The Department of Biomedicine
BBB seminar

BBB seminar: Timm Schubert

Synaptic integration in neurons of the outer retina: different synaptic sites for global feedforward and local feedback signaling

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Timm Schubert
Institute for Ophthalmic Research and Centre for Integrative Neuroscience, University of Tübingen, Germany

In the outer plexiform layer of the mammalian retina, two or three types of cone photoreceptors (cones) provide input to more than a dozen types of cone bipolar cells. In the mouse, this transmission is modulated by a single type of horizontal cell, the only interneuron in the outer retina. Horizontal cells form feedback synapses with cones and feedforward synapses with cone bipolar cells. However, the exact computational role of horizontal cells is still debated. Along with performing global signaling within their laterally coupled network, horizontal cells also provide local, cone-specific feedback. Specifically, it is not known, how horizontal cells provide local feedback to cones at the same time that they provide global feedforward signaling to cone bipolar cells and where the underlying synapses are located.

In this study we performed digital reconstructions within a serial block-face electron microscopy volume of the dendritic trees of five horizontal cells as well as the cone axon terminals and the cone bipolar cell dendrites to quantitatively analyze their connectivity. In addition to the fine horizontal cell dendritic tips invaginating cone axon terminals, we also identified “bulbs”, short segments of increased dendritic diameter on the primary dendrites of horizontal cells. These bulbs are located in a different stratum of the outer plexiform layer, well below the cone axon terminal base and make contact to other horizontal cells and cone bipolar cells. Taken together with results from immunolabeling of GABAergic synapses, glutamate and voltage imaging, and a biophysical-based computational model, our data suggest that the outer plexiform layer of the retina, rather than being homogeneous in its connectivity, can be divided into two synaptic strata that spatially separate cone-specific feedback and feedforward signaling to cone bipolar cells.

Chairperson: Meg Veruki, Department of Biomedicine