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V. J. Kefalov, J. Wang; Chromophore Recycling Within Rod-Dominant Vertebrate Retina. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1661. doi: https://doi.org/.
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Regeneration of bleached visual pigment during dark adaptation requires recycling of its chromophore. Biochemical studies and shortcomings of the canonical chromophore recycling pathway within the pigment epithelium suggest the existence of a second, cone-specific pigment regeneration pathway located within the retina and involving the Müller cells. To address this possibility, we tested whether under physiological conditions the rod-dominant salamander retina can promote cone pigment regeneration and dark adaptation independently of the pigment epithelium.
Using a suction electrode, membrane current was recorded from single photoreceptors. Retinas were removed from eyecups and cleaned from residual pigment epithelium tissue. One retina from each animal was used to determine the sensitivity of dark-adapted cells, while the other retina, intact or dissociated, was bleached and then allowed to recover in darkness for two hours prior to recording from single cells.
Exposure of dissociated retina to bleaching light induced a 50-fold decrease in flash sensitivity of red cones. This desensitization is consistent with a lack of pigment regeneration in dissociated cones and corresponds to loss of 86% of their pigment. The persistent bleaching adaptation in these cones was readily reversed upon treatment with exogenous 11-cis retinal. In contrast, identical bleach in intact retina produced only 1.6-fold decrease in cone sensitivity. This minimal desensitization indicates substantial cone pigment regeneration within the intact retina and corresponds to loss of only 7% cone pigment. Thus, the retina promoted the regeneration of the remaining 79% bleached cone pigment, largely dark adapting the cones. Sporadic pigment epithelium contamination was ruled out as the source of recycled chromophore by the uniform recovery of sensitivity in all cones, the lack of dark adaptation in rods from intact retina, and the dose-dependent inhibition of cone dark adaptation by the Müller cell-specific inhibitor alpha-aminoadipic acid. The persistent bleaching adaptation in the latter two cases was readily reversed by exogenous 11-cis retinal. Alpha-aminoadipic acid did not inhibit the pigment epithelium-driven cone dark adaptation in intact retina from eyecup.
Our results establish the physiological function of a cone-specific mechanism for recycling of chromophore and regeneration of visual pigment within the retina. This pathway promotes dark adaptation exclusively in cones; it involves the Müller cells; and is independent of the pigment epithelium.
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