Aversive associative memories formed by the association between a neutral conditioned stimulus (CS+) and an aversive unconditioned stimulus (US+) are progressively made permanent by a process of consolidation 1 . However upon retrieval, intervention by amnestic agents [2][3][4][5][6][7] , either prior to or immediately after retrieval, results in disruption of the previously consolidated fear memory. This suggests that a consolidated memory returns to a transient destabilized state shortly after reactivation necessitating a dynamic time-dependent process of reconsolidation in order to persist further. During this reconstruction, a memory is vulnerable to experimental intervention [8][9][10] leading to amnesia, but can also be enhanced [11][12][13] or modified on the long-term [14][15][16] , thereby updating the previous memory with new information [14][15][16][17] . In clinical terms, the bidirectional and adaptive nature of reconsolidation is ideally placed to mediate both the modification of memory strength 12 , as well as memory content 16,18 , rendering this process a promising therapeutical target to counteract the hyper-responsive fear system. In order to fully exploit reconsolidation-based therapies that adapt the content of fear memories, leading to a loss of fear response on the long term, it is crucial to elucidate the molecular underpinnings of reconsolidation, which to this date remain obscure.3 Long-lasting changes in synaptic efficacy brought about by gene transcription, protein synthesis and changes in strength of hippocampal glutamatergic synapses via AMPA receptor trafficking are believed to be the cellular substrates of learning and memory [19][20][21] . Although reconsolidation is not merely a recapitulation of the initial consolidation process 22 , it has been shown that transcription, de novo protein synthesis and synaptic protein degradation in the hippocampus are also necessary for memory remodeling after retrieval 4,7,17,[23][24][25] . Here, we investigated whether the temporal profile of reconsolidation that is hypothesized to be limited to a 6 h time window 5,8 actuates a sequential profile of defined dorsohippocampal AMPA receptor synaptic plasticity that is crucial to the synaptic remodeling that underlies subsequent fear expression (changes in memory strength) and reinterpretation of fear memory after retrieval (changes in memory content). Results Memory recall induces acute hippocampal AMPAR-endocytosisIn order to analyze whether glutamate receptors are regulated during reconsolidation in animals receiving the US+ and retrieval (US-R), we dissected the dorsal hippocampus at 1 and 4 h post-retrieval, and analyzed the synaptic membrane fraction, including membrane-bound proteins and associated proteins 26,27 , by immunoblotting for subunits of AMPA receptors. A no-shock group experiencing retrieval (NS-R) was used to control for the specificity of an aversive-associative memory (Supplementary Fig. S1). These two time points were chosen as they fall within the 6-h time window after ret...

Sparse populations of neurons in the dentate gyrus (DG) of the hippocampus are causally implicated in the encoding of contextual fear memories. However, engram-specific molecular mechanisms underlying memory consolidation remain largely unknown. Here we perform unbiased RNA sequencing of DG engram neurons 24 h after contextual fear conditioning to identify transcriptome changes specific to memory consolidation. DG engram neurons exhibit a highly distinct pattern of gene expression, in which CREB-dependent transcription features prominently ( P = 6.2 × 10 −13 ), including Atf3 ( P = 2.4 × 10 −41 ), Penk ( P = 1.3 × 10 −15 ), and Kcnq3 ( P = 3.1 × 10 −12 ). Moreover, we validate the functional relevance of the RNAseq findings by establishing the causal requirement of intact CREB function specifically within the DG engram during memory consolidation, and identify a novel group of CREB target genes involved in the encoding of long-term memory.

Encoding and retrieval of contextual memories is initially mediated by sparsely activated neurons, so-called engram cells, in the hippocampus. Subsequent memory persistence is thought to depend on network-wide changes involving progressive contribution of cortical regions, a process referred to as systems consolidation. Using a viral-based TRAP (targeted recombination in activated populations) approach, we studied whether consolidation of contextual fear memory by neurons in the medial prefrontal cortex (mPFC) is modulated by memory strength and CREB function. We demonstrate that activity of a small subset of mPFC neurons is sufficient and necessary for remote memory expression, but their involvement depends on the strength of conditioning. Furthermore, selective disruption of CREB function in mPFC engram cells after mild conditioning impairs remote memory expression. Together, our data demonstrate that memory consolidation by mPFC engram cells requires CREB-mediated transcription, with the functionality of this network hub being gated by memory strength.