Temporal characteristics of the protective effect of aminoguanidine on cerebral ischemic damage

We investigated the temporal profile of the reduction in focal cerebral ischemic damage exerted by aminoguanidine (AG), an inhibitor of inducible nitric oxide synthase (iNOS). In anesthetized spontaneously hypertensive rats, the middle cerebral artery (MCA) was occluded distal to the origin of the lenticulostriate arteries. Rats were treated with vehicle (saline) or AG (100 mg kg-1, i.p.) immediately after MCA occlusion and, thereafter, two times per day. Rats were sacrificed 1(n = 7), 2(n = 8), 3 (n = 6) or 4 days (n = 5) after MCA occlusion. Injury volume (mm3) was determined in thionin-stained sections using an image analyzer. Volumes were corrected for ischemic swelling. Administration of AG up to 2 days after MCA occlusion did not reduce cerebral ischemic damage (p < 0.05 from vehicle; t-test). Treatment for a longer period decreased injury volume, the reduction averaging 21 +/- 5% at 3 days (p < 0.05) and 30 +/- 9% at 4 days (p < 0.05). Aminoguanidine did not affect ischemic brain swelling (p > 0.05). Administration of AG did not substantially modify arterial pressure, arterial blood gases, pH, hematocrit, plasma glucose and rectal temperature. We conclude that the protective effect of AG is time-dependent and occurs only when the drug is administered for longer than 2 days, starting after induction of ischemia. Because iNOS enzymatic activity develops more than 24 h after MCA occlusion [C. Iadecola, X. Xu, F. Zhang, E.E. El-Fakahany, M.E. Ross, Marked induction of calcium-independent nitric oxide synthase activity after focal cerebral ischemia, J. Cereb. Blood Flow, Metab. 14 (1995) 52-59; C. Iadecola, F. Zhang, X. Xu, R. Casey, M.E. Ross, Inducible nitric oxide synthase gene expression in brain following cerebral ischemia, J. Cereb. Blood Flow Metab. 15 (1995) 378-384.], the data support the hypothesis that the protective effect of AG is medicated by inhibition of iNOS in the post-ischemic brain.