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Focal cerebral ischemia induces cellular responses that may result in secondary tissue damage. We recently demonstrated multi-facetted spatial and temporal patterns of neuroinflammation by multimodal imaging. In the present study, we especially focus on the separation of vital and necrotic tissue, which enabled us to define a demarcation zone. Focal cerebral ischemia was induced via macrosphere embolization of the middle cerebral artery in Wistar rats. Subsequent cellular processes were investigated immunohistochemically from 3 to 56 days after onset of ischemia. We detected several infarct subareas: a necrotic infarct core and its margin adjacent to a nerve/glial antigen 2 (NG2)+ zone delineating it from a vital peri-infarct zone. Initially transition from necrotic to vital tissue was gradual; later on necrosis was precisely separated from vital tissue by a narrow NG2+ belt that was devoid of astrocytes, oligodendrocytes or neurons. Within this demarcation zone NG2+ cells associate with ionized calcium binding adaptor molecule 1 (Iba1) but not with GFAP, neuronal nuclear antigen (NeuN) or 2', 3'-cyclic nucleotide 3'-phosphodiesterase (CNPase). During further infarct maturation NG2 seemed to be positioned in the extracellular matrix (ECM) of the demarcation zone, whereas Iba1+ cells invaded the necrotic infarct core and GFAP+ cells built a gliotic containing belt between the lesion and NeuN+ unaffected tissue. Overall, our data suggested that NG2 proteoglycan expression and secretion hallmarked demarcation as a process that actively separated necrosis from vital tissue and therefore decisively impacts secondary neurodegeneration after ischemic stroke., (© 2012 Japanese Society of Neuropathology.)

Focal cerebral ischemia elicits strong inflammatory responses involving activation of resident microglia and recruitment of monocytes/macrophages. These cells express peripheral benzodiazepine receptors (PBRs) and can be visualized by positron emission tomography (PET) using [(11)C]PK11195 that selectively binds to PBRs. Earlier research suggests that transient ischemia in rats induces increased [(11)C]PK11195 binding within the infarct core. In this study, we investigated the expression of PBRs during permanent ischemia in rats. Permanent cerebral ischemia was induced by injection of macrospheres into the middle cerebral artery. Multimodal imaging 7 days after ischemia comprised (1) magnetic resonance imaging that assessed the extent of infarcts; (2) [(18)F]-2-fluoro-2-deoxy-D-glucose ([(18)F]FDG)-PET characterizing cerebral glucose transport and metabolism; and (3) [(11)C]PK11195-PET detecting neuroinflammation. Immunohistochemistry verified ischemic damage and neuroinflammatory processes. Contrasting with earlier data for transient ischemia, no [(11)C]PK11195 binding was found in the infarct core. Rather, permanent ischemia caused increased [(11)C]PK11195 binding in the normoperfused peri-infarct zone (mean standard uptake value (SUV): 1.93+/-0.49), colocalizing with a 60% increase in the [(18)F]FDG metabolic rate constant with accumulated activated microglia and macrophages. These results suggest that after permanent focal ischemia, neuroinflammation occurring in the normoperfused peri-infarct zone goes along with increased energy demand, therefore extending the tissue at risk to areas adjacent to the infarct.

Ischemic stroke causes cell necrosis with the exposure of extracellular ribonucleic acid (RNA) and other intracellular material. As shown recently, extracellular RNA impaired the blood-brain-barrier and contributed to vasogenic edema-formation. Application of ribonuclease 1 (RNase 1) diminished edema-formation and also reduced lesion volume in experimental stroke. Here we investigate whether reduction of lesion volume is due to the reduction of edema or of other neuroprotective means. Neuroprotective and edema protective effects of RNase 1 pretreatment were assessed using a temporary middle cerebral artery occlusion (MCAO) model in rats. Lesion volume was assessed on magnetic resonance imaging (MRI). T2-relaxation-time and midline-shift as well as brain water content (wet-dry-method) were measured to quantify edema formation. The impact of edema formation on infarct volume was evaluated in craniectomized animals. Exogenous RNase 1 was well tolerated and reduced edema-formation and infarct size (26.7% +/- 10.7% vs. 41.0% +/- 10.3%; p<0.01) at an optimal dose of 42 microg/kg as compared to placebo. Craniectomized animals displayed a comparable edema reduction but no reduction in infarct size. The present study introduces a hitherto unrecognized mechanism of ischemic brain damage and a novel neuroprotective approach towards acute stroke treatment.