Luminal amino acid sensing in the rat gastric mucosa

Abstract

. Luminal amino acid sensing in the rat gastric mucosa. Am J Physiol Gastrointest Liver Physiol 291: G1163-G1170, 2006. First published June 29, 2006 doi:10.1152/ajpgi.00587.2005.-Recent advancements in molecular biology in the field of taste perception in the oral cavity have raised the possibility for ingested nutrients to be "tasted" in the upper gastrointestinal tract. The purpose of this study was to identify the existence of a nutrient-sensing system by the vagus in the rat stomach. Afferent fibers of the gastric branch increased their firing rate solely with the intragastric application of the amino acid glutamate. Other amino acids failed to have the same effect. This response to glutamate was blocked by the depletion of serotonin (5-HT) and inhibition of serotonin receptor 3 (5-HT3) or nitric oxide (NO) synthase enzyme. Luminal perfusion with the local anesthesia lidocaine abolished the glutamate-evoked afferent activation. The afferent response was also mimicked by luminal perfusion with a NO donor, sodium nitroprusside. In addition, the NO donorinduced afferent activation was abolished by 5-HT 3 blockade as well. Altogether, these results strongly suggest the existence of a sensing system for glutamate in the rat gastric mucosa. Thus luminal glutamate would enhance the electrophysiological firing rate of afferent fibers from the vagus nerve of the stomach through the production of mucosal bioactive substances such as NO and 5-HT. Assuming there is a universal coexistence of free glutamate with dietary protein, a glutamate-sensing system in the stomach could contribute to the gastric phase of protein digestion.visceral nutrient sensing; rat vagal gastric afferent; serotonin; nitric oxide THE NUMBER OF ARTICLES DEDICATED to the abdominal vagus nerve that were focused in gut nutrient sensing has increased dramatically over the last decade (33,36,42). This is due to the advancement of techniques that facilitated the study of visceral afferent fibers and their function and characteristic electrophysiological patterns and also due to recognition that these fibers are important for body nutrient homeostasis. Psychophysiological approaches for the understanding of ingestive behavior have demonstrated that the presence of food in the upper gastrointestinal (GI) tract plays a critical role in determining meal size. The vagus nerve is extensively distributed throughout the GI tract, from the posterior region of oral cavity and esophagus to the lowest part of the colon, and functions as the primary neuroanatomical circuit in the gut-brain axis to transmit meal-related signals from the GI mucosa to the central nervous system. This is where regulatory processes (e.g., ingestive behavior, nutrient absorption, GI secretion, and stomach emptying) as well as conscious sensations (e.g., satiety, nausea, and discomfort) take place (5, 22, 37).Recently, it has been shown that taste transduction-related molecules, such as ␣-gustducin and a family of bitter-sensing taste receptors (T 2 Rs), were also expressed at the GI mu...

Sense of Taste in the Gastrointestinal Tract

“…We speculate that sweeteners are not sensed by the stomach since we failed to detect T1R2 expression using T1R2-LacZ knock-in mice (19). On the other hand, we have observed activation of afferent fibers of the gastric branch by intragastric infusion of glutamate, indicating that umami sensors reside in the stomach (41). Therefore, detection of sweeteners by taste-like cells is presumably intestinal, while umami compounds may be both gastric and intestinal.…”

New Therapeutic Strategy for Amino Acid Medicine: Effects of Dietary Glutamate on Gut and Brain Function

“…Interestingly, among the 20 amino acids, only Glu stimulated the rat vagal gastric afferents (VGA) (Fig. 2) (10). Furthermore, this Glu response was blocked by the depletion of serotonin (5-HT) and by the inhibition of 5-HT 3 receptors or the nitric oxide (NO) synthase enzyme.…”

Nutraceuticals as Chemopreventive and Therapeutic agents in Gut Associated Pathogenesis

“…Glutamate, a derivative of glutamic acid is the major oxidative fuel for the gut. Glutamate is also a potent enteric neurotransmitter; important precursor for synthesis of bioactive molecules and a potential therapeutic agent for improving neonatal gut function [58][59][60][61]. The premature neonatal gut exhibits a high rate of cellular proliferation and replacement in gut epithelia.…”

“…Large number of transporter molecules are expressed in epithelial cells of gastric lining but only few of them are involved in glutamate transport system [65,66]. Glutamate activates contractile action in the gastric fundus possibly via cholinergic neurons [58, 59,67].…”