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Andrew Chervenak, Krystal Harrison, Kwoon Y Wong; Characterization of ipRGC-coupled amacrine cells. Invest. Ophthalmol. Vis. Sci. 2016;57(12):4668.
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© ARVO (1962-2015); The Authors (2016-present)
We previously showed that intrinsically photosensitive retinal ganglion cells (ipRGC) transmit their tonic photoresponses to three morphological classes of amacrine cells (AC) via gap junctions (Reifler et al. Curr Biol 2015). Here, we investigate the number and types of ACs coupled to each type of ipRGC.
We used sharp electrodes to inject the gap junction-permeable tracer Neurobiotin into GFP-labeled ipRGCs (Ecker et al. Neuron 2010) in isolated mouse retinas. Retinas were stained with streptavidin to identify injected cells and gap junction-coupled cells, as well as with antibodies against neuromodulators to identify specific types of AC. The number of cells coupled to each ipRGC was counted and the soma size for each coupled cell measured.
We injected all five well-known types of ipRGCs, M1-M5, as well as the recently discovered M6 type (Quattrochi et al. Soc Neurosci 2013). All six ipRGC types showed coupling to other cells. All these coupled cells’ somas were significantly smaller than the ipRGCs’, suggesting they were ACs. Although each ipRGC type had tracer-coupled ACs, coupled cells were not observed for every injected ipRGC. The percentages of injected ipRGCs that had coupled ACs were: M1, 25.0%; M2, 68.4%; M3, 57.9%; M4, 68.4%; M5, 85.7%; and M6, 66.7%. Coupled ACs were most often found in the ganglion cell layer (GCL), but occasionally were also found in the inner nuclear layer (INL). Within the GCL, coupled ACs exhibited different somatic morphologies. M1 had coupled ACs with the largest somas, while M2-coupled AC somas were the smallest. And ipRGCs with wider dendritic fields (M1, M2, M4) had more coupled ACs than ipRGCs with narrower dendritic fields (M3, M5, M6). Coupled ACs were immunonegative for ChAT and vasoactive intestinal peptide, but a few were positive for neuronal nitric oxide synthase (bNOS).
Müller et al. (J Comp Neurol 2010) previously found coupling between M1 – M3 ipRGCs with displaced ACs. We now report that all six types of ipRGCs show coupling to displaced ACs, and that a few INL ACs are also ipRGC-coupled. Different ipRGC types appear coupled to different types of displaced ACs with diverse soma sizes. Further, we have identified the first specific type of ipRGC-coupled AC, which contains bNOS. Gap-junction transmission from ipRGCs to ACs could trigger neuromodulator release from the latter, enabling ipRGCs to regulate retinal circuits.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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