The Relationship Between Ambient Lighting Conditions, Absolute Dark-Adapted Thresholds, and Rhodopsin in Black and Albino Mice

Abstract

Abstract: : Purpose: Strains of mice with differing pigmentation levels exhibit significant variation in absolute dark-adapted thresholds both within and between strains. Differences in sensitivity within a single strain appear to be related to the William’s photostasis, effect i.e., photoreceptor rhodopsin levels are dependent upon ambient lighting conditions. In this study we wondered if photostasis could explain the difference between different pigmented strains; we maintained albino and black mice under the appropriate lighting conditions to equalize their rhosopsin levels and then measured visual thresholds. Methods: We maintained albino mice (c ^2J /c ^2J) at 10 ^-4 cd/m² (dim) and black mice at 200 cd/m² (bright) in order to equalize their rhodopsin levels. We measured thresholds with a water maze screening test. Sub-retinal space calcium levels were measured in an in vivo eye cup preparation. Results:We found that the ocular rhodopsin level for albino mice (albino-dim) was 494 ±11 ρmoles/eye and the rhodopsin level for black mice (black-bright) was 506 ±25 ρmoles/eye. For comparison, rhodopsin levels in black mice maintained under dim light were 586 ±46 ρmoles/eye and 217 ±46 ρmoles/eye in albino mice maintained in bright light. Water maze tests showed that albino and black mice with equivalent rhodopsin levels had similar dark-adapted thresholds: -6.38 log cd/m² vs. -6.47 log cd/m² (respectively). This suggests that dark-adapted thresholds are directly related to rhodopsin levels regardless of the level ocular melanin pigmentation. Measurements of photoreceptor outer segment length (c ^2J 10% less than black-18µm vs. 20µm) and ONL cell counts (not significantly different) do not show sufficient differences to account for the reduction in rhodopsin. Thus, the density of rhodopsin within the outer segment must be increased. Conclusions: We hypothesize that the absolute dark-adapted threshold deficit that is observed in hypopigmented mutants is a result of abnormalities in rhodopsin level regulation. We have found that the sub-retinal space calcium concentration is elevated in albino animals compared to black controls (1.4mM ± 0.015 vs. 0.8 mM ± 0.025). The albino calcium elevation is not directly casual to the threshold elevation in albino animals, but may mimic light adaptation and influence the process of photostasis to down-regulate rhodopsin in hypo-pigmented animals.

Keywords: photoreceptors: visual performance • low vision • opsins