The brain is a truly remarkable organ. A recent study by research scientists at the Massachusetts Institute of Technology (MIT) in the United States has found that memories which cannot be retrieved by other methods can be recovered using an emerging technology known as optogenetics.
Though it might sound more like science fiction than science fact, optogenetics allows scientists to stimulate and control neurons – even in intact tissue – with light even to the point of switching on and off memories.
The method entails adding certain photosensitive proteins to neurons that act as light receptors and activate (or reactivate) neural activity.
The team at MIT – led by Professor Susumu Tonegawa, a Nobel Prize winning biologist specialising in learning and memory – implanted proteins into the brains of laboratory mice and stimulated them with optogenetics tools and monitored the neural activity with sensors.
They found that the process strengthened synaptic links between memory-related cells called engram cells and, thereby, memory.
However, the memory disappeared completely if scientists added another compound to the neuron, in this case one that blocks protein synthesis, immediately after the new memory was formed.
Professor Tonegawa says that when his team attempted to retrieve the memory a day later, there was no sign of it. “So even though the engram cells are there, without protein synthesis those cell synapses are not strengthened, and the memory is lost,” Tonegawa says.
Yet, astonishingly, when the researchers then reactivated the engram cells using optogenetics, the mice showed all the signs of recalling the memory in full.
“If you go directly to the engram-bearing cells and activate them with light, you can restore the memory,” says Professor Tonegawa.
And further work revealed that memories are not stored in the cells themselves but in the connective pathways between cells.
“We are proposing a new concept, in which there is an engram cell ensemble pathway, or circuit, for each memory,” he says. “This circuit encompasses multiple brain areas and the engram cell ensembles in these areas are connected specifically for a particular memory.”
What this all proves is that memory storage and memory retrieval are different processes, which opens up the idea that amnesia is a recovery problem, not a storage problem.
Neuroscience researchers have for many years debated whether retrograde amnesia — which follows traumatic injury, stress, or diseases such as Alzheimer’s — is caused by damage to specific brain cells, meaning a memory cannot be stored, or if access to that memory is somehow blocked, preventing its recall.
“The majority of researchers have favoured the storage theory, but we have shown that this majority theory is probably wrong,” Tonegawa says. “Amnesia is a problem of retrieval impairment.”
It may be many years before these findings are applied to human therapies, but they are considered a ground-breaking step forward in the future treatment of acquired brain injury and degenerative brain disease.
