Platelet thrombi in experimental cerebral infarction

Beneficial effects of GM1 ganglioside on photochemically-induced microvascular injury in cerebral cortex and hypophysis in rat

Gangliosides, the glycophospholipids which are abundantly present in the central nervous system, have been shown to stimulate neuronal regeneration and counteract the deleterious effects of ischemia on cerebral neurons. The further elucidate the mechanism of action of gangliosides in cerebral ischemia, we investigated the influence of GM1 ganglioside in the model of photochemically-induced microvascular injury in rat brain. The animals were injected with rose Bengal and illuminated through cranium with halogen lamp. This treatment resulted in the development of microthrombi and alterations in endothelial cells in the microvessels. Administration of 20 mg/kg GM1 ganglioside, 1 h before the photochemical reaction, largely reduced subsequent microvascular damage. In conclusion, the GM1 ganglioside is able to prevent microvascular damage in the central nervous system.

The role of microthrombi and microcirculatory factors in localization and evolution of focal cerebral ischemia

Ischemic cerebral infarction is caused primarily by occlusion of a large arterial vessel. Local circulatory disturbances in the center of ischemic tissue and in ischemic penumbra and the degree of sensitivity to ischemia in different brain regions influence subsequent ischemic progression. Postischemic recirculation is impaired by hemodynamic disturbances and formation of microthrombi, hemorrheologic changes and degeneratively altered vessels of microcirculation. Increased postischemic coagulation can be demonstrated in laboratory tests of few minutes extending up to two weeks after the onset of ischemia. Morphological observations on microthrombi after experimental focal ischemia as well as in patients with cerebral infarction show that formation of microthrombi is dependent on the duration of ischemia and the extent of infarcted tissue. Microthrombi are most prevalent in early stages of tissue damage. This suggests that microthrombi have an effect on the progression of ischemic necrosis. On the basis of our results, we can state that 1) microcirculatory disturbances are triggered by focal cerebral ischemia, 2) formation of microthrombi is a contributing factor to the evolution of postischemic microcirculatory disturbances, and 3) microthrombi promote the progression of ischemic necrosis.

Protective effect of beraprost sodium, a new chemically stable prostacyclin analogue, against the deterioration of baroreceptor reflex following transient global cerebral ischaemia in dogs

1. A possible cerebroprotective effect of a chemically stable prostacyclin analogue, beraprost sodium, was investigated in a canine model of cerebral ischaemia. Cerebral ischaemia was produced by the combined occlusions of the left subclavian and the brachiocephalic arteries with preceding ligations of the intercostal arteries. 2. The decrease in baroreceptor reflex sensitivity (BRS), measured by phenylephrine-induced reflex bradycardia, following 5 min ischaemia was used to assess the cerebroprotective effect. 3. Beraprost (1 microgram kg-1 min-1 i.v., infused for 15 min just before ischaemia) completely prevented the decrease in BRS. Although the lower dose of beraprost (0.1 microgram kg-1 min-1 i.v.) failed to show such a protective effect, its inhibitory effect on ADP-induced platelet aggregation was as potent as that of the higher dose. 4. The extent of decrease in BRS was inversely correlated with the extent of the residual blood flow in the medulla oblongata during ischaemia. Since beraprost did not affect the extent of the residual blood flow during ischaemia, its cerebroprotective effect could not be ascribed to the reduction of the degree of ischaemia by increasing collateral blood flow to the brain. 5. Post-ischaemic reduction of the regional blood flow in the medulla and the cerebral cortex was completely prevented by the higher dose of beraprost. 6. The present study suggests that the cerebroprotective effect of beraprost may be independent of its anti-aggregatory and vasodilator effects. It is possible that the protection may be due to a prostacyclin-like cytoprotective effect through membrane stabilization.

Cyclical carotid artery flow reduction in conscious dogs: effect of a new thromboxane receptor antagonist

Under sterile conditions, dogs were instrumented for continuous measurement of hemodynamics, and an Ameroid constrictor was positioned around the left carotid artery to produce slowly progressive narrowing of the vessel. In the awake, unsedated state, carotid cyclical blood flow reduction abruptly appeared on day 5.1 +/- 0.6 (mean +/- S.E.M.) at a frequency of 6.7 +/- 0.6 cycles per 30 minutes and usually remained present for only 1 day. The vessel was totally occluded on day 7.9 +/- 0.8. No focal neurologic deficit was observed. Following administration of the new thromboxane receptor antagonist, BM 13.505 (4-[2-(4-chlorobenzene-sulfonamide)-ethy]-benzene acetic acid), 50 and 100 micrograms/kg intravenously, the cyclical reduction in carotid blood flow was abolished for 30 +/- 5 and 45 +/- 15 minutes, respectively. No changes in systemic hemodynamics were observed over the course of the experiment. It is concluded that slowly progressive constriction of the carotid artery may provide an in vivo model for the study of perfusion deficits present in transient ischemic attacks. In addition, the results support the hypothesis that platelet aggregation plays a major role in cyclical carotid flow reduction, and the phenomenon can be eliminated by treatment with a new thromboxane receptor antagonist, BM 13.505.

Photochemically induced cerebral infarction. I. Early microvascular alterations

Cerebral ischemia leading to infarction was produced in rats by intravascular thrombosis induced by a photochemical reaction between systemically injected rose bengal and green light (560 nm) transmitted through the intact skull for a 2-min period. At 2 or 15 min following photochemical sensitization, animals were perfusion-fixed for scanning (SEM) and transmission (TEM) electron microscopic analyses of the cerebral vasculature. At 2 minutes, ultrastructural examination of cortical regions destined to undergo infarction revealed numerous platelet aggregates within both pial and intraparenchymal vessels. Platelets close to the endothelial walls were routinely degranulated with pseudopodia. Endothelial cells were frequently swollen and contained dilated mitochondria and granular endoplasmic reticulum. The endothelial luminal membrane structure was shown by high-power TEM to be focally damaged. If brain temperature was reduced by 4 degrees C during the photochemical sensitization period, the platelet response was inhibited without interfering with other ultrastructural changes. These results are consistent with the hypothesis that photochemically induced endothelial alterations stimulate platelet activation and implicate abnormal endothelial function as a primary event in the pathogenesis of photochemically induced cerebral infarction.

Dual effect of naloxone on blood platelet aggregation and cerebral blood flow in gerbils

The effect of naloxone on blood platelet aggregation and cerebral blood flow in gerbils was studied. Administration of naloxone in dose 1 mg/kg to intact gerbils resulted in a marked increase in platelet aggregability accompanied by 27% reduction in cerebral blood flow. Focal cerebral ischemic injury significantly enhanced platelet aggregatory response and treatment with naloxone was without any additional effect on platelet aggregation. Cerebral blood flow in ischemic hemisphere, however, increased following naloxone injection by 46%. In vitro naloxone in millimolar concentrations inhibited platelet aggregation in a dose-dependent way. Apparent decrease in fluorescence of platelet membranes tagged with fluorescence probe due to naloxone suggests conformational changes in platelet membrane as a primary mechanism for the antiaggregatory effect of naloxone in vitro.

Vascular aspects and hemodynamic consequences of central nervous system injury

We document here microvascular alterations occurring in models of mild, moderate, and severe cerebral ischemic injury. The relationship of the vascular abnormalities to the generation of hemodynamic alterations was also evaluated. Following periods of severe incomplete ischemia, scanning electron microscopic analysis of cerebral microvessels revealed the widespread production of cerebral endothelial microvilli. These microvilli increased in frequency as the ischemic insult was prolonged and remained prominent during periods of recirculation. Although these luminal projections would not be expected to inhibit reperfusion completely, they might increase microvascular resistance, leading to moderate hemodynamic impediments extending into the post-ischemic period. Similar periods of complete ischemia resulted in more severe microvascular alterations. Light and electron microscopic studies revealed a high frequency of compressed capillary lumina with vascular stasis. These compressed vessels were consistently surrounded by swollen astrocytic foot processes. When recirculation was instituted for 1 hr following 1 hr of complete ischemia, regions of non-perfusion were detected autoradiographically within brain regions destined to undergo ischemic infarction. Finally, in an attempt to determine the consequences of a primary microvascular insult on brain structure and function, the endothelial layer of microvessels in the cerebral cortex was injured using a noninvasive photochemical method. Endothelial damage led to platelet aggregation in both pial and intraparenchymal vessels. Occlusive thrombi were frequently detected with perivascular edema associated with vascular compression and severe focal ischemia. Ultrastructural blood-brain barrier studies using the horseradish peroxidase tracer demonstrated that protein leakage at the site of primary vascular injury resulted in tracer material in brain regions remote from the pathological lesion. The widespread leakage of protein tracer was associated with decreased blood flow in remote brain regions at several postirradiation periods. These data emphasize the importance of injury-induced microvascular dysfunction in the generation of brain lesions and hemodynamic abnormalities.

Induction of reproducible brain infarction by photochemically initiated thrombosis

We have used a photochemical reaction in vivo to induce reproducible thrombosis leading to cerebral infarction in rats. After the intravenous injection of rose bengal, a potent photosensitizing dye, an ischemic lesion was formed by irradiating the left parietal convexity of the exposed skull for 20 minutes with green light (560 nm) from a filtered xenon arc lamp. Animals were allowed to survive from 30 minutes to 15 days after irradiation. Early microscopic alterations within the irradiated zone included the formation of thrombotic plugs and adjacent red blood cell stasis within pial and parenchymal vessels. Scanning electron microscopy revealed frequent platelet aggregates adhering to the vascular endothelium, often resulting in vascular occlusion. Carbon-black brain perfusion demonstrated that occlusion of vascular channels progressed after irradiation and was complete within 4 hours. Histopathological examination at 1, 5, and 15 days revealed that the associated infarct evolved reproducibly through several characteristic stages, including a phase of massive macrophage infiltration. Although cerebral infarction in this model is initiated by thrombosis of small blood vessels, the fact that the main pathological features of stroke are consistently reproduced should permit its use in assessing treatment regimens. Further, the capability of producing infarction in preselected cortical regions may facilitate the study of behavioral, functional, and structural consequences of acute and chronic stroke.

Cerebral infarction due to carotid occlusion and carbon monoxide exposure. I. Pathophysiological and neuropathological investigations

Ligation of one common carotid artery and exposure to carbon monoxide has proved to be a reliable method of producing unilateral cerebral infarcts in the rat, allowing controlled experiments in any given sample size. Pathophysiological measurements in awake and narcotized rats has shown that, in contrast to hypoxic hypoxia, the carbon monoxide-induced functional anaemia (3000 ppm in room air) did not stimulate chemoreceptors, thus causing a severe systemic hypotension owing to peripheral vascular dilatation. This hypotension is likely to represent the main pathogenetic factor in this model. An inhibition of ferro-enzymes by carbon monoxide did not seem to be involved. The only cause of death was shown to be diffuse ipsilateral brain oedema with or without extra-vasation of serum proteins. The EEG, the systemic arterial pressure, rotational behaviour, and carotidal stump pressure proved to be reliable predictors of outcome.

Cerebral embolism: local CBF and edema measured by CT scanning and Xes inhalation

Serial CT scans were made in baboons after cerebral embolization during stable Xe inhalation for measuring local values for CBF and lambda (brain-blood partition or solubility coefficients), followed by iodine infusion for detecting blood-brain barrier (BBB) damage. Supplementary 133Xe CBF measurements were made at corresponding intervals. Persistent zones of zero flow surrounded by reduced flow were measured predominantly in subcortical regions, which showed gross and microscopic evidence of infarction at necropsy. Overlying cortex was relatively spared. Reduced lambda values attributed to edema appeared within 3--5 minutes and progressed up to 60 minutes. Damage to BBB with visible transvascular seepage of iodine began to appear 1--1 1/2 hours after embolism. In chronic animals, lambda values were persistently reduced in areas showing histologic infarction. Contralateral hemispheric CBF increased for the first 15 minutes after embolism, followed by progressive reduction after 30 minutes (diaschisis).