Yet another “impossible” finding from California—implanted memories.
Researchers from UCLA report a rather elegant study that showed the transfer of memory by transferring RNA from one snail to another . The study is pretty clear evidence that the RNA some how encodes the memory, which means that it likely is encoded in the genome of the neurons (i.e., as some form of epigenetic change in specific cells).
(Even back when I was an undergraduate psych major, I personally never believed that memories were stored in synapses, at least not entirely. I’m gratified, if astonished, to learn of this RNA mechanism. Told ya.)
The study itself used snails Aplysia californica, which have nice simple nerve systems. Essentially, some snails were “taught” to expect a shock, which made them pull into the shell for a period. The trained snails stay in longer in response to a shock than untrained snails.
RNA from the trained snails was injected into untrained snails, which acquired the signal behavior. Again: Whoa!
The report explains the methods, including the careful control conditions. Among other things, the study offers strong evidence that the RNA is modifying the DNA of the neurons (i.e., the memory is encoded in the DNA, and the RNA is transferring it from one neuron to another). Furthermore, the RNA affects only sensory neurons, not motor neurons (i.e., the sensory learning is encoded only in sensory neurons).
Together, the results indicate that this case of sensory learning is encoded via the DNA of a subset of neurons (the “sensory” neurons), and is transferred to other neurons via RNA. This would be a mechanism for memory “spreading” and strengthening, as whatever the trace is becomes encoded in more neurons.
There are many open questions here. For one thing, this study does not reveal what modifications are happening in the DNA, and how they result in the behavioral change. In the same vein, it isn’t known what controls which neurons are affected by this process.
Aside from the pure wonder of maybe identifying a DNA-based epigenetic mechanism for engrams, this study raises a lot of interesting questions to investigate.
First of all, this particular study is just one kind of ‘memory’, and a pretty simple one. However this case is encoding the engram, I would think that there are likely many ways that “memories” are encoded. There could be specific “codes” for different kinds of memories, and for different classes of neurons.
The localization of this effect is very intriguing. The RNA picks up the memory from a specific neuron, and even though the RNA may float everywhere, it is picked up only by certain other neurons. This suggests that neural specialization is tagged somehow in the DNA (epigenetically) in the neurons. How does this work?
I also wonder about how this process interacts with all the other possible epigenetic actions. There will be more than one “memory” floating around, and who knows what other signals, diseases, and what not. How do these interact? This complex of epigenetic process might ultimately have a role in forgetting, interference of different memories, and pathologies. Wow, this is going to get complicated!
As a software guy, I wonder about the encoding scheme(s). What is the “grammar” and other logical of this mechanism? What sort of error detection/correction might exist? Are there individual differences, i.e., idiomatic or group variations in memory encoding?
This study was short term. How does this mechanism operate over time. For that matter, does the mechanism change over time. (Learning to learn?) How does it unfold over the life of an animal? Is it possible for a mother’s RNA to cross into a fetus, and if so, does than potentially affect the growth of an infant’s brain?
Philosophically, it is interesting to think about this RNA transfer as part of a spectrum of methods for transferring “engrams” from one organism to another. Behavior and language might be seen as indirectly fiddling with the receiver’s genome (signal->perception->neural encoding->RNA->additional neural encodings).
It is also interesting to think about the subtle different between “genetic” and “epigenetic” mechanisms driving behavior. Nature vs. nurture is kind of moot in a case where “nurture” is directly modifying “nature”!
- Alexis Bédécarrats, Shanping Chen, Kaycey Pearce, Diancai Cai, and David L. Glanzman, RNA from Trained Aplysia; Can Induce an Epigenetic Engram for Long-Term Sensitization in Untrained Aplysia. eneuro, 2018. http://eneuro.org/content/early/2018/05/14/ENEURO.0038-18.2018.abstract
- Shivani Dave, ‘Memory transplant’ achieved in snails, in BBC News – Science & Environment. 2018. http://www.bbc.com/news/science-environment-44111476