Spectroscopy and Histology Demonstrate Lipids in Ischemic Rat Brain after Middle Cerebral Artery Occlusion with Reperfusion
Background: Elevated macromolecule signals, as measured by proton magnetic resonance spectroscopy (MRS) after human stroke, are hypothesized to arise from mobile lipids that accumulate within macrophages. However, their chemical identity, cellular location, and clinical significance remain uncertain. Methods: Adult Wistar rats underwent a 90 minute middle cerebral artery occlusion with reperfusion. Animals survived for 2 to 5 days prior to sacrifice. Immediately after decapitation, 400 μm slices from an infarcted region of forebrain were prepared and perfused in artificial CSF at 36 degrees C, and studied by gradient-enhanced one-dimensional (1D) MRS and by two-dimensional correlation spectroscopy (COSY). Cryostat coronal sections from other, identically prepared rats were stained for lipid (Nile Red) and immunostained for microglia/macrophages (OX42). Results: The 1D spectra from infarcted samples contained broad resonances arising from lipid methylene and other fatty acyl protons that were absent in control samples. These assignments were confirmed by the COSY crosspeaks. Increases in crosspeaks from mobile polypeptides were not detected. The corresponding histologic specimens showed abundant lipid droplets within the infarct that were not seen in contralateral, uninfarcted tissue. These droplets were primarily located within OX42 immunostaining cells. Conclusions: Our model studies support the hypothesis that macromolecule signals detected by MRS in subacute stroke arise from mobile lipids concentrated in macrophages. These results help to clarify the spectroscopic findings in humans. Further histologic study is warranted to determine whether cells containing lipid droplets that are not stained by OX42 are also present.