Involvement of the lateral hypothalamic peptide orexin in morphine dependence and withdrawal

Neurobehavioral meaning of pupil size

Pupil size is a widely used metric of brain state. It is one of the few signals originating from the brain that can be readily monitored with low-cost devices in basic science, clinical, and home settings. It is, therefore, important to investigate and generate well-defined theories related to specific interpretations of this metric. What exactly does it tell us about the brain? Pupils constrict in response to light and dilate during darkness, but the brain also controls pupil size irrespective of luminosity. Pupil size fluctuations resulting from ongoing "brain states" are used as a metric of arousal, but what is pupil-linked arousal and how should it be interpreted in neural, cognitive, and computational terms? Here, we discuss some recent findings related to these issues. We identify open questions and propose how to answer them through a combination of well-defined tasks, neurocomputational models, and neurophysiological probing of the interconnected loops of causes and consequences of pupil size.

The Role of Orexin Receptor Antagonists in Inhibiting Drug Addiction: A Review Article

The orexinergic system and its receptors are involved in many physiological processes. Their functions in energy homeostasis, arousal, cognition, stress processing, endocrine functions, and pain modulation have been investigated. Many studies have shown that the orexinergic system cooperates with the dopaminergic system in the addiction process. Emerging evidence suggests that the orexinergic system can be effective in the induction of drug dependence and tolerance. Therefore, several researches have been conducted on the effect of orexin receptor (OXR) antagonists on reducing tolerance and dependence caused by drug abuse. Due to the significant growth of the studies on the orexinergic system, the current literature was conducted to collect the findings of previous studies on orexin and its receptors in the induction of drug addiction. In addition, cellular and molecular mechanisms of the possible role of orexin in drug tolerance and dependence are discussed. The findings indicate that the administration of OXR antagonists reduces drug dependence. OXR blockers seem to counteract the addictive effects of drugs through multiple mechanisms, such as preventing neuronal adaptation. This review proposes the potential clinical use of OXR antagonists in the treatment of drug dependence.

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

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

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.

Oxycodone: A Current Perspective on Its Pharmacology, Abuse, and Pharmacotherapeutic Developments

Oxycodone, a semisynthetic derivative of naturally occurring thebaine, an opioid alkaloid, has been available for more than 100 years. Although thebaine cannot be used therapeutically due to the occurrence of convulsions at higher doses, it has been converted to a number of other widely used compounds that include naloxone, naltrexone, buprenorphine, and oxycodone. Despite the early identification of oxycodone, it was not until the 1990s that clinical studies began to explore its analgesic efficacy. These studies were followed by the pursuit of several preclinical studies to examine the analgesic effects and abuse liability of oxycodone in laboratory animals and the subjective effects in human volunteers. For a number of years oxycodone was at the forefront of the opioid crisis, playing a significant role in contributing to opioid misuse and abuse, with suggestions that it led to transitioning to other opioids. Several concerns were expressed as early as the 1940s that oxycodone had significant abuse potential similar to heroin and morphine. Both animal and human abuse liability studies have confirmed, and in some cases amplified, these early warnings. Despite sharing a similar structure with morphine and pharmacological actions also mediated by the μ-opioid receptor, there are several differences in the pharmacology and neurobiology of oxycodone. The data that have emerged from the many efforts to analyze the pharmacological and molecular mechanism of oxycodone have generated considerable insight into its many actions, reviewed here, which, in turn, have provided new information on opioid receptor pharmacology. SIGNIFICANCE STATEMENT: Oxycodone, a μ-opioid receptor agonist, was synthesized in 1916 and introduced into clinical use in Germany in 1917. It has been studied extensively as a therapeutic analgesic for acute and chronic neuropathic pain as an alternative to morphine. Oxycodone emerged as a drug with widespread abuse. This article brings together an integrated, detailed review of the pharmacology of oxycodone, preclinical and clinical studies of pain and abuse, and recent advances to identify potential opioid analgesics without abuse liability.

Orexin and MCH neurons: regulators of sleep and metabolism

Sleep-wake and fasting-feeding are tightly coupled behavioral states that require coordination between several brain regions. The mammalian lateral hypothalamus (LH) is a functionally and anatomically complex brain region harboring heterogeneous cell populations that regulate sleep, feeding, and energy metabolism. Significant attempts were made to understand the cellular and circuit bases of LH actions. Rapid advancements in genetic and electrophysiological manipulation help to understand the role of discrete LH cell populations. The opposing action of LH orexin/hypocretin and melanin-concentrating hormone (MCH) neurons on metabolic sensing and sleep-wake regulation make them the candidate to explore in detail. This review surveys the molecular, genetic, and neuronal components of orexin and MCH signaling in the regulation of sleep and metabolism.

Future perspectives of emerging novel drug targets and immunotherapies to control drug addiction

Substance Use Disorder (SUD) is one of the major mental illnesses that is terrifically intensifying worldwide. It is becoming overwhelming due to limited options for treatment. The complexity of addiction disorders is the main impediment to understanding the pathophysiology of the illness. Hence, unveiling the complexity of the brain through basic research, identification of novel signaling pathways, the discovery of new drug targets, and advancement in cutting-edge technologies will help control this disorder. Additionally, there is a great hope of controlling the SUDs through immunotherapeutic measures like therapeutic antibodies and vaccines. Vaccines have played a cardinal role in eliminating many diseases like polio, measles, and smallpox. Further, vaccines have controlled many diseases like cholera, dengue, diphtheria, Haemophilus influenza type b (Hib), human papillomavirus, influenza, Japanese encephalitis, etc. Recently, COVID-19 was controlled in many countries by vaccination. Currently, continuous effort is done to develop vaccines against nicotine, cocaine, morphine, methamphetamine, and heroin. Antibody therapy against SUDs is another important area where serious attention is required. Antibodies have contributed substantially against many serious diseases like diphtheria, rabies, Crohn's disease, asthma, rheumatoid arthritis, and bladder cancer. Antibody therapy is gaining immense momentum due to its success rate in cancer treatment. Furthermore, enormous advancement has been made in antibody therapy due to the generation of high-efficiency humanized antibodies with a long half-life. The advantage of antibody therapy is its instant outcome. This article's main highlight is discussing the drug targets of SUDs and their associated mechanisms. Importantly, we have also discussed the scope of prophylactic measures to eliminate drug dependence.

Targeting the orexin/hypocretin system for the treatment of neuropsychiatric and neurodegenerative diseases: from animal to clinical studies

Orexins (also known as hypocretins) are neuropeptides located exclusively in hypothalamic neurons that have extensive projections throughout the central nervous system and bind two different G protein-coupled receptors (OX1R and OX2R). Since its discovery in 1998, the orexin system has gained the interest of the scientific community as a potential therapeutic target for the treatment of different pathological conditions. Considering previous basic science research, a dual orexin receptor antagonist, suvorexant, was the first orexin agent to be approved by the US Food and Drug Administration to treat insomnia. In this review, we discuss and update the main preclinical and human studies involving the orexin system with several psychiatric and neurodegenerative diseases. This system constitutes a nice example of how basic scientific research driven by curiosity can be the best route to the generation of new and powerful pharmacological treatments.

Cross-talk between the HPA axis and addiction-related regions in stressful situations

Addiction is a worldwide problem that has a negative impact on society by imposing significant costs on health care, public security, and the deactivation of the community economic cycle. Stress is an important risk factor in the development of addiction and relapse vulnerability. Here we review studies that have demonstrated the diverse roles of stress in addiction. Term searches were conducted manually in important reference journals as well as in the Google Scholar and PubMed databases, between 2010 and 2022. In each section of this narrative review, an effort has been made to use pertinent sources. First, we will provide an overview of changes in the Hypothalamus-Pituitary-Adrenal (HPA) axis component following stress, which impact reward-related regions including the ventral tegmental area (VTA) and nucleus accumbens (NAc). Then we will focus on internal factors altered by stress and their effects on drug addiction vulnerability. We conclude that alterations in neuro-inflammatory, neurotrophic, and neurotransmitter factors following stress pathways can impact related mechanisms on craving and relapse susceptibility.

Nucleus accumbens circuit disinhibits lateral hypothalamus glutamatergic neurons contributing to morphine withdrawal memory in male mice

The lateral hypothalamus (LH) is physiologically critical in brain functions. The LH also plays an important role in drug addiction. However, neural circuits underlying LH involvement of drug addiction remain obscure. In the present study,our results showed that in male mice, during context-induced expression of morphine withdrawal memory, LH glutamatergic neurons played an important role; dopamine D1 receptor-expressing medium spiny neurons (D1-MSNs) projecting from the core of nucleus accumbens (NAcC) to the LH were an important upstream circuit to activate LH glutamatergic neurons; D1-MSNs projecting from the NAcC to the LH activated LH glutamatergic neurons through inhibiting LH local gamma-aminobutyric acid (GABA) neurons. These results suggest that disinhibited LH glutamatergic neurons by neural circuits from the NAcC importantly contribute to context-induced the expression of morphine withdrawal memory.

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.

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.

Blockade of orexin receptor type-1 by SB-334867 and activation of orexin receptor type-2 attenuate morphine tolerance in rats

Purpose

The interaction of orexinergic neurons with the opioidergic system and their effects on morphine analgesia and tolerance have not been fully elucidated. The purpose of the study was to evaluate the effects of the orexin-1 and orexin-2 receptor (OX1R and OX2R) agonist and antagonist on morphine analgesia and tolerance in rats.

Material and methods

A total of 90 Wistar albino male rats weighing 180–220 g were used in the experiments. To induce morphine tolerance, rats were injected with a single dose of morphine (50 mg kg−1, s.c.) for 3 days. Morphine tolerance was assessed on day 4 in randomly selected rats by analgesia tests. In order to evaluate morphine tolerance situation, orexin-A, SB-334867, orexin-B and TCS OX2 29 were administered together with morphine for 3 days. The analgesic effects of orexin-A (10 μg kg−1), OXR1 antagonist SB-334867 (10 mg kg−1), OXR2 agonist orexin-B (15 μg kg−1), OXR2 antagonist TCS OX2 29 (0.5 mg kg−1) and morphine (5 mg kg−1) were measured at 15 or 30-min intervals by tail-flick and hot-plate antinociceptive tests.

Results

The results suggested that the combination of orexin-1 receptor antagonist SB-334867 and orexin-B with morphine significantly increased the analgesic effect compared to morphine-tolerant rats. In addition, administration of orexin-A and -B alone showed significant analgesic effects compared to the saline group. However, co-administration of orexin-A and -B with morphine did not increase the analgesic efficacy of morphine.

Conclusions

The results of this study demonstrated that co-administration of SB-334867 and orexin-B with morphine attenuated morphine tolerance. Further studies are needed to elucidate the details of the interaction between orexin receptors and the opioidergic system.

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.

Neural correlates and potential targets for the contribution of orexin to addiction in cortical and subcortical areas

The orexin (hypocretin) is one of the hypothalamic neuropeptides that plays a critical role in some behaviors including feeding, sleep, arousal, reward processing, and drug addiction. This variety of functions can be described by a united function for orexins in translating states of heightened motivation, for example during physiological requirement states or following exposure to reward opportunities, into planned goal-directed behaviors. An addicted state is characterized by robust activation of orexin neurons from the environment, which triggers downstream circuits to facilitate behavior directed towards obtaining the drug. Two orexin receptors 1 (OX1R) and 2 (OX2R) are widely distributed in the brain. Here, we will introduce and describe the cortical and subcortical brain areas involved in addictive-like behaviors and the impact of orexin on 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.

OX2R-selective orexin agonism is sufficient to ameliorate cataplexy and sleep/wake fragmentation without inducing drug-seeking behavior in mouse model of narcolepsy

Acquired loss of hypothalamic orexin (hypocretin)-producing neurons causes the chronic sleep disorder narcolepsy-cataplexy. Orexin replacement therapy using orexin receptor agonists is expected as a mechanistic treatment for narcolepsy. Orexins act on two receptor subtypes, OX1R and OX2R, the latter being more strongly implicated in sleep/wake regulation. However, it has been unclear whether the activation of only OX2R, or both OX1R and OX2R, is required to replace the endogenous orexin functions in the brain. In the present study, we examined whether the selective activation of OX2R is sufficient to rescue the phenotype of cataplexy and sleep/wake fragmentation in orexin knockout mice. Intracerebroventricular [Ala¹¹, _D-Leu¹⁵]-orexin-B, a peptidic OX2R-selective agonist, selectively activated OX2R-expressing histaminergic neurons in vivo, whereas intracerebroventricular orexin-A, an OX1R/OX2R non-selective agonist, additionally activated OX1R-positive noradrenergic neurons in vivo. Administration of [Ala¹¹, _D-Leu¹⁵]-orexin-B extended wake time, reduced state transition frequency between wake and NREM sleep, and reduced the number of cataplexy-like episodes, to the same degree as compared with orexin-A. Furthermore, intracerebroventricular orexin-A but not [Ala¹¹, _D-Leu¹⁵]-orexin-B induced drug-seeking behaviors in a dose-dependent manner in wild-type mice, suggesting that OX2R-selective agonism has a lower propensity for reinforcing/drug-seeking effects. Collectively, these findings provide a proof-of-concept for safer mechanistic treatment of narcolepsy-cataplexy through OX2R-selective agonism.

From Molecule to Behavior: Hypocretin/orexin Revisited From a Sex-dependent Perspective

The hypocretin/orexin (Hcrt/Orx) system in the perifornical lateral hypothalamus has been recognized as a critical node in a complex network of neuronal systems controlling both physiology and behavior in vertebrates. Our understanding of the Hcrt/Orx system and its array of functions and actions has grown exponentially in merely 2 decades. This review will examine the latest progress in discerning the roles played by the Hcrt/Orx system in regulating homeostatic functions and in executing instinctive and learned behaviors. Furthermore, the gaps that currently exist in our knowledge of sex-related differences in this field of study are discussed.

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.

Actions of feeding-related peptides on the mesolimbic dopamine system in regulation of natural and drug rewards

The mesolimbic dopamine system is the primary neural circuit mediating motivation, reinforcement, and reward-related behavior. The activity of this system and multiple behaviors controlled by it are affected by changes in feeding and body weight, such as fasting, food restriction, or the development of obesity. Multiple different peptides and hormones that have been implicated in the control of feeding and body weight interact with the mesolimbic dopamine system to regulate many different dopamine-dependent, reward-related behaviors. In this review, we summarize the effects of a selected set of feeding-related peptides and hormones acting within the ventral tegmental area and nucleus accumbens to alter feeding, as well as food, drug, and social reward.

Administration of Orexin-A into the Rat Thalamic Paraventricular Nucleus Enhances the Naloxone Induced Morphine Withdrawal

OBJECTIVE

Orexin neuropeptides are implicated in physical dependence on opioids and expression of withdrawal symptoms in drug abuse. The paraventricular nucleus of the midline thalamus (PVT) has a high expression of orexin receptors. The current research studied the effect of orexin-A in the PVT area on the development of behavioral indices produced by morphine withdrawal in rats.

METHODS

Male Wistar rats weighing 250-300 gr were utilised. To produce drug dependence, morphine (6, 16, 26, 36, 46, 56, and 66 mg/kg, 2 ml/kg) was injected at an interval of 24 hrs for 7 days. To assess the involvement of the orexin in withdrawal syndrome, we injected orexin-A (100 μM, 200 nl) into the PVT for 7 days before each morphine injection. On the day after the last injection of morphine, naloxone (2.5 mg/kg, i.p.) was injected to elicit the morphine withdrawal symptoms which were observed and checked for 25 min.

RESULTS

The results of the current research showed that the orexin-A in PVT enhances the severity of behavioral symptoms prompted by the injection of naloxone in drug-dependent rats.

CONCLUSIONS

These observations imply that targeting the orexin receptors in PVT might exhibit a new therapeutic strategy for the future treatment of dependence.

Neuroimaging reveals distinct brain glucose metabolism patterns associated with morphine consumption in Lewis and Fischer 344 rat strains

Vulnerability to addiction may be given by the individual's risk of developing an addiction during their lifetime. A challenge in the neurobiology of drug addiction is understanding why some people become addicted to drugs. Here, we used positron emission tomography (PET) and statistical parametric mapping (SPM) to evaluate changes in brain glucose metabolism in response to chronic morphine self-administration (MSA) in two rat strains with different vulnerability to drug abuse, Lewis (LEW) and Fischer 344 (F344). Four groups of animals were trained to self-administer morphine or saline for 15 days. 2-deoxy-2-[¹⁸F]-fluoro-d-glucose (FDG)-PET studies were performed on the last day of MSA (acquisition phase) and after 15 days of withdrawal. PET data were analyzed using SPM12. LEW-animals self-administered more morphine injections per session than F344-animals. We found significant brain metabolic differences between LEW and F344 strains in the cortex, hypothalamus, brainstem, and cerebellum. In addition, the different brain metabolic patterns observed after the MSA study between these rat strains indicate differences in the efficiency of neural substrates to translate the drug effects, which could explain the differences in predisposition to morphine abuse between one individual and another. These findings have important implications for the use of these rat strains in translational morphine and opiate research.

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.

Role of the Orexinergic System Within the Ventral Tegmental Area in the Development of Sensitization to Morphine Induced by Lateral Hypothalamus Stimulation

Introduction

The Lateral Hypothalamus (LH) has long been known to implicate the addictive behaviors of drug abuse. The Ventral Tegmental Area (VTA) is a major area of the mesolimbic system that is strongly involved in developing morphine sensitization. The current study aimed to examine the role of intra-VTA orexin receptors in the LH stimulation-induced sensitization to the antinociceptive response of morphine.

Methods

A total of 114 adult male Wistar rats underwent unilateral implantation of two separate cannulae in the LH and VTA using the stereotaxic apparatus. Intra-VTA administration of the Orexin-1 (OX1) and Orexin-2 (OX2) receptor antagonists, SB334867 and TCS OX2 29 (1, 3, and 10 nM/0.3 μL DMSO), respectively, was performed 5 min before concurrent microinjection of carbachol (250 nM/0.5 μL saline) into the LH and an ineffective dose of morphine (0.5 mg/kg; SC) during a 3-day sensitization period. After a 5-day free drug period, on the ninth day, for assessing the morphine sensitization, the nociceptive response was measured before and after morphine injection (1 mg/kg; SC) using the tail-flick test.

Results

The results revealed that the concurrent administration of carbachol (250 nM) and an ineffective dose of morphine significantly induced morphine sensitization. Besides, the blockade of OX1 and OX2 receptors within the VTA before intra-LH carbachol injection attenuated morphine sensitization.

Conclusion

These findings suggest that LH stimulation potentiates the sensitization to morphine antinociceptive responses via affecting orexin receptors located in the VTA. However, OX1 receptors contribute more than OX2 receptors in the VTA to morphine sensitization in rats.

Highlights

LH stimulation enhances sensitization to the ineffective dose of morphineIntra-VTA OX1 receptor involves in morphine sensitization-induced by LH stimulationIntra-VTA OX2 receptor involves in morphine sensitization-induced by LH stimulation.

Plain Language Summary

Behavioral sensitization, such as sensitization to the antinociceptive response of drugs, which defines as an enhanced systemic reaction to the same dose of addictive drugs, occurs in response to continuous and intermittent administration of these drugs. The Lateral Hypothalamus (LH) sends the orexinergic projections to the various regions of the brain and stimulation of LH induces sensitization to the antinociceptive response of morphine. The Ventral tegmental area (VTA) is a region of the brain that is strongly involved in developing morphine sensitization and receives orexinergic projections of LH. The current study aimed to examine the role of orexin receptors within the VTA in the LH stimulation-induced sensitization to the antinociceptive response of morphine in rats. In this study orexin-1 (OX1) and orexin-2 (OX2) receptors within the VTA region were blocked using their antagonists. After five minutes chemical stimulation of LH was done using carbachol microinjection into this area and ineffective dose of morphine was injected subcutaneously. These interventions were done for three consecutive days as sensitization period. After a 5-day free drug period, on the ninth day, for assessing the morphine sensitization, the nociceptive response was measured. The results revealed that the concurrent administration of LH stimulation and an ineffective dose of morphine significantly induced morphine sensitization. Besides, the blockade of OX1 and OX2 receptors within the VTA before LH stimulation attenuated sensitization to the antinociceptive response of morphine. Therefore, the orexinergic system plays an important role in morphine sensitization and can be considered as one of the potential targets to increase the analgesic effect of morphine.

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.

Similar role of mPFC orexin-1 receptors in the acquisition and expression of morphine- and food-induced conditioned place preference in male rats

Self-control problems are a typical character of drug addiction and excessive food consumption and it has been shown that natural rewards and drugs of abuse share parts of the same neural substrate and reward processing in the brain. Different brain areas are involved in natural and drug reward processing including the mesolimbic pathway, amygdala, nucleus accumbens (NAc), and prefrontal cortex. Considering the important role of orexins in the addictive behavior and the presence of orexin-1 subtype receptors (Orx1R) in the medial prefrontal cortex (mPFC), this study investigated the role of mPFC in natural- and drug-reward seeking behaviors to deepen our understanding of possible similarities or differences. To induce food- or morphine-conditioned place preference (CPP), adult male Wistar rats underwent CPP testing and received intra-mPFC doses of SB334867 (3, 10, or 30 nM/0.5 μl DMSO 12%), as an Orx1R antagonist, during the acquisition or expression phases of the CPP test. Results indicated that microinjection of Orx1R antagonist into the mPFC had similar effects on both morphine- and food-induced CPP and attenuated CPP scores in the acquisition and expression phases of the CPP test. The data demonstrated that Orx1Rs in the mPFC regulate the reward-related effects of morphine- and food-induced reward.

Connections of the mouse subfornical region of the lateral hypothalamus (LHsf)

The lateral hypothalamus is a major integrative hub with a complex architecture characterized by intricate and overlapping cellular populations expressing a large variety of neuro-mediators. In rats, the subfornical lateral hypothalamus (LHsf) was identified as a discrete area with very specific outputs, receiving a strong input from the nucleus incertus, and involved in defensive and foraging behaviors. We identified in the mouse lateral hypothalamus a discrete subfornical region where a conspicuous cluster of neurons express the mu opioid receptor. We thus examined the inputs and outputs of this LHsf region in mice using retrograde tracing with the cholera toxin B subunit and anterograde tracing with biotin dextran amine, respectively. We identified a connectivity profile largely similar, although not identical, to what has been described in rats. Indeed, the mouse LHsf has strong reciprocal connections with the lateral septum, the ventromedial hypothalamic nucleus and the dorsal pre-mammillary nucleus, in addition to a dense output to the lateral habenula. However, the light input from the nucleus incertus and the moderate bidirectional connectivity with nucleus accumbens are specific to the mouse LHsf. A preliminary neurochemical study showed that LHsf neurons expressing mu opioid receptors also co-express calcitonin gene-related peptide or somatostatin and that the reciprocal connection between the LHsf and the lateral septum may be functionally modulated by enkephalins acting on mu opioid receptors. These results suggest that the mouse LHsf may be hodologically and functionally comparable to its rat counterpart, but more atypical connections also suggest a role in consummatory behaviors.

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.

Central blockade of orexin type 1 receptors reduces naloxone induced activation of locus coeruleus neurons in morphine dependent rats

Orexin neuropeptides are implicated in the expression of morphine dependence. Locus coeruleus (LC) nucleus is an important brain area involving in the development of withdrawal signs of morphine and contains high expression of orexin type 1 receptors (OX1Rs). Despite extensive considerations, effects of immediate inhibition of OX1Rs by a single dose administration of SB-334867 prior to the naloxone-induced activation of LC neurons remains unknown. Therefore, we examined the direct effects of OX1Rs acute blockade on the neuronal activity of the morphine-dependent rats which underwent naloxone administration. Adult male rats underwent subcutaneous administration of 10 mg/kg morphine (two times/day) for a ten-day period. On the last day of experiment, intra-cerebroventricular administration of 10 μg/μl antagonist of OX1Rs, SB-334867, was performed just before intra-peritoneal injection of 2 mg/kg naloxone. Thereafter, in vivo extracellular single unit recording was employed to evaluate the electrical activity of LC neuronal cells. The outcomes demonstrated that morphine tolerance developed following ten-day of injection. Then, naloxone administration causes hyperactivity of LC neuronal cells, whereas a single dose administration of SB-334867 prior to naloxone prevented the enhanced activity of neurons upon morphine withdrawal. Our findings indicate that increased response of LC neuronal cells to applied naloxone could be prevented by the acute inhibition of the OX1Rs just before the naloxone treatment.

Effect of suvorexant on morphine tolerance and dependence in mice: Role of NMDA, AMPA, ERK and CREB proteins

The major problems of morphine use in the clinic are its tolerance and dependence. This study aimed to investigate the effect of suvorexant, a dual orexin receptor antagonist, on morphine-induced dependence and tolerance in mice and evaluate the level of NMDA, AMPA, ERK, p-ERK, CREB and p-CREB proteins in the brain. Tolerance and dependence were induced by repeated injection of morphine in mice (three times a day for 3 days, 50, 50, and 75 mg/kg /day). To evaluate the effects of the drugs on morphine-induced tolerance and dependence, suvorexant (30, 60 and 90 mg/kg), clonidine (positive control, 0.1 mg/kg) and saline were injected intraperitoneally 30 min before each injection of morphine. Tolerance and locomotor activity were assessed by tail-flick and open-field tests, respectively. The effect of suvorexant on the naloxone (5 mg/kg, ip)-induced morphine withdrawal, was also evaluated. Finally, the expression of proteins in the brain of mice was measured by western blot. Administration of suvorexant with morphine significantly reduced morphine-induced tolerance. Also, suvorexant attenuated the naloxone-precipitated opioid withdrawal. Suvorexant decreased morphine-enhanced levels of CREB and p-ERK proteins but did not affect the expression of NMDA and AMPA proteins compared to the morphine group. Suvorexant reduced morphine-induced tolerance and dependence through the inhibition of orexin receptors as well as changes in CREB and p-ERK protein levels in the brain.

Drug addiction co-morbidity with alcohol: Neurobiological insights

Addiction is a chronic disorder that consists of a three-stage cycle of binge/intoxication, withdrawal/negative affect, and preoccupation/anticipation. These stages involve, respectively, neuroadaptations in brain circuits involved in incentive salience and habit formation, stress surfeit and reward deficit, and executive function. Much research on addiction focuses on the neurobiology underlying single drug use. However, alcohol use disorder (AUD) can be co-morbid with substance use disorder (SUD), called dual dependence. The limited epidemiological data on dual dependence indicates that there is a large population of individuals suffering from addiction who are dependent on more than one drug and/or alcohol, yet dual dependence remains understudied in addiction research. Here, we review neurobiological data on neurotransmitter and neuropeptide systems that are known to contribute to addiction pathology and how the involvement of these systems is consistent or divergent across drug classes. In particular, we highlight the dopamine, opioid, corticotropin-releasing factor, norepinephrine, hypocretin/orexin, glucocorticoid, neuroimmune signaling, endocannabinoid, glutamate, and GABA systems. We also discuss the limited research on these systems in dual dependence. Collectively, these studies demonstrate that the use of multiple drugs can produce neuroadaptations that are distinct from single drug use. Further investigation into the neurobiology of dual dependence is necessary to develop effective treatments for addiction to multiple drugs.

Wnt signaling contributes to withdrawal symptoms from opioid receptor activation induced by morphine exposure or chronic inflammation

Preventing and treating opioid dependence and withdrawal is a major clinical challenge, and the underlying mechanisms of opioid dependence and withdrawal remain elusive. We hypothesized that prolonged morphine exposure or chronic inflammation-induced μ-opioid receptor activity serves as a severe stress that elicits neuronal alterations and recapitulates events during development. Here, we report that Wnt signaling, which is important in developmental processes of the nervous system, plays a critical role in withdrawal symptoms from opioid receptor activation in mice. Repeated exposures of morphine or peripheral inflammation produced by intraplantar injection of complete Freund's adjuvant significantly increase the expression of Wnt5b in the primary sensory neurons in dorsal root ganglion (DRG). Accumulated Wnt5b in DRG neurons quickly transmits to the spinal cord dorsal horn (DH) after naloxone treatment. In the DH, Wnt5b, acts through the atypical Wnt-Ryk receptor and alternative Wnt-YAP/TAZ signaling pathways, contributing to the naloxone-precipitated opioid withdrawal-like behavioral symptoms and hyperalgesia. Inhibition of Wnt synthesis and blockage of Wnt signaling pathways greatly suppress the behavioral and neurochemical alterations after naloxone-precipitated withdrawal. These findings reveal a critical mechanism underlying naloxone-precipitated opioid withdrawal, suggesting that targeting Wnt5b synthesis in DRG neurons and Wnt signaling in DH may be an effective approach for prevention and treatment of opioid withdrawal syndromes, as well as the transition from acute to chronic pain.

Deep Brain Stimulation of the Lateral Hypothalamus Facilitates Extinction and Prevents Reinstatement of Morphine Place Preference in Rats

OBJECTIVES

We have previously shown that high-frequency (HF) deep brain stimulation (DBS) of the lateral hypothalamus (LH) during the acquisition phase of morphine-induced conditioned place preference (CPP) abolished the development of morphine reward. In the present study, we investigated the effect of DBS in the LH during the extinction phase of morphine CPP.

MATERIALS AND METHODS

Rats were implanted with electrodes in the LH and went through conditioning trials for morphine CPP (40 min each, for three days), followed by extinction trials (20 min, for nine days). DBS-like stimulation (square pulses at 13 or 130 Hz, 200 μA, 100 μsec) was applied during the extinction trials.

RESULTS

Rats that received HF-DBS (130 Hz) accomplished extinction of morphine place preference by day 5 of the phase, whereas those in sham-stimulation or low-frequency-DBS (LF-DBS, 13 Hz) groups reached the criterion for extinction at day 8. One day later, rats received a priming injection of morphine (2 mg/kg) to reinstate the extinguished preference. While rats in the sham-DBS and LF-DBS relapsed into the state of preferring morphine-associated context, those in the HF-DBS group did not show such preference. Rats were then proceeded into an additional phase of extinction training (20 min, once daily, three to five days) with DBS, followed by restraint stress-induced reinstatement test. Again, sham-DBS and LF-DBS had no effect on relapse to the morphine place preferring state, but HF-DBS completely prevented the relapse.

CONCLUSION

HF-DBS facilitated extinction of morphine place preference and disrupted drug priming- and stress-induced renewal of morphine place preference.

Pleasure, addiction, and hypocretin (orexin)

The hypocretins/orexins were discovered in 1998. Within 2 years, this led to the discovery of the cause of human narcolepsy, a 90% loss of hypothalamic neurons containing these peptides. Further work demonstrated that these neurons were not simply linked to waking. Rather these neurons were active during pleasurable behaviors in waking and were silenced by aversive stimulation. This was seen in wild-type mice, rats, cats, and dogs. It was also evident in humans, with increased Hcrt release during pleasurable activities and decreased release, to the levels seen in sleep, during pain. We found that human heroin addicts have, on average, an increase of 54% in the number of detectable Hcrt neurons compared to "control" human brains and that these Hcrt neurons are substantially smaller than those in control brains. We found that in mice, chronic morphine administration induced the same changes in Hcrt neuron number and size. Our studies in the mouse allowed us to determine the specificity, dose response relations, time course of the change in the number of Hcrt neurons, and that the increased number of Hcrt neurons after opiates was not due to neurogenesis. Furthermore, we found that it took a month or longer for these anatomical changes in the mouse brain to return to baseline. Human narcoleptics, despite their prescribed use of several commonly addictive drugs, do not show significant evidence of dose escalation or substance use disorder. Similarly, mice in which the peptide has been eliminated are resistant to addiction. These findings are consistent with the concept that an increased number of Hcrt neurons may underlie and maintain opioid or cocaine use disorders.

Drug Addiction: Hyperkatifeia/Negative Reinforcement as a Framework for Medications Development

Compulsive drug seeking that is associated with addiction is hypothesized to follow a heuristic framework that involves three stages (binge/intoxication, withdrawal/negative affect, and preoccupation/anticipation) and three domains of dysfunction (incentive salience/pathologic habits, negative emotional states, and executive function, respectively) via changes in the basal ganglia, extended amygdala/habenula, and frontal cortex, respectively. This review focuses on neurochemical/neurocircuitry dysregulations that contribute to hyperkatifeia, defined as a greater intensity of negative emotional/motivational signs and symptoms during withdrawal from drugs of abuse in the withdrawal/negative affect stage of the addiction cycle. Hyperkatifeia provides an additional source of motivation for compulsive drug seeking via negative reinforcement. Negative reinforcement reflects an increase in the probability of a response to remove an aversive stimulus or drug seeking to remove hyperkatifeia that is augmented by genetic/epigenetic vulnerability, environmental trauma, and psychiatric comorbidity. Neurobiological targets for hyperkatifeia in addiction involve neurocircuitry of the extended amygdala and its connections via within-system neuroadaptations in dopamine, enkephalin/endorphin opioid peptide, and γ-aminobutyric acid/glutamate systems and between-system neuroadaptations in prostress corticotropin-releasing factor, norepinephrine, glucocorticoid, dynorphin, hypocretin, and neuroimmune systems and antistress neuropeptide Y, nociceptin, endocannabinoid, and oxytocin systems. Such neurochemical/neurocircuitry dysregulations are hypothesized to mediate a negative hedonic set point that gradually gains allostatic load and shifts from a homeostatic hedonic state to an allostatic hedonic state. Based on preclinical studies and translational studies to date, medications and behavioral therapies that reset brain stress, antistress, and emotional pain systems and return them to homeostasis would be promising new targets for medication development. SIGNIFICANCE STATEMENT: The focus of this review is on neurochemical/neurocircuitry dysregulations that contribute to hyperkatifeia, defined as a greater intensity of negative emotional/motivational signs and symptoms during withdrawal from drugs of abuse in the withdrawal/negative affect stage of the drug addiction cycle and a driving force for negative reinforcement in addiction. Medications and behavioral therapies that reverse hyperkatifeia by resetting brain stress, antistress, and emotional pain systems and returning them to homeostasis would be promising new targets for medication development.

The orexin (hypocretin) neuropeptide system is a target for novel therapeutics to treat cocaine use disorder with alcohol coabuse

An estimated 50-90% of individuals with cocaine use disorder (CUD) also report using alcohol. Cocaine users report coabusing alcohol to 'self-medicate' against the negative emotional side effects of the cocaine 'crash', including the onset of anxiety. Thus, pharmaceutical strategies to treat CUD would ideally reduce the motivational properties of cocaine, alcohol, and their combination, as well as reduce the onset of anxiety during drug withdrawal. The hypothalamic orexin (hypocretin) neuropeptide system offers a promising target, as orexin neurons are critically involved in activating behavioral and physiological states to respond to both positive and negative motivators. Here, we seek to describe studies demonstrating efficacy of orexin receptor antagonists in reducing cocaine, alcohol- and stress-related behaviors, but note that these studies have largely focused on each of these phenomena in isolation. For orexin-based compounds to be viable in the clinical setting, we argue that it is imperative that their efficacy be tested in animal models that account for polysubstance use patterns. To begin to examine this, we present new data showing that rats' preferred level of cocaine intake is significantly increased following chronic homecage access to alcohol. We also report that cocaine intake and motivation are reduced by a selective orexin-1 receptor antagonist when rats have a history of cocaine + alcohol, but not a limited history of cocaine alone. In light of these proof-of-principle data, we outline what we believe to be the key priorities going forward with respect to further examining the orexin system in models of polysubstance use. This article is part of the special issue on Neurocircuitry Modulating Drug and Alcohol Abuse.

Median Nerve Stimulation as a Nonpharmacological Approach to Bypass Analgesic Tolerance to Morphine: A Proof-of-Concept Study in Mice

Analgesic tolerance to opioids contributes to the opioid crisis by increasing the quantity of opioids prescribed and consumed. Thus, there is a need to develop non-opioid-based pain-relieving regimens as well as strategies to circumvent opioid tolerance. Previously, we revealed a non-opioid analgesic mechanism induced by median nerve electrostimulation at the overlaying PC6 (Neiguan) acupoint (MNS-PC6). Here, we further examined the efficacy of MNS-PC6 in morphine-tolerant mice with neuropathic pain induced by chronic constriction injury (CCI) of the sciatic nerve. Daily treatments of MNS-PC6 (2 Hz, 2 mA), but not electrostimulation at a nonmedian nerve-innervated location, for a week post-CCI induction significantly suppressed established mechanical allodynia in CCI-mice in an orexin-1 (OX₁) and cannabinoid-1 (CB₁) receptor-dependent fashion. This antiallodynic effect induced by repeated MNS-PC6 was comparable to that induced by repeated gabapentin (50 mg/kg, i.p.) or single morphine (10 mg/kg, i.p.) treatments, but without tolerance, unlike repeated morphine-induced analgesia. Furthermore, single and repeated MNS-PC6 treatments remained fully effective in morphine-tolerant CCI-mice, also in an OX₁ and CB₁ receptor-dependent fashion. In CCI-mice receiving escalating doses of morphine for 21 days (10, 20 and 50 mg/kg), single and repeated MNS-PC6 treatments remained fully effective. Therefore, repeated MNS-PC6 treatments induce analgesia without tolerance, and retain efficacy in opioid-tolerant mice via a mechanism that involves OX₁ and CB₁ receptors. This study suggests that MNS-PC6 is an alternative pain management strategy that maybe useful for combatting the opioid epidemic, and opioid-tolerant patients receiving palliative care. PERSPECTIVE: Median nerve stimulation relieves neuropathic pain in mice without tolerance and retains efficacy even in mice with analgesic tolerance to escalating doses of morphine, via an opioid-independent, orexin-endocannabinoid-mediated mechanism. This study provides a proof of concept for utilizing peripheral nerve stimulating devices for pain management in opioid-tolerant patients.

Non-Opioid Treatments for Opioid Use Disorder: Rationales and Data to Date

Opioid use disorder (OUD) represents a major public health problem that affects millions of people in the USA and worldwide. The relapsing and recurring aspect of OUD, driven by lasting neurobiological adaptations at different reward centres in the brain, represents a major obstacle towards successful long-term remission from opioid use. Currently, three drugs that modulate the function of the opioidergic receptors, methadone, buprenorphine and naltrexone have been approved by the US Food and Drug Administration (FDA) to treat OUD. In this review, we discuss the limitations and challenges associated with the current maintenance and medication-assisted withdrawal strategies commonly used to treat OUD. We further explore the involvement of glutamatergic, endocannabinoid and orexin signaling systems in the development, maintenance and expression of addiction-like behaviours in animal models of opioid addiction, and as potential and novel targets to expand therapeutic options to treat OUD. Despite a growing preclinical literature highlighting the role of these potential targets in animal models of opioid addiction, clinical and translational studies for novel treatments of OUD remain limited and inconclusive. Further preclinical and clinical investigations are needed to expand the arsenal of primary treatment options and adjuncts to maximise efficacy and prevent relapse.

Alteration of orexin-A and PKCα in the postmortem brain of pure-opioid and multi-drug abusers

Finding changes induced by the drug of abuse is one of the most important approaches to design new drugs for the treatment of substance use disorders (SUD). Postmortem study is the most reliable method for detecting alteration in the brain of SUD patients. Recently, the role of orexinergic system in SUD is in consideration. In the current study, we evaluated the level of orexin-A in the CSF and protein kinase Cα (PKCα) in the brain of pure-opioid (POA) and multi-drug abusers (MDA). A total of 56 POA, 45 MDA, and 13 matched control brains were collected from the legal medicine center, Tehran, Iran. The CSF was gathered from the third ventricle immediately after opening the skull and kept at -80 °C. The medial prefrontal cortex (mPFC), lateral prefrontal cortex (lPFC), orbitofrontal cortex (OFC), nucleus accumbens (NAc), and amygdala were dissected from fresh brain, frozen with liquid nitrogen and kept at -80 °C. The level of orexin-A evaluated in the CSF. Using western blotting, the level of PKCα assessed in the brain. Obtained data revealed that the level of orexin-A increased in POA and MDA compared with the control group (p < 0.05). In addition, the level of PKCα increased in the prefrontal cortex and amygdala of the abusers compared with the control group, although we did not detect changes in the level of PKCα in the NAc. Along with animal studies, the current results showed that the level of orexin increased in the CSF of drug abusers, which might be related to increases in the activation of lateral hypothalamic orexinergic neurons faced with the drug of abuse. Enhancement in the level of PKCα in the drug reward circuits might be adaptational changes induced by orexin and drugs of abuse.

Bidirectional Relationship between Opioids and Disrupted Sleep: Putative Mechanisms

Millions of Americans suffer from opiate use disorder, and over 100 die every day from opioid overdoses. Opioid use often progresses into a vicious cycle of abuse and withdrawal, resulting in very high rates of relapse. Although the physical and psychologic symptoms of opiate withdrawal are well-documented, sleep disturbances caused by chronic opioid exposure and withdrawal are less well-understood. These substances can significantly disrupt sleep acutely and in the long term. Yet poor sleep may influence opiate use, suggesting a bidirectional feed-forward interaction between poor sleep and opioid use. The neurobiology of how opioids affect sleep and how disrupted sleep affects opioid use is not well-understood. Here, we will summarize what is known about the effects of opioids on electroencephalographic sleep in humans and in animal models. We then discuss the neurobiology interface between reward-related brain regions that mediate arousal and wakefulness as well as the effect of opioids in sleep-related brain regions and neurotransmitter systems. Finally, we summarize what is known of the mechanisms underlying opioid exposure and sleep. A critical review of such studies, as well as recommendations of studies that evaluate the impact of manipulating sleep during withdrawal, will further our understanding of the cyclical feedback between sleep and opioid use. SIGNIFICANCE STATEMENT: We review recent studies on the mechanisms linking opioids and sleep. Opioids affect sleep, and sleep affects opioid use; however, the biology underlying this relationship is not understood. This review compiles recent studies in this area that fill this gap in knowledge.

Characterization of the intergenerational impact of in utero and postnatal oxycodone exposure

Prescription opioid abuse during and after pregnancy is a rising public health concern. While earlier studies have documented that offspring exposed to opioids in utero have impaired neurodevelopment, a significant knowledge gap remains in comparing the overall development between offspring exposed in utero and postnatally. Adding a layer of complexity is the role of heredity in the overall development of these exposed offspring. To fill in these important knowledge gaps, the current study uses a preclinical rat model mimicking oxycodone (oxy) exposure in utero (IUO) and postnatally (PNO) to investigate comparative and intergenerational effects in the two different treatment groups. While significant phenotypic attributes were observed with the two treatments and across the two generations, RNA sequencing revealed alterations in the expression of key synaptic genes in the two exposed groups in both generations. RNA sequencing and post validation of genes using RT-PCR highlighted the differential expression of several neuropeptides associated with the hypocretin system, a system recently implicated in addiction. Further, behavior studies revealed anxiety-like behaviors and social deficits that persisted even in the subsequent generations in the two treatment groups. To summarize, our study for the first time reveals a new line of investigation on the potential risks associated with oxy use during and after pregnancy, specifically the disruption of neurodevelopment and intergenerational impact on behavior.

Persistent effects of the orexin-1 receptor antagonist SB-334867 on naloxone precipitated morphine withdrawal symptoms and nociceptive behaviors in morphine dependent rats

AIM OF THE STUDY

In this study, we investigated the effect of long-term administration of orexin receptor 1 (OXR1) antagonist on naloxone-precipitated morphine withdrawal symptoms and nociceptive behaviors in morphine-dependent rats.

MATERIALS AND METHODS

Wistar rats received subcutaneous (s.c.) injections of morphine (6, 16, 26, 36, 46, 56, and 66 mg/kg, 2 ml/kg) at an interval of 24 h for 7 days. In chronic groups, the OXR1 antagonist, SB-334867 (20 mg/kg, i.p.), or its vehicle, was injected repetitively from postnatal day 1 (PND1)-PND23 and then for the following seven days before each morphine injection. Meanwhile, in acute groups, SB-334867, or its vehicle, was administered before each morphine injection. In groups of rats that were designated for withdrawal experiments, naloxone (2.5 mg/kg, i.p.) was administered after the last injection of morphine. In the formalin-induced pain, the effect of OXR1 inhibition on the antinociceptive effects of morphine was measured by injecting formalin after the final morphine injection.

RESULTS

Animals that received long-term SB-334867 administration before morphine injection demonstrated a significant reduction in chewing, defecation, diarrhea, grooming, teeth chattering, wet-dog shake, and writhing. Inhibiting OXR1 for a long time increased formalin-induced nociceptive behaviors in interphase and phase II of the formalin-induced pain.

CONCLUSIONS

Our results indicated that the inhibition of OXR1 significantly reduces the development of morphine dependence and behavioral signs elicited by the administration of naloxone in morphine-dependent rats. Furthermore, the prolonged blockade of OXR1 might be involved in formalin-induced nociceptive behaviors.