Gut hormone PYY(3-36) physiologically inhibits food intake

Gut hormones in the control of appetite

Obesity is one of the greatest threats to the health of the developed world. In order to design effective drugs to treat the alarming increase in obesity, it is essential to understand the physiology of normal appetite control and the pathophysiology of obesity. The hypothalamus interprets and integrates neural and humoral inputs to provide a coordinated feeding and energy expenditure response. Recent evidence suggests that certain gut hormones - ghrelin, polypeptide YY, pancreatic polypeptide, glucagon-like-peptide 1 and oxyntomodulin - have a physiological role in governing satiety via the hypothalamus. Gut hormone appetite-regulatory systems represent a potential target for the design of antiobesity drugs.

Identification of hypothalamic genes implicated in the development of obesity in Psammomys obesus using differential display PCR

The hypothalamus is a key central controller of energy homeostasis and is the source and/or site of action of many neuropeptides involved in this process. The aim of this study was to isolate hypothalamic genes differentially expressed between lean and obese Psammomys obesus, a polygenic animal model of obesity and type 2 diabetes. Differential display PCR was used to compare hypothalamic gene expression profiles of lean and healthy, obese and hyperinsulinemic, and obese, diabetic P. obesus in both the fed and fasted states. We conducted differential display with 180 separate primer combinations to amplify approximately 9,000 expressed transcripts. Sixty differentially expressed bands were excised. Taqman PCR was performed on 36 of these transcripts to confirm differential gene expression in a larger sample population. Of these 36 transcripts, 9 showed homology to known genes, and 27 were considered to be novel sequences. Gene expression profiles for two of these genes are presented here. In conclusion, differential display PCR was successfully used to isolate several transcripts that may be involved in the central regulation of energy balance. We are currently conducting numerous studies to further investigate the role of these genes in the development of obesity in P. obesus.

NPY and Y receptors: lessons from transgenic and knockout models

Neuropeptide Y (NPY) in the central nervous system is a major regulator of food consumption and energy homeostasis. It also regulates blood pressure, induces anxiolysis, enhances memory retention, affects circadian rhythms and modulates hormone release. Five Y receptors (Y1, Y2, Y4, Y5 and Y6) are known to mediate the action of NPY and its two other family members, peptide YY (PYY) and pancreatic polypeptide (PP). Increased NPY signaling due to elevated NPY expression in the hypothalamus leads to the development of obesity and its related phenotypes, Type II diabetes and cardiovascular disease. Dysregulation in NPY signaling also causes alterations in bone formation, alcohol consumption and seizure susceptibility. The large number of Y receptors has made it difficult to delineate their individual contributions to these physiological processes. However, recent studies analysing NPY and Y receptor overexpressing and knockout models have started to unravel some of the different functions of these Y receptors. Particularly, the use of conditional knockout models has made it possible to pinpoint a specific function to an individual Y receptor in a particular location.

Molecular evolution of NPY receptor subtypes

The neuropeptide Y (NPY) system consists in mammals of three peptides and 4-5 G-protein-coupled receptors called Y receptors that are involved in a variety of physiological functions such as appetite regulation, circadian rhythm and anxiety. Both the receptor family and the peptide family display unexpected evolutionary complexity and flexibility as shown by information from different classes of vertebrates. The vertebrate ancestor most likely had a single peptide gene and three Y receptor genes, the progenitors of the Y1, Y2 and Y5 subfamilies. The receptor genes were probably located in the same chromosomal segment. Additional gene copies arose through the chromosome quadruplication that took place before the emergence of jawed vertebrates (gnathostomes) whereupon differential losses of the gene copies ensued. The inferred ancestral gnathostome gene repertoire most likely consisted of two peptide genes, NPY and PYY, and no less than seven Y receptor genes: four Y1-like (Y1, Y4/a, Y6, and Yb), two Y2-like (Y2 and Y7), and a single Y5 gene. Whereas additional peptide genes have arisen in various lineages, the most common trend among the Y receptor genes has been further losses. Mammals have lost Yb and Y7 (the latter still exists in frogs) and Y6 is a pseudogene in several mammalian species but appears to be still functional in some. One challenge is to find out if mammals have been deprived of any functions through these gene losses. Teleost fishes like zebrafish and pufferfish, on the other hand, have lost the two major appetite-stimulating receptors Y1 and Y5. Nevertheless, teleost fishes seem to respond to NPY with increased feeding why some other subtype probably mediates this effect. Another challenge is to deduce how Y2 and Y4 came to evolve an inhibitory effect on appetite. Changes in anatomical distribution of receptor expression may have played an important part in such functional switching along with changes in receptor structures and ligand preferences.

Distribution of NPY receptors in the hypothalamus

Neuropeptide Y (NPY) neurons abundantly innervate the hypothalamus, where NPY is involved in the regulation and integration of a broad range of homeostatic functions. In order to understand NPY-mediated behavioral, autonomic and neuroendocrine effects, it is important to characterize in detail the distribution of the hypothalamic NPY receptors. In this review, we briefly summarize the origin of NPY and its two related peptides, peptide YY and pancreatic polypeptide in the hypothalamus. Moreover, based on the results obtained with histological techniques such as in situ hybridization, immunohistochemistry and ligand binding, we summarize data on the hypothalamic distribution of the known NPY receptors, the Y1 Y2, Y4 and Y5 receptors as best characterized to date. These NPY receptors are found with individual distribution patterns in many hypothalamic neurons including neuroendocrine motoneurons, magnocellular neurosecretory neurons and numerous neurons connecting the hypothalamus with the limbic and the autonomic nervous systems. The histochemical analyses allow characterization of coexisting molecules and in this way definition of the neurochemistry of NPY circuitries. By showing coexistence of various NPY receptors they provide a morphological basis for in vitro studies showing heterodimerization of NPY receptors. The NPY neurons and their circuitries underlie the integrative role of NPY as a pleiotropic neuropeptide in the regulation of homeostasis.

Peptide YY release after colectomy in slow transit constipation

BACKGROUND

The gut hormone peptide YY is abundant in the colonic mucosa. Circulating PYY inhibits gastrointestinal motility and decreases food intake. The aim was to determine whether colectomy decreases PYY release in patients with slow transit constipation.

METHODS

Plasma PYY concentrations were measured in 10 patients with slow transit constipation before and 3-24 months after total abdominal colectomy with ileorectal anastomosis, and in 8 healthy controls. A liquid meal was infused intraduodenally to stimulate PYY release.

RESULTS

Postprandial PYY significantly (P < 0.05) increased from a basal value of 15.6 +/- 1.8 pM to a peak of 71.2 +/- 11.6 pM after colectomy. Basal and postprandial plasma PYY concentrations were not significantly different from the results before surgery. Fasting, but not postprandial, plasma peptide YY after colectomy was significantly higher than that in healthy volunteers, 10.9 +/- 0.9 pM.

CONCLUSION

Despite removal of a major source of PYY-secreting cells, colectomy with ileorectal anastomosis does not induce major impairment of PYY release in slow transit constipation.

PYY3-36 reinforces insulin action on glucose disposal in mice fed a high-fat diet

Peptide YY(3-36) (PYY(3-36)) is released by the gut in response to nutrient ingestion. It modulates the activities of orexigenic neuropeptide Y (NPY) neurons and anorexigenic proopiomelanocortin (POMC) neurons in the hypothalamus to inhibit food intake. Because both NPY and POMC have also been shown to impact insulin action, we wondered whether PYY(3-36) could improve insulin sensitivity. To address this question, we examined the acute effect of intravenous PYY(3-36) on glucose and free fatty acid (FFA) flux during a hyperinsulinemic-euglycemic clamp in mice maintained on a high-fat diet for 2 weeks before the experiment. We also evaluated the effects of PYY(3-36) infusion on glucose uptake in muscle and adipose tissue in this experimental context. Under basal conditions, none of the metabolic parameters were affected by PYY(3-36). Under hyperinsulinemic conditions, glucose disposal was significantly increased in PYY(3-36)-infused compared with vehicle-infused mice (103.8 +/- 10.9 vs. 76.1 +/- 11.4 micromol.min(-1).kg(-1), respectively; P = 0.001). Accordingly, glucose uptake in muscle and adipose tissue was greater in PYY(3-36)-treated animals, although the difference with controls did not reach statistical significance in adipose tissue (muscle: 2.1 +/- 0.5 vs. 1.5 +/- 0.5 micromol/g tissue, P = 0.049; adipose tissue: 0.8 +/- 0.4 vs. 0.4 +/- 0.3 micromol/g tissue, P = 0.08). In contrast, PYY(3-36) did not impact insulin action on endogenous glucose production or FFA metabolism. These data indicate that PYY(3-36) reinforces insulin action on glucose disposal in mice fed a high-fat diet, through a mechanism that is independent of food intake and body weight. In contrast, it leaves glucose production and lipid flux largely unaffected in this experimental context.

NPY and cohorts in regulating appetite, obesity and metabolic syndrome: beneficial effects of gene therapy

Neuropeptide Y is the most potent physiological appetite transducer known. The NPY network is the conductor of the hypothalamic appetite regulating orchestra in the arcuate nucleus-paraventricular nucleus (ARC-PVN) of the hypothalamus. NPY and cohorts, AgrP, GABA and adrenergic transmitters, initiate appetitive drive directly through Y1, Y5, GABAA and alpha1 receptors, co-expressed in the magnocellular PVN (mPVN) and ARC neurons and by simultaneously repressing anorexigenic melanocortin signaling in the ARC-PVN axis. The circadian and ultradian rhythmicities in NPY secretion imprint the daily circadian and episodic feeding patterns. Although a number of afferent hormonal signals from the periphery can directly modulate NPYergic signaling, the reciprocal circadian and ultradian rhythmicities of anorexigenic leptin from adipocytes and orexigenic ghrelin from stomach, encode a corresponding pattern of NPY discharge for daily meal patterning. Subtle and progressive derangements produced by environmental and genetic factors in this exquisitely intricate temporal relationship between the two opposing humoral signals and the NPY network promote hyperphagia and abnormal rate of weight gain culminating in obesity and attendant metabolic disorders. Newer insights at cellular and molecular levels demonstrate that a breakdown of the integrated circuit due both to high and low abundance of NPY at target sites, underlies hyperphagia and increased adiposity. Consequently, interruption of NPYergic signaling at a single locus with NPY receptor antagonists may not be the most efficacious therapy to suppress hyperphagia and obesity. Central leptin gene therapy in rodents has been shown to subjugate, i.e. bring under homeostatic control, NPYergic signaling and suppress the age-related and dietary obesity for extended periods and thus shows promise as a newer treatment modality to curb the pandemic of obesity and metabolic syndrome.

Physiology: does gut hormone PYY3-36 decrease food intake in rodents?

Batterham et al. report that the gut peptide hormone PYY3-36 decreases food intake and body-weight gain in rodents, a discovery that has been heralded as potentially offering a new therapy for obesity. However, we have been unable to replicate their results. Although the reasons for this discrepancy remain undetermined, an effective anti-obesity drug ultimately must produce its effects across a range of situations. The fact that the findings of Batterham et al. cannot easily be replicated calls into question the potential value of an anti-obesity approach that is based on administration of PYY3-36.

Neuropeptide Y and energy homeostasis: insights from Y receptor knockout models

A complex system has evolved to regulate food intake and to maintain energy homeostasis. A series of short-term hormonal and neural signals that derive from the gastrointestinal tract, such as cholecystokinin (CCK), pancreatic polypeptide (PP) and peptide YY-(3-36), recently discovered to regulate meal size. Others such as ghrelin initiate meals, and insulin and leptin, together with circulating nutrients, indicate long-term energy stores. All these signals act on central nervous system sites which converge on the hypothalamus, an area that contains a large number of peptide and other neurotransmitters that influence food intake with neuropeptide Y (NPY) being one of the most prominent ones. Five Y receptors are known which mediate the action of neuropeptide Y and its two other family members, peptide YY and pancreatic polypeptide. Elevated neuropeptide Y expression in the hypothalamus leads to the development of obesity and its related phenotypes, Type II diabetes and cardiovascular disease. The limited availability of specific pharmacological tools and the considerable number of Y receptors have made it difficult to delineate their individual contributions to the regulation of energy homeostasis. However, recent studies analysing transgenic and knockout neuropeptide Y and Y receptor mouse models have started to unravel some of the individual functions of these Y receptors potentially also helping to develop novel therapeutics for a variety of physiological disorders including obesity.

Brain circuits regulating energy homeostasis

For decades, increasingly sophisticated methods have been designed to address the problem of the involvement of the brain in the physiology of energy homeostasis and the pathogenesis of obesity. A vast number of experimental observations have been made from novel genetic and physiologic approaches that allowed the identification of metabolic hormones and their relationship to key peptidergic systems in the brain. Although the central integration of afferent signals reflecting acute and chronic energy requirements is becoming clearer, the blueprint of the central regulation of energy expenditure is not known. This review offers a look at central neuronal circuitries that are implicated in metabolism regulation and strongly suggests that without a blueprint, attempts to intervene and control energy balance will remain futile.

Effects of peptide YY(3-36) on PRL secretion: pituitary and extra-pituitary actions in the rat

Polypeptide YY(3-36) (PYY(3-36)) is a gastrointestinal secreted molecule, agonist of neuropeptide Y (NPY) receptor subtypes Y2 and Y5, that has been recently involved as anorexigenic signal in the network controlling food intake. Notably, several factors primarily involved in food intake control and energy homeostasis (as leptin, orexins, ghrelin and NPY) have been linked also to the regulation of anterior pituitary hormone secretion and carry out pleiotropic effects upon the reproductive axis. However, whether similar actions are conducted by PYY(3-36) remains so far largely unexplored. Present studies were undertaken to analyze the potential effects of PYY(3-36) in the control of prolactin (PRL) secretion in the rat. To this end, responses to PYY(3-36) in terms of PRL secretion were monitored in vitro, after pituitary exposure to 10(-8) to 10(-6) M concentrations, and in vivo, after i.p. administration of different doses of PYY(3-36) (3, 10 and 30 microg/kg) to prepubertal male and female rats. In addition, the in vivo effects of PYY(3-36) were tested after central (i.c.v.) administration of 3 nmol of the peptide to prepubertal rats, and in hyperprolactinaemic aged females. PYY(3-36) stimulated, in a dose-dependent manner, in vitro PRL secretion by pituitaries from prepubertal male and female rats. In contrast, systemic administration of PYY(3-36) failed to modify serum PRL levels, whereas central infusion of PYY(3-36) significantly inhibited PRL secretion in prepubertal rats. Finally, PRL secretion was stimulated in aged hyperprolactinaemic female rats by systemic administration of PYY(3-36). In conclusion, the anorexigenic peptide PYY(3-36) may participate in the control of PRL secretion in the prepubertal rat, acting at pituitary (stimulatory effect) and extra-pituitary (likely inhibitory action at the hypothalamus) sites of the lactotrope axis. Moreover, net actions of PYY(3-36) on PRL secretion may depend on the age and prevailing PRL levels.

Gut-derived signaling molecules and vagal afferents in the control of glucose and energy homeostasis

PURPOSE OF REVIEW

The control of glucose and energy homeostasis, including feeding behaviour, is tightly regulated by gut-derived peptidic and nonpeptidic endocrine mediators, autonomic nervous signals, as well as nutrients such as glucose. We will review recent findings on the role of the gastrointestinal tract innervation and of portal vein glucose sensors; we will review selected data on the action of gastrointestinally released hormones.

RECENT FINDINGS

The involvement of mechanosensory vagal afferents in postprandial meal termination has been clarified using mouse models with selective impairments of genes required for development of mechanosensory fibres. These activate central glucogen-like peptide-1/glucogen-like peptide-2 containing ascending pathways linking the visceroceptive brainstem neurons to hypothalamic nuclei. Mucosal terminals comprise the chemosensory vagal afferents responsive to postprandially released gastrointestinal hormones. The mechanism by which the hepatoportal glucose sensor stimulates glucose utilization by muscles was demonstrated, using genetically modified mice, to be insulin-independent but to require GLUT4 and AMP-kinase. This sensor is a key site of glucogen-like peptide-1 action and plays a critical role in triggering first phase insulin secretion. PeptideYY and ghrelin target intracerebral receptors as they are bidirectionally transported across the blood brain barrier. The anorectic functions of peripherally released peptideYY may however be mediated both via vagal afferents and intracerebral Y2 receptors in the brainstem and arcuate nucleus.

SUMMARY

These recent findings demonstrate that the use of improved anatomical and physiological techniques and animal models with targeted gene modifications lead to an improved understanding of the complex role of gastrointestinal signals in the control of energy homeostasis.

Pre-obese and obese agouti mice are sensitive to the anorectic effects of peptide YY(3-36) but resistant to ghrelin

OBJECTIVE

The role of the melanocortin system in the feeding effects of peripheral peptide YY(3-36) (PYY(3-36)) and ghrelin was investigated using the agouti (A(y)/a) mouse as a model of abnormal melanocortin signalling. Furthermore, we examined whether the ectopic expression of agouti protein in A(y)/a mice results in complete MC4-R inhibition, by studying the effects of peripheral alpha-melanocyte-stimulating hormone (alpha-MSH) and leptin on food intake.

DESIGN

Adult A(y)/a mice were studied in the pre-obese state (7-8 weeks) and obese state (14-15 weeks). Animals received PYY(3-36) (0.02 micromol/kg), NDP-alpha-MSH (0.2 micromol/kg), leptin (2 micromol/kg) (all 24 h fasted state) and ghrelin (0.2 micromol/kg) (fed state) by intraperitoneal (i.p.) injection. Age-matched A(y)/a controls received i.p. saline. A separate cohort of wild-type (WT), age-matched controls received the same peptide dose or saline. Food intake was measured at 1, 2, 4, 8 and 24 h post-injection and compared in all four groups. Plasma leptin-, ghrelin- and PYY-like immunoreactivity (IR) were measured using radioimmunoassay (RIA).

RESULTS

At 2 h post-injection, PYY(3-36) reduced food intake in pre-obese and obese A(y)/a mice, whereas ghrelin had no effect. Plasma ghrelin levels were significantly reduced in pre-obese and obese A(y)/a mice compared to WT controls. Peripheral administration of NDP-alpha-MSH and leptin acutely suppressed feeding (0-2 h) in pre-obese and obese A(y)/a mice.

CONCLUSIONS

Responsiveness of pre-obese and obese A(y)/a mice to PYY(3-36) suggests that the melanocortin system may not be essential for the anorectic effects of this peptide. Melanocortinergic antagonism by agouti protein in A(y)/a mice may be sufficient to block the effects of endogenous, but not exogenous PYY(3-36), alpha-MSH and leptin. The mechanism underlying ghrelin resistance in A(y)/a mice may result from antagonism of hypothalamic melanocortin receptors-4 by agouti protein, supporting a role for the melanocortin system in mediating ghrelin's actions.

Neuropeptides in eating disorders

The past decade has witnessed a dramatic acceleration in research on the role of the neuropeptides in the regulation of eating behavior and body weight homeostasis. This expanding research focus has been driven in part by increasing public health concerns related to obesity and the eating disorders anorexia nervosa (AN) and bulimia nervosa (BN). Preclinical advances have been facilitated by the development of new molecular and behavioral research methodologies. With a focus on clinical investigations in AN and BN, this article reviews research on selected hypothalamic and gut-related peptide systems with prominent effects on eating behavior. Studies of the orexigenic peptides neuropeptide Y and the opioid peptides have shown state-related abnormalities in patients with eating disorders. With respect to gut-related peptides, there appears to be substantial evidence for blunting in the meal-related release of the satiety promoting peptide cholecystokinin in BN. Fasting plasma levels of the orexigenic peptide ghrelin have been found to be elevated in patients with AN. As discussed in this review, additional studies will be needed to assess the role of nutritional and body weight changes in neuropeptide alterations observed in symptomatic eating disorder patients, and to identify stable trait-related abnormalities in neuropeptide regulation that persist in individuals who have recovered from an eating disorder.

Food Safety and Functional Foods in the European Union: Obesity as a Paradigmatic Example for Novel Food Development

Since 1997, the European Union has undergone deep changes in the scientific assessment of food-related problems. Among the immediate challenges in the near future are the scientific and technological developments and regulatory measures for the so-called "functional foods," which can positively affect the health and well- being of consumers. Obesity, whose genetic and molecular basis are increasingly understood, is now foreseen as one of the key targets for novel functional foods developments, and offers a paradigmatic example of the complexity of a biological system as a target to be controlled.

PPARs and the complex journey to obesity

Obesity and the related disorders of dyslipidemia and diabetes (components of syndrome X) have become global health epidemics. Over the past decade, the elucidation of key regulators of energy balance and insulin signaling have revolutionized our understanding of fat and sugar metabolism and their intimate link. The three 'lipid-sensing' peroxisome proliferator-activated receptors (PPAR-alpha, PPAR-gamma and PPAR-delta) exemplify this connection, regulating diverse aspects of lipid and glucose homeostasis, and serving as bona fide therapeutic targets. With molecular underpinnings now in place, new pharmacologic approaches to metabolic disease and new questions are emerging.

Gastrointestinal satiety signals IV. Apolipoprotein A-IV

The focus of this article is to review evidence that apolipoprotein A-IV (apo A-IV) acts as a satiety factor. Additionally, information regarding the general involvement of apo A-IV in the regulation of food intake and body weight is stated. Apo A-IV is a glycoprotein synthesized by the human intestine. In rodents, both the small intestine and liver secrete apo A-IV, but the small intestine is the major organ responsible for circulating apo A-IV. There is now solid evidence that the hypothalamus, especially the arcuate nucleus, is another active site of apo A-IV expression. Intestinal apo A-IV synthesis is markedly stimulated by fat absorption and does not appear to be mediated by the uptake or reesterification of fatty acids to form triglycerides. Rather, the local formation of chylomicrons acts as a signal for the induction of intestinal apo A-IV synthesis. Intestinal apo A-IV synthesis is also enhanced by a factor from the ileum, probably peptide tyrosine-tyrosine (PYY). The inhibition of food intake by apo A-IV is mediated centrally. The stimulation of intestinal synthesis and secretion of apo A-IV by lipid absorption are rapid; thus apo A-IV likely plays a role in the short-term regulation of food intake. Other evidence suggests that apo A-IV may also be involved in the long-term regulation of food intake and body weight, as it is regulated by both leptin and insulin. Chronic ingestion of a high-fat diet blunts the intestinal as well as the hypothalamic apo A-IV response to lipid feeding. It also suppresses apo A-IV gene expression in the hypothalamus. Whereas it is tempting to speculate that apo A-IV may play a role in diet-induced obesity, we believe the confirmation of such a proposal awaits further experimental evidence.

Visceral obesity without insulin resistance in late-onset obesity rats

We describe a line of transgenic rats in which the males develop a unique autosomal dominant, late-onset obesity (LOB) phenotype. LOB males gradually accumulate fat specifically in visceral, but not peripheral, fat depots despite a normal intake of a low fat diet. LOB females normally develop only mild obesity with advanced age. However, the phenotype can be induced rapidly in young females by ovariectomy and prevented by estrogen replacement. LOB males are highly sensitive to dietary fat. Young, nonobese LOB males gain more weight on a 30% fat diet and lose more weight when treated with the lipase inhibitor, Orlistat, than their nontransgenic littermates. Remarkably, despite severe visceral obesity, LOB rats have normal fasting blood glucose, insulin, and corticosterone; show normal or increased insulin sensitivity in glucose and insulin tolerance tests; have increased plasma adiponectin levels; and display a heightened response to treatment with rosiglitazone. Their visceral adiposity reflects a specific increase in visceral adipocyte number, not size. Analysis of the transgene in LOB rats revealed a deletion in the gene encoding the S26 subunit of the mitochondrial ribosome that results in the production of a truncated protein, which we show to be imported into mitochondria. However, the transgene integrant is complex, so whether this is the sole molecular disruption underlying this phenotype remains to be established. Nevertheless, LOB rats provide a valuable new model of late-onset, male-preponderant, visceral-specific obesity, clearly dissociated from insulin resistance.

Minireview: Gut peptides regulating satiety

The gastrointestinal tract and the pancreas release hormones regulating satiety and body weight. Ghrelin stimulates appetite, and glucagon-like peptide-1, oxyntomodulin, peptide YY, cholecystokinin, and pancreatic polypeptide inhibit appetite. These gut hormones act to markedly alter food intake in humans and rodents. Obesity is the current major cause of premature death in the United Kingdom, killing almost 1000 people per week. Worldwide, its prevalence is accelerating. There is currently no effective answer to the pandemic of obesity, but replacement of the low levels of peptide YY observed in the obese may represent an effective antiobesity therapy.

Enhancement of feeding suppression by PYY(3-36) in rats with area postrema ablations

We investigated suppression of food intake by intraperitoneal (IP) injections of peptide YY(3-36) (PYY(3-36)) (24, 60, or 150 microg/kg) in rats with ablations of the area postrema (APX) and in controls with sham ablations. In controls, PYY(3-36)-induced suppression was modest and short-lived, averaging 20% at most and persisting less than 6h. The highest dose tested (150 microg/kg) was even less effective than were the two lesser doses after 3h. APX did not diminish the potency of these effects of PYY(3-36). In fact, the magnitude of suppression produced by the greatest dose of PYY(3-36) in APX rats was significantly greater than in controls and PYY(3-36)-induced suppression was still present at 24h.

Ghrelin, appetite, and gastric motility: the emerging role of the stomach as an endocrine organ

Recent progress in the field of energy homeostasis was triggered by the discovery of adipocyte hormone leptin and revealed a complex regulatory neuroendocrine network. A late addition is the novel stomach hormone ghrelin, which is an endogenous agonist at the growth hormone secretagogne receptor and is the motilin-related family of regulatory peptides. In addition to its ability to stimulate GH secretion and gastric motility, ghrelin stimulates appetite and induces a positive energy balance leading to body weight gain. Leptin and ghrelin are complementary, yet antagonistic, signals reflecting acute and chronic changes in energy balance, the effects of which are mediated by hypothalamic neuropeptides such as neuropeptide Y and agouti-related peptide. Endocrine and vagal afferent pathways are involved in these actions of ghrelin and leptin. Ghrelin is a novel neuroendocrine signal possessing a wide spectrum of biological activities that illustrates the importance of the stomach in providing input into the brain. Defective ghrelin signaling from the stomach could contribute to abnormalities in energy balance, growth, and associated gastrointestinal and neuroendocrine functions.

Biomarkers of satiation and satiety

This review's objective is to give a critical summary of studies that focused on physiologic measures relating to subjectively rated appetite, actual food intake, or both. Biomarkers of satiation and satiety may be used as a tool for assessing the satiating efficiency of foods and for understanding the regulation of food intake and energy balance. We made a distinction between biomarkers of satiation or meal termination and those of meal initiation related to satiety and between markers in the brain [central nervous system (CNS)] and those related to signals from the periphery to the CNS. Various studies showed that physicochemical measures related to stomach distension and blood concentrations of cholecystokinin and glucagon-like peptide 1 are peripheral biomarkers associated with meal termination. CNS biomarkers related to meal termination identified by functional magnetic resonance imaging and positron emission tomography are indicators of neural activity related to sensory-specific satiety. These measures cannot yet serve as a tool for assessing the satiating effect of foods, because they are not yet feasible. CNS biomarkers related to satiety are not yet specific enough to serve as biomarkers, although they can distinguish between extreme hunger and fullness. Three currently available biomarkers for satiety are decreases in blood glucose in the short term (<5 min), which have been shown to be involved in meal initiation; leptin changes during longer-term (>2-4 d) negative energy balance; and ghrelin concentrations, which have been implicated in both short-term and long-term energy balance. The next challenge in this research area is to identify food ingredients that have an effect on biomarkers of satiation, satiety, or both. These ingredients may help consumers to maintain their energy intake at a level consistent with a healthy body weight.

Gastrointestinal satiety signals III. Glucagon-like peptide 1, oxyntomodulin, peptide YY, and pancreatic polypeptide

Many peptides are synthesized and released from the gastrointestinal tract and pancreas, including pancreatic polypeptide (PP) and the products of the gastrointestinal L cells, glucagon-like peptide 1 (GLP-1), oxyntomodulin, and peptide YY (PYY). Whereas their roles in regulation of gastrointestinal function have been known for some time, it is now evident that they also influence eating behavior. This review considers the anorectic peptides PYY, PP, GLP-1, and oxyntomodulin, which decrease appetite and promote satiety in both animal models and humans.

Characterization of neuropeptide Y Y2 and Y5 receptor expression in the mouse hypothalamus

Neuropeptide Y (NPY) neurons abundantly innervate the hypothalamus, where NPY is involved in the regulation of a broad range of homeostatic functions. In the present work we studied NPY Y2 and Y5 receptor (R) gene expression in the mouse hypothalamus by using immunohistochemical detection of beta-galactosidase (beta-gal), a gene reporter molecule for Y2R and Y5R in Y2R-knockout (KO) and Y5R-KO mice, respectively. With this approach, cells normally expressing Y2R or Y5R are immunopositive for beta-gal. In the hypothalamus of the Y2R-KO mouse, beta-gal immunoreactivity (-ir) was found in numerous neurons of the medial preoptic nucleus as well as in the lateral anterior, periventricular, dorsomedial, tuberal, perifornical, and arcuate nuclei. Most of the dopaminergic neurons in the A13 dorsal hypothalamic group were beta-gal positive, whereas other hypothalamic dopaminergic neurons rarely displayed beta-gal-ir. In the arcuate nucleus, most of the beta-gal-positive neurons expressed NPY, but colocalizations with beta-endorphin were also found; in the tuberal and perifornical nuclei, many beta-gal-positive neurons contained nitric oxide synthase. beta-Gal-ir was also found in other forebrain regions of the Y2R-KO mouse, including the amygdala, thalamic nuclei, hippocampal CA3 area, and cortex. In the hypothalamus of the Y5R-KO mouse, beta-gal-positive neurons were found mainly in the arcuate nucleus and contained beta-endorphin. The present data show that Y2R and Y5R are expressed in distinct groups of hypothalamic neurons. High levels of Y2R expression in the preoptic nuclei suggest an involvement of Y2R in the regulation of reproductive behavior, whereas Y2R expression in the arcuate, dorsomedial, and perifornical nuclei may be relevant to feeding and body weight control. The finding that A13 dopaminergic neurons express Y2R suggests a new mechanism putatively involved in the central control of feeding, in which NPY can modulate dopamine secretion. The distribution of Y5R expression supports earlier evidence for involvement of this receptor in control of feeding and body weight via NPY's action on proopiomelanocortin-expressing neurons. J. Comp. Neurol. 470:256-265, 2004.

Abnormal cholinergic and GABAergic vascular innervation in the hypothalamic arcuate nucleus of obesetub/tubmice

Tubby and tubby-like proteins (TULPs) are encoded by members of a small gene family. An autosomal recessive mutation in the mouse tub gene leads to blindness, deafness, and maturity-onset obesity. The mechanisms by which the mutation causes the obesity syndrome has not been established. We compared obese tub/tub mice and their lean littermates in order to find abnormalities within the mediobasal hypothalamus, a region intimately associated with the regulation of body weight. Using an antiserum to the vesicular acetylcholine transporter (VAChT), a marker for cholinergic neurons, many unusually large VAChT-immunoreactive (-ir) nerve terminals, identified by colocalization with the synaptic vesicle protein synaptophysin, were demonstrated in the hypothalamic arcuate nucleus of obese tub/tub mice. Double-labeling showed that VAChT-ir nerve endings also contained glutamic acid decarboxylase (GAD), a marker for gamma-aminobutyric acid (GABA) neurons. The VAChT- and GAD-ir nerve terminals were in close contact with blood vessels, identified with antisera to platelet endothelial cell adhesion molecule-1 (PECAM; also called CD31), laminin, smooth muscle actin (SMA), and glucose transporter-1 (GLUT1). Such large cholinergic and GABAergic nerve terminals surrounding blood vessels were not seen in the arcuate nucleus of lean tub/+ mice. The presence of abnormal cholinergic/GABAergic vascular innervation in the arcuate nucleus suggests that alterations in this region, which contains neurons that receive information from the periphery and which relays information about the energy status to other parts of the brain, may be central in the development of the obese phenotype in animals with an autosomal recessive mutation in the tub gene.

Anabolic neuropeptides

The hypothalamus and other brain regions that control energy homeostasis contain neuronal populations that produce specific neuropeptides which have experimental effects on feeding behavior and body weight. Here, we describe examples of neuropeptides that exert 'anabolic' effects, notably stimulation of feeding and increased body weight. Neuropeptide Y (NPY) neurons in the hypothalamic arcuate nucleus (ARC) are inhibited by leptin and insulin, and thus are stimulated in states of energy deficit and fat loss, e.g., underfeeding. NPY neuronal overactivity contributes to enhanced hunger and food-seeking activity under these conditions. The lateral hypothalamic area (LHA) contains specific neuronal populations that affect feeding in different ways. Neurons expressing the appetite-stimulating peptide orexin A are stimulated by starvation (but not food restriction) and by hypoglycemia, but only if food is withheld. Orexin neurons are apparently activated by low glucose but are promptly inhibited by visceral feeding signals, probably mediated via vagal sensory pathway and the nucleus of the solitary tract (NTS); a short-term role in initiating feeding seems most likely. Other LHA neurons express melanin-concentrating hormone (MCH), which transiently increases food intake when injected centrally. MCH neurons may be regulated by leptin, insulin and glucose. Glucose-sensing neurons in the hypothalamus and elsewhere are sensitive to other cues of nutritional state, including visceral satiety signals (transmitted via the vagus) and orexin A. Thus, long- and short-term humoral and neural signals interact with each other to meet diverse nutritional needs, and anabolic neuropeptides are important in the overall integration of energy homeostasis. Clarifying the underlying mechanisms will be essential to understanding normal energy balance and the pathogenesis and treatment of disorders, such as obesity and cachexia.

Mice lacking pro-opiomelanocortin are sensitive to high-fat feeding but respond normally to the acute anorectic effects of peptide-YY(3-36)

Inactivating mutations of the pro-opiomelanocortin (POMC) gene in both mice and humans leads to hyperphagia and obesity. To further examine the mechanisms whereby POMC-deficiency leads to disordered energy homeostasis, we have generated mice lacking all POMC-derived peptides. Consistent with a previously reported model, Pomc(-/-) mice were obese and hyperphagic. They also showed reduced resting oxygen consumption associated with lowered serum levels of thyroxine. Hypothalami from Pomc(-/-) mice showed markedly increased expression of melanin-concentrating hormone mRNA in the lateral hypothalamus, but expression of neuropeptide Y mRNA in the arcuate nucleus was not altered. Provision of a 45% fat diet increased energy intake and body weight in both Pomc(-/-) and Pomc(+/-) mice. The effects of leptin on food intake and body weight were blunted in obese Pomc(-/-) mice whereas nonobese Pomc(-/-) mice were sensitive to leptin. Surprisingly, we found that Pomc(-/-) mice maintained their acute anorectic response to peptide-YY(3-36) (PYY(3-36)). However, 7 days of PYY(3-36) administration had no effect on cumulative food intake or body weight in wild-type or Pomc(-/-) mice. Thus, POMC peptides seem to be necessary for the normal response of energy balance to high-fat feeding, but not for the acute anorectic effect of PYY(3-36) or full effects of leptin on feeding. The finding that the loss of only one copy of the Pomc gene is sufficient to render mice susceptible to the effects of high fat feeding emphasizes the potential importance of this locus as a site for gene-environment interactions predisposing to obesity.

Ghrelin secretion is modulated in a nutrient- and gender-specific manner

BACKGROUND

Ghrelin is a potent GH secretagogue that also plays an important role in appetite and weight regulation. Ghrelin increases hunger and food intake, and its levels decrease after a standard meal or glucose.

OBJECTIVE

To examine the effects of standard oral glucose, lipid and protein loads on ghrelin levels, investigating the possibility that these responses may be modulated by several anthropometric and metabolic factors.

SUBJECTS AND METHODS

There were 24 adult nondiabetic subjects (13 men/11 women; mean age 55.3 +/- 2.9 years, range 26-74 years). Each participant underwent one or more of the following nutrient loads: (i) a standard oral glucose (75 g) load (n = 18); (ii) an oral lipid load (40 g, with 24 g saturated fat; n = 13); (iii) an oral protein load (40 g; n = 11).

RESULTS

Fasting ghrelin levels were negatively related to body mass index (BMI; r =-0.47; P = 0.02), waist circumference (r = -0.58; P = 0.0028), waist/hip ratio (r = -0.56; P = 0.0046), fasting insulin (r = -0.44, P = 0.03), and homeostasis model assessment insulin resistance index (HOMA-R; r = -0.43, P = 0.034). Glucose load induced a decrease in ghrelin levels (P < 0.0001), and this response was modulated by sex (P < 0.0001), in that levels were significantly higher in females. The presence of obesity affected ghrelin response to glucose (< 0.0217), in that log-transformed ghrelin levels started to increase back to baseline after its initial decline earlier in obese than in lean subjects. Ghrelin levels after a glucose load were lower over time in subjects with more pronounced insulin resistance (P < 0.0001). Similarly, ghrelin levels decreased significantly following the lipid meal (P = 0.035), and were modulated by HOMA-R (P = 0.027) and gender (P = 0.029). Protein did not affect ghrelin levels.

CONCLUSIONS

This study demonstrates that ghrelin levels respond in a different manner to glucose, lipid and protein loads, and are subject to modulation according to gender, obesity and insulin sensitivity.

Characterization of N-(1-Acetyl-2,3-dihydro-1H-indol-6-yl)-3-(3-cyano-phenyl)-N-[1-(2-cyclopentyl-ethyl)-piperidin-4yl]acrylamide (JNJ-5207787), a small molecule antagonist of the neuropeptide Y Y2 receptor

The in vitro pharmacological properties of N-(1-Acetyl-2,3-dihydro-1H-indol-6-yl)-3-(3-cyano-phenyl)-N-[1-(2-cyclopentyl-ethyl)-piperidin-4yl]-acrylamide (JNJ-5207787), a novel neuropeptide Y Y(2) receptor (Y(2)) antagonist, were evaluated. JNJ-5207787 inhibited the binding of peptide YY (PYY) to human Y(2) receptor in KAN-Ts cells (pIC(50) = 7.00 +/- 0.10) and to rat Y(2) receptors in rat hippocampus (pIC(50) = 7.10 +/- 0.20). The compound was >100-fold selective versus human Y(1),Y(4), and Y(5) receptors as evaluated by radioligand binding. In vitro receptor autoradiography data in rat brain tissue sections confirmed the selectivity of JNJ-5207787. [(125)I]PYY binding sites sensitive to JNJ-5207787 were found in rat brain regions known to express Y(2) receptor (septum, hypothalamus, hippocampus, substantia nigra, and cerebellum), whereas insensitive binding sites were observed in regions known to express Y(1) receptor (cortex and thalamus). JNJ-5207787 was demonstrated to be an antagonist via inhibition of PYY-stimulated guanosine 5'-O-(3-[(35)S]thio)triphosphate binding ([(35)S]GTPgammaS) in KAN-Ts cells (pIC(50) corrected = 7.20 +/- 0.12). This was confirmed auto-radiographically in rat brain sections where PYY-stimulated guanosine 5'-O-(3-[(35)S]thio)triphosphate binding was inhibited by JNJ-5207787 (10 microM) in hypothalamus, hippocampus, and substantia nigra. After intraperitoneal administration in rats (30 mg/kg), JNJ-5207787 penetrated into the brain (C(max) = 1351 +/- 153 ng/ml at 30 min) and occupied Y(2) receptor binding sites as revealed by ex vivo receptor autoradiography. Hence, JNJ-5207787 is a potent and selective pharmacological tool available to establish the potential role of central and peripheral Y(2) receptors in vivo.

Éléments de physiologie et de neurobiologie de la prise alimentaire

Many methods and techniques have accumulated a considerable mass of data about the mechanisms which control food intake and energetic loss. After a presentation of the main experimental approaches in this respect, the most relevant signals sent by peripheral organs to the central nervous system are presented: Glucose, triglycerides; from adipocytes: Leptin; from pancreas: Insulin, pancreatic polypeptide, amylin, enterostatin; from digestive tract: Ghrelin, cholecystokinin, peptide Y Y 3-36. Then are considered, especially at the hypothalamic level, their interpretation by neurones whose transmitters are either neuropeptides such as: Neuropeptide Y, Agouti Related Peptide, Cocaine/Amphetamine Regulated Transcript, Melanin Concentrating Hormone, alpha Melanocyte Stimulating Hormone, orexins/hypocretins, octadecaneuropeptide, nociceptin/orphanin FQ, opioid peptides, Interleukin 1, galanin, urocortin 2, Neurotrophic ciliary factor, or monoamines such as: Glutamate, dopamine, Norepinephrine, serotonine, GABA, histamine, acetylcholine. In a last part are considered the likely relationships existing between feeding, pleasure and addiction, centered on dopamine transmission in the nucleus accumbens. After this brief synopsis one should not be surprised that this so complex system which regulates feeding may be affected by various disorders; however one may be amazed by such a scarcity of drugs to influence it; in any case, many new pharmacological strategies can be expected in the future.

Altered NPY and AgRP in membrane type-1 matrix metalloproteinase-deficient mice

Membrane-type-1 matrix metalloproteinase (MT1-MMP) knockout (KO) mice fail to gain weight and die 3-4 weeks after birth. To understand the wasting phenotype in MT1-MMP-KO mice we studied the expression of some hypothalamic neuropeptides involved in control of appetite and body weight. In MT1-MMP-KO mice, neuronal perikarya in the arcuate nucleus displayed accumulations of NPY and agouti-related protein (AgRP) immunoreactivity (-ir). In contrast, NPY-ir and AgRP-ir were reduced in the projection areas of the arcuate neurons. NPY and AgRP are known to relay metabolic signals from the periphery into the brain to stimulate body weight gain. Their altered subcellular distribution suggests that MT1-MMP is involved in postnatal development of the arcuate NPY/AgRP-system which may contribute to the generation of the wasting phenotype.

Obesity wars: molecular progress confronts an expanding epidemic

The worldwide prevalence of obesity is increasing at an alarming rate, with major adverse consequences for human health. This "obesity epidemic" is paralleled by a rapid and substantive increase in our understanding of molecular pathways and physiologic systems underlying the regulation of energy balance. While efforts to address the environmental factors that are responsible for the recent "epidemic" must continue, new molecular and physiologic insights into this system offer exciting possibilities for future development of successful therapies.

Investigation of the chronic effects of NPY by subcutaneous implantation of 6-23 cells producing NPY in WAG rats

OBJECTIVE

In this experiment, we studied the chronic effects of NPY, as there were no data on long-term effects of NPY in vivo.

METHODS

Complementary DNA encoding NPY was isolated, sequenced and cloned into the expression vector, pCEP4. The 6-23 clone 6 cell line was transfected with this clone. Two groups of 10 adult male WAG rats (180-250 g body weight) were injected with either untransfected 6-23 clone 6 or 6-23 clone 6 transfected with NPY cDNA [6-23 (NPY)]. After 8 weeks, the animals were killed, their plasma assayed for insulin. Pancreatic glucagon (PG), by RIA, and plasma glucose were measured.

RESULTS

The transfected cells were shown to be producing fully processed, bioactive NPY. The expression of NPY was also confirmed by Northern blot analysis. The animals injected with 6-23 (NPY) cells gained significantly more weight than the controls, (on day 54, 31.89 +/- 3.56 vs. 24.1 +/- 4.12 g, n = 10, P < 0.05). Plasma insulin and PG increased significantly in NPY animals compared to controls. The total RNA extracted from tumours was analysed by Northern blotting and showed NPY mRNA expression in NPY animals, but not in controls.

CONCLUSION

The long-term effects of NPY was confirmed by injection of the cells producing this peptide.

NPY presynaptic actions are reduced in the hypothalamic mpPVN of obese (fa/fa), but not lean, Zucker rats in vitro

1. Neuropeptide Y (NPY) profoundly enhances feeding when injected intracerebroventricularly, or directly into hypothalamic nuclei, such as the paraventricular nucleus (PVN). Paradoxically, NPY has a reduced action on feeding in obese Zucker rats relative to lean Zucker rats, although the obese rats have much higher levels of hypothalamic NPY expression. GABAergic inputs to a subpopulation of medial parvocellular PVN (mpPVN) neurons are sensitive to NPY. Here, we tested the hypothesis that the blunted eating response to NPY observed in obese Zucker rats will be reflected in a reduced NPY action at mpPVN GABAergic synapses. 2. 'Blind' whole-cell patch-clamp recordings made from mpPVN neurons in acute brain slices of lean and obese Zucker rats revealed GABAergic inhibitory postsynaptic currents (IPSC) responses which were inhibited by NPY. While the maximum response in the obese Zucker rats was significantly less than in lean Zucker or Sprague-Dawley rats, there was no difference in the EC(50). 3. Experiments using blocking concentrations of Y(1)- or Y(5)-receptor antagonists revealed no differences between lean and obese Zucker rats in the contributions of either of these receptors to the total NPY response in mpPVN. 4. NPY is less effective at the mpPVN GABA synapse in obese than in lean Zucker rats. This is not associated with a change in the proportion of Y(1) or Y(5) receptors mediating the NPY response, and is consistent with the downregulation of NPY receptors or a reduction in receptor-effector coupling, and with the reduced sensitivity of obese rats to NPY.

Jejunal administration of linoleic acid increases activity of neurons in the paraventricular nucleus of the hypothalamus

The present experiment examined whether neurons located in the paraventricular nucleus of the hypothalamus (PVN) respond to intestinal infusions of long-chain fatty acids. Single-unit recordings were made of neurons located in and adjacent to the PVN during jejunal administration of linoleic acid. Jejunal administration of linoleic acid increased single-unit activity of neurons located in the PVN but did not affect activity of neurons located in adjacent tissue outside the PVN. The largest increases in neuronal activity were observed in the anterior PVN (0.9-1.3 mm posterior to bregma) compared with the posterior PVN (1.8-2.1 mm posterior to bregma). Jejunal administration of saline failed to affect activity of neurons located either inside or outside the PVN. When the same neurons were subsequently tested for their response to intravenous administration of 2 microg/kg of CCK-8, excitatory responses were more frequently observed than inhibitory responses, but both types of responses were observed regardless of whether neurons were located inside or outside the PVN. In addition, there was no strong correlation between the magnitude of the neuronal response evoked by jejunal administration of linoleic acid compared with intravenous CCK-8. These data suggest that neurons located in the anterior PVN may play a role in the mediation of suppression of food intake produced by intestinal administration of lipids.

Acute effects of PYY3-36 on food intake and hypothalamic neuropeptide expression in the mouse

It has recently been suggested that gut-derived PYY(3-36) may be involved in the central mediation of post-prandial satiety signals. We have examined the acute effects of peripherally administered PYY(3-36) on food intake and hypothalamic gene expression of neuropeptides in mice. A single intraperitoneal injection of PYY(3-36) to mice that had been fasted for 24h resulted in a highly significant reduction in food intake at 6 and 24h post-injection but not at 48h. However, in freely fed mice, food intake was unaltered by PYY(3-36) administration. In the arcuate nucleus POMC mRNA expression was significantly elevated at 6h and remained elevated at 24h following PYY(3-36) injection. By contrast NPY mRNA expression in the arcuate nucleus was suppressed at 6h but not at 24h post-injection. In the lateral hypothalamus there were no differences in MCH mRNA expression at either time point. In conclusion, peripherally administered PYY(3-36) has a suppressive effect on food intake that is more prominent in recently fasted mice and lasts up to 24 h. This is associated with a short-lived suppression of NPY mRNA, a longer lasting increase in POMC mRNA but no change in MCH mRNA expression.

Insulin-like growth factor I increases rat peptide YY promoter activity through Sp1 binding sites

Studies in rodents demonstrate that the mitogen, IGF-I, stimulates intestinal peptide YY (PYY) expression. To investigate whether the stimulatory influence of IGF-I is exerted at the level of gene transcription, rat PYY 5'-upstream sequences (-2800/+37 bp, -770/+37 bp, -127/+37 bp) fused to the firefly luciferase (luc) reporter gene were transfected into rat pheochromocytoma cells (PC12) and luc activity measured after IGF-I treatment. IGF-I increased transcriptional activity of all constructs similarly; the PYY (-127/+37 bp)-luc construct was used in subsequent experiments. IGF-I increased PYY (-127/+37 bp)-luc activity in a time- and dose-dependent fashion. Sequence analysis detected five putative Sp1 binding sites in the -127/+37-bp sequence. EMSA and supershift experiments using two oligonucleotide fragments of the -127/+37 region showed that Sp1 and Sp3 proteins bound to putative Sp1 sites. Overexpression of Sp1 greatly increased PYY (-127/+37 bp)-luc activity and site-directed mutagenesis of putative Sp1 binding sites decreased basal and IGF-I-induced elevations in PYY (-127/+37 bp)-luc activity. IGF-I treatment also increased Sp1 protein levels and binding activity. Blockade of the IGF-I receptor (IGF-IR) with an IGF-IR antibody decreased the stimulatory influence of IGF-I on Sp1 protein levels and PYY (-127/+37 bp)-luc activity. Together, these findings indicate that IGF-I functions as a positive regulator of PYY gene expression and that the stimulatory effect may be mediated by Sp1 proteins that bind to the proximal PYY promoter region.

Energy homeostasis, obesity and eating disorders: recent advances in endocrinology

Health problems resulting from obesity could offset many of the recent health gains achieved by modern medicine, and obesity may replace tobacco as the number one health risk for developed societies. An estimated 300,000 deaths per year and significant morbidity are directly attributable to obesity, mainly due to heart disease, diabetes, cancer, asthma, sleep apnea, arthritis, reproductive complications and psychological disturbances. In parallel with the increasing prevalence of obesity, there has been a dramatic increase in the number of scientific and clinical studies on the control of energy homeostasis and the pathogenesis of obesity to further our understanding of energy balance. It is now recognized that there are many central and peripheral factors involved in energy homeostasis, and it is expected that the understanding of these mechanisms should lead to effective treatments for the control of obesity. This brief review discusses the potential role of several recently discovered molecular pathways involved in the control of energy homeostasis, obesity and eating disorders.

Individual subtypes of enteroendocrine cells in the mouse small intestine exhibit unique patterns of inositol 1,4,5-trisphosphate receptor expression

Enteroendocrine cells are a complex population of intestinal epithelial cells whose hormones play critical roles in regulating gastrointestinal and whole-animal physiology. There are many subpopulations of enteroendocrine cells based on the major hormone(s) produced by individual cells. Intracellular calcium plays a critical role in regulating hormone release. Inositol 1,4,5-trisphophate (IP3) receptors regulate calcium mobilization from endoplasmic reticulum-derived calcium stores in many endocrine and excitatory cells and are expressed in the intestine. However, the specific subtypes of enteroendocrine cells that express these receptors have not been reported. Immunohistochemical (IHC) studies revealed that enteroendocrine cells did not express detectable levels of type 2 IP3 receptors, whereas nearly all enteroendocrine cells that produced chromogranin A and/or serotonin expressed type 1 and type 3 IP3 receptors. Conversely, enteroendocrine cells that produced glucose-dependent insulinotropic polypeptide, glucagon-like peptide-1, cholecystokinin, or somatostatin did not express detectable levels of any IP3 receptors. Subsets of enteroendocrine cells that produced substance P or secretin expressed type 1 (33% or 18%, respectively) and type 3 (10% or 62%, respectively) IP3 receptors. Thus, different subtypes of enteroendocrine cells, as well as individual cells that express a particular hormone, exhibit remarkable heterogeneity in the molecular machineries that regulate hormone release in vivo. These results suggest that therapeutic agents can be developed that could potentially inhibit or promote secretion of hormones from specific subtypes of enteroendocrine cells.

AMP-activated protein kinase plays a role in the control of food intake

AMP-activated protein kinase (AMPK) is the downstream component of a protein kinase cascade that acts as an intracellular energy sensor maintaining the energy balance within the cell. The finding that leptin and adiponectin activate AMPK to alter metabolic pathways in muscle and liver provides direct evidence for this role in peripheral tissues. The hypothalamus is a key regulator of food intake and energy balance, coordinating body adiposity and nutritional state in response to peripheral hormones, such as leptin, peptide YY-(3-36), and ghrelin. To date the hormonal regulation of AMPK in the hypothalamus, or its potential role in the control of food intake, have not been reported. Here we demonstrate that counter-regulatory hormones involved in appetite control regulate AMPK activity and that pharmacological activation of AMPK in the hypothalamus increases food intake. In vivo administration of leptin, which leads to a reduction in food intake, decreases hypothalamic AMPK activity. By contrast, injection of ghrelin in vivo, which increases food intake, stimulates AMPK activity in the hypothalamus. Consistent with the effect of ghrelin, injection of 5-amino-4-imidazole carboxamide riboside, a pharmacological activator of AMPK, into either the third cerebral ventricle or directly into the paraventricular nucleus of the hypothalamus significantly increased food intake. These results suggest that AMPK is regulated in the hypothalamus by hormones which regulate food intake. Furthermore, direct pharmacological activation of AMPK in the hypothalamus is sufficient to increase food intake. These findings demonstrate that AMPK plays a role in the regulation of feeding and identify AMPK as a novel target for anti-obesity drugs.

Glucagon-like peptide 1 and gastric inhibitory polypeptide: potential applications in type 2 diabetes mellitus

Although the insulinotropic actions of gastric inhibitory polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) have been known for almost 2 decades, the incretin hormones have not yet become available for clinical application. This can be explained by their unfavourable pharmacological properties. Both hormones are rapidly inactivated by the enzyme dipeptidyl peptidase IV (DPP IV), yielding biologically inactive fragments. There have been several attempts to make use of the antidiabetogenic potential of the incretin hormones. Various analogues of GLP-1 and GIP have been generated in order to achieve resistance to DPP IV degradation. The natural GLP-1 receptor agonist exendin-4, found in the saliva of the Gila monster, has a longer biological half-life after subcutaneous injection than GLP-1, and inhibition of DPP IV using, for example, pyrrolidine derivatives provides elevated concentrations of intact, biologically active GIP and GLP-1 endogenously released from the gut. A continuous intravenous infusion of native GLP-1 for a limited time may be suitable in certain clinical situations. Numerous clinical studies are currently underway to evaluate these approaches. Therefore, an antidiabetic treatment based on incretin hormones may become available within the next 5 years.

Peripheral regulation of food intake: new insights

Appetite is controlled by a complicated system with hunger and satiety signals interacting in complex pathways both peripherally and centrally. Insulin, leptin and ghrelin are key hormonal regulators of food intake. Ghrelin enhances appetite while leptin is a satiety signal. A novel peripheral regulator of food intake, peptide YY(3-36), is released from the gastrointestinal tract postprandially. In this review old and new peripheral signals and their interaction in the control of food intake are briefly discussed.

Chronic neuropeptide Y infusion into the lateral ventricle induces sustained feeding and obesity in mice lacking either Npy1r or Npy5r expression

Neuropeptide Y (NPY) is a powerful orexigenic neurotransmitter. The NPY Y1 and Y5 receptors have been implicated in mediating the appetite-stimulating activity of NPY. To further investigate the importance of these two receptors in NPY-induced hyperphagia after chronic central administration, we used mice lacking either Npy1r or Npy5r expression. NPY infusion into the lateral ventricle of wild-type mice stimulated food intake and induced obesity over a 7-d period. Fat pad weight as well as plasma insulin, leptin, and corticosterone levels were strongly increased in NPY-treated mice. In addition, NPY infusion resulted in a significant decrease in hypothalamic NPY and proopiomelanocortin expression. Interestingly, the lack of either Npy1r or Npy5r expression in knockout mice did not affect such feeding response to chronic NPY infusion. Moreover, the obesity syndrome that developed in these animals was similar to that in wild-type animals. Taken together, these data strongly suggest biological redundancies between Y1 and Y5 receptor signaling in the NPY-mediated control of food intake.

Stimulation of NPY Y2 receptors by PYY3-36 reveals divergent cardiovascular effects of endogenous NPY in rats on different dietary regimens

In the present experiments the gut hormone peptide YY3-36 (PYY3-36), which inhibits neuropeptide Y (NPY) release, was used as a tool to study the cardiovascular effects of endogenous NPY under different dietary regimens in rats instrumented with a telemetry transmitter. In a first experiment, rats were placed on a standard chow diet ad libitum and in a second experiment on a high-fat diet ad libitum. After 6 wk, PYY3-36 (300 μg/kg) or vehicle was injected intraperitoneally. In a third experiment, PYY3-36 or vehicle was administered after 14 days of 50% restriction of a standard chow diet. In food-restricted rats, PYY3-36 increased mean arterial pressure (7 ± 1 mmHg, mean ± SE, P < 0.001 vs. saline, 1-way repeated-measures ANOVA with Bonferroni t-test) and heart rate (22 ± 4 beats/min, P < 0.001) during 3 h after administration. Conversely, PYY3-36 did not influence mean arterial pressure (0 ± 1 mmHg) and heart rate (-8 ± 5 beats/min) significantly in rats on a high-fat diet. Rats fed standard chow diet ad libitum showed an intermediate response (mean arterial pressure 4 ± 1 mmHg, P < 0.05, and heart rate 5 ± 2 beats/min, not significant). Thus, in our studies, divergent cardiovascular responses to PYY3-36 were observed in rats on different dietary regimens. These findings suggest that the cardiovascular effects of PYY3-36 depend on the hypothalamic NPY release, which is increased after chronic food restriction and decreased during a high-fat diet.