Abstract
Purpose:
Rapid regeneration of the visual pigment following its photoactivation is critical for the function of cone photoreceptors. We investigated the contributions of classic RPE visual cycle and novel cone-specific retina visual cycle to regeneration of mouse M-cone pigment during dark adaptation. We also examined the role of processing of visual chromophore (retinal) in cone outer segments (COS) by RDH8 and ABCA4 in cone dark adaptation.
Methods:
To test cone dark adaptation following >90% pigment bleach, we performed transretinal ERG recordings (a-wave) from cones in isolated Gnat1-/- mouse retinas lacking rod signaling in the presence of synaptic blockers or standard ERG (b-wave) in live animals. To separate effects of the two visual cycles, we applied intravitreal injections of light-inducible toxin Killer Red to destroy glial Müller cells that can regenerate 11-cis-retinol intraretinally or IP injections of retinylamine, a potent inhibitor of the RPE visual cycle. The role of retinal processing in COS was tested in Rdh8-/-Abca4-/-Gnat1-/- triple knockout (TKO) mice. Photopic visual acuity in Gnat1-/- and TKO mice was determined from optomotor responses.
Results:
M-cone dark adaptation was 4-6-fold slower in live Gnat1-/- mice treated with retinylamine indicating substantial contribution of the RPE visual pathway. In addition, cones in retinylamine-treated mice were unable to maintain their sensitivity in steady bright light. On the other hand, retinylamine only slightly reduced the final level of cone sensitivity recovery in retinas detached from the RPE. Mice injected with Killer Red exhibited up to 2-fold lower postbleach sensitivity in their isolated retinas showing the importance of the retina visual cycle. Simultaneous deletion of RDH8 and ABCA4 (whose expression in COS of Gnat1-/- mice was confirmed by IHC) retarded cone dark adaptation by 2-fold in both live animals and isolated retinas. Surprisingly, fully dark-adapted TKO cones were not chromophore-deficient, as evidenced by treatment with exogenous 11-cis-retinal, but had accelerated responses compared with control cones. Finally, the photopic visual acuity was reduced in adult TKO animals.
Conclusions:
Dark adaptation of mouse cones requires the combined action of both RPE and retina visual cycles. Efficient processing of retinal by RDH8 and ABCA4 in COS is essential for rapid and complete dark adaptation of cones.
Keywords: 508 electrophysiology: non-clinical •
648 photoreceptors •
705 retinoids/retinoid binding proteins