Connectomics of synaptic microcircuits: lessons from the outer retina

Abstract

Photoreceptors form a sophisticated synaptic complex with bipolar and horizontal cells, transmitting the signals generated by the phototransduction cascade to downstream retinal circuitry. The cone photoreceptor synapse shows several characteristic anatomical connectivity motifs that shape signal transfer: typically, ON-cone bipolar cells receive photoreceptor input through invaginating synapses; OFF-cone bipolar cells form basal synapses with photoreceptors. Both ON- and OFF-cone bipolar cells are believed to sample from all cone photoreceptors within their dendritic span. Electron microscopy and immunolabelling studies have established the robustness of these motifs, but have been limited by trade-offs in sample size and spatial resolution, respectively, constraining precise quantitative investigation to a few individual cells. 3D-serial electron microscopy overcomes these limitations and has permitted complete sets of neurons to be reconstructed over a comparatively large section of retinal tissue. Although the published mouse dataset lacks labels for synaptic structures, the characteristic anatomical motifs at the photoreceptor synapse can be exploited to identify putative synaptic contacts, which has enabled the development of a quantitative description of outer retinal connectivity. This revealed unexpected exceptions to classical motifs, including substantial interaction between rod and cone pathways at the photoreceptor synapse, sparse photoreceptor sampling and atypical contacts. Here, we summarize what was learned from this study in a more general context: we consider both the implications and limitations of the study and identify promising avenues for future research.

Keywords: bipolar cell; connectomics; electron microscopy; photoreceptor; retina; synapse.

Figure 1. Connectivity of cone photoreceptors and cone bipolar cells

A, vertical view of the mouse retina with five classes of retinal neurons organized by stratification. B, quantitative connectivity map between cones, rods and bipolar cells (BCs). Photoreceptor types are shown on the left, and BC types are shown on the right. Connectivity is scaled to show proportion of BC input received from each photoreceptor type. C, cone axon terminal (cyan) and contacted postsynaptic cone BC dendrites (red, orange), demonstrating the divergence at the photoreceptor synapse. D, two ribbon synapses from a single mouse cone axon terminal (cyan) with postsynaptic partners: invaginating ON‐cone bipolar cells (CBCs) (red) and horizontal cells (grey). OFF‐CBCs (yellow, orange indicate dendrites form different cells) forming basal contacts. Note that only two cone ribbon synapses are shown, from the total set of 10. AC, amacrine cell; GC, ganglion cell; GCL, ganglion cell layer; HC, horizontal cell; INL, inner nuclear layer; IPL, inner plexiform layer; ONL, outer nuclear layer; OPL, outer plexiform layer.

Figure 2. Crossover contacts between cones and rod photoreceptors and bipolar cells

A, vertical electron microscopy image (top), 3D‐volume‐rendered cells (middle) and representative scheme (bottom) showing a cone axon terminal (cyan) with an invaginating rod bipolar cell (RBC) dendrite (red). B, vertical electron microscopy image (top), 3D‐volume‐rendered cells (middle) and representative scheme (bottom) showing a rod axon terminal (purple) with invaginating RBC dendrite (red) and OFF‐CBC making a basal contact (yellow). Scale bars: 1 μm.