Discovery could explain why light keeps us awake and may lead to new treatments for disorders such as jet-lag and SAD.
Researchers from Imperial College London, Johns Hopkins University, USA and Brown University, USA have discovered that melanopsin, a recently identified protein, plays a key role in a completely new light detection system in the eye.
Professor Russell Foster, from Imperial College London at the Charing Cross Hospital comments: "It had long been assumed that the rod and cone cells of the retina are responsible for all light detection. However, over the last few years research from our group has led us to the inescapable conclusion that there is a third light detection system that has lain undiscovered over more than 100 years of intensive research on the eye. Although we have known of their existence for several years, it has proved difficult to discover much more about these new receptors".
Now, in an important breakthrough scientists from the UK and USA have provided a direct link between melanopsin and this system. Melanopsin is an opsin-like protein which is expressed in a small number of ganglion cells in the retina of the eye. The research team tested whether melanopsin is part of the new light detecting system by measuring light-induced pupillary constriction in genetically modified mice that lacked melanopsin. When the mice lacking melanopsin were exposed to low light, their pupillary response was the same as normal mice, but when they were exposed to bright light their pupil constriction was incomplete.
Dr Rob Lucas from Imperial College London at the Charing Cross Hospital comments: "Our results show melanopsin is a critical component of this novel photoreceptor. They also show that melanopsin is particularly important in the detection of bright light. This is important because we think that this new photoreceptor is responsible for telling our bodies that it is daytime – daylight is always bright light.
"Our research has led us to believe that, quite apart from regulating pupil size, the melanopsin photoreceptors may be responsible for a broad range of responses to light, including its ability to keep us awake and alert. It is also likely that these photoreceptors are responsible for resetting our internal body clocks to local time following a flight across time_zones."
A deeper understanding of these photoreceptors may provide new ways to overcome jet-lag and treatments for disorders such as SAD (Seasonally Affective Disorder) that can be caused by a lack of light, particularly during the winter months.
Imperial College, University of London. January 2003.