Writing in the June 15 advance online section of Nature, the team reports that rods, cones and special retinal cells that make a protein called melanopsin together account for the entirety of a mouse's reaction to light levels. Others have proposed a role for cells that make proteins called cryptochromes, but that doesn't seem to be the case for mice -- and probably not for man -- say the researchers.
"We're fairly confident the rod/cone system and the melanopsin system are the mammalian eye's only two systems for detecting light levels," says King-Wai Yau, Ph.D., professor of neuroscience in the Johns Hopkins School of Medicine's Institute for Basic Biomedical Science and a Howard Hughes Medical Institute investigator. "Never say never, but there's no evidence for a third system right now."
Producing visual images is the eye's most well-recognized job, but detecting and reacting to light levels is critical to well-being. With this accessory ability, the eye maintains the body's internal clock, the pupil's ability to react to light, and the motivation to be active during the appropriate part of the day. Without signals from the three cell types, mice lost these normal abilities, the scientists report.
For their experiments, Yau and postdoctoral fellow Samer Hattar, Ph.D., removed the genes for three key proteins, each of which helps pass along light information from rods, cones or melanopsin-producing cells. Previously, the traditional way to prevent information from rods and cones -- also the eye's vision-producing cells -- was by using a mouse model in which rods and cones degenerate.
"We wanted to avoid the uncertainty of the traditional 'rod-less, cone-less' mouse," says Hattar. "With that model, you can't be sure
Contact: Joanna Downer
Johns Hopkins Medical Institutions