Non-neuronal responses to short-term occlusion of the middle cerebral artery

MRI and neuropathological validations of the involvement of air pollutants in cortical selective neuronal loss

Vehicles are a major source of air pollution, especially particulate matter (PM) pollution, throughout the world and auto-rickshaws are considered main contributors to this air pollution. PM, in addition to causing respiratory and cardiovascular disorders, has potential to gain access to the brain and could induce neuroinflammation leading to different neurological disorders. Therefore, in the current project, MRI and immunohistochemistry techniques were adopted to ascertain the neurotoxic potential of the chronic exposure to different PM generated by two-stroke auto-rickshaws (TSA), four-stroke auto-rickshaws (FSA), and aluminum sulfate (AS) solution in rats. The results highlighted that all treated groups followed a pattern of dose-dependent increase in pure cortical neuronal loss, selective neuronal loss (SNL), nuclear pyknosis, karyolysis, and karyorrhexis. Mild to moderate areas of penumbra were also observed with increase in the population of activated microglia and astrocytes, while no alteration in the intensities of T2W MRI signals was perceived in any group. When comparing the findings, TSA possess more neurotoxic potential than FSA and AS, which could be associated with increased concentration of certain elements in TSA emissions. The study concludes that chronic exposure to PM from TSA, FSA, and AS solutions produces diverse neuropathies in the brain, which may lead to different life-threatening neurological disorders like stroke, Alzheimer's, and Parkinson's disorders. Government and environmental agencies should take serious notice of this alarming situation, and immediate steps should be implemented to improve the standards of PM emissions from auto-rickshaws.

Characterizing infarction and selective neuronal loss following temporary focal cerebral ischemia in the rat: a multi-modality imaging study

BACKGROUND AND PURPOSE

Current models dictate that, depending on occurrence of early reperfusion, the ischemic penumbra either undergoes or escapes infarction (i.e., "pan-necrosis"). However, tissue outcome following temporary middle-cerebral artery occlusion (tMCAo) in rodents can also include selective neuronal loss (SNL), which even if subtle may impede functional recovery. In order to explore the pathophysiology of ischemic stroke, determine potential therapeutic targets and monitor effects of therapy, in vivo imaging surrogates of these varied histopathological outcomes applicable in the clinical setting would be useful. Although hyperintense signal on T(2)-weighted MRI in the chronic post-stroke stage is considered a reliable surrogate of tissue infarction, SNL is not associated with T(2)W abnormal signal. In the clinical setting, the neuron-specific PET ligand (11)C-flumazenil (FMZ) has been used to identify both pan-necrosis and peri-infarct SNL, but this inference has not been histopathological confirmed so far. Here we investigated the late tissue sequelae of tMCAo in the rodent using in vivo T(2)W MRI and FMZ-PET against post mortem immunohistochemistry as gold standard.

METHODS

Adult spontaneously hypertensive rats (SHRs) underwent 45 min distal-clip middle-cerebral artery occlusion and, 28 days later, FMZ-PET and T(2)W-MRI, immediately followed by immunohistochemistry for neuronal loss (NeuN), activated microglia and astrocytosis. Based on standard histopathological definitions, ischemic lesions were classified into pan-necrosis, partial infarction or SNL. NeuN changes and FMZ binding across the whole hemisphere were quantified in the same set of 44 regions-of-interest according to previously validated protocols; linear regressions between these two measures were carried out both within and across subjects.

RESULTS

Both cortical pan-necrosis/partial infarction and SNL were present in all rats except one, where SNL was isolated and extensive. Infarction/partial infarction, but not SNL, was associated with T(2)W hyperintense signals and cortical atrophy. In contrast, FMZ binding was decreased in all types of lesions including SNL, in proportion with NeuN staining intensity both within (p<0.05 to <0.001) and across (p<0.001) subjects, including the subject that showed pure SNL (p=0.01).

CONCLUSION

This novel study revealed three main facts: i) long-term histopathological cortical changes following 45 min tMCAo in SHRs included all three of SNL, partial infarction and frank infarction; ii) T2W MRI showed conspicuous high signal lesions for complete or partial infarction, but no changes for SNL; and iii) FMZ-PET was sensitive to all three types of tMCAo-induced histopathological changes, including isolated SNL, suggesting it is a valid surrogate for the histological sequelae of focal cerebral ischemia. In addition, the finding of almost universal completed cortical infarction at 28 days differed from our previous findings at 14-day survival using the same model and rat strain, where SNL was the almost exclusive outcome, possibly representing delayed infarct maturation. Prospective studies are needed to investigate this interesting possibility.

Computed tomography perfusion imaging in spectacular shrinking deficit

Spectacular shrinking deficit (SSD) is characterized by abrupt onset of a major hemispheric stroke syndrome, followed by dramatic and rapid improvement. We retrospectively identified patients with SSD diagnosed at our institution between December 1, 2007, and June 30, 2009. We reviewed computed tomography perfusion (CTP) imaging to determine perfusion defect as a measure of initial ischemic penumbra, and magnetic resonance imaging diffusion-weighted imaging (DWI) to determine the final infarct core. Among the 472 consecutive ischemic stroke patients, 126 (27%) presented with major hemispheric ischemic stroke syndrome, defined as National Institutes of Health Stroke Scale score (NIHSS) ≥8 in the territory of the middle cerebral artery (MCA) or internal carotid artery (ICA). Out of these patients, we identified 8 SSD patients with available CTP data. In these 8 patients, the mean time to dramatic recovery was 3.4 hours (range, 0.75-7 hours), and the mean time from onset to CTP was 12.7 hours (range, 3-30 hours). All 8 patients had perfusion abnormalities in portions of the MCA territory (partial MCA territory in 5 patients and complete MCA territory in 3 patients). The mean time from onset to MRI DWI was 15.5 hours (range, 7.9-34 hours). Restricted diffusion was present in all patients in the corresponding MCA distribution. Vascular imaging revealed MCA occlusion in 2 patients. Cervical vascular imaging revealed carotid occlusion in 2 patients and high-grade carotid stenosis in 2 patients. The stroke mechanisms were cardioembolism in 2 patients, large artery in 4 patients, and unknown in 2 patients. Four patients had repeat CTP imaging available that demonstrated eventual resolution of the perfusion defect. SSD is associated with a "shrinking" clinical syndrome and a "shrinking" perfusion pattern on CTP that lags behind clinical recovery. CTP imaging corroborates that a larger territory is at risk in SSD and contributes to better understanding of SSD.

Defining the macroscopic and microscopic findings of experimental focal brain ischemia in rats from a forensic scientist's point of view

Approximately 10% of all deaths in the world occur as a result of stroke. Determination of the time schedule of the pathologic events in a stroke patient is invaluable for a forensic specialist. The aim of this study was to define the schedule of the macroscopic and microscopic changes that occurred in a rat model of permanent focal ischemia for providing useful clues for the evaluation of stroke patients. Male Wistar rats weighing 250 to 350 g were used in this study. Permanent focal brain ischemia was applied by the suture occlusion method. The animals were divided into 7 experimental groups (n = 6) with time schedules including 1.5, 3, 6, 12, 24, 72 hours, and the sham. Brains were harvested at the end of the determined time schedule. Lesions in the frontoparietal cortex were evaluated macroscopically first and later hematoxylin eosin stained sections from the infarct core were investigated microscopically. Macroscopically, enlargement of the ipsilateral hemisphere was mild at 6 hour, apparent at 12 and 24 hours, and mild again at 72 hours. Microscopically, ischemic changes were apparent even at 1.5 hour. Red neurons and infiltration of the parenchyma with neutrophil leukocytes were observed at 12 hours. Pannecrosis and massive leukocyte infiltration were observed at 72 hours. Macroscopic and microscopic findings obtained from a rat model may provide clues for determination of the time-dependent changes due to brain ischemia in human subjects. Finally, the benefits of determination of time course of pathologic changes in the brain for forensic scientists were discussed.

Periinfarct and remote excitability changes after transient middle cerebral artery occlusion

Transient middle cerebral artery (MCA) occlusion results in substantially smaller cortical infarcts than permanent MCA occlusion if reperfusion is initiated within the first few hours. Only little information is available on the long-term functional outcome of the cortical regions "salvaged" by early reperfusion. To address this issue we examined basic electrophysiologic parameters in vitro using standard extracellular recording techniques at 7 and 28 days after transient MCA occlusion (1- and 2-hour ischemia) in rats. Both neocortical areas ipsi- and contralateral to MCA occlusion were systematically mapped to delineate the extent of periinfarct and remote alterations. In the periinfarct region we found a significant reduction of field potential amplitudes up to 3 mm when measuring from the infarct border at 7 days and up to 7 mm at 28 days. Paired-pulse inhibition, an indicator of GABAergic transmission, was only moderately impaired in this region at 7 days and not significantly different from control at 28 days. Remote effects were observed both ipsi- and contralaterally. Ipsilaterally they were restricted to a region close to the midline (presumably motor cortex) and were most likely attributable to the degeneration of corticostriatal connections. The extent of the contralateral excitability changes was clearly related to the size of the neocortical infarcts with large infarcts resulting in the widespread reduction of field potential amplitudes and an impairment of paired-pulse inhibition. The results show that there is a relatively large periinfarct region with decreased overall excitability after transient MCA occlusion which is likely to have a profound effect on perilesional processes involved in functional recovery. Remote excitability changes may contribute to the functional deficit and are probably related to deafferentation.