Neurobiology of the Orexin System and Its Potential Role in the Regulation of Hedonic Tone

Graph convolutional network with attention mechanism improve major depressive depression diagnosis based on plasma biomarkers and neuroimaging data

BACKGROUND

The absence of clinically-validated biomarkers or objective protocols hinders effective major depressive disorder (MDD) diagnosis. Compared to healthy control (HC), MDD exhibits anomalies in plasma protein levels and neuroimaging presentations. Despite extensive machine learning studies in psychiatric diagnosis, a reliable tool integrating multi-modality data is still lacking.

METHODS

In this study, blood samples from 100 MDD and 100 HC were analyzed, along with MRI images from 46 MDD and 49 HC. Here, we devised a novel algorithm, integrating graph neural networks and attention modules, for MDD diagnosis based on inflammatory cytokines, neurotrophic factors, and Orexin A levels in the blood samples. Model performance was assessed via accuracy and F1 value in 3-fold cross-validation, comparing with 9 traditional algorithms. We then applied our algorithm to a dataset containing both the aforementioned protein quantifications and neuroimages, evaluating if integrating neuroimages into the model improves performance.

RESULTS

Compared to HC, MDD showed significant alterations in plasma protein levels and gray matter volume revealed by MRI. Our new algorithm exhibited superior performance, achieving an F1 value and accuracy of 0.9436 and 94.08 %, respectively. Integration of neuroimaging data enhanced our novel algorithm's performance, resulting in an improved F1 value and accuracy, reaching 0.9543 and 95.06 %.

LIMITATIONS

This single-center study with a small sample size requires future evaluations on a larger test set for improved reliability.

CONCLUSIONS

In comparison to traditional machine learning models, our newly developed MDD diagnostic model exhibited superior performance and showed promising potential for inclusion in routine clinical diagnosis for MDD.

Clinical usefulness of dual orexin receptor antagonism beyond insomnia: Neurological and psychiatric comorbidities

Orexin is a neurotransmitter produced by a small group of hypothalamic neurons. Besides its well-known role in the regulation of the sleep-wake cycle, the orexin system was shown to be relevant in several physiological functions including cognition, mood and emotion modulation, and energy homeostasis. Indeed, the implication of orexin neurotransmission in neurological and psychiatric diseases has been hypothesized via a direct effect exerted by the projections of orexin neurons to several brain areas, and via an indirect effect through orexin-mediated modulation of sleep and wake. Along with the growing evidence concerning the use of dual orexin receptor antagonists (DORAs) in the treatment of insomnia, studies assessing their efficacy in insomnia comorbid with psychiatric and neurological diseases have been set in order to investigate the potential impact of DORAs on both sleep-related symptoms and disease-specific manifestations. This narrative review aimed at summarizing the current evidence on the use of DORAs in neurological and psychiatric conditions comorbid with insomnia, also discussing the possible implication of modulating the orexin system for improving the burden of symptoms and the pathological mechanisms of these disorders. Target searches were performed on PubMed/MEDLINE and Scopus databases and ongoing studies registered on Clinicaltrials.gov were reviewed. Despite some contradictory findings, preclinical studies seemingly support the possible beneficial role of orexin antagonism in the management of the most common neurological and psychiatric diseases with sleep-related comorbidities. However, clinical research is still limited and further studies are needed for corroborating these promising preliminary results.

Orexins/Hypocretins: Gatekeepers of Social Interaction and Motivation

Ever since the discovery of the brain's orexin/hypocretin system, most research was directed toward unveiling its contribution to the normal functioning of individuals. The investigation of reward-seeking behaviors then gained a lot of attention once the distribution of orexinergic neurons was revealed. Here, we discuss findings on the involvement of orexins in social interaction, a natural reward type. While some studies have succeeded in defining the relationship between orexin and social interaction, the controversy regarding its nature (direct or inverse relation) raises questions about what aspects have been overlooked until now. Upon examining the literature, we identified a research gap concerning conditions influencing the impact of orexins on social behavior expression. In this review, we introduce a number of factors (e.g., stress, orexin's source) that must be considered while studying the role of orexins in social interaction. Furthermore, we refer to published research to investigate the stage at which orexins affect social interaction and we highlight the nucleus accumbens (NAc) shell's role in social interaction and other rewarding behaviors. Finally, the underlying orexin molecular pathway influencing social motivation in particular illnesses is proposed. We conclude that orexin's impact on social interaction is multifactorial and depends on specific conditions available at a time.

The Orexin/Hypocretin System, the Peptidergic Regulator of Vigilance, Orchestrates Adaptation to Stress

The orexin/hypocretin neuropeptide family has emerged as a focal point of neuroscientific research following the discovery that this family plays a crucial role in a variety of physiological and behavioral processes. These neuropeptides serve as powerful neuromodulators, intricately shaping autonomic, endocrine, and behavioral responses across species. Notably, they serve as master regulators of vigilance and stress responses; however, their roles in food intake, metabolism, and thermoregulation appear complementary and warrant further investigation. This narrative review provides a journey through the evolution of our understanding of the orexin system, from its initial discovery to the promising progress made in developing orexin derivatives. It goes beyond conventional boundaries, striving to synthesize the multifaceted activities of orexins. Special emphasis is placed on domains such as stress response, fear, anxiety, and learning, in which the authors have contributed to the literature with original publications. This paper also overviews the advancement of orexin pharmacology, which has already yielded some promising successes, particularly in the treatment of sleep disorders.

Orexin-A alleviates ferroptosis by activating the Nrf2/HO-1 signaling pathway in traumatic brain injury

BACKGROUND

Traumatic Brain Injury (TBI) has high disability and mortality rate. Oxidative stress and ferroptosis are important pathophysiological characteristics after TBI. Orexin-A (OXA) can alleviate neuronal damage in diverse neurological disorders. Nevertheless, the role and mechanism of OXA in TBI stay unknown.

OBJECTIVES

The research investigated protection influence of OXA on TBI and its potential mechanisms.

METHODS

Male Sprague-Dawley rats were randomly grouped into: sham, TBI, TBI + normal saline (NS) and TBI+OXA groups. TBI model was constructed in rat via modified Feeney's approach, and OXA treatment was administered following construction of TBI model.

RESULTS

Relative to TBI+NS group, TBI+OXA group displayed greatly recovered tissue damage and neurological deficits. Additionally, OXA eased oxidative stress as well as ferroptosis in cerebral cortex of rats following TBI. Furthermore, OXA increased Nrf2 expression and regulating factors HO-1 and NQO1 in cerebral cortex of TBI rats.

CONCLUSIONS

Our research found OXA may restrain ferroptosis via Nrf2/HO-1 signaling pathway activation, thereby reducing brain injury after TBI.

Identifying novel gene dysregulation associated with opioid overdose death: A meta-analysis of differential gene expression in human prefrontal cortex

Only recently have human postmortem brain studies of differential gene expression (DGE) associated with opioid overdose death (OOD) been published; sample sizes from these studies have been modest (N = 40-153). To increase statistical power to identify OOD-associated genes, we leveraged human prefrontal cortex RNAseq data from four independent OOD studies and conducted a transcriptome-wide DGE meta-analysis (N = 285). Using a unified gene expression data processing and analysis framework across studies, we meta-analyzed 20 098 genes and found 335 significant differentially expressed genes (DEGs) by OOD status (false discovery rate < 0.05). Of these, 66 DEGs were among the list of 303 genes reported as OOD-associated in prior prefrontal cortex molecular studies, including genes/gene families (e.g., OPRK1, NPAS4, DUSP, EGR). The remaining 269 DEGs were not previously reported (e.g., NR4A2, SYT1, HCRTR2, BDNF). There was little evidence of genetic drivers for the observed differences in gene expression between opioid addiction cases and controls. Enrichment analyses for the DEGs across molecular pathway and biological process databases highlight an interconnected set of genes and pathways from orexin and tyrosine kinase receptors through MEK/ERK/MAPK signaling to affect neuronal plasticity.

Unraveling the function and structure impact of deleterious missense SNPs in the human OX1R receptor by computational analysis

The orexin/hypocretin receptor type 1 (OX1R) plays a crucial role in regulating various physiological functions, especially feeding behavior, addiction, and reward. Genetic variations in the OX1R have been associated with several neurological disorders. In this study, we utilized a combination of sequence and structure-based computational tools to identify the most deleterious missense single nucleotide polymorphisms (SNPs) in the OX1R gene. Our findings revealed four highly conserved and structurally destabilizing missense SNPs, namely R144C, I148N, S172W, and A297D, located in the GTP-binding domain. Molecular dynamics simulations analysis demonstrated that all four most detrimental mutant proteins altered the overall structural flexibility and dynamics of OX1R protein, resulting in significant changes in the structural organization and motion of the protein. These findings provide valuable insights into the impact of missense SNPs on OX1R function loss and their potential contribution to the development of neurological disorders, thereby guiding future research in this field.

ADHD in narcolepsy: A closer look at prevalence and ties

The reported prevalence of attention deficit hyperactivity disorder (ADHD) in narcolepsy varies considerably, while the associated factors remain inadequately established. A systematic search of studies published in PubMed, EMBASE, and the Cochrane Library was performed from inception to March 2023. Ten studies with 839 patients with narcolepsy were included in the study. Utilizing a random effects model, the pooled prevalence of ADHD in narcolepsy was 25% (95% CI, 14-38%). Notably, patients with narcolepsy type 2 showed a significantly higher prevalence of ADHD than that of narcolepsy type 1 (46% vs. 20%, p = 0.045). Furthermore, the rate of ADHD was notably elevated in narcolepsy compared with the healthy controls (odds ratio 9.59, 95% CI, 4.06-22.63, p < 0.001). Several factors such as excessive daytime sleepiness (EDS), fatigue, insomnia severity, and the quality of life were significantly associated with ADHD in narcolepsy (all ps < 0.05). These findings highlight the importance of monitoring and managing ADHD in narcolepsy, and provide a clue to help reducing ADHD by intervening in these associated factors.

Association between peripheral orexin A/B levels and depression with childhood trauma

BACKGROUND

Orexin dysfunction has previously been demonstrated to be associated with depression. However, no studies reported the different effects of orexin A/B on depression with and without childhood trauma (CT). In this study，we assessed the correlation between expression of orexin A/B and depression severity in major depressive disorder (MDD) patients and healthy controls.

METHODS

A total of 97 MDD patients and 51 healthy controls were recruited in this study. According to the total scores of childhood trauma questionnaire (CTQ), the MDD patients were further divided into two subgroups, MDD with CT and MDD without CT. The 17-item Hamilton Depression Scale (HAMD-17), and plasma orexin A and orexin B concentrations were measured in all participants using enzyme-linked immunosorbent assay.

RESULTS

Orexin B plasma levels were significantly higher in MDD patients with CT and without CT than that in the healthy control group (P < 0.05), whereas there was no statistical difference between the two depression groups. After adjusting age and BMI for covariates, the LASSO regression revealed significant association between the plasma orexin B levels and the total scores of HAMD (β = 3.348), CTQ (β = 2.005). There was no difference in plasma orexin A levels among three groups (P > 0.05).

CONCLUSIONS

Although peripheral orexin B levels are associated with the depression, rather than orexin A, CT appear to play a role in the association between orexin B levels and depression. China Clinical Trial Registration Center (Registration No.: ChiCTR2000039692).

Restraint stress potentiates sensitivity to the antinociceptive effect of morphine through orexin receptors in the ventral tegmental area

Orexin signaling in the ventral tegmental area (VTA) plays a critical role in stress and addictive behaviors. On the other hand, exposure to stress potentiates behavioral sensitization to drugs of abuse such as morphine. This study aimed to elucidate the role of orexin receptors within the VTA in restraint stress (RS)-induced morphine sensitization. Adult male albino Wistar rats underwent stereotaxic surgery, and two stainless steel guide cannulae were bilaterally implanted into the VTA. Different doses of SB334867 or TCS OX2 29 as orexin-1 (OX1) and orexin-2 (OX2) receptor antagonists were microinjected into the VTA five min before exposure to RS, respectively. A duration of three hours was considered for applying the RS, and 10 min after RS exposure, animals received a subcutaneous injection of an ineffective dose of morphine (1 mg/kg) for three consecutive days followed by a five-day drug/stress-free period. On the ninth day, the tail-flick test evaluated the sensitivity to the antinociceptive effects of morphine. The results demonstrated that the sole application of RS or morphine (1 mg/kg) could not induce morphine sensitization; however, concurrent application of RS and morphine could induce morphine sensitization. Besides, intra-VTA administration of OX1 R or OX2 R antagonists before paired administration of morphine and RS blocked morphine sensitization. The role of OX1 R and OX2 R in the induction of stress-induced morphine sensitization was almost identical. This study provides new insight into the role of orexin signaling in the VTA in the potentiation of morphine sensitization induced by RS and morphine co-administration.

Daridorexant, an Orexin Receptor Antagonist for the Management of Insomnia

BACKGROUND

Insomnia is a common sleep disorder that is diagnosed primarily by patients' subjective reported symptoms. Daridorexant is a new dual orexin receptor antagonist that was recently approved by Food and Drug Administration for insomnia characterized by difficulty falling asleep and/or maintaining sleep.

MECHANISM OF ACTION, PHARMACODYNAMICS, AND PHARMACOKINETICS

The orexin neuropeptide signaling system plays a role in wakefulness, and blocking the wake-promoting neuropeptides results in diminished wake signaling, thus exerting a sedative effect using an entirely different mechanism of action than the classical sleep promoting agents. The drug has quick onset of action, high volume of distribution, and high protein binding. Pharmacokinetics and pharmacodynamic parameters were similar in patients of different sex and age and were not significantly affected by race, body size, or mild-to-moderate kidney impairment. Dose limitation to 25 mg in moderate liver impairment and no use in severe liver impairment are recommended. The drug undergoes hepatic CYP3A4 metabolism; thus, caution with strong CYP3A4 inhibitors and inducers is warranted.

CLINICAL TRIALS

The drug was approved based on phase 3 trials involving study 1 and study 2. Study 1 noted daridorexant at doses of 25 and 50 mg demonstrated a statistically significant improvement in wake time after sleep onset, latency to persistent sleep, and self-reported total sleep time against placebo at months 1 and 3. Similarly in study 2, compared with placebo, the 25 mg dose demonstrated statistically significant improvement in wake time after sleep onset, latency to persistent sleep, and self-reported total sleep time at months 1 and 3. Treatment-emergent adverse events were similar for daridorexant and placebo, with nasopharyngitis and headache most frequently reported.

THERAPEUTIC ADVANCE

Daridorexant is a novel agent with demonstrated efficacy in sleep onset and maintenance and decrease in daytime sedation. Preliminary results from a 1-year extension study note similar incidences of mild-to-moderate side effects as noted in previous trials. Further studies are needed to establish its place in the pharmacological treatment of insomnia.

Neuropeptides Modulate Feeding via the Dopamine Reward Pathway

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

Annual Research Review: Perspectives on progress in ADHD science - from characterization to cause

The science of attention-deficit/hyperactivity disorder (ADHD) is motivated by a translational goal - the discovery and exploitation of knowledge about the nature of ADHD to the benefit of those individuals whose lives it affects. Over the past fifty years, scientific research has made enormous strides in characterizing the ADHD condition and in understanding its correlates and causes. However, the translation of these scientific insights into clinical benefits has been limited. In this review, we provide a selective and focused survey of the scientific field of ADHD, providing our personal perspectives on what constitutes the scientific consensus, important new leads to be highlighted, and the key outstanding questions to be addressed going forward. We cover two broad domains - clinical characterization and, risk factors, causal processes and neuro-biological pathways. Part one focuses on the developmental course of ADHD, co-occurring characteristics and conditions, and the functional impact of living with ADHD - including impairment, quality of life, and stigma. In part two, we explore genetic and environmental influences and putative mediating brain processes. In the final section, we reflect on the future of the ADHD construct in the light of cross-cutting scientific themes and recent conceptual reformulations that cast ADHD traits as part of a broader spectrum of neurodivergence.

Dual Cannabinoid and Orexin Regulation of Anhedonic Behaviour Caused by Prolonged Restraint Stress

The endocannabinoid and orexin systems share many biological functions, including wakefulness, stress response, reward processing, and mood. While these systems work against one another with respect to arousal, chronic stress-induced downregulation of both systems often leads to anhedonia or the inability to experience pleasure from natural rewards. In the current study, a 24 h restraint stress test (24 h RST) reduced sucrose preference in adult male and female C57BL/6 mice. Prior to the stressor, subsets of mice were intraperitoneally administered cannabinoid and orexin receptor agonists, antagonists, and combinations of these drugs. Restraint mice that received the cannabinoid receptor type 1 (CB1R) antagonist SR141716A, orexin receptor type 2 (OX2R) agonist YNT-185, and the combination of SR141716A and YNT-185, exhibited less anhedonia compared to vehicle/control mice. Thus, the 24 h RST likely decreased orexin signaling, which was then restored by YNT-185. Receptor colocalization analysis throughout mesocorticolimbic brain regions revealed increased CB1R-OX1R colocalization from SR141716A and YNT-185 treatments. Although a previous study from our group showed additive cataleptic effects between CP55,940 and the dual orexin receptor antagonist (TCS-1102), the opposite combination of pharmacological agents proved additive for sucrose preference. Taken together, these results reveal more of the complex interactions between the endocannabinoid and orexin systems.

A Comprehensive Review of Daridorexant, a Dual-Orexin Receptor Antagonist as New Approach for the Treatment of Insomnia

Insomnia affects 4.4-4.8% of the world's population, but because the effect of hypnotic drugs is limited and may cause harmful side-effects, scientists are turning their attention to developing drugs that act on the orexin system. Daridorexant, a selective dual-orexin receptor antagonist (DORA), has exhibited promising results in both animal and human studies. Its activity was evaluated based on the physiology-based pharmacodynamic and pharmacokinetic model. The use of daridorexant is considered safe, with no clinically significant side-effects including deprivation of next-morning residual effects. In this manuscript we conducted a comprehensive review of daridorexant including pharmacodynamics, animal and human research, pharmacokinetics and safety.

Synergistic antidepressant effects of citalopram and SB-334867 in the REM sleep-deprived mice: Possible role of BDNF

This study was done to evaluate the effect of co-treatment of orexin agents along with citalopram on the modulation of depression-like behavior and the expression of BDNF in the prefrontal cortex (PFC) of sleep-deprived male mice. A sleep deprivation model was performed in which rapid eye movement (REM) sleep was completely prohibited, and non-REM sleep was intensely reduced for 24 h. For drug microinjection, the guide cannula was surgically fixed in the left lateral ventricle of mice. Furthermore, we used the open-field test (OFT), forced swim test (FST), tail suspension test (TST), and splash test for recording depression-like behavior as well as Real-Time PCR amplification for assessing the expression of BDNF in the PFC of REM sleep-deprived mice. Our results revealed that REM sleep deprivation did not change locomotor activity while increased depressive-like behavior in FST, TST, and splash tests. However, the expression of BDNF was decreased in the PFC. Intraperitoneally (i.p.) administration of citalopram induced antidepressant effect in the normal and REM sleep-deprived mice. Moreover, intracerebroventricular (i.c.v.) microinjection of a non-effective dose of SB-334867, an orexin antagonist, potentiated the antidepressant-like effect of citalopram. On the other hand, a non-significant dosage of orexin-1 reversed the antidepressant effect of citalopram in the normal and REM sleep-deprived animals. Furthermore, our results showed that injection of citalopram alone or with SB-334867 increased the mRNA expression level of BDNF in the PFC of REM sleep-deprived mice. These data suggest that REM sleep deprivation interferes with the neural systems underlying the depression-like process and supports a likely interaction of the orexin system with citalopram on the modulation of depression-like behavior in REM sleep-deprived mice.

What evidence is there for implicating the brain orexin system in neuropsychiatric symptoms in dementia?

Neuropsychiatric symptoms (NPS) affect people with dementia (PwD) almost universally across all stages of the disease, and regardless of its exact etiology. NPS lead to disability and reduced quality of life of PwD and their caregivers. NPS include hyperactivity (agitation and irritability), affective problems (anxiety and depression), psychosis (delusions and hallucinations), apathy, and sleep disturbances. Preclinical studies have shown that the orexin neuropeptide system modulates arousal and a wide range of behaviors via a network of axons projecting from the hypothalamus throughout almost the entire brain to multiple, even distant, regions. Orexin neurons integrate different types of incoming information (e.g., metabolic, circadian, sensory, emotional) and convert them into the required behavioral output coupled to the necessary arousal status. Here we present an overview of the behavioral domains influenced by the orexin system that may be relevant for the expression of some critical NPS in PwD. We also hypothesize on the potential effects of pharmacological interference with the orexin system in the context of NPS in PwD.