Widespread distribution of orexin in rat brain and its regulation upon fasting

Evaluating the efficacy of the selective orexin 1 receptor antagonist nivasorexant in an animal model of binge-eating disorder

OBJECTIVE

Test the efficacy of the selective orexin 1 receptor (OX1R) antagonist (SO1RA) nivasorexant in an animal model of binge-eating disorder (BED) and study its dose-response relationship considering free brain concentrations and calculated OX1R occupancy. Compare nivasorexant's profile to that of other, structurally diverse SO1RAs. Gain understanding of potential changes in orexin-A (OXA) neuropeptide and deltaFosB (ΔFosB) protein expression possibly underlying the development of the binge-eating phenotype in the rat model used.

METHOD

Binge-like eating of highly palatable food (HPF) in rats was induced through priming by intermittent, repeated periods of dieting and access to HPF, followed by an additional challenge with acute stress. Effects of nivasorexant were compared to the SO1RAs ACT-335827 and IDOR-1104-2408. OXA expression in neurons and neuronal fibers as well as ΔFosB and OXA-ΔFosB co-expression was studied in relevant brain regions using immuno- or immunofluorescent histochemistry.

RESULTS

All SO1RAs dose-dependently reduced binge-like eating with effect sizes comparable to the positive control topiramate, at unbound drug concentrations selectively blocking brain OX1Rs. Nivasorexant's efficacy was maintained upon chronic dosing and under conditions involving more frequent stress exposure. Priming for binge-like eating or nivasorexant treatment resulted in only minor changes in OXA or ΔFosB expression in few brain areas.

DISCUSSION

Selective OX1R blockade reduced binge-like eating in rats. Neither ΔFosB nor OXA expression proved to be a useful classifier for their binge-eating phenotype. The current results formed the basis for a clinical phase II trial in BED, in which nivasorexant was unfortunately not efficacious compared with placebo.

PUBLIC SIGNIFICANCE

Nivasorexant is a new investigational drug for the treatment of binge-eating disorder (BED). It underwent clinical testing in a phase II proof of concept trial in humans but was not efficacious compared with placebo. The current manuscript investigated the drug's efficacy in reducing binge-like eating behavior of a highly palatable sweet and fat diet in a rat model of BED, which initially laid the foundation for the clinical trial.

Blockade of the orexin-2 receptors within the ventral tegmental area facilitates the extinction and prevents the reinstatement of methamphetamine-seeking behavior

Repeated use of methamphetamine (METH) causes severe effects on the central nervous system, associated with an increased relapse rate. The orexinergic system is highly implicated in the reward circuitry and may be a promising target for treating psychostimulant dependency. The present study aimed to investigate the involvement of the orexin system, mainly the orexin-2 receptors (OX2R) in the ventral tegmental area (VTA) in the extinction and reinstatement of METH-seeking behavior using a conditioned place preference (CPP) paradigm. To this end, animals received METH (1 mg/kg; sc) for a 5-day conditioning period. Then, in the first set of experiments, different groups of rats were given intra-VTA TCS OX2 29 (1, 3, 10, or 30 nmol/0.3 μl DMSO) as an OX2R antagonist over a 10-day extinction period. In another experiment, after the extinction period, a different set of animals received a single dose of TCS OX2 29 (1, 3, 10, or 30 nmol) before the priming dose of METH (0.25 mg/kg; sc) on the reinstatement day. The results revealed that TCS OX2 29 (10 and 30 nmol) remarkably facilitated the extinction of rewarding properties of METH (P < 0.001 for both doses). Furthermore, TCS OX2 29 (3, 10, or 30 nmol) significantly suppressed the METH-induced reinstatement (3 nmol; P < 0.05, 10 nmol; P < 0.01, and 30 nmol; P < 0.001). In conclusion, the current study revealed that the orexinergic system, specifically the VTA OX2R, is involved in METH-seeking behaviors and that manipulation of this system can be considered a potential therapeutics in treating METH dependency.

Physiological Role of Orexin/Hypocretin in the Human Body in Motivated Behavior: A Comprehensive Review

Neurohormones are neurosecretory materials released by neurosecretory cells that serve both as neuromodulators in the brain and spinal cord and as circulating regulatory hormones. They serve a wide range of functions, including homeostasis, development, and modulation of neuronal and muscle activity. In the hypothalamus, neurohormones called hypocretins are created that were discovered in the late nineties. Orexin receptors (OXRs) have been shown to enhance synaptic signaling in the central nervous system at the cellular level. The orexins improve stimulated neural activity in the hippocampus, which, in turn, aids with spatial memory, learning, and mood. They present themselves as mediators for the hypothalamic functions. They have been shown to regulate sleep-wake cycles, arousal mechanisms, addiction, sympathetic nerve activity (SNA), blood pressure, and thermogenesis. Its role in storing brown adipose tissue has implications for thermal homeostasis. The significant role of orexins is seen in tumorigenesis when orexin A (OrxA) and orexin B (OrxB) induce apoptosis in fast-growing tumor cells. Orexin-null subjects show clinical narcolepsy, indicating that orexins were responsible for keeping them awake. Orexin microinjections in mice brains stimulated increased physical activity, thus possibly countering diet-induced obesity. Physical activity significantly increased plasma orexin-A levels, which facilitated the process of energy homeostasis. The amount of adrenocorticotropic hormone (ACTH) increases in stress conditions, which further facilitates the release of the stress hormone cortisol. No increase in the ACTH hormone is seen in stressed mice administered with orexin receptor 2 (OX2R) antagonists thus showing orexin's role in stress reaction. As a result of linking hypocretin/orexin to various physiological procedures, increased research into the medicinal potential of drugs targeting these receptors is emerging. We summed up in this review the recent advances in our understanding of how orexin and its receptor system play an essential role in clinical and pathological functions. This research summarizes a new area for research in human medicine, providing the possibility of controlling a vast array of physiological functions through intra-cerebroventricular injections of a single neuropeptide.

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.

Orexins (hypocretins): The intersection between homeostatic and hedonic feeding

Orexins are hypothalamic neuropeptides originally discovered to play a role in the regulation of feeding behaviour. The broad connections of orexin neurons to mesocorticolimbic circuitry suggest they may play a role in mediating reward-related behaviour beyond homeostatic feeding. Here, we review the role of orexin in a variety of eating-related behaviour, with a focus on reward and motivation, and the neural circuits driving these effects. One emerging finding is the involvement of orexins in hedonic and appetitive behaviour towards palatable food, in addition to their role in homeostatic feeding. This review discusses the brain circuitry and possible mechanisms underlying the role of orexins in these behaviours. Overall, there is a marked bias in the literature towards studies involving male subjects. As such, future work needs to be done to involve female subjects. In summary, orexins play an important role in driving motivation for high salient rewards such as highly palatable food and may serve as the intersection between homeostatic and hedonic feeding.

The orexin receptors in the ventral tegmental area are involved in the development of sensitization to expression of morphine-induced preference in rats

Recent studies have shown that orexin neurons in the lateral hypothalamus send a compelling project to the ventral tegmental area (VTA). Besides, orexin-1 (OX1) and orexin-2 (OX2) in the VTA are necessary for the development of morphine-induced place preference. Also, sensitivity to morphine can reinforce the rewarding effects of morphine. The current study aims to determine the role of VTAs orexin receptors in morphine sensitization in rats. In 84 adult male albino Wistar rats, two separate cannulae bilaterally implanted into the VTA. They received intra-VTA infusions of SB334867 (0.1, 1 and 10 nM) and TCS OX2 29 (1, 7 and 20 nM) as OX1 and OX2 receptor antagonists, respectively, 10 min before subcutaneous administration of morphine (5 mg/kg) during 3-day sensitization period. After a 5-day drug-free period, the conditioned place preference (CPP) paradigm induced by subthreshold doses of morphine (0.5 mg/kg), and CPP scores were measured by EthoVision software. The results revealed that the blockade of both OX1 and OX2 receptors within the VTA reduced the expression of morphine-induced CPP in the sensitized rats. It is plausible that VTAs orexin receptors are involved in the development/acquisition of sensitization to morphine-induced CPP in the rats.

The Important Role of Adiponectin and Orexin-A, Two Key Proteins Improving Healthy Status: Focus on Physical Activity

Exercise represents the most important integrative therapy in metabolic, immunologic and chronic diseases; it represents a valid strategy in the non-pharmacological intervention of lifestyle linked diseases. A large body of evidence indicates physical exercise as an effective measure against chronic non-communicable diseases. The worldwide general evidence for health benefits are both for all ages and skill levels. In a dysregulated lifestyle such as in the obesity, there is an imbalance in the production of different cytokines. In particular, we focused on Adiponectin, an adipokine producted by adipose tissue, and on Orexin-A, a neuropeptide synthesized in the lateral hypothalamus. The production of both Adiponectin and Orexin-A increases following regular and structured physical activity and both these hormones have similar actions. Indeed, they improve energy and glucose metabolism, and also modulate energy expenditure and thermogenesis. In addition, a relevant biological role of Adiponectin and Orexin A has been recently highlighted in the immune system, where they function as immune-suppressor factors. The strong connection between these two cytokines and healthy status is mediated by physical activity and candidates these hormones as potential biomarkers of the beneficial effects induced by physical activity. For these reasons, this review aims to underly the interconnections among Adiponectin, Orexin-A, physical activity and healthy status. Furthermore, it is analyzed the involvement of Adiponectin and Orexin-A in physical activity as physiological factors improving healthy status through physical exercise.

The in vitro effect of orexin a on the porcine myometrial transcriptomic profile during the early-implantation period

In pigs, early gestation is the most critical period deciding about the reproduction success, and it depends on many processes, involving a significant number of genes and their products. Myometrium was found to be an important source of factors pivotal for a proper course of gestation. The aim of the study was to determine the effect of orexin A (OXA) on the porcine transcriptome, and the determination of relationships among differentially expressed genes (DEG) in the porcine myometrium during implantation using microarray technology. The analyses of gene ontology (GO), DEG assays, biological pathways and networks were performed. OXA affected the expression of 461 genes with fold-change values greater than 1.2 (p < 0.05). The expression of 260 genes were up-regulated and 201 down-regulated in the OXA-treated myometrium. Twelve genes were selected for qPCR validation of differential expression based on their known role in angiogenesis, immune processes, steroid hormone signaling and prostaglandins synthesis. The analysis of relationship between DEG indicated that OXA interacts with genes involved in the inflammatory response, cytokine binding, cytokine activity, interleukin production, leukocyte migration, angiogenesis and embryonic hemopoiesis. The presented results suggest that OXA may play a key role in ensuring optimal conditions for implanting embryos.

Role of orexin-1 and -2 receptors within the nucleus accumbens in the acquisition of sensitization to morphine in rats

It has been reported that orexins A and B are involved in the mediation of drug reward. In addition, the nucleus accumbens (NAc) has an important role in the development of morphine-conditioned place preference (CPP) and morphine sensitization. In the present study, we aimed to evaluate the role of orexin receptors within the NAc in morphine sensitization using CPP paradigm. Adult male Wistar rats were used and were bilaterally implanted by two cannulae in the NAc. The animals received intra-accumbal administration of OX1 or OX2 receptor antagonists, SB-334867 (0.1, 1, and 10 nM/side) or TCS OX2 29 (2, 10, and 20 nM/side), 10 min before morphine injection during the sensitization period, during which the animals received repeated administration of morphine (5 mg/kg; s.c.) once daily for three days followed by 5 morphine injection-free days. Then the CPP paradigm was conducted for the evaluation of morphine rewarding properties by injecting a sub-threshold dose of morphine (0.5 mg/kg; s.c.). The results showed that bilateral administration of OX1 receptor antagonist into the NAc reduced acquisition of morphine sensitization in a dose-dependent manner, but OX2 receptor antagonist produced similar effect only at its highest dose, indicating that OX1 and OX2 receptors within the NAc are involved in the acquisition of morphine sensitization.

The effect of estrone and estradiol on the expression of the orexin/hypocretin system in the porcine uterus during early pregnancy

Orexin A and B (OXA, OXB) are hypothalamic neuropeptides acting via two receptors, type 1 (OX1R) and 2 (OX2R). Orexins, also known as hypocretins, take part in a common endocrine system regulating metabolism and reproductive functions. Changes in the orexin system expression during the estrous cycle and pregnancy suggest dependence on the local hormonal milieu. Estrogens are the key hormones controlling reproductive functions, including maternal recognition of pregnancy and implantation. We hypothesize that estrogens may affect orexin system expression in the early pregnant uterus. The aim of this study was to investigate the influence of estrogens on prepro-orexin (PPO), OX1R, and OX2R gene expression, OX1R and OX2R protein content in the porcine uterine tissue, as well as OXA and OXB secretion on days 10-11, 12-13, 15-16, and 27-28 of pregnancy and on days 10-12 of the estrous cycle (n = 5 per group). The expression of PPO, OX1R, and OX2R genes was examined using qPCR, OX1R and OX2R protein content was evaluated using western blotting, and orexins secretion was determined with ELISA. This is the first study to describe the influence of estrogens on orexin system expression in the porcine uterus. Obtained results revealed that estrogens significantly affect the expression of orexin system and orexins secretion. The influence of estrogens varied between different stages of early pregnancy and the estrous cycle. The steroids showed a tissue-specific and dose-dependent effect. Our findings suggest that orexins could act as a "molecular switch" for estrogen activation in the processes of endometrial decidualization and rapid uterine enlargement during early pregnancy.

Functional characterization of an orexin neuropeptide in amphioxus reveals an ancient origin of orexin/orexin receptor system in chordate

Amphioxus belongs to the subphylum cephalochordata, an extant representative of the most basal chordates, whose regulation of endocrine system remains ambiguous. Here we clearly demonstrated the existence of a functional orexin neuropeptide in amphioxus, which is able to interact with orexin receptor, activate both PKC and PKA pathways, decrease leptin expression, and stimulate lipogenesis. We also showed the transcription level of amphioxus orexin was affected by fasting or temperature, indicating a role of this gene in the regulation of energy balance. In addition, the expression of the amphioxus orexin was detected at cerebral vesicle, which has been proposed to be a homolog of the vertebrate brain. These data collectively suggest that a functional orexin neuropeptide has already emerged in amphioxus, which provide insights into the evolutionary origin of orexin in chordate and the functional homology between the cerebral vesicle and vertebrate brain.

The orexin neuropeptide system: physical activity and hypothalamic function throughout the aging process

There is a rising medical need for novel therapeutic targets of physical activity. Physical activity spans from spontaneous, low intensity movements to voluntary, high-intensity exercise. Regulation of spontaneous and voluntary movement is distributed over many brain areas and neural substrates, but the specific cellular and molecular mechanisms responsible for mediating overall activity levels are not well understood. The hypothalamus plays a central role in the control of physical activity, which is executed through coordination of multiple signaling systems, including the orexin neuropeptides. Orexin producing neurons integrate physiological and metabolic information to coordinate multiple behavioral states and modulate physical activity in response to the environment. This review is organized around three questions: (1) How do orexin peptides modulate physical activity? (2) What are the effects of aging and lifestyle choices on physical activity? (3) What are the effects of aging on hypothalamic function and the orexin peptides? Discussion of these questions will provide a summary of the current state of knowledge regarding hypothalamic orexin regulation of physical activity during aging and provide a platform on which to develop improved clinical outcomes in age-associated obesity and metabolic syndromes.

Function and dysfunction of hypocretin/orexin: an energetics point of view

The basic elements of animal behavior that are critical to survival include energy, arousal, and motivation: Energy intake and expenditure are fundamental to all organisms for the performance of any type of function; according to the Yerkes-Dodson law, an optimal level of arousal is required for animals to perform normal functions; and motivation is critical to goal-oriented behaviors in higher animals. The brain is the primary organ that controls these elements and, through evolution, has developed specialized structures to accomplish this task. The orexin/hypocretin system in the perifornical/lateral hypothalamus, which was discovered 15 years ago, is one such specialized area. This review summarizes a fast-growing body of evidence discerning how the orexin/hypocretin system integrates internal and external cues to regulate energy intake that can then be used to generate sufficient arousal for animals to perform innate and goal-oriented behaviors.

Physiology of the orexinergic/hypocretinergic system: a revisit in 2012

The neuropeptides orexins and their G protein-coupled receptors, OX(1) and OX(2), were discovered in 1998, and since then, their role has been investigated in many functions mediated by the central nervous system, including sleep and wakefulness, appetite/metabolism, stress response, reward/addiction, and analgesia. Orexins also have peripheral actions of less clear physiological significance still. Cellular responses to the orexin receptor activity are highly diverse. The receptors couple to at least three families of heterotrimeric G proteins and other proteins that ultimately regulate entities such as phospholipases and kinases, which impact on neuronal excitation, synaptic plasticity, and cell death. This article is a 10-year update of my previous review on the physiology of the orexinergic/hypocretinergic system. I seek to provide a comprehensive update of orexin physiology that spans from the molecular players in orexin receptor signaling to the systemic responses yet emphasizing the cellular physiological aspects of this system.

Intra-paragigantocellularis lateralis injection of orexin-A has an antinociceptive effect on hot plate and formalin tests in rat

In the present study, the effect of orexin-A (ORXA) microinjection into the paragigantocellularis lateralis (LPGI) on nociceptive behaviors, using hot-plate and formalin tests as thermal and chemical models of pain in rat, was examined. Also, we determined whether the pretreatment with SB-334867, a selective OX1-receptor antagonist, would prevent the antinociceptive effect of orexin-A. ORXA (0.1-100 nM/0.5 μL) microinjected into the LPGi nucleus, dose-dependently decreased the formalin induced nociceptive behaviors and also produced a dose-dependent antinociceptive effect in the hot-plate test. Pretreatment with a selective orexin receptor 1 (OX1R) antagonist, SB-334867, also inhibited the effect of ORXA on formalin induced nociceptive behaviors while the SB-334867 (100 μM) alone had no effect on formalin test. These data demonstrated that the ORXA-induced antinociception in formalin test is mainly mediated through the OX1R in LPGi which might play a potential role in processing the pain information associated with descending pain modulation.

Different distribution of neuromedin S and its mRNA in the rat brain: NMS peptide is present not only in the hypothalamus as the mRNA, but also in the brainstem

Neuromedin S (NMS) is a neuropeptide identified as another endogenous ligand for two orphan G protein-coupled receptors, FM-3/GPR66 and FM-4/TGR-1, which have also been identified as types 1 and 2 receptors for neuromedin U structurally related to NMS. Although expression of NMS mRNA is found mainly in the brain, spleen, and testis, the distribution of its peptide has not yet been investigated. Using a newly prepared antiserum, we developed a highly sensitive radioimmunoassay for rat NMS. NMS peptide was clearly detected in the rat brain at a concentration of 68.3 ± 3.4 fmol/g wet weight, but it was hardly detected in the spleen and testis. A high content of NMS peptide was found in the hypothalamus, midbrain, and pons-medulla oblongata, whereas abundant expression of NMS mRNA was detected only in the hypothalamus. These differing distributions of the mRNA and peptide suggest that nerve fibers originating from hypothalamic NMS neurons project into the midbrain, pons, or medulla oblongata. In addition, abundant expression of type 2 receptor mRNA was detected not only in the hypothalamus, but also in the midbrain and pons-medulla oblongata. These results suggest novel, unknown physiological roles of NMS within the brainstem.

Neuropeptides controlling energy balance: orexins and neuromedins

In this chapter, we review the feeding and energy expenditure effects of orexin (also known as hypocretin) and neuromedin. Orexins are multifunctional neuropeptides that affect energy balance by participating in regulation of appetite, arousal, and spontaneous physical activity. Central orexin signaling for all functions originates in the lateral hypothalamus-perifornical area and is likely functionally differentiated based on site of action and on interacting neural influences. The effect of orexin on feeding is likely related to arousal in some ways but is nonetheless a separate neural process that depends on interactions with other feeding-related neuropeptides. In a pattern distinct from other neuropeptides, orexin stimulates both feeding and energy expenditure. Orexin increases in energy expenditure are mainly by increasing spontaneous physical activity, and this energy expenditure effect is more potent than the effect on feeding. Global orexin manipulations, such as in transgenic models, produce energy balance changes consistent with a dominant energy expenditure effect of orexin. Neuromedins are gut-brain peptides that reduce appetite. There are gut sources of neuromedin, but likely the key appetite-related neuromedin-producing neurons are in the hypothalamus and parallel other key anorectic neuropeptide expression in the arcuate to paraventricular hypothalamic projection. As with other hypothalamic feeding-related peptides, hindbrain sites are likely also important sources and targets of neuromedin anorectic action. Neuromedin increases physical activity in addition to reducing appetite, thus producing a consistent negative energy balance effect. Together with the other various neuropeptides, neurotransmitters, neuromodulators, and neurohormones, neuromedin and orexin act in the appetite network to produce changes in food intake and energy expenditure, which ultimately influences the regulation of body weight.

Fasted zebrafish mimic genetic and physiological responses in mammals: a model for obesity and diabetes?

With worldwide rates of obesity and type-II diabetes increasing, it is essential to identify and understand the mechanisms involved during nutrient absorption and fuel allocation. Recent studies demonstrate that nutrients (e.g., lipids and carbohydrates) play a major regulatory role in gene transcription of glycolytic and lipogenic enzymes in addition to hormones, including insulin and glucagon. These nutrients generally exert their effects through key cellular nutrient/energy receptors. Fasting was used to identify these nutrient/energy receptors known from mammalian studies to ascertain if zebrafish (Danio rerio) are a suitable model for the study of metabolic disorders. Zebrafish were subjected to a fasting/re-feeding regime for 3 weeks, and gene expression of sterol responsive binding protein 1 and 2 (SREBP), the mammalian target of rapamycin (mTOR), cAMP response element binding protein 3-like 3 (CREB3l3), and AMP-activated protein kinase alpha (AMPKα) was assessed. Fasted zebrafish lost ∼10% of their body mass over the 3-week experiment, with an associated depression in oxygen consumption. Increases in liver AMPKα and CREB3l3 mRNA transcript level were noted, concurrent with increases in the activities of the β-oxidation and gluconeogenic markers β-hydroxyacyl CoA dehydrogenase and phosphoenolpyruvate carboxykinase, respectively. Conversely, a depression in liver mTOR and SREBP1 and 2 expression was noted, with a decrease in pyruvate kinase and alanine aminotransferase activities and decreases in liver lipid and glycogen contents. Twenty-four hours after re-feeding, zebrafish rapidly recover, and the majority of parameters return to control values. Taken together, these data suggest adult zebrafish are an appropriate model for the further study of human metabolic disorders.

Developmental programming of energy balance and its hypothalamic regulation

Developmental programming is an important physiological process that allows different phenotypes to originate from a single genotype. Through plasticity in early life, the developing organism can adopt a phenotype (within the limits of its genetic background) that is best suited to its expected environment. In humans, together with the relative irreversibility of the phenomenon, the low predictive value of the fetal environment for later conditions in affluent countries makes it a potential contributor to the obesity epidemic of recent decades. Here, we review the current evidence for developmental programming of energy balance. For a proper understanding of the subject, knowledge about energy balance is indispensable. Therefore, we first present an overview of the major hypothalamic routes through which energy balance is regulated and their ontogeny. With this background, we then turn to the available evidence for programming of energy balance by the early nutritional environment, in both man and rodent models. A wealth of studies suggest that energy balance can indeed be permanently affected by the early-life environment. However, the direction of the effects of programming appears to vary considerably, both between and within different animal models. Because of these inconsistencies, a comprehensive picture is still elusive. More standardization between studies seems essential to reach veritable conclusions about the role of developmental programming in adult energy balance and obesity.

Thyrotropin Releasing Hormone (TRH) in goldfish (Carassius auratus): role in the regulation of feeding and locomotor behaviors and interactions with the orexin system and cocaine- and amphetamine regulated transcript (CART)

TRH is a peptide produced by the hypothalamus which major function in mammals is the regulation of TSH secretion by the pituitary. In fish, TRH does not appear to affect TSH secretion, suggesting that it might regulate other functions. In this study, we assessed the effects of central (intracerebroventricular, icv) injections of TRH on feeding and locomotor behavior in goldfish. TRH at 10 and 100 ng/g, but not 1 ng/g, significantly increased feeding and locomotor behaviors, as indicated by an increase in food intake and in the number of total feeding acts as compared to saline-injected fish. In order to assess possible interactions between TRH and other appetite regulators, we examined the effects of icv injections of TRH on the hypothalamic expression of orexin, orexin receptor and CART. The mRNA expression levels of all three peptides were significantly increased in fish injected with TRH at 100 ng/g as compared to saline-injected fish. Fasting increased TRH, orexin, and orexin receptor hypothalamic mRNA levels and decreased CART hypothalamic mRNA levels. Our results suggest that TRH is involved in the regulation of feeding/locomotor activity in goldfish and that this action is associated with a stimulation of both the orexin and CART systems.

PI3K: An Attractive Candidate for the Central Integration of Metabolism and Reproduction

In neurons, as in a variety of other cell types, the enzyme phosphatidylinositol-3-kinase (PI3K) is a key intermediate that is common to the signaling pathways of a number of peripheral metabolic cues, including insulin and leptin, which are well known to regulate both metabolic and reproductive functions. This review article will explore the possibility that PI3K is a key integrator of metabolic and neural signals regulating gonadotropin releasing hormone (GnRH)/luteinizing hormone (LH) release and explore the hypothesis that this enzyme is pivotal in many disorders where gonadotropin release is at risk. Although the mechanisms mediating the influence of metabolism and nutrition on fertility are currently unclear, the strong association between metabolic disorders and infertility is undeniable. For example, women suffering from anorectic disorders experience amenorrhea as a consequence of malnutrition-induced impairment of LH release, and at the other extreme, obesity is also commonly co-morbid with menstrual dysfunction and infertility. Impaired hypothalamic insulin and leptin receptor signaling is thought to be at the core of reproductive disorders associated with metabolic dysfunction. While low levels of leptin and insulin characterize states of negative energy balance, prolonged nutrient excess is associated with insulin and leptin resistance. Metabolic models known to alter GnRH/LH release such as diabetes, diet-induced obesity, and caloric restriction are also accompanied by impairment of PI3K signaling in insulin and leptin sensitive tissues including the hypothalamus. However, a clear link between this signaling pathway and the control of GnRH release by peripheral metabolic cues has not been established. Investigating the role of the signaling pathways shared by metabolic cues that are critical for a normal reproductive state can help identify possible targets in the treatment of metabolic and reproductive disorders such as polycystic ovarian syndrome.

Endogenous orexin-A modulates gastric motility by peripheral mechanisms in rats

Orexin-A (OXA) and orexin receptor type 1 (OX1R) are found in enteric nervous system and smooth muscle cells in the digestive tract. Fasting is a stimulant for OXA synthesis. The aim of the present study was to investigate central and peripheral effects of endogenous OXA on gastric motility. Endogenous OXA synthesis was induced by 36h fasting. Vagotomy was used to evaluate N.vagus-mediated effects of OXA. Gastric emptying and interdigestive gastric motility were measured by spectrophotometric and manometric methods, respectively. Rats were pretreated with OX1R antagonist SB-334867 prior to measurements. Plasma OXA concentration was assayed with radioimmunoassay while preproorexin (PPO) expression was determined with Western blotting in gastric and hypothalamic tissues. OXA immunoreactivity in antrum was determined with immunohistochemistry. Plasma OXA level, PPO protein expression and OXA immunoreactivity were significantly increased in response to 36h fasting. Endogenous OXA facilitated gastric emptying and inhibited gastric interdigestive motility. As these effects were abolished with SB-334867, it is likely that gastrokinetic effects of OXA are mediated via OX1R. Vagotomy did not alter OXA-mediated effects. According to current data, OXA is up-regulated both centrally and peripherally upon fasting. Endogenous OXA accelerates gastric emptying while it inhibits interdigestive motility.

Orexin-1 receptor co-localizes with pancreatic hormones in islet cells and modulates the outcome of streptozotocin-induced diabetes mellitus

Recent studies have shown that orexins play a critical role in the regulation of sleep/wake states, feeding behaviour, and reward processes. The exocrine and endocrine pancreas are involved in the regulation of food metabolism and energy balance. This function is deranged in diabetes mellitus. This study examined the pattern of distribution of orexin-1 receptor (OX₁R) in the endocrine cells of the pancreas of normal and diabetic Wistar (a model of type 1 diabetes), Goto-Kakizaki (GK, a model of type 2 diabetes) rats and in orexin-deficient (OX^−/−) and wild type mice. Diabetes mellitus (DM) was induced in Wistar rats and mice by streptozotocin (STZ). At different time points (12 h, 24 h, 4 weeks, 8 months and 15 months) after the induction of DM, pancreatic fragments of normal and diabetic rats were processed for immunohistochemistry and Western blotting. OX₁R-immunoreactive nerves were observed in the pancreas of normal and diabetic Wistar rats. OX₁R was also discernible in the pancreatic islets of normal and diabetic Wistar and GK rats, and wild type mice. OX₁R co-localized with insulin (INS) and glucagon (GLU) in the pancreas of Wistar and GK rats. The number of OX₁R-positive cells in the islets increased markedly (p<0.0001) after the onset of DM. The increase in the number of OX₁R-positive cells is associated with a high degree of co-localization with GLU. The number of GLU- positive cells expressing OX₁R was significantly (p<0.0001) higher after the onset of DM. The tissue level of OX₁R protein increased with the duration of DM especially in type 1 diabetes where it co-localized with cleaved caspase 3 in islet cells. In comparison to STZ-treated wild type mice, STZ-treated OX^−/− animals exhibited reduced hyperglycemia and handled glucose more efficiently in glucose tolerance test. The findings suggest an important role for the OX-OX₁R pathway in STZ-induced experimental diabetes.

The Effect of Porcine Orexin a on Glucose-Dependent Insulinotropic Polypeptide Plasma Concentrations in Pigs

The orexigenic system was discovered in 1998. It consists of two neuropeptides, Orexin A and Orexin B. Researchers have focused more on Orexin A, since its intracerebroventricular injection into the lateral ventricle of the rat's brain causes an increase in the consumption of food. Besides, it seems that, apart from appetite, Orexin A regulates many other physiological functions with unknown regulatory and metabolic mechanisms. Orexin A is produced by a small group of neurons located in and around the lateral hypothalamic area. It has been known for decades that the latter is involved in regulating feeding in mammals. An intravenous injection of Orexin A causes changes in insulin and glucagon plasma concentrations in rats. In this study, we investigated the possible effects of the central administration of porcine Orexin A on glucose-dependent insulinotropic polypeptide plasma concentrations in pigs, and examined whether these changes are related to the possible effect of the neuropeptide on the enteroinsular axis.

Role of orexin/hypocretin in dependence and addiction

The orexins (or hypocretins) are hypothalamic neuropeptides that have been implicated in a variety of behaviors ranging from feeding to sleep and arousal. Evidence from animal models suggests a role for orexins in reward processing and drug addiction. In this review, we discuss orexin's interaction with the mesocorticolimbic reward pathway and the effects of drugs of abuse on the orexin system. We further review models of drug dependence and addiction and describe behavioral alterations that are seen when the orexin system is manipulated both pharmacologically and genetically. Based on the findings reported in the literature thus far, we posit that orexin functioning contributes to both drug reward and drug-related stress/aversive responsiveness; however, diverse anatomical substrates, and perhaps receptor specificity, contribute differentially to reward and stress components.

Orexin B/hypocretin 2 increases glutamatergic transmission to ventral tegmental area neurons

The orexins (hypocretins) play a crucial role in arousal, feeding and reward. Highly relevant to these functions, orexin-containing neurons from the lateral hypothalamus project densely to the ventral tegmental area (VTA), which is the origin of dopamine projections implicated in motivation and reward. Orexin A/hypocretin 1 (oxA/hcrt-1) can enable long-term changes associated with drugs of abuse; however, the effects of orexin B/hypocretin 2 (oxB/hcrt-2) on excitatory synaptic transmission in the VTA are unknown. We used whole-cell patch-clamp electrophysiology in rat horizontal midbrain slices to examine the effects of oxB/hcrt-2 on excitatory synaptic transmission. We observed that oxB/hcrt-2 has distinct effects from oxA/hcrt-1 in the VTA. oxB/Hcrt-2 (100 nM) increased presynaptic glutamate release in addition to a postsynaptic potentiation of NMDA receptors (NMDARs). The oxB/hcrt-2-mediated postsynaptic potentiation of NMDARs was mediated via activation of orexin/hypocretin 2 (OX2/Hcrt-2) receptors and protein kinase C (PKC). Furthermore, the increase in transmitter release probability was also PKC-dependent, but not through activation of orexin/hypocretin 1 (OX1/Hcrt-1) or OX2/Hcrt-2 receptors. Finally, oxB/hcrt-2 or the selective OX2/Hcrt-2 receptor agonist ala(11)-D-leu(15)-orexin B, significantly reduced spike-timing-induced long-term potentiation. Taken together, these results support a dual role for oxB/hcrt-2 in mediating enhanced glutamatergic transmission in the VTA, and suggest that oxA/hcrt-1 and oxB/hcrt-2 exert different functional roles in modulating the enhancement of the motivational components of arousal and feeding.

Distribution of orexins-containing fibers and contents of orexins in the rat olfactory bulb

Orexin-A and -B (identical to hypocretin-1 and -2) are hypothalamic neuropeptides that regulate appetite and arousal. Orexins-producing neurons project their axons to various brain regions, including the olfactory bulb. In the present study, to understand the relationship between orexins and olfaction, we investigated the distribution of the orexin-A- and -B-immunoreactive (ir) fibers in the rat olfactory bulb and the contents of orexin-A and -B in the rat olfactory bulb after food deprivation for 48 h by using immunohistochemistry and radioimmunoassay, respectively. Both orexin-A- and -B-ir fibers are similarly wide spread from the glomerular layer of the olfactory bulb where the terminals of the peripheral olfactory nerves make synapses with the mitral cells or the tufted cells, to the piriform cortex. Dense orexin-A- and -B-ir fibers were observed mainly in the granular cell layer and anterior olfactory nucleus. The contents of orexin-A and -B (pg/10 mg wet weight tissue) in fed rats (mean+/-S.E.M., n=6) were 2.72+/-0.24 and 6.31+/-0.63, respectively. Fasting for 48 h significantly reduced the contents of orexin-B, but not orexin-A. Orexins in the rat olfactory bulb may be involved in not only olfactory system but also energy balance.

Orphan G-protein-coupled receptors : strategies for identifying ligands and potential for use in eating disorders

G-protein-coupled receptors (GPCRs) are key regulators of intercellular interactions, participating in almost every physiological response. They exert their effects by being activated by a variety of endogenous ligands. Traditionally, these ligands were identified first, providing tools to characterise the receptors. However, since the late 1980s, homology screening approaches have allowed the GPCRs to be found first, and in turn used as orphan targets to identify their ligands. Over the last decade this method has led to the identification of 12 novel neuropeptide families. Interestingly, four of these deorphanised GPCR systems, melanin-concentrating hormone, ghrelin, orexin and neuropeptide B/neuropeptide W, have been found to play a role in the control of energy balance. This article reviews the role of these GPCR systems in the control of food intake and energy expenditure, and discusses their potential use in therapies directed at eating disorders. As obesity has reached epidemic proportions across the developed world, pharmacotherapy has focused on this condition. However, difficulties in weight control also characterise disorders of binge eating such as bulimia and binge-eating disorder. Consequently, hypophagic treatments may be of potential benefit in normal, overweight or obese individuals displaying aberrant (out of control) eating behaviour.

The Effect of Porcine Orexin a on C-Peptide Plasma Concentrations in Pigs

The hypothalamus and the neuropeptides that are produced and act within its neuronal circuits constitute an area of extensive laboratory research. In 1998, the neuropeptide, Orexin A, was discovered and isolated from the hypothalamus of the rat. An i.c.v. injection of Orexin A into the lateral ventricle of the rat's brain causes an increase in the consumption of food, and, apart from appetite, it also seems to be regulating many other normal functions of the organism, whose regulatory and metabolic mechanisms remain unknown to date. The neuropeptide is produced by a small cluster located in and round the lateral hypothalamic area. It has been known for decades that this area is involved in the regulation of feeding and energy homeostasis in mammals. The intravenous, subcutaneous, or i.c.v. injection of Orexin A causes changes in insulin and glucagon concentrations. The same effect is also seen under in vitro experimental conditions. In this study, we investigated the potential effects of i.c.v. administration of porcine Orexin A on c-peptide concentrations in the peripheral blood of pigs, and tested whether these changes are associated with the potential effect of the neuropeptide on the function of the pancreas.

Peptidomic identification and biological validation of neuroendocrine regulatory peptide-1 and -2

Recent advances in peptidomics have enabled the identification of previously uncharacterized peptides. However, sequence information alone does not allow us to identify candidates for bioactive peptides. To increase an opportunity to discover bioactive peptides, we have focused on C-terminal amidation, a post-translational modification shared by many bioactive peptides. We analyzed peptides secreted from human medullary thyroid carcinoma TT cells that produce amidated peptides, and we identified two novel amidated peptides, designated neuroendocrine regulatory peptide (NERP)-1 and NERP-2. NERPs are derived from distinct regions of the neurosecretory protein that was originally identified as a product of a nerve growth factor-responsive gene in PC12 cells. Mass spectrometric analysis of the immunoprecipitate using specific antibodies as well as reversed phase-high performance liquid chromatography coupled with radioimmunoassay analysis of brain extract demonstrated the endogenous presence of NERP-1 and NERP-2 in the rat. NERPs are abundant in the paraventricular and supraoptic nuclei of the rat hypothalamus and colocalized frequently with vasopressin but rarely with oxytocin. NERPs dose-dependently suppressed vasopressin release induced by intracerebroventricular injection of hypertonic NaCl or angiotensin II in vivo. NERPs also suppressed basal and angiotensin II-induced vasopressin secretion from hypothalamic explants in vitro. Bioactivity of NERPs required C-terminal amidation. Anti-NERP IgGs canceled plasma vasopressin reduction in response to water loading, indicating that NERPs could be potent endogenous suppressors of vasopressin release. These findings suggest that NERPs are novel modulators in body fluid homeostasis.

A comparative analysis of the distribution of immunoreactive orexin A and B in the brains of nocturnal and diurnal rodents

Background

The orexins (hypocretins) are a family of peptides found primarily in neurons in the lateral hypothalamus. Although the orexinergic system is generally thought to be the same across species, the orexins are involved in behaviors which show considerable interspecific variability. There are few direct cross-species comparisons of the distributions of cells and fibers containing these peptides. Here, we addressed the possibility that there might be important species differences by systematically examining and directly comparing the distribution of orexinergic neurons and fibers within the forebrains of species with very different patterns of sleep-wake behavior.

Methods

We compared the distribution of orexin-immunoreactive cell bodies and fibers in two nocturnal species (the lab rat, Rattus norvegicus and the golden hamster, Mesocricetus auratus) and two diurnal species (the Nile grass rat, Arvicanthis niloticus and the degu, Octodon degus). For each species, tissue from the olfactory bulbs through the brainstem was processed for immunoreactivity for orexin A and orexin B (hypocretin-1 and -2). The distribution of orexin-positive cells was noted for each species. Orexin fiber distribution and density was recorded and analyzed using a principal components factor analysis to aid in evaluating potential species differences.

Results

Orexin-positive cells were observed in the lateral hypothalamic area of each species, though there were differences with respect to distribution within this region. In addition, cells positive for orexin A but not orexin B were observed in the paraventricular nucleus of the lab rat and grass rat, and in the supraoptic nucleus of the lab rat, grass rat and hamster. Although the overall distributions of orexin A and B fibers were similar in the four species, some striking differences were noted, especially in the lateral mammillary nucleus, ventromedial hypothalamic nucleus and flocculus.

Conclusion

The orexin cell and fiber distributions observed in this study were largely consistent with those described in previous studies. However, the present study shows significant species differences in the distribution of orexin cell bodies and in the density of orexin-IR fibers in some regions. Finally, we note previously undescribed populations of orexin-positive neurons outside the lateral hypothalamus in three of the four species examined.

The Hypothalamic Orexinergic System: Pain and Primary Headaches

The primary headaches are a group of distinct individually characterized attack forms, which although varying in presentation, share some common anatomical basis responsible for the pain component of the attack. The hypothalamus is known to modulate a multitude of functions and has been shown to be involved in the pathophysiology of a variety of primary headaches including cluster headache and chronic migraine. It seems likely that it may be involved in other primary headache disorders due to their episodic nature and may underlie many of their diverse symptoms. We discuss the hypothalamic involvement in the modulation of trigeminovascular processing and examine the involvement of the hypothalamic orexinergic system as a key regulator of this function.

Central orexin sensitivity, physical activity, and obesity in diet-induced obese and diet-resistant rats

Nonexercise activity thermogenesis (NEAT), the most variable component of energy expenditure, can account for differential capacities for human weight gain. Also highly variable, spontaneous physical activity (SPA) may similarly affect weight balance in animals. In the following study, we utilized the rat model of obesity, the diet-induced obese (DIO) rat, as well as the diet-resistant (DR) rat strain, to investigate how access to a high-fat diet alters SPA and the associated energy expenditure (i.e., NEAT). DIO and DR rats showed no differences in the amount of SPA before access to the high-fat diet. After 29 days on a high-fat diet, the DIO rats showed significant decreases in SPA, whereas the DR rats did not. Next, we wanted to determine whether the DIO and DR rats showed differential sensitivity to microinjections of orexin into the paraventricular nucleus of the hypothalamus (PVN). Unilateral guide cannulae were implanted, aimed at the PVN. Orexin A (0, 0.125, 0.25, and 1.0 nmol in 500 nl) was microinjected through the guide cannula into the PVN, then SPA and energy expenditure were measured for 2 h. Using the response to vehicle as a baseline, the DR rats showed significantly greater increase in NEAT compared with the DIO rats. These data indicate that diet-induced obesity is associated with decreases in SPA and a lack of increase in NEAT. A putative mechanism for changes in NEAT that accompany obesity is a decreased sensitivity to the NEAT-activating effects of neuropeptides such as orexin.

Effect of intraperitoneal CCK-8 on food intake and brain orexin-A after 48 h of fasting in the rat

We investigated the interactions of the peripheral satiety peptide cholecystokinin and the brain orexin-A system in the control of food intake. The effect of an intraperitoneal (i.p.) injection of sulfated cholecystokinin octapeptide (in this article called CCK) (5 microg/kg, 4.4 nmol/kg) or of phosphate-buffered saline (PBS, vehicle control) on 48 h fasting-induced feeding and on orexin-A peptide content was analyzed in diverse brain regions innervated by orexin neurons and involved in the control of food intake. Administration of CCK after a 48 h fast reduced fasting-induced hyperphagia (P<0.05). I.p. CCK increased the orexin-A content in the posterior brainstem of 48 h fasted rats by 35% (P<0.05). Fed animals receiving CCK had 48% higher orexin-A levels in the posterior brainstem than fasted rats (P<0.05). In the lateral hypothalamus, fasting decreased orexin-A levels by 50% as compared to fed rats (P<0.05). In the septal nuclei, the combination of fasting and CCK administration reduced orexin-A contents compared to fed PBS and CCK animals by 13% and 17%, respectively (P<0.05). These results suggest a convergence of pathways activated by peripheral CCK and by fasting on the level of orexin-A released in the posterior brainstem and provide evidence for a novel interaction between peripheral satiety signaling and a brain orexigen in the control of food intake.

Stimulatory effect of endogenous orexin A on gastric emptying and acid secretion independent of gastrin

Orexin A (OXA) increases food intake and inhibits fasting small bowel motility in rats. The aim of this study was to examine the effect of exogenous OXA and endogenous OXA on gastric emptying, acid secretion, glucose metabolism and distribution of orexin immunoreactivity in the stomach. Rats equipped with a gastric fistula were subjected to intravenous (IV) infusion of OXA or the selective orexin-1 receptor (OX1R) antagonist SB-334867-A during saline or pentagastrin infusion. Gastric emptying was studied with a liquid non-nutrient or nutrient, using 51Cr as radioactive marker. Gastric retention was measured after a 20-min infusion of OXA or SB-334867-A. Plasma concentrations of OXA, insulin, glucagon, glucose and gastrin were studied. Immunohistochemistry against OXA, OX1R and gastrin in gastric tissue was performed. OXA alone had no effect on either acid secretion or gastric emptying. SB-334867-A inhibited both basal and pentagastrin-induced gastric acid secretion and increased gastric retention of the liquid nutrient, but not PEG 4000. Plasma gastrin levels were unchanged by IV OXA or SB-334867-A. Plasma OXA levels decreased after intake of the nutrient meal and infusion of the OX1R antagonist. Only weak effects were seen on plasma glucose and insulin by OXA. Immunoreactivity to OXA and OX1R were found in the mucosa, myenteric cells bodies and varicose nerve fibers in ganglia and circular muscle of the stomach. In conclusion, endogenous OXA influences gastric emptying of a nutrient liquid and gastric acid secretion independent of gastrin. This indicates a role for endogenous OXA, not only in metabolic homeostasis, but also in the pre-absorptive processing of nutrients in the gut.

Changes in orexin-A and neuropeptide y expression in the hypothalamus of obese and lean Zucker Diabetic Fatty rats

This study was carried out to investigate the changes of orexin-A (OXA) and neuropeptide Y (NPY) expression in the hypothalamus of the obese and lean Zucker Diabetic Fatty (ZDF) rats which have a missense mutation in the leptin receptor gene. The mean body weights (MBW) between the obese and lean ZDF rats were significantly different at 28 and 70 postnatal days. However, at 14 postnatal day, there was no significant difference in the MBW between the obese and lean ZDF rats in both male and female. The OXA immunoreactivities were not significantly different between the obese and lean ZDF rats in both sexes at 14, 28, and 70 postnatal days, respectively. The NPY immunoreactivity was higher in the obese than in the lean ZDF rats in both male and female at 28 and 70 postnatal days, whereas there was no significant difference between the obese and lean ZDF rats at 14 postnatal day. These results indicate that both OXA and NPY might halt their roles for food intake in the obese phenotype of the male and female ZDF rats in the preweaning period of 14 postnatal day, whereas NPY might play a main role in the obesity of these rats in the weaning period of 28 and 70 postnatal days.