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A. Polosa, J. Racine, E. Zimak, S. G. Rosolen, S. Chemtob, P. Lachapelle; Evidence Indicating an Enhanced Retinofugal Signal in Light-Induced Retinopathy (LIR). Invest. Ophthalmol. Vis. Sci. 2008;49(13):6097.
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In the adult form of LIR there is a selective loss of the Outer Nuclear Layer (ONL) while in the juvenile form, the thinning of the ONL is accompanied by a near equivalent thickening of the Inner Nuclear Layer (INL), with the superior hemiretina always showing more pathology compared to the inferior. We hypothesized that the INL thickening was triggered to maintain a near normal retinofugal signal. We proceeded to test this claim.
Juvenile Sprague Dawley rats exposed from P14-20 and P14-28 to a bright light (12D: 12L; 10 000 lux) were compared to adult rats exposed to the same light for 6 days. Photopic (intensity: 0.9 log cd.m-2.sec; background: 30 cd.m-2) flash ERGs (fERG), flash VEPs (fVEP) and multifocal ERGs (mfERG: VERIS 5.1; camera display unit, 37 hexagons; white: 200 cd.m-2; black: 0 cd.m-2; background: 100 cd.m-2; bandwidth: 10-100 Hz) were recorded at P30 and P50.
fERGs were significantly (p<.05) reduced in juvenile rats only for P14-28 exposure (88.86±17.93 µV) and in 6 day exposed adults (15.36±6.27µV) compared to control (260.97±38.31 µV at P30 and 194.84±47.89 µV at P50). In control rats, the mfERG superior/inferior ratios were 1.02±0.11 and 1.03±0.11 at P30 and P50 respectively. This ratio was reduced (0.71±0.01) in adult LIR (p<.01) while in juvenile LIR it remained within the normal range [P14-20 (0.96±0.36) and P14-28 (1.03±0.31); p>.05]. In juvenile LIR, fVEPs (P14-20: 36.6±9.9 µV and P14-28: 31.8±8.7 µV) were not significantly different (p>.05) from that of controls (36.7±4.4 µV), while significantly reduced amplitudes (22.19±2.48 µV; p<.05) were measured in adult LIR. In control rats, the superior/inferior mfVEP amplitude ratios were 0.91±0.31 and 1.12±0.58 at P30 and P50 respectively compared to 0.15±0.07 in adult LIR (p<.05) and 0.81±0.61 (p>.05) and 0.68±0.03 (P>.05) in juvenile LIR following P14-20 and P14-28 exposures respectively.
Our results thus indicate that despite significant retinal pathology as documented at histology and fERG, the retina of juvenile rats exposed postnatally to intense light does not display the hemiretinal functional difference observed in adult rats exposed to the same stress, suggesting that the functional difference trails the structural one. fVEP data does support our claim that, in juvenile LIR, the restructuring of the INL would take place in order to compensate for the weaker signal generated by the thinner ONL so that a near normal retinofugal signal is forwarded to the cortical visual areas. Supported by CIHR and Réseau Vision.
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