ABSTRACT
How an animal responds to a particular sensory stimulus will to a great extent depend on prior experience associated with that stimulus. For instance, aversive associative learning may lead to a change in the predicted outcomes, which suppresses the behavioural response to an otherwise rewarding stimulus. However, the neuronal mechanisms of how aversive learning can result in the suppression of even a vitally important innate behaviour is not well understood. Here we used the model system of Lymnaea stagnalis to address the question of how an anticipated aversive outcome can alter the behavioural response to a previously effective feeding stimulus. We found that aversive classical conditioning with sucrose as the CS (conditioned stimulus) and strong touch as the aversive US (unconditioned stimulus) reverses the decision so that the same salient feeding stimulus inhibits feeding, rather than activating it. Key to the understanding of the neural mechanism underlying this switch in the behavioural response is the PlB (pleural buccal) extrinsic interneuron of the feeding network whose modulatory effects on the feeding circuit inhibit feeding. After associative aversive training, PlB is excited by sucrose to reverse its effects on the feeding response. Aversive associative learning induces a persistent change in the electrical properties of PlB that is both sufficient and necessary for the switch in the behavioural output. In addition, the strong touch used as the US during the associative training protocol can also serve as a sensitizing stimulus to lead to an enhanced defensive withdrawal response to a mild touch stimulus. This non-associative effect of the strong touch is probably based on the facilitated excitatory output of a key identified interneuron of the defensive withdrawal network, PeD12.
Footnotes
Following advice from Dr Riccardo Mozzachiodi, Texas A&M University-Corpus Christi, we now have included in the Discussion a comparison between our findings and earlier findings from a paper by Walters et al. (1981) Science, 211:4481, 504-506 that first demonstrated conditioned fear in an invertebrate, Aplysia californica. In this new discussion section we state that our findings have some interesting similarities to early seminal findings in Aplysia that provided the first evidence for conditioned fear in an invertebrate. In that study, an initially neutral chemosensory CS was paired with electric shock to the head as the US. This aversive classical conditioning resulted both in the facilitation of a number of defensive responses to the CS and a concomitant depression of the feeding response to seaweed, a natural salient food stimulus for Aplysia. However, our aversive classical conditioning paradigm was based on the use of a salient food chemosensory CS, and its primary effect was a pairing-specific depression of the feeding response to this CS, in the absence of a defensive withdrawal response to it. At the same time, the strong tactile US used in our experiments caused a sensitized response to a mild tactile stimulus at test. Therefore, unlike the paradigm used in the Aplysia study, which resulted in a variety of different conditioned fear responses to the CS, including feeding depression, our paradigm seems to have resulted in a conditioned suppression of feeding and a non-associative enhancement of the withdrawal response without the emergence of conditioned fear. From this we conclude that unlike the noxious electrical stimulation used in Aplysia, the aversive tactile stimulation used in our experiments did not produce defensive arousal.