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Y. N. Li, J. I. Matsui, J. E. Dowling; Selectivity in the Horizontal Cell-Photoreceptor Connections in the Zebrafish Retina. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1042.
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Horizontal cells (HCs) are inhibitory neurons involved in establishing the center-surround receptive field organization of bipolar cells. In a number of species, HCs respond differentially to colors and are believed to play a role in color vision. An earlier study suggested four types of HCs exist in the adult zebrafish retina: three cone HCs (H1, H2 and H3) and one rod HC (Song et al., 2008). In this study, we describe the connections of the HCs with each of the five types of photoreceptors found in the zebrafish retina.
Pulled glass electrode tips loaded with DiI crystals were inserted and broken off into the whole-mounted retinas of wild-type AB-strain or SWS1-GFP transgenic zebrafish (Takechi et al., 1993). HCs were fully labeled by DiI within a week. Serial confocal images of the HCs were taken and the dendrites and their terminals analyzed.
Cones are arranged in a precise mosaic in the zebrafish retina: rows of alternating blue- (B) and ultraviolet-sensitive (UV) single cones alternate with rows of red- (R) and green-sensitive (G) double cones; G cones are adjacent to UV cones, and B cones adjacent to R cones (Robinson et al., 1993). Four types of HCs were observed: two small field cells (H1 and H2) and two large field cells (H3 and rod HC). Dendritic terminals of cone HCs connected to cones with single boutons, doublets, or rosettes made of several boutons, whereas those of rod HCs connected to rods with only single boutons. The single boutons/doublets/rosettes of cone HCs arranged into distinct patterns: double rows separated by single rows for H1 cells; pairs and singles for H2 cells; and rectilinear patterns for H3 cells, indicating their connections to cones were unique and selective. Referring to the cone mosaic and using the SWS1-GFP retinas whose UV cones are GFP positive, the HC cone connections were confirmed as follows: H1: R, G and B; H2: G, B and UV; H3: B and UV. About 20 rod HCs were clearly observed; each was adjacent to the soma or axon of a DiI-labeled cone HC and connected to 50-200 rods.
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