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Crystal Sigulinsky, J Scott Lauritzen, John Hoang, Carl Watt, Bryan Jones, James Anderson, Shoeb Mohammed, Robert Marc; Sparse network principles of GABAergic amacrine cell heterocellular coupling. Invest. Ophthalmol. Vis. Sci. 2013;54(15):2509.
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© ARVO (1962-2015); The Authors (2016-present)
Heterocellular coupling between specific classes of ganglion cells (GCs) and GABA (γ) containing amacrine cells (γACs) in the mammalian retina is well established, but the actual gap junctions supporting these connections have never been visualized. Given the abundance of γ+ GCs in the mammalian retina, we sought to comprehensively map the spatial distributions and sizes of gap junctions associated with γACs and their presynaptic bipolar cell (BC) inputs or postsynaptic BC, AC and GC targets.
γAC networks in the ultrastructural rabbit retinal connectome RC1 were annotated with the Viking viewer, and explored by 3D rendering and graph visualization of connectivity (Anderson et al., 2011 J Microscopy). Small molecule signals embedded in RC1 combined with morphological reconstruction and connectivity analysis allow robust cell classification. Gap junctions were validated by TEM re-imaging with tilt at 0.3 nm resolution.
Spatially infrequent yet routine gap junctions exist between γACs and γ+ GCs. GC 606 is strongly γ+ with a dendritic arbor spanning beyond the 0.25 mm diameter of RC1. GC 606 is driven by at least 10 ON cone (CBb) BCs, each contributing 1-3 ribbon synapses. It also receives over 200 AC synapses, most from γACs. Within the RC1 volume, GC 606 makes only 11 identified gap junctions, three with interstitial γAC 9769, one with γAC 5451, and others with unidentified AC processes. These gap junctions range in size from 90-355 nm and are widely dispersed along the major GC dendrites. We further discovered sparse γAC coupling with OFF cone (CBa) BCs. Indeed only one CBa in a 150 μm patch of homocellularly coupled CBa cells has been found to couple heterocellularly to a γAC, and these coupled CBa cells are only weakly γ+.
Coupling instances (::) in heterocellular γAC::GC and γAC::CBa networks are sparse. We calculate that a single gap junction serves a GC receptive field area of ≈ 4500 μm2: a patch ≈ 35 μm in diameter, roughly 2 times larger than the area served by a single ribbon. GC 606 is large and one of the most strongly coupled cells based on γ signal strength, so GCs with weaker γ signals or smaller fields may display markedly fewer gap junctions. Assuming these gap junctions are electrophysiologically active, there may not be a 1:1 correlation between γACs and their coupling targets, as a single gap junction appears to control a patch of coupled CBa cells spanning ≈ 150 μm.
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