Increased absolute light sensitivity in Himalayan mice with cold-induced ocular pigmentation

Abstract

Controversy over the relationship between ocular pigmentation and absolute dark-adapted light sensitivity has persisted for over two decades. Previous electrophysiological experiments in hypopigmented mammals (mice, rats, rabbits) show increased thresholds in the dark-adapted state proportional to the deficit in ocular melanin. Animals with the least amount of ocular melanin have the most elevated thresholds. Dark-adapted thresholds in hypopigmented mice show similar threshold elevations in behavioral tests. The present study extends these findings to show that a specific increase in ocular pigmentation results in the converse effect, lowered absolute dark-adapted thresholds. The increase in ocular melanin was accomplished by keeping Himalayan mice in the cold (4°C) for 6 weeks. Himalayan mice (C57BL/6J c^H/c^H) were compared to black mice (C57BL/6J +/+) and albino mice (C57BL/6J c^2J/c^2J) after 6 weeks at either 4°C or 20°C in 12-h cycling light (<1 cd/m²). The Himalayan mice that were kept in the cold exhibited a 44% increase in ocular melanin compared to Himalayan mice kept at room temperature. Cold rearing did not effect ocular melanin or visual thresholds in control animals (black mice = 10^−5.9 cd/m² and albino mice = 10^−4.4 cd/m²). In contrast, the Himalayan mice maintained at 4°C had thresholds of 10^−5.7 cd/m² compared to 10^−5.1 cd/m² for Himalayan mice kept at 20°C. This represents compelling evidence of a direct relationship between ocular melanin concentration and absolute dark-adapted light sensitivity.