Recently, the scientists realized that even when they interfered with their trained snails’ brain cells in a way that should have removed the memory completely, some vestige remained. They decided to see whether something beyond the brain cells’ connections to each other — namely, RNA — could be hanging on to the memory.

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You might remember RNA from high school biology: It is best known for ferrying messages between the genome and the rest of the cell. But scientists have gradually realized that there is more to RNA than playing messenger.

There are some kinds of RNA that, instead of carrying messages, help switch genes on and off. They have been shown to be involved in long-term memory in snails, mice and rats, through their ability to influence chemical tags on DNA. These tags in turn influence whether a gene will be turned on in an organism.

To understand what was happening in their snails, the researchers first extracted all the RNA from the brain cells of trained snails, and injected it into new snails. To their surprise, the new snails kept their siphons wrapped up much longer after a shock, almost as if they’d been trained.

Next, the researchers took the brain cells of trained snails and untrained snails and grew them in the lab. They bathed the untrained neurons in RNA from trained cells, then gave them a shock, and saw that they fired in the same way that trained neurons do. The memory of the trained cells appeared to have been transferred to the untrained ones.