ABSTRACT
Clinical studies have identified widespread white matter degeneration in ischemic stroke patients. However, contemporary research in stroke has predominately focused on the infarct and periinfarct penumbra regions. The involvement of white matter degeneration after ischemic stroke and its contribution to post-stroke cognitive impairment and dementia (PSCID) has remained less explored in experimental models. In this study, we examined the progression of locomotor and cognitive function up to 4 months after inducing ischemic stroke by middle cerebral artery occlusion in young adult rats. Despite evident ongoing locomotor recovery, long-term cognitive and affective impairments persisted after ischemic stroke, as indicated by Morris water maze, elevated plus maze, and open field performance. At 4 months after stroke, multimodal MRI was conducted to assess white matter degeneration. T2-weighted MRI (T2WI) unveiled bilateral cerebroventricular enlargement after ischemic stroke. Fluid Attenuated Inversion Recovery MRI (FLAIR) revealed white matter hyperintensities in the corpus callosum and fornix across bilateral hemispheres. A positive association between the volume of white matter hyperintensities and total cerebroventricular volume was noted in stroke rats. Further evidence of bilateral white matter degeneration was indicated by the reduction of fractional anisotropy and quantitative anisotropy at bilateral corpus callosum in diffusion-weighted MRI (DWI) analysis. Additionally, microglia and astrocyte activation were identified in the bilateral corpus callosum after stroke. Our study suggests that experimental ischemic stroke induced by MCAO in young rat replicate long-term cognitive impairment and bihemispheric white matter degeneration observed in ischemic stroke patients. This model provides an invaluable tool for unraveling the mechanisms underlying post-stroke secondary white matter degeneration and its contribution to PSCID.
ABSTRACT
Glutamate-mediated excitotoxicity has been extensively explored as a therapeutic target for the development of potential treatments of neurological disorders including stroke. However, the effect of glutamate on astrocytes under pathological conditions has been less studied. Using primary astrocyte culture, we determined the effect of glutamate on astrocytes against ischemic insult. Glutamate provided a cytoprotective effect and acted as an alternative substrate for ATP production in primary astrocytes against oxygen glucose deprivation reoxygenation insult, which was blocked by glutamate uptake inhibition. The cytoprotective effect of glutamate appears to be astrocyte-specific, as glutamate dose-dependently induces cytotoxic action in murine hippocampal HT-22 cell line. Interestingly, the cytoprotective effect of glutamate against glucose deprivation was short-last, as no protection was observed after 3-day glucose deprivation. We determined the metabolic phenotype of primary astrocyte cultured in glucose or glutamate. Primary astrocytes cultured in glutamate displayed a different metabolic phenotype when compared to those cultured in glucose, evidenced by higher basal and maximal oxygen consumption rate (OCR), higher ATP production and proton leak-coupled OCR, as well as lower glycolysis. Furthermore, glutamate exposure resulted in astrocyte activation, evidenced by an increase in astrocyte size and GFAP expression. Our study demonstrated that glutamate exerts a dual effect on astrocytes under ischemic condition. Glutamate provides an alternative substrate for energy metabolism in the absence of glucose, thereby protecting astrocytes against ischemic insults. On the other hand, glutamate exposure induces astrogliosis. Modulation of glutamate uptake and metabolism in astrocytes may provide novel targets for alleviating ischemic injury and improving function recovery after ischemic stroke.