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W Li, E Trexler, J Keung, SC Massey; Anatomical Evidence of Rod Input to Cone Bipolar Cells in the Rabbit Retina . Invest. Ophthalmol. Vis. Sci. 2002;43(13):740.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: In the mammalian retina, starting from photoreceptors, visual signals split into rod and cone pathways, which share some common bipolar neurons and then merge into the same sets of ganglion cells. In the rod pathway, there is only one type of second-order neuron, the rod bipolar cell. But rod signals may circumvent the rod bipolar cell and reach cone pathways via gap junctions between rods and cones. Thus rod/cone coupling accounts for an alternative rod pathway. However, rod signals can apparently bypass the rod bipolar cell in the "coneless" transgenic mouse, which suggested the possibility of direct connections between rods and cone bipolar cells (Soucy, et al., 1998). This has been confirmed by EM studies in normal rodent retina (Tsukamoto, et al., 2001; Heck, et al., 1999). But is this a common pathway for other mammals (Bloomfield and Dacheux, 2001)? This study was designed to search for anatomical evidence of direct contact between rods and cone bipolar cells in the rabbit retina. Methods: Different types of cone bipolar cells were dye-injected in DAPI-incubated isolated rabbit retina. The tissues were then labeled with antibodies to mgluR6 and synaptic proteins (Synaptophysin and SV2C) for triple-label analysis with the confocal microscope. Results: The mgluR6 antibody revealed clusters of ON cone bipolar dendritic tips and doublets of rod bipolar dendritic tips, which give a reliable index of rod vs. cone terminals. Antibodies to synaptic proteins outlined both rod spherules and cone pedicles. Analysis of the spatial relationship between the dendritic tips of dye-injected cone bipolar cells and signals from the above two antibodies showed some OFF cone bipolar cell dendrites contact rod spherules in addition to cone pedicles. The absence of mgluR6 signals on the bipolar dendritic tips indicates they are OFF cone bipolar cells. Reconstruction of the bipolar cells will be used to identify the OFF bipolar cell sub-types. Conclusion: In rabbit retina, certain OFF cone bipolar cells receive mixed input from both cones and rods. This alternative rod pathway may be a common feature of the mammalian retina.
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