Research from MIT and Boston University paves the way for researchers to monitor the activity of many neurons at once.
The team developed a new fluorescent probe that lights up as neurons send and receive electrical signals. Used in conjunction with a simple optical microscope, the probe enables neuroscientists to visualize the activity of circuits within the brains of mice and link them to specific behaviors, says Edward Boyden, the Y. Eva Tan Professor in Neurotechnology and a professor of biological engineering and of brain and cognitive sciences at MIT.
A bright idea
"If you want to study a behavior, or a disease, you need to image the activity of populations of neurons because they work together in a network," says Boyden, the paper's senior co-author.
The new approach has been shown to record electrical activity in many more neurons than any other similar method available today.
Neurons generate rapid electrical impulses to communicate with one another. These impulses (which are generated chemically in neurons) basically underpin the functionality of our brains -- and, as such, they create our thoughts, behaviors, and our perception of the world.
The traditional way of recording neural activity involves sticking an electrode into the brain to pick up on these signals. However, it's a very laborious process, and only yields limited data (from one neuron at a time). Multielectrode arrays can be used to look at several neurons at a time, but they still can't get all neurons in an area or volume of the brain. If researchers want to get activity data from a volume of the brain, they can use calcium imaging -- but this is an indirect and often slow way of measuring electrical activity in neurons.
Boyden's team previously developed a method of monitoring electrical activity in the brain by labeling neurons with a fluorescent probe. They also engineered a molecule (called Archon1) that can be genetically inserted into neurons and embeds itself in their membranes. When electrical activity in the cell increases, this molecule fluoresces bright enough to be seen with a standard light microscope.
Previously, the team showed that they can use their method to see electrical activity in the brains of transparent worms, zebrafish embryos, and in mouse brain slices. In the present study, they applied it to the brains of living, awake mice as the animals engaged in a specific behavior.
Clearing the noise
In order to achieve this, they modified their probe so that it would stay in a specific subregion of neurons' membranes. When the molecule stretches throughout the entire membrane, it makes the recordings blurry (the axons and dendrites of individual neurons extend far from its body). So the team attached a peptide to their probe that guides it specifically to the bodies of neurons (the 'soma'). They call their modified protein SomArchon.
"With SomArchon, you can see each cell as a distinct sphere," Boyden says. "Rather than having one cell's light blurring all its neighbors, each cell can speak by itself loudly and clearly, uncontaminated by its neighbors."
The team used this probe to investigate the striatum of mice, an area of the brain involved in planning movement, as the animals ran on a ball. They recorded the activity in several neurons at a time and correlated the readings with the movements of the animals. Some neurons' activity went up when the mice were running, some went down, and others showed no significant change.
The fluorescent probe obtained similar measurements to those recorded by an electrical probe, which the team says is an indication that it can pick up on quick changes in electrical activity levels -- making the fluorescent probe more informative than calcium imaging.
"Over the years, my lab has tried many different versions of voltage sensors, and none of them have worked in living mammalian brains until this one," says Xue Han, an associate professor of biomedical engineering at Boston University, and senior co-author of the study.
"We want to record electrical activity on a millisecond timescale. The timescale and activity patterns that we get from calcium imaging are very different. We really don't know exactly how these calcium changes are related to electrical dynamics."
The probe can pick up on very small fluctuations in activity that occur even when a neuron is not firing. This could allow neuroscientists to study how they impact a neuron’s overall behavior, which has previously been very difficult in living brains.
In the future, the team plans to work on combining this probe with expansion microscopy -- a technique that Boyden’s lab developed to expand brain tissue before imaging it, making it easier to see the anatomical connections between neurons in high resolution.
"One of my dream experiments is to image all the activity in a brain, and then use expansion microscopy to find the wiring between those neurons," Boyden says. "Then can we predict how neural computations emerge from the wiring."
The paper "Population imaging of neural activity in awake behaving mice" has been published in the journal Science.