Contextual Pavlovian conditioning in the crab Chasmagnathus.

In contextual conditioning, a complex pattern of information is processed to associate the characteristics of a particular place with incentive or aversive reinforcements. This type of learning has been widely studied in mammals, but studies of other taxa are scarce. The context-signal memory (CSM) paradigm of the crab Chasmagnathus has been extensively used as a model of learning and memory. Although initially interpreted as habituation, some characteristics of contextual conditioning have been described. However, no anticipatory response has been detected for animals exposed to the training context. Thus, CSM could be interpreted either as an associative habituation or as contextual conditioning that occurs without a context-evoked anticipatory response. Here, we describe a training protocol developed for contextual Pavlovian conditioning (CPC). For each training trial, the context (conditioned stimulus, CS) was discretely presented and finished together with the unconditioned stimulus (US). In agreement with the CSM paradigm, a robust freezing response was acquired during the 15 training trials, and clear retention was found when tested with the US presentation after short (2 and 4 h) and long (1-4 days) delays. This CPC memory showed forward but not simultaneous presentation conditioning and was context specific and protein synthesis dependent. Additionally, a weak CPC memory was enhanced during consolidation. One day after training, CPC was extinguished by repeated CS presentation, while one presentation induced a memory labilisation-reconsolidation process. Finally, we found an anticipatory conditioned response (CR) during the CS presentation for both short-term (4 h) and long-term memory (24 h). These findings support the conditioning nature of the new paradigm.