'Ischemic tolerance' phenomenon detected in various brain regions.

We investigated the effects of mild and non-lethal ischemic insult on neuronal death following subsequent lethal ischemic stress in various brain regions, using a gerbil model of bilateral cerebral ischemia. Single 10-min ischemia consistently caused neuronal damage in the hippocampal CA1, CA2, CA3 and CA4, layer III/IV of the cerebral cortex, dorsolateral part of the caudoputamen and ventrolateral part of the thalamus. On the other hand, in double ischemia groups, 2-min ischemic insult 2 days before 10-min ischemia exhibited significant protection in the CA1 and CA3 of the hippocampus, the cerebral cortex, the caudoputamen and the thalamus. Five-min ischemic insult 2 days before 10-min ischemia also showed protective effect in the same areas as those of 2-min ischemia except for the CA1 region of the hippocampus, while 1-min ischemic insult exhibited no protective effect in any brain regions. In the immunoblot analysis, both 2- and 5-min ischemia caused increased synthesis of heat shock protein 72 (HSP 72) in the hippocampus, but 1-min ischemia did not. The present study demonstrated that the 'ischemic tolerance' phenomenon was widely found in the brain and also suggested that ischemic treatment severe enough to cause HSP 72 synthesis might be needed for induction of 'ischemic tolerance'.     

‘Ischemic tolerance’ phenomenon found in the brain

We investigated the possibility that neuronal cells given a mild ischemic treatment sufficient to perturb the cellular metabolism acquired tolerance to a subsequent, and what would be lethal, ischemic stress in vivo. Cerebral ischemia was produced in the gerbils by occlusion of both common carotids for 5 min, which consistently resulted in delayed neuronal death in the CA1 region of the hippocampus. Minor 2-min ischemia in this model depletes high-energy phosphate compounds and perturbs the protein synthesis, but nerver causes neuronal necrosis, and therefore was chosen as mild ischemic treatment. Single 2-min ischemia 1 day or 2 days before 5 min ischemia exhibited only partial protective effects against delayed neuronal death. However, two 2-min ischemic treatments at 1 day intervals 2 days before 5 min ischemia exhibited drastically complete protection against neuronal death. The duration and intervals of ischemic treatment, enough to perturb cellular metabolism and cause protein syhthesis, were needed respectively, because neither 1-min ischemia nor 2-min ischemia received twice at short intervals exhibited protective effects. This ‘ischemic tolerance’ phenomenon induced by ischemic stress — which is unquestionably important — and frequent stress in clinical medicine, is intriguing and may open a new approach to investigate the pathophysiology of ischemic neuronal damage.     

Immunohistochemical investigation of ischemic and postischemic damage after bilateral carotid occlusion in gerbils

We investigated progression and recovery of neuronal damage during and after global cerebral ischemia in gerbils after bilateral occlusion of the common carotid arteries, using the immunohistochemical method (reaction for tubulin and creatine kinase BB-isoenzyme). The earliest, but reversible, ischemic lesions occurred after 3 minutes' ischemia in the subiculum-CA1 and CA2 regions of the hippocampus. The lesions became irreversible after 4 minutes' ischemia. The ischemic and postischemic lesions in the cerebral cortex, thalamus, and caudoputamen were partially or completely reversible if the ischemic period was 5 minutes, whereas delayed degeneration occurred in the pyramidal cells of the medial CA1 region after reperfusion for 48 hours (delayed neuronal death). After 10 minutes' ischemia and subsequent reperfusion, delayed neuronal death extended from the medial to the lateral CA1 region; the ischemic and postischemic lesions in the cerebral cortex, thalamus, and caudoputamen also expanded during reperfusion. Our investigation demonstrates that selective vulnerability existed in global cerebral ischemia as in incomplete or regional ischemia and suggests that neurons in many areas of the brain possessed the potential for recovery, progressive deterioration, and even delayed neuronal death depending on the severity and duration of cerebral ischemia.     

Repeated focal cerebral ischemia in gerbils is associated with development of infarction.

We induced repeated focal cerebral ischemia in gerbils. Single 5-min ischemia produced neuronal damage limited to the ipsilateral CA1 and CA4 hippocampus. Two 5-min ischemic insults spaced at a 1-h interval caused selective neuronal damage to the CA1, CA3 and CA4 hippocampus, striatum, neocortex, and thalamus. Three 5-min ischemic insults at 1-h intervals produced infarction. Thus, repeated focal ischemia produced cumulative brain damage by conversion of sublethal damage into selective neuronal damage and of the neuronal damage into infarction.     

Temporospatial expression of HSP72 and c-JUN, and DNA fragmentation in goat hippocampus after global cerebral ischemia

The role of gene induction (expression of HSP72 and c-JUN proteins) and delayed ischemic cell death (in situ labeling of DNA fragmentation) have been investigated in the goat hippocampus after transient global cerebral ischemia. The animals were subjected to 20-min ischemia (bilateral occlusion of the external carotid arteries plus bilateral jugular vein compression) and allowed to reperfuse for 2 h, and then 1, 3, and 7 days. Histological signs of cell loss were not found in the hippocampus at 2 h, 1 day, or 3 days of reperfusion. However, such an ischemic insult produced extensive, selective, and delayed degeneration in the hippocampus, as 68% of the neurons in CA1 had died at 7 days, but cell loss was not detected in CA3 and dentate gyrus fields. Concomitantly, a high percentage of TUNEL-positive CA1 neurons (60+/-9%, mean +/- SEM) was seen at 7 days, but not at the earlier time points. Mild induction of HSP72 was detected in the goat hippocampus after ischemia. The maximum percentage of HSP72-positive neurons (10-15%) was shown at 3 days of reperfusion and was concentrated mainly in the CA3 field, subiculum, and hilus, rather than in the CA1 field, whereas HSP72 expression was hardly detected at 7 days. At this later time point, scattered induction of nuclear c-JUN was found in a few neurons. The results show that: 1) postischemic delayed neuronal death selectively affects the CA1 field in the goat hippocampus, a phenomenon which seems to take longer to develop than in previously reported rodent models; and 2) postischemic expression of c-JUN does not appear to be related to cell death or survival, while the inability of most CA1 neurons to express HSP72 could contribute to neuronal death.     

Neuronal damage following non-lethal but repeated cerebral ischemia in the gerbil

Summary Brief, non-lethal transient forebrain ischemia in the gerbil can injure selectively vulnerable neurons when such ischemia is induced repeatedly. The influence of the number and interval of the ischemic insults on neuronal damage, as well as the time course of damage, following repeated 2-min forebrain ischemia were examined. A single 2-min forebrain ischemia were examined. A single 2-min ischemic insult caused no morphological neuronal damage. A moderate number of hippocampal CA1 neurons were destroyed following two ischemic insults with a 1-h interval, and destruction of almost all CA1 neurons resulted from three or five insults at 1-h intervals. Three and five insults also resulted in moderate to severe damage to the striatum and thalamus, depending on the number of episodes. Although three ischemic insults at 1-h intervals caused severe neuronal damage, this number of insults at 5-min and 4-h intervals caused destruction of relatively few neurons, and non neurons were destroyed at 12-h intervals. Following three ischemic insults at 1-h intervals, damage to the striatum, neocortex, hippocampal CA4 subfield and thalamus was observed at 6–24 h of survival, whereas damage to the hippocampal CA1 subfield appeared at 2–4 days. The results indicate that even a brief non-lethal ischemic insult can produce severe neuronal damage in selectively vulnerable regions when it is induced repeatedly at a certain interval. The severity of neuronal damage was dependent on the number and interval of ischemic episodes.     

Re-evaluation of ischemia-induced neuronal damage in hippocampal regions in the normothermic gerbil

Summary It has not been discussed whether transient forebrain ischemia of 5-min duration, which is a model frequently used to evaluate pharmacological protection against ischemic injury, is an optimal model in the CA1 field of this animal whose brain temperature is maintained at normothermic levels. The temperature of the brain during an ischemic insult strongly affects the extent of the resulting neuronal injury. If the brain temperature is not regulated, it usually falls in the gerbil by 2°–4°C during 5-min ischemia. However, the brain temperature during ischemic insult was not regulated in many previous studies. In the present study, the effects of transient (1 to 5 min) forebrain ischemia on the development of neuronal degeneration in hippocampal regions of the gerbil whose brain temperature was maintained at 37°C were examined. In the CA1 field of the hippocampus, transient ischemia of 3- and 4-min duration caused almost the same maximal damage (88%–91% neuronal loss) as observed in the gerbils subjected to 5-min ischemia. Transient ischemia of 2-and 2.5-min duration provoked substantial neuronal damage in 25% and 55% of experimental gerbils, respectively. These results indicate that 5-min bilateral forebrain ischemia is more than is necessary to examine ischemiainduced neuronal degeneration in hippocampal CA1 field of the gerbil whose brain temperature is maintained at normothermic levels. In the normothermic gerbil brain, an ischemic period of 3-min already induces extensive neuronal death in the CA1 and, thus, constitutes a sensitive model to evaluate faint protective effects of drugs against ischemic injury in the normothermic gerbil.Supported by Grant-in-Aid for Encouragement of Young Scientist (03857019) from the Ministry of Education, Science and Culture of Japan and the Sasakawa Health Science Foundation to A.M., and Grants-in-Aid for General Scientific Research (01400004 and 03557007) from the Ministry of Education, Science and Culture of Japan, Japan Foundation for Aging and Health and Mitsui Life Social Welfare Foundation to K.K.     

Calcium deposits in the thalamus following repeated cerebral ischemia and long-term survival in the gerbil

We investigated the long-term changes in the gerbil brain following three episodes of 2-min forebrain ischemia at 1-h intervals in comparison with a 6-min period of ischemia. The animals were sacrificed after 1 month and 6 months. Following either ischemic insult, the hippocampal CA1 region showed a loss of pyramidal neurons together with a diffuse calcium accumulation as shown by alizarin red S staining. Three 2-min ischemic insults additionally produced neuronal damage in the striatum and thalamus. The thalamic damage was accompanied by an accumulation of small calcium granules after 1 month and large calcium concretions after 6 months. Calcium staining in the striatum was weak. Thus, the thalamic neuronal damage was accompanied by an active process of calcification, which has not been described in experimental cerebral ischemia models. The observations show that repeated ischemic insults produce different long-term effects in different brain regions.     

沙鼠脑缺血耐受的组织学变化及HSP在其中的作用

目的 :观察脑缺血耐受时的组织学变化及 HSP在其中的作用。方法 :通过 HE染色观察脑缺血耐受时的组织学变化 ,并通过免疫组化染色 ,了解 HSP70及 HSP2 7在其中的作用。结果 :一次性 5分钟缺血后 7天海马 CA1区神经元大多坏死 ,若在缺血前给予 2分钟的缺血预处理 ,该区神经元大多保留 ,表现出明显的保护作用。只给一次性 5分钟缺血 ,海马 CA1区神经元无 HSP70染色。若在缺血前给予预处理 ,海马 CA1区神经元可见明显 HSP70染色。而HSP2 7主要在胶质细胞表达 ,海马区的神经元未见其表达。结论 :缺血前给予预处理对以后的缺血有保护作用 ;在缺血耐受过程中 ,HSP70表达出一定的保护作用。     

Induction of 27-kDa heat shock protein following cerebral ischemia in a rat model of ischemic tolerance

Preconditioning the brain with sublethal cerebral ischemia induces tolerance to subsequent lethal periods of ischemia (ischemic tolerance). The purpose of this study is to investigate the role of low-molecular weight stress proteins, 27-kDa heat shock protein (HSP27) and αB crystallin, in ischemic tolerance. We measured the content of these proteins with enzyme immunoassay in the rat hippocampus and cerebral cortex following 6 min of ischemia with and without preconditioning with 3 min of ischemia and 3 days of reperfusion. We also visualized the localization of HSP27 immunohistochemically in comparison with that of HSP70. A 3-min period of ischemia caused a 2.4-fold increase in HSP27 content in the hippocampus after 3 days. Immunohistochemical localization of HSP27 was found in glial cells in all subregions of the hippocampus, whereas HSP70 immunostaining was seen only in CA1 pyramidal neurons. HSP27 content in the hippocampus decreased 2 h after 6 min of ischemia. HSP27 content progressively increased in the unpreconditioned hippocampus after 1 and 3 days, but returned to preischemic levels in the preconditioned hippocampus. HSP27 and HSP70 immunostaining was seen in CA1 pyramidal neurons after 1 day both with and without preconditioning. After 3 and 7 days, an intense HSP27 staining was observed in reactive glial cells in the CA1 without preconditioning, whereas the staining decreased in the preconditioned hippocampus. HSP70 staining was seen only in neurons at these time points. We observed no significant changes in HSP27 content in the cerebral cortex although neurons in the third and fifth layers were immunostained after 1 and 3 days. We observed no alterations in αB crystallin content after ischemia both in the hippocampus and the cortex. The present study demonstrated that cerebral ischemia induces HSP27 expression but not αB crystallin. Both HSP27 and HSP70 induction had a good temporal correlation with the induction of ischemic tolerance. However, different sites of action were suggested because the localization and cell types of HSP27 induction were quite different from those of HSP70 induction. The result suggests that it is unlikely that HSP27 is directly involved in the protection afforded by ischemic preconditioning.     

Hippocampal neuronal damage after transient forebrain ischemia in monkeys

To investigate cerebral injury in the monkey due to transient ischemia, monkeys were each subjected to temporary occlusion of eight (bilateral common carotid, internal and external carotid, and vertebral) major arteries. After 0 (control), 5, 10, 13, 15, and 18 min occlusion, blood flow was restored. The monkeys were sacrificed by perfusion fixation 5 days after the operation, and all brain regions were then histologically examined for ischemic neuronal changes induced by the occlusion. The amplitude of EEG signals from skull and scalp became almost isoelectric within 1-6 min after the onset of occlusion. The EEG signals from the hippocampus were markedly attenuated within 1-4 min, although they did not become completely isoelectric. Blood pressure was significantly increased after 10-min ischemia. Five-min occlusion produced no ischemic neuronal changes except a slight increment of glial cells in the striatum and III, V, and VI layers of the neocortices. After 10- to 15-min occlusion, there were ischemic cell changes restricted exclusively to the CA1 subfield of the hippocampus. Eighteen-min occlusion produced more prominent ischemic neuronal damage in the CA1 subfield of the hippocampus, but ischemic neuronal damage was no longer confined to the hippocampus. These results suggest that only the CA1 subfield of the monkey hippocampus could be damaged by mild ischemic insult. We demonstrate that the limited lesion of the hippocampus, especially the CA1 subfield, after 10- to 15-min occlusion of eight arteries in the monkey, produces a model equivalent to human amnesia caused by transient ischemic insult.     

Effects of normothermic versus mild hyperthermic forebrain ischemia in rats

We compared the neuropathological consequences of global forebrain ischemia under normothermia versus mild hyperthermia. Twenty-one rats underwent 20 minutes of four-vessel occlusion during which brain temperature was maintained at either 37 degrees C (normothermia, n = 9) or 39 degrees C (hyperthermia, n = 12). Quantitative neuropathological assessment was conducted 1 or 3 days later. At 1 day following the ischemic insult, normothermic rats demonstrated neuronal injury mainly confined to the most dorsolateral striatum. By 3 days, ischemic cells were present throughout the striatum and CA1 hippocampus in normothermic animals. Compared with normothermic rats, intraischemic hyperthermia significantly increased the extent and severity of brain damage at 1 day after the ischemic insult. Areas of severe neuronal necrosis and frank infarction included the cerebral cortex, CA1 hippocampus, striatum, and thalamus. Morphologic damage was also detected in the cerebellum and pars reticulata of the substantia nigra. An overall mortality rate of 83% was demonstrated at 3 days in the hyperthermic ischemic group. We conclude that intraischemic hyperthermia 1) markedly augments ischemic brain damage and mortality compared with normothermia, 2) transforms ischemic cell injury into frank infarction, and 3) accelerates the morphological appearance of ischemic brain injury in regions usually demonstrating delayed neuronal necrosis. These observations on mild hyperthermia may have important implications for patients undergoing cardiac or cerebrovascular surgery as well as patients following cardiac arrest or those with stroke-in-evolution.     

Temporal profile of the effects of pretreatment with brief cerebral ischemia on the neuronal damage following secondary ischemic insult in the gerbil: cumulative damage and protective effects

We examined the response of the gerbil brain to secondary ischemic insult following pretreatment with brief ischemia at intervals of 5 min, 1 and 6 h, 1, 2, 4, 7 and 14 days. Two minutes of bilateral carotid artery occlusion produced no histopathological brain damage, whereas 3 min of occlusion caused a moderate to severe reduction in the number of hippocampal CA1 pyramidal cells. Two-minute occlusion followed by 3-min occlusion at 5-min, 1- and 6-h intervals resulted in almost complete destruction of CA1 neurons. Additional neuronal damage was observed in the striatum at a 1-h interval and in the thalamus and the neocortex at 1- and 6-h intervals. The neuronal damage was most severe at a 1-h interval. Two-minute ischemia followed by 3-min ischemia at intervals of 1, 2, 4 and 7 days, however, caused a marked protective effect, and the hippocampal CA1 neurons were preserved. The protective effect was not observed at a 14-day interval and following pretreatment with 1-min ischemia. Thus, pretreatment with brief ischemia leads to complex responses of the brain to secondary ischemic insult; cumulative damage at intervals of 1-6 h and protective effects at intervals of 1-7 days.     

During repetitive forebrain ischemia, post-ischemic hypothermia protects neurons from damage.

In rodents damage from repetitive transient cerebral ischemia is more severe than that seen with a single ischemic insult of similar duration. Mild hypothermia has been shown to be very effective in protecting the brain during single ischemic insults. We tested the protective effects of hypothermia in repetitive ischemic insults. We used the gerbil model of repetitive ischemia (three minutes ischemia repeated at one hourly intervals three times) and histological evaluation was done using the silver staining technique. Our study reveals that a decrease in body and scalp temperature by 1-2 degrees Celsius can significantly reduce neuronal damage in the cerebral cortex, CA1 region of the hippocampus and substantia nigra reticulata during repetitive ischemia. As the hypothermia was induced after the initial insult, we believe this offers an opportunity for intervention in the clinical settings.     

Emphasized selective vulnerability after repeated nonlethal cerebral ischemic insults in rats.

BACKGROUND AND PURPOSE: We examined the density and distribution of brain damage after repeated periods of nonlethal ischemic insult in rats in comparison with damage after single lethal periods of ischemic insult. METHODS: Transient cerebral ischemia was induced by four-vessel occlusion for 3, 10, 20, and 30 minutes, and 3-minute periods of ischemia were repeated two, three, or five times at 1-hour intervals, followed by 7 days of survival. RESULTS: Three minutes of ischemia produced no brain damage, but 10-30 minutes of ischemia produced neuronal damage, depending on the length of ischemia, to the selectively vulnerable forebrain regions such as hippocampal CA1 and CA4 subfields, neocortex, striatum, and ventral thalamus, as well as to the brain stem structures (medial geniculate body, substantia nigra, and inferior colliculus) and cerebellar Purkinje cells. Two 3-minute periods of ischemic insult produced neuronal damage to the hippocampal CA1 subfield. Three and five 3-minute insults produced neuronal damage extensively to the selectively vulnerable forebrain areas. An intense cumulative effect of damage was observed in the ventral thalamus, whereas the substantia nigra and the inferior colliculus were resistant to repeated ischemic insults. CONCLUSIONS: Our data indicate that the density and distribution of neuronal damage after repeated ischemic insults are altered as compared with after single ischemia.     

Neuronal damage and calcium accumulation following repeated brief cerebral ischemia in the gerbil

We investigated the distribution of neuronal damage following brief cerebral transient ischemia and repeated ischemia at 1-h intervals in the gerbil, using light microscopy and 45Ca autoradiography as a marker for detection of ischemic damage. The animals were allowed to survive for 7 days after ischemia induced by bilateral carotid artery occlusion. Following 2-min ischemia, neuronal damage determined by abnormal calcium accumulation was not observed in the forebrain regions. Following 3-min ischemia, however, abnormal calcium accumulation was recognized only in the hippocampal CA1 sector and part of the striatum. Two 2-min ischemic insults caused extensive abnormal calcium accumulation in the dorsolateral part of striatum, the hippocampal CA1 sector, the thalamus, the substantia nigra and the inferior colliculus. The ischemic insults were more severe than that of a single 3-min ischemia. However, three 1-min ischemic insults caused abnormal calcium accumulation only in the striatum. On the other hand, three 2-min ischemic insults caused severe abnormal calcium accumulation in the brain. The abnormal calcium accumulation was found in the dorsolateral part of striatum, the hippocampal CA1 sector, the thalamus, the medial geniculate body, the substantia nigra and the inferior colliculus. Gerbils subjected to three 3-min ischemic insults revealed most severe abnormal calcium accumulation. Marked calcium accumulation was seen not only in the above sites, but also spread in the neocortex, the septum and the hippocampal CA3 sector. Morphological study after transient or repeated ischemia indicated that the distribution and frequency of the neuronal damage was found in the sites corresponding to most of the regions of abnormal calcium accumulation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of intracarotid arterial injection of cyclosporin A and spontaneous hypothermia on brain damage incurred after a long period of global ischemia

A recent study showed that a single intracarotid arterial injection of cyclosporin A (CsA) can dramatically reduce infarct volume in rats subjected to transient focal ischemia. The present experiments were undertaken to investigate whether intracarotid arterial injection of CsA reduces brain damage after global ischemia. Since hypothermia is also an efficacious factor in preventing ischemic brain damage, in the second part of the experiments we tested whether a combination of hypothermia and CsA would provide additional brain protection. Global ischemia of a 30-min duration was induced in the rat. CsA (10 mg/kg) was injected into the carotid artery immediately after reperfusion. Hypothermia was instituted after ischemia by allowing spontaneous head temperature to fall to 30–32°C, while body temperature was upheld at 37°C. The results demonstrated that vehicle-treated animals could not survive beyond 1–2 days after reperfusion, and the histopathological outcome in a separate group of rats perfusion-fixed after 1 day reperfusion showed 80–100% brain damage in the caudoputamen, and in the hippocampal CA1, CA3, CA4 and dentate gyrus subregions. Microinfarction and grade 3 damage were frequently observed in the cingulate and parietal cortex and in the thalamus. CsA moderately prolonged animal survival to 3 days after reperfusion and reduced brain damage to grade 2 in the cortical areas and the thalamus. Hypothermia further increased animal survival to at least 6 days after reperfusion and reduced brain damage to 30% in the caudoputamen, to close to zero in the CA3, CA4, and dentate gyrus, and to grade 1–2 in the cortical areas and the thalamus. The combination of hypothermia and CsA did not give additional protection.     

Time course of early brain edema following reversible forebrain ischemia in rats

Cerebral ischemia is known to be accompanied by brain edema. This increase in brain tissue water content probably influences the final outcome of an ischemic insult negatively. Despite extensive investigations on different aspects of brain edema, information on edema development during the early recirculation period following ischemia is sparse. We assessed changes in brain water content, as reflected by changes in tissue density, during the early recirculation period following severe forebrain ischemia. Fasted rats were subjected to 5, 15, or 30 minutes of ischemia and 5 to 180 minutes of recirculation. The specific gravity of specimens from the caudoputamen, frontoparietal cortex, hippocampus, and mesencephalon were measured with a Percoll linear density gradient. Five minutes of ischemia followed by recirculation did not produce any significant regional brain edema. However, following 15 minutes of ischemia, transient edema developed in the caudoputamen, frontoparietal cortex, and hippocampus. This edema was maximal after 30 minutes of reperfusion and was normalized after 180 minutes of reperfusion. Similar edema was seen following 30 minutes of ischemia. In the mesencephalon (where blood flow is approximately 50% of control during the ischemic insult) no brain edema was noted following 5, 15, or 30 minutes of ischemia. We discuss to what extent this transient regional brain edema may influence the selective neuronal vulnerability and cell damage observed in rats subjected to reversible forebrain ischemia and how these findings may correlate with neurochemical alterations observed during the early recirculation period.     

Ischemic neuronal damage specific to monkey hippocampus: Histological investigation

We previously reported lesions confined specifically to the hippocampus when produced by occluding eight vessels (the bilateral vertebral, common, internal, and external carotid arteries), which supply blood to the brain. However, histopathological changes in the primate brain, caused by ischemic injury, have not previously been thoroughly investigated. In the present study, macaque monkeys were subjected to 5–18-min ischemia by occluding the eight vessels. After the brains were perfused and fixed 5 days after the occlusion, all regions were histologically investigated for ischemic cell changes. Ischemia for 5 min produced no ischemic cell change. Ischemia for 10–15 min produced cell death limited to the deeper portion of the pyramidal cell layer of the CA1 subfield in the hippocampus. In most monkeys, no cell death was observed in any brain region outside of the hippocampus after ischemia for up to 15 min. Ischemia for 18 min produced more widespread cell death in the CA1 subfield of the hippocampus, and cell death was no longer confined to the hippocampus, but was observed in layers III, V, and VI of the neocortices, the striatum, and some other regions. Brains that were perfused and fixed 1 year after 15-min ischemic insult revealed no ischemic cell morphological change in any region, but the number of pyramidal cells in the CA1 subfield was decreased to about half. The results indicate that the CA1 subfield of the monkey hippocampus is the precise region of the brain most susceptible to ischemic insult in the primate forebrain, and after a critical time (15-min ischemia in this procedure) ischemic cell changes occur suddenly and extensively. Ischemia due to occlusion of eight arteries for 10–15 min could produce a model of human amnesia caused by transient ischemic insult.     

Cerebral blood flow and neuronal damage during progressive cerebral ischemia in gerbils

A combined autoradiographic and immunohistochemical method was used to correlate the extent of focal cerebral ischemia and morphologic ischemic damage following unilateral carotid occlusion in 16 gerbils for 5-30 minutes. Immunohistochemical lesions detectable by the reaction for microtubule-associated proteins 1 and 2 were visible in the subiculum-CA1 and CA2 regions of the hippocampus and layer III/IV of the cerebral cortex after 5 minutes of ischemia (n = 4). Local blood flow was promptly reduced but still heterogeneous after 10 minutes of ischemia (n = 4); local blood flow in immunohistochemical lesions was less than 5 ml/100 g/min except in highly vulnerable regions, where flow values of 5-15 ml/100 g/min were observed. After 15 minutes of ischemia (n = 4) local blood flow in less vulnerable regions including the thalamus and caudoputamen also declined to less than 5 ml/100 g/min, and immunohistochemical lesions became visible in those regions after 30 minutes of ischemia (n = 4). On the other hand, many brain regions tolerated local blood flow of less than 5 ml/100 g/min without ischemic damage. The present study demonstrates that selective tissue vulnerability during progressive cerebral ischemia depends on the degree of hypoperfusion and on factors inherent to neurons in various brain regions.