A team from UCLA has reported the ability to transfer memory in the form of learned behavior in a recent publication in eNeuro. The work is carried out on the marine snail species Aplysia, a neurobiology model organism notable for its relatively small neuronal population and distinct behavioral patterns. And the mode of memory transfer, surprisingly, comes in the form of RNA.
The Aplysia snails were given mild electric shocks to their tails which induced a defensive withdrawal reflex. As a result of consistent stimuli—one every 20 minutes for a total of five times and again 24 hours later—the snails developed a simple type of learning known as “sensitization.” When given a tap, the trained snails would respond by defensively contracting for an average of 50 seconds, compared to only about one second by the unsensitized snails.
The team then extracted RNA from the central nervous system of trained snails and injected them into seven others that had not received shocks. A second control group received RNA from unsensitized snails. When the recipient snails were tested, the ones that received RNA from the trained snails remarkably also displayed the learned reaction. Behaving as if they themselves were trained with shocks, these new snails contracted for an average of 40 seconds. This reaction was not observed in the control group.
The team also isolated sensory and motor neurons from normal, non-shocked Aplysia. When they added RNA from the trained snails to the different types of neurons in vitro, they found increased excitability in sensory neurons but not in motor neurons. Control RNA from untrained snails produced no excitability.
In neuropsychology, the concept of “engrams” are believed to be the physical means by which memories are stored, such as through biochemical mechanisms. The team concluded that, as observed with their Aplysia model, RNA may be one such an engram, capable of transferring long-term, learned sensitization.
"It's as though we transferred the memory. If memories were stored at synapses, there is no way our experiment would have worked," noted senior co-author David Glanzman.
Glanzman believes in the possibility of RNA being used to awaken and restore memories that may have gone dormant, such as with neurological disorders. "I think in the not-too-distant future, we could potentially use RNA to ameliorate the effects of Alzheimer's disease or post-traumatic stress disorder,” says Glanzman.
Image: A schematic highlights the methodology of transferring RNA from a trained animal to elicit a learned response onto a new animal. Image courtesy of Bédécarrats et al., eNeuro (2018).