Taste discrimination in conditioned taste aversion of the pond snailLymnaea stagnalis

Abstract

Conditioned taste aversion (CTA) in the pond snailLymnaea stagnalis has been widely used as a model for gaining an understanding of the molecular and behavioral mechanisms underlying learning and memory. At the behavioral level, however, it is still unclear how taste discrimination and CTA interact. We thus examined how CTA to one taste affected the feeding response induced by another appetitive food stimulus. We first demonstrated that snails have the capacity to recognize sucrose and carrot juice as distinct appetitive stimuli. We then found that snails can become conditioned (i.e. CTA) to avoid one of the stimuli and not the other. These results show that snails can distinguish between appetitive stimuli during CTA, suggesting that taste discrimination is processed upstream of the site where memory consolidation in the snail brain occurs. Moreover, we examined second-order conditioning with two appetitive stimuli and one aversive stimulus. Snails acquired second-order conditioning and were still able to distinguish between the different stimuli. Finally, we repeatedly presented the conditional stimulus alone to the conditioned snails, but this procedure did not extinguish the long-term memory of CTA in the snails. Taken together, our data suggest that CTA causes specific, irreversible and rigid changes from appetitive stimuli to aversive ones in the conditioning procedure.

One-trial conditioned taste aversion inLymnaea: good and poor performers in long-term memory acquisition

“…Consistent with this hypothesis are the data showing that altered gene activity and new protein synthesis must occur in these and other neurons if LTM is to form following the CTA procedure Hatakeyama et al, 2004b;Hatakeyama et al, 2006;Azami et al, 2006;Wagatsuma et al, 2006). In addition, our recent findings showed that snails are both able to discriminate different tastes and continue to eat other foods following the CTA procedure (Sugai et al, 2006).…”

“…The CS used was either 5·ml of 10·mmol·l -1 sucrose solution (CS1) or 5·ml of 0.3% carrot juice solution (CS2), whereas the US was 5·ml of 10·mmol·l -1 KCl. Either CS elicits a reliable feeding response (number of bites·min -1 ) from snails deprived of food for 1·day, whereas the US reliably causes the snails to stop their feeding behavior (Sugai et al, 2006). Higher concentrations of KCl cause snails to withdraw into their shells for a long period (Kojima et al, 1996) and so were not used in our studies.…”

“…It is also much more difficult to cause extinction of a strong memory than a weaker memory (Sangha et al, 2003a). We have previously shown that the phenomenon of extinction did not occur in snails trained by the 10-trial conditioning procedure for CTA (Sugai et al, 2006). We, therefore, wished to determine, as a second means of assessing LTM strength, whether the LTM following our newly developed one-trial CTA procedure was also resistant to extinction.…”

“…In our previous experiments examining CTA as well as in the present study, sucrose or carrot juice, both of which increase the feeding response, was used as the conditioned stimulus (CS), whereas KCl, which inhibits feeding behavior, was used as the unconditioned stimulus (US) (Kojima et al, 1996;Sugai et al, 2006). Following the paired presentations of the CS and US, sucrose or carrot juice no longer acts as an appetitive stimulus, and this CTA persists as an LTM for more than a month (Kojima et al, 1996).…”

Memory block: A consequence of conflict resolution

“…Here, we look at the ability of Lymnaea to learn and remember for many days not to respond to a food substance that normally elicits a feeding response. This is referred to as conditioned taste aversion (CTA) (Ito et al, , 2012Kawai et al, 2004;Sugai et al, 2006;Takigami et al, 2014). To produce a CTA, an appetitive stimulus, such as sucrose [the conditioned stimulus (CS)], is repeatedly paired with an aversive stimulus, such as an electric shock [the unconditioned stimulus (US)] Ito et al, 2015).…”