Morphologic identification of the OFF-type blue cone bipolar cell in the rabbit retina

Purpose. Bipolar cells play major roles in transmitting visual signals from photoreceptors to ganglion cells and can be subdivided into at least 10 to 13 distinct types based on their morphology and physiology. This study aimed to morphologically identify the blue cone bipolar cells responsible for transmitting color signals in the rabbit retina. Methods. To find this cell type, bipolar cells were injected with a neuroanatomic tracer in the whole-mount rabbit retina and were subsequently labeled with peanut agglutinin and S-cone opsin to verify their cone selectivity. Results. Results indicated that all narrow- and medium-field bipolar cells showed no selectivity for cone type. Among widefield bipolar cells, one type made exclusive contact with Scones and was identified to be a blue cone bipolar cell. This cell type gave rise to four to five branchless primary dendrites that specifically contacted S-cone pedicles and their axons ramified in sublamina a of the inner plexiform layer. Conclusions. In addition to the ON-type blue cone bipolar cell found in all mammalian retinas, the authors have identified the OFF-type blue cone bipolar cell in this study. Therefore, both ON and OFF blue cone bipolar cells are responsible for color information transmission in the rabbit retina.