss some recent findings related to these issues. We identify open questions and propose how to answer them through a combination of well-defined tasks, neurocomputational models, and neurophysiological probing of the interconnected loops of causes and consequences of pupil size.

Orexin-mediated motivated arousal and reward seeking

The neuromodulator orexin has been identified as a key factor for motivated arousal including recent evidence that sleep deprivation-induced enhancement of reward behavior is modulated by orexin. While orexin is not necessary for either reward or arousal behavior, orexin neurons' broad projections, ability to sense the internal state of the animal, and high plasticity of signaling in response to natural rewards and drugs of abuse may underlie heightened drug seeking, particularly in a subset of highly motivated reward seekers. As such, orexin receptor antagonists have gained deserved attention for putative use in addiction treatments. Ongoing and future clinical trials are expected to identify individuals most likely to benefit from orexin receptor antagonist treatment to promote abstinence, such as those with concurrent sleep disorders or high craving, while attention to methodological considerations will aid interpretation of the numerous preclinical studies investigating disparate aspects of the role of orexin in reward and arousal.

TAK-861, a potent, orally available orexin receptor 2-selective agonist, produces wakefulness in monkeys and improves narcolepsy-like phenotypes in mouse models

Narcolepsy type 1 (NT1) is associated with severe loss of orexin neurons and characterized by symptoms including excessive daytime sleepiness and cataplexy. Current medications indicated for NT1 often show limited efficacy, not addressing the full spectrum of symptoms, demonstrating a need for novel drugs. We discovered a parenteral orexin receptor 2 (OX2R) agonist, danavorexton, and an orally available OX2R agonist, TAK-994; both improving NT1 phenotypes in mouse models and individuals with NT1. However, danavorexton has limited oral availability and TAK-994 has a risk of off-target liver toxicity. To avoid off-target-based adverse events, a highly potent molecule with low effective dose is preferred. Here, we show that a novel OX2R-selective agonist, TAK-861 [N-{(2S,3R)-4,4-Difluoro-1-(2-hydroxy-2-methylpropanoyl)-2-[(2,3',5'-trifluoro[1,1'-biphenyl]-3-yl)methyl]pyrrolidin-3-yl}ethanesulfonamide], activates OX2R with a half-maximal effective concentration of 2.5 nM and promotes wakefulness at 1 mg/kg in mice and monkeys, suggesting ~ tenfold higher potency and lower effective dosage than TAK-994. Similar to TAK-994, TAK-861 substantially ameliorates wakefulness fragmentation and cataplexy-like episodes in orexin/ataxin-3 and orexin-tTA;TetO DTA mice (NT1 mouse models). Compared with modafinil, TAK-861 induces highly correlated brain-wide neuronal activation in orexin-tTA;TetO DTA mice, suggesting efficient wake-promoting effects. Thus, TAK-861 has potential as an effective treatment for individuals with hypersomnia disorders including narcolepsy, potentially with a favorable safety profile.

Orexin neurons mediate temptation-resistant voluntary exercise

Despite the well-known health benefits of physical activity, many people underexercise; what drives the prioritization of exercise over alternative options is unclear. We developed a task that enabled us to study how mice freely and rapidly alternate between wheel running and other voluntary activities, such as eating palatable food. When multiple alternatives were available, mice chose to spend a substantial amount of time wheel running without any extrinsic reward and maintained this behavior even when palatable food was added as an option. Causal manipulations and correlative analyses of appetitive and consummatory processes revealed this preference for wheel running to be instantiated by hypothalamic hypocretin/orexin neurons (HONs). The effect of HON manipulations on wheel running and eating was strongly context-dependent, being the largest in the scenario where both options were available. Overall, these data suggest that HON activity enables an eat-run arbitration that results in choosing exercise over food.

A high-fat diet induced depression-like phenotype via hypocretin-HCRTR1 mediated inflammation activation

Background: A high-fat diet (HFD) is generally associated with an increased risk of mental disorders that constitute a sizeable worldwide health. A HFD results in the gut microbiota-brain axis being altered and linked to mental disorders. Hypocretin-1, which can promote appetite, has been previously confirmed to be associated with depression. However, no exact relationship has been found for hypocretin between depression and HFDs. Methods: Adult male SD rats were randomly assigned to either a HFD or a normal diet for eight weeks, followed by behavioral tests and plasma biochemical analyses. Then, we investigated the protein and mRNA levels of inflammation-related factors in the hippocampus. We also observed morphological changes in brain microglia and lipid accumulation. Additionally, metagenomic and metabolomic analyses of gut microbiomes were performed. 3T3-L1 cells were utilized in vitro to investigate the impact of hypocretin receptor 1 antagonists (SB334867) on lipid accumulation. To consider the connection between the brain and adipose tissue, we used a conditioned medium (CM) treated with 3T3-L1 cells to observe the activation and phagocytosis of BV2 cells. Following a 12-week period of feeding a HFD to C57BL/6 mice, a three-week intervention period was initiated during which the administration of SB334867 was observed. This was followed by a series of assessments, including monitoring of body weight changes and emotional problems, as well as attention to plasma biochemical levels and microglial cell phenotypes in the brain. Results: The HFD rats displayed anxiety and depressive-like behaviors. HFD rats exhibited increased plasma HDL, LDL, and TC levels. A HFD also causes an increase in hypocretin-1 and hypocretin-2 in the hypothalamus. Metagenomics and metabolomics revealed that the HFD caused an increase in the relative abundance of associated inflammatory bacteria and decreased the abundance of anti-inflammatory and bile acid metabolites. Compared with the CTR group, hippocampal microglia in the HFD group were significantly activated and accompanied by lipid deposition. At the same time, protein and mRNA expression levels of inflammation-related factors were increased. We found that SB334867 could significantly reduce lipid accumulation in 3T3-L1 cells after differentiation. The expression of inflammatory factors decreased in the SB334867 group. The administration of SB334867 was found to reverse the adverse effects of the HFD on body weight, depressive-like behaviour and anxiety-like mood. Furthermore, this treatment was associated with improvements in plasma biochemical levels and a reduction in the number of microglia in the brain. Conclusions: In summary, our results demonstrated that a HFD induced anxiety and depressive-like behaviors, which may be linked to the increased hypocretin-1 level and lipid accumulation. Supplementation with SB334867 improved the above. These observations highlight the possibility of hypocretin-1 inducing the risk of HFD-associated emotional dysfunctions.

International Union of Basic and Clinical Pharmacology CXIV: Orexin Receptor Function, Nomenclature and Pharmacology

The orexin system consists of the peptide transmitters orexin-A and -B and the G protein-coupled orexin receptors OX1 and OX2 Orexin receptors are capable of coupling to all four families of heterotrimeric G proteins, and there are also other complex features of the orexin receptor signaling. The system was discovered 25 years ago and was immediately identified as a central regulator of sleep and wakefulness; this is exemplified by the symptomatology of the disorder narcolepsy with cataplexy, in which orexinergic neurons degenerate. Subsequent translation of these findings into drug discovery and development has resulted to date in three clinically used orexin receptor antagonists to treat insomnia. In addition to sleep and wakefulness, the orexin system appears to be a central player at least in addiction and reward, and has a role in depression, anxiety and pain gating. Additional antagonists and agonists are in development to treat, for instance, insomnia, narcolepsy with or without cataplexy and other disorders with excessive daytime sleepiness, depression with insomnia, anxiety, schizophrenia, as well as eating and substance use disorders. The orexin system has thus proved an important regulator of numerous neural functions and a valuable drug target. Orexin prepro-peptide and orexin receptors are also expressed outside the central nervous system, but their potential physiological roles there remain unknown. SIGNIFICANCE STATEMENT: The orexin system was discovered 25 years ago and immediately emerged as an essential sleep-wakefulness regulator. This discovery has tremendously increased the understanding of these processes and has thus far resulted in the market approval of three orexin receptor antagonists, which promote more physiological aspects of sleep than previous hypnotics. Further, orexin receptor agonists and antagonists with different pharmacodynamic properties are in development since research has revealed additional potential therapeutic indications. Orexin receptor signaling is complex and may represent novel features.

The nucleus accumbens shell: a neural hub at the interface of homeostatic and hedonic feeding

Feeding behavior is a complex physiological process regulated by the interplay between homeostatic and hedonic feeding circuits. Among the neural structures involved, the nucleus accumbens (NAc) has emerged as a pivotal region at the interface of these two circuits. The NAc comprises distinct subregions and in this review, we focus mainly on the NAc shell (NAcSh). Homeostatic feeding circuits, primarily found in the hypothalamus, ensure the organism's balance in energy and nutrient requirements. These circuits monitor peripheral signals, such as insulin, leptin, and ghrelin, and modulate satiety and hunger states. The NAcSh receives input from these homeostatic circuits, integrating information regarding the organism's metabolic needs. Conversely, so-called hedonic feeding circuits involve all other non-hunger and -satiety processes, i.e., the sensory information, associative learning, reward, motivation and pleasure associated with food consumption. The NAcSh is interconnected with hedonics-related structures like the ventral tegmental area and prefrontal cortex and plays a key role in encoding hedonic information related to palatable food seeking or consumption. In sum, the NAcSh acts as a crucial hub in feeding behavior, integrating signals from both homeostatic and hedonic circuits, to facilitate behavioral output via its downstream projections. Moreover, the NAcSh's involvement extends beyond simple integration, as it directly impacts actions related to food consumption. In this review, we first focus on delineating the inputs targeting the NAcSh; we then present NAcSh output projections to downstream structures. Finally we discuss how the NAcSh regulates feeding behavior and can be seen as a neural hub integrating homeostatic and hedonic feeding signals, via a functionally diverse set of projection neuron subpopulations.

Sex-specific role of high-fat diet and stress on behavior, energy metabolism, and the ventromedial hypothalamus

BACKGROUND

Scientific evidence highlights the influence of biological sex on the relationship between stress and metabolic dysfunctions. However, there is limited understanding of how diet and stress concurrently contribute to metabolic dysregulation in both males and females. Our study aimed to investigate the combined effects of high-fat diet (HFD) induced obesity and repeated stress on fear-related behaviors, metabolic, immune, and hypothalamic outcomes in male and female mice.

METHODS

To investigate this, we used a highly reliable rodent behavioral model that faithfully recapitulates key aspects of post-traumatic stress disorder (PTSD)-like fear. We subjected mice to footshock stressor followed by a weekly singular footshock stressor or no stressor for 14 weeks while on either an HFD or chow diet. At weeks 10 and 14 we conducted glucose tolerance and insulin sensitivity measurements. Additionally, we placed the mice in metabolic chambers to perform indirect calorimetric measurements. Finally, we collected brain and peripheral tissues for cellular analysis.

RESULTS

We observed that HFD-induced obesity disrupted fear memory extinction, increased glucose intolerance, and affected energy expenditure specifically in male mice. Conversely, female mice on HFD exhibited reduced respiratory exchange ratio (RER), and a significant defect in glucose tolerance only when subjected to repeated stress. Furthermore, the combination of repeated stress and HFD led to sex-specific alterations in proinflammatory markers and hematopoietic stem cells across various peripheral metabolic tissues. Single-nuclei RNA sequencing (snRNAseq) analysis of the ventromedial hypothalamus (VMH) revealed microglial activation in female mice on HFD, while male mice on HFD exhibited astrocytic activation under repeated stress.

CONCLUSIONS

Overall, our findings provide insights into complex interplay between repeated stress, high-fat diet regimen, and their cumulative effects on health, including their potential contribution to the development of PTSD-like stress and metabolic dysfunctions, emphasizing the need for further research to fully understand these interconnected pathways and their implications for health.

Hits and misses with animal models of narcolepsy and the implications for drug discovery

INTRODUCTION

Narcolepsy is a chronic and rare neurological disorder characterized by disordered sleep. Based on animal models and further research in humans, the dysfunctional orexin system was identified as a contributing factor to the pathophysiology of narcolepsy. Animal models played a larger role in the discovery of some of the pharmacological agents with established benefit/risk profiles.

AREAS COVERED

In this review, the authors examine the phenotypes observed in animal models of narcolepsy and the characteristics of clinically used pharmacological agents in these animal models. Additionally, the authors compare the effects of clinically used pharmacological agents on the phenotypes in animal models with those observed in narcolepsy patients.

EXPERT OPINION

Research in canine and mouse models have linked narcolepsy to the O×R2mutation and orexin deficiency, leading to new diagnostic criteria and a drug development focus. Advancements in pharmacological therapies have significantly improved narcolepsy management, with insights from both clinical experience and from animal models having led to new treatments such as low sodium oxybate and solriamfetol. However, challenges persist in addressing symptoms beyond excessive daytime sleepiness and cataplexy, highlighting the need for further research, including the development of diurnal animal models to enhance understanding and treatment options for narcolepsy.

Interactions between Lateral Hypothalamic Orexin and Dorsal Raphe Circuitry in Energy Balance

Orexin/hypocretin terminals innervate the dorsal raphe nucleus (DRN), which projects to motor control areas important for spontaneous physical activity (SPA) and energy expenditure (EE). Orexin receptors are expressed in the DRN, and obesity-resistant (OR) rats show higher expression of these receptors in the DRN and elevated SPA/EE. We hypothesized that orexin-A in the DRN enhances SPA/EE and that DRN-GABA modulates the effect of orexin-A on SPA/EE. We manipulated orexin tone in the DRN either through direct injection of orexin-A or through the chemogenetic activation of lateral-hypothalamic (LH) orexin neurons. In the orexin neuron activation experiment, fifteen minutes prior to the chemogenetic activation of orexin neurons, the mice received either the GABA-agonist muscimol or antagonist bicuculline injected into the DRN, and SPA/EE was monitored for 24 h. In a separate experiment, orexin-A was injected into the DRN to study the direct effect of DRN orexin on SPA/EE. We found that the activation of orexin neurons elevates SPA/EE, and manipulation of GABA in the DRN does not alter the SPA response to orexin neuron activation. Similarly, intra-DRN orexin-A enhanced SPA and EE in the mice. These results suggest that orexin-A in the DRN facilitates negative energy balance by increasing physical activity-induced EE, and that modulation of DRN orexin-A is a potential strategy to promote SPA and EE.

Appetite- and Weight-Regulating Neuroendocrine Circuitry in Hypothalamic Obesity

Since hypothalamic obesity (HyOb) was first described over 120 years ago by Joseph Babinski and Alfred Fröhlich, advances in molecular genetic laboratory techniques have allowed us to elucidate various components of the intricate neurocircuitry governing appetite and weight regulation connecting the hypothalamus, pituitary gland, brainstem, adipose tissue, pancreas, and gastrointestinal tract. On a background of an increasing prevalence of population-level common obesity, the number of survivors of congenital (eg, septo-optic dysplasia, Prader-Willi syndrome) and acquired (eg, central nervous system tumors) hypothalamic disorders is increasing, thanks to earlier diagnosis and management as well as better oncological therapies. Although to date the discovery of several appetite-regulating peptides has led to the development of a range of targeted molecular therapies for monogenic obesity syndromes, outside of these disorders these discoveries have not translated into the development of efficacious treatments for other forms of HyOb. This review aims to summarize our current understanding of the neuroendocrine physiology of appetite and weight regulation, and explore our current understanding of the pathophysiology of HyOb.

Lesion of NPY Receptor-expressing Neurons in Perifornical Lateral Hypothalamus Attenuates Glucoprivic Feeding

Glucoprivic feeding is one of several counterregulatory responses (CRRs) that facilitates restoration of euglycemia following acute glucose deficit (glucoprivation). Our previous work established that glucoprivic feeding requires ventrolateral medullary (VLM) catecholamine (CA) neurons that coexpress neuropeptide Y (NPY). However, the connections by which VLM CA/NPY neurons trigger increased feeding are uncertain. We have previously shown that glucoprivation, induced by an anti-glycolygic agent 2-deoxy-D-glucose (2DG), activates perifornical lateral hypothalamus (PeFLH) neurons and that expression of NPY in the VLM CA/NPY neurons is required for glucoprivic feeding. We therefore hypothesized that glucoprivic feeding and possibly other CRRs require NPY-sensitive PeFLH neurons. To test this, we used the ribosomal toxin conjugate NPY-saporin (NPY-SAP) to selectively lesion NPY receptor-expressing neurons in the PeFLH of male rats. We found that NPY-SAP destroyed a significant number of PeFLH neurons, including those expressing orexin, but not those expressing melanin-concentrating hormone. The PeFLH NPY-SAP lesions attenuated 2DG-induced feeding but did not affect 2DG-induced increase in locomotor activity, sympathoadrenal hyperglycemia, or corticosterone release. The 2DG-induced feeding response was also significantly attenuated in NPY-SAP-treated female rats. Interestingly, PeFLH NPY-SAP lesioned male rats had reduced body weights and decreased dark cycle feeding, but this effect was not seen in female rats. We conclude that a NPY projection to the PeFLH is necessary for glucoprivic feeding, but not locomotor activity, hyperglycemia, or corticosterone release, in both male and female rats.

Metabolic underpinnings of cancer-related fatigue

Cancer-related fatigue (CRF) is one of the most prevalent and detrimental complications of cancer. Emerging evidence suggests that obesity and insulin resistance are associated with CRF occurrence and severity in cancer patients and survivors. In this narrative review, we analyzed recent studies including both preclinical and clinical research on the relationship between obesity and/or insulin resistance and CRF. We also describe potential mechanisms for these relationships, though with the caveat that because the mechanisms underlying CRF are incompletely understood, the mechanisms mediating the association between obesity/insulin resistance and CRF are similarly incompletely delineated. The data suggest that, in addition to their effects to worsen CRF by directly promoting tumor growth and metastasis, obesity and insulin resistance may also contribute to CRF by inducing chronic inflammation, neuroendocrinological disturbance, and metabolic alterations. Furthermore, studies suggest that patients with obesity and insulin resistance experience more cancer-induced pain and are at more risk of emotional and behavioral disruptions correlated with CRF. However, other studies implied a potentially paradoxical impact of obesity and insulin resistance to reduce CRF symptoms. Despite the need for further investigation utilizing interventions to directly elucidate the mechanisms of cancer-related fatigue, current evidence demonstrates a correlation between obesity and/or insulin resistance and CRF, and suggests potential therapeutics for CRF by targeting obesity and/or obesity-related mediators.

Orexinergic neurons contribute to autonomic cardiovascular regulation for locomotor exercise

While the hypothalamic orexinergic nervous system is established as having a pivotal role in the long-term regulation of various organismic functions, including wakefulness, metabolism and hypertensive states, whether this system contributes to the rapid autonomic cardiovascular regulation during physical activity remains elusive. This study aimed to elucidate the role of the orexinergic nervous system in transmitting volitional motor signals, i.e. central command, to drive somatomotor and sympathetic cardiovascular responses. We first found that this system is activated by voluntary locomotor exercise as evidenced by an increased expression of Fos, a marker of neural activation, in the orexinergic neurons of Sprague-Dawley rats engaged in spontaneous wheel running. Next, using transgenic Orexin-Cre rats for optogenetic manipulation of orexinergic neurons, we found that optogenetic excitation of orexinergic neurons caused sympathoexcitation on a subsecond timescale under anaesthesia. In freely moving conscious rats, this excitatory stimulation rapidly elicited exploration-like behaviours, predominantly locomotor activity, along with pressor and tachycardiac responses. Meanwhile, optogenetic inhibition of orexinergic neurons during spontaneous wheel running immediately suppressed locomotor activities and blood pressure elevation without affecting basal cardiovascular homeostasis. Taken together, these findings demonstrate the essential role of the orexinergic nervous system in the central circuitry that transmits central command signals for locomotor exercise. This study not only offers insights into the brain circuit mechanisms precisely regulating autonomic cardiovascular systems during voluntary exercise but also likely contributes to our understanding of brain mechanisms underlying abnormal cardiovascular adjustments to exercise in pathological conditions, such as hypertension. KEY POINTS: The hypothalamic orexinergic nervous system plays various roles in the long-term regulation of autonomic and endocrine functions, as well as motivated behaviours. We present a novel, rapid role of the orexinergic nervous system, revealing its significance as a crucial substrate in the brain circuit mechanisms that coordinate somatomotor and autonomic cardiovascular controls for locomotor exercise. Our data demonstrate that orexinergic neurons relay volitional motor signals, playing a necessary and sufficient role in the autonomic cardiovascular regulation required for locomotor exercise in rats. The findings contribute to our understanding of how the brain precisely regulates autonomic cardiovascular systems during voluntary exercise, providing insights into the central neural mechanisms that enhance physical performance moment-by-moment during exercise.

Transient targeting of hypothalamic orexin neurons alleviates seizures in a mouse model of epilepsy

Lateral hypothalamic (LH) hypocretin/orexin neurons (HONs) control brain-wide electrical excitation. Abnormally high excitation produces epileptic seizures, which affect millions of people and need better treatments. HON population activity spikes from minute to minute, but the role of this in seizures is unknown. Here, we describe correlative and causal links between HON activity spikes and seizures. Applying temporally-targeted HON recordings and optogenetic silencing to a male mouse model of acute epilepsy, we found that pre-seizure HON activity predicts and controls the electrophysiology and behavioral pathology of subsequent seizures. No such links were detected for HON activity during seizures. Having thus defined the time window where HONs influence seizures, we targeted it with LH deep brain stimulation (DBS), which inhibited HON population activity, and produced seizure protection. Collectively, these results uncover a feature of brain activity linked to seizures, and demonstrate a proof-of-concept treatment that controls this feature and alleviates epilepsy.

Hypothalamic Orexinergic Neurons Projecting to the Mesencephalic Locomotor Region Are Activated by Voluntary Wheel Running Exercise in Rats

Background

Cardiovascular changes during exercise are regulated by a motor volitional signal, called central command, which originates in the rostral portions of the brain and simultaneously regulates somatomotor and autonomic nervous systems. Whereas we recently elucidated mesencephalic locomotor region (MLR) neurons projecting to the rostral ventrolateral medulla as a crucial component of the central circuit responsible for transmitting central command signals, upstream circuits that regulate the MLR neurons remain unknown. Orexinergic neurons, which primarily originate from the perifornical area (PeFA) of the hypothalamus and reportedly play roles in eliciting locomotion and elevating sympathetic activity, send axonal projection to the MLR. The knowledge led us to investigate whether central command signals are relayed through orexinergic neurons projecting to the MLR.

Methods

We performed anterograde transsynaptic tagging with AAV1 encoding Cre to confirm the presence of MLR neurons postsynaptic to the PeFA in rats. We also conducted retrograde neural tracing with retrograde AAV, combined with immunohistochemical staining, to examine the excitability of MLR-projecting orexinergic neurons in rats that were allowed to freely run on the wheel for 90 min.

Results

A significant number of MLR neurons were labeled with Cre, indicating that PeFA neurons make synaptic contacts with MLR neurons. Moreover, immunoreactivities of Fos, a marker of neuronal excitation, were found in many MLR-projecting orexinergic neurons by voluntary wheel running exercise, compared to non-exercising control rats, especially in the intermediate-posterior, rather than anterior, and medial, rather than lateral, portions within the orexinergic neuron-distributing domain.

Conclusion

The findings suggest that specifically located orexinergic neurons transmit central command signals onto the MLR for running exercise. Elucidating the role of these MLR-projecting orexinergic neurons in somatomotor control and autonomic cardiovascular control deserves further study to unveil central circuit mechanisms responsible for central command function.

Novel orexin receptor agonists based on arene- or pyridine-fused 1,3-dihydro-2H-imidazole-2-imines

A structurally novel class of benzo- or pyrido-fused 1,3-dihydro-2H-imidazole-2-imines was designed and evaluated in an inositol phosphate accumulation assay for Gq signaling to measure agonistic activation of the orexin receptor type 2 (OX2R). These compounds were synthesized in 4-9 steps overall from readily available starting materials. Analogs that contain a stereogenic methyl or cyclopropyl substituent at the benzylic center, and a correctly configured alkyl ether, alkoxyalkyl ether, cyanoalkyl ether, or α-hydroxyacetamido substituted homobenzylic sidechain were identified as the most potent activators of OX2R coupled Gq signaling. Our results also indicate that agonistic activity was stereospecific at both the benzylic and homobenzylic stereogenic centra. We identified methoxyethoxy-substituted pyrido-fused dihydroimidazolimine analog 63c containing a stereogenic benzylic methyl group was the most potent agonist, registering a respectable EC50 of 339 nM and a maximal response (Emax) of 96 % in this assay. In vivo pharmacokinetic analysis indicated good brain exposure for several analogs. Our combined results provide important information towards a structurally novel class of orexin receptor agonists distinct from current chemotypes.

Orexin Receptor Antagonism: Normalizing Sleep Architecture in Old Age and Disease

Sleep is essential for human well-being, yet the quality and quantity of sleep reduce as age advances. Older persons (>65 years old) are more at risk of disorders accompanied and/or exacerbated by poor sleep. Furthermore, evidence supports a bidirectional relationship between disrupted sleep and Alzheimer's disease (AD) or related dementias. Orexin/hypocretin neuropeptides stabilize wakefulness, and several orexin receptor antagonists (ORAs) are approved for the treatment of insomnia in adults. Dysregulation of the orexin system occurs in aging and AD, positioning ORAs as advantageous for these populations. Indeed, several clinical studies indicate that ORAs are efficacious hypnotics in older persons and dementia patients and, as in adults, are generally well tolerated. ORAs are likely to be more effective when administered early in sleep/wake dysregulation to reestablish good sleep/wake-related behaviors and reduce the accumulation of dementia-associated proteinopathic substrates. Improving sleep in aging and dementia represents a tremendous opportunity to benefit patients, caregivers, and health systems.

Connecting the dots: An updated review of the role of autoimmunity in narcolepsy and emerging immunotherapeutic approaches

BACKGROUND

Narcolepsy type 1 (NT1) is a chronic disorder characterized by pathological daytime sleepiness and cataplexy due to the disappearance of orexin immunoreactive neurons in the hypothalamus. Genetic and environmental factors point towards a potential role for inflammation and autoimmunity in the pathogenesis of the disease. This study aims to comprehensively review the latest evidence on the autoinflammatory mechanisms and immunomodulatory treatments aimed at suspected autoimmune pathways in NT1.

METHODS

Recent relevant literature in the field of narcolepsy, its autoimmune hypothesis, and purposed immunomodulatory treatments were reviewed.

RESULTS

Narcolepsy is strongly linked to specific HLA alleles and T-cell receptor polymorphisms. Furthermore, animal studies and autopsies have found infiltration of T cells in the hypothalamus, supporting T cell-mediated immunity. However, the role of autoantibodies has yet to be definitively established. Increased risk of NT1 after H1N1 infection and vaccination supports the autoimmune hypothesis, and the potential role of coronavirus disease 2019 and vaccination in triggering autoimmune neurodegeneration is a recent finding. Alterations in cytokine levels, gut microbiota, and microglial activation indicate a potential role for inflammation in the disease's development. Reports of using immunotherapies in NT1 patients are limited and inconsistent. Early treatment with IVIg, corticosteroids, plasmapheresis, and monoclonal antibodies has seldomly shown some potential benefits in some studies.

CONCLUSION

The current body of literature supports that narcolepsy is an autoimmune disorder most likely caused by T-cell involvement. However, the potential for immunomodulatory treatments to reverse the autoinflammatory process remains understudied. Further clinical controlled trials may provide valuable insights into this area.

The immunopathogenesis of narcolepsy type 1

Narcolepsy type 1 (NT1) is a chronic sleep disorder resulting from the loss of a small population of hypothalamic neurons that produce wake-promoting hypocretin (HCRT; also known as orexin) peptides. An immune-mediated pathology for NT1 has long been suspected given its exceptionally tight association with the MHC class II allele HLA-DQB1*06:02, as well as recent genetic evidence showing associations with polymorphisms of T cell receptor genes and other immune-relevant loci and the increased incidence of NT1 that has been observed after vaccination with the influenza vaccine Pandemrix. The search for both self-antigens and foreign antigens recognized by the pathogenic T cell response in NT1 is ongoing. Increased T cell reactivity against HCRT has been consistently reported in patients with NT1, but data demonstrating a primary role for T cells in neuronal destruction are currently lacking. Animal models are providing clues regarding the roles of autoreactive CD4+ and CD8+ T cells in the disease. Elucidation of the pathogenesis of NT1 will allow for the development of targeted immunotherapies at disease onset and could serve as a model for other immune-mediated neurological diseases.

Brain Circuits Underlying Narcolepsy

Narcolepsy is a sleep disorder manifesting symptoms such as excessive daytime sleepiness and often cataplexy, a sudden and involuntary loss of muscle activity during wakefulness. The underlying neuropathological basis of narcolepsy is the loss of orexin neurons from the lateral hypothalamus. To date numerous animal models of narcolepsy have been produced in the laboratory, being invaluable tools for delineating the brain circuits of narcolepsy. This review will examine the evidence regarding the function of the orexin system, and how loss of this wake-promoting system manifests in excessive daytime sleepiness. This review will also outline the brain circuits controlling cataplexy, focusing on the contribution of orexin signaling loss in narcolepsy. Although our understanding of the brain circuits of narcolepsy has made great progress in recent years, much remains to be understood.

Neural Control of REM Sleep and Motor Atonia: Current Perspectives

PURPOSE OF REVIEW

Since the formal discovery of rapid eye movement (REM) sleep in 1953, we have gained a vast amount of knowledge regarding the specific populations of neurons, their connections, and synaptic mechanisms regulating this stage of sleep and its accompanying features. This article discusses REM sleep circuits and their dysfunction, specifically emphasizing recent studies using conditional genetic tools.

RECENT FINDINGS

Sublaterodorsal nucleus (SLD) in the dorsolateral pons, especially the glutamatergic subpopulation in this region (SLDGlut), are shown to be indispensable for REM sleep. These neurons appear to be single REM generators in the rodent brain and may initiate and orchestrate all REM sleep events, including cortical and hippocampal activation and muscle atonia through distinct pathways. However, several cell groups in the brainstem and hypothalamus may influence SLDGlut neuron activity, thereby modulating REM sleep timing, amounts, and architecture. Damage to SLDGlut neurons or their projections involved in muscle atonia leads to REM behavior disorder, whereas the abnormal activation of this pathway during wakefulness may underlie cataplexy in narcolepsy. Despite some opposing views, it has become evident that SLDGlut neurons are the sole generators of REM sleep and its associated characteristics. Further research should prioritize a deeper understanding of their cellular, synaptic, and molecular properties, as well as the mechanisms that trigger their activation during cataplexy and make them susceptible in RBD.

Daily treatment with the dual orexin receptor antagonist DORA-12 during oxycodone abstinence decreases oxycodone conditioned reinstatement

Chronic opioid use disturbs circadian rhythm and sleep, encouraging opioid use and relapse. The orexin (OX) system is recruited by opioids and regulates physiological processes including sleep. Dual OX receptor antagonists (DORAs), developed for insomnia treatment, may relieve withdrawal-associated sleep disturbances. This study investigated whether DORA-12, a recently developed DORA, reduces physiological activity disturbances during oxycodone abstinence and consequently prevents oxycodone-seeking behavior. Male and female Wistar rats were trained to intravenously self-administer oxycodone (0.15 mg/kg, 21 sessions; 8 h/session) in the presence of a contextual/discriminative stimulus (SD). The rats were subsequently housed individually (22 h/day) to monitor activity, food and water intake. They received DORA-12 (0-30 mg/kg, p.o.) after undergoing daily 1-h extinction training (14 days). After extinction, the rats were tested for oxycodone-seeking behavior elicited by the SD. Hypothalamus sections were processed to assess oxycodone- or DORA-12-associated changes to the OX cell number. In males, oxycodone-associated increases in activity during the light-phase, reinstatement, and decreases in the number of OX cells observed in the vehicle-treated group were not observed with DORA-12-treatment. Oxycodone-associated increases in light-phase food and water intake were not observed by day 14 of 3 mg/kg DORA-12-treatment and dark-phase water intake was increased across treatment days. In females, OX cell number was unaffected by oxycodone or DORA-12. Three and 30 mg/kg DORA-12 increased females' day 7 dark-phase activity and decreased reinstatement. Thirty mg/kg DORA-12 reduced oxycodone-associated increases in light-phase food and water intake. The results suggest that DORA-12 improves oxycodone-induced disruptions to physiological activities and reduces relapse.

The role of orexinergic system in the regulation of cataplexy

Loss of orexin/hypocretin causes serious sleep disorder; narcolepsy. Cataplexy is the most striking symptom of narcolepsy, characterized by abrupt muscle paralysis induced by emotional stimuli, and has been considered pathological activation of REM sleep atonia system. Clinical treatments for cataplexy/narcolepsy and early pharmacological studies in narcoleptic dogs tell us about the involvement of monoaminergic and cholinergic systems in the control of cataplexy/narcolepsy. Muscle atonia may be induced by activation of REM sleep-atonia generating system in the brainstem. Emotional stimuli may be processed in the limbic systems including the amygdala, nucleus accumbens, and medial prefrontal cortex. It is now considered that orexin/hypocretin prevents cataplexy by modulating the activity of different points of cataplexy-inducing circuit, including monoaminergic/cholinergic systems, muscle atonia-generating systems, and emotion-related systems. This review will describe the recent advances in understanding the neural mechanisms controlling cataplexy, with a focus on the involvement of orexin/hypocretin system, and will discuss future experimental strategies that will lead to further understanding and treatment of this disease.

Hypothalamic MCH Neurons: From Feeding to Cognitive Control

Modern neuroscience is progressively elucidating that the classic view positing distinct brain regions responsible for survival, emotion, and cognitive functions is outdated. The hypothalamus demonstrates the interdependence of these roles, as it is traditionally known for fundamental survival functions like energy and electrolyte balance, but is now recognized to also play a crucial role in emotional and cognitive processes. This review focuses on lateral hypothalamic melanin-concentrating hormone (MCH) neurons, producing the neuropeptide MCH-a relatively understudied neuronal population with integrative functions related to homeostatic regulation and motivated behaviors, with widespread inputs and outputs throughout the entire central nervous system. Here, we review early findings and recent literature outlining their role in the regulation of energy balance, sleep, learning, and memory processes.

Deficiency of orexin signaling during sleep is involved in abnormal REM sleep architecture in narcolepsy

Narcolepsy is a sleep disorder caused by deficiency of orexin signaling. However, the neural mechanisms by which deficient orexin signaling causes the abnormal rapid eye movement (REM) sleep characteristics of narcolepsy, such as cataplexy and frequent transitions to REM states, are not fully understood. Here, we determined the activity dynamics of orexin neurons during sleep that suppress the abnormal REM sleep architecture of narcolepsy. Orexin neurons were highly active during wakefulness, showed intermittent synchronous activity during non-REM (NREM) sleep, were quiescent prior to the transition from NREM to REM sleep, and a small subpopulation of these cells was active during REM sleep. Orexin neurons that lacked orexin peptides were less active during REM sleep and were mostly silent during cataplexy. Optogenetic inhibition of orexin neurons established that the activity dynamics of these cells during NREM sleep regulate NREM-REM sleep transitions. Inhibition of orexin neurons during REM sleep increased subsequent REM sleep in "orexin intact" mice and subsequent cataplexy in mice lacking orexin peptides, indicating that the activity of a subpopulation of orexin neurons during the preceding REM sleep suppresses subsequent REM sleep and cataplexy. Thus, these results identify how deficient orexin signaling during sleep results in the abnormal REM sleep architecture characteristic of narcolepsy.

Eat, seek, rest? An orexin/hypocretin perspective

Seeking and ingesting nutrients is an essential cycle of life in all species. In classical neuropsychology these two behaviours are viewed as fundamentally distinct from each other, and known as appetitive and consummatory, respectively. Appetitive behaviour is highly flexible and diverse, but typically involves increased locomotion and spatial exploration. Consummatory behaviour, in contrast, typically requires reduced locomotion. Another long-standing concept is "rest and digest", a hypolocomotive response to calorie intake, thought to facilitate digestion and storage of energy after eating. Here, we note that the classical seek➔ingest➔rest behavioural sequence is not evolutionarily advantageous for all ingested nutrients. Our limited stomach capacity should be invested wisely, rather than spent on the first available nutrient. This is because nutrients are not simply calories: some nutrients are more essential for survival than others. Thus, a key choice that needs to be made soon after ingestion: to eat more and rest, or to terminate eating and search for better food. We offer a perspective on recent work suggesting how nutrient-specific neural responses shape this choice. Specifically, the hypothalamic hypocretin/orexin neurons (HONs) - cells that promote hyperlocomotive explorative behaviours - are rapidly and differentially modulated by different ingested macronutrients. Dietary non-essential (but not essential) amino acids activate HONs, while glucose depresses HONs. This nutrient-specific HON modulation engages distinct reflex arcs, seek➔ingest➔seek and seek➔ingest➔rest, respectively. We propose that these nutri-neural reflexes evolved to facilitate optimal nutrition despite the limitations of our body.

Control of energy homeostasis by the lateral hypothalamic area

The lateral hypothalamic area (LHA) is a subcortical brain region that exerts control over motivated behavior, feeding, and energy balance across species. Recent single-cell sequencing studies have defined at least 30 distinct LHA neuron types. Some of these influence specific aspects of energy homeostasis; however, the functions of many LHA cell types remain unclear. This review addresses the rapidly emerging evidence from cell-type-specific investigations that the LHA leverages distinct neuron populations to regulate energy balance through complex connections with other brain regions. It will highlight recent findings demonstrating that LHA control of energy balance extends beyond mere food intake and propose outstanding questions to be addressed by future research.

Targeting orexin receptors: Recent advances in the development of subtype selective or dual ligands for the treatment of neuropsychiatric disorders

Orexin-A and orexin-B, also named hypocretin-1 and hypocretin-2, are two hypothalamic neuropeptides highly conserved across mammalian species. Their effects are mediated by two distinct G protein-coupled receptors, namely orexin receptor type 1 (OX1-R) and type 2 (OX2-R), which share 64% amino acid identity. Given the wide expression of OX-Rs in different central nervous system and peripheral areas and the several pathophysiological functions in which they are involved, including sleep-wake cycle regulation (mainly mediated by OX2-R), emotion, panic-like behaviors, anxiety/stress, food intake, and energy homeostasis (mainly mediated by OX1-R), both subtypes represent targets of interest for many structure-activity relationship (SAR) campaigns carried out by pharmaceutical companies and academies. However, before 2017 the research was predominantly directed towards dual-orexin ligands, and limited chemotypes were investigated. Analytical characterizations, including resolved structures for both OX1-R and OX2-R in complex with agonists and antagonists, have improved the understanding of the molecular basis of receptor recognition and are assets for medicinal chemists in the design of subtype-selective ligands. This review is focused on the medicinal chemistry aspects of small molecules acting as dual or subtype selective OX1-R/OX2-R agonists and antagonists belonging to different chemotypes and developed in the last years, including radiolabeled OX-R ligands for molecular imaging. Moreover, the pharmacological effects of the most studied ligands in different neuropsychiatric diseases, such as sleep, mood, substance use, and eating disorders, as well as pain, have been discussed. Poly-pharmacology applications and multitarget ligands have also been considered.

Orexin and MCH neurons: regulators of sleep and metabolism

Sleep-wake and fasting-feeding are tightly coupled behavioral states that require coordination between several brain regions. The mammalian lateral hypothalamus (LH) is a functionally and anatomically complex brain region harboring heterogeneous cell populations that regulate sleep, feeding, and energy metabolism. Significant attempts were made to understand the cellular and circuit bases of LH actions. Rapid advancements in genetic and electrophysiological manipulation help to understand the role of discrete LH cell populations. The opposing action of LH orexin/hypocretin and melanin-concentrating hormone (MCH) neurons on metabolic sensing and sleep-wake regulation make them the candidate to explore in detail. This review surveys the molecular, genetic, and neuronal components of orexin and MCH signaling in the regulation of sleep and metabolism.

Rethinking the Role of Orexin in the Regulation of REM Sleep and Appetite

Orexin plays a significant role in the modulation of REM sleep, as well as in the regulation of appetite and feeding. This review explores, first, the current evidence on the role of orexin in the modulation of sleep and wakefulness and highlights that orexin should be considered essentially as a neurotransmitter inhibiting REM sleep and, to a much lesser extent, a wake promoting agent. Subsequently, the relationship between orexin, REM sleep, and appetite regulation is examined in detail, shedding light on their interconnected nature in both physiological conditions and diseases (such as narcolepsy, sleep-related eating disorder, idiopathic hypersomnia, and night eating syndrome). Understanding the intricate relationship between orexin, REM sleep, and appetite regulation is vital for unraveling the complex mechanisms underlying sleep-wake patterns and metabolic control. Further research in this field is encouraged in order to pave the way for novel therapeutic approaches to sleep disorders and metabolic conditions associated with orexin dysregulation.