Interaction between the corticotropin-releasing factor system and hypocretins (orexins): a novel circuit mediating stress response

The diverse role of corticotropin-releasing factor (CRF) and its CRF1 and CRF2 receptors under pathophysiological conditions: Insights into stress/anxiety, depression, and brain injury processes

Corticotropin-releasing factor (CRF, corticoliberin) is a neuromodulatory peptide activating the hypothalamic-pituitary-adrenal (HPA) axis, widely distributed in the central nervous system (CNS) in mammals. In addition to its neuroendocrine effects, CRF is essential in regulating many functions under physiological and pathophysiological conditions through CRF1 and CRF2 receptors (CRF1R, CRF2R). This review aims to present selected examples of the diverse and sometimes opposite effects of CRF and its receptor ligands in various pathophysiological states, including stress/anxiety, depression, and processes associated with brain injury. It seems interesting to draw particular attention to the fact that CRF and its receptor ligands exert different effects depending on the brain structures or subregions, likely stemming from the varied distribution of CRFRs in these regions and interactions with other neurotransmitters. CRFR-mediated region-specific effects might also be related to brain site-specific ligand binding and the associated activated signaling pathways. Intriguingly, different types of CRF molecules can also influence the diverse actions of CRF in the CNS.

Causal relationship between narcolepsy and anxiety: A two-sample Mendelian randomization study

BACKGROUND

The aim of this study was to assess the causal relationship between narcolepsy and anxiety using Mendelian randomization (MR) methodology.

METHODS

Our research applied a bidirectional two-sample Mendelian Randomization strategy to explore the linkage between narcolepsy and anxiety. Utilizing summary data from GWAS on both conditions, we primarily employed the inverse-variance weighted technique for our analysis. To evaluate heterogeneity and horizontal pleiotropy, we utilized tools such as the MR Egger method, the weighted median method, Cochran's Q statistic, and the MR Egger intercept.

RESULTS

The analysis using the inverse variance-weighted method showed a clear positive link between narcolepsy and anxiety, with an odds ratio of 1.381 (95% CI: 1.161-1.642, p < 0.001). Tests for heterogeneity and horizontal pleiotropy, including MR Egger and IVW methods, indicated no significant findings (p-values 0.616 and 0.637, respectively, for heterogeneity; p = 0.463 for pleiotropy). Furthermore, no reverse causation was observed between anxiety and narcolepsy (odds ratio 1.034, 95% CI: 0.992-1.078, p = 0.111), with consistent findings across various analytical approaches.

CONCLUSION

This research suggests a possible causal link between narcolepsy and anxiety disorders. The results illuminate this connection and advocate additional studies to elucidate the mechanisms involved and to identify effective interventions.

Inhibiting CRF Projections from the Central Amygdala to Lateral Hypothalamus and Amygdala Deletion of CRF Alters Binge-Like Ethanol Drinking in a Sex-Dependent Manner

Background

Binge alcohol drinking is a dangerous pattern of consumption that can contribute to the development of more severe alcohol use disorders (AUDs). Importantly, the rate and severity of AUDs has historically differed between men and women, suggesting that there may be sex differences in the central mechanisms that modulate alcohol (ethanol) consumption. Corticotropin releasing factor (CRF) is a centrally expressed neuropeptide that has been implicated in the modulation of binge-like ethanol intake, and emerging data highlight sex differences in central CRF systems.

Methods

In the present report we characterized CRF+ neurocircuitry arising from the central nucleus of the amygdala (CeA) and innervating the lateral hypothalamus (LH) in the modulation of binge-like ethanol intake in male and female mice.

Results

Using chemogenetic tools we found that silencing the CRF+ CeA to LH circuit significantly blunted binge-like ethanol intake in male, but not female, mice. Consistently, genetic deletion of CRF from neurons of the CeA blunted ethanol intake exclusively in male mice. Furthermore, pharmacological blockade of the CRF type-1 receptor (CRF1R) in the LH significantly reduced binge-like ethanol intake in male mice only, while CRF2R activation in the LH failed to alter ethanol intake in either sex. Finally, a history of binge-like ethanol drinking blunted CRF mRNA in the CeA regardless of sex.

Conclusions

These observations provide novel evidence that CRF+ CeA to LH neurocircuitry modulates binge-like ethanol intake in male, but not female mice, which may provide insight into the mechanisms that guide known sex differences in binge-like ethanol intake.

Intra-accumbal orexinergic system contributes to the stress-induced antinociceptive behaviors in the animal model of acute pain in rats

Stress and pain are interleaved at numerous levels - influencing each other. Stress can increase the nociception threshold in animals, long-known as stress-induced analgesia (SIA). Orexin is known as a neuropeptide that modulates pain. The effect of stress on the mesolimbic system in the modulation of pain is known. The role of the intra-accumbal orexin receptors in the modulation of acute pain by forced swim stress (FSS) is unclear. In this study, 117 adult male albino Wistar rats (270-300 g) were used. The animals were unilaterally implanted with cannulae above the NAc. The antagonist of the orexin-1 receptor (OX1r), SB334867, and antagonist of the orexin-2 receptor (OX2r), TCS OX2 29, were microinjected into the NAc in different doses (1, 3, 10, and 30 nmol/0.5 µl DMSO) before exposure to FSS for a 6-min period. The tail-flick test was carried out as an assay nociception of acute pain, and the nociceptive threshold [tail-flick latency (TFL)] was measured for 60-minute. The findings demonstrated that exposure to acute stress could remarkably increase the TFLs and antinociceptive responses. Moreover, intra-accumbal microinjection of SB334867 or TCS OX2 29 blocked the antinociceptive effect of stress in the tail-flick test. The contribution of orexin receptors was almost equally modulating SIA. The present study's findings suggest that OX1r and OX2r within the NAc modulate stress-induced antinociceptive responses. The intra-accumbal microinjection of orexin receptors antagonists declares inducing antinociceptive responses by FSS in acute pain. Proposedly, intra-accumbla orexinergic receptors have a role in the development of SIA.

Role of corticotropin-releasing factor neurotransmission in the lateral hypothalamus on baroreflex impairment evoked by chronic variable stress in rats

Despite the importance of physiological responses to stress in a short-term, chronically these adjustments may be harmful and lead to diseases, including cardiovascular diseases. The lateral hypothalamus (LH) has been reported to be involved in expression of physiological and behavioral responses to stress, but the local neurochemical mechanisms involved are not completely described. The corticotropin-releasing factor (CRF) neurotransmission is a prominent brain neurochemical system implicated in the physiological and behavioral changes induced by aversive threats. Furthermore, chronic exposure to aversive situations affects the CRF neurotransmission in brain regions involved in stress responses. Therefore, in this study, we evaluated the influence of CRF neurotransmission in the LH on changes in cardiovascular function and baroreflex activity induced by chronic variable stress (CVS). We identified that CVS enhanced baseline arterial pressure and impaired baroreflex function, which were followed by increased expression of CRF2, but not CRF1, receptor expression within the LH. Local microinjection of either CRF1 or CRF2 receptor antagonist within the LH inhibited the baroreflex impairment caused by CVS, but without affecting the mild hypertension. Taken together, the findings documented in this study suggest that LH CRF neurotransmission participates in the baroreflex impairment related to chronic stress exposure.

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

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

Orexin receptors in the hippocampal dentate gyrus modulated the restraint stress-induced analgesia in the animal model of chronic pain

Previous studies have shown that stressful stimuli induced an adaptive response of reduced nociception, known as stress-induced analgesia (SIA). Since orexin neuropeptides are involved in pain modulation, and orexin neurons, primarily located in the lateral hypothalamus (LH), project to various hippocampal regions, such as the dentate gyrus (DG), the current study aimed to examine the role of orexin receptors within the DG region in the restraint SIA in the animal model of chronic pain. One hundred-thirty adult male Wistar rats (230-250 g) were unilaterally implanted with a cannula above the DG region. Animals were given SB334867 or TCS OX2 29 (1, 3, 10, and 30 nmol, 0.5 µl/rat) into the DG region as orexin-1 receptor (OX1r) and orexin-2 receptor (OX2r) antagonists, respectively, five min before exposure to a 3-hour restraint stress (RS) period. Animals were then undergone the formalin test to assess pain-related behaviors as the animal model of chronic pain. The results showed that RS produces an analgesic response during the early and late phases of the formalin test. However, intra-DG microinjection of OX1r and OX2r antagonists attenuated the restraint SIA. OX2r antagonist was more potent than OX1r antagonist in the early phase of the formalin test, while OX1r antagonist was little more effective in the late phase. Predominantly, it could be concluded that the orexinergic system in the DG region might act as a potential endogenous pain control system and a novel target for treating stress-related disorders.

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.

The integrative role of orexin-1 and orexin-2 receptors within the hippocampal dentate gyrus in the modulation of the stress-induced antinociception in the formalin pain test in the rat

The stressful experiences, by triggering a cascade of hormonal and neural changes, can produce antinociception commonly referred to as stress-induced antinociception (SIA). Orexin neuropeptides have an essential role in stress responses and pain modulation. The dentate gyrus receives orexinergic projections and has been shown to be involved in pain processing. The current study investigated the possible role of orexin-1 and orexin-2 receptors (OX1r and OX2r, respectively) within the dentate gyrus in SIA in a rat model of formalin-induced pain behavior in one hind paw. Male Wistar rats weighing 230-250 g underwent stereotaxic surgery and a cannula was implanted in their brains, above the dentate gyrus region. Either SB334867 or TCS OX2 29 (OX1r and OX2r antagonists, respectively) was microinjected into the dentate gyrus region at a range of doses at 1, 3, 10, and 30 nmol (control group received DMSO 12% as vehicle), 5 min before the forced swim stress (FSS) exposure. The formalin test was performed to assess pain-related behaviors. The results indicated that FSS exposure relieves pain-related behavior in the early and late phases of the formalin test. Blockade of intra-dentate gyrus OX1 or OX2 receptors reduced the antinociceptive responses induced by FSS in the formalin test, with more impact during the late phase. Our findings support the potential role of intra-dentate gyrus orexin receptors as target sites of orexin neurons in painful and stressful situations. Therefore, understanding the exact mechanisms of SIA and the role of the orexinergic system in this phenomenon can lead to identifying the strategies to guide future research and offer a new approach to discovering new pain therapeutic agents.

The potential role of the orexin system in premenstrual syndrome

Premenstrual syndrome (PMS) occurs recurrently during the luteal phase of a woman's menstrual cycle and disappears after menstruation ends. It is characterized by abnormal changes in both the body and mood, and in certain cases, severe disruptions in daily life and even suicidal tendencies. Current drugs for treating PMS, such as selective serotonin reuptake inhibitors, do not yield satisfactory results. Orexin, a neuropeptide produced in the lateral hypothalamus, is garnering attention in the treatment of neurological disorders and is believed to modulate the symptoms of PMS. This paper reviews the advancements in research on sleep disturbances, mood changes, and cognitive impairment caused by PMS, and suggests potential pathways for orexin to address these symptoms. Furthermore, it delves into the role of orexin in the molecular mechanisms underlying PMS. Orexin regulates steroid hormones, and the cyclic fluctuations of estrogen and progesterone play a crucial role in the pathogenesis of PMS. Additionally, orexin also modulates the gamma-aminobutyric acid (GABA) system and the inflammatory response involved in coordinating the mechanism of PMS. Unraveling the role of orexin in the pathogenesis of PMS will not only aid in understanding the etiology of PMS but also hold implications for orexin as a novel target for treating PMS.

The role of hypothalamic Orexin-A in stress-induced gastric dysmotility: An agonistic interplay with corticotropin releasing factor

BACKGROUND

Central Orexin-A (OXA) modulates gastrointestinal (GI) functions and stress response. This study aimed to investigate whether OXA and CRF interact at hypothalamic level.

METHODS

Solid gastric emptying (GE), fecal output (FO), plasma corticosterone (CORT), and postprandial antro-pyloric motility were assessed in rats that underwent acute restraint stress (ARS) and pretreated with central OX1R and/or CRF receptor antagonists SB-334867 and alpha-helical CRF9,41 . Microdialysis was performed to assess ARS-induced release of OXA and CRF in PVN and LHA, respectively. Immunofluorescence labeling was performed to detect the stress-induced changes in OXA and to assess the hypothalamic distribution of OX1R and CRF1/2 receptors. ARS-induced c-Fos immunoreactivity was evaluated in PVN and LHA of rats received OX1R and CRF receptor antagonists.

KEY RESULTS

ARS delayed GE by disturbing the coordination of antro-pyloric contractions while stimulating FO and CORT secretion. ARS-induced alterations in GE, FO, plasma CORT, and antro-pyloric motility were attenuated by OX1R and/or CRF receptor antagonists, however, these changes were completely restored in rats received both antagonists. ARS stimulated release of OXA and CRF which were significantly attenuated by α-CRF9,41 and SB-334867, respectively. The OX1R was detected in CRF-immunoreactive cells, whereas dense expression of CRF2 receptor but not CRF1 was observed in LHA. ARS remarkably increased OXA immunoreactivity in LHA. ARS-induced c-Fos expression in LHA and PVN was abolished by α-CRF9,41 and SB-334867, respectively.

CONCLUSIONS & INFERENCES

Our findings suggest a reciprocal contribution of OXA and CRF which seems to be involved in the mediation of stress-induced alterations in neuroendocrine and GI motor functions.

Neuroscience: Neurons that use a stress hormone increase eating

A neuropeptide called corticotropin-releasing hormone (CRH) is known for stress signaling in the brain. A study now shows that a small population of CRH-expressing neurons situated in the lateral hypothalamus area are involved in sensing olfactory food cues and promoting food consumption in mice.

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.

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.

Blockade of orexin receptors in the infralimbic cortex prevents stress-induced reinstatement of alcohol-seeking behaviour in alcohol-dependent rats

BACKGROUND AND PURPOSE

A major problem managing alcohol use disorder is the high vulnerability to relapse, even after long periods of abstinence. Chronic alcohol use dysregulates stress responsivity, rendering this system hyporesponsive and making individuals vulnerable to relapse. Orexin (hypocretin) plays a role in diverse physiological processes, including stress. Orexin neurons in the hypothalamus, project to the infralimbic cortex. This study asked does infralimbic cortex orexin transmission play a significant role in stress-induced reinstatement of alcohol-seeking behaviour in alcohol-dependent rats.

EXPERIMENTAL APPROACH

Male and female rats were trained to self-administer 10% alcohol (3 weeks) and then made dependent via chronic intermittent alcohol vapour exposure. Following extinction (5 days·week-1 at 8 h abstinence for 10 sessions), rats received an intra- infralimbic cortex microinfusion of the OX1/2 antagonist TCS 1102 (15 μg/0.5 μl per side) and then tested for footshock stress-induced reinstatement of alcohol seeking. In a separate cohort, orexin regulation of infralimbic cortex neuronal activity at the time of reinstatement was investigated using ex vivo electrophysiology.

KEY RESULTS

TCS 1102 prevented reinstatement in dependent animals only. Moreover, Hcrtr mRNA expression in the hypothalamus and Hcrtr1/2 in the infralimbic cortex increased in alcohol-dependent animals at the time of testing. Dependence dampened basal orexin/OX receptor influence over infralimbic cortex GABAergic synapses (using TCS 1102) allow for greater stimulated orexin effects.

CONCLUSION AND IMPLICATIONS

Infralimbic cortex transmission is implicate in stress-induced reinstatement of alcohol-seeking behaviour in subjects with a history of alcohol dependence and show maladaptive recruitment of infralimbic cortex transmission by alcohol dependence.

The stress of losing sleep: Sex-specific neurobiological outcomes

Sleep is a vital and evolutionarily conserved process, critical to daily functioning and homeostatic balance. Losing sleep is inherently stressful and leads to numerous detrimental physiological outcomes. Despite sleep disturbances affecting everyone, women and female rodents are often excluded or underrepresented in clinical and pre-clinical studies. Advancing our understanding of the role of biological sex in the responses to sleep loss stands to greatly improve our ability to understand and treat health consequences of insufficient sleep. As such, this review discusses sex differences in response to sleep deprivation, with a focus on the sympathetic nervous system stress response and activation of the hypothalamic-pituitary-adrenal (HPA) axis. We review sex differences in several stress-related consequences of sleep loss, including inflammation, learning and memory deficits, and mood related changes. Focusing on women's health, we discuss the effects of sleep deprivation during the peripartum period. In closing, we present neurobiological mechanisms, including the contribution of sex hormones, orexins, circadian timing systems, and astrocytic neuromodulation, that may underlie potential sex differences in sleep deprivation responses.

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

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

The stress-induced antinociceptive responses to the persistent inflammatory pain involve the orexin receptors in the nucleus accumbens

Stress suppresses the sense of pain, a physiological phenomenon known as stress-induced analgesia (SIA). Brain orexin peptides regulate many physiological functions, including wakefulness and nociception. The contribution of the orexinergic system within the nucleus accumbens (NAc) in the modulation of antinociception induced by forced swim stress (FSS) remains unclear. The present study addressed the role of intra-accumbal orexin receptors in the antinociceptive responses induced by FSS during the persistent inflammatory pain model in the rat. Stereotaxic surgery was performed unilaterally on 106 adult male Wistar rats weighing 250-305 g. Different doses (1, 3, 10, and 30 nmol/ 0.5 μl DMSO) of orexin-1 receptor (OX1r) antagonist (SB334867) or OX2 receptor antagonist (TCS OX2 29) were administered into the NAc five minutes before exposure to FSS for a 6-min period. The formalin test was carried out using formalin injection (50 μl; 2.5%) into the rat's hind paw plantar surface, which induces biphasic pain-related responses. The first phase begins immediately after formalin infusion and takes 3-5 min. Subsequently, the late phase begins 15-20 min after formalin injection and takes 20-40 min. The findings demonstrated that intra-accumbal microinjection of SB334867 or TCS OX2 29 attenuated the FSS-induced antinociception in both phases of the formalin test, with the TCS OX2 29 showing higher potency. Moreover, the effect of TCS OX2 29 was more significant during the early phase of the formalin test. The results suggest that OX1 and OX2 receptors in the NAc might modulate the antinociceptive responses induced by the FSS.

Activation of the nociceptin/orphanin-FQ receptor promotes NREM sleep and EEG slow wave activity

Sleep/wake control involves several neurotransmitter and neuromodulatory systems yet the coordination of the behavioral and physiological processes underlying sleep is incompletely understood. Previous studies have suggested that activation of the Nociceptin/orphanin FQ (N/OFQ) receptor (NOPR) reduces locomotor activity and produces a sedation-like effect in rodents. In the present study, we systematically evaluated the efficacy of two NOPR agonists, Ro64-6198 and SR16835, on sleep/wake in rats, mice, and Cynomolgus macaques. We found a profound, dose-related increase in non-Rapid Eye Movement (NREM) sleep and electroencephalogram (EEG) slow wave activity (SWA) and suppression of Rapid Eye Movement sleep (REM) sleep in all three species. At the highest dose tested in rats, the increase in NREM sleep and EEG SWA was accompanied by a prolonged inhibition of REM sleep, hypothermia, and reduced locomotor activity. However, even at the highest dose tested, rats were immediately arousable upon sensory stimulation, suggesting sleep rather than an anesthetic state. NOPR agonism also resulted in increased expression of c-Fos in the anterodorsal preoptic and parastrial nuclei, two GABAergic nuclei that are highly interconnected with brain regions involved in physiological regulation. These results suggest that the N/OFQ-NOPR system may have a previously unrecognized role in sleep/wake control and potential promise as a therapeutic target for the treatment of insomnia.

Orexin receptor antagonists in the pathophysiology and treatment of sleep disorders and epilepsy

The correlation between sleep and epilepsy has been argued over the past decades among scientists. Although the similarities and contrasts between sleep and epilepsy had been considered, their intertwined nature was not revealed until the nineteenth century. Sleep is recognized as a recurring state of mind and body through alternating brain electrical activities. It is documented that sleep disorders are associated with epilepsy. The origin, suppression, and spread of seizures are affected by sleep. As such, in patients with epilepsy, sleep disorders are a frequent comorbidity. Meanwhile, orexin, a wake-promoting neuropeptide, provides a bidirectional effect on both sleep and epilepsy. Orexin and its cognate receptors, orexin receptor type 1 (OX1R) and type 2 (OX2R), orchestrate their effects by activating various downstream signaling pathways. Although orexin was considered a therapeutic target in insomnia shortly after its discovery, its potential usefulness for psychiatric disorders and epileptic seizures has been suggested in the pre-clinical studies. This review aimed to discuss whether the relationship between sleep, epilepsy, and orexin is clearly reciprocal.

Orexinergic neurons modulate stress coping responses in mice

Stress is a critical precipitating factor for major depression. However, individual responses to the same stressor vary widely, possibly owing to individual variations in stress resilience. Nevertheless, the factors that determine stress susceptibility and resilience remain poorly understood. Orexin neurons have been implicated in the control of stress-induced arousal. Therefore, we investigated whether orexin-expressing neurons are involved in the regulation of stress resilience in male mice. We found that the level of c-fos expression was significantly different in susceptible versus resilient mice in the learned helplessness test (LHT). Furthermore, activating orexinergic neurons induced resilience in the susceptible group, and this resilience was also consistently observed in other behavioral tests. However, activating orexinergic neurons during the induction period (during inescapable stress exposure) did not affect stress resilience in the escape test. In addition, analyses using pathway-specific optic stimulation revealed that activating orexinergic projections to the medial part of the nucleus accumbens (NAc) alone mediated a decrease in anxiety but was not sufficient to induce resilience in the LHT. Collectively, our data suggest that orexinergic projections to multiple targets control diverse and flexible stress-related behaviors in response to various stressors.

Dual Cannabinoid and Orexin Regulation of Anhedonic Behaviour Caused by Prolonged Restraint Stress

The endocannabinoid and orexin systems share many biological functions, including wakefulness, stress response, reward processing, and mood. While these systems work against one another with respect to arousal, chronic stress-induced downregulation of both systems often leads to anhedonia or the inability to experience pleasure from natural rewards. In the current study, a 24 h restraint stress test (24 h RST) reduced sucrose preference in adult male and female C57BL/6 mice. Prior to the stressor, subsets of mice were intraperitoneally administered cannabinoid and orexin receptor agonists, antagonists, and combinations of these drugs. Restraint mice that received the cannabinoid receptor type 1 (CB1R) antagonist SR141716A, orexin receptor type 2 (OX2R) agonist YNT-185, and the combination of SR141716A and YNT-185, exhibited less anhedonia compared to vehicle/control mice. Thus, the 24 h RST likely decreased orexin signaling, which was then restored by YNT-185. Receptor colocalization analysis throughout mesocorticolimbic brain regions revealed increased CB1R-OX1R colocalization from SR141716A and YNT-185 treatments. Although a previous study from our group showed additive cataleptic effects between CP55,940 and the dual orexin receptor antagonist (TCS-1102), the opposite combination of pharmacological agents proved additive for sucrose preference. Taken together, these results reveal more of the complex interactions between the endocannabinoid and orexin systems.

Blockade of the orexin receptors in the ventral tegmental area could attenuate the stress-induced analgesia: A behavioral and molecular study

Exposure to stressful stimuli induces various physiological and behavioral responses, affects pain perception, and alters gene expression. Stress elicits an analgesic effect in laboratory animals, termed the "stress-induced analgesia" (SIA). Orexin neuropeptides, processed from pre-pro-orexin in the hypothalamus, release during stress and are known to be antinociceptive. The current study examined the modulatory role of the ventral tegmental area (VTA) orexinergic system in the restraint SIA and extracellular signal-regulated kinase (ERK) activation in the nucleus accumbens (NAc). Adult male Wistar rats were subjected to intra-VTA injection of orexin-1 and -2 receptor antagonists (SB334867 and TCS OX2 29; 1, 3, 10, and 30 nmol/0.3 μl, respectively) five min before a 3-h period of exposure to restraint stress (RS). Western blot analysis was also used to assess the levels of ERK and phosphorylated ERK (p-ERK) in the NAc tissues. RS exposure produced an analgesic response to the thermal pain model (Tail-flick test). RS-induced antinociception was inhibited by intra-VTA administration of SB334867 and TCS OX2 29. Moreover, in the molecular study, exposure to forced swim stress (FSS) and RS significantly enhanced the p-ERK/ERK ratio. Blockade of both orexin receptors diminished the p-ERK/ERK ratio, but this decrease was significant only in the FSS group of animals that received TCS OX2 29. Collectively, the present findings suggested the functional roles of intra-VTA orexin receptors and ERK signaling in the SIA.

Cross-Species Convergence of Brain Transcriptomic and Epigenomic Findings in Posttraumatic Stress Disorder: A Systematic Review

Introduction

Posttraumatic stress disorder (PTSD) is a complex multifactorial disorder influenced by the interaction of genetic and environmental factors. Analyses of epigenomic and transcriptomic modifications may help to dissect the biological factors underlying the gene-environment interplay in PTSD. To date, most human PTSD epigenetics studies have used peripheral tissue, and these findings have complex and poorly understood relationships to brain alterations. Studies examining brain tissue may help characterize the brain-specific transcriptomic and epigenomic profiles of PTSD. In this review, we compiled and integrated brain-specific molecular findings of PTSD from humans and animals.

Methods

A systematic literature search according to the PRISMA criteria was performed to identify transcriptomic and epigenomic studies of PTSD, focusing on brain tissue from human postmortem samples or animal-stress paradigms.

Results

Gene- and pathway-level convergence analyses revealed PTSD-dysregulated genes and biological pathways across brain regions and species. A total of 243 genes converged across species, with 17 of them significantly enriched for PTSD. Chemical synaptic transmission and signaling by G-protein-coupled receptors were consistently enriched across omics and species.

Discussion

Our findings point out dysregulated genes highly replicated across PTSD studies in humans and animal models and suggest a potential role for the corticotropin-releasing hormone/orexin pathway in PTSD's pathophysiology. Further, we highlight current knowledge gaps and limitations and recommend future directions to address them.

Estradiol-dependent hypocretinergic/orexinergic behaviors throughout the estrous cycle

RATIONALE

The female menstrual or estrous cycle and its associated fluctuations in circulating estradiol (E2), progesterone, and other gonadal hormones alter orexin or hypocretin peptide production and receptor activity. Depending on the estrous cycle phase, the transcription of prepro-orexin mRNA, post-translational modification of orexin peptide, and abundance of orexin receptors change in a brain region-specific manner. The most dramatic changes occur in the hypothalamus, which is considered the starting point of the hypothalamic-pituitary-gonadal axis as well as the hub of orexin-producing neurons. Thus, hypothalamus-regulated behaviors, including arousal, feeding, reward processing, and the stress response depend on coordinated efforts between E2, progesterone, and the orexin system. Given the rise of orexin therapeutics for various neuropsychiatric conditions including insomnia and affective disorders, it is important to delineate the behavioral outcomes of this drug class in both sexes, as well as within different time points of the female reproductive cycle.

OBJECTIVES

Summarize how the menstrual or estrous cycle affects orexin system functionality in animal models in order to predict how orexin pharmacotherapies exert varying degrees of behavioral effects across the dynamic hormonal milieu.

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.

Sleep disturbance as a therapeutic target to improve opioid use disorder treatment

Sleep health is an important factor across several physical and mental health disorders, and a growing scientific consensus has identified sleep as a critical component of opioid use disorder (OUD), both in the active disease state and during OUD recovery. The goal of this narrative review is to collate the literature on sleep, opioid use, and OUD as a means of identifying therapeutic targets to improve OUD treatment outcomes. Sleep disturbance is common and often severe in persons with OUD, especially during opioid withdrawal, but also in persons on opioid maintenance therapies. There is ample evidence that sleep disturbances including reduced total sleep time, disrupted sleep continuity, and poor sleep quality often accompany negative OUD treatment outcomes. Sleep disturbances are bidirectionally associated with several other factors related to negative treatment outcomes, including chronic stress, stress reactivity, low positive affect, high negative affect, chronic pain, and drug craving. This constellation of outcome variables represents a more comprehensive appraisal of the quality of life and quality of recovery than is typically assessed in OUD clinical trials. To date, there are very few clinical trials or experimental studies aimed at improving sleep health in OUD patients, either as a means of improving stress, affect, and craving outcomes, or as a potential mechanistic target to reduce opioid withdrawal and drug use behaviors. As such, the direct impact of sleep improvement in OUD patients is largely unknown, yet mechanistic and clinical research suggests that therapeutic interventions that target sleep are a promising avenue to improve OUD treatment. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

Antagonism of the orexin receptors in the ventral tegmental area diminished the stress-induced analgesia in persistent inflammatory pain

As a part of descending pain inhibitory system, orexin (OXs) in the ventral tegmental area (VTA) are implicated in nociceptive responses. The current study aimed to evaluate the role of OX receptors (OXRs) in the VTA in stress-induced analgesia in persistent inflammatory pain. Ninety-nine adult male Wistar rats underwent forced swim stress (FSS) following intra-VTA infusion of various doses of SB334867 or TCS OX2 29 (1, 3, 10, and 30 nmol/0.3 μL) as an OX1R or OX2R antagonist, respectively. The nociceptive threshold was evaluated using the formalin test as an animal model of persistent inflammatory pain. Current results demonstrated FSS as acute stress produced analgesic responses in the persistent inflammatory pain. Moreover, either OX1R or OX2R antagonist infusion in the VTA hindered the FSS-induced analgesia in both early and late phases. The inhibitory effect of SB334768 in the FSS-induced analgesia was stronger than TCS OX2 29 in both early and late phases of the formalin test. Neither SB334768 nor TCS OX2 29 alone affects pain-related behaviors in formalin tests. Intra-VTA microinjection of each treatment could not modify locomotion in rats. The findings suggest that OX1R and OX2R in the VTA are implicated in FSS-induced analgesia mechanisms.

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

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

Escitalopram alters the hypothalamic OX system but does not affect its up-regulation induced by early-life stress in adult rats

We hypothesized that there is a relationship between the orexinergic system (OX) alterations and changes elicited by escitalopram or venlafaxine in adult rats subjected to maternal separation (MS). This animal model of childhood adversity induces long-lasting consequences in adult physiology and behavior. Male Wistar rats from the control and MS groups were injected with escitalopram or venlafaxine (10 mg/kg) IP from postnatal day (PND) 69-89. Adult rats were subjected to behavioral assessment, estimation of hypothalamic-pituitary-adrenal (HPA) axis activity and analysis of the OX system (quantitative PCR and immunohistochemistry) in the hypothalamus and amygdala. MS caused anxiety- and depressive-like behavior, endocrine stress-related response, and up-regulation of the OX system in the hypothalamus. Escitalopram, but not venlafaxine, increased the activity of hypothalamic OX system in the control rats and both drugs had no effect on OXs in the MS group. The disturbed signaling of the OX pathway may be significant for harmful long-term consequences of early-life stress. Our data show that the normal brain and brain altered by MS respond differently to escitalopram. Presumably, anti-anxiety and antidepressant effects of this drug do not depend on the activity of hypothalamic OX system.

A Role for Neuropeptide S in Alcohol and Cocaine Seeking

The neuropeptide S (NPS) is the endogenous ligand of the NPS receptor (NPSR). The NPSR is widely expressed in brain regions that process emotional and affective behavior. NPS possesses a unique physio-pharmacological profile, being anxiolytic and promoting arousal at the same time. Intracerebroventricular NPS decreased alcohol consumption in alcohol-preferring rats with no effect in non-preferring control animals. This outcome is most probably linked to the anxiolytic properties of NPS, since alcohol preference is often associated with high levels of basal anxiety and intense stress-reactivity. In addition, NPSR mRNA was overexpressed during ethanol withdrawal and the anxiolytic-like effects of NPS were increased in rodents with a history of alcohol dependence. In line with these preclinical findings, a polymorphism of the NPSR gene was associated with anxiety traits contributing to alcohol use disorders in humans. NPS also potentiated the reinstatement of cocaine and ethanol seeking induced by drug-paired environmental stimuli and the blockade of NPSR reduced reinstatement of cocaine-seeking. Altogether, the work conducted so far indicates the NPS/NPSR system as a potential target to develop new treatments for alcohol and cocaine abuse. An NPSR agonist would be indicated to help individuals to quit alcohol consumption and to alleviate withdrawal syndrome, while NPSR antagonists would be indicated to prevent relapse to alcohol- and cocaine-seeking behavior.

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

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

A developmental switch between electrical and neuropeptide communication in the ventromedial hypothalamus

Dissecting neural connectivity patterns within local brain regions is an essential step to understanding the function of the brain.¹ Neural microcircuits in brain regions, such as the neocortex and the hippocampus, have been extensively studied.² By contrast, the microcircuit in the hypothalamus remains largely uncharacterized. The hypothalamus is crucial for animals' survival and reproduction.³ Knowledge of how different hypothalamic nuclei coordinate with each other and outside brain regions for hypothalamus-related functions has been significantly advanced.^4-9 Although there are limited studies on the neural microcircuit in the lateral hypothalamus (LHA)^10,11 and the suprachiasmatic nucleus (SCN),^12,13 the patterns of neural microcircuits in most of the given hypothalamic nuclei remain largely unknown. This study applied combinatory approaches to address the local neural circuit pattern in the ventromedial hypothalamus (VMH) and other hypothalamic nuclei. We discovered a unique neural circuit design in the VMH. Neurons in the VMH were electrically coupled at the early postnatal stage like ones in the neocortex.¹⁴ However, unlike neocortical neurons,^14,15 they developed very few chemical synapses after the disappearance of electrical synapses. Instead, VMH neurons communicated with neuropeptides. The similar scarceness of synaptic connectivity found in other hypothalamic nuclei further indicated that the lack of synaptic connections is a unique feature for local neural circuits in most adult hypothalamic nuclei. Thus, our findings provide a solid synaptic basis at the cellular level to understand hypothalamic functions better.

Hypothalamic deep brain stimulation as a strategy to manage anxiety disorders

SignificanceAnxiety disorders are among the most prevalent mental illnesses worldwide. Despite significant advances in their treatment, many patients remain treatment resistant. Thus, new treatment modalities and targets are much needed. Therefore, we developed a deep brain stimulation therapy that targets a recently identified anxiety center in the lateral hypothalamus. We show that this therapy rapidly silences anxiety-implicated neurons and immediately relieves diverse anxiety symptoms in a variety of stressful situations. This therapeutic effect occurs without acute or chronic side effects that are typical of many existing treatments, such as physical sedation or memory deficits. These findings identify a clinically applicable new therapeutic strategy for helping patients to manage treatment-resistant anxiety.

Role of CRF1 and CRF2 receptors in the lateral hypothalamus in cardiovascular and anxiogenic responses evoked by restraint stress in rats: Evaluation of acute and chronic exposure

We investigated the role of corticotropin-releasing factor (CRF) neurotransmission within the lateral hypothalamus (LH) in cardiovascular and anxiogenic-like responses evoked by acute and repeated restraint stress in rats. For this, animals were subjected to intra-LH microinjection of a selective CRF₁ (CP376395) or CRF₂ (antisauvagine-30) receptor antagonist before either an acute or the 10th session of restraint stress. Restraint-evoked arterial pressure and heart rate increases, tail skin temperature decrease and anxiogenic-like effect in the elevated plus maze (EPM) were evaluated. We also assessed the effect of 10 daily sessions of restraint on expression of CRF₁ and CRF₂ receptors within the LH. We identified that antagonism of either CRF₁ or CRF₂ receptor within the LH decreased the tachycardia during both the acute and 10th session of restraint, but the effect of the CRF₁ receptor antagonist was more pronounced during the 10th session. Acute restraint stress also caused anxiogenic-like effect, and this response was inhibited in animals treated with either CP376395 or antisauvagine-30. Anxiety-like behaviors were not changed following the 10th session of restraint, and pharmacological treatments did not affect the behavior in the EPM in chronically stressed animals. Repeated restraint also did not change the level of the CRF receptors within the LH. Taken together, the findings indicate that CRF₁ and CRF₂ receptors within the LH are involved in tachycardic and anxiogenic-like responses to aversive stimuli. Control of tachycardia by the CRF₁ receptor is sensitized by previous stressful experience, and this effect seems to be independent of changes in expression of the receptor.

EDCs trigger immune-neurotransmitter related gene expression, and cause histological damage in sensitive mud crab Macrophthalmus japonicus gills and hepatopancreas

Endocrine-disrupting chemicals (EDCs), distributed at various concentrations in freshwater and marine ecosystems, affect the survival, reproduction, and behavior of wide ranges organisms. Most toxicology studies on EDCs have focused on the endocrine system of invertebrates, and research on invertebrate neurotransmitters is limited. In the present study, we investigated the expression of Macrophthalmus japonicus genes encoding γ-aminobutyric acid transporter subtype 2 (GAT-2) and glutamine synthetase (GS), which play important roles as neurotransmitters at synapses. We observed differences in the mRNA expression levels of GAT-2 and GS as well as histological changes in various tissues after exposure to bisphenol-A (BPA) and di-(2-ethylhexyl) phthalate (DEHP). The amino acid sequences of M. japonicus GAT-2 and GS formed separate branches in crustaceans, fish, insects, and mammals. M. japonicus GAT-2 and GS expression levels were highest in the gills, hepatopancreas, and stomach, and showed different between DEHP or BPA treatments. In particular, hepatopancreas GS expression on Day 1, the first step in the presynaptic process, was upregulated after BPA and DEHP exposure, while GAT-2, sequential step in the presynaptic process, was significantly elevated only in DEHP. After BPA treatments, gill GS expression was increased at all concentrations, whereas GAT-2 expression was overall down regulations. In contrast, in DEHP treatment groups hepatopancreatic GS and GAT-2 expression at Day 1 was only significantly higher and all groups including gill GS and GAT-2 expression were downregulation. Histological changes in the gills and hepatopancreas were observed in a concentration-dependent manner. Accordingly, BPA and DEHP exposure in crabs could be stimulate neurotransmitter gene expression and alter the morphological structure of gill and hepatopancreas.

Chronic Sleep Deprivation Blocks Voluntary Morphine Consumption but Not Conditioned Place Preference in Mice

The opioid epidemic remains a significant healthcare problem and is attributable to over 100,000 deaths per year. Poor sleep increases sensitivity to pain, impulsivity, inattention, and negative affect, all of which might perpetuate drug use. Opioid users have disrupted sleep during drug use and withdrawal and report poor sleep as a reason for relapse. However, preclinical studies investigating the relationship between sleep loss and substance use and the associated underlying neurobiological mechanisms of potential interactions are lacking. One of the most common forms of sleep loss in modern society is chronic short sleep (CSS) (<7 h/nightly for adults). Here, we used an established model of CSS to investigate the influence of disrupted sleep on opioid reward in male mice. The CSS paradigm did not increase corticosterone levels or depressive-like behavior after a single sleep deprivation session but did increase expression of Iba1, which typically reflects microglial activation, in the hypothalamus after 4 weeks of CSS. Rested control mice developed a morphine preference in a 2-bottle choice test, while mice exposed to CSS did not develop a morphine preference. Both groups demonstrated morphine conditioned place preference (mCPP), but there were no differences in conditioned preference between rested and CSS mice. Taken together, our results show that recovery sleep after chronic sleep disruption lessens voluntary opioid intake, without impacting conditioned reward associated with morphine.

Orexin receptors in the CA1 region of hippocampus modulate the stress-induced antinociceptive responses in an animal model of persistent inflammatory pain

Stress activates multiple neural pathways and neurotransmitters that often suppress pain perception, the phenomenon called stress-induced analgesia (SIA). Orexin neurons from the lateral hypothalamus project to entire brain structures such as the hippocampus. The present study examined this hypothesis that orexinergic receptors in the CA1 region of the hippocampus may play a modulatory role in the development of SIA in formalin test as an animal model of persistent inflammatory pain. One hundred-two adult male Wistar rats were administered with intra-CA1 orexin-1 receptor (OX1r) antagonist, SB334867, at the doses of 3, 10, 30, and 100 nmol or TCS OX2 29 as orexin-2 receptor (OX2r) antagonist at the doses of 1, 3, 10, and 30 nmol. Five min later, rats were exposed to forced swim stress (FSS) for a 6-min period. Then, pain-related behaviors induced by formalin injection were measured at the 5-min blocks during a 60-min period of formalin test. The current study indicated that solely stress exposure elicits antinociception in the early and late phases of the formalin test. The FSS-induced analgesia was prevented by intra-CA1 administration of SB334867 or TCS OX2 29 during either phase of the formalin test. Moreover, the contribution of the OX2r in the mediation of analgesic effect of stress was more prominent than that of the OX1r during both phases of the formalin test. It is suggested that OX1r and OX2r in the CA1 region of the hippocampus are involved in stress-induced analgesia in the animal model of persistent inflammatory pain.

The Central Inflammatory Network: A Hypothalamic fMRI Study of Experimental Endotoxemia in Humans

INTRODUCTION

Inflammation is a mechanism of the immune system that is part of the reaction to pathogens or injury. The central nervous system closely regulates inflammation via neuroendocrine or direct neuroimmune mechanisms, but our current knowledge of the underlying circuitry is limited. Therefore, we aimed to identify hypothalamic centres involved in sensing or modulating inflammation and to study their association with known large-scale brain networks.

METHODS

Using high-resolution functional magnetic resonance imaging (fMRI), we recorded brain activity in healthy male subjects undergoing experimental inflammation from intravenous endotoxin. Four fMRI runs covered key phases of the developing inflammation: pre-inflammatory baseline, onset of endotoxemia, onset of pro-inflammatory cytokinemia, and peak of pro-inflammatory cytokinemia. Using masked independent component analysis, we identified functionally homogeneous subregions of the hypothalamus, which were further tested for changes in functional connectivity during inflammation and for temporal correlation with tumour necrosis factor and adrenocorticotropic hormone serum levels. We then studied the connection of these inflammation-associated hypothalamic subregions with known large-scale brain networks.

RESULTS

Our results show that there are at least 6 hypothalamic subregions associated with inflammation in humans including the paraventricular nucleus, supraoptic nucleus, dorsomedial hypothalamus, bed nucleus of the stria terminalis, lateral hypothalamic area, and supramammillary nucleus. They are functionally embedded in at least 3 different large-scale brain networks, namely a medial frontoparietal network, an occipital-pericentral network, and a midcingulo-insular network.

CONCLUSION

Measuring how the hypothalamus detects or modulates systemic inflammation is a first step to understand central nervous immunomodulation.

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

In this review study, we aimed to introduce the orexinergic system as an important signaling pathway involved in a variety of cognitive functions such as memory, motivation, and reward-related behaviors. This study focused on the role of orexinergic system in modulating reward-related behavior, with or without the presence of stressors. Cross-talk between the reward system and orexinergic signaling was also investigated, especially orexinergic signaling in the ventral tegmental area (VTA), the nucleus accumbens (NAc), and the hippocampus. Furthermore, we discussed the role of the orexinergic system in modulating mood states and mental illnesses such as depression, anxiety, panic, and posttraumatic stress disorder (PTSD). Here, we narrowed down our focus on the orexinergic signaling in three brain regions: the VTA, NAc, and the hippocampus (CA1 region and dentate gyrus) for their prominent role in reward-related behaviors and memory. It was concluded that the orexinergic system is critically involved in reward-related behavior and significantly alters stress responses and stress-related psychiatric and mood disorders.

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.

Changes in the Orexin System in Rats Exhibiting Learned Helplessness Behaviors

Orexin-A (OX-A) and orexin-B (OX-B) are neuropeptides produced in the hypothalamus. Preclinical and clinical studies suggest that depression and anxiety are associated with the orexin system. In the current study, we used the learned helplessness (LH) animal model of depression to identify rats displaying LH behaviors (LH rats) and those that did not (No-LH rats). We compared the number of orexin-containing neurons in the hypothalamus of LH, No-LH, and control rats. Orexin peptides, orexin receptor 1 (OXR1) and 2 (OXR2) in brain areas involved in major depression and serum OX-A and corticosterone (CORT) concentrations were quantified and compared between rat groups. We found that LH and No-LH rats displayed higher serum OX-A concentrations compared with control rats. Comparison between LH and No-LH rats revealed that No-LH rats had significantly higher OX-A levels in the brain, more OX-A neurons, and more OX-A neuron activation. LH rats had more OX-B neurons and more OX-B neuron activation. Orexin peptides and receptors in the brain areas involved in major depression exhibited different patterns in LH and NoLH rats. Our findings revealed that activation of OX-A neurons could promote resilient behaviors under stressful situations and OX-A and OX-B neuropeptides exhibit dissimilar functions in LH behaviors.

Do orexin/hypocretin neurons signal stress or reward?

Hypothalamic neurons that produce the peptide transmitters orexins/hypocretins (HONs) broadcast their predominantly neuroexcitatory outputs to the entire brain via their extremely wide axonal projections. HONs were originally reported to be activated by food deprivation, and to stimulate arousal, energy expenditure, and eating. This led to extensive studies of HONs in the context of nutrient-sensing and energy balance control. While activation of HONs by body energy depletion continues to be supported by experimental evidence, it has also become clear that HONs are robustly activated not only by nutrient depletion, but also by diverse sensory stimuli (both neutral and those associated with rewarding or aversive events), seemingly unrelated to each other or to energy balance. One theory that could unify these findings is that all these stimuli signal "stress" - defined either as a potentially harmful state, or an awareness of reward deficiency. If HON activity is conceptualized as a cumulative representation of stress, then many of the reported HONs outputs - including EEG arousal, sympathetic activation, place avoidance, and exploratory behaviours - could be viewed as logical stress-counteracting responses. We discuss evidence for and against this unifying theory of HON function, including the alterations in HON activity observed in anxiety and depression disorders. We propose that, in order to orchestrate stress-countering responses, HONs need to coactivate motivation and aversion brain systems, and the impact of HON stimulation on affective states may be perceived as rewarding or aversive depending on the baseline HON activity.

Effects of Sleep Deprivation by Olfactorily Induced Sexual Arousal Compared to Immobilization Stress and Manual Sleep Deprivation on Neuromessengers and Time Keeping Genes in the Suprachiasmatic Nuclei and Other Cerebral Entities of Syrian Hamsters-An Immunohistochemical Study

We investigated the effects of sexual arousal induced by olfactory stimuli on the expression of neuromodulators, neurotransmitters and sexual steroid receptors in the suprachiasmatic nucleus (SCN, the circadian pacemaker of mammals) and other cerebral entities of Syrian hamsters (Mesocricetus auratus) compared to manual sleep deprivation and immobilization stress. The hamsters kept under a 12:12 hours (h) light:dark cycle were deprived of sleep by sexual stimulation, gentle manual handling or immobilization stress for 1 h at the beginning of the light phase and subsequently sacrificed at zeitgeber time 01:00, respectively; for comparison, hamsters were manually sleep deprived for 6 or 20 h or sacrificed after completing a full sleep phase. As demonstrated by immunohistochemistry, apart from various alterations after manual sleep deprivation, sexual stimulation caused down-regulation of arginine-vasopressin (AVP), vasointestinal peptide (VIP), serotonin (5-HT), substance P (SP), and met-enkephalin (ME) in the SCN. Somatostatin (SOM) was diminished in the medial periventricular nucleus (MPVN). In contrast, an increase in AVP was observed in the PVN, that of oxytocin (OXY) in the supraoptic nucleus (SON), of tyrosine-hydroxylase (TH) in the infundibular nucleus (IN), and dopamine beta-hydroxylase (DBH) in the A7 neuron population of the brain stem (A7), respectively. Testosterone in plasma was increased. The results indicate that sexual arousal extensively influences the neuropeptide systems of the SCN, suggesting an involvement of the SCN in reproductive behavior.

The Central Role of Hypothermia and Hyperactivity in Anorexia Nervosa: A Hypothesis

Typically, the development of anorexia nervosa (AN) is attributed to psycho-social causes. Several researchers have recently challenged this view and suggested that hypothermia and hyperactivity (HyAc) are central to AN. The following hypothesis will attempt to clarify their role in AN. Anorexia nervosa patients (ANs) have significantly lower core temperatures (T_core) compared to healthy controls (HCs). This reduced temperature represents a reset T_core that needs to be maintained. However, ANs cannot maintain this T_core due primarily to a reduced basal metabolic rate (BMR); BMR usually supplies heat to sustain T_core. Therefore, to generate the requisite heat, ANs revert to the behavioral-thermoregulatory strategy of HyAc. The majority of ANs (~89%) are reportedly HyAc. Surprisingly, engagement in HyAc is not motivated by a conscious awareness of low T_core, but rather by the innocuous sensation of "cold- hands" frequently reported by ANs. That is, local hand-thermoreceptors signal the brain to initiate HyAc, which boosts perfusion of the hands and alters the sensation of "cold-discomfort" to one of "comfort." This "rewarding" consequence encourages repetition/habit formation. Simultaneously, hyperactivity increases the availability of heat to assist with the preservation of T_core. Additionally, HyAc induces the synthesis of specific brain neuromodulators that suppress food intake and further promote HyAc; this outcome helps preserve low weight and perpetuates this vicious cycle. Based on this hypothesis and supported by rodent research, external heat availability should reduce the compulsion to be HyAc to thermoregulate. A reduction in HyAc should decrease the production of brain neuromodulators that suppress appetite. If verified, hopefully, this hypothesis will assist with the development of novel treatments to aid in the resolution of this intractable condition.

Modulatory role of the orexin system in stress-induced analgesia: Involvement of the ventral tegmental area

BACKGROUND

Exposure to stressful experiences is often accompanied by suppressing pain perception, referred to as stress-induced analgesia. The neuropeptides orexins are essential in regulating the mechanism that responds to stressful and painful stimuli. Meanwhile, the ventral tegmental area (VTA), as a part of descending pain inhibitory system, responds to noxious stimuli. This study aimed to investigate the role of intra-VTA administration of orexin receptor antagonists on stress-induced antinociceptive responses in the animal model of acute pain.

METHOD

Ninety-three adult Wistar rats weighing 230-250 g were unilaterally implanted by a cannulae above the VTA. Animals were pretreated with different doses (1, 3, 10 and 30 nM/0.3 μl) of SB334867 as the orexin-1 receptor antagonist and TCS OX2 29 as the orexin-2 receptor antagonist into the VTA, just 5 min before 6 min exposure to forced swim stress (FSS). Nociceptive threshold was measured using the tail-flick test as a model of acute pain.

RESULTS

The results showed that exposure to FSS could significantly increase analgesic responses. Moreover, intra-VTA administration of SB334768 and TCS OX2 29 blocked the antinociceptive effect of FSS in the tail-flick test.

CONCLUSION

The findings suggest that OX1 and OX2 receptors in the VTA might modulate the antinociceptive behaviours induced by FSS in part.

SIGNIFICANCE

Acute exposure to physical stress suppresses pain-related behaviors in the animal model of acute pain. Blockade of the OX1 and OX2 receptors in the VTA attenuates antinociceptive responses induced by FSS. The contribution of the OX2 receptors in the VTA is more predominant than OX1 receptors in stress-induced analgesia.

Sex-Specific Effects of Stress on Respiratory Control: Plasticity, Adaptation, and Dysfunction

As our understanding of respiratory control evolves, we appreciate how the basic neurobiological principles of plasticity discovered in other systems shape the development and function of the respiratory control system. While breathing is a robust homeostatic function, there is growing evidence that stress disrupts respiratory control in ways that predispose to disease. Neonatal stress (in the form of maternal separation) affects "classical" respiratory control structures such as the peripheral O₂ sensors (carotid bodies) and the medulla (e.g., nucleus of the solitary tract). Furthermore, early life stress disrupts the paraventricular nucleus of the hypothalamus (PVH), a structure that has emerged as a primary determinant of the intensity of the ventilatory response to hypoxia. Although underestimated, the PVH's influence on respiratory function is a logical extension of the hypothalamic control of metabolic demand and supply. In this article, we review the functional and anatomical links between the stress neuroendocrine axis and the medullary network regulating breathing. We then present the persistent and sex-specific effects of neonatal stress on respiratory control in adult rats. The similarities between the respiratory phenotype of stressed rats and clinical manifestations of respiratory control disorders such as sleep-disordered breathing and panic attacks are remarkable. These observations are in line with the scientific consensus that the origins of adult disease are often found among developmental and biological disruptions occurring during early life. These observations bring a different perspective on the structural hierarchy of respiratory homeostasis and point to new directions in our understanding of the etiology of respiratory control disorders. © 2021 American Physiological Society. Compr Physiol 11:1-38, 2021.

Brain Circuit of Claustrophobia-like Behavior in Mice Identified by Upstream Tracing of Sighing

Emotions are distinct patterns of behavioral and physiological responses triggered by stimuli that induce different brain states. Elucidating the circuits is difficult because of challenges in interrogating emotional brain states and their complex outputs. Here, we leverage the recent discovery in mice of a neural circuit for sighing, a simple, quantifiable output of various emotions. We show that mouse confinement triggers sighing, and this "claustrophobic" sighing, but not accompanying tachypnea, requires the same medullary neuromedin B (Nmb)-expressing neurons as physiological sighing. Retrograde tracing from the Nmb neurons identified 12 forebrain centers providing presynaptic input, including hypocretin (Hcrt)-expressing lateral hypothalamic neurons. Confinement activates Hcrt neurons, and optogenetic activation induces sighing and tachypnea whereas pharmacologic inhibition suppresses both responses. The effect on sighing is mediated by HCRT directly on Nmbneurons. We propose that this HCRT-NMB neuropeptide relay circuit mediates claustrophobic sighing and that activated Hcrt neurons are a claustrophobia brain state that directly controls claustrophobic outputs.

Should I Stay or Should I Go? CRHPVN Neurons Gate State Transitions in Stress-Related Behaviors

Corticotropin-releasing hormone (CRH) neurons in the paraventricular nucleus of the hypothalamus are the canonical controllers of the endocrine response to stress. Here we propose a new role for these cells as a gate for state transitions that allow the organism to engage in stress-related behaviors. Specifically, we review evidence indicating that activation of these cells at critical times allows organisms to move to a state that is permissive for motor action. This is evident when the organism is under duress (defensive behavior), when the organism has successfully vanquished a threat (coping behavior), and when an organism initiates approach to a conspecific (social behavior). The motor behavior that follows from the activation of CRH neurons is not necessarily under the control of these cells but is determined by higher order circuits that discriminate more refined features of environmental context to execute the appropriate behavior.