It has long been believed that memories were stored in the synapses of neurons. So, when those synapses were destroyed, the memories they held must be lost as well. However, a new study involving marine snails known as Aplysia has found that this might not be the case. If true, this could lead to memory restoration for patients with early onset Alzheimer's. David Glanzman of UCLA was senior author of the paper, which was published in eLife.
"Long-term memory is not stored at the synapse," Glanzman said in a press release. "That's a radical idea, but that's where the evidence leads. The nervous system appears to be able to regenerate lost synaptic connections. If you can restore the synaptic connections, the memory will come back. It won't be easy, but I believe it's possible."
In order test the snail's memory, they needed to trigger its defense response. Aplysia have delicate gills, which they are very quick to shield from damage. This reflex was bolstered by administering small electrical shocks on the tail. The snails began to form short-term memories associated with the shock due to the release of the neurotransmitter serotonin. Over time, the serotonin and secreted proteins creates synaptic connections associated with the formation of long-term memories. Injuries (like concussions) or other factors can interrupt this from occurring normally, which prevents the formation of long-term memories. For this study, the researchers artificially interrupted the progress to explore the creation and preservation of long term memories.
"If you train an animal on a task, inhibit its ability to produce proteins immediately after training, and then test it 24 hours later, the animal doesn't remember the training," Glanzman explained. "However, if you train an animal, wait 24 hours, and then inject a protein synthesis inhibitor in its brain, the animal shows perfectly good memory 24 hours later. In other words, once memories are formed, if you temporarily disrupt protein synthesis, it doesn't affect long-term memory. That's true in the Aplysia and in human's brains."
The researchers removed the sensory and motor neurons from the snails that were associated with long term memories of the electric impulses on their tails, and placed them in a Petri dish. Serotonin and necessary proteins were introduced to the dish, which prompted the neurons to create new synaptic connections. Surprisingly, the snails made these connections in the same spots where they had been inside the live animal.
This seems to indicate that though synaptic connections are associated with long-term memory, that is not where they are stored. Otherwise, the connections would not have been able to re-form where they did. However, they aren't sure where exactly they are stored. Glanzman said his team suspects that memories are stored in the nucleus of the neurons, but they are currently not able to prove that.
If it is true that synaptic connections can re-grow in the same pattern as when the long-term memories were made and the memories themselves are found in the nucleus, this could possibly have future implications in restoring long-term memories that have been lost due to early Alzheimer's or other forms of dementia.
Of course, there is still quite a bit of work to be done before it can be asserted that this would ever be a functional solution for restoring human memory. First of all, humans have about 1 trillion neurons in the brain, while the snails have around 20,000. Though the snails are markedly easier to study with fewer neurons, the comparative complexity of the human brain could make the process much more difficult.