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B. V. Bui, V. H. Y. Wong, Z. He, A. J. Vingrys; Barium Chloride Sensitive Components in the Rat Electroretinogram. Invest. Ophthalmol. Vis. Sci. 2009;50(13):2180.
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Barium chloride is known to inhibit potassium channels in the eye. We employ this agent to characterize the role that potassium channels have in generating rat electroretinograms.
ERGs were measured in anaesthetized (60:5 mg/kg ketamine:xylazine, 50% top up/50 min) dark-adapted adult Long-Evan rats (n=7). One eye was randomly assigned to intravitreal injection (2 µl) of BaCl2 (2 mM) or vehicle (dH20). The ERG protocol consisted of dark-adapted (-5.66 to 2.28 log cd.s/m2) responses as well as a twin-flash (at 2.28 log cd.s/m2 with an interstimulus interval of 500 ms) to isolated cone mediated function. ERG recording was conducted once the effect was stable (~40 min after injection). At the completion of ERG recording both eyes received injections (4 µl) of L-amino-4-phosphonobutyric acid (L-AP4, 1 mM) and cis-2,3-piperidinedicarboxylic acid (PDA, 5 mM). ERG recordings were repeated 40 minutes following the second injection. ERG amplitudes were analyzed at fixed times following flash onset (A8, A110, A220 ms) to characterize the major ERG components, and expressed as a mean (±SEM) percentage change relative to the vehicle treated eye. Peak time and trough time were also extracted.
BaCl2 did not effect the dark-adapted photoreceptoral response (A8, -267 ± 37 vs -235 ± 35 µV, p = 0.23). At the dimmest light levels BaCl2 removed both the positive (A110, -91 ± 5%) and negative (A220) scotopic threshold response (STR), leaving a positive component (b-wave) with a longer peak time (control: 117 ± 4 vs BaCl2: 143 ± 3 ms, p<0.001). In contrast, at moderate to high intensities (>-2.37 log cd.s/m2) BaCl2 significantly enhanced the b-wave (146 ± 20%). At high intensities (>1.55 log cd.s/m2) BaCl2 produced a faster b-wave peak (control: 84 ± 4 vs BaCl2: 56 ± 5 ms, p<0.001). In contrast to the enhancement seen in the dark-adapted waveform, cone isolated b-waves were significantly reduced (A110, control: 310 ± 41 vs BaCl2: 263 ± 27 µV, p<0.01) and slowed (control: 64 ± 3 vs BaCl2: 91 ± 7 µV, p<0.001) by BaCl2. In terms of the L-AP4/PDA isolated response, BaCl2 did not affect the photoreceptoral response (A8) but removed a slow sustained component consistent with the slow P3, producing a -69 ± 10% reduction in amplitude (A110, -485 ± 30 vs BaCl2: -164 ± 41 µV, p<0.001).
BaCl2 sensitive mechanisms mediate the scotopic threshold response in the pigmented rat. Rod mediated responses were enhanced by BaCl2, whereas cone responses were reduced. Slow P3 removal by BaCl2 does not account for b-wave changes, suggesting potassium channels modify b-wave generators in both dark-adapted and cone isolated conditions.
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