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

Agouti-related protein-deficient mice display an age-related lean phenotype

Endogenous modulators of the central melanocortin system, such as the agouti-related protein (AgRP), should hold a pivotal position in the regulation of energy intake and expenditure. Despite this, AgRP-deficient mice were recently reported to exhibit normal food intake, body weight gain, and energy expenditure. Here we demonstrate that 2- to 3-month-old Agrp null mice do in fact exhibit subtle changes in response to feeding challenges (fasting and MCR agonists) but, of more significance and magnitude, exhibit reduced body weight and adiposity after 6 months of age. This age-dependent lean phenotype is correlated with increased metabolic rate, body temperature, and locomotor activity and increased circulating thyroid hormone (T4 and T3) and BAT UCP-1 expression. These results provide further proof of the importance of the AgRP neuronal system in the regulation of energy homeostasis.

Ghrelin, the same peptide for different functions: player or bystander?

Ghrelin is a recently discovered brain-gut peptide with two main physiological actions: growth hormone secretagogue activity and food intake inducer. Although its production is prevalently gastric, ghrelin is widely expressed in several tissues, where it might therefore act as a paracrine or autocrine factor. It is becoming clear that ghrelin is much more than a simple growth hormone secretagogue. In addition to its formerly envisaged role, ghrelin has other activities including stimulation of pituitary hormones secretion, modulation of food intake and control of energy metabolism, regulation of gastric and pancreatic activity, and cardiovascular and hemodynamic activities. In addition, modulation of cartilage and bone homeostasis, sleep and behavioral influences, and modulation of the immune system, as well as effects on cell proliferation, are other relevant actions of ghrelin. Thus, the peptide appears to be an important component of an integrated multifaceted regulatory system. In this review, we summarize several aspects of ghrelin biology and attempt to inform the reader with information regarding unexpected functions of this gastric peptide.

Effect of a selective OX1R antagonist on food intake and body weight in two strains of rats that differ in susceptibility to dietary-induced obesity

An orexin-1 receptor antagonist decreases food intake whereas orexin-A selectively induces hyperphagia to a high-fat diet. In the present study, we evaluated the effect of an orexin antagonist in two strains of rats that differ in their sensitivity to becoming obese while eating a high-fat diet. Male Osborne-Mendel (OM) and S5B/Pl (S5B) rats were treated acutely with an orexin-1 receptor antagonist (SB-334867), after adaptation to either a high-fat (56% fat energy) diet or a low-fat (10% fat energy) diet that were equicaloric for protein (24% energy). Ad libitum fed rats were injected intraperitoneally with SB-334867 at doses of 3, 10 or 30 mg/kg, or vehicle at the beginning of the dark cycle, and food intake and body weight were measured. Hypothalamic prepro-orexin and orexin-1 receptor mRNA expression were analyzed in OM and S5B rats fed at a high-fat or low-fat diet for two weeks. SB-334867 significantly decreased food intake in both strains of rats eating the high-fat diet but only in the OM rats eating the low fat diet. The effect was greatest at 12 and 24 h. Body weight was also reduced in OM rats 1d after injection of SB-334867 but not in the S5B rats. Prepro-orexin and orexin-1 receptor expression levels did not differ between strains or diets. These experiments demonstrate that an orexin antagonist (SB-334867) reduces food intake and has a greater effect in a rat strain that is susceptible to dietary-induced obesity, than in a resistant strain.

THE COW AS A MODEL TO STUDY FOOD INTAKE REGULATION

Animal models have been invaluable for studying aspects of food intake regulation that for various reasons cannot be observed in humans. The dairy cow is a unique animal model because of an unrivaled energy requirement; its great drive to eat results in feeding behavior responses to treatments within the physiological range. Cows' docile nature and large size make them ideal for measuring temporal treatment effects because digestion and absorption kinetics and responses in endocrine systems, gene expression, metabolite pools and fluxes, and feeding behavior can be measured simultaneously. Thus, cows are important models to investigate interactions of short-term signals regulating food intake. Furthermore, different physiological states throughout the lactation cycle provide powerful models to study how short- and long-term signals interact to affect long-term energy status. The use of the cow as a model can lead to breakthroughs in understanding the complex interactions of signals regulating food intake.

Exogenously administered leptin leads to weight loss and increased physical activity in the marsupial Sminthopsis crassicaudata

The adipose tissue derived cytokine leptin, modifies energy balance via effects on both food intake and energy expenditure. It is not clear, however, whether the component of energy expenditure accounted for by voluntary (nonexercise) physical activity is increased in response to leptin. The aim of this study was to investigate the effect of exogenously administered leptin on physical activity in the marsupial Sminthopsis crassicaudata. Body weight, tail width and food intake, were measured daily and physical activity was measured hourly in normal lean S. crassicaudata (n=8) with ad libitum access to standard laboratory diet. After 5 days baseline the animals were divided into two equal groups (n=4), and either human recombinant leptin (2.5 mg/kg) or placebo was administered twice daily intraperitoneally. Approximately 81% of the total daily activity during the baseline period occurred during the nocturnal phase. After 9 days of leptin administration, there were significant decreases in body weight (P<0.001) and fat content (P<0.01), which were not accompanied by a decrease in total energy intake. Overall daily physical activity increased (P=0.028); this effect was confined to the dark phase (P=0.033). We conclude that in lean S. crassicaudata the exogenous administration of human recombinant leptin results in a decrease in adiposity which occurs in the absence of a measurable effect on food intake and is associated with an increase in non-exercise physical activity at least over the duration of this study.

Obesity and the neuroendocrine control of energy homeostasis: the role of spontaneous locomotor activity

Obesity represents one of the most urgent global health threats as well as one of the leading causes of death throughout industrialized nations. Efficacious and safe therapies remain at large. Attempts to decrease fat mass via pharmacological reduction of energy intake have had limited potency or intolerable side effects. Increasingly widespread sedentary lifestyle is often cited as a major contributor to the increasing prevalence of obesity. Moreover, low levels of spontaneous physical activity (SPA) are a major predictor of fat mass accumulation during overfeeding in humans, pointing to a substantial role for SPA in the control of energy balance. Despite this, very little is known about the molecular mechanisms by which SPA is regulated. The overview will attempt to summarize available information on neuroendocrine factors regulating SPA.

Roles for ghrelin in the regulation of appetite and body weight

PURPOSE OF REVIEW

Ghrelin, the only known circulating appetite stimulant, has garnered widespread scientific interest and is currently featured in more than 2.3 new publications per day. Antagonists and agonists of its receptor are vigorously being developed by pharmaceutical companies to treat obesity and wasting conditions respectively. Here, the authors summarize the current state of knowledge regarding ghrelin's roles in energy homeostasis.

RECENT FINDINGS

Ghrelin is an acylated peptide produced primarily by the stomach and proximal small intestine. Circulating levels sharply increase before, and decrease after, meals. These and other findings implicate ghrelin in pre-meal hunger and meal initiation. Moreover, ghrelin satisfies established criteria for an adiposity-associated hormone involved in long-term body weight regulation. Blood levels correlate with energy stores and display compensatory changes in response to alterations of those stores. Ghrelin influences neuronal activity in brain areas critical to energy homeostasis. Excessive ghrelin signaling durably increases food intake and decreases energy expenditure, thereby promoting weight gain. Conversely, acute ghrelin blockade in adult animals reduces food intake and body weight, although the effects of lifelong genetic deletions are very subtle. Overproduction of ghrelin is etiologically implicated in Prader-Willi syndrome, whereas defective secretion may contribute to weight loss after gastric bypass surgery.

SUMMARY

Ghrelin appears to participate in mealtime hunger and meal initiation, as well as in long-term energy balance. Whether these roles are sufficiently important that ghrelin blockade will prove to be an effective antiobesity modality is a pivotal question that should soon be answered as ghrelin receptor antagonists are developed. Ghrelin agonists hold promise in the treatment of wasting conditions, for which few medications currently exist.

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.

Influencing the between-feeding and endocrine responses of plasma ghrelin in healthy dogs

OBJECTIVES

Ghrelin has recently been isolated from rat and human stomach as an endogenous ligand for the growth hormone (GH) secretagog receptor. Using beagle dogs, we investigated the distribution of ghrelin in the stomach and its possible role.

METHODS

We examined: (i) GH release in response to ghrelin injection (0.5 or 5 microg/kg, i.v.), (ii) gastric localization of ghrelin-immunostained cells, (iii) changes in daily food consumption after ghrelin injection (3, 10, and 20 microg/kg, i.v.), (iv) plasma ghrelin levels under regular, but restricted feeding conditions, and (v) variations in plasma ghrelin levels in relatively lean, normal and obese dogs.

RESULTS

Administration of ghrelin to dogs promptly increased circulating GH concentrations, although this effect was transitory and was maintained for only 20 min. Ghrelin was localized in the stomach fundus and body, but none was detected in either the pylorus or cardia. Administration of ghrelin at a dose of 20 microg/kg increased the daily food intake of beagle dogs. Plasma ghrelin levels peaked just before meal times, and then returned to basal levels. Obese dogs had higher plasma ghrelin levels than did normal and lean dogs.

CONCLUSIONS

These results indicate that ghrelin is a potent GH secretagog in dogs. The distribution of ghrelin-immunoreactive cells in the canine stomach resembles that of both the murine and human stomach. Ghrelin participates in the control of feeding behavior and energy homeostasis in dogs and may, therefore, be involved in the development of obesity.