Elevated Free Calcium Levels in the Subretinal Space Elevate the Absolute Dark-Adapted Threshold in Hypopigmented Mice

Abstract

Abundant evidence spanning 25 years demonstrates that hypopigmentation is associated with sensory abnormalities manifested most clearly as elevated absolute dark-adapted thresholds in hypopigmented mice. Here we show that when ocular melanin is increased in the himalayan mouse via α-melanocyte stimulating hormone (α-MSH) injections, dark-adapted thresholds drop in proportion to the change in ocular melanin. We further measured free calcium concentration with calcium-sensitive microelectrodes in both albino and black mouse retinal eyecups in living subjects. The recordings were done in anesthetized animals as the defect is not present in isolated retinas or in the superfused eye preparation. A double-barreled electrode—pCa and Vref—was used to simultaneously record the calcium concentration and the electroretinogram (ERG) at each of many depths as the electrode was driven through the retina. The position of the electrode was confirmed with ERG and 1,1′-dioctadecyl-3, 3,3′,3′-tetramethylindocarbocyanine perchlorate electrode tract reconstruction. Dark-adapted albinos ( n = 6) had 1.4 ± 0.015 mM calcium in the subretinal space compared with 0.80 ± 0.025 mM in black mice ( n = 6). The results of these experiments are consistent with the hypothesis that ocular hypopigmentation causes elevated calcium levels in the subretinal space that in turn mimic light adaptation in hypopigmented mice.