But this new study lends credence to the hypothesis that memory is also partly stored in physical traces called engrams.
Professor David Glanzman, one of the authors from UCLA, told the BBC the result was "as though we transferred the memory". The team's research is published May 14 in eNeuro, the online journal of the Society for Neuroscience.
The type of RNA relevant to these findings is believed to regulate a variety functions in the cell involved with the development and disease.
As it turned out, the RNA samples retained the memory of the electric shock, causing the untrained snails to exhibit a defense mechanism that lasted nearly as long as that of the donor snails. During each of two sessions separated by 24 hours, biologists five times he touched the electrodes to the foot of snails every 20 minutes. The shocks enhance the snail's defensive withdrawal reflex, a response it displays for protection from potential harm.
When the researchers subsequently tapped the snails, they found those that had been given the shocks displayed a defensive contraction lasting about 50 seconds, while those that had not received the shocks contracted for only about one second. Those that had not been given the shocks contracted for only about one second.
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In order to test the idea, scientists implanted wires into the tails of California sea hares and give them a series of electric shocks. Then the RNA from the first (sensitized) group was injected into seven marine snails that had not received any shocks, and the RNA from the second group was injected into a control group of seven other snails that also had not received any shocks. Like all mollusks, these snails have groups of neurons called ganglia, rather than brains.
The shocked snails had been "sensitised" to the stimulus. Some dishes had RNA from marine snails that had been given electric tail shocks, and some dishes contained RNA from snails that had not been given shocks. Sticking electrodes in the snail's tail and shocking it makes this defensive response last longer, tens of seconds, and sometimes up to nearly a minute. Zapping the culture with a bit of current excited the sensory neurons much more than neurons treated with RNA from nonshocked snails. He found that introducing the RNA directly to the neurons "increased (their) excitability".
But Prof Glanzman said: "If memories were stored at synapses, there is no way our experiment would have worked".
Long-term memory is thought to be housed within modified connections between brain cells.
Professor Glanzman stressed the marine snails were not hurt by the experiment, but they were alarmed. Even more impressive? That early research may someday pave the way for similar processes in humans. The paper might support hints from studies conducted decades ago that RNA was involved in memory.