INCREASED ABSOLUTE LIGHT SENSITIVITY IN HIMALAYAN MICE WITH COLD-INDUCED OCULAR PIGMENTATION

Controversy over the relationship between ocular pigmentation and abso lute 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 sta te proportional to the deficit in ocular melanin. Animals with the lea st amount of ocular melanin have the most elevated thresholds. Dark-ad apted thresholds in hypopigmented mice show similar threshold elevatio ns in behavioral tests. The present study extends these findings to sh ow that a specific increase in ocular pigmentation results in the conv erse effect, lowered absolute dark-adapted thresholds. The increase in ocular melanin was accomplished by keeping Himalayan mice in the cold (4 degrees C) for 6 weeks. Himalayan mice (C57BL/6J c(H)/c(H)) were c ompared to black mice (C57BL/6J +/+) and albino mice (C57BL/6J c(2J)/c (2J)) after 6 weeks at either 4 degrees C or 20 degrees C in 12-h cycl ing light (<1 cd/m(2)). 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 o r visual thresholds in control animals (black mice = 10(-5.9) cd/m(2) and albino mice = 10(-4.4) cd/m(2)). In contrast, the Himalayan mice m aintained at 4 degrees C had thresholds of 10(-5.7) cd/m(2) compared t o 10(-5.1) cd/m(2) for Himalayan mice kept at 20 degrees C. This repre sents compelling evidence of a direct relationship between ocular mela nin concentration and absolute dark-adapted light sensitivity.