Immunoreactive orexin-A in human plasma

Expression of orexin-A and orexin receptors in the kidney and the presence of orexin-A-like immunoreactivity in human urine

Orexin-A (hypocretin-1), a neuropeptide with stimulatory actions on arousal and appetite, was originally shown to be specifically expressed in the hypothalamus. We studied expression of orexin-A and orexin receptors in the kidney and the presence of orexin-A-like immunoreactivity in human urine. Immunocytochemistry showed that orexin-A-like immunoreactivity and two types of orexin receptors (types 1 and 2) were localized in the tubules of the human kidney obtained at autopsy. Orexin-A-like immunoreactivity was detected in human kidneys (21.3 +/- 6.2 fmol/g wet weight, mean +/- S.E.M., n = 4) and rat kidneys (16.2 +/- 1.6 fmol/g wet weight, n = 5) by radioimmunoassay, although the levels were much lower than the levels in the brain. Orexin-A-like immunoreactivity was present in the urine obtained from male healthy volunteers (67.8 +/- 4.5 pmol/l, n = 5). Reverse phase high-performance liquid chromatography showed that most of orexin-A-like immunoreactivity of the urine extract was eluted earlier than authentic orexin-A, suggesting that orexin-A-like immunoreactivity in urine was modified to hydrophilic forms. Reverse transcriptase polymerase chain reaction showed expression of orexin receptors 1 and 2 mRNAs in the human kidney. These findings suggest that orexin-A is produced by the renal tubular cells and secreted into urine. Orexin-A may act on the kidney in the autocrine or paracrine fashion, or via the urine (urocrine fashion).

Orexins in the regulation of the hypothalamic-pituitary-adrenal axis

Orexin-A and orexin-B are hypothalamic peptides that act via two G protein-coupled receptors, named orexin type 1 and type 2 receptors (OX1-Rs and OX2-Rs). The most studied biological functions of orexins are the central control of feeding and sleep, but in the past few years findings that orexin system modulates the hypothalamic-pituitary-adrenal (HPA) axis, acting on both its central and peripheral branches, have accumulated. Orexins and their receptors are expressed in the hypothalamic paraventricular nucleus and median eminence and orexin receptors in pituitary corticotropes, adrenal cortex, and medulla. Whereas the effects of orexins on adrenal aldosterone secretion are doubtful, compelling evidence indicates that these peptides enhance glucocorticoid production in rats and humans. This effect involves a 2-fold mechanism: 1) stimulation of the adrenocorticotropin-releasing hormone-mediated pituitary release of adrenocorticotropin, which in turn raises adrenal glucocorticoid secretion; and 2) direct stimulation of adrenocortical cells via OX1-Rs coupled to the adenylate cyclase-dependent cascade. The effects of orexins on catecholamine release from adrenal medulla are unclear and probably of minor relevance, but there are indications that orexins can stimulate in vitro secretion of human pheochromocytoma cells via OX2-Rs coupled to the phospholipase C-dependent cascade. Evidence is also available that orexins enhance the growth in vitro of adrenocortical cells, mainly acting via OX2-Rs. Moreover, findings suggest that the orexin system may favor HPA axis responses to stresses and play a role in the pathophysiology of cortisol-secreting adrenal adenomas.

CSF hypocretin-1/orexin-A concentrations in patients with subarachnoid hemorrhage (SAH)

The aim of this study was to examine the role of the hypothalamic hypocretin/orexin system in complications of delayed ischemic neuronal deficit (DIND) resulting from symptomatic vasospasm in patients with aneurysmal subarachnoid hemorrhage (SAH). CSF hypocretin-1/orexin-A levels were measured in 15 SAH patients. DIND complications occurred in seven patients with symptomatic vasospasm. Hypocretin-1/orexin-A levels were low in SAH patients during the 10 days following the SAH event. CSF hypocretin-1/orexin-A levels were lower in patients with DIND complications than in those who did not develop DIND. A significant transient decline in CSF hypocretin-1/orexin-A levels was also observed at the onset of DIND in all patients with symptomatic vasospasm. The reduced hypocretin/orexin production observed in SAH patients may reflect reduced brain function due to the decrease in cerebral blood flow. These results, taken together with recent experimental findings in rats that indicate hypocretin receptor 1 (orexin 1 receptor) mRNA and protein are elevated following middle cerebral artery occlusion, suggest that a reduction in hypocretin/orexin production in SAH and DIND patients is associated with alterations in brain hypocretin/orexin signaling in response to ischemia.

Enhanced antinociception by intracerebroventricularly administered orexin A in histamine H1 or H2 receptor gene knockout mice

Orexins are neuropeptides that are mostly expressed in the posterior and lateral hypothalamus, and related to the central control of appetite, arousal, and antinociception. Orexin neurons projected to the tuberomammillary nucleus and orexins may release histamine from the histamine neurons in this nucleus. Histamine is known to cause hypernociception. The roles of histamine H1 and H2 receptors in the orexin A-induced antinociception, however, have not been clarified yet. Here we studied the effects of histamine H1 and H2 receptors on orexin A-produced antinociception using histamine receptor knockout mice in four assays of nociception; the hot-plate, the tail-flick, the tail-pressure and the capsaicin tests. Furthermore we studied effects of histamine H1 and H2 receptor antagonists on orexin A-produced antinociception in C57BL/6 mice. The antinociceptive effects of i.c.v. orexin A were greater in histamine H1 receptor or H2 receptor knockout mice than in the wild-type mice in all four assays of pain. Furthermore, treatment of C57BL/6 mice with a combination of i.c.v. orexin A and d-chlorpheniramine (a histamine H1 receptor antagonist) or cimetidine (a histamine H2 receptor antagonist) showed a greater antinociception than i.c.v. orexin A alone in all four assays. These findings suggest the possibility that orexin A may activate H1 and H2 receptors in the supraspinal levels through the release of histamine from neurons, which might attenuate the antinociceptive effects of orexin A. Thus, the blocking of the histamine H1 or H2 receptor may produce antinociception and enhance the orexin A-induced antinociception.

Orexins stimulate glucocorticoid secretion from cultured rat and human adrenocortical cells, exclusively acting via the OX1 receptor

Orexins A and B are hypothalamic peptides, that act via two subtypes of receptors, named OX1-R and OX2-R. Rat and human adrenal cortexes are provided with both OX1-R and OX2-R, and we have previously shown that orexin-A, but not orexin-B, enhances glucocorticoid secretion from dispersed adrenocortical cells. Since OX1-Rs preferentially bind orexin-A and OX2-Rs are non-selective for both orexins, the hypothesis has been advanced that the secretagogue effect of orexin-A is exclusively mediated by the OX1-R. Here, we aimed to verify this contention and to gain insight into the signaling mechanism(s) underlying the secretagogue effect of orexins using primary cultures of rat and human adrenocortical cells. Reverse transcription-polymerase chain reaction showed that cultured cells, as freshly dispersed cells, expressed both OX1-R and OX2-R mRNAs. Orexin-A, but not orexin-B, concentration-dependently increased corticosterone and cortisol secretion from cultured rat and human adrenocortical cells, respectively. The blockade of OX1-Rs by selective antibodies abrogated the secretagogue effect of orexin-A, while the immuno-blockade of OX2-Rs was ineffective. The glucocorticoid response of cultured cells to orexin-A was annulled by the adenylate cyclase and protein kinase (PK) A inhibitors SQ-22536 and H-89, and unaffected by the phospholipase C and PKC inhibitors U-73122 and calphostin-C. Orexin-A, but not orexin-B, enhanced cyclic-AMP production from cultured cells, and did not alter inositol-3-phosphate release. Collectively, our present results allow us to conclude that orexins stimulate glucocorticoid secretion from rat and human adrenocortical cells, exclusively acting through OX1-Rs coupled to the adenylate cyclase/PKA-dependent signaling cascade.

Preproorexin and orexin receptors are expressed in cortisol-secreting adrenocortical adenomas, and orexins stimulate in vitro cortisol secretion and growth of tumor cells

Orexins A and B are hypothalamic peptides that originate from the proteolytic cleavage of preproorexin and act through two subtypes of receptors, named OX1-R and OX2-R. OX1-R almost exclusively binds orexin-A, whereas OX2-R is nonselective for both orexins. We previously found that orexin-A, via the OX1-R, stimulates cortisol secretion from dispersed human adrenocortical cells. In this study, we demonstrate that six of eight cortisol-secreting adenomas expressed preproorexin mRNA, and seven of 10 adenomas contained measurable amounts of orexin-A but not orexin-B. Normal adrenal cortexes neither expressed preproorexin nor contained orexins. All adenomas expressed OX1-R and OX2-R mRNAs, and real-time PCR showed that the expression of both receptors was up-regulated in adenomas, compared with normal adrenal cortex. Orexin-A concentration-dependently raised basal cortisol secretion from freshly dispersed normal and adenomatous cells, minimal and maximal effective concentrations being 10(-10) and 10(-8) m, and the peptide efficacy (percent increase elicited by 10(-8) m orexin-A) was significantly higher in adenomas than in the normal adrenal cortex. Orexin-B was ineffective, thereby indicating that orexin secretagogue action is mediated by the OX1-R. In contrast, both orexins (10(-8) m) raised the proliferative activity of cultured normal and adenomatous cells, suggesting that this effect is mediated by OX2-R or both receptor subtypes. Collectively, our findings allow us to conclude that the orexin system is overexpressed in cortisol-secreting adenomas and suggest that orexin-A may act as an autocrine-paracrine regulator of the secretory activity and growth of some of these adrenal tumors.

Decreased plasma levels of orexin-A in sleep apnea

BACKGROUND

Orexin-A, also known as hypocretin, is a neuropeptide implicated in appetite and sleep regulation. Because the obstructive sleep apnea syndrome (OSAS) is characterized by obesity and excessive daytime sleepiness, we hypothesized that orexin-A levels may be abnormal in patients with OSAS. Further, since treatment with continuous positive airway pressure (CPAP) in patients with OSAS is very effective in normalizing daytime sleepiness, we also hypothesized that the chronic use of CPAP may influence plasma levels of orexin-A in these patients.

OBJECTIVE

To evaluate plasma levels of orexin-A in patients with OSAS and the effect of CPAP treatment.

PATIENTS AND METHODS

We compared the plasma levels of orexin-A in 13 healthy controls, 27 untreated patients with OSAS and 14 patients treated with CPAP during at least 1 year (4.5 +/- 0.5 h/night; mean +/- SEM). All patients had severe OSAS (apnea-hypopnea index, 57 +/- 4 h(-1)).

RESULTS

Orexin-A plasma levels were significantly lower in untreated (9.4 +/- 1.9 pg.ml(-1), p < 0.01) and treated patients with OSAS (4.2 +/- 1.5 pg.ml(-1), p < 0.001) than in healthy subjects (20.6 +/- 4.5 pg.ml(-1)). In untreated patients, orexin-A levels were not significantly related to daytime somnolence assessed by Epworth scale (r = -0.18, p = 0.37) or the body mass index (r = -0.13, p = 0.52).

CONCLUSIONS

Orexin-A plasma levels are abnormally low in patients with OSAS, independently of the level of somnolence and/or presence of obesity. These results suggest that these low orexin-A levels may be related to the pathogenesis of OSAS.

Low Plasma Orexin-A Levels Were Improved by Continuous Positive Airway Pressure Treatment in Patients With Severe Obstructive Sleep Apnea-Hypopnea Syndrome

INTRODUCTION

We have previously shown that plasma levels of orexin-A, a neuropeptide with an arousal-stimulating action, were decreased in parallel with the severity of the disease in patients with obstructive sleep apnea-hypopnea syndrome (OSAHS).

OBJECTIVE

To clarify the effects of nasal continuous positive airway pressure (nCPAP) treatment on plasma orexin-A levels in patients with this syndrome.

METHOD

Sleep tests and blood sample collections were conducted at the sleep-related respiratory disorders clinic and the sleep laboratory of the Iwate Medical University Hospital. We studied 27 patients with OSAHS (apnea-hypopnea index [AHI], >/= 20 by polysomnography) who were treated with nCPAP for 3 to 6 months. These patients were divided into the following two groups according to the arousal index (AI): group A (n = 11), >/= 60; group B (n = 16), < 60. Plasma samples were obtained before and after the nCPAP treatment for 3 to 6 months. Plasma immunoreactive (IR)-orexin-A concentrations were measured by radioimmunoassay after the extraction using cartridges.

RESULTS

Plasma IR-orexin-A concentrations were inversely correlated with the AI (r = -0.807; p < 0.0001) and AHI (r = -0.661; p < 0.0001) in 27 patients before the nCPAP treatment. Mean (+/- SEM) plasma IR-orexin-A concentrations were significantly lower in group A (1.0 +/- 0.3 pmol/L) than in group B (4.6 +/- 0.4 pmol/L). Mean plasma IR-orexin-A concentrations were significantly increased after the nCPAP treatment in group A (to 3.4 +/- 1.2 pmol/L; p = 0.0069), whereas they were not significantly changed in group B. The increases in plasma IR-orexin-A concentrations after the nCPAP treatment were in parallel with the improvements in AI and Epworth sleepiness scale (a marker of severity of daytime excessive sleepiness) score in group A.

CONCLUSIONS

The low plasma orexin-A levels were increased by the nCPAP treatment in patients with severe OSAHS, suggesting that orexin-A is a plasma marker that reflects the severity of OSAHS and the response to treatment.

Effect of intestinal ischemia-reperfusion injury on protein levels of leptin and orexin-A in peripheral blood and central secretory tissues

AIM: To explore the effect of intestinal ischemia-reperfusion injury on protein levels of leptin and orexin-A in peripheral blood and their central secretory tissues and to find out the role leptin and orexin-A play in acute inflammatory responses.

METHODS: An intestinal ischemia-reperfusion (I/R) injury model of rats was established and rats were divided randomly into six groups: sham-operation group, 60 min ischemia/30 min reperfusion group (I60’R30’), I60’R90’, I60’R150’, I60’R240’ and I60’R360’, 9 rats each group. Two highly-sensitive radioimmunoassays for leptin and orexin-A were established and used to check the change of their concentrations in peripheral blood and central secretory tissues before and after intestinal I/R injury.

RESULTS: Compared with the serum leptin level before injury, it decreased significantly in I60’R30’ group and increased significantly in I60’R360’ group; compared to sham-operation group after injury, serum leptin level increased significantly in I60’R360’ group; compared to sham-operation group after injury, adipose leptin levels decreased significantly in I60’R30’ and I60’R90’ groups, while increased significantly in I60’R360’ group. There was no significant difference between the expression levels of orexin-A before and after I/R injury.

CONCLUSION: Leptin has a time-dependent response and orexin-A has a delayed response to acute inflammatory stimuli such as intestinal I/R injury and they may participate in metabolic disorders in injury as inflammatory cytokines.

Clinical significance of daytime plasma orexin-A-like immunoreactivity concentrations in patients with obstructive sleep apnea hypopnea syndrome

BACKGROUND

Polysomnography (PSG) tests are very complicated and time consuming, despite their clinical benefits in the diagnosis of patients with obstructive sleep apnea hypopnea syndrome (OSAHS). A plasma marker would be desirable to select patients suspected of OSAHS for further PSG studies. We have recently reported that orexin-A concentrations in plasma collected immediately after waking early in the morning were significantly lower in patients with OSAHS than in controls.

OBJECTIVES

We conducted the present study to assess the clinical usefulness of the measurement of orexin-A concentrations in plasma obtained in the daytime as a diagnostic predictor to screen patients with OSAHS.

METHODS

Blood samples were collected in the daytime from 19 male patients with suspected sleep-disordered breathing. Plasma orexin-A concentrations were measured by radioimmunoassay before performing PSG.

RESULTS

PSG was conducted in all 19 subjects. PSG showed that 14 subjects had OSAHS and 5 subjects did not. Plasma orexin-A concentrations were significantly lower in patients with OSAHS (4.9 +/- 0.8 pmol/l, mean +/- SE, n = 14) than in control subjects (12.3 +/- 1.9 pmol/l, n = 5) (p = 0.0004).

CONCLUSIONS

These findings suggest that the orexin-A concentration in plasma obtained even in the daytime may be a useful plasma marker for screening OSAHS.

Establishment and primary application of a highly-sensitive orexin-A radioimmunoassay

Orexin-A was labeled by 125I using the chloramine-T method, and was purified with a Sephadex G-25 chromatographic column. The reaction between antigen and antibody was carried out by a one-step balance method and was incubated at 4 degrees C for 24 hours, then bonded and free antigen were separated by PR reagent. The detection range of this RIA is 21-2000 pg/mL; the lowest detection level is 21 pg/mL. The intra-assay and inter-assay variations were 7.8% and 9.7%, respectively. Plasma orexin-A levels of 30 normal individuals and 30 patients with hyperlipidemia (serum triglyceride > 1.7 mmol/L and serum total cholesterol > 5.7 mmol/L) were detected by this RIA, while orexin-A levels of plasma and hypothalamus in rat intestinal ischemia-reperfusion injury model were also measured. Plasma orexin-A levels of normal individuals was 338.48 +/- 20.24 pg/mL, while those of patients with hyperlipidemia were 343.51 +/- 15.49 pg/mL; there were no significant differences between these two groups t = -0.1976; P = 0.8441. We also found that orexin-A levels of rat plasma and hypothalamus did not express a significant change during the early stages of intestinal ischemia-reperfusion injury. These results have shown that this orexin-A radioimmunoassay is stable, simple, and specific, being sensitive enough to test orexin-A levels in human plasma, rat plasma, and hypothalamus.

Plasma levels of orexin-a and leptin in obese children

The present study was designed to determine the plasma level of orexin and its relationship with other metabolic and anthropometric markers in obese children. Forty-seven obese Japanese children, consisting of 31 boys and 16 girls, were enrolled in the study. Their ages were 10.4 ± 0.5 (mean ± s.e.m.) yr, and their percentage overweight was 42.9 ± 1.9%. Blood was drawn after an overnight fast. The age-matched control group consisted of 26 nonobese children, 13 boys and 13 girls. Plasma orexin-A concentration was higher in obese children (17.0 ± 0.4 pg/ml; p<0.001) than in the control children (13.5 ± 1.1 pg/ml). Similarly, plasma leptin concentration was higher in obese children (12.0 ± 1.0 ng/ml; p<0.001) than in the control children (5.2 ± 0.4 ng/ml). There was a highly significant positive correlation between the two parameters in the obese children (r=0.49, p<0.001). Plasma orexin-A level was correlated significantly with waist-to-hip ratio, while leptin level was correlated with percentage overweight, waist circumference and percentage body fat in the obese children. These results suggest that high plasma orexin-A level parallels the leptin level in obese children.

Orexin 1 receptor messenger ribonucleic acid expression and stimulation of testosterone secretion by orexin-A in rat testis

Orexins are hypothalamic neuropeptides primarily involved in the regulation of food intake and arousal states. In addition, a role for orexins as central neuroendocrine modulators of reproductive function has recently emerged. Prepro-orexin and orexin type-1 receptor mRNAs have been detected in the rat testis. This raises the possibility of additional peripheral actions of orexins in the control of reproductive axis, which remains so far unexplored. To analyze the biological effects and mechanisms of action of orexins in the male gonad, we evaluated testicular expression of orexin receptor 1 (OX(1)R) and orexin receptor 2 (OX(2)R) mRNAs in different experimental settings and the effect of orexin-A on testicular testosterone (T) secretion. Persistent expression of OX(1)R mRNA was demonstrated in the rat testis throughout postnatal development. In contrast, OX(2)R transcript was not detected at any developmental stage. Expression of OX(1)R mRNA persisted after selective elimination of mature Leydig cells and was detected in isolated seminiferous tubules at defined stages of the seminiferous epithelial cycle. In addition, testicular OX(1)R mRNA expression appeared to be under hormonal regulation; it was reduced by long-term hypophysectomy and partially restored by FSH replacement, whereas down-regulation was observed after exposure to increasing doses of the ligand in vitro. Moreover, OX(1)R mRNA expression was sensitive to neonatal imprinting by estrogen. Finally, orexin-A, in a dose-dependent manner, significantly increased basal, but not human choriogonadotropin-stimulated, T secretion in vitro. A similar stimulatory effect was observed in vivo after intratesticular administration of orexin-A. In conclusion, our present results provide the first evidence for the regulated expression of OX(1)R mRNA and functional role of orexin-A in the rat testis. Overall, our data are suggestive of a novel site of action of orexins in the control of male reproductive axis.

Orexin-A expression in human peripheral tissues

Orexin-A is a neuropeptide present in the brain and is known to regulate feeding and sleeping. In this study, we examined the systemic distribution of orexin-A in human tissues. Immunoreactivity for orexin-A was detected in ganglion cells of the thoracic sympathetic trunk, myenteric plexuses and endocrine cells of the gastrointestinal tract, islet cells of the pancreas and syncytiotrophoblasts and decidual cells of the placenta. In the gastrointestinal tract, orexin-A immunoreactivity was detected in the myenteric plexuses from 26 gestational weeks to birth. In double immunostaining in the pancreas, a great majority of insulin-positive cells was simultaneously positive for orexin-A. mRNA expression for prepro-orexin was also detected in the kidney, adrenal gland, pancreas, placenta, stomach, ileum, colon and colorectal epithelial cells. These results suggest the production of orexin-A in various human peripheral tissues and orexin-A may also play important roles in some peripheral organs.

Stimulation of catecholamine synthesis by orexin-A in bovine adrenal medullary cells through orexin receptor 1

Orexin-A has recently been identified as a new hypothalamic peptide working as a mediator in the regulation of feeding behavior and sleep control. To determine the role of orexin-A in peripheral metabolic processes, we examined direct effects of orexin-A on catecholamine synthesis and secretion in cultured bovine adrenal medullary cells. Incubation of cells with orexin-A (100 pM) for 20 min caused a small but significant increase in 14C-catecholamine synthesis from [14C]tyrosine, but not from L-3,4-dihydroxyphenyl[3-14C]alanine. Orexin-A (100 pM) potentiated the stimulatory effects of acetylcholine (0.3 mM) on 14C-catecholamine synthesis. Orexin-A significantly increased tyrosine hydroxylase activity, which was evident at 1 pM and maximal at 100 pM. 4 beta-Phorbol-12 beta-myristate-13 alpha-acetate, an activator of protein kinase C, did not enhance the stimulatory effects of orexin-A on tyrosine hydroxylase activity, while H-7 and staurosporine, inhibitors of protein kinase C, nullified the effects of orexin-A. Orexin-A had little effect on catecholamine secretion from the cells. Orexin receptor 1 (OX(1)R) but not orexin receptor 2 (OX(2)R) mRNA was detected in bovine adrenal medullary cells by reverse transcriptase-polymerase chain reaction. These findings suggest that orexin-A activates tyrosine hydroxylase and then stimulates catecholamine synthesis, probably via activation of the OX(1)R-protein kinase C pathway in adrenal medullary cells.

Gonadal steroids differentially regulate the messenger ribonucleic acid expression of pituitary orexin type 1 receptors and adrenal orexin type 2 receptors

Hypothalamic prepro-orexin as well as pituitary and adrenal orexin receptors are gender-specifically expressed. To assess the regulation by gonadal steroids, we investigated the effect of 17beta-estradiol in female and of testosterone in male rats on prepro-orexin and orexin receptor mRNA expression. Rats were either sham-operated or gonadectomized and subsequently treated with placebo, 17beta-estradiol, or testosterone for 21 d. Tissue mRNA levels of prepro-orexin, orexin type-1 (OX(1)), and orexin type-2 (OX(2)) receptors were measured using quantitative real-time RT-PCR. In female rats, pituitary OX(1) receptor mRNA levels were increased 12-fold after ovariectomy compared with sham- operated rats. The increase of pituitary OX(1) receptor mRNA was inhibited by treatment with 17beta-estradiol. Adrenal mRNA levels of OX(2) receptors in ovariectomized rats were increased 2-fold compared with sham-operated rats and were also reduced by treatment with 17beta-estradiol. In male rats, orchidectomy increased the mRNA levels of pituitary OX(1) receptors compared with sham-operated rats. In contrast, adrenal OX(2) receptor mRNA was reduced after orchidectomy. Testosterone treatment reversed the effect of orchidectomy on pituitary OX(1) and adrenal OX(2) receptors. In the hypothalamus, no differences were found in the mRNA levels of prepro-orexin, OX(1), and OX(2) receptors between sham-operated, placebo-treated, and steroid-treated female or male rats. Our results indicate that gonadal steroids differentially regulate pituitary OX(1) receptors and adrenal OX(2) receptors in male and female rats and may contribute to specific sex- dependent neuroendocrine and endocrine actions of orexins.

Plasma orexin-A-like immunoreactivity in patients with sleep apnea hypopnea syndrome

Orexin-A (hypocretin-1), a neuropeptide produced in hypothalamus, stimulates arousal. We studied plasma concentrations of orexin-A-like immunoreactivity (orexin-A-LI) in 156 patients with sleep apnea hypopnea syndrome (SAHS) and 22 control subjects. Plasma orexin-A-LI levels were significantly decreased in 156 patients with SAHS (4.4+/-0.15 pmol/l, mean+/-S.E.) as compared with controls (5.3+/-0.45 pmol/l). The levels were decreased in parallel with the severity of sleep-related respiratory disturbance and magnitude of sleep fragmentation. These findings raise the possibility that a low plasma level of orexin-A-LI may be a marker to show the severity of the disease in patients with SAHS.

Glucose regulates the release of orexin-a from the endocrine pancreas

Orexins (hypocretins) are novel neuropeptides that appear to play a role in the regulation of energy balances. Orexin-A (OXA) increases food intake in rodents, and fasting activates OXA neurons in both the lateral hypothalamic area and gut. OXA is also found in the endocrine pancreas; however, little is known about its release or functional significance. In this study, we show that depolarizing stimuli evoke the release of OXA from rat pancreatic islets in a calcium-dependent manner. Moreover, OXA release is stimulated by low glucose (2.8 mmol/l), similar to glucagon secretion, and inhibited by high glucose (16.7 mmol/l). Fasting increases plasma OXA, supporting the idea that orexin is released in response to hypoglycemia. Cells that secrete glucagon and insulin contain OXA and both cell types express orexin receptors. OXA increases glucagon secretion and decreases glucose-stimulated insulin release from isolated islets. OXA infusion increases plasma glucagon and glucose levels and decreases plasma insulin in fasted rats. We conclude that orexin-containing islet cells, like those in the brain and gut, are glucosensitive and part of a network of glucose "sensing" cells that becomes activated when blood glucose levels fall. OXA may modulate islet hormone secretion to maintain blood glucose levels during fasting.

Orexins and the treatment of obesity

Orexin-A and -B are two peptides derived by proteolytic cleavage from a 130-amino acid precursor, prepro-orexin, which were recently isolated from the rat hypothalamus. Orexin-A is fully conserved across mammalian species, whilst rat and human orexin-B differ by two amino acids. These peptides bind to two Gq-coupled receptors, termed orexin-1 and orexin-2. The receptors are 64% homologous and highly conserved across species. Orexin-A is equipotent at orexin-1 and orexin-2 receptors, whilst orexin-B displays moderate (approximately 10 fold) selectivity for orexin-2 receptors. The distribution and pharmacology of the orexin peptides and their receptors indicate that they play a role in various regulatory systems including energy homeostasis and the regulation of feeding, the evidence for which is reviewed here.

The physiology and pharmacology of the orexins

Orexin-A and orexin-B are two peptides derived by proteolytic cleavage from a 130 amino acid precursor prepro-orexin, which recently were isolated from the rat hypothalamus. Orexin-A is fully conserved across mammalian species, whilst rat and human orexin-B differ by 2 amino acids. These peptides bind to two G(q)-coupled receptors, termed OX(1) and OX(2). The receptors are 64% homologous and highly conserved across species. Orexin-A is equipotent at OX(1) and OX(2), whilst orexin-B displays moderate ( approximately 10-fold) selectivity for OX(2). Prepro-orexin is found in the hypothalamus and, to a markedly lesser extent, the testes, adrenals, and myenteric plexus. However, orexin-A and orexin-B are found throughout the CNS, due to extrahypothalamic projections, as well as in the adrenals and small intestine. OX(1) is expressed mainly in the hypothalamus and locus coeruleus, as well as other brain regions and the spinal cord. OX(2) is expressed in the hypothalamus, cortex, spinal cord, and a few discrete brain nuclei. Both receptors are also expressed in the gut. The orexins modulate feeding behaviour and energy homeostasis, as well as associated drinking behaviours, and also regulate the sleep-wake cycle. Moreover, disruption of prepro-peptide expression or mutations in the gene encoding OX(2) result in a narcoleptic phenotye in various animal models, whilst several clinical studies have linked disruption of the orexin system to narcolepsy in humans. The orexins also have cardiovascular and neuroendocrine effects. This review further details the pharmacology and localisation of these peptides and summarises the evidence for their role in the physiology outlined above.

Anorectic, thermogenic and anti-obesity activity of a selective orexin-1 receptor antagonist in ob/ob mice

A single dose of the orexin-1 (OX1) receptor antagonist 1-(2-methylbenzoxazol-6-yl)-3-[1,5] naphthyridin-4-yl urea hydrochloride (SB-334867-A) reduces orexin-A-induced feeding and natural feeding in Sprague Dawley rats. In this study, the anti-obesity effects of SB-334867-A were determined in genetically obese (ob/ob) mice dosed with SB-334867-A (30 mg/kg, i.p.) once daily for 7 days, and then twice daily for a further 7 days. SB-334867-A reduced cumulative food intake and body weight gain over 14 days. Total fat mass gain, determined by Dual Emission X-ray Absorptiometry, was reduced, while gain in fat-free mass was unchanged. Fasting (5 h) blood glucose was also reduced at the end of the study, with a trend to reduced plasma insulin. Interscapular brown adipose tissue (BAT) weight was reduced, the tissue was noticeably darker in colour and quantitative PCR (TaqMan) analysis of this tissue showed a trend to an increase in uncoupling protein-1 mRNA expression, suggesting that SB-334867-A might stimulate thermogenesis. This was confirmed in a separate study in which a single dose of SB-334867-A (30 mg/kg, i.p.) increased metabolic rate over 4 h in ob/ob mice. OX1 receptor mRNA was detected in BAT, and its expression was increased by 58% by treatment with SB-334867-A. This is the first demonstration that OX1 receptor antagonists have potential as both anti-obesity and anti-diabetic agents.

Hypocretin/orexin suppresses corticotroph responsiveness in vitro

The hypocretin/orexins (Hcrts/ORXs) are peptides produced in neurons in the lateral hypothalamic area that project to neuroendocrine centers in the hypothalamus. Hcrt/ORX receptors are present in the hypothalamus and anterior pituitary gland. We examined the possibility that the Hcrts/ORXs, which we have demonstrated previously to act in the brain to stimulate sympathetic function, could alter stress hormone secretion by a direct pituitary action. In vitro studies revealed a dose-related inhibitory effect of the Hcrts/ORXs on corticotropin-releasing hormone-stimulated ACTH secretion that appeared to be mediated via the orexin-1 receptor and to be expressed at doses (threshold dose 1 nM orexin A) similar to the affinity constant for the receptor. The effect was not due to abrogation of the cAMP response of the corticotroph to corticotropin-releasing hormone and was not pertussis toxin sensitive, suggesting a non-G(i)-mediated mechanism. Instead, a G(q)-mediated signaling mechanism was indicated by the ability of protein kinase C blockade with calphostin C to reverse the inhibitory action of orexin A. Orexin A and orexin B did not significantly alter basal ACTH secretion in vitro and did not alter basal or releasing factor-stimulated secretion of luteinizing hormone, prolactin, thyroid-stimulating hormone or growth hormone from cells harvested from male or random-cycle female donors. Our data suggest a direct, pituitary action of the Hcrts/ORXs to modulate the endocrine response to stress and identify the potential cellular mechanism of a unique biological action of the peptides in the anterior pituitary gland.

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

Orexins are produced specifically by neurons located in the lateral hypothalamus. Recent results suggested peripheral actions of orexins. Therefore, we analyzed the mRNA expression of prepro-orexin and the orexin receptor subtypes OX(1) and OX(2) in peripheral rat tissues. Using real-time quantitative RT-PCR we detected significant amounts of prepro-orexin mRNA in testis, but not in ovaries. OX(1) receptor mRNA was highly expressed in the brain and at lower levels in the pituitary gland. Only small amounts of OX(1) receptor mRNA were found in other tissues such as kidney, adrenal, thyroid, testis, ovaries, and jejunum. Very high levels of OX(2) receptor mRNA, 4-fold higher than in brain, were found in adrenal glands of male rats. Low amounts of OX(2) receptor mRNA were present in lung and pituitary. In adrenal glands, OX(2) receptor mRNA was localized in the zona glomerulosa and reticularis by in situ hybridization, indicating a role in adrenal steroid synthesis and/or release. OX(1) receptor mRNA in the pituitary and OX(2) receptor mRNA in the adrenal gland were much higher in male than in female rats. In the hypothalamus, OX(1) receptor mRNA was slightly elevated in female rats. The differential mRNA expression of orexin receptor subtypes in peripheral organs indicates discrete peripheral effects of orexins and the existence of a peripheral orexin system. This is supported by the detection of orexin A in rat plasma. Moreover, the sexually dimorphic expression of OX(1) and OX(2) receptors in the hypothalamus, pituitary, and adrenal glands suggests gender-specific roles of orexins in the control of endocrine functions.