Delayed short-term secretory regulation of ghrelin in obese animals: evidenced by a specific RIA for the active form of ghrelin

GLP-1 and ghrelin inversely regulate insulin secretion and action in pancreatic islets, vagal afferents, and hypothalamus for controlling glycemia and feeding

Glucagon-like peptide-1 (GLP-1) was discovered as an incretin hormone, which is released from the intestine upon nutrient intake and stimulates insulin secretion from the pancreatic islet β-cells. Subsequently, its ability to suppress appetite was recognized. Ghrelin, discovered as the ligand for growth hormone secretagogue-receptor (GHS-R), is released from the stomach and produces appetite. Later, its ability to inhibit insulin secretion and elevate blood glucose was found. Thus, GLP-1 and ghrelin regulate insulin secretion and appetite toward opposite directions. The receptor agonists for GLP-1 and ghrelin have been developed and are now used to treat metabolic diseases, in which insulin plays a key role. However, underlying action mechanism and possible interplay of these hormones have remained elusive. Here, we describe that GLP-1 and ghrelin reciprocally regulate the insulin system. GLP-1 enhances and ghrelin suppresses insulin secretion in pancreatic β-cells. Moreover, GLP-1 cooperates with and ghrelin counteracts insulin action in the vagal afferent and hypothalamic arcuate nucleus (ARC) neurons, the interfaces between the peripheral metabolism and brain. Notably, ghrelin rises and works preprandially and GLP-1 rises and works postprandially. The interplay of ghrelin, GLP-1, and insulin leads to optimal circadian control of feeding, glycemia, and metabolism.

Ghrelin Bridges DMV Neuropathology and GI Dysfunction in the Early Stages of Parkinson's Disease

Ghrelin contributes to the communication between the brain and gastrointestinal (GI) tract. Both decreased ghrelin levels and functional GI disorders are early events in Parkinson's disease (PD) patients and animal models. However, the reason is not clear. Here it is found that choline acetyltransferase (ChAT)-positive neurons in the dorsal motor nucleus of the vagus nerve (DMV), are lost in PD transgenic mice. In response to the selective damaging of DMV neurons with mu p75-SAP, a rapid reduction both in plasma total and active ghrelin levels is observed. While by contrast, chemogenetic activation of DMV cholinergic neurons can increase the plasma ghrelin levels. Impairment of cholinergic neurons is accompanied by GI disorders, including decreased stool wet weight, stool dry weight, small intestine advancing rate, and gastric emptying rate, while exogenous ghrelin treatment can partially ameliorate GI dysfunction of A53T α-synuclein transgenic mice. Using pseudorabies virus retrograde trace method, the existence of a direct pathway from the stomach fundus to the DMV is shown. Taken together, the findings suggest that the reduction in plasma ghrelin levels in the early stages of PD may be the result of the lesion of cholinergic neurons in the DMV, thus linking neurodegeneration and GI dysfunction in PD.

Status of ghrelin as an islet hormone and paracrine/autocrine regulator of insulin secretion

Ghrelin is expressed in the pancreatic islet cells as well as the stomach. In the perfused pancreas and isolated islets, GHS-R antagonism, ghrelin immunoneutralization and ghrelin-knockout (Ghr-KO) all increase glucose-induced insulin release. Thus, pharmacological, immunological and genetic blockades of ghrelin in the pancreatic islets all markedly augment glucose-induced insulin release, showing that islet-derived ghrelin physiologically restricts insulin release in rodents. In this review, we focus on the current understanding of the following key questions: 1) from which islet cells ghrelin is released, 2) on which islet cells ghrelin acts, and 3) mechanisms by which the islet-derived ghrelin inhibits insulin secretion.

Protective and Healing Effects of Ghrelin and Risk of Cancer in the Digestive System

Ghrelin is an endogenous ligand for the ghrelin receptor, previously known as the growth hormone secretagogue receptor. This hormone is mainly produced by endocrine cells present in the gastric mucosa. The ghrelin-producing cells are also present in other organs of the body, mainly in the digestive system, but in much smaller amount. Ghrelin exhibits a broad spectrum of physiological effects, such as stimulation of growth hormone secretion, gastric secretion, gastrointestinal motility, and food intake, as well as regulation of glucose homeostasis and bone formation, and inhibition of inflammatory processes. This review summarizes the recent findings concerning animal and human data showing protective and therapeutic effects of ghrelin in the gut, and also presents the role of growth hormone and insulin-like growth factor-1 in these effects. In addition, the current data on the possible influence of ghrelin on the carcinogenesis, its importance in predicting the risk of developing gastrointestinal malignances, as well as the potential usefulness of ghrelin in the treatment of cancer, have been presented.

MECHANISMS IN ENDOCRINOLOGY: The gut-brain axis: regulating energy balance independent of food intake

Obesity is a global pandemic with a large health and economic burden worldwide. Bodyweight is regulated by the ability of the CNS, and especially the hypothalamus, to orchestrate the function of peripheral organs that play a key role in metabolism. Gut hormones play a fundamental role in the regulation of energy balance, as they modulate not only feeding behavior but also energy expenditure and nutrient partitioning. This review examines the recent discoveries about hormones produced in the stomach and gut, which have been reported to regulate food intake and energy expenditure in preclinical models. Some of these hormones act on the hypothalamus to modulate thermogenesis and adiposity in a food intake-independent fashion. Finally, the association of these gut hormones to eating, energy expenditure, and weight loss after bariatric surgery in humans is discussed.

The Effect of the Dietary Inclusion of Crude Glycerin in Pre-Starter and Starter Diets for Piglets

The aim of this study was to evaluate the effect of the inclusion of crude glycerin in post-weaning diets for piglets on growth performance and digestibility. The study was carried out with a total of 360 piglets over a 39 day period. Animals were blocked by body weight (7.7 ± 0.86 kg) and allotted randomly to one of three dietary treatments containing 0, 2.5 or 5% glycerin (G0, G2.5 and G5, respectively). Considering the whole period, glycerin did not affect the average daily gain. However, the average daily feed intake (ADFI) and the feed conversion ratio (FCR) tended to decrease or decreased linearly as the amount of glycerin increased, respectively (p = 0.060 and p = 0.039). The apparent total tract digestibility (ATTD) of dry and organic matter (DM, OM) increased linearly with increasing glycerin in both periods (p ≤ 0.05). At the end of the study, there were no differences between treatments for any of the hormones measured. In conclusion, the FCR and digestibility of DM and OM were improved although the ADFI tended to be lower when glycerin was included at 5%. Consequently, crude glycerin could be used as an alternative ingredient to partially replace sweet whey and wheat in post-weaning diets.

Mechanism of Action of Acupuncture in Obesity: A Perspective From the Hypothalamus

Obesity is a prevalent metabolic disease caused by an imbalance in food intake and energy expenditure. Although acupuncture is widely used in the treatment of obesity in a clinical setting, its mechanism has not been adequately elucidated. As the key pivot of appetite signals, the hypothalamus receives afferent and efferent signals from the brainstem and peripheral tissue, leading to the formation of a complex appetite regulation circuit, thereby effectively regulating food intake and energy homeostasis. This review mainly discusses the relationship between the hypothalamic nuclei, related neuropeptides, brainstem, peripheral signals, and obesity, as well as mechanisms of acupuncture on obesity from the perspective of the hypothalamus, exploring the current evidence and therapeutic targets for mechanism of action of acupuncture in obesity.

"A LEAP 2 conclusions? Targeting the ghrelin system to treat obesity and diabetes"

BACKGROUND

The hormone ghrelin stimulates food intake, promotes adiposity, increases body weight, and elevates blood glucose. Consequently, alterations in plasma ghrelin levels and the functioning of other components of the broader ghrelin system have been proposed as potential contributors to obesity and diabetes. Furthermore, targeting the ghrelin system has been proposed as a novel therapeutic strategy for obesity and diabetes.

SCOPE OF REVIEW

The current review focuses on the potential for targeting ghrelin and other proteins comprising the ghrelin system as a treatment for obesity and diabetes. The main components of the ghrelin system are introduced. Data supporting a role for the endogenous ghrelin system in the development of obesity and diabetes along with data that seemingly refute such a role are outlined. An argument for further research into the development of ghrelin system-targeted therapeutic agents is delineated. Also, an evidence-based discussion of potential factors and contexts that might influence the efficacy of this class of therapeutics is provided.

MAJOR CONCLUSIONS

It would not be a "leap to" conclusions to suggest that agents which target the ghrelin system - including those that lower acyl-ghrelin levels, raise LEAP2 levels, block GHSR activity, and/or raise desacyl-ghrelin signaling - could represent efficacious novel treatments for obesity and diabetes.

The Correlation Between Circulating Ghrelin and Insulin Resistance in Obesity: A Meta-Analysis

Background: Ghrelin, a peptide mainly produced by stomach X-A cells. It plays a pivotal role in the regulation of food intake and energy metabolism, including glucose metabolism and insulin sensitivity. However, the correlation between circulating ghrelin levels and insulin resistance in obesity remained uncertain. This meta-analysis aimed to clarify the association between ghrelin and IR in obesity.

Methods: A systematic literature search was performed using PubMed, EMBASE, Cochrane Library and Web of Science until April 18, 2018 with the keywords “ghrelin” and “insulin resistance.” Two independent reviewers selected studies and assessed data. Subgroup analyses were performed to search for sources of heterogeneity. Funnel plots and Egger's test were used to detect publication bias. A random-effects model was used to calculate the pooled effect size.

Results: Ten studies with 546 participants were included in this meta-analysis. We found that ghrelin levels were negatively correlated with IR in obese individuals. (r = −0.31; 95% CI: −0.45 to −0.18). Subgroup analysis revealed that circulating ghrelin levels were significantly negatively correlated with IR in people with normal fasting blood glucose (FBG) (<6.9 mmol/dl) (r = −0.28; 95% CI: −0.47 to −0.09, I² = 39.5%), while there was no relationship between circulating ghrelin levels and IR in the high FBG group (>6.9 mmol/dl) (r = −0.15; 95% CI: −0.33 to 0.03, I² = 0.0%). Publication bias was insignificant (Egger's test: P = 0.425).

Conclusion: In obesity, circulating ghrelin levels were significantly negative correlated with insulin resistance in individuals with normal fasting blood glucose.

Leptin Intake at Physiological Doses Throughout Lactation in Male Wistar Rats Normalizes the Decreased Density of Tyrosine Hydroxylase-Immunoreactive Fibers in the Stomach Caused by Mild Gestational Calorie Restriction

Introduction: Gestational under nutrition in rats has been shown to decrease expression of sympathetic innervation markers in peripheral tissues of offspring, including the stomach. This has been linked to lower gastric secretion and decreased circulating levels of ghrelin. Considering the critical role of leptin intake during lactation in preventing obesity and reversing adverse developmental programming effects, we aimed to find out whether leptin supplementation may reverse the above mentioned alterations caused by mild gestational calorie restriction. Methods: Three groups of male rats were studied at a juvenile age (25 days old) and during adulthood (3 and 6 months old): the offspring of ad libitum fed dams (controls), the offspring of dams that were diet restricted (20%) from days 1 to 12 of gestation (CR), and CR rats supplemented with a daily oral dose of leptin (equivalent to 5 times the average amount they could receive each day from maternal milk) throughout lactation (CR-Leptin). The density of TyrOH-immunoreactive (TyrOH⁺) fibers and the levels of Tyrosine hydroxylase (TyrOH)-used as potential markers of functional sympathetic innervation-were measured in stomach. Plasma leptin and ghrelin levels were also determined. Results: Twenty five-day-old CR rats, but not CR-Leptin rats, displayed lower density of TyrOH⁺ fibers (-46%) and TyrOH levels (-47%) in stomach compared to controls. Alterations in CR animals were mitigated at 6 months of age, and differences were not significant. Adult CR-Leptin animals showed higher plasma ghrelin levels than CR animals, particularly at 3 months (+16%), and a lower leptin/ghrelin ratio (-28 and -37% at 3 and 6 months, respectively). Conclusion: Leptin intake during lactation is able to reverse the alterations in the density of TyrOH⁺ fibers in the stomach and normalize the increased leptin/ghrelin ratio linked to a mild gestational calorie restriction in rats, supporting the relevance of leptin as an essential nutrient during lactation.

Intraventricular administration of Tenebrio molitor larvae extract regulates food intake and body weight in mice with high-fat diet-induced obesity

We recently reported the in vitro and in vivo antiobesity effects of Tenebrio molitor larvae, a traditional food in many countries, but it remains unknown how the larvae affect appetite regulation in mice with diet-induced obesity. We hypothesized that the extract of T molitor larvae mediates appetite by regulating neuropeptide expression. We investigated T molitor larvae extract's (TME's) effects on anorexigenesis and endoplasmic reticulum (ER) stress-induced orexigenic neuropeptide expression in the hypothalami of obese mice. Intracerebroventricular TME administration suppressed feeding by down-regulating the expression of the orexigenic neuropeptides neuropeptide Y and agouti-related protein. T molitor larvae extract significantly reduced the expression of ER stress response genes. These results suggest that TME and its bioactive components are potential therapeutics for obesity and ER stress-driven disease states.

Influence of feeding a fish oil-containing diet to mature, overweight dogs: Effects on lipid metabolites, postprandial glycaemia and body weight

The objective of this study was to determine the effect of feeding a fish oil (FO)-containing diet on lipid and protein metabolism, postprandial glycaemia and body weight (BW) of mature, overweight dogs. Seven female dogs were randomly assigned to one of two isonitrogenous and isocaloric diets, control (CO) or FO (FO), in a crossover design. Experimental periods were 69 day, separated by a washout period of 30 day. At the beginning of the experiment, and at 30 and 60 day of feeding the experimental diets, the dogs were infused with D-glucose (2 g/kg BW) through an intravenous catheter. Blood samples were collected for 3 hr to perform a glucose tolerance test. Nitrogen balance measurements began at 06:30 on d 63 of each experimental period and ended at 06:30 on d 69. On d 66 of each period, a single dose (7.5 mg/kg) of ¹⁵ N-glycine was administered orally for determination of protein turnover. Incremental area under the curve and glucose concentration at peak did not differ between treatments or among sampling days within treatment. Glucose half-life tended to decrease (p < .10) in the FO treatment on day 30 when compared to baseline (day 0). β-hydroxybutyrate, non-esterified fatty acid (NEFA) and triglycerides did not differ within or between treatments. Cholesterol decreased (p < .05) on the FO treatment on day 30, 60 and 69 when compared to day 0. High-density lipoprotein (HDL) decreased (p < .05) in the FO treatment on day 69 when compared to day 0. Body weight, food intake, faecal excretion, DM and N digestibilities, N balance and protein turnover were not different between diets. Overall, FO-containing diet decreases cholesterol in mature overweight dogs; however, further research is warranted to verify the effects of FO on glucose metabolism.

Ghrelin affects stopover decisions and food intake in a long-distance migrant

Billions of birds migrate long distances to either reach breeding areas or to spend the winter at more benign places. On migration, most passerines frequently stop over to rest and replenish their fuel reserves. To date, we know little regarding how they decide that they are ready to continue their journey. What physiological signals tell a bird's brain that its fuel reserves are sufficient to resume migration? A network of hormones regulates food intake and body mass in vertebrates, including the recently discovered peptide hormone, ghrelin. Here, we show that ghrelin reflects body condition and influences migratory behavior of wild birds. We measured ghrelin levels of wild garden warblers (Sylvia borin) captured at a stopover site. Further, we manipulated blood concentrations of ghrelin to test its effects on food intake and migratory restlessness. We found that acylated ghrelin concentrations of garden warblers with larger fat scores were higher than those of birds without fat stores. Further, injections of unacylated ghrelin decreased food intake and increased migratory restlessness. These results represent experimental evidence that appetite-regulating hormones control migratory behavior. Our study lays a milestone in migration physiology because it provides the missing link between ecologically dependent factors such as condition and timing of migration. In addition, it offers insights in the regulation of the hormonal system controlling food intake and energy stores in vertebrates, whose disruption causes eating disorders and obesity.

Novel approach to determine ghrelin analogs by liquid chromatography with mass spectrometry using a monolithic column

In our project, ghrelin analogs possessing enhanced stability and potential to significantly increase food intake were used. Three newly synthesized ghrelin analogs with fatty acid residues consisting of 8, 10, and 14 carbon atoms were studied. The main goal of this work was to develop a suitable analytical method for the determination of the stability of the novel ghrelin analogs in plasma. An appropriate liquid chromatography-mass spectrometry method was developed and optimized. The results obtained were compared with the data measured by using a commercial enzyme-linked immunosorbent assay kit, and a good correlation was found. A preparation strategy for plasma samples was optimized and consisted of simple dilution of the plasma samples followed by direct injection onto a very short monolithic column in combination with mass spectrometric detection. The developed analytical method was utilized for the determination of the stability of the prepared lipopeptides in plasma and for the quantification of the lipopeptides in a preliminary pharmacokinetic study. The feasibility of the developed separation method was clearly demonstrated. Accuracy and precision were within 80-120% and ±20% limits, respectively. Calibration curves were constructed in the range of 1-250 μg/mL.

Corticotropin-releasing factor overexpression in mice abrogates sex differences in body weight, visceral fat, and food intake response to a fast and alters levels of feeding regulatory hormones

BACKGROUND

Corticotropin-releasing factor overexpressing (CRF-OE) male mice showed an inhibited feeding response to a fast, and lower plasma acyl ghrelin and Fos expression in the arcuate nucleus compared to wild-type (WT) mice. We investigated whether hormones and hypothalamic feeding signals are impaired in CRF-OE mice and the influence of sex.

METHODS

Male and female CRF-OE mice and WT littermates (4-6 months old) fed ad libitum or overnight fasted were assessed for body, adrenal glands and perigonadal fat weights, food intake, plasma hormones, blood glucose, and mRNA hypothalamic signals.

RESULTS

Under fed conditions, compared to WT, CRF-OE mice have increased adrenal glands and perigonadal fat weight, plasma corticosterone, leptin and insulin, and hypothalamic leptin receptor and decreased plasma acyl ghrelin. Compared to male, female WT mice have lower body and perigonadal fat and plasma leptin but higher adrenal glands weights. CRF-OE mice lost these sex differences except for the adrenals. Male CRF-OE and WT mice did not differ in hypothalamic expression of neuropeptide Y (NPY) and proopiomelanocortin (POMC), while female CRF-OE compared to female WT and male CRF-OE had higher NPY mRNA levels. After fasting, female WT mice lost more body weight and ate more food than male WT, while CRF-OE mice had reduced body weight loss and inhibited food intake without sex difference. In male WT mice, fasting reduced plasma insulin and leptin and increased acyl ghrelin and corticosterone while female WT showed only a rise in corticosterone. In CRF-OE mice, fasting reduced insulin while leptin, acyl ghrelin and corticosterone were unchanged with no sex difference. Fasting blood glucose was higher in CRF-OE with female > male. In WT mice, fasting increased hypothalamic NPY expression in both sexes and decreased POMC only in males, while in CRF-OE mice, NPY did not change, and POMC decreased in males and increased in females.

CONCLUSIONS

These data indicate that CRF-OE mice have abnormal basal and fasting circulating hormones and hypothalamic feeding-related signals. CRF-OE also abolishes the sex difference in body weight, abdominal fat, and fasting-induced feeding and changes in plasma levels of leptin and acyl ghrelin.

Ghrelin-Reactive Immunoglobulins in Conditions of Altered Appetite and Energy Balance

Part of circulating ghrelin is bound to immunoglobulins (Ig) protecting it from degradation and preserving its functional activity. This review summarizes the data on ghrelin- and desacyl-ghrelin-reactive IgG in conditions of altered appetite and energy balance. Plasma levels and affinity kinetics of such IgG were compared in patients with obesity and anorexia nervosa (AN) and in animal models of obesity including ob/ob mice, high-fat diet-induced obese mice, and obese Zucker rats as well as in mice after chronic food restriction and activity-based anorexia and in rats with methotrexate-induced anorexia. We show that plasmatic IgG in both obese humans and animals are characterized by increased affinity for ghrelin. In contrast, patients with AN and anorectic rodents all show lower affinity of ghrelin- and desacyl-ghrelin-reactive IgG, respectively, the changes which were not observed in non-anorectic, chronically starved mice. We also show that affinity of ghrelin-reactive IgG correlate with plasma levels of ghrelin. These data point to common mechanisms underlying modifications of affinity kinetics properties of ghrelin-reactive IgG during chronic alterations of energy balance in humans and rodents and support a functional role of such autoantibodies in ghrelin-mediated regulation of appetite.

Lipopeptides as therapeutics: applications and in vivo quantitative analysis

A number of novel lipopeptides have been studied for their possible therapeutic potential. These studies should be supported by the appropriate analytical tools not only for novel potential drugs but also for their metabolites, precursors and side products. Lipopeptides have specific physicochemical properties that make them successful in medical applications. However, there are some difficulties with their qualitative and quantitative analyses in biological samples. Therefore, reliable, sensitive and robust analytical methods are in high demand. The main interest of our review is to describe a selection of specific and important properties of lipopeptides, and the analytical methods currently utilized for their characterization and determination in biological samples. A comparison of the pros and cons of immunomethods versus LC-MS methods is discussed in detail.

Influence of feeding a fish oil-containing diet to young, lean, adult dogs: effects on lipid metabolites, postprandial glycaemia and body weight

The aim of this study was to determine the effect of feeding a fish oil (FO)-containing diet on lipid and protein metabolism, postprandial glycaemia and body weight in young, lean, adult dogs. Eight female Beagles were randomly assigned to one of two isonitrogenous and isoenergetic diets, Control or FO, in a crossover design. At the beginning of the experiment and at 30 and 60 d, a baseline blood sample was collected and the dogs then were fed their daily ration. Nitrogen balance began at 07:00 h on day 63 of each experimental period and ended at 07:00 h on day 69. On day 66 of each period, a single dose (7.5 mg/kg) of (15)N-glycine was administered orally to each dog via gelatin capsule. Postprandial glycaemia did not differ between treatments or among sampling days within treatment. Cholesterol concentration was increased (p<0.05) on the Control treatment throughout the experiment when compared to values of day 0. Dogs fed the FO treatment had higher plasma triglyceride and ghrelin concentrations than those fed the Control treatment. Body weight and food intake did not differ between dietary treatments. Faecal excretion was increased (p<0.05) in the FO treatment. Dry matter digestibility was decreased (p<0.05) and fat digestibility tended (p<0.10) to decrease in the FO treatment. Overall, feeding a FO-containing diet showed a protective effect against the rise of plasma cholesterol and it increased plasma ghrelin concentration. However, FO supplementation did not appear to affect protein metabolism or postprandial glycaemia in adult lean dogs.

Allomyrina Dichotoma Larvae Regulate Food Intake and Body Weight in High Fat Diet-Induced Obese Mice Through mTOR and Mapk Signaling Pathways

Recent evidence has suggested that the Korean horn beetle (Allomyrina dichotoma) has anti-hepatofibrotic, anti-neoplastic, and antibiotic effects and is recognized as a traditional medicine. In our previous works, Allomyrina dichotoma larvae (ADL) inhibited differentiation of adipocytes both in vitro and in vivo. However, the anorexigenic and endoplasmic reticulum(ER) stress-reducing effects of ADL in obesity has not been examined. In this study, we investigated the anorexigenic and ER stress-reducing effects of ADL in the hypothalamus of diet-induced obese (DIO) mice. Intracerebroventricular (ICV) administration of ethanol extract of ADL (ADE) suggested that an antagonizing effect on ghrelin-induced feeding behavior through the mTOR and MAPK signaling pathways. Especially, ADE resulted in strong reduction of ER stress both in vitro and in vivo. These findings strongly suggest that ADE and its constituent bioactive compounds are available and valuable to use for treatment of various diseases driven by prolonged ER stress.

Nesfatin-130-59 Injected Intracerebroventricularly Differentially Affects Food Intake Microstructure in Rats Under Normal Weight and Diet-Induced Obese Conditions

Nesfatin-1 is well-established to induce an anorexigenic effect. Recently, nesfatin-1₃₀₋₅₉, was identified as active core of full length nesfatin-1₁₋₈₂ in mice, while its role in rats remains unclear. Therefore, we investigated the effects of nesfatin-1₃₀₋₅₉ injected intracerebroventricularly (icv) on the food intake microstructure in rats. To assess whether the effect was also mediated peripherally we injected nesfatin-1₃₀₋₅₉ intraperitoneally (ip). Since obesity affects the signaling of various food intake-regulatory peptides we investigated the effects of nesfatin-1₃₀₋₅₉ under conditions of diet-induced obesity (DIO). Male Sprague–Dawley rats fed ad libitum with standard diet were icv cannulated and injected with vehicle (5 μl ddH₂O) or nesfatin-1₃₀₋₅₉ at 0.37, 1.1, and 3.3 μg (0.1, 0.3, 0.9 nmol/rat) and the food intake microstructure assessed using a food intake monitoring system. Next, naïve rats were injected ip with vehicle (300 μl saline) or nesfatin-1₃₀₋₅₉ (8.1, 24.3, 72.9 nmol/kg). Lastly, rats were fed a high fat diet for 10 weeks and those developing DIO were icv cannulated. Nesfatin-1 (0.9 nmol/rat) or vehicle (5 μl ddH₂O) was injected icv and the food intake microstructure assessed. In rats fed standard diet, nesfatin-1₃₀₋₅₉ caused a dose-dependent reduction of dark phase food intake reaching significance at 0.9 nmol/rat in the period of 4–8 h post injection (−29%) with the strongest reduction during the fifth hour (−75%), an effect detectable for 24 h (−12%, p < 0.05 vs. vehicle). The anorexigenic effect of nesfatin-1₃₀₋₅₉ was due to a reduction in meal size (−44%, p < 0.05), while meal frequency was not altered compared to vehicle. In contrast to icv injection, nesfatin-1₃₀₋₅₉ injected ip in up to 30-fold higher doses did not alter food intake. In DIO rats fed high fat diet, nesfatin-1₃₀₋₅₉ injected icv reduced food intake in the third hour post injection (−71%), an effect due to a reduced meal frequency (−27%, p < 0.05), while meal size was not altered. Taken together, nesfatin-1₃₀₋₅₉ is the active core of nesfatin-1₁₋₈₂ and acts centrally to reduce food intake in rats. The anorexigenic effect depends on the metabolic condition with increased satiation (reduction in meal size) under normal weight conditions, while in DIO rats satiety (reduction in meal frequency) is induced.

Low glucose-induced ghrelin secretion is mediated by an ATP-sensitive potassium channel

Ghrelin is synthesized in X/A-like cells of the gastric mucosa, which plays an important role in the regulation of energy homeostasis. Although ghrelin secretion is known to be induced by neurotransmitters or hormones or by nutrient sensing in the ghrelin-secreting cells themselves, the mechanism of ghrelin secretion is not clearly understood. In the present study, we found that changing the extracellular glucose concentration from elevated (25  mM) to optimal (10 mM) caused an increase in the intracellular Ca2+ concentration ([Ca2+]i) in ghrelin-secreting mouse ghrelinoma 3-1 (MGN3-1) cells (n=32, P<0.01), whereas changing the glucose concentration from elevated to lowered (5 or 1 mM) had little effect on [Ca2+]i increase. Overexpression of a closed form of an ATP-sensitive K+ (KATP) channel mutant suppressed the 10  mM glucose-induced [Ca2+]i increase (n=8, P<0.01) and exocytotic events (n=6, P<0.01). We also found that a low concentration of a KATP channel opener, diazoxide, with 25  mM glucose induced [Ca2+]i increase (n=23, P<0.01) and ghrelin secretion (n≥3, P<0.05). In contrast, the application of a low concentration of a KATP channel blocker, tolbutamide, significantly induced [Ca2+]i increase (n=15, P<0.01) and ghrelin secretion (n≥3, P<0.05) under 5 mM glucose. Furthermore, the application of voltage-dependent Ca2+ channel inhibitors suppressed the 10 mM glucose-induced [Ca2+]i increase (n≥26, P<0.01) and ghrelin secretion (n≥5, P<0.05). These findings suggest that KATP and voltage-dependent Ca2+ channels are involved in glucose-dependent ghrelin secretion in MGN3-1 cells.

Fasting ghrelin but not PYY(3-36) is associated with insulin-resistance independently of body weight in Wistar rats

OBJECTIVE

The objective of this study was to evaluate the association between insulin-resistance and fasting levels of ghrelin and PYY in Wistar rats.

MATERIALS AND METHODS

A total of 25 male Wistar rats, weighing 200-300 g, was included in this study. The animals were maintained in cages with a 12/12h light-dark cycle and fed standard chow and water ad libitum. After 12-h overnight fasting, ghrelin, PYY, insulin and glucose values were determined. Insulin resistance was assessed by means of the HOMA-IR, which was ranked and the median was used as a cut-off value to categorize insulin-resistance. HOMA-IR values equal and above 2.62 were considered insulin-resistant (IR) while values below 2.62 were considered insulin sensitive (IS). Differences between means were determined using the Student t-test. Multiple regression and Pearson's correlation test were used to evaluate the association between variables.

RESULTS

HOMA-IR median IQ range values for IS and IR groups were, respectively, 1.56 (0.89 - 2.16) vs. [4.06 (3.50 - 4.61); p < 0.001]. The IR group presented increased levels of fasting ghrelin, PYY and insulin respectively: [50.35 (25.99 - 74.71) pg/mL vs. 12.33 (8.77 - 15.89) pg/mL; p = 0.001]; [54.38 (37.50 - 71.26) pg/mL vs. 33.17 (22.34 - 43.99) pg/mL; p = 0.016]; [18.04 (14.48 - 21.60) uU/mL vs. 7.09 (4.83 - 9.35) uU/mL; p = 0.001]. Ghrelin, but not PYY, correlated linearly and positively with HOMA-IR: ghrelin vs. HOMA-IR (r = 0.52; p = 0.008), and PYY vs. HOMA-IR (r = 0.22; p = 0.200). This correlation was independent of body weight.

CONCLUSION

Fasting ghrelin and PYY serum levels are increased in lean, relatively insulin resistant Wistar rats, and this increase is independent of weight.

The effects of separate and combined dietary weight loss and exercise on fasting ghrelin concentrations in overweight and obese women: a randomized controlled trial

OBJECTIVE

Compensatory metabolic changes that accompany weight loss, for example, increased ghrelin, contribute to weight regain and difficulty in long-term weight loss maintenance; however, the separate effects of long-term caloric restriction and exercise on total circulating ghrelin in humans are unknown.

DESIGN

A 12-month randomized controlled trial comparing: i) dietary weight loss with a 10% weight loss goal ('diet'; n = 118); ii) moderate-to-vigorous intensity aerobic exercise for 45 min/day, 5 days/week ('exercise'; n = 117); iii) dietary weight loss and exercise ('diet + exercise'; n = 117); or iv) no-lifestyle-change control (n = 87).

PARTICIPANTS

439 overweight or obese postmenopausal women (50-75 y).

MEASUREMENTS

Fasting total serum ghrelin was measured by radioimmunoassay at baseline and 12 months. Fasting serum leptin, adiponectin and insulin were also measured.

RESULTS

Fasting total ghrelin significantly increased in the diet + exercise arm (+7·4%, P = 0·008) but not in either the diet (+6·5%, P = 0·07) or exercise (+1·0%, P = 0·53) arms compared with control. Greater weight loss was associated with increased ghrelin concentrations, regardless of intervention. Neither baseline ghrelin nor body composition modified the intervention effects on changes in total ghrelin. The 12-month change in total ghrelin was inversely associated with changes in leptin, insulin and insulin resistance, and positively associated with change in adiponectin.

CONCLUSIONS

Greater weight loss, achieved through a reduced calorie diet or exercise, is associated with increased total ghrelin concentrations in overweight or obese postmenopausal women.

The regulation of circulating ghrelin - with recent updates from cell-based assays

Ghrelin is a stomach-derived orexigenic hormone with a wide range of physiological functions. Elucidation of the regulation of the circulating ghrelin level would lead to a better understanding of appetite control in body energy homeostasis. Earlier studies revealed that circulating ghrelin levels are under the control of both acute and chronic energy status: at the acute scale, ghrelin levels are increased by fasting and decreased by feeding, whereas at the chronic scale, they are high in obese subjects and low in lean subjects. Subsequent studies revealed that nutrients, hormones, or neural activities can influence circulating ghrelin levels in vivo. Recently developed in vitro assay systems for ghrelin secretion can assess whether and how individual factors affect ghrelin secretion from cells. In this review, on the basis of numerous human, animal, and cell-based studies, we summarize current knowledge on the regulation of circulating ghrelin levels and enumerate the factors that influence ghrelin levels.

The bone and fat connection in inflammatory bowel diseases

Osteopenia and osteoporosis are common manifestations in inflammatory bowel diseases (IBD) but the pathogenetic mechanism of bone loss in IBD is only partially understood. There is evidence that fat mass is an important determinant of the bone mineral density and adipose-derived factors seem to play an important role for the association between fat mass and bone mass. The association between adiposity and low bone density is rather poorly studied in IBD, but emerging data on adipokines in IBD in relation to osteoporosis provide a novel pathophysiological concept that may shed light on the etiology of bone loss in IBD. It could be suggested that adipokines interfere in bone metabolism by altering the sensitive balance between osteoblasts and osteoclasts although further studies in this setting are needed.

Hyperghrelinemia in Prader-Willi syndrome begins in early infancy long before the onset of hyperphagia

Circulating total ghrelin levels are elevated in older children and adults with Prader-Willi syndrome (PWS). However, the presence or absence of hyperghrelinemia in young children with PWS remains controversial. We hypothesized that a more robust way to analyze appetite-regulating hormones in PWS would be by nutritional phases rather than age alone. Our objectives were to compare total serum ghrelin levels in children with PWS by nutritional phase as well as to compare total ghrelin levels in PWS (5 weeks to 21 years of age) to normal weight controls and individuals with early-onset morbid obesity (EMO) without PWS. Fasting serum total ghrelin levels were measured in 60 subjects with PWS, 39 subjects with EMO of unknown etiology, and in 95 normal non-obese sibling controls of PWS or EMO subjects (SibC) in this 12 year longitudinal study. Within PWS, total ghrelin levels were significantly (P < 0.001) higher in earlier nutritional phases: phase 1a (7,906  ±  5,887); 1b (5,057 ± 2,624); 2a (2,905 ± 1,521); 2b (2,615 ± 1,370) and 3 (2,423 ± 1,350). Young infants with PWS also had significantly (P = 0.009) higher total ghrelin levels than did the sibling controls. Nutritional phase is an important independent prognostic factor of total ghrelin levels in individuals with PWS. Circulating ghrelin levels are elevated in young children with PWS long before the onset of hyperphagia, especially during the early phase of poor appetite and feeding. Therefore, it seems unlikely that high ghrelin levels are directly responsible for the switch to the hyperphagic nutritional phases in PWS.

Ghrelin signalling in β-cells regulates insulin secretion and blood glucose

Insulin secretion from pancreatic islet β-cells is stimulated by glucose. Glucose-induced insulin release is potentiated or suppressed by hormones and neural substances. Ghrelin, an acylated 28-amino acid peptide, was isolated from the stomach in 1999 as the endogenous ligand for the growth hormone (GH) secretagogue-receptor (GHS-R). Circulating ghrelin is produced predominantly in the stomach and to a lesser extent in the intestine, pancreas and brain. Ghrelin, initially identified as a potent stimulator of GH release and feeding, has been shown to suppress glucose-induced insulin release. This insulinostatic action is mediated by Gα(i2) subtype of GTP-binding proteins and delayed outward K⁺ (Kv) channels. Interestingly, ghrelin is produced in pancreatic islets. The ghrelin originating from islets restricts insulin release and thereby upwardly regulates the systemic glucose level. Furthermore, blockade or elimination of ghrelin enhances insulin release, which can ameliorate glucose intolerance in high-fat diet fed mice and ob/ob mice. This review focuses on the insulinostatic action of ghrelin, its signal transduction mechanisms in islet β-cells, ghrelin's status as an islet hormone, physiological roles of ghrelin in regulating systemic insulin levels and glycaemia, and therapeutic potential of the ghrelin-GHS-R system as the target to treat type 2 diabetes.

Effect of a test meal on meal responses of satiation hormones and their association to insulin resistance in obese adolescents

OBJECTIVE

The role of gastrointestinal (GI) hormones in the pathophysiology of obesity is unclear, although they are involved in the regulation of satiation and glucose metabolism. To (i) examine glucagon-like peptide 1 (GLP-1), amylin, ghrelin, and glucagon responses to a meal in obese adolescents and to (ii) test which GI peptides are associated with insulin resistance are presented.

METHODS

A total of 16 obese (body mass index (BMI) ≥ 97th percentile for age and gender) and 14 control (BMI between 25th and 75th percentiles) adolescents were included. Subjects were instructed to eat a test meal (490 kcal). Plasma samples were collected for hormone and glucose analysis.

RESULTS

Obese adolescents were insulin resistant as expressed by the Homeostasis Model Assessment (HOMA) index and had significantly increased fasting glucagon and amylin levels compared to the control group (P = 0.003 and 0.044, respectively). In response to the meal, the increase in GLP-1 levels was reduced in obese adolescents (P < 0.001). In contrast, amylin secretion was significantly increased in the obese population compared to the control group (P < 0.005).

CONCLUSIONS

Obese adolescents have increased fasting glucagon and amylin levels and attenuated post-prandial GLP-1 concentrations compared with the control group. These factors could contribute to the metabolic syndrome.

Hyperosmolarity in the small intestine contributes to postprandial ghrelin suppression

Plasma levels of the orexigenic hormone ghrelin are suppressed by meals with an efficacy dependent on their macronutrient composition. We hypothesized that heterogeneity in osmolarity among macronutrient classes contributes to these differences. In three studies, the impact of small intestinal hyperosmolarity was examined in Sprague-Dawley rats. In study 1, isotonic, 2.5×, and 5× hypertonic solutions of several agents with diverse absorption and metabolism properties were infused duodenally at a physiological rate (3 ml/10 min). Jugular vein blood was sampled before and at 30, 60, 90, 120, 180, 240, and 300 min after infusion. Plasma ghrelin was suppressed dose dependently and most strongly by glucose. Hyperosmolar infusions of lactulose, which transits the small intestine unabsorbed, and 3-O-methylglucose (3-O-MG), which is absorbed like glucose but remains unmetabolized, also suppressed ghrelin. Glucose, but not lactulose or 3-O-MG, infusions increased plasma insulin. In study 2, intestinal infusions of hyperosmolar NaCl suppressed ghrelin, a response that was not attenuated by coinfusion with the neural blocker lidocaine. In study 3, we reconfirmed that the low-osmolar lipid emulsion Intralipid suppresses ghrelin more weakly than isocaloric (but hypertonic) glucose. Importantly, raising Intralipid's osmolarity to that of the glucose solution by nonabsorbable lactulose supplementation enhanced ghrelin suppression to that seen after glucose. Hyperosmolar ghrelin occurred particularly during the initial 3 postinfusion hours. We conclude that small intestinal hyperosmolarity 1) is sufficient to suppress ghrelin, 2) may combine with other postprandial mechanisms to suppress ghrelin, 3) might contribute to altered ghrelin regulation after gastric bypass surgery, and 4) may inform dietary modifications for metabolic health.

Ghrelin and obestatin levels in children with failure to thrive and obesity

OBJECTIVES

Ghrelin and obestatin are 2 gastric hormones with opposite effects on food intake and body weight. We investigated plasma ghrelin and obestatin in children with failure to thrive (FTT) and obesity as compared with age-matched controls.

METHODS

A total of 63 children were included in the study: 13 with FTT, 17 with obesity, and 33 age-matched controls. Children fasted for at least 8 hours before specimen collection. Both hormones were measured using commercially available enzyme immunoassay kits.

RESULTS

Ghrelin and obestatin levels in children with FTT were not significantly different from that of the age-matched controls (P >0.05). In children with obesity, the total ghrelin levels were significantly lower (P = 0.0003) and the obestatin levels significantly higher (P = 0.029) compared with those in controls. In the control group, the fasting ghrelin level was significantly higher in the younger (<3 years) than in the older children (>3 years; P = 0.0004). Obestatin levels correlated positively with weight-for-age percentiles in the obese group (P = 0.011) and negatively in the control group >3 years (P = 0.019).

CONCLUSIONS

Compared with the levels in age-matched controls, fasting ghrelin and obestatin levels did not differ significantly in children with FTT. In the children with obesity, the decreased ghrelin and increased obestatin levels suggest a possible adaptive process to positive energy balance. Ghrelin had pronounced age-related changes, and obestatin was associated with the weight status. This may suggest that these 2 hormones use different mechanisms to regulate energy balance and weight.

Ghrelin increases vagally mediated gastric activity by central sites of action

BACKGROUND

Vagally dependent gastric reflexes are mediated through vagal afferent fibers synapsing upon neurons of the nucleus tractus solitarius (NTS) which, in turn modulate the preganglionic parasympathetic dorsal motor nucleus of the vagus (DMV) neurons within the medullary dorsal vagal complex (DVC). The expression and transport of ghrelin receptors has been documented for the afferent vagus nerve, and functional studies have confirmed that vagal pathways are integral to ghrelin-induced stimulation of gastric motility. However, the central actions of ghrelin within the DVC have not been explored fully.

METHODS

We assessed the responses to ghrelin in fasted rats using: (i) in vivo measurements of gastric tone and motility following IVth ventricle application or unilateral microinjection of ghrelin into the DVC and (ii) whole cell recordings from gastric-projecting neurons of the DMV.

KEY RESULTS

(i) IVth ventricle application or unilateral microinjection of ghrelin into the DVC-elicited contractions of the gastric corpus via excitation of a vagal cholinergic efferent pathway and (ii) ghrelin facilitates excitatory, but not inhibitory, presynaptic transmission to DMV neurons.

CONCLUSIONS & INFERENCES

Our data indicate that ghrelin acts centrally by activating excitatory synaptic inputs onto DMV neurons, resulting in increased cholinergic drive by way of vagal motor innervation to the stomach.

Update on ghrelin biology in birds

Ghrelin is a peptide found in the mucosal layer of the rat stomach that exhibits growth hormone-releasing and appetite-stimulating activities. Since the discovery of ghrelin in chicken in 2002, information on its structure, distribution, function, and receptors has been accumulated, mainly in poultry. Here, we summarize the following findings since 2008 in birds: (1) central ghrelin acts as an anorexigenic neuropeptide, but the effect of peripheral ghrelin differs depending on the chicken strain and light conditions the birds are kept in; (2) central ghrelin inhibits not only food intake but also water drinking, and it may be mediated by urocortin, a member of the corticotropin-releasing factor family; (3) peripheral ghrelin acts as an anti-lipogenic factor in broiler chickens but not in rats; (4) the enzyme involved in ghrelin acylation (ghrelin-O-acyltransferase [GOAT]) has been identified in chickens; (5) dietary lipids are used for ghrelin acylation; (6) des-acyl ghrelin administered alone or with ghrelin does not affect feeding behavior; (7) the existence and physiological function of obestatin must now be carefully examined in birds; (8) other than the growth hormone secretagogue receptors (GHS) R1a and 1b, GHS-R variants not found in mammals have been found in chicken and Japanese quail; and finally (9) little is known about the involvement of the ghrelin system in wild birds and in avian-specific behavior such as brooding and migration.

Early alterations in plasma ghrelin levels in offspring of calorie-restricted rats during gestation may be linked to lower sympathetic drive to the stomach

Serum ghrelin concentration is generally reduced in obesity. We aimed to assess whether this alteration is present in rats predisposed to obesity because of moderate undernutrition during gestation, and to explore whether this could be related with alterations in stomach sympathetic innervation, which is involved in gastric ghrelin secretion. Offspring of control and 20% gestational calorie-restricted dams (CR) exposed to normal-fat-diet from weaning onward were studied. Circulating ghrelin levels were measured at 25 days and 4 months of age. Morphometry, number of ghrelin-positive (ghrelin(+)) cells, ghrelin mRNA and protein levels, and tyrosine hydroxylase (TH) protein levels in stomach were determined at 25 days. Adult CR male animals, but not females, exhibited greater body-weight (BW) than their controls, but both males and females showed lower circulating ghrelin levels. This alteration in ghrelin levels was already present at 25 days, prior to any difference in BW. At this juvenile age, no differences in gastric morphometry, number of ghrelin(+) cells or ghrelin mRNA/protein levels were found between control and CR animals, however, CR animals showed lower TH stomach content. These results suggest that circulating ghrelin concentration is early altered in rats prenatally programmed to develop obesity. This does not seem to be associated with lower ghrelin production capacity but with specific alterations in sympathetic drive to the stomach.

A water-alcohol extract of Citrus grandis whole fruits has beneficial metabolic effects in the obese Zucker rats fed with high fat/high cholesterol diet

Epidemiological studies suggest that citrus fruits and compounds such as flavonoids, limonoids and pectins have health promoting effects. Our aim was to study the effects of Citrus grandis (L.) Osbeck var. tomentosa hort. fruit extract on the energy metabolism. A whole fruit powder from dry water and alcohol extracts of C. grandis containing 19% naringin flavonoid was prepared. The effects of the citrus extract were followed in the obese Zucker rats fed with the HFD. The circulatory levels of GLP-1 decreased significantly by the extract in comparison to the HFD group, whereas the decreased ghrelin levels were reversed. The levels of PYY were decreased in all HFD groups. The leptin amounts decreased but not significantly whereas insulin and amylin were unchanged. The cholesterol and glucose levels were somewhat but not systematically improved in the HFD fed rats. Further studies are needed to identify the active compounds and their mechanisms.

Enhanced gastric ghrelin production and secretion in rats with gastric outlet obstruction

BACKGROUND AND AIM

Ghrelin has distinct effects on gastrointestinal motility through the vagus nerve and gastric excitatory neural plexus. The objectives of this study were to investigate the dynamics of ghrelin and expression of neuromuscular markers in a newly established surgically manipulated rat model of gastric outlet obstruction (GOO), akin to the pyloric stricture associated with duodenal ulcer, advanced gastric cancer, and other conditions, in the clinical setting.

MATERIAL AND METHODS

The rats were divided into two groups, a control group (sham operation) and the GOO group (proximal duodenal stricture). The animals were sacrificed 2 weeks after the operation. Plasma and gastric ghrelin were measured by radioimmunoassay. mRNA expression in the stomach of neural choline acetyltransferase (ChAT), c-kit, and membrane-bound stem cell factor (SCF) were analyzed by quantitative RT-PCR. In addition, gastric mRNA expression of the aforementioned were also evaluated 60 min after intraperitoneal administration of a synthetic GHS-R1a antagonist ([D: -Lys3] GHRP-6 6.0 mg/kg).

RESULTS

Mechanical GOO induced increases of fasting plasma ghrelin levels and hyperplasia of the gastric muscle layers, with enhanced expression of the gastric neuromuscular markers. Administration of [D: -Lys3] GHRP-6 normalized the enhanced expression of c-kit and SCF.

CONCLUSION

GOO stimulates ghrelin dynamics and then enhances the mechanistic expression of gastric cellular communication network molecules between nerves and smooth muscle cells.

Circulating leptin and insulin in obese patients with and without type 2 diabetes mellitus: relation to ghrelin and oxidative stress

AIM

This case control study aimed to investigate relationship between appetite hormones (ghrelin and leptin) and body mass index (BMI), insulin and oxidative stress in simple obese and type 2 diabetes (T2DM) obese patients.

METHODS

Thirty healthy controls; 30 simple obese and 30 T2DM obese patients were enrolled. Demographic and clinical data of all participants were reported. Serum levels of fasting blood glucose (FBG), postprandial blood glucose (PBG), lipid peroxide (LPO) and nitric oxide (NO) were measured by chemical methods while, insulin, leptin and ghrelin by ELISA kits.

RESULTS

Serum levels of insulin, leptin, LPO were significantly higher while, ghrelin was significantly lower in simple obese and obese patients with diabetes versus controls. Insulin resistance was found in 76.67% simple obese and 93.33% obese patients with diabetes. Ghrelin showed a positive correlation with PBG in controls; but negative correlation with BMI in simple obese and with NO in obese patients with diabetes. Positive correlations were found between LPO and FBG, insulin, homeostasis model assessment of insulin resistance (HOMA-IR) and between leptin and FBG in obese patients with diabetes.

CONCLUSIONS

Our results suggested that hyperinsulinemia and hyperleptinemia may be most important mechanisms in decreasing ghrelin and inducing oxidative stress in simple obese and T2DM obese patients.

The regulation of food intake in mammalian hibernators: a review

One of the most profound hallmarks of mammalian hibernation is the dramatic reduction in food intake during the winter months. Several species of hibernator completely cease food intake (aphagia) for nearly 7 months regardless of ambient temperature and in many cases, whether or not food is available to them. Food intake regulation has been studied in mammals that hibernate for over 50 years and still little is known about the physiological mechanisms that control this important behavior in hibernators. It is well known from lesion experiments in non-hibernators that the hypothalamus is the main brain region controlling food intake and therefore body mass. In hibernators, the regulation of food intake and body mass is presumably governed by a circannual rhythm since there is a clear seasonal rhythm to food intake: animals increase food intake in the summer and early autumn, food intake declines in autumn and actually ceases in winter in many species, and resumes again in spring as food becomes available in the environment. Changes in circulating hormones (e.g., leptin, insulin, and ghrelin), nutrients (glucose, and free fatty acids), and cellular enzymes such as AMP-activated protein kinase (AMPK) have been shown to determine the activity of neurons involved in the food intake pathway. Thus, it appears likely that the food intake pathway is controlled by a variety of inputs, but is also acted upon by upstream regulators that are presumably rhythmic in nature. Current research examining the molecular mechanisms and integration of environmental signals (e.g., temperature and light) with these molecular mechanisms will hopefully shed light on how animals can turn off food intake and survive without eating for months on end.

Neuroendocrine and metabolic activities of ghrelin gene products

Acylated ghrelin (AG) is a 28 amino acid gastric peptide a natural ligand for the growth hormone secretagogue (GHS) receptor type 1a (GHS-R1a), endowed with GH-secreting and orexigenic properties. Besides, ghrelin exerts several peripheral metabolic actions, including modulation of glucose homeostasis and stimulation of adipogenesis. Notably, AG administration causes hyperglycemia in rodents as in humans. Ghrelin pleiotropy is supported by a widespread expression of the ghrelin gene, of GHS-R1a and other unknown ghrelin binding sites. The existence of alternative receptors for AG, of several natural ligands for GHS-R1a and of acylation-independent ghrelin non-neuroendocrine activities, suggests that there might be a complex 'ghrelin system' not yet completely explored. Moreover, the patho-physiological implications of unacylated ghrelin (UAG), and obestatin (Ob), the other two ghrelin gene-derived peptides, need to be clarified. Within the next few years, we may better understand the 'ghrelin system', where we might envisage clinical applications.

Understanding the functional significance of ghrelin processing and degradation

Post-translational modification, cleavage and processing of circulating hormones are common themes in the control of hormone activities. Full-length ghrelin is a 28 amino acid protein that exists in several modified and processed forms, including addition of an acyl moiety at the third serine of the N-terminus. When modified with octanoic acid, the first five N-terminal residues of ghrelin can modulate a signaling pathway via the ghrelin receptor GHSR1a. Although modification via a lipid moiety is essential for binding and activation of GHSR1a by ghrelin, many reports suggest that a desacyl form of ghrelin exists and has synergistic, opposing and distinct properties as compared to the acyl form. Therefore, it is important to clarify the physiological relevance of ghrelin derivatives. Based on lines of evidence from various studies, we propose that a larger proportion of secreted ghrelin is present in the deacylated form and furthermore, that circulating acyl and desacyl forms of ghrelin may be hydrolyzed to form short peptide fragments. Here, we summarize the results of studies aimed at understanding ghrelin processing and its implications for physiological function, as well as our recent findings regarding enzymes in the blood capable of generating processed forms of ghrelin.

Physiological roles of preproghrelin-derived peptides in GH secretion and feeding

Among the factors playing a crucial role in the regulation of energy metabolism, gastro-intestinal peptides are essential signals to maintain energy homeostasis as they relay to the central nervous system the informations about the nutritional status of the body. Among these factors, preproghrelin is a unique prohormone as it encodes ghrelin, a powerful GH secretagogue and the only orexigenic signal from the gastrointestinal tract and obestatin, a proposed functional ghrelin antagonist. These preproghrelin-derived peptides may contribute to balance energy intake, metabolism and body composition by regulating the activity of the GH/IGF-1 axis and appetite. Whereas the contribution of ghrelin has been well characterized, the role of the more recently identified obestatin, in this regulatory process is still controversial. In this chapter, we describe the contribution of these different preproghrelin-derived peptides and their receptors in the regulation of GH secretion and feeding. Data obtained from pharmacological approaches, mutant models and evaluation of the hormones in animal and human models are discussed.

Meal anticipatory rise in acylated ghrelin at dark onset is blunted after long-term fasting in rats

Ghrelin is a 28 amino acid acylated peptide originally characterised for its capacity to stimulate growth hormone secretion. Ghrelin is also an orexigenic and adipogenic hormone and is thought to be a signal to increase locomotor activity in anticipation of a scheduled meal. Although ghrelin is considered to be up-regulated during fasting, there are still conflicting data regarding the impact of starvation on ghrelin secretion. To test whether the secretory pattern of acylated ghrelin is altered during fasting, plasma levels were monitored every 20 min for 6 h in freely-behaving rats at the light/dark cycle transition, when animals initiate feeding and activity and use preferentially free fatty acids (FFA) as a source of energy. Rats were fed ad lib. or fasted at dark onset for 24, 48 or 72 h, with or without refeeding rate. The anticipatory rise in ghrelin levels, as well as home-cage activity at the onset of darkness, was significantly reduced from 48 h of fasting compared to ad lib. conditions. A delayed ghrelin peak, sensitive to renutrition, was observed in fasted animals. Although their motivation to eat appeared to be intact, rats fasted for 72 h showed the smallest compensatory refeeding rate after fasting, possibly reflecting altered gut function. Expression of agouti-related protein and neuropeptide Y, was significantly increased in 48- and 72-h fasted animals. Thus, following fasting, a blunted acylated ghrelin secretion at dark onset (i.e. a period when animals depend on FFA as a source of energy) is associated with reduced locomotor activity and refeeding and an up-regulation of anabolic neuropeptides. Such changes could be interpreted as compensatory mechanisms for helping to conserve energy under conditions where food is not available.

Is there altered sensitivity to ghrelin-receptor ligands in leptin-deficient mice?: importance of satiety state and time of day

BACKGROUND

Several fine-tuned and interconnected hypothalamic peptidergic systems orchestrate the regulation of energy homeostasis in the body. The orexigenic peptide ghrelin and the anorexigenic peptide leptin are among the most important, and both have been implicated in the development of eating disorders from obesity to anorexia nervosa.

OBJECTIVES

The goal of these studies was to examine the response of leptin-deficient ob/ob mice in ghrelin-receptor ligands in a food intake task.

METHODS

Changes in cumulative food intake were measured after peripheral administration of ghrelin (1 and 2 nmol/10 g) and the ghrelin-receptor antagonist (D-Lys(3))-GHRP-6 (66.6 and 133.3 nmol/10 g) in obese and lean control mice during the light and dark cycle as well as in a state of food restriction. Hypothalamic ghrelin and ghrelin-receptor expression was measured in ob/ob and lean mice at two different timepoints.

RESULTS

Ghrelin increased food intake in lean and obese mice in the light and dark cycle, whereas the ghrelin-receptor antagonist caused significantly stronger reduction in food intake in obese mice only in the dark cycle. After fasting, ob/ob mice displayed decreased light cycle sensitivity to the anorexigenic effects of the ghrelin-receptor antagonist. Hypothalamic expression levels of ghrelin were unaltered during the light cycle but decreased during the dark cycle in ob/ob mice; whereas, although unchanged in the light cycle, ghrelin-receptor expression was increased in the dark cycle in obese mice.

CONCLUSION

The functionality and sensitivity of the ghrelinergic system is dependent on the time of day and the satiety state in leptin-deficient ob/ob mice.

Effects of improving glycemic control with insulin on leptin, adiponectin, ghrelin and neuropeptidey levels in patients with type 2 diabetes mellitus: a pilot study

OBJECTIVE

Insulin therapy is associated with weight gain in patients with type 2 diabetes mellitus (T2DM). Several peptides are implicated in appetite control. We evaluated the effects of insulin-induced improved glycaemic control on leptin, adiponectin, ghrelin, neuropeptide Y (NPY) levels and patient characteristics.

METHOD

Consecutive T2DM patients (n = 90) were divided into 2 groups: Group A: 45 insulin-naïve uncontrolled (glycosylated haemoglobin A(1c); HbA(1c )>7%) patients on oral hypoglycaemic agents (OHAs) who converted to insulin monotherapy. Group B: 45 well-controlled (HbA(1c )<7%) patients on OHAs. Both groups were monitored at baseline, 3 and 6 months. Males and females were analyzed separately because some hormone levels differ between genders.

RESULTS

In both genders, insulin therapy (Group A) was associated with significant (p = 0.003 to <0.001) increases in weight, body mass index and leptin levels and significant decreases in glucose, HbA(1c) and NPY levels. In male insulin-treated patients a significant increase in adiponectin levels (p = 0.008) was observed. There were significant correlations (p = 0.016 to <0.001) between leptin levels, waist circumference and body fat in all patient groups, except group B males.

CONCLUSION

Changes in leptin, adiponectin and NPY levels may occur after insulin-induced improved glycaemic control. These changes may be influenced by gender, weight, body fat and HbA(1c).

Scheduled feeding results in adipogenesis and increased acylated ghrelin

Ghrelin, known to stimulate adipogenesis, displays an endogenous secretory rhythmicity closely related to meal patterns. Therefore, a chronic imposed feeding schedule might induce modified ghrelin levels and consequently adiposity. Growing Wistar rats were schedule-fed by imposing a particular fixed feeding schedule of 3 meals/day without caloric restriction compared with total daily control intake. After 14 days, their body composition was measured by DEXA and compared with ad libitum-fed controls and to rats daily intraperitoneal injection with ghrelin. Feeding patterns, circadian activity, and pulsatile acylated ghrelin variations were monitored. After 14 days, rats on the imposed feeding schedule displayed, despite an equal daily calorie intake, a slower growth rate compared with ad libitum-fed controls. Moreover, schedule-fed rats exhibiting a feeding pattern with intermittent fasting periods had a higher fat/lean ratio compared with ad libitum-fed controls. Interestingly, ghrelin-treated rats also showed an increase in fat mass, but the fat/lean ratio was not significantly increased compared with controls. In the schedule-fed rats, spontaneous activity and acylated ghrelin levels were increased and associated with the scheduled meals, indicating anticipatory effects. Our results suggest that scheduled feeding, associated with intermittent fasting periods, even without nutrient/calorie restriction on a daily basis, results in adipogenesis. This repartitioning effect is associated with increased endogenous acylated ghrelin levels. This schedule-fed model points out the delicate role of meal frequency in adipogenesis and provides an investigative tool to clarify any effects of endogenous ghrelin without the need for ghrelin administration.