ELEVATED DARK-ADAPTED THRESHOLDS IN HYPOPIGMENTED MICE MEASURED WITH A WATER MAZE SCREENING APPARATUS

In previous electrophysiological experiments from hypopigmented animal s (mice, rats, rabbits), single-unit recordings from both retinal gang lion axons and cells in the superior colliculus have demonstrated an i ncrease in threshold in the dark-adapted state which is roughly propor tional to the ocular melanin concentration. In the present study we co mpared an albino mouse strain which is relatively resistant to light d amage and the beige mouse mutant to their wild-type controls in a situ ation that involved unanesthetized, unrestrained mice as a control to the electrophysiological single unit experiments. We used a six-chambe red water maze. Animals were trained to swim to an illuminated ramp un til their performances leveled off (about 10 days). The animals were t hen dark-adapted for 24 h and tested after reducing the luminance leve l of the water maze. We found that the albino mice failed to find the ramp when the luminance fell to 1.58 x 10(-3) cd/M2 (p less-than-or-eq ual-to .0001), the beige mice failed at 2.00 x 10(-4) cd/M2 (p less-th an-or-equal-to .0001), and the normally pigmented controls performed t o 5.00 x 10(-5) cd/m2 (p less-than-or-equal-to .0001). These results s upport our previous findings that the sensitivity defect in hypopigmen ted animals is proportional to the degree of ocular hypopigmentation.