ARE UMAMI TASTE RECEPTOR-SITES STRUCTURALLY RELATED TO GLUTAMATE CNS RECEPTOR-SITES

Umami tasting substances, MSG (monosodium glutamate), HG (glutamic acid), LGDE (1-glutamic acid diethyl ester), DLHCA (dl-homocysteic acid), DLAAA (dl-aminoadipic acid) and 5'GMP, were tested on the hamster and the human. Ten mM MSG was routinely used in the hamster as it elicited strong chorda tympani responses. Similar response amplitudes were found for MSG, HG, LGDE, DLAAA 10 mM, DLHCA 8 mM and sucrose 100 mM. A 5-mu-M concentration of 5'GMP eventually was an efficient stimulus on a few preparations. Such a low concentration is very seldom efficient as a taste stimulus in rodents, indicating a higher specificity of receptor mechanisms than what is usually found for sweet taste, for example. The synergy between MSG and 5'GMP was found in the hamster CT only for concentrations lower than those of the literature, i.e., a mixture of 12-mu-M 5'GMP and 2.5 mM MSG showed a reinforcement of 50% in response amplitude equivalent to a 100% increase in concentration. We take this as an evidence of an umami component in the hamster CT response to glutamate; in accordance with literature data, we could not find reinforcement for higher concentrations which were in fact near saturation. Responses to MSG, HG, LGDE, DLAAA and DLHCA, among 38 other organic stimuli, were studied in 42 hamster chorda tympani. Responses to HG, LGDE and 5'GMP, among 51 other organic stimuli, were recorded in another run of 60 hamster CT. Results show that several receptors are involved in the chemoreception of these compounds used as umami tasting stimuli. The correlations between the CT response profiles obtained for these compounds indicate that the hamster tongue receptors classify the different agonists of the so-called NMDA- and non-NMDA receptors in the same way as brain receptors do. The question arises then as whether they could simply be the same in the brain and on the tongue.