The Function of Intestinal Afferent Neurons in Regulating Satiety During Health and Obesity

Background and aim: Within the gastrointestinal tract, vagal afferents regulate satiety and food intake via chemical and mechanical mechanisms. Cysteinyl Leukotrienes (CysLTs) are lipid mediators that are believed to regulate food intake and body weight. However, the involvement of vagal afferents in this effect remains to be established. Conversely, Glucagon like peptide-1 (GLP-1) is a satiety and incretin peptide hormone. The effect of obesity on GLP-1 mediated gut-brain signaling has yet to be investigated. Since intestinal vagal afferents’ activity is reduced during obesity, it is intriguing to investigate their responses to GLP-1 in such conditions. Methods: Extracellular recordings were performed on intestinal afferents from normal C57Bl6, low fat fed (LFF), and high fat fed (HFF) mice. To examine the effect on neuronal calcium signaling, calcium-imaging experiments were performed on isolated nodose ganglion neurons. Food intake experiments were conducted using LFF and HFF mice. Oral glucose tolerance tests (OGTT) were carried out. Whole cell patch clamp recordings were performed on nodose ganglion neurons from A) normal C57Bl mice to test the effect of CysLTs on membrane excitability, B) LFF and HFF mice to examine GLP-1 effect on membrane excitability during obesity. c-Fos immunohistochemical techniques were performed to measure the level of neuronal activation in the brainstem of both LFF and HFF mice in response to Ex-4. Results: CysLTs increased intestinal afferent firing rate and mechanosensitivity. In single nodose neuron experiments, CysLTs increased excitability. The GLP-1 agonist Ex-4 significantly decreased food intake in LFF but not HFF mice. However, Ex-4 markedly attenuated the rise in blood glucose in both LFF and HFF mice. The observed increase in nerve firing and mechanosensitivity following the application of GLP-1 and Ex-4 was abolished in HFF mice. Cell membrane excitability was significantly increased by Ex-4 in nodose from LFF but not HFF mice. Ex-4 significantly increased the number of activated neurons in the NTS area of LFF mice but not in their HFF counterparts. Conclusion: The previous observations indicate that the role CysLTs play in regulating satiety is likely to be vagally mediated. Also that satiety, but not incretin, effects of GLP-1 are impaired during obesity.

IN SITU AND IN VITRO IMMUNOLOCALIZATION OF OVIDUCTIN BINDING SITES ON HAMSTER UTERINE EPITHELIAL CELLS AND DETECTION OF A HAMSTER OVIDUCTIN HOMOLOGUE IN THE FEMALE RAT REPRODUCTIVE TRACT

Oviductin is an oviduct-specific and high-molecular-weight glycoprotein that has been suggested to play important roles in the early events of reproduction. The present study was undertaken to localize the oviductin binding sites in the uterine epithelial cells of the golden hamster (Mesocricetus auratus) both in situ and in vitro, and to detect a hamster oviductin homologue in the female rat reproductive tract. Immunohistochemical localization of oviductin in the hamster uterus revealed certain uterine epithelial cells reactive to the monoclonal anti-hamster oviductin antibody. In order to study the interaction between hamster oviductin and the endometrium in vitro, a method for culturing primary hamster uterine epithelial cells has been established and optimized. Study with confocal microscopy of the cell culture system showed a labeling pattern similar to what was observed using immunohistochemistry. Pre-embedding immunolabeling of cultured uterine epithelial cells also showed gold particles associated with the plasma membrane and microvilli. These results demonstrated that hamster oviductin can bind to the plasma membrane of certain hamster uterine epithelial cells, suggesting the presence of a putative oviductin receptor on the uterine epithelial cell surface. In the second part of the present study, using the monoclonal anti-hamster oviductin antibody that cross-reacts with the rat tissue, we have been able to detect an oviduct-specific glycoprotein, with a molecular weight of 180~300kDa, in the female rat reproductive tract. Immunohistochemical labeling of the female rat reproductive tract revealed a strong immunolabeling in the non-ciliated oviductal epithelial cells and a faint immunoreaction on the cell surface of some uterine epithelial cells. Ultrastructurally, immunogold labeling was restricted to the secretory granules, Golgi apparatus, and microvilli of the non-ciliated secretory cells of the oviduct. In the uterus, immunogold labeling was observed on the cell surface of some uterine epithelial cells. Furthermore, electron micrographs of ovulated oocytes showed an intense immunolabeling for rat oviductin within the perivitelline space surrounding the ovulated oocytes. The findings of the present study demonstrated that oviductin is present in the rat oviduct and uterus, and it appears that, in the rat, oviductin is secreted by the non-ciliated secretory cells of the oviduct.