Pharmacokinetic profile of orexin A and effects on plasma insulin and glucagon in the rat

Molecular and cellular mechanisms for differential effects of chronic social isolation stress in males and females

Chronic social isolation stress during adolescence induces susceptibility for neuropsychiatric disorders. Here we show that 5-week post-weaning isolation stress induces sex-specific behavioral abnormalities and neuronal activity changes in the prefrontal cortex (PFC), basal lateral amygdala (BLA), and ventral tegmental area (VTA). Chemogenetic manipulation, optogenetic recording, and in vivo calcium imaging identify that the PFC to BLA pathway is causally linked to heightened aggression in stressed males, and the PFC to VTA pathway is causally linked to social withdrawal in stressed females. Isolation stress induces genome-wide transcriptional alterations in a region-specific manner. Particularly, the upregulated genes in BLA of stressed males are under the control of activated transcription factor CREB, and CREB inhibition in BLA normalizes gene expression and reverses aggressive behaviors. On the other hand, neuropeptide Hcrt (Hypocretin/Orexin) is among the top-ranking downregulated genes in VTA of stressed females, and Orexin-A treatment rescues social withdrawal. These results have revealed molecular mechanisms and potential therapeutic targets for stress-related mental illness.

Subcutaneous Adipose Tissue Browning, Serum Orexin-A, and Insulin Resistance Following Aerobic Exercise in High-Fat Diet Obesity Male Wistar Rats

Background

Subcutaneous adipose tissue (SAT) relative to the other adipose tissues may have different roles in health and insulin resistance. The purpose of this study was to investigate the effectiveness of aerobic exercise on SAT thermogenesis indices, serum orexin-A (OXA), and insulin resistance in high-fat diet-induced obesity male Wistar rats.

Methods

Thirty-two male Wistar rats with an average weight of 180-200 g were randomly assigned into 4 equal groups: normal fat diet (NFD), high-fat diet obesity (HFDO), normal fat diet after high-fat diet obesity (HFDO-NFD), and aerobic exercise group with normal fat diet after high-fat diet obesity (HFDO-AEX). Fasting levels of serum OXA, insulin, FBS, high-density lipoproteins, low-density lipoproteins, cholesterol and gene expression of peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) and UCP1 in SAT were evaluated. Samples were taken in the HFDO group after obesity-induced and in other groups 48 h after 8 weeks of aerobic exercise.

Results

The results showed that HFD significantly decreased serum levels of OXA, HDL-c and gene expression of PGC1α and UCP1 in SAT. In addition, it caused a significant increase in Lee index, FBS, insulin resistance, and serum lipid profile in comparison with the NFD group (P ≤ 0.001). Aerobic exercise significantly modified the changes caused by HFD to the normal levels (P ≤ 0.001).

Conclusions

These data suggest that aerobic exercise caused an improvement in insulin resistance and blood lipid profiles through an increase in the serum level of OXA and alteration in the SAT phenotype from white to brown or beige.

Detection of neuropeptides in vivo and open questions for current and upcoming fluorescent sensors for neuropeptides

During a stress response, various neuropeptides are secreted in a spatiotemporally coordinated way in the brain. For a precise understanding of peptide functions in a stress response, it is important to investigate when and where they are released, how they diffuse, and how they are broken down in the brain. In the past two decades, genetically encoded fluorescent calcium indicators have greatly advanced our knowledge of the functions of specific neuronal activity in regulation of behavioral changes and physiological responses during stress. In addition, various kinds of structural information on G-protein-coupled receptors (GPCRs) for neuropeptides have been revealed. Recently, genetically encoded fluorescent sensors have been developed for detection of neurotransmitters by making use of conformational changes induced by ligand binding. In this review, we summarize the recent and upcoming advances of techniques for detection of neuropeptides and then present several open questions that will be solved by application of recent or upcoming technical advances in detection of neuropeptides in vivo.

Hindbrain orexin 1 receptors blunt intake suppression by gastrointestinal nutrients and cholecystokinin in male rats

Hypothalamic orexin neurons project to many brain areas, including hindbrain structures such as the nucleus of the solitary tract (NTS) and area postrema (AP), where orexin 1 receptors (OX1Rs) are expressed. Hindbrain administration of orexin-A increases feeding and meal size, and blockade of hindbrain OX1Rs with the selective antagonist SB334867 has the opposite effect. Here we asked whether hindbrain OX1R stimulation or blockade alter rats' sensitivity to gastrointestinal satiety signals. Rats received 4th intracerebroventricular (icv) injections of vehicle or orexin-A, at a dose with no effect on its own, prior to an intragastric (IG) infusion of saline or a satiating volume of Ensure. IG Ensure suppressed subsequent chow intake, but orexin-A pretreatment significantly attenuated this IG nutrient-induced satiety at 2 h into the dark phase. In a second experiment, rats received NTS injections of vehicle or orexin-A before intraperitoneal (IP) injection of vehicle or the satiation hormone cholecystokinin (CCK). NTS orexin-A pretreatment completely blocked the intake-suppressive effect of CCK on dark-phase chow intake. Finally, we investigated the role of endogenous hindbrain OX1R activation by pretreating rats with 4th-icv injection of vehicle or SB334867 followed by IG infusion of saline or Ensure just before a chocolate Ensure licking test session. IG nutrient infusion suppressed Ensure intake, and blockade of hindbrain OX1Rs significantly prolonged that intake-suppressive effect. We conclude that hindbrain OX1Rs are a mechanism though which hypothalamic orexin neurons can reduce animals' sensitivity to gastrointestinal nutrient load, allowing them to consume more food.

Chronic ketamine abuse is associated with orexin-A reduction and ACTH elevation

BACKGROUND

Ketamine has emerged as a major substance of abuse worldwide. Evidence suggests a role of orexin system in reward processing, withdrawal, and stress response. It also interacts with the stress mechanisms of hypothalamic-pituitary-adrenal (HPA) axis to regulate drug-taking behavior. The study aimed to explore the relevance of orexin and stress hormones to chronic ketamine abuse.

METHODS

We enrolled 67 ketamine-dependent (KD) patients and 64 controls. The levels of orexin-A, adrenocorticotropic hormone (ACTH), and cortisol were measured at baseline, 1 week, and 2 weeks after ketamine discontinuation. KD patients were assessed by Beck Depression Inventory, Beck Anxiety Inventory, and Visual Analogue Scale for ketamine craving at baseline.

RESULTS

Compared with the controls, KD patients had significantly lower orexin-A (0.65 ± 0.12 vs. 0.74 ± 0.10 ng/mL, p < 0.001) and increased ACTH (32.3 ± 16.3 vs. 22.3 ± 11.0 pg/mL, p = 0.008) levels at baseline, whereas cortisol levels were similar between two groups. Levels of the three markers did not correlate with ketamine use variables, craving, depression, or anxiety symptoms. The levels did not alter after 1 or 2 weeks of ketamine discontinuation. Notably, those with higher anxiety had lower orexin-A but increased cortisol levels than did those with lower anxiety.

CONCLUSIONS

This study showed that KD patients had persistent orexin-A reduction and stress hormone dysregulation in early abstinence. The anxious phenotype of KD might be associated with a lower orexin-A expression. These results point to a promising pathway to investigate the neurochemical mechanisms of ketamine addiction.

Plasma orexin-A-like immunoreactivity levels and renal function in patients in a geriatric ward

Orexin-A is a neuropeptide mainly produced by hypothalamic neurons with functions in the central nervous system such as regulation of the sleep-wake cycle. Recent studies suggest that orexin-A also plays major roles in peripheral tissues. Although a few studies have reported a role for the kidney in the dynamics of orexin-A, little is known about the association between plasma orexin-A-like immunoreactivity (orexin-A-LI) levels and renal function. We evaluated this association, and also explored other clinical characteristics associated with plasma orexin-A-LI levels. In this cross-sectional study, we included 70 consecutive patients aged ≥65 years admitted to the geriatric ward of Nagoya University Hospital from December 2017 to January 2018. Patients taking suvorexant (an orexin receptor antagonist) were excluded. On hospital days 2-4, fasting blood was collected in the morning. We evaluated associations between plasma orexin-A-LI levels and renal function and other clinical characteristics. Renal function was evaluated in two ways: the estimated glomerular filtration rate (eGFR) using serum creatinine, and estimated creatinine clearance (eCrCl) using the Cockroft-Gault formula. Pearson's correlation coefficient revealed that plasma orexin-A-LI levels were negatively correlated with the eGFR (r = -0.351, p = 0.003) and eCrCl (r = -0.342, p = 0.004). There were no significant associations between plasma orexin-A-LI levels and the primary diagnosis, body mass index, duration of fasting, or other clinical characteristics. In conclusion, plasma orexin-A-LI levels were negatively correlated with renal function in patients in a geriatric ward. Renal function may affect the study design and data interpretation in studies of plasma orexin-A-LI.

Ectopic expression of OX1R in ulcerative colitis mediates anti-inflammatory effect of orexin-A

Orexins (orexin-A and orexin-B) are hypothalamic peptides that are produced by the same precursor and are involved in sleep/wake control, which is mediated by two G protein-coupled receptor subtypes, OX1R and OX2R. Ulcerative colitis (UC) is an inflammatory bowel disease, (IBD) which is characterized by long-lasting inflammation and ulcers that affect the colon and rectum mucosa and is known to be a significant risk factor for colon cancer development. Based on our recent studies showing that OX1R is aberrantly expressed in colon cancer, we wondered whether orexin-A could play a role in UC. Immunohistochemistry studies revealed that OX1R is highly expressed in the affected colonic epithelium of most UC patients, but not in the non-affected colonic mucosa. Injection of exogenous orexin-A specifically improved the inflammatory symptoms in the two colitis murine models. Conversely, injection of inactive orexin-A analog, OxB7-28 or OX1R specific antagonist SB-408124 did not have anti-inflammatory effect. Moreover, treatment with orexin-A in DSS-colitis induced OX1R^-/- knockout mice did not have any protective effect. The orexin-A anti-inflammatory effect was due to the decreased expression of pro-inflammatory cytokines in immune cells and specifically in T-cells isolated from colonic mucosa. Moreover, orexin-A inhibited canonical NFκB activation in an immune cell line and in intestinal epithelial cell line. These results suggest that orexin-A might represent a promising alternative to current UC therapies.

Activation of orexin-1 receptors in the ventrolateral periaqueductal grey matter (vlPAG) modulates pulpal nociception and the induction of substance P in vlPAG and trigeminal nucleus caudalis

AIM

To characterize the role of orexin-1 receptors (OX1Rs) in ventrolateral periaqueductal grey matter (vlPAG) on modulation of capsaicin-induced pulpal nociception in rats.

METHODOLOGY

Sixty-six adult male Wistar rats (2 months old) weighing between 230 and 260 g were used. The animals were cannulated for microinjection of drugs into the vlPAG matter. Pulpalgia was induced by intradental application of capsaicin solution (100 μg) into the incisor teeth of the rats. Ten min prior to capsaicin application, orexin-A (50, 100 and 150 pmol L^-1 per rat) was administered. Orexin-A (150 pmol L^-1 ) was also co-administrated with SB-334867 (40 nmol L^-1 per rat), an OX1Rs antagonist; or bicuculline (1 μg per rat), a GABAA receptors antagonist. Moreover, treatment effects on the release of pro-nociceptive modulator substance P (SP) in vlPAG and trigeminal nucleus caudalis (Vc) of rats were explored using an immunofluorescence technique. One-way analysis of variance was used for the statistical analysis.

RESULTS

Orexin-A dose-dependently decreased capsaicin-induced nociceptive behaviour. However, SB-334867 (40 nmol L^-1 per rat) pretreatment (P < 0.05), but not bicuculline (1 μg per rat), attenuated the analgesic effect of orexin-A (150 pmol L^-1 ). The level of SP was significantly increased in Vc and decreased in vlPAG of capsaicin-treated rats (P < 0.05). Capsaicin-induced changes in SP levels, however, were prohibited by orexin-A treatment (150 pmol L^-1 ) (P < 0.05).

CONCLUSIONS

Orexin-A administration into the vlPAG was associated with an inhibitory effect on capsaicin-induced pulpal nociception and bidirectional effects on the induction of SP in vlPAG and Vc of rats. Central activation of OX1Rs is a potential therapeutic tool for pulpalgia.

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.

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.

A review of orexin's unprecedented potential as a novel, highly-specific treatment for various localized and metastatic cancers

A systematic review was conducted to categorize the types of cancerous tissues that express orexin receptors and also to examine the effect of in vitro administration of orexin A or B to corresponding cell samples. Comprehensive literature analyses of primary experimental studies were performed. The results of the review included an increased frequency of orexin receptor expression in many colon and prostate cancer tissues and an upward trend of pro-apoptotic activity in these aggressive cell types.

Inverse Association of Peripheral Orexin-A with Insulin Resistance in Type 2 Diabetes Mellitus: A Randomized Clinical Trial

AIMS

To investigate the association between serum orexin concentrations and insulin resistance/sensitivity in a sample of patients with type 2 diabetes mellitus, and to study the effects of anti-hyperglycemic treatment on orexin concentrations over three months.

METHODS

This study was designed as a randomized, open-label, clinical trial. Before allocation, sixty medication-naïve, newly-diagnosed, type 2 diabetes patients underwent a 75 g oral glucose tolerance test (OGTT). Afterwards, using a randomized trial design (IRCT201102275917N1) patients were allocated to either the metformin (1000 mg daily) or pioglitazone (30 mg daily) arm, and were reexamined after three months. Serum insulin, plasma glucose, and orexin concentrations were measured at baseline, during OGTT, and after three months.

RESULTS

Orexin concentrations significantly decreased after OGTT (0 vs. 120 min: 0.63 ± 0.07 vs. 0.31 ± 0.03 ng/ml, p < 0.001). Insulin resistance determined by homeostasis model assessment of insulin resistance (HOMA-IR) was significantly and negatively correlated with orexin (r = -0.301, p = 0.024). Furthermore, orexin concentrations were significantly and positively correlated with the insulin sensitivity index derived from OGTT (r = 0.326, p = 0.014). Three-month treatment with metformin and pioglitazone significantly improved insulin sensitivity and increased orexin concentrations by 26% (p = 0.025) and 14% (p = 0.076), respectively. Between-group analysis showed that changes in orexin concentrations with metformin and pioglitazone were not significantly different (p = 0.742).

CONCLUSIONS

There was a negative association between peripheral orexin concentrations and insulin resistance in type 2 diabetes patients. Three-month anti-hyperglycemic treatment with proportionate doses of metformin or pioglitazone increased orexin concentrations via amelioration of insulin resistance and improvement of glycemic control.

Impact of Orexin-A Treatment on Food Intake, Energy Metabolism and Body Weight in Mice

Orexin-A and -B are hypothalamic neuropeptides of 33 and 28-amino acids, which regulate many homeostatic systems including sleep/wakefulness states, energy balance, energy homeostasis, reward seeking and drug addiction. Orexin-A treatment was also shown to reduce tumor development in xenografted nude mice and is thus a potential treatment for carcinogenesis. The aim of this work was to explore in healthy mice the consequences on energy expenditure components of an orexin-A treatment at a dose previously shown to be efficient to reduce tumor development. Physiological approaches were used to evaluate the effect of orexin-A on food intake pattern, energy metabolism body weight and body adiposity. Modulation of the expression of brain neuropeptides and receptors including NPY, POMC, AgRP, cocaine- and amphetamine related transcript (CART), corticotropin-releasing hormone (CRH) and prepro-orexin (HCRT), and Y2 and Y5 neuropeptide Y, MC4 (melanocortin), OX1 and OX2 orexin receptors (Y2R, Y5R, MC4R, OX1R and OX2R, respectively) was also explored. Our results show that orexin-A treatment does not significantly affect the components of energy expenditure, and glucose metabolism but reduces intraperitoneal fat deposit, adiposity and the expression of several brain neuropeptide receptors suggesting that peripheral orexin-A was able to reach the central nervous system. These findings establish that orexin-A treatment which is known for its activity as an inducer of tumor cell death, do have minor parallel consequence on energy homeostasis control.

Orexin A regulates plasma insulin and leptin levels in a time-dependent manner following a glucose load in mice

AIMS/HYPOTHESIS

Orexin A (OXA) is a neuropeptide implicated in the regulation of arousal status and energy metabolism. Orexin receptors are expressed not only in the central nervous system but also in the pancreas and adipose tissue. However, little is known about the physiological function of orexins. This study investigated the role of exogenous OXA in blood glucose control after glucose load in mice. In addition, the effect of OXA on insulin secretion was also identified in mouse pancreatic beta cells.

METHODS

Insulin secretion and intracellular Ca(2+) levels were measured in perifused mouse islets. To investigate the effects of exogenous OXA on blood glucose levels in vivo, intraperitoneal glucose tolerance tests were performed after a subcutaneous injection of OXA in normal and high-fat diet-induced diabetic mice.

RESULTS

OXA significantly potentiated glucose-stimulated insulin secretion in vitro, which increased intracellular Ca(2+) levels, mainly through adenylate cyclase and ryanodine receptor activation. This Ca(2+)-dependent insulinotropic effect of OXA was blocked in Epac2 (Rapgef4)-deficient beta cells. After a glucose load in mice, exogenous OXA decreased blood glucose levels, compared with the control, by enhancing plasma insulin and decreasing plasma glucagon levels. Additionally, OXA caused a delayed increase in plasma leptin levels, resulting in lower plasma insulin levels when blood glucose levels fell to baseline.

CONCLUSIONS/INTERPRETATION

These results suggest that OXA might be a critical regulator of insulin, glucagon and leptin secretion in response to glucose. Thus, exogenous OXA might have therapeutic potential in improving blood glucose control in patients with type 2 diabetes.

Elevated plasma orexin A levels in a subgroup of patients with schizophrenia associated with fewer negative and disorganized symptoms

BACKGROUND

Orexin A and B, a pair of hypothalamic neuropeptides also named hypocretin 1 and 2, play a role in the regulation of arousal, appetite, reward, attention, and cognition. Animal studies showed that antipsychotics can activate orexin neurons in a manner correlated with their weight gain liability. However, little is known about the role of orexin in patients with schizophrenia. This study aimed to investigate the correlation of plasma orexin level with clinical symptom profile, neurocognitive functioning and weight gain liability of the antipsychotics taken in patients with schizophrenia.

METHODS

We measured plasma levels of orexin A in 127 patients with schizophrenia and 34 healthy controls by radioimmunoassay. In patients, we assessed clinical symptoms on the Positive and Negative Syndrome Scale and executive function by the Wisconsin Card Sorting test (WCST), and examined their associations with plasma orexin A level.

RESULTS

Patients with schizophrenia had a significantly higher mean orexin A level than healthy controls (60.7±37.9 vs. 38.8±15.5pg/ml). Patients were divided into two subgroups based on their orexin A levels that were distributed in two clusters divided by 80pg/ml. Patients in the high-orexin subgroup had significantly fewer negative and disorganized symptoms, and tended to have fewer perseverative errors, more failure to maintain set yet comparable category achieved on the WCST than the normal-orexin subgroup. There was no significant difference in orexin A levels among patients taking antipsychotics with different weight gain liabilities.

CONCLUSION

Higher level of orexin A seems to be related to favorable clinical symptom profiles of schizophrenia, but the causal relationship needs further clarification.

Glucagon regulates orexin A secretion in humans and rodents

AIMS/HYPOTHESIS

Orexin A (OXA) modulates food intake, energy expenditure, and lipid and glucose metabolism. OXA regulates the secretion of insulin and glucagon, while glucose regulates OXA release. Here, we evaluate the role of glucagon in regulating OXA release both in vivo and in vitro.

METHODS

In a double-blind crossover study, healthy volunteers and type 1 diabetic patients received either intramuscular glucagon or placebo. Patients newly diagnosed with type 2 diabetes underwent hyperinsulinaemic-euglycaemic clamp experiments, and insulin-hypoglycaemia tests were performed on healthy volunteers. The primary endpoint was a change in OXA levels after intramuscular glucagon or placebo administration in healthy participants and patients with type 1 diabetes. Secondary endpoints included changes in OXA in healthy participants during insulin tolerance tests and in patients with type 2 diabetes under hyperinsulinaemic-euglycaemic conditions. Participants and staff conducting examinations and taking measurements were blinded to group assignment. OXA secretion in response to glucagon treatment was assessed in healthy and obese mice, the streptozotocin-induced mouse model of type 1 diabetes, and isolated rat pancreatic islets.

RESULTS

Plasma OXA levels declined in lean volunteers and in type 1 diabetic patients injected with glucagon. OXA levels increased during hyperinsulinaemic hypoglycaemia testing in healthy volunteers and during hyperinsulinaemic euglycaemic conditions in type 2 diabetic patients. Plasma OXA concentrations in healthy lean and obese mice and in a mouse model of type 1 diabetes were lower after glucagon treatment, compared with vehicle control. Glucagon decreased OXA secretion from isolated rat pancreatic islets at both low and high glucose levels. OXA secretion declined in pancreatic islets exposed to diazoxide at high and low glucose levels, and after exposure to an anti-insulin antibody. Glucagon further reduced OXA secretion in islets pretreated with diazoxide or an anti-insulin antibody.

CONCLUSIONS/INTERPRETATION

Glucagon inhibits OXA secretion in humans and animals, irrespective of changes in glucose or insulin levels. Through modifying OXA secretion, glucagon may influence energy expenditure, body weight, food intake and glucose metabolism.

Orexin A stimulates glucose uptake, lipid accumulation and adiponectin secretion from 3T3-L1 adipocytes and isolated primary rat adipocytes

AIMS/HYPOTHESIS

Orexin A (OXA) modulates body weight, food intake and energy expenditure. In vitro, OXA increases PPARγ (also known as PPARG) expression and inhibits lipolysis, suggesting direct regulation of lipid metabolism. Here, we characterise the metabolic effects and mechanisms of OXA action in adipocytes.

METHODS

Isolated rat adipocytes and differentiated murine 3T3-L1 adipocytes were exposed to OXA in the presence or absence of phosphoinositide 3-kinase (PI3K) inhibitors. Pparγ expression was silenced using small interfering RNA. Glucose uptake, GLUT4 translocation, phosphatidylinositol (3,4,5)-trisphosphate production, lipogenesis, lipolysis, and adiponectin secretion were measured. Adiponectin plasma levels were determined in rats treated with OXA for 4 weeks.

RESULTS

OXA PI3K-dependently stimulated active glucose uptake by translocating the glucose transporter GLUT4 from cytoplasm into the plasma membrane. OXA increased cellular triacylglycerol content via PI3K. Cellular triacylglycerol accumulation resulted from increased lipogenesis as well as from a decrease of lipolysis. Adiponectin levels in chow- and high-fat diet-fed rats treated chronically with OXA were increased. OXA stimulated adiponectin expression and secretion in adipocytes. Both pharmacological blockade of peroxisome proliferator-activated receptor γ (PPARγ) activity or silencing Pparγ expression prevented OXA from stimulating triacylglycerol accumulation and adiponectin production.

CONCLUSIONS/INTERPRETATION

Our study demonstrates that OXA stimulates glucose uptake in adipocytes and that the evolved energy is stored as lipids. OXA increases lipogenesis, inhibits lipolysis and stimulates the secretion of adiponectin. These effects are conferred via PI3K and PPARγ2. Overall, OXA's effects on lipids and adiponectin secretion resemble that of insulin sensitisers, suggesting a potential relevance of this peptide in metabolic disorders.

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.

Impact of Porcine Orexin a on Glucagon Plasma Concentrations in Pigs

In 1998, Orexin A was added to the long list of orexigenic neuropeptides of the brain's physiology. Orexin A is involved in the central control of appetite and in energy homeostasis, as well as in the regulation of many other physiological functions. It is produced by a small cluster of the brain's neurons, located mainly in and around the lateral hypothalamic area. This site is known to be involved in regulating feeding in mammals. An intracerebroventricular injection of Orexin A into the rat's brain causes an impressive increase in the consumption of food, while an intravenous injection induces changes on glucagon plasma concentrations in rats. In addition, there are signs of changes on glucagon plasma concentrations when Orexin A acts on individual pancreatic islets of rats. In this study, we investigated the potential effects of the central administration of porcine Orexin A on glucagon plasma concentrations in pigs, and examined whether these changes are associated with the possible effect of the neuropeptide on the enteroinsular axis.

Orexin-A inhibits glucagon secretion and gene expression through a Foxo1-dependent pathway

Orexin-A (OXA) regulates food intake and energy homeostasis. It increases insulin secretion in vivo and in vitro, although controversial effects of OXA on plasma glucagon are reported. We characterized the effects of OXA on glucagon secretion and identify intracellular target molecules in glucagon-producing cells. Glucagon secretion from in situ perfused rat pancreas, isolated rat pancreatic islets, and clonal pancreatic A-cells (InR1-G9) were measured by RIA. The expression of orexin receptor 1 (OXR1) was detected by Western blot and immunofluorescence. The effects of OXA on cAMP, adenylate-cyclase-kinase (AKT), phosphoinositide-dependent kinase (PDK)-1, forkhead box O-1 (Foxo1), and cAMP response element-binding protein were measured by ELISA and Western blot. Intracellular calcium (Ca(2+)(i)) concentration was detected by fura-2and glucagon expression by real-time PCR. Foxo1 was silenced in InR1-G9 cells by transfecting cells with short interfering RNA. OXR1 was expressed on pancreatic A and InR1-G9 cells. OXA reduced glucagon secretion from perfused rat pancreas, isolated rat pancreatic islets, and InR1-G9 cells. OXA inhibited proglucagon gene expression via the phosphatidylinositol 3-kinase-dependent pathway. OXA decreased cAMP and Ca(2+)(i) concentration and increased AKT, PDK-1, and Foxo1 phosphorylation. Silencing of Foxo1 caused a reversal of the inhibitory effect of OXA on proglucagon gene expression. Our study provides the first in vitro evidence for the interaction of OXA with pancreatic A cells. OXA inhibits glucagon secretion and reduces intracellular cAMP and Ca(2+)(i) concentration. OXA increases AKT/PDK-1 phosphorylation and inhibits proglucagon expression via phosphatidylinositol 3-kinase- and Foxo-1-dependent pathways. As a physiological inhibitor of glucagon secretion, OXA may have a therapeutic potential to reduce hyperglucagonemia in type 2 diabetes.

Functions of orexins in peripheral tissues

Orexin A (OXA) and orexin B were originally isolated as hypothalamic peptides regulating sleep, wakefulness and feeding. However, growing evidence suggests that orexins have major functions also in the peripheral tissues. Central orexigenic pathways originating from medulla activate the hypothalamus-pituitary axis and can influence the sympathetic tone. Orexins and their receptors are widely dispersed throughout the intestine, where orexin receptors are regulated by the nutritional status, affect insulin secretion and intestinal motility. Although the primary source of the peptide has not been elucidated, OXA is detected in plasma and its level varies in response to the metabolic state. In this review, we focus on the current knowledge on peripheral functions of orexins and discuss possible endocrine, paracrine and neurocrine roles.

Do enteric neurons make hypocretin?

Hypocretins (orexins) are wake-promoting neuropeptides produced by hypothalamic neurons. These hypocretin-producing cells are lost in people with narcolepsy, possibly due to an autoimmune attack. Prior studies described hypocretin neurons in the enteric nervous system, and these cells could be an additional target of an autoimmune process. We sought to determine whether enteric hypocretin neurons are lost in narcoleptic subjects. Even though we tried several methods (including whole mounts, sectioned tissue, pre-treatment of mice with colchicine, and the use of various primary antisera), we could not identify hypocretin-producing cells in enteric nervous tissue collected from mice or normal human subjects. These results raise doubts about whether enteric neurons produce hypocretin.

Modulation of nociceptive dural input to the trigeminal nucleus caudalis via activation of the orexin 1 receptor in the rat

Migraine pathophysiology is thought to involve the trigeminal innervation of the dura mater and intracranial blood vessels. Electrical stimulation of dural blood vessels is painful in humans and causes activation of neurons in the caudal-most portion of the trigeminal nucleus in experimental animals. The hypothalamic neuropeptides orexin A and B are selectively synthesized in the lateral and posterior hypothalamus, and recent findings have implicated their involvement in nociceptive processing. To evaluate the potential for orexin receptor modulation of trigeminovascular nociceptive afferents, we examined the effects of intravenous orexin A and B on responses of neurons in the trigeminal nucleus caudalis. To dissect the receptor pharmacology of responses to stimulation we utilized the novel orexin 1 receptor (OX(1)R) antagonist N-(2-methyl-6-benzoxazolyl)-N''-1,5-naphthyridin-4-yl urea (SB-334867). Orexin A 30 microg/kg (F(1.9,9.8) = 21.93, P < 0.001) and 50 microg/kg (F(3.2,16.4) = 3.28, P < 0.045) inhibited the A-fibre responses to dural electrical stimulation over 60 min. Maximum inhibition was achieved at 25 min for both 30 microg/kg (t(5) = 19.83, n = 6, P < 0.001) and 50 microg/kg (t(5) = 7.74, n = 6, P < 0.001). The response with orexin A 30 microg/kg was reversed by pretreatment with the OX(1)R antagonist SB-334867 (F(3.5,17.5) = 0.49, P = 0.73), which had no effect when given alone. Orexin B and control vehicle administration had no significant effect on trigeminal neuronal firing. The current study demonstrates that orexin A is able to inhibit A-fibre responses to dural electrical stimulation via activation of the OX(1)R.

Hormonal regulation of food intake

Our knowledge of the physiological systems controlling energy homeostasis has increased dramatically over the last decade. The roles of peripheral signals from adipose tissue, pancreas, and the gastrointestinal tract reflecting short- and long-term nutritional status are now being described. Such signals influence central circuits in the hypothalamus, brain stem, and limbic system to modulate neuropeptide release and hence food intake and energy expenditure. This review discusses the peripheral hormones and central neuronal pathways that contribute to control of appetite.