Neuroendocrine Mediators, Food Intake and Obesity: A Narrative Review

Obesity is a chronic multifactorial disease caused by imbalance between caloric intake and energy expenditure. The Neuroendocrine system is one of the main factors regulating energy intake in humans. The Neuroendocrine system is made up of cells able to synthesize and secrete amines, peptides, growth factors and biological mediators, known as neurohormones, which modulate various biological functions by interacting with the nervous and immune system. In the central nervous system, neurosecretory elements are mainly located in the hypothalamus which is the anatomical site of the hunger (lateral nucleus) and satiety (ventromedial nucleus) centers; thus it plays a key role in chemical coding of food intake. Dopamine, Noradrenaline and Serotonin are historically considered key points in the regulation of feeding behavior. However, other neurohormones have been identified; these substances, also synthesized in peripheral tissues (especially adipose tissue and digestive tract), influence food intake. Some of these hormones have orexigenic activity; conversely, other substances have anorexigenic activity. A constant balance between orexigenic and anorexigenic neurohormones is essential to ensure a smooth feeding behavior, whereas a subtle and progressive disruption of neurochemical transmission is sufficient to induce hyperphagia or anorexia. Several factors affect the synthesis and release of neuropeptides: genetic, hormonal, psychological, environmental, receptorial, type of feeding and meal frequency. In the recent past some drugs, as Sibutramine and Rimonabant, modulating the activity of several neuroendocrine mediators (Serotonin, Noradrenaline, Endocannabinoids), have proven to be effective in reducing weight excess, even if they were withdrawn because of serious side effects. Recently, promising results in this way have been obtained with Glucagon like Peptide-1 analogs, showing significant efficacy in counteracting weight excess without side effects. Further knowledge developments on these complex neuroendocrine circuits and their hypothalamic interactions in food intake regulation could open new frontiers for effective pharmacological therapeutic approach to Obesity and other nutritional disorders

Participação dos prostanóides e do cálcio na contratilidade da vesícula biliar às endotelinas

Tese (doutorado) - Universidade Federal de Santa Catarina, Centro de Ciências Biológicas. Programa de Pós-Graduação em Farmacologia.O estudo avalia as ações contráteis das [endotelinas] (ETs) na [vesícula biliar] (VB) de [cobaia], comparando-as com aquelas da VB de [coelho], bem como o papel de [prostanóides] e do [cálcio intracelular] nas contrações induzidas através de receptores ETA e ETB. A ET-1 é um potente agonista contrátil da VB de coelho, que parece expressar um predomínio maior de receptores ETB sobre os ETA do que a VB de cobaia. Os prostanóides são importantes na mediação/modulação dos efeitos contráteis das ETs na VB de cobaia, pois a [indometacina] (INDO) inibe as contrações por elas induzidas, e esses peptídeos estimulam a produção de prostaciclina. As contrações tônicas induzidas pelas ETs na VB de cobaia correlacionam-se a aumentos proporcionais da concentração de cálcio intracelular livre. Os perfis inibitórios da INDO sobre as contrações e aumentos da fluorescência induzidas pelas ETs na VB de cobaia sugerem que receptores ETA e ETB sinalizam a produção de conjuntos distintos de prostanóides que exercem influências inibitórias e excitatórias na contratilidade do músculo liso, respectivamente. A relevância fisiológica e/ou fisiopatológica destes achados permanece a ser esclarecida

The Effect of Gut Hormones on Metabolism and Energy Homeostasis

The increasing prevalence of obesity, and its associated morbidity and mortality, together with limited treatment options, underscores an urgent need for investigation into effective therapeutic interventions. Gut hormones have been identified as integral factors in the regulation of appetite. This thesis examines two signalling systems involved in the gastrointestinal regulation of energy homeostasis: Peptide YY (PYY) and Proglucagon-derived peptides. PYY is a gut hormone released from the L cells of the intestine after the ingestion of food and elicits its effects via the Y2 receptor. The predominant circulating form of PYY, PYY3-36, has been shown to acutely reduce food intake when administered peripherally at physiological concentrations in both lean and obese rodents and humans. However, several groups have shown that continuous administration of PYY3-36 via osmotic mini-pumps results in a transient reduction in food intake. It has been suggested that an apparent desensitisation to the anorectic effects of PYY3-36 may be due to the physiological defence of body weight and counter regulatory mechanisms; however tolerance due to receptor downregulation may also occur. In this thesis, I aimed to elucidate if the transient anorectic effect of PYY3-36 is a result of direct tolerance to the peptide itself or if this is indirect, due to the homeostatic defence of body weight. I have shown that the anorectic effect of PYY3-36 is attenuated following prior exposure to a low dose, suggesting tolerance at the receptor level. In addition, animals which were food restricted preceding an infusion of PYY3-36 remained sensitive to the anorectic effects, suggesting that body weight change alone cannot result in the hyperphagic response. However, no change in Y2 mRNA receptor expression following PYY3-36 infusion was detected. The proglucagon peptide family includes the hormones Glucagon like peptide-1 (GLP-1) and Glucagon (GCG). GLP-1 is release from the L-cells of the intestine in response to food intake and has known actions as a satiety factor and incretin hormone. GCG is released under fasting conditions and in response to adrenergic stimulation to stimulate gluconeogenesis and glycogenolysis, as well as to increase energy expenditure. Recent evidence suggests that simultaneous co-agonism of the GLP-1 and GCG receptors may be beneficial to the treatment of obesity and diabetes. In this thesis, I explore the development of a GLP-1R and GCGR co-agonist which reduces food intake and increases energy expenditure in rodents. Furthermore, to investigate the mechanism of GCG agonism on energy expenditure, I evaluate the effect of the specific dual agonist, GX6, on metabolic gene expression in brown adipose tissue and the liver. Overall, this thesis evaluates the potential roles of PYY, GLP-1 and GCG receptor agonism as novel therapies for obesity

The metabolic sequelae of oesophago-gastric resection

Bypass or resection of the stomach and oesophagus, has long been recognised to result in profound changes in the handling of ingested nutrients. This results in significant morbidity after radical surgery for oesophago-gastric cancer, in particular post-prandial hypoglycaemia, altered appetite, early satiety and noxious post-prandial symptoms. By profiling and challenging the gut hormone axis in healthy volunteers and patients who had undergone total or subtotal gastrectomy, or oesophagectomy, this thesis explores the possible causative mechanisms for the challenges faced by this patient population. In the surgical groups, an oral glucose tolerance test (OGTT) resulted in enhanced secretion of satiety and incretin gut hormones (GLP-1, GIP, PYY) and insulin, followed by hypoglycaemia in a cohort of patients. Continuous glucose monitoring of gastrectomy participants over two weeks of normal lifestyle identified an increased incidence of day and night time hypoglycaemia. RNAseq and mass spectrometry based peptidomics of human and murine enteroendocrine cells in the pre- and post-operative populations revealed no significant change in the underlying cellular pathways for nutrient sensing and gut hormone secretion, indicating that the altered hormone secretion is primarily driven by accelerated nutrient transit, rather than adaptive changes in the gut. Finally, specific blockade of the GLP-1 receptor in post-gastrectomy patients using Exendin 9-39 normalised insulin secretion and prevented reactive hypoglycaemia after an OGTT. In conclusion, profound changes in gut hormone secretion as a result of enhanced nutrient transit after foregut surgery likely underlie the early and late post-prandial symptoms seen in this group, and therapies specifically targeting the gut hormone axis, and GLP-1 in particular, could be the first targeted treatments for post-gastrectomy syndromes.Research constituting this thesis was supported by: NIHR Evelyn Trust Royal College of Surgeons European Foundation for the Study of Diabetes Wellcome Trus

Investigating the physiological and pharmacological effects of the gut hormone peptide YY (PYY)

The obesity epidemic is a critical and global public health burden. Drugs that safely promote weight loss are urgently needed to halt the rising prevalence of obesity and its associated complications, such as type 2 diabetes (T2D). Gut hormones are important regulators in metabolism and have therapeutic potential as treatments for obesity and T2D. The gut hormone peptide YY (PYY) is released from the intestine after a meal. Exogenous PYY3–36 suppresses food intake in both rodents and humans, including in the obese state. PYY3–36 suppresses appetite by acting on its receptor, the Y2R. Y2R is expressed in brain appetite centres but also in the afferent vagus nerve, the main neuroanatomical link carrying information from the gut to the brain. However, the relevant contribution of the afferent vagus to the overall effects of PYY3–36 is unknown. Chemogenetic activation of vagal afferent neurones results in reduced food intake (surpassing the effects of PYY) and might have altered the immune landscape of the gastrointestinal tract. To dissect the role of the Y2R expressed in the afferent vagus, we have developed a novel microsurgical technique in the mouse. Our work suggests that vagal Y2R mediates the anorectic effect of low dose and endogenous PYY3–36 and that this vagal signalling pathway regulates short-term feeding. This anorectic effect was not caused by an aversive response. In vitro calcium imaging confirmed that PYY3–36 directly activates vagal afferents. Chronic treatment of diet-induced obese (DIO) mice with a long-acting PYY3–36 analogue, Y242, did not cause a significant body weight loss. Longitudinal tracking of individual islet function using a novel imaging platform allowed to study the effect of diet and Y242 treatment. Chronic Y242 did not improve or worsen islet function in obese mice. Therefore, PYY-based treatments might not be suitable as a single agent but have potential in combination with other gut-hormones. Vagus nerve neuromodulation has shown potential as an anti-obesity therapy and the work in this thesis adds to a better understanding of vagal afferent function which will help optimise therapeutic interventions.Open Acces

Oral and small intestinal sensitivity to fats in lean and obese humans : implications for energy intake regulation in obesity.

The research presented in this thesis focuses on the complex and interrelated oral and gastrointestinal mechanisms involved in the regulation of appetite and energy intake in lean and obese individuals. The three broad areas of research that have been investigated in the thesis include: i) the gastrointestinal motor and hormonal functions involved in the regulation of energy intake in healthy individuals; ii) the effects of oral and intraduodenal nutrients on gastrointestinal motility and hormone release, appetite and energy intake in obese compared with lean individuals; and iii) the effects of acute and prolonged energy restriction on gastrointestinal function, appetite and energy intake. Following ingestion of a meal, the interaction of nutrients with receptors in the small intestinal lumen modulates gastro-pyloroduodenal motility, stimulates the release of gastrointestinal hormones, and suppresses appetite and energy intake. It appears that modulation of gastrointestinal functions, that is, gastrointestinal motility and hormone release/suppression, mediate the regulation of appetite and acute energy intake in humans, at least in part. Changes in motility and hormone secretion occur concurrently with changes in appetite; however, there is little information regarding which, if any, of these factors are independent determinants of energy intake. In the study presented in Chapter 5, we determined independent predictors of energy intake and identified specific changes in gastrointestinal motor and hormone functions (i.e. stimulation of pyloric pressures and plasma cholecystokinin) that are associated with the suppression of acute energy intake in healthy lean males. The incidence of obesity is rapidly increasing and, currently, the therapies used for the prevention and management of obesity have limited long-term benefits. In addition, the available therapies have largely ignored the pivotal role of the gastrointestinal tract in the regulation of appetite. There is evidence that gastrointestinal function in obesity is modified, which may be the result of the eating habits of obese individuals and, in turn, may also contribute to the maintenance of obesity by causing insufficient suppression of energy intake. However, much of the literature relating to gastrointestinal function in the obese is inconclusive and controversial. A better understanding of any adaptations that occur in obesity is important, particularly in regards to treatment approaches for weight loss. There is also evidence that previous patterns of energy intake, in excess or in restriction, even when sustained for short periods, have the capacity to modify gastrointestinal function and energy intake. For example, in humans following a high fat diet for two weeks, gastric emptying and mouth-to-caecum transit in response to a high fat test meal were faster. In contrast, fasting had the opposite effect and a four-day fast slowed gastric emptying of a glucose drink in both lean and obese subjects, suggesting that a reduction in nutrient exposure may increase the sensitivity of gastrointestinal responses to nutrients in the obese. Although many studies have addressed aspects of gastrointestinal function in the obese, there is a lack of studies that have evaluated gastric emptying and gastrointestinal hormone release specifically GLP-1 and GIP, given the risk of diabetes in obesity, as well as previous patterns of nutrient intake concurrently. In the study presented in Chapter 6, we evaluated the effects of oral ingestion of a nutrient liquid on gastric emptying, oro-caecal transit, plasma GLP-1 and GIP, appetite and energy intake, as well as, habitual energy and fat intake in lean, overweight and obese individuals. We reported no differences in gastric emptying, intragastric distribution or oro-caecal transit between the lean, overweight and obese groups. After the drink, blood glucose and plasma insulin were greater in the obese, when compared with both the lean and overweight groups, however, there were no differences in plasma GLP-1 or GIP concentrations, appetite and energy intake at the buffet meal or habitual energy intake between the groups. In the obese, the magnitude of the rise in blood glucose was inversely related to the gastric emptying, suggesting that obesity per se, in the absence of differences in habitual energy intake, has no effect on gastric emptying or incretin hormone release and that gastric emptying influences postprandial blood glucose in the obese. In Chapter 7, we investigated the hypothesis that gastrointestinal and oral sensitivity to fat is compromised in the obese and directly related to their high fat/energy consumption. For this purpose, we investigated the effects of an intraduodenal infusion (to bypass gastric emptying), of a fatty acid (oleic acid) on gastrointestinal function, appetite and energy intake, and relationships with habitual energy intake and oral fatty acid detection threshold in lean and obese individuals. We report that pyloric pressure, which plays a major role in the regulation of gastric emptying, was lower in response to intraduodenal oleic acid infusion, with trends for reduced cholecystokinin stimulation and energy intake responses in the obese compared with lean. Oral fatty acid detection thresholds were higher in obese compared with lean subjects, and obese subjects also had greater habitual energy and fat intakes than lean subjects. The results suggest that the ability to detect fats both orally and within the gastrointestinal tract is compromised in obese males, probably due to their increased fat consumption. In the study presented in Chapter 8, we evaluated the hypothesis that in obese individuals, the effects of duodenal fat on gastrointestinal motor and hormone function, and appetite would be enhanced by a short period on a very low calorie diet. We demonstrated that following a 70% four-day very low calorie diet there was a significant increase in pyloric pressure and the stimulation of PYY and suppression of ghrelin was greater during an intraduodenal lipid infusion. In addition, following the four-day very low calorie diet, appetite perceptions and energy intake in response to intraduodenal lipid were reduced, indicating that gastrointestinal function, appetite and energy intake in obese can be enhanced over a short period. Given that gastrointestinal function is sensitive to changes even over short periods of dietary restriction, it is important to determine whether these changes are maintained in the long term in order to determine the efficacy of energy restriction therapies for obesity. To maintain dietary restriction and weight loss in the longer term, we used a 30%, as opposed to 70%, energy-restricted diet. In the study presented in Chapter 9, we evaluated the effects of an acute (in lean and obese) and prolonged (in obese only) 30% energy restriction on gastrointestinal function and appetite in response to an intraduodenal lipid infusion. In contrast to the previous 70% very low calorie diet study, there were no differences in gastrointestinal motor or hormonal function in the obese following the acute or prolonged 30% dietary restriction period, although there was a trend for energy intake to be reduced. However, in the lean, there was a decrease in plasma CCK and an increase in ghrelin concentrations following the acute period of dietary restriction with no differences in gastrointestinal motility or energy intake, suggesting that a 30% energy-restricted diet diminishes gastrointestinal hormone responses in lean, but not obese, which may suggest that obese are less sensitive to this caloric restriction. These observations will contribute to the advances in basic appetite physiology and will have clinical implications for further development of dietary interventions for the treatment of obesity.Thesis (Ph.D.) -- University of Adelaide, School of Medicine, 201

Enhancement of intestinal absorption of peptides

1. Peptide hormones, once available only through extraction from animal tissue, are now increasingly manufactured synthetically. However, the route of administration is usually by injection due to the low oral bioavailability of the hormones, as a consequence of proteolytic degradation in the small intestine, poor penetration of the intestinal mucosa, and clearance from the portal vein by the hepatocytes of the liver. 2. Since bile acids are efficiently absorbed by an active carrier system in the ileum, the present study investigated the degree to which transport of ileally-administered tetragastrin, a peptide composed of four amino acid residues, could be enhanced by coupling with cholic acid. Conjugation with cholic acid was at the position where - glycine and taurine attach naturally in the formation of conjugated bile salts. 3. In the fasted anaesthetised rat, gastric acid secretion was measured at 15 min intervals as a bioassay for the levels of tetragastrin in the blood stream. In all experiments, intravenous administration of tetragastrin or the tetragastrin-cholic acid conjugate (G-CA) was adopted as the first and final procedure, in order to demonstrate the continued viability of the animal preparation. 4. Intravenously administered tetragastrin (minimum effective dose, most commonly 15mug kg-1) as a first procedure was shown to evoke a mean peak increase in gastric acid levels of 0.50 +/- 0.06mumol 15min-1. Over a period of one hour following injection, a mean cumulative increase in gastric acid output of 1.07 +/- 0.39mumol hr-1 was demonstrated (n=31). G-CA (15ng kg-1) also demonstrated biological activity when administered intravenously: a mean peak increase in gastric acid levels of 0.50 +/- 0.18mumol 15min-1, and a mean cumulative increase of 0.91 +/- 0.47mumol hr-1 were obtained (n=32). In both cases, a broadly comparable increase in gastric acid secretion was also obtained in response to a second intravenous injection, indicating that the animal preparation was neither fatigued, nor that earlier procedures had resulted in potentiation of the response. 5. Intra-intestinal administration of tetragastrin (2500ng kg-1) was shown not to produce a measurable increase in gastric acid levels. By contrast, the present study demonstrated unequivocally that infusion of G-CA (600mug kg-1) into the ileum resulted in the stimulation of increased levels of gastric HCl secretion (means of small samples were weighted to give a true mean increase of 1.84 +/- 1.22mumol 180min-1). 6. Simultaneous infusion of tetragastrin and glycocholic acid (in a ratio of 3:2, total dose of 600mug kg-1) into the ileum did not elicit any measurable increases in gastric acid secretion, despite the solution being biologically active, as confirmed by a measurable increase in gastric acid secretion following intravenous injection (1.09 +/- 0.58mumol hr-1, P=0.033; n=5). 7. Surprisingly, instillation of tetragastrin (2500mug kg-1) into the ileum did evoke a significantly marked increase in gastric acid secretion (3.05 +/- 1.89mumol 180min-1, P=0.006; n=7), though only when tetragastrin was preceded by ileally-administered G-CA (600ng kg-1). This indicated that a possible lasting permeability change in the ileal mucosa was caused by ileal G-CA, thus permitting the permeation of the intestinal wall by tetragastrin. Nevertheless, the results of light microscopic analysis revealed little evidence of a pathological alteration to the intestinal mucosa. 8. The transmucosal movement of G-CA appeared to be specific to the ileum, as there was no evidence of transport across the jejunal mucosa following intraluminal instillation of G-CA (600mug kg-1). Subsequent administration of G-CA (600mug kg-1) in the ileum of the same rat produced a significant increase in gastric acid secretion. 9. The results of the present study are indicative of utilisation of the bile salt active transporter by G-CA. This is very encouraging in terms of a successful approach to address the facilitation of permeation of otherwise poorly absorbed oligopeptides across the intestinal mucosa and into the systemic circulation

Investigating PYY3-36 and PYY3-36 analogues in the development of an obesity therapy

Obesity, defined as a body mass index greater than 30kg/m2, has reached the scale of a pandemic. The increasing prevalence of obesity, and its associated morbidity and mortality, together with limited treatment options, underscores an urgent need for effective therapeutic interventions. Gut hormones have been identified as integral factors in the regulation of appetite. One such gut hormone is Peptide YY (PYY), a postprandial satiety hormone that communicates nutritional status to the central nervous system. PYY is processed to generate the principle circulating form PYY3-36, which acts on Y2 receptors in the brainstem and hypothalamus to reduce appetite. Chronic intravenous infusion of PYY3-36 induces weight loss in rodents and acute intravenous infusion to obese humans reduces food intake. Furthermore, obese humans have been reported to display a blunted postprandial rise in PYY3-36, suggesting PYY3-36 is a potential anti-obesity drug target. However, exogenous PYY3-36 is rapidly cleared and has a short circulating half-life. Additionally, at supraphysiological levels PYY3-36 can produce nausea in humans. The administration of long-acting PYY3-36 analogues to the obese may overcome these limitations. This thesis investigates the design and development of PYY3-36 analogues, and their potential in the treatment of obesity. I have investigated modifications to different domains within the primary structure of PYY3-36 in order to elucidate Y2R affinity, susceptibility to proteolytic degradation and biological activity. I demonstrated that a combination of substitutions in different domains can create a long acting analogue. I have examined sites at which PYY3-36 is susceptible to degradation by specific proteases and identified that inhibition of meprin metalloendopeptidases can prolong the plasma longevity and anorectic actions of PYY3-36. I also found that substitution of a section of the α-helix of PYY3-36 with a conserved α-helical epitope creates a long-acting analogue, that is more efficacious than PYY3-36 in chronically reducing food intake and body weight in a diet-induced obese rodent model. Finally, in investigating a slow-release delivery system for PYY3-36 analogues, I have shown that substituting His residues into the α-helix-substituted analogue promotes chelation with Zn in vitro, and facilitates a slow-release pharmacokinetic profile in vivo, that may circumvent the side effects associated with administering high levels of PYY3-36

Stable isotope tracer studies for the measurement of equine gastrointestinal motility

Abdominal disorders are a major cause of morbidity and mortality in horses, and abnormal gastrointestinal motility may be a significant factor in the aetiopathogenesis of many equine colic syndromes. The understanding of such conditions is hampered by the lack of suitable noninvasive tests for the quantitative measurement of intestinal transit. The overall objective of this work was to investigate the potential value of stable 13C-isotope breath tests for the assessment of specific parameters of equine gastrointestinal motility. A new method developed for the collection of equine expiratory breath and measurement of its ratio was shown to have excellent repeatability. Assessment of peripheral blood tracer content was also performed and correlated significantly to that of concurrent breath samples. In the first study, the 13C-octanoic acid breath test (13C-OABT) was evaluated for the measurement of solid phase gastric emptying rate in 12 healthy horses by direct comparison with the predicate method of gastric scintigraphy. Having shown that the 13C-OABT was a reliable diagnostic procedure for use in healthy horses, a further study was performed against scintigraphy in subjects with atropine-induced gastroparesis (n = 8) to determine whether the test remained accurate when emptying rate was markedly prolonged. In study 3, the 13C-OABT was applied to measure the relative and dose-related effects of common sedative agents on solid phase gastric emptying in 8 horses. The study results may have clinical significance for case selection when these agents are used for purposes of sedation and/or analgesia. The 13C-bicarbonate and sodium 13C-acetate breath tests were investigated in study 4 for the assessment of equine liquid phase gastric emptying, and elucidation of the pattern of 13CO2 recovery from the body bicarbonate pool. The lactose 13C-ureide breath test (13C-LUBT) was investigated in study 5 for estimation of orocaecal transit time (OCTT), and concurrent comparison made to the hydrogen breath test (H2BT). In study 6 the induced 13C-LUBT was evaluated in vivo for the measurement of OCTT and a mean (+/- s.d.) time of 3.24 (+/- 0.65) h was gained. In order to examine the relationship between gastric emptying of solid ingesta, small bowel transit and its arrival in the caecum, a combined test was developed and applied in study 7, incorporating both 13C-OA and 13C-LU. Mathematical modelling of 13C recovery after ingestion of the dual test meal allowed calculation of small bowel half transit time, in addition to gastric and caecal parameters. Finally, minimised test protocols were developed for the 13C-OABT and 13C-LUBT in order to increase their clinical utility. The effects of decreasing the duration or frequency of breath collection on generation of intestinal transit parameters were assessed and linear regression models produced for each test based on the collection of 5 breath samples. Gastric t1/2, tlag and OCTT estimates from the reduced model and the full sampling protocols were highly correlated. However, in each case the reduced models were likely to underestimate these parameters when significantly prolonged, decreasing their sensitivity for the detection of delayed intestinal transit. The stable isotope breath tests offer a novel means of investigating features of intestinal motility and physiology in the horse and have potential value as both diagnostic modalities and humane research tools in this species. As the tests are non-invasive, simple to perform and do not require extensive equipment, they may be performed on site and the samples then submitted for isotopic analysis. Unlike other techniques for assessment of equine gastrointestinal motility, the stable isotope breath tests also provide an indirect measure of the transit rate of ingesta itself, which is directly relevant to the clinical situation. (Abstract shortened by ProQuest.)