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Christopher Fortenbach, Christopher Kessler, Marie Burns; The effect of RGS9 overexpression on flicker responses of mouse bipolar cells. Invest. Ophthalmol. Vis. Sci. 2013;54(15):6337.
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© ARVO (1962-2015); The Authors (2016-present)
In retinal photoreceptors, changes in illumination result in changes in membrane current and voltage that are largely determined by the reaction rates of outer segment signal transduction mechanisms. In contrast, more downstream measures of visual function, including rod-driven ERG recordings and behavioral tests, suggest that phototransduction does not rate-limit performance of the rod pathway (Umino et al., 2012). To investigate the time course of signaling at the rod synapse, we have combined electroretinoraphy (ERG) and bipolar cell recordings in mice with altered phototransduction kinetics to better understand the mechanisms that limit the temporal resolution at the second stage of the rod pathway.
Mice were dark-adapted overnight prior to the experiment. Corneal ERGs, rod suction electrode, and whole cell recordings from On and Off bipolar cells in retinal slices were elicited by calibrated flashes, steps, and sinusoidal flickering stimuli. Light-evoked responses were subjected to Fourier analysis to generate power spectra and normalized to the peak response amplitude for comparison.
In the presence of a background light that suppressed ~40% of the rod a-wave amplitude, the normalized rod ERG flicker response showed frequency tuning, with a peak at 8-10 Hz. In whole cell recordings, the majority of Off bipolar cells and many On bipolar cells showed similar frequency tuning, optimally signaling at 8Hz when presented with a dim flickering stimulus (mean ~10R*/rod/s). In mice overexpressing the RGS9 complex, this increase in power was more dramatic. Both the rods and the rod-driven bipolar cells from RGS9-overexpressing mice showed greater modulation at higher frequency flicker than those of wild-type mice.
Both ERG and single cell recordings are consistent with behavioral measures of frequency tuning in the rod pathway. The overexpression of RGS9 and the resulting acceleration of the photoresponse recovery in rods exaggerates this tuning, improving the ability of bipolar cells to follow higher frequency flicker.
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