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In a new study in mice, a team of researchers at the University of Illinois Urbana-Champaign (Champaign, IL) has shown that specially tailored, ultrafast pulses of light can trigger neurons to fire, which is promising for helping patients with light-sensitive circadian or mood problems. 

Chemists have used such carefully crafted light beams, called coherent control, to regulate chemical reactions, but this study demonstrates using them to control function in a living cell. The study used optogenetic mouse neurons (cells that had a gene added to make them respond to light), but the researchers say the same technique could be used on cells that are naturally responsive to light, such as those in the retina.

Photoreceptors in our retinas connect to different parts in the brain that control mood, metabolic rhythms, and circadian rhythms, explains Stephen Boppart, who led the study. Boppart is an Illinois professor of electrical and computer engineering and of bioengineering, and also is a medical doctor.

The researchers used light to excite a light-sensitive channel in the membrane of neurons. When the channels were excited, they allowed ions through, which caused the neurons to fire. They used several <100 fs light pulses to deliver a lot of energy in a short period of time, exciting the molecules to different energy states. Along with controlling the length of the light pulses, the research team controls the order of wavelengths in each light pulse.

The researchers used ultrafast pulses of tailored light to make neurons fire in different patterns as an example of coherent control in a living cell. (Image courtesy of Stephen Boppart)

"When you have an ultrashort or ultrafast pulse of light, there's many colors in that pulse. We can control which colors come first and how bright each color will be," Boppart says. "For example, blue wavelengths are much higher energy than red wavelengths. If we choose which color comes first, we can control what energy the molecule sees at what time, to drive the excitement higher or back down to the base line. If we create a pulse where the red comes before the blue, it’s very different than if the blue comes before the red."

The researchers demonstrated using patterns of tailored light pulses to make the neurons fire in different patterns.

Boppart says coherent control could give optogenetics studies more flexibility, since changing properties of the light used can give researchers more avenues than having to engineer mice with new genes every time they want a different neuron behavior.

The group used advanced imaging techniques to monitor neurons and their activity: (L-R) Professor Stephen Boppart, professor Parijat Sengupta, graduate student Eugene Ark, and research scientist Kush Paul. (Photo by L. Brian Stauffer)

Outside of optogenetics, the researchers are working to test their coherent control technique with naturally light-responsive cells and processes—retinal cells and photosynthesis, for example. Using light and coherent control to regulate biological function, Boppart explains, could be a gene-free, drug-free way of regulating cell and tissue function.