THE DAPI-3 AMACRINE CELLS OF THE RABBIT RETINA

In the rabbit retina, the nuclear dye, 4,6,diamidino-2-phenylindole (D API), selectively labels a third type of amacrine cell, in addition to the previously characterized type a and type b cholinergic amacrine c ells. In this study, these ''DAPI-3'' amacrine cells have been charact erized with respect to their somatic distribution, dendritic morpholog y, and neurotransmitter content by combining intracellular injection o f biotinylated tracers with wholemount immunocytochemistry. There are about 100,000 DAPI-3 amacrine cells in total, accounting for 2% of all amacrine cells in the rabbit retina, and their cell density ranges fr om about 130 cells/mm(2) in far-peripheral retina to 770 cells/mm(2) i n the visual streak. The thin varicose dendrites of the DAPI-3 amacrin e cells form a convoluted dendritic tree that is symmetrically bistrat ified in S1/S2 and S4 of the inner plexiform layer. Tracer coupling sh ows that the DAPI-3 amacrine cells have a fivefold dendritic-field ove rlap in each sublamina, with the gaps in the arborization of each cell being occupied by dendrites from neighboring cells. The DAPI-3 amacri ne cells consistently show the strongest glycine immunoreactivity in t he rabbit retina and they also accumulate exogenous [H-3]-glycine to a high level. By contrast, the An amacrine cells, which are the best ch aracterized glycinergic cells in the retina, are amongst the most weak ly labelled of the glycine-immunopositive amacrine cells. The DAPI-3 a macrine cells costratify narrowly with the cholinergic amacrine cells and the On-Off direction-selective ganglion cells, suggesting that the y may play an important role in movement detection.