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D.K. Mojumder, L.J. Frishman; Effects of Blocking Spiking Activity on the Flash ERG of the Rat are Dependent Upon Background Illumination . Invest. Ophthalmol. Vis. Sci. 2004;45(13):812.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose:To determine the effect of background illumination on the role of spiking activity in shaping the waveform of the flash ERG in the rat Experiment:Ganzfeld ERG’s were recorded from Brown Norway rats anesthetized with Ketamine (86 mg/kg) and Xylazine (6.5 mg/kg). Recordings were made differentially between DTL fiber electrodes placed under contact lens on the two eyes. ERGs were recorded using brief flashes (λmax=462 nm; –5.8 to 1.94 log sc td.s) from darkness, and in the presence of backgrounds ranging from –1.7 to 2.3 log sc td (λmax=462 nm). The following pharmacological agents were injected intravitreally: TTX (3 µM vitreal conc.) to block all Na+–dependent spikes produced by retinal ganglion and amacrine cells, PDA (5.8 mM) to block postsynaptic transmission via ionotropic AMPA/KA (glutatmate) receptors to 3rd order neurons and cone–OFF bipolar cells, and GABA (76 mM) in an attempt to suppress all inner retinal activity. Results:For dark–adapted (scotopic) conditions, TTX greatly reduced both n– and p–STR, and slightly reduced the peak amplitude of b–wave. In presence of backgrounds that suppressed rods, a large early portion of the PhNR was removed, and b–wave amplitude was reduced to ∼50% of its control value. In presence of moderate backgrounds of –1.7 to 0.29 log sc td, the peak amplitude of b–wave was reduced by ∼90% by TTX. Results after PDA were similar to those after TTX for the moderate backgrounds, but PDA reduced the b–wave amplitude more under scotopic conditions, and less under photopic conditions than TTX. GABA eliminated inner retinal responses: STR and PhNR, but enhanced b–wave peak amplitude under all background conditions both before and after TTX. Conclusions:The effect of blocking spiking inner retinal contributions to the flash ERG is dependent on background illumination. Spiking activity contributes to formation of inner retinal components, n– and p–STR under scotopic conditions, and PhNR under photopic conditions. For mesopic conditions b–wave amplitude is affected differently by removal of spiking activity than by removal of GABA–sensitive activity.
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