Visual Neuroscience



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


GRANT W.  BALKEMA a1c1 and SHANNON  MacDONALD a1
a1 Biology Department, Boston College, Chestnut Hill MA 02167, USA

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 cH/cH) were compared to black mice (C57BL/6J +/+) and albino mice (C57BL/6J c2J/c2J) 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.

(Received September 22 1997)
(Accepted February 24 1998)


Key Words: Hypopigmentation; Albino; Light adaptation; Retinal degeneration.

Correspondence:
c1 Reprint requests to: Grant W. Balkema, Biology Department, Boston College, Chestnut Hill, MA 02467, USA.