Mouse Study Shows How Memories Survive the Brain Changes of Hibernation

Neuroscientists have long searched for the cells that generate and store memories, and a study of hibernating mice offers new clues. Over two days of hibernation, mice lost nearly half the connections between brain cells in the hippocampus, a brain region central to memory, according to the study, published Aug. 13 in the journal Science. Yet after this loss, the mice still retained their memories.

A research team led by Kazumasa Tanaka, a neuroscientist at the Okinawa Institute of Science and Technology in Japan, induced a form of artificial hibernation in mice and imaged their brains before, during and after using electron microscopy, which reveals synapses — the connections between brain cells — and related structures called dendritic spines in detail. During hibernation, activity in the mice's hippocampi dropped by 70%, and the mice's neurons withdrew more than half of their synaptic connections compared with mice that did not hibernate. Just two days after hibernation ended, these connections had been restored, and more than 80% of the lost dendritic spines reappeared in the same locations as before hibernation.

Many of the spines that persisted through hibernation were found in multisynaptic boutons, junctions where a single axon connects to several spines at once. In a separate experiment, anesthetized mice given a drug known to impair memory formation lost their multisynaptic boutons — and their memories, suggesting these junctions play a key role in preserving the brain's wiring.

After the mice emerged from hibernation, the researchers tested their ability to remember locations linked to food rewards or mild electric shocks that they had learned about before their rest. Despite the large changes in their brains, the mice could still remember the fear of a shock or the location of a treat. Tanaka's team is now working to identify whether the spared spines carry any unique molecular profile that sets them apart.