Leptin sensitive neurons in the hypothalamus.

A major paradigm in the field of obesity research is the existence of an adipose tissue-brain endocrine axis for the regulation of body weight. Leptin, the peptide mediator of this axis, is secreted by adipose cells. It lowers food intake and body weight by acting in the hypothalamus, a region expressing an abundance of leptin receptors and a variety of neuropeptides that influence food intake and energy balance. Among the most promising candidates for leptin-sensitive cells in the hypothalamus are arcuate nucleus neurons that co-express the anabolic neuropeptides, neuropeptide Y (NPY) and agouti-related peptide (AGRP), and those that express proopiomelanocortin (POMC), the precursor of the catabolic peptide, alphaMSH. These cell types contain mRNA encoding leptin receptors and show changes in neuropeptide gene expression in response to changes in food intake and circulating leptin levels. Decreased leptin signaling in the arcuate nucleus is hypothesized to increase the expression of NPY and AGRP. Levels of leptin receptor mRNA and leptin binding are increased in the arcuate nucleus during fasting, principally in NPY/AGRP neurons. These findings suggest that changes in leptin receptor expression in the arcuate nucleus are inversely associated with changes in leptin signaling, and that the arcuate nucleus is an important target of leptin action in the brain.     

Triiodothyronine (T3) stimulates food intake via enhanced hypothalamic AMP-activated kinase activity

Thyroid hormone regulates food intake. We previously reported that rats with triiodothyronine (T3)-induced thyrotoxicosis display hyperphagia associated with suppressed circulating leptin levels, increased hypothalamic neuropeptide Y (NPY) mRNA and decreased hypothalamic pro-opiomelanocortin (POMC) mRNA. AMP-activated kinase (AMPK) is a serine/threonine protein kinase that is activated when cellular energy is depleted. We hypothesized that T3 causes an increase in hypothalamic AMPK activity, which in turn contributes to the development of T3-induced hyperphagia. Rats that were given s.c. injections of T3 (4.5 nmol/kg) had increased food intake 2 h later without alterations in NPY and POMC mRNA levels, but with increased hypothalamic phosphorylated AMPK (169%) and phosphorylated acetyl-CoA carboxylase (194%). To determine the more chronic effects of T3, rats were given 6 daily s.c. injection of T3 or the vehicle. Food intake was significantly increased. Multiple T3 injections increased hypothalamic phosphorylated AMPK (278%) and phosphorylated acetyl-CoA carboxylase (335%) compared to the controls. Intracerebroventricular administration of compound C, an AMPK inhibitor, blocked the food intake induced by a single or multiple injections of T3. Taken together, these results suggest that enhanced hypothalamic AMPK phosphorylation contributes to T3-induced hyperphagia. Hypothalamic AMPK plays an important role in the regulation of food intake and body weight.     

Central alpha-MSH,energy balance,thermal balance,and antipyresis

Intracerebroventricular administration of alpha-MSH in young adult rats enhanced metabolic rate and caused a dose-dependent suppression of food intake, exhibiting a coordinated catabolic pattern. However, the thermoregulatory effects did not seem to be coordinated: the rising heat production was accompanied by a practically simultaneous tendency for rise in heat loss (skin vasodilatation), and the final core temperature either increased or decreased depending on which rise prevailed. The effect on heat loss possibly explains the antipyretic properties of the peptide.     

NPY Y1 receptor antagonist prevents NPY-induced torpor-like hypothermia in cold-acclimated Siberian hamsters

Siberian hamsters (Phodopus sungorus) undergo bouts of daily torpor during which body temperature decreases by as much as 20 degrees C and provides a significant savings in energy expenditure. Natural torpor in this species is normally triggered by winterlike photoperiods and low ambient temperatures. Intracerebroventricular injection of neuropeptide Y (NPY) reliably induces torporlike hypothermia that resembles natural torpor. NPY-induced torporlike hypothermia is also produced by intracerebroventricular injections of an NPY Y1 receptor agonist but not by injections of an NPY Y5 receptor agonist. In this research, groups of cold-acclimated Siberian hamsters were either coinjected with a Y1 receptor antagonist (1229U91) and NPY or were coinjected with a Y5 receptor antagonist (CGP71683) and NPY in counterbalanced designs. Paired vehicle + NPY induced torporlike hypothermia in 92% of the hamsters, whereas coinjection of Y1 antagonist + NPY induced torporlike hypothermia in 4% of the hamsters. In contrast, paired injections of vehicle + NPY and Y5 antagonist + NPY induced torporlike hypothermia in 100% and 91% of the hamsters, respectively. Although Y5 antagonist treatment alone had no effect on body temperature, Y1 antagonist injections produced hyperthermia compared with controls. Both Y1 antagonist and Y5 antagonist injections significantly reduced food ingestion 24 h after treatment. We conclude that activation of NPY 1 receptors is both sufficient and necessary for NPY-induced torporlike hypothermia.     

The role of NPY in the mediation of orexin-induced hypothermia

The mediation of orexin-A-induced hypothermia was investigated. Different doses of orexin-A (140-560 pmol) were administered intracerebroventricularly (i.c.v.) to adult male rats, and the colon temperature was used as an index of the thermoregulatory action. Orexin-A decreased both the basal colon temperature and the lipopolysaccharide-induced fever and exhibited a bell-shaped dose-response curve. I.c.v. pretreatment with neuropeptide Y (NPY) antiserum 24 h before orexin administration significantly decreased the hypothermic effect of orexin-A. These data strengthen the hypothesis that this appetite-regulating peptide might also play a role in thermoregulation, and its hypothermic effect seems to be mediated at least partially by NPY.     

Food restriction selectively increases hypothalamic orexin-B levels in lactating rats

Orexins are hypothalamic peptides implicated in the regulation of ingestive and other behaviours. Here we investigated prepro-orexin expression and hypothalamic orexin-A and -B levels in lactating rats, which display marked hyperphagia, with or without food restriction for 2 days or treatment with bromocriptine, which inhibits milk production and thus reduces the energy losses of lactation. Neither prepro-orexin gene expression nor hypothalamic orexin-A peptide levels were changed in any of these lactating groups compared with age-matched virgin controls. However, hypothalamic orexin-B levels were significantly higher in lactating rats that were food-restricted for 2 days (P<0.05) compared with non-lactating controls and with lactating rats that were either freely-fed or bromocriptine-treated. Thus, food restriction superimposed on lactation selectively increases hypothalamic orexin-B levels, suggesting that orexin-A and -B may be differentially released or cleared. Changes in orexin-B availability may influence physiological activities other than energy homeostasis, perhaps inducing arousal.     

Network of hypothalamic neurons that control appetite

The central nervous system (CNS) controls food intake and energy expenditure via tight coordinations between multiple neuronal populations. Specifically, two distinct neuronal populations exist in the arcuate nucleus of hypothalamus (ARH): the anorexigenic (appetite-suppressing) pro-opiomelanocortin (POMC) neurons and the orexigenic (appetite-increasing) neuropeptide Y (NPY)/agouti-related peptide (AgRP) neurons. The coordinated regulation of neuronal circuit involving these neurons is essential in properly maintaining energy balance, and any disturbance therein may result in hyperphagia/obesity or hypophagia/starvation. Thus, adequate knowledge of the POMC and NPY/AgRP neuron physiology is mandatory to understand the pathophysiology of obesity and related metabolic diseases. This review will discuss the history and recent updates on the POMC and NPY/AgRP neuronal circuits, as well as the general anorexigenic and orexigenic circuits in the CNS. [BMB Reports 2015; 48(4): 229-233]     

Hypothalamic NPY status during positive energy balance and the effects of the NPY antagonist, BW 1229U91, on the consumption of highly palatable energy-rich diet.

We have studied the hypothalamic activity of the neuropeptide Y (NPY) system in dietary-induced obese male Wistar rats and examined whether the NPY antagonist, BW1229U91, can inhibit the hyperphagia during positive energy balance associated with feeding rats an energy-rich, highly palatable diet. Rats given a highly palatable, high-fat diet became obese after 8 weeks and exhibited hyperinsulinemia and hyperleptinemia, as compared to lean rats fed on standard pellet laboratory diet. Hypothalamic NPY mRNA concentrations were significantly reduced by approximately 70% in dietary-obese rats compared with lean controls, and the former were hypersensitive to intracerebroventricular injections of NPY, possibly as a result of NPY receptor up-regulation. Intracerebroventricular injections of BW 1229U91, that inhibits food intake in starved rats, did not alter food intake in either control or obese rats fed either standard pellet diet or the highly palatable diet, respectively. We conclude that dietary-obese rats have underactive hypothalamic NPYergic neurons compared to lean controls, possibly as a result of increased plasma concentrations of leptin and/or insulin that directly inhibit the NPY neuronal activity. The lack of effect of BW1229U91 on the increased caloric intake of dietary-obese rats suggests that the hyperphagia is not NPY-driven and supports the data indicating reduced synaptic activity of the hypothalamic NPY system.     

Neuropeptides and the central regulation of body temperature during fever and hibernation

Neuropeptides, acting on structures within the central nervous system influence body temperature. Non-opioid peptides induce hypothermia usually, while opioid peptides are mostly hyperthermic. Neuropeptides exert their effect only when injected into specific brain areas.

Hypo- Or hyperthermic effect of neuropeptides may be either due to changes in threshold body temperatures for induction of thermoregulatory effectors or due to changes in hypothalamic thermosensitivity.

At the cellular level the opioid peptides also act differently than the non-opioid peptides. The opioid peptides mostly inhibit spontaneous neuronal firing, while the non-opioid peptides usually stimulate it. Neuropeptides exert their influence on all neurones in the hypothalamus, independently on their temperature characteristics.

Neuropeptides may play a role in the regulation of body temperature under stressful conditions and during fever or hibernation, in particular. Some neuropeptides, namely AVP, -MSH and ACTH, act as natural antipyretic substances by lowering the threshold for cold thermogenesis.

Neuropeptides also modulate food intake, reproduction and many other functions which are substantially changed during hibernation. There appears to be a correlation between the effect of peptides on the control of food intake and on the control of body temperature. Opioid peptides, which increase food intake, induce hyperthermia, while non-opioid peptides, which are appetite inhibiting, induce hypothermia. The exact role o neuropeptides in the regulation of body temperature, food intake and gonadal activity of hibernators remains unclear, however.     

Sustained NPY Overexpression in the PVN Results in Obesity via Temporarily Increasing Food Intake

Increasing neuropeptide Y (NPY) signaling in the paraventricular nucleus (PVN) by recombinant adeno‐associated virus (rAAV)‐mediated overexpression of NPY in rats, results in hyperphagia and obesity in rats. To determine the importance of hyperphagia in the observed obesity phenotype, we pair‐fed a group of AAV‐NPY‐injected rats to AAV‐control‐injected rats and compared parameters of energy balance to ad libitum fed AAV‐NPY‐injected rats. For 3 weeks, AAV‐NPY‐injected rats, received the same amount of food as ad libitum‐fed rats injected with control rAAV They did not gain more body weight than these controls. When allowed access to food ad libitum, these AAV‐NPY‐injected rats increased food intake, which subsequently decreased when rats reached the same body weight as AAV‐NPY‐injected rats that were fed ad libitum for the entire study. These data indicate that overexpression of NPY in the PVN results in obesity by increasing food intake until a certain body weight is achieved.     

The role of previous exposure in the appetitive and consummatory effects of orexigenic neuropeptides

The ingestion of foods is comprised of two distinct phases of behavior: appetitive and consummatory. While most food intake paradigms include both phases, the intraoral intake test emphasizes the stereotyped consummatory-phase by infusing a liquid food directly into the oral cavity. Several hypothalamic peptides have been shown to increase intake of chow in standard food intake paradigms and the current experiments sought to test whether these peptides would increase food intake in the intraoral intake paradigm. NPY, melanin-concentrating hormone (MCH) and orexin-A were infused into the third ventricle (i3vt) in a counterbalanced latin-square design just prior to rats getting 0.1M sucrose solution infused via indwelling intraoral catheters and compared it to intake on bottle tests with access to the same sucrose solution. On the first day, each peptide increased intraoral intake relative to saline in the between-subjects comparison. Moreover, intake of sucrose following i3vt saline increased as a function of training. By the final day of the experiment, rats receiving saline consumed as much sucrose as rats receiving NPY, MCH, or orexin-A. This finding was conceptually replicated in the second experiment in which rats drank sucrose freely from a bottle on the home cage. A third experiment directly assessed the role of previous exposure in the sucrose intake induced by NPY. Those results confirm that repeated exposure to sucrose increases baseline intake and attenuates the hyperphagic effect of NPY. These results are consistent with two conclusions: (1) NPY, MCH, and orexin-A increase both appetitive and consummatory-phase ingestive behaviors on initial exposures; (2) repeated training interacts with the effects of these orexigenic peptides.     

Knockdown of NPY expression in the dorsomedial hypothalamus promotes development of brown adipocytes and prevents diet-induced obesity

Hypothalamic neuropeptide Y (NPY) has been implicated in control of energy balance, but the physiological importance of NPY in the dorsomedial hypothalamus (DMH) remains unclear. Here we report that knockdown of NPY expression in the DMH by adeno-associated virus-mediated RNAi reduced fat depots in rats fed regular chow and ameliorated high-fat diet-induced hyperphagia and obesity. DMH NPY knockdown resulted in development of brown adipocytes in inguinal white adipose tissue through the sympathetic nervous system. This knockdown increased uncoupling protein 1 expression in both inguinal fat and interscapular brown adipose tissue (BAT). Consistent with the activation of BAT, DMH NPY knockdown increased energy expenditure and enhanced the thermogenic response to a cold environment. This knockdown also increased locomotor activity, improved glucose homeostasis, and enhanced insulin sensitivity. Together, these results demonstrate critical roles of DMH NPY in body weight regulation through affecting food intake, body adiposity, thermogenesis, energy expenditure, and physical activity.     

Effects of neuropeptide Y antagonists on food intake in rats: differences with cold-adaptation

Hyperphagia followed both central neuropeptide Y (NPY) administration and the presumed increase of endogenous NPY activity after food deprivation. NPY induced greater hyperphagia in cold-adapted than non-adapted rats; fasting of comparable severity caused similar hyperphagia in the two groups. NPY-receptor-antagonist D-Tyr(27,36), D-Thr32-NPY(27,36) or functional NPY-antagonist D-myo-inositol-1,2,6-trisphosphate attenuated the hyperphagic effect of both NPY and fasting in non-adapted rats. However, while completely preventing the NPY-hyperphagia, they did not influence the fasting-induced hyperphagia in cold-adapted rats. With cold-adaptation the sensitivity to NPY and to its antagonists increases, but the hypothalamic NPY loses from its fundamental role in the regulation of food intake, and the hyperphagia seen in cold-adaptation may need some other explanation.     

Regulation of food intake in the goldfish by interaction between ghrelin and orexin

Intracerebroventricular (ICV) administration of ghrelin, orexin and neuropeptide Y (NPY) stimulates food intake in goldfish. Orexin and NPY interact with each other in the regulation of feeding, while ghrelin-induced feeding has also shown to be mediated by NPY in the goldfish model. To investigate the interaction between ghrelin and orexin, we examined the effects of a selective orexin receptor-1 antagonist, SB334867, and a growth hormone secretagogue-receptor antagonist, [D-Lys(3)]-GHRP-6, on ghrelin- and orexin-A-induced feeding. Ghrelin-induced food intake was completely inhibited for 1h following ICV preinjection of SB334867, while [D-Lys(3)]-GHRP-6 attenuated orexin-A stimulated feeding. Furthermore, ICV administration of ghrelin or orexin-A at a dose sufficient to stimulate food intake increased the expression of each other's mRNA in the diencephalon. These results indicate that, in goldfish, ghrelin and orexin-A have interacting orexigenic effects in the central nervous system. This is the first report that orexin-A-induced feeding is mediated by the ghrelin signaling in any animal model.     

The effect of food deprivation and experimental diabetes on orexin and NPY mRNA levels

Although exogenous orexin can induce feeding, reports of increased orexin gene expression after caloric manipulations have been inconsistent. We hypothesized that orexin gene expression is increased only by extreme negative energy balance challenges. We measured hypothalamic orexin and NPY mRNA by in situ hybridization and orexin-A immunoreactivity in rats after food deprivation, streptozotocin-induced diabetes, and combined deprivation and diabetes. Neither food deprivation, nor diabetes, nor the combination affected orexin mRNA levels, although orexin-A immunoreactivity was increased by diabetes. NPY mRNA levels were increased by either treatment. These results suggest that increased orexin gene expression is not a consistent correlate of negative energy balance challenges.     

Amylin blunts hyperphagia and reduces weight and fat gain during recovery in socially stressed rats

During recovery from social stress in a visible burrow system (VBS), during which a dominance hierarchy is formed among the males, rats display hyperphagia and gain weight preferentially as visceral adipose tissue. By proportionally increasing visceral adiposity, social stress may contribute to the establishment of metabolic disorder. Amylin was administered to rats fed ad libitum during recovery from VBS stress in an attempt to prevent hyperphagia and the resultant gain in body weight and fat mass. Amylin treatment reduced food intake, weight gain, and accumulation of fat mass in male burrow rats, but not in male controls that spent time housed with a single female rather than in the VBS. Amylin did not alter neuropeptide Y (NPY), agouti-related peptide (AgRP), or proopiomelanocortin (POMC) mRNA expression in the arcuate nucleus of the hypothalamus as measured at the end of the recovery period, nor did it affect plasma corticosterone or leptin. Amylin exerted most of its effect on food intake during the first few days of recovery, possibly through antagonism of NPY and/or increasing leptin sensitivity. The potential for chronic social stress to contribute to metabolic disorder is diminished by amylin treatment, though the neuroendocrine mechanisms behind this effect remain elusive.     

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

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

ICV NPY Y1 receptor agonist but not Y5 agonist induces torpor-like hypothermia in cold-acclimated Siberian hamsters

The reduced metabolism derived from daily torpor enables numerous small mammals, including Siberian hamsters, to survive periods of energetic challenge. Little is known of the neural mechanisms underlying the initiation and expression of torpor. Hypothalamic neuropeptide Y (NPY) contributes to surviving energetic challenges by both increasing food ingestion and reducing metabolic expenditure. Intracerebroventricular injections of NPY in cold-acclimated Siberian hamsters induce torpor-like hypothermia comparable to natural torpor. Multiple NPY receptor subtypes have been identified, and the Y1 receptor and Y5 receptor both contribute to the orexigenic effect of NPY. The purpose of this research was to compare and contrast the effects of Y1 receptor activation by a specific Y1 agonist ([D-Arg25]-NPY) or Y5 receptor activation by a specific Y5 agonist ([D-Trp34]-NPY) on body temperature and subsequent food intake in cold-acclimated Siberian hamsters. Intracerebroventricular injections of Y1 agonist produced torporlike hypothermia closely resembling that induced by intracerebroventricular NPY. The intracerebroventricular Y5 agonist infrequently produced hypothermia reaching criterion for torpor and that failed to resemble either NPY-induced or natural torpor. Combined injections of Y1 and Y5 agonists resulted in hypothermia comparable to Y5 agonist treatments alone, negating the mimicry of NPY treatment seen with Y1 agonist alone. Prior treatment with Y1 agonist or Y5 agonist surprisingly had lingering effects on NPY-induced torpor expression, Y1 agonist enhanced and Y5 agonist inhibited the effect of NPY. The ability of NPY to induce torporlike hypothermia, especially its initiation, most likely involves activation of the NPY Y1 receptor subtype.