Memories transferred between molluscs

Scientists have been able to transfer a memory from one snail to another, providing a tantalising clue to the answer of one of most vexing questions in biology – how are memories stored?

In a paper published in the journal eNeuro, a team led by David Glanzman from the University of California, Los Angeles, in the US, reports an intriguing experiment using marine snails known as Californian sea hares (Aplysia californica).

The researchers trained one cohort of the molluscs to exhibit a defensive reflex when their tails were stimulated by mild electric shocks. A second cohort, untrained, did not show the reflex.

Once the reflex action had been established in the trained snails, they were euthanised and their abdominal ganglia removed. The process was repeated with some of the untrained animals. Through various procedures, RNA was extracted from the samples and prepared for introduction via injection into other snails.

Some of the new snails received RNA from the trained cohort, and some, as controls, from the untrained group.

Glanzman and his colleagues discovered that those receiving RNA from the trained snails exhibited the same reflex actions in response to tail simulation – even though they had not themselves been trained to do so.

The result is important, because it provides a clue to the nature of what is known as the “engram” – a word that functions a little like the term “dark matter” in that it denotes something that must exist but about which almost nothing is known.

“Engram” is the word used to denote the physical substrate of memory – the structure inside the brain that physically stores long term memories, broadly analogous to the way a hard-drive stores data on a computer.

The term was coined in the first decade of the twentieth century by German evolutionary biologist Richard Semon, but to date there has been no definitive evidence produced to conclusively show the mechanisms or biological structures involved.

The current favoured theory among neuroscientists, however, is that long-term memories are encoded in synapses – the functional interfaces between neurons through which electrical or chemical signals pass.

The work of Glanzman and his team, however, lends weight to an emerging counter-theory that suggests long-term memory is actually stored within the cell bodies of the neurons themselves.

This raises the possibility that RNA plays an important role in memory formation – an idea that some previous studies using the same species of snail seems to support.

In their paper, Glanzman and colleagues say their results raise many new questions about the mechanics of memory storage and the nature of the engram. However, they conclude that their findings offer “dramatic support” for the idea that memory does not have to be stored in synapses.