MOLECULAR MECHANISM OF SIGNAL TRANSDUCTION IN VISUAL AND NEURAL SYSTEM OF ASCIDIAN BRAIN

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Ascidian tadpole larvae change swimming behavior during the course of development. The photic behavior of the larvae of Ciona intestinalis was monitored by a computerized cell-tracking system with a time resolution of 0.1 s. Newly hatched larvae swim at an average speed of 1.4 mm/s, but show no response to light stimuli. The larvae were induced to swim more rapidly by a sudden decrease in light intensity 4 h after hatching. During the course of development the maximal speed of swimming behavior increased with time until 8h after hatching and then plateaued. The action spectrum for the step-down photophobic response of the larvae was determined at around 8 h after hatching and was fitted to Dartnall's nomogram with the absorbance maximum of the pigment located at 505 nm. These results suggest retinal proteins in the ocellus of the larvae are the photoreceptors for the photobehavior. Encephalic photoreceptors that are coupled to a biological clock for reproduction drive annual changes in gonadal activity of the preoptic-hypothalamic GnRH system in many animals.Photoreceptor and GnRH system in the cerebral ganglion of ascidians, primitive chordate, were examined and two light-evoked responses were recorded extracellularly from the cerebral ganglion of ascidian, Halocynthia roretzi. A light-evoked slow potential is assumed to originate from the photoreceptor cell and high frequency spontaneous discharges recorded in the dark were completely inhibited by light. Immunohistochemical study shows the cells bearing retinal proteins were found in the peripheral cellular cortex mainly at the dorsal surface. GnRH-immunoreactive cell bodies and fibers were distributed entire part of the cerebral ganglion and the part of those located close to the photoreceptor cells. Since the GnRH neuron exhibits a regular spontaneous beating discharge pattern, these results suggest photoreceptor cell coupled to GnRH neuron and pacemaker signal of GnRH neuron was controlled by light.