Glucose and insulin response to conditioned feeding in lean and genetically obese hyperglycemic (ob/ob) mice

Food anticipatory hormonal responses: A systematic review of animal and human studies

Food anticipatory hormonal responses (cephalic responses) are proactive physiological processes, that allow animals to prepare for food ingestion by modulating their hormonal levels in response to food cues. This process is important for digesting food, metabolizing nutrients and maintaining glucose levels within homeostasis. In this systematic review, we summarize the evidence from animal and human research on cephalic responses. Thirty-six animal and fifty-three human studies were included. The majority (88 %) of studies demonstrated that hormonal levels are changed in response to cues previously associated with food intake, such as feeding time, smell, and sight of food. Most evidence comes from studies on insulin, ghrelin, pancreatic polypeptide, glucagon, and c-peptide. Moreover, impaired cephalic responses were found in disorders related to metabolism and food intake such as diabetes, pancreatic insufficiency, obesity, and eating disorders, which opens discussions about the etiological mechanisms of these disorders as well as on potential therapeutic opportunities.

Modulation of behavioral and insulin responses by contextual stimuli paired with food

A context-like conditioned stimulus (CS) previously paired with a food unconditioned stimulus (US) enhanced both food-related activity and insulin secretion in rats. Experiment 1 documented the effectiveness of an appetitive conditioning procedure in which a 10 sec visual CS ("x") was followed by food when embedded within one 35 sec auditory CS ("A"), but not another ("B"). Approach to a food magazine during x was enhanced (facilitated) when x was within A, as compared to within B or alone. Experiment 2 documented the adequacy of blood sampling and insulin assay procedures that could be accommodated to the conditioning procedure. Plasma insulin was observably different after unconditioned deliveries of glucose solutions of different intensities. In Experiment 3, training as in Experiment 1 was followed by testing for food magazine approach and changes in insulin levels following x alone or in the presence of A or B. It was demonstrated that the A cue enhanced both responses. The results support the view [e.g., (18,19)] that insulin secretion is in part under the control of environmental cues, and findings (22) that show that conditioned context-like cues can modulate consummatory responding to punctate CSs.

Glucagon-like peptide-1 and the entero-insular axis in obese hyperglycaemic (ob/ob) mice

The intestines of obese hyperglycaemic (ob/ob) mice contain greatly increased amounts of glucagon-like immunoreactive peptides. To investigate their role in the increased activity of the entero-insular axis of these mice, the insulin-releasing effect of glucagon-like peptide-1 (GLP-1) was examined in 24 hour fasted 12-15 weeks old ob/ob mice under conditions of basal and elevated glycaemia. Compared with glucagon (100 micrograms/kg ip), which produce an approximately 3-fold increase in basal plasma glucose and insulin concentrations, GLP-1 (100 micrograms/kg ip) produce a very small (less than 1 fold) increase in plasma insulin, with no significant change in plasma glucose. The insulin-releasing effect of glucagon, but not GLP-1 was increased by administration in combination with glucose (2 g/kg ip). The results indicate that GLP-1, which exhibits considerable sequence homology with glucagon, exerts only a weak insulin-releasing effect without a significant hyperglycaemic effect in ob/ob mice. Thus GLP-1 is unlikely to be an important endocrine component of the two over-active entero-insular axis in ob/ob mice.

Glucoregulatory effects of bombesin in lean and genetically obese hyperglycaemic (ob/ob) mice

1. Plasma glucose and insulin responses to bombesin were examined in 12-15-week-old 12 hr fasted lean and genetically obese hyperglycaemic (ob/ob) mice. 2. Bombesin (1 mg/kg ip) produced a prompt but transient increase of plasma insulin in lean mice (maximum increase of 50% at 5 min), and a more slowly generated but protracted insulin response in ob/ob mice (maximum increase of 80% at 30 min). Plasma glucose concentrations of both groups of mice were increased by bombesin (maximum increases of 40 and 48% respectively in lean and ob/ob mice at 15 min). 3. When administered with glucose (2 g/kg ip), bombesin (1 mg/kg ip) rapidly increased insulin concentrations of lean and ob/ob mice (maximum increases of 39 and 63% respectively at 5 min). Bombesin did not significantly alter the rise of plasma glucose after exogenous glucose administration to these mice. 4. The results indicate that bombesin exerts an insulin-releasing effect in lean and ob/ob mice. The greater insulin-releasing effect in ob/ob mice renders bombesin a possible component of the overactive entero-insular axis in the ob/ob mutant, especially if it acts within the islets as a neurotransmitter or paracrine agent.

Substance P, neurokinin A, vasoactive intestinal polypeptide and gastrin releasing peptide in the intestine and pancreas of spontaneously obese-diabetic mice

The concentrations and contents of immunoreactive substance P (SP), neurokinin A (NKA), vasoactive intestinal polypeptide (VIP) and gastrin releasing peptide (GRP) were measured in acid-ethanol extracts of intestine (duodenum-jejunum-ileum) and pancreas of C57BL/KsJ diabetes-obese (db/db) mice, Aston obese-hyperglycaemic (ob/ob) mice, and their respective lean controls. The intestinal concentration of GRP and pancreatic concentrations of VIP and GRP were 36-57% lower in lean Aston mice than lean C57BL/KsJ mice, indicating the influence of genetic background in control mice. Intestinal concentrations of SP and NKA were reduced by 19-33% in the db/db and ob/ob mutants compared with their lean controls, but the intestinal contents of these peptides were normal or greater than normal due to intestinal hypertrophy of the mutant mice. The intestinal VIP concentration was not altered, but the content was increased by 87% and 25% respectively in db/db and ob/ob mice, whereas the intestinal GRP concentration was reduced by 51% in ob/ob mice. Pancreatic concentrations and contents of NKA, VIP and GRP were similar in lean and db/db C57BL/KsJ mice. However, pancreatic concentrations and contents of VIP and GRP were reduced by 51-55% in ob/ob mice compared with their lean controls. The sensitivity of the present assay did not permit accurate determination of the low pancreatic concentrations of SP. The results suggest that the spontaneous ob/ob and db/db syndromes of obesity and diabetes in mice are associated with reduced intestinal concentrations of SP and NKA. The ob/ob mouse also exhibited reductions of intestinal GRP and pancreatic GRP and VIP concentrations. These changes in regulatory peptides may relate to abnormalities of intestinal and possibly pancreatic function in obese and diabetic mutant mice.

Anorectic effect of fenfluramine, cholecystokinin and neurotensin in genetically obese (ob/ob) mice

To investigate the satiety defect of hyperphagic genetically obese (ob/ob) mice, acute feeding responses to three differently acting anorectic agents were determined in 7-9 weeks old lean (+/+) and ob/ob mice habituated to a restricted (0900-1230 hr) daily feeding routine. Fenfluramine (10 mg/kg), cholecystokinin (100 U/kg) and neurotensin (500 micrograms/kg), administered intraperitoneally 15 min before feeding, each produced a rapid but transient suppression of food consumption in ob/ob mice, similar to lean controls. The results suggest that neural satiety mechanisms triggered via serotoninergic pathways (fenfluramine), vagal afferents (cholecystokinin) and the hypothalamic paraventricular nucleus (neurotensin) are functional in ob/ob mice, supporting the view that the satiety defect of ob/ob mice resides outside of the nervous system.

Dietary components and plasma insulin responses to fasting and refeeding in genetically obese hyperglycaemic (ob/ob) mice

To investigate the role of dietary components in the hyperinsulinaemia of the obese hyperglycaemic (ob/ob) syndrome, plasma insulin responses to fasting and refeeding were examined in Aston ob/ob mice supplied with standard diet, non-digestible-carbohydrate test food, and isoenergetic test foods from which either carbohydrate, protein or fat was omitted. During fasting, plasma insulin concentrations fell more rapidly and to a greater extent than plasma glucose. Refeeding the standard diet raised insulin concentrations above normal, associated with a 25% compensatory increase in food intake over 24 h. Test foods supplied to previously fed or fasted mice produced glucose responses consistent with the available carbohydrate content. Carbohydrate-free food (protein and fat) provided a small insulinotropic stimulus; the effect of protein-free food (carbohydrate and fat) was greater; and the combination of carbohydrate with protein (fat-free food) evoked a marked insulin response. In contrast, insulin concentrations declined in mice given the non-digestible-carbohydrate food. Consumption of the standard diet was increased after 24 h feeding non-digestible-carbohydrate food, but was unaffected by a 30 h fast initiated 54 h previously. These results demonstrate that hyperinsulinaemia in ob/ob mice is not merely triggered by the ingestion of bulk, but depends on the type of nutrient ingested. Dietary carbohydrate appears to be the major stimulus to the hyperinsulinaemia, with an important augmentation in the presence of protein. Since direct glucose stimulation of insulin release is defective in ob/ob mice, the hyperinsulinaemia must be mediated by increased activity of the enteroinsular axis.