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First published online February 15, 2006
Journal of Experimental Biology 209, 826-833 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02069

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Taste discrimination in conditioned taste aversion of the pond snail Lymnaea stagnalis

Rio Sugai¹, Hatsuki Shiga¹, Sachiyo Azami¹, Takayuki Watanabe¹, Hisayo Sadamoto^1,2, Yutaka Fujito³, Ken Lukowiak⁴ and Etsuro Ito^1,5,*

¹ Division of Biological Sciences, Graduate School of Science, Hokkaido University, North 10, West 8, Kita-ku, Sapporo 060-0810, Japan
² Laboratory of Functional Biology, Faculty of Pharmaceutical Sciences at Kagawa Campus, Tokushima Bunri University, 1314-1 Shido, Sanuki 769-2193, Japan
³ Department of Physiology, School of Medicine, Sapporo Medical University, South 1, West 17, Chuo-ku, Sapporo 060-8556, Japan
⁴ Hotchkiss Brain Institute, University of Calgary, Calgary, Alberta T2N 4N1, Canada
⁵ Division of Innovative Research, Creative Research Initiative "Sousei" (CRIS), Hokkaido University, North 21, West 10, Kita-ku, Sapporo 001-0021, Japan

View larger version (10K): [in a new window]   Fig. 1. Feeding responses elicited by different tastes applied to the lips of snails. (A) The number of bites min–1 elicited in snails by increasing the concentration of D-sucrose (sweet taste). (B) As in A, but sodium-L-glutamic acid (Na-L-Glu, umami taste, closed squares) was used as the stimulant. The open circles represent experiments in which a cocktail consisting of 3 mmol l–1 sucrose and 10 mmol l–1 Glu was applied to the lips. Notice that based on the data obtained in A, it would be expected that this cocktail would elicit a response of at least 10 bites min–1. However, this cocktail only produced the same level of feeding as that produced by Glu alone or by sucrose alone at these concentrations. (C) Again as in A, but carrot juice (complex taste) was used as the stimulant to the lips. All data are means ± s.e.m. obtained from 20 snails. The x axis is in logarithmic scale. These results show that 10 mmol l–1 sucrose or 0.3% carrot juice induces a reliable feeding response in snails.

View larger version (13K): [in a new window]   Fig. 2. Conditioned taste aversion (CTA) in snails. The numbers of bites min–1 elicited by the CS [the conditional (taste) stimulus] in the test session following training. (A) Sucrose was used as the CS. The CS-elicited feeding response following training in CTA-trained snails (N=40) was significantly less (*P<0.01, one-way ANOVA followed by the post hoc Scheffé test) than in either snails given backward-conditioning (N=35) or naive (N=39) snails. Further, there was no significant difference in the elicited feeding response between the backward-conditioned group and the naive group. (B) Carrot juice was used as the CS. The data are plotted as in A, and similar results were obtained. The numbers of snails were as follows: 45 naive snails, 35 backward-conditioned snails, and 63 conditioned snails. All data are means ± s.e.m.

View larger version (15K): [in a new window]   Fig. 3. Taste discrimination following conditioned taste aversion (CTA) in snails. (A) Sucrose was used as the conditional stimulus (CS) in the CTA training procedure. In the pre-test session, sucrose (N=40, closed squares) and carrot juice (N=40, open circles) elicited statistically similar feeding responses. At specific times following CTA training, sucrose elicited a significantly smaller feeding response than did carrot juice. Additionally, the feeding response elicited by sucrose at each time point following CTA was significantly less than it was in the pre-test session, whereas the feeding response elicited by carrot juice did not change significantly over the course of the experiment. The memory for CTA following this training procedure persisted for at least 48 h (*P<0.01, Student's t-test). The numbers of snails was reduced to 37 for carrot juice and 36 for sucrose at 48 h, because 3 and 4 snails, withdrew their bodies into their shells or died, respectively. (B) As in A, except that carrot juice was used as the CS. Similar results were obtained. All data are means ± s.e.m. The numbers of snails became 38 for sucrose and 40 for carrot juice at 48 h because 2 snails for sucrose withdrew their bodies into the shells. The x-axes are logarithmic scale. Thus snails are able to taste discriminate following CTA learning, memory formation and memory persistence.

View larger version (21K): [in a new window]   Fig. 4. Second-order conditioning of feeding behavior in snails. (A) In the pre-test session, sucrose (CS1; conditional stimulus 1) and carrot juice (CS2) elicit similar feeding responses in naive snails (N=50). Conditioned taste aversion (CTA) training with sucrose as the CS (CS1) and KCl as the US was then performed. In the first post-test session, the CS1 elicited a significantly smaller feeding response whereas the CS2 (carrot juice) elicited a similar response as in the pre-test session. In the first post-test session, the response elicited by sucrose (CS1) was also significantly less (*P<0.01, Student's t-test) than that elicited by the CS2. In the next phase of training, the CS2 (carrot juice) was paired with the CS1 (sucrose). Following this training, a second post-test session was performed. In this session, the feeding response elicited by CS2 (carrot juice) was significantly less (*P<0.01, Student's t-test) than the response it elicited in either the pre-test session. Notice, however, that the response elicited by CS2 was still significantly greater (*P<0.01, Student's t-test) than the response elicited by CS1. The number of snails was reduced to 49 after the first-phase training because one snail withdrew its body into its shell, and then 50 snails were used for the second-phase training. (B) As in A (N=50), except that carrot juice was the CS1 and sucrose the CS2. Identical results were obtained as in A. The numbers of snails was reduced to 48 and 49 after the first-phase training because 2 and 1 snail withdrew their bodies into their shells for the CS1 and the CS2, respectively, and then 49 snails were used for each of the second-phase training. All data are means ± s.e.m.

View larger version (15K): [in a new window]   Fig. 5. Extinction and conditioned taste aversion memory. (A) Naive snails (N=50) were first challenged with the sucrose taste. This elicited robust feeding. Snails were then conditioned taste aversion (CTA) trained. 10 min after training, snails were given the conditional stimulus (CS) alone (i.e. an extinction session) and the elicited feeding response noted. Snails were designated as either good (N=40; closed boxes) or poor (N=10; open boxes) performers. All snails received two more (at 10 min intervals) extinction sessions. Again in each extinction session the elicited feeding response was noted. All snails were then challenged with the sucrose stimulus 1 and 24 h later. Extinction did not occur. The three extinction sessions did not result in a behavioral phenotype resembling the naive state in either the good or the poor performers. (B) As in A, except carrot juice was used as the CS. Similar results were obtained. All data are means ± s.e.m. The differences between the feeding response of the good performers and that of the poor performers in the both cases were maintained for at least 24 h with *P<0.01 (Student's t-test), showing that the memory formed by CTA training cannot be extinguished.