Figure 2. Outer retinal Mn²⁺ uptake visibly increases with age.

Representative maps of tissue R₁ (in units of s⁻¹) from baseline (no Mn²⁺) and Mn²⁺-injected rats. In Mn²⁺-injected rats, R₁ – which is linearly related to tissue Mn²⁺ concentration [58]– increases with age. As detailed in Supplemental Figure S2 (in File S1), this change could not be attributed to altered integrity of the blood-retinal barrier, which was fully intact at all ages. There was no relationship between age and baseline R₁s in dark-adapted or light-exposed retinas, nor between age and dark-light differences in R₁ (−0.18<r<0.17 with P>0.66; in dark, mean±SEM baseline R₁s were 0.64±0.01, 0.49±0.05, and 0.59±0.04 s⁻¹ respectively in young, mid-, and old adults; in light, 0.59±0.05, 0.50±0.03, and 0.60±0.03 s⁻¹ respectively). Given that baseline R₁s were stable, the average for outer retina (0.568 s⁻¹) was subtracted from Mn²⁺-injected R₁s to calculate ΔR₁s – which are directly proportional to Mn²⁺ uptake – shown in Figure 3. Note that these R₁ maps are for illustration only: Formal measurements used linearized and spatial-normalized retinal data. Also note that these R₁ maps are sub-optimal for displaying the fine structural detail (layering) of the retina, which we routinely observe at the present image resolution (see Fig. S3 in File S1).