A naturally occurring rat model of X-linked cone dysfunction.

PURPOSE To describe the electrophysiological, histologic, and hereditary features of a naturally occurring rat model of cone function loss. METHODS Dark- and light-adapted electroretinograms (ERGs) were used to evaluate retinal function. The thickness and architecture of the retina were observed by light microscopy. The cone density was investigated by wholemount immunocytochemistry. The inheritance pattern was defined by mating with control female rats. RESULTS In affected rats, light-adapted ERGs were nearly absent, whereas dark-adapted responses were of normal amplitude with delays in b-wave implicit time. Overall retinal structure was normal at the light microscopic level. There was no difference in cone density between control and affected rats. The cone function abnormality is inherited as an X-linked trait. CONCLUSIONS A spontaneous rat mutant was identified that has markedly affected cone function, whereas rod-mediated function is largely spared. The presence of the normal number of cone outer segments indicates that the defect does not involve cone photoreceptor degeneration. This rat model provides a model of X-linked cone dysfunction, and may also be used to examine aspects of rod-mediated visual function in the rat. Further studies are needed to identify the gene that is involved.