Retinal proteins in the cerebral ganglion of the ascidian, Halocynthia roretzi, were successfully visualized and their localization was determined by the time-resolved fluorescence difference imaging method. This visualizes retinal proteins in the tissue even though the concentration of the retinal proteins is low and there is considerable endogenous fluorescence. Heterogeneous retinal proteins in the same tissues could be distinguished by differential sensitivities to photobleaching, standardized by rhodopsin and retinochrome in octopus retina. Retinal proteins in the ocellus of ascidian larva, which is composed of only about 20 photoreceptor cells, were successfully visualized. Retinal proteins in the cerebral ganglion of adult Halocynthia roretzi, localized mainly at the surface of the anterodorsal root and the posterodorsal root. In the cross sections along the anteroposterio axis of the cerebral ganglion, the cells bearing retinal proteins were found in the peripheral cellular cortex mainly at the dorsal surface. Close localization of retinal protein and GnRH bearing cells suggests that retinal protein may trigger the biological clock for spawning in this ascidian.
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