Initially, memory is labile and requires consolidation to become stable. However, several studies support that consolidated memories can undergo a new period of lability after retrieval. The mechanistic differences of this process, termed reconsolidation, with the consolidation process are under debate, including the participation of hippocampus. Up to this point, few reports describe molecular changes and, in particular, transcription factor (TF) involvement in memory restabilization. Increasing evidence supports the participation of the TF nuclear factor-B (NF-B) in memory consolidation. Here, we demonstrate that the inhibition of NF-B after memory reactivation impairs retention of a hippocampal-dependent inhibitory avoidance task in mice. We used two independent disruptive strategies to reach this conclusion. First, we administered intracerebroventricular or intrahippocampal sulfasalazine, an inhibitor of IKK (IB kinase), the kinase that activates NF-B. Second, we infused intracerebroventricular or intrahippocampal B decoy, a direct inhibitor of NF-B consisting of a double-stranded DNA oligonucleotide that contains the B consensus sequence. When injected immediately after memory retrieval, sulfasalazine or B decoy (Decoy) impaired long-term retention. In contrast, a one base mutated B decoy (mDecoy) had no effect. Furthermore, we also found NF-B activation in the hippocampus, with a peak 15 min after memory retrieval. This activation was earlier than that found during consolidation. Together, these results indicate that NF-B is an important transcriptional regulator in memory consolidation and reconsolidation in hippocampus, although the temporal kinetics of activation differs between the two processes.

Several studies support that stored memories undergo a new period of consolidation after retrieval. It is not known whether this process, termed reconsolidation, requires the same transcriptional mechanisms involved in consolidation. Increasing evidence supports the participation of the transcription factor NF-B in memory. This was initially demonstrated in the crab Chasmagnathus model of associative contextual memory, in which re-exposure to the training context induces a well characterized reconsolidation process. Here we studied the role of NF-B in reconsolidation. NF-B was specifically activated in trained animals re-exposed to the training context but not to a different context. NF-B was not activated when animals were re-exposed to the context after a weak training protocol insufficient to induce long-term memory. A specific inhibitor of the NF-B pathway, sulfasalazine, impaired reconsolidation when administered 20 min before re-exposure to the training context but was not effective when a different context was used. These findings indicate for the first time that NF-B is activated specifically by retrieval and that this activation is required for memory reconsolidation, supporting the view that this molecular mechanism is required in both consolidation and reconsolidation.

Although it is generally accepted that memory consolidation requires regulation of gene expression, only a few transcription factors (TFs) have been clearly demonstrated to be specifically involved in this process. Increasing research data point to the participation of the Rel/nuclear factor-kappaB (NF-kappaB) family of TFs in memory and neural plasticity. Here we found that two independent inhibitors of NF-kappaB induced memory impairment in the one-trial step-through inhibitory avoidance paradigm in mice: post-training administration of the drug sulfasalazine and 2 h pretraining administration of a double-stranded DNA oligonucleotide containing the NF-kappaB consensus sequence (kappaB decoy). Conversely, one base mutation of the kappaB decoy (mut-kappaB decoy) injection did not affect long-term memory. Accordingly, the kappaB decoy inhibited NF-kappaB in hippocampus 2 h after injection but no inhibition was found with mut-kappaB decoy administration. A temporal course of hippocampal NF-kappaB activity after training was determined. Unexpectedly, an inhibition of NF-kappaB was found 15 min after training in shocked and unshocked groups when compared with the naïve group. Hippocampal NF-kappaB was activated 45 min after training in both shocked and unshocked groups, decreasing 1 h after training and returning to basal levels 2 and 4 h after training. On the basis of the latter results, we propose that activation of NF-kappaB in hippocampus is part of the molecular mechanism involved in the storage of contextual features that constitute the conditioned stimulus representation. The results presented here provide the first evidence to support NF-kappaB activity being regulated in hippocampus during consolidation, stressing the role of this TF as a conserved molecular mechanism for memory storage.