Characterization of spatial memory reconsolidation.

Reconsolidation is necessary for the restabilization of reactivated memory traces. However, experimental parameters have been suggested as boundary conditions for this process. Here we investigated the role of a spatial memory trace's age, strength, and update on the reconsolidation process in mice. We first found that protein synthesis is necessary for reconsolidation to occur in the hippocampal CA3 region after reactivation of partially acquired and old memories but not for strongly acquired and recent memories. We also demonstrated that the update of a previously stable memory required, again, a memory reconsolidation in the hippocampal CA3. Finally, we found that the reactivation of a strongly acquired memory requires an activation of the anterior cingulate cortex as soon as 24 h after acquisition. This study demonstrates the importance of the knowledge of the task on the dynamic nature of memory reconsolidation processing.

Pharmacological intervention of hippocampal CA3 NMDA receptors impairs acquisition and long-term memory retrieval of spatial pattern completion task.

Pattern completion is the ability to retrieve complete information on the basis of incomplete retrieval cues. Although it has been demonstrated that this cognitive capacity depends on the NMDA receptors (NMDA-Rs) of the hippocampal CA3 region, the role played by these glutamatergic receptors in the pattern completion process has not yet been specified. In the present study, we investigated the function of the CA3 NMDA-Rs during the different memory stages (acquisition, memory consolidation, and retrieval) in a spatial pattern completion task (when some visual cues were removed from the environment) in comparison to a standard spatial water maze task (when all visual cues were available in the environment). Thus, we coupled a massed training with the injection of NMDA-receptor antagonist (AP5) into the CA3 subfield of the dorsal hippocampus of C57BL/6 mice. Our results show that NMDA-Rs are not implicated in a standard situation but are crucial during both acquisition and long-term memory retrieval in pattern completion. This work provides the first evidence of a specific role of CA3 NMDA-Rs during memory process involved in the reactivation of incomplete memory trace, particularly when the amount of environmental information available is strongly limited.