Mechanisms of memory stabilization: are consolidation and reconsolidation similar or distinct processes?

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Consolidation of new memories depends on a crucial phase of protein synthesis. It is widely held that, once consolidated, memories are stable and resilient to disruption. However, established memories become labile when recalled and require another phase of protein synthesis to be maintained. Therefore, it has been proposed that when a memory is reactivated it must undergo additional consolidation (reconsolidation) to persist. To determine whether reconsolidation recapitulates consolidation, in the past few years several groups have investigated whether the same molecules and pathways mediate the formation of a memory and its maintenance after reactivation. At first glance, the results appear conflicting: although both processes appear to engage the same molecules and mechanisms, brain areas involved in consolidation after initial training are not required for reconsolidation. In addition, the formation of a memory and its maintenance after reactivation seem to have distinctive temporal molecular requirements. This review concludes with a working model that could explain the apparent controversy of memory vulnerability after reactivation.

Section snippets

Mechanisms of memory consolidation

Memory consolidation has been studied for more than a century 3, 9. It has been defined on the basis of observations that a newly formed memory undergoes a transformation process, becoming stronger and more resilient over time until it is insensitive to disruption. Factors shown to interfere with consolidation include cerebral trauma, electroconvulsive shock, protein synthesis inhibitors and several drugs [3]. In several species and memory systems, many molecular, anatomical and system-level

Molecular requirements of reconsolidation

The reconsolidation hypothesis, first suggested in the 1960s [10] and recently re-proposed following the work of Nader et al. [7], implies that every time a memory is reactivated it must undergo again a process of consolidation to be maintained. This idea contrasts with the classical view that a memory consolidates only once and, over time, becomes stronger and more stable.

To unravel this controversy, a great deal of effort has recently focused on determining whether reconsolidation, as implied

Temporal requirements of reconsolidation

Further distinctions between the two processes became evident when the temporal requirements for protein synthesis during reconsolidation were analyzed. In IA, the protein synthesis requirement induced by recall is a function of the age of the memory, with recent (2- and 7-day-old) but not remote (14- and 28-day-old) memories becoming sensitive to protein synthesis inhibitors upon reactivation [35]. Similar results, indicating a temporally graded requirement for protein synthesis after recall,

Concluding remarks and future directions

The knowledge accumulated so far indicates that reconsolidation of a reactivated memory and consolidation of an initial learning are characterized by distinctive features. First, they involve different brain areas and circuits. Consolidation appears to require several areas that are not essential for reconsolidation and, as will be suggested in this section, reconsolidation might involve mostly modulatory systems. Second, consolidation and reconsolidation also differ in their temporal dynamics.

Acknowledgements

I thank Maria Milekic and Stephen Taubenfeld for helpful comments and discussions.

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