A new microscope technology that allows real-time imaging of brain cell activity could have important applications in understanding the biology of the brain, and of brain diseases such as Parkinson’s.
The technology was described in Nature Methods, in a study titled “Kilohertz two-photon fluorescence microscopy imaging of neural activity in vivo.”
Within the brain, nerve cells called neurons communicate with each other through electrical and chemical signals. This signaling forms the basis for thoughts, emotions, and actions, both subconscious and conscious. As such, understanding how these signals work is a broad goal of neurological research, as it could lead to better understanding of how the brain works — both normally and in disease states.
However, measuring the activity of brain cells in living organisms poses many technological challenges.
A previously-developed technology, called two-photon fluorescence microscopy (2PFM), addressed some of these challenges: using a combination of special lasers and fluorescent markers in the brain, the technology was able to detect some of the chemical signals that pass between neurons.
However, conventional 2PFM can’t detect the electrical firing of neurons, which happens much more quickly than chemical changes — on the level of milliseconds (one-thousandth of a second). Simply, the frame rate at which 2PFM detects images is too slow to detect these rapid changes.
The new technology solves this problem by implementing a technique called FACED (free-space angular-chirp-enhanced delay). FACED uses parallel mirrors to create a series of super-fast laser pulses that act over 1,000 times faster than previous methods.
The new technology, in which FACED was applied to 2PFM, was dubbed FACED-2PFM. Like conventional 2PFM, this technology detects brain activity using specially engineered proteins in the brain.
“These engineered proteins will light up (or fluoresce) whenever there is a voltage signal pass[ing] through the neurons,” Kevin Tsia, PhD, a professor at the University of Hong Kong who helped develop FACED-2PFM, said in a press release. “The emitted light is then detected by the microscope and formed into a 2D image that visualizes the locations of these voltage changes.”
“This is really an exciting result as we now can peek into the neuronal activities, that were once obscured and could provide the fundamental clues to understanding brain functions and more importantly brain diseases,” Tsia added.
FACED-2PFM has other notable advantages, the scientists said. For instance, it was minimally invasive to the mice whose brains were imaged. Other methods rely on the insertion of a probe directly into the brain, but this can cause damage to the brain and obscure findings. Because the lasers used are able to penetrate living tissue up to about a millimeter deep, such probes aren’t required for FACED-2PFM.
Furthermore, FACED-2PFM was not only able to detect neurons firing: it could also detect so-called sub-threshold signals, where a neuron has some activity, but doesn’t generate a complete electrical impulse (called an action potential). These weak signals are thought to play important roles in the brain and in neurological diseases, but this has been hard to test, mostly because measuring these signals hasn’t been feasible.
Therefore, this new technology may hold potential for understanding the brain in greater detail than was possible before.
“Existing, standard … 2PFM can readily be transformed into kilohertz imaging systems by adding a FACED module,” the researchers wrote. As such, this technology might be easily adapted by other researchers who are already using 2PFM technolgies.
“This is so far a one-of-its-kind technology that could detect millisecond-changing activities of individual neurons in the living brain,” Tsia said. “So, this is, I would say, the cornerstone of neuroscience research to more accurately ‘decoding’ brain signals.”
“We are working to further combine other advanced microscopy techniques to achieve imaging at higher resolution, wider view and deeper into the brain in the neocortex, which is about 1 millimeter,” he added. “This will allow us to probe deeper into the brain for a better and more comprehensive understanding of the functions of the brain.”
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