Opposite actions of dopamine on aversive and appetitive memories in the crab

The understanding of how the reinforcement is represented in the central nervous system during memory formation is a current issue in neurobiology. Several studies in insects provide evidence of the instructive role of biogenic amines during the learning and memory process. In insects it was widely accepted that dopamine (DA) mediates aversive reinforcements. However, the idea of DA being exclusively involved in aversive memory has been challenged in recent studies. Here, we study the involvement of DA during aversive and appetitive memories in the crab Chasmagnathus. We found that DA-receptor antagonists impair aversive memory consolidation, in agreement with previous reports in insects, while administration of DA facilitates memory formation after a weak training protocol. In contrast, DA treatment during appetitive training was found to impair formation of long-term appetitive memory. In addition, as a first step in elucidating the neuroanatomical correlates of DA action on memory, we mapped dopaminergic neurons in the central nervous system of the crab. Results of the current study, together with those obtained in a previous work about the role of octopamine (OA), suggest that both amines (DA and OA) play a dual action in memory processes. On the one hand, DA and OA mediate the aversive and the appetitive signals, respectively, throughout training, while on the other hand, they interfere with the formation of memory of the opposite sign (DA in appetitive and OA in aversive). Our results support a new understanding about the way appetitive and aversive stimuli are processed during memory formation to ensure adaptive behavior.     

Social context shapes cognitive abilities: associative memories are modulated by fight outcome and social isolation in the crab Neohelice granulata

Animal Cognition - Cognitive abilities of an animal can be influenced by distinct social experiences. However, the extent of this modulation has not been addressed in different learning scenarios:...     

Aggressiveness and memory: subordinate crabs present higher memory ability than dominants after an agonistic experience

A relationship between aggressiveness and memory has been proposed in several studies with different animal species. Here, we study this possibility in the crab Chasmagnathus granulatus, using the context-signal memory model (CSM) that involves an association between the learning context and a visual danger stimulus. Each experiment consisted of an agonistic phase and a memory one. During the former, matched pairs of male crabs were staged in two 10-min encounters and the dominant or subordinate condition of each member of the dyad was determined. During the memory phase, crabs were trained to acquire CSM and tested 24 h later. Results showed that the agonistic encounter, staged 48 h before the acquisition of CSM, can modulate memory according to the dominance condition of the fighter; in such a way that memory retention of subordinates results higher than that of dominants. By contrast, when the memory phase preceded the agonist one, forthcoming dominants and subordinates did not differ in their memory ability. The memory modulation would not be linked to a dominance status but to a persistent dominance relationship fully reconstructed in each encounter between the same opponents. Therefore, the crab's CSM would not depend directly on predetermined intrinsic properties, but on the outcome of the fight, which would be determined in turn by the relative aggressiveness of the fighters. The finding that the agonistic episode modulates memory opens the possibility of using this episodic interference to probe the function of diverse phases of CSM.