To further understand the synaptic connections in the outer plexiform layer (OPL) of the cone-dominated ground squirrel retina, we used Serial Block-face Scanning Electron microscopy (SBEM) to acquire high resolution, large scale EM image series to obtain three-dimensional reconstructions of the complete S-cone connectome. One specific question we attempted to answer is whether there exists an S-cone specific OFF cone bipolar cell as suggested by a recent publication on the rabbit retina. In a patch of retina that contains 20 cones, we identified 2 S-cones and reconstructed the complete synaptic output of these two cone pedicles. We found that 1) the majority of the synaptic contacts at S-cone terminals are invaginating ON type contacts; 2) mostif not allof the ON bipolar dendrites that make invaginating synapses at S-cone pedicles belong to S-cone ON bipolar cells; and 3) a small number of dendrites, which do not belong to S-cone ON bipolar cells make either invaginating or flat synaptic contacts with S-cones. However, these processes are always found to also make contacts with neighbouring M-cones, thus they are NOT S-cone selective. We thus conclude that there are no S-cone specific OFF cone bipolar cells in the ground squirrel retina and that the S-cone signal is likely conveyed by S-cone ON bipolar cells exclusively. To further corroborate this conclusion, we performed ERG experiments to probe OFF cone bipolar cell contributions to S-cone signaling pathways. Using a 1-s long pulse of white or green light stimuli that activates both S- and M-cones, we found that the ground squirrel photopic ERG shows a prominent a-wave and d-wave, both of which are thought to originate from OFF cone bipolar cells. However, when we used a blue/green light pulse, which minimally activates M-cones while strongly activating S-cones, the a- and d-waves were almost completed abolished, indicating minimal, if any, input from OFF cone bipolar cells. We conclude that in the ground squirrel retina, the S-cone to bipolar synapse is highly selective, with the S-cone ON bipolar cell as its sole synaptic target. This work lays a foundation for our future work on identifying genetic/molecular mechanisms for such specific and precise synaptic targeting of S-cones.
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