Interaction between rod and cone inputs in mixed-input bipolar cells in goldfish retina.

One class of goldfish bipolar cells, the mixed-input bipolar cell, contacts both rods and cones. Although the morphology of the different mixed-input bipolar cell subtypes has been described, insight into the interaction between rods and cones at the bipolar cell level is scarce. The aim of this study was to characterize this interaction in the different physiological types of mixed-input bipolar cells. We found mixed-input bipolar cells that depolarized, hyperpolarized, or showed a combination of the two types of response after center stimulation. The relative contributions of rod and cone inputs varied strongly in these cell populations. Depolarizing mixed-input bipolar cells are rod-dominated, having the highest sensitivity and the smallest dynamic range. Hyperpolarizing mixed-input bipolar cells, on the other hand, have a more balanced rod-cone input ratio. This extends their dynamic range and decreases their sensitivity. Finally, opponent mixed-input bipolar cells seem to be mostly cone-dominated, although some rod input is present. The antagonistic photoreceptor inputs form a push-pull system that makes these mixed-input bipolar cells very sensitive to changes in light intensity. Our finding that spectral tuning changes with light intensity conflicts with the idea that the separate non-opponent and opponent channels are related to coding of brightness and color, respectively. The organization of mixed-input bipolar cells into various classes with different dynamic ranges and absolute sensitivities might be a strategy to transmit information about all visual aspects most efficiently, given the sustained nature of bipolar cell responses and their limited voltage range.