Neuron: The Brain May Fail to Consolidate Memories During Sleep Even When Sleep Appears Normal

A team of scientists led by Min Xu and Wanling Peng from the Institute of Neuroscience at the Chinese Academy of Sciences studied hippocampal neurons, cells located in the brain region responsible for learning and memory. These neurons are particularly active during slow-wave sleep and contribute to brief bursts of electrical activity known as hippocampal ripples. These events are associated with the reactivation and strengthening of newly formed memories.

In one experiment, mice were allowed to explore two identical objects and were then returned to their home cages for five hours. During this period, the animals spent most of their time asleep. Before the next test, researchers moved one of the objects. Control mice spent more time exploring the object that had been moved, indicating that they remembered its original location. Mice whose activity in the studied neurons had been suppressed after learning showed almost no such preference.

Suppressing these cells specifically during slow-wave sleep also impaired memory. Interventions during wakefulness or rapid eye movement (REM) sleep did not produce the same effect. In a separate experiment, the intervention also disrupted the mice's social memory: the animals became less able to distinguish familiar mice from unfamiliar ones.

The researchers found that neuronal activity was linked to acetylcholine, a chemical messenger whose levels decrease during slow-wave sleep. The Chrm2 gene encodes a receptor through which acetylcholine suppresses the activity of these cells. When acetylcholine levels fall, this inhibition weakens, allowing the neurons to play a more active role in generating hippocampal ripples.

An analysis of existing human brain data revealed a similar population of cells, although their role in human memory and sleep remains to be investigated. The authors believe the findings could help researchers explore the mechanisms underlying cognitive disorders. However, it is not yet known whether this mechanism could be used to develop new treatments.