Chem. Senses 28: 261-266,
2003
© Oxford University Press 2003
Synergistic Responses of the Chorda Tympani to Mixtures of Umami and Sweet Substances in Rats
Department of Oral Physiology, Asahi University School of Dentistry, 1851 Hozumi, Hozumi, Motosu, Gifu 501-0296 1 Department of Behavioral Physiology, Graduate School of Human Sciences, Osaka University, 1-2 Yamadaoka, Suita, Osaka 565-0871, Japan
Correspondence to be sent to: Noritaka Sako, Department of Oral Physiology, Asahi University School of Dentistry, 1851 Hozumi, Hozumi, Motosu, Gifu 501-0296 Japan. e-mail: sako{at}dent.asahi-u.ac.jp
It has been known that umami substances such as monosodium L-glutamate (MSG) and 5'-inosine monophosphate (IMP) elicit a unique taste called ‘umami’ in humans. One of the characteristics of the umami taste is synergism: the synergistic enhancement of the magnitude of response produced by the addition of 5'-ribonucleotides to MSG. In addition to this well-documented synergism, we report here for the first time on another type of synergism between a glutamate receptor agonist, L-AP4, and sweet substances, by analyzing the chorda tympani responses in rats. The results are as follows: (i) when L-AP4 was mixed with one of the sweet substances, such as sucrose, glucose, fructose and maltose, large synergistic responses were observed. (ii) These synergistic responses, except to L-AP4 + sucrose, were not suppressed by sweet taste suppressants, gurmarin and pronase E. (iii) These synergistic responses were not suppressed by either metabotropic or ionotropic glutamate receptor antagonists. (iv) Fibers that responded well to the binary mixtures of L-AP4 and sweet substances also responded well to NaCl and HCl, but very weakly to sucrose. These findings are different from the characteristics of synergism between glutamate and IMP. The multiple transduction mechanisms for the umami taste in rat taste cells are discussed.
Key words: chorda tympani, rats, sweet, synergism, umami
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