Physiological roles of glutamate signaling in gut and brain function

Some ingested nutrients have postingestive effects that modulate food intake and improve mood subconsciously. Here, we provide an overview of the positive postingestive effects of such nutrients, primarily L-glutamate, sugar, and lipids, with respect to behavior and brain function. We also discuss the mechanisms of brain activation resulting from signaling through the gut-brain axis. Recent studies have shown that rats prefer solutions paired with intragastric nutrients that have positive postingestive effects. Using functional magnetic resonance imaging (fMRI), we previously evaluated neural activation in response to ingested glucose, L-glutamate, and corn oil emulsion in rats and found that distinct forebrain regions were activated by these nutrients. Most of the areas activated by intragastric administration of L-glutamate were eliminated by abdominal vagotomy. On the other hand, the areas activated by intragastric administration of glucose were not affected by vagotomy. A behavioral study showed similar results for L-glutamate and glucose. These results indicate that brain activation in response to ingested nutrients occurs through distinct internal signals from the gut to the brain. Distinct regional and temporal activation in the brain determines the variety of postingestive behaviors and physiological responses.