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A. V. Kolesnikov, R. O. Parker, R. K. Crouch, V. J. Kefalov; Interphotoreceptor Retinoid-Binding Protein and Its Role in the Mouse Cone Visual Cycle. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1115.
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Interphotoreceptor retinoid-binding protein (IRBP) has been suggested to play an important role in maintaining cone structure and function by facilitating retinoid transport between cones and the RPE. However, its role in the cone-specific retina visual cycle is unknown. To address this issue we generated a double knockout Irbp-/-/Tα-/- (rod transducin alpha) mouse lacking rod signaling and developed a method of transretinal ERG recordings from mouse eyecups.
We recorded cone ERG responses (a-wave) from isolated mouse retinas or intact eyecups where the retina was attached to the RPE (1-2 m.o. animals). Contributions of the b-wave and slow glial component were blocked pharmacologically. We compared the kinetics of cone photoresponses and photosensitivity recovery following full M-cone pigment bleach in Irbp-/-/Tα-/- vs. Tα-/- control animals.
We found that Irbp-/-/Tα-/- cones were two times less sensitive and their responses had significantly slower shutoff (300 ms vs. 180 ms) compared to Tα-/- cones; both possibly due to mild degeneration. Surprisingly, the kinetics of cone photosensitivity recovery was not substantially different between Irbp-/-/Tα-/- and Tα-/- mice (time constants 240 s vs. 210 s for eyecups; and 100 s vs. 70 s for isolated retinas, respectively). Notably, in both mouse genotypes sensitivity recovery in eyecup was biphasic, with a rapid phase similar to that in isolated retina, followed by a slower phase that produced 2-fold higher final recovery of sensitivity in eyecup compared to the retina (60% vs. 34% by 30 min postbleach, on average). Background light adaptation function of Irbp-/-/Tα-/- cones was shifted ca. 2-fold to dimmer light intensities both in retina and eyecup, most likely due to their longer integration time.
Our results indicate that IRBP is apparently not critical for the timely recovery of mouse cone photosensitivity after bright illumination, that is for the operation of the cone-specific retina visual pathway. Cone dark adaptation in eyecup was biphasic, with a rapid phase presumably due to the retina visual cycle and a slow phase due to the RPE.
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