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J.Scott Lauritzen, Bryan W. Jones, Carl B. Watt, Shoeb Mohammed, James R. Anderson, Robert E. Marc; Bipolar Cell "Accessory ON" Input to Amacrine Cells in the OFF Inner Plexiform Layer. Invest. Ophthalmol. Vis. Sci. 2011;52(14):4552.
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© ARVO (1962-2015); The Authors (2016-present)
Recent confocal studies report en passant ribbon synapses from inner plexiform layer (IPL) 4/5-stratifying ON Cone Bipolar Cells (ONCBCs) onto dopaminergic amacrine cells (DACs), intrinsically photosensitive ganglion cells (ipRGCs) and bi-stratified diving ganglion cells (bsdGCs) in the OFF IPL of mammalian retina, thus comprising an "accessory on" input to the OFF layer. Our goal was to identify additional bipolar cell (BC) types suspected to participate in this accessory ON input, confirm the presence of en passant synapses with transmission electron microscopy (TEM), and identify the targets of such synapses.
BC networks in the ultrastructural rabbit retinal connectome RC1 were annotated with the Viking viewer, and explored via 3D rendering and graph visualization of connectivity (Anderson et al. 2011 The Viking viewer for connectomics: scalable multi-user annotation and summarization of large volume data sets. J Microscopy: [doi:10.1111/j.1365-2818.2010.03402.x]). Small molecule signals embedded in RC1, e.g. 4-aminobutyrate (y), glycine (G), and L-glutamate (E), combined with morphological reconstruction allow for robust BC classification.
Glycine positive bipolar cells (GBCs) with CBb3, CBb5, and Wide-Field morphology (MacNeil et al., 2004 classification scheme) form en passant ribbon synapses in IPL OFF layers, targeting either GABAergic amacrine cells (yACs), glycinergic amacrine cells (GACs), or both. TEM ultrastructural examination confirms the presence of both pre- and post-synaptic components necessary for functional synapses at the observed locations. Additional en passant ribbon synapses were confirmed with as yet unidentified targets, which may be the DACs, ipRGCs, and bsdGCs reported in the previous literature. Thus our results are consistent with and extend previous observations.
Multiple ONCBC types violate the IPL stratification rules, demonstrating a richer accessory ON network architecture than previously reported. yAC and GAC recipients of en passant synapses in the OFF layer represent novel targets of this accessory ON input, and GAC targets emerge as intriguing candidates for mediating crossover inhibition via accessory ON pathways.
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