Abstract
Purpose: :
Bipolar cells (BCs) convey luminance, spatial and color information from photoreceptors to amacrine and ganglion cells. As many as seventeen types of BCs exist in the adult zebrafish retina (Connaughton et al., 2004). In this study, we describe the connections of several BC types with the four types of cone photoreceptors found in the zebrafish retina.
Methods: :
Pulled glass electrode tips loaded with DiI crystals were inserted and broken off into the whole-mounted retinas of SWS1-GFP transgenic zebrafish (Takechi et al., 2003; Li et al., 2009). BCs were fully labeled by DiI within two weeks. Serial confocal images of BCs were taken and the dendrites, their terminals, somata and axons analyzed.
Results: :
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). Using the SWS1-GFP retinas whose UV cones are GFP positive, we are able to identify the positions of the four types of cones. BCs, identified by their morphology, were observed to make R,G; R,G,B; G,B,UV; G and UV cone connections. Many R,G and R,G,B BCs have rods input as well, whereas G,B,UV; G and UV BCs have only cones input. Certain cone-specific BCs have very small dendritic spreads; in contrast, G specific BCs have very large dendritic fields. Based on their axon terminal stratification, the BCs could be further sub-divided into ON-, OFF-, and ON/OFF-BCs.
Conclusions: :
BCs connect specifically to certain type(s) of photoreceptors in the zebrafish retina. Especially, R,G; R,G,B; G and UV specific BCs were observed, suggesting complex chromatic information transmission by bipolar cells in the zebrafish retina.
Keywords: bipolar cells • photoreceptors • retinal connections, networks, circuitry