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

Behavioral economics of polysubstance use: The role of orexin-1 receptors in nicotine-induced augmentation of synthetic opioid consumption

Nicotine and opioid use disorders are highly comorbid in clinical populations. Ongoing nicotine administration facilitates opioid consumption in both rodents and humans. Moreover, preclinical studies support that former exposure to nicotine solely during adolescence augments opioid consumption in adulthood similarly to acute nicotine administration. This suggests that developmental nicotine exposure persistently alters the neural substrates underlying motivation in a manner that resembles the acute pharmacological actions of nicotine. The orexin system mediates motivation to consume opioids in large part through signaling at orexin-1 receptors (ORX1Rs). Both developmental nicotine exposure and acute nicotine administration profoundly alter functioning of the orexin system which may mediate the reinforcing enhancing properties of nicotine. Here, we used behavioral economic procedures to generate demand curves for consumption of the synthetic, short-acting, μ-opioid receptor agonist remifentanil (RMF) in adulthood following prior adolescent nicotine exposure (ANE) and again following reintroduction of acute nicotine administration (ANA). We found that ANE was sufficient to augment multiple indices of the motivational value of RMF in adulthood and these effects were further exacerbated by ANA given during RMF self-administration sessions. Additionally, we demonstrate that systemic antagonism of ORX1Rs with SB-334867 is more efficacious in limiting motivation for RMF in nicotine-exposed rats relative to controls and this differential efficacy was even greater in ANA conditions relative to former ANE. These findings support that nicotine-induced facilitation of orexin signaling may mechanistically contribute to augmented opioid consumption offering critical insight for treatment options for a population that is particularly vulnerable to developing opioid use disorder.

Dual Hypocretin Receptor Antagonism Reduces Oxycodone Seeking During Abstinence

A major barrier in the treatment of opioid use disorder is persistent drug craving during abstinence. While opioid-based medications have been used to treat opioid use disorder for decades, there is an urgent need for novel, non-opioid-based pharmacotherapies. The hypocretin/orexin (hypocretin) system is a promising target for treating opioid use disorder due to its influence on motivation for drugs of abuse through actions on dopamine transmission. We recently showed that intermittent access (IntA) to oxycodone promoted sustained oxycodone seeking and alterations in dopamine transmission during abstinence. In the current studies, we investigated to what extent suvorexant, an FDA-approved dual hypocretin receptor antagonist, reduces oxycodone seeking and restores dopamine function during abstinence. Results indicated that IntA to oxycodone produced sustained cue-induced oxycodone seeking after a 14-day abstinence period, which was associated with reduced dopamine uptake in the nucleus accumbens core as we have previously shown. Treatment with suvorexant 24 h prior to a cue-induced seeking test significantly reduced oxycodone seeking and normalized aberrant dopamine uptake. These findings suggest that targeting hypocretin receptors may be a promising strategy for reducing opioid craving and associated neuroadaptations, thus lowering the risk of relapse.

Orexin receptors: possible therapeutic targets for psychiatric disorders

RATIONALE

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

OBJECTIVE

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

RESULTS

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

CONCLUSION

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

Suvorexant enhances oxycodone-induced respiratory depression in male rats

BACKGROUND

Recent studies have proposed the use of dual orexin receptor antagonists, such as suvorexant, for the treatment of opioid use disorder (OUD) and opioid-related sleep disturbances because of orexin's role in sleep-wake regulation and addiction. Accumulating evidence suggests that orexin is also an important modulator of respiratory function, raising the possibility of adverse respiratory events when combining orexin antagonists and opioids. The aim of the present study was to investigate the effects of suvorexant, alone or in combination with the opioid oxycodone, on pulmonary ventilation in male rats.

METHODS

Adult, male Sprague Dawley rats received treatments with vehicle, oxycodone (3 and 10mg/kg, i.p.) or suvorexant (10 and 18mg/kg, i.p.), and respiratory measures were obtained using whole-body plethysmography. We then tested the effects of a combination of suvorexant (10 and 18mg/kg, i.p.) and the highest dose of oxycodone that did not suppress respiration alone (3mg/kg, i.p).

RESULTS

Oxycodone induced respiratory depression at 10mg/kg, but not 3.0mg/kg; as evident by significant decreases in minute volume (mls/min) and tidal volume (mls). Suvorexant alone did not alter any respiratory measures at the doses tested. When combined, 18mg/kg (but not 10mg/kg) suvorexant plus an ineffective dose of oxycodone significantly decreased minute and tidal volume compared with vehicle and either drug alone, whereas respiratory frequency was significantly decreased compared with vehicle.

CONCLUSIONS

Our findings show that suvorexant, at a dose associated with sleep promotion and blockade of oxycodone self-administration, robustly enhanced oxycodone-induced respiratory depression in male rats.

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.

Endogenous opiates and behavior: 2023

This paper is the forty-sixth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2023 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug and alcohol abuse (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

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

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

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.

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

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

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.