Why do green rods of frog and toad retinas look green?

Abstract

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.

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Abbreviations

BSA:

Bovine serum albumin

IS:

Inner segment

LED:

Light-emitting diode

MSP:

Microspectrophotometer

NA:

Numerical aperture

OD:

Optical density

OLM:

Outer limiting membrane

OS:

Outer segment

ROS:

Rod outer segment

RPE:

Retinal pigment epithelium

VP:

Visual pigment

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Acknowledgments

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.

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Authors declare no conflict of interests.

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Correspondence to Victor I. Govardovskii.

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Govardovskii, V.I., Reuter, T. Why do green rods of frog and toad retinas look green?. J Comp Physiol A 200, 823–835 (2014). https://doi.org/10.1007/s00359-014-0925-z

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Keywords

  • Green rods
  • Amphibia
  • Photoreceptor optics
  • Color vision
  • Light adaptation