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X. Qiu, T. Ichinose, P. D. Lukasiewicz; GABAergic Surround Inhibition Differentially Modulates Distinct Types of Mouse Retinal Ganglion Cells. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1409.
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© ARVO (1962-2015); The Authors (2016-present)
The center-surround receptive field organization of retinal ganglion cells (GCs) is critical for extracting features of the visual scene. Previous studies suggest that inner plexiform layer (IPL) circuits, comprised of GABAergic amacrine cells, contribute to the surround organization of GCs. However, it is not known how these amacrine cell inputs affect the multiple subtypes of GCs. Here we measured the excitatory and inhibitory currents of GC layer neurons to determine how GABAergic inhibition contributes to morphologically distinct types of GCs.
Full-field light-evoked excitatory and inhibitory postsynaptic currents (L-EPSCs and L-IPSCs) were recorded from GC layer neurons, by voltage-clamping at -60 and 0 mV, respectively, in mouse retinal slice preparations. Strychnine was always present in the bath to block glycine receptors. The GABAA receptor-mediated L-IPSCs were identified by their sensitivity to bicuculline blockade. GCs were morphologically characterized by including sulforhodamine B in the recording pipette.
L- IPSCs in ON/OFF GCs were evoked both at the onset and the offset of light stimulation, similar to the pattern for L-EPSCs in these GCs. ON GCs, whose L-EPSCs were evoked only at light onset, were classified into 3 types, based on their L-IPSC properties. In the first class, L-IPSCs were evoked at light onset, similar to the L-EPSCs. In the second class, little or no GABA mediated L-IPSCs were evoked at either light onset or offset. The dendritic ramifications were comparable for both classes of ON GCs, stratifying in the inner IPL. In the third class of ON GCs, L-IPSCs were evoked at the light offset, the opposite of L-EPSCs at light onset. The dendrites of this class of ON GCs mainly ramified at the inner IPL, but also with a minor dendritic projection to the outer IPL. For all GC classes, we observed a general pattern; inhibition was transient for GCs stratifying in mid IPL strata near the ON and OFF border, and sustained for GCs stratifying in the innermost and outermost regions of the IPL.
We found that GABAAR-mediated amacrine cell inputs vary with GC type. The sign and the time course of the L-IPSCs correlated the IPL stratification of the GC dendrites. Similar to previous reports for L-EPSCs, our findings suggest that different layers of the IPL encode different forms of GABAergic L-IPSCs in GCs. Our findings also suggest that the contributions of GABAergic surround inhibition differ with ON GC class.
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