For many years it was believed that the human brain did not produce new neurons after birth, a process called neurogenesis. However, in the past few decades, researchers have discovered that distinct areas of the adult brain do produce new cells.
Studies have shown that it is these new cells forming the necessary links, or synapses, that allow the older cells to send signals to the new ones.
What remained unclear, though, was whether the new cells could not only receive signals, but also send signals to particular cells that exist within a circuit.
To explore this, researchers in the lab of Shaoyu Ge of Stony Brook University’s Department of Neurobiology and Behavior used a combination of techniques to track the formation of new cells and measure their activity.
Newborn neurons form from cells called progenitors. Progenitor cells are intermediate cells, in that they are only partially formed and different environments and chemicals can cause them to become one of many cell types. Think of them as a basic batter. The same batter can become a pancake, a waffle or a crepe depending on what is done to it.
Dr. Ge’s lab targeted these progenitor cells for a study published in Nature Neuroscience. They were infected with a virus which allowed for two things to happen. The first was that the virus caused the progenitors and the cells those progenitors became to glow green when observed under a microscope, which allowed the Ge lab to locate the cells. The second was that it gave the researchers in the Ge lab a way to control the cell through a technique called optogenetics.
In the eye, cells contain receptors that detect light and cause those cells to send a signal, which is the first step in the process of vision. Optogenetics allows this same light-detecting receptor to be placed in neurons, where it does not normally exist.
This allows researchers to tell the cell to send a signal by shining light on it, giving researchers control over the cell.
Once the progenitors infected with the virus matured into young neurons, the Ge lab shined light on the new cells and caused them to send a signal. If the new cells were successfully communicating with older ones, then this signal would be detected in the older cells.
The Ge lab found that when light was shined onto the new cells it caused them to send a signal to the older cells. The reception of this signal was able to be detected by electrodes placed in the old cells, meaning that the new cells were capable of not only receiving signals but sending them to a target as well.
Dr. Ge and his colleagues then wanted to know how important the new cells were to the circuit they had made connections to.
The area of the brain where these particular new cells are forming—an area called the hippocampus—is important for memory. So the Ge lab tested mouse memory while controlling the new cells.
The mice were injected with a virus, but this time the virus allowed the researchers to silence the cells rather than tell them to send a signal. The mice were then placed in a maze that required them to learn where a specific goal was.
While the mice were learning, half of them had their new cells silenced and the other half did not. Both groups of mice learned how to solve the maze equally well suggesting that the new cells were not needed to form new memories.
However, when mice learned how to solve the maze and were then retested some time later, they had a hard time remembering how to complete the maze if their new cells were silenced during retesting. So, while the new cells were not found to be necessary for memory formation, they were very important for memory retrieval.
With this study, Dr. Ge and his researchers answered many of the outstanding questions surrounding adult-born neurons. Now they are working to more fully understand the adult-born neuron formation process.
A better understanding of how new neurons form could potentially allow scientists to utilize new cell formation to help treat brain development problems and brain injuries in the future.