Mice Retain Fear Memory Despite Losing Half Their Synapses in Artificial Hibernation

In a new study published in *Science*, researchers at the Okinawa Institute of Science and Technology showed that mice can keep a learned fear memory even after more than half of the synapses in the engram neurons of their hippocampus disappear during an induced hibernation state. The team trained mice to associate an alcohol‑smelling chamber with mild electric shocks, then artificially slowed the animals’ metabolism with drugs and darkened the chamber for two days. Five days later, the mice still froze in the chamber, indicating the memory was intact. Brain imaging revealed that while many synapses vanished, those clustered closely on the surface of engram neurons remained, while non‑clustered synapses were lost but reappeared within a day after hibernation. Dr. Kazumasa Tanaka, the study’s lead author, said the findings suggest clustered synapses are essential for memory persistence, whereas non‑clustered ones may help access the memory. The results challenge the long‑held view that a memory requires a stable, specific synaptic pattern and open new avenues for understanding memory resilience in conditions such as amnesia or Alzheimer’s disease.