Why do green rods of frog and toad retinas look green?
- 329 Downloads
Amphibian “green” rods express a blue-sensitive cone visual pigment, and should look yellow. However, when observing them axially under microscope one sees them as green. We used single-cell microspectrophotometry (MSP) to reveal the basis of the perceived color of these photoreceptors. Conventional side-on MSP recording of the proximal cell segments reveals no selective long-wave absorbing pigment explaining the green color. End-on MSP recording shows, in addition to the green rod visual pigment, an extra 2- to 4-fold attenuation being almost flat throughout the visible spectrum. This attenuation is absent in red (rhodopsin) rods, and vanishes in green rods when the retina is bathed in high-refractive media, and at wide illumination aperture. The same treatments change the color from green to yellow. It seems that the non-visual pigment attenuation is a result of slender green rod myoids operating as non-selective light guides. We hypothesize that narrow myoids, combined with photomechanical movements of melanin granules, allow a wide range of sensitivity regulation supporting the operation of green rods as blue receptors at mesopic-to low-photopic illumination levels. End-on transmittance spectrum of green rods looks similar to the reflectance spectrum of khaki military uniforms. So their greenness is the combined result of optics and human color vision.
KeywordsGreen rods Amphibia Photoreceptor optics Color vision Light adaptation
Bovine serum albumin
Outer limiting membrane
Rod outer segment
Retinal pigment epithelium
Experimental animals were treated in accordance with the Guide for the Care and Use of Laboratory Animals (1996. National Academy of Sciences, Washington, DC) and with the rules approved by the local Institutional Animal Care and Use Committees. The Authors are thankful to Vadim Maximov, Oleg Orlov and Christa Neumeyer for many helpful discussions. The work was supported by the Grant # 13-04-00701 from The Russian Foundation for Basic Research to V.G., and by a grant from the Finnish Society of Sciences and Letters to T. R.
Conflict of interest
Authors declare no conflict of interests.
- Bäckström A-C, Reuter T (1974) Opponent colour interaction between two kinds of rod signals in the frog’s retina. Phys Nor 7:187–189Google Scholar
- Boll F (1877) Zur Anatomie und Physiologie der Retina. Arch Anat Physiol (Physiol Abt) 1877:4–36 (Translated into English and reprinted: Boll F (1977) On the anatomy and physiology of the retina. Vis Res 17:1249–1265)Google Scholar
- Dartnall HJA (1957) The visual pigments. Methuen, London, pp 1–216Google Scholar
- Kondrashev SL, Gnyubkin VF (1978) Contribution of green rods to the visual process in anurans. In: Orlov O Yu (ed) Mechanisms of vision in animals. Nauka, Moscow, pp 76-84 (In Russian)Google Scholar
- Maurer EY, Govardovskii VI (2013) Regeneration of visual pigments in the isolated retina of the frog Rana temporaria. Sens Syst 27:99–107 (in Russian)Google Scholar
- Muntz WRA (1962b) Effectiveness of different colors of light in releasing positive phototactic behavior of frogs, and a possible function of the retinal projection to the diencephalon. J Neurophysiol 25:712–720Google Scholar
- Orlov OY (1961) Difference in the frog optic nerve reactions depending on the colour of the stimulus. Biofizica 6:77–83 (In Russian)Google Scholar
- Orlov OY, Kondrashev SL (1978) Colour-discrimination functions of visual projections in frog. In: Orlov OY (ed) Mechanisms of vision in animals. Nauka, Moscow, pp 135–165 (In Russian)Google Scholar