The hypocretin/orexin system: an increasingly important role in neuropsychiatry

Orexin Receptor Antagonists for the Prevention and Treatment of Alzheimer's Disease and Associated Sleep Disorders

Orexins/hypocretins are neuropeptides produced by the hypothalamic neurons, binding two G-protein coupled receptors (orexin 1 and orexin 2 receptors) and playing a critical role in regulating arousal, wakefulness, and various physiological functions. Given the high prevalence of sleep disturbances in Alzheimer's disease (AD) and their reported involvement in AD pathophysiology, the orexin system is hypothesized to contribute to the disease pathogenesis. Specifically, recent evidence suggests that orexin's influence may extend beyond sleep regulation, potentially affecting amyloid-β and tau pathologies. Dual orexin receptor antagonists (DORAs), namely suvorexant, lemborexant, and daridorexant, demonstrated efficacy in treating chronic insomnia disorder across diverse clinical populations. Considering their stabilizing effects on sleep parameters and emerging evidence of a possible neuroprotective role, these agents represent a promising strategy for AD management. This leading article reviews the potential use of orexin receptor antagonists in AD, particularly focusing on their effect in modulating disease-associated sleep disturbances and clinical outcomes. Overall, clinical studies support the use of DORAs to enhance sleep quality in patients with AD with comorbid sleep and circadian sleep-wake rhythm disorders. Preliminary results also suggest that these compounds might influence AD pathology, potentially affecting disease progression. Conversely, research on selective orexin receptor antagonists in AD is currently limited. Further investigation is needed to explore orexin antagonism not only as a symptomatic treatment for sleep disturbances, but also for its broader implications in modifying AD neurodegeneration, emphasizing mechanisms of action and long-term outcomes.

Exploring the emotional and behavioural profile in paediatric narcolepsy type 1: A case-control study

Narcolepsy type 1 (NT1) is a central disorder of hypersomnolence often arising in childhood and adolescence. NT1 has a significant, but poorly defined, psychological impact. We aimed to investigate the psycho-social functioning of children and adolescents with NT1. We performed a cross-sectional, child and parent-reported questionnaire survey in 37 children and adolescents (6-17 years) with NT1, compared with age- and sex-matched controls. Questionnaires (SSHS, ESS-CHAD, CDI, MASC, CBCL, CRS-R, and SNAP-IV) evaluated various aspects of behavioural and emotional profiles, sleep habits, and daytime sleepiness. Subsequently, NT1 intra-group analysis was performed to investigate the effect of sex (males vs females) and pharmacological treatment (treated vs non-treated) on psychological features. The NT1 questionnaires total scores were then correlated with the clinical characteristics (age, body mass index [BMI], ESS-CHAD score, cerebrospinal hypocretin-1 [Hcrt-1] levels, and diagnostic delay). Patients with NT1 showed a higher tendency to depressive symptoms, anxiety, somatisation, inattention, hyperactivity, oppositional/defiant problems, and other maladaptive behaviours compared with controls. Among NT1 patients, females showed a higher propensity to anxiety, and non-treated patients displayed higher depressive symptoms. Psychological symptoms increased with age, BMI, and daytime sleepiness in patients with NT1, while a younger age was associated with more frequent somatisation symptoms. Lower cerebrospinal Hcrt-1 levels correlated with poorer social competencies, daily activities, and inattention. Diagnostic delay was associated with a higher impact of depressive symptoms and behavioural problems. NT1 in children and adolescents is associated with poorer functioning in multiple psychological domains calling for a multidisciplinary approach and monitoring to reduce disease burden and to prevent psychiatric consequences.

Executive function performance in children and adolescent patients with narcolepsy type 1

OBJECTIVE

The executive function profile in patients with narcolepsy type 1 (NT1) has been mentioned; however, limited research exists on children and adolescent patients with NT1.This study aims to assess executive function in children and adolescent patients with NT1 in China, examine potential influencing factors and evaluate the short-term treatment effect on executive function.

METHODS

53 NT1 patients (36 males, age 12.2 ± 3.4 years) and 37 healthy controls (23 males, age 12.2 ± 2.5 years) underwent self-reported measures assessing subjective sleepiness, depression, anxiety and sleep quality. A comprehensive neuropsychological test was administered to assess executive function domains, including processing speed, inhibitory control, cognitive flexibility and working memory. These assessments were repeated in NT1 patients after three-day regular drug treatment.

RESULTS

NT1 patients exhibited higher levels of excessive daytime sleepiness, depression, anxiety, and poor sleep quality compared to healthy controls. Patients showed impaired processing speed, inhibitory control and cognitive flexibility (p < 0.05), whereas working memory was unaffected (p > 0.05). Regression analysis revealed that parameters from sleep monitoring, such as sleep efficiency and sleep latency, were correlated with executive function performance after controlling for age, gender, and education years. The short-term treatment led to improvements in inhibitory control, cognitive flexibility, and working memory.

CONCLUSION

The findings showed that executive function was impaired among children and adolescent patients with NT1, which was associated with objective sleep parameters. Furthermore, this study emphasizes the necessity of neuropsychological assessments and early interventions among children and adolescent NT1 patients.

Orexins in apoptosis: a dual regulatory role

The orexins, also referred to as hypocretins, are neuropeptides that originate from the lateral hypothalamus (LH) region of the brain. They are composed of two small peptides, orexin-A, and orexin-B, which are broadly distributed throughout the central and peripheral nervous systems. Orexins are recognized to regulate diverse functions, involving energy homeostasis, the sleep-wake cycle, stress responses, and reward-seeking behaviors. Additionally, it is suggested that orexin-A deficiency is linked to sleepiness and narcolepsy. The orexins bind to their respective receptors, the orexin receptor type 1 (OX1R) and type 2 (OX2R), and activate different signaling pathways, which results in the mediation of various physiological functions. Orexin receptors are widely expressed in different parts of the body, including the skin, muscles, lungs, and bone marrow. The expression levels of orexins and their receptors play a crucial role in apoptosis, which makes them a potential target for clinical treatment of various disorders. This article delves into the significance of orexins and orexin receptors in the process of apoptosis, highlighting their expression levels and their potential contributions to different diseases. The article offers an overview of the existing understanding of the orexin/receptor system and how it influences the regulation of apoptosis.

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.

Inactivation of hypocretin receptor-2 signaling in dopaminergic neurons induces hyperarousal and enhanced cognition but impaired inhibitory control

Hypocretin/Orexin (HCRT/OX) and dopamine (DA) are both key effectors of salience processing, reward and stress-related behaviors and motivational states, yet their respective roles and interactions are poorly delineated. We inactivated HCRT-to-DA connectivity by genetic disruption of Hypocretin receptor-1 (Hcrtr1), Hypocretin receptor-2 (Hcrtr2), or both receptors (Hcrtr1&2) in DA neurons and analyzed the consequences on vigilance states, brain oscillations and cognitive performance in freely behaving mice. Unexpectedly, loss of Hcrtr2, but not Hcrtr1 or Hcrtr1&2, induced a dramatic increase in theta (7-11 Hz) electroencephalographic (EEG) activity in both wakefulness and rapid-eye-movement sleep (REMS). DAHcrtr2-deficient mice spent more time in an active (or theta activity-enriched) substate of wakefulness, and exhibited prolonged REMS. Additionally, both wake and REMS displayed enhanced theta-gamma phase-amplitude coupling. The baseline waking EEG of DAHcrtr2-deficient mice exhibited diminished infra-theta, but increased theta power, two hallmarks of EEG hyperarousal, that were however uncoupled from locomotor activity. Upon exposure to novel, either rewarding or stress-inducing environments, DAHcrtr2-deficient mice featured more pronounced waking theta and fast-gamma (52-80 Hz) EEG activity surges compared to littermate controls, further suggesting increased alertness. Cognitive performance was evaluated in an operant conditioning paradigm, which revealed that DAHcrtr2-ablated mice manifest faster task acquisition and higher choice accuracy under increasingly demanding task contingencies. However, the mice concurrently displayed maladaptive patterns of reward-seeking, with behavioral indices of enhanced impulsivity and compulsivity. None of the EEG changes observed in DAHcrtr2-deficient mice were seen in DAHcrtr1-ablated mice, which tended to show opposite EEG phenotypes. Our findings establish a clear genetically-defined link between monosynaptic HCRT-to-DA neurotransmission and theta oscillations, with a differential and novel role of HCRTR2 in theta-gamma cross-frequency coupling, attentional processes, and executive functions, relevant to disorders including narcolepsy, attention-deficit/hyperactivity disorder, and Parkinson's disease.

Improvement of autistic-like behaviors in adult rats prenatally exposed to valproic acid through early suppression of orexin receptor

Introduction

Autism spectrum disorder (ASD) is a disabling psychiatric disease characterized by impairments in communication and social skills. The pathophysiology of autism is complex and not fully known. Considering the incidence of sleep disorders in individuals with ASD and the important role of orexin in sleep, it is possible to hypothesize that an alteration of the orexinergic system could be implicated in the pathogenesis of autism symptoms. The present study was conducted to investigate the effect of suvorexant [dual orexin receptor antagonists (DORAs)] on autism-like behavior in prenatally valproic acid (VPA)-exposed rats].

Methods

Wistar female rats were administered VPA [600 mg/kg, intraperitoneally (i.p.)] or normal saline (10 ml/kg, i.p.; vehicle control) on gestational day 12.5. Thirty-two male offspring were divided into four groups: Control, VPA, Suvorexant+VPA, and VPA+Risperidone. The pups were given suvorexant [20 ml/kg, by mouth/orally (p.o.)] or risperidone (1 ml/kg, p.o.) daily from postnatal day (PND) (40-54). The offspring were tested for repetitive behaviors and cognitive ability with a Y-maze task on PND 55, and social interaction was assessed by play behavior in the open field on PND 56. And anxiety with using the three-chamber social assay on PND 56.

Results

In the Y-maze apparatus, spontaneous alteration significantly decreased in the prenatal VPA-treated rats compared to control rats showing autistic-like behavior, and 2-week suvorexant increased the alternation, indicating the beneficial effect of suvorexant. Prenatal treatment with VPA, impaired play behavior (sniffing, grooming, and darting), and increased anxiety-related behavior. Suvorexant treatment attenuated the problems in male offspring's social behavior.

Conclusion

Our results showed that suvorexant improved ASD-associated behaviors in the VPA-treated rats, and the orexinergic system may be associated with the pathogenesis of autism symptoms.

Decreased plasma neuropeptides in first-episode schizophrenia, bipolar disorder, major depressive disorder: associations with clinical symptoms and cognitive function

Background

There is an urgent need to identify differentiating and disease-monitoring biomarkers of schizophrenia, bipolar disorders (BD), and major depressive disorders (MDD) to improve treatment and management.

Methods

We recruited 54 first-episode schizophrenia (FES) patients, 52 BD patients, 35 MDD patients, and 54 healthy controls from inpatient and outpatient clinics. α-Melanocyte Stimulating Hormone (α-MSH), β-endorphin, neurotensin, orexin-A, oxytocin, and substance P were investigated using quantitative multiplex assay method. Psychotic symptoms were measured using the Brief Psychiatric Rating Scale (BPRS) and Positive and Negative Syndrome Scale (PANSS), manic symptoms using the Young Mania Rating Scale (YMRS), and depressive symptoms using 17 item-Hamilton Depression Rating Scale (HAMD). We additionally measured cognitive function by using a battery of tests given to all participants.

Results

α-MSH, neurotensin, orexin-A, oxytocin, and substance P were decreased in the three patient groups compared with controls. Neurotensin outperformed all biomarkers in differentiating patient groups from controls. There were no significant differences for 6 neuropeptides in their ability to differentiate between the three patient groups. Higher neurotensin was associated with better executive function across the entire sample. Lower oxytocin and higher substance p were associated with more psychotic symptoms in FES and BD groups. β-endorphin was associated with early morning wakening symptom in all three patient groups.

Conclusion

Our research shows decreased circulating neuropeptides have the potential to differentiate severe mental illnesses from controls. These neuropeptides are promising treatment targets for improving clinical symptoms and cognitive function in FES, BD, and MDD.

Nasal administration of orexin A partially rescues dizocilpine-induced cognitive impairments in female C57BL/6J mice

Sex difference has been reported in several behavioural endophenotypes of neuropsychiatric disorder in both rodents and humans. However, sex difference in cognitive symptoms associated with neuropsychiatric disorders has not been studied in detail. In this study, we induced cognitive impairment using the NMDA receptor antagonist, dizocilpine (MK-801), in male and female C57BL/6J mice and performed a visual discrimination task in an automated touchscreen system. We found that discrimination performance decreased with increased doses of MK-801 in both sexes. However, female mice showed stronger deficit in discrimination performance than the male mice especially after administration of low (0.01mg/kg) and high (0.15mg/kg) doses of MK-801. Furthermore, we tested if administration of orexin A, orexin-1 receptor antagonist SB-334867 or orexin-2 receptor antagonist EMPA rescued MK-801 (0.15mg/kg) induced cognitive impairment in visual discrimination. We found that nasal administration of orexin A partially rescued the cognitive impairment induced by MK-801 in females but not in males. Taken together, our data show that female C57BL/6J mice are more sensitive compared to males to some doses of MK-801 in discrimination learning task and that orexin A partially rescues this cognitive impairment in females.

Orexin receptor antagonists in the pathophysiology and treatment of sleep disorders and epilepsy

The correlation between sleep and epilepsy has been argued over the past decades among scientists. Although the similarities and contrasts between sleep and epilepsy had been considered, their intertwined nature was not revealed until the nineteenth century. Sleep is recognized as a recurring state of mind and body through alternating brain electrical activities. It is documented that sleep disorders are associated with epilepsy. The origin, suppression, and spread of seizures are affected by sleep. As such, in patients with epilepsy, sleep disorders are a frequent comorbidity. Meanwhile, orexin, a wake-promoting neuropeptide, provides a bidirectional effect on both sleep and epilepsy. Orexin and its cognate receptors, orexin receptor type 1 (OX1R) and type 2 (OX2R), orchestrate their effects by activating various downstream signaling pathways. Although orexin was considered a therapeutic target in insomnia shortly after its discovery, its potential usefulness for psychiatric disorders and epileptic seizures has been suggested in the pre-clinical studies. This review aimed to discuss whether the relationship between sleep, epilepsy, and orexin is clearly reciprocal.

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.

CO2 reactivity as a biomarker of exposure-based therapy non-response: study protocol

BACKGROUND

Exposure-based therapy is an effective first-line treatment for anxiety-, obsessive-compulsive, and trauma- and stressor-related disorders; however, many patients do not improve, resulting in prolonged suffering and poorly used resources. Basic research on fear extinction may inform the development of a biomarker for the selection of exposure-based therapy. Growing evidence links orexin system activity to deficits in fear extinction and we have demonstrated that reactivity to an inhaled carbon dioxide (CO₂) challenge-a safe, affordable, and easy-to-implement procedure-can serve as a proxy for orexin system activity and predicts fear extinction deficits in rodents. Building upon this basic research, the goal for the proposed study is to validate CO₂ reactivity as a biomarker of exposure-based therapy non-response.

METHODS

We will assess CO₂ reactivity in 600 adults meeting criteria for one or more fear- or anxiety-related disorders prior to providing open exposure-based therapy. By incorporating CO₂ reactivity into a multivariate model predicting treatment non-response that also includes reactivity to hyperventilation as well as a number of related predictor variables, we will establish the mechanistic specificity and the additive predictive utility of the potential CO₂ reactivity biomarker. By developing models independently within two study sites (University of Texas at Austin and Boston University) and predicting the other site's data, we will validate that the results are likely to generalize to future clinical samples.

DISCUSSION

Representing a necessary stage in translating basic research, this investigation addresses an important public health issue by testing an accessible clinical assessment strategy that may lead to a more effective treatment selection (personalized medicine) for patients with anxiety- and fear-related disorders, and enhanced understanding of the mechanisms governing exposure-based therapy.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT05467683 (20/07/2022).

Orexin/Hypocretin System Dysfunction in ESSENCE (Early Symptomatic Syndromes Eliciting Neurodevelopmental Clinical Examinations)

Early Symptomatic Syndromes Eliciting Neurodevelopmental Clinical Examinations (ESSENCE) is an umbrella term covering a wide range of neurodevelopmental difficulties and disorders. Thus, ESSENCE includes attention-deficit/hyperactivity disorder (ADHD), autism spectrum disorder (ASD), and other neurodevelopmental disorders (NDDs) and difficulties, with a variety of symptoms in cognitive, motor, sensory, social, arousal, regulatory, emotional, and behavioral developmental domains, frequently co-occurring and likely having partly common neurobiological substrates. The ESSENCE concept is a clinical paradigm that promotes organizing NDDs in everyday clinical practice according to their coexistence, symptom dimensions overlapping, and treatment possibilities. Despite increased knowledge regarding NDDs, the neurobiological mechanisms that underlie them and other ESSENCE-related problems, are not well understood. With its wide range of neural circuits and interactions with numerous neurotransmitters, the orexin/hypocretin system (Orx-S) is possibly associated with a variety of neurocognitive, psychobiological, neuroendocrine, and physiological functions and behaviors. Dysfunction of Orx-S has been implicated in various psychiatric and neurological disorders. This article provides an overview of Orx-S dysfunctions' possible involvement in the development, presentation, and maintenance of ESSENCE. We provide a focused review of current research evidence linking orexin neuropeptides with specific clinical NDDs symptoms, mostly in ADHD and ASD, within the Research Domain Criteria (RDoC) framework. We propose that Orx-S dysfunction might have an important role in some of these neurodevelopmental symptom domains, such as arousal, wakefulness, sleep, motor and sensory processing, mood and emotional regulation, fear processing, reward, feeding, attention, executive functions, and sociability. Our perspective is presented from a clinical point of view. Further, more thorough systematic reviews are needed as well as planning of extensive new research into the Orx-S's role in ESSENCE, especially considering RDoC elements.

Orexin/hypocretin system dysfunction in patients with Takotsubo syndrome: A novel pathophysiological explanation

Takotsubo syndrome (TTS) is an acute heart failure syndrome. Emotional or physical stressors are believed to precipitate TTS, while the pathophysiological mechanism is not yet completely understood. During the coronavirus disease (COVID-19) pandemic, an increased incidence of TTS has been reported in some countries; however, the precise pathophysiological mechanism for developing TTS with acute COVID-19 infection is unknown. Nevertheless, observing the symptoms of COVID-19 might lead to new perspectives in understanding TTS pathophysiology, as some of the symptoms of the COVID-19 infection could be assessed in the context of an orexin/hypocretin-system dysfunction. Orexin/hypocretin is a cardiorespiratory neuromodulator that acts on two orexin receptors widely distributed in the brain and peripheral tissues. In COVID-19 patients, autoantibodies against one of these orexin receptors have been reported. Orexin-system dysfunction affects a variety of systems in an organism. Here, we review the influence of orexin-system dysfunction on the cardiovascular system to propose its connection with TTS. We propose that orexin-system dysfunction is a potential novel explanation for the pathophysiology of TTS due to direct or indirect dynamics of orexin signaling, which could influence cardiac contractility. This is in line with the conceptualization of TTS as a cardiovascular syndrome rather than merely a cardiac abnormality or cardiomyopathy. To the best of our knowledge, this is the first publication to present a plausible connection between TTS and orexin-system dysfunction. We hope that this novel hypothesis will inspire comprehensive studies regarding orexin's role in TTS pathophysiology. Furthermore, confirmation of this plausible pathophysiological mechanism could contribute to the development of orexin-based therapeutics in the treatment and prevention of TTS.

Central Disorders of Hypersomnolence: Association with Fatigue, Depression and Sleep Inertia Prevailing in Women

Fatigue, depression, and sleep inertia are frequently underdiagnosed manifestations in narcolepsy and idiopathic hypersomnia. Our cross-sectional study design included diagnostic interview accompanied by assessment instruments and aimed to explore how these factors influence disease severity as well as to elucidate any sex predisposition. One hundred and forty-eight subjects (female 63%) were divided into narcolepsy type 1 (NT1; n = 87, female = 61%), narcolepsy type 2 (NT2; n = 22, female = 59%), and idiopathic hypersomnia (IH; n = 39, female = 69%). All subjects completed a set of questionnaires: Epworth Sleepiness Scale (ESS), Hospital Anxiety and Depression Scales (HADS), Fatigue Severity Scale (FSS), and Sleep Inertia Questionnaire (SIQ). In narcoleptic subjects, questionnaire data were correlated with the Narcolepsy Severity Scale (NSS), and in subjects with idiopathic hypersomnia, with the Idiopathic Hypersomnia Severity Scale (IHSS). The highest correlation in narcoleptic subjects was found between NSS and ESS (r = 0.658; p < 0.0001), as well as FSS (r = 0.506; p < 0.0001), while in subjects with idiopathic hypersomnia, the most prominent positive correlations were found between IHSS and SIQ (r = 0.894; p < 0.0001), FSS (r = 0.812; p < 0.0001), HADS depression scale (r = 0.649; p = 0.0005), and HADS anxiety scale (r = 0.528; p < 0.0001). ESS showed an analogic correlation with disease severity (r = 0.606; p < 0.0001). HADS anxiety and depression scores were higher in females (p < 0.05 and p < 0.01), with similar results for FSS and SIQ scales (p < 0.05 for both), and a trend toward higher ESS values in females (p = 0.057). Our study illustrates that more attention should be focused on pathophysiological mechanisms and associations of fatigue, depression, as well as sleep inertia in these diseases; they influence the course of both illnesses, particularly in women.

Orexin deficiency affects sensorimotor gating and its amphetamine-induced impairment

The orexin neuropeptides have an important role in the regulation of the sleep/wake cycle and foraging, as well as in reward processing and emotions. Furthermore, recent research implicates the orexin system in different behavioral endophenotypes of neuropsychiatric diseases such as social avoidance and cognitive flexibility. Utilizing orexin-deficient mice, the present study tested the hypothesis that orexin is involved in two further mouse behavioral endophenotypes of neuropsychiatric disorders, i.e., sensorimotor gating and amphetamine sensitivity. The data revealed that orexin-deficient mice expressed a deficit in sensorimotor gating, measured by prepulse inhibition of the startle response. Amphetamine treatment impaired prepulse inhibition in wildtype and heterozygous orexin-deficient mice, but had no effects in homozygous orexin-deficient mice. Furthermore, locomotor activity and center time in the open field was not affected by orexin deficiency but was similarly increased or decreased, respectively, by amphetamine treatment in all genotypes. These data indicate that the orexin system modulates prepulse inhibition and is involved in mediating amphetamine's effect on prepulse inhibition. Future studies should investigate whether pharmacological manipulations of the orexin system can be used to treat neuropsychiatric diseases associated with deficits in sensorimotor gating, such as schizophrenia or attention deficit hyperactivity disorder.

Dissociative Symptoms are Highly Prevalent in Adults with Narcolepsy Type 1

INTRODUCTION

The core symptoms of narcolepsy such as excessive daytime sleepiness and cataplexy are well known. However, there is mounting evidence for a much broader symptom spectrum, including psychiatric symptoms. Disordered sleep has previously been linked with dissociative symptoms, which may imply that patients with narcolepsy are more prone to develop such symptoms.

OBJECTIVES

To investigate the frequency of dissociative symptoms in adult patients with narcolepsy type 1 compared to population controls.

METHODS

In a retrospective case control study, sixty adult patients fulfilling the criteria for narcolepsy type 1 and 120 matched population control subjects received a structured interview using the Schedules for Clinical Assessment in Neuropsychiatry (SCAN) to assess dissociative symptoms and disorders.

RESULTS

A majority of narcolepsy patients reported dissociative symptoms, and even fulfilled the DSM-IV-TR criteria of a dissociative disorder (62% vs 1% in controls, p < .001). Most frequently reported symptoms were "dissociative amnesia" (37% vs 1%, p < .001) and "dissociative disorder of voluntary movement" (32% vs 1%, p < .001).

CONCLUSION

Dissociative symptoms are strikingly prevalent in adult patients with narcolepsy type 1. Although a formal diagnosis of dissociation disorder should not be made as the symptoms can be explained by narcolepsy as an underlying condition, the findings do illustrate the extent and severity of the dissociative symptoms. As for the pathophysiological mechanism, there may be symptom overlap between narcolepsy and dissociation disorder. However, there may also be a more direct link between disrupted sleep and dissociative symptoms. In either case, the high frequency of occurrence of dissociative symptoms should result in an active inquiry by doctors, to improve therapeutic management and guidance.

Efficacy and Safety of Seltorexant as Adjunctive Therapy in Major Depressive Disorder: A Phase 2b, Randomized, Placebo-Controlled, Adaptive Dose-Finding Study

BACKGROUND

Seltorexant, a selective antagonist of human orexin-2 receptors, demonstrated antidepressant effects in a previous exploratory study in patients with major depressive disorder (MDD).

METHODS

To replicate and extend this observation, a double-blind, adaptive dose-finding study was performed in patients with MDD who had an inadequate response to 1-3 selective serotonin/serotonin-norepinephrine reuptake inhibitors in the current episode. Patients were randomized (2:1:1) to placebo or seltorexant (20 mg or 40 mg) once-daily, administered adjunctively to the antidepressant the patient had been receiving at screening. After an interim analysis (6 weeks post-randomization of 160th patient), newly recruited patients randomly received (3:3:1) placebo or seltorexant 10 mg or 20 mg; the 40-mg dose was no longer assigned. Patients were stratified by baseline Insomnia Severity Index (ISI) scores (ISI ≥ 15 vs < 15). The primary endpoint was change from baseline Montgomery-Åsberg Depression Rating Scale (MADRS) total score at week 6.

RESULTS

Mixed-Model for Repeated Measures analysis showed a greater improvement in MADRS total score in the seltorexant 20-mg group vs placebo at weeks 3 and 6; least-square means difference (90% CI): -4.5 (-6.96; -2.07), P = .003; and -3.1 (-6.13; -0.16), P = .083, respectively. The improvement in MADRS score at week 6 for seltorexant 20 mg was greater in patients with baseline ISI ≥ 15 vs those with ISI < 15; least-square means difference (90% CI) vs placebo: -4.9 (-8.98; -0.80) and -0.7 (-5.16; 3.76), respectively. The most common (≥5%) adverse events with seltorexant were somnolence, headache, and nausea.

CONCLUSIONS

A clinically meaningful reduction of depressive symptoms was observed for seltorexant 20 mg. In the subset of patients with sleep disturbance (ISI ≥ 15), a larger treatment difference between seltorexant 20 mg and placebo was observed, warranting further investigation. No new safety signal was identified.

REGISTRATION

ClinicalTrials.gov Identifier: NCT03227224.

PREVIOUS PRESENTATION

Poster presented at 58th Annual Meeting of American College of Neuropsychopharmacology (ACNP), December 8-11, 2019, Orlando, FL.

Changes of Hypocretin (Orexin) System in Schizophrenia: From Plasma to Brain

Hypocretin (also called orexin) regulates various functions, such as sleep-wake rhythms, attention, cognition, and energy balance, which show significant changes in schizophrenia (SCZ). We aimed to identify alterations in the hypocretin system in SCZ patients. We measured plasma hypocretin-1 levels in SCZ patients and healthy controls and found significantly decreased plasma hypocretin-1 levels in SCZ patients, which was mainly due to a significant decrease in female SCZ patients compared with female controls. In addition, we measured postmortem hypothalamic hypocretin-1-immunoreactivity (ir), ventricular cerebrospinal fluid (CSF) hypocretin-1 levels, and hypocretin receptor (Hcrt-R) mRNA expression in the superior frontal gyrus (SFG) in SCZ patients and controls We observed a significant decrease in the amount of hypothalamic hypocretin-1 ir in SCZ patients, which was due to decreased amounts in female but not male patients. Moreover, Hcrt-R2 mRNA in the SFG was decreased in female SCZ patients compared with female controls, while male SCZ patients showed a trend of increased Hcrt-R1 mRNA and Hcrt-R2 mRNA expression compared with male controls. We conclude that central hypocretin neurotransmission is decreased in SCZ patients, especially female patients, and this is reflected in the plasma.

Increased orexin A concentrations in cerebrospinal fluid of patients with behavioural variant frontotemporal dementia

Orexins are hypothalamic neuropeptides that regulate several physiological functions, such as appetite, arousal, cognition, stress, sleep and metabolism. Emerging pieces of evidence suggest an orexinergic dysfunction in several neuropsychiatric disorders, including depression, anxiety and addiction. A syndromic overlap between behavioural variant frontotemporal dementia (bvFTD) and several psychiatric disorders was recently demonstrated. Therefore, we analysed cerebrospinal fluid (CSF) orexin A concentrations of 40 bvFTD and 32 non-demented patients, correlating neuropeptide concentrations with several clinical characteristics. A significant increase of orexin A concentrations was found in bvFTD patients when compared to controls (p<0.001). CSF orexin A concentration showed a correlation with Mini-Mental State Examination scores, drug assumption, history of compulsive behaviour and extrapyramidal signs. Moreover, we found a relationship between CSF markers of neurodegeneration, total tau and Aβ_1–42 and CSF orexin A concentrations. Our study provides evidence of an orexinergic dysfunction in bvFTD, correlating with several clinical symptoms. Further larger studies are needed to confirm our data.

Orexin deficiency affects sociability and the acquisition, expression, and extinction of conditioned social fear

Accumulating evidence indicates that the central orexin (hypocretin) system plays an important role in regulating emotional processes in both humans and rodents. Thus, the orexin system has been repeatedly implicated in the pathophysiology of several neuropsychiatric disorders, such as anxiety disorders. Among others, symptoms like social fear and social withdrawal are frequently observed in these disorders. Based on this, we investigated the role of orexin deficiency in social (fear) behavior. For that, female and male orexin-deficient mice were tested for (1) sociability and social novelty, and (2) acquisition, expression, and extinction of conditioned social fear. We found that female orexin-deficient mice displayed reduced sociability and decreased preference for social novelty compared to their wild-type littermates. These effects of orexin deficiency were not observed in males. Moreover, orexin deficiency facilitated the acquisition and/or expression of conditioned social fear and impaired the extinction of social fear in both sexes. Taken together, our results indicate an important, partly sex-dependent, regulatory role of the orexin system in social (fear) behavior. Our findings support the hypothesis of orexin being an integrator of motivation, affect, and emotion.

Orexin deficiency modulates cognitive flexibility in a sex-dependent manner

Cognitive flexibility is an important executive function and refers to the ability to adapt behaviors in response to changes in the environment. Of note, many brain disorders are associated with impairments in cognitive flexibility. Several classical neurotransmitter systems including dopamine, acetylcholine and noradrenaline are shown to be important for cognitive flexibility, however, there is not much known about the role of neuropeptides. The neuropeptide orexin, which is brain-widely released by neurons in the lateral hypothalamus, is a major player in maintaining sleep/wake cycle, feeding behavior, arousal, and motivational behavior. Recent studies showed a role of orexin in attention, cognition and stress-induced attenuation of cognitive flexibility by disrupting orexin signaling locally or systemically. However, it is not known so far whether brain-wide reduction or loss of orexin affects cognitive flexibility. We investigated this question by testing male and female orexin-deficient mice in the attentional set shifting task (ASST), an established paradigm of cognitive flexibility. We found that orexin deficiency impaired the intra-dimensional shift phase of the ASST selectively in female homozygous orexin-deficient mice and improved the first reversal learning phase selectively in male homozygous orexin-deficient mice. We also found that these orexin-mediated sex-based modulations of cognitive flexibility were not correlated with trait anxiety, narcoleptic episodes, and reward consumption. Our findings highlight a sexually dimorphic role of orexin in regulating cognitive flexibility and the need for further investigations of sex-specific functions of the orexin circuitry.

Orexin and Alzheimer's Disease: A New Perspective

Orexin's role in human cognition has recently been emphasized and emerging evidences indicate its close relationship with Alzheimer's disease (AD). This review aimed to demonstrate recent research on the relationship between orexin and AD. Orexin's role in stress regulation and memory is discussed, with significant findings related to sexual disparities in stress response, with potential clinical implications pertaining to AD pathology. There are controversies regarding the orexin levels in AD patients, but the role of orexin in the trajectory of AD is still emphasized in recent literatures. Orexin is also accentuated in the context of tau pathology, and orexin as a potential therapeutic target for AD is frequently discussed. Future directions with regard to the relationship between orexin and AD are suggested: 1) consideration for AD trajectory in the measurement of orexin levels, 2) the need for objective measure such as polysomnography and actigraphy, 3) the need for close observation of cognitive profiles of orexin-deficient narcolepsy patients, 4) the need for validation studies by neuroimaging 5) the need for taking account sexual disparities in orexinergic activiation, and 6) consideration for orexin's role as a stress regulator. The aforementioned new perspectives could help unravel the relationship between orexin and AD.

The Neurochemistry of Autism

Autism spectrum disorder (ASD) refers to complex neurobehavioral and neurodevelopmental conditions characterized by impaired social interaction and communication, restricted and repetitive patterns of behavior or interests, and altered sensory processing. Environmental, immunological, genetic, and epigenetic factors are implicated in the pathophysiology of autism and provoke the occurrence of neuroanatomical and neurochemical events relatively early in the development of the central nervous system. Many neurochemical pathways are involved in determining ASD; however, how these complex networks interact and cause the onset of the core symptoms of autism remains unclear. Further studies on neurochemical alterations in autism are necessary to clarify the early neurodevelopmental variations behind the enormous heterogeneity of autism spectrum disorder, and therefore lead to new approaches for the treatment and prevention of autism. In this review, we aim to delineate the state-of-the-art main research findings about the neurochemical alterations in autism etiology, and focuses on gamma aminobutyric acid (GABA) and glutamate, serotonin, dopamine, N-acetyl aspartate, oxytocin and arginine-vasopressin, melatonin, vitamin D, orexin, endogenous opioids, and acetylcholine. We also aim to suggest a possible related therapeutic approach that could improve the quality of ASD interventions. Over one hundred references were collected through electronic database searching in Medline and EMBASE (Ovid), Scopus (Elsevier), ERIC (Proquest), PubMed, and the Web of Science (ISI).

Orexin/Hypocretin Type 2 Receptor (HCRTR2) Gene as a Candidate Gene in Sertraline-Associated Insomnia in Depressed Patients

BACKGROUND

Selective serotonin reuptake inhibitors (SSRIs) are considered as first-line drugs for treating depressive disorders. Among the adverse effects reported with sertraline is sleep disturbances; however, the etiology lying beneath is obscure. Orexin, the most recently discovered neurotransmitter, is involved in the sleep cycle. It exerts its physiological actions through orexin or hypocretin type 1 and 2 receptors (HCRTR1 and HCRTR2). Dysfunction of the orexin system contributes to various psychiatric, neurologic and neuropsychiatric disorders. Thus, our study aimed to assess the possible association of genetic variation of HCRTR2 G1246A with hypersomnia reported with sertraline in a group of major depressive disorder (MDD) patients.

PATIENTS AND METHODS

Ninety-six newly diagnosed MDD patients were enrolled in our cohort study. MDD was assessed using DSM-V criteria. Insomnia Severity Index (ISI) was used to assess insomnia at baseline (week 0) and week 4. Blood samples were collected for further genotyping of HCRTR2 G1246A (rs2653349) using polymerase chain reaction-restriction fragment length polymorphism.

RESULTS

A significant association between G1264A polymorphism of HCRTR2 and insomnia was observed. Insomnia with sertraline happens by 2.5-fold (P=0.022; odds ratio (OR)=2.5; 95% confidence interval (CI): 1.1-5.7) in patients having GG genotype. Patients with G allele experience insomnia by 2.1-fold more than A allele carriers (P=0.022; OR=2.1; 95% CI= 1.1-4.0). Subgroup analysis showed a significant association between GG genotype as well as the G allele and insomnia only in female MDD patients (P=0.011; OR=4.0; 95% CI=1.3-12.0 and P=0.033; OR=2.4; 95% CI=1.02-5.7, respectively).

CONCLUSION

In conclusion, the G1246A variant might be a predictor for insomnia in MDD patients treated with sertraline. Our findings support the idea that some variants of the HCRTR might contribute to inter-individual variability in the sleep pattern of patients receiving antidepressants.

Effects of 5-HT2C, 5-HT1A receptor challenges and modafinil on the initiation and persistence of gambling behaviours

RATIONALE

Problematic patterns of gambling are characterised by loss of control and persistent gambling often to recover losses. However, little is known about the mechanisms that mediate initial choices to begin gambling and then continue to gamble in the face of losing outcomes.

OBJECTIVES

These experiments first assessed gambling and loss-chasing performance under different win/lose probabilities in C57Bl/6 mice, and then investigated the effects of antagonism of 5-HT_2CR with SB242084, 5-HT_1AR agonism with 8-OH-DPAT and modafinil, a putative cognitive enhancer.

RESULTS

As seen in humans and other species, mice demonstrated the expected patterns of behaviour as the odds for winning were altered increasing gambling and loss-chasing when winning was more likely. SB242084 decreased the likelihood to initially gamble, but had no effects on subsequent gambling choices in the face of repeated losses. In contrast, 8-OH-DPAT had no effects on choosing to gamble in the first place, but once started 8-OH-DPAT increased gambling choices in a dose-sensitive manner. Modafinil effects were different to the serotonergic drugs in both decreasing the propensity to initiate gambling and chase losses.

CONCLUSIONS

We present evidence for dissociable effects of systemic drug administration on different aspects of gambling behaviour. These data extend and reinforce the importance of serotonergic mechanisms in mediating discrete components of gambling behaviour. They further demonstrate the ability of modafinil to reduce gambling behaviour. Our work using a novel mouse paradigm may be of utility in modelling the complex psychological and neurobiological underpinnings of gambling problems, including the analysis of genetic and environmental factors.

Impaired social functioning in children with narcolepsy

Study Objectives

To explore impairments in social functioning in children with narcolepsy compared to healthy children.

Methods

Parents of 53 pediatric patients with narcolepsy type 1 and 64 matched healthy children completed the Social Responsiveness Scale (SRS) and the Child Behavior Checklist 6-18 (CBCL 6-18).

Results

Patients scored significantly higher on the total score of the SRS (median 56, interquartile range [IQR] 23.5) compared to controls (median 44.5, IQR 8.5, U = 797.0, p < 0.001). Patients also scored higher on the sum of the CBCL 6-18 subscales indicative of social functioning (Withdrawn/Depressed, Social Problems, and Thought Problems; median 183, IQR 30.5) compared to controls (median 155, IQR 13, U = 500.0, p < 0.001). A total of 24 patients (45.3%) reported at least mild-to-moderate difficulties in social functioning compared to seven controls (10.9%, χ2 = 17.165, p < 0.001). Eleven patients (20.8%) and only one control (1.6%) had T scores above 75, which points to severely impaired social functioning (χ2 = 11.602, p = 0.001). Within the patient group, girls reported mild-to-moderate difficulties in social functioning significantly more often compared to boys on the SRS (77.8% versus 28.6%, χ2 = 17.560, p < 0.001).

Conclusions

Impaired social functioning is common in children with narcolepsy type 1, especially in girls. Questionnaires such as the SRS and the CBCL 6-18 may help in early detection of social problems in pediatric narcolepsy. Recognition of these problems could be valuable in the management of young people with narcolepsy.

Plasma orexin-A-like immunoreactivity levels and renal function in patients in a geriatric ward

Orexin-A is a neuropeptide mainly produced by hypothalamic neurons with functions in the central nervous system such as regulation of the sleep-wake cycle. Recent studies suggest that orexin-A also plays major roles in peripheral tissues. Although a few studies have reported a role for the kidney in the dynamics of orexin-A, little is known about the association between plasma orexin-A-like immunoreactivity (orexin-A-LI) levels and renal function. We evaluated this association, and also explored other clinical characteristics associated with plasma orexin-A-LI levels. In this cross-sectional study, we included 70 consecutive patients aged ≥65 years admitted to the geriatric ward of Nagoya University Hospital from December 2017 to January 2018. Patients taking suvorexant (an orexin receptor antagonist) were excluded. On hospital days 2-4, fasting blood was collected in the morning. We evaluated associations between plasma orexin-A-LI levels and renal function and other clinical characteristics. Renal function was evaluated in two ways: the estimated glomerular filtration rate (eGFR) using serum creatinine, and estimated creatinine clearance (eCrCl) using the Cockroft-Gault formula. Pearson's correlation coefficient revealed that plasma orexin-A-LI levels were negatively correlated with the eGFR (r = -0.351, p = 0.003) and eCrCl (r = -0.342, p = 0.004). There were no significant associations between plasma orexin-A-LI levels and the primary diagnosis, body mass index, duration of fasting, or other clinical characteristics. In conclusion, plasma orexin-A-LI levels were negatively correlated with renal function in patients in a geriatric ward. Renal function may affect the study design and data interpretation in studies of plasma orexin-A-LI.

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

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

Health-Related Quality of Life in Patients With Narcolepsy: A Review of the Literature

Narcolepsy is a chronic disorder characterized by symptoms of excessive daytime sleepiness, irresistible sleep attacks that may be accompanied by cataplexy brought on by emotions, sleep paralysis, and hypnagogic hallucinations. This is a review of 32 empirical articles on health-related quality of life (HRQoL) published in peer-reviewed journals over the past 37 years. Deleterious implications on education, recreation, driving, sexual life, and personality are associated with the disease with a consequent negative psychosocial impact. Sleepiness has an important influence on HRQoL, more than the other symptoms of this disorder that have disrupting roles, too. Therefore, patients with narcolepsy need assistance not only for medication prescription but also in terms of psychological and social support. It is also of importance to assess patients with narcolepsy carefully in terms of depressive symptoms because they may have a major impact on HRQoL with important clinical implications.

Orexins and stress

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

The contribution of orexins to sex differences in the stress response

Women are twice as likely as men to suffer from stress-related psychiatric disorders, such as post-traumatic stress disorder (PTSD) and Major Depressive Disorder (MDD), however, the biological basis of these sex differences is not fully understood. Interestingly, orexins are known to be dysregulated in these disorders. This review first discusses the important role of orexins regulating the response to stress. Next, we review the evidence for sex differences in the orexin system, in which the majority of both preclinical and clinical studies have reported higher orexin system expression in females. Finally, we discuss the functional consequences of these sex differences in orexin expression. Most importantly, the preclinical literature reveals that higher orexin system activity in females contributes to exaggerated neuroendocrine and behavioral responses to stress. In sum, the available data suggests that orexins may be important in the etiology of stress-related psychiatric disorders that present differently in men and women. Thus, targeting orexins could potentially ameliorate many phenotypes of stress-related illness in a sex-specific way.

Cluster headache is associated with unhealthy lifestyle and lifestyle-related comorbid diseases: Results from the Danish Cluster Headache Survey

AIM

To compare the prevalence of unhealthy lifestyle factors and comorbid disorders in cluster headache patients with headache-free controls, in order to discuss pathophysiology and possible consequences.

METHODS

Cluster headache patients from the Danish cluster headache survey aged 18-65 years, diagnosed according to ICHD-II, were compared to sex- and age-matched headache-free controls. Participants completed questionnaires and structured interviews.

RESULTS

A total of 400 cluster headache patients and 200 controls participated. Patients had a more unhealthy lifestyle compared with controls in the form of current and current/former smoking (48.3% vs. 9.0%, p < 0.001 and 74.5% vs. 30.0%, p < 0.001, respectively), higher average alcohol intake per week (98.2 grams vs. 77.9 grams, p = 0.033) and BMI (26.1 vs. 24.2 kg/m², p < 0.001), whereas coffee and energy drink consumption was equally distributed. Further, lifestyle-related, psychiatric and pain-related diseases were much more prevalent in patients compared with controls, except for diabetes. Sub-group analyses revealed that current/former smokers had a worse clinical presentation than never smokers.

CONCLUSION

Unhealthy lifestyle factors and lifestyle-related diseases were more prevalent in cluster headache patients compared to controls. As lifestyle-related diseases might have serious consequences in the management of cluster headache, it is key to inform patients at an early time point about the possible risks of their lifestyle choices.

The hypocretin/orexin system as a target for excessive motivation in alcohol use disorders

The hypocretin/orexin (ORX) system has been repeatedly demonstrated to regulate motivation for drugs of abuse, including alcohol. In particular, ORX seems to be critically involved in highly motivated behaviors, as is observed in high-seeking individuals in a population, in the seeking of highly palatable substances, and in models of dependence. It seems logical that this system could be considered as a potential target for treatment for addiction, particularly alcohol addiction, as ORX pharmacological manipulations significantly reduce drinking. However, the ORX system also plays a role in a wide range of other behaviors, emotions, and physiological functions and is disrupted in a number of non-dependence-associated disorders. It is therefore important to consider how the ORX system might be optimally targeted for potential treatment for alcohol use disorders either in combination with or separate from its role in other functions or diseases. This review will focus on the role of ORX in alcohol-associated behaviors and whether and how this system could be targeted to treat alcohol use disorders while avoiding impacts on other ORX-relevant functions. A brief overview of the ORX system will be followed by a discussion of some of the factors that makes it particularly intriguing as a target for alcohol addiction treatment, a consideration of some potential challenges associated with targeting this system and, finally, some future directions to optimize new treatments.

Sex- and Age-dependent Effects of Orexin 1 Receptor Blockade on Open-Field Behavior and Neuronal Activity

Adolescence is a sensitive and critical period in brain development where psychiatric disorders such as anxiety, depression and post-traumatic stress disorder are more likely to emerge following a stressful life event. Females are two times more likely to suffer from psychiatric disorders than males. Patients with these disorders show alterations in orexins (also called hypocretins), important neuropeptides that regulate arousal, wakefulness and the hypothalamic-pituitary-adrenal axis activity. Little is known on the role of orexins in mediating arousal behaviors in male and female rats during adolescence or adulthood. Here, we examine the influence of orexin 1 receptor blockade by SB334867 in open-field behavior in male and female rats during early adolescence (PND 31-33) or adulthood (PND 75-77). Animals were injected with 0 (vehicle), 1, 10, or 30 mg/kg SB334867 (i.p.). Thirty minutes later, they were placed in an open field, and behavior and neuronal activity (c-Fos) were assessed. In adolescent males, SB334867 significantly increased immobility in the 10 mg/kg group compared to vehicle. However, this increase in immobility in adolescent males was not observed in adolescent females. In contrast to adolescent males, adult males in the 10 mg/kg dose group showed the opposite effect on immobility compared to vehicle. These results indicate that 10 mg/kg dose of SB334867 has opposing effects in adolescent and adult males, but few effects in adolescent and adult females. Differences in functional networks between limbic regions may underlie these effects of orexin receptor blockade that are sex- and age-dependent in rats.

Reduced Orexin System Function Contributes to Resilience to Repeated Social Stress

Exposure to stress increases the risk of developing affective disorders such as depression and post-traumatic stress disorder (PTSD). However, these disorders occur in only a subset of individuals, those that are more vulnerable to the effects of stress, whereas others remain resilient. The coping style adopted to deal with the stressor, either passive or active coping, is related to vulnerability or resilience, respectively. Important neural substrates that mediate responses to a stressor are the orexins. These neuropeptides are altered in the cerebrospinal fluid of patients with stress-related illnesses such as depression and PTSD. The present experiments used a rodent social defeat model that generates actively coping rats and passively coping rats, which we have previously shown exhibit resilient and vulnerable profiles, respectively, to examine if orexins play a role in these stress-induced phenotypes. In situ radiolabeling and qPCR revealed that actively coping rats expressed significantly lower prepro-orexin mRNA compared with passively coping rats. This led to the hypothesis that lower levels of orexins contribute to resilience to repeated social stress. To test this hypothesis, rats first underwent 5 d of social defeat to establish active and passive coping phenotypes. Then, orexin neurons were inhibited before each social defeat for three additional days using designer receptors exclusively activated by designer drugs (DREADDs). Inhibition of orexins increased social interaction behavior and decreased depressive-like behavior in the vulnerable population of rats. Indeed, these data suggest that lowering orexins promoted resilience to social defeat and may be an important target for treatment of stress-related disorders.

New techniques, applications and perspectives in neuropeptide research

Neuropeptides are one of the most diverse classes of signaling molecules and have attracted great interest over the years owing to their roles in regulation of a wide range of physiological processes. However, there are unique challenges associated with neuropeptide studies stemming from the highly variable molecular sizes of the peptides, low in vivo concentrations, high degree of structural diversity and large number of isoforms. As a result, much effort has been focused on developing new techniques for studying neuropeptides, as well as novel applications directed towards learning more about these endogenous peptides. The areas of importance for neuropeptide studies include structure, localization within tissues, interaction with their receptors, including ion channels, and physiological function. Here, we discuss these aspects and the associated techniques, focusing on technologies that have demonstrated potential in advancing the field in recent years. Most identification and structural information has been gained by mass spectrometry, either alone or with confirmations from other techniques, such as nuclear magnetic resonance spectroscopy and other spectroscopic tools. While mass spectrometry and bioinformatic tools have proven to be the most powerful for large-scale analyses, they still rely heavily on complementary methods for confirmation. Localization within tissues, for example, can be probed by mass spectrometry imaging, immunohistochemistry and radioimmunoassays. Functional information has been gained primarily from behavioral studies coupled with tissue-specific assays, electrophysiology, mass spectrometry and optogenetic tools. Concerning the receptors for neuropeptides, the discovery of ion channels that are directly gated by neuropeptides opens up the possibility of developing a new generation of tools for neuroscience, which could be used to monitor neuropeptide release or to specifically change the membrane potential of neurons. It is expected that future neuropeptide research will involve the integration of complementary bioanalytical technologies and functional assays.

The Discovery of Suvorexant, the First Orexin Receptor Drug for Insomnia

Historically, pharmacological therapies have used mechanisms such as γ-aminobutyric acid A (GABA_A) receptor potentiation to drive sleep through broad suppression of central nervous system activity. With the discovery of orexin signaling loss as the etiology underlying narcolepsy, a disorder associated with hypersomnolence, orexin antagonism emerged as an alternative approach to attenuate orexin-induced wakefulness more selectively. Dual orexin receptor antagonists (DORAs) block the activity of orexin 1 and 2 receptors to both reduce the threshold to transition into sleep and attenuate orexin-mediated arousal. Among DORAs evaluated clinically, suvorexant has pharmacokinetic properties engineered for a plasma half-life appropriate for rapid sleep onset and maintenance at low to moderate doses. Unlike GABA_A receptor modulators, DORAs promote both non-rapid eye movement (NREM) and REM sleep, do not disrupt sleep stage-specific quantitative electroencephalogram spectral profiles, and allow somnolence indistinct from normal sleep. The preservation of cognitive performance and the ability to arouse to salient stimuli after DORA administration suggest further advantages over historical therapies.

Involvement of the CXCL12 System in the Stimulatory Effects of Prenatal Exposure to High-Fat Diet on Hypothalamic Orexigenic Peptides and Behavior in Offspring

Exposure to a high fat diet (HFD) during gestation stimulates neurogenesis and expression of hypothalamic orexigenic neuropeptides that affect consummatory and emotional behaviors. With recent studies showing a HFD to increase inflammation, this report investigated the neuroinflammatory chemokine, CXCL12, and compared the effects of prenatal CXCL12 injection to those of prenatal HFD exposure, first, by testing whether the HFD affects circulating CXCL12 in the dam and the CXCL12 system in the offspring brain, and then by examining whether prenatal exposure to CXCL12 itself mimics the effects of a HFD on hypothalamic neuropeptides and emotional behaviors. Our results showed that prenatal exposure to a HFD significantly increased circulating levels of CXCL12 in the dam, and that daily injections of CXCL12 induced a similar increase in CXCL12 levels as the HFD. In addition, prenatal HFD exposure significantly increased the expression of CXCL12 and its receptors, CXCR4 and CXCR7, in the hypothalamic paraventricular nucleus (PVN) of the offspring. Finally, the results revealed strong similarities in the effects of prenatal HFD and CXCL12 administration, which both stimulated neurogenesis and enkephalin (ENK) expression in the PVN, while having inconsistent or no effect in other regions of the hypothalamus, and also increased anxiety as measured by several behavioral tests. These results focus attention specifically on the CXCL12 chemokine system in the PVN of the offspring as being possibly involved in the stimulatory effects of prenatal HFD exposure on ENK-expressing neurons in the PVN and their associated changes in emotional behavior.

Orexins Mediate Sex Differences in the Stress Response and in Cognitive Flexibility

BACKGROUND

Women are twice as likely as men to experience stress-related psychiatric disorders. The biological basis of these sex differences is poorly understood. Orexins are altered in anxious and depressed patients. Using a rat model of repeated stress, we examined whether orexins contribute to sex differences in outcomes relevant to stress-related psychiatric diseases.

METHODS

Behavioral, neural, and endocrine habituation to repeated restraint stress and subsequent cognitive flexibility was examined in adult male and female rats. In parallel, orexin expression and activation were determined in both sexes, and chromatin immunoprecipitation was used to determine transcription factors acting at the orexin promoter. Designer receptors exclusively activated by designer drugs were used to inhibit orexin activation throughout repeated restraint to determine if the stress-related impairments in female rats could be reduced.

RESULTS

Female rats exhibited impaired habituation to repeated restraint with subsequent deficits in cognitive flexibility compared with male rats. Increased orexin expression and activation were observed in female rats compared with male rats. The higher expression of orexin messenger RNA in female rats was due to actions of glucocorticoid receptors on the orexin promoter, as determined by chromatin immunoprecipitation. Inhibition of orexins using designer receptors exclusively activated by designer drugs in female rats throughout repeated restraint abolished their heightened hypothalamic-pituitary-adrenal responsivity and reduced stress-induced cognitive impairments.

CONCLUSIONS

Orexins mediate the impairments in adaptations to repeated stress and in subsequent cognitive flexibility exhibited by female rats and provide evidence for a broader role for orexins in mediating functions relevant to stress-related psychiatric diseases.

The Cellular Diversity of the Pedunculopontine Nucleus: Relevance to Behavior in Health and Aspects of Parkinson's Disease

The pedunculopontine nucleus (PPN) is a rostral brainstem structure that has extensive connections with basal ganglia nuclei and the thalamus. Through these the PPN contributes to neural circuits that effect cortical and hippocampal activity. The PPN also has descending connections to nuclei of the pontine and medullary reticular formations, deep cerebellar nuclei, and the spinal cord. Interest in the PPN has increased dramatically since it was first suggested to be a novel target for treating patients with Parkinson's disease who are refractory to medication. However, application of frequency-specific electrical stimulation of the PPN has produced inconsistent results. A central reason for this is that the PPN is not a heterogeneous structure. In this article, we review current knowledge of the neurochemical identity and topographical distribution of neurons within the PPN of both humans and experimental animals, focusing on studies that used neuronally selective targeting strategies to ascertain how the neurochemical heterogeneity of the PPN relates to its diverse functions in relation to movement and cognitive processes. If the therapeutic potential of the PPN is to be realized, it is critical to understand the complex structure-function relationships that exist here.

Distribution, physiology and pharmacology of relaxin-3/RXFP3 systems in brain

Relaxin-3 is a member of a superfamily of structurally-related peptides that includes relaxin and insulin-like peptide hormones. Soon after the discovery of the relaxin-3 gene, relaxin-3 was identified as an abundant neuropeptide in brain with a distinctive topographical distribution within a small number of GABAergic neuron populations that is well conserved across species. Relaxin-3 is thought to exert its biological actions through a single class-A GPCR - relaxin-family peptide receptor 3 (RXFP3). Class-A comprises GPCRs for relaxin-3 and insulin-like peptide-5 and other peptides such as orexin and the monoamine transmitters. The RXFP3 receptor is selectively activated by relaxin-3, whereas insulin-like peptide-5 is the cognate ligand for the related RXFP4 receptor. Anatomical and pharmacological evidence obtained over the last decade supports a function of relaxin-3/RXFP3 systems in modulating responses to stress, anxiety-related and motivated behaviours, circadian rhythms, and learning and memory. Electrophysiological studies have identified the ability of RXFP3 agonists to directly hyperpolarise thalamic neurons in vitro, but there are no reports of direct cell signalling effects in vivo. This article provides an overview of earlier studies and highlights more recent research that implicates relaxin-3/RXFP3 neural network signalling in the integration of arousal, motivation, emotion and related cognition, and that has begun to identify the associated neural substrates and mechanisms. Future research directions to better elucidate the connectivity and function of different relaxin-3 neuron populations and their RXFP3-positive target neurons in major experimental species and humans are also identified.

LINKED ARTICLES

This article is part of a themed section on Recent Progress in the Understanding of Relaxin Family Peptides and their Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.10/issuetoc.

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

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