Prepro-orexin and orexin receptor mRNAs are differentially expressed in peripheral tissues of male and female rats

Gender medicine and sleep disorders: from basic science to clinical research

Several pivotal differences in sleep and sleep disorders are recognized between women and men. This is not only due to changes in hormonal balance during women's reproductive life, such as in pregnancy and menopause. Women are more likely to report insomnia and non-specific symptoms of apneas, such as fatigue or mood disturbance, compared to men. Thus, it is important for clinicians and researchers to take sex and gender differences into account when addressing sleep disorders in order to acknowledge the biology unique to women. We present a narrative review that delves into the primary sleep disorders, starting from basic science, to explore the impact of gender differences on sleep and the current status of research on women's sleep health.

A direct estrogenic involvement in the expression of human hypocretin

Hypocretin is synthesized exclusively in the hypothalamus and distributes inputs to several areas of the brain, which may play an important role in depression. Our previous study showed that hypocretin-1 was increased in the lateral hypothalamus in female patients with depression compared to female controls. Estrogen acts through estrogen receptor (ER)α and ERβ. We studied the possibility of a direct action of estrogen receptors on the expression of human hypocretin. We found that hypocretin-1 plasma levels were significantly higher in female patients with depression than in female controls. Female depression estrogen receptors and hypocretin are colocalized in the human lateral hypothalamus, PC12, and SK-N-SH cells. The estrogen receptor response elements (ERE) that exist in the hypocretin promoter region may directly regulate the gene expression of hypocretin. The synchronicity of change of hypocretin and estradiol both in hypothalamus and plasma was verified in female rats. In the presence of estradiol, specific binding occurs between the recombinant human ER and hypocretin-ERE. Expression of ER combined with estradiol repressed hypocretin promoter activity via the ERE. In conclusion, we found that estradiol may directly affect hypocretin neurons in the human hypothalamus via ER binding to the hypocretin-ERE, which may lead to the sex-specific pathogenesis of depression.

Orexins/Hypocretins: Gatekeepers of Social Interaction and Motivation

Ever since the discovery of the brain's orexin/hypocretin system, most research was directed toward unveiling its contribution to the normal functioning of individuals. The investigation of reward-seeking behaviors then gained a lot of attention once the distribution of orexinergic neurons was revealed. Here, we discuss findings on the involvement of orexins in social interaction, a natural reward type. While some studies have succeeded in defining the relationship between orexin and social interaction, the controversy regarding its nature (direct or inverse relation) raises questions about what aspects have been overlooked until now. Upon examining the literature, we identified a research gap concerning conditions influencing the impact of orexins on social behavior expression. In this review, we introduce a number of factors (e.g., stress, orexin's source) that must be considered while studying the role of orexins in social interaction. Furthermore, we refer to published research to investigate the stage at which orexins affect social interaction and we highlight the nucleus accumbens (NAc) shell's role in social interaction and other rewarding behaviors. Finally, the underlying orexin molecular pathway influencing social motivation in particular illnesses is proposed. We conclude that orexin's impact on social interaction is multifactorial and depends on specific conditions available at a time.

Targeting orexin receptors: Recent advances in the development of subtype selective or dual ligands for the treatment of neuropsychiatric disorders

Orexin-A and orexin-B, also named hypocretin-1 and hypocretin-2, are two hypothalamic neuropeptides highly conserved across mammalian species. Their effects are mediated by two distinct G protein-coupled receptors, namely orexin receptor type 1 (OX1-R) and type 2 (OX2-R), which share 64% amino acid identity. Given the wide expression of OX-Rs in different central nervous system and peripheral areas and the several pathophysiological functions in which they are involved, including sleep-wake cycle regulation (mainly mediated by OX2-R), emotion, panic-like behaviors, anxiety/stress, food intake, and energy homeostasis (mainly mediated by OX1-R), both subtypes represent targets of interest for many structure-activity relationship (SAR) campaigns carried out by pharmaceutical companies and academies. However, before 2017 the research was predominantly directed towards dual-orexin ligands, and limited chemotypes were investigated. Analytical characterizations, including resolved structures for both OX1-R and OX2-R in complex with agonists and antagonists, have improved the understanding of the molecular basis of receptor recognition and are assets for medicinal chemists in the design of subtype-selective ligands. This review is focused on the medicinal chemistry aspects of small molecules acting as dual or subtype selective OX1-R/OX2-R agonists and antagonists belonging to different chemotypes and developed in the last years, including radiolabeled OX-R ligands for molecular imaging. Moreover, the pharmacological effects of the most studied ligands in different neuropsychiatric diseases, such as sleep, mood, substance use, and eating disorders, as well as pain, have been discussed. Poly-pharmacology applications and multitarget ligands have also been considered.

Interaction of Orexin and Bone Morphogenetic Proteins in Steroidogenesis by Human Adrenocortical Cells

Orexins are neuropeptides that play important roles in sleep-wake regulation and food intake in the central nervous system, but their receptors are also expressed in peripheral tissues, including the endocrine system. In the present study, we investigated the functions of orexin in adrenal steroidogenesis using human adrenocortical H295R cells by focusing on its interaction with adrenocortical bone morphogenetic proteins (BMPs) that induce adrenocortical steroidogenesis. Treatment with orexin A increased the mRNA levels of steroidogenic enzymes including StAR, CYP11B2, CYP17, and HSD3B1, and these effects of orexin A were further enhanced in the presence of forskolin. Interestingly, orexin A treatment suppressed the BMP-receptor signaling detected by Smad1/5/9 phosphorylation and Id-1 expression through upregulation of inhibitory Smad7. Orexin A also suppressed endogenous BMP-6 expression but increased the expression of the type-II receptor of ActRII in H295R cells. Moreover, treatment with BMP-6 downregulated the mRNA level of OX1R, but not that of OX2R, expressed in H295R cells. In conclusion, the results indicate that both orexin and BMP-6 accelerate adrenocortical steroidogenesis in human adrenocortical cells; both pathways mutually inhibit each other, thereby leading to a fine-tuning of adrenocortical steroidogenesis.

Hormonal regulation of visfatin and adiponectin system in quail muscle cells

Visfatin and adiponectin are two adipokines known to regulate energy homeostasis and stress response within different peripheral tissues. Their role and regulation in highly metabolically active tissue such as the muscle is of particular interest. As modern poultry exhibit insulin resistance, obesity, and hyperglycemia along with a lack of insight into the regulation of these avian adipokines, we undertook the present work to determine the regulation of visfatin and adiponectin system by cytokines and obesity-related hormones in a relevant in vitro model of avian muscle, quail muscle (QM7) cells. Cells were treated with pro-inflammatory cytokine IL-6 (5 and 10 ng/mL) and TNFα (5 and 10 ng/mL), as well as leptin (10 and 100 ng/mL) and both orexin-A and orexin-B (ORX-A/B) (5 and 10 ng/mL). Results showed significant increases in visfatin mRNA abundance under both cytokines (IL-6 and TNFα), and down regulation with ORX-B treatment. Adiponectin expression was also upregulated by pro-inflammatory cytokines (IL-6 and TNFα), but down regulated by leptin, ORX-A, and ORXB. High doses of IL-6 and TNFα up regulated the expression of adiponectin receptors AdipoR1 and AdipoR2, respectively. Leptin and orexin treatments also down regulated both AdipoR1 and AdipoR2 expression. Taken together, this is the first report showing a direct response of visfatin and the adiponectin system to pro-inflammatory and obesity-related hormones in avian muscle cells.

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

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

Cellular Localization of Orexin 1 Receptor in Human Hypothalamus and Morphological Analysis of Neurons Expressing the Receptor

The orexin system is related to food behavior, energy balance, wakefulness and the reward system. It consists of the neuropeptides orexin A and B, and their receptors, orexin 1 receptor (OX1R) and orexin 2 receptor (OX2R). OX1R has selective affinity for orexin A, and is implicated in multiple functions, such as reward, emotions, and autonomic regulation. This study provides information about the OX1R distribution in human hypothalamus. The human hypothalamus, despite its small size, demonstrates a remarkable complexity in terms of cell populations and cellular morphology. Numerous studies have focused on various neurotransmitters and neuropeptides in the hypothalamus, both in animals and humans, however, there is limited experimental data on the morphological characteristics of neurons. The immunohistochemical analysis of the human hypothalamus revealed that OX1R is mainly found in the lateral hypothalamic area, the lateral preoptic nucleus, the supraoptic nucleus, the dorsomedial nucleus, the ventromedial nucleus, and the paraventricular nucleus. The rest of the hypothalamic nuclei do not express the receptor, except for a very low number of neurons in the mammillary bodies. After identifying the nuclei and neuronal groups that were immunopositive for OX1R, a morphological and morphometric analysis of those neurons was conducted using the Golgi method. The analysis revealed that the neurons in the lateral hypothalamic area were uniform in terms of their morphological characteristics, often forming small groups of three to four neurons. A high proportion of neurons in this area (over 80%) expressed the OX1R, with particularly high expression in the lateral tuberal nucleus (over 95% of neurons). These results were analyzed, and shown to represent, at the cellular level, the distribution of OX1R, and we discuss the regulatory role of orexin A in the intra-hypothalamic areas, such as its special role in the plasticity of neurons, as well as in neuronal networks of the human hypothalamus.

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

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

Blockade of the orexin 1-receptors and 'cocaine- and amphetamine-regulated transcript' in the nucleus accumbens shell alters impulse control in rats

Impulsivity is a multifaceted construct and alterations in impulsiveness are often associated with psychiatric diseases, including drug addiction and binge eating disorder. Impulse control involves several brain regions. The present study assessed the role of the orexigenic, appetite stimulating neuropeptide orexin (OX) and the anorexigenic, appetite reducing neuropeptide cocaine- and amphetamine-regulated transcript (CART) within the nucleus accumbens shell (NAcSh) in impulse control in rats. The animals were ranked for their trait impulsivity based on a screening in the 5-choice serial reaction time task (5-CSRTT). The rats' performances were analysed after bilateral infusions of the OX 1-receptor antagonist SB-334867 (SB) and CART-antibodies (CART-ABs) into the NAcSh. After SB infusions, there was no change in premature responses observed on average. Further analysis revealed a negative linear correlation between the effect of intra-NAcSh SB infusions on premature responses and trait impulsivity. The effect of SB ranged from an increase, no change to a decrease in premature responses in the individual animals with increasing trait impulsivity. Infusions of CART-ABs led to consistently enhanced impulse control with fewer irrelevant actions, independent of trait impulsivity. These data suggest that both OX, especially OX A, and CART in the NAcSh, can be considered endogenous regulators of impulsive action, dependent on underlying impulsivity in the case of OX and independent from trait impulsivity in the case of CART.

Inhibition of the retinal orexin receptors affects the hypothalamic-pituitary-gonadal axis through retinal pituitary adenylate cyclase activating polypeptide (PACAP) in male Wistar rats

Orexins A and B (OXA and OXB) and their receptors are expressed in the retina of both human and rodents and play a vital role in regulating signal transmission circuits in the retina. There is an anatomical-physiological relationship between the retinal ganglion cells and suprachiasmatic nucleus (SCN) through glutamate as a neurotransmitter and retinal pituitary adenylate cyclase-activating polypeptide (PACAP) as a co-transmitter. SCN is the main brain center for regulating the circadian rhythm, which governs the reproductive axis. The impact of retinal orexin receptors on the hypothalamic-pituitary-gonadal axis has not been investigated. Retinal OX1R or/and OX2R in adult male rats by 3 µl of SB-334867 (1 µg) or/and 3 µl of JNJ-10397049 (2 µg) were antagonized via intravitreal injection (IVI). Four time-periods were considered (3, 6, 12, and 24 h) for the controls without any treatment, SB-334867, JNJ-10397049, and SB-334867 + JNJ-10397049 groups. Antagonizing retinal OX1R or/and OX2R resulted in a significant elevation of retinal PACAP expression compared to control animals. In addition, expression of GnRH increased non-significantly in the hypothalamus over the 6 h of the study, and the serum concentration of LH decreased significantly in the SB-334867 group after 3 h of injection. Furthermore, testosterone serum levels declined significantly, especially within 3 h of injection; serum levels of progesterone were also exposed to a significant rise at least within 3 h of injection. However, the retinal PACAP expression changes were mediated by OX1R more effectively than by OX2R. In this study, we report the retinal orexins and their receptors as light-independent factors by which the retina affects the hypothalamic-pituitary-gonadal axis.

Gender differences in narcolepsy: What are recent findings telling us?

Three papers currently published in SLEEP using two different mouse models of narcolepsy, including either Hcrt-tTa;TetO diptheria toxin-A (DTA) or Hypocretin knock-out (Hcrt-KO) mice, suggest important gender differences in narcolepsy expression. Specifically, these recent data corroborate previous findings in mice demonstrating that females show more cataplexy events and more total cataplexy expression than males. Moreover, in the neurotoxic DTA mouse model, females show earlier onset of cataplexy expression than males during active Hcrt cell loss. Finally, females show a doubling of cataplexy during estrous compared to other phases of the estrous cycle. These findings are reviewed in the broader context of prior published literature, including reported gender differences in Hcrt expression and hormonal influences on sleep and wakefulness. Although similar findings have not been reported in humans, a systematic evaluation of gender differences in human narcolepsy has yet to be performed. Taken together, these animal data suggest that more research exploring gender differences in human narcolepsy is warranted.

Orexin/hypocretin system dysfunction in patients with Takotsubo syndrome: A novel pathophysiological explanation

Takotsubo syndrome (TTS) is an acute heart failure syndrome. Emotional or physical stressors are believed to precipitate TTS, while the pathophysiological mechanism is not yet completely understood. During the coronavirus disease (COVID-19) pandemic, an increased incidence of TTS has been reported in some countries; however, the precise pathophysiological mechanism for developing TTS with acute COVID-19 infection is unknown. Nevertheless, observing the symptoms of COVID-19 might lead to new perspectives in understanding TTS pathophysiology, as some of the symptoms of the COVID-19 infection could be assessed in the context of an orexin/hypocretin-system dysfunction. Orexin/hypocretin is a cardiorespiratory neuromodulator that acts on two orexin receptors widely distributed in the brain and peripheral tissues. In COVID-19 patients, autoantibodies against one of these orexin receptors have been reported. Orexin-system dysfunction affects a variety of systems in an organism. Here, we review the influence of orexin-system dysfunction on the cardiovascular system to propose its connection with TTS. We propose that orexin-system dysfunction is a potential novel explanation for the pathophysiology of TTS due to direct or indirect dynamics of orexin signaling, which could influence cardiac contractility. This is in line with the conceptualization of TTS as a cardiovascular syndrome rather than merely a cardiac abnormality or cardiomyopathy. To the best of our knowledge, this is the first publication to present a plausible connection between TTS and orexin-system dysfunction. We hope that this novel hypothesis will inspire comprehensive studies regarding orexin's role in TTS pathophysiology. Furthermore, confirmation of this plausible pathophysiological mechanism could contribute to the development of orexin-based therapeutics in the treatment and prevention of TTS.

Plasticity of the hypocretinergic/orexinergic system after a chronic treatment with suvorexant in rats. Role of the hypocretinergic/orexinergic receptor 1 as an autoreceptor

The hypothalamic hypocretinergic/orexinergic (Hcrt/Ox) system is involved in many physiological and pathophysiological processes. Malfunction of Hcrt/Ox transmission results in narcolepsy, a sleep disease caused in humans by progressive neurodegeneration of hypothalamic neurons containing Hcrt/Ox. To explore the Hcrt/Ox system plasticity we systemically administered suvorexant (a dual Hcrt/Ox receptor antagonist) in rats to chronically block Hcrt/Ox transmission without damaging Hcrt/Ox cells. Three groups of eight rats (four males and four females) received daily i.p. injections of suvorexant (10 or 30 mg/kg) or vehicle (DMSO) over a period of 7 days in which the body weight was monitored. After the treatments cerebrospinal fluid (CSF) Hcrt1/OxA concentration was measured by ELISA, and hypothalamic Hcrt/OxR1 and Hcrt/OxR2 levels by western blot. The systemic blockade of the Hcrt/Ox transmission with the suvorexant high dose produced a significant increase in body weight at the end of the treatment, and a significant decrease in CSF Hcrt1/OxA levels, both features typical in human narcolepsy type 1. Besides, a significant overexpression of hypothalamic Hcrt/OxR1 occurred. For the Hcrt/OxR2 two very close bands were detected, but they did not show significant changes with the treatment. Thus, the plastic changes observed in the Hcrt/Ox system after the chronic blockade of its transmission were a decrease in CSF Hcrt1/OXA levels and an overexpression of hypothalamic Hcrt/OxR1. These findings support an autoregulatory role of Hcrt/OxR1 within the hypothalamus, which would induce the synthesis/release of Hcrt/Ox, but also decrease its own availability at the plasma membrane after binding Hcrt1/OxA to preserve Hcrt/Ox system homeostasis.

Orexin 2 receptor (OX2R) protein distribution measured by autoradiography using radiolabeled OX2R-selective antagonist EMPA in rodent brain and peripheral tissues

Orexin, a neuropeptide, performs various physiological functions, including the regulation of emotion, feeding, metabolism, respiration, and sleep/wakefulness, by activating the orexin 1 receptor and orexin 2 receptor (OX2R). Owing to the pivotal role of OX2R in wakefulness and other biological functions, OX2R agonists are being developed. A detailed understanding of OX2R protein distribution is essential for determining the mechanisms of action of OX2R agonists; however, this has been hindered by the lack of selective antibodies. In this study, we first confirmed the OX2R-selective binding of [³H]-EMPA in in vitro autoradiography studies, using brain slices from OX2R knockout mice and their wild-type littermates. Subsequently, OX2R protein distribution in rats was comprehensively assessed in 51 brain regions and 10 peripheral tissues using in vitro autoradiography with [³H]-EMPA. The widespread distribution of OX2R protein, including that in previously unrecognized regions of the retrosplenial cortex, was identified. In contrast, OX2R protein expression was negligible/very low in peripheral tissues, suggesting that orexin exerts OX2R-dependent physiological functions primarily through activation of the central nervous system. These findings will be useful for understanding the wide range of biological functions of OX2R and the application of OX2R agonists in various disorders.

Orexin Signaling: A Complex, Multifaceted Process

The orexin system comprises two G protein-coupled receptors, OX₁ and OX₂ receptors (OX₁R and OX₂R, respectively), along with two endogenous agonists cleaved from a common precursor (prepro-orexin), orexin-A (OX-A) and orexin-B (OX-B). For the receptors, a complex array of signaling behaviors has been reported. In particular, it becomes obvious that orexin receptor coupling is very diverse and can be tissue-, cell- and context-dependent. Here, the early signal transduction interactions of the orexin receptors will be discussed in depth, with particular emphasis on the direct G protein interactions of each receptor. In doing so, it is evident that ligands, additional receptor-protein interactions and cellular environment all play important roles in the G protein coupling profiles of the orexin receptors. This has potential implications for our understanding of the orexin system’s function in vivo in both central and peripheral environments, as well as the development of novel agonists, antagonists and possibly allosteric modulators targeting the orexin system.

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.

New Aspects of Corpus Luteum Regulation in Physiological and Pathological Conditions: Involvement of Adipokines and Neuropeptides

The corpus luteum is a small gland of great importance because its proper functioning determines not only the appropriate course of the estrous/menstrual cycle and embryo implantation, but also the subsequent maintenance of pregnancy. Among the well-known regulators of luteal tissue functions, increasing attention is focused on the role of neuropeptides and adipose tissue hormones—adipokines. Growing evidence points to the expression of these factors in the corpus luteum of women and different animal species, and their involvement in corpus luteum formation, endocrine function, angiogenesis, cells proliferation, apoptosis, and finally, regression. In the present review, we summarize the current knowledge about the expression and role of adipokines, such as adiponectin, leptin, apelin, vaspin, visfatin, chemerin, and neuropeptides like ghrelin, orexins, kisspeptin, and phoenixin in the physiological regulation of the corpus luteum function, as well as their potential involvement in pathologies affecting the luteal cells that disrupt the estrous cycle.

Understanding the Role of Orexin Neuropeptides in Drug Addiction: Preclinical Studies and Translational Value

Orexins (also known as hypocretins) are neuropeptides that participate in the regulation of energy metabolism, homeostasis, sleep, feeding, stress responses, arousal, and reward. Particularly relevant to the scope of the present review is the involvement of the orexin system in brain mechanisms that regulate motivation, especially highly motivated behavior, arousal, and stress, making it an ideal target for studying addiction and discovering treatments. Drug abuse and misuse are thought to induce maladaptive changes in the orexin system, and these changes might promote and maintain uncontrolled drug intake and contribute to relapse. Dysfunctional changes in this neuropeptidergic system that are caused by drug use might also be responsible for alterations of feeding behavior and the sleep-wake cycle that are commonly disrupted in subjects with substance use disorder. Drug addiction has often been associated with an increase in activity of the orexin system, suggesting that orexin receptor antagonists may be a promising pharmacological treatment for substance use disorder. Substantial evidence has shown that single orexin receptor antagonists that are specific to either orexin receptor 1 or 2 can be beneficial against drug intake and relapse. Interest in the efficacy of dual orexin receptor antagonists, which were primarily developed to treat insomnia, has grown in the field of drug addiction. Treatments that target the orexin system may be a promising strategy to reduce drug intake, mitigate relapse vulnerability, and restore “normal” physiological functions, including feeding and sleep. The present review discusses preclinical and clinical evidence of the involvement of orexins in drug addiction and possible beneficial pharmacotherapeutic effects of orexin receptor antagonists to treat substance use disorder.

Orexin/Hypocretin System Dysfunction in ESSENCE (Early Symptomatic Syndromes Eliciting Neurodevelopmental Clinical Examinations)

Early Symptomatic Syndromes Eliciting Neurodevelopmental Clinical Examinations (ESSENCE) is an umbrella term covering a wide range of neurodevelopmental difficulties and disorders. Thus, ESSENCE includes attention-deficit/hyperactivity disorder (ADHD), autism spectrum disorder (ASD), and other neurodevelopmental disorders (NDDs) and difficulties, with a variety of symptoms in cognitive, motor, sensory, social, arousal, regulatory, emotional, and behavioral developmental domains, frequently co-occurring and likely having partly common neurobiological substrates. The ESSENCE concept is a clinical paradigm that promotes organizing NDDs in everyday clinical practice according to their coexistence, symptom dimensions overlapping, and treatment possibilities. Despite increased knowledge regarding NDDs, the neurobiological mechanisms that underlie them and other ESSENCE-related problems, are not well understood. With its wide range of neural circuits and interactions with numerous neurotransmitters, the orexin/hypocretin system (Orx-S) is possibly associated with a variety of neurocognitive, psychobiological, neuroendocrine, and physiological functions and behaviors. Dysfunction of Orx-S has been implicated in various psychiatric and neurological disorders. This article provides an overview of Orx-S dysfunctions' possible involvement in the development, presentation, and maintenance of ESSENCE. We provide a focused review of current research evidence linking orexin neuropeptides with specific clinical NDDs symptoms, mostly in ADHD and ASD, within the Research Domain Criteria (RDoC) framework. We propose that Orx-S dysfunction might have an important role in some of these neurodevelopmental symptom domains, such as arousal, wakefulness, sleep, motor and sensory processing, mood and emotional regulation, fear processing, reward, feeding, attention, executive functions, and sociability. Our perspective is presented from a clinical point of view. Further, more thorough systematic reviews are needed as well as planning of extensive new research into the Orx-S's role in ESSENCE, especially considering RDoC elements.

An overview of the orexinergic system in different animal species

Orexin (hypocretin), is a neuropeptide produced by a subset of neurons in the lateral hypothalamus. From the lateral hypothalamus, the orexin-containing neurons project their fibres extensively to other brain structures, and the spinal cord constituting the central orexinergic system. Generally, the term ''orexinergic system'' usually refers to the orexin peptides and their receptors, as well as to the orexin neurons and their projections to different parts of the central nervous system. The extensive networks of orexin axonal fibres and their terminals allow these neuropeptidergic neurons to exert great influence on their target regions. The hypothalamic neurons containing the orexin neuropeptides have been implicated in diverse functions, especially related to the control of a variety of homeostatic functions including feeding behaviour, arousal, wakefulness stability and energy expenditure. The broad range of functions regulated by the orexinergic system has led to its description as ''physiological integrator''. In the last two decades, the orexinergic system has been a topic of great interest to the scientific community with many reports in the public domain. From the documentations, variations exist in the neuroanatomical profile of the orexinergic neuron soma, fibres and their receptors from animal to animal. Hence, this review highlights the distinct variabilities in the morphophysiological aspects of the orexinergic system in the vertebrate animals, mammals and non-mammals, its presence in other brain-related structures, including its involvement in ageing and neurodegenerative diseases. The presence of the neuropeptide in the cerebrospinal fluid and peripheral tissues, as well as its alteration in different animal models and conditions are also reviewed.

Elevated HB-EGF expression in neural stem cells causes middle age obesity by suppressing Hypocretin/Orexin expression

Obesity is common in the middle aged population and it increases the risks of diabetes, cardiovascular diseases, certain cancers, and dementia. Yet, its etiology remains incompletely understood. Here, we show that ectopic expression of HB-EGF, an important regulator of neurogenesis, in Nestin⁺ neuroepithelial progenitors with the Cre-LoxP system leads to development of spontaneous middle age obesity in male mice accompanied by hyperglycemia and insulin resistance. The Nestin-HB-EGF mice show decreases in food uptake, energy expenditure, and physical activity, suggesting that reduced energy expenditure underlies the pathogenesis of this obesity model. However, HB-EGF expression in appetite-controlling POMC or AgRP neurons or adipocytes fails to induce obesity. Mechanistically, HB-EGF suppresses expression of Hypocretin/Orexin, an orexigenic neuropeptide hormone, in the hypothalamus of middle aged Nestin-HB-EGF mice. Hypothalamus Orexin administration alleviates the obese and hyperglycemic phenotypes in Nestin-HB-EGF mice. This study uncovers an important role for HB-EGF in regulating Orexin expression and energy expenditure and establishes a midlife obesity model whose pathogenesis involves age-dependent changes in hypothalamus neurons.

Emerging therapeutic targets for narcolepsy

INTRODUCTION

Narcolepsy type 1 (NT1) and type 2 (NT2) are chronic sleep disorders primarily characterized by excessive daytime sleepiness (EDS), disturbed sleep-wake regulation, and reduced quality of life. The precise disease mechanism is unclear, but it is certain that in NT1 the hypocretin/orexin (Hcrt) system is affected. Current treatment options are symptomatic - they improve EDS and/or reduce cataplexy. Complete symptom control is relatively rare - particularly problematic is residual daytime sleepiness.

AREAS COVERED

This review discusses various emerging treatment targets for narcolepsy. The focus is on the Hcrt receptors but included are also wake-promoting pathways, and sleep-stabilization through GABAergic mechanisms. Additionally, we discuss the potential of targeting the likely autoimmune basis of narcolepsy. PubMed and ClinicalTrials.gov was searched through June 2021 for relevant information.

EXPERT OPINION

Targeting Hcrt receptors has the potential to alleviate narcolepsy symptoms. Results from ongoing drug development programs are promising, but care needs to be taken when evaluating potential side effects. It is still largely unknown what roles Hcrt receptors play in the periphery and how these might be affected by treatment. Immunotherapies could potentially target the core pathophysiology of narcolepsy, but more work is needed to identify the best therapeutic target for this approach.

Effects of Orexin B on Swine Granulosa and Endothelial Cells

Simple Summary

The follicle is the ovarian functional unit. It is mainly composed of granulosa cells and angiogenesis is crucial to guarantee its development till ovulation. Carrying on our previous studies on the orexin system in the ovary, we presently demonstrate a potential role of orexin B in the control of granulosa cells’ oxidative stress and of the angiogenesis event.

Abstract

In addition to the well-known central modulatory role of orexins, we recently demonstrated a peripheral involvement in swine granulosa cells for orexin A and in adipose tissue for orexin B (OXB). The aim of present research was to verify immunolocalization of OXB and its potential role in modulating the main features of swine granulosa cells. In particular, we explored the effects on granulosa cell proliferation (through the incorporation of bromodeoxyuridine), cell metabolic activity (as indirect evaluation by the assessment of ATP), steroidogenic activity (by immunoenzymatic examination) and redox status (evaluating the production of superoxide anion by means of the WST test, production of nitric oxide through the use of the Griess test and the non-enzymatic reducing power by FRAP test). Our data point out that OXB does not modify granulosa cell growth, steroidogenesis and superoxide anion generation. On the contrary, the peptide stimulates (p < 0.05) nitric oxide output and non-enzymatic reducing power. Since new vessel growth is crucial for ovarian follicle development, a further aim of this study was to explore the expression of prepro-orexin and the effects of OXB on swine aortic endothelial cells. We found that the peptide is ineffective in modulating cell growth, while it inhibits redox status parameters. In addition, we demonstrated a stimulatory effect on angiogenesis evaluated in fibrin gel angiogenesis assay. Taken together, OXB appears to be potentially involved in the modulation of redox status in granulosa and endothelial cells and we could argue an involvement of the peptide in the follicular angiogenic events.

Intranasal insulin and orexins to treat age-related cognitive decline

The intranasal (IN) administration of neuropeptides, such as insulin and orexins, has been suggested as a treatment strategy for age-related cognitive decline (ARCD). Because dysfunctional neuropeptide signaling is an observed characteristic of ARCD, it has been suggested that IN delivery of insulin and/or orexins may restore endogenous peptide signaling and thereby preserve cognition. IN administration is particularly alluring as it is a relatively non-invasive method that directly targets peptides to the brain. Several laboratories have examined the behavioral effects of IN insulin in young, aged, and cognitively impaired rodents and humans. These studies demonstrated improved performance on various cognitive tasks following IN insulin administration. Fewer laboratories have assessed the effects of IN orexins; however, this peptide also holds promise as an effective treatment for ARCD through the activation of the cholinergic system and/or the reduction of neuroinflammation. Here, we provide a brief overview of the advantages of IN administration and the delivery pathway, then summarize the current literature on IN insulin and orexins. Additional preclinical studies will be useful to ultimately uncover the mechanisms underlying the pro-cognitive effects of IN insulin and orexins, whereas future clinical studies will aid in the determination of the most efficacious dose and dosing paradigm. Eventually, IN insulin and/or orexin administration may be a widely used treatment strategy in the clinic for ARCD.

Pre-treatment of blood samples reveal normal blood hypocretin/orexin signal in narcolepsy type 1

The hypocretin/orexin system regulates arousal through central nervous system mechanisms and plays an important role in sleep, wakefulness and energy homeostasis. It is unclear whether hypocretin peptides are also present in blood due to difficulties in measuring reliable and reproducible levels of the peptides in blood samples. Lack of hypocretin signalling causes the sleep disorder narcolepsy type 1, and low concentration of cerebrospinal fluid hypocretin-1/orexin-A peptide is a hallmark of the disease. This measurement has high diagnostic value, but performing a lumbar puncture is not without discomfort and possible complications for the patient. A blood-based test to assess hypocretin-1 deficiency would therefore be of obvious benefit. We here demonstrate that heating plasma or serum samples to 65°C for 30 min at pH 8 significantly increases hypocretin-1 immunoreactivity enabling stable and reproducible measurement of hypocretin-1 in blood samples. Specificity of the signal was verified by high-performance liquid chromatography and by measuring blood samples from mice lacking hypocretin. Unspecific background signal in the assay was high. Using our method, we show that hypocretin-1 immunoreactivity in blood samples from narcolepsy type 1 patients does not differ from the levels detected in control samples. The data presented here suggest that hypocretin-1 is present in the blood stream in the low picograms per millilitres range and that peripheral hypocretin-1 concentrations are unchanged in narcolepsy type 1.

Hypocretin (Orexin) Replacement Therapies

Twenty-two years after their discovery, the hypocretins (Hcrts), also known as orexins, are two of the most studied peptidergic systems, involved in myriad physiological systems that range from sleep, arousal, motivation, homeostatic regulation, fear, anxiety and learning. A causal relationship between activity of Hcrt and arousal stability was established shortly after their discovery and have led to the development of a new class of drugs to treat insomnia. In this review we discuss the many faces of the Hcrt system and examine recent findings that implicate decreased Hcrt function in the pathogenesis of a number of neuropsychiatric conditions. We also discuss future therapeutic strategies to replace or enhance Hcrt function as a treatment option for these neuropsychiatric conditions.

Sex- and Brain Region-specific Changes in Gene Expression in Male and Female Rats as Consequences of Methamphetamine Self-administration and Abstinence

Sex differences in METH use exist among human METH users and in animal models of METH addiction. Herein, we tried to identify potential differences in gene expression between female and male rats after Methamphetamine self-administration (METH SA). Rats were trained to self-administer METH using two 3-hours daily sessions for 20 days. Cue-induced drug seeking was measured on withdrawal days 3 (WD3) and 30 (WD30). Rats were euthanized twenty-four hours after WD30. Prefrontal cortex (PFC) and hippocampus (HIP) were dissected to measure mRNA expression. Both female and male rats increased their METH intake and showed increased METH seeking during withdrawal. Female had higher basal level expression of hypocretin receptor 1 (Hcrtr1) and prodynorphin (Pdyn) mRNAs in the PFC and HIP. Basal corticotropin releasing hormone receptor 1 (Crhr1), Crh receptor 2 (Crhr2), hypocretin receptor 2 (Hcrtr2) and opioid receptor kappa 1 (Oprk1) mRNA levels were higher in the PFC of females. Male rats had higher basal levels of Crh and Crhr1 in HIP. METH SA was associated with increased Crh and Crhr1 in the HIP of both sexes and Crhr2 only in female HIP. Importantly, increased Crh and Crhr1 mRNA levels correlated positively with incubation of METH craving in both sexes, supporting their potential involvement, in part, in the regulation of this behavioral phenomenon. When taken together, our results identified sexual dimorphic baseline differences in rats. We also detected dimorphic responses in animals that had self-administered METH. These observations highlight the importance of understanding the molecular neurobiology of sex differences when therapeutic interventions are planned against METH addiction.

Blockade of Orexin Receptors in the Posterior Paraventricular Nucleus of the Thalamus Prevents Stress-Induced Reinstatement of Reward-Seeking Behavior in Rats With a History of Ethanol Dependence

Neural systems involved in processing natural rewards and drugs of abuse overlap and exposure to drugs of abuse induce neuroadaptations that can cause compulsive-like behavior. For example, the recruitment of the orexin (Orx) system by drugs of abuse has been proposed to induce neuroadaptations that in turn alter its function, reflected by maladaptive, compulsive, and addictive behavior. Orexin neurons project to the paraventricular nucleus of the thalamus (PVT)—particularly the posterior part (pPVT), a structure that plays a key role in stress regulation. This study investigated whether Orx transmission in the pPVT plays a role in stress-induced reinstatement of reward-seeking behavior toward ethanol (EtOH) and a highly palatable food reward [sweetened condensed milk (SCM)] in rats and whether this role changes with EtOH dependence. After being trained to orally self-administer EtOH or SCM, the rats were made dependent (EtOH_D and SCM_D) by chronic intermittent EtOH vapor exposure. The control nondependent groups (EtOH_ND and SCM_ND) were exposed to air. Following extinction, the rats were tested for stress-induced reinstatement of EtOH- and SCM-seeking behavior. Stress reinstated EtOH- and SCM-seeking behavior in all groups (EtOH_D/ND and SCM_D/ND). Administration of the dual Orx receptor (OrxR) antagonist TCS1102 (15 μg) in the pPVT prevented stress-induced reinstatement only in dependent rats (EtOH_D and SCM_D). In parallel, the qPCR analysis showed that Orx mRNA expression in the hypothalamus and OrxR1/R2 mRNA expression in the pPVT were increased at the time of testing in the EtOH_D and SCM_D groups. These results are the first to implicate Orx transmission in the pPVT in the stress-induced reinstatement of reward-seeking behavior in EtOH dependent rats and indicate the maladaptive recruitment of Orx transmission in the pPVT by EtOH dependence.

Associations of plasma hypocretin-1 with metabolic and reproductive health: Two systematic reviews of clinical studies

The hypocretin system consists of two peptides hypocretin-1 and hypocretin-2 (HCRT1 and HCRT2). Hypocretin-containing neurons are located in the posterior and lateral hypothalamus, and have widespread projections throughout the brain and spinal cord. In addition to its presence in the cerebrospinal fluid (CSF), peripheral HCRT1 has been detected in plasma. Robust experimental evidence demonstrates functions of hypothalamic-originated HCRT1 in regulation of multiple biological systems related to sleep-wake states, energy homeostasis and endocrine function. In contrast, HCRT1 studies with human participants are limited by the necessarily invasive assessment of CSF HCRT1 to patients with underlying morbidity. Regulation by HCRT1 of energy homeostasis and reproduction in animals suggests similar regulation in humans and prompts these two systematic reviews. These reviews translate prior experimental findings from animal studies to humans and examine associations between HCRT1 and: 1) metabolic risk factors; 2) reproductive function in men, women and children. A total of 21 studies and six studies met the inclusion criteria for the two searches, respectively. Research question, study design, study population, assessments of HCRT1, reproductive, cardiometabolic data and main findings were extracted. Associations between HCRT1, metabolic and reproductive function are inconsistent. Limitations of studies and future research directions are outlined.

Neurophysiology and Treatment of Disorders of Consciousness Induced by Traumatic Brain Injury: Orexin Signaling as a Potential Therapeutic Target

BACKGROUND

Traumatic brain injury (TBI) can cause disorders of consciousness (DOC) by impairing the neuronal circuits of the ascending reticular activating system (ARAS) structures, including the hypothalamus, which are responsible for the maintenance of the wakefulness and awareness. However, the effectiveness of drugs targeting ARAS activation is still inadequate, and novel therapeutic modalities are urgently needed.

METHODS

The goal of this work is to describe the neural loops of wakefulness, and explain how these elements participate in DOC, with emphasis on the identification of potential new therapeutic options for DOC induced by TBI.

RESULTS

Hypothalamus has been identified as a sleep/wake center, and its anterior and posterior regions have diverse roles in the regulation of the sleep/wake function. In particular, the posterior hypothalamus (PH) possesses several types of neurons, including the orexin neurons in the lateral hypothalamus (LH) with widespread projections to other wakefulness-related regions of the brain. Orexins have been known to affect feeding and appetite, and recently their profound effect on sleep disorders and DOC has been identified. Orexin antagonists are used for the treatment of insomnia, and orexin agonists can be used for narcolepsy. Additionally, several studies demonstrated that the agonists of orexin might be effective in the treatment of DOC, providing novel therapeutic opportunities in this field.

CONCLUSION

The hypothalamic-centered orexin has been adopted as the point of entry into the system of consciousness control, and modulators of orexin signaling opened several therapeutic opportunities for the treatment of DOC.

The hypocretin (orexin) system: from a neural circuitry perspective

Hypocretin/orexin neurons are distributed restrictively in the hypothalamus, a brain region known to orchestrate diverse functions including sleep, reward processing, food intake, thermogenesis, and mood. Since the hypocretins/orexins were discovered more than two decades ago, extensive studies have accumulated concrete evidence showing the pivotal role of hypocretin/orexin in diverse neural modulation. New method of viral-mediated tracing system offers the possibility to map the monosynaptic inputs and detailed anatomical connectivity of Hcrt neurons. With the development of powerful research techniques including optogenetics, fiber-photometry, cell-type/pathway specific manipulation and neuronal activity monitoring, as well as single-cell RNA sequencing, the details of how hypocretinergic system execute functional modulation of various behaviors are coming to light. In this review, we focus on the function of neural pathways from hypocretin neurons to target brain regions. Anatomical and functional inputs to hypocretin neurons are also discussed. We further briefly summarize the development of pharmaceutical compounds targeting hypocretin signaling. This article is part of the special issue on Neuropeptides.

Physiological Implications of Orexins/Hypocretins on Energy Metabolism and Adipose Tissue Development

Orexins/hypocretins and their receptors (OXRs) are ubiquitously distributed throughout the nervous system and peripheral tissues. Recently, various reports have indicated that orexins play regulatory roles in numerous physiological processes involved in obesity, energy homeostasis, sleep-wake cycle, analgesia, alcoholism, learning, and memory. This review aims to outline recent progress in the research and development of orexins used in biochemical signaling pathways, secretion pathways, and the regulation of energy metabolism/adipose tissue development. Orexins regulate a variety of physiological functions in the body by activating phospholipase C/protein kinase C and AC/cAMP/PKA pathways, through receptors coupled to Gq and Gi/Gs, respectively. The secretion of orexins is modulated by blood glucose, blood lipids, hormones, and neuropeptides. Orexins have critical functions in energy metabolism, regulating both feeding behavior and energy expenditure. Increasing the sensitivity of orexin-coupled hypothalamic neurons concurrently enhances spontaneous physical activity, non-exercise activity thermogenesis, white adipose tissue lipolysis, and brown adipose tissue thermogenesis. With this comprehensive review of the current literature on the subject, we hope to provide an integrated perspective for the prevention/treatment of obesity.

Molecular mechanism of tumour necrosis factor alpha regulates hypocretin (orexin) expression, sleep and behaviour

Hypocretin 1 and hypocretin 2 (orexin A and B) regulate sleep, wakefulness and emotion. Tumour necrosis factor alpha (TNF-α) is an important neuroinflammation mediator. Here, we examined the effects of TNF-α treatment on hypocretin expression in vivo and behaviour in mice. TNF-α decreased hypocretin 1 and hypocretin 2 expression in a dose-dependent manner in cultured hypothalamic neurons. TNF-α decreased mRNA stability of prepro-hypocretin, the single precursor of hypocretin 1 and hypocretin 2. Mice challenged with TNF-α demonstrated decreased expression of prepro-hypocretin, hypocretin 1 and hypocretin 2 in hypothalamus. In response to TNF-α, prepro-hypocretin mRNA decay was increased in hypothalamus. TNF-α neutralizing antibody restored the expression of prepro-hypocretin, hypocretin 1 and hypocretin 2 in vivo in TNF-α challenged mice, supporting hypocretin system can be impaired by increased TNF-α through decreasing hypocretin expression. Repeated TNF-α challenge induced muscle activity during rapid eye movement sleep and sleep fragmentation, but decreased learning, cognition and memory in mice. TNF-α neutralizing antibody blocked the effects of TNF-α; in contrast, hypocretin receptor antagonist enhanced the effects of TNF-α. The data support that TNF-α is involved in the regulation of hypocretin expression, sleep and cognition. The findings shed some lights on the role of neuroinflammation in neurodegenerative diseases including Alzheimer's disease and Parkinson's disease.

Neurophysiological and Behavioral Effects of Anti-Orexinergic Treatments in a Mouse Model of Huntington's Disease

Huntington's disease (HD) is associated with sleep and circadian disturbances in addition to hallmark motor and cognitive impairments. Electrophysiological studies on HD mouse models have revealed an aberrant oscillatory activity at the beta frequency, during sleep, that is associated with HD pathology. Moreover, HD animal models display an abnormal sleep-wake cycle and sleep fragmentation. In this study, we investigated a potential involvement of the orexinergic system dysfunctioning in sleep-wake and circadian disturbances and abnormal network (i.e., beta) activity in the R6/1 mouse model. We found that the age at which orexin activity starts to deviate from normal activity pattern coincides with that of sleep disturbances as well as the beta activity. We also found that acute administration of Suvorexant, an orexin 1 and orexin 2 receptor antagonist, was sufficient to decrease the beta power significantly and to improve sleep in R6/1 mice. In addition, a 5-day treatment paradigm alleviated cognitive deficits and induced a gain of body weight in female HD mice. These results suggest that restoring normal activity of the orexinergic system could be an efficient therapeutic solution for sleep and behavioral disturbances in HD.

Orexin A modulates prolactin production by regulating BMP-4 activity in rat pituitary lactotorope cells

The impact of orexins on anterior pituitary function has yet to be clarified. We studied the effects of orexin A and its interaction with the bone morphogenetic protein (BMP) system on the regulatory role of prolactin synthesis using rat lactotrope GH3 cells expressing BMP-4. Orexin type 1 receptor (OX1R), but not type 2 receptor (OX2R), was predominantly expressed in GH3 cells. Orexin A suppressed forskolin-induced, but not basal, prolactin mRNA expression without reducing cAMP levels. Of note, orexin A suppressed BMP-4-induced prolactin mRNA and cAMP synthesis. Impairment of the effects of orexin by chemical inhibitors suggested involvement of the P38 pathway in the OX1R activity that suppresses BMP-4-induced PRL expression. Given that inhibition of BMP-receptor signaling reduced prolactin mRNA levels, endogenous BMP action is likely to be linked to the activation of prolactin synthesis by GH3 cells. Orexin A was revealed to suppress Smad1/5/9 phosphorylation and Id-1 transcription induced by BMP-4, which was restored in the presence of orexin-receptor antagonists, suggesting that the inhibitory effect of orexin A occurred via OX1R. Orexin A also reduced ALK-3 expression but increased inhibitory Smad6/7 expression, while BMP-4 treatment downregulated OX1R expression. These results indicated that orexin A plays an inhibitory role in prolactin production through suppression of endogenous BMP activity in GH3 cells, suggesting that a new functional role of the interaction between orexin and BMP-4 is modulation of prolactin levels in lactotrope cells.

Influence of AT1 blockers on obesity and stress induced eating of cafeteria diet

Based on findings that treatment with AT1 receptor blocker (ARB) prevents diet-induced obesity and that the activity of the hypothalamic-pituitary-adrenal (HPA) axis is stimulated by AngII and blocked by ARBs, we aimed to investigate whether ARB treatment can reduce stress-induced eating of cafeteria diet (CD) , thus contributing to alterations in eating behavior. Sprague Dawley rats were fed with chow or CD and treated with telmisartan (TEL, 8mg/kg/d) or vehicle. At weeks 2 and 12, rats were stressed over 5 consecutive days by restraint stress (RS, 4h) and by additional shaking at d5. Tail blood was sampled during RS to determine hormone levels. During the first period of RS, ACTH and corticosterone responses were diminished at d5 in CD- compared to chow-fed rats. Independently of feeding, TEL did not reduce stress hormones. Compared to food behavior before RS, the stress-induced CD eating increased in controls but remained unchanged in TEL-treated rats. After 12 weeks, TEL reduced weight gain and energy intake, particularly in CD-fed rats. Similar to the first RS period, corticosterone response was reduced in CD-fed rats at d5 during the second RS period. TEL did not further reduce stress hormones and did not lessen the CD eating upon RS. We conclude that CD feeding compensates for stress reactions. However, stress-induced CD eating was only reduced by TEL after short-term, but not after long-term drug treatment. Thus, the potency of ARBs to lower HPA activity only plays a minor role in reducing energy intake to prevent obesity.

Neurobiological characteristics underlying metabolic differences between males and females

The hypothalamus is the main integrating center for metabolic control. Our understanding of how hypothalamic circuits function to control appetite and energy expenditure has increased dramatically in recent years, due to the rapid rise in the incidence of obesity and the search for effective treatments. Increasing evidence indicates that these treatments will most likely differ between males and females. Indeed, sex differences in metabolism have been demonstrated at various levels, including in two of the most studied neuronal populations involved in metabolic control: the anorexigenic proopiomelanocortin neurons and the orexigenic neuropeptide Y/Agouti-related protein neurons. Here we review what is known to date regarding the sex differences in these two neuronal populations, as well as other neuronal populations involved in metabolic control and glial cells.

The contribution of orexins to sex differences in the stress response

Women are twice as likely as men to suffer from stress-related psychiatric disorders, such as post-traumatic stress disorder (PTSD) and Major Depressive Disorder (MDD), however, the biological basis of these sex differences is not fully understood. Interestingly, orexins are known to be dysregulated in these disorders. This review first discusses the important role of orexins regulating the response to stress. Next, we review the evidence for sex differences in the orexin system, in which the majority of both preclinical and clinical studies have reported higher orexin system expression in females. Finally, we discuss the functional consequences of these sex differences in orexin expression. Most importantly, the preclinical literature reveals that higher orexin system activity in females contributes to exaggerated neuroendocrine and behavioral responses to stress. In sum, the available data suggests that orexins may be important in the etiology of stress-related psychiatric disorders that present differently in men and women. Thus, targeting orexins could potentially ameliorate many phenotypes of stress-related illness in a sex-specific way.

The role of orexin in controlling the activity of the adipo-pancreatic axis

Orexin A and B are two neuropeptides, which regulate a variety of physiological functions by interacting with central nervous system and peripheral tissues. Biological effects of orexins are mediated through two G-protein-coupled receptors (OXR1 and OXR2). In addition to their strong influence on the sleep-wake cycle, there is growing evidence that orexins regulate body weight, glucose homeostasis and insulin sensitivity. Furthermore, orexins promote energy expenditure and protect against obesity by interacting with brown adipocytes. Fat tissue and the endocrine pancreas play pivotal roles in maintaining energy homeostasis. Since both organs are crucially important in the context of pathophysiology of obesity and diabetes, we summarize the current knowledge regarding the role of orexins and their receptors in controlling adipocytes as well as the endocrine pancreatic functions. Particularly, we discuss studies evaluating the effects of orexins in controlling brown and white adipocytes as well as pancreatic alpha and beta cell functions.

The ghrelin and orexin activity in testicular tissues of patients with idiopathic non-obstructive azoospermia

The aim of the present study is to evaluate the presence of ghrelin and orexin in the testicular tissue of patients who have undergone microscopic testicular sperm extraction (micro-TESE) due to idiopathic non-obstructive azoospermia. Seventy azoospermic cases were included in this study; serum hormone levels were measured and genetic investigations were performed. The patients were divided into two groups: micro-TESE (+) and micro-TESE (-). The number of Leydig cells and stained cells in the seminiferous tubules were counted under a light microscope, and we analyzed ghrelin and orexin activity. The relationship between serum hormone levels and ghrelin and orexin distributions in testicular tissue was evaluated according to micro-TESE results. While sperm was found in 33 cases (47.1%), micro-TESE was negative in 37 cases (52.9%). Peptide hormone activity in testicular tissue was higher in micro-TESE (+) cases. However, interstitial orexin (p = 0.038) and ghrelin (p = 0.002) activity showed statistically meaningful differences. Many different peptides, genes, and other unknown mechanisms play important roles in testicular function. In particular, the peptides orexin and ghrelin may play regulatory roles in testicular function in humans.

Interaction between orexin A and bone morphogenetic protein system on progesterone biosynthesis by rat granulosa cells

The involvement of orexins in reproductive function has been gradually uncovered. However, the functional role of orexins in ovarian steroidogenesis remains unclear. In the present study, we investigated the effects of orexin A on ovarian steroidogenesis by using rat primary granulosa cells that express both OX1 and OX2 receptors for orexins. Treatment with orexin A enhanced progesterone, but not estradiol, biosynthesis induced by FSH, whereas it did not affect basal levels of progesterone or estradiol. In accordance with the effects on steroidogenesis, orexin A increased the mRNA levels of progesterogenic enzymes, including StAR, P450scc and 3βHSD, but not P450arom, and cellular cAMP synthesis induced by FSH. Under the condition of blockage of endogenous BMP actions by noggin or BMP-signaling inhibitors, orexin A failed to increase levels of progesterone synthesis induced by FSH treatment, suggesting that endogenous BMP activity in granulosa cells might be involved in the enhancement of progesterone synthesis by orexin A. Treatment with orexin A impaired Smad1/5/9 activation as well as Id-1 mRNA expression stimulated by BMP-6 and BMP-7, the latter of which was reversed by treatment with an OX1 antagonist. It was also found that orexin A suppressed the mRNA expression of both type-I and -II receptors for BMPs and increased that of inhibitory Smad6 and Smad7 in granulosa cells. On the other hand, treatments with BMP-6 and -7 suppressed the expression of OX1 and OX2. Collectively, the results indicated that orexin A enhances FSH-induced progesterone production, at least in part, by downregulating BMP signaling in granulosa cells. Thus, a new role of orexin A in facilitating progesterone synthesis and functional interaction between the orexin and BMP systems in granulosa cells were revealed.

Localization of orexin B and orexin-2 receptor in the rat epididymis

The peptides orexin A (OXA) and orexin B (OXB) derived from the proteolytic cleavage of a common precursor molecule, prepro-orexin, were originally described in the rat hypothalamus. Successively, they have been found in many other brain regions as well as in peripheral organs of mammals and other less evolved animals. The widespread localization of orexins accounts for the multiple activities that they exert in the body, including the regulation of energy homeostasis, feeding, metabolism, sleep and arousal, stress, addiction, and cardiovascular and endocrine functions. Both OXA and OXB peptides bind to two G-coupled receptors, orexin-1 (OX1R) and orexin-2 (OX2R) receptor, though with different binding affinity. Altered expression/activity of orexins and their receptors has been associated with a large number of human diseases. Though at present evidence highlighted a role for orexins and cognate receptors in mammalian reproduction, their central and/or local effects on gonadal functions remain poorly known. Here, we investigated the localization of OXB and OX2R in the rat epididymis. Immunohistochemical staining of sections from caput, corpus and cauda segments of the organ showed intense signals for both OXB and OX2R in the principal cells of the lining epithelium, while no staining was detected in the other cell types. Negative results were obtained from immunohistochemical analysis of hypothalamic and testicular tissues from OX2R knock-out mice (OX2R^-/-) and OX1R/OX2R double knock-out (OX1R^-/-; OX2R^-/-) mice, thus demonstrating the specificity of the rabbit polyclonal anti-OX2R antibody used in our study. On contrary, the same antibody clearly showed the presence of OX2R in sections from hypothalamus and testis of normal mice and rats which are well known to express the receptor. Thus, our results provide the first definite evidence for the immunohistochemical localization of OXB and OX2R in the principal cells of rat epididymis.

Role of orexins in the central and peripheral regulation of glucose homeostasis: Evidences & mechanisms

Orexins (A & B), neuropeptides of hypothalamic origin, act through G-protein coupled receptors, orexin 1 receptor (OX₁R) and orexin 2 receptor (OX₂R). The wide projection of orexin neurons in the hypothalamic region allows them to interact with the other neurons and regulate food intake, emotional status, sleep wake cycle and energy metabolism. The autonomic nervous system plays an important regulatory role in the energy metabolism as well as glucose homeostasis. Orexin neurons are also under the control of GABAergic neurons. Emerging preclinical as well as clinical research has reported the role of orexins in the glucose homeostasis since orexins are involved in hypothalamic metabolism circuitry and also rely on sensing peripheral metabolic signals such as gut, adipose derived and pancreatic peptides. Apart from the hypothalamic origin, integration and control in various physiological functions, peripheral origin in wide organs, raises the possibility of use of orexins as a therapeutic biomarker in the management of metabolic disorders. The present review focuses the central as well as peripheral roles of orexins in the glucose homeostasis.

Potential role of orexin A binding the receptor 1 for orexins in normal and cryptorchid dogs

Background

Cryptorchidism is one of the most common birth disorders of the male reproductive system identified in dogs and other mammals. This condition is characterised by the absence of one (unilateral) or both (bilateral) gonads from the scrotum. The peptides orexin A (OxA) and B (OxB) were obtained by post-transcriptional proteolytic cleavage of a precursor molecule, called prepro-orexin. These substances bind two types of G-coupled receptors called receptor 1 (OX1R) and 2 (OX2R) for orexins. OX1R is specific to OxA while OX2R binds the two peptides with equal affinity. Orexins modulate a great variety of body functions, such as the reproductive mechanism. The purpose of the present research was to study the presence of OxA and its receptor 1 and their possible involvement in the canine testis under healthy and pathological conditions.

Methods

This study was performed using adult male normal dogs and male dogs affected by unilateral cryptorchidism. Tissue samples were collected from testes and were divided into three groups: normal, contralateral and cryptic. The samples were used for immunohistochemistry, Western blot and in vitro tests for testosterone evaluation in normal and pathological conditions.

Results

OxA-immunoreactivity (IR) was described in interstitial Leydig cells of the normal gonad, and Leydig, Sertoli cells and gonocytes in the cryptic gonad. In the normal testis, OX1R-IR was described in Leydig cells, in pachytene and second spermatocytes and in immature and mature spermatids throughout the stages of the germ developing cycle of the male gonad. In the cryptic testis OX1R-IR was distributed in Leydig and Sertoli cells. The presence of prepro-orexin and OX1R was demonstrated by Western blot analysis. The incubation of fresh testis slices with OxA caused the stimulation of testosterone synthesis in the normal and cryptic gonad while the steroidogenic OxA-induced effect was cancelled by adding the selective OX1R antagonist SB-408124.

Conclusions

These results led us to hypothesise that OxA binding OX1R might be involved in the modulation of spermatogenesis and steroidogenesis in canine testis in healthy and pathological conditions.