The current findings are consistent with the hypothesis that IEG expression during resting states captures experience-specific changes in neuronal activity following spatial experience. These data also support recent studies showing that granule cells maintain key physiological distinctions relative to other hippocampal regions and provide novel insights into how the behavior of these cells changes with progressive age.
The current data show that a subpopulation of granule cells in the DG are predisposed to being repeatedly active (or at least being engaged in sufficient activity to induce plasticity that coincides with zif268 transcription) in any combination of two epochs, even in the absence of previous experience (i.e., in the immediate group). This is in striking contrast to unit recordings and IEG expression data from other hippocampal regions, both of which indicate that the probability that a single cell will be active during both rest and behavior under similar conditions is approximately equal to random chance with replacement. These data, however, are consistent with Shen et al.’s report  that ~50% of cells active during a single rest episode are also active during subsequent maze exploration. However, because these data were pooled from a large number of sessions after the rats were trained over several days, it is also possible that correlations between spatial behavior and preceding rest episodes reflects accumulated memories from the previous days’ experiences. In fact, such “preplay” has been previously reported in CA1 [27, 28]. Based on these data, and others [16, 17], it seems likely that the distribution of firing thresholds in the granule cell population is highly variable, and those cells with the lowest thresholds are predisposed to become active, both in multiple environments and between active behavior and resting states. Spatial experience, however, further increases this high “basal” correlation, making it even more likely that granule cells that are active during rest are the same cells that were active during spatial experience.
The current data show that this is also true in the aged DG – despite a decline of the number of granule cells transcribing zif268 during exploration behavior, the proportion of cells concurrently expressing zif268 in the foci and cytoplasm was not statistically different. This indicates that zif268 expression during resting states in general and expression during bouts of reactivation in particular are relatively preserved in the aged DG. This dissociation between the age-related changes in zif268 expression during rest and during exploratory behavior suggests that the source for these changes may be likewise dissociable. This notion, in turn, has implications both for the communication between CA3 and DG as well as how this pathway changes with progressive aging.
During exploratory behavior, the decrease in the number of granule cells transcribing zif268 in aged animals, relative to adult ones, solely during spatial exploration is likely caused by decreased innervation from the perforant path. The input to the DG from the perforant path declines by approximately one-third with progressive aging [29–31]. This decreased excitatory input to the aged DG is the likely reason for the attenuation of activity-related gene expression with age. In contrast, the reactivation-related expression of zif268 in the DG during rest is relatively preserved, and we hypothesize that this activity is mediated by the backprojection from CA3. This hypothesis is supported by the fact that both computational models (e.g., [32–34]) and experimental evidence (e.g., [34–38]) support CA3 as the most likely source of recapitulation of recent activity patterns. This is thought to stimulate DG either through direct backprojections from CA3c (nearest the hilus), or through indirect projections via mossy cells [39, 40]. The current data suggest that these backprojections from CA3 to the DG are functionally intact in the aged hippocampus, and remains equally capable of inducing reactivation of recent traces in the aged DG despite a decline in granule cell activity levels during exploratory behavior in senescent animals. This hypothesis is consistent both with recent anatomical  and functional imaging data  suggesting that intrinsic connections of the hippocampus are relatively intact in aged animals. Moreover, the dissociation between resting and behaviorally-driven zif268 expression in the aged DG is consistent with a wealth of data suggesting an increased emphasis in CA3 on the retrieval of previously stored patterns relative to input from the perforant path (see  for review). Although the preserved amount and pattern of zif268 expression during rest in aged animals is consistent with the notion that encoding failure (rather than impaired consolidation per se) is the major cause of deficits in hippocampus-dependent behaviors in aged animals , this conclusion must be made with caution.
Recent studies demonstrate that the reactivation deficits that occur in aged animals are actually quite subtle. That is, when pattern reactivation is quantified on the basis of cell pair correlations, no age-related deficit is observed . In contrast, when the temporal sequence of reactivation patterns across cell pairs is accounted for, a deficit emerges that correlates with memory deficits across animals . It was proposed by Gerrard et al. that only sequence reactivation relies on associative synaptic plasticity, and recent data  showing that reactivation critically depends on co-firing of cells during behaviour within brief (50 ms) time intervals are consistent with this claim. However, precise temporal order is not required to induce long-term potentiation (LTP) in at least some hippocampal synapses [43, 44], and reactivation still occurs if cells do not exhibit a temporal bias in their firing within the time required to induce spike timing-dependent plasticity . These data, combined with the fact that the expression of IEGs such as zif268 are required for enduring LTP , and long-term stability of place fields  suggests that neither zif268 expression, nor the enduring plasticity that it mediates, require cells to fire in a strictly preserved temporal sequence. Definitive assessment of the sensitivity of zif268 expression to the replay of specific sequences, however, would require further experiments using tasks involving more stereotyped paths of navigation. In open field exploration, as conducted here, animals take many different paths through the same physical space. Because the animal will not consistently cross the same path, cell pairs will not consistently fire in a set temporal sequence.