Retinal ganglion cell death induced by retinal ischemia. neuroprotective effects of two alpha-2 agonists

We have investigated in adult Sprague-Dawley rats the neuroprotective effects of two alpha-2-selective agonists [AGN 191,103 (AGN) and brimonidine tartrate (BMD)] on retinal ganglion cell (RGC) survival after transient retinal ischemia. RGCs were labelled with Fluorogold (FG) applied to both superior colliculi. Seven days later, 90 min of retinal ischemia were induced in the left eyes by ligature of the ophthalmic vessels (LOV). In one group of animals, vehicle or AGN (0.01 mg/kg) were administered systemically 1 hr before ischemia. In another group of animals, two 5 microl drops of vehicle, AGN (0.05%) or BMD (0.1%) were administered topically in the left eye 1 hr before ischemia. The animals were processed 7 or 21 days later. RGC survival was estimated by counting FG-labelled cells in 12 standard areas of each retina. In control retinas of systemically pretreated animals, mean densities of labelled RGCs were 2372 +/- 49 cells/mm(2) (mean +/- SEM; n = 6). In experimental retinas of systemically pretreated animals, mean RGC densities had decreased 7 days after ischemia to 53% (n = 6) or 81% (n = 6) of control in the groups treated with vehicle or AGN, respectively. Twenty-one days after ischemia, mean RGC densities had decreased to 38% (n = 6) or 79% (n = 6) of control in the groups treated with vehicle or AGN, respectively. In control retinas of topically pretreated animals, mean densities of labelled RGCs were 2208 +/- 29 cells/mm(2) (n = 6). In experimental retinas of topically pretreated animals, mean RGC densities had decreased 7 days after ischemia to 54% (n = 6), 95% (n = 6) or 96% (n = 6) of control in the groups treated with vehicle, AGN or BMD, respectively. These results indicate that pretreatment with a single systemic or topical dose of AGN or BMD can prevent completely the early rapid phase of RGC loss and abolish the delayed RGC loss observed after 90 min of retinal ischemia induced by ligature of the ophthalmic vessels.