Scientists have long since thought that memories lie in synapses. A synapse is the junction where one nerve cell connects to another; it has a gap where substances known as neurotransmitters are released for information to travel from neurone to neurone. According to their theories, memories are stored there, such that if a synapse is destroyed, the memory disappears as well. A new study has however yielded results that contest this long-held ‘fact’.
The authors of the new scientific paper worked on marine snails called the Aplysia. The latter memory was tested relative to its synapses. Aplysia are endowed with fragile gills that are made to retreat when a potential hazard is spotted. The researchers subjected the animals to small electric shocks so that their reflex action of shielding the gills from damage is strengthened. This led to the formation of short-term memories with respect to the shocks. As such, the neurotransmitter serotonin was released. Serotonin and other secreted proteins thereafter formed synaptic connections that built long-term memory.
This is a process that can be obstructed, either by injuries or other factors including artificial methods. The researchers made use of the latter to interrupt the process of long-term memory formation. The results proved to be extremely surprising.
How memory works?
The lead author, David Glanzman, said: “If you train an animal on a task, inhibit its ability to produce proteins immediately after training, and then test it 24 hours later, the animal doesn’t remember the training. However, if you train an animal, wait 24 hours, and then inject a protein synthesis inhibitor in its brain, the animal shows perfectly good memory 24 hours later. In other words, once memories are formed, if you temporarily disrupt protein synthesis, it doesn’t affect long-term memory. That’s true in the Aplysia and in human’s brains.”
The snails’ sensory and motor neurones linked to long-term memories were later removed. They were then placed onto a Petri dish, and added serotonin and other proteins to trigger the neurones to make new synaptic connections. As a matter of fact, the snails did produce the connections, and in the very same patterns as before. This suggests that although the connections are linked with this type of memory, they are not their storage compartment.
Where are memories stored then? The scientists are as yet unsure.
“Long-term memory is not stored at the synapse. That’s a radical idea, but that’s where the evidence leads. The nervous system appears to be able to regenerate lost synaptic connections. If you can restore the synaptic connections, the memory will come back. It won’t be easy, but I believe it’s possible,” said the lead author.
This study is specially pertinent for Alzheimer’s disease patients. It might pave the way for ways to restore the memory of the affected people. If synaptic connections regrow in the same positions as before when the long-term memories were made, perhaps, the memories themselves can be brought back.