Gerhard Rosner

Dynamic Penumbra Demonstrated by Sequential Multitracer PET after Middle Cerebral Artery Occlusion in Cats

Experimental models of focal cerebral ischemia have provided important data on early circulatory and biochemical changes, but typically their correspondence with metabolic and hemodynamic findings in stroke patients has been poor. To fill the gap between experimental studies at early time points and rather late clinical studies, we repeatedly measured CBF, CMRO2, oxygen extraction fraction (OEF), cerebral blood volume (CBV), and CMRglc in six cats before and up to 24 h after permanent middle cerebral artery (MCA) occlusion (MCAO), using the 15O steady state and [18F]fluorodeoxyglucose methods and a high-resolution positron emission tomography (PET) scanner. Likewise, three sham-operated control cats were studied during the same period. Final infarct size was determined on serial histologic sections. In the areas of final glucose metabolic depression that were slightly larger than the histologic infarcts, mean CBF dropped to ∼40% of control values immediately on arterial occlusion. It further decreased to <20% during the course of the experiment. This progressive ischemia was most conspicuous in border zones. CMRO2 fell to a lesser degree (55%), eventually reaching ∼25% of its control level. At early stages, OEF increased mainly in the center of ischemia. With time, areas of increased OEF moved from the center to the periphery of the MCA territory. Concurrently, progressive secondary decreases in OEF in conjunction with further reductions of CBF and CMRO2 indicated the development of central necrosis. The findings are highly suggestive of a dynamic penumbra. In five cats with complete MCA infarcts, CBF decreased and OEF increased in the contralateral hemisphere after 24 h, suggesting whole-brain damage. This effect may be explained by the widespread brain edema found histologically in addition to the nonspecific CBF reductions and OEF elevations observed also in the sham-operated controls after 1 day in the experimental condition. In one cat, cortical OEF increased only transiently. Normal CMRO2 and CMRglc were eventually restored, and the final infarct was small. This study demonstrates that acute regional pathophysiologic changes can be repeatedly assessed by multivariate PET in cats. Viable tissue can be detected up to several hours after MCA occlusion, and the transition of misery-perfused regions into necrosis or preserved tissue can be followed over time. The present results support the concept of a dynamic penumbra, in which for up to 24 h tissue damage spreads progressively from the center to the periphery of ischemia. Sequential high-resolution PET provides insight into the dynamics of regional pathophysiology and may thus further the development of rational therapeutic strategies.

Flow thresholds for extracellular purine catabolite elevation in cat focal ischemia.

Ischemic glutamate excitotoxicity may be counteracted by adenosine which appears extracellularly during ischemia as an intermediate purine catabolite and has the potential to modulate glutamate release and its receptor action. The present study was conducted to evaluate the flow threshold for purine catabolite accumulation in relation to that for glutamate elevation in focal ischemia which was induced by middle cerebral artery (MCA) occlusion in halothane anesthetized cats. Assemblies of platinum electrodes and microdialysis probes were inserted into the somatosensory (SF, n = 13) and the auditory (A, n = 9) cortices to assess local cerebral blood flow (CBF) using hydrogen clearance and purine catabolite (adenosine, inosine and hypoxanthine) as well as glutamate concentrations in the dialysate using high-performance liquid chromatography (HPLC). In both investigated areas, purine catabolites were elevated if CBF fell below 25 ml/100 g/min, while glutamate increased at a flow threshold below 20 ml/100 g/min. Maximum elevations of adenosine, inosine and hypoxanthine were 76-, 29- and 11-fold, respectively, that of glutamate was 24-fold. In the range between 20 and 25 ml/100 g/min, significant increases of adenosine (5-15-fold) were measured, while glutamate did not markedly increase. The elevation of adenosine was transient whereas that of inosine, hypoxanthine and glutamate persisted over an ischemic period of 3 h. The higher flow threshold for adenosine may reflect an inherent but time limited protective mechanism against glutamate excitotoxicity.

Elevation of neuroactive substances in the cortex of cats during prolonged focal ischemia.

Sustained accumulation of excitatory amino acids and other neuroactive substances may contribute to the delayed progression of infarction in focal ischemia. Following occlusion of the left middle cerebral artery (MCAO), extracellular amino acid and purine catabolite concentrations as well as local CBF were repeatedly monitored for up to 15 h in auditory (A) and somatosensory (SF) cortices of seven halothane-anesthetized cats using microdialysis/HPLC and hydrogen clearance. MCAO resulted in persistent reduction of local CBF, which was more severe in A (n = 6) than in SF (n = 6). Accordingly, higher transmitter amino acid and purine catabolite concentrations were found in A than in SF during ischemia. Aspartate, glutamate, and γ-aminobutyrate (GABA) as well as hypoxanthine and inosine reached maximum levels 1–2 h after onset of ischemia (15-, 7-, 31-, 8-, and 14-fold increases, respectively). Maximum levels remained almost constant, with the exception of inosine, which decreased subsequently. Glycine seemed to increase with prolonged ischemia and reached maximum levels (10-fold) 15 h after occlusion. Adenosine peaked 30 min after occlusion (54-fold) and decreased thereafter to control levels within 1–2 h. One hour after occlusion, CBF thresholds for amino acid elevation were lower (glutamate and GABA ∼20 ml 100 g−1 min−1 and glycine ∼10 ml 100 g−1 min−1) than 6 and 15 h after occlusion (thresholds for all amino acids at ∼30 ml 100 g−1 min−1). These results indicate that in prolonged ischemia, excitotoxicity is an important factor, particularly in border zones of ischemic foci. It may be enhanced by an increase of glycine and the early disappearance of adenosine, which are considered to facilitate and inhibit, respectively, the deleterious effects of excitatory amino acids.

Hemodynamic and metabolic effects of decompressive hemicraniectomy in normal brain: An experimental PET-study in cats

Hemicraniectomy is increasingly used as treatment option in stroke and in head trauma, but little is known on the (patho)physiological regional effects of hemicraniectomy in the normal brain. A standard left-sided craniectomy was performed in three cats. Regional cerebral blood flow (CBF), cerebral metabolic rate of oxygen (CMRO(2)) and cerebral metabolic rate of glucose (CMR(glc)) were measured from the brain tissue underneath the craniectomy at 2, 20 and 28 h after hemicraniectomy. CBF significantly decreased (P<0.01) and oxygen extraction fraction (OEF) (P<0.05) significantly increased. CMRO(2) and CMR(glc) decreased only in regions with most severe CBF reduction. These effects remained for at least a day irrespective of corrective sustaining cranioplasty. The authors demonstrated for the first time that decompressive hemicraniectomy in the cat decreases CBF, and to a lesser extent CMR02 and CMR(glc) 2 h after hemicraniectomy in normal brain tissue that last for at least 1 day. Even though the underlying basis of these phenomena are not fully understood, this finding implies that persisting pathophysiological processes are induced by hemicraniectomy and should be taken into consideration for surgical indications.

Differences in ischemia-induced accumulation of amino acids in the cat cortex.

It is well established that excitatory amino acid neurotransmitters are extensively liberated during ischemia and that they have neurotoxic properties contributing to neuronal injury. To study changes in the liberation of excitatory and other amino acids during cerebral ischemia, we measured their extracellular concentrations and related them to blood flow levels and electrophysiologic activity (electrocorticogram and auditory evoked potentials) before and for up to 2 hours after multiple cerebral vessel occlusion in 14 anesthetized cats. Blood flow levels between 0 and 43 ml/100 g/min were reached. Concentrations of the excitatory amino acid neurotransmitters increased most (aspartate 10-fold, glutamate 30-fold, and gamma-aminobutyric acid 300-fold compared with control values) below a blood flow threshold of 20 ml/100 g/min. The total power of the electrocorticogram and the amplitude of the auditory evoked potentials were affected below the same blood flow threshold. In contrast, concentrations of the nontransmitter amino acids taurine, alanine, asparagine, serine, and glutamine increased 1.5-5-fold as blood flow decreased, while concentrations of the essential amino acids phenylalanine, valine, leucine, and isoleucine did not change during cerebral ischemia. The great increases in concentrations of the excitatory amino acid neurotransmitters below a blood flow threshold close to that for functional disturbance is in accordance with the role of these amino acids in ischemic cell damage. Their release at blood flow levels compatible with cell survival and the increase in their concentrations with severity and duration of cerebral ischemia imply that excitotoxic antagonists may have potential as therapeutic agents.

Experimental focal ischemia in cats: changes in multimodality evoked potentials as related to local cerebral blood flow and ischemic brain edema.

Somatosensory and auditory evoked cortical potentials (SEPs and AEPs), regional cerebral blood flow, regional brain water content, and alteration of the blood-brain barrier were investigated in 3 cortical areas during permanent and 1- and 2-hour transient occlusion of the left middle cerebral artery and after restoration of blood flow in cats. During occlusion, blood flow in the auditory cortex was severely suppressed. In the fore limb projection area of the somatosensory cortex, blood flow was moderately reduced while it was nearly unaffected in the hind limb projection area. Despite different degrees of ischemia in the 3 cortical areas, all evoked responses were completely abolished within 10 minutes after occlusion. During permanent occlusion, the pattern of blood flow reduction persisted, and all evoked potentials stayed abolished. Recirculation after occlusion restored blood flow rapidly. AEPs recovered poorly after both 1 and 2 hours of ischemia. SEPs regained normal amplitudes soon after recirculation in the group with 1-hour occlusion. After 2 hours of ischemia, the recovery of SEPs was variable but better than that of the AEPs. Remarkable water accumulation was observed in the auditory cortex of all 3 groups and was accompanied in the 2-hour ischemia group by a disruption of the blood-brain barrier. In the 2-hour group, water accumulation was also found in the subcortical white matter radiation, whereas significant changes in regional water content were not observed in the somatosensory areas. The present study indicates that abolition of SEPs during middle cerebral artery occlusion in cats is caused by lesions in the afferent pathway leading to cortical deafferentation rather than by cortical ischemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Extracellular correlates of glutamate toxicity in short-term cerebral ischemia and reperfusion: A direct in vivo comparison between white and gray matter

Glutamate toxicity and cellular calcium overload are thought to be pathophysiological key factors not only in gray matter (GM) but also in white matter (WM) ischemia. Correlates of excitotoxicity have never been directly investigated in vivo in GM and WM ischemia and reperfusion. We measured simultaneously amino acids, purines, and calcium in relation to tissue depolarization using microdialysis and ion-selective electrodes and regional CBF using hydrogen clearance in GM and WM of cats during 10 min of global ischemia and 120 min of reperfusion. CBF ceased during ischemia. Reperfusion was followed by hyperperfusion that turned into hypoperfusion within 60 min in both GM and WM. Direct current potential decreased in ischemia to around -15 mV in GM and -10 mV in WM and shifted back after reperfusion towards control levels in both compartments. Extracellular calcium decreased in GM during ischemia, whereas it increased in WM. After reperfusion, calcium returned to control levels in both GM and WM. Glutamate, aspartate, GABA, and taurine increased in GM but not in WM during ischemia and reperfusion. Adenosine increased transiently in both compartments peaking during the first minutes of reperfusion, and returned thereafter to control levels. Contrasting with GM, deleterious processes such as glutamate accumulation and cellular Ca(2+) influx do not occur in WM during short-term ischemia and reperfusion. Rather, an intrinsic neuroprotective role of adenosine may be discussed. In our view, therefore, therapeutic strategies against glutamate toxicity in short-term ischemia and reperfusion should be mainly focused on GM.

Peri-Infarct Depolarizations Reveal Penumbra-Like Conditions in Striatum

Spreading depression-like peri-infarct depolarizations not only characterize but also worsen penumbra conditions in cortical border zones of experimental focal ischemia. We intended to investigate the relevance of ischemic depolarization in subcortical regions of ischemic territories. Calomel electrodes measured DC potentials simultaneously in the lateral and medial portions of the caudate nucleus (CN) of 11 anesthetized cats after permanent occlusion of the middle cerebral artery. Additionally, platinum electrodes measured cerebral blood flow (CBF) in the CN, and laser Doppler probes CBF in the cortex. Depolarizations (negative DC shifts >10 mV) were obtained in 10 of 11 cats. Further differentiation revealed that short-lasting spreading depression-like depolarizations (SDs; 5 of 10 cats: 5.24 ± 1.22 min total duration; 23.3 ± 4.2 mV amplitude) were predominantly found in medial and longer depolarizations (LDs; 4 of 10 cats: 64.7 ± 47.5 min; 25.0 ± 11.3 mV) in the lateral CN. Terminal depolarizations (TDs; 6 of 10 cats; without repolarization) occurred immediately after occlusion or at later stages, being then accompanied by elevations of intracranial pressure presumably inducing secondary CBF reduction. CBF tended to be lower in regions with TDs (33.3 ± 29.9% of control) and LDs (37.3 ± 22.8%) than in regions with SDs (51.5 ± 48.0%). We conclude that in focal ischemia, transient peri-infarct depolarizations emerge not only in cortical but also in striatal gray matter, thereby demonstrating the existence of subcortical zones of ischemic penumbra. The generation of these ischemic depolarizations is a multifocal process possibly linked to brain swelling and intracranial pressure rise in the later course of focal ischemia, and therefore a relevant correlate of progressively worsening conditions.

Prolonged Transient Ischemia Results in Impaired CBF Recovery and Secondary Glutamate Accumulation in Cats

Effects of prolonged focal ischemia [middle cerebral artery occlusion (MCAO)] of 1, 2, and 4 h followed by 15-h reperfusion on CBF, extracellular amino acids, purine catabolites, evoked potentials, and infarction were studied in core (A: auditory cortex) and border zone (SF: somatosensory cortex) areas of halothane-anesthetized cats. Following MCAO, CBF reduction was severe in A(<15 ml 100 g−1 min−1) and mild to moderate in SF. Prominent elevation of glutamate and abolition of evoked potentials in A contrasted with milder and more variable disturbances in SF. After reperfusion, recovery of CBF, glutamate, and evoked potentials was fast and persistent in the 1- and 2-h groups. In the 4-h group, immediate recovery of CBF, glutamate, and evoked potentials was incomplete, and secondary deterioration of all parameters was obtained at the end of the experiments. Infarction in the 4-h group was significantly larger than in the 1- and 2-h groups. Persistent recovery of extracellular glutamate concentration and electrical function and salvage of neuronal tissue from infarction therefore seem to depend on successful restoration of CBF, which in turn depends on the magnitude and the duration of CBF reduction and of exposure to potentially harmful substances such as glutamate during the ischemic attack.

Functional impairment due to white matter ischemia after middle cerebral artery occlusion in cats.

We recorded regional cerebral blood flow, somatosensory evoked potentials, and auditory evoked potentials in the thalamic relay nuclei (ventral posterior lateral nucleus and medial geniculate body) and in the somatosensory and auditory cortices during and after 1 hour of transient left middle cerebral artery occlusion in nine cats. Regional cerebral blood flow was also measured in the thalamocortical tracts of five of these cats. Additionally, the integrity of thalamocortical connections was tested by retrograde labeling of the thalamic nuclei with horseradish peroxidase in eight cats (three of which experienced no ischemia). Regional cerebral blood flow was severely reduced during middle cerebral artery occlusion in the left primary auditory cortex (8.5 ml/100 g/min) and in white matter pathways (6.4-7.6 ml/100 g/min). In contrast, regional cerebral blood flow did not change significantly in the somatosensory cortex or in either thalamic nucleus. Evoked potentials were abolished in both cortices but remained unchanged in the thalamic nuclei. Cortical somatosensory evoked potentials disappeared 5-8 minutes later than auditory evoked potentials. Recirculation after 1 hour of ischemia resulted in rapid and almost complete recovery (94%) of somatosensory evoked potentials and little recovery (18.4%) of auditory evoked potentials. We conclude that in the auditory pathway both cortical and fiber tract ischemia are (perhaps synergistically) responsible for dysfunction, while in the somatosensory cortex evoked potentials are abolished due to white matter ischemia. The delayed disappearance and better recovery of somatosensory than of auditory evoked potentials indicate that ischemic tolerance is higher in fiber tracts than in cortex.