There is good news from the Massachusetts Institute of Technology (MIT). Memory loss is not necessarily inevitable with age, results from a recent mouse study suggest.
It was serendipity at work when scientists discovered that the adult brain is packed with millions of immature connections between neurons, called silent synapses. They are ready and waiting to form and store your new memories.
It was thought that silent synapses were active only in babies and very young children, soaking up new information. The new MIT study showed, however, that about 30 percent of the synapses in the cerebral cortex of adult mice were silent.
“These silent synapses are looking for new connections, and when important new information is presented, connections between the relevant neurons are strengthened. This lets the brain create new memories without overwriting the important memories stored in mature synapses, which are harder to change,” says Dimitra Vardalaki, a graduate student at MIT, in a statement.
In this new study, the MIT team wasn’t looking for silent synapses. They were studying the dendrites (extensions protruding from the “body” of neurons) of singular neurons and the way they responded to new information. They tried to measure neurotransmitter receptors (these bind to chemical messengers in different dendritic branches.)
While they were studying the dendrites, they made a surprising discovery. “The first thing we saw, which was super bizarre and we didn’t expect, was that there were filopodia everywhere,” says Mark Harnett, associate professor of brain and cognitive sciences. Filopodia are thin membrane protrusions that extend from dendrites. They have been seen before, but their functions were unknown.
The researchers searched for filopodia in other parts of the adult mice brain. They found them in the visual cortex and other parts of the brain, in much greater numbers than had been seen previously. They had neurotransmitter receptors called NMDA receptors, but no AMPA receptors.
Synapses that have only NMDA receptors cannot pass along an electric current and are referred to as “silent.” The researchers found a method to “unsilence” the synapses, converting them to active synapses that can form new memories. It was much more efficient than altering mature synapses.
“If you start with an already functional synapse, that plasticity protocol doesn’t work,” Harnett says. “The synapses in the adult brain have a much higher threshold, presumably because you want those memories to be pretty resilient. You don’t want them constantly being overwritten. Filopodia, on the other hand, can be captured to form new memories.”
“It’s entirely possible that by changing the amount of flexibility you’ve got in a memory system, it could become much harder to change your behaviors and habits or incorporate new information,” Harnett explains. “You could also imagine finding some of the molecular players that are involved in filopodia and trying to manipulate some of those things to try to restore flexible memory as we age.”
This research is published in Nature.
