Involvement of apolipoprotein A-IV and cholecystokinin1 receptors in exogenous peptide YY3 36-induced stimulation of intestinal feedback

Capsaicin-induced visceral deafferentation does not attenuate flavor conditioning by intragastric fat infusions in mice

The postoral actions of sugar and fat can rapidly stimulate the intake of and preference for flavors associated with these nutrients via a process known as appetition. Prior findings revealed that postoral glucose appetition is not attenuated following capsaicin-induced visceral deafferentation. The present experiment determined if capsaicin treatment altered fat appetition in C57BL/6 mice. Following capsaicin (Cap) or control (Con) treatment, mice were fitted with chronic intragastric (IG) catheters. They were then given 1-h sessions with a flavored saccharin solution (CS-) paired with IG water infusion or a different flavor (CS+) paired with IG 6.4% fat infusion. IG fat stimulated CS+ intakes in both Cap and Con mice, and the groups displayed similar preferences for CS+ over CS- in two-choice tests. These results confirm prior reports of normal fat conditioning in rats exposed to capsaicin or vagal deafferentation surgery. In contrast, other recent findings indicate that total or selective vagotomy alters the preference of mice for dilute vs. concentrated fat sources.

The PYY/Y2R-Deficient Mouse Responds Normally to High-Fat Diet and Gastric Bypass Surgery

BACKGROUND/GOALS

The gut hormone peptide YY (PYY) secreted from intestinal L-cells has been implicated in the mechanisms of satiation via Y2-receptor (Y2R) signaling in the brain and periphery and is a major candidate for mediating the beneficial effects of bariatric surgery on appetite and body weight.

METHODS

Here we assessed the role of Y2R signaling in the response to low- and high-fat diets and its role in the effects of Roux-en-Y gastric bypass (RYGB) surgery on body weight, body composition, food intake, energy expenditure and glucose handling, in global Y2R-deficient (Y2RKO) and wildtype (WT) mice made obese on high-fat diet.

RESULTS

Both male and female Y2RKO mice responded normally to low- and high-fat diet in terms of body weight, body composition, fasting levels of glucose and insulin, as well as glucose and insulin tolerance for up to 30 weeks of age. Contrary to expectations, obese Y2RKO mice also responded similarly to RYGB compared to WT mice for up to 20 weeks after surgery, with initial hypophagia, sustained body weight loss, and significant improvements in fasting insulin, glucose tolerance, insulin resistance (HOMA-IR), and liver weight compared to sham-operated mice. Furthermore, non-surgical Y2RKO mice weight-matched to RYGB showed the same improvements in glycemic control as Y2RKO mice with RYGB that were similar to WT mice.

CONCLUSIONS

PYY signaling through Y2R is not required for the normal appetite-suppressing and body weight-lowering effects of RYGB in this global knockout mouse model. Potential compensatory adaptations of PYY signaling through other receptor subtypes or other gut satiety hormones such as glucagon-like peptide-1 (GLP-1) remain to be investigated.

High temporal resolution gastric emptying breath tests in mice

BACKGROUND

Gastric emptying is a complex physiological process regulating the division of a meal into smaller partitions for the small intestine. Disrupted gastric emptying contributes to digestive disease, yet current measures may not reflect different mechanisms by which the process can be altered.

METHODS

We have developed high temporal resolution solid and liquid gastric emptying breath tests in mice using [13 C]-octanoic acid and off axis- integrated cavity output spectroscopy (OA-ICOS). Stretched gamma variate and 2-component stretched gamma variate models fit measured breath excretion data.

KEY RESULTS

These assays detect acceleration and delay using pharmacological (7.5 mg/kg atropine) or physiological (nutrients, cold exposure stress, diabetes) manipulations and remain stable over time. High temporal resolution resolved complex excretion curves with 2 components, which was more prevalent in mice with delayed gastric emptying following streptozotocin-induced diabetes. There were differences in the gastric emptying of Balb/c vs C57Bl6 mice, with slower gastric emptying and a greater occurrence of two-phase gastric emptying curves in the latter strain. Gastric emptying of C57Bl6 could be accelerated by halving the meal size, but with no effect on the occurrence of two-phase gastric emptying curves. A greater proportion of two-phase gastric emptying was induced in Balb/c mice with the administration of PYY (8-80 nmol) 60 min following meal ingestion.

CONCLUSIONS AND INFERENCES

Collectively, these results demonstrate the utility of high temporal resolution gastric emptying assays. Two-phase gastric emptying is more prevalent than previously reported, likely involves intestinal feedback, but contributes little to the overall rate of gastric emptying.

Role of the gut, melanocortin system and malonyl-CoA in control of feed intake in non-ruminant animals

Regulation of feed intake is under complex control, involving physical, chemical, hormonal and neuronal responses. Understanding the regulation of feed intake in farm animals is key to optimisation of intake to meet production and profitability goals. Fundamental mechanisms regulating feed intake include constraints imposed by the gut, systems monitoring current and long-term energy status to increase or decrease intake, and hedonic, reward-related drives. Feed intake is closely related to the rate of passage of digesta and the capacity of the gastrointestinal tract. Indigestible fibre increases the rate of digesta passage and feed intake until excess distension sends signals of satiety to the brain. The presence of partially digested nutrients and products of microbial fermentation in the distal intestines releases peptides (PYY, OXM, GPL-1, Apo A-IV, amylin) from gut and pancreas to activate the intestinal brake, which slows the rate of passage and reduces feed intake. These peptides also act on orexigenic (NPY, AgRP) and anorexigenic (POMC, CART) peptides of the melanocortin system of the hypothalamus to reduce intake over the long term. Immediate energy status of the animal is monitored through the ratio of AMP : ATP via adenosine monophosphate-activated kinase and mammalian target of rapamycin, whereas the overall animal energy status is monitored by insulin, leptin and ghrelin. These energy-monitoring systems control short- and long-term intakes through the melanocortin system of the hypothalamus, primarily via malonyl-CoA, to alter the relative expression of orexigenic and anorexigenic peptides. Gut and hypothalamic control of feed intake can be over-ridden by hedonic, reward-related centres of the brain, predominantly through the release of dopamine. These hedonic responses can lead to over-consumption and obesity under some circumstances or reduced feed intake under stressful or other negative environmental situations. Knowledge of these mechanisms can be used to identify practical strategies for either increasing or decreasing voluntary intake in pigs.

Metabolic Actions of the Type 1 Cholecystokinin Receptor: Its Potential as a Therapeutic Target

Cholecystokinin (CCK) regulates appetite and reduces food intake by activating the type 1 CCK receptor (CCK1R). Attempts to develop CCK1R agonists for obesity have yielded active agents that have not reached clinical practice. Here we discuss why, along with new strategies to target CCK1R more effectively. We examine signaling events and the possibility of developing agents that exhibit ligand-directed bias, to dissociate satiety activity from undesirable side effects. Potential allosteric sites of modulation are also discussed, along with desired properties of a positive allosteric modulator (PAM) without intrinsic agonist action as another strategy to treat obesity. These new types of CCK1R-active drugs could be useful as standalone agents or as part of a rational drug combination for management of obesity.

Enteroendocrine cells: a review of their role in brain-gut communication

BACKGROUND

Specialized endoderm-derived epithelial cells, that is, enteroendocrine cells (EECs), are widely distributed throughout the gastrointestinal (GI) tract. Enteroendocrine cells form the largest endocrine organ in the body and play a key role in the control of GI secretion and motility, the regulation of food intake, postprandial glucose levels and metabolism. EECs sense luminal content and release signaling molecules that can enter the circulation to act as classic hormones on distant targets, act locally on neighboring cells and on distinct neuronal pathways including enteric and extrinsic neurons. Recent studies have shed light on EEC sensory transmission by showing direct connections between EECs and the nervous system via axon-like processes that form a well-defined neuroepithelial circuits through which EECs can directly communicate with the neurons innervating the GI tract to initiate appropriate functional responses.

PURPOSE

This review will highlight the role played by the EECs in the complex and integrated sensory information responses, and discuss the new findings regarding EECs in the brain-gut axis bidirectional communication.

Role of Y(2) receptors in the regulation of gastric tone in rats

We set out to determine the effect of peptide YY(3-36) (PYY(3-36)) on the gastric muscle tone in conscious rats by measuring intragastric pressure (IGP) during intragastric nutrient drink infusion. After an overnight fast, a chronically implanted gastric fistula was connected to a custom-made nutrient drink infusion system and a catheter to measure IGP. IGP was measured before and during the infusion of a nutrient drink (Nutridrink; 0.5 ml/min) until 10 ml was infused. Rats were treated with PYY(3-36) (0, 33, and 100 pmol·kg(-1)·min(-1)) in combination with a subcutaneous injection of the Y(2) receptor antagonists JNJ31020028 (10 mg/kg) or BIIE0246 (2 mg/kg). Experiments were also performed after subdiaphragmatic vagotomy and after pretreatment with 3 ml of nutrient drink (to mimic a fed state). IGP was compared as the average IGP during nutrient infusion, represented as means ± SE and compared using ANOVA. PYY(3-36) dose dependently increased the IGP during nutrient infusion (4.7 ± 0.3, 5.7 ± 0.5 and 7.3 ± 0.7 mmHg; P < 0.01) while JNJ31020028 and BIIE0246 could block this increase [4.4 ± 0.5 (P < 0.001) and 4.8 ± 0.4 (P < 0.05) mmHg, respectively]. Also in vagotomized rats, PYY(3-36) was able to significantly increase the IGP during, an effect attenuated by JNJ31020028. BIIE0246 and JNJ31020028 were not able to decrease the IGP when no PYY(3-36) was administered. PYY(3-36) increased gastric tone through an Y(2) receptor-mediated mechanism that does not involve the vagus nerve. Y(2) receptor antagonists were not able to decrease gastric tone without exogenous administration of PYY(3-36), indicating that Y(2) receptors do not play a crucial role in the determination of gastric tone in physiological conditions.

Rapid post-oral stimulation of intake and flavor conditioning by glucose and fat in the mouse

Although widely assumed to have only satiating actions, nutrients in the gut can also condition increases in intake in some cases. Here we studied the time course of post-oral nutrient stimulation of ingestion in food-restricted C57BL/6J mice. In experiment 1, mice adapted to drink a 0.8% sucralose solution 1 h/day, rapidly increased their rate of licking (within 4-6 min) when first tested with an 8% glucose solution and even more so in tests 2 and 3. Other mice decreased their licking rate when switched from sucralose to 8% fructose, a sugar that is sweet like glucose but lacks positive post-oral effects in mice. The glucose-stimulated drinking is due to the sugar's post-oral rather than taste properties, because sucralose is highly preferred to glucose and fructose in brief choice tests. A second experiment showed that the glucose-stimulated ingestion is associated with a conditioned flavor preference in both intact and capsaicin-treated mice. This indicates that the post-oral stimulatory action of glucose is not mediated by capsaicin-sensitive visceral afferents. In experiment 3, mice consumed flavored saccharin solutions as they self-infused water or glucose via an intragastric (IG) catheter. The glucose self-infusion stimulated ingestion within 13-15 min in test 1 and produced a conditioned increase in licking that was apparent in the initial minute of tests 2 and 3. Experiment 4 revealed that IG self-infusions of a fat emulsion also resulted in post-oral stimulation of licking in test 1 and conditioned increases in tests 2 and 3. These findings indicate that glucose and fat can generate stimulatory post-oral signals early in a feeding session that increase ongoing ingestion and condition increases in flavor acceptance and preference revealed in subsequent feeding sessions. The test procedures developed here can be used to investigate the peripheral and central processes involved in stimulation of intake by post-oral nutrients.

Impact of murine intestinal apolipoprotein A-IV expression on regional lipid absorption, gene expression, and growth

Apolipoprotein A-IV (apoA-IV) is synthesized by intestinal enterocytes during lipid absorption and secreted into lymph on the surface of nascent chylomicrons. A compelling body of evidence supports a central role of apoA-IV in facilitating intestinal lipid absorption and in regulating satiety, yet a longstanding conundrum is that no abnormalities in fat absorption, feeding behavior, or weight gain were observed in chow-fed apoA-IV knockout (A4KO) mice. Herein we reevaluated the impact of apoA-IV expression in C57BL6 and A4KO mice fed a high-fat diet. Fat balance and lymph cannulation studies found no effect of intestinal apoA-IV gene expression on the efficiency of fatty acid absorption, but gut sac transport studies revealed that apoA-IV differentially modulates lipid transport and the number and size of secreted triglyceride-rich lipoproteins in different anatomic regions of the small bowel. ApoA-IV gene deletion increased expression of other genes involved in chylomicron assembly, impaired the ability of A4KO mice to gain weight and increase adipose tissue mass, and increased the distal gut hormone response to a high-fat diet. Together these findings suggest that apoA-IV may play a unique role in integrating feeding behavior, intestinal lipid absorption, and energy storage.

Ileal interposition attenuates the satiety responses evoked by cholecystokinin-8 and -33

One of the possible mechanisms by which the weight-reducing surgical procedure ileal interposition (II) works is by increasing circulating levels of lower gut peptides that reduce food intake, such as glucagon like peptide-1 and peptide YY. However, since this surgery involves both lower and upper gut segments, we tested the hypothesis that II alters the satiety responses evoked by the classic upper gut peptide cholecystokinin (CCK). To test this hypothesis, we determined meal size (MS), intermeal interval (IMI) and satiety ratio (SR) evoked by CCK-8 and -33 (0, 1, 3, 5nmol/kg, i.p.) in two groups of rats, II and sham-operated. CCK-8 and -33 reduced MS more in the sham group than in the II group; CCK-33 prolonged IMI in the sham group and increased SR in both groups. Reduction of cumulative food intake by CCK-8 in II rats was blocked by devazepide, a CCK(1) receptor antagonist. In addition, as previously reported, we found that II resulted in a slight reduction in body weight compared to sham-operated rats. Based on these observations, we conclude that ileal interposition attenuates the satiety responses of CCK. Therefore, it is unlikely that this peptide plays a significant role in reduction of body weight by this surgery.

Electrophysiological evidence for distinct vagal pathways mediating CCK-evoked motor effects in the proximal versus distal stomach

Intravenous cholecystokinin octapeptide (CCK-8) elicits vago-vagal reflexes that inhibit phasic gastric contractions and reduce gastric tone in urethane-anaesthetized rats. A discrete proximal subdivision of the ventral gastric vagus nerve (pVGV) innervates the proximal stomach, but the fibre populations within it have not been characterized previously.We hypothesized that I.V. CCK-8 injection would excite inhibitory efferent outflow in the pVGV, in contrast to its inhibitory effect on excitatory efferent outflow in the distal subdivision (dVGV), which supplies the distal stomach. In each VGV subdivision, a dual-recording technique was used to record afferent and efferent activity simultaneously, while also monitoring intragastric pressure (IGP). CCK-8 dose dependently (100-1000 pmol kg(-1), I.V.) reduced gastric tone, gastric contractile activity and multi-unit dVGV efferent discharge, but increased pVGV efferent firing. Single-unit analysis revealed a minority of efferent fibres in each branch whose response differed in direction from the bulk response. Unexpectedly, efferent excitation in the pVGV was significantly shorter lived and had a significantly shorter decay half-time than did efferent inhibition in the dVGV, indicating that distinct pathways drive CCK-evoked outflow to the proximal vs. the distal stomach. Efferent inhibition in the dVGV began several seconds before, and persisted significantly longer than, simultaneously recorded dVGV afferent excitation.Thus, dVGV afferent excitation could not account for the pattern of dVGV efferent inhibition. However, the time course of dVGV afferent excitation paralleled that of pVGV efferent excitation. Similarly, the duration of CCK-8-evoked afferent responses recorded in the accessory celiac branch of the vagus (ACV) matched the duration of dVGV efferent responses. The observed temporal relationships suggest that postprandial effects on gastric complicance of CCK released from intestinal endocrine cells may require circulating concentrations to rise to levels capable of exciting distal gastric afferent fibres, in contrast to more immediate effects on distal gastric contractile activity mediated via vago-vagal reflexes initiated by paracrine excitation of intestinal afferents.

Cocaine- and amphetamine-regulated transcript mediates the actions of cholecystokinin on rat vagal afferent neurons

BACKGROUND & AIMS

Cholecystokinin (CCK) acts on vagal afferent neurons to inhibit food intake and gastric emptying; it also increases expression of the neuropeptide cocaine- and amphetamine-regulated transcript (CART), but the significance of this is unknown. We investigated the role of CARTp in vagal afferent neurons.

METHODS

Release of CART peptide (CARTp) from cultured vagal afferent neurons was determined by enzyme-linked immunosorbent assay. Expression of receptors and neuropeptides in rat vagal afferent neurons in response to CARTp was studied using immunohistochemistry and luciferase promoter reporter constructs. Effects of CARTp and CCK were studied on food intake.

RESULTS

CCK stimulated CARTp release from cultured nodose neurons. CARTp replicated the effect of CCK in stimulating expression of Y2R and of CART itself in these neurons in vivo and in vitro, but not in inhibiting cannabinoid-1, melanin-concentrating hormone, and melanin-concentrating hormone-1 receptor expression. Effects of CCK on Y2R and CART expression were reduced by CART small interfering RNA or brefeldin A. Exposure of rats to CARTp increased the inhibitory action of CCK on food intake after short-, but not long-duration, fasting.

CONCLUSIONS

The actions of CCK in stimulating CART and Y2R expression in vagal afferent neurons and in inhibiting food intake are augmented by CARTp; CARTp is released by CCK from these neurons, indicating that it acts as an autocrine excitatory mediator.

Peripheral peptide YY inhibits propulsive colonic motor function through Y2 receptor in conscious mice

Peptide YY (PYY) antisecretory effect on intestinal epithelia is well established, whereas less is known about its actions to influence colonic motility in conscious animals. We characterized changes in basal function and stimulated colonic motor function induced by PYY-related peptides in conscious mice. PYY(3-36), PYY, and neuropeptide Y (NPY) (8 nmol/kg) injected intraperitoneally inhibited fecal pellet output (FPO) per hour during novel environment stress by 90%, 63%, and 57%, respectively, whereas the Y(1)-preferring agonists, [Pro(34)]PYY and [Leu(31),Pro(34)]NPY, had no effect. Corticotrophin-releasing factor 2 receptor antagonist did not alter PYY(3-36) inhibitory action. PYY and PYY(3-36) significantly reduced restraint-stimulated defecation, and PYY(3-36) inhibited high-amplitude distal colonic contractions in restrained conscious mice for 1 h, by intraluminal pressure with the use of a microtransducer. PYY suppression of intraperitoneal 5-hydroxytryptophan induced FPO and diarrhea was blocked by the Y(2) antagonist, BIIE0246, injected intraperitoneally and mimicked by PYY(3-36), but not [Leu(31),Pro(34)]NPY. PYY(3-36) also inhibited bethanechol-stimulated FPO and diarrhea. PYY(3-36) inhibited basal FPO during nocturnal feeding period and light phase in fasted/refed mice for 2-3 h, whereas the reduction of food intake lasted for only 1 h. PYY(3-36) delayed gastric emptying after fasting-refeeding by 48% and distal colonic transit time by 104%, whereas [Leu(31),Pro(34)]NPY had no effect. In the proximal and distal colon, higher Y(2) mRNA expression was detected in the mucosa than in muscle layers, and Y(2) immunoreactivity was located in nerve terminals around myenteric neurons. These data established that PYY/PYY(3-36) potently inhibits basal and stress/serotonin/cholinergic-stimulated propulsive colonic motor function in conscious mice, likely via Y(2) receptors.

Cholecystokinin and gut-brain signalling

Enteroendocrine cells of the gastrointestinal tract act as a luminal surveillance system responding to either the presence or absence of food in the gut lumen. Collectively, their secretory products regulate the course of digestion and determine the delivery of nutrient to the gut by controlling food intake. Afferent neurons of the vagus nerve are an important target of gut hormones, particularly for control of food intake. The intestinal hormone cholecystokinin (CCK) stimulates vagal afferent neuron discharge and also controls the expression of both G-protein coupled receptors and peptide neurotransmitters in these neurons. When plasma CCK concentrations are low, for example in fasting, vagal afferent neurons express cannabinoid CB1 and melanin concentrating hormone (MCH)-1 receptors, both of which are associated with stimulation of food intake. Post-prandial release of CCK rapidly down-regulates the expression of both receptors but stimulates the expression of Y2 receptors in neurons projecting to the stomach. In fasting, there is also increased expression in these neurons of the appetite-stimulating neuropeptide transmitter MCH, and depressed expression of the satiety-peptide cocaine and amphetamine regulated transcript (CART). Secretion of CCK decreases expression of MCH and increases expression of CART. The neurochemical phenotype of vagal afferent neurons therefore encodes whether or not there has been nutrient ingestion over the previous period. At low plasma concentrations of CCK vagal afferent neurons exhibit increased capacity for appetite-stimulation, while post-prandial concentrations of CCK lead to enhanced capacity for satiety signalling. A gatekeeper function can therefore be attributed to CCK in that its presence or absence influences the capacity of vagal afferent neurons to respond to other neurohormonal signals.

Metabolic surgery and gut hormones - a review of bariatric entero-humoral modulation

The global pandemic of obesity is increasing. Inappropriate food intake relative to energy expenditure results in increased adiposity. These factors are partly regulated by signals through the gut-brain and adipose-brain axes. Metabolic operations (otherwise known as Bariatric surgery) offer the most effective results for sustained metabolic improvement and weight loss. They modulate a number of gut hormones that constitute the gut-brain axis. This review summarizes the literature to-date reporting the gut hormone changes associated with these operations and their subsequent effects on appetite. Understanding the anatomical differences between each operation and how these can differentially regulate gut hormonal release can provide new treatments and targets for obesity, appetite and metabolic disorders.

The versatility of the vagus

The gut is one of several organs contributing to the peripheral signalling network that controls food intake. Afferent neurons of the vagus nerve provide an important pathway for gut signals that act by triggering ascending pathways from the brain stem to hypothalamus. Recent work indicates the existence of mechanisms operating at the level of vagal afferent neurons to modulate the effect of gastrointestinal satiety signals. Thus, the well known satiety hormone cholecystokinin (CCK) not only stimulates the discharge of these neurons but also controls their expression of both G-protein coupled receptors and peptide neurotransmitters known to influence food intake. When plasma CCK concentrations are low e.g. in fasting, the expression by vagal afferent neurons of cannabinoid (CB)-1 and melanin concentrating hormone (MCH)-1 receptors is increased. Release of CCK by feeding leads to a rapid down-regulation of expression of both receptors and to increased expression of Y2 receptors. In fasting, there is also increased expression in these neurons of the appetite-stimulating neuropeptide transmitter MCH, and depressed expression of the satiety-peptide cocaine and amphetamine regulated transcript (CARTp); endogenous CCK decreases MCH expression and stimulates CART expression. The gastric orexigenic hormone ghrelin blocks these actions of CCK at least in part by excluding phosphoCREB from the nucleus. The data suggest that CCK acts as a gatekeeper to determine the capacity of other neuroendocrine signals to act via vagal afferent neurons to influence food intake.

Cholecystokinin regulates expression of Y2 receptors in vagal afferent neurons serving the stomach

The intestinal hormones CCK and PYY3-36 inhibit gastric emptying and food intake via vagal afferent neurons. Here we report that CCK regulates the expression of Y2R, at which PYY3-36 acts. In nodose ganglia from rats fasted up to 48 h, there was a fivefold decrease of Y2R mRNA compared with rats fed ad libitum; Y2R mRNA in fasted rats was increased by administration of CCK, and by refeeding through a mechanism sensitive to the CCK1R antagonist lorglumide. Antibodies to Y2R revealed expression in both neurons and satellite cells; most of the former (89 +/- 4%) also expressed CCK1R. With fasting there was loss of Y2R immunoreactivity in CCK1R-expressing neurons many of which projected to the stomach, but not in satellite cells or neurons projecting to the ileum or proximal colon. Expression of a Y2R promoter-luciferase reporter (Y2R-luc) in cultured vagal afferent neurons was increased in response to CCK by 12.3 +/- 0.1-fold and by phorbol ester (16.2 +/- 0.4-fold); the response to both was abolished by the protein kinase C inhibitor Ro-32,0432. PYY3-36 stimulated CREB phosphorylation in rat nodose neurons after priming with CCK; in wild-type mice PYY3-36 increased Fos labeling in brainstem neurons but in mice null for CCK1R this response was abolished. Thus Y2R is expressed by functionally distinct subsets of nodose ganglion neurons projecting to the stomach and ileum/colon; in the former expression is dependent on stimulation by CCK, and there is evidence that PYY3-36 effects on vagal afferent neurons are CCK dependent.

Activation of hindbrain neurons in response to gastrointestinal lipid is attenuated by high fat, high energy diets in mice prone to diet-induced obesity

Food intake is controlled by peripheral signals from the gastrointestinal tract and adipocytes, which are integrated within the central nervous system. There is evidence that signals from the GI tract are modulated by long term changes in diet, possibly leading to hyperphagia and increased body weight. We tested the hypothesis that diet-induced obese-prone (DIO-P) and obese-resistant (DIO-R) mice strains differ in the long term adaptive response of the gut-brain pathway to a high fat diet. Immunochemical detection of Fos protein was used as a measure of neuronal activation in the nucleus of the solitary tract (NTS) in response to intragastric administration of lipid in DIO-P (C57Bl6) and DIO-R (129sv) mouse strains maintained on chow or high fat, high energy diets (45% or 60% kcal from fat). Intragastric lipid administration activated neurons in the NTS in both DIO-P and DIO-R mice; the number of activated neurons was significantly greater in DIO-P than in DIO-R mice (P<0.001). However, lipid-induced activation of NTS neurons in DIO-P mice was attenuated by approximately 30% after maintenance on either 45% or 60% HF diet, for 4 or 8 weeks, compared to chow fed controls (P<0.05). In contrast, in DIO-R mice, maintenance on a HF diet (45% or 60%) had no effect on lipid-induced activation of NTS neurons. These results demonstrate that DIO-P and DIO-R mice strains differ in the adaptation of the pathway to long term ingestion of high fat diets, which may contribute to decrease satiation and increased food intake.

Peptide YY

PURPOSE OF REVIEW

This review discusses recent studies examining the effects of peptide YY on energy homeostasis, highlights the emerging hedonic effects of peptide YY and evaluates the therapeutic potential of the peptide YY system.

RECENT FINDINGS

A role for exogenous PYY3-36 as an anorectic agent in obese humans and rodents has been established and weight loss effects demonstrated in obese rodents. New lines of evidence support a role for endogenous peptide YY in regulating energy homeostasis. The NPY-Y2 receptor mediates the anorectic actions of PYY3-36 with rodent studies implicating the hypothalamus, vagus and brainstem as key target sites. Functional imaging in humans has confirmed that PYY3-36 activates brainstem and hypothalamic regions. The greatest effects, however, were observed within the orbitofrontal cortex, a brain region involved in reward processing. Further evidence for a hedonic role for PYY3-36 is supported by rodent studies showing that PYY3-36 decreases the motivation to seek high-fat food. Rodent studies using selective Y2 agonists and strategies combining PYY3-36/Y2 agonists with other anorectic agents have revealed increased anorectic and weight-reducing effects.

SUMMARY

Peptide YY plays a role in the integrative regulation of metabolism. The emerging hedonic effects of peptide YY together with the weight-reducing effects observed in obese rodents suggest that targeting the peptide YY system may offer a therapeutic strategy for obesity.

Mechanisms of CCK signaling from gut to brain

Following the observation that exogenous peripheral injection of CCK could inhibit food intake, the mechanisms by which CCK influences the gut-brain pathway have been the subject of intense study for nearly 30 years. Recently, it has become evident that the system is more complex and that the consequences of CCK's action on the gut-brain pathway are more far reaching than previously recognized. This review will examine the recent evidence showing the role of CCK and CCK1Rs in modulating expression of other receptors for orexigenic and anorexigenic regulatory peptides at the level of vagal afferent neurons. In addition, new evidence showing the importance of the action of CCK at the level of the vagus nerve in the regulation of food intake, body weight, and in activation of an anti-inflammatory pathway will be reviewed.