Pre-treatment of adult rats with high doses of erythropoietin induces caspase-9 but prevents light-induced retinal injury

Erythropoietin (Epo) had been shown to have a neuroprotective effect independent from its erythropoietic properties. In this study, we tested whether Epo could protect the retina from damage induced by a long period of moderate light insult and how it protected. First, rats were injected intraperitoneally (i.p.) by human recombinant Epo at 5000 or 30,000 U/kg to assess Epo concentration in plasma and retina. Second, rats were untreated or injected i.p. with Epo at 30,000 U/kg, 1 or 4 h before being placed in constant light (24 h; 2200 lux). Electroretinograms (ERG) were recorded before treatment, 1 day and 15 days (D15) after light exposure. After the last ERG, eyes were taken for histology. In parallel, we tested Epo protection against oxidative stressors on isolated retinas and its effect on caspase-9 activity. Epo injected at 30,000 U/kg body weight, 4 h before exposure to the damaging light, protected retinal function and structure against light damage and induced an increase in caspase-9 activity and expression. Epo had no direct or indirect protective effect against free radicals-induced death on isolated retinas. Epo protected the retina from a long period of moderate light exposure through a mechanism independent from a free radical scavenging property or an antioxidant facilitating activity. The activation of caspase-9, 4 h after Epo injection, corresponding to the start of light exposure, suggests that caspase-9 plays a role in neuroprotection.