Eyes Aren't Only for Seeing,
Two New Studies Show
Excerpt By, Merritt McKinney, Reuters Health

NEW YORK (Reuters Health) - New research adds to increasing evidence that our eyes are good for more than just seeing.

Independent research teams have shown that proteins found in the retina help the pupils adjust to light. These proteins may be important for setting the body's internal clock, according to the head of one of the teams, Dr. Russell N. Van Gelder.

"Our model of what the eye does is evolving," Van Gelder, who is an ophthalmologist at Washington University in St. Louis, Missouri, told Reuters Health.

It is clear, Van Gelder said, that the notion that the only function of the eye is for vision "needs to be revised."

It has long been known that receptors called rods and cones play a crucial role in vision. These light-sensitive receptors also are involved in regulating the size of the pupil, which opens wider in dim light and gets smaller as light gets brighter. For many years, rods and cones were thought to be the only light-sensitive cells in the retina.

According to Van Gelder, however, scientists have suspected that something else is involved in regulating the pupil, since it responds to changes in light even in blind mice. In a study published last year, researchers found that proteins in a part of the retina called the ganglion are also responsive to light, or photosensitive.

Now, scientists have identified two different types of proteins that seem to be involved in detecting light in the retina.

One team, led by Van Gelder, focused on proteins called cryptochromes. Another group, led by Dr. Robert J. Lucas of Imperial College London in the UK and Dr. King-Wai Yau at Johns Hopkins University in Baltimore, Maryland, have uncovered signs that a protein called melanopsin helps control the pupil's response to light.

The research is reported in separate articles in the January 10th issue of the journal Science.

Van Gelder's team found that in mice that lacked cryptochromes, the pupil's response to light was much lower than normal.

In the research involving melanopsin, the pupil's ability to respond to light was also diminished, but only when the mice were exposed to bright light. This suggests, according to the researchers, that melanopsin works in tandem with rods and cones to control the pupil's response to light.

The study "provides ultimate proof that there is indeed a distinct, functioning light-detection pathway in the eye originating from photoreceptors other than rods and cones," Yau told Reuters Health.

In the interview, Van Gelder said that just a few years ago, no one had any idea that there was a second system of photoreceptors in the eye.

"No one had a clue that there was an equivalent of a light meter in the eye," he said.

According to Van Gelder, both melanopsin and cryptochromes seem to be involved in sensing light, but the differences in how they work still needs to be figured out. The two "don't completely overlap," he said.

He noted that cryptochrome genes regulate the internal clock, or circadian rhythm, of fruit flies, so studying the cryptochromes may lead to ways to help people whose body clocks are out of kilter, including people who are jet lagged, who work odd shifts or who experience seasonal affective disorder.

If cryptochromes are involved in circadian rhythms in people, it is possible that people with certain kinds of eye disease are more or less likely to have problems with their internal clocks, according to Van Gelder. For instance, he said that people who have glaucoma or optic nerve disease may lose this pathway, while it may be preserved in people with other eye diseases, including retinitis pigmentosa or macular degeneration.

The research shows that "eye disease has ramifications beyond vision," he said.

SOURCE: Science 2003;299:222,245-247.

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