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M. Tsuda, T. Horie, N. Takimoto, Y. Toda, T. Kusakabe, H. Horiguchi, T. Hariyama; Origin of the Vertebrate Retina: On–Type and Off–Type Photoreceptors in Ascidian Larva . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2834.
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© ARVO (1962-2015); The Authors (2016-present)
Ascidians are considered chordates, because their tadpole–like larvae share a basic body plan with that of vertebrates. The larva of Ciona intestinalis is composed of only 2600 cells and has a primitive nervous system which is comprised of only about 100 neurons. The brain vesicle contains two sensory organs, a gravity sense organ (otolith) and an eyespot (ocellus).
The origin of vertebrate retina was studied using ascidian larva. Morphology of ascidians retina were visualized with eye specific antibodies. We found 34 photoreceptors were directly projected to the brain vesicles. In the vertebrate retina, ON and OFF–responses take place at the bipolar cells. It is expected that ascidian photoreceptors could show ON and OFF–response to light.
Morphology of ascidians retina was characterized with the eye specific antibodies, rhodopsin, arrestin and several visual cycle proteins. Transmitters of photoreceptors were characterized with anti–VGLUT antibody and anti–VGAT antibody. Light–evoked responses recorded from the ascidian photoreceptors with electrophysiological method. Swimming behavior of photic response were analyzed with Motion analysis software.
Immunohistochemical studies showed there are three different groups of photoreceptors in the brain vesicle. Group I and Group II located in the ocellus and Group III located closed to otolith. Laser ablation experiments showed that Group I and Group II photoreceptors are responsible for the photic swimming behavior of the larvae. Immunohistochemical studies showed that two third of photoreceptors in the ocellus contain glutamate as neurotransmitters and one third of the photoreceptors contains GABA as neurotransmitters. Electrophysiological experiments show there are two different light–evoked responses. One group of the photoreceptors was hyperpolarized with actinic light, and the other group of the photoreceptors was depolarized with actinic light.
Though the vertebrate retina shows ON–type response and OFF–type response at bipolar cells, ascidian larva show ON–type response and OFF–type response at the photoreceptors cells.
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