Adaptations in gastrointestinal satiety during pregnancy

Abstract

Background: Increased food intake during pregnancy is associated with reduced central satiety, the development of leptin resistance and changes in sex hormones. Meal termination occurs partly via gastrointestinal vagal afferents (VAs) which sense food-related mechanical stimuli, including distension of the stomach and intestine and through nutrient-induced release of satiety hormones from intestinal enteroendocrine cells (EECs). These pathways both signal to the central nervous system to stop eating. Currently it is unknown how gastrointestinal (GI) satiety signalling adapts to permit increased food intake during pregnancy. This PhD project explored pregnancy-related adaptations in gastric VA (GVA) signalling in standard laboratory diet (SLD) fed and western diet fed-mice, intestinal expression of nutrient chemosensors and satiety hormones and the circadian timing of behaviour. Aims: 1) a. To determine the mechanosensitivity of GVAs and food intake behaviours at different pregnancy stages compared to non-pregnant mice. b. To determine the effects of pregnancy-related hormones on GVA tension receptors in non-pregnant mice, as potential mechanisms underlying pregnancy related adaptations. 2) To investigate the expression of protein, fatty acid and carbohydrate nutrient receptors and satiety hormones in the duodenum, jejunum and ileum in different stages of pregnancy compared to non-pregnant mice. 3) To assess food intake and mechanosensitivity of GVAs throughout pregnancy in mice fed a SLD or western high-fat high-sugar diet (HFHSD). 4) To determine effects of pregnancy on circadian rhythms of food and water intake, sleep and activity behaviour. Methods and results: The study in Chapter 2 characterised the response of GVAs to stretch and changes in food intake parameters in early-, mid- and late-pregnant mice compared to non-pregnant mice. This work showed that the mechanosensitivity of GVAs was attenuated during mid- and late-pregnancy. Furthermore, addition of growth hormone (GH) to the in vitro organ bath decreased GVA responses to stretch in non-pregnant mice. Chapter 3 focussed on the intestinal nutrient-sensing repertoire during pregnancy. Fatty acid (GPR84, FFAR1,2,3,4), protein (GPR93, CaSR, mGLUR4, T1R1), carbohydrate (TRPM5, T1R2, T1R3) receptors and gut hormones (GCG, CCK) were characterised in the small intestine of early-, mid- and late-pregnant mice compared to non-pregnant mice. In addition, immunofluorescence experiments were used to determine the number of FFAR4, GPR93, CCK and GLP-1 positive cells within the duodenum and jejunum of late-pregnant compared to non-pregnant mice. There were selective changes in nutrient-sensor mRNA expression during pregnancy. FFAR4 expression was lower in late-compared to non-pregnant mice in all regions, but jejunal FFAR4 positive cells were more abundant in late- than non-pregnant mice. Duodenal GPR93 expression was lower in late- than non-pregnant mice. In the ileum, FFAR1 expression was greater in mid- than early- and late-pregnant mice and FFAR2 expression was greater in mid- than early-pregnant mice. GCG and CCK expression at the transcript level and numbers of GPR93, CCK and GLP-1 immunopositive cells were unaffected by pregnancy. Chapter 4 aimed to determine how maternal HFHSD feeding impacts adaptations in GVA function and food intake behaviours during pregnancy. The response of tension sensitive GVAs to stretch was attenuated in pregnancy within SLD-fed mice, consistent with results of Chapter 2, and was lower in HFHSD than SLD-fed non-pregnant mice. However, GVA responses to stretch were similar in HFHSD-fed pregnant and non-pregnant mice. Light-phase food intake (g) and meal size (g) within each study day was higher in SLD-fed mice than HFHSD-fed mice and was greater in pregnant mice than non-pregnant mice from d 8.5 onwards. In addition, pregnant HFHSD-fed mice ate larger light-phase meals on d 14.5-16.5 than non-pregnant HFHSD-fed mice, but this was not preserved on the final study day before electrophysiology recording. Lastly, the study reported in Chapter 5 determined the rhythms of food and water intake, activity and wakefulness across weeks 1, 2 and 3 of pregnancy compared to age-matched non-pregnant mice. From week 1, pregnant mice moved and were awake significantly less than non-pregnant mice during the dark-phase. Furthermore, the timing of peak food intake and activity late in the light-phase (time period of interest I: ZT8-ZT12) was delayed and the pregnant group ate more during the same time period in week 3 compared to the non-pregnant group. Food intake was also increased early in the dark-phase (time period of interest II: ZT12-ZT15) from pregnancy week 2. Conclusion: The mechanosensitivity of GVAs are attenuated during pregnancy and associated with increased food intake. These GVA adaptations are likely to support increases in food intake to meet the energy demands of the growing fetus, and may be driven by increases in circulating levels of GH, but this is yet to be determined. Within the intestine, there were specific alterations in nutrient sensor FFAR1, 2 and 4 and GPR93. Future research should be directed at understanding whether pregnant mice are less sensitive to luminal nutrients and whether nutrient-induced secretion of GI tract hormones changes during pregnancy. Energy balance was also altered through behaviour, where pregnant mice increased food consumption during the inactive phase and decreased movement during the active phase, when food intake was the highest. Lastly, both pregnancy and HFHSD feeding attenuated the mechanosensitivity of GVA, however, pregnancy did not further reduce GVA mechanosensitivity in HFHSD-fed mice. Further studies are required to increase understanding of food intake regulation across pregnancy to inform strategies to improve pregnancy outcomes.