Opiates increase the number of hypocretin-producing cells in human and mouse brain and reverse cataplexy in a mouse model of narcolepsy

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

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

Treatment of insomnia associated with alcohol and opioid use: a narrative review

Substance use disorders (SUDs) are associated with profound sleep disturbances, including insomnia, sleep fragmentation, and circadian rhythm dysfunction resulting in serious mental and physical consequences. This minireview presents an overview of the neurocircuitry underlying sleep disturbances in SUDs and elaborates on treatment options with emphasis on alcohol use disorder (AUD) and opioid use disorder (OUD). A PubMed, Embase, CINAHL Plus, Cochrane, and Scopus search were conducted using sleep- and AUD/OUD related keywords from January 1st, 2000, to January 31st, 2023, with preferences for recent publications and randomized-controlled trials. A bidirectional relationship exists between insomnia and addiction with the status of each condition impacting the other in dictating clinical outcome. Existing evidence points to a resurgence of insomnia during detoxification, and unless treated satisfactorily, insomnia may lead to relapse. The discussion summarizes the strengths and limitations of cognitive behavioral therapy and pharmacological treatment for insomnia in SUDs covering evidence from both animal and clinical studies. The assumption of reestablishing normal sleep patterns by attaining and maintaining sobriety is misguided. Comorbid insomnia in patients with SUDs should be approached as an independent condition that requires its own treatment. Future clinical trials are needed with the aim of providing a resource for guiding clinical management of the many patients with insomnia and SUD.

Orexin-mediated motivated arousal and reward seeking

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

Single-Cell Profiling Uncovers Evolutionary Divergence of Hypocretin/Orexin Neuronal Subpopulations

Brain nuclei are traditionally defined by their anatomy, activity, and expression of specific markers. The hypothalamus contains discrete neuronal populations that coordinate fundamental behavioral functions, including sleep and wakefulness, in all vertebrates. Particularly, the diverse roles of hypocretin/orexin (Hcrt)-releasing neurons suggest functional heterogeneity among Hcrt neurons. Using single-cell RNA sequencing (scRNA-seq) and high-resolution imaging of the adult male and female zebrafish hypothalamic periventricular zone, we identified 21 glutamatergic and 28 GABAergic cell types. Integration of zebrafish and mouse scRNA-seq revealed evolutionary conserved and divergent hypothalamic cell types. The expression of specific genes, including npvf, which encodes a sleep-regulating neuropeptide, was enriched in subsets of glutamatergic Hcrt neurons in both larval and adult zebrafish. The genetic profile, activity, and neurite processing of the neuronal subpopulation that coexpresses both Hcrt and Npvf (Hcrt+Npvf+) differ from other Hcrt neurons. These interspecies findings provide a unified annotation of hypothalamic cell types and suggest that the heterogeneity of Hcrt neurons enables multifunctionality, such as consolidation of both wake and sleep by the Hcrt- and Npvf-releasing neuronal subpopulation.

Evaluating the efficacy of the selective orexin 1 receptor antagonist nivasorexant in an animal model of binge-eating disorder

OBJECTIVE

Test the efficacy of the selective orexin 1 receptor (OX1R) antagonist (SO1RA) nivasorexant in an animal model of binge-eating disorder (BED) and study its dose-response relationship considering free brain concentrations and calculated OX1R occupancy. Compare nivasorexant's profile to that of other, structurally diverse SO1RAs. Gain understanding of potential changes in orexin-A (OXA) neuropeptide and deltaFosB (ΔFosB) protein expression possibly underlying the development of the binge-eating phenotype in the rat model used.

METHOD

Binge-like eating of highly palatable food (HPF) in rats was induced through priming by intermittent, repeated periods of dieting and access to HPF, followed by an additional challenge with acute stress. Effects of nivasorexant were compared to the SO1RAs ACT-335827 and IDOR-1104-2408. OXA expression in neurons and neuronal fibers as well as ΔFosB and OXA-ΔFosB co-expression was studied in relevant brain regions using immuno- or immunofluorescent histochemistry.

RESULTS

All SO1RAs dose-dependently reduced binge-like eating with effect sizes comparable to the positive control topiramate, at unbound drug concentrations selectively blocking brain OX1Rs. Nivasorexant's efficacy was maintained upon chronic dosing and under conditions involving more frequent stress exposure. Priming for binge-like eating or nivasorexant treatment resulted in only minor changes in OXA or ΔFosB expression in few brain areas.

DISCUSSION

Selective OX1R blockade reduced binge-like eating in rats. Neither ΔFosB nor OXA expression proved to be a useful classifier for their binge-eating phenotype. The current results formed the basis for a clinical phase II trial in BED, in which nivasorexant was unfortunately not efficacious compared with placebo.

PUBLIC SIGNIFICANCE

Nivasorexant is a new investigational drug for the treatment of binge-eating disorder (BED). It underwent clinical testing in a phase II proof of concept trial in humans but was not efficacious compared with placebo. The current manuscript investigated the drug's efficacy in reducing binge-like eating behavior of a highly palatable sweet and fat diet in a rat model of BED, which initially laid the foundation for the clinical trial.

Pivotal role of orexin signaling in the posterior paraventricular nucleus of the thalamus during the stress-induced reinstatement of oxycodone-seeking behavior

BACKGROUND

The orexin (OX) system has received increasing interest as a potential target for treating substance use disorder. OX transmission in the posterior paraventricular nucleus of the thalamus (pPVT), an area activated by highly salient stimuli that are both reinforcing and aversive, mediates cue- and stress-induced reinstatement of reward-seeking behavior. Oral administration of suvorexant (SUV), a dual OX receptor (OXR) antagonist (DORA), selectively reduced conditioned reinstatement of oxycodone-seeking behavior and stress-induced reinstatement of alcohol-seeking behavior in dependent rats.

AIMS

This study tested whether OXR blockade in the pPVT with SUV reduces oxycodone or sweetened condensed milk (SCM) seeking elicited by conditioned cues or stress.

METHODS

Male Wistar rats were trained to self-administer oxycodone (0.15 mg/kg, i.v., 8 h/day) or SCM (0.1 ml, 2:1 dilution [v/v], 30 min/day). After extinction, we tested the ability of intra-pPVT SUV (15 µg/0.5 µl) to prevent reinstatement of oxycodone or SCM seeking elicited by conditioned cues or footshock stress.

RESULTS

The rats acquired oxycodone and SCM self-administration, and oxycodone intake correlated with signs of physical opioid withdrawal, confirming dependence. Following extinction, the presentation of conditioned cues or footshock elicited reinstatement of oxycodone- and SCM-seeking behavior. Intra-pPVT SUV blocked stress-induced reinstatement of oxycodone seeking but not conditioned reinstatement of oxycodone or SCM seeking or stress-induced reinstatement of SCM seeking.

CONCLUSIONS

The results indicate that OXR signaling in the pPVT is critical for stress-induced reinstatement of oxycodone seeking, further corroborating OXRs as treatment targets for opioid use disorder.

Insomnia symptoms and neurofunctional correlates among adults receiving buprenorphine for opioid use disorder

OBJECTIVES

Insomnia symptoms are negatively related to opioid use disorder (OUD) treatment outcomes, possibly reflecting the influence of sleep on neurofunctional domains implicated in addiction. Moreover, the intersection between OUD recovery and sleep represents an area well-suited for the development of novel, personalized treatment strategies. This study assessed the prevalence of clinically significant insomnia symptoms and characterized its neurofunctional correlates among a clinical sample of adults with OUD receiving buprenorphine.

METHODS

Adults (N = 129) receiving buprenorphine for OUD from an outpatient clinic participated in a cross-sectional survey. Participants completed an abbreviated version of NIDA's Phenotyping Assessment Battery, which assessed 6 neurofunctional domains: sleep, negative emotionality, metacognition, interoception, cognition, and reward. Bivariate descriptive statistics compared those with evidence of clinically significant insomnia symptoms (Insomnia Severity Index [ISI] score of ≥11) to those with minimal evidence of clinically significant insomnia symptoms (ISI score of ≤10) across each of the neurofunctional domains.

RESULTS

Roughly 60% of participants reported clinically significant insomnia symptoms (ISI score of ≥11). Experiencing clinically significant insomnia symptoms was associated with reporting greater levels of depression, anxiety, post-traumatic stress, stress intolerance, unhelpful metacognition, and interoceptive awareness (ps<0.05). Participants with evidence of clinically significant insomnia were more likely to report that poor sleep was interfering with their OUD treatment and that improved sleep would assist with their treatment (ps<0.05).

CONCLUSIONS

Insomnia was prevalent among adults receiving buprenorphine for OUD. Insomnia was associated with neurofunctional performance, which may impact OUD treatment trajectories. Our findings indicate potential targets in the development of personalized treatment plans for patients with co-morbid insomnia and OUD. To inform the development of novel treatment strategies, more research is needed to understand the potential mechanistic links between sleep disturbances and substance use.

Opiate anticipation, opiate induced anatomical changes in hypocretin (Hcrt, orexin) neurons and opiate induced microglial activation are blocked by the dual Hcrt receptor antagonist suvorexant, while opiate analgesia is maintained

We previously found that heroin addiction in humans is accompanied by an increase in the number of detected Hcrt neurons and a decrease in their soma size. We now show that the increased number of Hcrt cells visible after morphine treatment is likely the result of increased Hcrt production in neurons having sub-detection levels of the peptides. We find that morphine increases Hcrt projections to the ventral tegmental area (VTA), the level of tyrosine hydroxylase enzyme (TH) and the number of TH positive cells in VTA, with no changes in the adjacent substantia nigra. We find that the dual Hcrt receptor antagonist suvorexant prevents morphine-induced changes in the number and size of Hcrt neurons, microglial activation and morphine anticipatory behavior, but does not diminish morphine analgesia. These findings suggest that combined administration of opiates and suvorexant may be a less addictive way of administering opiates for pain relief in humans.

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

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

Sleep and the hypothalamus

Neural substrates of wakefulness, rapid eye movement sleep (REMS), and non-REMS (NREMS) in the mammalian hypothalamus overlap both anatomically and functionally with cellular networks that support physiological and behavioral homeostasis. Here, we review the roles of sleep neurons of the hypothalamus in the homeostatic control of thermoregulation or goal-oriented behaviors during wakefulness. We address how hypothalamic circuits involved in opposing behaviors such as core body temperature and sleep compute conflicting information and provide a coherent vigilance state. Finally, we highlight some of the key unresolved questions and challenges, and the promise of a more granular view of the cellular and molecular diversity underlying the integrative role of the hypothalamus in physiological and behavioral homeostasis.

Utility of the Zebrafish Model for Studying Neuronal and Behavioral Disturbances Induced by Embryonic Exposure to Alcohol, Nicotine, and Cannabis

It is estimated that 5% of pregnant women consume drugs of abuse during pregnancy. Clinical research suggests that intake of drugs during pregnancy, such as alcohol, nicotine and cannabis, disturbs the development of neuronal systems in the offspring, in association with behavioral disturbances early in life and an increased risk of developing drug use disorders. After briefly summarizing evidence in rodents, this review focuses on the zebrafish model and its inherent advantages for studying the effects of embryonic exposure to drugs of abuse on behavioral and neuronal development, with an emphasis on neuropeptides known to promote drug-related behaviors. In addition to stimulating the expression and density of peptide neurons, as in rodents, zebrafish studies demonstrate that embryonic drug exposure has marked effects on the migration, morphology, projections, anatomical location, and peptide co-expression of these neurons. We also describe studies using advanced methodologies that can be applied in vivo in zebrafish: first, to demonstrate a causal relationship between the drug-induced neuronal and behavioral disturbances and second, to discover underlying molecular mechanisms that mediate these effects. The zebrafish model has great potential for providing important information regarding the development of novel and efficacious therapies for ameliorating the effects of early drug exposure.

Activated Wake Systems in Narcolepsy Type 1

OBJECTIVE

Narcolepsy type 1 (NT1) is assumed to be caused solely by a lack of hypocretin (orexin) neurotransmission. Recently, however, we found an 88% reduction in corticotropin-releasing hormone (CRH)-positive neurons in the paraventricular nucleus (PVN). We assessed the remaining CRH neurons in NT1 to determine whether they co-express vasopressin (AVP) to reflect upregulation. We also systematically assessed other wake-systems, since current NT1 treatments target histamine, dopamine, and norepinephrine pathways.

METHODS

In postmortem tissue of people with NT1 and matched controls, we immunohistochemically stained and quantified neuronal populations expressing: CRH and AVP in the PVN, and CRH in the Barrington nucleus; the key neuronal histamine-synthesizing enzyme, histidine decarboxylase (HDC) in the hypothalamic tuberomammillary nucleus (TMN); the rate-limited-synthesizing enzyme, tyrosine hydroxylase (TH), for dopamine in the mid-brain and for norepinephrine in the locus coeruleus (LC).

RESULTS

In NT1, there was: a 234% increase in the percentage of CRH cells co-expressing AVP, while there was an unchanged integrated optical density of CRH staining in the Barrington nucleus; a 36% increased number of histamine neurons expressing HDC, while the number of typical human TMN neuronal profiles was unchanged; a tendency toward an increased density of TH-positive neurons in the substantia nigra compacta; while the density of TH-positive LC neurons was unchanged.

INTERPRETATION

Our findings suggest an upregulation of activity by histamine neurons and remaining CRH neurons in NT1. This may explain earlier reports of normal basal plasma cortisol levels but lower levels after dexamethasone suppression. Alternatively, CRH neurons co-expressing AVP neurons are less vulnerable. ANN NEUROL 2023;94:762-771.

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.

Effects of sodium oxybate on hypocretin/orexin and locus coeruleus neurons

Long-term use of sodium oxybate (SXB), (also called gamma-hydroxybutyrate [GHB]) attenuates the cataplexy and sleepiness of human narcolepsy. We had previously found that chronic opiate usage in humans and long-term opiate administration to mice significantly increased the number of detected hypocretin/orexin (Hcrt) neurons, decreased their size, and increased Hcrt level in the hypothalamus. We also found that opiates significantly decreased cataplexy in human narcoleptics as well as in narcoleptic mice and that cessation of locus coeruleus neuronal activity preceded and was tightly linked to cataplectic attacks in narcoleptic dogs. We tested the hypothesis that SXB produces changes similar to opiates and now report that chronic SXB administration significantly increased the size of Hcrt neurons, the reverse of what we had seen with opiates in humans and mice. Levels of Hcrt in the hypothalamus were nonsignificantly lower, in contrast to the significant increase in hypothalamic Hcrt level after opiates. SXB decreased tyrosine hydroxylase levels in the locus coeruleus, the major descending projection of the hypocretin system, also the reverse of what we saw with opioids. Therefore despite some similar effects on narcoleptic symptomatology, SXB does not produce anatomical changes similar to those elicited by opiates. Analysis of changes in other links in the cataplexy pathway might further illuminate SXB's mechanism of action on narcolepsy.

The therapeutic potential of opioids in narcolepsy type 1: A systematic literature review and questionnaire study

OBJECTIVE

Narcolepsy type 1 is a primary sleep disorder caused by deficient hypocretin transmission leading to excessive daytime sleepiness and cataplexy. Opioids have been suggested to increase the number of hypocretin-producing neurons. We aimed to assess opioid use and its self-reported effect on narcolepsy type 1 symptom severity through a literature review and questionnaire study.

METHODS

We systematically reviewed literature on opioid use in narcolepsy. We also recruited 100 people with narcolepsy type 1 who completed an online questionnaire on opioid use in the previous three years. The main questionnaire topics were the indication for use, and the possible effects on narcolepsy symptom severity. Structured follow-up interviews were conducted when opioid use was reported.

RESULTS

The systematic literature review mainly showed improvements in narcolepsy symptom severity. Recent opioid use was reported by 16/100 questionnaire respondents, who had used 20 opioids (codeine: 7/20, tramadol: 6/20, oxycodone: 6/20, fentanyl: 1/20). Narcolepsy symptom changes were reported in 11/20. Positive effects on disturbed nocturnal sleep (9/20), excessive daytime sleepiness (4/20), hypnagogic hallucinations (3/17), cataplexy (2/18), and sleep paralysis (1/13) were most pronounced for oxycodone (4/6) and codeine (4/7).

CONCLUSIONS

Opioids were relatively frequently used compared to a similarly young general Dutch sample. Oxycodone and, to a lesser extent, codeine were associated with self-reported narcolepsy symptom severity improvements. Positive changes in disturbed nocturnal sleep and daytime sleepiness were most frequently reported, while cataplexy effects were less pronounced. Randomised controlled trials are now needed to verify the potential of opioids as therapeutic agents for narcolepsy.

Sensitivity of Hypocretin System to Chronic Alcohol Exposure: A Human and Animal Study

Human heroin addicts and mice administered morphine for a 2 week period show a greatly increased number of hypothalamic hypocretin (Hcrt or orexin) producing neurons with a concomitant reduction in Hcrt cell size. Male rats addicted to cocaine similarly show an increased number of detectable Hcrt neurons. These findings led us to hypothesize that humans with alcohol use disorder (AUD) would show similar changes. We now report that humans with AUD have a decreased number and size of detectable Hcrt neurons. In addition, the intermingled melanin concentrating hormone (MCH) neurons are reduced in size. We saw no change in the size and number of tuberomammillary histamine neurons in AUD. Within the Hcrt/MCH neuronal field we found that microglia cell size was increased in AUD brains. In contrast, male rats with 2 week alcohol exposure, sufficient to elicit withdrawal symptoms, show no change in the number or size of Hcrt, MCH and histamine neurons, and no change in the size of microglia. The present study indicates major differences between the response of Hcrt neurons to opioids and that to alcohol in human subjects with a history of substance abuse.

A review of physiological functions of orexin: From instinctive responses to subjective cognition

Orexin, also known as hypocretin, is an excitatory neuropeptide secreted by the hypothalamus. Orexin is divided into orexin-A (OXA) and orexin-B (OXB), which are derived from a common precursor secreted by hypothalamic neurons. Orexin acts on orexin receptor-1 (OX1R) and orexin receptor-2 (OX2R). Orexin neurons, as well as receptors, are widely distributed in various regions of the brain as well as in the peripheral system and have a wider range of functions. This paper reviews the latest research results of orexin in the aspects of food intake, sleep, addiction, depression and anxiety. Because orexin has certain physiological functions in many systems, we further explored the possibility of orexin as a new target for the treatment of bulimia, anorexia nervosa, insomnia, lethargy, anxiety and depression. It is precisely because orexin has physiological functions in multiple systems that orexin, as a new target for the treatment of the above diseases, has potential contradictions. For example, it promotes the function of 1 system and may inhibit the function of another system. How to study a new drug, which can not only treat the diseases of this system, but also do not affect other system functions, is what we need to focus on.

Characteristics and outcomes of cases with methylphenidate abuse, dependence or withdrawal: an analysis of spontaneous reports in EudraVigilance

Methylphenidate (MPH) is a central nervous stimulant, which is mainly used in attention deficit hyperactivity disorder (ADHD) and narcolepsy. In recent years, rising MPH prescription volumes have drawn attention to possible misuse. We analyzed data on suspected MPH abuse, dependence or withdrawal reported to the EudraVigilance database (1996-2019), comparing case characteristics (e.g. age, indications and outcome). In 1531 cases from 35 countries (median: 29 years, 57.6% male), 42.3% had been diagnosed with ADHD and 8.0% with narcolepsy. Narcolepsy cases were older than ADHD cases (mean age: 47 vs. 22 years), and used co-medications more frequently (79.4 vs. 47.8%). Intravenous MPH administration was most common among individuals not diagnosed with ADHD or narcolepsy. A history of abuse, dependence or withdrawal of any substance was more often documented in fatal than in nonfatal cases (49.0 vs. 22.5%), whereas differences regarding the route of administration were less pronounced. Minors, who presumably received MPH for ADHD treatment, presented less frequently with serious outcomes than older cases or those without an approved indication. Prescribers should exercise caution in adult MPH users and should make a thorough co-medication assessment. Finally, more research on substance abuse in narcolepsy patients is required, which should include comorbidities and co-medication.

Immortal orexin cell transplants restore motor-arousal synchrony during cataplexy

Waking behaviors such as sitting or standing require suitable levels of muscle tone. But it is unclear how arousal and motor circuits communicate with one another so that appropriate motor tone occurs during wakefulness. Cataplexy is a peculiar condition in which muscle tone is involuntarily lost during normal periods of wakefulness. Cataplexy therefore provides a unique opportunity for identifying the signaling mechanisms that synchronize motor and arousal behaviors. Cataplexy occurs when hypothalamic orexin neurons are lost in narcolepsy; however, it is unclear if motor-arousal decoupling in cataplexy is directly or indirectly caused by orexin cell loss. Here, we used genomic, proteomic, chemogenetic, electrophysiological, and behavioral assays to determine if grafting orexin cells into the brain of cataplectic (i.e., orexin^-/-) mice restores normal motor-arousal behaviors by preventing cataplexy. First, we engineered immortalized orexin cells and found that they not only produce and release orexin but also exhibit a gene profile that mimics native orexin neurons. Second, we show that engineered orexin cells thrive and integrate into host tissue when transplanted into the brain of mice. Next, we found that grafting only 200-300 orexin cells into the dorsal raphe nucleus-a region densely innervated by native orexin neurons-reduces cataplexy. Last, we show that real-time chemogenetic activation of orexin cells restores motor-arousal synchrony by preventing cataplexy. We suggest that orexin signaling is critical for arousal-motor synchrony during wakefulness and that the dorsal raphe plays a pivotal role in coupling arousal and motor behaviors.

The abuse potential of lemborexant, a dual orexin receptor antagonist, according to the 8 factors of the Controlled Substances Act

RATIONALE

Lemborexant (LEM) is a dual orexin receptor antagonist (DORA) approved in multiple countries including the USA, Japan, Canada, Australia, and several Asian countries for the treatment of insomnia in adults. As a compound with central nervous system activity, it is important to understand the abuse potential of LEM with respect to public health.

OBJECTIVES

This review discusses data for LEM relevant to each of the 8 factors of the United States Controlled Substances Act.

RESULTS

LEM did not demonstrate abuse potential in nonclinical testing and was associated with a low incidence of abuse-related adverse events in clinical study participants with insomnia disorder. Similar to other DORAs that have been evaluated (eg., almorexant, suvorexant (SUV), and daridorexant), LEM and the positive controls (zolpidem and SUV) also showed drug liking in a phase 1 abuse potential study that enrolled subjects who used sedatives recreationally. However, internet surveillance of SUV and the FDA Adverse Events Reporting System suggests that drugs in the DORA class display very low abuse-related risks in the community. Additionally, as described in FDA-approved labeling, it does not carry physical dependence and withdrawal risks.

CONCLUSIONS

LEM, similar to most other prescription insomnia medications, was placed into Schedule IV. However, LEM and other drugs in the DORA class may have a lower potential for abuse as suggested by real-world postmarketing data from federal surveys and internet surveillance, and thus may have lower risks to public health than Schedule IV benzodiazepines and nonbenzodiazepine hypnotics that potentiate GABA signaling.

Increased muscle activity during sleep and more RBD symptoms in H1N1-(Pandemrix)-vaccinated narcolepsy type 1 patients compared with their non-narcoleptic siblings

STUDY OBJECTIVES

Narcolepsy type 1 (NT1) is characterized by unstable sleep-wake and muscle tonus regulation during sleep. We characterized dream enactment and muscle activity during sleep in a cohort of post-H1N1 NT1 patients and their siblings, and analyzed whether clinical phenotypic characteristics and major risk factors are associated with increased muscle activity.

METHODS

RBD symptoms and polysomnography m. tibialis anterior electromyographical signals [long (0.5-15 s); short (0.1-0.49 s)] were compared between 114 post-H1N1 NT1 patients and 89 non-narcoleptic siblings. Association sub-analyses with RBD symptoms, narcoleptic symptoms, CSF hypocretin-1 levels, and major risk factors [H1N1-(Pandemrix)-vaccination, HLA-DQB1*06:02-positivity] were performed.

RESULTS

RBD symptoms, REM and NREM long muscle activity indices and REM short muscle activity index were significantly higher in NT1 patients than siblings (all p < 0.001). Patients with undetectable CSF hypocretin-1 levels (<40 pg/ml) had significantly more NREM periodic long muscle activity than patients with low but detectable levels (40-150 pg/ml) (p = 0.047). In siblings, REM and NREM sleep muscle activity indices were not associated with RBD symptoms, other narcolepsy symptoms, or HLA-DQB1*06:02-positivity. H1N1-(Pandemrix)-vaccination status did not predict muscle activity indices in patients or siblings.

CONCLUSION

Increased REM and NREM muscle activity and more RBD symptoms is characteristic of NT1, and muscle activity severity is predicted by hypocretin deficiency severity but not by H1N1-(Pandemrix)-vaccination status. In the patients' non-narcoleptic siblings, neither RBD symptoms, core narcoleptic symptoms, nor the major NT1 risk factors is associated with muscle activity during sleep, hence not indicative of a phenotypic continuum.

Mu-opioid receptor-expressing neurons in the paraventricular thalamus modulate chronic morphine-induced wake alterations

Disrupted sleep is a symptom of many psychiatric disorders, including substance use disorders. Most drugs of abuse, including opioids, disrupt sleep. However, the extent and consequence of opioid-induced sleep disturbance, especially during chronic drug exposure, is understudied. We have previously shown that sleep disturbance alters voluntary morphine intake. Here, we examine the effects of acute and chronic morphine exposure on sleep. Using an oral self-administration paradigm, we show that morphine disrupts sleep, most significantly during the dark cycle in chronic morphine, with a concomitant sustained increase in neural activity in the Paraventricular Nucleus of the Thalamus (PVT). Morphine binds primarily to Mu Opioid Receptors (MORs), which are highly expressed in the PVT. Translating Ribosome Affinity Purification (TRAP)-Sequencing of PVT neurons that express MORs showed significant enrichment of the circadian entrainment pathway. To determine whether MOR + cells in the PVT mediate morphine-induced sleep/wake properties, we inhibited these neurons during the dark cycle while mice were self-administering morphine. This inhibition decreased morphine-induced wakefulness but not general wakefulness, indicating that MORs in the PVT contribute to opioid-specific wake alterations. Overall, our results suggest an important role for PVT neurons that express MORs in mediating morphine-induced sleep disturbance.

Sleep duration, hypnotic drug use, and risk factors: cross- sectional study

Both short sleep duration (SSD) and long sleep duration (LSD) are associated with an increased risk of morbidity and mortality. Here, we aimed to assess the prevalence of sleep duration disturbances among adults in association with demographic, medication use, personal habits, and chronic diseases, while also considering the impact of hypnotic drug use. We performed a cross-sectional study of 9991 adult participants of the Rafsanjan Cohort Study (RCS), as part of the Prospective epidemiological research studies in Iran (PERSIAN). Multivariate logistic regression analyses were conducted to assess the association between short (< 6 h) and long (> 9 h) sleep duration with demographic and lifestyle parameters and common non-communicable diseases. Additionally, we performed stratified analysis to investigate the association of sleep duration with the abovementioned factors and diseases, in groups with and without hypnotic drug use. We found higher odds of SSD significantly associated with age (P < 0.001), BMI (P < 0.001), physical activity (P < 0.001), and depression (P = 0.023). LSD displayed a positive association with the female sex (P < 0.001), opium consumption (P < 0.001), and history of MI (P = 0.045), and a reverse connection with education (P = 0.007), physical activity (P < 0.001) and alcohol consumption (P = 0.027). Stratifying for the hypnotic drug use, our sensitivity analyses indicated that in hypnotic drug users, education (P = 0.034) and physical activity (P < 0.001) were associated with LSD, in this group, significantly increased odds ratio of LSD were associated with opium consumption (P = 0.046) and thyroid dysfunction (P = 0.037). Our findings demonstrated the demographic and lifestyle factors and diseases associated with long and short sleep duration in the population of the RCS. Additionally, after stratifying for hypnotic drug use, our results indicated that some diseases are only associated with abnormal sleep duration upon using hypnotic drugs.

Targeting Orexin Receptors for the Treatment of Insomnia: From Physiological Mechanisms to Current Clinical Evidence and Recommendations

After a detailed description of orexins and their roles in sleep and other medical disorders, we discuss here the current clinical evidence on the effects of dual (DORAs) or selective (SORAs) orexin receptor antagonists on insomnia with the aim to provide recommendations for their further assessment in a context of personalized and precision medicine. In the last decade, many trials have been conducted with orexin receptor antagonists, which represent an innovative and valid therapeutic option based on the multiple mechanisms of action of orexins on different biological circuits, both centrally and peripherally, and their role in a wide range of medical conditions which are often associated with insomnia. A very interesting aspect of this new category of drugs is that they have limited abuse liability and their discontinuation does not seem associated with significant rebound effects. Further studies on the efficacy of DORAs are required, especially on children and adolescents and in particular conditions, such as menopause. Which DORA is most suitable for each patient, based on comorbidities and/or concomitant treatments, should be the focus of further careful research. On the contrary, studies on SORAs, some of which seem to be appropriate also in insomnia in patients with psychiatric diseases, are still at an early stage and, therefore, do not allow to draw definite conclusions.

Suvorexant, an FDA-approved dual orexin receptor antagonist, reduces oxycodone self-administration and conditioned reinstatement in male and female rats

Background: The Department of Health and Human Services reports that prescription pain reliever (e.g., oxycodone) misuse was initiated by 4,400 Americans each day in 2019. Amid the opioid crisis, effective strategies to prevent and treat prescription opioid use disorder (OUD) are pressing. In preclinical models, the orexin system is recruited by drugs of abuse, and blockade of orexin receptors (OX receptors) prevents drug-seeking behavior. The present study sought to determine whether repurposing suvorexant (SUV), a dual OX receptor antagonist marketed for the treatment of insomnia, can treat two features of prescription OUD: exaggerated consumption and relapse. Methods: Male and female Wistar rats were trained to self-administer oxycodone (0.15 mg/kg, i. v., 8 h/day) in the presence of a contextual/discriminative stimulus (S^D) and the ability of SUV (0-20 mg/kg, p. o.) to decrease oxycodone self-administration was tested. After self-administration testing, the rats underwent extinction training, after which we tested the ability of SUV (0 and 20 mg/kg, p. o.) to prevent reinstatement of oxycodone seeking elicited by the S^D. Results: The rats acquired oxycodone self-administration and intake was correlated with the signs of physical opioid withdrawal. Additionally, females self-administered approximately twice as much oxycodone as males. Although SUV had no overall effect on oxycodone self-administration, scrutiny of the 8-h time-course revealed that 20 mg/kg SUV decreased oxycodone self-administration during the first hour in males and females. The oxycodone S^D elicited strong reinstatement of oxycodone-seeking behavior that was significantly more robust in females. Suvorexant blocked oxycodone seeking in males and reduced it in females. Conclusions: These results support the targeting of OX receptors for the treatment for prescription OUD and repurposing SUV as pharmacotherapy for OUD.

Repeated measures of hypocretin-1 in Danish and Italian patients with narcolepsy and in controls

STUDY OBJECTIVES

The assay currently used worldwide to measure cerebrospinal fluid hypocretin-1 (CSF-hcrt-1) for diagnosing narcolepsy uses a competitive radioimmunoassay with polyclonal anti-hcrt-1 antibodies. This assay detects multiple hypocretin-1 immunoreactive species in the CSF that are all derived from full-length hcrt-1. We aimed to revalidate CSF-hcrt-1 cut-offs for narcolepsy type 1 (NT1) diagnosis and to evaluate temporal changes in CSF-hcrt-1 levels in patients suspected of having central hypersomnia.

METHOD

We carried out a repeat lumbar puncture with a mean follow-up of 4.0 years, to measure CSF-hcrt-1 in patients suspected of having central hypersomnia in a follow-up study. Data from CSF samples of patients with NT1 and of controls without known hypersomnia, from the Italian-Stanford and Danish populations, were examined using a receiver-operating characteristic analysis.

RESULTS

The optimal CSF-hcrt-1 cut-offs for identifying NT1 were 129 pg/ml and 179 pg/ml for the Italian-Stanford and Danish populations, respectively. The sensitivity was 0.93-0.99 and the specificity was 1. Follow-up lumbar puncture measurements of CSF-hcrt-1 were obtained from 73 patients. 30 of 32 patients with low CSF-hcrt-1 levels continued to be categorized as low, with an unaltered diagnosis; two patients showed a marked increase in CSF-hcrt-1, attaining normal values at follow-up. One of these patients relapsed to low CSF-hcrt-1 after follow-up. All 41 patients with normal CSF-hcrt-1 at baseline had normal CSF-hcrt-1 at follow-up.

CONCLUSION

CSF-hcrt-1 measurement can provide an accurate test for diagnosing NT1, although it is important to validate the CSF-hcrt-1 cut-off for specific testing locations. Stable CSF-hcrt-1 levels support the already established prognosis of narcolepsy as permanent once the disorder has fully developed.

Physiological Role of Orexin/Hypocretin in the Human Body in Motivated Behavior: A Comprehensive Review

Neurohormones are neurosecretory materials released by neurosecretory cells that serve both as neuromodulators in the brain and spinal cord and as circulating regulatory hormones. They serve a wide range of functions, including homeostasis, development, and modulation of neuronal and muscle activity. In the hypothalamus, neurohormones called hypocretins are created that were discovered in the late nineties. Orexin receptors (OXRs) have been shown to enhance synaptic signaling in the central nervous system at the cellular level. The orexins improve stimulated neural activity in the hippocampus, which, in turn, aids with spatial memory, learning, and mood. They present themselves as mediators for the hypothalamic functions. They have been shown to regulate sleep-wake cycles, arousal mechanisms, addiction, sympathetic nerve activity (SNA), blood pressure, and thermogenesis. Its role in storing brown adipose tissue has implications for thermal homeostasis. The significant role of orexins is seen in tumorigenesis when orexin A (OrxA) and orexin B (OrxB) induce apoptosis in fast-growing tumor cells. Orexin-null subjects show clinical narcolepsy, indicating that orexins were responsible for keeping them awake. Orexin microinjections in mice brains stimulated increased physical activity, thus possibly countering diet-induced obesity. Physical activity significantly increased plasma orexin-A levels, which facilitated the process of energy homeostasis. The amount of adrenocorticotropic hormone (ACTH) increases in stress conditions, which further facilitates the release of the stress hormone cortisol. No increase in the ACTH hormone is seen in stressed mice administered with orexin receptor 2 (OX2R) antagonists thus showing orexin's role in stress reaction. As a result of linking hypocretin/orexin to various physiological procedures, increased research into the medicinal potential of drugs targeting these receptors is emerging. We summed up in this review the recent advances in our understanding of how orexin and its receptor system play an essential role in clinical and pathological functions. This research summarizes a new area for research in human medicine, providing the possibility of controlling a vast array of physiological functions through intra-cerebroventricular injections of a single neuropeptide.

Habitual behaviour associated with exposure to high-calorie diet is prevented by an orexin-receptor-1 antagonist

Habitual actions, which are associated with addictive behaviours, contribute to the loss of control of food seeking seen following exposure to calorie-dense foods in rats. Antagonism of orexin-receptor-1 (ORX-R1) has been shown to reduce a range of stimulus-driven feeding behaviours, but have yet to be implicated in the regulation of habitual actions. In the current study, male Long-Evans rats were given 'binge-like' access to high-calorie diet (HCD) or standard chow diet, and were subsequently trained to press a lever for food outcome. When lever responses were tested following outcome devaluation, chow-fed rats displayed goal-directed actions, whereas HCD-exposed rats displayed habitual actions. In study 1, it was shown that systemic administration of the ORX-R1 antagonist, SB-334867, prior to test restored goal-directed behaviour in HCD-exposed rats. In study 2, intra-nigral administration of SB-334867 similarly restored goal-directed behaviour, thereby implicating the substantia nigra as an important site for this effect. This study demonstrates that targeting ORX-R1 reduces habitual food seeking in male rats which may be important for understanding and treating compulsive feeding, obesity and binge eating disorder. This study also implicates the lateral hypothalamus, where ORX is produced, in mediating the expression of habits for the first time, and thus extends on the neurocircuits known to regulate habitual actions. Further investigation is required to determine whether the same effects are also seen in female rats, given that there are recognised sexual dimorphisms in feeding behaviour and a higher incidence of disordered eating in female than male populations.

Sleep disturbance as a therapeutic target to improve opioid use disorder treatment

Sleep health is an important factor across several physical and mental health disorders, and a growing scientific consensus has identified sleep as a critical component of opioid use disorder (OUD), both in the active disease state and during OUD recovery. The goal of this narrative review is to collate the literature on sleep, opioid use, and OUD as a means of identifying therapeutic targets to improve OUD treatment outcomes. Sleep disturbance is common and often severe in persons with OUD, especially during opioid withdrawal, but also in persons on opioid maintenance therapies. There is ample evidence that sleep disturbances including reduced total sleep time, disrupted sleep continuity, and poor sleep quality often accompany negative OUD treatment outcomes. Sleep disturbances are bidirectionally associated with several other factors related to negative treatment outcomes, including chronic stress, stress reactivity, low positive affect, high negative affect, chronic pain, and drug craving. This constellation of outcome variables represents a more comprehensive appraisal of the quality of life and quality of recovery than is typically assessed in OUD clinical trials. To date, there are very few clinical trials or experimental studies aimed at improving sleep health in OUD patients, either as a means of improving stress, affect, and craving outcomes, or as a potential mechanistic target to reduce opioid withdrawal and drug use behaviors. As such, the direct impact of sleep improvement in OUD patients is largely unknown, yet mechanistic and clinical research suggests that therapeutic interventions that target sleep are a promising avenue to improve OUD treatment. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

Orexin Reserve: A Mechanistic Framework for the Role of Orexins (Hypocretins) in Addiction

In 2014, we proposed that orexin signaling transformed motivationally relevant states into adaptive behavior directed toward exploiting an opportunity or managing a threat, a process we referred to as motivational activation. Advancements in animal models since then have permitted higher-resolution measurements of motivational states; in particular, the behavioral economics approach for studying drug demand characterizes conditions that lead to the enhanced motivation that underlies addiction. This motivational plasticity is paralleled by persistently increased orexin expression in a topographically specific manner-a finding confirmed across species, including in humans. Normalization of orexin levels also reduces drug motivation in addiction models. These new advancements lead us to update our proposed framework for the orexin function. We now propose that the capacity of orexin neurons to exhibit dynamic shifts in peptide production contributes to their role in adaptive motivational regulation and that this is achieved via a pool of reserve orexin neurons. This reserve is normally bidirectionally recruited to permit motivational plasticity that promotes flexible, adaptive behavior. In pathological states such as addiction, however, we propose that the orexin system loses capacity to adaptively adjust peptide production, resulting in focused hypermotivation for drug, driven by aberrantly and persistently high expression in the orexin reserve pool. This mechanistic framework has implications for the understanding and treatment of several psychiatric disorders beyond addiction, particularly those characterized by motivational dysfunction.

Utility of 'substance use disorder' as a heuristic for understanding overeating and obesity

Rates of obesity and obesity-associated diseases have increased dramatically in countries with developed economies. Substance use disorders (SUDs) are characterized by the persistent use of the substance despite negative consequences. It has been hypothesized that overconsumption of palatable energy dense food can elicit SUD-like maladaptive behaviors that contribute to persistent caloric intake beyond homeostatic need even in the face of negative consequences. Palatable food and drugs of abuse act on many of the same motivation-related circuits in the brain, and can induce, at least superficially, similar molecular, cellular, and physiological adaptations on these circuits. As such, applying knowledge about the neurobiological mechanisms of SUDs may serve as useful heuristic to better understand the persistent overconsumption of palatable food that contributes to obesity. However, many important differences exist between the actions of drugs of abuse and palatable food in the brain. This warrants caution when attributing weight gain and obesity to the manifestation of a putative SUD-related behavioral disorder. Here, we describe similarities and differences between compulsive drug use in SUDs and overconsumption in obesity and consider the merit of the concept of "food addiction".

Sleep loss and addiction

Reducing sleep hours is a risk factor for developing cardiovascular, metabolic, and psychiatric disorders. Furthermore, previous studies have shown that reduction in sleep time is a factor that favors relapse in addicted patients. Additionally, animal models have demonstrated that both sleep restriction and sleep deprivation increase the preference for alcohol, methylphenidate, and the self-administration of cocaine. Therefore, the present review discusses current knowledge about the influence of sleep hours reduction on addictivebehaviors; likewise, we discuss the neuronal basis underlying the sleep reduction-addiction relationship, like the role of the orexin and dopaminergic system and neuronal plasticity (i.e., delta FosB expression). Potentially, chronic sleep restriction could increase brain vulnerability and promote addictive behavior.

A retinal contribution to opioid-induced sleep disorders?

Chronic opioid use is linked to persistent and severe sleep/wake disturbances in patients. These opioid-related sleep problems increase risk for developing opioid dependence, mood disorders and in turn overdose in chronic pain patients receiving opioid therapy. Despite the well-established link between long-term opioid use and sleep disorders, the mechanism by which opioids perturb sleep remains unclear. Interestingly, animal studies indicate that opioids disrupt sleep/wake behaviors by altering an animal’s ability to synchronize their circadian rhythms to environmental light cycles (i.e., photoentrainment). A specific subset of retinal cells known as intrinsically photosensitive retinal ganglion cells (ipRGCs) that express μ-opioid receptors are exclusively responsible for transmitting environmental light information to sleep/circadian centers in the brain. Thus, this review will focus on the effect of opioids on ipRGCs and their projection regions that are involved in the photoentrainment of sleep/wake behaviors. Lastly, we discuss the viability of ipRGCs as a potential therapeutic target for treating opioid-related sleep/wake problems.

Suvorexant ameliorated sleep disturbance, opioid withdrawal, and craving during a buprenorphine taper

Increased orexin/hypocretin signaling is implicated in opioid withdrawal, sleep disturbances, and drug-seeking behaviors. This study examined whether a dual-orexin receptor antagonist would improve sleep and withdrawal outcomes when compared with placebo during a buprenorphine/naloxone taper. Thirty-eight participants with opioid use disorder were recruited to a clinical research unit and maintained on 8/2 to 16/4 mg of buprenorphine/naloxone treatment for 3 days before being randomized to 20 mg of suvorexant (n = 14), 40 mg of suvorexant (n = 12), or placebo (n = 12); 26 individuals completed the study. After randomization, participants underwent a 4-day buprenorphine/naloxone taper and 4-day post-taper observation period. Total sleep time (TST) was collected nightly with a wireless electroencephalography device and wrist-worn actigraphy; opioid withdrawal symptoms were assessed via the Subjective Opiate Withdrawal Scale (SOWS); and abuse potential was assessed on a 0- to 100-point visual analog scale of "High" every morning. A priori outcomes included two-group (collapsing suvorexant doses versus placebo) and three-group comparisons of area-under-the-curve (AUC) scores for TST, SOWS, and High. In two-group comparisons, participants receiving suvorexant displayed increased TST during the buprenorphine/naloxone taper and decreased SOWS during the post-taper period. In three-group comparisons, participants receiving 20 mg of suvorexant versus placebo displayed increased AUC for TST during the buprenorphine/naloxone taper, but there was no difference in SOWS among groups. There was no evidence of abuse potential in two- or three-group analyses. The results suggest that suvorexant might be a promising treatment for sleep and opioid withdrawal in individuals undergoing a buprenorphine/naloxone taper.

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

Orexin peptides comprise two neuropeptides, orexin A and orexin B, that bind two G-protein coupled receptors (GPCRs), orexin receptor 1 (OXR1) and orexin receptor 2 (OXR2). Although cell bodies that produce orexin peptides are localized in a small area comprising the lateral hypothalamus and adjacent regions, orexin-containing fibres project throughout the neuraxis. Although orexins were initially described as peptides that regulate feeding behaviour, research has shown that orexins are involved in diverse functions that range from the modulation of autonomic functions to higher cognitive functions, including reward-seeking, behaviour, attention, cognition, and mood. Furthermore, disruption in orexin signalling has been shown in mood disorders that are associated with low hedonic tone or anhedonia, including depression, anxiety, attention deficit hyperactivity disorder, and addiction. Notably, projections of orexin neurons overlap circuits involved in the modulation of hedonic tone. Evidence shows that orexins may potentiate hedonic behaviours by increasing the feeling of pleasure or reward to various signalling, whereas dysregulation of orexin signalling may underlie low hedonic tone or anhedonia. Further, orexin appears to play a key role in regulating behaviours in motivationally charged situations, such as food-seeking during hunger, or drug-seeking during withdrawal. Therefore, it would be expected that dysregulation of orexin expression or signalling is associated with changes in hedonic tone. Further studies investigating this association are warranted.

Understanding the Role of Orexin Neuropeptides in Drug Addiction: Preclinical Studies and Translational Value

Orexins (also known as hypocretins) are neuropeptides that participate in the regulation of energy metabolism, homeostasis, sleep, feeding, stress responses, arousal, and reward. Particularly relevant to the scope of the present review is the involvement of the orexin system in brain mechanisms that regulate motivation, especially highly motivated behavior, arousal, and stress, making it an ideal target for studying addiction and discovering treatments. Drug abuse and misuse are thought to induce maladaptive changes in the orexin system, and these changes might promote and maintain uncontrolled drug intake and contribute to relapse. Dysfunctional changes in this neuropeptidergic system that are caused by drug use might also be responsible for alterations of feeding behavior and the sleep-wake cycle that are commonly disrupted in subjects with substance use disorder. Drug addiction has often been associated with an increase in activity of the orexin system, suggesting that orexin receptor antagonists may be a promising pharmacological treatment for substance use disorder. Substantial evidence has shown that single orexin receptor antagonists that are specific to either orexin receptor 1 or 2 can be beneficial against drug intake and relapse. Interest in the efficacy of dual orexin receptor antagonists, which were primarily developed to treat insomnia, has grown in the field of drug addiction. Treatments that target the orexin system may be a promising strategy to reduce drug intake, mitigate relapse vulnerability, and restore “normal” physiological functions, including feeding and sleep. The present review discusses preclinical and clinical evidence of the involvement of orexins in drug addiction and possible beneficial pharmacotherapeutic effects of orexin receptor antagonists to treat substance use disorder.

Hypocretin/Orexin Interactions with Norepinephrine Contribute to the Opiate Withdrawal Syndrome

We previously found that human heroin addicts and mice chronically exposed to morphine exhibit a significant increase in the number of detected hypocretin/orexin (Hcrt)-producing neurons. However, it remains unknown how this increase affects target areas of the hypocretin system involved in opioid withdrawal, including norepinephrine containing structures locus coeruleus (LC) and A1/A2 medullary regions. Using a combination of immunohistochemical, biochemical, imaging, and behavioral techniques, we now show that the increase in detected hypocretin cell number translates into a significant increase in hypocretin innervation and tyrosine hydroxylase (TH) levels in the LC without affecting norepinephrine-containing neuronal cell number. We show that the increase in TH is completely dependent on Hcrt innervation. The A1/A2 regions were unaffected by morphine treatment. Manipulation of the Hcrt system may affect opioid addiction and withdrawal.SIGNIFICANCE STATEMENT Previously, we have shown that the hypothalamic hypocretin system undergoes profound anatomic changes in human heroin addicts and in mice exposed to morphine, suggesting a role of this system in the development of addictive behaviors. The locus coeruleus plays a key role in opioid addiction. Here we report that the hypothalamic hypocretin innervation of the locus coeruleus increases dramatically with morphine administration to mice. This increase is correlated with a massive increase in tyrosine hydroxylase expression in locus coeruleus. Elimination of hypocretin neurons prevents the tyrosine hydroxylase increase in locus coeruleus and dampens the somatic and affective components of opioid withdrawal.

Reduced numbers of corticotropin‐releasing hormone neurons in narcolepsy type 1

Narcolepsy type 1 (NT1) is a chronic sleep disorder correlated with loss of hypocretin(orexin). In NT1 post‐mortem brains, we observed 88% reduction in corticotropin‐releasing hormone (CRH)‐positive neurons in the paraventricular nucleus (PVN) and significantly less CRH‐positive fibers in the median eminence, whereas CRH‐neurons in the locus coeruleus and thalamus, and other PVN neuronal populations were spared: that is, vasopressin, oxytocin, tyrosine hydroxylase, and thyrotropin releasing hormone‐expressing neurons. Other hypothalamic cell groups, that is, the suprachiasmatic, ventrolateral preoptic, infundibular, and supraoptic nuclei and nucleus basalis of Meynert, were unaffected. The surprising selective decrease in CRH‐neurons provide novel targets for diagnostics and therapeutic interventions. ANN NEUROL 2022;91:282–288

New directions in modelling dysregulated reward seeking for food and drugs

Behavioral models are central to behavioral neuroscience. To study the neural mechanisms of maladaptive behaviors (including binge eating and drug addiction), it is essential to develop and utilize appropriate animal models that specifically focus on dysregulated reward seeking. Both food and cocaine are typically consumed in a regulated manner by rodents, motivated by reward and homeostatic mechanisms. However, both food and cocaine seeking can become dysregulated, resulting in binge-like consumption and compulsive patterns of intake. The speakers in this symposium for the 2021 International Behavioral Neuroscience Meeting utilize behavioral models of dysregulated reward-seeking to investigate the neural mechanisms of binge-like consumption, enhanced cue-driven reward seeking, excessive motivation, and continued use despite negative consequences. In this review, we outline examples of maladaptive patterns of intake and explore recent animal models that drive behavior to become dysregulated, including stress exposure and intermittent access to rewards. Lastly, we explore select behavioral and neural mechanisms underlying dysregulated reward-seeking for both food and drugs.

Reduction of Orexin-A Is Associated With Anxiety and the Level of Depression of Male Methamphetamine Users During the Initial Withdrawal Period

BACKGROUND

Orexin has been linked to the regulation of reward and motivation in animals, but there have been few human studies to validate its regulatory effects. We aimed to determine how orexin-A levels changed during different stages of withdrawal, as well as the relationship between orexin-A levels and withdrawal symptoms in male METH users.

METHODS

This study included 76 METH users and 35 control participants. The METH users were divided into three groups: group 1 (abstinence within 1 week, n = 23), group 2 (abstinence between 1 week and 3 months, n = 38), and group 3 (abstinence over 3 months, n = 15). At baseline and 1 month of enrollment, the plasma orexin-A level was examined. To track the withdrawal symptoms, self-report questionnaires (anxiety, depression, craving, and sleep quality) were collected at two points.

RESULTS

The orexin-A levels of groups 1 (p < 0.001) and 2 (p < 0.001) were lower than that of the controls at baseline but not group 3. One month later, the orexin-A levels of group 2 (p < 0.05) significantly increased, while no significant changes in those of groups 1 and 3 were observed. Furthermore, the orexin-A levels of group 1 were positively linked with depression (p < 0.01) and anxiety (p < 0.01) at baseline.

CONCLUSIONS

The decrease in orexin-A levels was only transitory during the initial abstinence phase, and it was eventually restored near to normal with continued abstinence among the male METH users. Furthermore, a lower concentration of orexin-A may serve as a risk factor for negative emotions during METH withdrawal.

Potential therapeutic targets for the treatment of opioid abuse and pain

Although μ-opioid peptide (MOP) receptor agonists are effective analgesics available in clinical settings, their serious adverse effects put limits on their use. The marked increase in abuse and misuse of prescription opioids for pain relief and opioid overdose mortality in the past decade has seriously impacted society. Therefore, safe analgesics that produce potent analgesic effects without causing MOP receptor-related adverse effects are needed. This review highlights the potential therapeutic targets for the treatment of opioid abuse and pain based on available evidence generated through preclinical studies and clinical trials. To ameliorate the abuse-related effects of opioids, orexin-1 receptor antagonists and mixed nociceptin/MOP partial agonists have shown promising results in translational aspects of animal models. There are several promising non-opioid targets for selectively inhibiting pain-related responses, including nerve growth factor inhibitors, voltage-gated sodium channel inhibitors, and cannabinoid- and nociceptin-related ligands. We have also discussed several emerging and novel targets. The current medications for opioid abuse are opioid receptor-based ligands. Although neurobiological studies in rodents have discovered several non-opioid targets, there is a translational gap between rodents and primates. Given that the neuroanatomical aspects underlying opioid abuse and pain are different between rodents and primates, it is pivotal to investigate the functional profiles of these non-opioid compounds compared to those of clinically used drugs in non-human primate models before initiating clinical trials. More pharmacological studies of the functional efficacy, selectivity, and tolerability of these newly discovered compounds in non-human primates will accelerate the development of effective medications for opioid abuse and pain.

Sleep dysregulation in binge eating disorder and "food addiction": the orexin (hypocretin) system as a potential neurobiological link

It has been proposed that binge eating reflects a pathological compulsion driven by the "addictive" properties of foods. Proponents of this argument highlight the large degree of phenomenological and diagnostic overlap between binge eating disorder (BED) and substance use disorders (SUDs), including loss of control over how much is consumed and repeated unsuccessful attempts to abstain from consumption, as well as commonalities in brain structures involved in food and drug craving. To date, very little attention has been given to an additional behavioral symptom that BED shares with SUDs-sleep dysregulation-and the extent to which this may contribute to the pathophysiology of BED. Here, we review studies examining sleep outcomes in patients with BED, which collectively point to a heightened incidence of sleep abnormalities in BED. We identify the orexin (hypocretin) system as a potential neurobiological link between compulsive eating and sleep dysregulation in BED, and provide a comprehensive update on the evidence linking this system to these processes. Finally, drawing on evidence from the SUD literature indicating that the orexin system exhibits significant plasticity in response to drugs of abuse, we hypothesize that chronic palatable food consumption likewise increases orexin system activity, resulting in dysregulated sleep/wake patterns. Poor sleep, in turn, is predicted to exacerbate binge eating, contributing to a cycle of uncontrolled food consumption. By extension, we suggest that pharmacotherapies normalizing orexin signaling, which are currently being trialed for the treatment of SUDs, might also have utility in the clinical management of BED.

Orexin (hypocretin) and addiction

Although originally implicated in appetite and sleep/wakefulness, the hypothalamic orexin (hypocretin) system has now been demonstrably linked with motivated behavior. This highly plastic system responds to reward-associated environmental stimuli and becomes pathologically overactive in addicted states. Here, we provide a brief overview of the roles of the orexin system in reward-seeking and addiction, as well as potential therapeutic opportunities for substance use disorders based on normalizing orexin function.

Nuclear organization of orexinergic neurons in the hypothalamus of a lar gibbon and a chimpanzee

Employing orexin-A immunohistochemical staining we describe the nuclear parcellation of orexinergic neurons in the hypothalami of a lar gibbon and a chimpanzee. The clustering of orexinergic neurons within the hypothalamus and the terminal networks follow the patterns generally observed in other mammals, including laboratory rodents, strepsirrhine primates and humans. The orexinergic neurons were found within three distinct clusters in the ape hypothalamus, which include the main cluster, zona incerta cluster and optic tract cluster. In addition, the orexinergic neurons of the optic tract cluster appear to extend to a more rostral and medial location than observed in other species, being observed in the tuberal region in the anterior ventromedial aspect of the hypothalamus. While orexinergic terminal networks were observed throughout the brain, high density terminal networks were observed within the hypothalamus, medial and intralaminar nuclei of the dorsal thalamus, and within the serotonergic and noradrenergic regions of the midbrain and pons, which is typical for mammals. The expanded distribution of orexinergic neurons into the tuberal region of the ape hypothalamus, is a feature that needs to be investigated in other primate species, but appears to correlate with orexin gene expression in the same region of the human hypothalamus, but these neurons are not revealed with immunohistochemical staining in humans. Thus, it appears that apes have a broader distribution of orexinergic neurons compared to other primate species, but that the neurons within this extension of the optic tract cluster in humans, while expressing the orexin gene, do not produce the neuropeptide.

Sexually dimorphic and asymmetric effects of embryonic ethanol exposure on hypocretin/orexin neurons as related to behavioral changes in zebrafish

Neurons expressing the neuropeptide hypocretin/orexin (Hcrt) in the hypothalamus promote reward-related behaviors including alcohol consumption and are shown in rodents and zebrafish to be stimulated by embryonic exposure to ethanol (EtOH). We used here in zebrafish three-dimensional analyses of the entire population of Hcrt neurons to examine how embryonic EtOH exposure at low-moderate concentrations (0.1% or 0.5% v/v) alters these neurons in relation to behavior. We found that EtOH in the water for 2 h (22-24 h post fertilization) increases the number of Hcrt neurons on the left but not right side of the brain through a stimulation of cell proliferation, this is accompanied by a decrease in locomotor activity under novel conditions but not after habituation, and these effects are evident in both larvae and adults indicating they are long lasting. Our analyses in adults revealed sexually dimorphic effects, with females consuming more EtOH-gelatin and exhibiting more freezing behavior along with an asymmetric increase in Hcrt neurons and males exhibiting increased aggression with no change in Hcrt. These findings suggest that a long lasting, asymmetric increase in Hcrt neurons induced by EtOH results from an asymmetric increase in proliferation specific to Hcrt and contributes to behavioral changes in females.

Cerebrospinal fluid orexin in Alzheimer's disease: a systematic review and meta-analysis

OBJECTIVE/BACKGROUND

A growing body of evidence suggests that sleep and Alzheimer's disease (AD) have a bi-directional relationship. Emerging research also suggests that orexin, a key neurotransmitter involved in sleep-wake regulation, may be altered in persons with AD, however results have not been consistent across prior studies. This investigation was conducted to both evaluate the aggregate literature to minimize the risk of bias and identify potential factors associated with heterogeneity across studies.

METHODS

Systematic review identified relevant investigations that compared cerebrospinal fluid orexin in persons with AD and controls. Meta-analysis (random effects model) compared effect size (Hedge's g) for orexin between AD and controls. Meta-regression was additionally performed for key variables of interest to evaluate potential causes of heterogeneity among studies.

RESULTS

17 studies were identified that met inclusion/exclusion criteria. Evidence of publication bias was not identified. Non-significant increases in orexin were observed in AD relative to controls, with moderate to large heterogeneity among studies (Hedge's g = 0.20, p = 0.136, I² = 72.6%). Meta-regression demonstrated both year of publication (β = 0.055, p = 0.020) and effect size for phosphorylated tau in AD versus controls (β = 0.417, p = 0.031) were associated with differences in orexin.

CONCLUSIONS

Results do not support broad differences in orexin in AD compared to controls, however, evolving diagnostic criteria may have affected findings across studies. Future research that examines orexin in AD over the longitudinal course of the disorder and explores potential links between phosphorylated tau and orexin are indicated.

Glucagon-like peptide-1 receptor agonist, exendin-4, reduces reinstatement of heroin-seeking behavior in rats

Opioid use disorder (OUD) causes the death of nearly 130 Americans daily. It is evident that new avenues for treatment are needed. To this end, studies have reported that 'satiety' agents such as the glucagon-like peptide-1 receptor (GLP-1R) agonist, exendin-4 (Ex-4), decreases responding for addictive drugs such as cocaine, nicotine, alcohol, and oxycodone, but no work has been done with heroin. In this study, we used a reward devaluation model in which rats avoid ingesting a saccharin solution that predicts drug availability to test the effects of 2.4 μg/kg Ex-4 on responding for a natural reward cue (i.e., saccharin) and on cue- and drug-induced heroin seeking. The results showed that treatment with Ex-4 during the 16-day abstinence period and on the test day decreased cue-induced heroin seeking. Drug-induced heroin seeking also was reduced by Ex-4, but only when using a 1 h, but not a 6 h, pretreatment time. Treatment with Ex-4 did not alter intake of the saccharin cue when the drug was on board, but a history of treatment with Ex-4 increased acceptance of the saccharin cue in later extinction trials. Finally, treatment with Ex-4 did not alter body weight, but was associated with increased Orexin 1 receptor (OX1) mRNA expression in the nucleus accumbens shell. Taken together, these findings are the first to show that treatment with a GLP-1R agonist can reduce both cue-induced seeking and drug-induced reinstatement of heroin seeking. As such, a GLP-1R agonist may serve as an effective treatment for OUD in humans.

The Insomnia-Addiction Positive Feedback Loop: Role of the Orexin System

Significant sleep impairments often accompany substance use disorders (SUDs). Sleep disturbances in SUD patients are associated with poor clinical outcomes and treatment adherence, emphasizing the importance of normalizing sleep when treating SUDs. Orexins (hypocretins) are neuropeptides exclusively produced by neurons in the posterior hypothalamus that regulate various behavioral and physiological processes, including sleep-wakefulness and motivated drug taking. Given its dual role in sleep and addiction, the orexin system represents a promising therapeutic target for treating SUDs and their comorbid sleep deficits. Here, we review the literature on the role of the orexin system in sleep and drug addiction and discuss the therapeutic potential of orexin receptor antagonists for SUDs. We argue that orexin receptor antagonists may be effective therapeutics for treating addiction because they target orexin's regulation of sleep (top-down) and motivation (bottom-up) pathways.