Summary: Researchers report neurogenesis underlies the brain’s capacity to create and store new memories. Exercise, researchers report, promotes neurogenesis and increases hippocampal capacity.
The ability to obtain new memories in adulthood may depend on neurogenesis — the generation of new neurons in the hippocampus — to clear out old memories that have been safely stored in the cortex, according to research in male rats published in Journal of Neuroscience.
Previous research suggests that the hippocampus has a finite capacity to acquire and store new memories. It is unknown how the brain compensates for this limitation to facilitate learning throughout life.
Kaoru Inokuchi and colleagues show that reducing neurogenesis in rats impairs recovery of learning capacity while promoting neurogenesis through physical activity on a running wheel increased hippocampal capacity. This finding implies that neurogenesis, which can be reduced by stress and aging, underlies the brain’s capacity for new memories.
The study may also explain why exercise is especially important for patients with memory disorders such as Alzheimer’s disease as well as for healthy people to help maintain memory as they age.
Funding: Japan Science and Technology Agency, Japan Society for the Promotion of Science, Ministry of Education, Culture, Sports, Science and Technology, Mitsubishi Foundation, Uehara Memorial Foundation, Takeda Science Foundation funded this study.
Source: David Barnstone – SfN
Publisher: Organized by NeuroscienceNews.com.
Image Source: NeuroscienceNews.com image is credited to Alam et al., JNeurosci (2018).
Original Research: Open access research for “Adult Neurogenesis Conserves Hippocampal Memory Capacity” by Md Jahangir Alam, Takashi Kitamura, Yoshito Saitoh, Noriaki Ohkawa, Takashi Kondo and Kaoru Inokuchi in Journal of Neuroscience. Published July 9 2018.
Adult Neurogenesis Conserves Hippocampal Memory Capacity
The hippocampus is crucial for declarative memories in humans and encodes episodic and spatial memories in animals. Memory coding strengthens synaptic efficacy via a long-term potentiation (LTP)-like mechanism. Given that animals store memories of everyday experiences, the hippocampal circuit must have a mechanism that prevents saturation of overall synaptic weight for the preservation of learning capacity. Long-term depression (LTD) works to balance plasticity and prevent saturation. In addition, adult neurogenesis in the hippocampus is proposed to be involved in the down-scaling of synaptic efficacy. Here, we show that adult neurogenesis in male rats plays a crucial role in the maintenance of hippocampal capacity for memory (learning and/or memory formation). Neurogenesis regulated the maintenance of LTP, with decreases and increases in neurogenesis prolonging or shortening LTP persistence, respectively. Artificial saturation of hippocampal LTP impaired memory capacity in contextual fear conditioning, which completely recovered after 14 days, which was the time required for LTP to decay to the basal level. Memory capacity gradually recovered in parallel with neurogenesis-mediated gradual decay of LTP. Ablation of neurogenesis by X-ray irradiation delayed the recovery of memory capacity, while enhancement of neurogenesis using a running wheel sped up recovery. Thus, one benefit of ongoing adult neurogenesis is the maintenance of hippocampal memory capacity through homeostatic renewing of hippocampal memory circuits. Decreased neurogenesis in aged animals may be responsible for the decline in cognitive function with age.
Learning many events each day increases synaptic efficacy via long-term potentiation (LTP), which can prevent the storage of new memories in the hippocampal circuit. In this study, we demonstrate that hippocampal capacity for the storage of new memories is maintained by ongoing adult neurogenesis through homoeostatic renewing of hippocampal circuits in rats. A decrease or an increase in neurogenesis, respectively, delayed or sped up the recovery of memory capacity, suggesting that hippocampal adult neurogenesis plays a critical role in reducing LTP saturation and keeps the gate open for new memories by clearing out the old memories from the hippocampal memory circuit.