The Brain-Gut-Islet Connection

Abstract

Peptide signals from the pancreatic islets and the gastrointestinal tract influence the regulation of energy homeostasis by the brain, and the brain in turn influences the secretions of both the islets and the gut. This article focuses on how insulin interacts with the brain to influence food intake, blood glucose, and cognitive behavior. Insulin is secreted in response to changes of ambient glucose, and the levels achieved are directly proportional to body adiposity. Hence, insulin, like leptin, is an adiposity signal. An increased insulin signal in the mediobasal hypothalamus indicates that ample or excess energy is available in the body and elicits responses that limit food intake and reduce hepatic glucose secretion. Increased insulin (and leptin as well) locally within the brain complements other signals that indicate a surfeit of energy in the body, including satiety signals generated by the gut during meals, glucose, and some fatty acids. There is compelling evidence that overlapping intracellular signaling pathways within the mediobasal hypothalamus mediate the overall catabolic response to these diverse metabolic signals. Insulin receptors are also densely expressed in the hippocampus, and insulin acts there to facilitate learning and memory. The function of insulin receptors in other brain areas is poorly understood. Obesity and/or the consumption of diets high in fat render the brain as well as the body insulin resistant. In the hypothalamus, this is manifest as a reduced ability of insulin to reduce food intake and body weight, and in the hippocampus, it is manifest as a reduced ability of insulin to improve learning and/or memory.