Orexinergic innervation of POMC-containing neurons in the rat arcuate nucleus

Orexin and MCH neurons: regulators of sleep and metabolism

Sleep-wake and fasting-feeding are tightly coupled behavioral states that require coordination between several brain regions. The mammalian lateral hypothalamus (LH) is a functionally and anatomically complex brain region harboring heterogeneous cell populations that regulate sleep, feeding, and energy metabolism. Significant attempts were made to understand the cellular and circuit bases of LH actions. Rapid advancements in genetic and electrophysiological manipulation help to understand the role of discrete LH cell populations. The opposing action of LH orexin/hypocretin and melanin-concentrating hormone (MCH) neurons on metabolic sensing and sleep-wake regulation make them the candidate to explore in detail. This review surveys the molecular, genetic, and neuronal components of orexin and MCH signaling in the regulation of sleep and metabolism.

Metabolic disturbances in children with narcolepsy: a retrospective study

STUDY OBJECTIVES

To determine the prevalence of metabolic syndrome (MS) in children with narcolepsy and to evaluate their clinical and sleep characteristics according to the different components of MS.

METHODS

This retrospective study consisted of 58 de novo children with narcolepsy (median age: 12.7 years, 48.3% of boys). The recently published MS criteria in a French population of children were used. Clinical and sleep characteristics were compared between groups with different components of MS.

RESULTS

MS was present in 17.2% of children with narcolepsy, among whom 79.3% presented with high homeostasis model assessment for insulin resistance (HOMA-IR), 25.9% with high body mass index, 24.1% with low high-density lipoprotein cholesterol (HDL-C), and 12.1% with high triglycerides. Patients with at least two MS components had more night eating behaviors and tended to have lower percentage of slow-wave sleep and more fragmented sleep. On multiple sleep latency test, they had shorter mean sleep latencies to rapid eye movement (REM), non-REM sleep and tended to have more sleep onset REM periods (SOREMPs) than those with less than two MS components.

CONCLUSIONS

Insulin resistance was found to be the core metabolic disturbance in obese as well as in nonobese children with narcolepsy. Children with narcolepsy with at least two MS components presented a more severe daytime sleepiness and a higher prevalence of night-eating behaviors than those with less than two MS components. Such children might benefit from early evaluation and management in order to prevent future complications.

The Lateral Hypothalamus: An Uncharted Territory for Processing Peripheral Neurogenic Inflammation

The roles of the hypothalamus and particularly the lateral hypothalamus (LH) in the regulation of inflammation and pain have been widely studied. The LH consists of a parasympathetic area that has connections with all the major parts of the brain. It controls the autonomic nervous system (ANS), regulates feeding behavior and wakeful cycles, and is a part of the reward system. In addition, it contains different types of neurons, most importantly the orexin neurons. These neurons, though few in number, perform critical functions such as inhibiting pain transmission and interfering with the reward system, feeding behavior and the hypothalamic pituitary axis (HPA). Recent evidence has identified a new role for orexin neurons in the modulation of pain transmission associated with several inflammatory diseases, including rheumatoid arthritis and ulcerative colitis. Here, we review recent findings on the various physiological functions of the LH with special emphasis on the orexin/receptor system and its role in mediating inflammatory pain.

To eat or to sleep: That is a lateral hypothalamic question

The lateral hypothalamus (LH) is a functionally and anatomically complex brain region that is involved in the regulation of many behavioral and physiological processes including feeding, arousal, energy balance, stress, reward and motivated behaviors, pain perception, body temperature regulation, digestive functions and blood pressure. Despite noteworthy experimental efforts over the past decades, the circuit, cellular and synaptic bases by which these different processes are regulated by the LH remains incompletely understood. This knowledge gap links in large part to the high cellular heterogeneity of the LH. Fortunately, the rapid evolution of newer genetic and electrophysiological tools is now permitting the selective manipulation, typically genetically-driven, of discrete LH cell populations. This, in turn, permits not only assignment of function to discrete cell groups, but also reveals that considerable synergistic and antagonistic interactions exist between key LH cell populations that regulate feeding and arousal. For example, we now know that while LH melanin-concentrating hormone (MCH) and orexin/hypocretin neurons both function as sensors of the internal metabolic environment, their roles regulating sleep and arousal are actually opposing. Additional studies have uncovered similarly important roles for subpopulations of LH GABAergic cells in the regulation of both feeding and arousal. Herein we review the role of LH MCH, orexin/hypocretin and GABAergic cell populations in the regulation of energy homeostasis (including feeding) and sleep-wake and discuss how these three cell populations, and their subpopulations, may interact to optimize and coordinate metabolism, sleep and arousal. This article is part of the Special Issue entitled 'Hypothalamic Control of Homeostasis'.

Neuropeptide W-Induced Hypophagia is Mediated Through Corticotropin-Releasing Hormone-Containing Neurons

Neuropeptide W (NPW), which was originally isolated from the porcine hypothalamus, has been identified as the endogenous ligand for both the NPBWR1 (GPR7) and NPBWR2 (GPR8) receptors. These receptors, which belong to the orphan G protein-coupled receptor (GPCR) family, share a high sequence homology with the opioid and somatostatin receptor families. NPW and NPBWR1 are widely distributed in the rat central nervous system (CNS). While the intracerebroventricular (i.c.v.) injection of NPW elevates plasma corticosterone levels, the intravenous administration of NPW in conjunction with a corticotropin-releasing hormone (CRH) antagonist blocks NPW-induced corticosterone secretion. It has been reported that NPW is involved in regulating the hypothalamus-pituitary-adrenal cortex (HPA) axis and that i.c.v. administration of NPW decreases feeding behavior. The aim of the present study was to ascertain if NPW's role in feeding regulation is mediated (or not) through corticotropin-releasing hormone (CRH)-containing neurons. We found that NPW-containing axon terminals make synapses with CRH-immunoreactive cell bodies and dendritic processes in the hypothalamic paraventricular nucleus (PVN). The central infusion of NPW significantly induced c-Fos expression in CRH-immunoreactive neurons in the mouse PVN, but not in vasopressin- or oxytocin-immunoreactive neurons. To determine if NPW regulates feeding behavior through CRH neurons, the feeding behavior of mice was studied following the i.c.v. administration NPW in the presence or absence of pretreatment with a CRH antagonist. While NPW administration decreased feeding activity, the CRH antagonist inhibited this effect. These results strongly suggest that NPW regulates feeding behavior through CRH neurons in the mouse brain.

Hypocretin (orexin) regulates glutamate input to fast-spiking interneurons in layer V of the Fr2 region of the murine prefrontal cortex

We studied the effect of hypocretin 1 (orexin A) in the frontal area 2 (Fr2) of the murine neocortex, implicated in the motivation-dependent goal-directed tasks. In layer V, hypocretin stimulated the spontaneous excitatory postsynaptic currents (EPSCs) on fast-spiking (FS) interneurons. The effect was accompanied by increased frequency of miniature EPSCs, indicating that hypocretin can target the glutamatergic terminals. Moreover, hypocretin stimulated the spontaneous inhibitory postsynaptic currents (IPSCs) on pyramidal neurons, with no effect on miniature IPSCs. This action was prevented by blocking 1) the ionotropic glutamatergic receptors; 2) the hypocretin receptor type 1 (HCRTR-1), with SB-334867. Finally, hypocretin increased the firing frequency in FS cells, and the effect was blocked when the ionotropic glutamate transmission was inhibited. Immunolocalization confirmed that HCRTR-1 is highly expressed in Fr2, particularly in layer V-VI. Conspicuous labeling was observed in pyramidal neuron somata and in VGLUT1+ glutamatergic terminals, but not in VGLUT2+ fibers (mainly thalamocortical afferents). The expression of HCRTR-1 in GABAergic structures was scarce. We conclude that 1) hypocretin regulates glutamate release in Fr2; 2) the effect presents a presynaptic component; 3) the peptide control of FS cells is indirect, and probably mediated by the regulation of glutamatergic input onto these cells.

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

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

G-protein-gated inwardly rectifying K+ channel 4 (GIRK4) immunoreactivity in chemically defined neurons of the hypothalamic arcuate nucleus that control body weight

G-protein-gated inwardly rectifying K(+) channels (GIRKs; also called Kir3) are a family of K(+) channels, which are activated (opened) via a signal transduction cascade starting with ligand-stimulated G-protein-coupled receptors (GPCRs). Four GIRK genes have been identified (GIRK1-4). GIRK4 (Kir3.4) has a role in regulating energy homeostasis, since mice with a targeted mutation in the GIRK4 gene exhibit a predisposition to late-onset obesity. GIRK4 mRNA is expressed in hypothalamic regions that harbor neurons involved in the regulation of food intake and body weight. Using goat and rabbit antisera to the GIRK4 protein, the cellular localization and transmitter content of GIRK4-immunoreactive neurons was determined in the hypothalamic arcuate nucleus, a region that contains neurons which are accessible to circulating hormones and is intimately associated with the control of body weight. GIRK4-immunoreactive large cell bodies were demonstrated in the ventrolateral part of the arcuate nucleus, with smaller neuronal cell bodies in the ventromedial part of the nucleus. Double-labeling showed presence of GIRK4 immunoreactivity in large neurons of the ventrolateral arcuate nucleus containing the peptides α-melanocyte-stimulating hormone (α-MSH), a marker for pro-opiomelanocortin (POMC) neurons, and cocaine- and amphetamine-regulated transcript (CART). GIRK4 immunoreactivity was also seen in neurons of the ventromedial part of the arcuate nucleus containing agouti-regulated peptide (AgRP) and neuropeptide Y (NPY). The results suggest that the GIRK4 channel protein plays a role in regulating membrane excitability in chemically defined neurons of the arcuate nucleus that control body weight.

Electrophysiological analysis of circuits controlling energy homeostasis

Since the discovery of leptin and the central melanocortin circuit, electrophysiological studies have played a major role in elucidating mechanisms underlying energy homeostasis. This review highlights the contribution of findings made by electrophysiological measurements to the current understanding of hypothalamic neuronal networks involved in energy homeostasis with a specific focus on the arcuate-paraventricular nucleus circuit.

Developmental programming of energy balance and its hypothalamic regulation

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

Loss of leptin receptors on hypothalamic POMC neurons alters synaptic inhibition

Adaptive changes in hypothalamic neural circuitry occur in response to alterations in nutritional status. This plasticity at hypothalamic synapses contributes to the control of food intake and body weight. Here we show that genetic ablation of leptin receptor gene expression in pro-opiomelanocortin (POMC) neurons (POMC: Lepr(-/-) GFP) induces alterations at synapses on POMC neurons in the arcuate nucleus of the hypothalamus. Our studies reveal that POMC: Lepr(-/-) GFP mice have decreased frequency of spontaneous GABAergic, but not glutamatergic, postsynaptic currents at synapses on POMC neurons. The decay time course of GABAergic spontaneous inhibitory postsynaptic currents (sIPSCs) onto POMC neurons in POMC: Lepr(-/-) GFP mice is significantly slower than that of sIPSCs in control animals. While analysis of individual miniature IPSCs shows lowered baseline activity, this tonic decrease is associated with an increased amplitude and slow decay of mini-IPSCs onto POMC neurons in POMC: Lepr(-/-) GFP mice. Moreover, POMC neurons receive greater total ionic flux per GABAergic event in the absence of leptin receptor signaling. In addition, treatment with the alpha 3 subunit-containing GABA(A) receptor modulator SB-205384 enhances GABAergic transmission only onto POMC neurons in POMC: Lepr(-/-) GFP mice. Single-cell RT-PCR analysis further supports the expression of the alpha 3 subunit of the GABA(A) receptor on POMC neurons in POMC: Lepr(-/-) GFP mice. Finally, the responses to the GABA(A) receptor agonist isoguvacine of POMC neurons are significantly smaller in POMC: Lepr(-/-) GFP than in control animals. Therefore our present work demonstrates that loss of leptin signaling in POMC neurons induces synaptic alterations at POMC synapses that may play an essential role in energy homeostasis.

Integrating GHS into the Ghrelin System

Oligopeptide derivatives of metenkephalin were found to stimulate growth-hormone (GH) release directly by pituitary somatotrope cells in vitro in 1977. Members of this class of peptides and nonpeptidyl mimetics are referred to as GH secretagogues (GHSs). A specific guanosine triphosphatate-binding protein-associated heptahelical transmembrane receptor for GHS was cloned in 1996. An endogenous ligand for the GHS receptor, acylghrelin, was identified in 1999. Expression of ghrelin and homonymous receptor occurs in the brain, pituitary gland, stomach, endothelium/vascular smooth muscle, pancreas, placenta, intestine, heart, bone, and other tissues. Principal actions of this peptidergic system include stimulation of GH release via combined hypothalamopituitary mechanisms, orexigenesis (appetitive enhancement), insulinostasis (inhibition of insulin secretion), cardiovascular effects (decreased mean arterial pressure and vasodilation), stimulation of gastric motility and acid secretion, adipogenesis with repression of fat oxidation, and antiapoptosis (antagonism of endothelial, neuronal, and cardiomyocyte death). The array of known and proposed interactions of ghrelin with key metabolic signals makes ghrelin and its receptor prime targets for drug development.

Effect of anorexinergic peptides, cholecystokinin (CCK) and cocaine and amphetamine regulated transcript (CART) peptide, on the activity of neurons in hypothalamic structures of C57Bl/6 mice involved in the food intake regulation

The hypothalamus plays an important role in food consumption, receiving information about energy balance via hormonal, metabolic, and neural inputs. Its neurons produce neuropeptides influencing energy balance. Especially important to regulation of food consumption are certain hypothalamic structures, including the arcuate (ARC) and ventromedial (VMN) nuclei and the lateral hypothalamic area (LHA). We determined the impact of cholecystokinin (CCK) and cocaine and amphetamine regulated transcript (CART) peptide, on activity of ARC and VMN neurons and hypocretin (Hcrt) synthesizing neurons in LHA. ARC is an integrative nucleus regulating food consumption, VMN is considered to be a satiety centre, and LHA a hunger sensing centre. After overnight fasting, male C57Bl/6 mice received intraperitoneal injection of (i.p.) saline (SAL) or CCK (4microg/kg) or intracerebroventricular injection of (i.c.v.) CART peptide (0.1microg/mice) or CCK (i.p.) followed by CART peptide (i.c.v.) 5min later. Sixty minutes later, the presence of Fos or Fos/Hcrt immunostaining indicated activity of ARC and VMN neurons, as well as of Hcrt cells in LHA. CCK alone did not influence neuronal activity in any of the nuclei studied. CART peptide stimulated neurons in ARC and VMN (p<0.01) but decreased Hcrt neuronal activity in LHA (p<0.05). Co-administration of both peptides synergistically stimulated ARC neurons (p<0.01) and asynergistically inhibited LHA Hcrt neurons (p<0.01). Results indicate that CCK may modify the effect of CART peptide and thus substantially influence activity of neurons in hypothalamic structures involved in regulation of food intake.

Role of central melanocortin pathways in energy homeostasis

The rise in the global prevalence of human obesity has emphasized the need for a greater understanding of the physiological mechanisms that underlie energy homeostasis. Numerous circulating nutritional cues and central neuromodulatory signals are integrated within the brain to regulate both short- and long-term nutritional state. The central melanocortin system represents a crucial point of convergence for these signals and, thus, has a fundamental role in regulating body weight. The melanocortin ligands, synthesized in discrete neuronal populations within the hypothalamus and brainstem, modulate downstream homeostatic signalling via their action at central melanocortin-3 and -4 receptors. Intimately involved in both ingestive behaviour and energy expenditure, the melanocortin system has garnered much interest as a potential therapeutic target for human obesity.

Orexin-A hyperphagia: hindbrain participation in consummatory feeding responses

Orexin-A (ORXA) is an orexigenic neuropeptide produced by the lateral hypothalamus that increases food intake when injected into the brain ventricles or forebrain nuclei. We used a licking microstructure analysis to evaluate hindbrain and forebrain ORXA effects in intact and hindbrain-lesioned rats, to identify the motivational and anatomical bases of ORXA hyperphagia. Intact rats with cannulas in the fourth brain ventricle (4V) received vehicle (artificial cerebrospinal fluid) or ORXA (0.1, 0.4, 1, or 10 nm) injections before 90 min access to 0.1 m sucrose. Meal size and frequency were increased in a double-dissociated manner by the 1 and 10 nm doses, respectively. In experiment 2, 4V 1 nm ORXA was applied to rats offered solutions varied in caloric and gustatory intensity (water and 0.1 and 1 m sucrose). ORXA increased meal frequency for all tastants. ORXA increased meal size only for 0.1 m sucrose, by prolonging the meal without affecting early ingestion rate or lick burst size, suggesting that 4V ORXA influenced inhibitory postingestive feedback rather than taste evaluation. In experiment 3, rats with cannulas in the third ventricle (3V) received dorsal medullary lesions centered on the area postrema (APX group) or sham procedures, and licking for water and 0.1 and 1 m sucrose was evaluated after 1 nm 3V ORXA/artificial cerebrospinal fluid injections. The 3V ORXA increased 0.1 m sucrose meal size and meal frequency for all tastants in the sham group, as observed after 4V ORXA in experiment 2. In the APX group, 3V ORXA injections influenced meal frequency, but they no longer increased meal size. However, the APX rats increased meal size for 0.1 m sucrose after food and water deprivation and after 3V angiotensin II injection. They also showed meal size suppression after 3V injection of the melanocortin-3/4 receptor agonist melanotan II (1 nm). These findings suggest that the area postrema and subjacent nucleus of the solitary tract are necessary for increases in consummatory (meal size) but not appetitive (meal frequency) responses to 3V ORXA. The meal size increases may be due to reduced postingestive feedback inhibition induced by ORXA delivered to either the hindbrain or forebrain ventricles.

Neuroendocrine proopiomelanocortin neurons are excited by hypocretin/orexin

Hypocretin/orexin, produced by a group of neurons in the lateral hypothalamus/perifornical area, enhances cognitive arousal and also may play a crucial role in modulating the neuroendocrine system. How hypocretin modulates the endocrine system remains an open question. Hypocretin cells innervate the mediobasal hypothalamus where they can potentially influence the activity of specific cell populations within the arcuate nucleus. Here, we examine whether hypocretin modulates the median eminence-projecting proopiomelanocortin (POMC) neurons identified by selective green fluorescent protein expression and antidromic stimulation or retrograde Evans blue dye tracing in transgenic mice. We find that POMC neurons, in general, and, in addition, those that project their axons to the median eminence, were robustly activated by hypocretin in a dose-dependent manner. These excitatory actions included a threefold increase in spike frequency and direct membrane depolarization of up to 22 mV (mean, 17.9+/-7.2 mV). Direct postsynaptic depolarization was decreased at more positive membrane potentials, inhibited by the sodium-calcium exchanger antagonist KB-R7943, and reduced by lowering the bath temperature, or by buffering the postsynaptic calcium with BAPTA, suggesting that the primary mechanism for hypocretin-mediated excitation is the activation of the sodium-calcium exchanger. Hypocretin also enhanced excitatory inputs to POMC cells via a presynaptic mechanism and indirectly increased the release of GABA onto these cells in a spike-dependent manner. However, these synaptic actions were not necessary to cause postsynaptic membrane depolarization and spiking. Thus, in contrast to previous suggestions that hypocretin inhibited POMC cells, our results demonstrate robust direct excitation of POMC neurons by hypocretin.

High prevalence of eating disorders in narcolepsy with cataplexy: a case-control study

STUDY OBJECTIVES

To study the prevalence of and symptoms of eating disorders in patients with narcolepsy.

DESIGN

We performed a case-control study comparing symptoms of eating disorders in patients with narcolepsy versus healthy population controls, using the Schedules for Clinical Assessment in Neuropsychiatry (SCAN 2.1). To study whether an increased body mass index (BMI) could be responsible for symptoms of an eating disorder, we also compared patients with BMI-matched controls, using the SCAN as well as the Eating Disorder Examination-Questionnaire.

SETTING

University hospital.

PATIENTS AND PARTICIPANTS

Patients with narcolepsy/cataplexy (n = 60) were recruited from specialized sleep centers. Healthy controls (n = 120) were drawn from a population study previously performed in the Netherlands. Separately, 32 BMI-matched controls were recruited.

INTERVENTIONS

N/A.

MEASUREMENTS AND RESULTS

In total, 23.3% of the patients fulfilled the criteria for a clinical eating disorder, as opposed to none of the control subjects. Most of these were classified as Eating Disorder-Not Otherwise Specified, with an incomplete form of binge eating disorder. On the symptom level, half of the patients reported a persistent craving for food, as well as binge eating. Twenty-five percent of patients even reported binging twice a week or more often. When compared with BMI-matched controls, the significant increases persisted in symptoms of eating disorders among patients with narcolepsy. Except for a higher level of interference in daily activities due to eating problems in patients using antidepressants, medication use did not influence our findings.

CONCLUSIONS

The majority of patients with narcolepsy experience a number of symptoms of eating disorders, with an irresistible craving for food and binge eating as the most prominent features. Eating disorder symptomatology interfered with daily activities. These findings justify more attention for eating disorders in the treatment of patients with narcolepsy.

Neurocircuits integrating hormone and nutrient signaling in control of glucose metabolism

As obesity, diabetes, and associated comorbidities are on a constant rise, large efforts have been put into better understanding the cellular and molecular mechanisms by which nutrients and metabolic signals influence central and peripheral energy regulation. For decades, peripheral organs as a source and a target of such cues have been the focus of study. Their ability to integrate metabolic signals is essential for balanced energy and glucose metabolism. Only recently has the pivotal role of the central nervous system in the control of fuel partitioning been recognized. The rapidly expanding knowledge on the elucidation of molecular mechanisms and neuronal circuits involved is the focus of this review.

Orexin signaling in the ventral tegmental area is required for high-fat appetite induced by opioid stimulation of the nucleus accumbens

The overriding of satiety and homeostatic control mechanisms by cognitive, rewarding, and emotional aspects of palatable foods may contribute to the evolving obesity crisis, but little is known about neural pathways and mechanisms responsible for crosstalk between the "cognitive" and "metabolic" brain in the control of appetite. Here we show that neural connections between the nucleus accumbens and hypothalamus might be part of this link. Using the well known model of selective stimulation of high-fat intake induced by intra-accumbens injection of the mu-opioid receptor agonist D-Ala2-N-Me-Phe4-gly5-ol-enkephalin (DAMGO), we demonstrate that orexin signaling in the ventral tegmental area is important for this reward-driven appetite to override metabolic repletion signals in presatiated rats. We further show that accumbens DAMGO in the absence of food selectively increases the proportion of orexin neurons expressing c-Fos in parts of the perifornical hypothalamus and that neural projections originating in DAMGO-responsive sites of the nucleus accumbens make close anatomical contacts with hypothalamic orexin neurons. These findings suggest that direct accumbens-hypothalamic projections can stimulate hypothalamic orexin neurons, which in turn through orexin-1 receptor signaling in the ventral tegmental area and possibly other sites interfaces with the motivational and motor systems to increase intake of palatable food.

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

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

Electrical inhibition of identified anorexigenic POMC neurons by orexin/hypocretin

Proopiomelanocortin (POMC) neurons of the hypothalamic arcuate nucleus (ARC) suppress appetite, and lack of POMC-derived peptides or electrical silencing of POMC neurons causes obesity. ARC POMC neurons are surrounded by nerve terminals containing the wakefulness-promoting peptides orexins/hypocretins, but whether orexin affects their electrical activity has not been tested directly. Here we identify living ARC POMC cells in mouse brain slices by targeted expression of green fluorescent protein. Using whole-cell patch-clamp recordings, we show that orexin suppresses the spontaneous action potential firing of these neurons. Orexin-induced inhibition involves membrane hyperpolarization, a decreased excitatory synaptic drive, and an increased frequency of GABAergic inputs. Our results suggest a reduction in the electrical activity of ARC POMC neurons, which is mediated by changes in presynaptic inputs, contributes to the appetite-enhancing action of orexins.

Galanin-like peptide promotes feeding behaviour via activation of orexinergic neurones in the rat lateral hypothalamus

Galanin-like peptide (GALP) is produced in neurones in the hypothalamic arcuate nucleus and is implicated in the neural control of feeding behaviour. Previously, we have reported that GALP immunoreactive fibres were in direct contact with orexin/hypocretin immunoreactive neurones in the rat lateral hypothalamus using double-immunofluorescence. Centrally administered GALP is known to stimulate feeding behaviour. However, the target neurones of this action have not been clarified. The present study aimed to determine features of the GALP-mediated neuronal feeding pathway in rat. Accordingly, at the ultrastructural level, GALP-immunoreactive axon terminals were found to make synapses on orexin/hypocretin immunoreactive cell bodies and dendritic processes in the lateral hypothalamus. c-Fos immunoreactivity was expressed in orexin/hypocretin-immunoreactive neurones but not in melanin concentrating hormone-immunoreactive neurones in the lateral hypothalamus at 90 min after the application of GALP by i.c.v. infusion. Furthermore, to determine whether GALP regulates feeding behaviour via orexin/hypocretin neurones, the feeding behaviour of rats was studied following GALP i.c.v. injection with or without anti-orexin A and B immunoglobulin (IgG) pretreatment. The anti-orexin IgGs markedly inhibited GALP-induced hyperphagia. These results suggest that orexin/hypocretin-containing neurones in the lateral hypothalamus are targeted by GALP, and that GALP-induced hyperphagia is mediated via orexin/hypocretin neurones in the rat hypothalamus.

Centrally administered orexin A increases motivation for sweet pellets in rats

RATIONALE

Centrally administered orexin A induces both feeding and locomotion in rats. Thus, the feeding response following orexin A administration may be secondary to general increases in activity rather than a specific motivation to eat.

OBJECTIVE

The aim of the study is to determine whether orexin A increases the motivation to eat.

METHODS

The effect of orexin A (0, 31.25, 62.5, 125, 250, and 500 pmol) on breakpoint was determined in male Sprague-Dawley rats with rostro-lateral hypothalamic cannulae under a progressive ratio of five schedule (PR5). The effect of orexin A (0, 31.25, 125, and 500 pmol) on pressing rate under a fixed ratio (20) schedule was obtained to analyze the time course of orexin-A-induced pressing. The effect of 24-h food deprivation on breakpoint under PR5 and the effect of orexin A (125 pmol) on free feeding (sweet pellets) and on open-field locomotor activity (0, 100, 500, and 1,000 pmol) were also tested.

RESULTS

Orexin A significantly augmented free feeding of sweet pellets, open-field locomotor activity, rate of pressing (FR20 schedule), and breakpoint (PR5 schedule), although compared to 24-h deprivation, the effect of orexin A on breakpoint was mild. However, there was a differential dose response relationship and time course of stimulation between orexin A's effects on locomotion and lever pressing.

CONCLUSION

These data indicate that infusion of orexin A enhances free feeding by enhancing and possibly prolonging motivation to eat.

Characterization of orexin A immunoreactivity in the rat area postrema

The distribution of orexin A immunoreactivity and the synaptic relationships of orexin A-positive neurons in the rat area postrema were studied using both light and electron microscopy techniques. At the light microscope level, numerous orexin A-like immunoreactive fibers were found within the area postrema. Using electron microscopy, immunoreactivity within fibers was confined primarily to the axon terminals, most of which contained dense-cored vesicles. Both axo-somatic and axo-dendritic synapses made by orexin A-like immunoreactive axon terminals were found, with these synapses being both symmetric and asymmetric in form. Orexin A-like immunoreactive axon terminals could be found presynaptic to two different immunonegative profiles including the perikarya and dendrites. Occasionally, some orexin A-like immunoreactive profiles, most likely to be dendrites, could be seen receiving synaptic inputs from immunonegative or immunopositive axon terminals. The present results suggest that the physiological function of orexin A in the area postrema depends on synaptic relationships with other immunopositive and immunonegative neurons, with the action of orexin A mediated via a self-modulation feedback mechanism.

The orexinergic synaptic innervation of serotonin- and orexin 1-receptor-containing neurons in the dorsal raphe nucleus

Orexin/hypocretin has been well demonstrated to excite the serotonergic neurons in the dorsal raphe nucleus (DRN). We studied the morphological relationships between orexin-containing axon terminals and serotonin- as well as orexin-receptor-containing neurons in the dorsal raphe nucleus. Using immunohistochemical techniques at the light microscopic level, orexin A (OXA)-like immunoreactive neuronal fibers in the DRN were found to make close contact with serotonergic neurons, while some of the serotonergic neurons also expressed the orexin 1 receptor (OX1R). At the electron microscopic level, double-immunostaining experiments showed that the orexin A-like immunoreactive fibers were present mostly as axon terminals that made synapses on the serotonin- and orexin 1-receptor-containing neurons. While only axodendritic synapses between orexin A-containing axon terminals and serotonergic neurons were detected, the synapses made by orexin A-containing axon terminals on the orexin 1-receptor-containing neurons were both axodendritic and axosomatic. The present study suggests that excitation effect of orexin A on dorsal raphe serotonergic neurons is via synaptic communication through orexin 1 receptor.

Orexin-A projections to the caudal medulla and orexin-induced c-Fos expression, food intake, and autonomic function

Orexin-expressing neurons in the hypothalamus project throughout the neuraxis and are involved in regulation of the sleep/wake cycle, food intake, and autonomic functions. Here we specifically analyze the anatomical organization of orexin projections to the dorsal vagal complex (DVC) and raphe pallidus and effects on ingestive behavior and autonomic functions of local orexin-A administration in nonanesthetized rats. Retrograde tracing experiments revealed that as many as 20% of hypothalamic orexin neurons project to the DVC, where they form straight varicose axon profiles, some of which are in close anatomical apposition with tyrosine hydroxylase (TH)-, glucagon-like peptide-1-, gamma-aminobutyric acid-, and nitric oxide synthase-immunoreactive neurons in a nonselective manner. Similar contacts were frequently observed with neurons of the nucleus of the solitary tract whose activation by gastrointestinal food stimuli was demonstrated by the expression of nuclear c-Fos immunoreactivity. Orexin-A administration to the fourth ventricle induced significant Fos-expression throughout the DVC compared with saline control injections, with about 20-25% of TH-ir neurons among the stimulated ones. Fourth ventricular orexin injections also significantly stimulated chow and water intake in nonfood-deprived rats. Direct bilateral injections of orexin into the DVC increased intake of palatable high-fat pellets. Orexin-ir fibers also innervated raphe pallidus. Fourth ventricular orexin-A (1 nmol) activated Fos expression in the raphe pallidus and C1/A1 catecholaminergic neurons in the ventral medulla and increased body temperature, heart rate, and locomotor activity. The results confirm that hypothalamomedullary orexin projections are involved in a variety of physiological functions, including ingestive behavior and sympathetic outflow.

The electrophysiology of feeding circuits

Obesity is quickly becoming one of the most common and debilitating disorders of the developed world. More than 60% of American adults are now overweight or obese, predisposing them to a host of chronic diseases. To understand the etiology of obesity, and to discover new therapies for obesity, we must understand the components of energy balance. In simple terms, energy intake (feeding) must equal energy expenditure (physical activity, basal metabolism and adaptive thermogenesis) for body weight homeostasis. To maintain homeostasis, neurocircuitry must sense both immediate nutritional status and the amount of energy stored in adipose tissue, and must be able to provide appropriate output to balance energy intake and energy expenditure. The brain receives various signals that carry information about nutritional and metabolic status including neuropeptide PYY(3-36), ghrelin, cholecystokinin, leptin, glucose and insulin. Circulating satiety signals access the brain either by "leakage" across circumventricular organs or transport across the blood-brain barrier. Signals can also activate sensory vagal terminals that innervate the whole gastrointestinal tract.

Orexins (hypocretins) directly interact with neuropeptide Y, POMC and glucose-responsive neurons to regulate Ca 2+ signaling in a reciprocal manner to leptin: orexigenic neuronal pathways in the mediobasal hypothalamus

Orexin-A and -B (hypocretin-1 and -2) have been implicated in the stimulation of feeding. Here we show the effector neurons and signaling mechanisms for the orexigenic action of orexins in rats. Immunohistochemical methods showed that orexin axon terminals contact with neuropeptide Y (NPY)- and proopiomelanocortin (POMC)-positive neurons in the arcuate nucleus (ARC) of the rats. Microinjection of orexins into the ARC markedly increased food intake. Orexins increased cytosolic Ca(2+) concentration ([Ca(2+)](i)) in the isolated neurons from the ARC, which were subsequently shown to be immunoreactive for NPY. The increases in [Ca(2+)](i) were inhibited by blockers of phospholipase C (PLC), protein kinase C (PKC) and Ca(2+) uptake into endoplasmic reticulum. The stimulation of food intake and increases in [Ca(2+)](i) in NPY neurons were greater with orexin-A than with orexin-B, indicative of involvement of the orexin-1 receptor (OX(1)R). In contrast, orexin-A and -B equipotently attenuated [Ca(2+)](i) oscillations and decreased [Ca(2+)](i) levels in POMC-containing neurons. These effects were counteracted by pertussis toxin, suggesting involvement of the orexin-2 receptor and Gi/Go subtypes of GTP-binding proteins. Orexins also decreased [Ca(2+)](i) levels in glucose-responsive neurons in the ventromedial hypothalamus (VMH), a satiety center. Leptin exerted opposite effects on these three classes of neurons. These results demonstrate that orexins directly regulate NPY, POMC and glucose-responsive neurons in the ARC and VMH, in a manner reciprocal to leptin. Orexin-A evokes Ca(2+) signaling in NPY neurons via OX(1)R-PLC-PKC and IP(3) pathways. These neural pathways and intracellular signaling mechanisms may play key roles in the orexigenic action of orexins.

Hypothalamic control of energy balance: different peptides, different functions

Energy balance is maintained via a homeostatic system involving both the brain and the periphery. A key component of this system is the hypothalamus. Over the past two decades, major advances have been made in identifying an increasing number of peptides within the hypothalamus that contribute to the process of energy homeostasis. Under stable conditions, equilibrium exists between anabolic peptides that stimulate feeding behavior, as well as decrease energy expenditure and lipid utilization in favor of fat storage, and catabolic peptides that attenuate food intake, while stimulating sympathetic nervous system (SNS) activity and restricting fat deposition by increasing lipid metabolism. The equilibrium between these neuropeptides is dynamic in nature. It shifts across the day-night cycle and from day to day and also in response to dietary challenges as well as peripheral energy stores. These shifts occur in close relation to circulating levels of the hormones, leptin, insulin, ghrelin and corticosterone, and also the nutrients, glucose and lipids. These circulating factors together with neural processes are primary signals relaying information regarding the availability of fuels needed for current cellular demand, in addition to the level of stored fuels needed for long-term use. Together, these signals have profound impact on the expression and production of neuropeptides that, in turn, initiate the appropriate anabolic or catabolic responses for restoring equilibrium. In this review, we summarize the evidence obtained on nine peptides in the hypothalamus that have emerged as key players in this process. Data from behavioral, physiological, pharmacological and genetic studies are described and consolidated in an attempt to formulate a clear statement on the underlying function of each of these peptides and also on how they work together to create and maintain energy homeostasis.

Leptin signaling in the hypothalamus: emphasis on energy homeostasis and leptin resistance

Leptin, the long-sought satiety factor of adipocytes origin, has emerged as one of the major signals that relay the status of fat stores to the hypothalamus and plays a significant role in energy homeostasis. Understanding the mechanisms of leptin signaling in the hypothalamus during normal and pathological conditions, such as obesity, has been the subject of intensive research during the last decade. It is now established that leptin action in the hypothalamus in regulation of food intake and body weight is mediated by a neural circuitry comprising of orexigenic and anorectic signals, including NPY, MCH, galanin, orexin, GALP, alpha-MSH, NT, and CRH. In addition to the conventional JAK2-STAT3 pathway, it has become evident that PI3K-PDE3B-cAMP pathway plays a critical role in leptin signaling in the hypothalamus. It is now established that central leptin resistance contributes to the development of diet-induced obesity and ageing associated obesity. Central leptin resistance also occurs due to hyperleptinimia produced by exogenous leptin infusion. A defective nutritional regulation of leptin receptor gene expression and reduced STAT3 signaling may be involved in the development of leptin resistance in DIO. However, leptin resistance in the hypothalamic neurons may occur despite an intact JAK2-STAT3 pathway of leptin signaling. Thus, in addition to defective JAK2-STAT3 pathway, defects in other leptin signaling pathways may be involved in leptin resistance. We hypothesize that defective regulation of PI3K-PDE3B-cAMP pathway may be one of the mechanisms behind the development of central leptin resistance seen in obesity.

Synaptic interactions between ghrelin- and neuropeptide Y-containing neurons in the rat arcuate nucleus

Morphological relationships between neuropeptide Y- (NPY) like and ghrelin-like immunoreactive neurons in the arcuate nucleus (ARC) were examined using light and electron microscopy techniques. At the light microscope level, both neuron types were found distributed in the ARC and could be observed making contact with each other. Using a preembedding double immunostaining technique, some NPY-immunoreactive axon terminals were observed at the electron microscope level to make synapses on ghrelin-immunoreactive cell bodies and dendrites. While the axo-somatic synapses were mostly symmetric in nature, the axo-dendritic synapses were both symmetric and asymmetric. In contrast, ghrelin-like immunoreactive (ghrelin-LI) axon terminals were found to make synapses on NPY-like immunoreactive (NPY-LI) dendrites although no NPY-like immunoreactive perikarya were identified receiving synapses from ghrelin-LI axon terminals. NPY-like axon terminals were also found making synapses on NPY-like neurons. Axo-axonic synapses were also identified between NPY- and ghrelin-like axon terminals. The present study shows that NPY- and ghrelin-LI neurons could influence each other by synaptic transmission through axo-somatic, axo-dendritic and even axo-axonic synapses, and suggests that they participate in a common effort to regulate the food-intake behavior through complex synaptic relationships.

Immunoelectron microscopic examination of orexin-like immunoreactive fibers in the dorsal horn of the rat spinal cord

The ultrastructure and synaptic relationships of orexin A-like immunoreactive neuronal fibers in the dorsal horn of the rat cervical spinal cord were examined at both the light and electron microscopic levels. At the light microscopic level, many intensely immunostained orexin A-like fibers were found, while at the electron microscopic level, immunoreactivity in these fibers was mostly confined to axon terminals. Most of the axon terminals contained dense-cored vesicles. Immunoreactive and immunonegative dense-cored vesicles were occasionally found within the same orexin A-like immunoreactive axon terminals, which were often found making synapses with immunonegative dendrites. These synapses were both asymmetric and symmetric, with the asymmetric ones predominant. Orexin A-like immunoreactive processes that contained no synaptic vesicles were also found with less frequency. These processes were also observed receiving synaptic inputs from immunonegative axon terminals, but the synapses were mostly asymmetric. Sometimes, such processes were found to receive multiple synaptic inputs for which the presynaptic immunonegative axon terminals could make synapses on other immunonegative dendrites simultaneously. Occasionally, synapses between the orexin A-like immunoreactive axon terminals and orexin A-like immunoreactive processes containing no synaptic vesicles were also found. The present results provide solid morphological evidence that orexin A may be involved in pain-inhibition mechanisms in the spinal cord and suggest that this function may be complex and occur in conjunction with the regulatory effects of other neurotransmitters.

Distribution, function, and properties of leptin receptors in the brain

Leptin, a peptide hormone, is implicated in the modulation of food intake and maintenance of energy balance in many vertebrates including humans. It is considered to act via its receptor mainly through several hypothalamic nuclei that play critical roles in the regulation of appetite. This article looks mainly at the functional significance of leptin in rat brain by drawing on published reports of morphological and physiological analyses. Our immunohistochemical observations indicate that the leptin receptor is distributed throughout the brain, including the hypothalamus, and interestingly, is found in the hippocampus and neocortex. Physiological experiments with single living cells isolated from fresh rat hypothalamus clearly demonstrate that leptin has a significant effect on feeding-regulating neurons in the hypothalamus. Studies to date support a role for leptin not only in modulating food intake and appetite in rats and humans, but also in relation to learning and memory processes.

Hypothalamic neuronal networks and feeding-related peptides involved in the regulation of feeding

The hypothalamus is a region of the brain that plays a critical role in feeding regulation. It has been revealed by various physiological experiments that the feeding-regulating center is confined to the ventromedial hypothalamus, lateral hypothalamus (LH) and arcuate nucleus (ARC). Many kinds of neurons in these areas of the hypothalamus express factors such as melanin-concentrating hormone (MCH), neuropeptide Y (NPY), proopiomelanocortin (POMC), orexin (OX) and ghrelin, which have been implicated in feeding regulation. In tissues of the periphery, two critical factors involved in feeding regulation, leptin and ghrelin, have been identified. Both hormone peptides are secreted mainly from adipose and stomach tissue, respectively, and are considered to function via their receptors mainly through several hypothalamic nuclei that play important roles in the regulation of appetite. The present review looks mainly at the functional significance of feeding-regulation factors, such as those described above, and the humoral and neuronal interactions among these compounds in the hypothalamus by drawing on published reports of morphological and physiological analyses. Immunohistochemical and in situ hybridization experiments indicate that both leptin and ghrelin receptors are distributed in the hypothalamus and that there are reciprocal interactions between MCH and OX neurons in the LH. Morphological and physiological studies on single living cells isolated from fresh rat hypothalamus or with receptor agonist and antagonist combined with immunohistochemisry clearly demonstrate that both leptin and OX reciprocally regulate NPY- and POMC-containing neurons in the ARC and that ghrelin may regulate feeding status independently through direct OX and NPY pathways. In this way, cross-talking systems in the hypothalamus play a role in determining feeding states.

Orexin excites GABAergic neurons of the arcuate nucleus by activating the sodium--calcium exchanger

The neuropeptides orexins/hypocretins are essential for normal wakefulness and energy balance, and disruption of their function causes narcolepsy and obesity. Although much is known of the role of orexins in sleep/wake behavior, it remains unclear how they stimulate feeding and metabolism. One of the main targets of orexinergic neurons is the arcuate nucleus (ARC) of the hypothalamus, which plays a key role in feeding and energy homeostasis. By combining patch-clamp and RT-multiplex PCR analysis of individual neurons in mouse brain slices, we show that an electrophysiologically distinct subset of ARC neurons coexpress orexin receptors and glutamate decarboxylase-67 and are excited by orexin. Acting on postsynaptic orexin type 2 receptors, orexin activates a sodium-calcium exchange current, thereby depolarizing the cell and increasing its firing frequency. Because GABA is a potent stimulus for feeding, in both the ARC and its main projection site, these results suggest a mechanism for how orexin may control appetite.

Electron microscopic examination of the orexin immunoreactivity in the dorsal raphe nucleus

The ultrastructure and the synaptic relationships of the orexin-A-like immunoreactive fibers in the dorsal raphe nucleus were examined with an immunoelectron microscopic method. At the electron microscopic level, most of the immunoreactive fibers, a varicosity appearance at the light microscopic level, were found as axon terminals. The large dense-cored vesicles contained in the immunoreactive axon terminals were the most intensely immunostained organellae. These axon terminals were often found to make synapses. While the axo-dendritic synapses were usually asymmetric in appearance, the axo-somatic synapses were symmetric. Orexin-A-like immunoreactive processes with no synaptic vesicles were also found. These processes often received asymmetric synapses. With less frequency, the synapses were found between the orexin-like immunoreactive processes. The results suggest that the orexin peptides are stored in the large dense-cored vesicles; the orexin-containing fibers may have influences on the physiological activities of the dorsal raphe nucleus through direct synaptic relationships.

Peptides that regulate food intake: appetite-inducing accumbens manipulation activates hypothalamic orexin neurons and inhibits POMC neurons

Corticolimbic circuits involving the prefrontal cortex, amygdala, and ventral striatum determine the reward value of food and might play a role in environmentally induced obesity. Chemical manipulation of the nucleus accumbens shell (AcbSh) has been shown to elicit robust feeding and Fos expression in the hypothalamus and other brain areas of satiated rats. To determine the neurochemical phenotype of hypothalamic neurons receiving input from the AcbSh, we carried out c-Fos/peptide double-labeling immunohistochemistry in various hypothalamic areas known to contain feeding peptides, from rats that exhibited a significant feeding response after AcbSh microinjection of the GABA(A) agonist muscimol. In the perifornical area, a significantly higher percentage of orexin neurons expressed Fos after muscimol compared with saline injection. In contrast, Fos expression was not induced in melanin-concentrating hormone and cocaine-amphetamine-related transcript (CART) neurons. In the arcuate nucleus, Fos activation was significantly lower in neurons coexpressing CART and proopiomelanocortin, and there was a tendency for higher Fos expression in neuropeptide Y neurons. In the paraventricular nucleus, no significant activation of oxytocin and CART neurons was found. Thus AcbSh manipulation may elicit food intake through coordinated stimulation of hypothalamic neurons expressing orexigenic peptides and suppression of neurons expressing anorexigenic peptides. However, activation of many neurons not expressing these peptides suggests that additional peptides/transmitters in the lateral hypothalamus and accumbens projections to other brain areas might also be involved.

Peptides that regulate food intake: orexin gene expression is increased during states of hypertriglyceridemia

Previous reports implicate the orexins in eating and body weight regulation. This study investigated possible functional relationships between hypothalamic orexins and circulating hormones or metabolites. In situ hybridization and quantitative PCR were used to examine orexin expression in the perifornical hypothalamus (PF) of rats and mice on diets varying in fat content and with differential propensity toward obesity. The results showed that orexin gene expression was stimulated by a high-fat diet in close association with elevated triglyceride levels, suggesting a functional relationship between these measures. Results obtained in obesity-prone rats and mice revealed a similar increase in orexin in close relation to triglycerides. A direct test of this orexin-triglyceride link was performed with Intralipid, which increased PF orexin expression along with circulating triglycerides. Whereas PF galanin is similarly stimulated by dietary fat, double-labeling immunofluorescence studies showed that orexin and galanin neurons are anatomically distinct. This evidence suggests that the orexins, like galanin, are "fat-responsive" peptides that respond to circulating lipids.

Orexin-A-sensitive site for energy expenditure localized in the arcuate nucleus of the hypothalamus

Orexin-A is a unique hypothalamic neuropeptide that stimulates both food intake and energy expenditure, although orexigenic peptides usually have coordinated effects on fat storage by increasing food intake and decreasing energy expenditure. Here we investigated the site of action of orexin-A-induced thermogenesis in urethane-anesthetized rats. Microinjection of 1-10 pmol orexin-A into the arcuate nucleus (Arc) specifically increased whole-body O(2) consumption (VO(2)), an index of energy expenditure; whereas it had no effect on VO(2) when injected into the paraventricular nucleus (PVN), dorsomedial nucleus (DMH), lateral hypothalamus (LH), ventromedial nucleus (VMH) or medial preoptic nucleus (MPO) of the hypothalamus or into in the paraventricular thalamic nucleus (PVT) or pontine locus coeruleus (LC). VO(2) increased immediately after an orexin-A injection into the Arc, and this increase was accompanied by a simultaneous tachycardiac response and a gradual increase in colonic temperature (T(co)), whereas an injection of the saline vehicle into the Arc had no effect. The effective dose of orexin-A into the Arc was 10 times less than that into the cerebral ventricle to induce a similar level of response. In addition, intracerebroventricular administration of orexin-A (100 pmol) elicited a significantly smaller VO(2) response in Arc-lesioned rats than that in sham-operated control rats. These results suggest that the orexin-induced energy expenditure is mediated, at least in part, by the Arc.

Co-existence of leptin- and orexin-receptors in feeding-regulating neurons in the hypothalamic arcuate nucleus-a triple labeling study

The arcuate nucleus (ARC) of the hypothalamus has been identified as a prime feeding regulating center in the brain. Several feeding regulating peptides, such as neuropeptide Y (NPY) and proopiomelanocortin (POMC), are present in neurons of the ARC, which also serves as a primary targeting site for leptin, a feeding inhibiting hormone secreted predominantly by adipose tissues, and for orexin (OX)-containing neurons. OX is expressed exclusively around the lateral hypothalamus, an area also established as a feeding regulating center. Some recent physiological analyses have shown that NPY- and POMC-containing neurons are activated or inactivated by leptin and OX. Moreover, we have already shown, using double immunohistochemical staining techniques, that NPY- and POMC-containing neurons express leptin receptors (LR) and orexin type 1 receptors (OX-1R). However, no morphological study has yet described the possibility of whether or not these arcuate neurons are influenced by both leptin and OX simultaneously. In order to address this issue, we performed histochemistry on ARC neurons using a triple immunofluorescence method. We found that 77 out of 213 NPY- and 99 out of 165 POMC-immunoreactive neurons co-localized with both LR- and OX-1R-immunoreactivities. These findings strongly suggest that both NPY- and POMC-containing neurons are regulated simultaneously by both leptin and OX.

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.

Orexins and appetite regulation

Initial research on the functional significance of two novel hypothalamic neuropeptides, orexin-A and orexin-B, suggested an important role in appetite regulation. Since then, however, these peptides have also been shown to influence a wide range of other physiological and behavioural processes. In this paper, we review the now quite extensive literature on orexins and appetite control, and consider their additional effects within this context. Although the evidence for orexin (particularly orexin-A and the orexin-1 receptor) involvement in many aspects of ingestive physiology and behaviour is incontrovertible, central administration of orexins is also associated with increased EEG arousal and wakefulness, locomotor activity and grooming, sympathetic and HPA activity, and pain thresholds. Since the orexin system is selectively activated by signals indicating severe nutritional depletion, it would be highly adaptive for a hungry animal not only to seek sustenance but also to remain fully alert to dangers in the environment. Crucial evidence indicates that orexin-A increases food intake by delaying the onset of a behaviourally normal satiety sequence. In contrast, a selective orexin-1 receptor antagonist (SB-334867) suppresses food intake and advances the onset of a normal satiety sequence. These data suggest that orexin-1 receptors mediate the episodic signalling of satiety and appear to bridge the transition from eating to resting in the rats' feeding-sleep cycle. The argument is developed that the diverse physiological and behavioural effects of orexins can best be understood in terms of an integrated set of reactions which function to rectify nutritional status without compromising personal survival. Indeed, many of the non-ingestive effects of orexin administration are identical to the cluster of active defences mediated via the lateral and dorsolateral columns of the midbrain periaqueductal gray matter, i.e., somatomotor activation, vigilance, tachycardia, hypertension and non-opioid analgesia. In our view, therefore, the LH orexin system is very well placed to orchestrate the diverse subsystems involved in foraging under potentially dangerous circumstances, i.e., finding and ingesting food without oneself becoming a meal for someone else.

Ultrastructural localization of orexin-1 receptor in pre- and post-synaptic neurons in the rat arcuate nucleus

The distribution and ultrastructural localization of the orexin-1 receptor (OX(1)R) in the rat arcuate nucleus were studied using immunocytochemical techniques. OX(1)R-containing neurons were found throughout the nucleus, but were concentrated in its posterior region. Both OX(1)R-positive perikarya and dendrites were found to receive synapses from other unknown axon terminals. In addition, a small number of OX(1)R-positive axon terminals were observed. These OX(1)R-immunoreactive axon terminals were often found to make synapses on unidentified immunonegative dendritic processes. The present results indicate that orexin may act on food intake regulating neurons through both pre- and post-synaptic OX(1)R.

Multiple neural systems controlling food intake and body weight

Discovery of the leptin receptor and its downstream peptidergic pathways has reconfirmed the crucial role of the hypothalamus in the regulation of food intake and energy balance. Strategically located in the midst of the mammalian neuraxis, the hypothalamus receives at least three distinct types of relevant information via direct or indirect neural connections as well as hormone receptors and substrate sensors bestowed on hypothalamic neurons. First, the medial and to a lesser extent the lateral hypothalamus receive a rich mix of information pertaining to the internal state of relative energy repletion/depletion. Second, specific hypothalamic nuclei receive information about the behavioral state, such as diurnal clock, physical activity-level, reproductive cycle, developmental stage, as well as imminent (e.g. fight and flight) and chronic (e.g. infection) stressors, that can potentially impact on short-term availability of fuels and long-term energy balance. Third, the hypothalamus, particularly its lateral aspects, receives information from areas in the forebrain involved in the acquisition, storage, and retrieval of sensory representations of the external food space and internal food experience, as well as from the executive forebrain involved in behavior selection and initiation. In addition, rich intrahypothalamic connections facilitate further distribution of incoming information to various hypothalamic nuclei. On the other hand, the hypothalamus has widespread neural projections to the same cortical areas it receives inputs, and many hypothalamic neurons are one synapse away from most endocrine systems and from both sympathetic and parasympathetic effector organs involved in the flux, storage, mobilization, and utilization of fuels. It is argued that processing within cortico-limbic areas and communication with hypothalamic areas are particularly important in human food intake control that is more and more guided by cognitive rather than metabolic aspects in the obesigenic environment of affluent societies. A distributed neural network for the control of food intake and energy balance consisting of a central processor and several parallel processing loops is hypothesized. Detailed neurochemical, anatomical, and functional analysis of reciprocal connections of the numerous peptidergic neuron populations in the hypothalamus with extrahypothalamic brain areas will be necessary to better understand what hypothalamus, forebrain, and brainstem tell each other and who is in charge under specific conditions of internal and external nutrient availability.