Strain Differences in Light-Induced Retinopathy

Abstract

The purpose of this study was to better understand the role of ocular pigmentation and genetics in light-induced retinal damage. Adult pigmented [Long Evans (LE) and Brown Norway (BN)] and albino [Sprague Dawley (SD) and Lewis (LW)] rats were exposed to a bright cyclic light for 6 consecutive days and where compared with juvenile animals exposed to the same bright light environment from postnatal age 14 to 28. Flash ERGs and retinal histology were performed at predetermined days (D) post-light exposure. At D1, ERGs were similar in all adult groups with no recordable a-waves and residual b-waves. A transient recovery was noticed at D30 in the LW and LE only [b-wave: 18% and 25% of their original amplitude respectively]. Histology revealed that BN retina was the most damaged, while LE retina was best preserved. SD and LW rats were almost as damaged as BN rats. In contrast, the retina of juvenile BN was almost as resistant to the bright light exposure as that of juvenile LE rats. Our results strongly suggest that, although ocular pigmentation and genetic background are important factors in regulating the severity of light-induced retinal damage, the age of the animal at the onset of light exposure appears to be the most important determining factor.

Fig 1. Representative scotopic [rod-cone responses (A) and rodVmax (B)] and photopic (C) ERGs recorded from four different control and light-exposed rat strains (BN, SD, LW, LE) 1, 15 and 31 days following the end of the bright light exposure as well as at long term (LW and LE only).

In order to better appreciate residual responses, some tracings were amplified 10 times (gray waveforms). In addition, in LW and LE rats (at D15 and at long term), individual waveforms from each animal are presented separately to highlight variations in amplitude within the same group. Abbreviations: Brown Norway (BN), Sprague-Dawley (SD), Lewis (LW), Long Evans (LE), a-wave (a), b-wave (b) and days (D). Calibration: horizontal: rod-cone response: 200μV, rod Vmax: 100 μV and photopic response: 50 μV, for black waveforms; vertical: 40ms. A 20ms stimulus baseline is included in all tracings. Vertical arrows indicate the flash onset.

Fig 2. Graphic representation of the global retinal function (scotopic a-wave, b-wave and rodVmax and photopic b-wave) in four different strains of rats (BN, SD, LW and LE) at different time points following the cessation of the bright light insult.

Asterisks represent statistically significant differences (p<0.05) between light-exposed and aged-matched control groups, while the symbol # represent statistically significant differences between LW and other strains. Amplitudes are reported as mean 1±SD. Abbreviations: Brown Norway (BN), Sprague-Dawley (SD), Lewis (LW) and Long Evans (LE).

Fig 3. Representative reconstruction of the inferior (left) and superior (right) retina (composed of 12–13 consecutive histological segments of 75μm in width, each sectioned at every 340μm from the ONH to the ora serrate of each hemiretina) obtained from four different strains of adult light-exposed rats 1 day and 31 days following light exposure.

Abbreviations: Optic nerve head (ONH), outer nuclear layer (ONL), Brown Norway (BN), Sprague-Dawley (SD), Lewis (LW) and Long Evans (LE). Red arrows indicate the portion of the retina devoid of photoreceptors.

Fig 4. Representative reconstruction of the inferior (left) and superior (right) retina (composed of 12–13 consecutive histological segments of 75μm in width, each sectioned at every 340μm from the ONH to the ora serrate of each hemiretina) obtained from two different strains (LW and LE) of adult light-exposed rats over 1 year following light exposure.

Abbreviations: Optic nerve head (ONH), Lewis (LW) and Long Evans (LE). Red arrows indicate the portion of the retina devoid of photoreceptors.

Fig 5. Spidergraph representation of ONL loss along the inferior (right) and superior (left) axis in control (blue) and light exposed (pink) rats shown for the four stains and at different time points following bright light exposure.

Measurements were taken at every 340μm from the optic nerve head towards the ora serrate in both hemiretinas. Abbreviations: Optic nerve head (ONH), Brown Norway (BN), Sprague-Dawley (SD), Lewis (LW) and Long Evans (LE).

Fig 6

(A) Representative scotopic (rod-cone responses; top waveforms) and photopic (cone responses; bottom waveforms) ERGs recorded from juvenile BN and LE control and light-exposed rats at P30 and P60. Abbreviations: Brown Norway (BN), Long Evans (LE), a-wave (a) and b-wave (b). Calibration: horizontal: 200μV and 50μV; vertical: 40ms and 20ms, for scotopic and photopic waveforms respectively. A 20ms stimulus baseline is included in all tracings. Vertical arrows indicate the flash onset. (B) Graphic representation of the global retinal function (scotopic a-wave, b-wave and photopic b-wave) recorded juvenile BN and LE control and light-exposed rats at P30 and P60. Asterisks represent statistically significant differences (p<0.05) between light-exposed and aged-matched control groups. Amplitudes are reported as mean 1±SD. Abbreviations: Brown Norway (BN), Long Evans (LE), control (c) and exposed (e).

Fig 7. Representative reconstruction of the ONL of the inferior (left) and superior (right) retina (composed of 12–13 consecutive histological segments of 75μm in width, each sectioned at every 340μm from the ONH to the ora serrate of each hemiretina) obtained from control BN and LE rats at P30 and P60.

Abbreviations: Optic nerve head (ONH), Brown Norway (BN), Long Evans (LE) and postnatal day (P).

Fig 8. Spidergraph representation of ONL loss along the inferior (left) and superior (right) axis in control (blue) and light exposed (pink) BN and LE rats at P30 and P60.

Measurements were taken at every 340μm from the optic nerve head towards the ora serrate in both hemiretinas. Abbreviations: Optic nerve head (ONH), Brown Norway (BN) and Long Evans (LE).

Fig 9. Representative histological samples of melanin pigment distribution along the inferior (left) and superior (right) hemiretinas in control and exposed juvenile (A) and adult (B) BN rats and juvenile (C) and adult (D) LE rats.

Abbreviations: Segments (S), postnatal day (P), days after light exposure (D) and segment (S). Images were taken at every second segment. Each segment is 340μm in width. Red arrows identify the melanin pigment layer in both hemiretinas and red lines indicate the portion of the retina almost completely lacking the melanin pigment, as defined in the method section.