Astrocytic swelling in the ipsilateral substantia nigra after occlusion of the middle cerebral artery in rats.

BACKGROUND AND PURPOSE Focal cerebral ischemia results in neuronal changes in remote areas that have fiber connections with the ischemic area. We reported previously that a high-signal-intensity lesion was observed in the substantia nigra after striatal infarction on T2-weighted MR images in both clinical and experimental cases. However, the origin of these changes in signal intensity remains unclear. The aim of this study was to investigate the nigral changes by examining the correlation between the apparent diffusion coefficient (ADC) and the tissue structure. METHODS Sprague-Dawley rats were subjected to middle cerebral artery occlusion. Four days after the occlusion, when T2-weighted images revealed the presence of an area of high signal intensity in the ipsilateral substantia nigra, diffusion-weighted imaging was performed using a 4.7-T superconductive MR unit, and the ADCs were calculated and imaged. Histopathologic examination by both light and electron microscopy was performed on day 4 after surgery. RESULTS Diffusion-weighted images showed an area of high signal intensity in the ipsilateral substantia nigra, and the ADC map revealed uniform reduction of the ADC in this area. Swelling of astrocytic end-feet was observed, especially in the pars reticulata. CONCLUSION These findings suggest that MR changes in the ipsilateral substantia nigra after striatal injury consist mainly of swelling in the astrocytic end-feet.