Chronic intraparaventricular nuclear administration of orexin A in male rats does not alter thyroid axis or uncoupling protein-1 in brown adipose tissue

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.

The effects of orexin-A and orexin receptors on anxiety- and depression-related behaviors in a male rat model of post-traumatic stress disorder

Orexin neurons play an important role in stress-related mental disorders including post-traumatic stress disorder (PTSD). Anxiety- and depression-related symptoms commonly occur in combination with PTSD. However, the role of the orexin system in mediating alterations in these affective symptoms remains unclear. The medial prefrontal cortex (mPFC) is implicated in both cognitive and emotional processing. In the present study, we investigated anxiety- and depression-related behavioral changes using the elevated plus maze, the sucrose preference test, and the open field test in male rats with single prolonged stress (SPS) induced-PTSD. The expression of orexin-A in the hypothalamus and orexin receptors (OX1R and OX2R) in the mPFC was detected and quantified by immunohistochemistry, western blotting, and real-time polymerase chain reaction. We found that the SPS rats exhibited enhanced levels of anxiety, reduced exploratory activities, and anhedonia. Furthermore, SPS resulted in reductions in the expression of orexin-A in the hypothalamus and the increased the expression of OX1R in the mPFC. The intracerebroventricular administration of orexin-A alleviated behavioral changes in SPS rats and partly restored the increased levels of OX1R in the mPFC. These results suggest that the orexin system plays a role in the anxiety- and depression-related symptoms observed in PTSD.

Control of Adipose Cell Browning and Its Therapeutic Potential

Adipose tissue is the largest endocrine organ in humans and has an important influence on many physiological processes throughout life. An increasing number of studies have described the different phenotypic characteristics of fat cells in adults. Perhaps one of the most important properties of fat cells is their ability to adapt to different environmental and nutritional conditions. Hypothalamic neural circuits receive peripheral signals from temperature, physical activity or nutrients and stimulate the metabolism of white fat cells. During this process, changes in lipid inclusion occur, and the number of mitochondria increases, giving these cells functional properties similar to those of brown fat cells. Recently, beige fat cells have been studied for their potential role in the regulation of obesity and insulin resistance. In this context, it is important to understand the embryonic origin of beige adipocytes, the response of adipocyte to environmental changes or modifications within the body and their ability to transdifferentiate to elucidate the roles of these cells for their potential use in therapeutic strategies for obesity and metabolic diseases. In this review, we discuss the origins of the different fat cells and the possible therapeutic properties of beige fat cells.

Orexins/Hypocretins: Key Regulators of Energy Homeostasis

Originally described to be involved in feeding regulation, orexins/hypocretins are now also considered as major regulatory actors of numerous biological processes, such as pain, sleep, cardiovascular function, neuroendocrine regulation, and energy expenditure. Therefore, they constitute one of the most pleiotropic families of hypothalamic neuropeptides. Although their orexigenic effect is well documented, orexins/hypocretins also exert central effects on energy expenditure, notably on the brown adipose tissue (BAT) thermogenesis. A better comprehension of the underlying mechanisms and potential interactions with other hypothalamic molecular pathways involved in the modulation of food intake and thermogenesis, such as AMP-activated protein kinase (AMPK) and endoplasmic reticulum (ER) stress, is essential to determine the exact implication and pathophysiological relevance of orexins/hypocretins on the control of energy balance. Here, we will review the actions of orexins on energy balance, with special focus on feeding and brown fat function.

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.

Traveling from the hypothalamus to the adipose tissue: The thermogenic pathway

Brown adipose tissue (BAT) is a specialized tissue critical for non-shivering thermogenesis producing heat through mitochondrial uncoupling; whereas white adipose tissue (WAT) is responsible of energy storage in the form of triglycerides. Another type of fat has been described, the beige adipose tissue; this tissue emerges in existing WAT depots but with thermogenic ability, a phenomenon known as browning. Several peripheral signals relaying information about energy status act in the brain, particularly the hypothalamus, to regulate thermogenesis in BAT and browning of WAT. Different hypothalamic areas have the capacity to regulate the thermogenic process in brown and beige adipocytes through the sympathetic nervous system (SNS). This review discusses important concepts and discoveries about the central control of thermogenesis as a trip that starts in the hypothalamus, and taking the sympathetic roads to reach brown and beige fat to modulate thermogenic functions.

A screen for modulators reveals that orexin-A rapidly stimulates thyrotropin releasing hormone expression and release in hypothalamic cell culture

In the paraventricular nucleus of the mammalian hypothalamus, hypophysiotropic thyrotropin releasing hormone (TRH) neurons integrate metabolic information and control the activity of the thyroid axis. Additional populations of TRH neurons reside in various hypothalamic areas, with poorly defined connections and functions, albeit there is evidence that some may be related to energy balance. To establish extracellular modulators of TRH hypothalamic neurons activity, we performed a screen of neurotransmitters effects in hypothalamic cultures. Cell culture conditions were chosen to facilitate the full differentiation of the TRH neurons; these conditions had permitted the characterization of the effects of known modulators of hypophysiotropic TRH neurons. The major end-point of the screen was Trh mRNA levels, since they are generally rapidly (0.5-3h) modified by synaptic inputs onto TRH neurons; in some experiments, TRH cell content or release was also analyzed. Various modulators, including histamine, serotonin, β-endorphin, met-enkephalin, and melanin concentrating hormone, had no effect. Glutamate, as well as ionotropic agonists (kainate and N-Methyl-d-aspartic acid), increased Trh mRNA levels. Baclofen, a GABA_B receptor agonist, and dopamine enhanced Trh mRNA levels. An endocannabinoid receptor 1 inverse agonist promoted TRH release. Somatostatin increased Trh mRNA levels and TRH cell content. Orexin-A rapidly increased Trh mRNA levels, TRH cell content and release, while orexin-B decreased Trh mRNA levels. These data reveal unaccounted regulators, which exert potent effects on hypothalamic TRH neurons in vitro.

Hypothalamus and thermogenesis: Heating the BAT, browning the WAT

Brown adipose tissue (BAT) has been also considered as the main thermogenic organ responsible of maintenance body temperature through heat production. However, a new type of thermogenic fat has been characterized during the last years, the beige or brite fat, that is developed from white adipose tissue (WAT) in response to different stimuli by a process known as browning. The activities of brown and beige adipocytes ameliorate metabolic disease, including obesity in mice and correlate with leanness in humans. Many genes and pathways that regulate brown and beige adipocyte biology have now been identified, providing a variety of promising therapeutic targets for metabolic disease. The hypothalamus is the main central place orchestrating the outflow signals that drive the sympathetic nerve activity to BAT and WAT, controlling heat production and energy homeostasis. Recent data have revealed new hypothalamic molecular mechanisms, such as hypothalamic AMP-activated protein kinase (AMPK), that control both thermogenesis and browning. This review provides an overview of the factors influencing BAT and WAT thermogenesis, with special focus on the integration of peripheral information on hypothalamic circuits controlling thermoregulation.

Contribution of adaptive thermogenesis to the hypothalamic regulation of energy balance

Obesity and its related disorders are among the most pervasive diseases in contemporary societies, and there is an urgent need for new therapies and preventive approaches. Given (i) our poor social capacity to correct unhealthy habits, and (ii) our evolutionarily genetic predisposition to store excess energy as fat, the current environment of caloric surplus makes the treatment of obesity extremely difficult. During the last few decades, an increasing number of methodological approaches have increased our knowledge of the neuroanatomical basis of the control of energy balance. Compelling evidence underlines the role of the hypothalamus as a homeostatic integrator of metabolic information and its ability to adjust energy balance. A greater understanding of the neural basis of the hypothalamic regulation of energy balance might indeed pave the way for new therapeutic targets. In this regard, it has been shown that several important peripheral signals, such as leptin, thyroid hormones, oestrogens and bone morphogenetic protein 8B, converge on common energy sensors, such as AMP-activated protein kinase to modulate sympathetic tone on brown adipose tissue. This knowledge may open new ways to counteract the chronic imbalance underlying obesity. Here, we review the current state of the art on the role of hypothalamus in the regulation of energy balance with particular focus on thermogenesis.

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

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

Orexins (hypocretins) and energy balance: More than feeding

Initially implicated in the regulation of feeding, orexins/hypocretins are now acknowledged to play a major role in the control of a wide variety of biological processes, such as sleep, energy expenditure, pain, cardiovascular function and neuroendocrine regulation, a feature that makes them one of the most pleiotropic families of hypothalamic neuropeptides. While the orexigenic effect of orexins is well described, their central effects on energy expenditure and particularly on brown adipose tissue (BAT) thermogenesis are not totally unraveled. Better understanding of these actions and their possible interrelationship with other hypothalamic systems controlling thermogenesis, such as AMP-activated protein kinase (AMPK) and endoplasmic reticulum (ER) stress, will help to clarify the exact role and pathophysiological relevance of these neuropeptides have on energy balance.

Circulating Levels of Orexin-A, Nesfatin-1, Agouti-Related Peptide, and Neuropeptide Y in Patients with Hyperthyroidism

BACKGROUND

There is insufficient information about the appetite-related hormones orexin-A, nesfatin-1, agouti-related peptide (AgRP), and neuropeptide Y (NPY) in hyperthyroidism. The aim of the present study was to investigate the effects of hyperthyroidism on the basal metabolic rate (BMR) and energy intake, orexin-A, nesfatin-1, AgRP, NPY, and leptin levels in the circulation, and their relationship with each other and on appetite.

METHODS

In this prospective study, patients were evaluated in hyperthyroid and euthyroid states in comparison with healthy subjects. Twenty-one patients with overt hyperthyroidism and 33 healthy controls were included in the study.

RESULTS

Daily energy intake in the hyperthyroid state was found to be higher than that in the euthyroid state patient group (p=0.039). BMR was higher in hyperthyroid patients than the control group (p=0.018). Orexin-A was lower and nesfatin-1 was higher in hyperthyroid patients compared to the controls (p<0.001), whereas orexin-A increased and nesfatin-1 decreased after euthyroidism (p=0.003, p<0.001). No differences were found in the AgRP, NPY, and leptin levels between the hyperthyroid and euthyroid states and controls (p>0.05). Orexin-A correlated negatively with nesfatin-1 (p=0.042), BMR (p=0.013), free triiodothyronine (fT3; p<0.001), and free thyroxine (fT4; p<0.001) and positively with thyrotropin (TSH; p<0.001). Nesfatin-1 correlated negatively with orexin-A (p=0.042) and TSH (p<0.001) and positively with fT3 (p=0.005) and fT4 (p=0.001). In the regression analysis, "diagnosis of hyperthyroidism" was the main factor affecting orexin-A (p<0.001).

CONCLUSIONS

Although it seems that no relationship exists among orexin-A, nesfatin-1, and increased appetite in hyperthyroidism, the orexin-A and nesfatin-1 levels are markedly affected by hyperthyroidism.

The brain and brown fat

Brown adipose tissue (BAT) is a specialized organ responsible for thermogenesis, a process required for maintaining body temperature. BAT is regulated by the sympathetic nervous system (SNS), which activates lipolysis and mitochondrial uncoupling in brown adipocytes. For many years, BAT was considered to be important only in small mammals and newborn humans, but recent data have shown that BAT is also functional in adult humans. On the basis of this evidence, extensive research has been focused on BAT function, where new molecules, such as irisin and bone morphogenetic proteins, particularly BMP7 and BMP8B, as well as novel central factors and new regulatory mechanisms, such as orexins and the canonical ventomedial nucleus of the hypothalamus (VMH) AMP- activated protein kinase (AMPK)-SNS-BAT axis, have been discovered and emerged as potential drug targets to combat obesity. In this review we provide an overview of the complex central regulation of BAT and how different neuronal cell populations co-ordinately work to maintain energy homeostasis.

The hypocretins/orexins: integrators of multiple physiological functions

The hypocretins (Hcrts), also known as orexins, are two peptides derived from a single precursor produced in the posterior lateral hypothalamus. Over the past decade, the orexin system has been associated with numerous physiological functions, including sleep/arousal, energy homeostasis, endocrine, visceral functions and pathological states, such as narcolepsy and drug abuse. Here, we review the discovery of Hcrt/orexins and their receptors and propose a hypothesis as to how the orexin system orchestrates these multifaceted physiological functions.

Orexin restores aging-related brown adipose tissue dysfunction in male mice

The aging process causes an increase in percent body fat, but the mechanism remains unclear. In the present study we examined the impact of aging on brown adipose tissue (BAT) thermogenic activity as potential cause for the increase in adiposity. We show that aging is associated with interscapular BAT morphologic abnormalities and thermogenic dysfunction. In vitro experiments revealed that brown adipocyte differentiation is defective in aged mice. Interscapular brown tissue in aged mice is progressively populated by adipocytes bearing white morphologic characteristics. Aged mice fail to mobilize intracellular fuel reserves from brown adipocytes and exhibit deficiency in homeothermy. Our results suggest a role for orexin (OX) signaling in the regulation of thermogenesis during aging. Brown fat dysfunction and age-related assimilation of fat mass were accelerated in mice in which OX-producing neurons were ablated. Conversely, OX injections in old mice increased multilocular morphology, increased core body temperature, improved cold tolerance, and reduced adiposity. These results argue that BAT can be targeted for interventions to reverse age-associated increase in fat mass.

The orexigenic effect of orexin-A revisited: dependence of an intact growth hormone axis

Fifteen years ago orexins were identified as central regulators of energy homeostasis. Since then, that concept has evolved considerably and orexins are currently considered, besides orexigenic neuropeptides, key modulators of sleep-wake cycle and neuroendocrine function. Little is known, however, about the effect of the neuroendocrine milieu on orexins' effects on energy balance. We therefore investigated whether hypothalamic-pituitary axes have a role in the central orexigenic action of orexin A (OX-A) by centrally injecting hypophysectomized, adrenalectomized, gonadectomized (male and female), hypothyroid, and GH-deficient dwarf rats with OX-A. Our data showed that the orexigenic effect of OX-A is fully maintained in adrenalectomized and gonadectomized (females and males) rats, slightly reduced in hypothyroid rats, and totally abolished in hypophysectomized and dwarf rats when compared with their respective vehicle-treated controls. Of note, loss of the OX-A effect on feeding was associated with a blunted OX-A-induced increase in the expression of either neuropeptide Y or its putative regulator, the transcription factor cAMP response-element binding protein, as well as its phosphorylated form, in the arcuate nucleus of the hypothalamus of hypophysectomized and dwarf rats. Overall, this evidence suggests that the orexigenic action of OX-A depends on an intact GH axis and that this neuroendocrine feedback loop may be of interest in the understanding of orexins action on energy balance and GH deficiency.

Orexin is required for brown adipose tissue development, differentiation, and function

Orexin (OX) neuropeptides stimulate feeding and arousal. Deficiency of orexin is implicated in narcolepsy, a disease associated with obesity, paradoxically in the face of reduced food intake. Here, we show that obesity in orexin-null mice is associated with impaired brown adipose tissue (BAT) thermogenesis. Failure of thermogenesis in OX-null mice is due to inability of brown preadipocytes to differentiate. The differentiation defect in OX-null neonates is circumvented by OX injections to OX-null dams. In vitro, OX, triggers the full differentiation program in mesenchymal progenitor stem cells, embryonic fibroblasts and brown preadipocytes via p38 mitogen activated protein (MAP) kinase and bone morphogenetic protein receptor-1a (BMPR1A)-dependent Smad1/5 signaling. Our study suggests that obesity associated with OX depletion is linked to brown-fat hypoactivity, which leads to dampening of energy expenditure. Thus, orexin plays an integral role in adaptive thermogenesis and body weight regulation via effects on BAT differentiation and function.

Cross-talk between orexins (hypocretins) and the neuroendocrine axes (hypothalamic-pituitary axes)

Lesioning and electrical stimulation experiments carried out during the first half of the twentieth century showed that the lateral hypothalamic area (LHA) is involved in the neuroendocrine control of hormone secretion. However, the molecular basis of this phenomenon remained unclear until fifty years later when in 1998, two different laboratories discovered a new family of hypothalamic neuropeptides, the orexins or hypocretins (OX-A/Hcrt1 and OX-B/Hcrt2). Since then, remarkable evidence has revealed that orexins/hypocretins play a prominent role in regulating virtually all the neuroendocrine axes, acting as pivotal signals in the coordination of endocrine responses with regards to sleep, arousal and energy homeostasis. The clinical relevance of these actions is supported by human data showing impairment of virtually all the neuroendocrine axes in orexin/hypocretin-deficient narcoleptic patients. Here, we summarize more than ten years of knowledge about the orexins/hypocretins with particular focus on their role as neuroendocrine regulators. Understanding this aspect of orexin/hypocretin physiology could open new therapeutic possibilities in the treatment of sleep, energy homeostasis and endocrine pathologies.

Role of orexin in the regulation of glucose homeostasis

Orexin-A (hypocretin-1) and orexin-B (hypocretin-2) are hypothalamic neuropeptides that play key roles in the regulation of wakefulness, feeding, reward, autonomic functions and energy homeostasis. To control these functions indispensable for survival, orexin-expressing neurones integrate peripheral metabolic signals, interact with many types of neurones in the brain and modulate their activities via the activation of orexin-1 receptor or orexin-2 receptor. In addition, a new functional role of orexin is emerging in the regulation of insulin and leptin sensitivities responsible for whole-body glucose metabolism. Recent evidence indicates that orexin efficiently protects against the development of peripheral insulin resistance induced by ageing or high-fat feeding in mice. In particular, the orexin receptor-2 signalling appears to confer resistance to diet-induced obesity and insulin insensitivity by improving leptin sensitivity. In fact, the expression of orexin gene is known to be down-regulated by hyperglycaemia in the rodent model of diabetes, such as ob/ob and db/db mice. Moreover, the levels of orexin receptor-2 mRNA have been shown to decline in the brain of mice along with ageing. These suggest that hyperglycaemia due to insulin insensitivity during ageing or by habitual consumption of a high-fat diet leads to the reduction in orexin expression in the hypothalamus, thereby further exacerbating peripheral insulin resistance. Therefore, orexin receptor controlling hypothalamic insulin/leptin actions may be a new target for possible future treatment of hyperglycaemia in patients with type 2 diabetes.

Daily intraparaventricular orexin-A treatment induces weight loss in rats

The neuropeptide orexin (hypocretin) increases energy expenditure partially through increasing spontaneous physical activity. The ability of exogenous orexin to alter body weight has never been established, however. We sought to determine whether orexin-A microinjected into the paraventricular nucleus of the hypothalamus (PVN) induced weight loss in rats. Chronic guide cannulae were implanted into rats, aimed at the PVN. Rats were given daily microinjections of orexin (0.5 nmol) or vehicle into the PVN for 6 days; food intake and body weight were measured daily. In a separate group of rats, we injected orexin-A and vehicle intra-PVN and measured daily activity levels. Daily orexin treatment induced weight loss: orexin-A-treated rats lost significantly more weight than their vehicle-injected counterparts without a significant difference in food intake. Rats were significantly more active after intra-PVN orexin compared to vehicle. These results support the concept that orexinergic agents have the potential to produce negative energy balance through increasing physical activity. This presents a promising, untapped potential resource for weight loss.

Food intake in rhesus monkeys following central administration of orexins

Orexin A has been reported to stimulate food intake in rats while orexin B does not. The purpose of this study was to determine the role of orexin A or orexin B administration on food intake in adult, male rhesus monkeys. Food intake was measured at 2 and 8 h after the morning feeding following central injections of vehicle, orexin A (10, or 20 microg) or orexin B (10, 30, or 100 microg). When compared to vehicle injections, the 10 and 20 microg doses of orexin A decreased food intake at 2 h post-dose by 45% and 64%, respectively. Eight-hour food intake was decreased at only the 20 microg orexin A dose. Orexin B at all doses and time points did not alter food intake when compared to vehicle. These results indicate that orexin A exhibits anorectic activity while orexin B does not affect food intake in the rhesus monkey.

Dual regulatory effects of orexins on sympathetic nerve activity innervating brown adipose tissue in rats

This study examined how orexin regulates the activity of the sympathetic nerves that innervate brown adipose tissue (BAT) in rats. Infusion of orexin A at a dose of 0.3 nmol into the third cerebral ventricle decreased BAT sympathetic nerve activity, compared with the effect of PBS (P < 0.05), whereas infusion of orexin B at the same dose caused a significant increase (P < 0.05). Pretreatment with a third cerebral ventricle injection of 2.24 micromol/kg alpha-fluoromethylhistidine, an irreversible inhibitor of the histamine-synthesizing enzyme histidine decarboxylase, attenuated the orexin B-induced response of BAT sympathetic nerve activity, but not that induced by orexin A. These results indicate that orexins may regulate both BAT energy expenditure and thermogenesis through their dual effects on sympathetic nerve activity. In particular, orexin B regulates BAT sympathetic nerve activity via neuronal histamine in the hypothalamus.

Altered setting of the pituitary-thyroid ensemble in hypocretin-deficient narcoleptic men

Narcolepsy is a sleep disorder caused by disruption of hypocretin (orexin) neurotransmission. Injection of hypocretin-1 acutely suppresses TRH and TSH release in rats. In contrast, subchronic administration does not appear to affect the hypothalamo-pituitary-thyroid ensemble in animals. We explored (in 7 patients and 7 controls) whether hypocretin deficiency impacts circulating TSH levels and circadian timing of TSH release in narcoleptic humans. Plasma TSH concentration profiles (blood samples taken at 10-min intervals during 24 h) and TSH levels in response to TRH injection were analyzed by Cluster, robust regression, approximate entropy (ApEn), and deconvolution. Circulating TSH levels were lower in patients, which was primarily attributable to lower pulse amplitude and nadir concentrations. TSH secretion correlated positively with mean 24-h leptin levels (R2 = 0.46, P = 0.02) and negatively with amount of sleep (R2 = 0.29, P = 0.048). Pattern-synchrony between 24-h leptin and TSH concentrations was demonstrated by significant cross-correlation and cross-ApEn analyses with no differences between controls and patients. Sleep onset was closely associated with a fall in circulating TSH. Features of diurnal rhythmicity of circulating TSH fluctuations were similar in patients and controls, with the acrophase occurring shortly after midnight. Thyroxine and triiodothyronine concentrations were similar in patients and controls and did not display a diurnal rhythm. The response of plasma TSH levels to TRH was also similar in both groups. Sleep patterns in narcoleptics were significantly disorderly compared with controls, as measured by ApEn (P = 0.006). In summary, circulating TSH concentrations are low in hypocretin-deficient narcoleptic men, which could be attributable to their low plasma leptin levels and/or their abnormal sleep-wake cycle.

Chronic administration of NMU into the paraventricular nucleus stimulates the HPA axis but does not influence food intake or body weight

Hypothalamic neuromedin U (NMU) appears to have a role in the regulation of appetite and the hypothalamo-pituitary-adrenal (HPA) axis. Acute administration of NMU into the paraventricular nuclei (iPVN) increases plasma adrenocorticotrophic hormone and corticosterone, and inhibits food intake in fasted rats. No studies have as yet investigated the chronic effects of centrally administered NMU. We investigated the effect of twice-daily iPVN injections of 0.3 nmol NMU for 7 days on food intake, body weight, the HPA axis, and behavior in freely fed rats. Chronic iPVN NMU was not associated with a decrease in food intake or body weight. Chronic iPVN NMU produced a typical behavioral response on day 1 and day 4 of the study, and resulted in the elevation of plasma corticosterone present 18 h after the final injection. These results suggest NMU may have a role in the regulation of the HPA axis and behavior.

Repeated administration of the anorectic factor prolactin-releasing peptide leads to tolerance to its effects on energy homeostasis

Central administration of a single dose of prolactin-releasing peptide (PrRP) causes a reduction in both fast-induced and nocturnal food intake and body weight gain. The aim of this study was to examine the effect of repeated administration of PrRP on energy homeostasis, including a measure of the expression of the mitochondrial uncoupling protein-1 (UCP-1) in brown adipose tissue. Conscious, free-feeding animals received central injections of PrRP (4 nmol icv) or vehicle. A single injection at 1000 caused a sustained hyperthermia over the 4-h test period and an increase in the expression of UCP-1 mRNA. Repeated, twice daily injection caused a reduction in body weight gain greater than that seen in pair-fed animals for the first 48-72 h. After 72 h, the animals became refractory to the actions of PrRP. The pair-fed group showed a reduction in UCP-1 mRNA expression at 48 h, which was reversed by PrRP treatment. This study indicates that PrRP exerts its effects on energy homeostasis in the short-medium term by reducing food intake and increasing energy expenditure.

Functions of the orexinergic/hypocretinergic system

Orexin A and orexin B are hypothalamic peptides that act on their targets via two G protein-coupled receptors (OX1 and OX2 receptors). In the central nervous system, the cell bodies producing orexins are localized in a narrow region within the lateral hypothalamus and project mainly to regions involved in feeding, sleep, and autonomic functions. Via putative pre- and postsynaptic effects, orexins increase synaptic activity in these regions. In isolated neurons and cells expressing recombinant receptors orexins cause Ca2+ elevation, which is mainly dependent on influx. The activity of orexinergic cells appears to be controlled by feeding- and sleep-related signals via a variety of neurotransmitters/hormones from the brain and other tissues. Orexins and orexin receptors are also found outside the central nervous system, particularly in organs involved in feeding and energy metabolism, e.g., gastrointestinal tract, pancreas, and adrenal gland. In the present review we focus on the physiological properties of the cells that secrete or respond to orexins.