purpose. To test the hypothesis that the extent of outer retina uptake of manganese, measured noninvasively with manganese-enhanced MRI (MEMRI), is a quantitative biomarker of photoreceptor ion channel regulation by visual cycle activity.

methods. Four groups of animals were studied: control rats adapted to three different background light intensities, dark-adapted control mice systemically pretreated with retinylamine, and dark-adapted mice with a nonsense mutation in exon 3 of the RPE65 gene (RPE65 rd12) with and without systemic 11-cis-retinal pretreatment. In all cases, rodents were anesthetized and studied with MEMRI 4 hours after manganese administration IP. Central retinal thickness and intraretinal ion channel regulation were measured from the MEMRI data.