Study: Memories Survive Even When Most Synapses Vanish
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- Kazumasa Tanaka and colleagues at the Okinawa Institute of Science and Technology induced artificial hibernation in mice for two days, wiping out more than half of the synapses within engram neurons in the hippocampus — yet the mice still froze in fear when re-exposed to the alcohol scent previously paired with foot shocks.
- Clustered synapses on the surface of engram neurons were specifically retained during hibernation while non-clustered ones disappeared, leading the team to conclude that these tightly grouped connections are what actually encode and preserve long-term memories.
- Non-clustered synapses erased during hibernation reappeared within a day post-hibernation, which Tanaka's team interprets as evidence that these connections may be important for accessing or recalling memories rather than storing them.
- Steve Ramirez at Boston University, who was not involved in the study, called the findings "remarkable" and said they "absolutely, substantially advance our understanding of how memory works."
- George Dragoi at Yale University said the findings are relevant to people, noting prior research shows human brains store information very similarly to mice.
- Priyanka Rao-Ruiz at Vrije University Amsterdam called the work "a remarkable step forward" in addressing the long-standing puzzle of how the brain retains long-term memories.
- Ramirez said the findings offer hope that even in conditions like amnesia or Alzheimer's disease, where information seems lost, "memory may nonetheless persist" if the underlying clustered synapses survive.
Why it matters: The finding reframes memory as anchored in a small clustered subset of synapses rather than the full neural network, suggesting memory loss from Alzheimer's or amnesia could be more reversible than assumed. Tanaka's team is now tracing the molecular pathways that maintain these clustered connections — a step toward drug targets for memory impairment.
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