Spikes in mammalian bipolar cells support temporal layering of the inner retina

In the mammalian retina, 10-12 different cone bipolar cell (BC) types decompose the photoreceptor signal into parallel channels, providing the basis for the functional diversity of retinal ganglion cells (RGCs). BCs differing in their temporal properties appear to project to different strata of the retina's inner synaptic layer, based on somatic recordings of BCs and excitatory synaptic currents measured in RGCs. However, postsynaptic currents in RGCs are influenced by dendritic morphology and receptor types, and the BC signal can be transformed at the axon terminals both through interactions with amacrine cells and through the generation of all-or-nothing spikes. Therefore, the temporal properties of the BC output have not been analyzed systematically across different types of mammalian BCs. We recorded calcium signals directly within axon terminals using two-photon imaging and show that BCs can be divided into ≥eight functional clusters. The temporal properties of the BC output were directly reflected in their anatomical organization within the retina's inner synaptic layer: faster cells stratified closer to the border between ON and OFF sublamina. Moreover, ≥three fastest groups generated clear all-or-nothing spikes. Therefore, the systematic projection pattern of BCs provides distinct temporal "building blocks" for the feature extracting circuits of the inner retina.