Scientists Removed Crucial Structures from a Brain. The Memories Remained.

3 min readHere’s what you’ll learn when you read this story:Long-term memory was thought to depend on strong synapses, but memories survive the widespread synaptic loss of hibernation.Mice trained in specific skills still remembered them after two days of artificial hibernation.Imaging revealed that certain synaptic clusters resisted the loss, suggesting only those clusters matter for memory retention.In a very real sense, we are the sum of our memories. From them, we gain a sense of self, form skills, and learn about the world. Yet although long-term memories are a core feature of existence, scientists still don’t quite understand all of the biological processes that establish and maintain them.For example, we know that long-term potentiation, or LTP, strengthens connections between brain cells—specifically axons and dendrites—that repeatedly fire together. This forms physical trace memories known as engrams, which are known as the building blocks of long-term memory. If these cellular bonds are repeatedly activated, they strengthen over time as dendritic spines located on dendrites send neurotransmitters to transmit signals between brain cells. If the cells are inactive, the opposite is true, proving the old maxim of “if you don’t use it, you lose it.”As a result, it would be reasonable to suggest that these strong connections are what power our long-term memory. But past research has shown that even though the number of cells and engrams in a particular brain structure can change over time, the ability to recall memories still persists. So, how exactly are these memories sticking around if the brain structures they’re made of dissolve?To answer this question, a team of researchers led by scientists at the Okinawa Institute of Science and Technology turned to an atypical tool for studying memory—hibernation. When a mammal (such as a mouse) hibernates, neuronal activity reduces significantly, and some of those all-important dendrite spines even disappear entirely. So, researchers wondered: If they taught mice certain skills and then induced hibernation, would the mice still retain those memories when they awoke?In a 2020 study, researchers (some of whom were involved with this new study) discovered a method for inducing artificial hibernation. Borrowing that technique, the OIST researchers instilled certain memories—such as where to find sugar pellets in a maze—in mice, and then placed them in a two-day hibernation. Imaging the mice’s brains before, during, and after hibernation, Lin and Tanaka’s team found that brain activity dropped by 70 percent, and the size of dendritic spines didn’t dictate which synapses were removed. However, once the mice awoke from their artificial slumber, they accurately recalled the previously instilled memories.“It was astonishing,” Yu-Ju Lin, the lead author of the study from OIST, said in a press statement. “Logically, if all our engram synapses were essential in memory retention as traditionally thought, memory should have massively deteriorated.”To test this idea further, the team placed similarly trained mice under anesthesia and treated them with a molecule that blocks the strengthening of synapses. In this version of the experiment, the team found that the mice had lost their memories when they woke up. To dig deeper, researchers investigated the brains of the mice using correlative light and electron microscopy (CLEM)—a process that allows scientists to pinpoint important proteins and zoom in on specific synaptic structures. Through this method, they found that while synapses were lost regardless of whether their dendritic spines were large or small, synapses organized into clusters between engram cells were selectively spared.“This suggests that for long-term memory, only particular clusters of synapses matter—the rest may be dispensable,” Kazumasa Tanaka, senior author of the study from OIST, said in a press statement. “Interestingly, dendritic spine size, which has been shown to increase in initial memory encoding, doesn’t seem to play a factor in memory retention.”This study proves that memories can be retained despite widespread synaptic loss, and it is these specific clustered connections that make long-term memories so resilient. Speaking with Live Science, Tanaka said that the next step is to examine these clusters more closely and manipulate them while charting the impact on memory.If memories make us who we are, then these synaptic clusters are the engines of our self-identity.Darren lives in Portland, has a cat, and writes/edits about sci-fi and how our world works. You can find his previous stuff at Gizmodo and Paste if you look hard enough.