Biological Significance of Glutamate Signaling during Digestion of Food through the Gut-Brain Axis

Monosodium <i>L</i>-glutamate (MSG) elicits a unique taste termed umami and is widely used as a flavor enhancer in various cuisines. In addition, recent studies suggest the existence of sensors for <i>L</i>-glutamate (Glu) and transduction molecules in the gut mucosa as well as in the oral cavity. The vagal gastric afferent responds specifically to the luminal stimulation of Glu in the stomach and regulates the autonomic reflexes. The intragastric infusion of Glu also activates several brain areas (insular cortex, limbic system, and hypothalamus) and is able to induce flavor-preference learning in rats. These results suggest that umami signaling via gustatory and visceral pathways plays an important role in the processes of digestion, absorption, metabolism, and other physiological functions via activation of the brain.