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M. Kaneko, H. Maeda, L.J. Frishman; Effects of Prolonged Light Adaptation on the Amplitude of the A–Wave of the Flash ERG of the Mouse . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3095.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate the slow growth of the a–wave during prolonged adaptation to rod–suppressing backgrounds in order to determine whether the increase in a–wave amplitude is rod or cone dominated, and whether postreceptoral signals contribute to the growth.
Ganzfeld ERGs were recorded using DTL fiber electrodes from adult C57BL/6 mice anesthetized with ketamine and xylazine. Standard stimuli were brief saturating xenon photostrobe flashes (3.6 log sc td s) on rod–suppressing backgrounds. In some experiments weaker flashes were used. Before imposing the background, animals were kept in dim room light (1.1 log sc td). Single flash ERGs were recorded every 6 min. for 120 min. after onset of the background. In other experiments, paired flashes with a 1 sec interval between were used to suppress rod signals with the first flash, and isolate cone signals with the second. In order to evaluate postreceptoral contributions to the a–wave from 2nd and 3rd order neurons, the iGluR receptoral blocker, PDA (cis–2, 3–piperidine–dicarboxylic acid, 5.2 mM), was injected intravitreally in one eye of several mice. A–wave amplitude was measured at the a–wave trough, and measurements were made at fixed times on the leading edge of the a–wave (e.g. 7 ms after the flash).
After onset of a commonly used rod–suppressing background of 2.6 log sc td, surprisingly the a–wave amplitude (but not b–wave amplitude) in response to the saturating flashes grew very substantially for about an hour after introduction of the background. The a–wave increased to as much as 4.5 times its initial amplitude, after which the amplitude fell to slowly over the next hour ∼25% of the way to baseline. The growth was less when the background was increased to 3.2 log sc td or higher. For much weaker flashes (e.g. 40 times), there was no growth. After flash suppression of rods, to better isolate cone signals, the growth was greatly reduced, but not eliminated. When responses were measured early on the leading edge of the a–wave, where photoreceptor signals dominated, the growth was less than that of the a–wave amplitude for the 2.6 log sc td background and almost eliminated for the 3.2 log sc background. Use of PDA to remove late postreceptoral contributions reduced, but did not eliminate the growth of the a–wave.
Results of these studies suggest that the surprising growth of a–wave amplitude that occurs in the presence of putatively rod saturating backgrounds in the mouse ERG involves both receptoral and postreceptoral contributions, and probably involves both rod and cone signals.
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