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, CD15. The CD15-positive bipol ar 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 t o 50 mu m in diameter. The axonal arbors are narrowly stratified in sublami na b of the inner plexiform layer. Double label experiments using an antibo dy against the calcium binding protein, calbindin, or an antibody against p rotein kinase C, demonstrate that the CD15-positive bipolar cells are a sep arate population from the previously identified calbindin-positive cone bip olar cells and the rod bipolar cells. Labeling the processes of starburst a macrine 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 re tinal wholemounts showed that the axons of the CD15-positive bipolar cells follow the pattern of the ON-starburst cells' processes. Axonal varicositie s of the CD15-positive bipolar cells penetrate the bundles formed by the pr ocesses of the ON-starburst cells. This finding suggests that the CD15-posi tive bipolar cell provides input to the ON-starburst amacrine cells and/or the ON-plexus of the ON-OFF direction-selective ganglion cells. (C) 1999 Wi ley-Liss, Inc.