Multiple roles for orexin/hypocretin in addiction

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.

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.

Prediction of adverse events risk in patients with comorbid post-traumatic stress disorder and alcohol use disorder using electronic medical records by deep learning models

BACKGROUND

Identifying co-occurring mental disorders and elevated risk is vital for optimization of healthcare processes. In this study, we will use DeepBiomarker2, an updated version of our deep learning model to predict the adverse events among patients with comorbid post-traumatic stress disorder (PTSD) and alcohol use disorder (AUD), a high-risk population.

METHODS

We analyzed electronic medical records of 5565 patients from University of Pittsburgh Medical Center to predict adverse events (opioid use disorder, suicide related events, depression, and death) within 3 months at any encounter after the diagnosis of PTSD+AUD by using DeepBiomarker2. We integrated multimodal information including: lab tests, medications, co-morbidities, individual and neighborhood level social determinants of health (SDoH), psychotherapy and veteran data.

RESULTS

DeepBiomarker2 achieved an area under the receiver operator curve (AUROC) of 0.94 on the prediction of adverse events among those PTSD+AUD patients. Medications such as vilazodone, dronabinol, tenofovir, suvorexant, modafinil, and lamivudine showed potential for risk reduction. SDoH parameters such as cognitive behavioral therapy and trauma focused psychotherapy lowered risk while active veteran status, income segregation, limited access to parks and greenery, low Gini index, limited English-speaking capacity, and younger patients increased risk.

CONCLUSIONS

Our improved version of DeepBiomarker2 demonstrated its capability of predicting multiple adverse event risk with high accuracy and identifying potential risk and beneficial factors.

Orexin-1 Receptor Antagonist SB-334867 Enhances Formalin-Induced Nociceptive Behaviors in Adult Male Rats

Background

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. Neurons that produce orexin are scattered mediolaterally within the Dorsomedial Hypothalamus (DMH) and the lateral hypothalamus. In the current research, we assessed the impact of prolonged application of the antagonist of Orexin Receptor 1 (OXR1) on nociceptive behaviors in adult male rats.

Methods

Sixteen Wistar rats received subcutaneous (s.c.) injections of the OXR1 antagonist, SB-334867 (20 mg/kg, i.p.), or its vehicle repetitively from Postnatal Day 1 (PND1)-PND30. On the 30th day following the final application of the OXR1 antagonist formalin-provoked pain was evaluated by injecting formalin.

Results

Administration of the OXR1 antagonist in the long-term augmented the formalin-provoked nociceptive behaviors in interphase and phase II of the formalin-induced pain.

Conclusion

Current results showed that the continued inhibiting OXR1 might be implicated in formalin-induced nociceptive behaviors. Therefore, the present study highlighted the effect of orexin on analgesia.

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.

Prediction of Adverse Events Risk in Patients with Comorbid Post- Traumatic Stress Disorder and Alcohol Use Disorder Using Electronic Medical Records by Deep Learning Models

Background

Prediction of high-risk events in mental disorder patients is crucial. In our previous study, we developed a deep learning model: DeepBiomarker by using electronic medical records (EMR) to predict suicide related event (SRE) risk in post-traumatic stress disorder (PTSD) patients.

Methods

We applied DeepBiomarker2 through data integration of multimodal information: lab test, medication, co-morbidities, and social determinants of health. We analyzed EMRs of 5,565 patients from University of Pittsburgh Medical Center with a diagnosis of PTSD and alcohol use disorder (AUD) on risk of developing an adverse event (opioid use disorder, SREs, depression and death).

Results

DeepBiomarker2 predicted whether a PTSD + AUD patient will have a diagnosis of any adverse events (SREs, opioid use disorder, depression, death) within 3 months with area under the receiver operator curve (AUROC) of 0.94. We found piroxicam, vilazodone, dronabinol, tenofovir, suvorexant, empagliflozin, famciclovir, veramyst, amantadine, sulfasalazine, and lamivudine to have potential to reduce risk.

Conclusions

DeepBiomarker2 can predict multiple adverse event risk with high accuracy and identify potential risk and beneficial factors. Our results offer suggestions for personalized interventions in a variety of clinical and diverse populations.

Theoretical Frameworks and Mechanistic Aspects of Alcohol Addiction: Alcohol Addiction as a Reward Deficit/Stress Surfeit Disorder

Alcohol use disorder (AUD) can be defined by a compulsion to seek and take alcohol, the loss of control in limiting intake, and the emergence of a negative emotional state when access to alcohol is prevented. Alcohol use disorder impacts multiple motivational mechanisms and can be conceptualized as a disorder that includes a progression from impulsivity (positive reinforcement) to compulsivity (negative reinforcement). Compulsive drug seeking that is associated with AUD can be derived from multiple neuroadaptations, but the thesis argued herein is that a key component involves the construct of negative reinforcement. Negative reinforcement is defined as drug taking that alleviates a negative emotional state. The negative emotional state that drives such negative reinforcement is hypothesized to derive from the dysregulation of specific neurochemical elements that are involved in reward and stress within basal forebrain structures that involve the ventral striatum and extended amygdala, respectively. Specific neurochemical elements in these structures include decreases in reward neurotransmission (e.g., decreases in dopamine and opioid peptide function in the ventral striatum) and the recruitment of brain stress systems (e.g., corticotropin-releasing factor [CRF]) in the extended amygdala, which contributes to hyperkatifeia and greater alcohol intake that is associated with dependence. Glucocorticoids and mineralocorticoids may play a role in sensitizing the extended amygdala CRF system. Other components of brain stress systems in the extended amygdala that may contribute to the negative motivational state of withdrawal include norepinephrine in the bed nucleus of the stria terminalis, dynorphin in the nucleus accumbens, hypocretin and vasopressin in the central nucleus of the amygdala, and neuroimmune modulation. Decreases in the activity of neuropeptide Y, nociception, endocannabinoids, and oxytocin in the extended amygdala may also contribute to hyperkatifeia that is associated with alcohol withdrawal. Such dysregulation of emotional processing may also significantly contribute to pain that is associated with alcohol withdrawal and negative urgency (i.e., impulsivity that is associated with hyperkatifeia during hyperkatifeia). Thus, an overactive brain stress response system is hypothesized to be activated by acute excessive drug intake, to be sensitized during repeated withdrawal, to persist into protracted abstinence, and to contribute to the compulsivity of AUD. The combination of the loss of reward function and recruitment of brain stress systems provides a powerful neurochemical basis for a negative emotional state that is responsible for the negative reinforcement that at least partially drives the compulsivity of AUD.

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.

Use of experimental medicine approaches for the development of novel psychiatric treatments based on orexin receptor modulation

Despite progress in understanding the pathological mechanisms underlying psychiatric disorders, translation from animal models into clinical use remains a significant bottleneck. Preclinical studies have implicated the orexin neuropeptide system as a potential target for psychiatric disorders through its role in regulating emotional, cognitive, and behavioral processes. Clinical studies are investigating orexin modulation in addiction and mood disorders. Here we review performance-outcome measures (POMs) arising from experimental medicine research methods which may show promise as markers of efficacy of orexin receptor modulators in humans. POMs provide objective measures of brain function, complementing patient-reported or clinician-observed symptom evaluation, and aid the translation from preclinical to clinical research. Significant challenges include the development, validation, and operationalization of these measures. We suggest that collaborative networks comprising clinical practitioners, academics, individuals working in the pharmaceutical industry, drug regulators, patients, patient advocacy groups, and other relevant stakeholders may provide infrastructure to facilitate validation of experimental medicine approaches in translational research and in the implementation of these approaches in real-world clinical practice.

The Role of Orexin-1 Receptors Within the Hippocampal CA1 Area in the Extinction and Reinstatement of Methamphetamine-Seeking Behaviors

Psychostimulant addiction is a chronic brain disorder with high relapse rates, requiring new therapeutic strategies. The orexin system is highly implicated in processing reward and addiction through connections with critical areas such as the hippocampus. This study investigated the role of orexin-1 receptors (OX1R) within the CA1 subregion of the hippocampus in the extinction and reinstatement of the methamphetamine-induced conditioned place preference. After cannulae implantation, recovery, and establishing the methamphetamine place preference, 98 male Wistar rats received different doses of bilateral intra-CA1 selective OX1R antagonist, SB334867 (1, 3, 10, and 30 nmol/0.5 μl DMSO per side) during the 10-day extinction period (daily) or after extinction phase, just on the reinstatement day (single dose) in separate experimental and control groups. The findings indicated that bilateral microinjection of SB334867 into the CA1 area during the extinction period could significantly reduce the extinction latency and maintenance of rewarding aspects of methamphetamine dose-dependently (3, 10, and 30 nmol). In another set of experiments, a single dose of bilateral intra-CA1 SB334867 administration on the reinstatement phase prevented the methamphetamine-induced reinstatement of drug-seeking behaviors at the high doses (10, and 30 nmol). The present study provided more evidence for the implication of hippocampal OX1R in the maintenance of rewarding and reinforcing properties of methamphetamine and its role in the relapse of methamphetamine-seeking behavior. Further investigations on the role of the orexin system, including the orexin-2 receptors in treating addiction, are needed to introduce its antagonists as effective therapeutic options for psychostimulant addiction.

Sleep-mediated regulation of reward circuits: implications in substance use disorders

Our modern society suffers from both pervasive sleep loss and substance abuse-what may be the indications for sleep on substance use disorders (SUDs), and could sleep contribute to the individual variations in SUDs? Decades of research in sleep as well as in motivated behaviors have laid the foundation for us to begin to answer these questions. This review is intended to critically summarize the circuit, cellular, and molecular mechanisms by which sleep influences reward function, and to reveal critical challenges for future studies. The review also suggests that improving sleep quality may serve as complementary therapeutics for treating SUDs, and that formulating sleep metrics may be useful for predicting individual susceptibility to SUDs and other reward-associated psychiatric diseases.

Alternative use of suvorexant (Belsomra®) for the prevention of alcohol drinking and seeking in rats with a history of alcohol dependence

Alcohol use disorder (AUD) is one of the most treatment-resistant medical conditions globally. The orexin (Orx) system regulates diverse physiological processes, including stress, and is a system of interest for the development of pharmaceuticals to treat substance use disorders, particularly AUD. The present study tested the ability of the dual orexin receptor antagonist suvorexant (SUV), marketed by Merck as Belsomra®, for the treatment of insomnia, to decrease alcohol self-administration and the stress-induced reinstatement of alcohol-seeking behavior in male Wistar rats with a history of alcohol dependence. Rats were trained to orally self-administer 10% alcohol (30 min/day for 3 weeks) and were either made dependent via chronic intermittent alcohol vapor exposure (14 h ON, 10 h OFF) for 6 weeks or exposed to air (non-dependent). Starting on week 7, the effect of SUV (0-20 mg/kg, p.o.) was tested on alcohol self-administration at acute abstinence (8 h after vapor was turned OFF) twice weekly. A separate cohort of rats that were prepared in parallel was removed from alcohol vapor exposure and then subjected to extinction training for 14 sessions. Once extinction was achieved, the rats received SUV (0 and 5 mg/kg, p.o.) and were tested for the footshock stress-induced reinstatement of alcohol-seeking behavior. Suvorexant at 5, 10, and 20 mg/kg selectively decreased alcohol intake in dependent rats. Furthermore, 5 mg/kg SUV prevented the stress-induced reinstatement of alcohol-seeking behavior in dependent rats only. These results underscore the significance of targeting the Orx system for the treatment of substance use disorders generally and suggest that repurposing SUV could be an alternative approach for the treatment of AUD.

Rapid Eye Movement Sleep Engages Melanin-Concentrating Hormone Neurons to Reduce Cocaine Seeking

BACKGROUND

Persistent sleep disruptions following withdrawal from abused drugs may hold keys to battle drug relapse. It is posited that there may be sleep signatures that predict relapse propensity, identifying which may open new avenues for treating substance use disorders.

METHODS

We trained male rats (approximately postnatal day 56) to self-administer cocaine. After long-term drug withdrawal (approximately postnatal day 100), we examined the correlations between the intensity of cocaine seeking and key sleep features. To test for causal relationships, we then used behavioral, chemogenetic, or optogenetic methods to selectively increase rapid eye movement sleep (REMS) and measured behavioral and electrophysiological outcomes to probe for cellular and circuit mechanisms underlying REMS-mediated regulation of cocaine seeking.

RESULTS

A selective set of REMS features was preferentially associated with the intensity of cue-induced cocaine seeking after drug withdrawal. Moreover, selectively increasing REMS time and continuity by environmental warming attenuated a withdrawal time-dependent intensification of cocaine seeking, or incubation of cocaine craving, suggesting that REMS may benefit withdrawal. Warming increased the activity of lateral hypothalamic melanin-concentrating hormone (MCH) neurons selectively during prolonged REMS episodes and counteracted cocaine-induced synaptic accumulation of calcium-permeable AMPA receptors in the nucleus accumbens-a critical substrate for incubation. Finally, the warming effects were partly mimicked by chemogenetic or optogenetic stimulations of MCH neurons during sleep, or intra-accumbens infusions of MCH peptide during the rat's inactive phase.

CONCLUSIONS

REMS may encode individual vulnerability to relapse, and MCH neuron activities can be selectively targeted during REMS to reduce drug relapse.

Exploring Addictive Online Behaviors in Patients with Narcolepsy Type 1

BACKGROUND

Narcolepsy type 1 (NT1) is a rare neurological sleep disorder caused by the loss of neurons that produce hypocretin-a peptide that plays a crucial role in addictive behaviors. We aimed to compare, for the first time, levels of problematic online gaming, problematic social media use, and compulsive Internet use between NT1 patients and healthy controls (HC), and to evaluate the association between anxiety, depression, and emotion dysregulation with addictive online behaviors in NT1 patients.

METHODS

A total of 43 patients with NT1 and 86 sex- and age-matched HC participated in an online cross-sectional survey.

RESULTS

NT1 patients did not differ from HC in terms of problematic social media use and compulsive Internet use but displayed higher levels of problematic online gaming compared to HC. Higher levels of emotion dysregulation were significantly associated with higher levels of problematic social media use and compulsive Internet use, while none of the tested factors were associated with problematic online gaming.

CONCLUSION

NT1 patients and HC had similar levels of problematic social media use and compulsive Internet use, but NT1 patients showed higher levels of problematic online gaming. Emotion dysregulation might be an intervention target for reducing compulsive Internet use and problematic social media use.

Effects of single and dual hypocretin-receptor blockade or knockdown of hypocretin projections to the central amygdala on alcohol drinking in dependent male rats

Hypocretin/Orexin (HCRT) is a neuropeptide that is associated with both stress and reward systems in humans and rodents. The different contributions of signaling at hypocretin-receptor 1 (HCRT-R1) and hypocretin-receptor 2 (HCRT-R2) to compulsive alcohol drinking are not yet fully understood. Thus, the current studies used pharmacological and viral-mediated targeting of HCRT to determine participation in compulsive alcohol drinking and measured HCRT-receptor mRNA expression in the extended amygdala of both alcohol-dependent and non-dependent male rats. Rats were made dependent through chronic intermittent exposure to alcohol vapor and were tested for the acute effect of HCRT-R1-selective (SB-408124; SB-R1), HCRT-R2-selective (NBI-80713; NB-R2), or dual HCRT-R1/2 (NBI-87571; NB-R1/2) antagonism on alcohol intake. NB-R2 and NB-R1/2 antagonists each dose-dependently decreased overall alcohol drinking in alcohol-dependent rats, whereas, SB-R1 decreased alcohol drinking in both alcohol-dependent and non-dependent rats at the highest dose (30 mg/kg). SB-R1, NB-R2, and NB-R1/2 treatment did not significantly affect water drinking in either alcohol-dependent or non-dependent rats. Additional PCR analyses revealed a significant decrease in Hcrtr1 mRNA expression within the central amygdala (CeA) of dependent rats under acute withdrawal conditions compared to nondependent rats. Lastly, a shRNA-encoding adeno-associated viral vector with retrograde function was used to knockdown HCRT in CeA-projecting neurons from the lateral hypothalamus (LH). LH-CeA HCRT knockdown significantly attenuated alcohol self-administration in alcohol-dependent rats. These observations suggest that HCRT signaling in the CeA is necessary for alcohol-seeking behavior during dependence. Together, these data highlight a role for both HCRT-R1 and -R2 in dependent alcohol-seeking behavior.

Potential biomarkers of addiction identified by real-time PCR in human peripheral blood lymphocytes: a narrative review

Addiction-related neurobiological factors could be considered as potential biomarkers. The concentration of peripheral biomarkers in tissues like blood lymphocytes may mirror their brain levels. This review is focused on the mRNA expression of potential addiction biomarkers in human peripheral blood lymphocytes (PBLs). PubMed, EMBASE, Web of Science, Scopus and Google Scholar were searched using the keywords 'addiction', 'biomarker', 'peripheral blood lymphocyte', 'gene expression' and 'real-time PCR'. The results showed the alterations in the regulation of genes such as dopamine receptors, opioid receptors, NMDA receptors, cannabinoid receptors, α-synuclein, DYN, MAO-A, FosB and orexin-A as PBLs biomarkers in addiction stages. Such variations could also be found during abstinence and relapse. PBLs biomarkers may help in drug development and have clinical implications.

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.

Orexin enhances neuronal synchronization in adult rat hypothalamic culture: a model to study hypothalamic function

The regulation of sleep/wake behavior and energy homeostasis is maintained in part by the hypothalamic neuropeptide orexin A (OXA, hypocretin). Reduction in orexin signaling is associated with sleep disorders and obesity, whereas higher lateral hypothalamic (LH) orexin signaling and sensitivity promotes obesity resistance. Similarly, dysregulation of hypothalamic neural networks is associated with onset of age-related diseases, including obesity and several neurological diseases. Despite the association of obesity and aging, and that adult populations are the target for the majority of pharmaceutical and obesity studies, conventional models for neuronal networks utilize embryonic neural cultures rather than adult neurons. Synchronous activity describes correlated changes in neuronal activity between neurons and is a feature of normal brain function, and is a measure of functional connectivity and final output from a given neural structure. Earlier studies show alterations in hypothalamic synchronicity following behavioral perturbations in embryonic neurons obtained from obesity-resistant rats and following application of orexin onto embryonic hypothalamic cultures. Synchronous network dynamics in adult hypothalamic neurons remain largely undescribed. To address this, we established an adult rat hypothalamic culture in multi-electrode-array (MEA) dishes and recorded the field potentials. Then we studied the effect of exogenous orexin on network synchronization of these adult hypothalamic cultures. In addition, we studied the wake promoting effects of orexin in vivo when directly injected into the lateral hypothalamus (LH). Our results showed that the adult hypothalamic cultures are viable for nearly 3 mo in vitro, good quality MEA recordings can be obtained from these cultures in vitro, and finally, that cultured adult hypothalamus is responsive to orexin. These results support that adult rat hypothalamic cultures could be used as a model to study the neural mechanisms underlying obesity. In addition, LH administration of OXA enhanced wakefulness in rats, indicating that OXA enhances wakefulness partly by promoting neural synchrony in the hypothalamus.NEW & NOTEWORTHY This study, for the first time, demonstrates that adult hypothalamic cultures are viable in vitro for a prolonged duration and are electrophysiologically active. In addition, the study shows that orexin enhances neural synchronization in adult hypothalamic cultures.

Effects of selective orexin receptor-2 and cannabinoid receptor-1 antagonists on the response of medial prefrontal cortex neurons to tramadol

Tramadol is widely used to control pain in various diseases, but the relevant mechanisms are less known despite the severe risks of abuse. The medial prefrontal cortex (mPFC) is one of the critical centers of the reward system. Studies have shown that orexins and endocannabinoids are likely to play an important role in addiction. In this study, the effect of orexin receptor-2 (OX2R) and endocannabinoid receptor-1 (CB1R) blockade on the neuronal activity of mPFC was investigated in response to tramadol in male rats. Tramadol was injected intraperitoneally, and its effects on the firing of mPFC pyramidal neurons were investigated using in vivo extracellular single-unit recording. Tramadol affected the pyramidal neuronal activity of the mPFC. AM251 (18 nmol/4 μl), as a selective CB1R antagonist, and TCS-OX2-29 (50 nmol/4 μl), as a selective OX2R antagonist, individually or simultaneously were microinjected into the lateral ventricle of the brain (intracerebroventricular, ICV). The results showed that the ratio of neurons with the excitatory/inhibitory or no responses was significantly changed by tramadol (p < .05). These changes were prevented by blockade of CB1Rs alone or blockade of OX2Rs and CB1Rs simultaneously (p < .05). However, blockade of these receptors in the vehicle group had no significant effect on neuronal activity. The findings of this study indicate the potential role of orexin and endocannabinoid systems in mediating the effects of tramadol in mPFC and the possible interaction between the two systems via OX2 and CB1 receptors. However, further studies are needed to identify these effects by examining intracellular signaling.

Orexin-1 receptor signaling in ventral tegmental area mediates cue-driven demand for cocaine

Drug-associated sensory cues increase motivation for drug and the orexin system is importantly involved in this stimulus-enhanced motivation. Ventral tegmental area (VTA) is a major target by which orexin signaling modulates reward behaviors, but it is unknown whether this circuit is necessary for cue-driven motivation for cocaine. Here, we investigated the role of VTA orexin signaling in cue-driven motivation for cocaine using a behavioral economics (BE) paradigm. We found that infusion of the orexin-1 receptor (Ox1R) antagonist SB-334867 (SB) into VTA prior to BE testing reduced motivation when animals were trained to self-administer cocaine with discrete cues and tested on BE with those cues. SB had no effect when animals were trained to self-administer cocaine without cues or tested on BE without cues, indicating that learning to associate cues with drug delivery during self-administration training was necessary for cues to recruit orexin signaling in VTA. These effects were specific to VTA, as injections of SB immediately dorsal had no effect. Moreover, intra-VTA SB did not have an impact on locomotor activity, or low- or high-effort consumption of sucrose. Finally, we microinjected a novel retrograde adeno-associated virus (AAVretro) containing an orexin-specific short hairpin RNA (OxshRNA) into VTA to knock down orexin in the hypothalamus-VTA circuit. These injections significantly reduced orexin expression in lateral hypothalamus (LH) and decreased cue-driven motivation. These studies demonstrate a role for orexin signaling in VTA, specifically when cues predict drug reward.

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.

Orexin neuropeptides modulate the hippocampal-dependent memory through basolateral amygdala interconnections

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Orexin neuropeptides roles in hippocampal-dependent memory formation.

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Orexin neuropeptides activate the neural circuits of the basolateral amygdala.

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The power of memorization is modulated by the level of orexin neuropeptides.

Orexin neuropeptides have functional roles in hippocampal-dependent memory formation via the consolidation and retrieval of passive avoidance and spatial memories. The effects of these neuropeptides have been confirmed on the induction of long-term potentiation (LTP). The orexinergic system seems to have modulatory effects by sending projection fibers to several brain parts, such as the hippocampus and amygdala. Orexin neuropeptides activate the neural circuits of the basolateral amygdala during different arousal events with various emotional loads. Therefore, this system plays a vital role in creating appropriate behavioral reactions and responses particular to the situation. This review aimed to report new progression and advances in the hippocampus function in memory by focusing on its relationship with the amygdala through the orexinergic system.

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.

Orexin receptor blockers: A tool for lowering alcohol intake and alcohol addictive behavior in the light of preclinical studies

Alcohol use disorder (AUD) is a severe and globally widespread neurological and psychiatric problem. The treatment with currently used drugs often does not bring the expected effect. New optimization methods or directions in pharmacotherapy are still being sought. The group of bioactive ligands, targeted at neuropeptides called orexins (OXs) and their receptors (OXRs), affects a number of functions including ingestion, sleep-wake regulation, as well as the brain reward system which is the basis of addiction.

The purpose of this paper is to systematize the knowledge in the field of preclinical behavioral studies on rodents (rats and mice) in several models of alcohol consumption using the OXRs antagonists.

The results of the experiments indicated a potential efficacy of particular OXRs antagonists in the AUD treatment, especially those selectively blocking the OX1R. Among them, SB-334867 in the lowest effective dose of 3 mg/kg i.p. was most studied, as shown in the model of two-bottle choice using C57BL/6 mice. Moreover, this compound did not affect the reduction of cognitive functions. GSK1059865 was also involved in the selective reduction of ethanol intake, and simultaneously did not alter the consumption of sugar solution. The other group of selective OX2R antagonists, such as TCS-OX2-29 and LSN2424100, was less efficient.

In summary, the OX1R antagonists proved to have the potential in AUD therapy, not only through the reduction of ethanol consumption but also in the treatment of coexisting behavioral and physiological disorders, such as insomnia and anxiety.

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.

Choline-Sigma-1R as an Additional Mechanism for Potentiation of Orexin by Cocaine

Orexin A, an endogenous peptide involved in several functions including reward, acts via activation of orexin receptors OX1 and OX2, Gq-coupled GPCRs. We examined the effect of a selective OX1 agonist, OXA (17-33) on cytosolic calcium concentration, [Ca2+]i, in neurons of nucleus accumbens, an important area in the reward circuit. OXA (17-33) increased [Ca2+]i in a dose-dependent manner; the effect was prevented by SB-334867, a selective OX1 receptors antagonist. In Ca2+-free saline, the OXA (17-33)-induced increase in [Ca2+]i was not affected by pretreatment with bafilomycin A1, an endo-lysosomal calcium disrupter, but was blocked by 2-APB and xestospongin C, antagonists of inositol-1,4,5-trisphosphate (IP3) receptors. Pretreatment with VU0155056, PLD inhibitor, or BD-1047 and NE-100, Sigma-1R antagonists, reduced the [Ca2+]i response elicited by OXA (17-33). Cocaine potentiated the increase in [Ca2+]i by OXA (17-33); the potentiation was abolished by Sigma-1R antagonists. Our results support an additional signaling mechanism for orexin A-OX1 via choline-Sigma-1R and a critical role for Sigma-1R in the cocaine-orexin A interaction in nucleus accumbens neurons.

Should we or shouldn't we?: Preface to the Special Issue "Neurochemistry of Reward-Seeking"

Reward-seeking is critical for survival. Learning about the relationship between actions and outcomes helps us to make decisions and select behaviours that result in a specific outcome. This special issue entitled "The Neurochemistry of Reward-Seeking" addresses this crucial facet of behaviour and brings together a number of key thought-leaders to provide a timely update on the circuitry, chemistry and mechanisms underlying different aspects of reward-seeking. The reviews in this issue canvass unanswered questions in the field and provide a degree of forethought about how we may advance our understanding of reward-seeking by embracing novel technology alongside existing scholarship. This issue also highlights the neurochemical complexity of reward-seeking, and the reader will uncover both distinct and shared circuits and transmitters driving various forms of reward-seeking. Accordingly, we hope that this special issue will provide a valuable resource for the field and trigger future research on this topic.

Recent Perspectives on Sex Differences in Compulsion-Like and Binge Alcohol Drinking

Alcohol use disorder remains a substantial social, health, and economic problem and problem drinking levels in women have been increasing in recent years. Understanding whether and how the underlying mechanisms that drive drinking vary by sex is critical and could provide novel, more targeted therapeutic treatments. Here, we examine recent results from our laboratories and others which we believe provide useful insights into similarities and differences in alcohol drinking patterns across the sexes. Findings for binge intake and aversion-resistant, compulsion-like alcohol drinking are considered, since both are likely significant contributors to alcohol problems in humans. We also describe studies regarding mechanisms that may underlie sex differences in maladaptive alcohol drinking, with some focus on the importance of nucleus accumbens (NAcb) core and shell regions, several receptor types (dopamine, orexin, AMPA-type glutamate), and possible contributions of sex hormones. Finally, we discuss how stressors such as early life stress and anxiety-like states may interact with sex differences to contribute to alcohol drinking. Together, these findings underscore the importance and critical relevance of studying female and male mechanisms for alcohol and co-morbid conditions to gain a true and clinically useful understanding of addiction and neuropsychiatric mechanisms and treatment.

Does endogenous cholecystokinin modulate alcohol intake?

Alcohol use disorder or alcoholism is characterized by uncontrollable alcohol use and intoxication, as well as a heightened state of anxiety after alcohol withdrawal. Ethanol-associated stimuli also drive the urge to drink by means of classical conditioning. Alcoholism has been considered a dopamine (DA) dysregulation syndrome that involves the activity of the central amygdala circuitry of anxiety. Cholecystokinin (CCK) is the most abundant neuropeptide in the mammal brain, where it activates two receptors, CCK₁ and CCK₂. Genetic evidence relates CCK₁ receptors to alcoholism in humans. CCK₂ activity has been associated with the onset of human anxiety. CCK modulates DA release in the nucleus accumbens (NAc) and it is expressed in the γ-aminobutyric acid (GABA)-expressing basket interneurons in the cerebral cortex. CCK interacts with serotonin (5-HT) neurotransmission through 5-HT₃ receptors to regulate mesocorticolimbic pathways and with GABA to attenuate anxiety in the amygdala. Finally, CCK stimulates the release of orexins and oxytocin in the hypothalamus, two relevant hypothalamic neuropeptides involved in signaling satiety for ethanol and well-being respectively. Given the "dimmer-switch" function of endogenous CCK in the neurotransmission by 5-HT, DA, GABA, and glutamate in normal and pathological behaviors (Ballaz and Bourin, 2020), we hypothesize that CCK adjusts functioning of the reward and anxiety circuitries altered by ethanol. This review gathers data supporting this hypothesis, and suggests mechanisms underlying a role for endogenous CCK in alcoholism.

Orexins (hypocretins): The intersection between homeostatic and hedonic feeding

Orexins are hypothalamic neuropeptides originally discovered to play a role in the regulation of feeding behaviour. The broad connections of orexin neurons to mesocorticolimbic circuitry suggest they may play a role in mediating reward-related behaviour beyond homeostatic feeding. Here, we review the role of orexin in a variety of eating-related behaviour, with a focus on reward and motivation, and the neural circuits driving these effects. One emerging finding is the involvement of orexins in hedonic and appetitive behaviour towards palatable food, in addition to their role in homeostatic feeding. This review discusses the brain circuitry and possible mechanisms underlying the role of orexins in these behaviours. Overall, there is a marked bias in the literature towards studies involving male subjects. As such, future work needs to be done to involve female subjects. In summary, orexins play an important role in driving motivation for high salient rewards such as highly palatable food and may serve as the intersection between homeostatic and hedonic feeding.

Generational Effects of Opioid Exposure

The inheritance of substance abuse, including opioid abuse, may be influenced by genetic and non-genetic factors related to the environment, such as stress and socioeconomic status. These non-genetic influences on the heritability of a trait can be attributed to epigenetics. Epigenetic inheritance can result from modifications passed down from the mother, father, or both, resulting in either maternal, paternal, or parental epigenetic inheritance, respectively. These epigenetic modifications can be passed to the offspring to result in multigenerational, intergenerational, or transgenerational inheritance. Human and animal models of opioid exposure have shown generational effects that result in molecular, developmental, and behavioral alterations in future generations.

Differential importance of nucleus accumbens Ox1Rs and AMPARs for female and male mouse binge alcohol drinking

Alcohol use disorder exhausts substantial social and economic costs, with recent dramatic increases in female problem drinking. Thus, it is critically important to understand signaling differences underlying alcohol consumption across the sexes. Orexin-1 receptors (Ox1Rs) can strongly promote motivated behavior, and we previously identified Ox1Rs within nucleus accumbens shell (shell) as crucial for driving binge intake in higher-drinking male mice. Here, shell Ox1R inhibition did not alter female mouse alcohol drinking, unlike in males. Also, lower dose systemic Ox1R inhibition reduced compulsion-like alcohol intake in both sexes, indicating that female Ox1Rs can drive some aspects of pathological consumption, and higher doses of systemic Ox1R inhibition (which might have more off-target effects) reduced binge drinking in both sexes. In contrast to shell Ox1Rs, inhibiting shell calcium-permeable AMPA receptors (CP-AMPARs) strongly reduced alcohol drinking in both sexes, which was specific to alcohol since this did not reduce saccharin intake in either sex. Our results together suggest that the shell critically regulates binge drinking in both sexes, with shell CP-AMPARs supporting intake in both sexes, while shell Ox1Rs drove drinking only in males. Our findings provide important new information about sex-specific and -general mechanisms that promote binge alcohol intake and possible targeted therapeutic interventions.

Synchronous neuronal interactions in rat hypothalamic culture: a novel model for the study of network dynamics in metabolic disorders

Synchronous neural activity is a feature of normal brain function, and altered synchronization is observed in several neurological diseases. Dysfunction in hypothalamic pathways leads to obesity, suggesting that hypothalamic neural synchrony is critical for energy homeostasis. The lateral hypothalamic orexin neurons are extensively interconnected with other brain structures and are important for energy balance. Earlier studies show that rats with higher orexin sensitivity are obesity resistant. Similarly, topiramate, an anti-epileptic drug, has been shown to reduce weight in humans. Since orexin enhances neuronal excitation, we hypothesized that obesity-resistant rats with higher orexin sensitivity may exhibit enhanced hypothalamic synchronization. We further hypothesized that anti-obesity agents such as orexin and topiramate will enhance hypothalamic synchronization. To test this, we examined neural synchronicity in primary embryonic hypothalamic cell cultures, obtained from embryonic day 18 (E18) obesity-susceptible Sprague-Dawley (SD) and obesity-resistant rats. Hypothalamic tissue was cultured in multielectrode array (MEA), and recordings were performed twice weekly, from 4th to 32nd day in vitro (DIV). Next, we tested the effects of orexin and topiramate application on neural synchronicity of hypothalamic cultures obtained from SD rat embryos. Signals were analyzed for synchronization using cross correlation. Our results showed that (1) obesity-resistant hypothalamus exhibits significantly higher synchronization compared to obesity-sensitive hypothalamus; and (2) orexin and topiramate enhance hypothalamic synchronization. These results support that enhanced orexin sensitivity is associated with greater neural synchronization, and that anti-obesity treatments enhance network synchronization, thus constrain variability in hypothalamic output signals, to extrahypothalamic structures involved in energy homeostasis.

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 lateral hypothalamus and orexinergic transmission in the paraventricular thalamus promote the attribution of incentive salience to reward-associated cues

RATIONALE

Prior research suggests that the neural pathway from the lateral hypothalamic area (LHA) to the paraventricular nucleus of the thalamus (PVT) mediates the attribution of incentive salience to Pavlovian reward cues. However, a causal role for the LHA and the neurotransmitters involved have not been demonstrated in this regard.

OBJECTIVES

To examine (1) the role of LHA in the acquisition of Pavlovian conditioned approach (PavCA) behaviors, and (2) the role of PVT orexin 1 receptors (OX1r) and orexin 2 receptors (OX2r) in the expression of PavCA behaviors and conditioned reinforcement.

METHODS

Rats received excitotoxic lesions of the LHA prior to Pavlovian training. A separate cohort of rats characterized as sign-trackers (STs) or goal-trackers (GTs) received the OX1r antagonist SB-334867, or the OX2r antagonist TCS-OX2-29, into the PVT, to assess their effects on the expression of PavCA behavior and on the conditioned reinforcing properties of a Pavlovian reward cue.

RESULTS

LHA lesions attenuated the development of sign-tracking behavior. Administration of either the OX1r or OX2r antagonist into the PVT reduced sign-tracking behavior in STs. Further, OX2r antagonism reduced the conditioned reinforcing properties of a Pavlovian reward cue in STs.

CONCLUSIONS

The LHA is necessary for the development of sign-tracking behavior; and blockade of orexin signaling in the PVT attenuates the expression of sign-tracking behavior and the conditioned reinforcing properties of a Pavlovian reward cue. Together, these data suggest that LHA orexin inputs to the PVT are a key component of the circuitry that encodes the incentive motivational value of reward cues.

The modulation of the spatial reference memory by the orexinergic system of the dorsal raphe nucleus

The dorsal raphe nucleus (DRN) is a brainstem nucleus involved in the pathophysiology of the depression, through its serotoninergic innervation. Furthermore, depressive symptoms in patients are also associated with some memory and sleep complaints. Anatomical evidence confirmed the presence of projections from the lateral hypothalamus to serotonergic neurons of the dorsal raphe nucleus (DRN). These projection fibers release orexin neuropeptides which play roles in the spatial memory. Both of the orexinergic receptors are widely distributed in dorsal raphe nucleus. Therefore, the present work was aimed to assess the probable roles of orexin 1 and 2 receptors using an orexin 1 receptor antagonist, SB-334867-A, and an orexin 2 receptor antagonist, TCS-OX2-29 in the DRN on the retrieval, and consolidation phases of spatial reference memory in the Morris water maze (MWM) task. The results demonstrated that blocking orexin 1 receptors in the DRN impairs the process of memory consolidation in the spatial MWM via increasing in the time of the escape latency of the probe day. Blocking these receptors did not affect the retrieval phase of MWM learning. Furthermore, blocking of the orexin 2 receptors in this area did not affect neither consolidation nor retrieval phases of the memory. In conclusion, orexin 1 receptors in the DRN play major roles in the consolidation of the spatial reference memory in rats.

Orexin-A up-regulates dopamine D2 receptor and mRNA in the nucleus accumbens Shell

Orexins-A (OrxA) and -B (OrxB) neuropeptides are synthesized by a group of neurons located in the lateral hypothalamus and adjacent perifornical area, which send their projections to the mesolimbic dopaminergic (DAergic) system including ventral tegmental area and nucleus accumbens (NAc), where orexin receptors are expressed. NAc plays a central role in reward-seeking behavior and drug abuse. NAc-neurons express dopamine-1 (D1R) and dopamine-2 (D2R) receptors. Orexins bind to their two cognate G-protein-coupled receptors, orexin-receptor type-1 (Orx₁R) and type-2 (Orx₂R). Orexin receptor signaling is involved in behaviors such as motivation and addiction. Orexin-containing neurons modulate DAergic activity that is key in synaptic plasticity induced by addictive drugs. However, the effect of OrxA on expression and content of DAergic receptors in NAc is unknown. The purpose of this study was to investigate whether OrxA can alter gene expression and protein levels of D1R/D2R in NAc. Gene expression was evaluated by real-time PCR analysis and protein levels by western blot in rats. The results show that intracerebroventricular (i.c.v.) injection of OrxA increases both gene transcription and protein content of D2R but fails to modify D1R. This effect was also confirmed with OrxA infusion in NAc/Shell. Our results demonstrate for the first time that OrxA induces up-regulation of gene and protein of D2R in NAc. These findings support the hypothesis that OrxA modulates the DAergic transmission and this may serve to understand how orexin signaling enhances DA responses at baseline conditions and in response to psychostimulants.

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.

Evaluation of the relationship between the gene expression level of orexin-1 receptor in the rat blood and prefrontal cortex, novelty-seeking, and proneness to methamphetamine dependence: A candidate biomarker

BACKGROUND

previous studies have suggested that methamphetamine (METH) abuse may affect orexin regulation. However, the data regarding the relationship between the current level of orexin and the vulnerability to METH abuse are minimal. Here, we have investigated the correlation between the gene expression level of the orexin-1 receptor (OX1R) in the rat prefrontal cortex (PFC) and blood lymphocytes and susceptibility to METH dependence and its impact on novelty-seeking behavior.

METHODS

male Wistar rats were first examined for novelty-seeking behavior by the novel object recognition test, and the expression level of OX1R in their blood lymphocytes was evaluated by real-time PCR. Then, the susceptibility to METH abuse was investigated by voluntary METH oral consumption test. According to the amounts of METH consumption, the animals were divided into two groups of METH preferring and non-preferring. Half of the rats in each group were sacrificed, and the level of OX1R in their blood lymphocytes and PFC tissue was measured. The other half were sacrificed for the same reason after two weeks of drug abstinence.

RESULTS

The indexes of novelty-seeking behavior were significantly higher in the METH- preferring group compared to the non-preferring animals. Furthermore, the expression level of OX1R in the blood lymphocytes and PFC in the preferring group was considerably higher than the non-preferring group.

CONCLUSION

Up-regulation of the mRNA expression level of OX1R in the lymphocytes and PFC may predict vulnerability to the METH consumption and novelty-seeking, which may serve as a potential biomarker for METH abuse.

Cyclic tailor-made amino acids in the design of modern pharmaceuticals

Tailor-made AAs are indispensable components of modern medicinal chemistry and are becoming increasingly prominent in new drugs. In fact, about 30% of small-molecule pharmaceuticals contain residues of tailor-made AAs or structurally related diamines and amino-alcohols. Cyclic tailor-made AAs present a particular value to rational structural design by virtue of their local conformational constraints and are widely used in lead optimization programs. The present review article highlights 34 compounds, all of which are derived from cyclic AAs, representing recently-approved, small-molecule pharmaceuticals as well as promising drug candidates currently in various phases of clinical study. For each compound, the discussion includes the discovery, therapeutic profile and optimized synthesis, with a focus on the preparation of cyclic tailor-made AA as the principal structural feature. The present review article is intended to serve as a reference source for organic, medicinal and process chemists along with other professionals working in the fields of drug design and pharmaceutical discovery.

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.

The number of lateral hypothalamus orexin/hypocretin neurons contributes to individual differences in cocaine demand

Lateral hypothalamus (LH) orexin neuron signaling has been implicated in the motivation to seek and take drugs of abuse. The number of LH orexin neurons has been shown to be upregulated with exposure to drugs of abuse. We sought to determine if the number of LH orexin neurons related to individual differences in motivation (demand) for cocaine in our behavioral economics (BE) paradigm, and whether knockdown of these cells predicted changes in economic demand. We quantified LH orexin cell numbers in animals immediately following our BE paradigm, as well as after a 2-week period of abstinence, to relate the number of LH orexin cells to economic demand for cocaine. We also knocked down LH orexin expression with an orexin morpholino antisense to determine how reduced orexin numbers impacted cocaine demand. We found that animals with greater baseline motivation for cocaine (lower demand elasticity) had more LH orexin neurons. Following a 2-week abstinence from cocaine, the number of LH orexin neurons predicted economic demand for cocaine prior to abstinence, indicating that orexin expression is a persistent marker for demand. Reducing LH orexin cell numbers with antisense decreased motivation for cocaine (increased demand elasticity) without affecting baseline consumption. In addition, the number of spared LH orexin neurons after antisense treatment correlated with individual motivation for cocaine. These studies point to a role for the endogenous number of LH orexin neurons in individual differences in motivation for cocaine.

Targeting the Orexin System for Prescription Opioid Use Disorder

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

Effect of suvorexant on event-related oscillations and EEG sleep in rats exposed to chronic intermittent ethanol vapor and protracted withdrawal

STUDY OBJECTIVES

Insomnia is a prominent complaint in patients with alcohol use disorders (AUD). However, despite the importance of sleep in the maintenance of sobriety, treatment options for sleep disturbance associated with a history of AUD are currently limited. Recent clinical trials have demonstrated that suvorexant, a dual Hct/OX receptor antagonist, normalizes sleep in patients with primary insomnia; yet, its potential for the treatment of sleep pathology associated with AUD has not been investigated in either preclinical or clinical studies.

METHODS

This study employed a model whereby ethanol vapor exposure or control conditions were administered for 8 weeks to adult rats. Waking event-related oscillations (EROs) and EEG sleep were evaluated at baseline before exposure and again following 24 hr of withdrawal from the exposure. Subsequently, the ability of vehicle (VEH) and two doses (10, 30 mg/kg IP) of suvorexant to modify EROs, sleep, and the sleep EEG was investigated.

RESULTS

After 24 hr following EtOH withdrawal, the ethanol-treated group had increases in waking ERO θ and β activity, more fragmented sleep (shorter duration and increased frequency of slow wave (SW) and rapid eye movement [REM] sleep episodes), and increased θ and β power in REM and SW sleep. Suvorexant induced a dose-dependent decrease in the latency to REM and SW sleep onsets but also produced REM and SW sleep fragmentation and increased β energy in waking EROs when compared with VEH.

CONCLUSIONS

Taken together, these studies suggest that suvorexant has overall sleep-promoting effects, but it may exacerbate some aspects of sleep and EEG pathology.

Involvement of orexin-A in the regulation of neuronal activity and emotional behaviors in central amygdala in rats

The amygdala is a complex structure involved in the regulation of emotional behaviors including fear and anxiety. The central amygdala is the main output of the amygdala and plays an important role in emotional processing. Recent studies indicate that orexin, a kind of neuropeptides responsible for maintaining wakefulness, is also associated with emotion-related behaviors, such as depression- and anxiety-like behaviors. Central amygdala receives orexinergic fibers originating from the lateral hypothalamus and expresses OX₁ receptors in rats. To test the electrophysiological and behavioral effects of orexins in the central amygdala, single unit in vivo extracellular recordings, open field and elevated plus maze tests were performed in rats. Micro-pressure administration of orexin-A (0.01 mmol/L) increased the firing rate in 18 out of the 31 central amygdala neurons, while the other 13 neurons were not excited by orexin-A. The excitatory effects of orexin-A on central amygdala neurons were mainly mediated by OX₁ receptors rather than OX₂ receptors. Orexin-B (0.01 mmol/L) did not change the firing activity in all recorded central amygdala neurons. Selectively blocking OX₁ receptors by SB-334867 (0.01 mmol/L) significantly decreased the spontaneous firing rate in 14 out of the 33 central amygdala neurons, leaving the remaining 19 neurons were not affected. However, blocking OX₂ receptors by TCS-OX2-29 (0.01 mmol/L) did not change the firing activity. Finally, both open field test and elevated plus maze test showed that bilateral microinjection of orexin-A into the central amygdala induced significantly anxiolytic-like behaviors. The specific OX₁ receptor antagonist tended to produce opposite effects although there was no statistical difference. The present electrophysiological and behavioral studies suggested that orexin-A participates in anxiety-like behaviors by modulating the spontaneous firing activity of central amygdala neurons.

Activation of medial hypothalamic orexin neurons during a Go/No-Go task

Orexin neurons (Orx; also referred to as hypocretin) are found exclusively in the hypothalamus, and release the neuropeptides orexin A and orexin B (also referred to as hypocretin 1 and 2) throughout the CNS. With its widespread targets, the orexin system is involved in a number of functions including, but not limited to stress, reward, wakefulness, and food seeking. Our laboratory has previously proposed that the dorsomedial hypothalamus (DMH) and perifornical (PFA) orexin neurons function in stress and arousal whereas those in lateral hypothalamus (LH) participate in reward processes (Harris and Aston-Jones, 2006). In the current study, we compared Fos activation in orexin neurons located in medial hypothalamus (DMH and PFA) to those in LH during a Go/No-Go task for a highly palatable food reward, a task that would likely activate regions for arousal/attention as well as reward. The Go/No-Go paradigm is a useful behavioral tool to measure behavioral inhibition, impulsivity, learning, and reaction time. Our results revealed increased activation of medial hypothalamic orexin neurons correlated with greater accuracy on the Go/No-Go task. No correlation was found between Go/No-Go accuracy and activation of lateral hypothalamic orexin neurons. This study supports a functional dichotomy of medial vs lateral orexin neurons, and indicates a role for medial orexin neurons in behavioral performance that requires response inhibition.

A New Positron Emission Tomography Probe for Orexin Receptors Neuroimaging

The orexin receptor (OX) is critically involved in motivation and sleep−wake regulation and holds promising therapeutic potential in various mood disorders. To further investigate the role of orexin receptors (OXRs) in the living human brain and to evaluate the treatment potential of orexin-targeting therapeutics, we herein report a novel PET probe ([¹¹C]CW24) for OXRs in the brain. CW24 has moderate binding affinity for OXRs (IC₅₀ = 0.253 μM and 1.406 μM for OX₁R and OX₂R, respectively) and shows good selectivity to OXRs over 40 other central nervous system (CNS) targets. [¹¹C]CW24 has high brain uptake in rodents and nonhuman primates, suitable metabolic stability, and appropriate distribution and pharmacokinetics for brain positron emission tomography (PET) imaging. [¹¹C]CW24 warrants further evaluation as a PET imaging probe of OXRs in the brain.

Recent perspectives on orexin/hypocretin promotion of addiction-related behaviors

The neuropeptide hypocretin/orexin plays a broad and important role in physiological functions ranging from addiction, stress, and anxiety to sleep, energy metabolism, and homeostatic regulation. A number of recent reviews addressing the importance of orexin for different addictive behaviors, especially the contribution of orexin-1-receptors (Ox1Rs) in responding for intoxicants in higher-motivation individuals and situations, and orexin-2-receptor (Ox2Rs) in stress-related aspects of addictive responding. This may parallel the importance of more lateral orexin neurons in the hypothalamus for reward and more medial for stress and arousal. However, there is clearly also some crossover, which may reflect, in part, where positive and negative conditioning (reward- and relief-seeking) are both present concurrently in established addiction, and also where orexin signaling can differ in subregions of a particular brain region. Here, we attempt to examine and synthesize some of the most recent work addressing orexin functions in addiction, including a particular role for Ox1Rs for driving responding in higher-motivation individuals and under higher levels of effort. While there are some commonalities across addictive substances addressed here (alcohol, cocaine, opiates), there are also some differences, which may relate to several factors including the speed of intoxication with a given substance. Together, recent findings have shed important insight and clues into what a more unified role of Ox1Rs might entail, and critical areas for future work. In addition, these many studies support the development of Ox1R blockers for use in humans to counteract addiction and other disorders of motivation. This article is part of the special issue on Neuropeptides.