I.c.v. administration of orexin-A induces an antidepressive-like effect through hippocampal cell proliferation

Repetitive Transcranial Magnetic Stimulation (rTMS) Improves Cognitive Impairment and Intestinal Microecological Dysfunction Induced by High-Fat Diet in Rats

Consuming a high-fat diet (HFD) is widely recognized to cause obesity and result in chronic brain inflammation that impairs cognitive function. Repetitive transcranial magnetic stimulation (rTMS) has shown effectiveness in both weight loss and cognitive improvement, although the exact mechanism is still unknown. Our study examined the effects of rTMS on the brain and intestinal microecological dysfunction. rTMS successfully reduced cognitive decline caused by an HFD in behavioral assessments involving the Y maze and novel object recognition. This was accompanied by an increase in the number of new neurons and the transcription level of genes related to synaptic plasticity (spindlin 1, synaptophysin, and postsynaptic protein-95) in the hippocampus. It was reached that rTMS decreased the release of high mobility group box 1, activation of microglia, and inflammation in the brains of HFD rats. rTMS also reduced hypothalamic hypocretin levels and improved peripheral blood lipid metabolism. In addition, rTMS recovered the HFD-induced gut microbiome imbalances, metabolic disorders, and, in particular, reduced levels of the microvirus. Our research emphasized that rTMS enhanced cognitive abilities, resulting in positive impacts on brain inflammation, neurodegeneration, and the microbiota in the gut, indicating the potential connection between the brain and gut, proposing that rTMS could be a new approach to addressing cognitive deficits linked to obesity.

Pain-Insomnia-Depression Syndrome: Triangular Relationships, Pathobiological Correlations, Current Treatment Modalities, and Future Direction

Pain-insomnia-depression syndrome (PIDS) is a complex triad of chronic pain, insomnia, and depression that has profound effects on an individual's quality of life and mental health. The pathobiological context of PIDS involves complex neurobiological and physiological mechanisms, including alterations in neurotransmitter systems and impaired pain processing pathways. The first-line therapeutic approaches for the treatment of chronic pain, depression, and insomnia are a combination of pharmacological and non-pharmacological therapies. In cases where patients do not respond adequately to these treatments, additional interventions such as deep brain stimulation (DBS) may be required. Despite advances in understanding and treatment, there are still gaps in knowledge that need to be addressed. To improve our understanding, future research should focus on conducting longitudinal studies to uncover temporal associations, identify biomarkers and genetic markers associated with PIDS, examine the influence of psychosocial factors on treatment responses, and develop innovative interventions that address the complex nature of PIDS. The aim of this study is to provide a comprehensive overview of these components and to discuss their underlying pathobiological relationships.

The role of orexin in Parkinson's disease

Emerging evidence has implicated the orexin system in non-motor pathogenesis of Parkinson's disease. It has also been suggested the orexin system is involved in the modulation of motor control, further implicating the orexin system in Parkinson's disease. Parkinson's disease is the second most common neurodegenerative disease with millions of people suffering worldwide with motor and non-motor symptoms, significantly affecting their quality of life. Treatments are based solely on symptomatic management and no cure currently exists. The orexin system has the potential to be a treatment target in Parkinson's disease, particularly in the non-motor stage. In this review, the most current evidence on the orexin system in Parkinson's disease and its potential role in motor and non-motor symptoms of the disease is summarized. This review begins with a brief overview of Parkinson's disease, animal models of the disease, and the orexin system. This leads into discussion of the possible roles of orexin neurons in Parkinson's disease and levels of orexin in the cerebral spinal fluid and plasma in Parkinson's disease and animal models of the disease. The role of orexin is then discussed in relation to symptoms of the disease including motor control, sleep, cognitive impairment, psychological behaviors, and the gastrointestinal system. The neuroprotective effects of orexin are also summarized in preclinical models of the disease.

Possible role of exercise therapy on depression: Effector neurotransmitters as key players

About 280 million people suffer from depression as the most common neurological disorder and the most common cause of death worldwide. Exercise with serotonin released in the brain by the 5-HT3-IGF-1 mechanism can lead to antidepressant effects. Swimming exercise has antidepressant effects by increasing the sensitivity of serotonin 5-HT2 receptors and postsynaptic 5-HT1A receptors, increasing 5-HT and 5HIAA levels, increasing TPH and serotonin, and decreasing inflammatory levels of IFN-γ and TNF-α. Anaerobic and aerobic exercises increase beta-endorphin, enkephalin, and dynorphin and have antidepressant effects. Exercise by increasing dopamine, D1R, and D2R leads to the expression of BDNF and activation of TrkB and has antidepressant behavior. Exercise leads to a significant increase in GABAAR (γ2 and α2 subunits) and reduces neurodegenerative disorders caused by GABA imbalance through anti-inflammatory pathways. By increasing glutamate and PGC1α and reducing glutamatergic neurotoxicity, exercise enhances neurogenesis and synaptogenesis and prevents neurodegeneration and the onset of depression. Irisin release during exercise shows an important role in depression by increasing dopamine, BDNF, NGF, and IGF-1 and decreasing inflammatory mediators such as IL-6 and IL-1β. In addition, exercise-induced orexin and NPY can increase hippocampal neurogenesis and relieve depression. After exercise, the tryptophan to large neutral amino acids (TRP/LNAA) ratio and the tryptophan to branched-chain amino acids (BCAA) ratio increase, which may have antidepressant effects. The expression of M5 receptor and nAChR α7 increases after exercise and significantly increases dopamine and acetylcholine and ameliorates depression. It appears that during exercise, muscarinic receptors can reduce depression through dopamine in the absence of acetylcholine. Therefore, exercise can be used to reduce depression by affecting neurotransmitters, neuromodulators, cytokines, and/or neurotrophins.

The potential role of the orexin system in premenstrual syndrome

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

Fasting diets: what are the impacts on eating behaviors, sleep, mood, and well-being?

Fasting diets (FDs) have drawn great attention concerning their contribution to health and disease over the last decade. Despite considerable interest in FDs, the effect of fasting diets on eating behaviors, sleep, and mood-essential components of diet satisfaction and mental health- has not been addressed comprehensively. Understanding the critical role that fasting plays in these elements will open up potential treatment avenues that have not yet been explored. The aim of the present paper was to conduct a comprehensive critical review exploring the effects of fasting on eating behaviors, sleep, and mood. There is currently a lack of clarity regarding which fasting option yields the most advantageous effects, and there is also a scarcity of consistent trials that assess the effects of FDs in a comparable manner. Similarly, the effects and/or treatment options for utilizing FDs to modify eating and sleep behaviors and enhance mood are still poorly understood. Further researches aiming at understanding the impacts of various fasting regimes, providing new insights into the gut-brain axis and offering new treatment avenues for those with resistant anxiety and depression, are warranted. Alteration of eating behaviors can have lasting effects on various physiological parameters. The use of fasting cures can underpin ancient knowledge with scientific evidence to form a new approach to the prevention and treatment of problems associated with co-morbidities or challenges pertaining to eating behaviors. Therefore, a thorough examination of the various fasting regimens and how they impact disease patterns is also warranted.

Mini review of molecules involved in altered postnatal neurogenesis in autism

The neurobiology of autism is complex, but emerging research points to potential abnormalities and alterations in neurogenesis. The aim of the present review is to describe the advances in the understanding of the role of selected neurotrophins, neuropeptides, and other compounds secreted by neuronal cells in the processes of postnatal neurogenesis in conjunction with autism. We characterize the fundamental mechanisms of neuronal cell proliferation, generation of major neuronal cell types with special emphasis on neurogenic niches - the subventricular zone and hippocampal areas. We also discuss changes in intracellular calcium levels and calcium-dependent transcription factors in the context of the regulation of neurogenesis and cell fate determination. To sum up, this review provides specific insight into the known association between alterations in the function of the entire spectrum of molecules involved in neurogenesis and the etiology of autism pathogenesis.

Hypocretin/orexin neurons encode social discrimination and exhibit a sex-dependent necessity for social interaction

The hypothalamus plays a crucial role in the modulation of social behavior by encoding internal states. The hypothalamic hypocretin/orexin neurons, initially identified as regulators of sleep and appetite, are important for emotional and motivated behaviors. However, their role in social behavior remains unclear. Using fiber photometry and behavioral analysis, we show here that hypocretin neurons differentially encode social discrimination based on the nature of social encounters. The optogenetic inhibition of hypocretin neuron activity or blocking of hcrt-1 receptors reduces the amount of time mice are engaged in social interaction in males but not in females. Reduced hcrt-1 receptor signaling during social interaction is associated with altered activity in the insular cortex and ventral tegmental area in males. Our data implicating hypocretin neurons as sexually dimorphic regulators within social networks have significant implications for the treatment of neuropsychiatric diseases with social dysfunction, particularly considering varying prevalence among sexes.

Neural Progenitor Cells and the Hypothalamus

Neural progenitor cells (NPCs) are multipotent neural stem cells (NSCs) capable of self-renewing and differentiating into neurons, astrocytes and oligodendrocytes. In the postnatal/adult brain, NPCs are primarily located in the subventricular zone (SVZ) of the lateral ventricles (LVs) and subgranular zone (SGZ) of the hippocampal dentate gyrus (DG). There is evidence that NPCs are also present in the postnatal/adult hypothalamus, a highly conserved brain region involved in the regulation of core homeostatic processes, such as feeding, metabolism, reproduction, neuroendocrine integration and autonomic output. In the rodent postnatal/adult hypothalamus, NPCs mainly comprise different subtypes of tanycytes lining the wall of the 3^rd ventricle. In the postnatal/adult human hypothalamus, the neurogenic niche is constituted by tanycytes at the floor of the 3^rd ventricle, ependymal cells and ribbon cells (showing a gap-and-ribbon organization similar to that in the SVZ), as well as suprachiasmatic cells. We speculate that in the postnatal/adult human hypothalamus, neurogenesis occurs in a highly complex, exquisitely sophisticated neurogenic niche consisting of at least four subniches; this structure has a key role in the regulation of extrahypothalamic neurogenesis, and hypothalamic and extrahypothalamic neural circuits, partly through the release of neurotransmitters, neuropeptides, extracellular vesicles (EVs) and non-coding RNAs (ncRNAs).

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

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

Chronic orexin-1 receptor blockage attenuates depressive behaviors and provokes PSD-95 expression in a rat model of depression

Depression is a devastating mood disorder affecting more than 300 million people worldwide. Almost 30 % of patients still suffer from treatment resistant depression. Although many reports support the involvement of orexin in the pathophysiology of depression, the precise role of orexin is still unclear. In this study, we evaluated the role of the orexin 1 receptor (Orx₁R) on depressive behaviors and the alterations in postsynaptic density-95 (PSD-95) protein in the chronic mild stress (CMS) model of depression. Fifty-four male Wistar rats were randomly allocated to 6 groups; Control, CMS, acute SB-334867 (SB), CMS+SB, chronic SB (CSB) and CMS+CSB. Rats were exposed to one or two unpredictable stressors each day for three weeks for the induction of CMS. Intracerebroventricular (icv) injection of SB-334867, a selective Orx₁R antagonist, was performed either 30 min before behavioral tests (acute) or once daily for 14 days (chronic). Behavioral despair was assessed by immobility time in the forced swim test (FST), sucrose consumption in sucrose preference test (SPT), and the number of crosses in the open field test (OFT) on days 1, 11, and 22 of the experiment. Finally, rats were decapitated, and brain tissue of the hippocampus (HPC) and prefrontal cortex (PFC) were collected, and the relative expression of PSD-95 was evaluated by western blotting. The CMS model rats showed a significant increase in FST immobility time (P = 0.001) and a decrease in locomotion (P = 0.04) and sucrose preference (P = 0.039). Chronic application of SB decreased immobility time to the control values (P = 0.001) and diminished locomotion (P = 0.047) and sucrose preference (P = 0.042) in comparison to the CMS group. Acute SB reversed just the immobility time (P ≤ 0.006). Chronic SB treatment increased the relative PSD-95 expression in PFC (P = 0.001). Hence, chronic antagonism of Orx₁R alleviates depressive behaviors induced by CMS and improves PSD-95 expression in PFC.

Orexin Receptor Antagonists in the Treatment of Depression: A Leading Article Summarising Pre-clinical and Clinical Studies

The orexin (hypocretin) system comprises two neuropeptides (orexin-A and orexin-B) and two G-protein coupled receptors (the orexin type 1 and the orexin type 2 receptor). The system regulates several biological functions including appetite, the sleep-wake cycle, the stress response, and motivation and reward processing. Dysfunction of the orexin system has been implicated in the pathophysiology of depression in human and animal studies, although the exact nature of this dysfunction remains unclear. Orexin receptor antagonists (ORAs) are a class of compounds developed for the treatment of insomnia and have demonstrated efficacy in this area. Three dual orexin receptor antagonists (DORAs) have received licences for treatment of primary insomnia and some ORAs have since been investigated as potential treatments for major depressive disorder (MDD). In this leading article, we summarise the existing literature on use of ORAs in depression, in pre-clinical and clinical studies. In rodent models of depression, investigated ORAs have included the DORA almorexant and TCS1102, the selective orexin 1 receptor antagonists SB334867 and SB674042 and the selective orexin 2 receptor antagonists LSN2424100, MK-1064 and TCS-OX2-29. These pre-clinical studies suggest a possible antidepressant effect of systemic DORA treatment, however the evidence from selective ORAs is conflicting. To date, four published RCTs (one with the DORA filorexant and three with the selective orexin 2 receptor antagonist seltorexant), have compared an ORA with placebo in the treatment of MDD. Only one of these demonstrated a statistically significant difference relative to placebo.

From Molecule to Behavior: Hypocretin/orexin Revisited From a Sex-dependent Perspective

The hypocretin/orexin (Hcrt/Orx) system in the perifornical lateral hypothalamus has been recognized as a critical node in a complex network of neuronal systems controlling both physiology and behavior in vertebrates. Our understanding of the Hcrt/Orx system and its array of functions and actions has grown exponentially in merely 2 decades. This review will examine the latest progress in discerning the roles played by the Hcrt/Orx system in regulating homeostatic functions and in executing instinctive and learned behaviors. Furthermore, the gaps that currently exist in our knowledge of sex-related differences in this field of study are discussed.

Hypocretins (orexins): The ultimate translational neuropeptides

The hypocretins (Hcrts), also known as orexins, are two neuropeptides produced exclusively in the lateral hypothalamus. They act on two specific receptors that are widely distributed across the brain and involved in a myriad of neurophysiological functions that include sleep, arousal, feeding, reward, fear, anxiety and cognition. Hcrt cell loss in humans leads to narcolepsy with cataplexy (narcolepsy type 1), a disorder characterized by intrusions of sleep into wakefulness, demonstrating that the Hcrt system is nonredundant and essential for sleep/wake stability. The causal link between Hcrts and arousal/wakefulness stabilisation has led to the development of a new class of drugs, Hcrt receptor antagonists to treat insomnia, based on the assumption that blocking orexin-induced arousal will facilitate sleep. This has been clinically validated: currently, two Hcrt receptor antagonists are approved to treat insomnia (suvorexant and lemborexant), with a New Drug Application recently submitted to the US Food and Drug Administration for a third drug (daridorexant). Other therapeutic applications under investigation include reduction of cravings in substance-use disorders and prevention of neurodegenerative disorders such as Alzheimer's disease, given the apparent bidirectional relationship between poor sleep and worsening of the disease. Circuit neuroscience findings suggest that the Hcrt system is a hub that integrates diverse inputs modulating arousal (e.g., circadian rhythms, metabolic status, positive and negative emotions) and conveys this information to multiple output regions. This neuronal architecture explains the wealth of physiological functions associated with Hcrts and highlights the potential of the Hcrt system as a therapeutic target for a number of disorders. We discuss present and future possible applications of drugs targeting the Hcrt system for the treatment of circuit-related neuropsychiatric and neurodegenerative conditions.

Hippocampal orexin receptors: Localization and function

Orexin (hypocretin) is secreted from the perifornical/lateral hypothalamus and is well known for sleep regulation. Orexin has two, orexin A and B, transcripts and two receptors, type 1 and 2 (OX1R and OX2R), located in the plasma membrane of neurons in different brain areas, including the hippocampus involved in learning, memory, seizures, and epilepsy, as physiologic and pathologic phenomena. OX1R is expressed in the dentate gyrus and CA1 and the OX2R in the CA3 areas. Orexin enhances learning and memory as well as reward, stress, seizures, and epilepsy, partly through OX1Rs, while either aggravating or alleviating those phenomena via OX2Rs. OX1Rs activation induces long-term changes of synaptic responses in the hippocampus, an age and concentration-dependent manner. Briefly, we will review the localization and functions of hippocampal orexin receptors, their role in learning, memory, stress, reward, seizures, epilepsy, and hippocampal synaptic plasticity.

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

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Orexin neuropeptides roles in hippocampal-dependent memory formation.

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Orexin neuropeptides activate the neural circuits of the basolateral amygdala.

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

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

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

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

Changes in the Orexin System in Rats Exhibiting Learned Helplessness Behaviors

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

Orexin-A and endocannabinoids are involved in obesity-associated alteration of hippocampal neurogenesis, plasticity, and episodic memory in mice

The mammalian brain stores and distinguishes among episodic memories, i.e. memories formed during the personal experience, through a mechanism of pattern separation computed in the hippocampal dentate gyrus. Decision-making for food-related behaviors, such as the choice and intake of food, might be affected in obese subjects by alterations in the retrieval of episodic memories. Adult neurogenesis in the dentate gyrus regulates the pattern separation. Several molecular factors affect adult neurogenesis and exert a critical role in the development and plasticity of newborn neurons. Orexin-A/hypocretin-1 and downstream endocannabinoid 2-arachidonoylglycerol signaling are altered in obese mice. Here, we show that excessive orexin-A/2-arachidonoylglycerol/cannabinoid receptor type-1 signaling leads to the dysfunction of adult hippocampal neurogenesis and the subsequent inhibition of plasticity and impairment of pattern separation. By inhibiting orexin-A action at orexin-1 receptors we rescued both plasticity and pattern separation impairment in obese mice, thus providing a molecular and functional mechanism to explain alterations in episodic memory in obesity.

The authors show that adult hippocampal neurogenesis is altered in the dentate gyrus of obese mice with subsequent inhibition of long-term potentiation and impairment of pattern separation. Inhibition of orexin-A action at orexin-1 receptors rescued both impairments in obese mice.

Hypocretin/Orexin Receptor Pharmacology and Sleep Phases

The hypocretins/orexins are two excitatory neuropeptides, alternately called HCRT1 or orexin-A and HCRT2 or orexin-B, that are the endogenous ligands for two G-protein-coupled receptors, HCRTR1/OX1R and HCRTR2/OX2R. Shortly after the discovery of this system, degeneration of hypocretin/orexin-producing neurons was implicated in the etiology of the sleep disorder narcolepsy. The involvement of this system in a disorder characterized by the loss of control over arousal state boundaries also suggested its role as a critical component of endogenous sleep-wake regulatory circuitry. The broad projections of the hypocretin/orexin-producing neurons, along with differential expression of the two receptors in the projection fields of these neurons, suggest distinct roles for these receptors. While HCRTR1/OX1R is associated with regulation of motivation, reward, and autonomic functions, HCRTR2/OX2R is strongly linked to sleep-wake control. The association of hypocretin/orexin with these physiological processes has led to intense interest in the therapeutic potential of compounds targeting these receptors. Agonists and antagonists for the hypocretin/orexin receptors have shown potential for the treatment of disorders of excessive daytime somnolence and nocturnal hyperarousal, respectively, with the first antagonists approved by the US Food and Drug Administration (FDA) in 2014 and 2019 for the treatment of insomnia. These and related compounds have also been useful tools to advance hypocretin/orexin neurobiology.

Orexin-A Attenuates Inflammatory Responses in Lipopolysaccharide-Induced Neural Stem Cells by Regulating NF-KB and Phosphorylation of MAPK/P38/Erk Pathways

Background

Neuronal damage is the main cause of neurological diseases. Neural stem cells (NSCs) have the functions of cell repair and replacement of neurons, secretion of neurotrophic factors, and immune regulation of the neural microenvironment.

Objective

Previous study found that Orexin-A had a protective effect on neurons in the central nervous system, but it is lacking in making great efforts on the function of Orexin-A on NSCs. This study aimed to investigate the anti-inflammatory responses and signaling mechanisms of Orexin-A on lipopolysaccharide (LPS)-induced NSCs.

Methods

Quantitative real-time polymerase chain reaction was used to detect the mRNA level. Signaling pathway-related protein expression was detected by Western blot. The proliferation and migration of NSCs were investigated by Cell Counting Kit-8 (CCK-8) detection kit and transwell assay. Besides, the staining of hematoxylin and eosin (HE) was performed to study the morphology of cell.

Results

Orexin-A decreased the pro-inflammatory cytokines of IL-1β, TNF-α, and IL-6 induced by LPS by regulating nuclear factor-k-gene binding (NF-kB) and phosphorylation of P38/Erk-mitogen-activated protein kinases (MAPKs) pathways, but not p-JNK signaling.

Conclusion

Our findings indicate that Orexin-A can alleviate the inflammatory response of NSC. It can provide beneficial help in neural stem cell therapy applications.

Central regulatory mechanisms of visceral sensation in response to colonic distension with special reference to brain orexin

Visceral hypersensitivity is a major pathophysiology in irritable bowel syndrome (IBS). Although brain-gut interaction is considered to be involved in the regulation of visceral sensation, little had been known how brain controls visceral sensation. To improve therapeutic strategy in IBS, we should develop a novel approach to control visceral hypersensitivity. Here, we summarized recent data on central control of visceral sensation by neuropeptides in rats. Orexin, ghrelin or oxytocin in the brain is capable of inducing visceral antinociception. Dopamine, cannabinoid, adenosine, serotonin or opioid in the central nervous system (CNS) plays a role in the visceral hyposensitivity. Central ghrelin, levodopa or morphine could induce visceral antinociception via the orexinergic signaling. Orexin induces visceral antinociception through dopamine, cannabinoid, adenosine or oxytocin. Orexin nerve fibers are identified widely throughout the CNS and orexins are implicated in a number of functions. With regard to gastrointestinal functions, in addition to its visceral antinociception, orexin acts centrally to stimulate gastrointestinal motility and improve intestinal barrier function. Brain orexin is also involved in regulation of sleep/awake cycle and anti-depressive action. 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 IBS who are frequently accompanied with sleep disturbance, depressive state and disturbed gut functions such as gut motility disturbance, leaky gut and visceral hypersensitivity.

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.

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.

Involvement of orexinergic system in psychiatric and neurodegenerative disorders: A scoping review

Orexin is a neuropeptide secreted from lateral hypothalamus and pre-frontal cortex concerned in the wakefulness and excitement. This study aimed to review the possible neurobiological effect of orexin. A diversity of search strategies was adopted and assumed which included electronic database searches of Medline and PubMed using MeSH terms, keywords, and title words during the search. Orexin plays a vital role in activation of learning, memory acquisition, and consolidation through activation of monoaminergic system, which affect cognitive flexibility and cognitive function. Orexin stimulates adrenocorticotropin and corticosteroid secretions via activation of central corticotropin-releasing hormone. Cerebrospinal fluid (CSF) and serum orexin serum levels are reduced in depression, schizophrenia, and narcolepsy. However, high orexin serum levels are revealed in drug addictions. Regarding neurodegenerative brain diseases, CSF and serum orexin serum levels are reduced Parkinson disease, Alzheimer dementia, Huntington's disease, amyotrphic lateral sclerosis, and multiple sclerosis. Orexin antagonist leads to significant reduction of sympathetic over-activity during withdrawal syndrome. As well, orexin antagonist improves sleep pattern. Orexinergic system is involved in the different psychiatric and neurological disorders; therefore, targeting of this system could be possible novel pathway in the management of these disorders. In addition, measurement of CSF and serum orexin levels might predict the relapse and withdrawal of addict patients.

Orexin A alleviates neuroinflammation via OXR2/CaMKKβ/AMPK signaling pathway after ICH in mice

Background

Orexins are two neuropeptides (orexin A, OXA; orexin B, OXB) secreted mainly from the lateral hypothalamus, which exert a wide range of physiological effects by activating two types of receptors (orexin receptor 1, OXR1; orexin receptor 2, OXR2). OXA has equal affinity for OXR1 and OXR2, whereas OXB binds preferentially to OXR2. OXA rapidly crosses the blood-brain barrier by simple diffusion. Many studies have reported OXA’s protective effect on neurological diseases via regulating inflammatory response which is also a fundamental pathological process in intracerebral hemorrhage (ICH). However, neuroprotective mechanisms of OXA have not been explored in ICH.

Methods

ICH models were established using stereotactic injection of autologous arterial blood into the right basal ganglia of male CD-1 mice. Exogenous OXA was administered intranasally; CaMKKβ inhibitor (STO-609), OXR1 antagonist (SB-334867), and OXR2 antagonist (JNJ-10397049) were administered intraperitoneally. Neurobehavioral tests, hematoma volume, and brain water content were evaluated after ICH. Western blot and ELISA were utilized to evaluate downstream mechanisms.

Results

OXA, OXR1, and OXR2 were expressed moderately in microglia and astrocytes and abundantly in neurons. Expression of OXA decreased whereas OXR1 and OXR2 increased after ICH. OXA treatment significantly improved not only short-term but also long-term neurofunctional outcomes and reduced brain edema in ipsilateral hemisphere. OXA administration upregulated p-CaMKKβ, p-AMPK, and anti-inflammatory cytokines while downregulated p-NFκB and pro-inflammatory cytokines after ICH; this effect was reversed by STO-609 or JNJ-10397049 but not SB-334867.

Conclusions

OXA improved neurofunctional outcomes and mitigated brain edema after ICH, possibly through alleviating neuroinflammation via OXR2/CaMKKβ/AMPK pathway.

Orexin-1 receptor blockade differentially affects spatial and visual discrimination memory facilitation by intracranial self-stimulation

Intracranial self-stimulation (ICSS) of the medial forebrain bundle is an effective treatment to facilitate memory. Performance in both explicit and implicit memory tasks has been improved by ICSS, and this treatment has even been capable of recovering loss of memory function due to lesions or old age. Several neurochemical systems have been studied in regard to their role in ICSS effects on memory, however the possible involvement of the orexinergic system in this facilitation has yet to be explored. The present study aims to examine the relationship between the OX1R and the facilitative effects of ICSS on two different types of memory tasks, both carried out in the Morris Water Maze: spatial and visual discrimination. Results show that the OX1R blockade, by intraventricular administration of SB-334867, partially negates the facilitating effect of ICSS on spatial memory, whereas it hinders ICSS facilitation of the discrimination task. However, ICSS treatment was capable of compensating for the severe detrimental effects of OX1R blockade on both memory paradigms. These results suggest different levels of involvement of the orexinergic system in the facilitation of memory by ICSS, depending on the memory task.

Significance of the orexinergic system in modulating stress-related responses in an animal model of post-traumatic stress disorder

Converging evidence indicates that orexins (ORXs), the regulatory neuropeptides, are implicated in anxiety- and depression-related behaviors via the modulation of neuroendocrine, serotonergic, and noradrenergic systems. This study evaluated the role of the orexinergic system in stress-associated physiological responses in a controlled prospective animal model. The pattern and time course of activation of hypothalamic ORX neurons in response to predator-scent stress (PSS) were examined using c-Fos as a marker for neuronal activity. The relationship between the behavioral response pattern 7 days post-exposure and expressions of ORXs was evaluated. We also investigated the effects of intracerebroventricular microinfusion of ORX-A or almorexant (ORX-A/B receptor antagonist) on behavioral responses 7 days following PSS exposure. Hypothalamic levels of ORX-A, neuropeptide Y (NPY), and brain-derived neurotrophic factor (BDNF) were assessed. Compared with rats whose behaviors were extremely disrupted (post-traumatic stress disorder [PTSD]-phenotype), those whose behaviors were minimally selectively disrupted displayed significantly upregulated ORX-A and ORX-B levels in the hypothalamic nuclei. Intracerebroventricular microinfusion of ORX-A before PSS reduced the prevalence of the PTSD phenotype compared with that of artificial cerebrospinal fluid or almorexant, and rats treated with almorexant displayed a higher prevalence of the PTSD phenotype than did untreated rats. Activated ORX neurons led to upregulated expressions of BDNF and NPY, which might provide an additional regulatory mechanism for the modulation of adaptive stress responses. The study indicates that the activated ORX system might promote adaptive responses to PSS probably via stimulation of BDNF and NPY secretion, and early intervention with ORX-A reduces the prevalence of the PTSD phenotype and increases the prevalence of adaptive phenotypes. The findings provide some insights into the mechanisms underlying the involvement of the ORX system in stress-related disorders.

Hypocretin Mediates Sleep and Wake Disturbances in a Mouse Model of Traumatic Brain Injury

Traumatic brain injury (TBI) is a major cause of disability worldwide. Post-TBI sleep and wake disturbances are extremely common and difficult for patients to manage. Sleep and wake disturbances contribute to poor functional and emotional outcomes from TBI, yet effective therapies remain elusive. A more comprehensive understanding of mechanisms underlying post-TBI sleep and wake disturbance will facilitate development of effective pharmacotherapies. Previous research in human patients and animal models indicates that altered hypocretinergic function may be a major contributor to sleep-wake disturbance after TBI. In this study, we further elucidate the role of hypocretin by determining the impact of TBI on sleep-wake behavior of hypocretin knockout (HCRT KO) mice. Adult male C57BL/6J and HCRT KO mice were implanted with electroencephalography recording electrodes, and pre-injury baseline recordings were obtained. Mice were then subjected to either moderate TBI or sham surgery. Additional recordings were obtained and sleep-wake behavior determined at 3, 7, 15, and 30 days after TBI or sham procedures. At baseline, HCRT KO mice had a significantly different sleep-wake phenotype than control C57BL/6J mice. Post-TBI sleep-wake behavior was altered in a genotype-dependent manner: sleep of HCRT KO mice was not altered by TBI, whereas C57BL/6J mice had more non-rapid eye movement sleep, less wakefulness, and more short wake bouts and fewer long wake bouts. Numbers of hypocretin-positive cells were reduced in C57BL/6J mice by TBI. Collectively, these data indicate that the hypocretinergic system is involved in the alterations in sleep-wake behavior that develop after TBI in this model, and suggest potential therapeutic interventions.

A pilot study of metabolic fitness effects of weight-supported walking in women with obesity

Background

This is an exploratory pilot study of novel technology enabling people with mobility disability to walk with minimal effort, in the “sedentary range”. The study’s premise is that impairment of the leading physical activity of daily living, walking, is a major contributor to a dysmetabolic state driving many prevalent “civilization diseases” associated with insulin resistance.

Methods

We explore within-subject changes in standard oral glucose tolerance (OGT) tests including metabotropic molecules after 22 twice-weekly, 30-minute bouts of weight-supported light-moderate physical activity in 16 non-diabetic obese, otherwise healthy, reproductive-age, volunteer women walking on an “anti-gravity” lower-body positive pressure (LBPP) treadmill.

Results

Subjects had reference base-line fasting plasma glucose and triglycerides (TG) but 2-hr OGT insulin levels of 467 ± 276 pmol • liter^-1 (mean± S.D.) indicating nascent insulin resistance, compared to post-study 308 ± 179 (p = 0.002). Fasting TG decreased from 0.80 ± 0.30 mmol • liter^-1 to 0.71 ± 0.25 (p = 0.03). Concomitantly plasma total ghrelin decreased from 69.6 ± 41.6 pmol • liter^-1 to 56.0 ± 41.3 (p = 0.008). There were no statistically significant changes in body weight or any correlations between weight change and cardiometabolic markers. However, there were robust positive correlations between changes among different classes of peptides including C-reactive protein–Interleukin 6, leptin–adiponectin, β-endorphin–oxytocin and orexin A (r ² = 0.48–0.88).

Conclusion

We conclude that brief, low-dose physical activity, walking on an anti-gravity LBPP treadmill may improve cardiometabolic risk, exhibiting favorable changes in neuro-regulatory peptides without weight loss in people with problems walking.

Orexin prevents depressive-like behavior by promoting stress resilience

Hypothalamic neuropeptide orexin has been implicated in the pathophysiology of psychiatric disorders and accumulating clinical evidence indicates a potential link between orexin and depression. However, the exact role of orexin in depression, particularly the underlying neural substrates and mechanisms, remains unknown. In this study, we reveal a direct projection from the hypothalamic orexinergic neurons to the ventral pallidum (VP), a structure that receives an increasing attention for its critical position in rewarding processing, stress responses, and depression. We find that orexin directly excites GABAergic VP neurons and prevents depressive-like behaviors in rats. Two orexin receptors, OX1R and OX2R, and their downstream Na⁺-Ca²⁺ exchanger and L-type Ca²⁺ channel co-mediate the effect of orexin. Furthermore, pharmacological blockade or genetic knockdown of orexin receptors in VP increases depressive-like behaviors in forced swim test and sucrose preference test. Intriguingly, blockage of orexinergic inputs in VP has no impact on social proximity in social interaction test between novel partners, but remarkably strengthens social avoidance under an acute psychosocial stress triggered by social rank. Notably, a significantly increased orexin level in VP is accompanied by an increase in serum corticosterone in animals exposed to acute stresses, including forced swimming, food/water deprivation and social rank stress, rather than non-stress situations. These results suggest that endogenous orexinergic modulation on VP is especially critical for protecting against depressive reactions to stressful events. The findings define an indispensable role for the central orexinergic system in preventing depression by promoting stress resilience.

Reduced plasma orexin-A levels in patients with bipolar disorder

PURPOSE

Orexins are hypothalamic neuropeptides involved in the regulation of sleep, appetite and arousal. An altered orexin system has been implicated in the pathophysiology of psychiatric disorders. This study aimed to examine whether plasma orexin-A levels differ in patients with schizophrenia, major depressive disorder (MDD), or bipolar disorder (BD) compared to in healthy controls. We also examined the possible correlations between plasma orexin-A levels and clinical variables.

PATIENTS AND METHODS

All participants were Japanese. The sample consisted of 80 patients with schizophrenia (42 women, 52.5%; mean age 36.8 years), 80 patients with MDD (43 women, 53.8%; 43.7 years), and 40 patients with BD (24 women, 60%; 41.1 years), as well as 80 healthy controls (48 women, 60%; 47.0 years). Plasma orexin-A levels were quantified by an enzyme-linked immunosorbent assay.

RESULTS

Mean orexin-A levels were significantly different across the four diagnostic groups (F=4.09; df=3; p=0.007, η² =0.06). In particular, the patients with BD showed significantly lower orexin-A levels than did the controls. When the median value of the control group (109.8 pg/ml) was set as a cut-off value, subjects whose orexin-A levels were below the cut-off were more common in all psychiatric groups (schizophrenia: 73.8%, x² =9.56, df=1, p=0.003, OR=2.81, 95% CI: 1.45 to 5.45, d=0.57; MDD: 78.5%, x² =14.02, df=1, p<0.001, OR=3.65, 95% CI: 1.82 to 7.29, d=0.72; BD: 87.5%, x² =16.0, df=1, p<0.001, OR=7.00, 95% CI: 2.49 to 19.70, d=1.07). We found no association between plasma orexin-A levels and any clinical symptoms, depression severity, or medication doses.

CONCLUSION

Our results suggest that plasma orexin-A levels are reduced in patients with BD.

Brain and Cognition for Addiction Medicine: From Prevention to Recovery Neural Substrates for Treatment of Psychostimulant-Induced Cognitive Deficits

Addiction to psychostimulants like cocaine, methamphetamine, and nicotine poses a continuing medical and social challenge both in the United States and all over the world. Despite a desire to quit drug use, return to drug use after a period of abstinence is a common problem among individuals dependent on psychostimulants. Recovery for psychostimulant drug-dependent individuals is particularly challenging because psychostimulant drugs induce significant changes in brain regions associated with cognitive functions leading to cognitive deficits. These cognitive deficits include impairments in learning/memory, poor decision making, and impaired control of behavioral output. Importantly, these drug-induced cognitive deficits often impact adherence to addiction treatment programs and predispose abstinent addicts to drug use relapse. Additionally, these cognitive deficits impact effective social and professional rehabilitation of abstinent addicts. The goal of this paper is to review neural substrates based on animal studies that could be pharmacologically targeted to reverse psychostimulant-induced cognitive deficits such as impulsivity and impairment in learning and memory. Further, the review will discuss neural substrates that could be used to facilitate extinction learning and thus reduce emotional and behavioral responses to drug-associated cues. Moreover, the review will discuss some non-pharmacological approaches that could be used either alone or in combination with pharmacological compounds to treat the above-mentioned cognitive deficits. Psychostimulant addiction treatment, which includes treatment for cognitive deficits, will help promote abstinence and allow for better rehabilitation and integration of abstinent individuals into society.

Orexin/hypocretin receptor modulation of anxiolytic and antidepressive responses during social stress and decision-making: Potential for therapy

Hypothalmic orexin/hypocretin (Orx) neurons in the lateral and dorsomedial perifornical region (LH-DMH/PeF) innervate broadly throughout the brain, and receive similar inputs. This wide distribution, as well as two Orx peptides (Orx_A and Orx_B) and two Orx receptors (Orx₁ and Orx₂) allow for functionally related but distinctive behavioral outcomes, that include arousal, sleep-wake regulation, food seeking, metabolism, feeding, reward, addiction, and learning. These are all motivational functions, and tie the orexin systems to anxiety and depression as well. We present evidence, that for affective behavior, Orx₁ and Orx₂ receptors appear to have opposing functions. The majority of research on anxiety- and depression-related outcomes has focused on Orx₁ receptors, which appear to have primarily anxiogenic and pro-depressive actions. Although there is significant research suggesting contrary findings, the primary potential for pharmacotherapies linked to the Orx₁ receptor is via antagonists to block anxious and depressive behavior. Dual orexin receptor antagonists have been approved for treatment of sleep disorders, and are likely candidates for adaptation for affect disorder treatments. However, we present evidence here that demonstrates the Orx₂ receptors are anxiolytic and antidepressive. Using a new experimental pre-clinical model of anxious and depressive behavior stimulated by social stress and decision-making that produces two stable behavioral phenotypes, Escape/Resilient and Stay/Susceptible, we tested the effects of intracerebroventricular injections of Orx₂ agonist and antagonist drugs. Over ten behavioral measures, we have demonstrated that Orx₂ agonists promote resilience, as well as anxiolytic and antidepressive behavior. In contrast, Orx₂ antagonists or knockdown kindle anxious and pro-depressive behavior plus increase susceptibility. The results suggest that the Orx₂ receptor may be a useful target for pharmacotherapies to treat anxiety and depression.

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 Orexin/Receptor System: Molecular Mechanism and Therapeutic Potential for Neurological Diseases

Orexins, also known as hypocretins, are two neuropeptides secreted from orexin-containing neurons, mainly in the lateral hypothalamus (LH). Orexins orchestrate their effects by binding and activating two G-protein–coupled receptors (GPCRs), orexin receptor type 1 (OX1R) and type 2 (OX2R). Orexin/receptor pathways play vital regulatory roles in many physiological processes, especially feeding behavior, sleep–wake rhythm, reward and addiction and energy balance. Furthermore several reports showed that orexin/receptor pathways are involved in pathological processes of neurological diseases such as narcolepsy, depression, ischemic stroke, drug addiction and Alzheimer’s disease (AD). This review article summarizes the expression patterns, physiological functions and potential molecular mechanisms of the orexin/receptor system in neurological diseases, providing an overall framework for considering these pathways from the standpoints of basic research and clinical treatment of neurological diseases.

Reduced Orexin System Function Contributes to Resilience to Repeated Social Stress

Exposure to stress increases the risk of developing affective disorders such as depression and post-traumatic stress disorder (PTSD). However, these disorders occur in only a subset of individuals, those that are more vulnerable to the effects of stress, whereas others remain resilient. The coping style adopted to deal with the stressor, either passive or active coping, is related to vulnerability or resilience, respectively. Important neural substrates that mediate responses to a stressor are the orexins. These neuropeptides are altered in the cerebrospinal fluid of patients with stress-related illnesses such as depression and PTSD. The present experiments used a rodent social defeat model that generates actively coping rats and passively coping rats, which we have previously shown exhibit resilient and vulnerable profiles, respectively, to examine if orexins play a role in these stress-induced phenotypes. In situ radiolabeling and qPCR revealed that actively coping rats expressed significantly lower prepro-orexin mRNA compared with passively coping rats. This led to the hypothesis that lower levels of orexins contribute to resilience to repeated social stress. To test this hypothesis, rats first underwent 5 d of social defeat to establish active and passive coping phenotypes. Then, orexin neurons were inhibited before each social defeat for three additional days using designer receptors exclusively activated by designer drugs (DREADDs). Inhibition of orexins increased social interaction behavior and decreased depressive-like behavior in the vulnerable population of rats. Indeed, these data suggest that lowering orexins promoted resilience to social defeat and may be an important target for treatment of stress-related disorders.

The Discovery of Suvorexant, the First Orexin Receptor Drug for Insomnia

Historically, pharmacological therapies have used mechanisms such as γ-aminobutyric acid A (GABA_A) receptor potentiation to drive sleep through broad suppression of central nervous system activity. With the discovery of orexin signaling loss as the etiology underlying narcolepsy, a disorder associated with hypersomnolence, orexin antagonism emerged as an alternative approach to attenuate orexin-induced wakefulness more selectively. Dual orexin receptor antagonists (DORAs) block the activity of orexin 1 and 2 receptors to both reduce the threshold to transition into sleep and attenuate orexin-mediated arousal. Among DORAs evaluated clinically, suvorexant has pharmacokinetic properties engineered for a plasma half-life appropriate for rapid sleep onset and maintenance at low to moderate doses. Unlike GABA_A receptor modulators, DORAs promote both non-rapid eye movement (NREM) and REM sleep, do not disrupt sleep stage-specific quantitative electroencephalogram spectral profiles, and allow somnolence indistinct from normal sleep. The preservation of cognitive performance and the ability to arouse to salient stimuli after DORA administration suggest further advantages over historical therapies.

Orexin/hypocretin treatment restores hippocampal-dependent memory in orexin-deficient mice

Orexin A is produced in neurons of the lateral, perifornical and dorsomedial regions of the lateral hypothalamic area, which then project widely throughout the central nervous system to regulate arousal state, sleep-wake architecture, energy homeostasis and cognitive processes. Disruption of orexin signaling leads to sleep disturbances and increased body mass index, but recent studies also indicate that orexin neuron activation improves learning and memory. We hypothesized that hippocampal orexin receptor activation improves memory. To test this idea, we obtained orexin/ataxin-3 (O/A3) mice, which become deficient in orexin neurons by about 12 weeks of age. We first measured hippocampal orexin receptor 1 (OX1R) gene expression and protein levels, then tested acquisition and consolidation of two-way active avoidance (TWAA) memory, a hippocampal-dependent learning and memory task. Finally, we determined if exogenous intra-hippocampal OXA treatment could reverse cognitive impairment (as determined by TWAA) in OA/3 mice. We showed that OX1R mRNA expression and protein levels were significantly elevated in O/A3 mice, indicating the potential for preserved orexin responsiveness. The O/A3 mice were significantly impaired in TWAA memory vs. control mice, but OXA treatment (both acute and chronic) reversed these memory deficits. These results demonstrate that orexin plays an important role in hippocampal-dependent consolidation of two-way active avoidance memory, and orexin replacement can rescue the cognitive impairment.

Kososan, a Kampo medicine, prevents a social avoidance behavior and attenuates neuroinflammation in socially defeated mice

Background

Kososan, a Kampo (traditional Japanese herbal) medicine, has been used for the therapy of depressive mood in humans. However, evidence for the antidepressant efficacy of kososan and potential mechanisms are lacking. Recently, it has been recognized that stress triggers neuroinflammation and suppresses adult neurogenesis, leading to depression and anxiety. Here, we examined whether kososan extract affected social behavior in mice exposed to chronic social defeat stress (CSDS), an animal model of prolonged psychosocial stress, and neuroinflammation induced by CSDS.

Methods

In the CSDS paradigm, C57BL/6J mice were exposed to 10 min of social defeat stress from an aggressive CD-1 mouse for 10 consecutive days (days 1–10). Kososan extract (1.0 g/kg) was administered orally once daily for 12 days (days 1–12). On day 11, the social avoidance test was performed to examine depressive- and anxious-like behaviors. To characterize the impacts of kososan on neuroinflammation and adult neurogenesis, immunochemical analyses and ex vivo microglial stimulation assay with lipopolysaccharide (LPS) were performed on days 13–15.

Results

Oral administration of kososan extract alleviated social avoidance, depression- and anxiety-like behaviors, caused by CSDS exposure. CSDS exposure resulted in neuroinflammation, as indicated by the increased accumulation of microglia, the resident immune cells of the brain, and their activation in the hippocampus, which was reversed to normal levels by treatment with kososan extract. Additionally, in ex vivo studies, CSDS exposure potentiated the microglial pro-inflammatory response to a subsequent LPS challenge, an effect that was also blunted by kososan extract treatment. Indeed, the modulatory effect of kososan extract on neuroinflammation appears to be due to a hippocampal increase in an anti-inflammatory phenotype of microglia while sparing an increased pro-inflammatory phenotype of microglia caused by CSDS. Moreover, reduced adult hippocampal neurogenesis in defeated mice was recovered by kososan extract treatment.

Conclusions

Our findings suggest that kososan extract prevents a social avoidant behavior in socially defeated mice that is partially mediated by the downregulation of hippocampal neuroinflammation, presumably by the relative increased anti-inflammatory microglia and regulation of adult hippocampal neurogenesis. Our present study also provides novel evidence for the beneficial effects of kososan on depression/anxiety and the possible underlying mechanisms.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-017-0876-8) contains supplementary material, which is available to authorized users.

Exercise Influence on Hippocampal Function: Possible Involvement of Orexin-A

In the present article, we provide a brief review of current knowledge regarding the effects induced by physical exercise on hippocampus. Research involving animals and humans supports the view that physical exercise, enhancing hippocampal neurogenesis and function, improves cognition, and regulates mood. These beneficial effects depend on the contribute of more factors including the enhancement of vascularization and upregulation of growth factors. Among these, the BDNF seems to play a significant role. Another putative factor that might contribute to beneficial effects of exercise is the orexin-A. In support of this hypothesis there are the following observations: (1) orexin-A enhances hippocampal neurogenesis and function and (2) the levels of orexin-A increase with physical exercise. The beneficial effects of exercise may represent an important resource to hinder the cognitive decline associated with the aging-related hippocampal deterioration and ameliorate depressive symptoms.

Role of the Orexin System on the Hypothalamus-Pituitary-Thyroid Axis

Hypocretin/orexin (ORX) are two hypothalamic neuropeptides discovered in 1998. Since their discovery, they have been one of the most studied neuropeptide systems because of their projecting fields innervating various brain areas. The orexinergic system is tied to sleep-wakefulness cycle, and narcolepsy is a consequence of their system hypofunction. Orexinergic system is also involved in many other autonomic functions such as feeding, thermoregulation, cardiovascular and neuroendocrine regulation. The main aim of this mini review article is to investigate the relationship between ORX and thyroid system regulation. Although knowledge about the ORX system is evolving, its putative effects on hypothalamic-pituitary-thyroid (HPT) axis still appear unclear. We analyzed some studies about ORX control of HPT axis to know better the relationship between them. The studies that were analyzed suggest Hypocretin/ORX to modulate the thyroid regulation, but the nature (excitatory or inhibitory) of this possible interaction remains actually unclear and needs to be confirmed.

Orexin A-mediated AKT signaling in the dentate gyrus contributes to the acquisition, expression and reinstatement of morphine-induced conditioned place preference

Accumulating evidence indicates that the hippocampal dentate gyrus (DG), a critical brain region contributing to learning and memory, is involved in the addiction and relapse to abused drugs. Emerging studies also suggest the role of orexin signaling in the rewarding behavior induced by repeated exposure to opiates. In the present study, we investigated the dynamic adaptation of orexin signaling in the DG and its functional significance in the acquisition, expression, maintenance of and relapse to rewarding behavior induced by morphine. Repeated place conditioning with morphine significantly increased the orexin A content released from the lateral hypothalamic area projecting neurons into the DG. Local infusions of orexin A into the DG sensitized the acquisition of and relapse to the conditioned place preference induced by morphine. The application of the orexin receptor type 1 (OXR1) antagonist SB334867 significantly abolished the acquisition, expression and maintenance of the conditioned place preference induced by repeated exposure to morphine. Furthermore, the significant increase of the phosphorylation of AKT in the DG was associated with preference for the morphine-paired chamber in rats, which was reversed by the local administration of an OXR1 antagonist. Thus, these findings suggested that the dynamic upregulation of orexin A signaling, via the AKT pathway in the DG, may promote the acquisition and maintenance of opioid-induced craving behaviors and may increase sensitivity to the rewarding effect of subsequent opioids.

The Role of Hypothalamic Neuropeptides in Neurogenesis and Neuritogenesis

The hypothalamus is a source of neural progenitor cells which give rise to different populations of specialized and differentiated cells during brain development. Newly formed neurons in the hypothalamus can synthesize and release various neuropeptides. Although term neuropeptide recently undergoes redefinition, small-size hypothalamic neuropeptides remain major signaling molecules mediating short- and long-term effects on brain development. They represent important factors in neurite growth and formation of neural circuits. There is evidence suggesting that the newly generated hypothalamic neurons may be involved in regulation of metabolism, energy balance, body weight, and social behavior as well. Here we review recent data on the role of hypothalamic neuropeptides in adult neurogenesis and neuritogenesis with special emphasis on the development of food intake and social behavior related brain circuits.

Comprehensive Behavioral Analysis of Male Ox1r (-/-) Mice Showed Implication of Orexin Receptor-1 in Mood, Anxiety, and Social Behavior

Neuropeptides orexin A and orexin B, which are exclusively produced by neurons in the lateral hypothalamic area, play an important role in the regulation of a wide range of behaviors and homeostatic processes, including regulation of sleep/wakefulness states and energy homeostasis. The orexin system has close anatomical and functional relationships with systems that regulate the autonomic nervous system, emotion, mood, the reward system, and sleep/wakefulness states. Recent pharmacological studies using selective antagonists have suggested that orexin receptor-1 (OX1R) is involved in physiological processes that regulate emotion, the reward system, and autonomic nervous system. Here, we examined Ox1r (-/-) mice with a comprehensive behavioral test battery to screen additional OX1R functions. Ox1r (-/-) mice showed increased anxiety-like behavior, altered depression-like behavior, slightly decreased spontaneous locomotor activity, reduced social interaction, increased startle response, and decreased prepulse inhibition. These results suggest that OX1R plays roles in social behavior and sensory motor gating in addition to roles in mood and anxiety.

Understanding how discrete populations of hypothalamic neurons orchestrate complicated behavioral states

A major question in systems neuroscience is how a single population of neurons can interact with the rest of the brain to orchestrate complex behavioral states. The hypothalamus contains many such discrete neuronal populations that individually regulate arousal, feeding, and drinking. For example, hypothalamic neurons that express hypocretin (Hcrt) neuropeptides can sense homeostatic and metabolic factors affecting wakefulness and orchestrate organismal arousal. Neurons that express agouti-related protein (AgRP) can sense the metabolic needs of the body and orchestrate a state of hunger. The organum vasculosum of the lamina terminalis (OVLT) can detect the hypertonicity of blood and orchestrate a state of thirst. Each hypothalamic population is sufficient to generate complicated behavioral states through the combined efforts of distinct efferent projections. The principal challenge to understanding these brain systems is therefore to determine the individual roles of each downstream projection for each behavioral state. In recent years, the development and application of temporally precise, genetically encoded tools has greatly improved our understanding of the structure and function of these neural systems. This review will survey recent advances in our understanding of how these individual hypothalamic populations can orchestrate complicated behavioral states due to the combined efforts of individual downstream projections.