Differential synaptic organization of GABAergic bipolar cells and non-GABAergic (glutamatergic) bipolar cells in the tiger salamander retina.

The synaptic organizations of gamma-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 ( approximately 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.