Conditioned food aversion elicited by the temperature of drinking water as a conditioned stimulus in rats.

It is known that taste can act as a conditioned stimulus (CS) for conditioned food aversion. In the present study, in order to examine whether or not the temperature of drinking water can be a CS, we conducted behavioral experiments in Wistar rats. The following results were obtained: (1) The rats subjected to aversive conditioning to 5 or 40 degrees C distilled water could learn to avoid these CSs, but they did not avoid any taste stimuli. (2) The rats subjected to aversive conditioning to 5 or 40 degrees C 0.1 M sucrose developed a generalized avoidance to sucrose at any temperature. (3) When rats familiarized to 25 degrees C 5 mM saccharin-Na (Sacc) were subjected to aversive conditioning to 5 or 40 degrees C Sacc, they avoided the respective CS, but they did not generalize it to any other stimuli even if having the same temperature as the CS. (4) The rats which had undergone transection of the taste nerves (chorda tympani and glossopharyngeal nerves) could acquire the conditioned response to the temperature of the CS. These results suggest that rats can be conditioned to temperature aversion and that the taste nerves are not needed in the formation of this conditioning.

Synergistic responses of the chorda tympani to mixtures of umami and sweet substances in rats.

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.