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T. Jehle, D. Kunze, K. Wingert, M. Bach, W. A. Largrèze; Modulation of Contrast Depth and Flash Frequencies: Effects on Visual Evoked Potentials Recorded in Awake, Freely Moving Brown Norway Rats. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3756.
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© ARVO (1962-2015); The Authors (2016-present)
The assessment of visual pathways can be achieved over time by visual evoked potential (VEP) recordings. The aim of the study was to establish a chronic implant model, which enables us to measure steady-state VEP from awake, freely moving rats. Stimulating the entire visual field has the advantage of being independent on refractive and attention. We wanted to test influences of anaesthetics, optic nerve crush and retinal ischemia.
Potentials were recorded in Brown-Norway rats by screw electrodes implanted epidurally over the visual cortex. Fourier analysis was used to extract the VEP from background EEG activity. Stimulation modalities comprised series of flashes with a frequency range of 1.8-38 Hz with luminance difference of 100% and series of 7.5 Hz-flicker with a modulation depth from 1%-80%, stimulating the entire visual field of the rat with a ganzfeld bowl. To evaluate anaesthetics, Isoflurane was applied vaporized in oxygen in concentrations of 0.2% to 1.0%. Choralhydrate 200 and 400 mg/kg and the combination of Ketamine 65 or 130 mg/kg and Xyaline at 7 or 14 mg/kg respectively were given intraperitoneally. To evaluate effects of retinal ganglion cell damage the optic nerve was crushed approximately 2 mm behind the eye after performing a partial orbitotomy. Retinal ischemia was achieved by elevating the intraocular pressure for 60 minutes above systolic blood pressure.
VEPs with responses up to 16.1±1,1 µV (mean±SEM) depending on stimulation parameters were detectable over at least 10 days. With decreasing modulation depth the VEP amplitudes decreased; the frequence tuning curve showed a bimodal shape with an maximum amplitude at 19 Hz. Isoflurane depressed the VEP with increasing concentrations. Chloralhydrate and Ketamine/Xyaline acted differently depending on concentrations. After 60 minutes of retinal ischemia the potentials remain detectable, but markedly decreased in comparison to the control. After optic nerve crush no stimulation paradigm evoked significant potentials.
This new VEP paradigm allows monitoring the visual system of rats over an extended period. Compared to checkerboard stimuli our paradigm has the advantage of being independent on the refraction of the eye. VEP after retinal ischemia is able to quantify functional damage of retinal ganglion cells. Hence, this new technique allows the evaluation neuroprotective substances or treatments on visual function.
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