MOLECULAR MECHANISM OF LIGHT REGULATED BIOLOGICAL CLOCK FOR REPRODUCTION OF ASCIDIANS.

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Since ascidians, a primitive chordate, spawn at a fixed latency period after sunrise, light must regulate a biological clock for reproduction in the ascidians. A retinal protein found in the cerebral ganglion of the ascidian is a candidate for the photoreceptor that drives the changes in gonadal activity of the gonadotropin-releasing hormone (GnRH) system. Photoresponses of the cerebral ganglion of ascidian, Halocynthia roretzi, were examined and two light-evoked responses recorded extracellularly, a light-evoked slow potential and light inhibition of high frequency spontaneous discharges. These results suggest that pacemaker signals of GnRH neurons are regulated by photoreceptor activation. 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. 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.