For weary-eyed jet laggers and overtaxed students, fatigue might all just be a matter of light. As published in the July 3 issue of Nature, a U of T research team has helped demonstrate that a light-sensitive pigment in the eyes of mice, called melanopsin, controls a mouse’s day and night time patterns of activity. In other words, it is because of melanopsin that mice feel awake during the day and sleepy at night.
The main light receptors in the eye are rods and cones, special cells that line the back of the eye in a layer called the retina. Rods allow us to see black and white while cones pick up on colour. It has been known for roughly a decade that blind mice, which lack rods and cones, can still synchronize their activity with the light patterns they are exposed to, said Nicholas Mrosovsky, Professor Emeritus of Zoology. “One would think they wouldn’t be able to synchronize,” said Mrosovsky, “therefore there [we figured] must be another receptor in the eye.”
Mrosovsky, who has been studying biological rhythms for over twenty years, led a U of T group as part of an international effort to find that receptor. Melanopsin, a photosensitive pigment contained in cells in the inner retina of the eye, was a prime suspect.
The research team tested the behaviour of a breed of mice that lack rods, cones and melanopsin. It was found that such mice “could no longer synchronize [their] biological rhythms to light,” explained Mrosovsky.
The team also found that together, rods, cones and melanopsin-producing cells control all light reactions in mice, such as a pupil’s contraction, the suppression of sleep-inducing hormones during the day, and the release of sleep-inducing hormones at night. Humans also have melanopsin-producing cells in the eye, but “whether this would apply to human beings has not yet been ascertained,” said Mrosovsky.
Further experimentation is likely to continue, but human applications can already be surmised. It may be possible, for instance, to alter light conditions for people to better cope with jet lag and the adverse effects of night-shift work.
