Costratification of a population of bipolar cells with the direction- selective circuitry of the rabbit retina

We have stained a new population of bipolar cells in rabbit retina by using antibodies against the carbohydrate epitope, CD 15. The CD 15-positive bipolar cells comprise 6-8% of the total cone bipolar cells in peripheral retina. Their axonal and dendritic arbors are similar in size and range from 15 to 50 μm in diameter. The axonal arbors are narrowly stratified in sublamina b of the inner plexiform layer. Double label experiments using an antibody against the calcium binding protein, calbindin, or an antibody against protein kinase C, demonstrate that the CD15-positive bipolar cells are a separate population from the previously identified calbindin-positive cone bipolar cells and the rod bipolar cells. Labeling the processes of starburst amacrine cells with antibodies against choline acetyltransferase showed that the CD15-positive bipolar cells stratify within and slightly more distally to the processes of the ON-starburst amacrine cells. Confocal images of retinal wholemounts showed that the axons of the CD15-positive bipolar cells follow the pattern of the ON-starburst cells' processes. Axonal varicosities of the CD15-positive bipolar cells penetrate the bundles formed by the processes of the ON-starburst cells. This finding suggests that the CD 15-positive bipolar cell provides input to the ON-starburst amacrine cells and/or the ON-plexus of the ON-OFF direction-selective ganglion cells.