Abstract
The synaptic organizations of γ‐aminobutyric acid–immunoreactive (GABA‐IR, GABAergic) and non–GABA‐IR (non‐IR, glutamatergic) bipolar cells in salamander retina were compared by postembedding immunoelectron microscopy. A total of 238 presynaptic bipolar cell synapses were studied; 61 were GABA‐IR and 177 were non‐IR. Both groups were similar in that (1) they made asymmetrical ribbon synapses as well as asymmetrical non‐ribbon synapses; (2) they made ribbon synapses at dyads, triads, and monads; and (3) the vast majority of ribbon synapses (∼90%) were with dyads. The differences were that synapses of GABA‐IR bipolar cells had a higher proportion of (1) direct contact with ganglion cells, (2) non‐ribbon synapses, (3) output to GABA‐IR amacrine cells, and (4) output in sublamina a. Overall, the output of GABA‐IR ribbons was equally split between amacrine and ganglion cell processes, whereas for non‐IR ribbons, it was approximately 2:1 in favor of amacrine cells. The ribbon:non‐ribbon synapse ratio was approximately 1.2:1 (33:28) for GABA‐IR but approximately 2:1 (118:59) for non‐IR terminals. Thus, GABA‐IR bipolar cells made more direct contacts with ganglion cells and used a higher proportion of non‐ribbon synapses. GABA‐IR dyads were more likely to contact GABA‐IR amacrine profiles (52% vs. 38%). Finally, GABA‐IR ribbon synapses were more common in sublamina a than sublamina b (2:1), whereas non‐IR synapses were equally present in sublaminas a and b. This differential targeting of ganglion cells and amacrine cells in the OFF vs. ON layers indicates a difference in the role of bipolar cells in the generation of receptive field properties, depending on whether or not they use GABA as well as glutamate for their transmitter. J. Comp. Neurol. 455:187–197, 2003. © 2002 Wiley‐Liss, Inc.