DISTRIBUTION AND COVERAGE OF A-TYPE AND B-TYPE HORIZONTAL CELLS STAINED WITH NEUROBIOTIN IN THE RABBIT RETINA

Both A- and B-type horizontal cells in the rabbit retina were labeled by brief in vitro incubations of the isolated retina in the blue fluor escent dye 4,6-diamino-2-phenylindole. Intracellular injection of Luci fer Yellow into the somata revealed the morphology of the individual c ells. Dye-coupling with Lucifer Yellow was seen only between A-type ho rizontal cells. By contrast, injection of the tracer Neurobiotin showe d dye-coupling between both A- and B-type horizontal cells. There also appeared to be coupling between the axon terminals of B-type horizont al cells. The extensive dye-coupling seen following injection of Neuro biotin into a single horizontal cell soma can be used to obtain popula tion counts of each cell type. Staining of large numbers of each cell type across the retina showed that each type increased in number and d eclined in dendritic diameter as the visual streak was approached, suc h that relatively constant coverage across the retina was maintained. In the visual streak, A-type horizontal cells numbered 555 cells/mm(2) and averaged 120 mu m in diameter, compared to 1375 cells/mm(2) and 1 00 mu m for B-type horizontal cells. In the periphery, the A- and B-ty pes numbered 250 cells/mm(2) and 400 cells/mm(2), respectively. The av erage diameters of the dendritic trees at these locations were 225 mu m for the A-type and 175 mu m for the B-type. Coverage across the reti na averaged almost six for A-type horizontal cells and 8-10 for the B- type. A-type horizontal cells in the visual streak whose elliptical de ndritic fields were shown by Bloomfield (1992) to correlate physiologi cally with orientation bias were shown to be dye-coupled to cells with symmetrical dendritic fields.