Chromatic Properties of Bipolar Cells in the Mouse Retina

The retina performs a wide range of computations to process visual signals. Feature extrac-tion, such as the detection of edges, motion, and color originate in specialized retinal circuits. In this study we investigated the circuits underlying chromatic processing in the mouse retina. Although color vision is wide spread among mammals, its research tends to focus on primates. Studying non-primate mammals can be advantageous in understanding the general principles of retinal chromatic processing. Like most mammals, mice feature dichromatic color vision based on short (S) and medium (M) wavelength-sensitive cone types. It is thought that mammals share a common retinal circuit that compares S- and M-cone output (in trichromats S- and M+L-cone) to generate blue/green (blue/yellow) opponent signals, with distinct bipolar cells providing separate chromatic channels. While S cone selective ON-bipolar cells (in mouse “type 9”) have been anatomi-cally identified, little is known about other cone selective channels, such as, for instance, M-cone selective OFF-bipolar cells. Here, we characterized cone connectivity and light responses of selected mouse bipolar cell types using immunohistochemical and electrophysiological methods. Our anatomical data indicate that four of the five mouse OFF-bipolar cell types (types 2, 3a/b and 4) as well as type 7 (as an example for ON-bipolar cells) indiscriminately contact both S- and M-cones. Using a marker that labels dendrites of both type-1 and -2 OFF-bipolar cells we found reduced immunofluorescence at S-cone, suggesting that type 1 avoids S-cones. Recordings of light responses showed that the chromatic tuning of bipolar cells strongly depended on their position along the dorso-ventral axis – due to the dorso-ventral gradient in S-opsin co-expression in mouse M-cones. In dorsal retina, where co-expression is low, most type-2 (and type-7) cells were green-biased, with a fraction of cells (≈ 14 %) displaying strongly blue-biased responses, likely reflecting S-cone input. Type 1 cells were also green biased but did not include blue-biased “outliers”, consistent with type-1 cells avoiding S-cones. We therefore suggest that type 1 represents the greenOFF pathway in mouse. In addition, we confirmed that type-9 bipolar cells display blueON responses. In ventral retina, all bipolar cell types studied here displayed similar blue-biased responses, suggesting that color vision may be only supported in the dorsal mouse retina. In conclusion, our data supports an antagonistically organized blue/green circuit with bipolar cells functioning as chromatically defined channels, which form the common basis for mammalian dichromatic color vision