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M. R. Behrend, A. K. Ahuja, M. S. Humayun, R. H. Chow, J. D. Weiland; Threshold Mapping of Electrical Stimulation Using Calcium Indicators. Invest. Ophthalmol. Vis. Sci. 2008;49(13):5873. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate the stimulation thresholds of retinal ganglion cell (RGC) somata and axon bundles using an in vitro model for epiretinal prosthesis.
Isolated retina from larval tiger salamander was placed, ganglion-cell-side-down onto a transparent multielectrode array of indium-tin-oxide, having four 200-µm diameter electrodes. Forty-six smaller 10-µm electrodes are interspersed for stimulation and recording. RGCs were retrograde labeled in vitro with Oregon-Green-BAPTA-1-dextran 10,000 MW for 2 hours, before mounting. Trains of 400-µs biphasic pulses with varying amplitude were applied via Multichannel Systems STG2008 connected to a voltage-to-current converter. Epifluorescence images of intracellular calcium were captured by Andor Ixon camera on an inverted microscope. Extracellular recordings confirmed that the detected calcium transients were correlated with neural activity. Fluorescence intensities of soma and axon regions were extracted with Metamorph and post processed in Matlab by correlation analysis and fitting the response profile to a sigmoid.
Thresholds of RGC somata (n=189) varied with distance from center of the 200-µm electrode as follows: within 40 µm -5.2 +/- 0.3 µA (SE); 80-100 µm -6.8 +/- 0.7 µA; 100-120 µm -11 +/- 1.6 µA; 160-200 µm -12.6 +/- 2 µA. Somata >200 µm from the center, threshold measurements were influenced by antidromic stimulation. Axon bundle (n=21) threshold varied with displacement of the axons from the center of the electrode as follows: less than 40 µm -5.9 +/- 1.5µA; 40-80 µm -11 +/- 2.6µA; 80-120µm -14 +/- 5µA; 250-300 µm, -20 +/- 2.7µA.
Axonal stimulation thresholds are similar to those values for somatic stimulation of retinal ganglion cells, but depend on the displacement of the axon from the center of the electrode. Axons crossing the center of the 200-µm electrode have nearly the same threshold value as RGC soma on the center of the electrode. Axons passing tangent to the electrode are stimulated at amplitudes higher than soma in this region. Antidromic stimulation of RGCs was observed along the path of axon bundles crossing the stimulating electrode.
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