Leptin Resistance in Vagal Afferent Neurons Inhibits Cholecystokinin Signaling and Satiation in Diet Induced Obese Rats

Background and Aims: The gastrointestinal hormone cholecystokinin (CCK) plays an important role in regulating meal size and duration by activating CCK1 receptors on vagal afferent neurons (VAN). Leptin enhances CCK signaling in VAN via an early growth response 1 (EGR1) dependent pathway thereby increasing their sensitivity to CCK. In response to a chronic ingestion of a high fat diet, VAN develop leptin resistance and the satiating effects of CCK are reduced. We tested the hypothesis that leptin resistance in VAN is responsible for reducing CCK signaling and satiation. Results: Lean Zucker rats sensitive to leptin signaling, significantly reduced their food intake following administration of CCK8S (0.22 nmol/kg, i.p.), while obese Zucker rats, insensitive to leptin, did not. CCK signaling in VAN of obese Zucker rats was reduced, preventing CCK-induced up-regulation of Y2 receptor and down-regulation of melanin concentrating hormone 1 receptor (MCH1R) and cannabinoid receptor (CB1). In VAN from diet-induced obese (DIO) Sprague Dawley rats, previously shown to become leptin resistant, we demonstrated that the reduction in EGR1 expression resulted in decreased sensitivity of VAN to CCK and reduced CCK-induced inhibition of food intake. The lowered sensitivity of VAN to CCK in DIO rats resulted in a decrease in Y2 expression and increased CB1 and MCH1R expression. These effects coincided with the onset of hyperphagia in DIO rats. Conclusions: Leptin signaling in VAN is required for appropriate CCK signaling and satiation. In response to high fat feeding

Modulation of gut-brain signalling : CCK effects on vagal afferent neurons

EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Ability of GLP-1 to decrease food intake is dependent on nutritional status

UnlabelledGut-derived glucagon like peptide-1 (GLP-1) acts in the postprandial period to stimulate insulin secretion and inhibit gastrointestinal motor and secretory function; whether endogenous peripheral GLP-1 inhibits food intake is less clear. We hypothesized that GLP-1 inhibits food intake in the fed, but not fasted, state. There is evidence that GLP-1 acts via stimulation of vagal afferent neurons (VAN); we further hypothesized that the satiating effects of endogenous GLP-1 in the postprandial period is determined either by a change in GLP-1 receptor (GLP-1R) expression or localization to different cellular compartments in VAN.MethodsFood intake was recorded following administration of GLP-1 (50μg/kg or 100μg/kg) or saline (IP) in Wistar rats fasted for 18h or fasted then re-fed with 3g chow. GLP-1R protein expression and localization on VAN were determined by immunocytochemistry and immunoblots in animals fasted for 18h or fasted then re-fed for 40min. GLP-1R mRNA level was detected in animals fasted for 18h or fasted and re-fed ad libitum for 2h.ResultsGLP-1 (100μg/kg) significantly reduced 40min food intake by 38% in re-fed but not fasted rats (p<0.05). GLP-1R mRNA or protein levels in VAN were unchanged in re-fed compared to fasted rats. However, GLP-1R localization to the plasma membrane was significantly increased in VAN by feeding.ConclusionFeeding changes the ability of peripheral GLP-1 to inhibit food intake. GLP-1Rs are trafficked to the plasma membrane in response to a meal. GLP-1 may play a role in regulating food intake in the postprandial period

Chronic exposure to Low dose bacterial lipopolysaccharide inhibits leptin signaling in vagal afferent neurons

Bacterially derived factors are implicated in the causation and persistence of obesity. Ingestion of a high fat diet in rodents and obesity in human subjects is associated with chronic elevation of low plasma levels of lipopolysaccharide (LPS), a breakdown product of Gram-negative bacteria. The terminals of vagal afferent neurons are positioned within the gut mucosa to convey information from the gut to the brain to regulate food intake and are responsive to LPS. We hypothesized that chronic elevation of LPS could alter vagal afferent signaling. We surgically implanted osmotic mini-pumps that delivered a constant, low-dose of LPS into the intraperitoneal cavity of rats (12.5 μg/kg/hr for 6 weeks). LPS-treated rats developed hyperphagia and showed marked changes in vagal afferent neuron function. Chronic LPS treatment reduced vagal afferent leptin signaling, characterized by a decrease in leptin-induced STAT3 phosphorylation. In addition, LPS treatment decreased cholecystokinin-induced satiety. There was no alteration in leptin signaling in the hypothalamus. These findings offer a mechanism by which a change in gut microflora can promote hyperphagia, possibly leading to obesity

Progressive increase in large intestine transcellular but not paracellular permeability correlates with plasma endotoxemia in diet-induced obese rats

Background: High fat diet-induced obesity is characterized by increased intestinal paracellular permeability, increased plasma lipopolysaccharide (LPS), low-grade inflammation and metabolic alterations. Whether the increase in plasma LPS is due to the increased paracellular permeability is unclear. Moreover, the contribution of altered paracellular versus transcellular transport in different regions of the intestine to the obese phenotype has not been determined. Since LPS is a large molecule (.10 kDa), we hypothesized that it crosses the epithelium via a transcellular route. The aims of this study aims were to determine 1) the route of LPSpassage and 2) the temporal relationship between alterations in small and large intestinal permeability and endotoxemia in rats fed a high fat diet. Methods: Route of LPS passage was evaluated in cell culture models (Caco-2 and T84) and in rat intestinal tissues mounted in Ussing chambers using LPS-FITC, horseradish peroxidase (HRP, marker of transcellular permeability) and electrical conductance (marker of paracellular permeability). Diet-induced obesity was induced by feeding rats a high fat diet (HF, 45% fat) or normal chow (NC, 10% fat) for 1, 3 or 6 weeks. FITC-dextran 4000 (FD-4, marker of paracellular permeability) and HRP fluxes across intestinal tissues were evaluated in Ussing chambers. Plasma LPSbinding protein (LBP) was measured by ELISA. Results: In Caco-2 and T84 cells and rat intestine, FITC-LPS flux correlated significantly with HRP flux (P ,0.05) but not with electrical conductance (P.0.05), suggesting LPS crosses the gut epithelium through the transcellular rather than paracellular route. In the small intestine, FD-4 flux was higher inHF than NC rats at wk1 (jejunum P=0.08, ileum P=0.004) then returned to normal values ; HRP flux was not affected by HF diet (Table 1). In the large intestine, FD-4 flux was increased in the HF rats (Table 1). HRP flux was increased in the caecum and colon of HF compared to NC rats at wk3 (P=0.03 and 0.04) and wk6 (P=0.009 and 0.02) but not wk1 (Table 1). Plasma LBP was increased in HF rats at wk3 and wk6 (P=0.03 and 0.04, Table 1). Plasma LBP significantly correlated with caecum and colon HRP flux (P=0.006 and 0.01),which correlated significantly with adiposity (P,0.001). Conclusion: HF feeding induces an early increase in paracellular permeability in both small and large intestine. HF feeding induces a later onset (wk3) increase in transcellular transport in the large intestine that correlates with plasma LBP and adiposity. These data support the hypothesis that alteration of transcellular but not paracellular transport in the large intestine is involved in the metabolic endotoxemia and phenotype of diet-induced obesity in rats