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

海鞘蝌蚪幼虫在发育过程中会改变游泳行为。采用时间分辨率为0.1 s的计算机细胞跟踪系统监测了黄颡鱼幼虫的光行为。新孵化的幼虫平均游泳速度为1.4 mm/s，但对光刺激没有反应。孵化后4 h，光照强度突然降低，可使幼虫游得更快。在发育过程中，游泳行为的最大速度随着时间的增加而增加，直到孵化后8小时才趋于平稳。在孵化后8 h左右测定了幼虫的畏光反应减弱的作用谱，并拟合到Dartnall’s图中，色素的最大吸光度位于505 nm处。这些结果表明，幼虫细胞中的视网膜蛋白是光行为的光感受器。在许多动物中，与生殖生物钟耦合的脑光感受器驱动视前-下丘脑GnRH系统的性腺活动的年度变化。研究了原始脊索动物海鞘神经节的光感受器和GnRH系统，并在细胞外记录了两种光诱发反应。假设光诱发的慢电位来源于光感受器细胞，在黑暗中记录的高频自发放电完全被光抑制。免疫组化研究显示，携带视网膜蛋白的细胞主要分布在背表面的外周细胞皮层。gnrh免疫反应的细胞体和纤维分布于整个神经节和靠近感光细胞的部分。由于GnRH神经元表现出规律的自发跳动放电模式，这些结果表明光感受器细胞偶联于GnRH神经元，GnRH神经元的起搏器信号受光控制。

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.