Reduced orexin levels in the cerebrospinal fluid of suicidal patients with major depressive disorder

The hypocretin/orexin system: an increasingly important role in neuropsychiatry

Hypocretins, also named as orexins, are excitatory neuropeptides secreted by neurons specifically located in lateral hypothalamus and perifornical areas. Orexinergic fibers are extensively distributed in various brain regions and involved in a number of physiological functions, such as arousal, cognition, stress, appetite, and metabolism. Arousal is the most important function of orexin system as dysfunction of orexin signaling leads to narcolepsy. In addition to narcolepsy, orexin dysfunction is associated with serious neural disorders, including addiction, depression, and anxiety. However, some results linking orexin with these disorders are still contradictory, which may result from differences of detection methods or the precision of tools used in measurements; strategies targeted to orexin system (e.g., antagonists to orexin receptors, gene delivery, and cell transplantation) are promising new tools for treatment of neuropsychiatric disorders, though studies are still in a stage of preclinical or clinical research.

Orexin regulates bone remodeling via a dominant positive central action and a subordinate negative peripheral action

Orexin neuropeptides promote arousal, appetite, reward, and energy expenditure. However, whether orexin affects bone mass accrual is unknown. Here, we show that orexin functions centrally through orexin receptor 2 (OX2R) in the brain to enhance bone formation. OX2R null mice exhibit low bone mass owing to elevated circulating leptin, whereas central administration of an OX2R-selective agonist augments bone mass. Conversely, orexin also functions peripherally through orexin receptor 1 (OX1R) in the bone to suppress bone formation. OX1R null mice exhibit high bone mass owing to a differentiation shift from marrow adipocyte to osteoblast that results from higher osseous ghrelin expression. The central action is dominant because bone mass is reduced in orexin null and OX1R2R double null mice but enhanced in orexin-overexpressing transgenic mice. These findings reveal orexin as a critical rheostat of skeletal homeostasis that exerts a yin-yang dual regulation and highlight orexin as a therapeutic target for osteoporosis.

Attenuated orexinergic signaling underlies depression-like responses induced by daytime light deficiency

Light has profound effects on mood, as exemplified by seasonal affective disorder (SAD) and the beneficial effects of bright light therapy. However, the underlying neural pathways through which light regulates mood are not well understood. Our previous work has developed the diurnal grass rat, Arvicanthis niloticus, as an animal model of SAD (Leach et al., 2013a,b). By utilizing a 12:12-h dim light:dark (DLD) paradigm that simulates the lower light intensity of winter, we showed that the animals housed in DLD exhibited increased depression-like behaviors in the forced swim test (FST) and sweet solution preference (SSP) compared to animals housed in bright light during the day (BLD). The objective of the present study was to test the hypothesis that light affects mood by acting on the brain orexinergic system in the diurnal grass rat model of SAD. First, orexin A immunoreactivity (OXA-ir) was examined in DLD and BLD grass rats. Results revealed a reduction in the number of OXA-ir neurons in the hypothalamus and attenuated OXA-ir fiber density in the dorsal raphe nucleus of animals in the DLD compared to those in the BLD group. Then, the animals in BLD were treated systemically with SB-334867, a selective orexin 1 receptor (OX1R) antagonist, which led to a depressive phenotype characterized by increased immobility in the FST and a decrease in SSP compared to vehicle-treated controls. Results suggest that attenuated orexinergic signaling is associated with increased depression-like behaviors in grass rats, and support the hypothesis that the orexinergic system mediates the effects of light on mood.

OX1 and OX2 orexin/hypocretin receptor pharmacogenetics

Orexin/hypocretin peptide mutations are rare in humans. Even though human narcolepsy is associated with orexin deficiency, this is only extremely rarely due to mutations in the gene coding prepro-orexin, the precursor for both orexin peptides. In contrast, coding and non-coding variants of the OX₁ and OX₂ orexin receptors have been identified in many human populations; sometimes, these have been associated with disease phenotype, although most confer a relatively low risk. In most cases, these studies have been based on a candidate gene hypothesis that predicts the involvement of orexins in the relevant pathophysiological processes. In the current review, the known human OX₁/HCRTR1 and OX₂/HCRTR2 genetic variants/polymorphisms as well as studies concerning their involvement in disorders such as narcolepsy, excessive daytime sleepiness, cluster headache, polydipsia-hyponatremia in schizophrenia, and affective disorders are discussed. In most cases, the functional cellular or pharmacological correlates of orexin variants have not been investigated—with the exception of the possible impact of an amino acid 10 Pro/Ser variant of OX₂ on orexin potency—leaving conclusions on the nature of the receptor variant effects speculative. Nevertheless, we present perspectives that could shape the basis for further studies. The pharmacology and other properties of the orexin receptor variants are discussed in the context of GPCR signaling. Since orexinergic therapeutics are emerging, the impact of receptor variants on the affinity or potency of ligands deserves consideration. This perspective (pharmacogenetics) is also discussed in the review.

Relaxin-3/RXFP3 networks: an emerging target for the treatment of depression and other neuropsychiatric diseases?

Animal and clinical studies of gene-environment interactions have helped elucidate the mechanisms involved in the pathophysiology of several mental illnesses including anxiety, depression, and schizophrenia; and have led to the discovery of improved treatments. The study of neuropeptides and their receptors is a parallel frontier of neuropsychopharmacology research and has revealed the involvement of several peptide systems in mental illnesses and identified novel targets for their treatment. Relaxin-3 is a newly discovered neuropeptide that binds, and activates the G-protein coupled receptor, RXFP3. Existing anatomical and functional evidence suggests relaxin-3 is an arousal transmitter which is highly responsive to environmental stimuli, particularly neurogenic stressors, and in turn modulates behavioral responses to these stressors and alters key neural processes, including hippocampal theta rhythm and associated learning and memory. Here, we review published experimental data on relaxin-3/RXFP3 systems in rodents, and attempt to highlight aspects that are relevant and/or potentially translatable to the etiology and treatment of major depression and anxiety. Evidence pertinent to autism spectrum and metabolism/eating disorders, or related psychiatric conditions, is also discussed. We also nominate some key experimental studies required to better establish the therapeutic potential of this intriguing neuromodulatory signaling system, including an examination of the impact of RXFP3 agonists and antagonists on the overall activity of distinct or common neural substrates and circuitry that are identified as dysfunctional in these debilitating brain diseases.

Orexin antagonists for neuropsychiatric disease: progress and potential pitfalls

The tight regulation of sleep/wake states is critical for mental and physiological wellbeing. For example, dysregulation of sleep/wake systems predisposes individuals to metabolic disorders such as obesity and psychiatric problems, including depression. Contributing to this understanding, the last decade has seen significant advances in our appreciation of the complex interactions between brain systems that control the transition between sleep and wake states. Pivotal to our increased understanding of this pathway was the description of a group of neurons in the lateral hypothalamus (LH) that express the neuropeptides orexin A and B (hypocretin, Hcrt-1 and Hcrt-2). Orexin neurons were quickly placed at center stage with the demonstration that loss of normal orexin function is associated with the development of narcolepsy—a condition in which sufferers fail to maintain normal levels of daytime wakefulness. Since these initial seminal findings, much progress has been made in our understanding of the physiology and function of the orexin system. For example, the orexin system has been identified as a key modulator of autonomic and neuroendocrine function, arousal, reward and attention. Notably, studies in animals suggest that dysregulation of orexin function is associated with neuropsychiatric states such as addiction and mood disorders including depression and anxiety. This review discusses the progress associated with therapeutic attempts to restore orexin system function and treat neuropsychiatric conditions such as addiction, depression and anxiety. We also highlight potential pitfalls and challenges associated with targeting this system to treat these neuropsychiatric states.

Neural circuit interactions between the dorsal raphe nucleus and the lateral hypothalamus: an experimental and computational study

Orexinergic/hypocretinergic (Ox) neurotransmission plays an important role in regulating sleep, as well as in anxiety and depression, for which the serotonergic (5-HT) system is also involved in. However, little is known regarding the direct and indirect interactions between 5-HT in the dorsal raphe nucleus (DRN) and Ox neurons in the lateral hypothalamus (LHA). In this study, we report the additional presence of 5-HT1BR, 5-HT2AR, 5-HT2CR and fast ligand-gated 5-HT3AR subtypes on the Ox neurons of transgenic Ox-enhanced green fluorescent protein (Ox-EGFP) and wild type C57Bl/6 mice using single and double immunofluorescence (IF) staining, respectively, and quantify the colocalization for each 5-HT receptor subtype. We further reveal the presence of 5-HT3AR and 5-HT1AR on GABAergic neurons in LHA. We also identify NMDAR1, OX1R and OX2R on Ox neurons, but none on adjacent GABAergic neurons. This suggests a one-way relationship between LHA's GABAergic and Ox neurons, wherein GABAergic neurons exerts an inhibitory effect on Ox neurons under partial DRN's 5-HT control. We also show that Ox axonal projections receive glutamatergic (PSD-95 immunopositive) and GABAergic (Gephyrin immunopositive) inputs in the DRN. We consider these and other available findings into our computational model to explore possible effects of neural circuit connection types and timescales on the DRN-LHA system's dynamics. We find that if the connections from 5-HT to LHA's GABAergic neurons are weakly excitatory or inhibitory, the network exhibits slow oscillations; not observed when the connection is strongly excitatory. Furthermore, if Ox directly excites 5-HT neurons at a fast timescale, phasic Ox activation can lead to an increase in 5-HT activity; no significant effect with slower timescale. Overall, our experimental and computational approaches provide insights towards a more complete understanding of the complex relationship between 5-HT in the DRN and Ox in the LHA.

The hypocretin system and psychiatric disorders

The hypocretin system is constituted by a small group of hypothalamic neurons with widespread connections within the entire central nervous system producing two neuropeptides involved in several key physiological functions such as the regulation of sleep and wakefulness, motor control, autonomic functions, metabolism, feeding behavior, and reward. Narcolepsy with cataplexy is a neurological disorder regarded as a disease model for the selective hypocretin system damage, and also shares several psychopatological traits and comorbidities with psychiatric disorders. We reviewed the available literature on the involvement of the hypocretin system in psychiatric nosography. Different evidences such as cerebrospinal hypocretin-1 levels, genetic polymorphisms of the neuropeptides or their receptors, response to treatments, clinical, experimental and functional data directly or indirectly linked the hypocretin system to schizophrenia, mood, anxiety and eating disorders, as well as to addiction. Future genetic and pharmacological studies will disentangle the hypocretin system role in the field of psychiatry.

LSN2424100: a novel, potent orexin-2 receptor antagonist with selectivity over orexin-1 receptors and activity in an animal model predictive of antidepressant-like efficacy

We describe a novel, potent and selective orexin-2 (OX2)/hypocretin-2 receptor antagonist with in vivo activity in an animal model predictive of antidepressant-like efficacy. N-biphenyl-2-yl-4-fluoro-N-(1H-imidazol-2-ylmethyl) benzenesulfonamide HCl (LSN2424100) binds with high affinity to recombinant human OX2 receptors (Ki = 4.5 nM), and selectivity over OX1 receptors (Ki = 393 nM). LSN2424100 inhibited OXA-stimulated intracellular calcium release in HEK293 cells expressing human and rat OX2 receptors (Kb = 0.44 and 0.83 nM, respectively) preferentially over cells expressing human and rat OX1 (Kb = 90 and 175 nM, respectively). LSN2424100 exhibits good exposure in Sprague–Dawley rats after IP, but not PO, administration of a 30 mg/kg dose (AUC_{0–6 h} = 1300 and 269 ng^*h/mL, respectively). After IP administration in rats and mice, LSN2424100 produces dose-dependent antidepressant-like activity in the delayed-reinforcement of low-rate (DRL) assay, a model predictive of antidepressant-like efficacy. Efficacy in the DRL model was lost in mice lacking OX2, but not OX1 receptors, confirming OX2-specific activity. Importantly, antidepressant-like efficacy of the tricyclic antidepressant, imipramine, was maintained in both OX1 and OX2 receptor knock-out mice. In conclusion, the novel OX2 receptor antagonist, LSN2424100, is a valuable tool compound that can be used to explore the role of OX2 receptor-mediated signaling in mood disorders.

Nitric oxide mediates selective degeneration of hypothalamic orexin neurons through dysfunction of protein disulfide isomerase

We addressed the role of nitric oxide (NO) in orexin neuron degeneration that has been observed under various pathological conditions. Administration of an NO donor NOC18 (50 nmol) into the third ventricle of mice resulted in a significant decrease of orexin-immunoreactive (-IR) neurons, in contrast to a modest change in melanin-concentrating hormone-IR neurons. In addition, NOC18 promoted formation of orexin-A-IR aggregates within orexin neurons. An endoplasmic reticulum stress inducer tunicamycin replicated the effect of NOC18 with regard to decrease of orexin-IR neurons and formation of aggregates. We also found that NOC18 caused an increase in S-nitrosation of protein disulfide isomerase (PDI) and a decrease in PDI activity in hypothalamic tissues. Moreover, PDI inhibitors, such as cystamine and securinine, caused a selective decrease of orexin neurons and promoted formation of orexin-A-IR aggregates. Aggregate formation in orexin-IR neurons was also induced by local injection of small interfering RNA targeting PDI. Interestingly, sleep deprivation for 7 consecutive days induced a selective decrease of orexin-IR neurons, which was preceded by aggregate formation in orexin-IR neurons and an increase in S-nitrosated PDI in the hypothalamus. Activity of neuronal NO synthase (nNOS)-positive neurons in the lateral hypothalamus as assessed by c-Fos expression was elevated in response to sleep deprivation. Finally, sleep deprivation-induced decrease of orexin-IR neurons, formation of aggregates, and S-nitrosation of PDI were not observed in nNOS knock-out mice. These results indicate that nNOS-derived NO may mediate specific pathological events in orexin neurons, including neuropeptide misfolding via S-nitrosation and inactivation of PDI.

The role of neuropeptides in suicidal behavior: a systematic review

There is a growing evidence that neuropeptides may be involved in the pathophysiology of suicidal behavior. A critical review of the literature was conducted to investigate the association between neuropeptides and suicidal behavior. Only articles from peer-reviewed journals were selected for the inclusion in the present review. Twenty-six articles were assessed for eligibility but only 22 studies were included. Most studies have documented an association between suicidality and some neuropeptides such as corticotropin-releasing factor (CRF), VGF, cholecystokinin, substance P, and neuropeptide Y (NPY), which have been demonstrated to act as key neuromodulators of emotional processing. Significant differences in neuropeptides levels have been found in those who have attempted or completed suicide compared with healthy controls or those dying from other causes. Despite cross-sectional associations between neuropeptides levels and suicidal behavior, causality may not be inferred. The implications of the mentioned studies were discussed in this review paper.

Selective orexin receptor antagonists

The orexin, or hypocretin, neuropeptides (orexin-A and orexin-B) are produced on neurons in the hypothalamus which project to key areas of the brain that control sleep-wake states, modulation of food intake, panic, anxiety, emotion, reward and addictive behaviors. These neuropeptides exert their effects on a pair of G-protein coupled receptors termed the orexin-1 (OX1) and orexin-2 (OX2) receptors. Emerging biology suggests the involvement of these receptors in psychiatric disorders as they are thought to play a key role in the regulation of multiple systems. This review is intended to highlight key selective OX1 or OX2 small-molecule antagonists.

Hypothalamic orexin prevents hepatic insulin resistance induced by social defeat stress in mice

Depression is associated with insulin resistance and type 2 diabetes, although the molecular mechanism behind the pathological link remains unclear. Orexin, a hypothalamic neuropeptide regulating energy and glucose homeostasis, has been implicated in the endogenous antidepressant mechanism. To clarify whether orexin is involved in the coordination between mental and metabolic functions, we investigated the influence of orexin deficiency on social interaction behavior and glucose metabolism in mice subjected to chronic social defeat stress. Chronic stress-induced glucose intolerance and systemic insulin resistance as well as social avoidance were ameliorated by calorie restriction in an orexin-dependent manner. Moreover, orexin-deficient mice maintained under ad libitum-fed conditions after defeat stress exhibited hyperinsulinemia and elevated HOMA-IR (homeostasis model assessment for insulin resistance), despite normal fasting blood glucose levels. In a pyruvate tolerance test to evaluate hepatic insulin sensitivity, chronic stress-induced abnormal glucose elevation was observed in orexin-deficient but not wild-type mice, although both types of mice were susceptible to chronic stress. In addition, insulin-induced phosphorylation of Akt in the liver was impaired in orexin-deficient but not wild-type mice after chronic stress. These results demonstrate that the central physiological actions of orexin under ad libitum-fed conditions are required for the adaptive response to chronic defeat stress, which can prevent the development of hepatic insulin resistance but not social avoidance behavior. Moreover, calorie restriction, a paradigm to strongly activate orexin neurons, appears to prevent the persistence of depression-like behavior per se, leading to the amelioration of impaired glucose metabolism after chronic stress; therefore, we suggest that hypothalamic orexin system is the key for inhibiting the exacerbating link between depression and type 2 diabetes.

Role of orexin in the pathophysiology of depression: potential for pharmacological intervention

Depression is a devastating mental disorder with an increasing impact throughout the world, whereas the efficacy of currently available pharmacological treatment is still limited. Growing evidence from preclinical and clinical studies suggests that orexins (neuropeptides that are also known as hypocretins) and their receptors are involved in the physiopathology of depression. Indeed, the orexinergic system regulates functions that are disturbed in depressive states such as sleep, reward system, feeding behavior, the stress response and monoaminergic neurotransmission. Nevertheless, the precise role of orexins in behavioral and neurophysiological impairments observed in depression is still unclear. Both hypoactivity and hyperactivity of orexin signaling pathways have been found to be associated with depression. These discrepancies in the literature prompted the necessity for additional investigations, as the orexinergic system appears to be a promising target to treat the symptoms of depression. This assumption is underlined by recent data suggesting that pharmacological blockade of orexin receptors induces a robust antidepressant-like effect in an animal model of depression. Further preclinical and clinical studies are needed to progress the overall understanding of the orexinergic alterations in depression, which will eventually translate preliminary observations into real therapeutic potential. The aim of this paper is to provide an overview of human and animal research dedicated to the study of the specific involvement of orexins in depression, and to propose a framework in which disturbances of the orexinergic system are regarded as an integral component of the etiology of depression.

Hypersomnia and depressive symptoms: methodological and clinical aspects

The associations between depressive symptoms and hypersomnia are complex and often bidirectional. Of the many disorders associated with excessive sleepiness in the general population, the most frequent are mental health disorders, particularly depression. However, most mood disorder studies addressing hypersomnia have assessed daytime sleepiness using a single response, neglecting critical and clinically relevant information about symptom severity, duration and nighttime sleep quality. Only a few studies have used objective tools such as polysomnography to directly measure both daytime and nighttime sleep propensity in depression with normal mean sleep latency and sleep duration. Hypersomnia in mood disorders, rather than a medical condition per se, is more a subjective sleep complaint than an objective finding. Mood symptoms have also been frequently reported in hypersomnia disorders of central origin, especially in narcolepsy. Hypocretin deficiency could be a contributing factor in this condition. Further interventional studies are needed to explore whether management of sleep complaints improves mood symptoms in hypersomnia disorders and, conversely, whether management of mood complaints improves sleep symptoms in mood disorders.

Hyperstable regulation of vigilance in patients with major depressive disorder

OBJECTIVES

This study tested the hypothesis that patients with depression show less and later declines into lower EEG vigilance stages (different global functional brain states) under resting conditions than healthy controls, as proposed by the vigilance theory of affective disorders.

METHODS

Thirty patients with Major Depressive Disorder (19 female; mean age: 37.2 years, SD: 12.6) without psychotropic medication and 30 carefully age- and sex-matched controls (19 female; mean age: 37.3 years, SD: 12.8) without past or present mental disorders underwent a 15-min resting EEG. EEG-vigilance regulation was determined with a computer-based vigilance classification algorithm (VIGALL, Vigilance Algorithm Leipzig), allowing a classification of vigilance stages A (with substages A1, A2 and A3), B (with substages B1 and B2/3) and C.

RESULTS

Depressive patients spent significantly more time in the highest EEG vigilance substage A1, and less time in substages A2, A3 and B2/3 than controls. In depressive patients, a significantly longer latency until the occurrence of substages A2, A3 and B2/3 was observed. No significant group differences in the percentage of B1 segments or the latency until occurrence of B1 were found.

CONCLUSIONS

The results confirm the hypothesis that patients with depression show less (and later) declines into lower EEG vigilance stages under resting conditions than healthy controls, and support the vigilance theory of affective disorders linking a hyperstable vigilance regulation to depression.

Neurogenesis-independent antidepressant-like effects on behavior and stress axis response of a dual orexin receptor antagonist in a rodent model of depression

Growing evidence indicates that an increase of orexin (or hypocretin) signaling is involved in the pathophysiology of major depression, but little is known regarding the causal link between the orexinergic system and depressive-like states. Here we blocked orexin receptors in mice subjected to unpredictable chronic mild stress (UCMS) to investigate putative antidepressant-like effects of this treatment, as well as the underlying mechanisms. BALB/c mice were exposed to 9 weeks of UCMS and from the third week onward treated daily with fluoxetine (20 mg/kg per day, per os) or with the dual orexin receptor antagonist almorexant (100 mg/kg per day, per os). The effects of UCMS regimen and pharmacological treatments were assessed by physical measures and behavioral testing. The dexamethasone suppression test was performed to examine the integrity of the negative feedback of the hypothalamic-pituitary-adrenal (HPA) axis, and immunohistochemical markers were used to assess cell proliferation (Ki-67), immature newborn neurons (doublecortin), and mature newborn neurons (5-bromo-2'-deoxyuridine/NeuN) in the dorsal and ventral parts of the hippocampus. Our results show that 7 weeks of fluoxetine or almorexant treatments counteract the UCMS-induced physical and behavioral alterations. Both treatments prevented the HPA axis dysregulation caused by UCMS, but only fluoxetine reversed the UCMS-induced decrease of hippocampal cell proliferation and neurogenesis, while chronic almorexant treatment decreased cell proliferation and neurogenesis specifically in the ventral hippocampus. Taken together, this is the first evidence that pharmacological blockade of the orexinergic system induces a robust antidepressant-like effect and the restoration of stress-related HPA axis defect independently from a neurogenic action.

OX2R activation induces PKC-mediated ERK and CREB phosphorylation

Deficiencies in brain orexins and components of mitogen activated protein kinase (MAPK) signaling pathway have been reported in either human depression or animal model of depression. Brain administration of orexins affects behaviors toward improvement of depressive symptoms. However, the documentation of endogenous linkage between orexin receptor activation and MAPK signaling pathway remains to be insufficient. In this study, we report the effects of orexin 2 receptor (OX2R) activation on cell signaling in CHO cells over-expressing OX2R and in mouse hypothalamus cell line CLU172. Short-term extracellular signal-regulated kinase (ERK) phosphorylation and long-term cyclic adenosine monophosphate (cAMP) response element binding protein (CREB) phosphorylation were subsequently observed in CHO cells that over-express OX2R while 20 min of ERK phosphorylation was significantly detected in mouse adult hypothalamus neuron cell line CLU172. Orexin A, which can also activate OX2R, mediated ERK phosphorylation was as the same as orexin B in CHO cells. A MAPK inhibitor eliminated ERK phosphorylation but not CREB phosphorylation in CHO cells. Also, ERK and CREB phosphorylation was not mediated by protein kinase A (PKA) or calmodulin kinase (CaMK). However, inhibition of protein kinase C (PKC) by GF 109203X eliminated the phosphorylation of ERK and CREB in CHO cells. A significant decrease in ERK and CREB phosphorylation was observed with 1 μM GF 109203X pre-treatment indicating that the conventional and novel isoforms of PKC are responsible for CREB phosphorylation after OX2R activation. In contrast, ERK phosphorylation induced by orexin B in CLU172 cells cannot be inhibited by 1 μM of protein kinase C inhibitor. From above observation we conclude that OX2R activation by orexin B induces ERK and CREB phosphorylation and orexin A played the same role as orexin B. Several isoforms of PKC may be involved in prolonged CREB phosphorylation. Orexin B induced ERK phosphorylation in mouse hypothalamus neuron cells differs from CHO cell line and cannot be inhibited by PKC inhibitor GF 109203X. And hypothalamus neuron cells may use different downsteam pathway for orexin B induced ERK phosphorylation. This result supports findings that orexins might have anti-depressive roles.

Orexin-A induces anxiety-like behavior through interactions with glutamatergic receptors in the bed nucleus of the stria terminalis of rats

The hypothalamic neuropeptide orexin (ORX) has been implicated in anxiety, and anxiety-like behaviors. The purpose of these studies was to determine the role of ORX, specifically orexin-A (ORX-A) in the bed nucleus of the stria terminalis (BNST) on anxiety-like behaviors in rats. Rats injected with ORX-A into the BNST displayed greater anxiety-like measures in the social interaction and elevated plus maze tests compared to vehicle treated controls. Such anxiety-like behaviors were not observed when the ORX-A injections were adjacent to the BNST, in the medial septum. The anxiety-inducing effects of direct infusions of ORX-A into the BNST may be a consequence of increased activation of BNST neurons. In BNST slice preparations using patch-clamp techniques, ORX-A induced membrane depolarization and generation of action potentials in a subset of BNST neurons. The anxiety-inducing effects of ORX-A in the BNST also appear to be dependent on NMDA-type glutamate receptor activity, as pre-injecting the NMDA antagonist AP5 into the BNST blocked anxiogenic effects of local ORX-A injections. Injections of AMPA-type receptor antagonists into the BNST prior to ORX-A resulted in only a partial attenuation of anxiety-like behaviors.

Activation of central orexin/hypocretin neurons by dietary amino acids

Hypothalamic orexin/hypocretin (orx/hcrt) neurons regulate energy balance, wakefulness, and reward; their loss produces narcolepsy and weight gain. Glucose can lower the activity of orx/hcrt cells, but whether other dietary macronutrients have similar effects is unclear. We show that orx/hcrt cells are stimulated by nutritionally relevant mixtures of amino acids (AAs), both in brain slice patch-clamp experiments, and in c-Fos expression assays following central or peripheral administration of AAs to mice in vivo. Physiological mixtures of AAs electrically excited orx/hcrt cells through a dual mechanism involving inhibition of K(ATP) channels and activation of system-A amino acid transporters. Nonessential AAs were more potent in activating orx/hcrt cells than essential AAs. Moreover, the presence of physiological concentrations of AAs suppressed the glucose responses of orx/hcrt cells. These results suggest a new mechanism of hypothalamic integration of macronutrient signals and imply that orx/hcrt cells sense macronutrient balance, rather than net energy value, in extracellular fluid.

[Suicide and nutrition: a bio-psychosocial approach]

This article proposes a new bio-psychosocial perspective on the links between mental health, more specifically suicide, and nutrition. We first discuss the links between nutrition, its social role and suicide. The act of eating is studied as a social integrator and regulator, in the light of Durkheim's theorization. Nutrition is also examined as self-destruction, with particular cases of "diet-related suicide." De-structuring of meals and alienating foods are identified as contributing factors to the de-structuring of "nutrition models." We then discuss the place of food within the psychopathology, and finally, the links between biological parameters reflected in food and suicide risk. Avenues of research and intervention along this bio-psychosocial approach are also proposed.

Orexins: tissue localization, functions, and its relation to insulin secretion and diabetes mellitus

Orexins play a role in many biological functions include sleep, feeding, and energy balance. They also regulate circadian rhythms and the way that we feel pain. Orexins have been identified in a variety of tissues including the cerebrospinal fluid, blood, hypothalamus, spinal cord, sensory ganglion, enteric nervous system, pituitary, adrenal, salivary and lacrimal glands, testis, vestibular gland, and skin. Orexins play a role in a variety of biological functions including arousal, sleeping, food and fluid intake, pain, memory, perception of odor, and sexual activity. Orexins have also been implicated in the regulation of glucose metabolism. The expression of orexin is induced by hypoglycemia, low food, pregnancy, and hemodialysis. In contrast, factors that inhibit the expression of orexins include obstructive sleep apnea, aging, depression, obesity, traumatic brain injury, and inflammatory molecules such as liposaccharide. In conclusion, orexins are widely distributed and involved in a large variety of biological activities.

Orexin, stress, and anxiety/panic states

A panic response is an adaptive response to deal with an imminent threat and consists of an integrated pattern of behavioral (aggression, fleeing, or freezing) and increased cardiorespiratory and endocrine responses that are highly conserved across vertebrate species. In the 1920s and 1940s, Philip Bard and Walter Hess, respectively, determined that the posterior regions of the hypothalamus are critical for a "fight-or-flight" reaction to deal with an imminent threat. Since the 1940s it was determined that the posterior hypothalamic panic area was located dorsal (perifornical hypothalamus: PeF) and dorsomedial (dorsomedial hypothalamus: DMH) to the fornix. This area is also critical for regulating circadian rhythms and in 1998, a novel wake-promoting neuropeptide called orexin (ORX)/hypocretin was discovered and determined to be almost exclusively synthesized in the DMH/PeF perifornical hypothalamus and adjacent lateral hypothalamus. The most proximally emergent role of ORX is in regulation of wakefulness through interactions with efferent systems that mediate arousal and energy homeostasis. A hypoactive ORX system is also linked to narcolepsy. However, ORX role in more complex emotional responses is emerging in more recent studies where ORX is linked to depression and anxiety states. Here, we review data that demonstrates ORX ability to mobilize a coordinated adaptive panic/defense response (anxiety, cardiorespiratory, and endocrine components), and summarize the evidence that supports a hyperactive ORX system being linked to pathological panic and anxiety states.

CSF-hypocretin-1 levels in patients with major depressive disorder compared to healthy controls

Depressive patients exhibit symptoms of impaired regulation of wakefulness with hyperarousal and agitation as well as difficulties to falling asleep and preserving sleep continuity. Changes in hypocretin (hcrt) levels as polypeptides with impact on arousal and sleep-wake-regulation have been discussed in affective disorders but have not been investigated in patients with solely unipolar depression in comparison to healthy controls. In the present study, cerebrospinal fluid (CSF) levels of hcrt-1 for the first time were analyzed in patients with major depressive disorder (MDD) without psychiatric comorbidities and compared with levels in healthy controls. In 17 inpatients with MDD (mean Hamilton Depression Rating Scale 13.9 ± 7.4) and 10 healthy controls, CSF-hcrt-1 levels were measured using a fluorescence immunoassay (FIA). The mean hcrt-1 CSF levels in patients with MDD (74.3 ± 17.8pg/ml) did not differ compared to that of healthy controls (82.8 ± 22.1pg/ml). Hcrt-1 levels did not correlate with the severity of depressive episode, the symptoms of depression or the number of episodes. Although autonomic and neurohumoral signs of hyperarousal are common in MDD, hcrt-1 levels in CSF were not found to be altered in MDD compared to healthy controls. Whether hcrt-1 levels are altered in depressive patients exhibiting impaired vigilance regulation has to be investigated in further studies combining measures of CSF-hcrt-1 with electroencephalography.

Increased numbers of orexin/hypocretin neurons in a genetic rat depression model

The Flinders Sensitive Line (FSL) rat is a genetic animal model of depression that displays characteristics similar to those of depressed patients including lower body weight, decreased appetite and reduced REM sleep latency. Hypothalamic neuropeptides such as orexin/hypocretin, melanin-concentrating hormone (MCH) and cocaine and amphetamine regulated transcript (CART), that are involved in the regulation of both energy metabolism and sleep, have recently been implicated also in depression. We therefore hypothesized that alterations in these neuropeptide systems may play a role in the development of the FSL phenotype with both depressive like behavior, metabolic abnormalities and sleep disturbances. In this study, we first confirmed that the FSL rats displayed increased immobility in the Porsolt forced swim test compared to their control strain, the Flinders Resistant Line (FRL), which is indicative of depressive-like behavior. We then examined the number of orexin-, MCH- and CART-immunopositive neurons in the hypothalamus using stereological analyses. We found that the total number of orexin-positive neurons was higher in the hypothalamus of female FSL rats compared to female FRL rats, whereas no changes in the MCH or CART populations could be detected between the strains. Chronic treatment with the selective serotonin reuptake inhibitor (SSRI) escitalopram reduced immobility only in the FRL rats where it also increased the number of MCH positive neurons compared to untreated rats. These findings support the view that orexin may be involved in depression and strengthen the notion that the "depressed" brain responds differently to pharmacological interventions than the normal brain.

CSF biomarkers in suicide attempters--a principal component analysis

OBJECTIVE

The objective of the present study was to identify biological patterns (factors) among 20 cerebrospinal fluid (CSF) biomarkers in suicide attempters and subsequently analyse their association with suicidal behaviour.

METHOD

We measured kynurenic acid, orexin, homovanillic acid (HVA), 5-hydroxyindoleacetic acid (5-HIAA), 3-methoxy-4-hydroxyphenylglycol, chemokines, matrix metalloproteases and cytokines in the CSF of 124 drug-free suicide attempters. Patients were evaluated for suicidality and psychiatric symptoms using well-defined psychiatric rating scales and followed-up regarding future suicide. We used principal component analysis to identify factors among the biological substances.

RESULTS

Four factors were extracted from the 20 biomarkers, explaining 52.4% of the total variance. Factors 1 and 2 were characterized by high loadings of chemokines and cytokines respectively. They were both associated with severe depressive symptoms. Factor 2 was also associated with a high suicidal intent. Factor 4 was characterized by strong loadings of the monoamine metabolites 5-HIAA and HVA, as well as orexin and interleukin-6. High scores on this factor were found in patients who performed a violent suicide attempt and in patients who subsequently completed suicide.

CONCLUSION

Our results suggest that specific combinations of CSF biomarkers may discriminate between types of suicidal behaviour and indicate increased risk for future suicide.

Dichotomous cellular properties of mouse orexin/hypocretin neurons

Hypothalamic hypocretin/orexin (Hcrt/Orx) neurons recently emerged as critical regulators of sleep–wake cycles, reward seeking and body energy balance. However, at the level of cellular and network properties, it remains unclear whether Hcrt/Orx neurons are one homogeneous population, or whether there are several distinct types of Hcrt/Orx cells. Here, we collated diverse structural and functional information about individual Hcrt/Orx neurons in mouse brain slices, by combining patch-clamp analysis of spike firing, membrane currents and synaptic inputs with confocal imaging of cell shape and subsequent 3-dimensional Sholl analysis of dendritic architecture. Statistical cluster analysis of intrinsic firing properties revealed that Hcrt/Orx neurons fall into two distinct types. These two cell types also differ in the complexity of their dendritic arbour, the strength of AMPA and GABAA receptor-mediated synaptic drive that they receive, and the density of low-threshold, 4-aminopyridine-sensitive, transient K+ current. Our results provide quantitative evidence that, at the cellular level, the mouse Hcrt/Orx system is composed of two classes of neurons with different firing properties, morphologies and synaptic input organization.

Decreased aldosterone in the plasma of suicide attempters with major depressive disorder

Hormones and neurobiological factors may be regulated differently in suicidal versus non-suicidal depressive patients. There is currently limited knowledge about the relation of substances in the Renin-Angiotensin-Aldosterone system to depression and suicidality. We therefore investigated whether plasma levels of renin and aldosterone differ between suicide attempters, non-suicidal depressive patients and healthy controls. Furthermore, we analyzed the relation of renin and aldosterone to psychiatric symptoms in the patients. Suicidal patients with MDD, adjustment disorder and dysthymia, as well as two control groups consisting of non-suicidal MDD patients and healthy subjects, were rated using the Comprehensive Psychopathological Rating Scale (CPRS), including the Montgomery-Åsberg Depression Rating Scale (MADRS) and the Brief Scale for Anxiety (BSA). Plasma samples were frozen immediately after collection and stored at -80°C for 5-18years. Aldosterone and renin levels were analyzed using radioactive- and chemiluminescent immunoassays. We found that suicide attempters with MDD had significantly lower plasma levels of aldosterone than the other patient groups, as well as than the healthy controls. Moreover, increasing severity of psychiatric symptoms was associated with lower aldosterone levels in the suicide attempters with MDD. Non-suicidal patients with MDD did not differ significantly compared to healthy controls with respect to aldosterone and renin levels. These findings may indicate that low aldosterone levels could be a marker of suicidality in patients with MDD.

Association between major mood disorders and the hypocretin receptor 1 gene

BACKGROUND

Recent studies suggested a role for hypocretins in the neurobiology of Major Mood Disorders (MMD). The purpose of this study was to investigate hypocretin involvement in MMD evaluating whether particular alleles or genotypes of the hypocretin pathway genes (HCRT, HCRTR1 and HCRTR2) would modify the occurrence and clinical features of the disease.

METHODS

We selected for the study 229 MMD patients and 259 healthy age-, sex- and ethnicity-matched controls. Cases and controls were genotyped for several single-nucleotide polymorphisms (SNPs) of the HCRT, HCRTR1, and HCRTR2 genes.

RESULTS

We found that allelic and genotypic frequencies of the rs2271933 G>A polymorphism (Ile408Val) in the HCRTR1 gene were significantly different between cases and controls (p=0.003 and p=0.0004, respectively). The carriage of the A allele was associated with a significantly increased disease risk (OR:1.60, 95% C.I. 1.22-2.10). In addition, we found a significant association between HCRTR1 haplotypes and the disease (permutation p<0.0001). In the analysis of subgroups we confirmed the association only in patients with unipolar depression.

LIMITATIONS

Our sample was relatively small and included only cases and controls recruited from Northern Italy. Analysis of the disease subgroups warrants reexamination with more subjects. Finally, the effects of the rs2271933 G>A polymorphism on the hypocretin-1 receptor function are unknown.

CONCLUSIONS

Our study suggests that the HCRTR1 gene or a linked locus may modulate the risk for Major Mood Disorders and supports recent studies suggesting an involvement of hypocretin neurotransmitter system in affective disorders.

Activation of orexin neurons in dorsomedial/perifornical hypothalamus and antidepressant reversal in a rodent model of depression

Chronic stressful life events are risk factors for depression often accompanied by homeostatic disturbances. Hypothalamic neuropeptides, such as orexins (OXs) and melanin-concentrating hormone (MCH), are involved in regulation of several autonomic functions that are altered in depression. However, little is known about the link between orexinergic or MCH-ergic systems and depression. Using double immunohistochemical labeling for OX- or MCH-containing neurons and Fos protein, we studied the effects of a chronic selective serotonin reuptake inhibitor antidepressant treatment (fluoxetine) on the OX and MCH neuronal activation in mice exposed to unpredictable chronic mild stress (UCMS), a rodent model of depression. Western blot was also performed to assess OX and MCH receptor expression in various brain areas. Finally, almorexant, a dual OX receptor antagonist, was assessed in the tail suspension test. UCMS induced physical and behavioral disturbances in mice reversed by 6-week fluoxetine treatment. Orexinergic neurons were more activated in the dorsomedial and perifornical hypothalamic area (DMH-PFA) of UCMS-subjected mice compared to the lateral hypothalamus (LH), and this increase was reversed by 6-week fluoxetine treatment. UCMS also reduced expression of OX-receptor 2 in the thalamus and hypothalamus, but not in animals chronically treated with fluoxetine. MCH neurons were neither affected by UCMS nor by antidepressant treatment, while UCMS modulated MCH receptor 1 expression in thalamus and hippocampus. Finally, chronic but not acute administration of almorexant, induced antidepressant-like effect in the tail suspension test. These data suggest that OX neurons in the DMH-PFA and MCH-ergic system may contribute to the pathophysiology of depressive disorders.

Role of brain orexin in the pathophysiology of functional gastrointestinal disorders

BACKGROUND AND AIM

Orexins are neuropeptides that are localized in neurons within the lateral hypothalamic area and regulate feeding behavior. The lateral hypothalamic area plays an important role in not only feeding but the central regulation of other functions including gut physiology. Accumulating evidence have shown that orexins acts in the brain to regulate a wide variety of body functions including gastrointestinal functions.

METHOD

The purpose of this review is to summarize relevant findings on brain orexins and a digestive system, and discuss the pathophysiological roles of the peptides with special reference to functional gastrointestinal disorders.

RESULTS

Exogenously administered orexin or endogenously released orexin in the brain potently stimulates gastric acid secretion in pylorus-ligated conscious rats. The vagal cholinergic pathway is involved in the orexin-induced stimulation of acid secretion, suggesting that orexin-containing neurons in lateral hypothalamic area activates neurons in the dorsal motor nucleus in medulla oblongata, followed by increasing vagal outflow, thereby stimulating gastric acid secretion. In addition, brain orexin stimulates gastric motility, pancreatic secretion and induce gastroprotective action. On the other hand, brain orexin is involved in a number of physiological functions other than gut physiology, such as control of sleep/awake cycle and anti-depressive action in addition to increase in appetite.

CONCLUSIONS

From these evidence, we would like to make a hypothesis that decreased orexin signaling in the brain may play a role in the pathophysiology in a part of patients with functional gastrointestinal disorders who are frequently accompanied with appetite loss, sleep disturbance, depressive state and the inhibition of gut function.

Direct and indirect control of orexin/hypocretin neurons by glycine receptors

Hypothalamic hypocretin/orexin (hcrt/orx) neurons promote arousal and reward seeking, while reduction in their activity has been linked to narcolepsy, obesity and depression. However, the mechanisms influencing the activity of hcrt/orx networks in situ are not fully understood. Here we show that glycine, a neurotransmitter best known for its actions in the brainstem and spinal cord, elicits dose dependent postsynaptic Cl⁻ currents in hcrt/orx cells in acute mouse brain slices. The effect was blocked by the glycine receptor (GLyR) antagonist strychnine and mimicked by the GlyR agonist alanine. Postsynaptic GlyRs on hcrt/orx cells remained functional during both early postnatal and adult periods, and gramicidin-perforated patch-clamp recordings revealed that they progressively switch from excitatory to inhibitory during the first two postnatal weeks. The pharmacological profile of the glycine response suggested that developed hcrt/orx neurons contain α/β-heteromeric GlyRs that lack α2-subunits, whereas α2-subunits, whereas α2-subunits are present in early postnatal hcrt/orx neurons. All postsynaptic currents (PSCs) in developed hcrt/orx cells were blocked by inhibitors of GABA and glutamate receptors, with no evidence of GlyR-mediated PSCs. However, the frequency but not amplitude of miniature PSCs was reduced by strychnine and increased by glycine in ~50% of hcrt/orx neurons. Together, these results provide the first evidence for functional GlyRs in identified hcrt/orx circuits and suggest that the activity of developed hcrt/orx cells is regulated by two GlyR pools: inhibitory extrasynaptic GlyRs located on all hcrt/orx cells and excitatory GlyRs located on presynaptic terminals contacting some hcrt/orx cells.

Tobacco dependence, the insular cortex and the hypocretin connection

Tobacco use is a major cause of disease and premature death in the United States. Nicotine is considered the key component of tobacco responsible for addiction in human smokers. Accumulating evidence supports an important role for the hypocretin (orexin) neuropeptide system in regulating the reinforcing properties of most major drugs of abuse, including nicotine. Here, data showing that nicotine activates hypocretin-producing neurons in the lateral hypothalamus, and that disruption of hypocretin transmission decreases nicotine self-administration behavior in rats will be reviewed. Recent findings suggesting that plasma hypocretin levels may be related to the magnitude of cigarette craving in abstinent smokers will be discussed. Finally, the data suggesting that hypocretin transmission in the insular cortex may play an important role in regulating nicotine self-administration behavior in rats will be reviewed. This latter finding may provide mechanistic insight into the apparent disruption of tobacco addiction reported in human smokers with stroke-associated damage to the insular cortex.

Multiple hypothalamic circuits sense and regulate glucose levels

The hypothalamus monitors body energy status in part through specialized glucose sensing neurons that comprise both glucose-excited and glucose-inhibited cells. Here we discuss recent work on the elucidation of neurochemical identities and physiological significance of these hypothalamic cells, including caveats resulting from the currently imprecise functional and molecular definitions of glucose sensing and differences in glucose-sensing responses obtained with different experimental techniques. We discuss the recently observed adaptive glucose-sensing responses of orexin/hypocretin-containing neurons, which allow these cells to sense changes in glucose levels rather than its absolute concentration, as well as the glucose-sensing abilities of melanin-concentrating hormone, neuropeptide Y, and proopiomelanocortin-containing neurons and the recent data on the role of ventromedial hypothalamic steroidogenic factor-1 (SF-1)/glutamate-containing cells in glucose homeostasis. We propose a model where orexin/hypocretin and SF-1/glutamate neurons cooperate in stimulating the sympathetic outflow to the liver and pancreas to increase blood glucose, which in turn provides negative feedback inhibition to these cells. Orexin/hypocretin neurons also stimulate feeding and reward seeking and are activated by hunger and stress, thereby providing a potential link between glucose sensing and goal-oriented behavior. The cell-type-specific neuromodulatory actions of glucose in several neurochemically distinct hypothalamic circuits are thus likely to be involved in coordinating higher brain function and behavior with autonomic adjustments in blood glucose levels.

Low cerebrospinal fluid and plasma orexin-A (hypocretin-1) concentrations in combat-related posttraumatic stress disorder

The hypothalamic neuropeptide, orexin-A has a number of regulatory effects in humans and pre-clinical evidence suggests a link to neuroendocrine systems known to be pathophysiologically related to posttraumatic stress disorder (PTSD). However, there are no reports of central nervous system (CNS) or peripheral orexin-A concentrations in patients with PTSD, or any anxiety disorder. Cerebrospinal fluid (CSF) and plasma levels of orexin-A were serially determined in patients with PTSD and healthy comparison subjects to characterize the relationships between orexin-A (in the CNS and peripheral circulation) and central indices of monoaminergic neurotransmission and to determine the degree to which CNS orexin-A concentrations reflect those in the circulating blood. CSF and plasma samples were obtained serially over a 6-h period in 10 male combat veterans with chronic PTSD and 10 healthy male subjects through an indwelling subarachnoid catheter. Orexin-A concentrations were determined in plasma and CSF and CSF levels of the serotonin metabolite, 5-hydroxyindolacetic acid (5-HIAA), and the dopamine metabolite, homovanillic acid (HVA), were determined over the sampling period. CSF and plasma orexin-A concentrations were significantly lower in the patients with PTSD as compared with healthy comparison subjects at all time points. In addition, CSF orexin-A concentrations strongly and negatively correlated with PTSD severity as measured by the Clinician-Administered PTSD Scale (CAPS) in patients with PTSD. Peripheral and CNS concentrations of orexin-A were correlated in the healthy comparison subjects and peripheral orexin-A also correlated with CNS serotonergic tone. These findings suggest low central and peripheral orexin-A activity in patients with chronic PTSD are related to symptom severity and raise the possibility that orexin-A is part of the pathophysiological mechanisms of combat-related PTSD.

Role of orexin in the regulation of glucose homeostasis

Orexin-A (hypocretin-1) and orexin-B (hypocretin-2) are hypothalamic neuropeptides that play key roles in the regulation of wakefulness, feeding, reward, autonomic functions and energy homeostasis. To control these functions indispensable for survival, orexin-expressing neurones integrate peripheral metabolic signals, interact with many types of neurones in the brain and modulate their activities via the activation of orexin-1 receptor or orexin-2 receptor. In addition, a new functional role of orexin is emerging in the regulation of insulin and leptin sensitivities responsible for whole-body glucose metabolism. Recent evidence indicates that orexin efficiently protects against the development of peripheral insulin resistance induced by ageing or high-fat feeding in mice. In particular, the orexin receptor-2 signalling appears to confer resistance to diet-induced obesity and insulin insensitivity by improving leptin sensitivity. In fact, the expression of orexin gene is known to be down-regulated by hyperglycaemia in the rodent model of diabetes, such as ob/ob and db/db mice. Moreover, the levels of orexin receptor-2 mRNA have been shown to decline in the brain of mice along with ageing. These suggest that hyperglycaemia due to insulin insensitivity during ageing or by habitual consumption of a high-fat diet leads to the reduction in orexin expression in the hypothalamus, thereby further exacerbating peripheral insulin resistance. Therefore, orexin receptor controlling hypothalamic insulin/leptin actions may be a new target for possible future treatment of hyperglycaemia in patients with type 2 diabetes.

A possible mechanism underlying an antidepressive-like effect of Kososan, a Kampo medicine, via the hypothalamic orexinergic system in the stress-induced depression-like model mice

Kososan, a Kampo (Japanese herbal) medicine, has an antidepressive-like effect in behavioral animal models of depression and has been used clinically for the improvement of depressive mood. However, mechanism(s) underlying the antidepressive-like effect of kososan remain unknown. Previous studies showed that orexin-A (OX-A), a neuropeptide that is involved in feeding and arousal, exhibits an antidepressive-like property via hippocampal cell proliferation. Here, we used immunohistochemical analysis with bromodeoxyuridine (BrdU), a marker of proliferating cells, to investigate the effect of long-term treatment with kososan on the orexinergic system and on hippocampal cell proliferation. Oral administration of kososan (1.0 g/kg) or milnacipran (60 mg/kg), a serotonin and noradrenaline reuptake inhibitor, for 28 d led to an antidepressive-like effect in the stress-induced depression-like model mice and reversed the stress-induced decrease in the number of OX-A-positive cells in the lateral hypothalamic area. In addition, both kososan and milnacipran alleviated the stress-induced decrease in the number of BrdU-positive cells in the hippocampal dentate gyrus. Moreover, the antidepressive-like effect and the increase in cell proliferation and in the number of neuropeptide Y (NPY, which is closely associated with orexinergic system)-positive cells in the dentate gyrus induced by kososan were blocked by treatment with SB-334867, an orexin receptor 1 antagonist. These results suggest that kososan exerts an antidepressive-like effect via the improvement of the stress-induced decrease in hippocampal cell proliferation and that the mechanism underlying the antidepressive-like effect of kososan, but not of milnacipran, may be associated with the regulation of orexinergic and/or NPYergic transmission.

Altered sleep homeostasis after restraint stress in 5-HTT knock-out male mice: a role for hypocretins

Restraint stress produces changes in the sleep pattern that are mainly characterized by a delayed increase in rapid eye movement sleep (REMS) amounts. Because the serotonin (5-HT) and the hypocretin (hcrt) systems that regulate REMS are interconnected, we used mutant mice deficient in the 5-HT transporter (5-HTT(-/-)) to examine the role of 5-HT and hcrt neurotransmissions in the sleep response to stress. In contrast to wild-type mice, restraint stress did not induce a delayed increase in REMS amounts in 5-HTT(-/-) mice, indicating impaired sleep homeostasis in mutants. However, pharmacological blockade of the hcrt type 1 receptor (hcrt-R1) before restraint stress restored the REMS increase in 5-HTT(-/-) mice. In line with this finding, 5-HTT(-/-) mutants displayed after restraint stress higher long-lasting activation of hypothalamic preprohcrt neurons than wild-type mice and elevated levels of the hcrt-1 peptide and the hcrt-R1 mRNA in the anterior raphe area. Thus, hypocretinergic neurotransmission was enhanced by stress in 5-HTT(-/-) mice. Furthermore, in 5-HTT(-/-) but not wild-type mice, hypothalamic levels of the 5-HT metabolite 5-hydroxyindole acetic acid significantly increased after restraint stress, indicating a marked enhancement of serotonergic neurotransmission in mutants. Altogether, our data show that increased serotonergic -and in turn hypocretinergic- neurotransmissions exert an inhibitory influence on stress-induced delayed REMS. We propose that the direct interactions between hcrt neurons in the hypothalamus and 5-HT neurons in the anterior raphe nuclei account, at least in part, for the adaptive sleep-wakefulness regulations triggered by acute stress.

A key role for orexin in panic anxiety

Panic disorder is a severe anxiety disorder with recurrent, debilitating panic attacks. In subjects with panic disorder there is evidence of decreased central GABAergic activity as well as marked increases in autonomic and respiratory responses following intravenous infusions of 0.5M sodium lactate1–3. In an animal model of panic disorder, chronic inhibition of GABA synthesis in the dorsomedial/perifornical hypothalamus of rats produces anxiety-like states and a similar vulnerability to sodium lactate-induced cardioexcitatory responses4–9. The dorsomedial/perifornical hypothalamus is enriched in orexin (ORX, also known as hypocretin)-containing neurons10 that play a critical role in arousal10,11, vigilance10 and central autonomic mobilization12, all of which are key components of panic. Here, we demonstrate that activation of the ORX neurons is necessary for developing a panic-prone state in the animal model, and either silencing the hypothalamic ORX gene (Hcrt) product with RNA interference or systemic ORX1 antagonists blocks the panic responses. Moreover, we show that subjects with panic anxiety have elevated levels of ORX in the cerebrospinal fluid compared to subjects without panic anxiety. Taken together our results suggest that the ORX system may be involved in the pathophysiology of panic anxiety, and that ORX antagonists constitute a potential novel treatment strategy for panic disorder.

Comorbidity of insomnia and depression

During the last decade, several studies have shown that insomnia, rather than a symptom of depression, could be a medical condition on its own, showing high comorbidity with depression. Epidemiological research indicates that insomnia could lead to depression and/or that common causalities underlie the two disorders. Neurobiological and sleep EEG studies suggest that a heightened level of arousal may play a common role in both conditions and that signs of REM sleep disinhibition may appear in individuals prone to depression. The effects of antidepressant drugs on non-REM and REM sleep are discussed in relation to their use in insomnia comorbid with depression. Empirical treatment approaches are behavioral management of sleep combined with prescription of a sedative antidepressant alone, co-prescription of two antidepressants, or of an antidepressant with a hypnotic drug.

The brain hypocretins and their receptors: mediators of allostatic arousal

The hypocretins (abbreviated 'Hcrts' - also called 'orexins') are two neuropeptides secreted exclusively by a small population of neurons in the lateral hypothalamus. These peptides bind to two receptors located throughout the brain in nuclei associated with diverse cognitive and physiological functions. Initially, the brain Hcrt system was found to have a major role in the regulation of sleep/wake transitions. More recent studies indicate Hcrts may play a role in other physiological functions, including food intake, addiction, and stress. Taken together, these studies suggest a general role for Hcrts in mediating arousal, especially when an organism must respond to unexpected stressors and challenges in the environment.

The effect of clomipramine on wake/sleep and orexinergic expression in rats

We have previously found that neonatal treatment with clomipramine (CLI) induced a decrease in brain orexins during the juvenile period and that these changes were reversed at adulthood. This study investigated the effect of CLI on the orexinergic component and sleep/wake states. Two groups of adult male rats were conducted for 48-h polysomnographic recording. One group of rats was treated with CLI (20 mg/kg every 12 h), and a second group was treated with equivolume of saline (SAL) simultaneously after the first 24 h of polysomnographic recording. Rats were killed 2 h after the third dose of treatment. Brain tissues were collected for radioimmunoassay quantification of orexins and real-time PCR analysis of prepro-orexin and orexin receptor mRNA. The CLI group had significantly shorter rapid eye movement (REM) sleep and longer REM latency compared with both the baseline day and the SAL group and had significantly less active wake and more quiet wake. Compared with the control rats, the CLI rats had significantly higher mRNA expression of prepro-orexin in the hypothalamus and the frontal cortex, but not in the hippocampus. The CLI rats also had significantly less orexin B in the hypothalamus than the control rats. These results suggest that suppression of active wake and orexin B by CLI may be a factor responsible for CLI-induced depression and that the increase of prepro-orexin mRNA may be a sign of increased brain orexins found in this model.