Effect of leptin on hypothalamic GLP-1 peptide and brain-stem pre-proglucagon mRNA

Hypothalamic AMP-Activated Protein Kinase as a Whole-Body Energy Sensor and Regulator

5´-Adenosine monophosphate (AMP)-activated protein kinase (AMPK), a cellular energy sensor, is an essential enzyme that helps cells maintain stable energy levels during metabolic stress. The hypothalamus is pivotal in regulating energy balance within the body. Certain neurons in the hypothalamus are sensitive to fluctuations in food availability and energy stores, triggering adaptive responses to preserve systemic energy equilibrium. AMPK, expressed in these hypothalamic neurons, is instrumental in these regulatory processes. Hypothalamic AMPK activity is modulated by key metabolic hormones. Anorexigenic hormones, including leptin, insulin, and glucagon-like peptide 1, suppress hypothalamic AMPK activity, whereas the hunger hormone ghrelin activates it. These hormonal influences on hypothalamic AMPK activity are central to their roles in controlling food consumption and energy expenditure. Additionally, hypothalamic AMPK activity responds to variations in glucose concentrations. It becomes active during hypoglycemia but is deactivated when glucose is introduced directly into the hypothalamus. These shifts in AMPK activity within hypothalamic neurons are critical for maintaining glucose balance. Considering the vital function of hypothalamic AMPK in the regulation of overall energy and glucose balance, developing chemical agents that target the hypothalamus to modulate AMPK activity presents a promising therapeutic approach for metabolic conditions such as obesity and type 2 diabetes mellitus.

Obesity and its comorbidities, current treatment options and future perspectives: Challenging bariatric surgery?

Obesity and its comorbidities, including type 2 diabetes mellitus, cardiovascular disease, heart failure and non-alcoholic liver disease are a major health and economic burden with steadily increasing numbers worldwide. The need for effective pharmacological treatment options is strong, but, until recently, only few drugs have proven sufficient efficacy and safety. This article provides a comprehensive overview of obesity and its comorbidities, with a special focus on organ-specific pathomechanisms. Bariatric surgery as the so far most-effective therapeutic strategy, current pharmacological treatment options and future treatment strategies will be discussed. An increasing knowledge about the gut-brain axis and especially the identification and physiology of incretins unfolds a high number of potential drug candidates with impressive weight-reducing potential. Future multi-modal therapeutic concepts in obesity treatment may surpass the effectivity of bariatric surgery not only with regard to weight loss, but also to associated comorbidities.

Examining weight suppression as a transdiagnostic factor influencing illness trajectory in bulimic eating disorders

Recent research indicates that weight suppression (WS: defined as the difference between highest lifetime and current weight) prospectively predicts illness trajectory across eating disorders characterized by binge eating, including AN binge-purge subtype (ANbp), bulimia nervosa (BN), and binge eating disorder (BED), collectively referred to as bulimic eating disorders. Through a series of studies, we have developed a model to explain the link between WS and illness trajectory in bulimic eating disorders. Our model posits that WS contributes to reduced circulating leptin, which leads to reduced postprandial glucagon-like peptide 1 (GLP-1) response. Diminished leptin and GLP-1 function contribute to alterations in two reward-related constructs in the Research Domain Criteria (RDoC): reward value/effort and reward satiation. Respectively, these changes increase drive/motivation to consume food and decrease ability for food consumption to lead to a state of satiation/satisfaction. Combined, these alterations increase risk for experiencing large, out-of-control binge-eating episodes. The following review presents evidence that contributed to the development of this model as well as preliminary findings from an on-going project funded to test this model.

Hypothalamic AMPK as a Mediator of Hormonal Regulation of Energy Balance

As a cellular energy sensor and regulator, adenosine monophosphate (AMP)-activated protein kinase (AMPK) plays a pivotal role in the regulation of energy homeostasis in both the central nervous system (CNS) and peripheral organs. Activation of hypothalamic AMPK maintains energy balance by inducing appetite to increase food intake and diminishing adaptive thermogenesis in adipose tissues to reduce energy expenditure in response to food deprivation. Numerous metabolic hormones, such as leptin, adiponectin, ghrelin and insulin, exert their energy regulatory effects through hypothalamic AMPK via integration with the neural circuits. Although activation of AMPK in peripheral tissues is able to promote fatty acid oxidation and insulin sensitivity, its chronic activation in the hypothalamus causes obesity by inducing hyperphagia in both humans and rodents. In this review, we discuss the role of hypothalamic AMPK in mediating hormonal regulation of feeding and adaptive thermogenesis, and summarize the diverse underlying mechanisms by which central AMPK maintains energy homeostasis.

Physiology, pathophysiology and therapeutic implications of enteroendocrine control of food intake

With the increasing prevalence of obesity and its associated comorbidities, strides to improve treatment strategies have enhanced our understanding of the function of the gut in the regulation of food intake. The most successful intervention for obesity to date, bariatric surgery effectively manipulates enteroendocrine physiology to enhance satiety and reduce hunger. Areas covered: In the present article, we provide a detailed overview of the physiology of enteroendocrine control of food intake, and discuss its pathophysiologic correlates and therapeutic implications in both obesity and gastrointestinal disease. Expert commentary: Ongoing research in the field of nutrient sensing by L-cells, as well as understanding the role of the microbiome and bile acid signaling may facilitate the development of novel strategies to combat the rising population health threat associated with obesity. Further refinement of post-prandial satiety gut hormone based therapies, including the development of chimeric peptides exploiting the pleiotropic nature of the gut hormone response, and identification of novel methods of delivery may hold the key to optimization of therapeutic modulation of gut hormone physiology in obesity.

Stressing diabetes? The hidden links between insulinotropic peptides and the HPA axis

Diabetes mellitus exerts metabolic stress on cells and it provokes a chronic increase in the long-term activity of the hypothalamus-pituitary-adrenocortical (HPA) axis, perhaps thereby contributing to insulin resistance. GLP-1 receptor (GLP-1R) agonists are pleiotropic hormones that not only affect glycaemic and metabolic control, but they also produce many other effects including activation of the HPA axis. In fact, several of the most relevant effects of GLP-1 might involve, at least in part, the modulation of the HPA axis. Thus, the anorectic activity of GLP-1 could be mediated by increasing CRF at the hypothalamic level, while its lipolytic effects could imply a local increase in glucocorticoids and glucocorticoid receptor (GC-R) expression in adipose tissue. Indeed, the potent activation of the HPA axis by GLP-1R agonists occurs within the range of therapeutic doses and with a short latency. Interestingly, the interactions of GLP-1 with the HPA axis may underlie most of the effects of GLP-1 on food intake control, glycaemic metabolism, adipose tissue biology and the responses to stress. Moreover, such activity has been observed in animal models (mice and rats), as well as in normal humans and in type I or type II diabetic patients. Accordingly, better understanding of how GLP-1R agonists modulate the activity of the HPA axis in diabetic subjects, especially obese individuals, will be crucial to design new and more efficient therapies for these patients.

Hypothalamic AMPK as a Regulator of Energy Homeostasis

Activated in energy depletion conditions, AMP-activated protein kinase (AMPK) acts as a cellular energy sensor and regulator in both central nervous system and peripheral organs. Hypothalamic AMPK restores energy balance by promoting feeding behavior to increase energy intake, increasing glucose production, and reducing thermogenesis to decrease energy output. Besides energy state, many hormones have been shown to act in concert with AMPK to mediate their anorexigenic and orexigenic central effects as well as thermogenic influences. Here we explore the factors that affect hypothalamic AMPK activity and give the underlying mechanisms for the role of central AMPK in energy homeostasis together with the physiological effects of hypothalamic AMPK on energy balance restoration.

GLP-1 and weight loss: unraveling the diverse neural circuitry

Glucagon-like peptide-1 (GLP-1) is currently one of the most promising biological systems for the development of effective obesity pharmacotherapies. Long-acting GLP-1 analogs potently reduce food intake and body weight, and recent discoveries reveal that peripheral administration of these drugs reduces food intake largely through humoral pathways involving direct action on brain GLP-1 receptors (GLP-1R). Thus, it is of critical importance to understand the neural systems through which GLP-1 and long-acting GLP-1 analogs reduce food intake and body weight. In this review, we discuss several neural, physiological, cellular and molecular, as well as behavioral mechanisms through which peripheral and central GLP-1R signaling reduces feeding. Particular attention is devoted to discussion regarding the numerous neural substrates through which GLP-1 and GLP-1 analogs act to reduce food intake and body weight, including various hypothalamic nuclei (arcuate nucleus of the hypothalamus, periventricular hypothalamus, lateral hypothalamic area), hindbrain nuclei (parabrachial nucleus, medial nucleus tractus solitarius), hippocampus (ventral subregion; vHP), and nuclei embedded within the mesolimbic reward circuitry [ventral tegmental area (VTA) and nucleus accumbens (NAc)]. In some of these nuclei [VTA, NAc, and vHP], GLP-1R activation reduces food intake and body weight without concomitant nausea responses, suggesting that targeting these specific pathways may be of particular interest for future obesity pharmacotherapy. The widely distributed neural systems through which GLP-1 and GLP-1 analogs act to reduce body weight highlight the complexity of the neural systems regulating energy balance, as well as the challenges for developing effective obesity pharmacotherapies that reduce feeding without producing parallel negative side effects.

Regulation of body fat mass by the gut microbiota: Possible mediation by the brain

New insight suggests gut microbiota as a component in energy balance. However, the underlying mechanisms by which gut microbiota can impact metabolic regulation is unclear. A recent study from our lab shows, for the first time, a link between gut microbiota and energy balance circuitries in the hypothalamus and brainstem. In this article we will review this study further.

PPG neurons of the lower brain stem and their role in brain GLP-1 receptor activation

Within the brain, glucagon-like peptide-1 (GLP-1) affects central autonomic neurons, including those controlling the cardiovascular system, thermogenesis, and energy balance. Additionally, GLP-1 influences the mesolimbic reward system to modulate the rewarding properties of palatable food. GLP-1 is produced in the gut and by hindbrain preproglucagon (PPG) neurons, located mainly in the nucleus tractus solitarii (NTS) and medullary intermediate reticular nucleus. Transgenic mice expressing glucagon promoter-driven yellow fluorescent protein revealed that PPG neurons not only project to central autonomic control regions and mesolimbic reward centers, but also strongly innervate spinal autonomic neurons. Therefore, these brain stem PPG neurons could directly modulate sympathetic outflow through their spinal inputs to sympathetic preganglionic neurons. Electrical recordings from PPG neurons in vitro have revealed that they receive synaptic inputs from vagal afferents entering via the solitary tract. Vagal afferents convey satiation to the brain from signals like postprandial gastric distention or activation of peripheral GLP-1 receptors. CCK and leptin, short- and long-term satiety peptides, respectively, increased the electrical activity of PPG neurons, while ghrelin, an orexigenic peptide, had no effect. These findings indicate that satiation is a main driver of PPG neuronal activation. They also show that PPG neurons are in a prime position to respond to both immediate and long-term indicators of energy and feeding status, enabling regulation of both energy balance and general autonomic homeostasis. This review discusses the question of whether PPG neurons, rather than gut-derived GLP-1, are providing the physiological substrate for the effects elicited by central nervous system GLP-1 receptor activation.

Weight suppression in bulimia nervosa: Associations with biology and behavior

Bulimia nervosa (BN) is a serious eating disorder that can persist for years and contribute to medical complications and increased mortality, underscoring the need to better understand factors maintaining this disorder. Higher levels of weight suppression (WS) have been found to predict both the onset and maintenance of BN; however, no studies have examined mechanisms that may account for the effects of WS on BN. We hypothesized that high WS would lead to reduced leptin levels, which may increase risk of binge eating by modulating reward responses to food. The current study examined the relationship between WS, leptin levels, and the reinforcing value of food in women with BN (n = 32) and noneating disorder controls (n = 30). Participants provided information on WS, completed a fasting blood draw to obtain serum leptin, and completed a progressive ratio task to measure the reinforcing value of food. Individuals with BN had greater WS (p < .01) and reinforcing food value (p < .05) compared with controls. Additionally, higher WS was associated with both lower leptin (p < .05) and increased reinforcing value of food (p < .05). Contrary to hypotheses, BN and control participants did not differ on leptin levels, and leptin levels were not significantly associated with the reinforcing value of food. Findings support that efforts to conform to the thin ideal may alter drive to consume rewarding foods and leave women vulnerable to binge episodes. However, mechanisms through which WS contributes to food reward and binge eating remain unknown.

Physiology of proglucagon peptides: role of glucagon and GLP-1 in health and disease

The preproglucagon gene (Gcg) is expressed by specific enteroendocrine cells (L-cells) of the intestinal mucosa, pancreatic islet α-cells, and a discrete set of neurons within the nucleus of the solitary tract. Gcg encodes multiple peptides including glucagon, glucagon-like peptide-1, glucagon-like peptide-2, oxyntomodulin, and glicentin. Of these, glucagon and GLP-1 have received the most attention because of important roles in glucose metabolism, involvement in diabetes and other disorders, and application to therapeutics. The generally accepted model is that GLP-1 improves glucose homeostasis indirectly via stimulation of nutrient-induced insulin release and by reducing glucagon secretion. Yet the body of literature surrounding GLP-1 physiology reveals an incompletely understood and complex system that includes peripheral and central GLP-1 actions to regulate energy and glucose homeostasis. On the other hand, glucagon is established principally as a counterregulatory hormone, increasing in response to physiological challenges that threaten adequate blood glucose levels and driving glucose production to restore euglycemia. However, there also exists a potential role for glucagon in regulating energy expenditure that has recently been suggested in pharmacological studies. It is also becoming apparent that there is cross-talk between the proglucagon derived-peptides, e.g., GLP-1 inhibits glucagon secretion, and some additive or synergistic pharmacological interaction between GLP-1 and glucagon, e.g., dual glucagon/GLP-1 agonists cause more weight loss than single agonists. In this review, we discuss the physiological functions of both glucagon and GLP-1 by comparing and contrasting how these peptides function, variably in concert and opposition, to regulate glucose and energy homeostasis.

Liraglutide, leptin and their combined effects on feeding: additive intake reduction through common intracellular signalling mechanisms

AIM

To investigate the behavioural and intracellular mechanisms by which the glucagon like peptide-1 (GLP-1) receptor agonist, liraglutide, and leptin in combination enhance the food intake inhibitory and weight loss effects of either treatment alone.

METHODS

We examined the effects of liraglutide (a long-acting GLP-1 analogue) and leptin co-treatment, delivered in low or moderate doses subcutaneously (s.c.) or to the third ventricle, respectively, on cumulative intake, meal patterns and hypothalamic expression of intracellular signalling proteins [phosphorylated signal transducer and activator of transcription-3 (pSTAT3) and protein tyrosine phosphatase-1B (PTP1B)] in lean rats.

RESULTS

A low-dose combination of liraglutide (25 µg/kg) and leptin (0.75 µg) additively reduced cumulative food intake and body weight, a result mediated predominantly through a significant reduction in meal frequency that was not present with either drug alone. Liraglutide treatment alone also reduced meal size; an effect not enhanced with leptin co-administration. Moderate doses of liraglutide (75 µg/kg) and leptin (4 µg), examined separately, each reduced meal frequency, cumulative food intake and body weight; only liraglutide reduced meal size. In combination these doses did not further enhance the anorexigenic effects of either treatment alone. Ex vivo immunoblot analysis showed elevated pSTAT3 in the hypothalamic tissue after liraglutide-leptin co-treatment, an effect which was greater than that of leptin treatment alone. In addition, s.c. liraglutide reduced the expression of PTP1B (a negative regulator of leptin receptor signalling), revealing a potential mechanism for the enhanced pSTAT3 response after liraglutide-leptin co-administration.

CONCLUSIONS

Collectively, these results show novel behavioural and molecular mechanisms underlying the additive reduction in food intake and body weight after liraglutide-leptin combination treatment.

Endogenous glucagon-like peptide-1 reduces drinking behavior and is differentially engaged by water and food intakes in rats

Glucagon-like peptide-1 (GLP-1) is produced in the ileum and the nucleus of the solitary tract. It is well known that GLP-1 controls food intake, but there is a growing literature indicating that GLP-1 also is involved in fluid intake. It is not known, however, if the observed effects are pharmacological or if endogenous GLP-1 and its receptor contribute to physiological fluid intake control. Accordingly, we blocked endogenous GLP-1 by application of a receptor antagonist and measured subsequent drinking. Furthermore, we measured changes in GLP-1-associated gene expression after water intake, and compared the effects of fluid intake to those caused by food intake. Rats injected with the antagonist exendin-9 (Ex-9) drank more fluid in response to either subcutaneous hypertonic saline or water deprivation with partial rehydration than did vehicle-treated rats. Analysis of licking behavior showed that Ex-9 increased fluid intake by increasing the number of licking bursts, without having an effect on the number of licks per burst, suggesting that endogenous GLP-1 suppresses fluid intake by influencing satiety. Subsequent experiments showed that water intake had a selective effect on central GLP-1-related gene expression, unlike food intake, which affected both central and peripheral GLP-1. Although water and food intakes both affected central GLP-1-relevant gene expression, there were notable differences in the timing of the effect. These results show a novel role of the endogenous GLP-1 system in fluid intake, and indicate that elements of the GLP-1 system can be engaged separately by different forms of ingestive behavior.

Delineating the regulation of energy homeostasis using hypothalamic cell models

Attesting to its intimate peripheral connections, hypothalamic neurons integrate nutritional and hormonal cues to effectively manage energy homeostasis according to the overall status of the system. Extensive progress in the identification of essential transcriptional and post-translational mechanisms regulating the controlled expression and actions of hypothalamic neuropeptides has been identified through the use of animal and cell models. This review will introduce the basic techniques of hypothalamic investigation both in vivo and in vitro and will briefly highlight the key advantages and challenges of their use. Further emphasis will be place on the use of immortalized models of hypothalamic neurons for in vitro study of feeding regulation, with a particular focus on cell lines proving themselves most fruitful in deciphering fundamental basics of NPY/AgRP, Proglucagon, and POMC neuropeptide function.

The gut microbiota reduces leptin sensitivity and the expression of the obesity-suppressing neuropeptides proglucagon (Gcg) and brain-derived neurotrophic factor (Bdnf) in the central nervous system

The gut microbiota contributes to fat mass and the susceptibility to obesity. However, the underlying mechanisms are not completely understood. To investigate whether the gut microbiota affects hypothalamic and brainstem body fat-regulating circuits, we compared gene expression of food intake-regulating neuropeptides between germ-free and conventionally raised (CONV-R) mice. We found that CONV-R mice had decreased expression of the antiobesity neuropeptide glucagon-like peptide-1 (GLP-1) precursor proglucagon (Gcg) in the brainstem. Moreover, in both the hypothalamus and the brainstem, CONV-R mice had decreased expression of the antiobesity neuropeptide brain-derived neurotrophic factor (Bdnf). CONV-R mice had reduced expression of the pro-obesity peptides neuropeptide-Y (Npy) and agouti-related protein (Agrp), and increased expression of the antiobesity peptides proopiomelanocortin (Pomc) and cocaine- and amphetamine-regulated transcript (Cart) in the hypothalamus. The latter changes in neuropeptide expression could be secondary to elevated fat mass in CONV-R mice. Leptin treatment caused less weight reduction and less suppression of orexigenic Npy and Agrp expression in CONV-R mice compared with germ-free mice. The hypothalamic expression of leptin resistance-associated suppressor of cytokine signaling 3 (Socs-3) was increased in CONV-R mice. In conclusion, the gut microbiota reduces the expression of 2 genes coding for body fat-suppressing neuropeptides, Gcg and Bdnf, an alteration that may contribute to fat mass induction by the gut microbiota. Moreover, the presence of body fat-inducing gut microbiota is associated with hypothalamic signs of Socs-3-mediated leptin resistance, which may be linked to failed compensatory body fat reduction.

Decreased Glucagon-like Peptide 1 Release after Weight Loss in Overweight/Obese Subjects

OBJECTIVE

Postprandial glucagon-like peptide 1 (GLP-1) release seems to be attenuated in obese subjects. Results on whether weight loss improves GLP-1 release are contradictory. The aim of this study was to further investigate the effect of weight loss on basal and postprandial GLP-1 release in overweight/obese subjects.

RESEARCH METHODS AND PROCEDURES

Thirty-two overweight/obese subjects participated in a repeated measurement design before (BMI, 30.3 +/- 2.8 kg/m2; waist circumference, 92.6 +/- 7.8 cm; hip circumference, 111.1 +/- 7.4 cm) and after a weight loss period of 6 weeks (BMI, 28.2 +/- 2.7 kg/m2; waist circumference, 85.5 +/- 8.5 cm; hip circumference, 102.1 +/- 9.2 cm). During weight loss, subjects received a very-low-calorie diet (Optifast) to replace three meals per day. Subjects came to the laboratory fasted, and after a baseline blood sample, received a standard breakfast (1.9 MJ). Postprandially, blood samples were taken every one-half hour relative to intake for 120 minutes to determine GLP-1, insulin, glucose, and free fatty acids from plasma. Appetite ratings were obtained with visual analog scales.

RESULTS

After weight loss, postprandial GLP-1 concentrations at 30 and 60 minutes were significantly lower than before weight loss (p < 0.05). Glucose concentrations were also lower, and free fatty acids were higher compared with before weight loss. Ratings of satiety were increased, and hunger scores were decreased after weight loss (p < 0.05).

DISCUSSION

In overweight/obese subjects, GLP-1 concentrations after weight loss were decreased compared with before weight loss, and nutrient-related stimulation was abolished. This might be a response to a proceeding negative energy balance. Satiety and GLP-1 seem to be unrelated in the long term.

Direct regulation of the proglucagon gene by insulin, leptin, and cAMP in embryonic versus adult hypothalamic neurons

The proglucagon gene is expressed not only in the pancreas and intestine but also in the hypothalamus. Proglucagon-derived peptides have emerged as potential regulators of energy homeostasis. Whether leptin, insulin, or cAMP activation controls proglucagon gene expression in the hypothalamus is not known. A key reason for this has been the inaccessibility of hypothalamic proglucagon-expressing neurons and the lack of suitable neuronal cell lines. Herein we describe the mechanisms involved in the direct regulation of the proglucagon gene by insulin, leptin, and cAMP in hypothalamic cell models. Insulin, through an Akt-dependent manner, significantly induced proglucagon mRNA expression by 70% in adult-derived mHypoA-2/10 neurons and significantly suppressed it by 45% in embryonic-derived mHypoE-39 neurons. Leptin, via the Janus kinase-2/ signal transducer and activator of transcription-3 pathway, caused an initial increase by 66 and 43% at 1 h followed by a decrease by 45 and 34% at 12 h in mHypoA-2/10 and mHypoE-39 cells, respectively. Furthermore, cAMP activation by forskolin up-regulated proglucagon expression by 87% in mHypoE-39 neurons and increased proglucagon mRNA, through Epac activation, in the mHypoE-20/2 neurons. Specific regions of the proglucagon promoter were regulated by cAMP signaling, as determined by transient transfections, whereas mRNA stability assays demonstrate that insulin and leptin increase proglucagon mRNA stability in the adult cells. These findings suggest that insulin, leptin, and cAMP act directly, but differentially, on specific hypothalamic neurons to regulate proglucagon gene expression. Because proglucagon-derived peptides are potential regulators of energy homeostasis, an understanding of hypothalamic proglucagon neurons is important to further expand our knowledge of alternative feeding circuits.

GLP-1 and energy balance: an integrated model of short-term and long-term control

Glucagon-like peptide 1 (GLP-1), a peptide secreted from the intestine in response to nutrient ingestion, is perhaps best known for its effect on glucose-stimulated insulin secretion. GLP-1 is also secreted from neurons in the caudal brainstem, and it is well-established that, in rodents, central administration of GLP-1 potently reduces food intake. Over the past decade, GLP-1 has emerged not only as an essential component of the system that regulates blood glucose levels but also as a viable therapeutic target for the treatment of type 2 diabetes mellitus. However, although GLP-1 receptor agonists are known to produce modest but statistically significant weight loss in patients with diabetes mellitus, our knowledge of how endogenous GLP-1 regulates food intake and body weight remains limited. The purpose of this Review is to discuss the evolution of our understanding of how endogenous GLP-1 modulates energy balance. Specifically, we consider contributions of both central and peripheral GLP-1 and propose an integrated model of short-term and long-term control of energy balance. Finally, we discuss this model with respect to current GLP-1-based therapies and suggest ongoing research in order to maximize the effectiveness of GLP-1-based treatment of obesity.

Hyperphagia and increased fat accumulation in two models of chronic CNS glucagon-like peptide-1 loss of function

Central administration of glucagon-like peptide-1 (GLP-1) causes a dose-dependent reduction in food intake, but the role of endogenous CNS GLP-1 in the regulation of energy balance remains unclear. Here, we tested the hypothesis that CNS GLP-1 activity is required for normal energy balance by using two independent methods to achieve chronic CNS GLP-1 loss of function in rats. Specifically, lentiviral-mediated expression of RNA interference was used to knock down nucleus of the solitary tract (NTS) preproglucagon (PPG), and chronic intracerebroventricular (ICV) infusion of the GLP-1 receptor (GLP-1r) antagonist exendin (9-39) (Ex9) was used to block CNS GLP-1r. NTS PPG knockdown caused hyperphagia and exacerbated high-fat diet (HFD)-induced fat accumulation and glucose intolerance. Moreover, in control virus-treated rats fed the HFD, NTS PPG expression levels correlated positively with fat mass. Chronic ICV Ex9 also caused hyperphagia; however, increased fat accumulation and glucose intolerance occurred regardless of diet. Collectively, these data provide the strongest evidence to date that CNS GLP-1 plays a physiologic role in the long-term regulation of energy balance. Moreover, they suggest that this role is distinct from that of circulating GLP-1 as a short-term satiation signal. Therefore, it may be possible to tailor GLP-1-based therapies for the prevention and/or treatment of obesity.

Glucagon-like peptide 1 and the brain: central actions-central sources?

Glucagon-like peptide 1(GLP-1) is both an incretin released postprandially from the gut and a neuropeptide produced by select brainstem neurons. Its principal role is in the control of metabolic and cardiovascular functions, acting both in the periphery and within the central nervous system (CNS). Specifically, GLP-1 functions that involve the CNS include the suppression of food intake, the regulation of glucose homeostasis and the modulation of heart rate and blood pressure. Thus far, relatively little is known about the exact interplay between gut-derived and neuronally-produced GLP-1. This is partially due to the difficulty of identifying and targeting GLP-1 producing cells in vitro. This obstacle has recently been overcome by the generation of transgenic mice with fluorescently-tagged GLP-1 cells (mGLU-YFP mice). This review revisits what has been discovered about the central actions of GLP-1 during the past decade and puts it into context of the emerging findings from the mGLU-YFP mice.

Leptin directly depolarizes preproglucagon neurons in the nucleus tractus solitarius: electrical properties of glucagon-like Peptide 1 neurons

OBJECTIVE

Glucagon-like peptide (GLP)-1 inhibits food intake, acting both in the periphery and within the central nervous system. It is unclear if gut-derived GLP-1 can enter the brain, or whether GLP-1 from preproglucagon (PPG) cells in the lower brainstem is required to activate central GLP-1 receptors. Brainstem PPG neurons, however, have been poorly characterized, due to the difficulties in identifying these cells while viable. This study provides data on the electrical properties of brainstem PPG cells and their regulation by orexigenic and anorexigenic peptides.

RESEARCH DESIGN AND METHODS

Transgenic mice expressing Venus under control of the PPG promoter were used to identify PPG neurons in vitro in brainstem slice preparations for electrophysiological recordings. RESULTS The majority of PPG neurons were spontaneously active. Further electrical and molecular characterization revealed that GLP-1 receptor activation had no pre- or postsynaptic effect and that PPG neurons lack GLP-1 receptors. Similarly, they were unresponsive to PYY and ghrelin. In contrast, leptin rapidly and reversibly depolarized these neurons. Responses to electrical stimulation of the solitary tract suggest that PPG cells are mostly second-order neurons, receiving direct input from vagal afferent fibers. Both evoked and spontaneous excitatory postsynaptic currents were predominantly glutamatergic.

CONCLUSIONS

The study introduces PPG-promoter-Venus transgenic mice as a viable and important tool to study brainstem PPG cells. PPG neuron activity is directly modulated by leptin but was unaffected by other satiety or hunger peptides. Direct synaptic input from the solitary tract suggests that peripheral signals (including GLP-1) could modulate PPG cells via vagal afferents.

Preproglucagon derived peptides GLP-1, GLP-2 and oxyntomodulin in the CNS: role of peripherally secreted and centrally produced peptides

The scientific understanding of preproglucagon derived peptides has provided people with type 2 diabetes with two novel classes of glucose lowering agents, the dipeptidyl peptidase IV (DPP-IV) inhibitors and GLP-1 receptor agonists. For the scientists, the novel GLP-1 agonists, and DPP-IV inhibitors have evolved as useful tools to understand the role of the preproglucagon derived peptides in normal physiology and disease. However, the overwhelming interest attracted by GLP-1 analogues as potent incretins has somewhat clouded the efforts to understand the importance of preproglucagon derived peptides in other physiological contexts. In particular, our neurobiological understanding of the preproglucagon expressing neuronal pathways in the central nervous system as well as the degree to which central GLP-1 receptors are targeted by peripherally administered GLP-1 receptor agonists is still fairly limited. The role of GLP-1 as an anorectic neurotransmitter is well recognized, but clarification of the neuronal targets and physiological basis of this response is further warranted, as is the mapping of GLP-1 sensitive neurons involved in a variety of neuroendocrine and behavioral responses. Further recent evidence points to GLP-1 as a central neuropeptide with neuroprotective capabilities potentially mitigating a wide array of neurodegenerative conditions. It is the aim of the present review to summarize our current understanding of preproglucagon derived peptides as neurotransmitters in the central nervous system.

Role of the glucagon-like-peptide-1 receptor in the control of energy balance

The peripheral and central glucagon-like-peptide-1 (GLP-1) systems play an essential role in glycemic and energy balance regulation. Thus, pharmacological targeting of peripheral and/or central GLP-1 receptors (GLP-1R) may represent a potential long-term treatment option for both obesity and type-II diabetes mellitus (T2DM). Uncovering and understanding the neural pathways, physiological mechanisms, specific GLP-1R populations, and intracellular signaling cascades that mediate the food intake inhibitory and incretin effects produced by GLP-1R activation are vital to the development of these potential successful therapeutics. Particular focus will be given to the essential role of the nucleus tractus solitarius (NTS) in the caudal brainstem, as well as the gut-to-brain communication by vagal afferent fibers in mediating the physiological and behavioral responses following GLP-1R activation. The paper represents an invited review by a symposium, award winner or keynote speaker at the Society for the Study of Ingestive Behavior [SSIB] Annual Meeting in Portland, July 2009.

Pleiotropic actions of the incretin hormones

The insulin secretory response to a meal results largely from glucose stimulation of the pancreatic islets and both direct and indirect (autonomic) glucose-dependent stimulation by incretin hormones released from the gastrointestinal tract. Two incretins, Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), have so far been identified. Localization of the cognate G protein-coupled receptors for GIP and GLP-1 revealed that they are present in numerous tissues in addition to the endocrine pancreas, including the gastrointestinal, cardiovascular, central nervous and autonomic nervous systems (ANSs), adipose tissue, and bone. At these sites, the incretin hormones exert a range of pleiotropic effects, many of which contribute to the integration of processes involved in the regulation of food intake, and nutrient and mineral processing and storage. From detailed studies at the cellular and molecular level, it is also evident that both incretin hormones act via multiple signal transduction pathways that regulate both acute and long-term cell function. Here, we provide an overview of current knowledge relating to the physiological roles of GIP and GLP-1, with specific emphasis on their modes of action on islet hormone secretion, β-cell proliferation and survival, central and autonomic neuronal function, gastrointestinal motility, and glucose and lipid metabolism. However, it is emphasized that despite intensive research on the various body systems, in many cases there is uncertainty as to the pathways by which the incretins mediate their pleiotropic effects and only a rudimentary understanding of the underlying cellular mechanisms involved, and these are challenges for the future.

Minireview: finding the sweet spot: peripheral versus central glucagon-like peptide 1 action in feeding and glucose homeostasis

Glucagon-like peptide 1 (GLP-1) is both a gut-derived hormone and a neurotransmitter synthesized in the brain. Early reports suggested that GLP-1 acts in the periphery to promote insulin secretion and affect glucose homeostasis, whereas central GLP-1 reduces food intake and body weight. However, current research indicates that in fact, GLP-1 in each location plays a role in these functions. This review summarizes the evidence for involvement of peripheral and brain GLP-1 in food intake regulation and glucose homeostasis and proposes a model for the coordinated actions of GLP-1 at multiple sites.

The effects of two different hypocaloric diets on glucagon-like peptide 1 in obese adults, relation with insulin response after weight loss

OBJECTIVE

Few studies have investigated the effect of type of diets on GLP-1 concentrations. The aim of this study was to compare the effect of two diets on circulating GLP-1 levels and the relation with insulin response after weight loss.

METHODS

A population of 118 obese patients were analyzed. Patients were randomly allocated to two groups: (a) Diet I (low carbohydrate) and (b) Diet II (low fat). Biochemical and anthropometric parameters were measured before and after 3 months of hypocaloric diet.

RESULTS

Fifty-two patients (12 male/40 female) were treated with Diet I and 66 patients (21 male/45 female) with Diet II. In Group I, basal GLP-1 levels did not change after dietary treatment (9.4+/-3.3 vs. 9.9+/-3.1 ng/ml; ns). In Group II, GLP-1 levels decreased significantly (8.4%) (9.2+/-3.3 vs 8.7+/-3.1 ng/ml; P<.05). In the multivariate analysis with a dependent variable (levels of GLP-1), only insulin levels remained as an independent predictor in the model (F=5.9; P<.05), with an increase of 0.6 ng/ml (95% CI 0.1-1.1) GLP-1 concentrations with each increase of 1 mUI/ml of insulin.

CONCLUSION

A hypocaloric diet with a low fat percentage decreased GLP-1 levels with a direct correlation with insulin levels. Nevertheless, patients with a hypocaloric diet with a low carbohydrate percentage treatment did not change GLP-1 levels. Diet macronutrient manipulation on GLP-1 response could be useful in an obesity nutrition therapy.

The role of incretins in glucose homeostasis and diabetes treatment

Incretins are gut hormones that are secreted from enteroendocrine cells into the blood within minutes after eating. One of their many physiological roles is to regulate the amount of insulin that is secreted after eating. In this manner, as well as others to be described in this review, their final common raison d'être is to aid in disposal of the products of digestion. There are two incretins, known as glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide-1 (GLP-1), that share many common actions in the pancreas but have distinct actions outside of the pancreas. Both incretins are rapidly deactivated by an enzyme called dipeptidyl peptidase 4 (DPP4). A lack of secretion of incretins or an increase in their clearance are not pathogenic factors in diabetes. However, in type 2 diabetes (T2DM), GIP no longer modulates glucose-dependent insulin secretion, even at supraphysiological (pharmacological) plasma levels, and therefore GIP incompetence is detrimental to beta-cell function, especially after eating. GLP-1, on the other hand, is still insulinotropic in T2DM, and this has led to the development of compounds that activate the GLP-1 receptor with a view to improving insulin secretion. Since 2005, two new classes of drugs based on incretin action have been approved for lowering blood glucose levels in T2DM: an incretin mimetic (exenatide, which is a potent long-acting agonist of the GLP-1 receptor) and an incretin enhancer (sitagliptin, which is a DPP4 inhibitor). Exenatide is injected subcutaneously twice daily and its use leads to lower blood glucose and higher insulin levels, especially in the fed state. There is glucose-dependency to its insulin secretory capacity, making it unlikely to cause low blood sugars (hypoglycemia). DPP4 inhibitors are orally active and they increase endogenous blood levels of active incretins, thus leading to prolonged incretin action. The elevated levels of GLP-1 are thought to be the mechanism underlying their blood glucose-lowering effects.

Differential secretion of satiety hormones with progression of obesity in JCR:LA-corpulent rats

OBJECTIVE

To characterize the gastrointestinal tract at the onset and in well-established obesity.

METHODS AND PROCEDURES

Lean (+/?) and obese (cp/cp) male JCR:LA-cp rats lacking a functional leptin receptor were killed at 3.5 weeks and 9 months of age and plasma concentrations of satiety hormones determined. The small intestine, colon, and stomach were measured, weighed, and mRNA levels of satiety genes quantified.

RESULTS

At the onset of obesity, obese rats had greater intestine, colon, and liver mass when adjusted for body weight compared to lean rats. Conversely, adult rats with established obesity had lower intestine and colon mass and length after adjustment for body weight. Early changes in gene expression included decreased ghrelin mRNA levels in stomach and increased peptide YY (PYY) mRNA levels in duodenum of young obese rats. After massive accumulation of adipose tissue had occurred, adult obese rats had increased proglucagon and ghrelin mRNA expression in the proximal intestine. In the distal small intestine, obese rats had lower proglucagon, ghrelin, and PYY mRNA levels. Finally, at the onset and in well-established obesity, obese rats had higher plasma insulin, amylin, glucagon like peptide-1 (GLP-1), and PYY, a finding, with the exception of insulin, unique to this model. Plasma total ghrelin levels were significantly lower at the onset of obesity and established obesity compared to the lean rats.

DISCUSSION

Several defects are manifested in the obese gut early on in the disease before the accumulation of large excesses of body fat and represent potential targets for early intervention in obesity.

Divergent leptin signaling in proglucagon neurons of the nucleus of the solitary tract in mice and rats

The central targets mediating the anorectic and other actions of leptin have yet to be fully identified. Although previous studies focused on the hypothalamus, leptin also acts on neurons in extrahypothalamic sites, including the nucleus of the solitary tract (NTS). Moreover, injection of leptin into the NTS of rats suppresses food intake. Within the central nervous system, glucagon-like peptide (GLP-1), a product of proglucagon, is synthesized almost exclusively in neurons of the NTS. Intracerebroventricular administration of GLP-1 inhibits energy intake, and GLP-1 receptor antagonists attenuate the anorexic effects of leptin in rats. To examine whether NTS proglucagon neurons are directly regulated by leptin, we performed double GLP-1 and phosphorylation of signal transducer and activator of transcription-3 immunohistochemistry on brain sections from ip leptin-treated mice and rats. Leptin induced phosphorylation of signal transducer and activator of transcription-3 in 100% of GLP-1 cells in the caudal brainstem of mice. In striking contrast, 0% of GLP-1-positive neurons in rats responded to leptin. We then measured regulation of NTS proglucagon mRNA using real-time RT-PCR in mice and rats fed ad libitum, fasted, or fasted and treated ip with leptin. In mice, proglucagon mRNA fell by fasting, and this was prevented by leptin administration. In rats, by contrast, proglucagon mRNA was unaffected by either fasting or leptin. Taken together, our studies reveal direct regulation of proglucagon neurons by leptin in mice but not rats along with corresponding species differences in the regulation of proglucagon mRNA expression. These data, combined with previous results, suggest a different mechanism of interaction between leptin and NTS proglucagon neurons in mice and rats.

Leptin but not neuropeptide Y up-regulated glucagon-like peptide 1 receptor expression in GT1-7 cells and rat hypothalamic slices

In an attempt to gain better insight into the central effects of glucagon-like peptide (GLP-1), we studied the action of glucose and of regulatory peptides on the expression of its receptor (GLP-1R) in hypothalamic GT1-7 cells and in ventromedial (VMH) and lateral (LH) rat hypothalamus slices. The promoter activity of GLP-1R in transfected GT1-7 cells increased with leptin, whereas neuropeptide Y (NPY) did not modify it. Interestingly, when cells were incubated with both NPY and leptin, NPY blocked the stimulating effect of leptin. The effects of leptin and NPY were also confirmed at messenger RNA levels. In hypothalamic slices, GLP-1R messenger RNA levels increased at higher glucose concentrations in the VMH. In addition, leptin exerted a stimulating effect; and NPY did not modify receptor expression. By contrast, in the LH, the opposite effects were found for those parameters, except at 20 mmol/L glucose. These findings suggest that the stimulating effect of leptin on GLP-1R expression, with no changes in NPY-induced activity, could enhance the anorexic actions generated through this receptor. In addition, the different responses of the VMH and LH may be related to specific functions of these structures, as already known in vivo, highlighting the interest of hypothalamic slices for this kind of study.

Upregulation of the brainstem preproglucagon system in the obese Zucker rat

A group of neurons in the caudal nucleus of the solitary tract (NTS) processes preproglucagon to glucagon-like peptide 1 (GLP-1), GLP-2 and oxyntomodulin. Whereas the anorectic capacity of all three neuropeptides has been demonstrated, only relatively little is known of preproglucagon mRNA regulation in the brain stem. Using in situ hybridization and fluorescence immunohistochemistry, we examined hindbrain preproglucagon expression in lean and obese Zucker rats under different metabolic perturbations. First, the effect of an acute 48-h fast was examined in male Sprague-Dawley as well as in lean and obese Zucker rats. Whereas fasting had no effect on preproglucagon expression in either genotype, mRNA levels were strongly up regulated in obese Zucker rats. Using a direct immunostaining procedure and a monoclonal GLP-2 antibody, we found a doubling of the immunofluorescence signal emanating from the preproglucagon neurons in caudal brainstem suggesting that indeed the high mRNA levels observed using in situ hybridization histochemistry also reflect a higher translational activity. To investigate the effects of long-term body weight perturbations, lean and obese Zucker rats were either free-fed, voluntarily overfed (chocolate spread enriched chow) or food restricted for 35 days. Preproglucagon levels remained high in the obese Zucker rats irrespective of diet. Finally, in order to functionally validate the apparent hyperactivity in the preproglucagon system in the Zucker rat, we examined the effect of central GLP-1 receptor blockade. ICV administration of 20 microg of the GLP-1 receptor antagonist Des-His-Exendin-9-39 in the morning increased 4-h food intake in obese but not in lean Zucker rats, pointing to an increased activity in central preproglucagon containing pathways in leptin receptor deficient rats. Our data suggest that the preproglucagon neurons in the brainstem are influenced by leptin signaling and point to a role of preproglucagon neurons in the integration of metabolic signals that occurs in the nucleus of the solitary tract.

Decreased basal levels of glucagon-like peptide-1 after weight loss in obese subjects

OBJECTIVE

Basal glucagon-like peptide-1 (GLP-1) concentrations seem to be attenuated in obese subjects, although statistical significance is unclear. Only a few studies have investigated the effect of weight reduction on GLP-1 concentrations and have found unclear results. The aim of the present study was to determine whether subjects who lose weight on a hypocaloric diet experience the same change in circulating GLP-1 levels as subjects who do not lose weight.

MATERIAL AND METHODS

A population of 99 obese nondiabetic outpatients was analyzed in a prospective way. Weight and blood pressure were determined. Basal glucose, C-reactive protein, insulin, total cholesterol, LDL cholesterol, HDL cholesterol, triglycerides, and basal GLP-1 blood levels were measured before and after 3 months of hypocaloric diet. Bipolar impedance examination was performed in all patients to assess body composition. The lifestyle modification program consisted of a hypocaloric diet (1,520 kcal, 52% carbohydrates, 25% lipids and 23% proteins). The exercise program consisted of aerobic exercise for at least 3 times per week (60 min each).

RESULTS

Ninety-nine patients (20 male/79 female) gave informed consent and were enrolled in the study. Fourteen patients (2 male/12 female) did not lose weight (group I: weight increase of 2 +/- 1.1 kg, NS) and 75 patients (18 male/67 female) lost weight (group II, weight loss of 4 +/- 1.6 kg, p < 0.05). Weight, body mass index, fat mass, waist circumference, insulin, HOMA, LDL cholesterol, triglycerides and systolic blood pressure improved in group II, without significant statistical changes in group I. In group I, basal GLP-1 levels remained unchanged (7.4 +/- 3.1 vs. 7.15 +/- 3.6 ng/ml, NS). In group II, GLP-1 levels decreased significantly (8.4%, 6.88 +/- 2.5 vs. 6.3 +/- 2.4 ng/ml, p < 0.05). In the multivariate analysis with a dependent variable (levels of GLP-1 after hypocaloric diet adjusted by age and sex), only insulin levels remained as an independent predictor in the model (F = 5.9; p < 0.05), with an increase of 0.6 ng/ml (CI 95%: 0.1-1.1) in GLP-1 concentrations with each 1-mIU/ml increase of insulin.

CONCLUSION

Hypocaloric diet decreased GLP-1 levels in patients with weight loss with a significant improvement in anthropometric parameters and cardiovascular risk factors. Nevertheless, patients without weight loss after dietary treatment exhibited unchanged GLP-1 levels. Basal insulin correlates with basal GLP-1.

The incretins: a link between nutrients and well-being

The glucoincretins, glucagon-like peptide-1 (GLP-1) and gastric inhibitory peptide (GIP), are intestinal peptides secreted in response to glucose or lipid intake. Data on isolated intestinal tissues, dietary treatments and knockout mice strongly suggest that GIP and GLP-1 secretion requires glucose and lipid metabolism by intestinal cells. However, incretin secretion can also be induced by non-digestible carbohydrates and involves the autonomic nervous system and endocrine factors such as GIP itself and cholecystokinin. The classical pharmacological approach and the recent use of knockout mice for the incretin receptors have shown that a remarkable feature of incretins is the ability to stimulate insulin secretion in the presence of hyperglycaemia only, hence avoiding any hypoglycaemic episode. This important role is the basis of ongoing clinical trials using GLP-1 analogues. Since most of the data concern GLP-1, we will focus on this incretin. In addition, GLP-1 is involved in glucose sensing by the autonomic nervous system of the hepato-portal vein controlling muscle glucose utilization and indirectly insulin secretion. GLP-1 has been shown to decrease glucagon secretion, food intake and gastric emptying, preventing excessive hyperglycaemia and overfeeding. Another remarkable feature of GLP-1 is its secretion by the brain. Recently, elegant data showed that cerebral GLP-1 is involved in cognition and memory. Experiments using knockout mice suggest that the lack of the GIP receptor prevents diet-induced obesity. Consequently, macronutrients controlling intestinal glucose and lipid metabolism would control incretin secretion and would consequently be beneficial for health. The control of incretin secretion represents a major goal for new therapeutic as well as nutrition strategies for treating and/or reducing the risk of hyperglycaemic syndromes, excessive body weight and thus improvement of well-being.

Gut peptides in the regulation of food intake and energy homeostasis

Gut hormones signal to the central nervous system to influence energy homeostasis. Evidence supports the existence of a system in the gut that senses the presence of food in the gastrointestinal tract and signals to the brain via neural and endocrine mechanisms to regulate short-term appetite and satiety. Recent evidence has shown that specific gut hormones administered at physiological or pathophysiological concentrations can influence appetite in rodents and humans. Gut hormones therefore have an important physiological role in postprandial satiety, and gut hormone signaling systems represent important pharmaceutical targets for potential antiobesity therapies. Our laboratory investigates the role of gut hormones in energy homeostasis and has a particular interest in this field of translational research. In this review we describe our initial studies and the results of more recent investigations into the effects of the gastric hormone ghrelin and the intestinal hormones peptide YY, pancreatic polypeptide, glucagon-like peptide-1, and oxyntomodulin on energy homeostasis. We also speculate on the role of gut hormones in the future treatment of obesity.

The hypothalamus, hormones, and hunger: alterations in human obesity and illness

Obesity is a major global epidemic, with over 300 million obese people worldwide, and nearly 1 billion overweight adults. Being overweight carries significant health risks, reduced quality of life, and impaired socioeconomic success, with profound consequences for health expenditure. The most successful treatment for obesity is gastric bypass surgery, which acts in part by reducing appetite through alterations in gut hormones. Circulating gut hormones, secreted or suppressed after eating food, act in the brain, particularly the hypothalamus, to alter hunger and fullness. Stomach-derived ghrelin increases food intake even in those with anorexia from chronic illness, while pancreatic polypeptide (PP), intestinal peptide YY 3-36 (PYY), oxyntomodulin, and other hormones reduce food intake and appetite. While obese subjects have appropriate reductions in orexigenic ghrelin, other gut-hormone disturbances may contribute to obesity such as reduced anorexigenic PYY and PP. Prader-Willi syndrome (PWS) arises from the loss of paternally inherited genes on chromosome 15q11-13, leading to life-threatening insatiable hunger and obesity from early childhood, through developmental brain, particularly hypothalamic defects. The study of genetically homogenous causes of abnormal-feeding behavior helps our understanding of appetite regulation. PWS subjects have inappropriately elevated plasma ghrelin for their obesity, at least partly explained by preserved insulin sensitivity. It remains unproven if their hyperghrelinemia or other gut-hormone abnormalities contribute to the hyperphagia in PWS, in addition to brain defects. Postmortem human hypothalamic studies and generation of animal models of PWS can also provide insight into the pathophysiology of abnormal-feeding behavior. Changes in orexigenic NPY and AGRP hypothalamic neurons, or anorexigenic oxytocin neurons have been found in illness and PWS. Functional neuroimaging studies, using PET and fMRI, will also allow us to tease apart the hormonal and brain pathways responsible for controlling human appetite, and their defects in obesity.

Brain glucagon-like peptide-1 increases insulin secretion and muscle insulin resistance to favor hepatic glycogen storage

Intestinal glucagon-like peptide-1 (GLP-1) is a hormone released into the hepatoportal circulation that stimulates pancreatic insulin secretion. GLP-1 also acts as a neuropeptide to control food intake and cardiovascular functions, but its neural role in glucose homeostasis is unknown. We show that brain GLP-1 controlled whole-body glucose fate during hyperglycemic conditions. In mice undergoing a hyperglycemic hyperinsulinemic clamp, icv administration of the specific GLP-1 receptor antagonist exendin 9-39 (Ex9) increased muscle glucose utilization and glycogen content. This effect did not require muscle insulin action, as it also occurred in muscle insulin receptor KO mice. Conversely, icv infusion of the GLP-1 receptor agonist exendin 4 (Ex4) reduced insulin-stimulated muscle glucose utilization. In hyperglycemia achieved by i.v. infusion of glucose, icv Ex4, but not Ex9, caused a 4-fold increase in insulin secretion and enhanced liver glycogen storage. However, when glucose was infused intragastrically, icv Ex9 infusion lowered insulin secretion and hepatic glycogen levels, whereas no effects of icv Ex4 were observed. In diabetic mice fed a high-fat diet, a 1-month chronic i.p. Ex9 treatment improved glucose tolerance and fasting glycemia. Our data show that during hyperglycemia, brain GLP-1 inhibited muscle glucose utilization and increased insulin secretion to favor hepatic glycogen stores, preparing efficiently for the next fasting state.

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.

Glucagon like peptide-1 (7-36) amide (GLP-1) nerve terminals densely innervate corticotropin-releasing hormone neurons in the hypothalamic paraventricular nucleus

Glucagon like peptide-1 (7-36) amide (GLP-1), a potent regulator of glucose homeostasis, is also produced in the central nervous system and has been implicated in the control of hypothalamic-pituitary function and food intake. GLP-1 immunoreactive (IR) fibers and terminals are widely distributed in the septum, hypothalamus, thalamus and brainstem, likely originating from GLP-1-IR neuronal cell bodies from the nucleus of the solitary tract of the medulla oblongata. Central administration of GLP-1 increases plasma corticosterone levels and elicits c-fos expression in corticotropin releasing hormone (CRH) neurons of the hypothalamic paraventricular nucleus (PVN). To identify the endogenous neurocircuitry that may underlie this response, the present study determined whether there is an innervation of PVN CRH neurons by GLP-1-containing nerve terminals. GLP-1-IR fibers and nerve terminals were found in the parvocellular parts of the PVN, with highest concentrations in the anterior and medial parvocellular subdivisions. The magnocellular divisions of the PVN also showed moderate numbers of GLP-1-IR nerve fibers. Double immunolabelling revealed numerous GLP-1-IR nerve fibers in close apposition to approximately 65% of detectable CRH neurons in the medial parvocellular subdivision of the rat PVN. At the ultrastructural level, GLP-1-IR terminals were observed to establish synapses on both perikarya and dendrites of CRH neurons. These findings support the hypothesis that the GLP-1-induced activation of CRH neurons and the associated pituitary-adrenocortical activation may be accomplished by GLP-1's direct action on hypophysiotropic CRH neurons. Since central CRH is also thought to be an anorexigenic factor and GLP-1 neurons contain leptin receptors, activation of CRH neurons in the PVN by GLP-1 may contribute to the complex neuroendocrine and metabolic actions by the adipostatic hormone, leptin.

Effect of leptin on the acute feeding-induced hypothalamic serotonergic stimulation in normal rats

Both hypothalamic serotonin and leptin reduce energy intake and stimulate expenditure. There are evidence that increased serotonin metabolism may be involved in leptin actions. Using microdialysis, we directly assessed the effect of an intracerebroventricular leptin injection on 5-HT release in the lateral hypothalamus of normal rats. When LH microdialysates were collected in the absence of food intake, neither artificial cerebrospinal fluid (CSF) nor 10 microg leptin i.c.v. caused significant variations in 5-HT release, measured for 2 h post-injection, at 20-min periods. When food was ingested after CSF, 5-HT release increased significantly, with a maximal elevation of 51+/-16% above baseline occurring at the 100-120 min post-injection interval. Leptin inhibited food intake (-75% at 0-20 min and -73% at 20-40 min) while it accentuated the food-induced serotonergic activation. At the 0-20 min interval, serotonin release was significantly higher after leptin (42+/-12% above baseline) than after CSF (6+/-5%) and the maximal increase after leptin was of 126+/-53% above baseline (100-120 min, p>0.05 vs. CSF). These observations indicate that leptin probably interacts with the serotonergic-stimulating mechanisms elicited by food intake, intensifying them. The additional serotonergic activation induced by leptin may be significant for the hormone effects on energy balance.

Anorectic brainstem peptides: more pieces to the puzzle

Eating a meal is a mechanical process involving autonomous pathways that relay sensory and motor information between the whole length of the digestive tract and the central nervous system. This circuitry is able to initiate and terminate the meal, primarily by gut-brainstem-gut reflex arcs, and is independent of the caloric content of a meal. However, as part of our ability to regulate body weight over time, we must be able to modulate the amount of energy that we take in as food and the amount of energy that we expend. Thus, the gut-brainstem axis must be coupled to other systems that take account of factors such as food availability and preference, changing energy requirements and our social habits. Here, we review the importance of the brainstem nucleus of the tractus solitarius as a site of integration and the routes by which it connects the gut-brainstem axis with regulatory neuronal and endocrine networks that allow for strict body weight management.

The diverse roles of specific GLP-1 receptors in the control of food intake and the response to visceral illness

Intracerebroventricular administration of glucagon-like peptide-1 (7-36) amide (GLP-1) reduces food intake and produces symptoms of visceral illness, such as a conditioned taste aversion (CTA). The central hypothesis of the present work is that separate populations of GLP-1 receptors mediate the anorexia and taste aversion associated with GLP-1 administration. To test this hypothesis, we first compared the ability of various doses of GLP-1 to induce anorexia or CTA when administered into either the lateral or fourth ventricle. Lateral and fourth ventricular GLP-1 resulted in reduction of food intake at similar doses, whereas only lateral ventricular GLP-1 resulted in a CTA. Such data indicate that both hypothalamic and caudal brainstem GLP-1 receptors are likely to participate in the ability of GLP-1 to reduce food intake. We also hypothesized that the site that must mediate the ability of GLP-1 to induce visceral illness is in the central nucleus of the amygdala (CeA). Administration of 0.2 or 1.0 microg of GLP-1 (7-36) but not the inactive GLP-1 (9-36) resulted in a strong CTA with no accompanying anorexia. In addition, bilateral CeA administration of 2.5 microg of a GLP-1 receptor antagonist before intraperitoneal administration of the toxin lithium chloride resulted in a diminished CTA. Together, these data indicate that separate GLP-1 receptor populations mediate the multiple responses to GLP-1. These results indicate that GLP-1 is a flexible system that can be activated under various circumstances to alter the ingestion of nutrients and/or produce other visceral illness responses, depending on the ascending pathways of the GLP-1 system that are recruited.

Neuropeptides, food intake and body weight regulation: a hypothalamic focus

Energy homeostasis is controlled by a complex neuroendocrine system consisting of peripheral signals like leptin and central signals, in particular, neuropeptides. Several neuropeptides with anorexigenic (POMC, CART, and CRH) as well as orexigenic (NPY, AgRP, and MCH) actions are involved in this complex (partly redundant) controlling system. Starvation as well as overfeeding lead to changes in expression levels of these neuropeptides, which act downstream of leptin, resulting in a physiological response. In this review the role of several anorexigenic and orexigenic (hypothalamic) neuropeptides on food intake and body weight regulation is summarized.

Ups and downs for neuropeptides in body weight homeostasis: pharmacological potential of cocaine amphetamine regulated transcript and pre-proglucagon-derived peptides

Although most humans experience an underlying upwards drift of the body-weight set-point, body weight appears tightly regulated throughout life. The present review describes the structural basis of the adipostat and hypothesise, which components may constitute available targets for pharmacotherapy of excess body weight. Hypothalamic neurones constitute the major components of the body weight homeostasis maintaining device. Together with neurones of the nucleus of the solitary tract, neurones of the hypothalamic arcuate nucleus constitute the sensory components of the adipostat. The arcuate nucleus neurones respond to circulating levels of leptin and insulin, both of which reflect the levels of energy stored as triacylglycerol in adipocytes. The arcuate nucleus projects heavily to the hypothalamic paraventricular nucleus. Neurones of the hypothalamic paraventricular nucleus are hypothesised to constitute, at least partly, the adipostat motor pattern generator, which upon stimulation activates either net anabolic or catabolic physiological responses. The overall sensitivity of the adipostat is influenced by gain setting neurones hypothesised to be located in the dorsomedial hypothalamic nucleus and lateral hypothalamic area. Cocaine amphetamine regulated transcript (CART) peptides and pre-proglucagon derived peptides, glucagon-like peptide-1 (GLP-1) and glucagon-like peptide-2 (GLP-2) are catabolic neurotransmitters synthesised in neurones of the arcuate nucleus and the nucleus of the solitary tract, respectively. The present review summarises the available evidence that both families of peptides constitute endogenous transmitters mediating satiety and touch upon potential pharmacological exploitation of this knowledge.

Orexins and feeding: special occasions or everyday occurrence?

Neurons expressing prepro-orexin, the precursor of orexin-A and -B, are found in the lateral hypothalamic area, a region classically implicated in driving feeding. Orexin-A induces feeding transiently when injected centrally, and food intake can be decreased when orexin action is disrupted by immunoneutralization of orexin-A, or by pharmacological blockade of orexin receptors, or by transgenic knockout of orexin. Here, we argue that orexin neurons may act to stimulate feeding in the short term, and that important regulatory signals may be a fall in plasma glucose (stimulatory), countered by satiety signals generated by eating, such as gastric distention (inhibitory).

The gut and food intake: an update for surgeons

Food intake is the simplest and most obvious measure of gastrointestinal function, yet it rarely receives more than cursory attention from surgeons. In this review we cover recent findings on relationships between gut function and appetite regulation mediated via neuropeptides influenced by afferent and efferent vagal activity. Evidence from the new discipline known as neurogastroenterology elucidates gastric and intestinal signals involved in the elicitation of hunger, satiety, and aversion. Discovery of the adipose-tissue-derived hormone, leptin, has energized the field of metabolism spawning increasing numbers of publications related to interactions between leptin and insulin release and glucose disposal, as well as appetitive behavior. Peptides such as cholecystokinin (CCK), the proglucagon-derived peptides, glucagon-like peptides 1 and 2 (GLP-1 and GLP-2), and the recently identified powerful intake-stimulating molecule, orexin, are examples of potential targets for drug development and studies of surgical pathophysiology. A major conclusion of this work is that the considerable redundancy and overlap between mediators of caloric intake subserving survival of the species, while beneficial after foregut surgery, contribute to the complexity of treating the global epidemic of obesity. Possibly knowledge derived from basic research in neurogastroenterology can translate into advances in surgical treatment of obesity.

Degradation and glycemic effects of His(7)-glucitol glucagon-like peptide-1(7-36)amide in obese diabetic ob/ob mice

Glucagon-like peptide-1(7-36)amide (tGLP-1) has attracted considerable potential as a possible therapeutic agent for type 2 diabetes. However, tGLP-1 is rapidly inactivated in vivo by the exopeptidase dipeptidyl peptidase IV (DPP IV), thereby terminating its insulin releasing activity. The present study has examined the ability of a novel analogue, His(7)-glucitol tGLP-1 to resist plasma degradation and enhance the insulin-releasing and antihyperglycemic activity of the peptide in 20-25-week-old obese diabetic ob/ob mice. Degradation of native tGLP-1 by incubation at 37 degrees C with obese mouse plasma was clearly evident after 3 h (35% intact). After 6 h, more than 87% of tGLP-1 was converted to GLP-1(9-36)amide and two further N-terminal fragments, GLP-1(7-28) and GLP-1(9-28). In contrast, His(7)-glucitol tGLP-1 was completely resistant to N-terminal degradation. The formation of GLP-1(9-36)amide from native tGLP-1 was almost totally abolished by addition of diprotin A, a specific inhibitor of DPP IV. Effects of tGLP-1 and His(7)-glucitol tGLP-1 were examined in overnight fasted obese mice following i.p. injection of either peptide (30 nmol/kg) together with glucose (18 mmol/kg) or in association with feeding. Plasma glucose was significantly lower and insulin response greater following administration of His(7)-glucitol tGLP-1 as compared to glucose alone. Native tGLP-1 lacked antidiabetic effects under the conditions employed, and neither peptide influenced the glucose-lowering action of exogenous insulin (50 units/kg). Twice daily s.c. injection of ob/ob mice with His(7)-glucitol tGLP-1 (10 nmol/kg) for 7 days reduced fasting hyperglycemia and greatly augmented the plasma insulin response to the peptides given in association with feeding. These data demonstrate that His(7)-glucitol tGLP-1 displays resistance to plasma DPP IV degradation and exhibits antihyperglycemic activity and substantially enhanced insulin-releasing action in a commonly used animal model of type 2 diabetes.