Updating stored memory requires adult hippocampal neurogenesis

Adult hippocampal neurogenesis appears to influence hippocampal functions, such as memory formation for example. While adult hippocampal neurogenesis is known to be involved in hippocampal-dependent learning and consolidation processes, the role of such immature neurons in memory reconsolidation, a process involved in the modification of stored memories, remains unclear. Here, using a novel fast X-ray ablation protocol to deplete neurogenic cells, we have found that adult hippocampal neurogenesis is required to update object recognition stored memory more than to reinforce it. Indeed, we show that immature neurons were selectively recruited to hippocampal circuits during the updating of stored information. Thus, our data demonstrate a new role for neurogenesis in cognitive processes, adult hippocampal neurogenesis being required for the updating of stored OR memories. These findings suggest that manipulating adult neurogenesis may have a therapeutic application in conditions associated with traumatic stored memory, for example.

: X-ray irradiation impairs neurogenesis for at least 3 days. We show that irradiation protocol provoke a transient neurogenesis depletion without affect to mature neurons of dentate gyrus. Figure S2: X-ray irradiation doesn't provoke a significant inflammatory response in the hippocampus. Figure S3: Effect of the depletion of adult neurogenesis provoked by X-ray irradiation on object recognition memory reconsolidation when object recognition memory reactivation was performed in a novel context.

Table S1
Object exploration time (seconds) per session in the ORM experiments. This table demonstrates that X-ray irradiation does not affect to mice exploratory activity. This table is related with Figure 1. Table S2 Primer sequences used for RT-PCR gene expression analysis.

Supplemental Experimental Procedures
Immunohistochemistry, immunofluorescence and histological analysis showed in Supplementary figures S1 y S2.
RT-PCR gene expression. We perform a detailed description of gene expression experiments presented in Figure S2 of supplemental material.
Object recognition reconsolidation when reactivation and training sessions were performed in different contexts.

Figure S3
Supplemental Figure 3. Effect of adult neurogenesis depletion on object recognition memory reconsolidation when reactivation is performed in a novel context. Reconsolidation in sham and irradiated mice was compared in three different circumstances: A, reactivation without novelty; B, reactivation with novelty; and C, no reactivation. In all cases, irradiation was performed 3 days after OR training and after reactivation in a context different (circle) to that used in the training session (rectangle). In each graph, the letters A, B and C represent the different objects used: * represent significant differences between the sessions and with the training session in the same experimental group; + represent significant differences between the LTM sessions and the reactivation session in the same experimental group; and • represent significant differences between the LTM session of each irradiated group with respect to the sham mice (one symbol, p<0.05, two symbols, p<0.01 and three symbols, p<0.001).

Reverse transcription-PCR analysis of mRNA
Total RNA was extracted using the Tripure reagent (Roche Products) from the brain tissue of at least of six animals per group, collected from at least two different experimental sessions. More detailed information on the primers used can be found in Table S2. The values obtained were normalized with respect glyceraldehyde-3-phosphate dehydrogenase (GAPDH) expression and they are expressed relative to those of the non-irradiated animals to which a value of 1 was assigned.

Object recognition reconsolidation when reactivation and training sessions were performed in different contexts
The effect of depleted neurogenesis on OR memory reconsolidation was assessed using a 10 minute reactivation session performed in a circular arena (new context) three days after the end of the training session performed in a rectangular arena. Irradiation was administered 4h after the end of a reactivation session, in which the mice were exposed to: 1, reactivation without novelty (the same object as that used during the training session); or 2, reactivation with novelty (with a familiar object used in the training session and then a novel object); or no object reactivation in the circular arena. Three days after this reactivation session finished, one object in the rectangular arena was changed for a novel one in order to test neurogenesis dependent post-reactivation of long-term memory (PR-LTM).