The Primordial, Blue Cone Color System of the Mouse Retina

Abstract

Abstract: : Purpose: Primates have trichromatic color vision based on L–, M– and S–cones. All other placental mammals are dichromates and their color vision depends on the comparison of L– and S–cone signals, however, their cone selective wiring is still unknown. Here we identified the S–cone selective (blue cone) bipolar cells of the mouse retina. Methods: Blue cone bipolar cells were labeled in a transgenic mouse expressing Clomeleon, a chloride–sensitive fluorescent protein under the control of the thy1 promoter. They were studied in sections and whole mounts of the mouse retina immunostained for the different cone opsins and further cone–specific proteins. Results: Blue cone bipolar cells comprise only 1–2% of the bipolar cell population, their axons descend deep into the inner plexiform layer and their long, meandering dendrites selectively contact S–opsin expressing cones. Thus, mouse blue cone bipolar cells are very similar to monkey blue cone bipolar cells. They contact between 1 and 5 S–cone pedicles (convergence). Individual S–cone pedicles are contacted by dendrites of 1 to 5 blue cone bipolar cells (divergence). In the dorsal half of the mouse retina only 3–5% of the cones express S–opsin and they are all contacted by blue cone bipolar cells, while all L–opsin expressing cones (approx. 95%) are avoided. In the ventral mouse retina the great majority of cones express both S– and L–opsin. They are not contacted by blue cone bipolar cells. A minority of ventral cones express S–opsin only and they are selectively contacted by blue cone bipolar cells. We suggest that these are true blue cones. In contrast to the other cones, their pedicles do not contain cone arrestin and thus represent a distinct type. Conclusions: The blue cone bipolar cells of the mouse retina and their cone selectivity are closely similar to primate blue cone bipolars and we suggest that they both represent the phylogenetically ancient color system of the mammalian retina.

Keywords: bipolar cells • color vision • retinal connections, networks, circuitry