Dark adaptation is faster in pigmented than albino rats

Previous reports have raised the possibility that, compared to pigmented rats, albino rats might be night blind. The purpose of this study was to reinvestigate this issue by comparing the dark-adaptation process of the pigmented Long-Evans (LE) and albino Sprague Dawley (SD) rats.

Scotopic ERGs obtained from LE and SD rats were recorded following periods of dark adaptation 0.5, 3 and 12 h. Intensity response functions were generated with flashes of white light spanning over a 7 log-unit range with a maximal intensity of 8 cd x s x m(-2) in energy.

SD rats showed a gradual increase in the amplitude of the scotopic b-wave Vmax (maximal 'saturated' rod b-wave amplitude) and retinal sensitivity (k) as the duration of the dark-adaptation period increased. In contrast, LE rats did not demonstrate any further significant gain in retinal function (Vmax or k) beyond 30 min of dark-adaptation. Thus for periods of dark-adaptation of 30 min or less, the rod function of the LE rats is superior to that of the SD rats while both strains have comparable retinal functions following 3 h or more of dark-adaptation.

Our results indicate that LE and SD rats differ in their rapidity to dark-adapt, a finding that could explain the previous claim that SD rats were night blind. The reduced bio-availability of calcium ions in eyes lacking melanin could explain this difference. Calcium was previously shown to play a key role in retinal adaptation processes.