Reviewing the role of the orexinergic system and stressors in modulating mood and reward-related behaviors

Restraint stress-induced antinociceptive effects in acute pain: Involvement of orexinergic system in the nucleus accumbens

The complicated relevance between stress and pain has been identified. Neurotransmitters and neuropeptides of various brain areas play a role in this communication. Pain inhibitory response is known as stress-induced analgesia (SIA). The studies demonstrated that the nucleus accumbens (NAc) is critical in modulating pain. As a neuropeptide, orexin is crucially involved in initiating behavioral and physiological responses to threatening and unfeeling stimuli. However, the role of the orexin receptors of the NAc area after exposure to restraint stress (RS) as acute physical stress in the modulation of acute pain is unclear. One hundered twenty adult male albino Wistar rats (230-250 g) were used. Animals were unilaterally implanted with cannulae above the NAc. The SB334867 and TCS OX2 29 were used as antagonists for OX1r and OX2r, respectively. Different doses of the antagonists (1, 3, 10, and 30 nmol/0.5 µl DMSO) were microinjected intra-NAc five minutes before exposure to RS (3 hours). Then, the tail-flick test as a model of acute pain was performed, and the nociceptive threshold (Tail-flick latency; TFL) was measured in 60-minute time set intervals. According to this study's findings, the antinociceptive effects of RS in the tail-flick test were blocked during intra-NAc administration of SB334867 or TCS OX2 29. The RS as acute stress increased TFL and deceased pain-like behavior responses. The 50 % effective dose values of the OX1r and OX2r antagonists were 12.82 and 21.64 nmol, respectively. The result demonstrated contribution of the OX1r into the NAc was more remarkable than that of the OX2r on antinociceptive responses induced by the RS. Besides, in the absence of RS, the TFL was attenuated. The current study's data indicated that OX1r and OX2r into the NAc induced pain modulation responses during RS in acute pain. In conclusion, the findings revealed the involvement of intra-NAc orexin receptors in improving SIA.

Prokineticin-2 Participates in Chronic Constriction Injury-Triggered Neuropathic Pain and Anxiety via Regulated by NF-κB in Nucleus Accumbens Shell in Rats

Neuropathic pain (NP) is an intractable pain that results from primary nervous system injury and dysfunction. Herein, we demonstrated in animal models that peripheral nerve injury induced enhanced pain perception and anxiety-like behaviors. According to previous reports, nucleus accumbens (NAc) shell is required for complete expression of neuropathic pain behaviors and mood alternations, we found the elevated mRNA and protein level of Prokineticin-2 (Prok2) in the NAc shell after Chronic Constriction Injury (CCI). Prok2 knockdown in the NAc shell reversed NP and anxiety-like behaviors in rats, indicating that Prok2 might play a fundamental role in NP and anxiety co-morbidity. CCI significantly enhanced Prok2 co-expression with NF-κB P-p65 in comparison with control animals. In addition to reversing the established nociceptive hypersensitivities and anxiety simultaneously, NAc microinjection of NF-κB siRNA or specific inhibitor PDTC reversed Prok2 upregulation. Besides, Prok2 was significantly decreased in vitro when co-transfected with si-NF-κB. Dual-Luciferase assay showed NF-κB directly activated Prok2 gene transcriptional activity. Overall, these findings provide new insights into the neurobiological mechanisms behind NP and comorbid anxiety. The NF-κB/Prok2 pathway could be a potential therapeutic target for NP and anxiety disorders.

The role of orexin receptors within the CA1 area in the acquisition and expression of methamphetamine place preference

Treatment of Methamphetamine (METH) use disorder has become a crucial public health issue. The orexin system manipulation has provided promising evidence to attenuate addictive-like behaviors. This study explored the role of the orexin 1 receptor and orexin 2 receptor (OX1R and OX2R) in the CA1 area of the hippocampal formation in the acquisition and expression of METH-induced place preference. Animals were subjected to bilateral administration of different dosages (1, 3, 10, and 30 nmol/0.5 μl DMSO per side) of a selective OX1R antagonist, SB334867, or selective OX2R antagonist, TCS OX2 29 into the CA1 area throughout the conditioning phase or once on the post-conditioning phase in separate control and experimental groups. Behavioral data revealed that both OX1R (10 nmol; P < 0.01 and 30 nmol; P < 0.001) and OX2R (10 nmol; P < 0.05 and 30 nmol; P < 0.001) antagonism during the conditioning phase could block the formation of METH place preference dose-dependently. In addition, intra-CA1 microinjection of SB334867 on the post-conditioning phase attenuated the expression of METH place preference in a dose-dependent manner (3 nmol; P < 0.05, 10 nmol; P < 0.01 and 30 nmol; P < 0.001) whereas intra-CA1 administration of TCS OX2 29 only at the highest dosage (30 nmol) declined the expression of METH place preference (P < 0.01). It was also indicated that the suppressive effects of orexin receptor blockade on the METH-seeking behavior in the CA1 area were anatomically specific to this area. These findings support the possibility of targeting the orexin system to develop novel and successful pharmacological options for the treatment of METH dependence.

Lateral hypothalamus orexinergic projection to the medial prefrontal cortex modulates chronic stress-induced anhedonia but not anxiety and despair

Chronic stress-induced anxiodepression is a common health problem, however its potential neurocircuitry mechanism remains unclear. We used behavioral, patch-clamp electrophysiology, chemogenetic, and optogenetic approaches to clarify the response of the lateral hypothalamus (LH) and the medial prefrontal cortex (mPFC) to stress, confirmed the structural connections between the LH and mPFC, and investigated the role of the LH-mPFC pathway in chronic stress-induced anxiodepression symptoms. Unpredictable chronic mild stress (UCMS) caused anxiodepression-like behaviors, including anxiety, anhedonia, and despair behaviors. We discovered that the activity of the LH and mPFC was both increased after restraint stress (RS), a stressor of UCMS. Then we found that the orexinergic neurons in the LH predominantly project to the glutamatergic neurons in the mPFC, and the excitability of these neurons were increased after UCMS. In addition, overactivated LH orexinergic terminals in the mPFC induced anhedonia but not anxiety and despair behaviors in naive mice. Moreover, chemogenetically inhibited LH-mPFC orexinergic projection neurons and blocked the orexin receptors in the mPFC alleviated anhedonia but not anxiety and despair behaviors in UCMS-treated mice. Our study identified a new neurocircuit from LH orexinergic neurons to mPFC and revealed its role in regulating anhedonia in response to stress. Overactivation of LHOrx-mPFC pathway selectively mediated chronic stress-induced anhedonia. In normal mice, the LHOrx-mPFC pathway exhibits relatively low activity. However, after chronic stress, the activity of orexinergic neuron in LH is overactivated, leading to an increased release of orexin into the mPFC. This heightened orexin concentration results in increased excitability of the mPFC through OX1R and OX2R, consequently triggering anhedonia.

The Orexin/Hypocretin System, the Peptidergic Regulator of Vigilance, Orchestrates Adaptation to Stress

The orexin/hypocretin neuropeptide family has emerged as a focal point of neuroscientific research following the discovery that this family plays a crucial role in a variety of physiological and behavioral processes. These neuropeptides serve as powerful neuromodulators, intricately shaping autonomic, endocrine, and behavioral responses across species. Notably, they serve as master regulators of vigilance and stress responses; however, their roles in food intake, metabolism, and thermoregulation appear complementary and warrant further investigation. This narrative review provides a journey through the evolution of our understanding of the orexin system, from its initial discovery to the promising progress made in developing orexin derivatives. It goes beyond conventional boundaries, striving to synthesize the multifaceted activities of orexins. Special emphasis is placed on domains such as stress response, fear, anxiety, and learning, in which the authors have contributed to the literature with original publications. This paper also overviews the advancement of orexin pharmacology, which has already yielded some promising successes, particularly in the treatment of sleep disorders.

Pyrazole derivatives as selective orexin-2 receptor antagonists (2-SORA): synthesis, structure-activity-relationship, and sleep-promoting properties in rats

Selective orexin 2 receptor antagonists (2-SORA) such as seltorexant (15) are in clinical development for the treatment of insomnia and other conditions such as depression. Herein, we report our structure-activity-relationship (SAR) optimization efforts starting from an HTS hit (1) (N-(1-((5-acetylfuran-2-yl)methyl)-1H-pyrazol-4-yl)-5-(m-tolyl)oxazole-4-carboxamide) that was derived from an unrelated in-house GPCR-agonist program. Medicinal chemistry efforts focused on the optimization of orexin 2 receptor (OX2R) antagonistic activity, stability in liver microsomes, time dependent CYP3A4 inhibition, and aqueous solubility. Compounds were assessed for their brain-penetrating potential in in vivo experiments to select the most promising compounds for our in vivo sleep model. Our lead optimization efforts led to the discovery of the potent, brain penetrating and orally active, 2-SORA (N-(1-(2-(5-methoxy-1H-pyrrolo[3,2-b]pyridin-3-yl)ethyl)-1H-pyrazol-4-yl)-5-(m-tolyl)oxazole-4-carboxamide) 43 with efficacy in a sleep model in rats comparable to 15.

Association between peripheral orexin A/B levels and depression with childhood trauma

BACKGROUND

Orexin dysfunction has previously been demonstrated to be associated with depression. However, no studies reported the different effects of orexin A/B on depression with and without childhood trauma (CT). In this study，we assessed the correlation between expression of orexin A/B and depression severity in major depressive disorder (MDD) patients and healthy controls.

METHODS

A total of 97 MDD patients and 51 healthy controls were recruited in this study. According to the total scores of childhood trauma questionnaire (CTQ), the MDD patients were further divided into two subgroups, MDD with CT and MDD without CT. The 17-item Hamilton Depression Scale (HAMD-17), and plasma orexin A and orexin B concentrations were measured in all participants using enzyme-linked immunosorbent assay.

RESULTS

Orexin B plasma levels were significantly higher in MDD patients with CT and without CT than that in the healthy control group (P < 0.05), whereas there was no statistical difference between the two depression groups. After adjusting age and BMI for covariates, the LASSO regression revealed significant association between the plasma orexin B levels and the total scores of HAMD (β = 3.348), CTQ (β = 2.005). There was no difference in plasma orexin A levels among three groups (P > 0.05).

CONCLUSIONS

Although peripheral orexin B levels are associated with the depression, rather than orexin A, CT appear to play a role in the association between orexin B levels and depression. China Clinical Trial Registration Center (Registration No.: ChiCTR2000039692).

Restraint stress potentiates sensitivity to the antinociceptive effect of morphine through orexin receptors in the ventral tegmental area

Orexin signaling in the ventral tegmental area (VTA) plays a critical role in stress and addictive behaviors. On the other hand, exposure to stress potentiates behavioral sensitization to drugs of abuse such as morphine. This study aimed to elucidate the role of orexin receptors within the VTA in restraint stress (RS)-induced morphine sensitization. Adult male albino Wistar rats underwent stereotaxic surgery, and two stainless steel guide cannulae were bilaterally implanted into the VTA. Different doses of SB334867 or TCS OX2 29 as orexin-1 (OX1) and orexin-2 (OX2) receptor antagonists were microinjected into the VTA five min before exposure to RS, respectively. A duration of three hours was considered for applying the RS, and 10 min after RS exposure, animals received a subcutaneous injection of an ineffective dose of morphine (1 mg/kg) for three consecutive days followed by a five-day drug/stress-free period. On the ninth day, the tail-flick test evaluated the sensitivity to the antinociceptive effects of morphine. The results demonstrated that the sole application of RS or morphine (1 mg/kg) could not induce morphine sensitization; however, concurrent application of RS and morphine could induce morphine sensitization. Besides, intra-VTA administration of OX1 R or OX2 R antagonists before paired administration of morphine and RS blocked morphine sensitization. The role of OX1 R and OX2 R in the induction of stress-induced morphine sensitization was almost identical. This study provides new insight into the role of orexin signaling in the VTA in the potentiation of morphine sensitization induced by RS and morphine co-administration.

The role of orexin-1 receptors within the ventral tegmental area in the extinction and reinstatement of methamphetamine place preference

Targeting the orexin system has recently been identified as one of the promising options for treating drug addiction. It may be more feasible and achievable if we investigate the accurate function of the orexin system in brain areas implicated in reward and addiction, such as the ventral tegmental area (VTA) by animal reward models. This study investigated the contribution of the orexin system, mainly the orexin-1 receptors (OX1R) in the VTA, in the extinction and reinstatement of methamphetamine (METH) related memories in the conditioned place preference (CPP) model. Animals after the acquisition of METH place preference were subjected to two separate sets of extinction and reinstatement experiments to receive various concentrations of selective OX1R antagonist, SB334867 into the bilateral VTA before extinction sessions (1, 3, and 10 nmol/0.3 μl DMSO per side) or only on the reinstatement phase (3, 10, and 30 nmol/0.3 μl DMSO per side), respectively. Intra-VTA infusion of SB334867 throughout the extinction phase could remarkably facilitate the extinction process and decrease the maintenance of reinforcing effects of METH at the highest dosage (10 nmol; p < 0.0001). Data also indicated a single microinfusion of SB334867 into the VTA before reinstatement of the METH-seeking behavior could considerably prevent the relapse of previously formed reward-context memories (10 nmol; p < 0.01 and 30 nmol; p < 0.001). The present study provided evidence supporting the potential therapeutic effects of the orexin system modulation, specifically in the VTA, on different stages of METH-induced place preference.

The blockade of orexin receptors within the dentate gyrus of the hippocampus attenuated methamphetamine-induced reward learning during conditioning place preference

Orexins and orexinergic receptors have been shown to play a critical role in reward processing and drug addiction. Previous studies showed that the orexinergic system in the dentate gyrus (DG) region of the hippocampus affects the conditioning (acquisition) and post-conditioning (expression) phases of morphine-induced conditioned place preference (CPP). The action of each orexin receptor within the DG during conditioning and expression phases for methamphetamine (METH)-induced CPP remains unclear. The present study aimed to determine the role of orexin-1 and -2 receptors in the hippocampal DG in METH CPP acquisition and expression. During the 5-day conditioning phase, rats received an intra-DG microinjection of SB334867, a selective orexin-1 receptor (OX1R) antagonist, or TCS OX2-29, a selective orexin-2 receptor (OX2R) antagonist, before injection of METH (1 mg/kg; sc). In different sets of animals on the expression day, rats received each antagonist before the CPP test. The results showed that SB334867 (3, 10, and 30 nmol) and TCS OX2-29 (3, 10, and 30 nmol) significantly decreased the acquisition of METH CPP during the conditioning phase. Furthermore, administration of SB 334867 (10 and 30 nmol) and TCS OX2-29 (3 and 10 nmol) on the post-conditioning day significantly reduced METH-induced CPP expression. The results also indicated that orexin receptors play a more critical role in the conditioning phase than in the expression phase. In summary, the orexin receptors in the DG play a crucial role in drug learning and memory and are essential for METH reward acquisition and expression.

Blockade of orexin receptor 1 attenuates morphine protracted abstinence-induced anxiety-like behaviors in male mice

One negative emotional state from morphine protracted abstinence is anxiety which can drive craving and relapse risk in opioid addicts. Although the orexinergic system has been reported to be important in mediating emotion processing and addiction, the role of orexinergic system in anxiety from drug protracted abstinence remains elusive. In this study, by using behavioral test, western blot, electrophysiology and virus-mediated regulation of orexin receptor 1 (OX1R), we found that: (1) Intraperitoneal and intra-VTA administration of a selective OX1R antagonist SB334867 alleviated anxiety-like behaviors in open field test (OFT) but not in elevated plus maze test (EPM) in morphine protracted abstinent male mice. (2) OX1R expression in the VTA was upregulated by morphine withdrawal. (3) Virus-mediated knockdown of OX1R in the VTA prevented morphine abstinence-induced anxiety-like behaviors and virus-mediated overexpression of OX1R in the VTA was sufficient to produce anxiety-like behaviors in male mice. (4) The VTA neuronal activity was increased significantly induced by morphine protracted abstinence, which was mediated by OX1R. (5) OX1R was widely distributed in the neuronal soma and processes of dopaminergic and non-dopaminergic neurons in the VTA. The findings revealed that the OX1R mediates morphine abstinence-induced anxiety-like behaviors and the VTA plays a critical role in this effect.

Neuroinflammatory Response in Reward-Associated Psychostimulants and Opioids: A Review

Substance abuse is one of the significant problems in social and public health worldwide. Vast numbers of evidence illustrate that motivational and reinforcing impacts of addictive drugs are primarily attributed to their ability to change dopamine signaling in the reward circuit. However, the roles of classic neurotransmitters, especially dopamine and neuromodulators, monoamines, and neuropeptides, in reinforcing characteristics of abused drugs have been extensively investigated. It has recently been revealed that central immune signaling includes cascades of chemokines and proinflammatory cytokines released by neurons and glia via downstream intracellular signaling pathways that play a crucial role in mediating rewarding behavioral effects of drugs. More interestingly, inflammatory responses in the central nervous system modulate the mesolimbic dopamine signaling and glutamate-dependent currents induced by addictive drugs. This review summarized researches in the alterations of inflammatory responses accompanied by rewarding and reinforcing properties of addictive drugs, including cocaine, methamphetamine, and opioids that were evaluated by conditioned place preference and self-administration procedures as highly common behavioral tests to investigate the motivational and reinforcing impacts of addictive drugs. The neuroinflammatory responses affect the rewarding properties of psychostimulants and opioids.

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.

Anxiety and Depression: What Do We Know of Neuropeptides?

In modern society, there has been a rising trend of depression and anxiety. This trend heavily impacts the population’s mental health and thus contributes significantly to morbidity and, in the worst case, to suicides. Modern medicine, with many antidepressants and anxiolytics at hand, is still unable to achieve remission in many patients. The pathophysiology of depression and anxiety is still only marginally understood, which encouraged researchers to focus on neuropeptides, as they are a vast group of signaling molecules in the nervous system. Neuropeptides are involved in the regulation of many physiological functions. Some act as neuromodulators and are often co-released with neurotransmitters that allow for reciprocal communication between the brain and the body. Most studied in the past were the antidepressant and anxiolytic effects of oxytocin, vasopressin or neuropeptide Y and S, or Substance P. However, in recent years, more and more novel neuropeptides have been added to the list, with implications for the research and development of new targets, diagnostic elements, and even therapies to treat anxiety and depressive disorders. In this review, we take a close look at all currently studied neuropeptides, their related pathways, their roles in stress adaptation, and the etiology of anxiety and depression in humans and animal models. We will focus on the latest research and information regarding these associated neuropeptides and thus picture their potential uses in the future.

What evidence is there for implicating the brain orexin system in neuropsychiatric symptoms in dementia?

Neuropsychiatric symptoms (NPS) affect people with dementia (PwD) almost universally across all stages of the disease, and regardless of its exact etiology. NPS lead to disability and reduced quality of life of PwD and their caregivers. NPS include hyperactivity (agitation and irritability), affective problems (anxiety and depression), psychosis (delusions and hallucinations), apathy, and sleep disturbances. Preclinical studies have shown that the orexin neuropeptide system modulates arousal and a wide range of behaviors via a network of axons projecting from the hypothalamus throughout almost the entire brain to multiple, even distant, regions. Orexin neurons integrate different types of incoming information (e.g., metabolic, circadian, sensory, emotional) and convert them into the required behavioral output coupled to the necessary arousal status. Here we present an overview of the behavioral domains influenced by the orexin system that may be relevant for the expression of some critical NPS in PwD. We also hypothesize on the potential effects of pharmacological interference with the orexin system in the context of NPS in PwD.