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Boshuo Wang, James D Weiland; Electrical Impedance of wild-type and rd1 Mouse Retina. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):770.
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© ARVO (1962-2015); The Authors (2016-present)
Electrical impedance of the retina is a critical factor in retinal prostheses, determining the intraretinal current flow and potential distribution of electrical stimulation. This experimental study measured the resistivity profiles of wild type and rd1 mice, providing basis for computational simulations and predictive modeling studies.
Female wild type and rd1 mice (Jackson Lab) were euthanized at ca. 12-14 wks postnatal. After enucleation of the eye, the retina was dissected in Ames solution and placed on top of agar in a custom-made recording chamber with the ganglion cell side up. A bipolar microelectrode with a pencil-tip profile (FHC, Inc.) was used for impedance measurement. The inner pole of Pt/Ir is 25 microns in diameter and height. A microcontroller lowered the electrode into the saline, approaching the retina and penetrating through the tissue. Impedance spectra were recorded at regular intervals depth and more frequently when impedance began to increase when the tip neared and entered the retina. The Peak Resistance Frequency (PRF) method was used to extract the resistance of the tissue and/or electrolyte, and the converted to resistivity using the cell constant of the electrode. Measurement was terminated when the impedance decreased to baseline as the electrode reached the agar beneath the retina.
The wild type retina profile showed an increase in resistivity starting from the retina ganglion cell side inward, from 1.4±0.4 Ω●m to a peak resistivity of 4.8±0.4 Ω●m, and then decreasing down to initial values on the photoreceptor side. The peak is located at about 75% thickness, which is consistent with literature data on rat and embryonic chick retina. Compared to the wild type retina, the rd1 resistivity profile is thinner by about 80 μm, as expected due to degeneration. The peak resistivity is lower (3.4±0.6 Ω●m), but still in the nominal range of neural tissue, while its location is at about 60% thickness.
Peak retinal resistivity decreased as retina thinned due to degeneration. The apparent resistivity determined with the above-described method is on a “mesoscopic” scale comparable to the electrode size. Therefore it was mostly influenced by geometric factors, while tissue structure at the cellular level, e.g. retinal remodeling and glial hypertrophy, could not be detected. Measurements with higher spatial resolution will be needed to assess the impact of these phenomena.
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