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J. Racine, P. Lachapelle; Understanding the Negative ERG of Dark-Adapted Guinea Pigs . Invest. Ophthalmol. Vis. Sci. 2003;44(13):1892.
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Purpose: Analysis of the scotopic ERG luminance-response function of Guinea pigs reveals that for intensities above that used to yield the rod Vmax, the amplitude of the a-wave gradually becomes larger than that of the b-wave (e.g. a/b wave ratio > 1) resulting, at maximum intensity (mixed rod-cone response), in a waveform of negative morphology; a feature normally never observed in other animal species. The purpose of this study was to investigate the possible cause of this unique ERG morphology. Methods: Scotopic ERG luminance-response functions (intensity : -6.3 to 0.9 log cd.m-2.sec; 14 steps) were obtained from normal Guinea pigs (n=6) following periods of dark-adaptation (DA) of 1, 3 and 12 hours in length and compared to that obtained at the onset of DA following exposure to a photopic background of 30 or 100 cd.m-2. Results: As the duration of the DA period increased, there was a gradual increase in the amplitude of the rod Vmax from 28.1 ± 3.1 µvolts (at 0 minute) to a maximum of 71.7 ± 4 .6 µvolts (> 150% increase) measured following 3 hours of DA. In comparison, the amplitude of the mixed rod-cone ERG increased from 68.0 ± 4.7 µvolts (at 0 minute) to 89.7 ± 2.8 µvolts (at 3 hours) or less than 50% increase while the a/b wave ratio did not appear to be influenced by the length of the dark-adaptation period (0 minute = 1.06; 1 hour = 1.02; 3 hours = 1.03; 12 hours = 1.02). Interestingly, following the intravitrial injection of cis- 2,3 piperidine dicarboxylic acid (PDA, 20 mM) or kynurenic acid (KYN, 10 mM), to block the retinal OFF pathway, the morphology of the scotopic mixed cone-rod ERG became positive and thus of a shape closer to that recorded in other animal species. Conclusions: The gradual increase in rod Vmax amplitude with length of the dark adaptation period indicates that Guinea pigs’ rods do gain in sensitivity with the dark-adaptation process. The fact that no similar gain is noted for the mixed rod-cone ERG might suggest that the latter is mostly driven by cones, which in Guinea pigs account for more than 15% of the photoreceptors. Given the above, we postulate that the negative scotopic ERG, which is characteristic of responses beyond Vmax , would result from the overstimulation of the OFF cone pathway in a way similar to that observed in photopic condition with the decay of the Photopic Hill. Results obtained with OFF pathway blockers would support this claim. Supported by CIHR, Réseau-Vision.
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