[3H]L-NG-Nitroarginine Binding after Transient Focal Ischemia and NMDA-Induced Excitotoxicity in Type I and Type III Nitric Oxide Synthase Null Mice

Abstract

We investigated the density and distribution of nitric oxide synthase (NOS) binding by quantitative autoradiography using [³H]L- N^G-nitroarginine ([³H]L-NNA) after transient focal ischemia or intrastriatal injection of N-methyl-D-aspartate (NMDA) in wild-type (SV-129 and C57black/6) and type I (neuronal) and type III (endothelial) NOS-deficient mice. The middle cerebral artery (MCA) was occluded by an intraluminal filament for 3 h followed by 10 min to 7 days of reperfusion. Specific [³H]L-NNA binding, observed in the wild-type and type III mutant mouse at baseline, increased by 50–250% in the MCA territory during ischemia and the first 3 h of reperfusion. The density of binding sites ( B_max), but not the dissociation constant ( K_d), increased significantly during the ischemic period as did type I NOS mRNA as detected by quantitative reverse transcription polymerase chain reaction. [³H]L-NNA binding after intrastriatal NMDA injection also increased by 20–230%. In the type I NOS-deficient mouse, [³H]L-NNA binding was low and only a very small increase was observed after ischemia or excitotoxicity. Under conditions of this study, [³H]L-NNA did not bind to type II NOS as there was no difference in the distribution or density of [³H]L-NNA binding in the rat spleen obtained after lipopolysaccharide treatment despite induction of NOS type II catalytic activity. Our data suggest that an ischemic/excitotoxic insult up-regulates type I NOS gene expression and [³H]L-NNA binding and that this up-regulation may play a pivotal role in the pathogenesis of ischemic/excitotoxic diseases.