The dual orexin receptor antagonist almorexant, alone and in combination with morphine, cocaine and amphetamine, on conditioned place preference and locomotor sensitization in the rat

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.

Factors associated with hypnotics polypharmacy in the Japanese population

OBJECTIVE

Insomnia disorder is a global public health issue, commonly treated with hypnotics. However, long-term use of benzodiazepine derivatives (BZDs), especially polypharmacy with this kind of drug, carries risks for dependence and abuse. This study using large-scale medical insurance records investigated the causes of polypharmacy through the treatment of insomnia disorder.

METHODS

A cross-sectional study analyzed anonymized medical record data from July 2014 to March 2018 provided by a nationwide Japanese health insurance association covering 405,952 individuals. Outpatients prescribed at least one sleep medication were included. Demographic data, pharmacological classification of the drugs, and comorbidities were assessed using hierarchical logistic regression analysis to explore their associations with polypharmacy.

RESULTS

Of the 33,212 outpatients who were prescribed sleep medications, 32.5 % were prescribed multiple types. After adjusting for demographics and type of sleep medications as covariates, hypnotic polypharmacy was significantly associated with younger age, the presence of certain kinds of comorbidities, and using BZD anxiolytics before bedtime with the highest adjusted odds ratios (8.01-9.39) when referenced with BZD hypnotics. On the other hand, usage of orexin receptor antagonists, melatonin receptor agonists, and Z-drugs indicated lower odds ratios (0.74-0.87).

CONCLUSIONS

Hypnotic polypharmacy is relatively common in the Japanese general population. With the introduction of non-pharmacological therapy in mind, assessing patients' comorbidities and avoiding the use of benzodiazepines, especially BZD anxiolytics, before bedtime would be recommended to prevent polypharmacy.

The orexin story and orexin receptor antagonists for the treatment of insomnia

Insomnia is present in up to one third of the adult population worldwide, and it can present independently or with other medical conditions such as mental, metabolic, or cardiovascular diseases, which highlights the importance of treating this multifaceted disorder. Insomnia is associated with an abnormal state of hyperarousal (increased somatic, cognitive, and cortical activation) and orexin has been identified as a key promotor of arousal and vigilance. The current standards of care for the treatment of insomnia recommend non-pharmacological interventions (cognitive behavioural therapy) as first-line treatment and, if behavioural interventions are not effective or available, pharmacotherapy. In contrast to most sleep medications used for decades (benzodiazepines and 'Z-drugs'), the new orexin receptor antagonists do not modulate the activity of γ-aminobutyric acid receptors, the main inhibitory mechanism of the central nervous system. Instead, they temporarily block the orexin pathway, causing a different pattern of effects, e.g., less morning or next-day effects, motor dyscoordination, and cognitive impairment. The pharmacokinetic/pharmacodynamic properties of these drugs are the basis of the different characteristics explained in the package inserts, including the recommended starting dose. Orexin receptor antagonists seem to be devoid of any dependence and tolerance-inducing effects, rendering them a viable option for longer-term treatment. Safety studies did not show exacerbation of existing respiratory problems, but more real-world safety and pharmacovigilance experience is needed. This review provides an overview of the orexin history, the mechanism of action, the relation to insomnia, and key features of available drugs mediating orexin signalling.

Abuse potential assessment of the dual orexin receptor antagonist daridorexant in rats

BACKGROUND

Drugs that act on the central nervous system (CNS) and have sedative effects can lead to abuse in humans. New CNS-active drugs often require evaluation of their abuse potential in dedicated animal models before marketing approval. Daridorexant is a new dual orexin receptor antagonist (DORA) with sleep-promoting properties in animals and humans. It was approved in 2022 in the United States and Europe for the treatment of insomnia disorder.

AIMS

Nonclinical evaluation of abuse potential of daridorexant using three specific rat models assessing reinforcement, interoception, and withdrawal.

METHODS

Reinforcing effects of daridorexant were assessed in an operant rat model of intravenous drug self-administration. Similarity of interoceptive effects to those of the commonly used sleep medication zolpidem was tested in an operant drug discrimination task. Withdrawal signs indicative of physical dependence were evaluated upon sudden termination of chronic daridorexant treatment. Rat experiments were conducted at a dose range resulting in daridorexant plasma concentrations equaling or exceeding those achieved at the clinically recommended dose of 50 mg in humans.

RESULTS

Daridorexant had no reinforcing effects, was dissimilar to zolpidem in the drug discrimination task, and did not induce any withdrawal-related signs upon treatment discontinuation that would be indicative of physical dependence.

OUTCOMES

Daridorexant showed no signs of abuse or dependence potential in rats. Our data indicate that daridorexant, like other DORAs, has a low potential for abuse in humans.

Current models of insomnia disorder: a theoretical review on the potential role of the orexinergic pathway with implications for insomnia treatment

Insomnia disorder is considered as a stress-related disorder associated with hyperarousal, stress and emotion dysregulation and the instability of the 'flip-flop' switch system. The orexinergic system is well known for its key role in sleep and arousal processes but also in the allostatic system regulating stress and emotions and may thus be of major interest for insomnia and its treatment. Accordingly, we discuss the potential role of orexins on sleep processes, brain systems modulating stress and emotions with potential implications for insomnia pathophysiology. We reviewed available data on the effect of dual orexin receptor antagonists (DORAs) on sleep and brain systems modulating stress/emotions with implications for insomnia treatment. We present our findings as a narrative review. Few data in animals and humans have reported that disrupted sleep and insomnia may be related to the overactivation of orexinergic system, while some more consistent data in humans and animals reported the overactivation of orexins in response to acute stress and in stress-related disorders. Taken together these findings may let us hypothesise that an orexins overactivation may be associated with stress-related hyperarousal and the hyperactivation of arousal-promoting systems in insomnia. On the other hand, it is possible that by rebalancing orexins with DORAs we may regulate both sleep and allostatic systems, in turn, contributing to a 'switch off' of hyperarousal in insomnia. Nevertheless, more studies are needed to clarify the role of the orexin system in insomnia and to evaluate the effects of DORAs on sleep, stress and emotions regulating systems.

Involvement of orexin-2 receptors in the CA1 region of the hippocampus in the extinction and reinstatement of methamphetamine-induced conditioned place preference in the rats

The present study, regarding the orexin receptors having a pivotal role in reward-related psychostimulant use disorder (PUD), aimed to investigate the role of orexin-2 (OX2) receptors in the CA1 region of the hippocampus (HPC) in the extinction and reinstatement of methamphetamine (METH)-induced conditioned place preference (CPP). In the first set of investigations, to determine the role of OX2 receptors in the extinction of METH-induced CPP, rats were daily given (during the extinction) bilaterally intra-CA1 region different doses of TCS OX2 29 (1, 3, 10, and 30 nmol/0.5 μl 12% DMSO) as the selective OX2 receptor antagonist. Then, to demonstrate the role of OX2 receptors in the reinstatement of METH-induced CPP after the extinction was established, each rat bilaterally received TCS OX2 29 at the same doses in the CA1 region before injection of the sub-threshold (priming) dose of METH (0.25 mg/kg, sc) on the reinstatement day. The data revealed that the administration of TCS OX2 29 in the CA1 region reduces the mean extinction latency and suppresses the reinstatement of METH-seeking behavior in extinguished rats. Additionally, the potency of TCS OX2 29 to inhibit the reinstatement phase was higher compared to the potency of this drug to modulate the extinction phase of METH-induced CPP. Accordingly, it could be concluded that the blockade of the OX2 receptors in this area might be an essential application and potential therapeutics in treating METH use disorder.

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.

Oxytocin and orexin systems bidirectionally regulate the ability of opioid cues to bias reward seeking

As opioid-related fatalities continue to rise, the need for novel opioid use disorder (OUD) treatments could not be more urgent. Two separate hypothalamic neuropeptide systems have shown promise in preclinical OUD models. The oxytocin system, originating in the paraventricular nucleus (PVN), may protect against OUD severity. By contrast, the orexin system, originating in the lateral hypothalamus (LH), may exacerbate OUD severity. Thus, activating the oxytocin system or inhibiting the orexin system are potential therapeutic strategies. The specific role of these systems with regard to specific OUD outcomes, however, is not fully understood. Here, we probed the therapeutic efficacy of pharmacological interventions targeting the orexin or oxytocin system on two distinct metrics of OUD severity in rats-heroin choice (versus choice for natural reward, i.e., food) and cued reward seeking. Using a preclinical model that generates approximately equal choice between heroin and food reward, we examined the impact of exogenously administered oxytocin, an oxytocin receptor antagonist (L-368,899), and a dual orexin receptor antagonist (DORA-12) on opioid choice. Whereas these agents did not alter heroin choice when rewards (heroin and food) were available, oxytocin and DORA-12 each significantly reduced heroin seeking in the presence of competing reward cues when no rewards were available. In addition, the number of LH orexin neurons and PVN oxytocin neurons correlated with specific behavioral economic variables indicative of heroin versus food motivation. These data identify a novel bidirectional role of the oxytocin and orexin systems in the ability of opioid-related cues to bias reward seeking.

Suvorexant maintenance enhances the reinforcing but not subjective and physiological effects of intravenous cocaine in humans

Preclinical research has sought to understand the role of the orexin system in cocaine addiction given the connection between orexin producing cells in the lateral hypothalamus and brain limbic areas. Exogenous administration of orexin peptides increased cocaine self-administration whereas selective orexin-1 receptor antagonists reduced cocaine self-administration in non-human animals. The first clinically available orexin antagonist, suvorexant (a dual orexin-1 and orexin-2 receptor antagonist), attenuated motivation for cocaine and cocaine conditioned place preference, as well as cocaine-associated impulsive responding, in rodents. This study aimed to translate those preclinical findings and determine whether suvorexant maintenance altered the pharmacodynamic effects of cocaine in humans. Seven non-treatment seeking subjects with cocaine use disorder completed this within-subject human laboratory study, and a partial data set was obtained from one additional subject. Subjects were maintained for at least three days on 0, 5, 10 and 20 mg oral suvorexant administered at 2230 h daily in random order. Subjects completed experimental sessions in which cocaine self-administration of 0, 10 and 30 mg/70 kg of intravenous cocaine was evaluated on a concurrent progressive ratio drug versus money choice task. Subjective and physiological effects of cocaine were also determined. Cocaine functioned as a reinforcer and produced prototypic dose-related subjective and physiological effects (e.g., increased ratings of "Stimulated" and heart rate). Suvorexant (10, 20 mg) increased self-administration of 10 mg/70 kg cocaine and decreased oral temperature but did not significantly alter any other effects of cocaine. Future research may seek to evaluate the effects of orexin-1 selective antagonists in combination with cocaine.

Daridorexant for the Treatment of Insomnia

Purpose of Review

Insomnia is a complex sleeping disorder that affects the lives of many individuals worldwide. Insomnia often occurs in the presence of coexisting comorbidities making it a complex disorder that requires a multifactorial approach to therapy. First-line therapy is cognitive-behavioral therapy for insomnia (CBT-I). Pharmacotherapy for insomnia falls into four classes based on mechanism of action: benzodiazepine receptor agonists (BZRAs), histamine receptor antagonists, melatonin receptor agonists, and dual orexin receptor antagonists (DORAs).

Recent Findings

Daridorexant is a dual orexin type 1 and types 2 (OX₁ and OX₂) receptor antagonist that was recently approved by the US FDA for the treatment of adults suffering from insomnia. It was shown to be effective in reducing insomnia symptoms, increasing daytime functioning, and improving the overall quality of sleep. Daridorexant offers patients relief from insomnia while avoiding the severe side effects and dependency issues of traditional treatments like benzodiazepines and sedatives.

Summary

In this article, we review the most recent data on insomnia treatments and summarize the safety and efficacy of daridorexant in treating insomnia.

Blockade of corticotropin-releasing factor-1 receptors in the infralimbic cortex prevents stress-induced reinstatement of alcohol seeking in male Wistar rats: Evidence of interaction between CRF1 and orexin receptor signaling

Alcohol use dysregulates responsivity to stress, which is mediated by corticotropin-releasing factor (CRF). With repeated cycles of alcohol use, the hypothalamic-pituitary-adrenal axis becomes hyporesponsive, rendering individuals vulnerable to the reinstatement of alcohol-seeking behavior during stressful episodes. Orexin (Orx; also called hypocretin) plays a well-established role in regulating diverse physiological processes, including stress, and interacts with CRF. The infralimbic cortex (IL) is a CRF-rich region. Anatomical evidence suggests that CRF and Orx interact in this area. To test the behavioral implication of CRF and Orx transmission in the IL during the stress-induced reinstatement of alcohol-seeking behavior, male Wistar rats were trained to self-administer 10% alcohol for 3 weeks. The rats then underwent two weeks of extinction training (identical to the alcohol self-administration sessions, but alcohol was withheld). The day after the last extinction session, the rats received a bilateral intra-IL injection of the CRF1 receptor antagonist CP154,526 (0.6 μg/0.5 μl/side), the dual Orx receptor antagonist TCS1102 (15 μg/0.5 μl/side), or their combination and then were tested for the footshock stress-induced reinstatement of alcohol-seeking behavior. CP154,526 significantly prevented reinstatement, but TCS1102 did not produce such an effect. Interestingly, the co-administration of TCS1102 and CP154,526 reversed the effect of CP154,526 alone, and footshock stress induced a significant increase in Crhr1 and Hcrtr2 mRNA expression in the IL. These results demonstrate a functional interaction between Orx receptor and CRF1 receptor signaling and suggest that CRF1 receptor antagonism may ameliorate stress-induced alcohol-seeking behavior.

Hypocretins (orexins): The ultimate translational neuropeptides

The hypocretins (Hcrts), also known as orexins, are two neuropeptides produced exclusively in the lateral hypothalamus. They act on two specific receptors that are widely distributed across the brain and involved in a myriad of neurophysiological functions that include sleep, arousal, feeding, reward, fear, anxiety and cognition. Hcrt cell loss in humans leads to narcolepsy with cataplexy (narcolepsy type 1), a disorder characterized by intrusions of sleep into wakefulness, demonstrating that the Hcrt system is nonredundant and essential for sleep/wake stability. The causal link between Hcrts and arousal/wakefulness stabilisation has led to the development of a new class of drugs, Hcrt receptor antagonists to treat insomnia, based on the assumption that blocking orexin-induced arousal will facilitate sleep. This has been clinically validated: currently, two Hcrt receptor antagonists are approved to treat insomnia (suvorexant and lemborexant), with a New Drug Application recently submitted to the US Food and Drug Administration for a third drug (daridorexant). Other therapeutic applications under investigation include reduction of cravings in substance-use disorders and prevention of neurodegenerative disorders such as Alzheimer's disease, given the apparent bidirectional relationship between poor sleep and worsening of the disease. Circuit neuroscience findings suggest that the Hcrt system is a hub that integrates diverse inputs modulating arousal (e.g., circadian rhythms, metabolic status, positive and negative emotions) and conveys this information to multiple output regions. This neuronal architecture explains the wealth of physiological functions associated with Hcrts and highlights the potential of the Hcrt system as a therapeutic target for a number of disorders. We discuss present and future possible applications of drugs targeting the Hcrt system for the treatment of circuit-related neuropsychiatric and neurodegenerative conditions.

Maternal deprivation effect on morphine-induced CPP is related to changes in Opioid receptors in selected rat brain regions (hippocampus, prefrontal cortex, and nucleus accumbens)

Early-life environmental conditions affect offspring's development. Maternal deprivation (MD) can induce persistent changes that give rise to neuropsychiatric diseases including substance abuse disorders. However, long-lasting mechanisms that determine vulnerability to drug addiction remain unknown. We hypothesized that MD could induce changes in Opioid system, HPA (hypothalamic-pituitary-adrenal) axis, and BDNF (brain-derived neurotrophic factor), so may be involved in the drug abuse in later life. Male offspring of Wistar rats (n=8 per group) were subjected to 3h of daily MD during postnatal days 1-14. In adulthood, morphine-induced CPP (conditioned place preference) was investigated using two doses of morphine (3 and 5mg/kg). Serum corticosterone level was measured by ELISA method. The expression level of genes in selected brain regions (hippocampus, prefrontal cortex, and nucleus accumbens) was determined by qPCR (quantitative PCR). A greater morphine-induced CPP was observed in MD rats with 3 and 5mg/kg morphine compared to controls. MD group had a higher corticosterone level. A significant decrease was observed in the expression of BDNF gene (in all of the selected brain regions) and GR (glucocorticoid receptor) gene (in the hippocampus and nucleus accumbens) in MD rats. Also, a significant increase in the expression of μ Opioid receptor (in all of the selected brain regions) and κ Opioid receptor (in the prefrontal cortex and nucleus accumbens) was observed in MD rats. Our results suggest that MD induces alterations in the HPA axis function, BDNF level, and Opioid receptors system that enhance vulnerability to morphine at adulthood.

Nonclinical evaluation of abuse liability of the dual orexin receptor antagonist lemborexant

Lemborexant is a dual orexin receptor antagonist (DORA) approved in multiple countries including the United States, Japan, Canada and Australia for the treatment of adults with insomnia. As required for marketing approval of new compounds with central nervous system activity with sedating effects, the abuse potential of lemborexant was assessed in accordance with regulatory guidelines, which included three nonclinical studies. These assessments comprised physical dependence and drug discrimination studies in rats and a self-administration study in rhesus monkeys. There was no evidence of withdrawal signs following abrupt drug discontinuation, indicating that lemborexant does not induce physical dependence. In the drug discrimination study, lemborexant at doses up to 1000 mg/kg administered orally did not cross-generalize to the zolpidem training stimulus, although another DORA included in the same experiment, suvorexant, showed partial generalization with zolpidem. In rhesus monkeys, lemborexant treatment did not induce any gross behavioral changes, and there was no increase in self-administration rates compared with control, indicative of a lack of reinforcing effects of lemborexant. Collectively, these nonclinical studies support the position that lemborexant, which has been placed in Schedule IV by the United States Drug Enforcement Administration, has a low risk of abuse in humans.

The dual orexin receptor antagonist almorexant blocks the sleep-disrupting and daytime stimulant effects of methamphetamine in rhesus monkeys

BACKGROUND

The present study investigated the effects of the dual orexin receptor antagonist (DORA) almorexant, a sleep-modulating drug, on the sleep-disrupting effects of methamphetamine in adult rhesus monkeys.

METHODS

Monkeys were fitted with primate collars to which actigraphy monitors were attached. To determine the effects of methamphetamine on daytime activity and sleep-like parameters, monkeys were given acute injections of vehicle or methamphetamine (0.03, 0.1 or 0.3 mg/kg, i.m.) in the morning (9:00 h) (n = 4 males). We then determined the ability of almorexant to alter the daytime and/or sleep-like effects of the largest (effective) dose of methamphetamine. Vehicle or almorexant (1, 3 or 10 mg/kg, i.m.) were administered in the evening (16:30 h, 1.5 h before "lights off") following morning (9:00 h) administration of methamphetamine (0.3 mg/kg, i.m.), or as a pretreatment (8:30 h) before methamphetamine injections (9:00 h) (n = 4 males). The ability of almorexant (10 mg/kg) to improve sleep-like behaviors also was assessed in a group of monkeys quantitatively identified with short-duration sleep (n = 2 males, 2 females).

RESULTS

Morning methamphetamine administration dose-dependently impaired sleep in rhesus monkeys (0.3 mg/kg significantly increased sleep latency and decreased sleep efficiency). Administration of almorexant, both as a pretreatment or as an evening treatment, improved methamphetamine-induced sleep impairment in a dose dependent manner. Morning pretreatment with almorexant also blocked the daytime stimulant effects of methamphetamine. Evening, but not morning, treatment with almorexant in a group of monkeys with baseline short-duration sleep improved sleep measures.

CONCLUSIONS

Our findings indicate that orexin receptor systems are involved in methamphetamine-induced hyperarousal and sleep disruption.

Interaction between cocaine use and sleep behavior: A comprehensive review of cocaine's disrupting influence on sleep behavior and sleep disruptions influence on reward seeking

Dopamine, orexin (hypocretin), and adenosine systems have dual roles in reward and sleep/arousal suggesting possible mechanisms whereby drugs of abuse may influence both reward and sleep/arousal. While considerable variability exists across studies, drugs of abuse such as cocaine induce an acute sleep loss followed by an immediate recovery pattern that is consistent with a normal response to loss of sleep. Under more chronic cocaine exposure conditions, an abnormal recovery pattern is expressed that includes a retention of sleep disturbance under withdrawal and into abstinence conditions. Conversely, experimentally induced sleep disturbance can increase cocaine seeking. Thus, complementary, sleep-related therapeutic approaches may deserve further consideration along with development of non-human models to better characterize sleep disturbance-reward seeking interactions across drug experience.

Nonclinical pharmacology of daridorexant: a new dual orexin receptor antagonist for the treatment of insomnia

Dual orexin receptor antagonists (DORAs) represent a novel type of sleep medication that provide an alternative to the traditionally used positive allosteric gamma-aminobutyric acid (GABA)-A receptor modulators. Daridorexant is a new DORA that exhibited in phase 3 trials in insomnia not only a beneficial effect on sleep variables, measured objectively and assessed subjectively, but also an improvement in daytime functioning. Daridorexant was discovered through a tailored research program aimed at identifying an optimized sleep-promoting molecule with pharmacokinetic properties appropriate for covering the whole night while avoiding next-morning residual activity at efficacious doses. By specific binding to both orexin receptors, daridorexant inhibits the actions of the wake-promoting orexin (also called hypocretin) neuropeptides. This mechanism avoids a more widespread inhibition of neuronal pathways and associated side effects that are intrinsic to positive allosteric GABA-A receptor modulators. Here, we review the general pharmacology of daridorexant, based on nonclinical pharmacology studies of daridorexant, unpublished or already described, or based on work with other DORAs. Some unique features of daridorexant will be highlighted, such as the promotion of natural and surmountable sleep, the preservation of memory and cognition, the absence of tolerance development or risk of physical dependence, and how it can benefit daytime functioning.

Drug-Induced Conditioned Place Preference and Its Practical Use in Substance Use Disorder Research

The conditioned place preference (CPP) paradigm is a well-established model utilized to study the role of context associations in reward-related behaviors, including both natural rewards and drugs of abuse. In this review article, we discuss the basic history, various uses, and considerations that are tied to this technique. There are many potential takeaway implications of this model, including negative affective states, conditioned drug effects, memory, and motivation, which are all considered here. We also discuss the neurobiology of CPP including relevant brain regions, molecular signaling cascades, and neuromodulatory systems. We further examine some of our prior findings and how they integrate CPP with self-administration paradigms. Overall, by describing the fundamentals of CPP, findings from the past few decades, and implications of using CPP as a research paradigm, we have endeavored to support the case that the CPP method is specifically advantageous for studying the role of a form of Pavlovian learning that associates drug use with the surrounding environment.

Augmentation of morphine-conditioned place preference by food restriction is associated with alterations in the oxytocin/oxytocin receptor in rat models

BACKGROUND

Studies indicate that food restriction (FR) reinforces the effects of morphine. The exact mechanisms by which FR influences the reward circuitry of morphine have not yet been determined.

OBJECTIVES

We hypothesized that the effects of FR on the oxytocin (OXT) system and HPA axis can be associated with substance abuse disorders. In this study, the serum levels of OXT and corticosterone, and the expression of OXT/OXT receptor (OXTR), glucocorticoid receptor (GR), and brain-derived neurotrophic factor (BDNF) in the hippocampus, prefrontal cortex, and nucleus accumbens were investigated in an FR model.

METHODS

First, the male rats (n = 8 per group) were subjected to FR for 3 weeks. Then, morphine-induced conditioned place preference (CPP) was observed using two doses of morphine (3 and 5 mg/kg). The serum concentrations of corticosterone and OXT were determined by ELISA and the expression of genes was examined by qPCR.

RESULTS

FR induced an enhanced preference in the animals for the 5 mg/kg dose of morphine compared to the controls. Serum corticosterone levels increased after FR but OXT levels decreased. Meanwhile, FR actuated downregulation of GR, BDNF, and OXT genes, while inducing the overexpression of OXTR.

CONCLUSION

We propose the inclusion of OXT and OXTR alterations in the enhancement of morphine-induced CPP and addiction vulnerability following FR. Moreover, we conclude that altered BDNF levels and HPA axis activity may be the mechanisms involved in the effects of FR on morphine-induced behavior.

Targeting the Orexin System for Prescription Opioid Use Disorder

Prescription opioids are potent analgesics that are used for clinical pain management. However, the nonmedical use of these medications has emerged as a major concern because of dramatic increases in abuse and overdose. Therefore, effective strategies to prevent prescription opioid use disorder are urgently needed. The orexin system has been implicated in the regulation of motivation, arousal, and stress, making this system a promising target for the treatment of substance use disorder. This review discusses recent preclinical studies that suggest that orexin receptor blockade could be beneficial for the treatment of prescription opioid use disorder.

Orexin Receptor Antagonists as Emerging Treatments for Psychiatric Disorders

Orexins comprise two neuropeptides produced by orexin neurons in the lateral hypothalamus and are released by extensive projections of these neurons throughout the central nervous system. Orexins bind and activate their associated G protein-coupled orexin type 1 receptors (OX1Rs) and OX2Rs and act on numerous physiological processes, such as sleep-wake regulation, feeding, reward, emotion, and motivation. Research on the development of orexin receptor antagonists has dramatically increased with the approval of suvorexant for the treatment of primary insomnia. In the present review, we discuss recent findings on the involvement of the orexin system in the pathophysiology of psychiatric disorders, including sleep disorders, depression, anxiety, and drug addiction. We discuss the actions of orexin receptor antagonists, including selective OX1R antagonists (SORA1s), selective OX2R antagonists (SORA2s), and dual OX1/2R antagonists (DORAs), in the treatment of these disorders based on both preclinical and clinical evidence. SORA2s and DORAs have more pronounced efficacy in the treatment of sleep disorders, whereas SORA1s may be promising for the treatment of anxiety and drug addiction. We also discuss potential challenges and opportunities for the application of orexin receptor antagonists to clinical interventions.

Orexin-mediated restoration of hippocampal synaptic potentiation in mice with established cocaine-conditioned place preference

Orexins (also called hypocretins) are implicated in reward and addiction, but little is known about their role(s) in the association between hippocampal synaptic plasticity and drug preference. Previously, we found that exogenous orexin via OX₁ and OX₂ receptors can impair low frequency stimulation-induced depotentiation, i.e. restoring potentiation of excitatory synaptic transmission (re-potentiation) in mouse hippocampal slices. Here, we found this re-potentiation in hippocampal slices from mice that had acquired conditioned place preference (CPP) to cocaine. Both 10 and 20 mg/kg of cocaine induced similar magnitudes of CPP in mice and re-potentiation in their hippocampal slices, but differed in their susceptibility to TCS1102, a dual (OX₁ and OX₂ ) orexin receptor antagonist. TCS1102 significantly attenuated CPP and hippocampal re-potentiation induced by cocaine at 10 mg/kg but not at 20 mg/kg. Nonetheless, SCH23390, an antagonist of dopamine D1-like receptors (D1-likeRs), inhibited the effects induced by both doses of cocaine. SKF38393, a D1-likeR-selective agonist, also induced hippocampal re-potentiation in vitro. Interestingly, this effect was attenuated by TCS1102. Conversely, SCH23390 prevented orexin A-induced hippocampal re-potentiation. These results suggest that endogenous orexins are released in mice during cocaine-CPP acquisition, which sustains potentiated hippocampal transmission via OX₁ /OX₂ receptors and may contribute to the addiction memory of cocaine. This effect of endogenous orexins, however, may be substituted by dopamine that may dominate hippocampal re-potentiation and CPP via D1-likeRs when the reinforcing effect of cocaine is high.

Demand elasticity predicts addiction endophenotypes and the therapeutic efficacy of an orexin/hypocretin-1 receptor antagonist in rats

Behavioral economics is a powerful, translational approach for measuring drug demand in both humans and animals. Here, we asked if demand for cocaine in rats with limited drug experience could be used to identify individuals most at risk of expressing an addiction phenotype following either long- or intermittent access self-administration schedules, both of which model the transition to uncontrolled drug-seeking. Because the orexin-1 receptor antagonist SB-334867 (SB) is particularly effective at reducing drug-seeking in highly motivated individuals, we also asked whether demand measured after prolonged drug experience could predict SB efficacy. Demand elasticity (α) measured immediately following acquisition of cocaine self-administration ('baseline α') was positively correlated with α assessed after 2w of long- or intermittent access. Baseline α also predicted the magnitude of compulsive responding for cocaine, drug-seeking in initial abstinence and cued reinstatement following long-, intermittent- or standard short access. When demand was measured after these differential access conditions, α predicted the same addiction endophenotypes predicted by baseline α, as well as primed reinstatement and the emergence of negative emotional mood behavior following abstinence. α also predicted the efficacy of SB, such that high demand rats showed greater reductions in motivation for cocaine following SB compared to low demand rats. Together, these findings indicate that α might serve as a behavioral biomarker to predict individuals most likely to progress from controlled to uncontrolled drug use, and to identify individuals most likely to benefit from orexin-based therapies for the treatment of addiction.

Therapeutics development for addiction: Orexin-1 receptor antagonists

The orexin system includes the neuropeptides orexin A and B and the cognate receptors of orexin-1 (OX1) and -2 (OX2) and has been indicated in a number of important physiological processes. It is generally accepted that the OX1 receptor is mainly involved in motivation and reward and the OX2 receptor in the modulation of sleep/wake cycle and energy homeostasis. A variety of OX1 selective antagonists (1-SORAs) have been disclosed in the literature and some of them have been evaluated as potential therapeutics for addiction treatment. In this review we summarize all OX1 antagonists reported thus far based on their core structure. Several dual orexin receptor antagonists (DORAs) and OX2 selective antagonist (2-SORAs) have also been recently evaluated in reward and addiction models. While DORAs may seem pharmacologically advantageous for alcohol addiction given the recent findings on the OX2 receptor in reward and alcohol consumption, 1-SORAs are the better options for other drugs of addiction such as cocaine due to the absence of the sedative effects inherently associated with dual antagonists.

SB-334867 (an Orexin-1 Receptor Antagonist) Effects on Morphine-Induced Sensitization in Mice-a View on Receptor Mechanisms

The present study focused upon the role of SB-334867, an orexin-1 receptor antagonist, in the acquisition of morphine-induced sensitization to locomotor activity in mice. Behavioral sensitization is an enhanced systemic reaction to the same dose of an addictive substance, which assumingly increases both the desire for the drug and the risk of relapse to addiction. Morphine-induced sensitization in mice was achieved by sporadic doses (five injections every 3 days) of morphine (10 mg/kg, i.p.), while a challenge dose of morphine (10 mg/kg) was injected 7 days later. In order to assess the impact of orexin system blockade on the acquisition of sensitization, SB-334867 was administered before each morphine injection, except the morphine challenge dose. The locomotor activity test was performed on each day of morphine administration. Brain structures (striatum, hippocampus, and prefrontal cortex) were collected after behavioral tests for molecular experiments in which mRNA expression of orexin, dopamine, and adenosine receptors was explored by the qRT-PCR technique. Additionally, the mRNA expression of markers, such as GFAP and Iba-1, was also analyzed by the same technique. SB-334867 inhibited the acquisition of morphine-induced sensitization to locomotor activity of mice. Significant alterations were observed in mRNA expression of orexin, dopamine, and adenosine receptors and in the expression of GFAP and Iba-1, showing a broad range of interactions in the mesolimbic system among orexin, dopamine, adenosine, and glial cells during behavioral sensitization. Summing up, the orexin system may be an effective measure to inhibit morphine-induced behavioral sensitization.

Orexin/Hypocretin System: Role in Food and Drug Overconsumption

The neuropeptide orexin/hypocretin (OX), while largely transcribed within the hypothalamus, is released throughout the brain to affect complex behaviors. Primarily through the hypothalamus itself, OX homeostatically regulates adaptive behaviors needed for survival, including food intake, sleep-wake regulation, mating, and maternal behavior. However, through extrahypothalamic limbic brain regions, OX promotes seeking and intake of rewarding substances of abuse, like palatable food, alcohol, nicotine, and cocaine. This neuropeptide, in turn, is stimulated by the intake of or early life exposure to these substances, forming a nonhomeostatic, positive feedback loop. The specific OX receptor involved in these behaviors, whether adaptive behavior or substance seeking and intake, is dependent on the particular brain region that contributes to them. Thus, we propose that, while the primary function of OX is to maintain arousal for the performance of adaptive behaviors, this neuropeptide system is readily co-opted by rewarding substances that involve positive feedback, ultimately promoting their abuse.

The dual orexin receptor antagonist TCS1102 does not affect reinstatement of nicotine-seeking

The orexin/hypocretin system is important for appetitive motivation towards multiple drugs of abuse, including nicotine. Both OX1 and OX2 receptors individually have been shown to influence nicotine self-administration and reinstatement. Due to the increasing clinical use of dual orexin receptor antagonists in the treatment of disorders such as insomnia, we examined whether a dual orexin receptor antagonist may also be effective in reducing nicotine seeking. We tested the effect of intracerebroventricular (i.c.v.) administration of the potent and selective dual orexin receptor antagonist TCS1102 on orexin-A-induced food self-administration, nicotine self-administration and reinstatement of nicotine-seeking in rats. Our results show that 30 μg of TCS1102 i.c.v. abolishes orexin-A-induced increases in food self-administration but does not reduce nicotine self-administration. Neither i.c.v. 10 μg nor 30 μg of TCS1102 reduced compound reinstatement after short-term (15 days) self-administration nicotine, but 30 μg transiently reduced cue/nicotine compound reinstatement after chronic self-administration (29 days). These results indicate that TCS1102 has no substantial effect on motivation for nicotine seeking following chronic self-administration and no effect after shorter periods of intake. Orexin receptor antagonists may therefore have little clinical utility against nicotine addiction.

Preclinical assessment of the abuse potential of the orexin receptor antagonist, suvorexant

Suvorexant (Belsomra^®) is a dual orexin receptor antagonist approved for the treatment of insomnia. Because of its pharmacology within the central nervous system, intended therapeutic indication, and first-in-class status, an assessment of suvorexant abuse liability potential was required prior to marketing approval. The nonclinical abuse liability potential studies for suvorexant included: 1) rat drug-dependence model to assess physical dependence following abrupt cessation; 2) rat drug-discrimination model to examine the potential similarity of the interoceptive or subjective effects of suvorexant to those elicited by zolpidem and morphine; 3) self-administration model to assess the relative reinforcing efficacy of suvorexant in rhesus monkeys conditioned to self-administer methohexital. No significant signs of spontaneous drug withdrawal or 'discontinuation syndrome' were observed in rats following abrupt discontinuation of suvorexant. Suvorexant did not elicit complete cross-generalization to either a zolpidem or morphine training/reference stimuli in rats, and suvorexant was devoid of behavioral evidence of positive reinforcing efficacy in monkeys. These nonclinical findings suggested that suvorexant will have low abuse potential in humans. In the final regulatory risk assessment, suvorexant was placed into Schedule IV, likely due to its first-in-class status, its sedative properties, and the outcome of the clinical abuse potential assessment.

A Decade of Orexin/Hypocretin and Addiction: Where Are We Now?

One decade ago, our laboratory provided the first direct evidence linking orexin/hypocretin signaling with drug seeking by showing that activation of these neurons promotes conditioned morphine-seeking behavior. In the years since, contributions from many investigators have revealed roles for orexins in addiction for all drugs of abuse tested, but only under select circumstances. We recently proposed that orexins play a fundamentally unified role in coordinating "motivational activation" under numerous behavioral conditions, and here we unpack this hypothesis as it applies to drug addiction. We describe evidence collected over the past 10 years that elaborates the role of orexin in drug seeking under circumstances where high levels of effort are required to obtain the drug, or when motivation for drug reward is augmented by the presence of external stimuli like drug-associated cues/contexts or stressors. Evidence from studies using traditional self-administration and reinstatement models, as well as behavioral economic analyses of drug demand elasticity, clearly delineates a role for orexin in modulating motivational, rather than the primary reinforcing aspects of drug reward. We also discuss the anatomical interconnectedness of the orexin system with wider motivation and reward circuits, with a particular focus on how orexin modulates prefrontal and other glutamatergic inputs onto ventral tegmental area dopamine neurons. Last, we look ahead to the next decade of the research in this area, highlighting the recent FDA approval of the dual orexin receptor antagonist suvorexant (Belsomra®) for the treatment of insomnia as a promising sign of the potential clinical utility of orexin-based therapies for the treatment of addiction.

Discovery and optimisation of 1-acyl-2-benzylpyrrolidines as potent dual orexin receptor antagonists

The evolution of our lead compound1into thein vivoactive, competitive, dual orexin receptor antagonist27, is described.

Endogenous opiates and behavior: 2013

This paper is the thirty-sixth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2013 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Orexin/hypocretin based pharmacotherapies for the treatment of addiction: DORA or SORA?

Addiction is a chronic relapsing disorder which presents a significant global health burden and unmet medical need. The orexin/hypocretin system is an attractive potential therapeutic target as demonstrated by the successful clinical trials of antagonist medications like Suvorexant for insomnia. It is composed of two neuropeptides, orexin-A and orexin-B and two excitatory and promiscuous G-protein coupled receptors, OX1 and OX2. Orexins are known to have a variety of functions, most notably in regulating arousal, appetite and reward. The orexins have been shown to have a role in mediating the effects of several drugs of abuse, such as cocaine, morphine and alcohol via projections to key brain regions such as the ventral tegmental area, nucleus accumbens and prefrontal cortex. However, it has not yet been demonstrated whether the dual orexin receptor antagonists (DORAs) under development for insomnia are ideal drugs for the treatment of addiction. The question of whether to use a DORA or single orexin receptor antagonist (SORA) for the treatment of addiction is a key question that will need to be answered in order to maximize the clinical utility of orexin receptor antagonists. This review will examine the role of the orexin/hypocretin system in addiction, orexin-based pharmacotherapies under development and factors affecting the selection of one or both orexin receptors as drug targets for the treatment of addiction.

Assessment of the abuse liability of a dual orexin receptor antagonist: a crossover study of almorexant and zolpidem in recreational drug users

BACKGROUND

Dual orexin receptor antagonists (DORAs) enable initiation and maintenance of sleep in patients with primary insomnia. Blockade of the orexin system has shown reduction of drug-seeking behavior in animal studies, supporting the role of orexin antagonism as a novel approach for treating substance abuse. Since hypnotics are traditionally associated with misuse, a lack of abuse liability of DORAs would offer significant benefits over current therapies for sleep disorders.

METHODS

In this randomized, crossover, proof-of-concept study, single oral doses of the DORA almorexant (200, 400, and 1,000 mg) were administered to healthy subjects with previous non-therapeutic experience with central nervous system depressants and were compared with placebo and single oral doses of zolpidem (20 and 40 mg), a benzodiazepine-like drug. Subjective measures of abuse potential (visual analog scales [VAS], Addiction Research Center Inventory, and Subjective Drug Value) and objective measures (divided attention [DA]) were evaluated over 24 h post-dose in 33 evaluable subjects.

RESULTS

Drug Liking VAS peak effect (E max; primary endpoint) was significantly higher for all doses of almorexant and zolpidem compared with placebo (p<0.001). Almorexant 200 mg showed significantly less 'Drug Liking' than both zolpidem doses (p<0.01), and almorexant 400 mg had smaller effects than zolpidem 20 mg (p<0.05), while almorexant 1,000 mg was not different from either zolpidem dose. Results were similar for other subjective measures, although almorexant generally showed smaller negative and perceptual effects compared with zolpidem. Almorexant also showed less cognitive impairment compared with zolpidem on most DA endpoints.

CONCLUSION

This study in humans investigating single doses of almorexant is the first to explore and show abuse liability of a DORA, a class of compounds that is not only promising for the treatment of sleep disorders, but also of addiction.

Orexin antagonists for neuropsychiatric disease: progress and potential pitfalls

The tight regulation of sleep/wake states is critical for mental and physiological wellbeing. For example, dysregulation of sleep/wake systems predisposes individuals to metabolic disorders such as obesity and psychiatric problems, including depression. Contributing to this understanding, the last decade has seen significant advances in our appreciation of the complex interactions between brain systems that control the transition between sleep and wake states. Pivotal to our increased understanding of this pathway was the description of a group of neurons in the lateral hypothalamus (LH) that express the neuropeptides orexin A and B (hypocretin, Hcrt-1 and Hcrt-2). Orexin neurons were quickly placed at center stage with the demonstration that loss of normal orexin function is associated with the development of narcolepsy—a condition in which sufferers fail to maintain normal levels of daytime wakefulness. Since these initial seminal findings, much progress has been made in our understanding of the physiology and function of the orexin system. For example, the orexin system has been identified as a key modulator of autonomic and neuroendocrine function, arousal, reward and attention. Notably, studies in animals suggest that dysregulation of orexin function is associated with neuropsychiatric states such as addiction and mood disorders including depression and anxiety. This review discusses the progress associated with therapeutic attempts to restore orexin system function and treat neuropsychiatric conditions such as addiction, depression and anxiety. We also highlight potential pitfalls and challenges associated with targeting this system to treat these neuropsychiatric states.

The effects of amphetamine injections on feeding behavior and the brain expression of orexin, CART, tyrosine hydroxylase (TH) and thyrotropin releasing hormone (TRH) in goldfish (Carassius auratus)

In this study, the effects of peripheral (intraperitoneal) injections of D-amphetamine on feeding behavior were assessed in goldfish. Compared with the saline-injected group, amphetamine injections decreased food intake at doses ranging from 1 to 75 μg/g, but not 0.5 μg/g, but increased locomotor behavior, as indicated by the increased number of total feeding and non-feeding acts, at doses ranging from 2.5 to 25 μg/g. Amphetamine at high doses inhibited both food intake (at 25, 50 and 75 μg/g) and feeding behavior (at 75 μg/g). In the hypothalamus, the expression of orexin was down-regulated, and both CART 1 and CART 2 expressions were up-regulated in amphetamine-treated fish (50 μg/g) as compared to saline-injected fish, but amphetamine treatment had no effect on either hypothalamic TH or TRH expression. In the telencephalon, amphetamine treatment (50 μg/g) up-regulated CART 1, CART 2 and TH mRNA expressions but had no effect on either orexin or TRH. Our results suggest that, as in mammals, the orexin, CART and TH systems might be involved in amphetamine-induced feeding/locomotor responses in goldfish.

Examining the role of endogenous orexins in hypothalamus-pituitary-adrenal axis endocrine function using transient dual orexin receptor antagonism in the rat

The orexin neuropeptide system regulates wakefulness and contributes to physiological and behavioral stress responses. Moreover, a role for orexins in modulating hypothalamus-pituitary-adrenal (HPA) axis activity has been proposed. Brain penetrating dual orexin receptor (OXR) antagonists such as almorexant decrease vigilance and have emerged as a novel therapeutic class for the treatment of insomnia. Almorexant was used here as a pharmacological tool to examine the role of endogenous orexin signaling in HPA axis endocrine function under natural conditions. After confirming the expression of prepro-orexin and OXR-1 and OXR-2 mRNA in hypothalamus, pituitary and adrenal glands, the effects of systemic almorexant were investigated on peripheral HPA axis hormone release in the rat under baseline, stress and pharmacological challenge conditions. Almorexant did not alter basal or stress-induced corticosterone release despite affecting wake and sleep stages (detected by radiotelemetric electroencephalography/electromyography) during the stress exposure. Moreover, almorexant did not affect the release of adrenocorticotropin (ACTH) and corticosterone at different time points along the diurnal rhythm, nor corticotrophin-releasing hormone (CRH)- and ACTH-stimulated neuroendocrine responses, measured in vivo under stress-free conditions. These results illustrate that dual OXR antagonists, despite modulating stress-induced wakefulness, do not interfere with endocrine HPA axis function in the rat. They converge to suggest that endogenous orexin signaling plays a minor role in stress hormone release under basal conditions and under challenge.