This is Your Brain…on Sunlight

Light-sensing cells deep inside brain tissue play a larger role in human growth, development and health than even ardent fans of the outdoors would expect

Wednesday, September 02, 2020

So consider this: sunlight doesn’t need to use your eyes to get inside your head.

While many people understand that invisible X-rays can pierce all the way through our bodies, it often surprises people to realize that photons from visible light also can zip right through our skulls to reach deep inside our brains.

Now, a multi-institutional team of scientists led by Richard Lang, PhD, from Cincinnati Children’s reports in the journal Nature that certain neurons inside our brains express a protein called Opsin 5 that can detect a specific wavelength of light from the sun. Those neurons, in turn, send signals that influence a number of our body functions—including our metabolism.

This discovery adds to our understanding of how sunlight plays a fundamental role in human health. Combined with lack of physical activity and poor food choices this may explain why so many more people in the modern world are becoming obese and battling metabolic disorders, the co-authors say.

“In this study, we provide evidence that the visual violet light found in normal sunlight, but not found in most indoor lighting, plays a direct role in regulating the fat-burning process in mice,” says Lang, Director of Visual Systems Group at Cincinnati Children’s.

In essence, too many of us are literally spending too much time inside, under the wrong kind of light.

“We also discovered where these light-sensing cells live—in the preoptic area of the hypothalamus,” says Kevin Zhang, the study’s lead author. “And most importantly, we know from other studies that the same type of light-sensing cells that affects metabolism in mice are also present in human brains.”

Zhang worked with two collaborators from Yale University—Amanda Holt, PhD and Alison Sweeney, PhD—to reveal that violet photons could penetrate into the mouse brain at levels sufficient to activate Opsin 5.

The team’s discoveries have wide ranging implications that could affect how “light therapy” might be applied to metabolic disorders, how women live during pregnancy, how preterm infants are cared for in hospitals, and someday may have everyone switching to different light bulbs in their homes and offices.

“It may sound corny, but this could be a true ‘Eureka’ moment,” Lang says. “Scientists at Cincinnati Children’s, and likely many other institutions, will be studying ways to translate these findings into real-world applications for years to come.”

Light Detection Outside the Eye

Today’s finding, Lang says, is one of three related discoveries published in the past year. Each revealed a piece of a large puzzle indicating that the body has ways of sensing light that have nothing to do with our eyes.

In concept, the idea isn’t new. Many scientists have documented how plants, insects, fish and other animals use sunlight in myriad ways, from drawing energy to survive, to setting their internal body clocks and timing their breeding seasons. But only in more recent years have scientists shown that mammals—including humans--also use “extraocular light detection.”

In October 2019, Lang and colleagues Ethan Buhr, PhD, and Russell Van Gelder, MD, PhD, from the University of Washington, made their first major contribution by reporting in Current Biology that Opsin 5 is expressed in the skin of mice. This opsin synchronizes the skin’s circadian clock to the light-dark cycle, independent of light input to the eyes or brain.

Then in January, the team, including Randy Seely, PhD, from the University of Michigan, and Cincinnati Children's researchers Juan Sanchez-Gurmaches, PhD, and Takahisa Nakamura, PhD, demonstrated in Cell Reports that Opsin 3, a blue-sensitive opsin, is expressed in white fat cells and plays a vital role in balancing how fast the body burns its layers of white fat to maintain body temperature.

Now, the new study in Nature shows that Opsin 5 neurons in the brain play a master regulatory role in these metabolic processes.

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