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Robert Marc, Felix Vazquez-Chona, John Hoang, Crystal Sigulinsky, Carl Watt, Bryan Jones, James Anderson, J Scott Lauritzen; Pure feedforward amacrine cells. Invest. Ophthalmol. Vis. Sci. 2013;54(15):2503.
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© ARVO (1962-2015); The Authors (2016-present)
Pure feedforward amacrine cells may exist, but no proof has ever emerged. We mined 100 GABA+ ACs (γACs) and glycine+ ACs in rabbit retinal connectome RC1 (Anderson et al., MolVis, 2011) to query the existence and attributes of feedforward inhibitory elements.
AC, bipolar cell (BC), and ganglion cell (GC) processes were traced throughout the full volume of the ultrastructural rabbit retinal connectome RC1, annotated with the Viking viewer, and explored by 3D rendering and graph visualization of connectivity (Anderson et al. 2009 PLoS Biology; Anderson et al. 2011, J Microscopy; Anderson et al. 2011 MolVis). RC1 contains embedded small molecule signals for cell classification and has a resolution of 2 nm, enabling identification of all synapses and gap junctions. Gap junctions were validated by TEM re-imaging at 0.3 nm resolution.
We discovered a single class of γAC that is exclusively feedforward. The interstitial AC (IAC) is a GABA+ cell with its soma positioned in the middle of the inner plexiform layer (IPL) rather than the AC layer. One instance of the IAC is cell 9769 in RC1. Its soma is located at the edge of the 0.243 mm diameter volume and its dendrites span the volume, suggesting the full cell is over 0.5 mm in diameter. It receives input from virtually every ON cone BC (CBb) it encounters classes CBb3, CBb4, CBb5, CBb5w and CBbw. Cell 9769 is postsynaptic to 31 CBbs at 75 ribbon synapses and is presynaptic to none of the them. Cell 9769 is presynaptic to several wide-field ON cone feedback and cone-rod crossover γACs and two classes of ON GCs. It is also coupled to a large GABA+ GC. No other kinds of pure feedforward ACs have yet been found.
IACs are large cells that appear to copy the aggregate ON cone BC signal pool (via sign-inverting GABA synapses) onto a select set of ON driven targets, including two classes of retinal GCs. IACs also provide nested feedforward to γACs. IACs tile rather than cover the image plane, so they may effectively measure the size of looming background structures and provide a framework for spatial and temporal antagonism that can refine the effects of feedback γACs. They are also coupled to a distinct class of GABA+ GCs and may transfer a very large sign-conserving, but non-amplified ON signal.
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