The gut microbiome has profound effects on development and function of the nervous system. Recent evidence indicates that disruption of the gut microbiome leads to altered hippocampal neurogenesis. Here, we examined whether the effects of gut microbiome disruption on neurogenesis are age-dependent, given that both neurogenesis and the microbiome show age-related changes. Additionally, we examined memory induced functional connectivity of hippocampal networks. Control and germ-free mice at three different ages (4, 8, and 12 weeks) were trained in contextual fear-conditioning, then subsequently tested the following day. Hippocampal neurogenesis, quantified via BrdU and doublecortin, exhibited age-dependent changes relative to controls, with the established age-dependent decrease in neurogenesis being delayed in germ-free mice. Moreover, we found sex-dependent effects of germ-free status on neurogenesis, with 4 week old male germ-free mice having decreased neurogenesis and 8 week old female germ-free mice having increased neurogenesis. To assess systems-level consequences of disrupted neurogenesis, we assessed functional connectivity of hippocampal networks by inducing c-Fos expression with contextual memory retrieval and applying a previously described network analysis. Our results indicate impaired connectivity of the dentate gyrus in germ-free mice in a pattern highly correlated with adult neurogenesis. In control but not germ-free mice, functional connectivity became more refined with age, indicating that age dependent network refinement is disrupted in germ-free mice. Overall, the results show that disruption of the gut microbiome affects hippocampal neurogenesis in an age-and sex-dependent manner and that these changes are also related to changes in the dentate gyrus functional network.

Postnatal hippocampal neurogenesis has been demonstrated to affect learning and memory in numerous ways. Several studies have now demonstrated that increased neurogenesis can induce forgetting of memories acquired prior to the manipulation of neurogenesis and, as a result of this forgetting can also facilitate new learning. However, the mechanisms mediating neurogenesis-induced forgetting are not well understood. Here, we used a subregion-based analysis of the immediate early gene c-Fos as well as in vivo fiber photometry to determine changes in activity corresponding with neurogenesis induced forgetting. We found that increasing neurogenesis led to reduced CA1 activity during context memory retrieval. We also demonstrate here that perineuronal net expression in areas CA1 is bidirectionally altered by the levels or activity of postnatally generated neurons in the dentate gyrus. These results suggest that neurogenesis may induce forgetting by disrupting perineuronal nets in CA1 which may otherwise protect memories from degradation.

Hippocampal neurogenesis has a role in many essential learning and memory processes, including forgetting. This forgetting process is important because it prevents proactive interference between old and new memories. While several studies have now established the role of neurogenesis in forgetting, the specific mechanisms mediating neurogenesis-induced forgetting have not been elucidated. The goal of this study was to examine how increased neurogenesis affects the recall of context fear memory in addition to its effects on population activity within hippocampal subregions. We trained mice in contextual fear conditioning and then increased neurogenesis via 4 weeks of voluntary wheel running. Increased neurogenesis led to a reduction in freezing behaviour during context testing, replicating previous studies showing that increased neurogenesis causes forgetting of context fear memories. Additionally, we mapped the expression of the immediate early gene c-Fos within hippocampal subregions and found that increasing neurogenesis led to reduced CA1 c-Fos expression during context testing. The results suggest that reduced CA1 population activity may underlie the association between increased neurogenesis and forgetting.