NEW YORK (Reuters Health) - New research in mice confirms that a light-sensitive protein in the retina called melanopsin is essential in setting the "body clock" in mammals. Scientists say the protein now stands as a potential target of therapy to normalize disturbed day/night schedules.

Previous research had suggested that melanopsin, found in a special network of cells in the retina, might be the main transmitter of light-and-dark messages to the central body clock in the brain. Melanopsin-containing cells appear to be separate from rods and cones, the retinal cells that allow people to see; instead, they provide a less-specialized perception of changes in light.

It's these daily shifts between day and night, perceived by the retina, that help set the body clock, or circadian rhythm. Besides governing the sleep-wake cycle, circadian rhythm plays an integral role in a range of body processes such as hormone production, blood pressure and body temperature.

Scientists have long sought to understand how the body "resets" this clock when the rhythm gets thrown off--by, for example, modern-day situations such as shift-work and travel across time zones.

Now two new studies, reported in the December 13th issue of Science, confirm that melanopsin plays a vital role in synchronizing circadian rhythm with the outside world--although, researchers add, other light-sensitive molecules must also be involved.

These findings are the first to "nail down a relationship between melanopsin and circadian rhythms," the lead author of one of the studies, Dr. Norman F. Ruby of Stanford University in California, told Reuters Health.

To do this, Ruby and his colleagues studied "knockout" mice that had been engineered to lack the gene for melanopsin, allowing the researchers to gauge the importance of the protein in adjusting to changes in the light/dark cycle. They found that, compared with normal mice, the melanopsin-deficient animals showed less of a response to changes in light--their body clocks did not "reset" to the same magnitude that those of normal mice did.

"The clock is not getting all of the light information because the retina lacks melanopsin, and the circadian clock 'sees' less light," Ruby explained.

Similarly, in the second study, an international team of researchers found diminished clock resetting in melanopsin-deficient mice. The study authors, led by Satchidananda Panda of Scripps Research Institute in San Diego, California, conclude that melanopsin is needed for normal body-clock function, but that "other mechanisms for light input to the clock also play a role."

Some scientists hope to use this growing understanding of how the body sets its clock to develop ways to restore these natural rhythms when they are disrupted by things like shift-work or insomnia. A growing body of research suggests that chronic body-clock disturbances can have health consequences, from stomach upset and ulcers to heart disease.

Ruby noted that for many blind people, circadian rhythm "cannot synchronize to the outside world," throwing off the normal sleep/wake cycle.

"Now that we know that melanopsin conveys light information to the clock," he said, "it is a new 'target' for developing...methods to keep people on a normal day/night schedule."

Some co-authors on his study are with Deltagen, Inc., a Redwood City, California-based biopharmaceutical company.

SOURCE: Science 2002;298:2211-2213,2213-2216.

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