The research represents an important step in understanding how light resets the internal clock, or circadian rhythm, and how the pupil opens and closes in response to light, the scientists say.

To learn how light reaches those brain centers, postdoctoral fellow Samer Hattar, Ph.D., created a mouse whose melanopsin protein was partially replaced with another, easy-to-detect protein. Melanopsin is suspected to be light-sensitive but not involved in forming visual images.

The scientists discovered that only a tiny fraction of nerve cells in the retina make melanopsin. These melanopsin-expressing nerve cells, which reach deep into the brain to areas that control the clock and the pupil, join image-producing rods and cones as the only retinal cells that can detect light, the researchers report.

The clock regulates the body's daily cycles, including sleep, hormone production, body temperature and blood pressure. While an individual's natural cycle may be more or less than 24 hours, the 24-hour cycle of day and night keeps the body's rhythm in tune with the environment. Light adjusts the cycle when it gets out of whack, as with jet-lag or workers switching to the late shift.

"The melanopsin-containing cells create a light-detecting network across the retina in the mice," says King-Wai Yau, Ph.D., a Howard Hughes Medical Institute investigator and professor of neuroscience and ophthalmology at Johns Hopkins. "The cells seem sensitive to how much light there is and how long it lasts, unlike the cells involved in vision, which detect borders between light and dark."

The findings support the idea that there are two primary groups of light-detecting cells in the eye: one respo
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Contact: Joanna Downer
jdowner1@jhmi.edu
410-955-1534
Johns Hopkins Medical Institutions
21-Feb-2002