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Not Blinded by the Light: Rods in the Retina Contribute to Daylight Vision

28. November 2017

Findings of international research team may be beneficial for new treatments for patients suffering from loss of vision in bright light

Two types of photoreceptor in the retina, rods and cones allow you to see over a wide range of brightness, from starlight to brightest sunlight. Rods were considered to be largely inoperable in bright light. A recent study by Tikidji-Hamburyan et al. shows that this is not the case and that rods can also contribute to vision in bright environments. The picture on the right shows a cross section of the retina in which different cells are marked in color. Photoreceptors are the green cells on the right. Moonpicture by John French, Abrams Planetarium. Picture of the reina by Hartwig Seitter, © AG Münch, Universität Tübingen

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.

Original Publication

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

Contact:

Dr. Thomas Münch

Retinal Circuits and Optogenetics
University of Tuebingen
Werner Reichardt Centre for Integrative Neuroscience

and

Institute for Ophthalmic Research
Centre for Ophthalmology

Otfried-Müller-Str. 25,
72076 Tübingen
Germany

Phone: +49 (0)7071 29 89182
Email: thomas.muench[at]cin.uni-tuebingen.de