For the first time, two-photon imaging was used to demonstrate the generation and elimination of synapses between neurons in the brain of living mice

A researcher at the University of Minnesota-Twin Cities is part of an international team that has used imaging techniques to demonstrate for the first time the production and elimination
of synapses between neurons in the brain of living mice.

This research provides insight into the process of memory generation and forgetting, helping scientists better understand and treat conditions
such as post-traumatic stress disorder (PTSD).

The study was published in Current Biology
.

Hye Yoon Park, co-first author of the study and associate professor in the Department of Electrical and Computer Engineering at the University of Minnesota, said: "Researchers have always wondered what happens
to synapses that form after we experience fear.
Previously, researchers were only able to detect these synapses after sacrificing mice, making it difficult to track them down over time
.
But now, we've been able to image synapses in the brains of living mice for several days, so we can better understand what happens
to them over the long term.
This is the first time this has been done in a living mouse brain, so this is very exciting news
in this area.
”

The study builds on Park's previous research, which used her lab's expertise in imaging to visualize memory-related nerve cells, or neurons and mRNA molecules, in the brains
of living mice.

Now, the researchers have added more detail
by imaging synapses, or connections, between neurons.
On average, there are about 7,000 synaptic connections between each neuron in the brain and other neurons that allow cells to transmit signals to each other and drive cognitive functions
such as learning and memory.

The University of Minnesota team, in collaboration with researchers at Seoul National University in South Korea, developed a technique called eGRASP to detect synapses
in the brain.
Combined with Park's imaging technology, the researchers were able to see the dynamics
of synapses in the brain of a living mouse as it remembered scary experiences and experienced "memory loss" or fear memory suppression.

"There are two different hypotheses in the field of neuroscience," Park explains
.
"When memories disappear, some believe that synapses formed during fearful conditioning may disappear, also known as 'forgetting'
of pre-acquired memories.
Others think they're still there, but another set of synapses may have formed, suggesting that mice now know the environment is safe, which is known as 'new learning'
of contingency.
" ”

The researchers' data support the forgetting hypothesis
.
They found that some of the new synapses formed during the fear experience were eliminated
during memory loss.
The findings could help scientists better understand brain activity
in patients with conditions such as post-traumatic stress disorder (PTSD).

Hippocampal engram networks for fear memory recruit new synapses and modify pre-existing synapses in vivo