RESUMO
Significant variation in absolute dark-adapted thresholds is observed both within and between strains of mice with differing ocular pigmentation levels. Differences in threshold within a single strain are related to the Williams' photostasis effect, that is, photoreceptor rhodopsin levels are dependent upon ambient lighting conditions. To examine threshold differences among strains, we equalized rhodopsin levels by maintaining albino mice (c2J/c2J) at 2 x 10-4 cd/m2 (dim light) and black mice at 2 x 102 cd/m2 (bright light). This resulted in ocular rhodopsin levels for albino mice (albino--dim) of 494 +/- 11 pmoles/eye and rhodopsin levels for black mice (black--bright) of 506 +/- 25 pmoles/eye. For comparison, rhodopsin levels in black mice maintained in dim light are 586 +/- 46 pmoles/eye and 217 +/- 46 pmoles/eye in albino mice maintained in bright light. We found similar dark-adapted thresholds (6.38 log cd/m2 vs. 6.47 log cd/m2)) in albino and black mice with equivalent rhodopsin determined with a water maze test. This suggests that dark-adapted thresholds are directly related to rhodopsin levels regardless of the level of ocular melanin. The number of photoreceptors, photoreceptor layer thickness, and outer segment length did not differ significantly between albino (dark) and black mice (bright). These results demonstrate that the visual sensitivity defect found in hypopigmented animals is secondary to abnormal rhodopsin regulation and that hypopigmented animals have either an improper input to the photostasis mechanism or that the photostasis mechanism is defective.
