Abstract
Purpose :
Cone photoreceptors enable vision over a wide range of light intensities, however, the processes enabling cone photopic vision are not adequately understood. Cones may receive the chromophore 11-cis-retinal (11cRAL) from non-photopic and photopic visual cycle pathways involving the retinal pigment epithelium and Müller glia, respectively. The contribution of each pathway to cone photopic vision is unknown. We tested visual cycle inhibitors in zebrafish to understand the mechanisms regulating immediate (0 minutes light adaptation (LA)), early (30 minutes LA) or late phases (~4-6 hours of LA) phases of cone photopic vision.
Methods :
Wildtype (Tü) zebrafish larvae were treated with 50 μM emixustat and/or 10 μM fenretinide at 3 days post fertilisation (dpf) and drug replaced at 4 dpf. Larvae were raised under 14 h light, 10 h dark or placed in complete darkness from initial drug treatment until analysis at 5 dpf. 10 μM 9-cis-retinal was added four hours following drug treatment. Immediate, early or late phases of cone photopic vision were assessed by optokinetic response (OKR). Retinoid levels were quantified by normal-phase HPLC with a photodiode-array detector. Retinal integrity was evaluated using transmission electron microscopy and zpr-1 immunohistochemistry. Three biological replicates were performed for each experiment.
Results :
Retinal morphology appears normal following treatment with fenretinide or emixustat. Notably, late photopic vision is reduced in fenretinide-treated larvae but not following emixustat treatment. Emixustat and fenretinide significantly (~35% and ~90%, respectively) impair immediate photopic vision correlating with significantly reduced 11cRAL levels (p<0.0001). Fenretinide in combination with emixustat, completely abolished immediate photopic vision (~85% reduction, p<0.001). Following 30 minutes of light, early photopic vision is restored (p<0.01) in emixustat but not fenretinide-treated larvae (p=0.32).
Conclusions :
Immediate photopic vision depends on the non-photopic visual cycle, whereas early-and late-photopic vision depend on the photopic visual cycle. Rpe65 appears redundant to this light-dependent regeneration of visual pigments, a process whereby diminished 11cRAL levels are sufficient for photopic vision.
This is a 2020 ARVO Annual Meeting abstract.