Inducing Hibernation in Mice Wipes Out Most Synapses but Leaves Memories Intact

The leading hypothesis for memory storage holds that when you learn something, the connections among neurons involved get stronger and physically larger, and that constitutes the memory. But these connections change significantly over time—they are plastic. “If you compare the arrangement of these connections on day one with the same on day four or five, it's very, very different,” Tanaka told Ars Technica. To learn how a memory that can last for years can sit on hardware that shifts every few days, his team made the shift a bit more dramatic.

Hibernation is a specialty of squirrels, hamsters, and bears, but the neural circuit that triggers it is conserved across mammals and is present in species that never hibernate in the wild, such as mice. In June 2020, a team led by Takeshi Sakurai, a neuroscientist at the University of Tsukuba and a collaborator on Tanaka’s study, developed a technique to artificially activate this hibernation circuit by activating a population called Q neurons in a region of the hypothalamus.

Using that technique, Tanaka’s team induced a hibernation-like state in mice. The procedure essentially erased the state of more than half of their synapses. Yet the mice apparently retained their memories. The study does not explain exactly how memories survive such extensive synaptic disruption, but it suggests that memory storage may not depend solely on the strength or configuration of individual synaptic connections. The researchers say more work is needed to understand the underlying mechanisms.