Not Blinded by the Light: Rods in the Retina Contribute to Daylight Vision
Findings of international research team may be beneficial for new treatments for patients suffering from loss of vision in bright light
An international research team headed by Thomas Münch of the Institute for Ophthalmic Research and Werner Reichardt Centre for Integrative Neuroscience of the University of Tübingen found the contribution of rod photoreceptors in mouse retinas to be much greater than previously assumed. One cannot distinguish colors with rods. They were thought to become useless as light levels increase, while vision in daylight conditions is based on cone photoreceptors. The new study – published in Nature Communications – shows that rod function can even increase in bright light.
The photoreceptors in the retina, at the back of the eyes, are the primary light sensitive cells that allow us to see: they convert light into electrical signals. There are two types, rods and cones, and it has generally been assumed that rods are responsible for seeing in dim light conditions, whereas cones allow vision in bright light. This division of labor between rods and cones can be found in virtually every biological and medical text book.
A new study challenges this traditional view: A group of researchers from the universities of Tübingen, Manchester and Helsinki led by Thomas Münch from Tübingen shows that rod photoreceptors do, in fact, contribute to daylight vision. Most surprisingly, their contribution even increases when the daylight becomes brighter, up to the brightest light levels that would ever be encountered in natural environs.
Using transgenic mice without functional cones, the investigators first measured rod-driven signals and could reliably detect them both in the retina and in the brain even at high light levels. Furthermore, they were subsequently able to show this bright-light rod contribution in mice with fully functional cones as well.
With this new data, it seemed obvious that the models used by most scientists beforehand must be incomplete. And indeed, much was already known about rod physiology that had not been included in the big picture, as it has been understood up to now. Adding this information, the research team arrived at a model that not only explains how rods see so well in dim light, but also their own findings: that rods continue to function in bright light.
“We showed that rods can function at bright light”, says Thomas Münch, “but it is true that cones can do this much better and much more reliably. However, these new insights may still open new avenues towards treatments for patients without functional cones, so-called rod monochromats.” People in modern times are exposed to artificial bright light for many hours each day. In the old paradigm, this has made reliance on rods for the development of treatments counterintuitive. With these new mechanistic insights into bright rod vision, however, there may be new possibilities for therapies for patients without functional cone vision.
Alexandra Tikidji-Hamburyan, Katja Reinhard, Riccardo Storchi, Johannes Dietter, Hartwig Seitter, Katherine E. Davis, Saad Idrees, Marion Mutter, Lauren Walmsley, Robert A. Bedford, Marius Ueffing, Petri Ala-Laurila, Timothy M. Brown, Robert J. Lucas, Thomas A. Münch: Rods Progressively Escape Saturation to Drive Visual Responses in Daylight Conditions. Nature Communications 2017 Nov 27; 8(1813). DOI: 10.1038/s41467-017-01816-6
Dr. Thomas Münch
University of Tübingen
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In the retina, there are two types of photoreceptors, the rods and the cones, that support vision over a huge brightness range, from dim starlight to bright sunlight. It has been thought that rods function exclusively in dim light conditions. A new study by Tikidji-Hamburyan et al updates this concept and shows that rod photoreceptors can contribute to vision also in bright environs. The image on the right shows a cross section of a retina, in which different cell types are labeled with different color. The photoreceptors are the green cells on the right.
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