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S Machida, PA Sieving, RA Bush; Contribution of the Photopic Negative Response to the Photopic ERG in Rats With Inherited Photoreceptor Degeneration . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3489.
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Purpose:The photopic negative response (PhNR) was previously described in the primate ERG as a corneal negative potential following the photopic b-wave (Viswanathan et al., 1999). This study investigated the PhNR and its origin in rats with inherited photoreceptor degeneration. Methods:The scotopic and photopic ERG was recorded from dystrophic RCS (p+) and P23H rhodopsin transgenic (line1, heterozygote) rats. Changes in ERG waveform were examined at various stages of degeneration. NMA (N-methyl-DL-aspartic acid) was injected intravitreally into RCS (p+) rats to block inner retinal responses. Recovery time of the PhNR after an intense bleach (500 lux, 10 minutes) was measured to distinguish cone from rod origin. Results:The PhNR made up an increasingly larger proportion of the photopic ERG as degeneration progressed in RCS (p+) rats between 4 and 14 weeks. Its amplitude and threshold were maintained despite a decline in b-wave amplitude. A strong bleach completely abolished the dark adapted ERG, which showed little recovery in 100 minutes (11.5±6.2 % of pre-bleach level). The PhNR was not extinguished and recovered to 84.5±4.0 % of pre-bleach level in less than 10 minutes, indicating that the PhNR is driven by cones. NMA eliminated most of the PhNR, implicating third-order neurons as the source of this ERG potential. In contrast to RCS rats, the PhNR and b-wave amplitude declined in parallel in P23H rhodopsin transgenic rats (R=0.761, p<0.0001). Conclusion:The PhNR in rats, as in primates, is generated by third-order neurons in the cone pathway. The PhNR is better preserved than the photopic b-wave in RCS degeneration. However, this is not true in all inherited photoreceptor degenerations.
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