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Todd Lynn Stincic, William Rowland Taylor; Time-course of EPSCs in On-type starburst amacrine cells is independent of dendritic location. Invest. Ophthalmol. Vis. Sci. 2016;57(12):2753.
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© ARVO (1962-2015); The Authors (2016-present)
Direction-selective ganglion cells compute the direction of motion on the retina. Direction-selectivity depends on directional inhibitory GABAergic transmission from starburst amacrine cells (SBACs). A contrast edge moving outward from the soma to the dendritic tips of SBACs produces greater release than the same stimulus moving inwards, but underlying biophysical mechanisms remain unclear. A high-resolution anatomical reconstruction of OFF SBACs in the mouse retina suggested directional release of GABA results from an asymmetric arrangement of bipolar cell inputs (Kim et al., 2014). A transient bipolar cell (BC3a) tended to make input preferentially at dendritic tips, while a delayed, more sustained cell (BC2) innervated proximal dendrites. Directionally asymmetric superposition of these inputs to OFF SBACs was proposed to generate directionally asymmetric depolarization of the dendritic tips. Here we test the hypothesis, that a similar arrangement generates directional signals in ON SBACs. The prediction is that EPSCs should be sustained at proximal locations, and more transient at distal locations.
Voltage clamp recordings were performed on flat-mounted mouse retinas at 34°C. The extent of the excitatory receptive fields were mapped by recording excitatory postsynaptic currents (EPSCs) in response to 50μm light bars. The time-course of EPSCs as a function of distance from the soma was assessed by recording EPSCs in response to 50μm wide annuli. EPSCs were recorded for a series of annuli with diameters increasing in 50μm steps.
Annuli within the RF centre produced sustained EPSCs in ON SBACs, which became progressively smaller as more distal inputs were activated. Peak EPSC amplitude at the soma was 148±21 pA and 45±8 pA at radii of 175 and 275 (n=14). Responses were inverted in an antagonistic surround (annulus diameter > 375), where the light stimulus suppressed a tonic inward current. After normalizing for amplitude, the time-course of the EPSCs within the centre were essentially identical regardless of distance.
The time-course of light-evoked EPSCs in ON SBACs are uniform as a function of distance from the soma. Dendritic filtering appeared to reduce the amplitude of the EPSCs, but had little effect on the rise-time or decay. The results suggest that asymmetries in EPSC time-course are an unlikely explanation for directional release of GABA from ON SBACs.
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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