Instead of relying on many individual, strong links between neurons — which would typically be pared down during hibernation — long-lasting memories seem to require higher-level patterns in connectivity, the study found.
The findings, published Thursday (Aug. 13) in the journal Science, may complicate the picture of how memory retention works.
Despite this brain shrinkage, hibernating animals, such as alpine marmots (Marmota marmota) and European ground squirrels (Spermophilus citellus), still retain memories they formed before they went into hibernation. "Some studies report their memories are intact, even after, so they can remember conspecifics [members of the same species], like their friends, or they can remember the locations of their food," Tanaka told Live Science.
A central dogma of neuroscience holds that memories are made when connections between neurons grow stronger.
An old adage in neuroscience sums up the idea: "Neurons that fire together, wire together."
A typical synapse (left) where one brain cell connects to another (the axon of one connecting to the dendrites of the other). Each dendrite is covered in tiny protrusions called dendritic spines, which detect signals released from axons. When a particular synapse is very active, it can lead to physical changes (right). (Image credit: Luo-chu Yang)Recent studies have found that the structures of memories in the brain are actually very dynamic. Rather than relying on a strict and consistent set of connections between specific neurons, these physical traces of memories "drift" over time, within days, Tanaka noted. As a memory drifts, the original LTP-strengthened connections that made that memory can be lost — and yet, the memory persists.
The new study zoomed in on the hippocampus, a major memory processing hub in the brain. It focused on episodic memories, meaning those concerned with specific events and personal experiences. These memories are born in the hippocampus, remain there for a time, and then get transferred to other parts of the brain for super-long-term storage. That transfer can take weeks, months or more, and some neuroscientists think contextual details of the memories — such as the environment where they were formed — remain forever embedded in the hippocampus.
Then, they pushed those mice into artificial hibernation for two days, by switching on a special set of neurons in the brain that had been identified in past work. In mere minutes, the mice's brains started to change.
Completely intact memories
Despite this drastic change in the hippocampus, the mice remembered what they'd learned prior to hibernation. After hibernation, the mice froze in place when they were placed in the setting where they were shocked, and in the maze, they navigated to the food as easily as they did before hibernation — indications that they remembered both events.
To get to the bottom of how that could be possible, the team compared the hibernating mice to another set of lab mice, which were instead put under long-term anesthesia and treated with a molecule that blocks neurons from strengthening their synapses. The latter group of mice also saw dramatic synapse loss, but unlike the hibernating mice, their memories didn't linger after the treatment. What made the difference?
These spared connections tended to be hubs where a single neuron sending outgoing messages linked up with several neighboring cells, broadcasting its messages. They also appeared in places where many dendritic spines — the points where neurons receive messages — were bunched close together and received messages from multiple neurons at once.
This diagram shows two characteristic patterns found in the study. On the left, one neuron sends messages to multiple dendritic spines on different cells. On the right, we see "clustered engram patterns," in which dendritic spines that all activate in relation to the same memory are located close together and connect to various axons. These two patterns were less likely to disappear during hibernation. (Image credit: Luo-chu Yang)The size of the dendritic spines didn't seem to matter, he added. It was this clustering pattern that seemed to be key for memories' survival.
RELATED STORIESLooking forward, there's much more to learn about these clusters and their relationship to memory retention. The scientists are now working to characterize the clusters at a molecular level. Eventually, they hope to manipulate the clusters' structure and formation to see what that does to memory. They are also running experiments to figure out how the brain preferentially spares these clusters while pruning away so many other connections in the hippocampus, Tanaka said.
"With this study," Tanaka said, "we just opened up another door to tackle this problem."
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