Eyes provide glimpse of circadian rhythms

PROVIDENCE, R.I., Feb. 6 (UPI) -- It is said the eyes are the "windows to the soul" and new research out of Brown University suggests they also provide a glimpse of circadian rhythms.

Circadian rhythms also are referred to as sleep-wake cycles, the 24-hour cycles that govern physiological and mental function. People notice them most as jet lag.

David Berson, associate professor of Neuroscience at Brown University in Providence, has discovered a cell in the retina of rats that may have direct control over circadian rhythms.

The cells of the visual system are among the best-characterized physiological features of the mammalian nervous system. Rods and cones -- specialized cells found in the retina -- have long been described as the only photoreceptors found in the eye. Photoreceptors are cells that translate energy from light into electrical energy. Visual cues are transmitted to the brain through the translation process.

The new cell found by the Brown University research team overturns almost 150 years of knowledge about the retina. The researchers describe it as "an intrinsically photosensitive ganglion cell."

Like rods and cones, the new cell also turns light energy directly into brain signals. However, the electrical signals from the new cell has a very different pattern from that seen with rods and cones.

The main difference, the researchers said, is "the delay between the onset of stimulus and the beginning of the response is extraordinarily long compared with conventional photoreceptors."

They believe that difference in the response is due to the fact the new cell does not have to translate information about visual cues, and thus speed is not an important aspect of the signaling properties.

"There is a strong likelihood that there are identical cells in humans," Berson said. "This could explain why certain people who are functionally blind due to retinal degeneration continue to set their biological clock according to the day-night cycle."

It was, in fact, the question of why people who are functionally blind are able to set their biological clocks that led Berson and colleagues to their discovery.

To further investigate the connection between the visual system and circadian rhythms, the researchers inserted a dye into the part of the brain that controls sleep-wake cycles. The dye allowed them to follow the connections between this part of the brain and the visual system. It was by following the trail of connections that they found the new type of photoreceptor.

Mary Carskadon, a professor of psychiatry at Brown University, made the statement that the finding is "important in providing another link in understanding this vital system."

(Written by Gargi Talukder in Seattle)