Cephalic phase of pancreatic secretion

Ghrelin: A link between memory and ingestive behavior

Feeding is a highly complex behavior that is influenced by learned associations between external and internal cues. The type of excessive feeding behavior contributing to obesity onset and metabolic deficit may be based, in part, on conditioned appetitive and ingestive behaviors that occur in response to environmental and/or interoceptive cues associated with palatable food. Therefore, there is a critical need to understand the neurobiology underlying learned aspects of feeding behavior. The stomach-derived "hunger" hormone, ghrelin, stimulates appetite and food intake and may function as an important biological substrate linking mnemonic processes with feeding control. The current review highlights data supporting a role for ghrelin in mediating the cognitive and neurobiological mechanisms that underlie conditioned feeding behavior. We discuss the role of learning and memory on food intake control (with a particular focus on hippocampal-dependent memory processes) and provide an overview of conditioned cephalic endocrine responses. A neurobiological framework is provided through which conditioned cephalic ghrelin secretion signals in neurons in the hippocampus, which then engage orexigenic neural circuitry in the lateral hypothalamus to express learned feeding behavior.

Control of pancreatic secretion in humans

Studies on the control of pancreatic secretion in humans of all ages have been a difficult task over the years because of patients' availability and ethic committee rules. Nevertheless, studies were performed and the objectives of this review are to summarize our knowledge on the development of secretory process in newborns, on the different phases of the pancreatic responses to a meal, on the pancreatic responses to the different components of the diet, on the mechanisms involved in the control of the pancreatic responses, and finally on the receptors involved in these controls.

Chemosensory dysfunction is a primary factor in the evolution of declining nutritional status and quality of life in patients with advanced cancer

Alterations in taste and smell functions have been reported in cancer patients. Although these senses are known to be particularly affected by chemotherapy, many features of chemosensory perception in cancer patients remain obscure. The relative importance of chemosensory changes in the etiology of malnutrition and wasting is not known. To assess this relationship, self-perceived taste and smell function were evaluated using a validated questionnaire in 66 patients with advanced cancer receiving palliative care (median survival 7.4 months). Participants also completed 3-day food records to assess dietary intake, and the Functional Assessment of Anorexia/Cachexia Therapy questionnaire to assess quality of life (QOL). Total chemosensory complaint scores ranged from 0 to 14 on a 16-point scale. Only 14% of the subjects reported no chemosensory complaints of any kind, whereas 86% reported some degree of chemosensory abnormality. The most common complaints were persistent bad taste in the mouth, taste distortion, and heightened sensitivity to odors. Subjects with severe chemosensory complaints showed substantially lower energy intakes (by 900-1,100 kcal/day), higher rates of weight loss, and lower QOL scores than subjects with mild or moderate chemosensory complaints. Severe chemosensory dysfunction is persistent well beyond the window of active therapy in patients with advanced cancer and represents a primary factor relating to malnutrition, wasting, and poor QOL. Further research is required to identify appropriate strategies to alleviate this important group of symptoms, to determine whether intervention will improve QOL, and to match foods and diet to the unique chemosensory profile of advanced cancer patients.

Food perception and postprandial lipid metabolism

The postprandial response to macronutrients in the diet, particularly carbohydrates and fats, underpins the detrimental changes in metabolism (impaired glucose tolerance or postprandial hyperlipaemia) and later pathology (insulin resistance, type 2 diabetes or atherosclerosis) associated with Westernised diets. Research has shown that in addition to what and how much we eat, eating behaviour in itself may be implicated in postprandial regulation. The process of ingestion stimulates cholinergic-vagal activity, irrespective of what is eaten, important in determining both the absorption and subsequent utilisation of nutrients but also potentially food intake through effects on hunger and satiety. Modifications in this aspect of physiology have the potential to influence all aspects of postprandial metabolism and subsequent disease risk in humans.

Orosensory considerations

The chemical senses (taste, smell, and chemical irritation) convey information from the external to the internal environment. This information influences an organism's quality of life, safety, reproductive function, and, to the present point, nutritional status. To illustrate this role, the effects of chemosensory stimulation on food choice, gastrointestinal function, and energy balance will be briefly reviewed. Each role is achieved, in part, by the chemosensory cue initiating anticipatory responses for an impending homeostatic challenge, a meal.

First-phase insulin secretion: does it exist in real life? Considerations on shape and function

To fulfill its preeminent function of regulating glucose metabolism, insulin secretion must not only be quantitatively appropriate but also have qualitative, dynamic properties that optimize insulin action on target tissues. This review focuses on the importance of the first-phase insulin secretion to glucose metabolism and attempts to illustrate the relationships between the first-phase insulin response to an intravenous glucose challenge and the early insulin response following glucose ingestion. A clear-cut first phase occurs only when the beta-cell is exposed to a rapidly changing glucose stimulus, like the one induced by a brisk intravenous glucose administration. In contrast, peripheral insulin concentration following glucose ingestion does not bear any clear sign of biphasic shape. Coupling data from the literature with the results of a beta-cell model simulation, a close relationship between the first-phase insulin response to intravenous glucose and the early insulin response to glucose ingestion emerges. It appears that the same ability of the beta-cell to produce a pronounced first phase in response to an intravenous glucose challenge can generate a rapidly increasing early phase in response to the blood glucose profile following glucose ingestion. This early insulin response to glucose is enhanced by the concomitant action of incretins and neural responses to nutrient ingestion. Thus, under physiological circumstances, the key feature of the early insulin response seems to be the ability to generate a rapidly increasing insulin profile. This notion is corroborated by recent experimental evidence that the early insulin response, when assessed at the portal level with a frequent sampling, displays a pulsatile nature. Thus, even though the classical first phase does not exist under physiological conditions, the oscillatory behavior identified at the portal level does serve the purpose of rapidly exposing the liver to elevated insulin levels that, also in virtue of their up-and-down pattern, are particularly effective in restraining hepatic glucose production.

Nonparallel secretion of antibacterial activity and protein in porcine pancreatic juice

The antimicrobial activity of exocrine pancreatic juice is an important component of gastrointestinal tract innate defenses, yet little is known about whether secretion is regulated in parallel with digestive enzymes. In this study, we used 8 pigs with pancreatic catheters to quantify antibacterial activity and measure protein content (indicator of enzyme secretion) of pancreatic juice collected hourly from 0700 to 1900, with the animals adapted to being fed at 0800 and 1600. Antibacterial activity in the samples of pancreatic juice was quantified by comparing the growth inhibition of Staphylococcus aureus subsp. aureus strain ATCC 6538P relative to a known concentration of gentamicin. Antibacterial activity (U/mL and /min) was highest in samples collected 1 hour prior to feeding (equivalent to 0.6 microgram gentamicin/mL), declined as the meal was consumed, and was lower (P < 0.05) in samples collected while the meals were being digested (0.41 microgram gentamicin/mL). Protein content was negatively correlated with antibacterial activity, with protein secretion lower (mg/mL and /min) before feeding, with an increase as the pigs ate and digested the meals (P < 0.05). The results indicate that the antibacterial activity in pancreatic juice is not secreted in parallel with protein secretion, suggesting that regulation involves alternative signaling pathways or contrasting responses to shared signals.

Exogenous leptin inhibits the secretion of pancreatic juice via a duodenal CCK1-vagal-dependent mechanism in anaesthetized rats

Leptin originally described as product of the ob gene has been shown to be expressed in various tissues including the gastrointestinal tract. In this study, we investigated the influence of leptin on the secretion of pancreatic juice in biliary-pancreatic duct cannulated anaesthetised rats and in dispersed rat pancreatic acini in vitro. Exogenous leptin was given in boluses intravenously with or without CCK-8 (12 pmol kg(-1) body weight) in the presence or absence pharmacological CCK(1) receptor blockade, cervical vagotomy, and capsaicin pre-treatment. Administration of leptin (0.1, 1 and 10 microg kg(-1) body weight) did not affect the volume of bile and pancreatic juice while the protein and trypsin outputs were reduced in a dose-dependent manner. In the rats, leptin inhibited CCK-8 stimulated protein and trypsin outputs stronger than the basal pancreatic secretion. The inhibition by leptin was abolished by the pharmacological CCK(1) receptor blockade, cervical vagotomy, and capsaicin pre-treatment. In contrast, leptin did not affect basal and CCK-8-stimulated amylase release from the dispersed rat pancreatic acini in vitro. In conclusion, the results of the present study suggest that leptin does not act directly on the rat pancreatic acinar cells but inhibits the secretion of pancreatic enzymes acting indirectly via a neurohormonal CCK-vagal-dependent mechanism.

Leptin protects the pancreas from damage induced by caerulein overstimulation by modulating cytokine production

BACKGROUND

Recent identification of specific leptin receptors in the pancreas suggests that this peptide may also play some role in this gland.

AIM

To examine the effect of intraperitoneal (i.p.) or intracerebroventricular (i.c.v.) administration of leptin in rats on caerulein-induced pancreatitis (CIP), pancreatic gene expression of leptin and inflammatory cytokine production.

METHODS

Caerulein (25 micrograms/kg) was infused subcutaneously into conscious rats over 5 h to produce CIP. Leptin (1, 5, or 10 micrograms/kg) was injected i.p. or i.c.v. 30 min prior to the CIP induction. The plasma level of TNF alpha and IL-4 was determined by ELISA, while plasma leptin was measured by RIA and leptin gene expression in pancreas by RT-PCR.

RESULTS

CIP was characterized by the usual pancreatic edema, reduction in pancreatic blood flow (PBF) and an increase in serum levels of amylase, TNF alpha and IL-4. Pretreatment with i.p. or i.c.v. leptin of the CIP rats partially reversed the harmful effects of CIP on the pancreas, and reduced pancreatic inflammation and the fall in PBF. This was accompanied by a dose-dependent reduction in serum levels of amylase and TNF alpha, while serum IL-4 in the CIP rats pretreated with leptin rose dose-dependently as compared to control rats with CIP alone. Pretreatment with leptin resulted in the dose-dependent rise in plasma leptin level over that observed in vehicle-treated controls. Leptin mRNA expression in the pancreas was dose-dependently increased after infusion of caerulein. Leptin content in isolated pancreatic acini was also increased dose-dependently by caerulein added to the incubation medium bathing these acini.

CONCLUSIONS

(1) Exogenous leptin protects the pancreas against damage by CIP; (2) endogenous leptin seems to limit the extend of pancreatic damage, and (3) these protective effects of leptin could be attributed to the reduction in TNF alpha and to the increase in IL-4 production.

Timing of vagal stimulation affects postprandial lipid metabolism in humans

BACKGROUND

Vagal stimulation combined with an oral fat load enhances postprandial lipemia in animals and humans.

OBJECTIVE

We assessed whether the observed postprandial increase in plasma lipids could be explained by changes in exogenous (chylomicron) or endogenous (VLDL) lipid metabolism and whether the timing of vagal stimulation in relation to fat intake was important.

DESIGN

Vagal stimulation was achieved by using the modified sham feeding (MSF) technique, in which food is tasted and chewed but not swallowed. Seven healthy men consumed an oral fat load (50 g) on one occasion (control protocol). On 2 other occasions, they consumed an oral fat load combined with MSF of an appetizing meal. MSF was performed for either 1 h before or 1 h after the oral fat load. Blood was collected for 7 h and was analyzed for hormones and metabolites.

RESULTS

The postprandial triacylglycerol response differed significantly (P < 0.001) between the 3 protocols. Both MSF studies resulted in significantly higher plasma pancreatic polypeptide concentrations compared with the control. Compared with MSF for 1 hour after fat intake, MSF for 1 h before fat intake resulted in significantly higher plasma insulin concentrations (P = 0.013), a more rapid rise in chylomicron triacylglycerol concentrations (P = 0.04), and higher VLDL triacylglycerol and apoliprotein B-100 concentrations.

CONCLUSIONS

Vagal stimulation enhanced postprandial lipemia via effects on both chylomicron and VLDL metabolism. MSF before fat intake had more dramatic effects on postprandial lipemia than did MSF after fat intake, possibly because of increased parasympathetic activity at the time of ingestion.

Modulation of exocrine pancreatic secretion by leptin through CCK(1)-receptors and afferent vagal fibres in the rat

In this report, we determined whether leptin could modify the exocrine pancreatic secretion of anaesthetized rats in vivo. Intravenous injection of recombinant murine leptin resulted in a time- and dose-dependent stimulation of exocrine pancreatic secretion, maximally observed with 30 nmol/kg of leptin. This stimulation of pancreatic water, bicarbonate, and protein output was abolished by atropine, hexamethonium, L364,718 ([3S(-)-N-(1,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzodiazepine]), a cholecystokinin CCK(1) receptor antagonist or perivagal capsaicin pretreatment, but unaffected by the CCK(2) receptor antagonist L365,260 ([3R(+)-N-(2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzodiazepin-3yl)-N'-(3-methylphenyl)urea]). In addition, the physiological dose of 3 nmol/kg leptin, ineffective per se, potentiated the secretory effect of 45 pmol/kg of cholecystokinin octapeptide (CCK-8) on exocrine pancreatic secretion. Furthermore, intraperitoneal leptin induced a rapid increase in plasma CCK levels in vivo in the rat. In conclusion, exogenous leptin can modulate exocrine pancreatic secretion through mechanisms involving CCK(1) receptors and capsaicin-sensitive afferent fibres in the rat. Whether this may have a physiological relevance in the postprandial regulation of exocrine pancreatic secretion and thus in nutrient digestion will require further investigations.

Role of leptin in the stomach and the pancreas

Leptin, a 16 kDa protein encoded by the ob gene, is known mainly for its role in the regulation of food intake, body composition and energy expenditure through a central feedback mechanism. Initially leptin was considered as an ob gene product of adipocytes but recently the presence of leptin and its receptors have been revealed in other organs including gastric mucosa and the pancreas and found to be released from these organs by cholecystokinin (CCK), gastrin and ordinary feeding. Furthermore, leptin was found to mimic the action of CCK on gastric and pancreatic integrity, while reducing the food intake and to affect gastric and pancreatic secretion. This report emphasizes the role of leptin originating from the gastrointestinal tract acting synergistically with CCK at the hypothalamus level on the mechanism of food intake and locally on the protection of gastric mucosa and the pancreas against noxious agents and to maintain tissue integrity.