Distinctive synaptic structural motifs link excitatory retinal interneurons to diverse postsynaptic partner types

Neurons make converging and diverging synaptic connections with distinct partner types. Whether synapses involving separate partners demonstrate similar or distinct structural motifs is not yet well understood. We thus used serial electron microscopy in mouse retina to map output synapses of cone bipolar cells (CBCs) and compare their structural arrangements across bipolar types and postsynaptic partners. Three presynap-tic configurations emerge-single-ribbon, ribbonless, and multiribbon synapses. Each CBC type exploits these arrangements in a unique combination, a feature also found among rabbit ON CBCs. Though most synapses are dyads, monads and triads are also seen. Altogether, mouse CBCs exhibit at least six motifs, and each CBC type uses these in a stereotypic pattern. Moreover, synapses between CBCs and particular partner types appear biased toward certain motifs. Our observations reveal synaptic strategies that diversify the output within and across CBC types, potentially shaping the distinct functions of retinal microcircuits.

Automated summary

Neurons form converging and diverging synaptic connections with different partner types. The structural arrangements of synapses involving separate partners are still not well understood. In this study, mouse retina was used to map and compare the structural arrangements of cone bipolar cells (CBCs) synapses across different bipolar types and postsynaptic partners. The observations revealed that CBCs exhibit at least six motifs, with each CBC type using these motifs in a stereotypic pattern. The synapses between CBCs and specific partner types show a bias towards certain motifs. These findings provide insights into the diversification of synaptic output within and across CBC types.