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G. Shaw, F. Montiani–Ferreira, S.M. Petersen–Jones; Pharmacological Dissection of the Electroretinogram of the Retinopathy, Globe Enlarged (rge) Chicken . Invest. Ophthalmol. Vis. Sci. 2005;46(13):5260.
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Purpose: The rge chick maintains electroretinographic responses well after functional vision loss. From hatch the ERG has several abnormalities including elevated response threshold, increased a– and b–wave implicit times and in response to brighter flashes of light a supernormal b–wave amplitude lacking in OPs. The purpose of this study was to use intravitreal drugs to dissect the ERG and thus investigate the origins of the supernormal b–wave. Methods: The following drugs were injected into the vitreous both alone and in various combinations: PDA (cis–2,3–piperidinedicarboxylic acid), APB (2–amino–4–phosphonobutyric acid), barium and ornithine. Either dark or light adapted ERGs were recorded following injection. Results: In control birds, the PDA decreased the a–wave amplitude and broadened the b–wave. After both the APB, and PDA combined with APB, the b–wave of control birds was completely eliminated. This unmasked the PIII response. In contrast, in the rge birds neither PDA, APB nor PDA combined with APB had much effect on the ERG waveforms. Barium decreased a– and b–wave amplitudes and increased a– and b–wave implicit times of the control birds’ ERG, whereas it increased the a– and b–wave amplitudes and implicit times in the rge birds. Ornithine was the only drug that was found to suppress the supernormal b–wave of the rge bird ERG, this also had the effect of increasing the implicit time of the a–wave. In the control birds ornithine also resulted in a reduction in b–wave amplitude and increased implicit time of the a–wave. Conclusions: Failure of APB to block the abnormally shaped and supernormal "b" wave of the rge chick suggests that this waveform may not be generated by the action of ON bipolar cells, or alternatively that for some reason in the rge bird they are resistant to the effect of APB. Barium blocks conductance of potassium channels in Müller cells. The fact that Barium decreases responses in control birds but increases the amplitudes in rge chicks supplies further evidence of abnormalities of function within the retina of the rge chick. Interpretation of the results of the ornithine study are difficult because the cell type affected by ornithine has not been elucidated. Further studies are necessary to elucidate the origin of the unusual ERG waveform of the rge chick.
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