In Vivo Excitotoxicity Induced by Ouabain, a Na+/K+-ATPase Inhibitor

Abstract

The susceptibility of immature rat brain to neurotoxicity of N-methyl-D-aspartate (NMDA) has provided a widely used in vivo paradigm to study excitotoxicity relevant to acute neurodegenerative diseases such as cerebral ischemia. In this study, in vivo excitotoxicity was induced via injection of ouabain (1 mM/0.5 μL), a Na+/K+-ATPase-inhibitor, into neonatal rat brain and compared with NMDA injection. The aim of the study was to induce excitotoxicity secondary to cellular membrane depolarization, thereby more closely mimicking the pathophysiologic processes of ischemia-induced brain injury where NMDA-receptor overstimulation by glutamate follows, not precedes, membrane depolarization. Na+/K+-ATPase-inhibition caused an acute, 40% ± 8% decrease of the apparent diffusion coefficient (ADC) of water, as measured using diffusion-weighted magnetic resonance imaging (MRI), and resulted in infarctlike lesions as measured using T2-weighted MRI and histology up to 2 weeks later. Localized one- and two-dimensional 1H-magnetic resonance spectroscopy (MRS) demonstrated that the early excitotoxic diffusion changes were not accompanied by an overall metabolic disturbance. Furthermore, 31P-MRS demonstrated that energy depletion is not a prerequisite for ADC decrease or excitotoxic cell death. Treatment with the NMDA-antagonist MK-80 (1 mg/kg) attenuated the volume of tissue exhibiting a decreased ADC ( P < 0.005), demonstrating that the ouabain-induced injury is indeed excitotoxic in nature. The authors argue that, compared with NMDA-injection, ouabain-induced excitotoxicity elicits more appropriate glutamate-receptor overstimulation and is better suited to detect relevant neuroprotection in that it is more sensitive to attenuation of synaptic glutamate levels.