Every second of every day, about 100 billion neurons in the brain are capable of firing off a burst of electricity called an 'action potential' up to 100 times per second. Researchers at Duke and Stanford Universities have devised a way to watch the details of neurons in action with a time resolution of about 0.2 milliseconds - a speed just fast enough to capture the action potentials in mammalian brains.
The paper appeared online in Science
‘Existing techniques for monitoring neurons are too slow or too tightly focused to generate a holistic view. Now, a new technique has made it possible to watch the details of neurons in real-time action.’
Yiyang Gong, assistant professor of biomedical engineering at Duke and first author on the paper, said, "We set out to combine a protein that can quickly sense neural voltage potentials with another protein that can amplify its signal output. The resulting increase in sensor speed matches what is needed to read out electrical spikes in the brains of live animals."
Gong did the work as a postdoctoral fellow in the laboratory of Mark Schnitzer, associate professor of biological sciences and applied physics at Stanford, and an investigator of the Howard Hughes Medical Institute. Gong and his colleagues sought out a voltage sensor fast enough to keep up with neurons. After several trials, the group landed on one found in algae, and engineered a version that is both sensitive to voltage activity and responds to the activity very quickly.
The amount of light it puts out, however, wasn't bright enough to be useful in experiments. It needed an amplifier.
To meet this engineering challenge, Gong fused the newly engineered voltage sensor to the brightest fluorescing protein available at the time. He linked the two close enough to interact optically without slowing the system down.
Gong explained, "When the voltage sensing component we engineered detects a voltage potential, it absorbs more light. And by absorbing more of the bright fluorescent protein's light, the overall fluorescence of the system dims in response to a neuron firing."
The new sensor was delivered to the brains of mice using a virus and incorporated into fruit flies through genetic modification. In both cases, the researchers were able to express the protein in selected neurons and observe voltage activity. They were also able to read voltage movements in different sub-compartments of individual neurons, which is very difficult to do with other techniques.
Gong said, "Being able to read voltage spikes directly from the brain and also see their specific timing is very helpful in determining how brain activity drives animal behavior. Our hope is that the community will explore those types of questions in more detail using this particular sensor. Already I've received multiple emails from groups interested in trying the technique in their own labs."