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B.G. Jeffrey, R. Duvoisin; Abnormal Photopic ERG Oscillatory Potentials in mGluR4 and mGluR8 Glutamate Receptor Deficient Mice . Invest. Ophthalmol. Vis. Sci. 2005;46(13):2221.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To determine the location of the metabotropic glutamate receptors, mGluR4 and mGluR8 within the retina and to determine the functional significance of these receptors as measured in–vivo with the electroretinogram (ERG). Methods: For immunolocalization of mGluR4 and mGluR8 in the retina, eyes were fixed post–mortem for 10–15 min in 4% paraformaldehyde in PBS. Eyecups were cryoprotected and sectioned at ∼15 um in a cryostat. Primary antibodies were applied overnight to sections, washed and fluorescent secondary antibodies were incubated 1–2 hours at room temperature and washed. Immunolabelings were visualized using a confocal microscope. After overnight dark–adaptation, scotopic full–field ERGs were recorded from 7 mice (4WT, 3 double mutant mGluR4–/––R8–/–) to brief flashes (–4.25 to 3.24 log sc cd–s/m2). Photopic ERGs (–0.53 to 3.46 log cd–s/m2) were then recorded against a 30 cd/m2 background. Maximal amplitudes were determined from the fits of the Naka–Rushton function to the plots of scotopic P3, P2 and photopic b–wave amplitudes against flash intensity. Scotopic and photopic oscillatory potentials (OPs) were isolated with a bandpass filter (–3dB at 65 and 300 Hz). Results: Punctate mGluR4 and mGluR8 labeling, presumably presynaptic in bipolar cell terminals was observed in the IPL with a sublaminar banding pattern. mGluR4 was more predominant in OFF sublaminae, especially at the outer edge of the IPL. mGluR8 distribution was more symmetrical throughout the IPL but absent from the rod bipolar cell terminal area. Photopic OPs were grossly abnormal in the mGluR4–/––R8–/– mice. The obvious peaks of the first 2 OPs seen in the WT were missing from the mGluR4–/––R8–/– mice and OPs were much smaller in amplitude. The phase of the photopic OPs differed markedly between each mGluR4–/––R8–/– mouse. In contrast, although photopic OP amplitudes varied between WT mice, all had nearly identical phase. There were no qualitative differences in scotopic ERG OPs, a– or b–waves or photopic ERG b–waves (amplitude, latency or waveform) between WT and mGluR4–/––R8–/– mice. Conclusions: These results indicate that 1) either mGluR8 and/or mGluR4 receptors are important in the temporal processing of light stimuli and are necessary for the signal processing within the proximal retina that generates the photopic OPs; 2) scotopic and photopic OPs are produced by separate mechanisms; and 3) since the scotopic rod pathway signal is transmitted through cone bipolar cells to ganglion cells, this transit does not recruit mGluR4 and mGluR8 receptors.
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