As they learn to navigate the world, newborns must swiftly retain enormous amounts of fresh knowledge. Silent synapses, which are immature connections between neurons and do not yet produce neurotransmitters, are assumed to be the hardware that enables this early-life quick information storage.
These possible neurological intersections were first identified in newborn mice decades ago, and it was believed that as the animal’s age, they would fade. This vanishing act may not be as extreme as first thought, according to a recent study by scientists from MIT in the US.
The group had not intended to focus on these potential linkages in particular. Instead, they were carrying on earlier research on the sites of dendritic extensions of nerve cells.
They received a little bit more than they anticipated. They photographed the dendrites as well as many tiny, filopodia-like protrusions that emerged from them.
The principal author of the study and MIT neurologist Mark Harnett explains, “The first thing we saw, which was really weird and we didn’t expect, was that there were filopodia everywhere.”
The researchers employed a unique imaging method created only last year called epitope-preserving magnified analysis of the proteome to visualize structures that are typically obscured by fluorescence used to illuminate the cell for imaging (eMAP).
In order to better analyze fragile cellular structures and proteins when tissues are altered, this novel imaging technique uses a gel to help lock them into place.
In order to help illuminate the necessary tissues for imaging, viruses producing a green fluorescent protein were injected into two male and two female adult mice. After being removed, their main visual cortex was cut into one-millimeter slices, which were then incubated in the eMAP hydrogel monomer solution and put between glass plates.
This gives the eMAP solution time to solidify the cellular structure, enabling the scientists to capture photos with extremely high resolution of the fluorescing dendrites.
The researchers were able to see for the first time that adult mice’s brains contained concentrations of filopedia never before observed in mature mice thanks to the magnified images of 2,234 dendritic protrusions.
Furthermore, many of the structures lacked both neurotransmitter receptors that are present in fully developed, functional synapses. They served as silent connections between neurons without the second.
The scientists then questioned if adult quiet synapses might be stimulated.
By releasing the neurotransmitter glutamate at the tips of the filopodia threads and producing a modest electrical current ten milliseconds later, they demonstrated that this was feasible.
Within minutes, this approach “unsilenced” the synapses, encouraging the growth of the missing receptors and enabling the filopodia to interact with the nearby nerve fibers.
The current unblocks these receptors, which are typically inhibited by magnesium ions, and allows the filopodia to communicate with another neuron.
The scientists discovered that it was considerably simpler to activate quiet synapses than it was to alter the activity of dendritic spines on a mature neuron.
Currently, scientists are looking at whether adult human brain tissue has silent synapses.
According to Harnett, “this paper is the first concrete proof that this is how it actually works in a mammalian brain.”
“A memory system can be both adaptable and reliable thanks to filopodia. You need stability to retain the crucial information, but you also need the flexibility to pick up new information.”