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F. R. Postma, J. Keung, D. Paul, S. C. Massey; Cone Telodendria Form the Substrate for Photoreceptor Coupling. Invest. Ophthalmol. Vis. Sci. 2010;51(13):2046.
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Connexin36 (Cx36) is abundantly expressed in cones photoreceptors (Feigenspan et al., 2004). It is presumed to be the major component of gap junctions between adjacent cones which are located at the tips of telodendria in cone pedicles. Here we report the use of several transgenic mouse lines designed to study the coupling between cone photoreceptors and visualize telodendria morphology.
We constructed transgenic mice in which the human red opsin promoter drives cone-specific expression of glycine transporter GlyT1. We also created a cone-specific Cx36 knockout and generated mouse lines in which eGFP is expressed specifically in blue cones or in green cones.
Immuno-labeling for GlyT1 revealed exogenous expression in almost every cone and that the cone pedicles are densely connected by a network of fine telodendria. Some of the telodendria extend upward towards the layer of rod spherules which are slightly above the pedicles. Double labeling for transporter and for Cx36 localizes Cx36 at the adjoining tips of telodendria from neighboring cones. In addition, Cx36 was detected in telodendria not in contact with other cones. Plausibly, these telodendria could be interacting with overlying rod spherules, consistent with the notion that rods and cones form gap junctions. However, double-labeling for GlyT1 and glycine revealed robust uptake of glycine in cones but no detectable signal in rods.To further explore possible junctional interactions between rods and cones, we performed RT-PCR on whole retina, dissected outer retina and individual rods and cones. Our results confirm that Cx36 is expressed by cones. However, rod photoreceptors express neither Cx36 nor any other known connexin. In the cone-Cx36KO, Cx36 labeling was abolished in the cone telodendria but present immediately beneath each cone pedicle, which we attribute to Cx36 expression in subsets of cone bipolar cells.
We interpret these results to show that the network of cone telodendria is the substrate for at least cone-cone photoreceptor coupling in the mouse retina. However we did not observe functional rod-cone coupling or find any connexins expressed in rods. The blue or green cone-eGFP mouse lines should allow us to determine whether Cx36 is associated with all cone photoreceptors or just green ones.
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