Title: Impact of adult hippocampal neurogenesis on behavior
Description: The role of adult hippocampal neurogenesis in behavior, especially contextual fear conditioning (CFC), is debated. Several studies demonstrated that blocking adult hippocampal neurogenesis in rodents impairs CFC, while several other studies failed to observe impairment. We sought to determine whether different CFC methods vary in their sensitivity to the arrest of adult neurogenesis. Adult neurogenesis was arrested in mice using low-dose, targeted x-irradiation, and the effects of x-irradiation were assayed in conditioning procedures that varied in the use of a discrete conditioned stimulus, the number of trials administered, and the final level of conditioning produced. We demonstrate that x-irradiation impairs CFC in mice when a single-trial CFC procedure is used but not when multiple-trial procedures are used, regardless of the final level of contextual fear produced. In addition, we show that the x-irradiation-induced deficit in single-trial CFC can be rescued by providing pre-exposure to the conditioning context.

These results indicate that adult hippocampal neurogenesis is required for CFC in mice only when brief training is provided. We next sought to determine the age at which adult-born hippocampal neurons contribute to behaviors such as CFC and novel object recognition (NOR). NOR was assessed in mice after neurogenesis was arrested using focal x-irradiation of the hippocampus, or a reversible, genetic method in which glial fibrillary acidic protein-positive neural progenitor cells are ablated with ganciclovir. Arresting neurogenesis did not alter general activity or object investigation during four exposures with two constant objects. However, when a novel object replaced a constant object, mice with neurogenesis arrested by either ablation method showed increased exploration of the novel object when compared with control mice. The increased novel object exploration did not manifest until 4-6 weeks after x-irradiation or 6 weeks following a genetic ablation, indicating that exploration of the novel object is increased specifically by the elimination of 4- to 6-week-old adult born neurons.

The increased novel object exploration was also observed in older mice, which exhibited a marked reduction in neurogenesis relative to young mice. Mice with neurogenesis arrested by either ablation method were also impaired in one-trial contextual fear conditioning (CFC) at 6 weeks but not at 4 weeks following ablation, further supporting the idea that 4- to 6-week-old adult born neurons are necessary for specific forms of hippocampus-dependent learning, and suggesting that the NOR and CFC effects have a common underlying mechanism. These data suggest that the transient enhancement of plasticity observed in young adult-born neurons contributes to cognitive functions. Finally, we sought to understand how a memory trace is formed and retrieved in the DG and in CA3, and whether adult hippocampal neurogenesis modulates these events. Our hypothesis is that the cells reactivated during recall of a memory are a component of the memory trace. We have designed a novel tool to test this hypothesis not just in a short time period but also on a longer timescale.

We, therefore, created an ArcCreERT2 BAC transgenic mouse that allows for the permanent labeling of cells expressing the IEG Arc/Arg3.1 and allows for a comparison between the cells that are activated during the encoding of an experience and those that are activated during the retrieval of the corresponding memory. To test our hypothesis of the memory trace, we have performed various manipulations that affect hippocampus-dependent memory: 1) contextual differences (fearful versus non-fearful context), 2) time (recent versus remote memory), and 3) arrest of adult hippocampal neurogenesis (x-irradiation and social defeat). We find that levels of reactivation in CA3, both in the presence and in the absence of neurogenesis, are correlated with the strength of the memory, which suggests that the cells reactivated in CA3, are a component of the memory trace.