Abstract
Purpose:
To test the hypothesis that subnormal intraretinal ion channel activity and visual performance measured in diabetic mice are due to inadequate availability of chromophore.
Methods:
Diabetic and non-diabetic age-matched C57Bl/6 mice and rd12 mice (RPE65 mutant) were dark adapted for either 1 day (1dDA) or 5 days (5dDA). 5dDA groups: controls (without or with matrigel s.q.) or treated (matrigel and 9-cis-retinal (PMID: 20574023) mice. Intraretinal L-type voltage gated calcium channel (VGCC) activity in vivo was evaluated using manganese-enhanced MRI (MEMRI). Spatial frequency threshold (SFT) and contrast sensitivity (CS) were measured with optokinetic tracking. Parallel ex vivo transretinal electrophysiological studies were also performed from 1dDA mice. A subset of the diabetic retinas was treated with 100 µM 11-cis retinal prior to recordings. Steady-state levels of 11-cis-retinal, all-trans-retinal, and retinyl esters were also measured.
Results:
In 5dDA controls, retinal VGCCs on MEMRI are not sustained in the open position (i.e., they are closed or “dark desensitized”) but were opened upon treatment with the chromophore surrogate 9-cis-retinal. However, visual performance did not decrease following extended darkness and was not altered by 9-cis-retinal treatment. In contrast, overnight dark adapted diabetic mice (before the appearance of microangiopathy) had inappropriately closed photoreceptor L-VGCCs on MEMRI which partly opened with five days of darkness. The ion channels in photoreceptors from diabetic mice were regulated by darkness in a manner similar to that in normoglycemic rd12 mice (an “equivalent light” model). Ex vivo transretinal ERG sensitivity was reduced about 2-fold in diabetic mice (and accordingly, I1/2 is increased about 2-fold). The effects were largely reversed by the application of 11-cis retinal. Changes in retinoid metabolism (particularly chromophore production) were found consistent with increased production of free opsin. In vivo, 5dDA diabetic mice treated with 9-cis-retinal had significantly opened outer retinal channels (MEMRI) and corrected diabetic visual impairment.
Conclusions:
Diabetes-evoked reduction in photoreceptor chromophore availability contributes to the associated visual performance dysfunction.
Keywords: 499 diabetic retinopathy •
648 photoreceptors •
705 retinoids/retinoid binding proteins