Published Online September 15, 2015
Proceedings of the National Academy of Sciences

The retina, the light-sensitive inner layer of the eye, always knows what time is it. The enduring question has been: How?

That question led to a light-bulb moment for researchers, who determined that the retina’s own biological clock functions independently from the one in the brain.

They also found that the retinal clock uses daylight as a time-setting signal, a process called photoentrainment.

“In other words,” says Richard Lang, PhD, “as long as the retina continues to receive daily light stimulation, it will maintain time.”

In the multi-institutional study, published in Proceedings of the National Academy of Sciences, researchers made a second significant finding. They revealed that this light-dependent time-setting mechanism uses a new opsin molecule called neuropsin as a light detector.

Lang directs the Visual Systems Group and the Center for Chronobiology at Cincinnati Children’s. Other investigators included Shruti Vemaraju, PhD, and Minh-Thanh Nguyen, PhD.

In mammals, behavioral circadian rhythms are synchronized to light and dark cycles through rods, cones, and photosensitive cells in the retina.

These molecular circadian rhythms in the retina are themselves synchronized to light and dark signals, but the study was the first to show how this photoentrainment, in an ex vivo setting, requires neuropsin.

“Remarkably,” researchers wrote, “the circadian clocks in the cornea are also photoentrained ex vivo in an OPN5-dependent manner.”

Many tissues in the body have their own biological clocks. But this study suggests that these tissues may function independently of the brain’s biological clock, located in the suprachiasmatic nucleus.

“We are investigating this possibility,” Lang says.