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 [1-8], providing the basis for the functional diversity of retinal ganglion cells (RGCs) [9]. BCs differing in their temporal properties appear to project to different strata of the retina's inner synaptic layer [10, 11], based on somatic recordings of BCs [1, 2, 4, 12-14] and excitatory synaptic currents measured in RGCs [10]. However, postsynaptic currents in RGCs are influenced by dendritic morphology [15, 16] and receptor types [17], and the BC signal can be transformed at the axon terminals both through interactions with amacrine cells [18, 19] and through the generation of all-or-nothing spikes [20-24]. 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 [25, 26] 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.