CONNECTIONS OF 2 TYPES OF FLAT CONE BIPOLARS IN THE RABBIT RETINA

The synaptic connections of two types of cone bipolar cells in the rab bit retina were studied with the electron microscope after labeling in vitro with 4',6-diamidino-2-phenylindole (DAPI), intracellular inject ion with Lucifer Yellow, and photooxidation (Mills and Massey [1992] J . Comp. Neurol, 321:133). Both types of bipolars belong to the flat va riety, because they make basal junctions with a group of four to ten n eighboring cone pedicles. One cell type has an axonal arborization tha t occupies strata 1 through 3 of the inner plexiform layer (IPL). At r ibbon synaptic junctions, it is presynaptic to ganglion cell dendrites and to reciprocal dendrites belonging to narrow-field bistratified (A II) amacrine cells. In addition, it contacts and is contacted by other amacrine cell processes of unknown origin, The other cell type has an axonal arborization entirely confined to stratum 2 of the IPL; it is pre- or postsynaptic to a pleomorphic population of amacrine cell proc esses, and, in particular, it receives input from the lobular appendag es of AII. Thus, these two bipolar types probably belong to the off-va riety because they make basal junctions with cone photoreceptors and s end their axon to sublamina alpha of the IPL, which is occupied by the dendrites of off-ganglion cells, They are also part of the rod pathwa y because they receive input from AII amarine cells. (C) 1996 Wiley-Li ss, Inc.