Nitric oxide mediates cerebral ischemic tolerance in a neonatal rat model of hypoxic preconditioning

Neuroprotection against cerebral ischemia can be realized if the brain is p reconditioned by previous exposure to a brief period of sublethal ischemia. The present study was undertaken to test the hypothesis that nitric oxide (NO) produced from the neuronal isoform of NO synthase (NOS) serves as a ne cessary signal for establishing an ischemia-tolerant state in brain. A newb orn rat model of hypoxic preconditioning was used, wherein exposure to subl ethal hypoxia (8% oxygen) for 3 hours renders postnatal day (PND) 6 animals completely resistant to a cerebral hypoxic-ischemic insult imposed 24 hour s later. Postnatal day 6 animals were treated 0.5 hour before preconditioni ng hypoxia with the nonselective NOS inhibitor L-nitroaginine (2 mg/kg intr aperitoneally). This treatment, which resulted in a 67 to 81% inhibition of calcium-dependent constitutive NOS activity 0.5 to 3.5 hours after its adm inistration, completely blocked preconditioning-induced protection. However , administration of the neuronal NOS inhibitor 7-nitroindazole (40 mg/kg in traperitoneally) before preconditioning hypoxia, which decreased constituti ve brain NOS activity by 58 to 81%, was without effect on preconditioning-i nduced cerebroprotection, as was pretreatment with the inducible NOS inhibi tor aminoguanidine (400 mg/kg intraperitoneally). The protective effects of preconditioning were also not blocked by treating animals with competitive [3-(2-carboxypiperazin-4-yl)propyl-1-phosphonate; 5 mg/kg intraperitoneall y] or noncompetitive (MK-801; 1 mg/kg intraperitoneally) N-methyl-D-asparta te receptor antagonists prior to preconditioning hypoxia. These findings in dicate that NO production and activity are critical to the induction of isc hemic tolerance in this model. However, the results argue against the invol vement of the neuronal NOS isoform, activated secondary to a hypoxia-induce d stimulation of N-methyl-D-aspartate receptors, and against the involvemen t of the inducible NOS isoform, but rather suggest that NO produced by the endothelial NOS isoform is required to mediate this profound protective eff ect.