Acute astrocyte activation in brain detected by MRI: new insights into T(1) hypointensity.

Increases in the T(1) of brain tissue, which give rise to dark or hypointense areas on T(1)-weighted images using magnetic resonance imaging (MRI), are common to a number of neuropathologies including multiple sclerosis (MS) and ischaemia. However, the biologic significance of T(1) increases remains unclear. Using a multiparametric MRI approach and well-defined experimental models, we have experimentally induced increases in tissue T(1) to determine the underlying cellular basis of such changes. We have shown that a rapid acute increase in T(1) relaxation in the brain occurs in experimental models of both low-flow ischaemia induced by intrastriatal injection of endothelin-1 (ET-1), and excitotoxicity induced by intrastriatal injection of N-methyl-D-aspartate (NMDA). However, there appears to be no consistent correlation between increases in T(1) relaxation and changes in other MRI parameters (apparent diffusion coefficient, T(2) relaxation, or magnetisation transfer ratio of tissue water). Immunohistochemically, one common morphologic feature shared by the ET-1 and NMDA models is acute astrocyte activation, which was detectable within 2 h of intracerebral ET-1 injection. Pretreatment with an inhibitor of astrocyte activation, arundic acid, significantly reduced the spatial extent of the T(1) signal change induced by intrastriatal ET-1 injection. These findings suggest that an increase in T(1) relaxation may identify the acute development of reactive astrocytes within a central nervous system lesion. Early changes in T(1) may, therefore, provide insight into acute and reversible injury processes in neurologic patients, such as those observed before contrast enhancement in MS.

Endothelin-1-induced spreading depression in rats is associated with a microarea of selective neuronal necrosis.

Two different theories of migraine aura exist: In the vascular theory of Wolff, intracerebral vasoconstriction causes migraine aura via energy deficiency, whereas in the neuronal theory of Leão and Morison, spreading depression (SD) initiates the aura. Recently, it has been shown that the cerebrovascular constrictor endothelin-1 (ET-1) elicits SD when applied to the cortical surface, a finding that could provide a bridge between the vascular and the neuronal theories of migraine aura. Several arguments support the notion that ET-1-induced SD results from local vasoconstriction, but definite proof is missing. If ET-1 induces SD via vasoconstriction/ischemia, then neuronal damage is likely to occur, contrasting with the fact that SD in the otherwise normal cortex is not associated with any lesion. To test this hypothesis, we have performed a comprehensive histologic study of the effects of ET-1 when applied topically to the cerebral cortex of halothane-anesthetized rats. Our assessment included histologic stainings and immunohistochemistry for glial fibrillary acidic protein, heat shock protein 70, and transferase dUTP nick-end labeling assay. During ET-1 application, we recorded (i) subarachnoid direct current (DC) electroencephalogram, (ii) local cerebral blood flow by laser-Doppler flowmetry, and (iii) changes of oxyhemoglobin and deoxyhemoglobin by spectroscopy. At an ET-1 concentration of 1 muM, at which only 6 of 12 animals generated SD, a microarea with selective neuronal death was found only in those animals demonstrating SD. In another five selected animals, which had not shown SD in response to ET-1, SD was triggered at a second cranial window by KCl and propagated from there to the window exposed to ET-1. This treatment also resulted in a microarea of neuronal damage. In contrast, SD invading from outside did not induce neuronal damage in the absence of ET-1 (n = 4) or in the presence of ET-1 if ET-1 was coapplied with BQ-123, an ET(A) receptor antagonist (n = 4). In conclusion, SD in presence of ET-1 induced a microarea of selective neuronal necrosis no matter where the SD originated. This effect of ET-1 appears to be mediated by the ET(A) receptor.

Accumulation of quinolinic acid with neuroinflammation: does it mean excitotoxicity?

The quinolinic acid (QUIN) accumulation that is associated with neuroinflammation is often considered capable of promoting excitotoxic neuronal damage, but QUIN is a relatively weak agonist of N-methyl-D-aspartate (NMDA) receptors. Our study aimed to determine, in vivo, which extracellular concentrations of QUIN must be reached to initiate electrophysiological changes indicative of excitotoxic stress in the cerebral cortex of rats, under normal conditions and when superimposed to a challenge involving NMDA-receptor activation, i.e. repeated cortical spreading depression (CSD). Our experimental strategy relied on microdialysis probes incorporating an electrode, implanted in the brain of halothane-anaesthetised rats. These devices were used to apply QUIN or NMDA locally to the cortical area under study (with or without co-perfusion of high K+ for repetitive induction of CSD), and to record the associated changes in the extracellular DC potential (for information on the membrane polarisation of the cellular population surrounding the probe) and lactate (for the detection of increased local energy demand). The extracellular EC50 for induction of local depolarisation in the normal cortex was around 30 times higher than the extracellular QUIN levels measured in the immunoactivated brain of gerbils. Within the range of concentrations 0.03 to 0.3 mM in the perfusion medium, QUIN suppressed concentration-dependently the elicitation of CSD by K+, presumably because of NMDA-receptor desensitisation. Finally, on-line monitoring of changes in extracellular lactate with local application of QUIN indicated that extracellular concentration of QUIN in the low micromolar range are well tolerated by the brain parenchyma, at least in cortical regions. All these data do not support the notion that QUIN accumulation adds an excitotoxic component to neuroinflammation.