Dietary
glutamate (Glu) stimulates to evoke the umami taste, one of the five basic tastes, enhancing food palatability. But it is also the main gut energy source for the absorption and metabolism for each nutrient, thus, only a trace amount of Glu reaches the general circulation. Recently, we demonstrated a unique gut sensing system for free Glu (
glutamate signaling). Glu is the only nutrient among
amino acids,
sugars and
electrolytes that activates rat gastric vagal afferents from the
luminal side specifically via metabotropic Glu receptors type 1 on mucosal cells releasing
mucin and
nitrite mono-
oxide (NO), then NO stimulates
serotonin (5HT) release at the enterochromaffin cell. Finally released 5HT stimulates 5HT3 receptor at the nerve end of the vagal afferent fiber. Functional magnetic resonance imaging (f-MRI, 4.7 T) analysis revealed that
luminal sensing with 1 % (w/v) monosodium
L-glutamate (
MSG) in rat stomach activates both the medial preoptic area (body temperature controller) and the dorsomedial hypothalamus (basic metabolic regulator), resulting in diet-induced thermogenesis during mealing without changes of appetite for food. Interestingly, rats were forced to eat a high fat and high
sugar diet with free access to 1 % (w/w)
MSG and water in a choice paradigm and showed the strong preference for the
MSG solution and subsequently, they displayed lower fat deposition,
weight gain and blood
leptin. On the other hand, these brain functional changes by the f-MRI signal after 60 mM
MSG intubation into the stomach was abolished in the case of total vagotomized rats, suggesting that
luminal glutamate signaling contributes to control digestion and thermogenesis without
obesity.