Origin of the vertebrate visual cycle: II. Visual cycle proteins are localized in whole brain including photoreceptor cells of a primitive chordate.

The visual cycle system in a primitive chordate, ascidian Ciona intestinalis, was studied by whole-mount in situ hybridization and by whole-mount immunohistochemistry. Three visual cycle proteins, Ciona homologue of RGR (Ci-opsin3), CRALBP (Ci-CRALBP), and BCO/RPE65 (Ci-BCO/RPE65) were widely distributed in the brain vesicle and visceral ganglion. To identify the visual cycle system in a primitive chordate, we compared the localization of photoreceptor-specific proteins (visual pigment and arrestin) and visual cycle proteins (Ci-opsin3 and Ci-CRALBP). The ascidian visual cycle is composed of two cellular compartments, the photoreceptors and the brain vesicle, but some photoreceptor cells also contain visual cycle proteins.