PYY3-36 inhibits the action potential firing activity of POMC neurons of arcuate nucleus through postsynaptic Y2 receptors

Gut Microbiome Regulation of Gut Hormone Secretion

The gut microbiome, comprising bacteria, viruses, fungi, and bacteriophages, is one of the largest microbial ecosystems in the human body and plays a crucial role in various physiological processes. This review explores the interaction between the gut microbiome and enteroendocrine cells (EECs), specialized hormone-secreting cells within the intestinal epithelium. EECs, which constitute less than 1% of intestinal epithelial cells, are key regulators of gut-brain communication, energy metabolism, gut motility, and satiety. Recent evidence shows that gut microbiota directly influence EEC function, maturation, and hormone secretion. For instance, commensal bacteria regulate the production of hormones like glucagon-like peptide 1 and peptide YY by modulating gene expression and vesicle cycling in EE cells. Additionally, metabolites such as short-chain fatty acids, derived from microbial fermentation, play a central role in regulating EEC signaling pathways that affect metabolism, gut motility, and immune responses. Furthermore, the interplay between gut microbiota, EECs, and metabolic diseases, such as obesity and diabetes, is examined, emphasizing the microbiome's dual role in promoting health and contributing to disease states. This intricate relationship between the gut microbiome and EECs offers new insights into potential therapeutic strategies for metabolic and gut disorders.

Obesogens and Energy Homeostasis: Definition, Mechanisms of Action, Exposure, and Adverse Effects on Human Health

BACKGROUND

Obesity is a major risk factor for noncommunicable diseases and is associated with a reduced life expectancy of up to 20 years, as well as with other consequences such as unemployment and increased economic burden for society. It is a multifactorial disease, and physiopathology of obesity involves dysregulated calorie utilization and energy balance, disrupted homeostasis of appetite and satiety, lifestyle factors including sedentary lifestyle, lower socioeconomic status, genetic predisposition, epigenetics, and environmental factors. Some endocrine-disrupting chemicals (EDCs) have been proposed as "obesogens" that stimulate adipogenesis leading to obesity. In this review, definition of obesogens, their adverse effects, underlying mechanisms, and metabolic implications will be updated and discussed.

SUMMARY

Disruption of lipid homeostasis by EDCs involves multiple mechanisms including increase in the number and size of adipocytes, disruption of endocrine-regulated adiposity and metabolism, alteration of hypothalamic regulation of appetite, satiety, food preference and energy balance, and modification of insulin sensitivity in the liver, skeletal muscle, pancreas, gastrointestinal system, and the brain. At a cellular level, obesogens can exert their endocrine disruptive effects by interfering with peroxisome proliferator-activated receptors and steroid receptors. Human exposure to chemical obesogens mainly occurs by ingestion and, to some extent, by inhalation and dermal uptake, usually in an unconscious manner. Persistent pollutants are lipophilic features; thus, they bioaccumulate in adipose tissue.

KEY MESSAGES

Although there are an increasing number of reports studying the effects of obesogens, their mechanisms of action remain to be elucidated. In addition, epidemiological studies are needed in order to evaluate human exposure to obesogens.

The emerging role of gut hormones

The gut is traditionally recognized as the central organ for the digestion and absorption of nutrients, however, it also functions as a significant endocrine organ, secreting a variety of hormones such as glucagon-like peptide 1, serotonin, somatostatin, and glucocorticoids. These gut hormones, produced by specialized intestinal epithelial cells, are crucial not only for digestive processes but also for the regulation of a wide range of physiological functions, including appetite, metabolism, and immune responses. While gut hormones can exert systemic effects, they also play a pivotal role in maintaining local homeostasis within the gut. This review discusses the role of the gut as an endocrine organ, emphasizing the stimuli, the newly discovered functions, and the clinical significance of gut-secreted hormones. Deciphering the emerging role of gut hormones will lead to a better understanding of gut homeostasis, innovative treatments for disorders in the gut, as well as systemic diseases.

Agrp-negative arcuate NPY neurons drive feeding under positive energy balance via altering leptin responsiveness in POMC neurons

Neuropeptide Y (NPY) in the arcuate nucleus (ARC) is known as one of the most critical regulators of feeding. However, how NPY promotes feeding under obese conditions is unclear. Here, we show that positive energy balance, induced by high-fat diet (HFD) or in genetically obese leptin-receptor-deficient mice, leads to elevated Npy2r expression especially on proopiomelanocortin (POMC) neurons, which also alters leptin responsiveness. Circuit mapping identified a subset of ARC agouti-related peptide (Agrp)-negative NPY neurons that control these Npy2r expressing POMC neurons. Chemogenetic activation of this newly discovered circuitry strongly drives feeding, while optogenetic inhibition reduces feeding. Consistent with that, lack of Npy2r on POMC neurons leads to reduced food intake and fat mass. This suggests that under energy surplus conditions, when ARC NPY levels generally drop, high-affinity NPY2R on POMC neurons is still able to drive food intake and enhance obesity development via NPY released predominantly from Agrp-negative NPY neurons.

Role of brain-gut-muscle axis in human health and energy homeostasis

The interrelationship between brain, gut and skeletal muscle plays a key role in energy homeostasis of the body, and is becoming a hot topic of research. Intestinal microbial metabolites, such as short-chain fatty acids (SCFAs), bile acids (BAs) and tryptophan metabolites, communicate with the central nervous system (CNS) by binding to their receptors. In fact, there is a cross-talk between the CNS and the gut. The CNS, under the stimulation of pressure, will also affect the stability of the intestinal system, including the local intestinal transport, secretion and permeability of the intestinal system. After the gastrointestinal tract collects information about food absorption, it sends signals to the central system through vagus nerve and other channels to stimulate the secretion of brain-gut peptide and produce feeding behavior, which is also an important part of maintaining energy homeostasis. Skeletal muscle has receptors for SCFAs and BAs. Therefore, intestinal microbiota can participate in skeletal muscle energy metabolism and muscle fiber conversion through their metabolites. Skeletal muscles can also communicate with the gut system during exercise. Under the stimulation of exercise, myokines secreted by skeletal muscle causes the secretion of intestinal hormones, and these hormones can act on the central system and affect food intake. The idea of the brain-gut-muscle axis is gradually being confirmed, and at present it is important for regulating energy homeostasis, which also seems to be relevant to human health. This article focuses on the interaction of intestinal microbiota, central nervous, skeletal muscle energy metabolism, and feeding behavior regulation, which will provide new insight into the diagnostic and treatment strategies for obesity, diabetes, and other metabolic diseases.

Neuropeptide Y modulates excitatory synaptic transmission and promotes social behavior in the mouse nucleus accumbens

Social interactions define the human experience, but these integral behaviors are disrupted in many psychiatric disorders. Social behaviors have evolved over millennia, and neuromodulatory systems that promote social behavior in invertebrates are also present in mammalian brains. One such conserved neuromodulator, neuropeptide Y (NPY), acts through several receptors including the Y1r, Y2r, and Y5r. These receptors are present in brain regions that control social behavior, including the nucleus accumbens (NAc). However, whether NPY modulates NAc neurotransmission is unknown. Using whole-cell patch-clamp electrophysiology of NAc neurons, we find that multiple NPY receptors regulate excitatory synaptic transmission in a cell-type specific manner. At excitatory synapses onto D1+ MSNs, Y1r activity enhances transmission while Y2r suppresses transmission. At excitatory synapses onto D1- MSNs, Y5r activity enhances transmission while Y2r suppresses transmission. Directly infusing NPY or the Y1r agonist [Leu³¹, Pro³⁴]-NPY into the NAc significantly increases social interaction with an unfamiliar conspecific. Inhibition of an enzyme that breaks down NPY, dipeptidyl peptidase IV (DPP-IV), shifts the effect of NPY on D1+ MSNs to a Y1r dominated phenotype. Together, these results increase our understanding of how NPY regulates neurotransmission in the NAc and identify a novel mechanism underlying the control of social behavior. Further, they reveal a potential strategy to shift NPY signaling for therapeutic gain.

Propionate stimulates the secretion of satiety hormones and reduces acute appetite in a cecal fistula pig model

Short-chain fatty acids (SCFA) can regulate appetite by stimulating the secretion of satiety hormones. However, the impact of short-chain fatty acid propionate on the release of gut satiety hormones and appetite regulation in pigs is not completely understood. In this study, 16 pigs were infused with saline or sodium propionate through a fistula in the caecum during a 28-day experimental period. We characterized the effects of propionate administration on peptide YY (PYY) and glucagon-like peptide 1 (GLP-1) secretion from colonic tissue, and investigated the role of propionate infusion on the expression of appetite-related genes in the colon and hypothalamus. Further, the direct impact of propionate administration on the expression of orexigenic neuropeptide agouti-related protein (AgRP) in hypothalamic N38 cells was also examined. The results showed that intra-cecal infusion of propionate reduced the short-term feed intake (P < 0.05) but not the long-term feed intake in pigs (P > 0.05). Propionate administration stimulated PYY and GLP-1 release from colon tissue in vivo and ex vivo (P < 0.05). It also upregulated PYY expression in the colonic mucosa (P < 0.05). Meanwhile, the GLP-1 and PYY levels in the blood were increased after intra-cecal infusion of propionate at d 28 (P < 0.05). Additionally, intra-cecal infusion of propionate upregulated the mRNA and protein expression of free fatty acid receptor 2/3 (FFAR2/FFAR3) in the colonic mucosa (P < 0.05). Propionate infusion also downregulated the orexigenic AgRP mRNA expression (P < 0.05) and upregulated the anorexigenic cocaine-and amphetamine-regulated transcript (CART) mRNA expression (P = 0.09) in the hypothalamus. Moreover, propionate administration directly downregulated AgRP expression in hypothalamic N38 cells in a dose-dependent manner (P < 0.05). Collectively, these findings demonstrated that cecal propionate stimulated colonic secretion of satiety hormones and suppressed appetite to reduce the short-term feed intake in pigs. This study highlights that microbial-derived propionate exerts an important role in regulating the physical functions of the host.

Arcuate Nucleus-Dependent Regulation of Metabolism-Pathways to Obesity and Diabetes Mellitus

The central nervous system (CNS) receives information from afferent neurons, circulating hormones, and absorbed nutrients and integrates this information to orchestrate the actions of the neuroendocrine and autonomic nervous systems in maintaining systemic metabolic homeostasis. Particularly the arcuate nucleus of the hypothalamus (ARC) is of pivotal importance for primary sensing of adiposity signals, such as leptin and insulin, and circulating nutrients, such as glucose. Importantly, energy state-sensing neurons in the ARC not only regulate feeding but at the same time control multiple physiological functions, such as glucose homeostasis, blood pressure, and innate immune responses. These findings have defined them as master regulators, which adapt integrative physiology to the energy state of the organism. The disruption of this fine-tuned control leads to an imbalance between energy intake and expenditure as well as deregulation of peripheral metabolism. Improving our understanding of the cellular, molecular, and functional basis of this regulatory principle in the CNS could set the stage for developing novel therapeutic strategies for the treatment of obesity and metabolic syndrome. In this review, we summarize novel insights with a particular emphasis on ARC neurocircuitries regulating food intake and glucose homeostasis and sensing factors that inform the brain of the organismal energy status.

NPY derived from AGRP neurons controls feeding via Y1 and energy expenditure and food foraging behaviour via Y2 signalling

Objective

Aguti-related protein (AGRP) neurons in the arcuate nucleus of the hypothalamus (ARC), which co-express neuropeptide Y (NPY), are key regulators of feeding and energy homeostasis. However, the precise role NPY has within these neurons and the specific pathways that it control are still unclear. In this article, we aimed to determine what aspects of feeding behaviour and energy homeostasis are controlled by NPY originating from AGRP neurons and which Y-receptor pathways are utilised to fulfil this function.

Methods

Novel conditional Agrp^cre/+;Npy^lox/lox knockout mice were generated and comprehensively phenotyped, both under standard chow as well as high-fat-diet conditions. Designer receptor exclusively activated by designer drugs (DREADD) technology was used to assess the altered responses on feeding and energy homeostasis control in the absence of NPY in these neurons. Rescue experiments utilising Npy1r- and Npy2r-selective NPY ligands were performed to assess which component of the energy homeostasis control is dependent by which specific Y-receptor pathway.

Results

We show that the specific deletion of Npy only in AGRP neurons leads to a paradoxical mild obese phenotype associated with reduced locomotion and energy expenditure and increased feeding and Respiratory Quotient (RQ) that remain elevated under a positive energy balance. The activation of Npy-deficient AGRP neurons via DREADD's is still able to drive feeding, yet with a delayed onset. Additionally, Clozapine-N-oxide (CNO) treatment reduces locomotion without impacting on energy expenditure. Rescue experiments re-introducing Npy1r- and Npy2r-selective NPY ligands revealed that the increased feeding and RQ are mostly driven by Npy1r, whereas energy expenditure and locomotion are controlled by Npy2r signalling.

Conclusion

Together, these results demonstrate that NPY originating from AGRP neurons is not only critical to initiate but also for continuously driving feeding, and we for the first time identify which Y-receptor controls which pathway.

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Neuropeptide Y (NPY) in AGRP neurons is required to initiate and maintain a feeding drive.

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NPY controls feeding and RQ via Npy1r signalling.

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NPY controls energy expenditure and activity via Npy2r signalling.

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Reduced NPY levels in AGRP neurons result in increased Npy2r expression in POMC neurons.

Obesity-An Update on the Basic Pathophysiology and Review of Recent Therapeutic Advances

Obesity represents a major public health problem with a prevalence increasing at an alarming rate worldwide. Continuous intensive efforts to elucidate the complex pathophysiology and improve clinical management have led to a better understanding of biomolecules like gut hormones, antagonists of orexigenic signals, stimulants of fat utilization, and/or inhibitors of fat absorption. In this article, we will review the pathophysiology and pharmacotherapy of obesity including intersection points to the new generation of antidiabetic drugs. We provide insight into the effectiveness of currently approved anti-obesity drugs and other therapeutic avenues that can be explored.

Ligands of the Neuropeptide Y Y2 Receptors as a Potential Multitarget Therapeutic Approach for the Protection of the Neurovascular Unit Against Acute Ischemia/Reperfusion: View from the Perspective of the Laboratory Bench

Ischemic stroke is the third leading cause of death and disability worldwide, with no available satisfactory prevention or treatment approach. The current treatment is limited to the use of “reperfusion methods,” i.e., an intravenous or intra-arterial infusion of a fibrinolytic agent, mechanical removal of the clot by thrombectomy, or a combination of both methods. It should be stressed, however, that only approximately 5% of all acute strokes are eligible for fibrinolytic treatment and fewer than 10% for thrombectomy. Despite the tremendous progress in understanding of the pathomechanisms of cerebral ischemia, the promising results of basic research on neuroprotection are not currently transferable to human stroke. A possible explanation for this failure is that experiments on in vivo animal models involve healthy young animals, and the experimental protocols seldom consider the importance of protecting the whole neurovascular unit (NVU), which ensures intracranial homeostasis and is seriously damaged by ischemia/reperfusion. One of the endogenous protective systems activated during ischemia and in neurodegenerative diseases is represented by neuropeptide Y (NPY). It has been demonstrated that activation of NPY Y2 receptors (Y2R) by a specific ligand decreases the volume of the postischemic infarction and improves performance in functional tests of rats with arterial hypertension subjected to middle cerebral artery occlusion/reperfusion. This functional improvement suggests the protection of the NVU. In this review, we focus on NPY and discuss the potential, multidirectional protective effects of Y2R agonists against acute focal ischemia/reperfusion injury, with special reference to the NVU.

The melanocortin pathway and energy homeostasis: From discovery to obesity therapy

BACKGROUND

Over the past 20 years, insights from human and mouse genetics have illuminated the central role of the brain leptin-melanocortin pathway in controlling mammalian food intake, with genetic disruption resulting in extreme obesity, and more subtle polymorphic variations influencing the population distribution of body weight. At the end of 2020, the U.S. Food and Drug Administration (FDA) approved setmelanotide, a melanocortin 4 receptor agonist, for use in individuals with severe obesity due to either pro-opiomelanocortin (POMC), proprotein convertase subtilisin/kexin type 1 (PCSK1), or leptin receptor (LEPR) deficiency.

SCOPE OF REVIEW

Herein, we chart the melanocortin pathway's history, explore its pharmacology, genetics, and physiology, and describe how a neuropeptidergic circuit became an important druggable obesity target.

MAJOR CONCLUSIONS

Unravelling the genetics of the subset of severe obesity has revealed the importance of the melanocortin pathway in appetitive control; coupling this with studying the molecular pharmacology of compounds that bind melanocortin receptors has brought a new obesity drug to the market. This process provides a drug discovery template for complex disorders, which for setmelanotide took 25 years to transform from a single gene into an approved drug.

Impact of Gut and Metabolic Hormones on Feeding Reward

Ingestion of food activates a cascade of endocrine responses (thereby reflecting a contemporaneous feeding status) that include the release of hormones from the gastrointestinal (GI) tract, such as cholecystokinin (CCK), glucagonlike peptide YY (PYY), peptide PP, and oleoylethanolamide, as well as suppression of ghrelin secretion. The pancreas and adipose tissue, on the other hand, release hormones that serve as a measure of the current metabolic state or the long-term energy stores, that is, insulin, leptin, and adiponectin. It is well known and intuitively understandable that these hormones target either directly (by crossing the blood-brain barrier) or indirectly (e.g., via vagal input) the "homeostatic" brainstem-hypothalamic pathways involved in the regulation of appetite. The current article focuses on yet another target of the metabolic and GI hormones that is critical in inducing changes in food intake, namely, the reward system. We discuss the physiological basis of this functional interaction, its importance in the control of appetite, and the impact that disruption of this crosstalk has on energy intake in select physiological and pathophysiological states. We conclude that metabolic and GI hormones have a capacity to strengthen or weaken a response of the reward system to a given food, and thus, they are fundamental in ensuring that feeding reward is plastic and dependent on the energy status of the organism. © 2021 American Physiological Society. Compr Physiol 11:1425-1447, 2021.

The gut-brain axis in vertebrates: implications for food intake regulation

The gut and brain are constantly communicating and influencing each other through neural, endocrine and immune signals in an interaction referred to as the gut-brain axis. Within this communication system, the gastrointestinal tract, including the gut microbiota, sends information on energy status to the brain, which, after integrating these and other inputs, transmits feedback to the gastrointestinal tract. This allows the regulation of food intake and other physiological processes occurring in the gastrointestinal tract, including motility, secretion, digestion and absorption. Although extensive literature is available on the mechanisms governing the communication between the gut and the brain in mammals, studies on this axis in other vertebrates are scarce and often limited to a single species, which may not be representative for obtaining conclusions for an entire group. This Review aims to compile the available information on the gut-brain axis in birds, reptiles, amphibians and fish, with a special focus on its involvement in food intake regulation and, to a lesser extent, in digestive processes. Additionally, we will identify gaps of knowledge that need to be filled in order to better understand the functioning and physiological significance of such an axis in non-mammalian vertebrates.

The infundibular peptidergic neurons and glia cells in overeating, obesity, and diabetes

Dysfunctional regulation of energy homeostasis results in increased bodyweight and obesity, eventually leading to type 2 diabetes mellitus. The infundibular nucleus (IFN) of the hypothalamus is the main regulator of energy homeostasis. The peptidergic neurons and glia cells of the IFN receive metabolic cues concerning energy state of the body from the circulation. The IFN can monitor hormones like insulin and leptin and nutrients like glucose and fatty acids. All these metabolic cues are integrated into an output signal regulating energy homeostasis through the release of neuropeptides. These neuropeptides are released in several inter- and extrahypothalamic brain regions involved in regulation of energy homeostasis. This review will give an overview of the peripheral signals involved in the regulation of energy homeostasis, the peptidergic neurons and glial cells of the IFN, and will highlight the main intra-hypothalamic projection sites of the IFN.

Pancreatic Polypeptide but Not Other Members of the Neuropeptide Y Family Shows a Moderate Association With Perceived Anxiety in Obese Men

Neuropeptide Y (NPY), peptide tyrosine tyrosine (PYY), and pancreatic polypeptide (PP) are important mediators in the bidirectional communication along the gut-brain-axis. Best known for their role in the regulation of appetite and food intake they are considered to play a crucial role in the development of obesity. Additionally, mounting evidence indicates a regulatory function in anxiety, mood and stress resilience with potential sex differences. In the present study, we examined the associations of NPY, PYY, and PP plasma levels with anxiety, depressiveness and perceived stress in obese patients. We analyzed 144 inpatients (90 female, 54 male, BMI mean: 49.4 kg/m²) in a naturalistic treatment setting for obesity and its somatic and mental comorbidities. Fasting blood samples were taken, and patients completed psychometric self-assessment questionnaires (GAD-7, PHQ-9, PSQ-20) within the first week after admission and before discharge. Plasma concentrations of the peptides were measured by ELISA. Women showed significant higher anxiety (GAD-7: 8.13 ± 5.67 vs. 5.93 ± 5.42, p = 0.04) and stress scores (PSQ-20: 52.62 ± 23.5 vs. 41.23 ± 22.53, p = 0.01) than men. In the longitudinal analysis women with a clinically relevant improvement of anxiety (≥ 5 points on GAD-7, p < 0.001) also showed significant improvements in depression (PHQ-9: 38%, p = 0.002) and PSQ-20 scores (23%, p = 0.005) while anxiety-improved male patients only improved in the subscale tension of the PSQ-20 (34%, p = 0.02). In men we observed a positive correlation of PP with anxiety scores (GAD-7: r = 0.41, p = 0.007) and with age (r = 0.49, p = 0.001) on admission while NPY negatively correlated with age (r = -0.38, p = 0.01). In contrast, there were no significant associations (p > 0.05) in female subjects in the cross-sectional as well as in the longitudinal analysis. In conclusion, women suffering from morbid obesity showed greater psychological comorbidity and considerable interactions among them. Despite that we solely observed associations of PP with anxiety and age with NPY and PP in men, suggesting a possible influence of sex hormones on the NPY system. However, improvement of anxiety scores did not lead to significant changes in NPY.

The Neuropeptide Y Y2 Receptor Is Coexpressed with Nppb in Primary Afferent Neurons and Y2 Activation Reduces Histaminergic and IL-31-Induced Itch

Itch stimuli are detected by specialized primary afferents that convey the signal to the spinal cord, but how itch transmission is regulated is still not completely known. Here, we investigated the roles of the neuropeptide Y (NPY)/Y2 receptor system on scratch behavior. The inhibitory Y2 receptor is expressed on mouse primary afferents, and intrathecal administration of the Y2 agonist peptide YY (PYY)3-36 reduced scratch episode frequency and duration induced by compound 48/80, an effect that could be reversed by intrathecal preadministration of the Y2 antagonist BIIE0246. Also, scratch episode duration induced by histamine could be reduced by PYY3-36 In contrast, scratch behavior induced by α-methyl-5HT, protease-activated receptor-2-activating peptide SLIGRL, chloroquine, topical dust mite extract, or mechanical itch induced by von Frey filaments was unaffected by stimulation of Y2 Primary afferent neurons expressing the Npy2r gene were found to coexpress itch-associated markers such as natriuretic peptide precursor b, oncostatin M receptor, and interleukin (IL) 31 receptor A. Accordingly, intrathecal PYY3-36 reduced the scratch behavior induced by IL-31. Our findings imply that the NPY/Y2 system reduces histaminergic and IL-31-associated itch through presynaptic inhibition of a subpopulation of itch-associated primary afferents. SIGNIFICANCE STATEMENT: The spinal neuropeptide Y system dampens scratching behavior induced by histaminergic compounds and interleukin 31, a cytokine involved in atopic dermatitis, through interactions with the Y2 receptor. The Y2 receptor is expressed by primary afferent neurons that are rich in itch-associated neurotransmitters and receptors such as somatostatin, natriuretic peptide precursor b, and interleukin 31 receptors.

Hyperlipidemias and Obesity

The organs require oxygen and other types of nutrients (amino acids, sugars, and lipids) to function, the heart consuming large amounts of fatty acids for oxidation and adenosine triphosphate (ATP) generation.

Hypothalamic γ-melanocyte stimulating hormone gene delivery reduces fat mass in male mice

γ-Melanocyte stimulating hormone (γ-MSH) is an endogenous agonist of the melanocortin 3-receptor (MC3R). Genetic disruption of MC3Rs increases adiposity and blunts responses to fasting, suggesting that increased MC3R signaling could be physiologically beneficial in the long term. Interestingly, several studies have concluded that activation of MC3Rs is orexigenic in the short term. Therefore, we aimed to examine the short- and long-term effects of γ-MSH in the hypothalamic arcuate nucleus (ARC) on energy homeostasis and hypothesized that the effect of MC3R agonism is dependent on the state of energy balance and nutrition. Lentiviral gene delivery was used to induce a continuous expression of γ-Msh only in the ARC of male C57Bl/6N mice. Parameters of body energy homeostasis were monitored as food was changed from chow (6 weeks) to Western diet (13 weeks) and back to chow (7 weeks). The γ-MSH treatment decreased the fat mass to lean mass ratio on chow, but the effect was attenuated on Western diet. After the switch back to chow, an enhanced loss in weight (−15% vs −6%) and fat mass (−37% vs −12%) and reduced cumulative food intake were observed in γ-MSH-treated animals. Fasting-induced feeding was increased on chow diet only; however, voluntary running wheel activity on Western diet was increased. The γ-MSH treatment also modulated the expression of key neuropeptides in the ARC favoring weight loss. We have shown that a chronic treatment intended to target ARC MC3Rs modulates energy balance in nutritional state-dependent manner. Enhancement of diet-induced weight loss could be beneficial in treatment of obesity.

Appetite-associated responses to central neuropeptide Y injection in quail

The appetite-associated effects of neuropeptide Y (NPY) have been extensively studied in mammalian models. Less knowledge exists for other vertebrate species including birds. The aim of this study was to determine the effects of central injection of NPY on feeding behavior and hypothalamic physiology in 7 day-old Japanese quail (Coturnix japonica). During the light cycle, intracerebroventricular injection of 1.9 pmol, 0.5, and 1.0 nmol doses of NPY did not affect food intake, 0.031 to 0.13 nmol increased food intake, and 2.0 nmol NPY decreased food intake, in comparison to vehicle injection. Multiple doses of NPY stimulated water intake, but when food was not available, water intake was not affected. When injected during the dark cycle, NPY did not influence food intake. NPY-injected chicks had more c-Fos immunoreactive cells in the arcuate nucleus of the hypothalamus (ARC) and greater hypothalamic agouti-related peptide and neuropeptide Y receptors 1 and 2 (NPYR1 and NPYR2, respectively) mRNA than vehicle-injected chicks. Within the ventromedial hypothalamus, NPY-treated chicks expressed less NPYR1 mRNA, within the dorsomedial hypothalamus less NPY mRNA, and in the ARC greater NPYR2 mRNA than vehicle-injected chicks. Lastly, quail injected with NPY increased feeding pecks, escape attempts, and time spent preening, while locomotion, the number of steps, and time spent perching decreased compared to chicks injected with the vehicle. Results demonstrate that NPY stimulates food intake in quail, consistent with mammals and other avian species, but with some unique responses at the molecular level that are not documented in other species.

Hypothalamic Integration of the Endocrine Signaling Related to Food Intake

Hypothalamic integration of gastrointestinal and adipose tissue-derived hormones serves as a key element of neuroendocrine control of food intake. Leptin, adiponectin, oleoylethanolamide, cholecystokinin, and ghrelin, to name a few, are in a constant "cross talk" with the feeding-related brain circuits that encompass hypothalamic populations synthesizing anorexigens (melanocortins, CART, oxytocin) and orexigens (Agouti-related protein, neuropeptide Y, orexins). While this integrated neuroendocrine circuit successfully ensures that enough energy is acquired, it does not seem to be equally efficient in preventing excessive energy intake, especially in the obesogenic environment in which highly caloric and palatable food is constantly available. The current review presents an overview of intricate mechanisms underlying hypothalamic integration of energy balance-related peripheral endocrine input. We discuss vulnerabilities and maladaptive neuroregulatory processes, including changes in hypothalamic neuronal plasticity that propel overeating despite negative consequences.

Dietary macronutrient composition and central neuropeptide Y injection affect dietary preference and hypothalamic gene expression in chicks

OBJECTIVE

The objective of this study was to determine the influence of dietary macronutrient composition on central NPY's orexigenic effect in chicks.

METHODS

Day-of-hatch chicks were fed one of three diets (3000 kcal ME/kg) ad libitum from hatch: high carbohydrate (HC), high fat (HF; 30% ME derived from soybean oil), and high protein (HP; 25 vs. 22% CP). In Experiment 1, chicks received intracerebroventricular injections of 0 (vehicle), 0.2, or 2.0 nmol NPY on day 4 and food intake was recorded for 6 hours. In Experiment 2, chicks were given all three diets before and after injection. In Experiment 3, hypothalamus was collected at 1-hour post-injection for gene expression analysis.

RESULTS

The HC diet-fed chicks responded with a greater increase, while the chicks fed the HF diet had a lower threshold response in food intake to NPY. Neuropeptide Y dose-dependently increased food intake in chicks fed the HC and HP diets. Chicks administered 0.2 nmol NPY preferred the HC and HP diets over the HF diet. Relative quantities of hypothalamic NPYR1 and MC4R mRNA were reduced by NPY in chicks that consumed the HP and HC diets, respectively.

DISCUSSION

Consumption of the HC diet was associated with the most robust NPY-induced increase in food intake. Injection of NPY accentuated differences among dietary groups in hypothalamic gene expression of several appetite-associated factors, results suggesting that the NPY/agouti-related peptide and melanocortin pathways are associated with some of the diet- and NPY-induced differences observed in this study.

Peptide YY: more than just an appetite regulator

Replenishment of beta cell mass is a key aim of novel therapeutic interventions for diabetes, and the implementation of new strategies will be aided by understanding the mechanisms employed to regulate beta cell mass under normal physiological conditions. We have recently identified a new role for the gut hormone peptide YY (PYY) and the neuropeptide Y (NPY) receptor systems in the control of beta cell survival. PYY is perhaps best known for its role in regulating appetite and body weight, but its production by islet cells, the presence of NPY receptors on islets and the demonstration that Y1 activation causes proliferation of beta cells and protects them from apoptosis, suggest a role for this peptide in modulating beta cell mass. This review introduces PYY and its potential role in glucose homeostasis, then focuses on evidence supporting the concept that PYY and NPY receptors are exciting new targets for the preservation of beta cells.

Targeting the intestinal L-cell for obesity and type 2 diabetes treatment

Degradation-resistant glucagon-like peptide-1 (GLP-1) mimetics and GLP-1 enhancers (inhibitors of dipeptidyl peptidase-4, the enzyme which degrades and inactivates GLP-1) have been used for treatment of type 2 diabetes mellitus since 2005-2006. Cutting-edge research is now focusing on uncovering the secretory mechanisms of the GLP-1-producing cells (L-cells) with the purpose of developing agonists that enhance endogenous hormone secretion. Since GLP-1 co-localizes with other anorectic peptides, cholecystokinin, oxyntomodulin/glicentin and peptide YY, L-cell targeting might cause release of several hormones at the same time, providing additive effects on appetite and glucose regulation. In this review, we explore the role of proglucagon-derived peptides and other L-cell co-localizing hormones, in appetite regulation and the mechanism regulating their secretion.

Teasaponin reduces inflammation and central leptin resistance in diet-induced obese male mice

Chronic inflammation is involved in the pathogenesis of obesity and type 2 diabetes. Recently teasaponin, an extract from tea, has been shown to have antiinflammatory effects. We examined the effect of teasaponin on obesity, inflammation, glucose metabolism, and central leptin sensitivity in obese mice fed a high-fat (HF) diet for 16 weeks. Intraperitoneal injections of teasaponin (10 mg/kg, daily) for 21 days significantly decreased the food intake and body weight of HF diet-induced obese mice. Teasaponin treatment also reduced the protein levels of proinflammatory cytokines (TNF-α, IL-6, and/or IL-1β) and nuclear factor-κB signaling (phosphorylated inhibitory-κB kinase and phosphorylated inhibitory-κBα) in adipose tissue and the liver. The antiinflammatory effects of teasaponin were associated with improved glycemic status in the treated animals, evidenced by improved glucose tolerance, homeostasis model assessment, and fasting plasma insulin. In the hypothalamus, teasaponin decreased both proinflammatory cytokines and inflammatory signaling in the mediobasal hypothalamus. Teasaponin treatment also enhanced the anorexigenic effect of central leptin administration, restored leptin phosphorylated signal transducer and activator of transcription-3 (p-STAT3) signaling in the arcuate nucleus, and increased hypothalamic expression of the anorexigenic peptide proopiomelanocortin. These results identify a potential novel application for teasaponin as an antiobesity and antiinflammatory agent.

Neuronal circuits that regulate feeding behavior and metabolism

Neurons within the central nervous system receive humoral and central (neurotransmitter or neuropeptide) signals that ultimately regulate ingestive behavior and metabolism. Recent advances in mouse genetics combined with neuroanatomical and electrophysiological techniques have contributed to a better understanding of these central mechanisms. This review integrates recently defined cellular mechanisms and neural circuits relevant to the regulation of feeding behavior, energy expenditure, and glucose homeostasis by metabolic signals.

Central nervous system neuropeptide Y signaling via the Y1 receptor partially dissociates feeding behavior from lipoprotein metabolism in lean rats

Elevated plasma triglyceride (TG) levels contribute to an atherogenic dyslipidemia that is associated with obesity, diabetes, and metabolic syndrome. Numerous models of obesity are characterized by increased central nervous system (CNS) neuropeptide Y (NPY) tone that contributes to excess food intake and obesity. Previously, we demonstrated that intracerebroventricular (icv) administration of NPY in lean fasted rats also elevates hepatic production of very low-density lipoprotein (VLDL)-TG. Thus, we hypothesize that elevated CNS NPY action contributes to not only the pathogenesis of obesity but also dyslipidemia. Here, we sought to determine whether the effects of NPY on feeding and/or obesity are dissociable from effects on hepatic VLDL-TG secretion. Pair-fed, icv NPY-treated, chow-fed Long-Evans rats develop hypertriglyceridemia in the absence of increased food intake and body fat accumulation compared with vehicle-treated controls. We then modulated CNS NPY signaling by icv injection of selective NPY receptor agonists and found that Y1, Y2, Y4, and Y5 receptor agonists all induced hyperphagia in lean, ad libitum chow-fed Long-Evans rats, with the Y2 receptor agonist having the most pronounced effect. Next, we found that at equipotent doses for food intake NPY Y1 receptor agonist had the most robust effect on VLDL-TG secretion, a Y2 receptor agonist had a modest effect, and no effect was observed for Y4 and Y5 receptor agonists. These findings, using selective agonists, suggest the possibility that the effect of CNS NPY signaling on hepatic VLDL-TG secretion may be relatively dissociable from effects on feeding behavior via the Y1 receptor.

Bowels control brain: gut hormones and obesity

Peptide hormones are released from the gastrointestinal tract in response to nutrients and communicate information regarding the current state of energy balance to the brain. These hormones regulate appetite, energy expenditure and glucose homeostasis. They can act either via the circulation at target peripheral tissues, by activation of the vagus nerve or by acting on key brain regions implicated in energy homeostasis such as the hypothalamus and brainstem. This review gives an overview of the main gut hormones implicated in the regulation of food intake and how some of these are being targeted to develop anti obesity treatments.

The receptive function of hypothalamic and brainstem centres to hormonal and nutrient signals affecting energy balance

The hypothalamic arcuate nucleus (ARC) and the area postrema (AP) represent targets for hormonal and metabolic signals involved in energy homoeostasis, e.g. glucose, amylin, insulin, leptin, peptide YY (PYY), glucagon-like peptide 1 (GLP-1) and ghrelin. Orexigenic neuropeptide Y expressing ARC neurons are activated by food deprivation and inhibited by feeding in a nutrient-dependent manner. PYY and leptin also reverse or prevent fasting-induced activation of the ARC. Interestingly, hypothalamic responses to fasting are blunted in different models of obesity (e.g. diet-induced obesity (DIO) or late-onset obesity). The AP also responds to feeding-related signals. The pancreatic hormone amylin acts via the AP to control energy intake. Amylin-sensitive AP neurons are also glucose-responsive. Furthermore, diet-derived protein attenuates amylin responsiveness suggesting a modulation of AP sensitivity by macronutrient supply. This review gives an overview of the receptive function of the ARC and the AP to hormonal and nutritional stimuli involved in the control of energy balance and the possible implications in the context of obesity. Collectively, there is consistency between the neurophysiological actions of these stimuli and their effects on energy homoeostasis under experimental conditions. However, surprisingly little progress has been made in the development of effective pharmacological approaches against obesity. A promising way to improve effectiveness involves combination treatments (e.g. amylin/leptin agonists). Hormonal alterations (e.g. GLP-1 and PYY) are also considered to mediate body weight loss observed in obese patients receiving bariatric surgery. The effects of hormonal and nutritional signals and their interactions might hold the potential to develop poly-mechanistic therapeutic strategies against obesity.

Intracerebroventricular administration of neuronostatin delays gastric emptying and gastrointestinal transit in mice

Neuronostatin is a 13-amino acid amidated peptide widely distributed in various organs including gastrointestinal tract. However, the effect of neuronostatin on gastrointestinal motility has not been well characterized. In the present work, effects of central administration of neuronostatin on gastric emptying and gastrointestinal transit were investigated. The results indicated that intracerebroventricular (i.c.v.) administration of neuronostatin (1, 5, 10 or 20nmol/mouse) delayed gastric emptying and gastrointestinal transit in a dose-related manner in mice. The effects were significantly reversed by melanocortin 3/4 receptor antagonist SHU9119 or classical opioid receptor antagonist naloxone, suggesting that the central melanocortin system and opioid system may be involved in the gastrointestinal effects elicited by i.c.v. administration of neuronostatin. In addition, we found that C-terminal amidation modification of neuronostatin is essential to exert its gastrointestinal effects. These results indicated that neuronostatin may play an important role in regulating gastrointestinal function.

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.

The role of the gut/brain axis in modulating food intake

Peptide hormones released from the gastrointestinal tract communicate information about the current state of energy balance to the brain. These hormones regulate appetite and energy expenditure via the vagus nerve or by acting on key brain regions implicated in energy homeostasis such as the hypothalamus and brainstem. This review gives an overview of the main gut hormones implicated in the regulation of food intake. Research in this area has provided novel targets for the pharmacological treatment of obesity. This article is part of a Special Issue entitled 'Central Control Food Intake'

The role of NPY in hypothalamic mediated food intake

Neuropeptide Y (NPY) is a highly conserved neuropeptide with orexigenic actions in discrete hypothalamic nuclei that plays a role in regulating energy homeostasis. NPY signals via a family of high affinity receptors that mediate the widespread actions of NPY in all hypothalamic nuclei. These actions are also subject to tight, intricate regulation by numerous peripheral and central energy balance signals. The NPY system is embedded within a densely-redundant network designed to ensure stable energy homeostasis. This redundancy may underlie compensation for the loss of NPY or its receptors in germline knockouts, explaining why conventional knockouts of NPY or its receptors rarely yield a marked phenotypic change. We discuss insights into the hypothalamic role of NPY from studies of its physiological actions, responses to genetic manipulations and interactions with other energy balance signals. We conclude that numerous approaches must be employed to effectively study different aspects of NPY action.

The Endocannabinoid System as Pharmacological Target Derived from Its CNS Role in Energy Homeostasis and Reward. Applications in Eating Disorders and Addiction

The endocannabinoid system (ECS) has been implicated in many physiological functions, including the regulation of appetite, food intake and energy balance, a crucial involvement in brain reward systems and a role in psychophysiological homeostasis (anxiety and stress responses). We first introduce this important regulatory system and chronicle what is known concerning the signal transduction pathways activated upon the binding of endogenous cannabinoid ligands to the G_i/0-coupled CB1 cannabinoid receptor, as well as its interactions with other hormones and neuromodulators which can modify endocannabinoid signaling in the brain. Anorexia nervosa (AN) and bulimia nervosa (BN) are severe and disabling psychiatric disorders, characterized by profound eating and weight alterations and body image disturbances. Since endocannabinoids modulate eating behavior, it is plausible that endocannabinoid genes may contribute to the biological vulnerability to these diseases. We present and discuss data suggesting an impaired endocannabinoid signaling in these eating disorders, including association of endocannabinoid components gene polymorphisms and altered CB1-receptor expression in AN and BN. Then we discuss recent findings that may provide new avenues for the identification of therapeutic strategies based on the endocannabinod system. In relation with its implications as a reward-related system, the endocannabinoid system is not only a target for cannabis but it also shows interactions with other drugs of abuse. On the other hand, there may be also a possibility to point to the ECS as a potential target for treatment of drug-abuse and addiction. Within this framework we will focus on enzymatic machinery involved in endocannabinoid inactivation (notably fatty acid amide hydrolase or FAAH) as a particularly interesting potential target. Since a deregulated endocannabinoid system may be also related to depression, anxiety and pain symptomatology accompanying drug-withdrawal states, this is an area of relevance to also explore adjuvant treatments for improving these adverse emotional reactions.

Moderate long-term modulation of neuropeptide Y in hypothalamic arcuate nucleus induces energy balance alterations in adult rats

Neuropeptide Y (NPY) produced by arcuate nucleus (ARC) neurons has a strong orexigenic effect on target neurons. Hypothalamic NPY levels undergo wide-ranging oscillations during the circadian cycle and in response to fasting and peripheral hormones (from 0.25 to 10-fold change). The aim of the present study was to evaluate the impact of a moderate long-term modulation of NPY within the ARC neurons on food consumption, body weight gain and hypothalamic neuropeptides. We achieved a physiological overexpression (3.6-fold increase) and down-regulation (0.5-fold decrease) of NPY in the rat ARC by injection of AAV vectors expressing NPY and synthetic microRNA that target the NPY, respectively. Our work shows that a moderate overexpression of NPY was sufficient to induce diurnal over-feeding, sustained body weight gain and severe obesity in adult rats. Additionally, the circulating levels of leptin were elevated but the immunoreactivity (ir) of ARC neuropeptides was not in accordance (POMC-ir was unchanged and AGRP-ir increased), suggesting a disruption in the ability of ARC neurons to response to peripheral metabolic alterations. Furthermore, a dysfunction in adipocytes phenotype was observed in these obese rats. In addition, moderate down-regulation of NPY did not affect basal feeding or normal body weight gain but the response to food deprivation was compromised since fasting-induced hyperphagia was inhibited and fasting-induced decrease in locomotor activity was absent.These results highlight the importance of the physiological ARC NPY levels oscillations on feeding regulation, fasting response and body weight preservation, and are important for the design of therapeutic interventions for obesity that include the NPY.

Countervailing modulation of Ih by neuropeptide Y and corticotrophin-releasing factor in basolateral amygdala as a possible mechanism for their effects on stress-related behaviors

Stress and anxiety-related behaviors controlled by the basolateral amygdala (BLA) are regulated in vivo by neuropeptide Y (NPY) and corticotrophin-releasing factor (CRF): NPY produces anxiolytic effects, whereas CRF produces anxiogenic effects. These opposing actions are likely mediated via regulation of excitatory output from the BLA to afferent targets. In these studies, we examined mechanisms underlying the effects of NPY and CRF in the BLA using whole-cell patch-clamp electrophysiology in rat brain slices. NPY, even with tetrodotoxin present, caused a dose-dependent membrane hyperpolarization in BLA pyramidal neurons. The hyperpolarization resulted in the inhibition of pyramidal cells, despite arising from a reduction in a voltage-dependent membrane conductance. The Y(1) receptor agonist, F(7)P(34) NPY, produced a similar membrane hyperpolarization, whereas the Y(1) antagonist, BIBO3304 [(R)-N-[[4-(aminocarbonylaminomethyl)-phenyl]methyl]-N(2)-(diphenylacetyl)-argininamide trifluoroacetate], blocked the effect of NPY. The NPY-inhibited current was identified as I(h), which is active at and hyperpolarized to rest. Responses to NPY were occluded by either Cs(+) or ZD7288 (4-ethylphenylamino-1,2-dimethyl-6-methylaminopyrimidinium chloride), but unaffected by the G(IRK)-preferring blockers Ba(2+) and SCH23390 [(R)-(+)-7-chloro-8-hydroxy-3-methyl-l-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride]. Application of CRF, with or without TTX present, depolarized NPY-sensitive BLA pyramidal neurons, resulting from an increase in I(h). Electrophysiological and immunocytochemical data were consistent with a major role for the HCN1 subunit. Our results indicate that NPY, via Y(1) receptors, directly inhibits BLA pyramidal neurons by suppressing a postsynaptic I(h), whereas CRF enhances resting I(h), causing an increased excitability of BLA pyramidal neurons. The opposing actions of these two peptides on the excitability of BLA output cells are consistent with the observed behavioral actions of NPY and CRF in the BLA.

Dietary resistant starch increases hypothalamic POMC expression in rats

Resistant starch (RS) is fermentable dietary fiber. Inclusion of RS in the diet causes decreased body fat accumulation and altered gut hormone profile. This study investigates the effect of feeding RS on the neuropeptide messenger RNA (mRNA) expressions in the arcuate nucleus (ARC) of the hypothalamus and whether vagal afferent nerves are involved. The rats were injected intraperitoneally with capsaicin to destroy unmyelinated small vagal afferent nerve fibers. The cholecystokinin (CCK) food suppression test was performed to validate the effectiveness of the capsaicin treatment. Then, capsaicin-treated rats and vehicle-treated rats were subdivided into a control diet or a RS diet group, and fed the corresponding diet for 65 days. At the end of study, body fat, food intake, plasma peptide YY (PYY) and glucagon-like peptide 1 (GLP-1), and hypothalamic pro-opiomelanocortin (POMC), neuropeptide Y (NPY), agouti-related peptide (AgRP) gene expressions were measured. RS-fed rats had decreased body fat, increased POMC expression in the hypothalamic ARC, and elevated plasma PYY and GLP-1 in both the capsaicin and vehicle-treated rats. Hypothalamic NPY and AgRP gene expressions were not changed by RS or capsaicin. Therefore, destruction of the capsaicin-sensitive afferent nerves did not alter the response to RS in rats. These findings suggest that dietary RS might reduce body fat through increasing the hypothalamic POMC expression and vagal afferent nerves are not involved in this process. This is the first study to show that dietary RS can alter hypothalamic POMC expression.

Brain regulation of appetite and satiety

Interest in the control of feeding has increased as a result of the obesity epidemic and rising incidence of metabolic diseases. The brain detects alterations in energy stores and triggers metabolic and behavioral responses designed to maintain energy balance. Energy homeostasis is controlled mainly by neuronal circuits in the hypothalamus and brainstem, whereas reward and motivation aspects of eating behavior are controlled by neurons in limbic regions and the cerebral cortex. This article provides an integrated perspective on how metabolic signals emanating from the gastrointestinal tract, adipose tissue, and other peripheral organs target the brain to regulate feeding, energy expenditure, and hormones. The pathogenesis and treatment of obesity and abnormalities of glucose and lipid metabolism are discussed.

Y eat?

Feeding behavior is tightly regulated by peptidergic transmission within the hypothalamus. Neuropeptide Y (NPY) is one of the most potent known stimulators of food intake and has robust effects on the hypothalamic feeding neuronal networks. A vast body of literature has documented the substantial effects of NPY on feeding behavior. However, the cellular mechanisms underlying the actions of NPY have only recently begun to be explored. The NPYergic signal, including its expression in hypothalamic neurons, its release into the synaptic space, and its direct or indirect receptor-mediated actions, is highly responsive to decreases in the metabolic state. The orexigenic NPY signal can suppress the anorexigenic drive to restore energy balance homeostasis when energy levels are low, such as after food deprivation. The NPY signal interacts with glucose- and fat-sensitive signals arriving in the hypothalamus and effects changes in anorexigenic pathways, such as those mediated by the melanocortins. Recent applications of electrophysiological methods to examine the neuronal activity and pathways engaged by NPY-mediated signaling have advanced our understanding of this orexigenic system. Furthermore, crucial roles for NPY pathways in the development of hypothalamic feeding circuitry have been identified by these means. Orexigenic NPY signaling is critical during development and its absence is lethal in adults, thus reflecting the essential role of NPY for the regulation of energy homeostasis.

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.

Gastrointestinal satiety signals

The increasing prevalence of obesity worldwide has imparted renewed impetus to the study of the mechanisms of appetite regulation. Digestion and nutrient absorption take place in the gastrointestinal (GI) tract, whereas food intake is controlled by neuronal circuits in the central nervous system. The need for gut-brain cross talk is therefore clear. It is now recognized that hormones released into the circulation from the GI tract in response to nutritional stimuli form a key component of this gut-brain axis. Peptides such as glucagon-like peptide-1, oxyntomodulin, pancreatic polypeptide, and peptide YY3-36 reduce food intake in both animal models and in humans. Physiologically, such peptides are thought to act as satiety signals and meal terminators. Here, we review the current state of the field of the effects of gut hormone action on appetite control.

The role of proopiomelanocortin (POMC) neurones in feeding behaviour

The precursor protein, proopiomelanocortin (POMC), produces many biologically active peptides via a series of enzymatic steps in a tissue-specific manner, yielding the melanocyte-stimulating hormones (MSHs), corticotrophin (ACTH) and β-endorphin. The MSHs and ACTH bind to the extracellular G-protein coupled melanocortin receptors (MCRs) of which there are five subtypes. The MC3R and MC4R show widespread expression in the central nervous system (CNS), whilst there is low level expression of MC1R and MC5R. In the CNS, cell bodies for POMC are mainly located in the arcuate nucleus of the hypothalamus and the nucleus tractus solitarius of the brainstem. Both of these areas have well defined functions relating to appetite and food intake. Mouse knockouts (ko) for pomc, mc4r and mc3r all show an obese phenotype, as do humans expressing mutations of POMC and MC4R. Recently, human subjects with specific mutations in β-MSH have been found to be obese too, as have mice with engineered β-endorphin deficiency. The CNS POMC system has other functions, including regulation of sexual behaviour, lactation, the reproductive cycle and possibly central cardiovascular control. However, this review will focus on feeding behaviour and link it in with the neuroanatomy of the POMC neurones in the hypothalamus and brainstem.

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.

Y2, Y4 receptors and obesity

The neuropeptide Y system - comprising neuropeptide Y, peptide YY, pancreatic polypeptide and the Y receptors through which they act (Y1, Y2, Y4, Y5 and y6) - has been at the center of attention with regards to regulation of feeding behavior and its possible involvement in obesity. In the past, research has focused mainly on the orexigenic and obesogenic action of this system, with Y1 and Y5 receptors being prime candidates as mediators of neuropeptide Y-induced hyperphagia and obesity. However, in recent years, the role of other members of the neuropeptide Y family, peptide YY, pancreatic polypeptide and the Y2 and Y4 receptors through which they predominantly act, have commanded increasing attention on account of their effects to mediate satiety and promote weight loss via actions in key brain structures, such as the arcuate nucleus of the hypothalamus and the brain stem. This review focuses on the role of peptide YY- and pancreatic polypeptide-like compounds as possible antiobesity drugs, taking into account their effects, not only on energy balance, but also in the regulation of bone formation, and highlights potential benefits of using Y2 and/or Y4 antagonists (as opposed to agonists such as peptide YY or pancreatic polypeptide) in the treatment of obesity.