CONE BIPOLAR CELLS AS INTERNEURONS IN THE ROD PATHWAY OF THE RABBIT RETINA

In the mammalian retina, rod signals are transmitted by rod bipolars t o the narrow-field, bistratified (AII) amacrine cell. This neuron, in turn, makes gap junctions with the axonal arborization of cone bipolar cells that reside in the vitreal half (sublamina b) of the inner plex iform layer (IPL). After examining rod bipolars and AII amacrines in t he rabbit retina, we have now reconstructed from electron micrographs of continuous series of thin sections the synaptic connections of the axonal arborizations of cone bipolar cells that make the highest numbe r of gap junctions with AII amacrines. These axonal arborizations were narrowly confined to stratum 4 (S4) of the IPL and made ribbon synaps es to dyads of postsynaptic dendrites that belonged to either ganglion or amacrine cells. In the population of postsynaptic processes, 30% w ere ganglion cell dendrites. These dendrites were probably originating , at least in part, from on-center ganglion cells because their course was confined to sublamina b of the IPL. Of the remaining postsynaptic processes, 51.7% belonged to amacrine cells and 18.3% were not identi fied. Among the postsynaptic amacrine cell processes, 33.3% returned a reciprocal synapse onto the cone bipolar endings. These reciprocal sy napses represented 21.3% of the total input onto the axonal arborizati ons, the remaining fraction (78.7%) arising from a heterogeneous popul ation of amacrine dendrites that were purely presynaptic to the cone b ipolars endings. Pre- and postsynaptic amacrines were part of several distinct microcircuits which suggest complex local processing of both rod and cone signals. Thus, the cone bipolars that make gap junctions with AII amacrines in sublamina b of the rabbit IPL exhibit a substant ial output onto ganglion cells. This fact, in conjunction with our pre vious observations that in this sublamina ganglion cells receive negli gible input from rod bipolars and AII amacrines, demonstrates that in the rabbit cone bipolars represent a necessary link in the pathway fol lowed by rod signals to enter on-center ganglion cells. Thus, rod and cone signals ultimately share the same integrating mechanisms and conv erge onto the same set of ganglion cells. (C) 1994 Wiley-Liss, Inc.