We have studied the microglial reaction that accompanies cortical infarction induced by middle cerebral artery occlusion (MCAO). Lectin histochemistry with the B4-isolectin from Griffonia simplicifolia as well as immunocytochemistry with a panel of monoclonal antibodies directed against major histocompatibility complex (MHC) and lymphocytic antigens were performed. Principal attention was focused on neocortical and thalamic regions, representative of primary and secondary ischemic damage, respectively. With the lectin procedure, activated microglial cells were abundant in the neocortex 24 hours after MCAO. In contrast, microglial activation in the thalamus was not apparent until day 2 after MCAO. On day 5, MHC class II antigen was expressed by reactive microglia in fiber tracts traversing the striatum, but was absent from activated microglia in the primary cortical infarction area. MHC class I and lymphocytic antigens were expressed differentially on microglia with class I antigens appearing early and lymphocytic antigens appearing late in the time course after focal ischemia. The findings are compatible with previous studies during global ischemia and confirm the early activation and the progressive nature of immunomolecule expression on activated microglia after an ischemic insult. In addition to neocortical and thalamic sites, our results showed an early microglial activation to be present also in forebrain regions outside of the middle cerebral artery (MCA) territory, such as the contralateral cortex and hippocampus. A unilateral microglial reaction was also detectable after long-term survival (> or = 4 weeks) in the pyramidal tracts, as well as in the corticospinal tracts at cervical but not lumbar spinal cord levels. Ischemia-induced neuronal damage, as evaluated by Nissl staining, was found only in cortical and thalamic regions. We conclude that the demonstration of reactive microglia indicates not only imminent ischemic neuronal damage within MCA territory but can also delineate extra-focal disturbances, possibly reflecting subtle and transitory changes in neuronal activity.

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We have examined the distribution and time course of the microglial reaction in the rat dorsal hippocampus after 25-min transient forebrain ischemia (four-vessel occlusion model). Microglial cells were visualized in brain sections using lectin staining with the Griffonia simplicifolia B4-isolectin following intervals of reperfusion ranging from 20 min to 4 weeks. Increased staining of microglial cells was detected in the dentate hilus and area CA1 as early as 20 min after reperfusion. These same regions demonstrated more intense microglial staining after 24 h. The strongest microglial reaction was observed 4-6 days after reperfusion when reactive microglia were abundant throughout all laminae of CA1 and the dentate hilus. Following longer reperfusion intervals, the microglial reaction became less intense; however, it remained above normal levels until the end of the fourth week. At all time points examined, microglial reactivity in the CA3 pyramidal and dentate granule cell layers was considerably lower than that observed in CA1 and dentate hilus. Our results are consistent with the known serial pathological changes associated with cerebral ischemia, but, in addition, show that the examination of the microglial reaction provides an extremely sensitive indicator of subtle and morphologically nonapparent neuronal damage during the early stages of injury.

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Magnetoencephalography (MEG) is considered clinically useful in localizing the epileptogenic focus in partial epilepsy. However, the relationship between the extent of the brain involved in paroxysmal activities and the magnetic field changes at the scalp has not been fully clarified. Furthermore, whether paroxysmal activities generated in deep brain structures such as the hippocampus can be detected magnetically is uncertain. Eight patients with temporal lobe epilepsy and two with extratemporal lobe epilepsy underwent chronic recording from subdural electrodes. Magnetic and electrocorticographic discharges representing epileptic activity were recorded simultaneously. MEG recorded magnetic field changes originating from paroxysmal activity in the superiolateral cerebral cortex when the amplitudes of the electrical paroxysmal activities exceeded 100 microV and extended over more than 3 cm2 of cortical surface. MEG failed to record paroxysmal activity localized to the medial temporal lobe. MEG is often useful in identifying a spike focus in the superiolateral aspects of the cerebral hemisphere, but not discharges arising from the medial temporal lobe. Rapid decay of the magnetic field is likely to be the reason for this limited sensitivity to medial discharges.