Regular Mosaics of Gap Junctionally Connected Bipolar Cells, Amacrine Cells and Ganglion Cells in the Color-Coded Retina of Japanese Dace(Issue Commemorating the Directorship of Prof. Yoko HASHIMOTO, at the Department of Physiology)

Harosi and Hashimoto (1983) demonstrated the presence of four types of color-sensitive cone photoreceptor cells (i.e. red-, green-, blue- and ultraviolet-sensitive cones) in the retina of Japanese dace (Tribolodon hakonensis), based on the absorption spectra of their outer segments with microspectrophotometry. The results suggest that well-developed color discrimination is processed in the teleost. The four types of cone photoreceptors are distributed in a regular row mosaic where rows of double cones and single cones are mutually arranged in the peripheral retina (Maehara et al., 1997) . In the present study, an attempt was made to investigate green-sensitive cones in the dace retina, using a specific monoclonal antibody against turtle cone opsin protein (Gaur et al., 1988; Ohtsuka and Kawamata, 1990). Two types of cones were immunopositive: one was the short member of the double cones, which was mutually arranged in the row of double cones, and the other was a single cone which was distributed in the row of single cones and between immunonegative long member of the double cones. These results suggest that the morphological mosaics of cones represent a functional arrangement for the absorption of color signals in cones, in order to perform color discrimination in the dace retina. Color signals of cones are vertically conveyed via retinal bipolar cells to retinal ganglion cells. Retinal amacrine cells laterally modify the excitatory vertical pathway in the inner plexiform layer. The present study was further taken to analyze the distribution of bipolar cells, amacrine cells and ganglion cells of the dace with relation to the regular row arrangement of cone photoreceptors. Homotypic gap junctions between these retinal cells were identified by intracellular injection of biotinylated compound, biocytin or Neurobiotin and electron microscopy. The present study demonstrated that gap junctionally connected retinal neurons are distributed in a regular mosaic. Quantitative analysis of cell distribution showed that OFF-center bipolar cells were more regularly arranged than ON-center cells. Three types of tip-contact amacrine cells: transient ON-OFF, sustained depolarizing and sustained hyperpolarizing, possessed regular mosaics arranged hexagonally, while two types of cross-contact amacrine cells: sustained depolarizing and sustained hyperpolarizing, were distributed in another regular mosaic. Three types of alpha-like large ganglion cells: sustained center-ON, sustained center-OFF and transient ON-OFF, were regularly distributed in a hexagonal array. These results show that bipolar cells, amacrine cells and ganglion cells of the dace are regularly arranged so that spatial information in the form of visual signals can be relayed from color-sensitive cone photoreceptors to visual centers of the brain