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D.K. Mojumder, L.J. Frishman; TTX–Blockade of Spiking Activity in the Rat Retina Reduces the Amplitude of the ERG b–wave Originating From Cone Bipolar Cells . Invest. Ophthalmol. Vis. Sci. 2005;46(13):2252.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To determine whether the effects on the rat ERG b–wave of blocking voltage–gated Na+–channels with TTX involve signals from rod or cone bipolar cells. Methods: Ganzfeld ERG’s recorded from Brown Norway rats anesthetized with Ketamine (86 mg/kg) and Xylazine (6.5 mg/kg) were made differentially between DTL fiber electrodes placed under contact lens on the two eyes using brief flashes ( max=462 nm; –5.8 to 1.94 log sc td.s) from darkness, and in the presence of steady backgrounds ranging from –1.7 to 2.3 log–sc–td ( max=462 nm). TTX (3 µM vitreal conc.) was used to block Na+–dependent spikes in the retina. Cone signals were isolated before and after TTX by silencing rod inputs with a transient rod suppressing background and probing for the residual cone input with a test flash under dark adapted conditions and in the presence of mesopic backgrounds (–0.7 and 0.3 log sc td). To investigate the role of rod vs cone circuits more quantitatively, b–wave amplitudes were measured at fixed times after the stimulus flash corresponding to the peak time of the dark adapted rod–driven b–wave, (110 ms) and the cone–isolated b–wave (65 ms). Results: Under dark–adapted conditions, TTX greatly reduced both n– and p–STR, and slightly reduced the saturated peak amplitude of b–wave. In presence of steady backgrounds that suppressed rods, TTX removed a large early portion of the PhNR, and reduced b–wave amplitude to ∼50% of its control value, and under mesopic conditions, TTX reduced the saturated b–wave amplitude by ∼90%. TTX did not cause any significant change in the amplitude or sensitivity of the dark–adapted b–wave until near its Vmax. Cone isolation experiments showed that the dark–adapted b–wave was affected by TTX only after cone signals were present. The cone isolated b–wave showed severe attenuation after TTX for all backgrounds. Under mesopic conditions, rod–driven b–waves also were reduced by TTX. However, at these backgrounds, based on results of studies using similar stimulus conditions in Cx36 KO mice lacking a distal connection between rods and cones, it is likely that in the rat rod signals were traveling in cone pathways. Conclusions: These findings are consistent with the hypothesis that blockade of TTX–sensitive spiking activity affects the b–wave originating from the cone rather than rod bipolar cells under all background conditions.
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