We first examined the effect of light at the 24-hour time point following exposure and assessed the effects on the retina using cryosections stained with H&E or immunolabeled. Following both fundus photography and Ganzfeld exposure with the two highest doses of light (0.5 and 1 mW/cm
2 for 1 minute), an increase in photoreceptor internuclear spacing was seen centrally, causing an apparent increase in ONL thickness (
Figs. 2B
1–B
3). The RPE layer also showed damage and apparent cell loss centrally as reflected by focal loss of RPE65 immunolabeling (
Figs. 3A
2,
3A
4). Staining with rod-OS specific markers (rhodopsin, peripherin, and GRK1) showed significant disruption of the OS centrally (
Figs. 3B
2,
3B
4,
3C
2,
3C
4,
3D
2,
3D
4) but not in a WT dog (
Supplementary Fig. S1C). Under the same exposure protocols, the ONL, OS, and RPE were well preserved in the periphery (
Figs. 3B
3,
3C
3,
3D
3), and the pattern of immunolabeling was like that seen in the central region of a shielded
RHO T4R retina (
Figs. 3A
1,
3B
1,
3C
1,
3D
1) or in the exposed retinas of WT controls (
Supplementary Fig. S1D
2, data not shown). At this time point, no obvious changes in the structure of bipolar cells were seen, although some distortion of the lamination of the inner nuclear layer (INL) and shrinkage of the outer plexiform layer (OPL) was observed in the central/exposed
RHO T4R retina (
Figs. 3E
1–E
4). These alterations were likely the consequence of the swelling of the overlying ONL. Müller cells, whose radial extension through the retinal thickness was clearly immunolabeled with vimentin, showed no increase in glial fibrillary acid protein (GFAP) staining, a marker of reactive gliosis; however, there was an increase in expression of EdnRB in the central exposed retinas (
Figs. 3F
1–F
4,
3G
1–G
4). Immunohistochemical alterations were also found in
RHO T4R retinas exposed to lower doses of light (0.5 mW/cm
2 and 0.3 mW/cm
2) for 1 minute, but no obvious changes were seen with exposure to 0.2 mW/cm
2 (
Table 4).