Elevated dark-adapted thresholds in albino rodents.

Albino mice and rats have elevated dark-adapted thresholds compared to normally pigmented animals. The absolute dark-adapted incremental threshold for black mice is about 1.5 log units lower than the threshold for albino mice when measured by single-unit recordings from the superior colliculus. Cell counts from the outer nuclear layer in albino mice are not significantly different from those in black mice, indicating that the elevated dark-adapted thresholds are not due to light damage of photoreceptor cells. No photoreceptor outer segment damage was found in these albino animals at the light or electron microscopic level. These experiments have been repeated in hooded and albino rats. The thresholds from albino rats were about 2 log units higher than the thresholds from pigmented rats in the dark-adapted state. The proximity of the retinal pigment epithelium (RPE) and the pigmented choroid to the photoreceptors in these animals suggests that a reduction in ocular melanin in hypopigmented animals may be causal to their elevated thresholds.