considered to be ‘mixed’ with contributions from rods and cones. Responses to the test flash preceded, by 400 msec, by a conditioning flash (12 cds/m2) were considered those of cones. By subtracting the cone response from the ‘mixed’ response, the rod response was isolated. The standard flash stimulus was sufficient to elicit saturated a-wave and b-wave responses.

54.2.3 Immunohistochemistry and TUNEL Labeling

Enucleated eyes were fixed, processed, cryoembedded and cryosectioned as described previously (Chrysostomou et al. 2008). Immunohistochemical labelling of retinal cryosections was performed using antibodies specific for glial fibrillary acidic protein (GFAP). Retinal cryosections were labeled using the TUNEL technique to identify the fragmentation of DNA characteristic of apoptosis.

54.3 Results

54.3.1 Rod and Cone Components of the ERG after Hyperoxia

Full field ERG responses were recorded in LE and SD adult rats before and after 14 days exposure to hyperoxia. In the pigmented LE strain, exposure to hyperoxia for 14 days significantly (P<0.05) reduced the amplitude of rod a-wave (Fig. 54.1a), rod b-wave (Fig. 54.1b) and cone b-wave (Fig. 54.1c) components of the ERG, by 29, 46 and 49% respectively. In the albino SD strain the effect of hyperoxic exposure on the ERG was much less marked, and did not reach statistical significance for any of these three ERG components (Figs. 54.1a–c).

54.3.2 Impact of Hyperoxia on the Rate of Photo receptor Death

In both strains, 14 days hyperoxic exposure increased the frequency of TUNEL+ cells in the retina; the increases were confined to the outer nuclear layer (data not shown). In the LE strain, hyperoxia resulted in a 55-fold increase in the frequency of TUNEL+ cells. By contrast, the same hyperoxic exposure resulted in a much smaller (5-fold) increase in TUNEL labeling of SD photoreceptors (Fig. 54.2a). Hyperoxiainduced DNA damage to LE and SD photoreceptors was not uniform across the retina. Photoreceptors in the peripheral retina were relatively unaffected, and there was a strong concentration of TUNEL+ cells in the central retina (Fig. 54.2b).

54.3.3 Impact of Hyperoxia on GFAP Expression

In normoxic LE and SD retinas, GFAP expression was confined to astrocyte cell bodies and processes at the inner surface of the retina (left panels of Figs. 54.3a and b). After 14 days hyperoxic exposure, GFAP was prominent in radially-oriented

Fig. 54.1 ERG responses were recorded in LE and SD rats before (normoxia) and immediately after (hyperoxia) 14 days exposure to 75% oxygen. Exposure to hyperoxia significantly reduced the amplitudes of rod (a,b) and cone (c) components of the ERG in the LE but not the SD strain

of rat. Histograms show mean

± SEM (n=7). P < 0.05 using a student’s t-test

Müller cell processes in the LE retina (right panel of Fig. 54.3a). Raised GFAP expression was most prominent in the central LE retina, co-localising with areas of high TUNEL labeling. In the SD retina, hyperoxic exposure had no effect on GFAP expression (right panel of Fig. 54.3b).