Abstract
Monosodium L-glutamate (MSG), a sodium salt of amino acid L-glutamate (Glu), elicits umami taste. Recent studies suggest the post-oral effect of Glu. Consumption of Glu is decreased in gastric vagotomized rats. Gastric vagal afferent fibers specifically respond to the intragastric (luminal) administration of Glu, but not to the other amino acids. These results raised hypotheses that ingested Glu is detected by chemoreceptors in the gastrointestinal tract and the signal is transmitted to the brain via vagus nerve. The goal of present study was to demonstrate spatiotemporal activation of the brain following intragastric Glu administration (in the absence of taste) using functional magnetic resonance imaging. Rats were anesthetized with α-chloralose and 60 mM MSG solution was infused in the stomach for 10 min. To exclude effects with sodium ion, NaCl (60 mM) was used as control. Second, we assessed a role of the vagal afferent nerve in rats under subdiaphragmatic total vagotomy (TVX). The results showed that only MSG elicited significant activation on several brain regions including the medial preoptic area, dorthomedial hypothalamus, habenular nucleus and amygdala. Both MSG and NaCl activated the anterior cingulate, caudate-putamen, insular cortex and hippocampus. TVX eliminated substantially the MSG or NaCl-induced activation. Our data are the first evidences demonstrating that post-oral Glu can activate the brain via vagus nerve. The brain activation caused by post-oral Glu may link to various physiological responses. [J Physiol Sci. 2008;58 Suppl:S167]
