Morphology and the Distribution Pattern of the Triphasic Horizontal Cells in the Dace Retina(Issue Commemorating the Directorship of Prof. Yoko HASHIMOTO, at the Department of Physiology)

Three monophasic (RHC, L1HC, L2HC) and three chromaticity (biphasic, triphasic and tetraphasic) horizontal cells in the dace retina were classified by their spectral properties. Usually, horizontal cells are electrically coupled with cells of the same category. This study is to verify if triphasic hotizontal cells could be classified morphologically into sub-groups. The fish was dark-adapted for at least 1 hr, and the retina was isolated and put on dry filter paper (millipore filter), receptor side up. Microelectrodes, filled with 6% lucifer yellow (LY) in 0.5 M LiCl, were used to record photoresponses and to inject LY. The cells were viewed in the flat mounted retina under a fluorescent microscope. The results suggest as follows. 1) Two types of triphasic horizontal cells can be classified morphologically. The first type of cell had thin, short dendrites and round somas. In contrast, second type had rather thick, long dentrites with elliptical soma. In addition, other subtypes were found in the second type. 2) Both types of triphasic horizontal cells were connected with tip-contact dendritic gap junctions. Dye injected cells made an hexagonal array of contacts with the same type of horizontal cells, making a single layer of homotypic cells with the same orientation in the retina. This orientation was shown to be at right angles to the rows of cones; the same orientation was also found in the non-mosaic area of photoreceptors. As yet, the results do not suggest the functional role of this organization, but it may be related to the ecology of vision. Further study is required