Rod photopigment deficits in albinos are specific to mammals and arise during retinal development

Adult albino mammals have specific retinal defects, including reduced numbers of rod photoreceptors. To examine when this rod deficit arises and whether it exists in nonmammalian albinos, we have used absorbance spectrophotometry to measure photopigment levels in dark-adapted eyes taken from three groups of pigmented and albino animals: adult rodents (rats and mice), developing rats, and mature Xenopus frogs. Rhodopsin concentrations were consistently and significantly reduced in mammalian albinos compared to their wild-type counterparts from before the time of eye opening, but photopigment levels were similar in frogs of both pigmentation phenotypes. The results strongly suggest that deficits in the rod cell population arise early in development of the mammalian albino retina, but do not generalize to nonmammalian mutants lacking retinal melanin.