The vagal afferent pathway does not play a major role in the induction of satiety by intestinal fatty acid in rats

Plasticity of gastro-intestinal vagal afferent endings

Vagal afferents are a vital link between the peripheral tissue and central nervous system (CNS). There is an abundance of vagal afferents present within the proximal gastrointestinal tract which are responsible for monitoring and controlling gastrointestinal function. Whilst essential for maintaining homeostasis there is a vast amount of literature emerging which describes remarkable plasticity of vagal afferents in response to endogenous as well as exogenous stimuli. This plasticity for the most part is vital in maintaining healthy processes; however, there are increased reports of vagal plasticity being disrupted in pathological states, such as obesity. Many of the disruptions, observed in obesity, have the potential to reduce vagal afferent satiety signalling which could ultimately perpetuate the obese state. Understanding how plasticity occurs within vagal afferents will open a whole new understanding of gut function as well as identify new treatment options for obesity.

Intact neural system of the portal vein is important for maintaining normal glucose metabolism by regulating glucagon-like peptide-1 and insulin sensitivity

The portal neural system may have an important role on the regulation of glucose homeostasis since activation of the gut-brain-liver neurocircuit by nutrient sensing in the proximal intestine reduces hepatic glucose production through enhanced liver insulin sensitivity. Although there have been many studies investigating the role of portal neural system, surgical denervation of the sole portal vein has not been reported to date. The aim of this study was to clarify the role of the portal neural system on the regulation of glucose homeostasis and food intake in the physiological condition. Surgical denervation of portal vein (DV) was performed in 10 male 12 week-old Wistar rats. The control was a sham operation (SO). One week after surgery, food intake and body weight were monitored; an oral glucose tolerance test (OGTT) was performed; and glucagon-like peptide-1 (GLP-1) and insulin levels during OGTT were assayed. In addition, insulinogenic index, homeostatic model assessment, and Matsuda index were calculated. All rats regained the preoperative body weight at one week after surgery. There was no significant difference in food intake between DV and SO rats. DV rats exhibited increased blood glucose levels associated with decreased insulin sensitivity but increased GLP-1 and insulin secretion during OGTT. In summary, in the physiological state, loss of the portal neural system leads to decreased insulin sensitivity and increased blood glucose levels but does not affect food intake. These data indicate that an intact portal neural system is important for maintaining normal glucose metabolism.

Nutrient-sensing mechanisms in the gut as therapeutic targets for diabetes

The small intestine is traditionally viewed as an organ that mediates nutrient digestion and absorption. This view has recently been revised owing to the ability of the duodenum to sense nutrient influx and trigger negative feedback loops to inhibit glucose production and food intake to maintain metabolic homeostasis. Further, duodenal nutrient-sensing defects are acquired in diabetes and obesity, leading to increased glucose production. In contrast, jejunal nutrient sensing inhibits glucose production and mediates the early antidiabetic effect of bariatric surgery, and gut microbiota composition may alter intestinal nutrient-sensing mechanisms to regain better control of glucose homeostasis in diabetes and obesity in the long term. This perspective highlights nutrient-sensing mechanisms in the gut that regulate glucose homeostasis and the potential of targeting gut nutrient-sensing mechanisms as a therapeutic strategy to lower blood glucose concentrations in diabetes.

From gut changes to type 2 diabetes remission after gastric bypass surgeries

Increasing evidence suggests that the gut may influence the host's metabolism and ultimately change the outcomes of type 2 diabetes mellitus (T2DM). We review the evidence on the relationship between the gut and T2DM remission after gastric bypass surgery, and discuss the potential mechanisms underlying the above relationship: gut anatomical rearrangement, microbial composition changes, altered gut cells, and gut hormone modulation. However, the exact changes and their relative importance in the metabolic improvements after gastric bypass surgery remain to be further clarified. Elucidating the precise metabolic mechanisms of T2DM resolution after bypass surgery will help to reveal the molecular mechanisms of pathogenesis, and facilitate the development of novel diagnoses and preventative interventions for this common disease.

Intestinal fatty acid infusion modulates food preference as well as calorie intake via the vagal nerve and midbrain-hypothalamic neural pathways in rats

The intestine plays important roles in the regulation of feeding behavior by sensing macronutrients. Intestinal fatty acids strongly suppress food intake, but little is known about whether intestinal fatty acids affect food preference. We investigated the effects of jejunal fatty acids infusion on food preference by conducting two-diet choice experiments in rats fed a high-fat diet (HFD) and a high-carbohydrate diet (HCD). Jejunal linoleic acid (18:2) infusion reduced HFD intake dose-dependently, while HCD intake increased with the middle dose of the infusion we examined (100 μL/h) and reduced to the control level with the higher doses (150 and 200 μL/h). α-Linolenic acid (18:3), but not caprylic acid (8:0), altered the food preference and total calorie intake in the same manner as linoleic acid. Linoleic acid infusion dose-dependently increased plasma glucagon-like peptide-1, peptide YY and cholecystokinin levels, but not ghrelin levels. Subdiaphragmatic vagotomy or midbrain transection prevented the change in food preference and total calorie intake by linoleic acid infusion. Jejunal linoleic acid infusion increased norepinephrine turnover in the paraventricular hypothalamic nucleus, while intracerebroventricular injection of idazoxan, an α2-adrenergic receptor (AR) antagonist, suppressed the increased HCD intake, but did not affect the decreased HFD intake. These findings indicated that intestinal long-chain fatty acids modulated food preference as well as total calorie intake via the vagal nerve and midbrain-hypothalamic neural pathways. The effects of the α2-AR antagonist in the brain suggested that the brain distinctly controlled HCD and HFD intake in response to jejunal linoleic acid infusion.

Jejunal nutrient sensing is required for duodenal-jejunal bypass surgery to rapidly lower glucose concentrations in uncontrolled diabetes

Gastrointestinal bypass surgeries restore metabolic homeostasis in patients with type 2 diabetes and obesity(1), but the underlying mechanisms remain elusive. Duodenal-jejunal bypass surgery (DJB), an experimental surgical technique that excludes the duodenum and proximal jejunum from nutrient transit(1,2), lowers glucose concentrations in nonobese type 2 diabetic rats(2–5). Given that DJB redirects and enhances nutrient flow into the jejunum and that jejunal nutrient sensing affects feeding(6,7), the repositioned jejunum after DJB represents a junction at which nutrients could regulate glucose homeostasis. Here we found that intrajejunal nutrient administration lowered endogenous glucose production in normal rats through a gut-brain-liver network in the presence of basal plasma insulin concentrations. Inhibition of jejunal glucose uptake or formation of long chain fatty acyl-coA negated the metabolic effects of glucose or lipid, respectively, in normal rats, and altered the rapid (2 d) glucose-lowering effect induced by DJB in streptozotocin (STZ)-induced uncontrolled diabetic rats during refeeding. Lastly, in insulin-deficient autoimmune type 1 diabetic rats and STZ-induced diabetic rats, DJB lowered glucose concentrations in 2 d independently of changes in plasma insulin concentrations, food intake and body weight. These data unveil a glucoregulatory role of jejunal nutrient sensing and its relevance in the early improvement of glycemic control after DJB in rat models of uncontrolled diabetes.

The role of the autonomic nervous liver innervation in the control of energy metabolism

Despite a longstanding research interest ever since the early work by Claude Bernard, the functional significance of autonomic liver innervation, either sympathetic or parasympathetic, is still ill defined. This scarcity of information not only holds for the brain control of hepatic metabolism, but also for the metabolic sensing function of the liver and the way in which this metabolic information from the liver affects the brain. Clinical information from the bedside suggests that successful human liver transplantation (implying a complete autonomic liver denervation) causes no life threatening metabolic derangements, at least in the absence of severe metabolic challenges such as hypoglycemia. However, from the benchside, data are accumulating that interference with the neuronal brain-liver connection does cause pronounced changes in liver metabolism. This review provides an extensive overview on how metabolic information is sensed by the liver, and how this information is processed via neuronal pathways to the brain. With this information the brain controls liver metabolism and that of other organs and tissues. We will pay special attention to the hypothalamic pathways involved in these liver-brain-liver circuits. At this stage, we still do not know the final destination and processing of the metabolic information that is transferred from the liver to the brain. On the other hand, in recent years, there has been a considerable increase in the understanding which brain areas are involved in the control of liver metabolism via its autonomic innervation. However, in view of the ever rising prevalence of type 2 diabetes, this potentially highly relevant knowledge is still by far too limited. Thus the autonomic innervation of the liver and its role in the control of metabolism needs our continued and devoted attention.