Summary: Scientists at the University of Tsukuba found that a very small population of adult-born neurons (ABNs) in the hippocampus are reactivated during REM sleep and that this reactivation locks waking experiences into long-term memory. Using genetically modified mice that allowed monitoring and manipulation of these neurons, the team observed that ABNs replay the same activity patterns during REM sleep as they exhibited during learning. Preventing that replay impaired later memory recall, showing that ABN reactivation during REM is essential for memory consolidation.
The study also showed that ABN activity must be precisely timed with ongoing theta oscillations for effective consolidation. These results clarify how a scarce class of neurons can have outsized influence on memory stability and suggest mechanisms that may help explain memory decline in disorders such as Alzheimer’s disease.
Key Facts:
- ABN Reactivation: Small ensembles of adult-born neurons replay learning-related activity during REM sleep to consolidate memories.
- Theta Rhythm Link: Successful memory storage requires ABN firing to synchronize with hippocampal theta waves.
- Memory Impairment: Disrupting ABN reactivation during REM sleep prevents proper memory recall.
Source: University of Tsukuba
Researchers examined the role of adult-born neurons located in the dentate gyrus region of the hippocampus, an area well known for generating strong theta oscillations. Although ABNs form a small minority of hippocampal principal neurons, they have been implicated in memory performance and are reduced in conditions such as Alzheimer’s disease. The present work aimed to determine whether specific ABN ensembles active during learning are transiently reactivated during REM sleep and whether that reactivation is necessary for memory consolidation.
In the experiments, genetically modified mice were exposed to a fear-conditioning task while ABN activity was recorded. The investigators then tracked the same neurons during subsequent sleep, with a particular focus on REM periods. They observed that ABNs spontaneously reactivated in brief, transient ensembles that reproduced key features of the original learning-related patterns. Remarkably, ensembles as small as approximately three ABNs were sufficient to support consolidation of the fear memory.
To test causality, the team used targeted manipulations to block ABN reactivation specifically during REM sleep. When those reactivations were suppressed, the mice showed clear deficits in memory recall, establishing a direct functional link between ABN replay in REM and later memory performance. Additional experiments revealed that not only reactivation but also precise timing relative to theta oscillations mattered: ABN firing needed to be coordinated to a particular theta phase to drive associative consolidation.
These findings provide causal evidence that memory consolidation can depend on transient reactivation of minimal neuronal populations and on the temporal coordination of those neurons with ongoing network rhythms. The work advances understanding of how fleeting neural events during REM sleep can translate into long-term storage of experience, and it highlights ABNs as a potential focal point for research into age-related and pathological memory loss.
Funding: This work was partially supported by the World Premier International Research Center Initiative (WPI) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan Agency for Medical Research and Development (JP21zf0127005, JP23wm0525003), Japan Society for the Promotion of Science (JSPS) (24H00894, 23H02784, 22H00469, JP16H06280, 20H03552, 21H05674, 21F21080), Takeda Science Foundation, Uehara Memorial Foundation, The Mitsubishi Foundation to M.S.; JSPS (19F19310) to M.S. and S.S.; JSPS (21F21080) to M.S. and P.V.; Goho Life Sciences International Fund, The Tokyo Biochemical Research Foundation Postdoctoral Fellowships for Asian Researchers in Japan, Takeda Science Foundation Scholarship for Foreign Researchers to S.S.; JSPS (21J11746, 24K18212, 23K19393 and DC2 fellowship) to I.K.
About this memory and sleep research news
Author: YAMASHINA Naoko
Source: University of Tsukuba
Contact: YAMASHINA Naoko – University of Tsukuba
Image: Image credit: Neuroscience News
Original Research: Open access. “Transient reactivation of small ensembles of adult-born neurons during REM sleep supports memory consolidation in mice” by Sakaguchi, Masanori et al., published in Nature Communications.
Abstract
Transient reactivation of small ensembles of adult-born neurons during REM sleep supports memory consolidation in mice
Memory consolidation has long been associated with reactivation of learning-related activity during REM sleep, coordinated with theta oscillations. However, direct causal links between specific small neuronal ensembles and consolidation were previously unresolved. Strong theta oscillations arise in the hippocampal dentate gyrus, where adult-born neurons are continuously generated. Although ABNs modulate hippocampal circuits, whether their specific information content drives memory behavior was unclear. This study shows that REM reactivation of ensembles comprising as few as ~3 ABNs is necessary for fear memory consolidation and that precise synchronization of ABN firing with a particular theta phase is essential for associative memory formation. These results provide causal evidence that consolidation depends on both reactivation of minimal neuronal populations and their precise coordination within theta-defined temporal windows.