Millions of ‘silent synapses’ could be the key to lifelong learning: ScienceAlert

Newborns need to store large amounts of new information quickly as they learn to navigate the world. Silent synapses, the immature connections between neurons that do not yet have neurotransmitter activity, are thought to be the hardware that allows this rapid storage of information to occur early in life.

First discovered decades ago in newborn mice, these possible neurological intersections were thought to disappear as the animals aged. A recent study by researchers at MIT in the US has found that this disappearing act might not be as extreme as first thought.

The team had not set out to look specifically at these potential connections. Rather, they were continuing earlier work on the location of nerve cell extensions called dendrites.

They got a little more than they bargained for. Not only did they capture images of the dendrites, but also countless tiny thread-like protrusions emerging from them called filopodia.

“The first thing we saw, which was super weird, and we didn’t expect, was that there were filopodia all over the place.” he says MIT neuroscientist Mark Harnett, lead author of the paper.

Usually hidden from the fluorescence light used to illuminate the cell for imaging, the researchers used a special imaging technique developed just last year called extended epitope-preserving proteome (eMAP) analysis.

This new imaging process uses a gel to help lock the delicate cellular structures and proteins in place, allowing researchers to better study them as the tissues are manipulated.

virus that expressed a green fluorescent protein were inserted into two male and two female adult mice, to help illuminate tissues relevant to imaging. Their primary visual cortex was later dissected and sliced ​​into one-millimeter slices before being incubated in the eMAP hydrogel monomer solution and mounted between glass slides.

This gives the eMAP solution time to cement the cell structure in place, which allowed the researchers to take super-high-resolution images of the fluorescent dendrites.

Armed with the magnified images of 2,234 dendritic spines, the researchers were able to see, for the first time, that the brains of adult mice had concentrations of filopedia never before seen in adult mice.

In addition, many of the structures had only one of the two neurotransmitter receptors expected from a mature, functional synapse. Without the second, they were effectively “silent” junctions between neurons.

The researchers then asked whether the silent synapses of adults could be activated.

They showed that this was possible by releasing the neurotransmitter glutamate at the tips of filopodia threads and producing a small electrical current ten milliseconds later.

This procedure “unsilenced” the synapses within minutes, stimulating the accumulation of the missing receptors and allowing the filopodia to form a connection with neighboring nerve fibers.

These receptors are usually blocked by magnesium ions, but the current releases them, allowing the filopodia to receive a message from another neuron.

The team found that it was much easier to activate silent synapses than to change the activity of dendritic spines in a mature neuron.

Researchers are now investigating whether silent synapses exist in adult human brain tissue.

“This paper is, as far as I know, the first real evidence that this is how it actually works in a mammalian brain,” Harnett. he says.

“Filopodia allow a memory system to be flexible and robust. You need flexibility to acquire new information, but you also need stability to retain important information.”

This article was published in Nature.