Food intake and the regulation of body weight

Measuring meals: structure of prandial food and water intake of rats

Attempts to understand ingestion have sought to understand the control of meals. The present study evaluated a meal definition that included prandial drinking (drinking-explicit meals). The spontaneous nocturnal intake of male Wistar rats was studied. The meal breakpoint was defined as the interval between feeding or drinking events providing the most stable estimate of meal structure. Alternative breakpoints derived from prevailing methodology, log-survivorship, or frequency histogram analysis of interfeeding intervals without respect to drinking were compared (drinking-naive meals). Threshold interfeeding intervals that accounted for drinking indirectly were evaluated as surrogate breakpoints (drinking-implicit meals). Definitions were compared by determining which criterion better conformed to predictions of satiety. Microstructural differences resulting from the definitions were also studied. Under the drinking-explicit definition, rats averaged nine or ten 13-min meals/night, during which they consumed food and water equally in duration (5–6 min) and quantity (2.3 g). Individual differences were observed in microstructure measures. Meals defined by drinking-informed, but not drinking-naive, methods were followed by the behavioral satiety sequence and by an initially low likelihood of resuming feeding that monotonically increased with time. The drinking-explicit definition uniquely revealed preprandial and postprandial correlations of similar magnitude. Under drinking-informed definitions, food restriction increased meal size, whereas drinking-naive definitions increased meal frequency. Drinking-implicit definitions provided workable approximations of meal frequency and size but inferior estimates of feeding duration, eating rate, and the preprandial correlation. Thus log-survivorship analysis is not appropriate for identifying meal breakpoints, and the consideration of drinking in meal definitions can provide a better estimate of meal structure.

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

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

Activation of hypothalamic ATP-sensitive K+ channels by the aminoguanidine carboxylate BVT.12777

Derivatives of 3-guanidinopropionic acid, such as leptin, reduce body weight in obese, diabetic mice. We have assessed whether one of these analogues, BVT.12777 activates intracellular signalling pathways in the arcuate nucleus in a manner analogous to leptin and insulin. In addition, because these hormones have been shown to activate K(ATP) channels in a subset of arcuate neurones, we examined whether this channel is also a functional endpoint for BVT.12777 in the arcuate nucleus. BVT.12777 transiently increased phosphorylation of MAPK, STAT3, PKB and GSK3, in a manner identical to that observed for leptin and insulin. BVT.12777 also hyperpolarized glucose-responsive neurones by increasing the activity of K(ATP) channels. The increase in K(ATP) activity driven by BVT.12777 was PI3-kinase independent, unlike leptin and insulin activation of this channel, and could also be elicited in isolated patches. However, K(ATP) activity induced by BVT.12777 was dependent on actin filament dynamics, both in intact neurones and isolated patches. Thus, BVT.12777 modulates arcuate neurone K(ATP) activity by re-organization of the cytoskeleton, a mechanism that has also been ascribed to leptin and insulin. Consequently, BVT.12777 appears to act as a leptin and insulin mimetic with respect to at least some elements of arcuate neurone intracellular signalling and the activation of K(ATP) channels. Resistance to leptin and insulin, associated with obesity has, at least in part, been postulated to be due to aberrant intracellular signalling in arcuate neurones. The data presented here indicate that it may be possible to develop drugs, which by-pass up-stream signalling components associated with adiposity hormone resistance, such as PI3-kinase, but can still induce functional outputs from arcuate neurones by targeting downstream components of the leptin and insulin signalling cascades.

Fat in the intestine as a regulator of appetite--role of CCK

The present review summarizes the appetite-suppressing effects of intestinal fat in the regulation of food intake in humans, with a special focus on the role of cholecystokinin (CCK). Current evidence supports a role for intestinal fat (especially long-chain free fatty acids) acting via the peptide CCK as a physiological satiety pathway. The regulation of satiety is, however, complex and it is not surprising that multiple control systems exist. It is interesting to note that nutrients, such as hydrolysis products of fat in the small intestine, stimulate the release of satiety peptides, such as CCK or PYY, that serve as satiety signals. CCK, released from the gastrointestinal tract by the local action of digested food, exerts various functions: stimulation of gallbladder contraction and exocrine pancreatic secretion, inhibition of gastric emptying, and inhibition of appetite. CCK functions therefore (1) as a positive feedback signal to stimulate digestive processes and (2) as negative feedback signal to limit the amount of food consumed during an individual meal.

Emerging antiobesity drugs

The healthcare burden that the obesity epidemic now poses in highly significant, in part due to increased risk of secondary chronic diseases such as hypertension. A lack of physical activity and high fat diets are major factors contributing to this condition. However, increasingly apparent is the genetic predisposition of individuals and ethnic groups to obesity. Present treatment strategies are currently inadequate and unlikely to have a major effect on the future prevalence of obesity. To slow the obesity epidemic, the source needs to be tackled now through fundamental research into the mechanisms by which obesity is manifest, and education on the risks and how to prevent it. This article will describe current and emerging treatments for obesity and review the recent advances in research that may provide the antiobesity treatments of the future. Research into obesity has escalated at considerable pace, catalysed by the discovery of the obese gene product leptin. Leptin is secreted by adipose tissue and acts via specific receptors in the brain to engage central neural pathways involved in regulating energy homeostasis. Since this discovery, numerous significant advances have been made in our understanding of how the brain integrates and responds to central and peripheral signals involved in maintaining energy homeostasis, and how disruption of these signalling mechanisms can manifest as obesity. As a consequence of these findings, numerous potential sites for therapeutic intervention into this condition have and are materializing. The aim of this review is to highlight current treatment strategies for obesity, recent advances in our understanding of the central neural control of energy balance, and what the authors consider to be the most promising targets for the development of novel antiobesity drugs in the future. Thus, the review focuses on leptin, neuropeptide Y, melanocortin and ghrelin signalling at the level of the CNS, and strategies targeting the sympathetic innervation of fat cells at the periphery.

Unlearned appetite controls: Watersnakes (Nerodia) take smaller meals when they have the choice

Studies of appetite in mammals emphasize that meal size is learned, but lactation and parental care constrain testing of naive individuals. Neonatal reptiles, in contrast, are self-sufficient foragers. The authors examined the effect of prey size on meal size in primivorous (at first feeding) northern watersnakes (Nerodia sipedon). When offered an excess of small prey (2%-20% of snake mass), neonates ate significantly smaller meals (M = 23.5% of snake mass) than when offered a single huge item (range = 32%-55%). The authors conclude that (a) the taking of smaller meals is not a learned effect, (b) there may be a satiety threshold for meal size rather than a target, (c) oropharyngeal stimuli may provide satiety cues, and (d) huge meals may have fitness costs.

Gastrointestinal hormones and food intake

Despite dramatic fluctuations in calorie intake, animals maintain a very stable body weight. The reason is that energy intake and expenditure are precisely matched. Long-term regulation of energy balance is dependent on the coordination and interpretation of signals such as those given by insulin and leptin indicating sufficient long-term energy stores as well as short-term, meal-related signals such as those given by cholecystokinin (CCK). Within the last 30 years, our knowledge of short-term signals has increased dramatically. Throughout the cephalo-caudal axis of the gastrointestinal system, discrete enteroendocrine cells respond to both mechanical and chemical stimulation. Meal-associated hormone release is dependent on the concentration and composition of the nutrients ingested. Released signals are transmitted neurally through vagal afferents or humorally as circulating ligands for specific receptor populations in the periphery and central nervous system. These signals are interpreted by the CNS and manifested as a behavioral modification of feeding. This review will present past and recent literature in support of gut hormones and their roles as mediators of satiety. Evidence from pharmacologic and physiologic studies involving both humans and rodents will be presented, along with a short section outlining the knowledge gained through the use of murine knockout models. Last, the contribution of satiety hormones as likely mediators of the effectiveness seen following obesity surgery will be reviewed. Although traditionally thought of as short-term, meal-related signals, enhanced, chronic hormone secretion and signaling resulting from gut reconstruction as seen with gastric bypass surgery most likely contributes to the superior efficacy of surgery as a treatment for obesity.

Leptin and post-prandial satiety: acute central leptin more potently reduces meal frequency than meal size in the rat

RATIONALE

Many attempts to understand ingestion have sought to clarify the control of meals. Little is known about the effects of the anorexogenic hormone leptin on meal patterning.

OBJECTIVE

The present study sought to perform a dose-response analysis of the effects of acute central leptin administration on meal patterning using a validated, objective meal definition and to compare these results to those obtained with a previously used, subjective meal definition.

METHODS

To validate the objective meal definition pharmacologically, the microstructural effects of the well-studied compound fenfluramine (SC 0, 1, 2, 4 mg/kg) on spontaneous nocturnal intake were determined in mature, non-deprived male Wistar rats (n=8) using a full Latin square design. The effects of intracerebroventricular leptin administration (0, 0.3, 1, 3, 6.25 microg; n=10) were also examined, and perceived meal patterns obtained from the objective and subjective definitions were compared.

RESULTS

Fenfluramine reduced meal size and eating rate at doses that did not reduce meal frequency or duration. In contrast, comparably anorectic doses of leptin had potent post-meal satiety-like effects, reducing meal frequency and prolonging the intermeal interval without reducing average meal size, a finding opposite to that suggested by the previously used subjective meal definition. Unlike comparably and more anorectic doses of fenfluramine, leptin non-specifically reduced both prandial and non-prandial drinking.

CONCLUSIONS

Acute increases in central leptin levels may potently augment post-prandial satiety and influence body-fluid homeostasis. The results reveal unappreciated central modes of action for the ob protein which qualitatively differ from the intra-meal satiating-like effects of fenfluramine.

Control of food intake through regulation of cAMP

The 3', 5'-cyclic adenosine monophosphate (cAMP) is a classic second messenger that is intimately involved in the regulation of food intake at the hypothalamus. cAMP can mediate the orexigenic and anorectic effects of various peripheral hormones or neuropeptides in a region-specific and neuron-specific manner. The importance of cAMP is particularly highlighted in a series of findings about cAMP transducing the anorectic signals of leptin and alpha-msh. This chapter provides an overview of several studies on how regulation of food intake takes place with cAMP as the second messenger in the hypothalamus.

Consumption of a high-fat diet alters the homeostatic regulation of energy balance

Humans in many countries are currently experiencing what has been called an epidemic of obesity. That is, the average body weight (and amount of fat stored in the body) is increasing over years, carrying with it a multitude of associated medical, psychological, and economic problems. While there is no shortage of possible causes of this epidemic, increased availability and consumption of high-fat (HF), calorically dense and generally quite palatable food is often touted as a likely culprit. In order to better assess the impact of consuming a diet with those qualities, we have developed a well-controlled animal model in which the effects of chronic consumption of a high-fat diet can be dissociated from those of becoming obese per se. Long-Evans rats are fed one of two semipurified pelleted diets, a HF diet that contains 20% fat by weight and a low-fat (LF) diet that contains 4% fat by weight. Pair-fed animals consume the HF diet but are limited to the daily caloric intake of LF rats. Another group receives pelleted chow. Relative to animals consuming diets low in fat, HF animals weigh more, have more carcass fat, are hyperinsulinemic and hyperleptinemic, and are insulin resistant. HF-fed animals, independent of whether they become obese or not, also have central insulin and MTII insensitivity. Finally, HF rats have a down-regulated hypothalamic apo A-IV system that could contribute to their hyperphagia.

Clinical endocrinology and metabolism. Regulation of energy homeostasis by peripheral signals

The increased incidence of obesity makes it imperative to understand the regulation of food intake and body weight. We review the signals that interact with the brain to control energy homeostasis, i.e. energy intake and expenditure. Three broad categories can be distinguished. Signals generated in the gastrointestinal tract during meals ('satiety' signals, e.g. cholecystokinin) elicit satiation and contribute to stopping the meal. The potency of these acutely acting signals must be increased if they are to be used therapeutically. Hormonal signals whose secretion is proportional to body fat (adiposity signals, leptin and insulin) robustly reduce food intake and body weight by directly stimulating receptors locally in the brain. Therapeutic applications will have to find ways to circumvent the systemic actions of these hormones, targeting only the brain. Satiety and adiposity signals interact with neuronal circuits in the brain that utilize myriad neurotransmitters to cause net catabolic or anabolic responses. Considerable effort is being directed towards finding ways to intervene in specific circuits to help accomplish weight loss.

Short-term effects of gastric bypass surgery on circulating ghrelin levels

OBJECTIVE

To prospectively evaluate the short-term effects of Roux-en-Y gastric bypass (RYGBP) on ghrelin secretion and its relevance on food intake and body weight changes.

RESEARCH METHODS AND PROCEDURES

Ghrelin response to a standardized test meal was evaluated in eight obese patients (BMI, 43.5 to 59.1 kg/m2) before and 6 weeks after RYGBP. Ghrelin response was compared with that of an age-matched group of six normal weight individuals (BMI, 19.6 to 24.9 kg/m2).

RESULTS

Fasting serum ghrelin levels were lower in obese subjects compared with controls (p < 0.05). Meal ingestion significantly suppressed ghrelin concentration in controls (p < 0.05) and obese subjects (p < 0.05), albeit to a lesser degree in the latter group (p < 0.05). Despite a 10.3 +/- 1.5% weight loss, fasting serum ghrelin levels were paradoxically further decreased in obese subjects 6 weeks after RYGBP (p < 0.05). Moreover, at this time-point, food intake did not elicit a significant ghrelin suppression. The changes in ghrelin secretion after RYGBP correlated with changes in insulin sensitivity (p < 0.05) and caloric intake (p < 0.05).

DISCUSSION

This study showed that the adaptive response of ghrelin to body weight loss was already impaired 6 weeks after RYGBP. Our study provides circumstantial evidence for the potential role of ghrelin in the negative energy balance in RYGBP-operated patients.

Psychotropic drugs in the treatment of obesity: what promise?

Obesity is a chronic and highly prevalent medical condition associated with increased risk for the development of numerous and sometimes fatal diseases. Despite its severity, there are few anti-obesity agents available on the market. Although psychotropic agents are not approved for the treatment of obesity, they have been used by clinicians as a therapeutic tool in daily clinical practice. The purpose of this article is to review the rationale, as well as the evidence, for the potential use of these agents in obesity treatment. Evidence for the efficacy of psychotropic agents in obesity treatment comes from different sources. The first type of evidence is weight loss observed with treatment in clinical trials of patients with neuropsychiatric syndromes (e.g. mood disorders, epilepsy). A recent example of such findings is the weight reduction reported in clinical trials involving obese patients with binge eating disorder. While randomised, controlled trials specifically designed to investigate the weight loss properties of psychotropic agents in obese patients are the most appropriate source of evidence of anti-obesity action, such trials remain scarce. The most studied psychotropic agents in obesity trials are drugs used in the treatment of mood disorders, i.e. mainly antidepressants and antiepileptics. SSRIs (e.g. fluoxetine, sertraline and fluvoxamine) were amongst the first psychotropic agents investigated in the treatment of obesity. Additional data have also been published for other antidepressants (e.g. venlafaxine, citalopram and bupropion) and antiepileptics (e.g. topiramate and zonisamide). Based on the available data for the efficacy of psychotropic agents in obesity and other related conditions, SSRIs may be considered for the management of certain subgroups of obese individuals with comorbid conditions such as depression, binge eating disorder and type 2 diabetes mellitus. In addition, some newer agents, such as bupropion, topiramate and zonisamide, appear to be promising candidates for selective use in the treatment of obesity. However, further studies are needed to define their possible role as new pharmacological options in the treatment of obesity.

Central administration of ghrelin and agouti-related protein (83-132) increases food intake and decreases spontaneous locomotor activity in rats

Ghrelin was recently identified as an endogenous ligand of the GH secretagogue receptor. The novel peptide hormone is produced by gastric A-like cells, and circulating levels rise before feeding, suggestive of ghrelin as an endogenous hunger factor. ghrelin stimulates food intake and promotes adiposity after peripheral or central administration, likely by activating hypothalamic neurons expressing the orexigenic neuropeptides neuropeptide Y (NPY) and agouti-related protein (AGRP). To examine whether ghrelin-induced feeding resembles NPY and AGRP [AGRP fragment (83-132)] induced orexia, we compared the short- and long-term orexigenic capacity of the three peptides. A single intracerebroventricular injection of ghrelin (0.2, 1.0, and 5.0 microg) increased food intake in a dose-dependent manner. A prolonged and uncompensated increase in feeding was seen after the highest dose of ghrelin. The prolonged effects on feeding (+72 h) closely resembled those of AGRP (83-132) but not NPY. Surprisingly, ghrelin injections reduced overall locomotor activity by 20% during the first 24-h observation period. AGRP (83-132) had similar effects on locomotor behavior, whereas NPY had no effect. In summary, ghrelin causes long-term increases of food intake and, like AGRP, plays a previously unknown role as a suppressor of spontaneous physical activity. Expanding the current model of food intake control to include mechanisms regulating physical activity may promote our understanding of two major etiological factors causing obesity.

Glucagon-like peptide 1 (GLP-1) and eating

New information regarding gastrointestinal mechanisms that participate in the control of food intake has extended our understanding of appetite control. Although each new signaling pathway discovered in the gut is a potential target for drug development in the treatment of obesity, the growing number of such signaling molecules indicates that a highly complex process controls food intake. The present summary focuses on the role of glucagon-like peptide 1 (GLP-1) in this regulatory process. The different biological effects of GLP-1 (glucose-lowering properties, inhibition of appetite and food intake) provide a powerful impetus for development of GLP-1-based new drugs.

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.

Effects of diabetes and insulin on the expression of galanin-like peptide in the hypothalamus of the rat

Galanin-like peptide (GALP) is produced in a small population of neurons in the arcuate nucleus of the hypothalamus, and leptin stimulates the hypothalamic expression of GALP mRNA. Because insulin and leptin share common signaling pathways in the brain, we reasoned that GALP neurons might also be responsive to changes in circulating concentrations of insulin. To test this hypothesis, we first studied the effect of insulin deficiency on the expression of GALP by comparing levels of GALP mRNA between normal and diabetic animals. Streptozotocin-induced diabetes was associated with a significant reduction in the expression of GALP mRNA, which was reversed by treatment with either insulin or leptin. Second, we examined the effect of insulin administered directly into the brain on the expression of GALP mRNA in fasted rats. Hypothalamic levels of GALP mRNA were lower in animals after a 48-h fast, and central treatment with insulin reversed this effect. These results suggest that GALP neurons are direct targets for regulation by insulin and implicate these cells for a role in the metabolic and behavioral sequelae of type 1 diabetes.

Appetite and Cancer-Associated Anorexia: A Review

Appetite is governed by peripheral hormones and central neurotransmitters that act on the arcuate nucleus of the hypothalamus and nucleus tactus solitarius of the brainstem. Cancer anorexia appears to be the result of an imbalance between neuropeptide-Y and pro-opiomelanocortin signals favoring pro-opiomelanocortin. Many of the appetite stimulants redress this imbalance. Most of our understanding of appetite neurophysiology and tumor-associated anorexia is derived from animals and has not been verified in humans. There have been few clinical trials and very little translational research on anorexia despite its prevalence in cancer.

Influence of social norms and palatability on amount consumed and food choice

In two parallel studies, we examined the effect of social influence and palatability on amount consumed and on food choice. In Experiment 1, which looked at amount consumed, participants were provided with either palatable or unpalatable food; they were also given information about how much previous participants had eaten (large or small amounts) or were given no information. In the case of palatable food, participants ate more when led to believe that prior participants had eaten a great deal than when led to believe that prior participants had eaten small amounts or when provided with no information. This social-influence effect was not present when participants received unpalatable food. In Experiment 2, which looked at food choice, some participants learned that prior participants had chosen the palatable food, others learned that prior participants had chosen the unpalatable food, while still others received no information about prior participants' choices. The social-influence manipulation had no effect on participants' food choices; nearly all of them chose the palatable food. The results were discussed in the context of Churchfield's (1995) distinction between judgments about matters of fact and judgments about preferences. The results were also used to illustrate the importance of palatability as a determinant of eating behavior.

Brain insulin and feeding: a bi-directional communication

Insulin and specific insulin receptors are found widely distributed in the central nervous system (CNS) networks related in particular to energy homeostasis. This review highlights the complex regulatory loop between dietary nutrients, brain insulin and feeding. It is well documented that brain insulin has a negative, anorexigenic effect on food intake. At present, a specific role for brain insulin on cognitive functions related to feeding is emerging. The balance between orexigenic and anorexigenic pathways in the hypothalamus is crucial for the maintenance of energy homeostasis in animals and humans. The ingestion of nutrients triggers neurochemical events that signal nutrient and energy availability in the CNS, down regulate stimulators, activate anorexigenic factors, including brain insulin, and result in reduced eating. The effects of insulin in the CNS are under a multilevel control of food-intake peripherally and in the CNS, via the metabolic, endocrine and neural modifications induced by nutrients. Single meals as well as glucose and serotonin are able to regulate insulin release directly in the hypothalamus and may be of importance for its biological effects. Central mechanisms operating in glucose-induced insulin release show some analogy with the mechanisms operating in the pancreas. Leptin and melanocortins, peptides that down regulate food intake and are largely affected by nutrients, are highly interactive with insulin in the CNS probably via the neurotransmitter serotonin. In the hypothalamus, insulin and leptin share a common signaling pathway involved in food intake, namely the insulin receptor substrate, phosphatidylinositol 3-kinase pathway. Over or under-feeding, unbalanced single meals or diets, in particular diets enriched in fat, modify the amount of insulin actively transported into the brain, the release of brain insulin, the expression of insulin messenger RNA and potentially disrupt insulin signaling in the CNS. This impairment may result in disorders related to feeding behavior and energy homeostasis leading to profound dysregulations, obesity or diabetes.

Evidence that paraventricular nucleus oxytocin neurons link hypothalamic leptin action to caudal brain stem nuclei controlling meal size

Hindbrain projections of oxytocin neurons in the parvocellular paraventricular nucleus (pPVN) are hypothesized to transmit leptin signaling from the hypothalamus to the nucleus of the solitary tract (NTS), where satiety signals from the gastrointestinal tract are received. Using immunocytochemistry, we found that an anorectic dose of leptin administered into the third ventricle (3V) increased twofold the number of pPVN oxytocin neurons that expressed Fos. Injections of fluorescent cholera toxin B into the NTS labeled a subset of pPVN oxytocin neurons that expressed Fos in response to 3V leptin. Moreover, 3V administration of an oxytocin receptor antagonist, [d-(CH2)5,Tyr(Me)2,Orn8]-vasotocin (OVT), attenuated the effect of leptin on food intake over a 0.5- to 4-h period (P < 0.05). Furthermore, to determine whether oxytocin contributes to leptin's potentiation of Fos activation within NTS neurons in response to CCK, we counted the number of Fos-positive neurons in the medial NTS (mNTS) after 3V administration of OVT before 3V leptin and intraperitoneal CCK-8 administration. OVT resulted in a significant 37% decrease (P < 0.05) in the potentiating effect of leptin on CCK activation of mNTS neuronal Fos expression. Furthermore, 4V OVT stimulated 2-h food intake by 43% (P < 0.01), whereas 3V OVT at the same dose was ineffective. These findings suggest that release of oxytocin from a descending pPVN-to-NTS pathway contributes to leptin's attenuation of food intake by a mechanism that involves the activation of pPVN oxytocin neurons by leptin, resulting in increased sensitivity of NTS neurons to satiety signals.

C75, a fatty acid synthase inhibitor, reduces food intake via hypothalamic AMP-activated protein kinase

Energy homeostasis and feeding are regulated by the central nervous system. C75, a fatty acid synthase (FAS) inhibitor, causes weight loss and anorexia, implying a novel central nervous system pathway(s) for sensing energy balance. AMP-activated protein kinase (AMPK), a sensor of peripheral energy balance, is phosphorylated and activated when energy sources are low. Here, we identify a role for hypothalamic AMPK in the regulation of feeding behavior and in mediating the anorexic effects of C75. 5-Aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR), an activator of AMPK, increased food intake, whereas compound C, an inhibitor of AMPK, decreased food intake. C75 rapidly reduced the level of the phosphorylated AMPK alpha subunit (pAMPKalpha) in the hypothalamus, even in fasted mice that had elevated hypothalamic pAMPKalpha levels. Furthermore, AICAR reversed both the C75-induced anorexia and the decrease in hypothalamic pAMPKalpha levels. C75 elevated hypothalamic neuronal ATP levels, which may contribute to the mechanism by which C75 decreased AMPK activity. C75 reduced the levels of pAMPKalpha and phosphorylated cAMP response element-binding protein (pCREB) in the arcuate nucleus neurons of the hypothalamus, suggesting a mechanism for the reduction in NPY expression seen with C75 treatment. These data indicate that modulation of FAS activity in the hypothalamus can alter energy perception via AMPK, which functions as a physiological energy sensor in the hypothalamus.

Oxytocin innervation of caudal brainstem nuclei activated by cholecystokinin

The integration of 'long-term' adiposity signaling with the 'short-term' meal-related signal cholecystokinin (CCK) is proposed to involve descending hypothalamic projections to areas of the caudal brainstem (CBS) that regulate the amount of food consumed during a single meal. One such projection extends from cell bodies in the hypothalamic paraventricular nucleus (PVN) to the nucleus tractus solitarius (NTS), where cells that respond to peripheral CCK are concentrated. Candidate neuronal cell types that may comprise this PVN-NTS projection includes those expressing corticotropin-releasing hormone (CRH) or oxytocin. We therefore sought to determine whether oxytocin or CRH axons are preferentially located in close anatomical proximity to neurons of the NTS that are activated by peripheral administration of CCK, as determined by immunocytochemical staining for Fos protein. Rats received injections of either an anorexic dose of CCK (8 nmol/kg, i.p.) or vehicle and were perfused 2 h later with 4% paraformaldehyde. Immunocytochemistry was performed on cryostat sections (14 microm) of caudal brainstem, using a polyclonal antibody to Fos protein and either a monoclonal antibody to oxytocin or a polyclonal antibody to CRH. As expected, CCK administration significantly increased the numbers of Fos-positive neurons by 489% (p<0.01) and 400% (p<0.01), respectively, in the medial and gelatinosus subdivisions of the NTS. These same regions received dense oxytocin axon innervation, whereas CRH immunoreactivity was not as prevalent in these areas. In areas outside the NTS, such as the dorsal motor nucleus of the vagus (DMV), Fos activation was absent despite a dense oxytocin and CRH innervation. To investigate whether CCK-induced reductions of food intake require intact oxytocin signaling, we performed a separate study in which CCK injection was preceded by injection into the fourth ventricle of an oxytocin receptor antagonist [d(CH(2))(5), Tyr (Me)(2), Orn(8)]-vasotocin (OVT). This study showed CCK was 23% and 22% less effective at inhibiting food intake at 30 min (p<0.05) and 1 h (p<0.05) food intake, respectively, in the presence of OVT. Taken together, the data indicate that oxytocin axons within the descending pathway from the PVN to the NTS are anatomically positioned to interact with NTS neurons that respond to vagally mediated peripheral CCK signals such as those that occur following ingestion of a meal. These findings support the hypothesis that oxytocin exerts a tonic stimulatory effect on the response of key neurons within the NTS to CCK and further reduce meal size.

The Timing of Meals

In most individuals, food intake occurs as discrete bouts or meals, and little attention has been paid to the factors that normally determine when meals will occur when food is freely available. On the basis of experiments using rats, the authors suggest that when there are no constraints on obtaining food and few competing activities, 3 levels of interacting controls normally dictate when meals will start. The first is the genetically determined circadian activity pattern on which nocturnal animals tend to initiate most meals in the dark. The second is the regularly occurring changing of the light cycle: These changes provide temporal anchors. The third relates to the size of the preceding meal, such that larger meals cause a longer delay until the onset of the next meal. Superimposed on these 3 are factors related to learning, convenience, and opportunity.

Circuitries Involved in the Control of Energy Homeostasis and the Hypothalamic-Pituitary-Adrenal Axis Activity

The regulation of bodyweight is a complex process involving the interplay of neuronal circuitries controlling food intake and energy expenditure (thermogenesis) with endocrine secretions modulating the activity of the neurons making up those circuitries. The neurons controlling food intake and thermogenesis also modulate the hypothalamic-pituitary-adrenal axis, the role of which in the regulation of energy balance has been acknowledged for some time. These neurons secrete various neuromolecules or neuropeptides including endocannabinoids, neuropeptide Y, agouti-related protein, melanin-concentrating hormone, orexins (hypocretins), melanocortins, cocaine- and amphetamine-regulated transcript, thyrotropin-releasing hormone, corticotropin-releasing hormone, and urocortins. Among those peptides, neuropeptide Y, agouti-related peptide, melanin-concentrating hormone, orexins, and endocannabinoids have been classified as being anabolic molecules whereas melanocortins, cocaine- and amphetamine-regulated transcript, thyrotropin-releasing hormone, and corticotropin-releasing hormone are referred to as catabolic peptides. The expression and secretion of these neuromolecules are known to be affected by the anabolic (corticosteroids and ghrelin) and catabolic (leptin, insulin, and glucagon-like peptide 1) peripheral hormones. A link is made between the pathways regulating energy balance and those modulating the activity of the hypothalamic-pituitary-adrenal axis.

Gastrointestinal satiety signals I. An overview of gastrointestinal signals that influence food intake

An overview is presented of those signals generated by the gastrointestinal (GI) tract during meals that interact with the central nervous system to create a sensation of fullness and satiety. Although dozens of enzymes, hormones, and other factors are secreted by the GI tract in response to food in the lumen, only a handful are able to influence food intake directly. Most of these cause meals to terminate and hence are called satiety signals, with CCK being the most investigated. Only one GI signal, ghrelin, that increases meal size has been identified. The administration of exogenous CCK or other satiety signals causes smaller meals to be consumed, whereas blocking the action of endogenous CCK or other satiety signals causes larger meals to be consumed. Satiety signals are relayed to the hindbrain, either indirectly via nerves such as the vagus from the GI tract or else directly via the blood. Most factors that influence how much food is eaten during individual meals act by changing the sensitivity to satiety signals. This includes adiposity signals as well as habits and learning, the social situation, and stressors.

Cardiovascular function and circadian patterns in rats after area postrema lesions or prolonged food restriction

The role of the area postrema (AP) in baseline cardiovascular function was evaluated by radio telemetry in rats with lesions of AP (APX), along with the effect of APX on circadian patterns of cardiovascular function and ingestive behavior. Despite lesion-induced hypophagia and body weight loss, APX did not affect circadian patterns of mean arterial pressure (MAP), heart rate (HR), or ingestion but did cause a substantial decrease (-60 bpm) in resting HR. The bradycardia was not attributable to weight loss, as resting HR in weight-matched controls, though decreased from baseline, was significantly greater than that in rats with APX. In weight-matched controls, HR increased slightly when weight returned to baseline; however, the bradycardia persisted in rats with APX. These results suggest that AP contributes to resting HR, independent of weight loss or changes in circadian patterns of HR or MAP.

Gastric leptin: a putative role in the short-term regulation of food intake

The discovery of the production of leptin by the stomach, in addition to its production by adipose tissue, has initiated new investigation into the possible role of this protein in the digestive physiology, in particular in the short-term control of energy balance. Leptin has been identified in the lower half of the stomach glands both in the pepsinogen granules of chief cells and in the granules of a specific endocrine cell type, suggesting that leptin action is exerted by both exocrine and endocrine pathways. Gastric leptin is sensitive to the nutritional state, being rapidly mobilized in response to food intake following fasting, or after the administration of satiety factors; this suggests a role for this protein in the short-term regulation of feeding, acting in collaboration with satiety peptides such as cholecystokinin. Leptin, produced by gastric cells and by adipocytes, could act on both acute and chronic regulation of feeding behaviour respectively, giving information to the brain on the availability of external (food) and internal (fat depots) energy resources, thus participating in short- and long-term satiation.

The relationship between eating behaviors and obesity in African American and Caucasian women

The primary goal of this study was to determine the relevance of four self-reported eating behaviors (eating before bedtime, eating between meals, feeling hungry within 3 h of eating, and eating beyond satiation) as risk factors for overweight and obesity. The sample consisted of 35- to 49-year-old, premenopausal African American (n=580) and Caucasian (n=398) women, randomly selected from the membership of a large urban prepaid health plan. Eating beyond satiation was the only behavior associated with body mass index (BMI). The odds of becoming obese increased 6-fold for Caucasian women and 15-fold for African American women who ate beyond satiation everyday compared to those who rarely or never ate beyond satiation. Additionally, eating beyond satiation was also the only eating behavior associated with the age of obesity onset. Focusing on this eating behavior in weight loss programs may be important.

Duodenal leptin stimulates cholecystokinin secretion: evidence of a positive leptin-cholecystokinin feedback loop

Some of the actions of leptin depend on cholecystokinin (CCK). However, it is unknown whether leptin modulates the release of CCK. Here, we demonstrate in vitro that leptin induces the phosphorylation of extracellular signal-related kinase (ERK)-1/2 proteins and increases CCK release (EC(50) = 0.23 nmol/l) in CCK-secreting STC-1 cells. We showed that rat duodenal juice contains leptin that circulates free and bound to macromolecules, suggesting that leptin has a lumenal action on the intestine. In vivo in the rat, duodenal infusion of leptin increased plasma CCK at levels comparable to those induced by feeding. Moreover, meal-induced increases in plasma CCK were markedly reduced in obese fa/fa rats, whereas the mobilization of the gastric leptin pool was similar in lean and obese Zucker rats. The release of CCK by leptin presumably generates a positive feedback loop. Indeed, the blockade of CCK receptors reversed the meal reduction of the stomach leptin pool and the meal-increased plasma insulin, consistent with the previous concept of an entero-insular axis. Collectively, these data support a novel mode of action of leptin where leptin and CCK may potentiate their own effects by cross-stimulating their secretion. The impairment of this leptin-CCK loop may have pathological implications related to obesity and diabetes.

Parameters for quantification of hunger in broiler breeders

The commercial restricted feeding programme of broiler breeders has a major negative effect on welfare, as the birds are continuously hungry. Objective parameters of hunger are needed to evaluate new management or feeding systems that may alleviate hunger and thus improve broiler breeder welfare. The aim of this experiment was to develop such parameters. Female broiler breeders (Hybro G), n=10 per treatment, were subjected to different levels of feed restriction and we assumed that we thus induced different levels of hunger. Treatment groups were fed ad libitum and at 90%, 70%, 50%, 35% and 25% of ad libitum. In addition, female broiler breeders that were less intensively selected on growth and food conversion (JA57, n=10 per treatment) were fed ad libitum and at 70% of ad libitum. At 6-7 weeks of age, home pen behaviour, behaviour in the open field and baseline plasma corticosterone, glucose and nonesterified fatty acid (NEFA) concentrations were determined. Thereafter, birds were subjected to the feed intake motivation (FIM) test that measures compensatory feed intake. Linear relationships between the level of restriction and the compensatory feed intake on the first days of the FIM test and the glucose/NEFA ratio were found which indicate that these parameters can be used to quantify the level of hunger. Curvilinear relationships between sitting and standing in the home pen, walking in the open field and level of restriction were found. A third-grade hyperbolic relationship between the level of restriction and the baseline plasma corticosterone concentration was found. Thus, these parameters do not seem to represent a straightforward relation with the level of hunger. Intensive selection on growth and food conversion seems to have affected the behaviour but not the physiological responses to feed restriction, because the JA57 birds responded behaviourally, but not physiologically, different to feed restriction as compared with the Hybro birds.

Differential sensitivity to central leptin and insulin in male and female rats

The distribution of fat in the body differs between the male and female sexes and is associated with the relative secretion of the two "adiposity" hormones leptin and insulin. We now report that the brains of male and female rats are differentially sensitive to the catabolic actions of small doses of these two hormones. Leptin (1 or 3.5 microg/2 microl) or saline (2 microl) was administered into the third cerebral ventricle of age- and weight-matched male and female rats. Leptin significantly reduced food intake in female and male rats over 4 h; however, leptin reduced 24-h intake in female but not in male rats. When the same rats were administered insulin (1 or 4 mU/2 microl) or saline (2 microl), male but not female rats had a robust reduction in food intake over 24 h. Previous research demonstrates the melanocortins are a central mediator of the effects of both leptin and insulin. However, we found no sex differences in sensitivity to the melanocortin agonist MTII (0.01, 0.1, 0.3, and 1.0 nmol/2 microl). These results suggest that the sex differences in sensitivity to leptin and insulin at the doses that we injected occur upstream of the melanocortin receptors. Because insulin and leptin reflect different fat beds and are differentially distributed in the male and female sexes, the implication is that the male and female sexes regulate adiposity-relevant parameters differently.

Brain insulin: regulation, mechanisms of action and functions

1. While many questions remained unanswered, it is now well documented that, contrary to earlier views, insulin is an important neuromodulator, contributing to neurobiological processes, in particular energy homeostasis and cognition. A specific role on cognitive functions related to feeding is proposed, and it is suggested that brain insulin from different sources might be involved in the above vital functions in health and disease. 2. A molecule identical to pancreatic insulin, and specific insulin receptors, are found widely distributed in the central nervous system networks related to feeding, reproduction, or cognition. 3. The actions of insulin in the central nervous system may be under both multilevel and multifactorial controls. The amount of blood insulin reaching the brain, brain insulin stores and secretion, potential local biosynthesis and degradation of the peptide, and insulin receptors and signal transduction can be affected by metabolic factors induced by nutrients, hormones, neurotransmitters, and regulatory peptides, peripherally or in the central nervous system. 4. Glucose and serotonin regulate insulin directly in the hypothalamus and may be of importance for its biological effects. Central mechanisms regulating glucose-induced insulin secretion show some analogy with the mechanisms operating in the pancreas. 5. A cross-talk between insulin and leptin receptors has been observed in the brain, and a regulation of central insulin actions, potentially via serotonin modulation, by leptin, galanin, melancortins, and neuropeptide Y (NPY) is suggested. 6. A more complete knowledge of the biological role of insulin in brain function and dysfunction, and of the regulatory mechanisms involved in these processes, constitutes a real advancement in the understanding of the pathophysiology of metabolic and mental diseases and could lead to important medical benefits.

Altered feeding responses in mice with targeted disruption of the dopamine-3 receptor gene

Dopamine signaling has been implicated in the control of food intake and body weight. In particular, dopamine is important in the control of meal size and number and is thought to mediate the response to metabolic deprivation states. In the present experiments, the authors assessed the role of the dopamine-3 receptor (D3R) in the feeding responses to 2-deoxy-D-glucose, mercaptoacetate, and peripheral insulin. All 3 compounds increased food intake in wild-type mice, but the hyperphagic responses were blunted in D3R-/- mice. In other experiments, D3R-/- mice were hyperresponsive to the administration of amylin and leptin relative to wild-type mice. These results support the hypothesis that D3Rs chronically inhibit the effects of adiposity hormones, thereby contributing to a net anabolic state.

Estradiol treatment increases CCK-induced c-Fos expression in the brains of ovariectomized rats

The ovarian hormone estradiol reduces meal size and food intake in female rats, at least in part by increasing the satiating potency of CCK. Here we used c-Fos immunohistochemistry to determine whether estradiol increases CCK-induced neuronal activation in several brain regions implicated in the control of feeding. Because the adiposity signals leptin and insulin appear to control feeding in part by increasing the satiating potency of CCK, we also examined whether increased adiposity after ovariectomy influences estradiol's effects on CCK-induced c-Fos expression. Ovariectomized rats were injected subcutaneously with 10 microg 17beta-estradiol benzoate (estradiol) or vehicle once each on Monday and Tuesday for 1 wk (experiment 1) or for 5 wk (experiment 2). Two days after the final injection of estradiol or vehicle, rats were injected intraperitoneally with 4 microg/kg CCK in 1 ml/kg 0.9 M NaCl or with vehicle alone. Rats were perfused 60 min later, and brain tissue was collected and processed for c-Fos immunoreactivity. CCK induced c-Fos expression in the nucleus of the solitary tract (NTS), area postrema (AP), paraventricular nucleus of the hypothalamus (PVN), and central nucleus of the amygdala (CeA) in vehicle- and estradiol-treated ovariectomized rats. Estradiol treatment further increased this response in the caudal, subpostremal, and intermediate NTS, the PVN, and the CeA, but not in the rostral NTS or AP. This action of estradiol was very similar in rats tested before (experiment 1) and after (experiment 2) significant body weight gain, suggesting that adiposity does not modulate CCK-induced c-Fos expression or interact with estradiol's ability to modulate CCK-induced c-Fos expression. These findings suggest that estradiol inhibits meal size and food intake by increasing the central processing of the vagal CCK satiation signal.

Leptin in horses: tissue localization and relationship between peripheral concentrations of leptin and body condition

Obesity has been a major concern in the horse industry for many years, and the recent discovery of leptin and leptin receptors in numerous nonequine species has provided a basis for new approaches to study this problem in equine. The objectives were to: 1) clone a partial sequence ofthe equine leptin and leptin receptor genes so as to enable the design of primers for RT-PCR determination of leptin and leptin receptor gene presence and distribution in tissues, 2) develop a radioimmunoassay to quantify peripheral concentrations of leptin in equine, 3) determine if peripheral concentrations of leptin correlate with body condition scores in equine, and 4) determine if changing body condition scores would influence peripheral concentrations of leptin in equine. In Experiment 1, equine leptin (GenBank accession number AF179275) and the long-form of the equine leptin receptor (GenBank accession number AF139663) genes were partially sequenced. Equine leptin receptor mRNA was detected in liver, lung, testis, ovary, choroid plexus, hypothalamus, and subcutaneous adipose tissues using RT-PCR. In Experiment 2, 71 horses were categorized by gender, age, and body condition score and blood samples were collected. Sera were assayed for leptin using a heterologous leptin radioimmunoassay developed for equine sera. Serum concentrations of leptin increased in horses with body condition score (1 = thin to 9 = fat; r = 0.64; P = 0.0001). Furthermore, serum concentrations of leptin were greater in geldings and stallions than in mares (P = 0.0002), and tended to increase with age of the animal (P = 0.08). In Experiment 3, blood samples, body weights, and body condition scores were collected every 14 d from 18 pony mares assigned to gain or lose weight over a 14-wk interval based on initial body condition score. Although statistical changes (P = 0.001) in body condition scores were achieved, congruent statistical changes in peripheral concentrations of leptin were not observed, likely due to the small range of change that occurred. Nonetheless, serum concentrations of leptin tended to be greater in fat-restricted mares than in thin-supplemented mares (P = 0.09). We conclude that leptin and leptin receptors are present in equine tissues and that peripheral concentrations of leptin reflect a significant influence of fat mass in equine.

A general model of intake regulation

Previously proposed models of intake regulation focus on specific variables thought to influence overall intake, and include factors involved in negative feedback loops with intake as well as genetic influences on intake. Recent evidence, however, suggests that these models although informative, are incomplete. They cannot account for the observations of prolonged and increasing deviations from defended levels, weakness and transitoriness of compensatory responses, the presence of powerful factors that are not compensated, and behavioral genetic data suggesting that there are a wide variety of independent genetic influences on numerous factors that influence intake. As a result we propose a new general model of intake regulation in which intake is influenced by both a set of uncompensated factors that are not influenced by intake and by a set of compensated factors that are. The preferred levels of intake and both sets of factors are specified as influenced by heredity. Further, the model includes impact factors, weights, which specify the magnitude of the effect each factor has on intake. The weights are assumed to be different for different individuals and their values are determined by heredity. A computer simulation of the new model demonstrated that it maintains different levels depending upon the external and internal environments, that changes in these environments result in new levels, and that inherited individual differences in responsiveness to these factors can markedly influence the levels obtained. The proposed general model appears to fit existing knowledge and is parsimonious and widely applicable. Future work should be directed to testing the general model and further developing specific models within the conceptual framework employing known physiological systems and uncompensated stimuli.

Integration of the regulation of reproductive function and energy balance: lactation as a model

Lactation is a physiological model for studying how the hypothalamus integrates peripheral signals, such as sensory signals (suckling stimulus) and those denoting energy balance (leptin), to alter hypothalamic function regulating food intake/energy balance and reproduction. The characteristics of food intake/energy balance during lactation are extreme hyperphagia, coupled with negative energy balance. The arcuate nucleus Neuropeptide Y (ARH-NPY) system is activated by: (1) brainstem projections specifically activated by the suckling stimulus, and (2) the decrease in leptin in response to the metabolic drain of milk production. NPY neurons from the ARH make direct contact with GnRH neurons and with CRH neurons in the PVH. NPY neurons also make contact with orexin and MCH neurons in the LHA, which, in turn, make contacts with GnRH neurons. Thus, the ARH-NPY system provides a neuroanatomical framework by which to integrate changes in food intake/energy with the regulation of cyclic reproductive function.

Weight loss at high altitude

Loss of appetite and weight are frequently observed at altitudes above 5000 m. However, the pathophysiology behind changes in body composition at extreme altitude is still not fully understood. Proper acclimatization to altitude and high caloric intake minimizes, but can not completely prevent significant weight loss under the influence of hypobaric hypoxia. The discovery of leptin in 1994 has initiated a new research area investigating molecular networks that connect peripheral organs with the central nervous system to sense and regulate energy intake as well as energy expenditure. Since then, a whole microcosm of new hormones, neurotransmitters and receptors has been discovered and studied with respect to body weight control. Those agents include neuropeptide Y (NPY), agouti-related protein (AGRP), melanocortin receptors (MC-R), cocaine-amphetamine regulated transcript (CART), pro-opiomelanocortin (POMC), orexin A and B (hypocretins), melanin-concentrating hormone (MCH) and ghrelin (endogenous ligand of the growth hormone secretagogue receptor). This overview will introduce the current concepts of the molecular control of energy homeostasis and attempt to reexamine the effects of altitude on appetite and body composition in light of these concepts. An overview of studies on changes of appetite and body composition at high altitude will be followed by the presentation of recent data on changes of endocrine parameters at hypobaric hypoxia that could be involved in the pathophysiology of weight loss.

The temporal organization of ingestive behaviour and its interaction with regulation of energy balance

Body weight of man and animals is under homeostatic control mediated by the adjustment of food intake. It is discussed in this review that besides signals reporting energy deficits, optimized programs of body clocks take part in feeding behaviour as well. Circadian light- and food-entrainable clocks determine anticipatory adaptive behavioural and physiological mechanisms, promoting or inhibiting food intake. In fact these clocks form the constraints within which the homeostatic regulation of feeding behaviour is operating. Therefore, a strong interaction between circadian and homeostatic regulation must occur. In this homeostatic control, a wide variety of regulatory negative feedback mechanisms, or satiety signals, play a dominant role. In this respect several gut hormones and body temperature function as 'short-term' satiety factors and determine meal sizes and intermeal intervals. Leptin, secreted by fat cells in proportion to the size of adipose tissue mass, is probably an important determinant of the 'long-term' regulation of feeding behaviour by setting the motivational background level for feeding behaviour. Thus, initiation or termination of meals at any particular point in time, depends on the resultant of all satiety signals and on constraints imposed by circadian light- and food-entrainable oscillators.

Mitochondrial ATP synthase--a possible target protein in the regulation of energy metabolism in vitro and in vivo

The increasing prevalence of obesity in the Western world has stimulated an intense search for mechanisms regulating food intake and energy balance. A number of appetite-regulating peptides have been identified, their receptors cloned and the intracellular events characterized. One possible energy-dissipating mechanism is the mitochondrial uncoupling of ATP-synthesis from respiratory chain oxidation through uncoupling proteins, whereby energy derived from food could be dissipated as heat, instead of stored as ATP. The exact role of the uncoupling proteins in energy balance is, however, uncertain. We show here that mitochondrial F1F0-ATP synthase itself is a target protein for an anorectic peptide, enterostatin, demonstrated both after affinity purification of rat brain membranes and through a direct physical interaction between enterostatin and purified F1-ATP synthase. In insulinoma cells (INS-1) enterostatin was found to target F1F0-ATP synthase, causing an inhibition of ATP production, an increased thermogenesis and increased oxygen consumption. The experiments suggest a role of mitochondrial F1F0-ATP synthase in the suppressed insulin secretion induced by enterostatin. It could be speculated that this targeting mechanism is involved in the decreased energy efficiency following enterostatin treatment in rat.

Peptide signals regulating food intake and energy homeostasis

The adiposity hormone leptin has been shown to decrease food intake and body weight by acting on neuropeptide circuits in the hypothalamus. However, it is not clear how this primary hypothalamic action of leptin is translated into a change in food intake. We hypothesize that the behavioral effect of leptin ultimately involves the integration of neuronal responses in the forebrain with those in the nucleus tractus solitarius in the caudal brainstem, where ingestive behavior signals are received from the gastrointestinal system and the blood. One example is the peptide cholecystokinin, which is released from the gut following ingestion of a meal and acts via vagal afferent nerve fibers to activate medial nucleus tractus solitarius neurons and thereby decrease meal size. While it is established that leptin acts in the arcuate nucleus in the hypothalamus to stimulate anorexigenic neurons that inhibit food intake while simulataneously inhibiting orexigenic neurons that increase food intake, the mechanisms linking these effects with regions of the caudal brainstem that integrate cues related to meal termination are unclear. Based on an increasing body of supportive data, we hypothesize that this integration involves a pathway comprising descending projections from neurons from the paraventricular nucleus to neurons within the nucleus tractus solitarius that are activated by meal-related satiety factors. Leptin's anorexic effect comprises primarily decreased meal size, and at subthreshold doses for eliciting an effect on food intake, leptin intensifies the satiety response to circulating cholecystokinin. The location of neurons subserving the effects of intracerebroventricular administration of leptin and intraperitoneal injection of cholecystokinin on food intake has been identified by analysis of Fos expression. These studies reveal a distribution that includes the paraventricular nucleus and regions within the caudal brainstem, with the medial nucleus tractus solitarius having the most pronounced Fos expression in response to leptin and cholecystokinin, and support the hypothesis that the long-term adiposity signal leptin and the short-term satiety signal cholecystokinin act in concert to maintain body weight homeostasis.

Orexins and the treatment of obesity

Orexin-A and -B are two peptides derived by proteolytic cleavage from a 130-amino acid precursor, prepro-orexin, which were recently isolated from the rat hypothalamus. Orexin-A is fully conserved across mammalian species, whilst rat and human orexin-B differ by two amino acids. These peptides bind to two Gq-coupled receptors, termed orexin-1 and orexin-2. The receptors are 64% homologous and highly conserved across species. Orexin-A is equipotent at orexin-1 and orexin-2 receptors, whilst orexin-B displays moderate (approximately 10 fold) selectivity for orexin-2 receptors. The distribution and pharmacology of the orexin peptides and their receptors indicate that they play a role in various regulatory systems including energy homeostasis and the regulation of feeding, the evidence for which is reviewed here.

Single nucleotide polymorphism identification in candidate gene systems of obesity

We have constructed a large panel of single nucleotide polymorphisms (SNP) identified in 68 candidate genes for obesity. Our panel combines novel SNP identification methods based on EST data, with public SNP data from largescale genomic sequencing, to produce a total of 218 SNPs in the coding regions of obesity candidate genes, 178 SNPs in untranslated regions, and over 1000 intronic SNPs. These include new non-conservative amino acid changes in thyroid receptor beta, esterase D, acid phosphatase 1. Our data show evidence of negative selection among these polymorphisms implying functional impacts of the non-conservative mutations. Comparison of overlap between SNPs identified independently from EST data vs genomic sequencing indicate that together they may constitute about one half of the actual total number of amino acid polymorphisms in these genes that are common in the human population (defined here as a population allele frequency above 5%). We have analyzed our polymorphism panel to construct a database of detailed information about their location in the gene structure and effect on protein coding, available on the web at http://www.bioinformat ics.ucla.edu/snp/obesity. We believe this panel can serve as a valuable new resource for genetic and pharmacogenomic studies of the causes of obesity.

The effect of Type I diabetes on the eating patterns of free-living French: a diet diary study

In order to investigate the changes produced by Type I diabetes on the ad libitum eating behavior of free-living humans, 56 French participants with diabetes and 28 healthy controls were paid to maintain detailed food intake diaries for four 7-day periods. The participants with diabetes ate more protein and more frequent meals, ate slightly later and with fewer other people, were less hungry, thirsty, and depressed, but more elated than the healthy participants. Responses to social facilitation, the diurnal rhythm, subjective hunger, the palatability of the food, and the weekend were not affected by the presence of diabetes. The participants with diabetes had significantly larger correlations and regression coefficients for the relationship between meal size and the duration of the before-meal interval. The relationship between meal size and the after-meal interval was strong and positive in all participants. The recorded behavioral characteristics of diabetics may, in part, be accounted for by the nutritional education that is provided as an integral part of treatment. Based upon these results, it was theorized that the glucose regulatory system is an influence on intake, but only one of many that are responsible for the coordination, control, and regulation of nutrient intake in free-living humans.

Obesity pharmacology: past, present, and future

Over the past several years, the pharmacologic treatment of obesity has undergone changes in safety, efficacy, and therapeutic targeting. The prevalence of cardiac valvulopathy associated with treatment with phentermine, fenfluramine, and dexfenfluramine is now becoming clarified with the publication of longer-term studies. Phenylpropanolamine, a well-known over-the-counter appetite suppressant, was recently removed from the market in the United States because of an increased risk of hemorrhagic stroke in women. In contrast, two currently approved medications, sibutramine and orlistat, have been shown to be safe and moderately effective for weight loss with documented beneficial effects on cardiovascular risk factors. Three other drugs, bupropion, topiramate, and ciliary neurotrophic factor, are undergoing clinical trials for obesity based on empirical observations. Most promising are the advances in genetics and molecular biology that are beginning to elucidate new targets for controlling appetite and energy utilization. These therapeutic agents will likely herald a second generation of anti-obesity medications over the next decade.

Small molecule insulin mimetics reduce food intake and body weight and prevent development of obesity

Obesity and insulin resistance are major risk factors for a number of metabolic disorders, such as type 2 diabetes mellitus. Insulin has been suggested to function as one of the adiposity signals to the brain for modulation of energy balance. Administration of insulin into the brain reduces food intake and body weight, and mice with a genetic deletion of neuronal insulin receptors are hyperphagic and obese. However, insulin is also an anabolic factor; when administered systemically, pharmacological levels of insulin are associated with body weight gain in patients. In this study, we investigated the efficacy and feasibility of small molecule insulin mimetic compounds to regulate key parameters of energy homeostasis. Central intracerebroventricular (i.c.v.) administration of an insulin mimetic resulted in a dose-dependent reduction of food intake and body weight in rats, and altered the expression of hypothalamic genes known to regulate food intake and body weight. Oral administration of a mimetic in a mouse model of high-fat diet-induced obesity reduced body weight gain, adiposity and insulin resistance. Thus, insulin mimetics have a unique advantage over insulin in the control of body weight and hold potential as a novel anti-obesity treatment.