Memory reconsolidation engages only a subset of immediate-early genes induced during consolidation

The relationship between memory consolidation and reconsolidation at the molecular level is poorly understood. Here, we identify three immediate-early genes that are differentially regulated in the mouse hippocampus after contextual fear conditioning and reactivation of the context-shock memory: serum- and glucocorticoid-induced kinase 1 (SGK1), SGK3, and nerve growth factor-inducible gene B (NGFI-B). The upregulation of SGK1 expression was not specific for the context-shock association and therefore not suitable for a comparison of contextual memory consolidation and reconsolidation. SGK3 expression was upregulated during both consolidation and reconsolidation. Analysis of SGK3 expression showed that expression changes elicited by a context-shock association during consolidation can subsequently be recapitulated during reconsolidation and that the transcriptional changes induced by retrieval depend on the remoteness of the memory. On the other hand, we found that NGFI-B is regulated during consolidation but not reconsolidation. This consolidation-specific regulation occurs in hippocampal area CA1. Our discovery of a consolidation-specific transcription indicates that reconsolidation is only a partial recapitulation of consolidation at the transcriptional level. Such partial rather than total recapitulation may have evolved as a more economic and reliable mechanism for organisms to modify memory.