Orexin administration to mice that underwent chronic stress produces bimodal effects on emotion-related behaviors

Exploring the role of orexins in the modulation of social reward

RATIONALE

positive social interactions are essential for mental health, by offering emotional support, reducing stress levels, and promoting resilience against drugs of abuse effects. However, not all individuals perceive social interaction as rewarding.

OBJECTIVES

the goal of this study was to investigate whether the modulation of the orexin system can shift passive coping and non-social behavior (vulnerable) to active coping and social behavior (resilient). This knowledge is primordial for stress- and addiction-related disorders, and for other psychiatric disorders involving impairment in social interaction.

METHODS

male C57/BL6N mice categorized into social and non-social groups, received injections of SB334867, a selective orexin 1 receptor (OX1R) antagonist, before the conditioning sessions with a male conspecific of the same weight and age.

RESULTS

our results from the conditioned place preference test (CPP) show that SB334867 has no effect on social preference in non-social mice, but it reduces their stress levels and depression-like behavior. These effects appear to be due to a higher OX1R expression in the basolateral amygdala (BLA), a stress-related brain area, of non-social mice compared to their social counterparts.

CONCLUSIONS

these data suggest that the orexin system may be a target to alleviate stress and depression-like behavior in non-social individuals rather than to promote social reward.

Hypocretin modulation of behavioral coping strategies for social stress

Best known for promoting wakefulness and arousal, the neuropeptide hypocretin (Hcrt) also plays an important role in mediating stress responses, including social stress. However, central and systemic manipulation of the Hcrt system has produced diverse behavioral outcomes in animal models. In this review, we first focus on studies where similar manipulations of the Hcrt system led to divergent coping behaviors. We hypothesize that Hcrt differentially facilitates active and passive coping behaviors in response to social stress by acting in different brain regions and on different cell types. We then focus on region and cell type-specific effects of Hcrt in the ventral pallidum, lateral habenula, ventral tegmental area, nucleus accumbens, amygdala, and bed nucleus of the stria terminalis. Overall, the evidence suggests that rather than enhancing or inhibiting behavioral responses to social stress, Hcrt may signal the heightened arousal associated with stressful contexts. The resulting behavioral effects depend on which circuits Hcrt release occurs in and which receptor types are activated. Further study is needed to determine how and why circuit specific activation of Hcrt neurons occurs.

Hypocretin in the nucleus accumbens shell modulates social approach in female but not male California mice

The hypocretin (Hcrt) system modulates arousal and anxiety-related behaviors and has been considered as a novel treatment target for stress-related affective disorders. We examined the effects of Hcrt acting in the nucleus accumbens shell (NAcSh) and anterodorsal bed nucleus of the stria terminalis (adBNST) on social behavior in male and female California mice (Peromyscus californicus). In female but not male California mice, infusion of Hcrt1 into NAcSh decreased social approach. Weak effects of Hcrt1 on social vigilance were observed in both females and males. No behavioral effects of Hcrt1 infused into the adBNST were observed. Analyses of sequencing data from California mice and Mus musculus NAc showed that Hcrtr2 was more abundant than Hcrtr1, so we infused the selective Hcrt receptor 2 antagonist into the NAcSh, which increased social approach in females previously exposed to social defeat. A calcium imaging study in the NAcSh of females before and after stress exposure showed that neural activity increased immediately following the expression of social avoidance but not during freezing behavior. This observation is consistent with previous studies that identified populations of neurons in the NAc that drive avoidance. Intriguingly, calcium transients were not affected by stress. These data suggest that hypocretin acting in the NAcSh plays a key role in modulating stress-induced social avoidance.

Finding biomarkers of experience in animals

At a time when there is a growing public interest in animal welfare, it is critical to have objective means to assess the way that an animal experiences a situation. Objectivity is critical to ensure appropriate animal welfare outcomes. Existing behavioural, physiological, and neurobiological indicators that are used to assess animal welfare can verify the absence of extremely negative outcomes. But welfare is more than an absence of negative outcomes and an appropriate indicator should reflect the full spectrum of experience of an animal, from negative to positive. In this review, we draw from the knowledge of human biomedical science to propose a list of candidate biological markers (biomarkers) that should reflect the experiential state of non-human animals. The proposed biomarkers can be classified on their main function as endocrine, oxidative stress, non-coding molecular, and thermobiological markers. We also discuss practical challenges that must be addressed before any of these biomarkers can become useful to assess the experience of an animal in real-life.

Contribution of changes in the orexin system and energy sensors in the brain in depressive disorder - a study in an animal model

BACKGROUND

Maternal elevated glucocorticoid levels during pregnancy can affect the developing fetus, permanently altering the structure and function of its brain throughout life. Excessive action of these hormones is known to contribute to psychiatric disorders, including depression.

MATERIALS

The study was performed in a rat model of depression based on prenatal administration of dexamethasone (DEX) in late pregnancy (0.1 mg/kg, days 14-21). We evaluated the effects of prenatal DEX treatment on the cognition and bioenergetic signaling pathways in the brain of adult male rats, in the frontal cortex and hippocampus, and in response to stress in adulthood, using behavioral and biochemical test batteries.

RESULTS

We revealed cognitive deficits in rats prenatally treated with DEX. At the molecular level, a decrease in the orexin A and orexin B levels and downregulation of the AMPK-SIRT1-PGC1α transduction pathway in the frontal cortex of these animals were observed. In the hippocampus, a decreased expression of orexin B was found and changes in the MR/GR ratio were demonstrated. Furthermore, an increase in HDAC5 level triggered by the prenatal DEX treatment in both brain structures and a decrease in MeCP2 level in the hippocampus were reported.

CONCLUSIONS

Our study demonstrated that prenatal DEX treatment is associated with cognitive dysfunction and alterations in various proteins leading to metabolic changes in the frontal cortex, while in the hippocampus adaptation mechanisms were activated. The presented results imply that different pathophysiological metabolic processes may be involved in depression development, which may be useful in the search for novel therapies.

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.

Central regulation of stress-evoked peripheral immune responses

Stress-linked psychiatric disorders, including anxiety and major depressive disorder, are associated with systemic inflammation. Recent studies have reported stress-induced alterations in haematopoiesis that result in monocytosis, neutrophilia, lymphocytopenia and, consequently, in the upregulation of pro-inflammatory processes in immunologically relevant peripheral tissues. There is now evidence that this peripheral inflammation contributes to the development of psychiatric symptoms as well as to common co-morbidities of psychiatric disorders such as metabolic syndrome and immunosuppression. Here, we review the specific brain and spinal regions, and the neuronal populations within them, that respond to stress and transmit signals to peripheral tissues via the autonomic nervous system or neuroendocrine pathways to influence immunological function. We comprehensively summarize studies that have employed retrograde tracing to define neurocircuits linking the brain to the bone marrow, spleen, gut, adipose tissue and liver. Moreover, we highlight studies that have used chemogenetic or optogenetic manipulation or intracerebroventricular administration of peptide hormones to control somatic immune responses. Collectively, this growing body of literature illustrates potential mechanisms through which stress signals are conveyed from the CNS to immune cells to regulate stress-relevant behaviours and comorbid pathophysiology.

Characteristics of Seltorexant-Innovative Agent Targeting Orexin System for the Treatment of Depression and Anxiety

Twenty-five years have passed since the discovery of the orexin system, during which time we have learned more and more about it. A number of studies have been conducted showing the role of the orexin system in insomnia, as well as its potential use in the treatment of obesity and depression. In this review, we present the role of the orexin system in the development of depressive illness and show the characteristics of seltorexant, a potential drug for the treatment of depression. This review describes the structure and synthesis of the compound as well as its pharmacodynamics and pharmacokinetics. Pre-clinical and clinical studies are also described, including side effects. There is evidence that the use of seltorexant is considered safe, with no clear or major clinically significant side effects, which makes it a promising candidate for the treatment of depression and anxiety disorders.

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.

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

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

The sex-dependent response to psychosocial stress and ischaemic heart disease

Stress is an important risk factor for modern chronic diseases, with distinct influences in males and females. The sex specificity of the mammalian stress response contributes to the sex-dependent development and impacts of coronary artery disease (CAD). Compared to men, women appear to have greater susceptibility to chronic forms of psychosocial stress, extending beyond an increased incidence of mood disorders to include a 2- to 4-fold higher risk of stress-dependent myocardial infarction in women, and up to 10-fold higher risk of Takotsubo syndrome-a stress-dependent coronary-myocardial disorder most prevalent in post-menopausal women. Sex differences arise at all levels of the stress response: from initial perception of stress to behavioural, cognitive, and affective responses and longer-term disease outcomes. These fundamental differences involve interactions between chromosomal and gonadal determinants, (mal)adaptive epigenetic modulation across the lifespan (particularly in early life), and the extrinsic influences of socio-cultural, economic, and environmental factors. Pre-clinical investigations of biological mechanisms support distinct early life programming and a heightened corticolimbic-noradrenaline-neuroinflammatory reactivity in females vs. males, among implicated determinants of the chronic stress response. Unravelling the intrinsic molecular, cellular and systems biological basis of these differences, and their interactions with external lifestyle/socio-cultural determinants, can guide preventative and therapeutic strategies to better target coronary heart disease in a tailored sex-specific manner.

Dual orexin receptor antagonist drug suvorexant can help in amelioration of predictable chronic mild stress-induced hyperalgesia

AIMS

This study aimed to evaluate the involvement of the orexin system in predictable chronic mild stress (PCMS) and the effects of suvorexant, a dual orexin receptor antagonist, on nociceptive behavior in PCMS.

MATERIALS AND METHODS

Male C57BL/6 J mice were separated into various PCMS groups: a control group with sawdust on the floor of the rearing cage (C), a group with mesh wire on the floor (M), and a group with water just below the mesh wire (W). Activation of lateral hypothalamic orexin neurons was assessed using immunofluorescence. In another experiment, half of the mice in each group were administered an intraperitoneal injection of suvorexant (10 mg/kg), and the remaining mice were injected with the same amount of vehicle (normal saline). Thermal hyperalgesia was examined using tail immersion and hot plate tests, while mechanical hyperalgesia was investigated using the tail pinch test after 21 days of PCMS.

KEY FINDINGS

Animals subjected to PCMS showed an increased percentage of activated orexin neurons in the lateral hypothalamic region after 21 days. Mice raised in the PCMS environment showed increased pain sensitivity in several pain tests; however, the symptoms were significantly reduced by suvorexant administration.

SIGNIFICANCE

The findings revealed that PCMS activates hypothalamic orexin neuronal activity, and the use of suvorexant can help attenuate PCMS-induced thermal and mechanical hyperalgesia.

Resilience to the effects of social stress on vulnerability to developing drug addiction

We review the still scarce but growing literature on resilience to the effects of social stress on the rewarding properties of drugs of abuse. We define the concept of resilience and how it is applied to the field of drug addiction research. We also describe the internal and external protective factors associated with resilience, such as individual behavioral traits and social support. We then explain the physiological response to stress and how it is modulated by resilience factors. In the subsequent section, we describe the animal models commonly used in the study of resilience to social stress, and we focus on the effects of chronic social defeat (SD), a kind of stress induced by repeated experience of defeat in an agonistic encounter, on different animal behaviors (depression- and anxiety-like behavior, cognitive impairment and addiction-like symptoms). We then summarize the current knowledge on the neurobiological substrates of resilience derived from studies of resilience to the effects of chronic SD stress on depression- and anxiety-related behaviors in rodents. Finally, we focus on the limited studies carried out to explore resilience to the effects of SD stress on the rewarding properties of drugs of abuse, describing the current state of knowledge and suggesting future research directions.

Multi-Level Processes and Retina-Brain Pathways of Photic Regulation of Mood

Light exerts powerful biological effects on mood regulation. Whereas the source of photic information affecting mood is well established at least via intrinsically photosensitive retinal ganglion cells (ipRGCs) secreting the melanopsin photopigment, the precise circuits that mediate the impact of light on depressive behaviors are not well understood. This review proposes two distinct retina–brain pathways of light effects on mood: (i) a suprachiasmatic nucleus (SCN)-dependent pathway with light effect on mood via the synchronization of biological rhythms, and (ii) a SCN-independent pathway with light effects on mood through modulation of the homeostatic process of sleep, alertness and emotion regulation: (1) light directly inhibits brain areas promoting sleep such as the ventrolateral preoptic nucleus (VLPO), and activates numerous brain areas involved in alertness such as, monoaminergic areas, thalamic regions and hypothalamic regions including orexin areas; (2) moreover, light seems to modulate mood through orexin-, serotonin- and dopamine-dependent pathways; (3) in addition, light activates brain emotional processing areas including the amygdala, the nucleus accumbens, the perihabenular nucleus, the left hippocampus and pathways such as the retina–ventral lateral geniculate nucleus and intergeniculate leaflet–lateral habenula pathway. This work synthetizes new insights into the neural basis required for light influence mood

Lateral hypothalamus orexinergic inputs to lateral habenula modulate maladaptation after social defeat stress

Social stress, a common stressor, causes multiple forms of physical and mental dysfunction. Prolonged exposure to social stress is associated with a higher risk of psychological disorders, including anxiety disorders and major depressive disorder (MDD). The orexinergic system is involved in the regulation of multiple motivated behaviors. The current study examined the regulatory effect of orexinergic projections from the lateral hypothalamic area (LHA) to the lateral habenula (LHb) in depression- and anxiety-like behaviors after chronic social defeat stress. When mice were defeated during social interaction, both orexinergic neurons in the LHA and glutamatergic neurons in the LHb were strongly activated, as indicated by the FosTRAP strategy. Infusion of orexin in the LHb significantly alleviated social avoidance and depression-like behaviors induced by chronic social defeat stress. Administration of an orexin receptor 2 antagonist in the LHb further aggravated the depressive phenotype. Photoactivation of orexinergic cell bodies in the LHA or terminals in the LHb relieved anxiety-like behaviors induced by chronic social defeat stress. Collectively, we identified the antidepressant and anxiolytic effects of the circuit from LHA orexinergic neurons to the LHb in response to chronic social stress, providing new evidence of the antidepressant properties of LHA orexin circuits.

Depression and stress levels increase risk of liver cancer through epigenetic downregulation of hypocretin

Recent studies suggest that Hypocretin (HCRT, Orexin) are involved in stress regulation of depression through the hypothalamic-pituitary-adrenal (HPA) axis. However, the molecular mechanism by which Hypocretin regulate neurobiological responses is unknown. Herein, the effects of chronic stress on the epigenetic modification of HCRT and its association with depression were explored with regard to a potential role in cancer progression. In the study, Sprague Dawley (SD) rats were used to establish an animal model of cancer with depression by administrating n-nitrosodiethylamine (DEN) and chronic unpredictable mild stress (CUMS). RNA-sequencing was used to detect differentially expressed genes in the hippocampus of rats and quantitative real-time polymerase chain reaction (qRT-PCR) was used to validate the results of RNA-sequencing. The status of HCRT promoter methylation was assessed by methylation specific polymerase chain reaction. Behavioral tests showed that rats exposed to CUMS had significant depressive-like behaviors. The number of liver tumors and tumor load in depressed rats exposed to CUMS was higher than in SD rats without CUMS. RNA-sequencing revealed that HCRT was one of the most siginificantly downregulated gene in the hippocampus of SD rats with CUMS compared to non-stressed group, which was validated by qRT-PCR. HCRT mRNA expression was downregulated and the promoter for HCRT was hyper-methylated in those with depression. These results identified a critical role for chronic psychological stressors in tumorigenesis and cancer progression, via epigenetic HCRT downregulation. Such epigenetic downregulation may be the molecular basis for the association of cancer with depression.

Orexin A injection into the ventral medial prefrontal cortex induces antidepressant-like effects: Possible involvement of local Orexin-1 and Trk receptors

RATIONALE

Reduced levels of orexin-A (OXA) in the central nervous system (CNS) have been associated with the pathophysiology of depression and its exogenous administration promotes antidepressant-like effect. The mechanisms associated with these effects are, however, not yet known. Herein, we investigated the hypothesis that OXA effects could be associated with orexin 1 receptor (OX1R) and tyrosine receptor kinase B (TrkB) activation, in the ventromedial prefrontal cortex (vmPFC), a brain region that is central to depression neurobiology.

OBJECTIVES

1. To Investigate the effects induced by OXA administration into the vmPFC; 2. Evaluate the participation of OX1R and TrkB in behavioral responses induced by OXA.

METHODS

Male Wistar rats received intra-vmPFC injections of OXA (10, 50 and 100 pmol/0.2 μL) and were exposed to the forced swimming test (FST) or the open field test (OFT). Independent groups received an intra-vmPFC injection of SB334867 (OX1R antagonist, 10 nmol/0.2 μL) or K252a (non-selective Trk antagonist, 10 pmol/0.2 μL), before local injection of OXA, and were exposed to the same tests.

RESULTS

OXA injection (100 pmol/0.2 μL) into the vmPFC induced antidepressant-like effect, which was prevented by SB334867 and K252a pretreatments.

CONCLUSION

OXA signaling in the vmPFC induces antidepressant-like effect in the FST which is dependent on OX1R and Trk receptors.

Orexin A in adolescents with anxiety disorders

Objectives: The relationships between orexins and stress-related conditions have been well documented in animal studies. However, human studies confirming this relationship are limited. The aim of this study was to investigate the association between orexin-A and anxiety disorders in adolescents. Additionally, we aimed to examine the relationship between orexin-A and cortisol levels in those with anxiety disorders.Methods: A total of 56 medication-free adolescents diagnosed with any anxiety disorder, except for specific phobias, and 32 healthy controls were included in this study. Depression, state and trait anxiety levels of the participants were measured using self-report scales. Orexin-A and cortisol levels were measured by an enzyme-linked immunosorbent assay (ELISA).Results: Analysis of covariance (ANCOVA) indicated that serum orexin-A levels were significantly higher in the anxiety disorder group than in the control group while controlling for age, sex and depression levels. After controlling for age and sex, orexin-A levels were positively and negatively correlated to depression and cortisol levels, respectively. In addition, a positive correlation trend between trait anxiety and orexin-A was found.Conclusions: Orexin-A levels are higher in adolescents with anxiety disorder; however, depressive symptoms should be considered when investigating this relationship.

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

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

Involvement of orexin/hypocretin in the expression of social play behaviour in juvenile rats

Social play is a highly rewarding and motivated behaviour displayed by juveniles of many mammalian species. We hypothesized that the orexin/hypocretin (ORX) system is involved in the expression of juvenile social play behaviour because this system is interconnected with brain regions that comprise the social behaviour and mesocorticolimbic reward networks. We found that exposure to social play increased recruitment of ORX-A neurons in juvenile rats. Furthermore, central administration of ORX-A decreased social play duration, while central blockade of ORX-1 receptors differentially altered social play duration in juvenile rats with low versus high baseline levels of social play (increasing social play in low baseline social play individuals and decreasing social play in high baseline social play individuals). Together, our results provided the first evidence of a role for the ORX system in the modulation of juvenile social play behaviour.

Evaluating the role of orexins in the pathophysiology and treatment of depression: A comprehensive review

Orexins are neuropeptides that are postulated to play a central role in the regulation of the sleep-wake cycle, appetite, affect, and reward circuitry. The objectives of the current review are to comprehensively evaluate (1) the potential role of orexins in the pathophysiology of major depressive disorders (MDD) and (2) the orexin system as a novel target in the treatment of MDD. Dysfunction of the sleep-wake cycle is observed as a central feature of MDD pathophysiology. Orexin system disturbances produce sleep-wake dysfunction, as observed in MDD. Orexin antagonists have been shown to treat insomnia effectively without disrupting normal sleep architecture in both preclinical (e.g., animal models) and clinical studies. Orexin antagonists are generally safe, well-tolerated, and associated with an acceptable long-term adverse effect profile with relatively low propensity for tolerance or dependence. Orexin antagonists have also been shown to possess antidepressant-like properties in some animal models of MDD. Extant evidence indicates that orexin-modulating treatments exert pleiotropic effects on multiple neural systems implicated in the phenomenology of mood disorders and suggests orexins as a promising target for investigation and intervention in mood disorders. To date, no human clinical trials evaluating the antidepressant effects of orexin antagonists in MDD have been completed. Given the promising results from preclinical studies, clinical trials are merited to evaluate the antidepressant effects of orexin antagonists in MDD.

Are neuropeptides relevant for the mechanism of action of SSRIs?

Selective serotonin reuptake inhibitors (SSRIs) are drugs of first choice in the therapy of moderate to severe depression and anxiety disorders. Their primary mechanism of action is via influence of the serotonergic (5-HT) system, but a growing amount of data provides evidence for other non-monoaminergic players in SSRI effects. It is assumed that neuropeptides, which play a role as neuromodulators in the CNS, are involved in their mechanism of action. In this review we focus on six neuropeptides: corticotropin-releasing factor - CRF, galanin - GAL, oxytocin - OT, vasopressin - AVP, neuropeptide Y - NPY, and orexins - OXs. First, information about their roles in depression and anxiety disorders are presented. Then, findings describing their interactions with the 5-HT system are summarized. These data provide background for analysis of the results of published preclinical and clinical studies related to SSRI effects on the neuropeptide systems. We also report findings showing how modulation of neuropeptide transmission influences behavioral and neurochemical effects of SSRIs. Finally, future research necessary for enriching our knowledge of SSRI mechanisms of action is proposed. Recognition of new molecular targets for antidepressants will have a significant effect on the development of novel therapeutic strategies for mood-related disorders.

The intersection of stress and reward: BNST modulation of aversive and appetitive states

The bed nucleus of the stria terminalis (BNST) is widely acknowledged as a brain structure that regulates stress and anxiety states, as well as aversive and appetitive behaviours. The diverse roles of the BNST are afforded by its highly modular organisation, neurochemical heterogeneity, and complex intrinsic and extrinsic circuitry. There has been growing interest in the BNST in relation to psychopathologies such as anxiety and addiction. Although research on the human BNST is still in its infancy, there have been extensive preclinical studies examining the molecular signature and hodology of the BNST and their involvement in stress and reward seeking behaviour. This review examines the neurochemical phenotype and connectivity of the BNST, as well as electrophysiological correlates of plasticity in the BNST mediated by stress and/or drugs of abuse.

Orexins and stress

The neuropeptides orexins are important in regulating the neurobiological systems that respond to stressful stimuli. Furthermore, orexins are known to play a role many of the phenotypes associated with stress-related mental illness such as changes in cognition, sleep-wake states, and appetite. Interestingly, orexins are altered in stress-related psychiatric disorders such as Major Depressive Disorder and Anxiety Disorders. Thus, orexins may be a potential target for treatment of these disorders. In this review, we will focus on what is known about the role of orexins in acute and repeated stress, in stress-induced phenotypes relevant to psychiatric illness in preclinical models, and in stress-related psychiatric illness in humans. We will also briefly discuss how orexins may contribute to sex differences in the stress response and subsequent phenotypes relevant to mental health, as many stress-related psychiatric disorders are twice as prevalent in women.

The role of the orexin system in stress response

Orexins are neuropeptides that are exclusively produced by hypothalamic neurons, which project throughout the entire brain. Orexin, also known as hypocretins, were initially identified to play a fundamental role in food intake, arousal and the regulation of sleep and wakefulness. Recent studies identified orexins to be critical for diverse physiological processes including motivation, reward, attention, emotional regulation, stress and anxiety. Here, I review recent findings that indicate orexin has an important role in acute and chronic stress. I also summarize the recent optogenetic and chemogenetic studies that have advanced our understanding of the orexin system. I will conclude by discussing clinical studies that implicate orexins in mental health disorders. This article is part of the Special Issue entitled 'Hypothalamic Control of Homeostasis'.

The hypocretin/orexin system as a target for excessive motivation in alcohol use disorders

The hypocretin/orexin (ORX) system has been repeatedly demonstrated to regulate motivation for drugs of abuse, including alcohol. In particular, ORX seems to be critically involved in highly motivated behaviors, as is observed in high-seeking individuals in a population, in the seeking of highly palatable substances, and in models of dependence. It seems logical that this system could be considered as a potential target for treatment for addiction, particularly alcohol addiction, as ORX pharmacological manipulations significantly reduce drinking. However, the ORX system also plays a role in a wide range of other behaviors, emotions, and physiological functions and is disrupted in a number of non-dependence-associated disorders. It is therefore important to consider how the ORX system might be optimally targeted for potential treatment for alcohol use disorders either in combination with or separate from its role in other functions or diseases. This review will focus on the role of ORX in alcohol-associated behaviors and whether and how this system could be targeted to treat alcohol use disorders while avoiding impacts on other ORX-relevant functions. A brief overview of the ORX system will be followed by a discussion of some of the factors that makes it particularly intriguing as a target for alcohol addiction treatment, a consideration of some potential challenges associated with targeting this system and, finally, some future directions to optimize new treatments.

Chronic light exposure alters serotonergic and orexinergic systems in the rat brain and reverses maternal separation-induced increase in orexin receptors in the prefrontal cortex

Maternal separation (MS) is a well-established rodent model of depression. Chronic constant light (CCL) treatment during adolescence has been shown to reverse the depression-like behaviour induced by MS. We aimed to further delineate the antidepressant effect of light by investigating the involvement of the dopaminergic, serotonergic and orexinergic systems. MS was used to induce changes in adult male Sprague-Dawley rats, some of whom were also treated with CCL for 3 weeks during adolescence. At P80, rats were decapitated and brain tissue collected for analysis of glutamate- and potassium-stimulated dopamine release in the nucleus accumbens (NAc) using an in vitro superfusion technique. Enzyme-linked immunosorbent assays were employed to measure 5-hydroxytryptamine (5-HT) levels in the hypothalamus and prefrontal cortex (PFC). Western blotting was used to measure orexin receptor 1 (OXR-1) and 2 (OXR-2) in the PFC. MS did not affect 5-HT levels in these rats. However, CCL increased hypothalamic 5-HT and reduced 5-HT levels in the PFC. CCL had opposite effects on OXR levels in the PFC of maternally separated and non-separated rats. MS increased OXR-1 and OXR-2 levels in the PFC, an effect that was normalized by CCL treatment. MS reduced glutamate-stimulated dopamine release in the NAc, an effect that was not reversed by CCL. The present results suggest that CCL treatment affects 5-HT and orexinergic systems in the MS model while not affecting the MS-induced decrease in dopamine release in the NAc. The reversal of changes in the orexinergic system may be of particular relevance to the antidepressant effect of CCL in depression.

Role of the Orexin/Hypocretin System in Stress-Related Psychiatric Disorders

Orexins (hypocretins) are critically involved in coordinating appropriate physiological and behavioral responses to aversive and threatening stimuli. Acute stressors engage orexin neurons via direct projections from stress-sensitive brain regions. Orexin neurons, in turn, facilitate adaptive behavior via reciprocal connections as well as via direct projections to the hypophysiotropic neurons that coordinate the hypothalamic-pituitary-adrenal (HPA) axis response to stress. Consequently, hyperactivity of the orexin system is associated with increased motivated arousal and anxiety, and is emerging as a key feature of panic disorder. Accordingly, there has been significant interest in the therapeutic potential of pharmacological agents that antagonize orexin signaling at their receptors for the treatment of anxiety disorders. In contrast, disorders characterized by inappropriately low levels of motivated arousal, such as depression, generally appear to be associated with hypoactivity of the orexin system. This includes narcolepsy with cataplexy, a disorder characterized by the progressive loss of orexin neurons and increased rates of moderate/severe depression symptomology. Here, we provide a comprehensive overview of both clinical and preclinical evidence highlighting the role of orexin signaling in stress reactivity, as well as how perturbations to this system can result in dysregulated behavioral phenotypes.