Millions of ‘Silent Synapses’ Could Be The Key to Lifelong Learning : ScienceAlert


Newborns must quickly store large amounts of new information as they learn to navigate the world. Silent synapses — the immature connections between neurons that don’t yet have neurotransmitter activity — are believed to be the hardware that enables this rapid information storage at a young age.

First discovered decades ago in newborn mice, it was thought that these potential neurological junctions would disappear as the animals aged. A recent study by researchers at MIT in the US has shown that this vanishing act may not be as extreme as initially believed.

The team had no intention of looking specifically at these possible connections. Rather, they continued previous work on the locations of nerve cell extensions called dendrites.

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

“The first thing we saw, which was super bizarre and we didn’t expect, was that there was filopodia everywhere,” said MIT neuroscientist Mark Harnett, the paper’s senior author.

Usually hidden in the glare of fluorescence used to illuminate the cell for imaging, the researchers used a special imaging technique developed only last year called epitope-preserving magnified analysis of the proteome (eMAP).

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

Viruses expressing a green fluorescent protein were introduced into two male and two female adult mice to illuminate the relevant tissues for imaging. Their primary visual cortex was later dissected and sectioned 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 cellular structure in place, allowing the researchers to take super-high-resolution images of the fluorescent dendrites.

Armed with the magnified images of 2,234 dendritic processes, the researchers were able to see – for the first time – that adult mouse brains contained concentrations of filopia never before seen in adult mice.

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

Next, the researchers asked whether mature silent synapses could be activated.

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

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

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

It was much easier to activate silent synapses than to change the activity of the dendritic spines on a mature neuron, the team found.

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

“This paper is, to my knowledge, the first real evidence that this is how it actually works in the mammalian brain,” says Harnett.

“Filopodia makes a memory system both flexible and robust. You need flexibility to acquire new information, but you also need stability to hold the important information.”

This article was published in Nature.

The Valley Voice
The Valley Voice
Christopher Brito is a social media producer and trending writer for The Valley Voice, with a focus on sports and stories related to race and culture.


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