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M. E. Estevez, J.-S. Wang, J. C. Corbo, M. C. Cornwall, V. J. Kefalov; The rd7 Mouse Retina Supports Rod Pigment Regeneration and Dark Adaptation. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1118.
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We recently characterized a cone visual cycle within the mouse retina. The cone-specific enzyme that oxidizes 11-cis retinol supplied by Müller cells to 11-cis retinal as part of this cycle is unknown. As a first step in identifying this enzyme, we examined the Nr2e3 knockout (rd7) mouse with hybrid rods expressing a subset of cone genes.
Mouse retinas were dissected from eyecups and any residual pigment epithelium was removed. Wild type and rd7 rod electroretinogram (ERG) responses were recorded from isolated retina after pharmacologically blocking synaptic transmission. Cone ERG responses were recorded from rod transducin-deficient (Gnat1 knockout) retinas. Photoreceptor sensitivity was measured in dark-adapted conditions as well as during and following recovery from bright bleaching light. In parallel experiments, rod optical density was measured with microspectrophotometry from wild type and rd7 retinas.
Exposure to bleaching light desensitized WT rods 143 ± 14 (n=16) fold from their dark-adapted state. In contrast, following an identical bleach cone sensitivity recovered substantially to within 3.1 ± 0.5 (n=9) fold from dark-adapted levels. Surprisingly, rd7 rods were also able to partially recover their sensitivity following a bleach in the isolated retina to within 14.1 ± 1.4 fold (n=5) fold from dark-adapted levels. The recovery of rd7 rod sensitivity following a bleach was significantly slower than that of cones, with time constants of 6.2 ± 1.3 min (n=3) and 1.4 ± 0.2 min (n=5), respectively. Microspectrophotometric measurements of the optical density of isolated rd7 retina also revealed substantial pigment recovery in bleached rd7 rods despite the absence of pigment epithelium.
Our results show that the rd7 retina promotes pigment regeneration and dark adaptation in rods in the absence of pigment epithelium. We conclude that the cone enzyme driving the oxidation of 11-cis retinol is likely expressed in rd7 rods.
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