Hibernation in ground squirrels induces state and species-specific tolerance to hypoxia and aglycemia: an in vitro study in hippocampal slices.

Hibernation in mammals is associated with a regulated depression of global cellular functions accompanied by reductions of cerebral blood flow that would render the brain profoundly ischemic under normal conditions. Homeostatic control is preserved, however, and brain damage does not occur. We investigated the possibility that hibernation not only confers tolerance to profound hypothermia, but also to hypoxia and aglycemia independent of temperature. Hippocampal slices from ground squirrels Citellus tridecemlineatus in both the active and hibernating states and from rats were subjected to in vitro hypoxia and aglycemia at incubation temperatures of 36 degrees C, 20 degrees C, and 7 degrees C and evaluated histologically. A binary bioassay was used to determine the duration of hypoxia/aglycemia tolerated in each group. At all temperatures, slices from hibernating animals were most tolerant compared with both active squirrels and rats. Slices from active ground squirrels were more tolerant than rat at 20 degrees C and 7 degrees C but not at 36 degrees C indicating a species-specific difference that becomes manifest at lower temperatures. These results indicate that hibernation is associated not only with tolerance to profound hypothermia but also to deprivation of oxygen and glucose. Because tolerance was already demonstrable at the shortest duration of hibernation studied, rapid therapeutic induction of a similar state may be possible. Therefore, identification of the regulatory mechanisms underlying this tolerance may lead to novel neuroprotective strategies.

Local cerebral blood flow during hibernation, a model of natural tolerance to "cerebral ischemia".

The breakdown of cellular homeostasis and progressive neuronal destruction in cerebral ischemia appears to be mediated by a complex network of causes that are intricately interrelated. We have investigated a physiological state existing normally in nature in which mammals appear to tolerate the ordinarily detrimental effects of ischemia with reduced oxygen availability and to resist activation of self-destructive processes, i.e., mammalian hibernation. Ground squirrels (Spermophilus tridecemlineatus) were chronically implanted with arterial and venous catheters and telemetry devices for electroencephalography, electrocardiography, and monitoring of body temperature. The animals were placed in an environmental chamber at an ambient temperature of 5 degrees C. Entrance into hibernation was characterized by a drop in heart rate followed by a gradual decline in body temperature and an isoelectric electroencephalogram. Cold-adapted active animals that were not hibernating served as controls. Cerebral blood flow (CBF) was measured in both groups with the autoradiographic [14C]iodoantipyrine method. Mean (+/- SD) mass-weighted CBF in the brain was 62 +/- 18 ml 100 g(-1) min (-1) (n = 4) in the control group but was reduced to ischemic levels, 7 +/- 4 ml 100 g(-1) min (-1) (n = 4), in the hibernating animals (p < 0.001) [corrected]. No neuropathological changes were found in similarly hibernating animals aroused from hibernation. Hibernation appears to be actively regulated, and hormonal factors may be involved. The identification and characterization of such factors and of the mechanisms used by hibernating species to increase ischemic tolerance and to blunt the destructive effects of ischemia may enable us to prevent or minimize the loss of homeostatic control during and after cerebral ischemia in other species.

Heat-shock protein and C-fos expression in focal microvascular brain damage.

Cortical brain damage was produced in rats by a focal pulse from a Nd-YAG laser, and evolution of the lesion was evaluated at 30 min, and 2, 8, and 24 h with respect to microvascular perfusion, blood-brain barrier (BBB) permeability, and expression of both the heat-shock/stress protein, hsp72, and the c-fos proto-oncogene transcription factor. A double-labeling fluorescence technique employing intravenously injected Evans blue albumin (EBA) and fluorescein-labeled dextran was used to map and measure BBB damage and microvascular perfusion in fresh frozen brain sections. Hsp72 and c-fos mRNAs were localized by in situ hybridization, and the respective proteins were identified by immunocytochemistry. Parallel sections were stained for glial fibrillary acidic protein and for routine histologic examination. Striking hsp72 mRNA expression was evident by 2 h in an approximately 300 microns wide rim surrounding an area of expanding BBB damage. Increased hsp72 mRNA was observed only in regions of preserved microcirculation, where the hsp72 protein was subsequently localized exclusively in the vasculature at 24 h after the insult. Hsp72-positive endothelial cells spanned the narrow margin between the lesion and histologically normal, glial fibrillary acidic protein (GFAP)-positive cortical tissue. There was no hsp72 expression in the area of subcortically migrating edema fluid. Inductions of c-fos mRNA and Fos protein were not strikingly evident around the focal brain lesion, but were observed transiently throughout the injured hemisphere at 30 min and 2.5 h, respectively, indicating that spreading depression was triggered by the focal injury. These results are in striking contrast to those previously obtained from studies of models of focal ischemic or traumatic brain injury, which are characterized by a complex pattern of glial and neuronal hsp72 expression in the periphery of an infarct, and which suggest that the tightly demarcated lesion produced by the Nd-YAG laser lacks these components of graded injury that are evident following other types of focal brain damage.

Cerebral sinus and venous thrombosis in rats induces long-term deficits in brain function and morphology--evidence for a cytotoxic genesis.

The pathophysiology of cerebral venous infarctions is poorly understood, due partially to the lack of a suitable experimental model. Therefore, we developed a model in rats to study acute and long-term changes of brain function and morphology following thrombosis of the superior sagittal sinus. The superior sagittal sinus of rats was exposed, ligated, and injected with thrombogenic material. Thrombosis of the longitudinal sinus and ascending cortical veins was monitored by intravital fluorescence angiography. Histology was studied at 24 h and 4 weeks after thrombosis and changes in intracranial pressure, electroencephalogram (EEG), and tissue impedance were noted. Spontaneous locomotor activity was followed for 4 weeks after thrombosis. The effect of heparin treatment on tissue impedance was evaluated. Thrombosis of the superior sagittal sinus could be regularly induced, although pathological sequelae developed only if ascending veins were affected. Sinus and venous thrombosis was histologically characterized by bilateral, parasagittal infarctions. Thrombosis induction was followed by an increase in intracranial pressure from 4.7 +/- 1.6 to 12.8 +/- 2.4 mm Hg (n = 4) at 1 h after thrombosis, associated with an exponential rise in tissue impedance to 165 +/- 14% (n = 8) of the control. EEG changes were similar to those following global cerebral ischemia and remained pathological for up to 6 months after thrombosis (n = 6). As a permanent behavioral deficit spontaneous locomotor activity was reduced to 60 +/- 10% (n = 6) of the control. Finally, the administration of heparin (1 IU/g body weight) after thrombosis induction was found to reverse the pathological tissue impedance response of the brain. In conclusion, involvement of ascending cortical veins following sinus thrombosis appears to be critical for the development of irreversible tissue damage, such as infarction. Changes in intracranial pressure and tissue impedance suggest that the venous thrombosis was followed by brain edema of a predominantly cytotoxic nature. Venous thrombosis led to long-term changes of brain function, as demonstrated by persistent disturbances of the EEG or of the spontaneous locomoter drive. These deficits may be amenable to treatment with heparin.

Cortical microcirculation in a new model of focal laser-induced secondary brain damage.

To study the causes of spatial and temporal evolution of progressive neuro-injury in focal brain ischemia, models with consistent lesion topography are required. In such models, continuous monitoring of the microcirculation in a penumbral area undergoing progressive damage could be possible. We used a fixed-pulse (1.0 s, 40 W) Nd-YAG laser (NYL) to produced discrete brain lesions in rats and monitored the cerebral blood flow (CBF) with laser-Doppler flowmetry (LDF) in nonirradiated areas directly adjacent to the maturing lesion. We also examined the time evolution of the lesion topography over a 4 day period. The lesion volume determined by histopathological methods increased from 3.1 +/- 0.5 to 4.5 +/- 0.5 mm3 (p less than 0.05) during the first 2 h. Simultaneously, LDF indicated severe hypoperfusion (-60 +/- 21%, p less than 0.01) at a zone (1 mm distance from the laser lesion) where progressive neuronal degeneration and increased tissue water content (80.0 +/- 3.3% versus 76.8 +/- 2.1% in normal tissue, n = 7, p less than 0.05) were also observed. At a 4 mm distance from the lesion, hyperemic CBF responses were observed, but no histopathological signs or edema. Secondary brain damage progressed up to 4 days (lesion volume of 6.0 +/- 0.7 mm3). The NYL-induced brain lesion produced a highly reproducible focal injury and progressive neuronal death in a spatial relationship with microcirculatory failure and edema formation. The model allows prospective study of tissue state at a discrete zone, which is separate from the initial injury, but susceptible to secondary brain damage.

Long-term sampling of cerebral venous blood in conscious rats.

The mechanisms of secondary brain damage following cerebral ischemia or CNS trauma are still unknown. A variety of mediators, released by the injured brain, are held to be responsible for delayed neuronal cell damage. No technique is available yet for repeated sampling of cerebral venous blood (CVB) on a long-term basis, which allows to assess the neurobiochemical responses to brain injury chronically in conscious laboratory animals. This is a report on the establishment of a method to sample CVB from the superior sagittal sinus (SSS) for at least 7 days in conscious freely moving rats. The torcular was exposed for implantation of a catheter into the caudal SSS. Patency was preserved by perfusion with a nonsystemic dose of heparin by an Alzet osmotic pump and verified daily by aspirating of 0.5 ml CVB for blood analysis. No neuropathology was seen in histological examination. The rats appeared comfortable and without distress, and the transient weight loss was rapidly regained. The present model provides the opportunity to evaluate acute or chronic neurobiochemical responses to stroke, CNS trauma, or any other typ of brain injury in the conscious rat.

Stimulation of tyrosine phosphorylation of a brain protein by hibernation.

Mammalian hibernation is a state of natural tolerance to severely decreased brain blood flow. As protein tyrosine phosphorylation is believed to be involved in the development of resistance to potentially cell-damaging insults, we used immunoblotting for the phosphotyrosine moiety to analyze extracts from various tissues of hibernating and nonhibernating ground squirrels. A single, hibernation-specific phosphoprotein was detected in the brain, but not in any other tissue tested. This protein, designated pp98 to reflect its apparent molecular weight, is distributed throughout the brain, and is associated with the cellular membrane fraction. The presence of the protein is tightly linked to the hibernation state; it is not present in contemporaneously assayed animals that are exposed to the same cold temperature as the hibernators, is present for the duration of a hibernation bout (tested from 1 to 14 days), and disappears within 1 hour of arousal from hibernation. The close association of pp98 with the hibernation state, its presence in cellular membranes, and the known properties of membrane phosphotyrosine proteins suggest that it may transduce a signal for adaptation to the limited availability of oxygen and glucose and low cellular temperature that characterizes hibernation in the ground squirrel.

Ascorbate and glutathione regulation in hibernating ground squirrels.

Ground squirrels withstand up to 90% reductions in cerebral blood flow during hibernation as well as rapid reperfusion upon periodic arousals from torpor. Metabolic suppression likely plays a primary adaptive role which allows hibernating species to tolerate such phenomena. However, several other aspects of hibernation physiology are also consistent with tolerance to dramatic fluctuations in cerebral blood flow, suggesting that multiple neuroprotective adaptations may work in concert during hibernation. The purpose of the present work was to study the dynamics of the low molecular weight antioxidants, ascorbate and glutathione (GSH), during hibernation. Alterations in concentrations of ascorbate during hibernation and arousal in two species of hibernating ground squirrels suggest that it could play a protective role during hibernation or arousal. Samples were collected during the hibernation season from arctic ground squirrels (AGS; Spermophilus parryii) and 13-lined ground squirrels (TLS; S. tridecemlineatus) during prolonged torpor and in squirrels that did not hibernate or had not been hibernating for several weeks. We determined antioxidant levels in plasma, cerebrospinal fluid (CSF), and in frontal cortex, hippocampus and cerebellum using high-performance liquid chromatography (HPLC). Plasma ascorbate concentrations increased dramatically (3-4-fold) in both species during hibernation and rapidly returned to prehibernation levels upon arousal. By contrast, plasma GSH concentrations fell slightly or remained stable during hibernation. Ascorbate levels in the CSF doubled in hibernating AGS (not determined in TLS), while brain ascorbate content fell slightly (10-15%) in both species. Substantial increases in plasma and CSF ascorbate concentrations suggest that this antioxidant could play a protective role during hibernation and reperfusion upon arousal from hibernation.

Platelet-activating factor and progressive brain damage following focal brain injury.

The effects of a platelet-activating factor (PAF) antagonist on brain edema, cortical microcirculation, blood-brain barrier (BBB) disruption, and neuronal death following focal brain injury are reported. A neodymium:yttrium-aluminum-garnet (Nd:YAG) laser was used to induce highly reproducible focal cortical lesions in anesthetized rats. Secondary brain damage in this model was characterized by progressive cortical hypoperfusion, edema, and BBB disruption in the vicinity of the hemispheroid lesion occurring acutely after injury. The histopathological evolution was followed for up to 4 days. Neuronal damage in the cortex and the hippocampus (CA-1) was assessed quantitatively, revealing secondary and progressive loss of neuronal tissue within the first 24 hours following injury. Pretreatment with the PAF antagonist BN 50739 ameliorated the severe hypoperfusion in 12 rats (increasing local cerebral blood flow from a mean +/- standard error of the mean of 40.5% +/- 8.3% to 80.2% +/- 7.8%, p less than 0.01) and reduced edema by 70% in 10 rats (p less than 0.05) acutely after injury. The PAF antagonist also reduced the progression of neuronal damage in the cortex and the CA-1 hippocampal neurons (decrease of neuronal death from 88.0% +/- 3.9% to 49.8% +/- 4.2% at 24 hours in the cortex and from 40.2 +/- 5.0% to 13.2% +/- 2.1% in the hippocampus in 30 rats; p less than 0.05). This study provides evidence to support progressive brain damage following focal brain injury, associated with secondary loss of neuronal cells. In this latter process, PAF antagonists may provide significant therapeutic protection in arresting secondary brain damage following cerebral ischemia and neurological trauma.

Neuroprotective strategies in nature--novel clues for the treatment of stroke and trauma.

A myriad of mediators and mechanisms have been implicated as participants in the propagation of damage following stroke and traumatic brain injury. Effective neuroprotection for these conditions, however, remains elusive at the clinical level. Adaptive strategies of animal species that naturally endure severe reductions in nutrient perfusion to the brain may reveal new mechanisms of homeostatic control and tolerance with potential clinical usefulness. A variety of species appear to qualify as models of tolerance, including those that are anoxia tolerant and species capable of hibernation. Mammalian hibernation represents a state in which global physiologic functions are virtually arrested and delivery of glucose and oxygen is minimal, yet homeostatic control is maintained. The profound reduction of cerebral perfusion in hibernation would lead to rapid autolysis of brain tissue in an unprotected state, but has no adverse effects on hibernators and brain damage does not occur. In fact, even hippocampal slices from hibernating ground squirrels and cerebellar slices from anoxia-tolerant turtles show increased tolerance to a superimposed insult of aglycemia and hypoxia. Surprisingly, the cellular mechanisms and signals that trigger and maintain these adaptations remain unknown. Main targets of current investigations are the regulation of the controlled metabolic suppression in hibernation and the mechanisms of preservation of cell structure and membrane functions and integrity despite reduced energy supplies. The possibility of induction of a similar tolerant state in humans by activation of natural mechanisms of reversible cellular arrest employed by hibernators and other tolerant states would have potentially far-reaching clinical implications. This includes prevention of secondary brain damage following brain trauma and ischemia as well as induction of a state of neuroprotection under conditions of anticipated reduction in cerebral perfusion pressure, such as arterial vasospasm after subarachnoid hemorrhage, or during surgical procedures that require temporary circulatory arrest. Induction of a resistant state could also provide additional time until specialized treatment to re-open occluded blood vessels in stroke patients could be administered.

The safety of dedicated-team catheter-based diagnostic cerebral angiography in the era of advanced noninvasive imaging.

BACKGROUND AND PURPOSE: Given the current high quality and usefulness of noninvasive cerebrovascular imaging, invasive angiographic evaluation of the cerebrovascular system is justified if the procedural risk for a neurologic complication is far below the anticipated benefit. The purpose of this study was to evaluate the safety of diagnostic cerebral angiography provided by a dedicated neurointerventional team in a high-volume university hospital. MATERIALS AND METHODS: A consecutive cohort of 1715 patients undergoing diagnostic cerebral angiography at our institution from 2000 to 2008 was retrospectively assessed for incidence of stroke or TIA related to cerebral angiography. In the subgroup of patients (n = 40) who serendipitously underwent DWI within the first 30 days after cerebral angiography, the presence of new DWI hyperintensities found in territories explored during angiography was tabulated. Complications related to the catheter technique and sheath placement were also studied. RESULTS: No stroke or permanent neurologic deficit was seen in any of the 1715 patients undergoing diagnostic neuroangiography. One patient experienced a TIA. Nonneurologic complications without long-term sequelae occurred in 9 patients. Two patients had punctate areas of restricted diffusion in territories that had been angiographically explored. CONCLUSIONS: Within a high-volume neurointerventional practice, the risk for neurologic complications related to catheter-based diagnostic cerebral angiography can approach zero. As the absolute number of invasive diagnostic procedures diminishes with time, diagnostic cerebral angiography remains a useful tool while providing a foundation for neuroendovascular interventions, and should preferably be performed in institutions with high-volume operators also capable of managing unanticipated complicating adverse events.

Laser-Doppler flowmetry. A review of its application for measuring cerebral and spinal cord blood flow.

Laser-Doppler flowmetry is a new technique for noninvasive and continuous measurement of local microcirculatory cerebral and spinal-cord blood flow. The flow estimate by this technique is based on the assessment of the Doppler shift of low-power laser light, which is scattered by moving red blood cells. Laser-Doppler flowmetry has been validated for various organs, including the central nervous system. These studies revealed a linear relationship between relative changes of the Doppler signal and blood flow over a wide range of pharmacological as well as pathological flow alterations, including cerebral ischemia. The usefulness of laser-Doppler flowmetry in experimental as well as clinical applications has received growing attention. The superiority of the technique lies in its high spatial and temporal resolution. Disadvantages are the difficulty of obtaining absolute flow values and the sensitivity to artifacts. The versatility and on-line capacity of laser-Doppler flowmetry might allow new insights into the pathophysiology of alterations of the cerebral and spinal-cord microcirculation.

Platelet-activating factor--key mediator in neuroinjury?

A growing body of evidence supports the hypothesis that platelet-activating factor (PAF) may be a key mediator in neuroinjury. PAF, originally isolated from stimulated basophils, can be produced by a variety of cells, such as polymorphonuclear leukocytes (PMNLs), platelets, monocytes, macrophages, and endothelial cells and has been suggested as a mediator of inflammation, platelet and neutrophil activation, plasma extravasation, and anaphylactic shock. Enhanced phospholipid metabolism in the ischemic penumbral zone has been reported and provides opportunity for production of PAF. A possible involvement of this lipid mediator in processes associated with cerebral ischemia and neurotrauma has been suggested by an increasing number of reports. PAF exerts cytotoxic effects on neuronal cells, causes vasoconstriction, and increases the blood-brain barrier permeability. Beneficial effects of PAF antagonists have been shown in various models of cerebral ischemia: pre- as well as postischemic application of the PAF antagonist resulted in reduction of edema and improved neurological outcome and improved cerebral microcirculation. These effects were correlated with improved neuronal survival and reduced accumulation of PMNLs, supporting a link and positive feedback between PAF and PMNLs in these processes. Since PAF appears to be uniquely involved in various pathophysiological events, it may function as a key mediator in ischemic and traumatic neuroinjury. The current review summarizes the current understanding of the function and biochemistry of PAF with respect to CNS physiology and pathology.

Rates of glucose utilization in brain of active and hibernating ground squirrels.

Rates of glucose utilization (CMRGlc) were determined in some cerebral structures of active warm- and cold-adapted ground squirrels and hibernating ground squirrels with [14C]deoxyglucose (DG) by direct chemical measurement of precursor and products in samples dissected from funnel-frozen brain. The rate of supply relative to demand of glucose and [14C]DG in brain of hibernating animals was similar to or greater than that of controls. [14C]DG cleared from the plasma in hibernators much more slowly than in active animals, and the level of unmetabolized [14C]DG in brain and the integrated specific activity of the precursor pool in plasma exceeded those of the active animals by 4- to 10-fold. At 45 min after an intravenous pulse of [14C]DG, the unmetabolized [14C]DG remaining in the brains of the hibernators accounted for approximately 96% of the total 14C compared with approximately 10-15% in the active animals. The value of lambda, a factor contained in the lumped constant of the operational equation of the [14C]DG method, was estimated for each animal and found to be relatively constant over the sixfold range of glucose levels in the brains of all animals. Calculated CMRGlc in squirrels in deep hibernation was only 1-2% of the values in active animals.

Increased cerebral lactate output to cerebral venous blood after forebrain ischemia in rats.

Increased cerebral lactate levels are a well-known aspect of the sequelae of the metabolic derangements that follow cerebral ischemia. A new technique has recently become available to sample cerebral venous blood from the superior sagittal sinus on a long-term basis in conscious rats. We report the applicability of this method to assess serial biochemical responses to brain injury. Serum samples were obtained from the superior sagittal sinus, the common carotid artery, and the external jugular vein of nine anesthetized rats before and up to 7 days after 10 minutes of forebrain ischemia was produced by carotid occlusion and hypovolemic hypotension (mean arterial blood pressure 50 +/- 4 mm Hg). The cerebral venous-arterial difference in serum lactate concentration was increased for up to 3 hours after ischemia, while there was no significant change in the difference in serum lactate concentrations in the common carotid artery and the external jugular vein. This indicates an elevated output of lactate from brain tissue to blood, detectable only in the superior sagittal sinus, which underlines the usefulness of the technique. We observed a persistent elevation in brain lactate production after virtually complete recovery from the acute insult.

Evidence for platelet-activating factor as a novel mediator in experimental stroke in rabbits.

Platelet-activating factor is a potent mediator of inflammation, which has untoward effects on cerebrovascular and neural elements. While several investigators have reported attenuation of ischemic damage after treatment with antagonists of platelet-activating factor, no study has proved endogenous production of platelet-activating factor in ischemia of the central nervous system. We hypothesized that endogenous production of platelet-activating factor participates in the early pathologic manifestations of deteriorating stroke. In 12 rabbits, we found tissue levels of platelet-activating factor measured by the release of serotonin from washed platelets to be elevated by approximately 20-fold in spinal cord injured by 25 minutes of ischemia and 2 hours of reperfusion (2.80 +/- 0.98 ng/g) compared with that in normal spinal cord (0.15 +/- 0.06 ng/g, p less than 0.01). Given during ischemia to seven rabbits, 10 mg/kg i.p. of a highly selective and potent antagonist of platelet-activating factor (BN 50739) accentuated the early postischemic hyperemia and prevented the delayed hypoperfusion measured by on-line laser-Doppler flowmetry (-35 +/- 7% of baseline [n = 7] without versus 33 +/- 14% with treatment, p less than 0.01) and the edema formation measured as the increase in tissue water content (4.4 +/- 0.7% without [n = 6] versus 2.1 +/- 0.6% with [n = 7]treatment, p less than 0.05) after 2 hours of reperfusion. This neurochemical and pharmacologic evidence emphasizes a new perspective of ischemia-induced phospholipid degradation and suggests an important role for platelet-activating factor in the early manifestations of stroke.

The onset of postischemic hypoperfusion in rats is precipitous and may be controlled by local neurons.

BACKGROUND AND PURPOSE: Reperfusion following transient global cerebral ischemia is characterized by an initial hyperemic phase, which precedes hypoperfusion. The pathogenesis of these flow derangements remains obscure. Our study investigates the dynamics of postischemic cerebral blood flow changes, with particular attention to the role of local neurons. METHODS: We assessed local cortical blood flow continuously by laser Doppler flowmetry to permit observation of any rapid flow changes after forebrain ischemia induced by four-vessel occlusion for 20 minutes in rats. To investigate the role of local cortical neurons in the regulation of any blood flow fluctuations, five rats received intracortical microinjections of a neurotoxin (10 micrograms ibotenic acid in 1 microliter; 1.5-mm-depth parietal cortex) 24 hours before ischemia to induce selective and localized neuronal depletion in an area corresponding to the sample volume of the laser Doppler probe (1 mm3). Local cerebral blood flow was measured within the injection site and at an adjacent control site. RESULTS: Ischemia was followed by marked hyperemia (235 +/- 23% of control, n = 7), followed by secondary hypoperfusion (45 +/- 3% of control, n = 7). The transition from hyperemia to hypoperfusion occurred not gradually but precipitously (maximal slope of flow decay: 66 +/- 6%/min; n = 7). In ibotenic acid-injected rats, hyperemia was preserved at the injection site, but the sudden decline of blood flow was abolished (maximal slope of flow decay: 5 +/- 3%/min compared with 53 +/- 8%/min at the control site; n = 5, p less than 0.001) and no significant hypoperfusion developed (103 +/- 20% of control at 60 minutes). CONCLUSIONS: These data suggest that the rapid transition to cortical hypoperfusion after forebrain ischemia may be triggered locally by a neuronal mechanism but that this mechanism does not underlie the initial hyperemia.

Mechanisms for increased levels of phosphorylation of elongation factor-2 during hibernation in ground squirrels.

Previously, eEF-2 phosphorylation has been identified as a reversible mechanism involved in the inhibition of the elongation phase of translation. In this study, an increased level of phosphorylation of eukaryotic elongation factor-2 (eEF-2) was observed in the brains and livers of hibernating ground squirrels. In brain and liver from hibernators, eEF-2 kinase activity was increased relative to that of active animals. The activity of protein phosphatase 2A (PP2A), a phosphatase that dephosphorylates eEF-2, was also decreased in brain and liver from hibernators. This was associated with an increase in the level of inhibitor 2 of PP2A (I(2)(PP2A)), although there was an increase in the level of the catalytic subunit of PP2A (PP2A/C) in hibernating brains and livers. These results indicate that eEF-2 phosphorylation represents a specific and previously uncharacterized mechanism for inhibition of the elongation phase of protein synthesis during hibernation. Increased levels of eEF-2 phosphorylation in hibernators appear to be a component of the regulated shutdown of cellular functions that permits hibernating animals to tolerate severe reductions in cerebral blood flow and oxygen delivery capacity.