As they navigate the world, newborns must be able to quickly store large amounts of information. Silent synapses – the immature connections between neurons that have no neurotransmitter activity yet – are thought to be the hardware that allow this rapid information storage to occur early in life.
These potential neurological intersections were first discovered in newborn mice decades ago. They are thought to disappear with age. Recent research by MIT researchers in the US found that this disappearing act might not be quite as drastic as first thought.
These connections were not the goal of the team. Instead, they were continuing work on nerve-cell extensions known as dendrites.
They got more than they bargained. They not only captured images of dendrites but also numerous tiny thread-like protrusions from them, called filopodia.
“We saw filopodia everywhere, which was a super strange and surprising thing. SaysMark Harnett (MIT neuroscientist), is the senior author.
Researchers developed an imaging technique to capture the cells that were often hidden from the flash of fluorescence. Only last yearcalled epitope preservation magnified analysis of proteome (eMAP).
This new imaging method uses a gel to lock delicate cellular structures into place. Researchers can then study these tissues more effectively as they are being manipulated.
VirusesTo help with imaging, two adult male and two female mice were infected with a green fluorescent protein. After dissection of their primary visual cortex, the cells were cut into slices measuring one millimeter in length. Then, the cells were placed in eMAP Hydrogel Monomer Solution and mounted between glass slides.
This gives the eMAP solution time to cement the cellular structure into place, which allowed the researchers to take super-high-resolution images of the fluorescing dendrites.
Armed with the magnified images of 2,234 dendritic protrusions, the researchers could see – for the first time – that adult mice brains had concentrations of filopedia never seen before in adult mice.
Furthermore, most structures only had one out of two neurotransmitter receivers that would be expected from a mature functioning synapse. Without the second, they were effectively a’silent junction’ between neurons.
The researchers then asked whether adult silent synapses might be activated.
The neurotransmitter glutamate was released at the tips and produced a small electrical current 10 milliseconds later.
This procedure “unsilenced” the synapses in minutes. It stimulated the accumulation of missing receptors and allowed the filopodia form a connection to the nearby nerve fibers.
The magnesium ions block these receptors, but the current allows them to be freed, allowing filopodia to receive a message.
The team discovered it was much easier than changing the activity of dendritic spines in a mature neuron to activate silent ones.
Researchers are currently investigating whether there are silent synapses in adult brain tissue.
Harnett stated, “This paper is, to my knowledge, the first evidence that it is actually how it works in a mammalian mind.” Says.
“Filopodia are a way to make a memory system both flexible, and also robust.” Flexibility is necessary to obtain new information but stability is required to preserve the most important information.
This paper was published by Nature.
