Hibernating mice tore down nearly half the connections in their memory center and still remembered what they had learned — a clue to how memories can outlast the very structures that seem to hold them.
What the researchers did
Kazumasa Tanaka, a neuroscientist at the Okinawa Institute of Science and Technology in Japan, and colleagues induced a hibernation-like state in mice and imaged their brains before, during and after dormancy using electron microscopy, which resolves individual synapses and the dendritic spines that receive them. The work was published August 13 in Science.
During hibernation, activity in the mice's hippocampi — the brain region central to episodic memory — fell by 70 percent. Neurons withdrew more than half of their synaptic connections compared with mice that did not hibernate. The team had expected large dendritic spines and their stronger synapses to hold out through the shutdown. They did not.
What survived were hub-like structures called multisynaptic boutons, where a single axon connects to several dendritic spines at once, like a power strip feeding multiple devices. In separate experiments, anesthetized mice given a drug that impairs memory formation lost these boutons — and lost their learned memories along with them.
Two days after hibernation ended, the connections had been rebuilt, and more than 80 percent of the lost dendritic spines reappeared in their original locations. "That says to me that although the spine is lost, the synaptic machinery … is still there," says Cliff Abraham, a retired neuroscientist at the University of Otago in New Zealand who was not involved in the study. Mice that had learned to fear a mild shock or to find a food reward still remembered both after waking.
Why it matters
These results challenge a long-standing assumption: that a specific memory depends on one specific set of synaptic connections staying intact for life. Instead, the wider architecture of the network — especially those multisynaptic hubs — appears to carry the memory, while individual spines come and go. That matters for anyone thinking about their own cognition, because it reframes what "preserving memory" means. If memories live in network patterns rather than fixed wiring, then keeping those patterns active may be more important than protecting any single connection. It also helps explain how the brain can stay plastic enough to keep learning without erasing what it already knows.
What you can do
- Revisit important information at spaced intervals (days, then weeks) rather than cramming once. Reactivation is what keeps network patterns stable.
- Vary how you recall a memory — write it, say it aloud, draw it. Multiple routes into the same memory may mimic the redundancy these hub connections provide.
- Protect sleep. Synaptic reorganization is an active process, and restorative sleep is when much of it happens.
- Treat forgetting details as normal maintenance, not failure. The structure often remains; the retrieval path is what needs practice.
Source: Science News
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