How food tastes greatly influences eating behavior. The tastes of monosodium glutamate (MSG), other amino acids, and sweet stimuli signal general food types and influence dietary intake. MSG, an amino acid that occurs naturally in foods such as vegetables and meats, elicits a taste called "umami" that increases the palatability of food and, thus, food intake. Unlike other primary tastes, sweet, sour, salty, and bitter, the biological mechanisms for detecting umami are not well understood. Although MSG has a prototypical umami quality, it has multiple perceptual qualities, possibly by stimulation of several taste receptors. Behavioral studies have shown that MSG also elicits taste qualities similar to sweet stimuli when the sodium taste is minimized. MSG is detected in part by a glutamate receptor called "taste-mGluR4". In rats, behavioral studies show that MSG and L-AP4 (an mGluR4 agonist) have nearly identical tastes. Nerve fiber recordings reveal that some fibers respond synergistically to mixtures of L-AP4 and sweet stimuli but, curiously, only fibers that respond weakly to sucrose alone. These same fibers do not show synergy to mixtures of MSG and sweet stimuli, suggesting that L-AP4 evokes a sweet taste through mGluR4 receptors. Molecular studies report that another potential umami receptor, T1R1/T1R3, responds broadly to L-amino acids, including MSG, but not to sweet stimuli. Together, these data suggest that there are multiple taste receptors for MSG and at least some L-amino acids but critical behavioral data are lacking. Using rats, Dr. Delay will combine pharmacological antagonism of specific receptors with behavioral methods, such as conditioned taste aversion (in which rats attend to perceptual similarities) and discrimination (in which rats attend to differences), to address three important questions: 1) Are taste-mGluR4 receptors involved in the sweet taste of MSG? 2) Does L-AP4 have a sweet quality and do taste-mGluR4 receptors contribute to this quality? 3) Is the taste of glutamate similar to other L-amino acids? Knowledge gained from these studies will help identify the taste receptors responsible for the perception of glutamate and amino acids. The project will provide opportunities for undergraduate students to gain knowledge and experience about research and scientific processes, and prepare for careers in science. The results may also help improve dietary intake of people who have deficiencies as a result of not eat well, e.g., elderly, chemotherapy patients, etc.
