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M. C. Cornwall, K. I. Miyagashima, A. C. Arman, S. Nymark, A. P. Sampath; Rod Responses Persist and Traverse the Retina Despite Substantial Bleaching of the Visual Pigment. Invest. Ophthalmol. Vis. Sci. 2010;51(13):2044. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
During daylight a substantial fraction of the visual pigment in our photoreceptor cells is bleached, conditions under which rods are presumed saturated and incapable of contributing visual information. Both psychophysical studies from rod monochromats and physiological recordings from mammalian retina, however, suggest that rod vision may extend into a range were > 105 rhodopsins are activated. Here we probe the upper limits of rod vision by measuring the influence of bleached pigment on the sensitivity of rods and downstream cells.
Whole-retina electroretinograms (ERGs) were used to assess the sensitivity of rods (a-wave) and downstream cells (b-waves) from mice lacking cone photoresponses (Gnat2-/-) and retinal gap junctions (Gnat2-/- Cx36-/-), allowing the separation of the rod bipolar pathway from the rod-cone pathway. Retinas were superfused with Ames’ media at 35-370C in the presence of pharmacological agents (picrotoxin, strychnine, CNQX) to isolate ERG components. Isolated retinas were bleached by full-field illumination (max ~ 520 nm) and allowed to recover to steady-state. Sensitivity then was measured in responses to 30 ms flashes of light, and analyzed offline to separate a-waves and b-waves.
The loss of sensitivity in mouse rods (a-wave) following bleaching was well described by a Weber-Fechner relationship where bleached pigment produced an equivalent background. The loss of sensitivity of downstream signals (b-wave) did not follow the loss of rod sensitivity. When 80% of the visual pigment was bleached, b-waves were ~ 10-fold more sensitive than their a-wave counterparts. This increased sensitivity was abolished in mice also lacking Cx36 gap junctions.
The persistence of a-waves at steady-state after bright bleaching light exposures indicates that rods were capable of encoding visual information even though they were greatly desensitized by the bleaching light. The higher sensitivity of b-waves in Gnat2-/- mice, but not Gnat2-/- Cx36-/- mice, suggests that the rod-cone pathway can improve sensitivity by pooling rod signals. The rod bipolar pathway thus appears to be desensitized significantly at higher bleach levels. Future studies will assess the sensitivity of inner retinal cells in retinal slices following pigment bleaching.
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