Adult Neurogenesis Conserves Hippocampal Learning Capacity

Memory coding strengthens synaptic efficacy in the hippocampus via a long-term potentiation (LTP)-like mechanism. Given that animals are able to store memories of everyday experiences, hippocampal circuits should be able to avoid saturation of overall synaptic weight to preserve learning capacity. However, the underlying mechanism for this is still poorly understood. Here, we show that adult neurogenesis in rats plays a crucial role in the maintenance of the hippocampal learning capacity for learning. Artificial saturation with hippocampal LTP impaired learning capacity in contextual fear conditioning, which then completely recovered after 14 days, when LTP had decayed to the basal level. Ablation of neurogenesis by X-ray irradiation delayed the recovery of learning capacity, while enhancement of neurogenesis using running wheel sped up the recovery. Thus, one benefit of ongoing adult neurogenesis is the maintenance of hippocampal learning capacity through homeostatic renewing of hippocampal memory circuits. Decreased neurogenesis in aged animals may underlie declines in cognitive function with aging.