Postischemic hypothermia. A critical appraisal with implications for clinical treatment

Cryoanethesia on postnatal day 1, but not day 10, affects adult behavior and cortical morphology in rats

Hypothermia was used to induce anesthesia in infant rats on postnatal days 1 or 10. In adulthood measures of spatial learning and activity were taken before the brains were harvested for measurement of cortical thickness and dendritic arborization in layer III pyramidal cells in parietal cortex. Cryoanethesia on day 1, but not day 10, produced a small, but statistically significant, impairment in learning a spatial navigation task as well as a reduction in cortical thickness and dendritic arborization. This study confirms that cryoanesthesia is not a benign treatment in newborn pups but appears to be without effect in older animals. It is important that all studies using cryoanesthesia have sham control animals exposed to the same degree of hypothermia.

Long-term activation of the glutamatergic system associated with N-methyl-D-aspartate receptors after postischemic hypothermia in gerbils

OBJECTIVE

The objective of this study was to investigate whether hypothermia would suppress secondary damage in the chronic postischemic stage, in terms of glutamate excitotoxicity.

METHODS

Gerbils underwent 5 minutes of ischemia via bilateral common carotid artery occlusion. Seven groups were studied, as follows: 1) ischemia without treatment group; 2) intraischemic hypothermia group; 3) postischemic hypothermia group (32 degrees C for 4 h); 4) MK-801 treatment group (2 mg/kg, every other day for 1 mo); 5) postischemic hypothermia with MK-801 treatment for 1 week group (2 mg/kg, every other day); 6) postischemic hypothermia with MK-801 treatment for 1 month group (2 mg/kg, every other day); and 7) sham-treated control group. One month after ischemia, histological changes in hippocampal CA1 neurons (assessed using hematoxylin and eosin staining) and memory function (assessed using an eight-arm radial maze) were studied. Extracellular glutamate concentrations were monitored by microdialysis during ischemia and hypothermia. Staining of microglia was performed 1 week and 1 month after ischemia.

RESULTS

MK-801 alone, postischemic hypothermia alone, and postischemic hypothermia with MK-801 treatment for 1 week failed to prevent ischemic neuronal damage and memory function decreases 1 month after the insult (P < 0.05 versus control). However, the postischemic hypothermia with MK-801 treatment for 1 month group exhibited significant protective effects (not significant [P > 0.05] compared with the control group). Extracellular glutamate levels for the intraischemic hypothermia group were significantly low, compared with the postischemic hypothermia group. There was no microglial activation in the postischemic hypothermia at 1 week and 1 month after ischemia groups.

CONCLUSION

Postischemic hypothermia and long-term intermittent administration of MK-801 demonstrated significant neuronal protection, indicating that long-term glutamatergic activation, with changes in N-methyl-D-aspartate receptors, plays a role in neuronal damage in the chronic postischemic stage.

The therapeutic potential of regulated hypothermia

Reducing body temperature of rodents has been found to improve their survival to ischaemia, hypoxia, chemical toxicants, and many other types of insults. Larger species, including humans, may also benefit from a lower body temperature when recovering from CNS ischaemia and other traumatic insults. Rodents subjected to these insults undergo a regulated hypothermic response (that is, decrease in set point temperature) characterised by preference for cooler ambient temperatures, peripheral vasodilatation, and reduced metabolic rate. However, forced hypothermia (that is, body temperature forced below set point) is the only method used in the study and treatment of human pathological insults. The therapeutic efficacy of the hypothermic treatment is likely to be influenced by the nature of the reduction in body temperature (that is, forced versus regulated). Homeostatic mechanisms counter forced reductions in body temperature resulting in physiological stress and decreased efficacy of the hypothermic treatment. On the other hand, regulated hypothermia would seem to be the best means of achieving a therapeutic benefit because thermal homeostatic systems mediate a controlled reduction in core temperature.

Therapeutic implications of hypothermic and hyperthermic temperature conditions in stroke patients

Brain temperature is an important variable in determining the outcome of cerebral ischemia; increases in core temperature escalate neural damage whereas decreases in core temperature reduce damage. Fever induction often occurs in patients prior to or as a direct or indirect result of the ischemic insult, with a worsened stroke outcome, compared with non-febrile ischemic patients. Most importantly, post-ischemic hypothermia reduces long term neural damage and associated behavioral deficits in animals studied for up to a year after the ischemic insult. This review discusses the importance of monitoring the brain temperature of stroke patients and implemention of therapeutic thermoregulatory strategies to reduce the temperature of ischemic patients.Key words: hypothermia, neuroprotection, fever, neural and behavioral outcomes.

Brain hypothermia relieves severe brain swelling following acute major cerebral artery occlusion

Seven patients were treated with brain hypothermia following acute major cerebral artery occlusion to utilize the suppressive effect against brain swelling. Five patients had internal carotid and two had proximal middle cerebral artery occlusion. Except for the first two cases, hypothermia was introduced early and the temperature reached 35.0 degrees C within 6 hours after the onset. The core temperature finally stabilized between 32 degrees C to 34 degrees C. Hypothermia had a suppressive effect against brain swelling and the temperature showed a significant correlation to intracranial pressure. Recurrence of brain swelling was observed during the rewarming process, but two patients became independent and three patients were moderately disabled in wheelchairs. Only two patients died. Brain hypothermia is an effective treatment for acute major cerebral artery occlusion through the relief of brain swelling. The overall outcome may be improved by combining brain hypothermia with other conventional therapies such as osmotherapy and external decompression implemented with an extended period of rewarming.

NXY-059, a free radical--trapping agent, substantially lessens the functional disability resulting from cerebral ischemia in a primate species

BACKGROUND AND PURPOSE

NXY-059 is a novel nitrone with free radical-trapping properties that has a considerable neuroprotective effect in rats. We have now examined the efficacy of this drug at reducing long-term functional disability in a primate model of stroke.

METHODS

Twelve monkeys were trained and tested on a variety of behavioral tasks used to dissociate and quantify motor and spatial deficits. Five minutes after permanent occlusion of the right middle cerebral artery, monkeys received a 1-mL intravenous infusion of either saline or NXY-059 (28 mg x kg(-1)), and osmotic minipumps, model 2001D, were implanted subcutaneously to provide continuous drug or saline infusion for 48 hours. Drug-filled pumps released NXY-059 at 16 mg x kg(-1) x h(-1). The monkeys were retested 3 and 10 weeks after surgery to assess functional disability. Surgery, behavioral testing, and histology were all done blinded to treatment condition.

RESULTS

NXY-059-treated monkeys were significantly better at reaching with their hemiparetic arm than were saline-treated monkeys when retested 3 weeks (P:<0.01) and 10 weeks (P:<0.01) after surgery. Drug treatment also significantly lessened the degree of spatial perceptual neglect (P:<0.01), a debilitating though ameliorating consequence of this infarct. NXY-059 treatment reduced the overall amount of brain damage by >50% of saline-treatment values, with similar levels of protection afforded to both white and gray matter.

CONCLUSIONS

This novel drug has a substantial protective effect, lessening the disability caused by an experimentally induced stroke in a primate species. These findings provide considerable encouragement for the clinical development of NXY-059.

Postischemic temperature as a modulator of the stress response in brain: dissociation of heat shock protein 72 induction from ischemic tolerance after bilateral carotid artery occlusion in the gerbil

Brief ischemia induces tolerance to subsequent more severe insults, and induction of the 70 kDa heat shock/stress protein, hsp72, has been suggested to play a role. This study tested the requirement for hsp72 expression in a gerbil tolerance model in which postischemic temperature was varied to modulate the level of hsp72 induction. Gerbils were subjected to 2 min bilateral common carotid artery occlusion and kept under halothane anesthesia for 90 min, during which rectal temperature was either maintained at 37 degrees C (normothermic, NT) or elevated to 39.5 degrees C (hyperthermic, HT) during 15-60 min recirculation. Hsp72 mRNA expression was determined by in situ hybridization with a (35)S-labeled oligonucleotide probe at 3, 24 and 48 h. Separate groups were subjected to a test challenge of 5 min ischemia 48 h after the priming insult, and CA1 neuron counts were obtained at 1 week. Significant protection was observed in both NT and HT groups. However, while 90% of hippocampi from NT animals showed detectable protection of CA1 neurons, less than half showed detectable hsp72 mRNA induction. These results indicate that, within the limits of experimental detection, hsp72 expression is not required for induction of ischemic tolerance.

Prolonged but delayed postischemic hypothermia: a long-term outcome study in the rat middle cerebral artery occlusion model

Delayed but prolonged hypothermia persistently decreases cell death and functional deficits after global cerebral ischemia in rodents. Postischemic hypothermia also reduces infarction after middle cerebral artery occlusion (MCAO) in rat. Because initial neuroprotection is sometimes transient and may not subserve functional recovery, especially on demanding tasks, the authors examined whether postischemic cooling would persistently reduce infarction and forelimb reaching deficits after MCAO. Male spontaneously hypertensive rats were trained to retrieve food pellets in a staircase test that measures independent forelimb reaching ability. Later, rats underwent 90 minutes of normothermic MCAO, through a microclip, or sham operation. In some rats, prolonged cooling (33 degrees C for 24 hours and then 35 degrees C for 24 hours) began 2.5 hours after the onset of ischemia (60 minutes after the start of reperfusion; n = 17 with subsequently 1 death) or sham procedures (n = 4), whereas untreated sham (n = 4) and ischemic (n = 16 with subsequently 1 death) rats maintained normothermia. An indwelling abdominal probe continually measured core temperature, and an automated fan and water spray system was used to produce hypothermia. One month later rats were reassessed in the staircase test over five days and then killed. The contralateral limb impairment in food pellet retrieval was completely prevented by hypothermia (P = 0.0001). Hypothermia reduced an infarct volume of 67.5 mm3 after untreated ischemia to 35.8 mm3 (P < 0.0001). These findings of persistent benefit encourage the clinical assessment of hypothermia.

Hyperbaric oxygen therapy for treatment of postischemic stroke in adult rats

The efficacy of hyperbaric oxygen (HBO) therapy for treatment of stroke remains to be validated in the laboratory. We report here that adult rats subjected to occlusion of the middle cerebral artery and subsequently exposed to HBO (3 atm, 2 x 90 min at a 24-h intervals; animals terminated shortly after the second treatment) or hyperbaric pressure (HBP; 3 atm, 2 x 90 min at a 24-h interval; animals terminated shortly after the second treatment) immediately after the ischemia or after a 60-min delay generally displayed recovery from motor deficits at 2.5 and 24 h of reperfusion, as well as a reduction in cerebral infarction at 24 h of reperfusion compared to ischemic animals subjected to normal atmospheric pressure. While both HBO and HBP treatments promoted beneficial effects, HBO produced more consistent protection than HBP. Treatment with HBO immediately or 60 min after reperfusion equally produced significant attenuations of cerebral infarction and motor deficits. In contrast, protective effects of HBP treatment against ischemia were noted only when administered immediately after ischemia, which resulted in a significantly reduced infarction volume, but only produced a trend toward decreased behavioral deficits. The present results demonstrate that HBO and, to some extent, HBP reduced ischemic brain damage and behavioral dysfunctions.

No additional neuroprotection provided by barbiturate-induced burst suppression under mild hypothermic conditions in rats subjected to reversible focal ischemia

OBJECT

Mild-to-moderate hypothermia is increasingly used for neuroprotection in humans. However, it is unknown whether administration of barbiturate medications in burst-suppressive doses-the gold standard of neuroprotection during neurovascular procedures-provides an additional protective effect under hypothermic conditions. The authors conducted the present study to answer this question.

METHODS

Thirty-two Sprague-Dawley rats were subjected to 90 minutes of middle cerebral artery occlusion and randomly assigned to one of four treatment groups: 1) normothermic controls; 2) methohexital treatment (burst suppression); 3) induction of mild hypothermia (33 degrees C); and 4) induction of mild hypothermia plus methohexital treatment (burst suppression). Local cerebral blood flow was continuously monitored using bilateral laser Doppler flowmetry and electroencephalography. Functional deficits were quantified and recorded during daily neurological examinations. Infarct volumes were assessed histologically after 7 days. Methohexital treatment, mild hypothermia, and mild hypothermia plus methohexital treatment reduced infarct volumes by 32%, 71%, and 66%, respectively, compared with normothermic controls. Furthermore, mild hypothermia therapy provided the best functional outcome, which was not improved by additional barbiturate therapy.

CONCLUSIONS

The results of this study indicate that barbiturate-induced burst suppression is not required to achieve maximum neuroprotection under mild hypothermic conditions. The magnitude of protection afforded by barbiturates alone appears to be modest compared with that provided by mild hypothermia.

Ischemic tolerance in the rat neocortex following hypothermic preconditioning

OBJECT

Ischemic neuronal damage associated with neurological and other types of surgery can have severe consequences for functional recovery after surgery. Hypothermia administered during and/or after ischemia has proved to be clinically beneficial and its effects often rival or exceed those of other therapeutic strategies. In the present study the authors examined whether transient hypothermia is an effective preconditioning stimulus for inducing ischemic tolerance in the brain.

METHODS

Adult rats were subjected to a 20-minute period of hypothermic preconditioning followed by an interval ranging from 6 hours to 7 days. At the end of this interval, the animals were subjected to transient focal ischemia induced by clamping one middle cerebral artery and both carotid arteries for 1 hour. The volume of cerebral infarction was assessed 1 or 7 days postischemia. In the first series of experiments, hypothermic preconditioning (28.5 degrees C) with a postconditioning interval of 1 day reduced the extent of cerebral infarction measured 1 and 7 days postischemia. In the second series, hypothermic preconditioning (31.5 degrees C) with postconditioning intervals of 6 hours, 1 day, or 2 days (but not 7 days) reduced the extent of cerebral infarction measured 1 day postischemia. Treatment with the protein synthesis inhibitor anisomycin blocked the protective effect of hypothermic preconditioning. In a final series of experiments, in vitro brain slices prepared from hypothermia-preconditioned (nonischemic) animals were shown to tolerate a hypoxic challenge better than slices prepared from unconditioned animals.

CONCLUSIONS

These findings indicate that hypothermic preconditioning induces a form of delayed tolerance to focal ischemic damage. The time course over which tolerance occurs and the ability of a protein synthesis inhibitor to block tolerance suggest that increased expression of one or more gene products is necessary to establish tissue tolerance following hypothermia. The attenuation of hypoxic injury in vitro following in vivo preconditioning indicates that tolerance is due, at least in part, to direct effects on the brain neuropil. Hypothermic preconditioning could provide a relatively low-risk approach for improving surgical outcome after invasive surgery, including high-risk neurological and cardiovascular procedures.

Post-ischemic hypothermia delayed neutrophil accumulation and microglial activation following transient focal ischemia in rats

Following ischemia, inflammation has been demonstrated to be involved in the progression of the tissue damage. Intra-ischemic hypothermia has been shown to attenuate the adverse activities of neutrophils and microglia. We investigated whether neutrophil accumulation and/or microglial activation is attenuated in post-ischemic hypothermia following transient focal ischemia in rats. After 1 h of ischemia, the neutrophil accumulation and the microglial activation was evaluated immunohistochemically. Percent infarct area was compared at 1, 2, 3, 5, and 7 days after ischemia/reperfusion. In hypothermia, the neutrophil accumulation was delayed but not attenuated. In normothermia, the accumulation reached the peak at 2 days after ischemia. The peak shifted to 3 days in hypothermia. Similarly, the microglial activation was delayed in hypothermia. Comparison of the infarct area showed significant protection by hypothermia at 1 and 2 days after reperfusion. However, hypothermia failed to show significant protection after 3 days and later. These results show that the delayed neutrophil accumulation and the microglial activation can be responsible for the loss of persistent protection in post-ischemic hypothermia.

A post-ischaemic single administration of galanthamine, a cholinesterase inhibitor, improves learning ability in rats

Transient forebrain ischaemia is widely observed in clinical practice. We have examined the effect of a single administration of the cholinesterase inhibitor galanthamine (2mg kg(-1) i.p.) 25 min after reperfusion in male Sprague-Dawley rats (180 +/- 20 g) after a 20-min common carotid artery occlusion. Twenty-four-hours post-ischaemia there was no difference in motor co-ordination or muscle tonus of the rats treated with or without galanthamine as assessed by the rota-rod test. Learning ability was examined using the shuttle-box test, evaluating the latency time and the number of errors for six days in succession. The performance of the ischaemic saline-injected rats was significantly impaired on days 4, 5, 6 (latency time) compared with the non-ischaemic rats and with the ischaemic animals administered galanthamine (P < 0.05). Similar results were obtained when counting the number of errors (failure to cross the cage during conditioned or unconditioned stimulus). The monitoring of body temperature during the first 12-h post-ischaemia did not show any significant difference between the groups. The data showed a beneficial effect of galanthamine on the recovery of learning ability when administered once only post-ischaemia. This suggests a direct effect on the early pathologic mechanisms of CNS damage. Cholinesterase inhibitors may prove useful in the early clinical treatment of ischaemic conditions.

Depression of long term potentiation in gerbil hippocampus following postischemic hypothermia

To investigate the mechanism of chronic cell death following postischemic hypothermia, the change of N-methyl-D-aspartate receptor (NMDAR) were examined by immunohistochemistry of NMDAR1 and long-term potentiation (LTP) in the CA1 subfield of the gerbil hippocampus. At 1 week following postischemic hypothermia (32 degrees Cx4 h), all CA1 neurons survived; however, immunoreactivity of NMDAR1 increased in neuronal perikarya whereas decreased in dendrites in the CA1 neurons. The abnormality was still observed in remaining CA1 neurons at 1 month after hypothermia. LTP was also significantly depressed at 1 week after hypothermia. These results suggest that some abnormalities in the glutamate receptor may be caused by ischemia; such abnormality would persist in spite of hypothermia treatment, resulting in the depression of LTP.

Hypothermia and hyperthermia in children after resuscitation from cardiac arrest

OBJECTIVE

In experimental models of ischemic-anoxic brain injury, changes in body temperature after the insult have a profound influence on neurologic outcome. Specifically, hypothermia ameliorates whereas hyperthermia exacerbates neurologic injury. Accordingly, we sought to determine the temperature changes occurring in children after resuscitation from cardiac arrest.

STUDY DESIGN

The clinical records of 13 children resuscitated from cardiac arrest were analyzed. Patients were identified through the emergency department and pediatric intensive care unit arrest logs. Only patients surviving for > or =12 hours after resuscitation were considered for analysis. Charts were reviewed for body temperatures, warming or cooling interventions, antipyretic and antimicrobial administration, and evidence of infection.

RESULTS

Seven patients had a minimum temperature (T min) of < or =35 degrees C and 11 had a maximum temperature (T max) of > or =38.1 degrees C. Hypothermia often preceded hyperthermia. All 7 patients with T min < or =35 degrees C were actively warmed with heating lamps and 5 of 7 responded to warming with a rebound of body temperatures > or =38.1 degrees C. None of the 6 patients with T min >35 degrees C were actively warmed but all developed T max > or =38.1 degrees C. Six patients received antipyretics and 11 received antibiotics. Fever was not associated with a positive culture in any case. Conclusion. Spontaneous hypothermia followed by hyperthermia is common after resuscitation from cardiac arrest. Temperature should be closely monitored after cardiac arrest and fever should be managed expectantly.

Jugular vein temperature reflects brain temperature during hypothermia

PURPOSE

The neuroprotective properties of mild to moderate hypothermia are well recognized but may not be employed correctly because brain temperature cannot usually be measured directly. This study investigated the jugular vein as a more accessible site that accurately reflects the actual brain temperature during mild, induced hypothermia.

METHODS

We selected ten mongrel dogs (mean weight 12 +/- 2 kg) and measured temperatures of the brain, jugular vein, cisterna magna, pulmonary artery and rectum during hypothermia, including cooling and rewarming. The brain temperature needle probe was inserted 2.0 cm into the parenchyma. A temperature probe was placed in the cisterna magna with an epidural needle. Swan-Ganz thermistor probes measured the jugular venous and pulmonary artery blood temperatures.

RESULT

The brain temperature decreased from 37.5 +/- 0.3 to 33.0 +/- 0.3 degrees C over an average 150 +/- 45 min cooling period. Stable cool was maintained for 245 +/- 32 min, followed by 165 +/- 50 min for rewarming from 33.5 +/- 0.3 to 37.5 +/- 0.3 degrees C. Jugular, cisterna magna and pulmonary arterial blood (PAB), but not rectal temperature, were close to brain temperature during stable cool. The mean jugular and cisterna magna temperatures were near the brain temperature at 0.1 degrees C higher and 0.1 degrees C lower, respectively. No significant effects of hypothermia were noted on hemodynamics in any phase.

CONCLUSION

Jugular vein temperature, along with cisterna magna and pulmonary artery blood and rectal temperature, reflected brain temperature during hypothermia. The jugular vein and cisterna magna sites more sensitively reflected brain temperature than other sites.

Hippocampal anatomy and water maze performance are affected by neonatal cryoanesthesia in rats of both sexes

There is recent evidence that cryoanesthesia, commonly used during neonatal hormone manipulations (e.g., gonadectomy), has deleterious effects on the morphology of the splenium of the corpus callosum and primary visual cortex in adult rats of both sexes. (Nuñez and Juraska, 1998; Nuñez, Kim, and Juraska, 1998). In the present study, the effect of neonatal cryoanesthesia on the morphology of the hippocampus and dentate gyrus and on performance in the Morris water maze was investigated. Cold exposure for as brief as 30 min (5 degrees C) on Postnatal Day 1 resulted in a significant decrease in the volume of the hippocampus and in brain weight of adults. Performance on the water maze was also impaired in cold-exposed animals. This study indicates that not only morphology but also behavioral performance in adulthood are affected by neonatal cryoanesthesia.

The corticosterone synthesis inhibitor metyrapone prevents hypoxia/ischemia-induced loss of synaptic function in the rat hippocampus

BACKGROUND AND PURPOSE

Ischemia is accompanied by abundant corticosterone secretion, which could potentially exacerbate brain damage via activation of glucocorticoid receptors. We addressed whether manipulating steroid levels during ischemia affects hippocampal synaptic function along with neuronal structure. Moreover, we established whether pretreatment with the glucocorticoid receptor antagonist RU38486 is as effective in preventing deleterious effects after ischemia as is the steroid synthesis inhibitor metyrapone.

METHODS

Rats underwent 20 minutes of unilateral hypoxia/ischemia (HI). Convulsions were monitored after HI, and 24 hours later, field potentials were recorded in vitro in the hippocampal CA1 area in response to stimulation of the Schaffer collateral/commissural fibers. Morphological alterations were determined in brain slices from the same animals. Data were correlated with steroid treatment before HI.

RESULTS

Metyrapone suppressed plasma corticosteroid levels during HI, whereas corticosterone treatment significantly elevated plasma steroid levels. These treatments affected the incidence of visible seizures after HI: corticosterone treatment resulted in the highest incidence, whereas metyrapone attenuated the occurrence of seizures. Moreover, the HI-induced impairment in synaptic transmission in the CA1 area in vitro was exacerbated by concomitant corticosteroid treatment and alleviated by pretreatment with metyrapone. In parallel, degenerative changes in the hippocampus after HI were most pronounced after corticosterone treatment, whereas metyrapone reduced these alterations. RU38486 was effective only in reducing the incidence of seizures shortly after ischemia.

CONCLUSIONS

We tentatively conclude that synaptic function along with cellular integrity is preserved after HI by preventing the ischemia-evoked rise in corticosteroid levels rather than blocking the glucocorticoid receptor.

Persistent neuroprotection with prolonged postischemic hypothermia in adult rats subjected to transient middle cerebral artery occlusion

Postischemic hypothermia provides long-lasting neuroprotection against global cerebral ischemia in adult rats and gerbils. Studies indicate that hypothermia must be prolonged (e.g., 24 h) to indefatigably salvage hippocampal CA1 neurons. Delayed hypothermia also reduces focal ischemic injury. However, no study has examined long-term outcome following postischemic hypothermia in adult animals. Furthermore, most studies examined only brief hypothermia (e.g., 3 h). Since previous studies may have overestimated long-term benefit and have likely used suboptimal durations of hypothermia, we examined whether prolonged cooling would attenuate infarction at a 2-month survival time following middle cerebral artery occlusion (MCAo) in rats. Adult male Wistar rats were implanted with telemetry brain temperature probes and later subjected to 30 min of normothermic MCAo (contralateral to side of probe placement) or sham operation. Ischemia was produced by the insertion of an intraluminal suture combined with systemic hypotension (60 mm Hg). Sham rats and one ischemic group controlled their own postischemic temperature while another ischemic group was cooled to 34 degrees C for 48 h starting at 30 min following the onset of reperfusion. The infarct area was quantified after a 2-month survival time. Normothermic MCAo resulted in almost complete striatal destruction (91% loss +/- 12 SD) with extensive cortical damage (36% +/- 16 SD). Delayed hypothermia treatment significantly reduced cortical injury to 10% +/- 10 SD (P < 0.001) while striatal injury was marginally reduced to 79% loss +/- 17 SD (P < 0.05). Delayed hypothermia of only 34 degrees C provided long-lasting cortical and striatal protection in adult rats subjected to a severe MCAo insult. These results strongly support the clinical assessment of hypothermia in acute stroke.

Clomethiazole protects against hemineglect in a primate model of stroke

Permanent occlusion of the M1 segment of the middle cerebral artery (pMCAO) in the marmoset, a New World species of monkey, produces unilateral functional deficits, including motor neglect with the contralesional arm and contralesional spatial hemineglect. In this study we examined whether clomethiazole, a drug which modulates the gamma-aminobutyric acid(A) receptor, reduced the severity of the hemineglect and other deficits in this primate model of stroke. Nine monkeys received pMCAO; 1 h later four of the nine were administered clomethiazole by intraperitoneal injection and subcutaneous implantation of osmotic mini-pumps, which released clomethiazole for 48 h. The monkeys had been trained and tested on a number of behavioral tasks prior to surgery and were re-tested 3 and 10 weeks later. Three weeks after pMCAO, monkeys treated with clomethiazole had a significantly reduced degree of spatial neglect compared to untreated controls. Clomethiazole was not effective against the severe contralesional motor impairment in the current study, although it ameliorated a somewhat less severe motor deficit in a previous study in which the more distal, M2 segment of the middle cerebral artery had been occluded. Postmortem analysis of the brains showed that clomethiazole treatment had significantly reduced the area of damage in part of the parietal cortex. These data suggest that clomethiazole may reduce the neglect that can be a debilitating consequence of right-sided stroke in man.

Hypothermia-induced ischemic tolerance

Delayed resistance to ischemic injury can be induced by a variety of conditioning stimuli. This phenomenon, known as delayed ischemic tolerance, is initiated over several hours or a day, and can persist for up to a week or more. The present paper describes recent experiments in which transient hypothermia was used as a conditioning stimulus to induce ischemic tolerance. A brief period of hypothermia administered 6 to 48 hours prior to focal ischemia reduces subsequent cerebral infarction. Hypothermia-induced ischemic tolerance is reversed by 7 days postconditioning, and is blocked by the protein synthesis inhibitor anisomycin. Electrophysiological studies utilizing in vitro brain slices demonstrate that hypoxic damage to synaptic responses is reduced in slices prepared from hypothermia-preconditioned animals. Taken together, these findings indicate that transient hypothermia induces tolerance in the brain parenchyma, and that increased expression of one or more gene products contributes to this phenomenon. Inasmuch as hypothermia is already an approved clinical procedure for intraischemic and postischemic therapy, it is possible that hypothermia could provide a clinically useful conditioning stimulus for limiting injury elicited by anticipated periods of ischemia.

Neuroprotection against ischemia by metabolic inhibition revisited. A comparison of hypothermia, a pharmacologic cocktail and magnesium plus mexiletine

Previous studies have suggested that metabolic inhibition is neuroprotective, but little evidence has been provided to support this proposal. Using the in vitro rabbit retina preparation as an established model of the central nervous system (CNS), we measured the rate of glucose utilization and lactate production, and the light-evoked compound action potentials (CAPs) as indices of neuronal energy metabolism and electrophysiologic function, respectively. We examined the effect of three (3) treatments options: hypothermia (i.e., 33 degrees C and 30 degrees C), a six-member pharmacologic "cocktail" (tetrodotoxin (0.1 microM), 2-amino-4-phosphonobutyric acid (20 microM), 2-amino-5-phosphonovaleric acid (1 mM), amiloride (1 mM), magnesium (10 mM) and lithium (10 mM) and the combination of magnesium (Mg2+ 1 mM) and mexiletine (Mex, 300 microM) on in vitro rabbit retinas, to see if there is a correlation between neuronal energy metabolism during ischemia (simulated by the reduction of oxygen from 95% to 15% and glucose from 6 mM to 1 mM), and the subsequent recovery of function. Hypothermia and the "cocktail" significantly inhibited both the rate of glucose utilization and lactate production, whereas Mg2+ and/or Mex showed only a nonsignificant tendency toward a reduction, compared to control retinas. Recovery of light-evoked CAPs was significantly improved in hypothermia- and cocktail-treated retinas, as well as with retinas exposed to the combination of Mg2+ plus Mex, but not with Mg2+ or Mex alone, relative to control retinas. A linear regression analysis of the % recovery of function versus the % reduction in the rate of glucose utilization during ischemia showed a significant correlation (r2 = 0.80, correlation coefficient = 0.9, p < 0.05) between these two parameters. This and other data discussed provide convincing evidence that there is a correlation between metabolic inhibition, achieved during ischemia, and neuroprotection.

Hypercarbia and mild hypothermia, only when not combined, improve postischemic bioenergetic recovery in neonatal rat brain slices

In the immature brain, postischemic metabolism may be influenced beneficially by the effect of inducing hypercarbia or hypothermia. With use of 31P nuclear magnetic resonance spectroscopy, intracellular pH (pHi) and cellular energy metabolites in ex vivo neonatal rat cerebral cortex were measured before, during, and after substrate and oxygen deprivation in in vitro ischemia. Early postischemic hypothermia (fall in temperature -3.2 +/- 1.0 degrees C) delayed the normalization of pHi after ischemia by inducing an acid shift in pHi (P < 0.01). Postischemic hypercarbia (Krebs-Henseleit bicarbonate buffer equilibrated with 10% carbon dioxide in oxygen) and hypothermia induced separate, but potentially additive, reversible decreases in pHi, each of approximately -0.16 pH unit (P < 0.05). When these postischemic perturbations were applied in isolation, there was significant improvement of approximately 20% in the recovery of beta-ATP (P < 0.05). In combination, however, hypercarbia and hypothermia worsened recovery in ATP by approximately 20% (P < 0.05). In control tissue, which had not been exposed to ischemia, ATP content was also significantly reduced by co-administration of the two treatments (P < 0.05), an effect that persisted even after discontinuing the perturbing conditions. Therefore, in this vascular-independent neonatal preparation, early postischemic modulation of metabolism by hypercarbia or hypothermia appears to confer improved bioenergetic recovery, but only if they are not administered together.

Cooling the newborn after asphyxia - physiological and experimental background and its clinical use

Many years of experimental work on hypoxic-ischaemic injury have supported the hypothesis that cooling the body and brain after the primary injury offers permanent neuroprotection. Clinically, the question of how late cooling can start after the insult and still have a protective effect is important and not fully investigated. Pilot studies in human adults initiated cooling after 10-18 h (trauma, stroke), however animal data suggest cooling is not effective if started later than 6 h. There might be a threshold for 'cooling dose' - by depth or duration - to achieve permanent protection. Hypothermia must be administered with understanding of the extensive physiological effects. Different enzymes have different sensitivity to changes in temperature, hence some effects may be beneficial and some deleterious. Hypothermia and cardiovascular responses and coagulation needs careful monitoring.

Comparative neuroprotective efficacy of prolonged moderate intraischemic and postischemic hypothermia in focal cerebral ischemia

OBJECT

The purpose of this study was to compare the effects of prolonged hypothermia on ischemic injury in a highly reproducible model of middle cerebral artery (MCA) occlusion in rats.

METHODS

Male Sprague-Dawley rats were anesthetized with halothane and subjected to 120 minutes of temporary MCA occlusion by retrograde insertion of an intraluminal nylon suture coated with poly-L-lysine through the external carotid artery into the internal carotid artery and the MCA. Two levels of prolonged postischemic cranial hypothermia (32 degrees C and 27 degrees C) and one level of intraischemic cranial hypothermia (32 degrees C) were compared with the ischemic normothermia (37 degrees C) condition. Target cranial temperatures were maintained for 3 hours and then gradually restored to 35 degrees C over an additional 2-hour period. The animals were evaluated using a quantitative neurobehavioral battery of tests before inducing MCA occlusion, during occlusion (at 60 minutes postonset in all rats except those in the intraischemic hypothermia group), and at 24, 48, and 72 hours after reperfusion. The rat brains were perfusion fixed at 72 hours after ischemia, and infarct volumes and brain edema were determined. Both intraischemic and postischemic cooling to 32 degrees C led to similar significant reductions in cortical infarct volume (by 89% and 88%, respectively) and total infarct volume (by 54% and 69%, respectively), whereas postischemic cooling to 27 degrees C produced lesser reductions (64% and 49%, respectively), which were not statistically significant. All three hypothermic regimens significantly lessened hemispheric swelling and improved the neurological score at 24 hours. The authors' data confirm that a high degree of histological neuroprotection is conferred by postischemic cooling to 32 degrees C, which is virtually equivalent to that observed with intraischemic cooling to the same level.

CONCLUSIONS

These results may be relevant to the design of future clinical trials of therapeutic hypothermia for acute ischemic stroke.

Hypothermia inhibits pentylenetetrazol kindling and prevents kindling-induced deficit in shuttle-box avoidance

In this study, we evaluated the effects of hypothermic exposure on pentylenetetrazol (PTZ) kindling and the resulting deficit of shuttle-box avoidance learning in rats. Additionally, to acknowledge neuronal cell loss, we estimated the number of toluidine blue-positive cells in different brain regions after PTZ kindling and hypothermia exposure in comparison to different normothermic and hypothermic controls. To obtain hypothermic conditions over a period of up to about 3 h, 30 min after PTZ application the animals were treated with 5 mg/kg chlorpromazine (CP) and 25 min later exposed to 15 degrees C cold water for 5 min. Under these conditions the rectal and the striatal temperature were reduced up to a maximum of 5 degrees C. The additional injection of CP did not influence the development of PTZ kindling. Animals treated with PTZ/CP and exposed to hypothermia did not reach the criterion for kindling. Furthermore, this group of animals did not demonstrate any learning deficit. Forty-eight hours after the last kindling application the number of toluidine blue-stained cells was decreased in the investigated brain regions (hippocampal CA1 and CA3 sector, hilus, and cingular cortex) of kindled rats. Hypothermia protected from cell damage in the hippocampal CA3 sector and in the hilus. Results suggest that the inhibiting effect of hypothermia on the development of kindling and the following learning deficit possibly resulted from the suppression of cell damage in distinct brain structures on PTZ-kindled rats.

Caspase inhibitors reduce neuronal injury after focal but not global cerebral ischemia in rats

BACKGROUND AND PURPOSE

Studies show that blocking the activation of caspases by the caspase inhibitors z-VAD.FMK and z-DEVD.FMK can reduce ischemic neuronal injury after cerebral ischemia. Because the severity of ischemia was mild in some studies, we tested the efficacy of these caspase inhibitors on moderately severe but transient forebrain and focal ischemic insults in the rat.

METHODS

Various regimens of z-VAD, z-DEVD, and control DMSO were given to rats subjected to either 4-vessel occlusion ischemia (4-VO, 10-minute occlusion, 7-day survival) or distal middle cerebral artery occlusion (MCAo, 90-minute occlusion, 22.5-hour survival). In global ischemia, treatments were given immediately after ischemia (experiment 1) or as preischemic and postischemic treatments (experiment 2). Three focal ischemia experiments were done. Injection times were 60 minutes into ischemia (experiment 1) and 60 minutes into ischemia plus 30 and 120 minutes after ischemia (experiment 2). Experiment 3 was identical to experiment 2 except that a 30-minute preischemia treatment was instituted. Core normothermia was maintained in all experiments during ischemia. However, in the last focal and global experiments, core and brain temperatures, respectively, were also measured after ischemia with telemetry probes. Because hyperthermia accompanied z-DEVD treatment, an extra z-DEVD-treated group (MCAo) was included with temperature clamped at normothermia.

RESULTS

Neither z-VAD nor z-DEVD significantly reduced CA1 injury after global ischemia. In focal ischemia, both drugs significantly reduced infarction, but only in the third experiment, and the prevention of hyperthermia that accompanied z-DEVD treatment did not alter this.

CONCLUSIONS

These results suggest a detrimental role of caspases in moderately severe focal but not global cerebral ischemia.

Temperature modulation (hypothermic and hyperthermic conditions) and its influence on histological and behavioral outcomes following cerebral ischemia

Core temperature (T(C)) is a critical determinant of the severity of neural damage that results from focal or global ischemia. Former studies indicated that especially intra-ischemic but also post ischemic mild hypothermia significantly decreased necrotic neural damage of a focal or global insult, as assessed between 3-7 days post-insult. More recent work shows that prolonged post-ischemic hypothermia reduces neural damage and inhibits associated behavioral deficits for up to one year after the insult (i.e. true neuroprotection with behavioral preservation). Alternatively, increases in core temperature via external heating or with pyrogens resulting from bacterial infections, at the time of the global ischemia insult worsen the neural damage of ischemic animals from those of respective normothermic controls given the same insult. This is paralleled in the clinical setting whereby approximately 50% of ischemic patients develop fevers within 2 days of the insult and have worsened neurological outcomes than non-febrile patients. The review discusses the possible mechanisms of neuroprotection of hypothermic therapy from cerebral ischemia as well as mechanisms involved in the exacerbation of neural damage of hypoxic ischemia under hyperthermic conditions. Questions are raised as to whether the medical community has sufficient evidence to begin appropriate hypothermic therapy of acute stroke patients. The importance of accurate monitoring core temperatures of all suspected stroke patients is emphasized, noting the differences in temperature that can occur with age, sex, medication or lifestyle so that appropriate temperature treatment could be implemented, if required.

A model of cochlear function assessment during reversible ischemia in the Mongolian gerbil

An in vivo model in the Mongolian gerbil is used to assess the auditory changes during reversible cochlear ischemia. This model allows the recordings of cochlear potentials (microphonics, summating potential and compound action potential) and otoacoustic emissions (cubic difference tones, CDTs), together with laser Doppler cochlear blood flow (CBF) measurements, over reversible cochlear ischemia. Ischemia is achieved by compression of the eighth nerve complex at the porus of the internal acoustical meatus, whereas the compression release allows the reperfusion to occur. While CBF monitoring permits to objectively determine the degree of ischemia and reperfusion, the combined analysis of cochlear potentials and CDTs makes it possible to point out the preferential site of main functional damage within the cochlea, i.e., outer hair cells (OHCs), inner hair cells, or ganglia cells. At least three ischemia-reperfusion sequences can be used on one side, and sometimes both ears can serve for experiments. This model could be applicable for testing drugs with alleged protective effects against cochlear ischemia and/or reperfusion, in treated vs. non-treated animals.

Control of brain temperature during experimental global ischemia in rats

Temperature control during experimental ischemia continues to be of major interest. However, if exposure of brain tissue is necessary during the experiment, regional heat loss may occur even when the core temperature is maintained. Furthermore, valid non-invasive brain temperature monitoring is difficult in small rodents. This paper describes a method for both monitoring and maintenance of brain temperature during small animal preparations in a stereotaxic frame. The device used includes an ear-bar thermocouple probe and a small near-infrared radiator. The new equipment permitted to maintain peri-ischemic brain temperature at a desired level while carrying out non-invasive continuous recordings of cerebral blood flow (laser Doppler-flowmetry) and of electrical brain function (EEG). In contrast, without extracranial heat application, superficial and basal brain temperatures decreased during global cerebral ischemia by 4.1 +/- 0.1 and 4.6 +/- 0.4 degrees C (mean +/- SEM), respectively, returning to baseline values at 15-30 min of reperfusion while rectal (core) temperature remained stable at baseline values. The ear-bar thermocouple probe (tympanic membrane) reliably reflected basal brain temperature, and temperature in superficial brain areas correlated well with that in the temporal muscle. Our data show that the new system allows to exclude unwanted hypothermic neuroprotection, and does not interfere with optical and electrical measurement techniques.

Combination drug therapy and mild hypothermia: a promising treatment strategy for reversible, focal cerebral ischemia

BACKGROUND AND PURPOSE

Hypothermia has been suggested to be the most potent therapeutic approach to reduce experimental ischemic brain injury identified to date, and mild hypothermia is increasingly used for neuroprotection during neurovascular surgery. We have recently demonstrated that combined administration of tirilazad mesylate and magnesium provides for an overall enhanced neuroprotective effect. The present study was designed to determine whether the efficacy of mild hypothermia (33 degrees C) can be increased by combination pharmacotherapy with tirilazad and magnesium (MgCl(2)).

METHODS

Forty Sprague-Dawley rats were subjected to transient, middle cerebral artery occlusion and were randomly assigned to 1 of 4 treatment arms (n=10 each): (1) normothermia+vehicle, (2) normothermia+tirilazad+MgCl(2), (3) hypothermia+vehicle, or (4) hypothermia+tirilazad+MgCl(2). Cortical blood flow was monitored by a bilateral laser-Doppler flowmeter, and the electroencephalogram was continuously recorded. Functional deficits were quantified by daily neurological examinations. Infarct volume was assessed after 7 days.

RESULTS

Tirilazad+MgCl(2), hypothermia, and hypothermia+tirilazad+MgCl(2) reduced total infarct volume by 56%, 63%, and 77%, respectively, relative to controls. In animals treated with both hypothermia and combination pharmacotherapy, cortical infarction was almost completely abolished (-99%), and infarct volume in the basal ganglia was significantly reduced by 55%. In addition, this treatment provided for the best electrophysiological recovery and functional outcome.

CONCLUSIONS

The neuroprotective efficacy of hypothermia can be increased by pharmacological antagonism of excitatory amino acids and free radicals by using clinically available drugs. This treatment strategy could be of great benefit when applied during temporary artery occlusion in cerebrovascular surgery.

Postischemic hypothermia and IL-10 treatment provide long-lasting neuroprotection of CA1 hippocampus following transient global ischemia in rats

Experimental studies have demonstrated that postischemic therapeutic interventions may delay rather than provide long-lasting neuroprotection. The purpose of this study was to determine whether mild hypothermia (33-34 degrees C) combined with the anti-inflammatory cytokine interleukin-10 (IL-10) would protect the CA1 hippocampus 2 months after ischemia. Rats were subjected to 12.5 min of normothermic (37 degrees C) forebrain ischemia by two-vessel occlusion followed immediately by: (a) 4 h of normothermic (37 degrees C) reperfusion (n = 5); (b) 4 h of postischemic hypothermia (33-34 degrees C) (n = 5); (c) 4 h of normothermia plus IL-10 (5 micrograms) treatment 30 min after ischemia and at 3 days (n = 5); or (d) 4 h of hypothermia plus IL-10 treatment (n = 5). Rats survived for 2 months and were perfusion fixed for quantitative histopathological assessment of CA1 hippocampus. Postischemic normothermia and hypothermia, as well as normothermia plus IL-10 treatment led to severe damage of the CA1 hippocampus. In contrast, the combined treatment of hypothermia with IL-10 treatment improved overall neuronal survival by 49% compared to normothermic ischemia (P < 0.01). These data emphasize the detrimental consequences of secondary inflammatory responses on ischemic neuronal damage after transient global ischemia. In postinjury settings where restricted durations of mild hypothermia can be induced, anti-inflammatory treatments, including IL-10, may promote chronic neuroprotection.

Indefatigable CA1 sector neuroprotection with mild hypothermia induced 6 hours after severe forebrain ischemia in rats

Considerable controversy exists about whether postischemic hypothermia can permanently salvage hippocampal CA1 neurons or just postpone injury. Studies of very brief cooling in rat have found transient benefit, whereas experiments in gerbil using protracted hypothermia report lasting protection. This discrepancy might be because of the greater efficacy of longer cooling or it might, for example, represent an important species difference. In the present study, a 48-hour period of mild hypothermia was induced starting 6 hours after 10 minutes of severe four-vessel occlusion ischemia in rats. Untreated normothermic ischemia resulted in total CA1 cell loss (99%), whereas delayed hypothermia treatment reduced neuronal loss to 14% at a 28-day survival. In unregulated rats, brain temperature spontaneously fell during ischemia, and stayed subnormal for an extended period after ischemia. This mild cooling resulted in more variable and less severe CA1 injury (75%). Finally, vertebral artery cauterization under halothane anesthesia caused an approximately 2 degrees C drop in brain temperature for 1 hour, but prevention of this hypothermia did not significantly affect CA1 damage. In summary, protracted postischemic hypothermia provided robust and long-term CA1 protection in rat. These results encourage the clinical assessment of prolonged hypothermia and its use as a model to understand ischemic CA1 injury.

Influence of rewarming conditions after hypothermia in gerbils with transient forebrain ischemia

OBJECT

Recently, several studies have demonstrated that hypothermia has a beneficial effect on clinical outcome; however, it is difficult to determine the appropriate rewarming conditions in clinical use. The purpose of the present study was to examine the influence of rewarming conditions in gerbils with transient forebrain ischemia.

METHODS

Ischemia was induced in the gerbils by a 5-minute bilateral common carotid artery occlusion, after which the animals were immediately subjected to moderate or deep hypothermia. After moderate hypothermia (30.5 degrees C for 4 hours) the animals were rewarmed over standard, fast, or slow time periods. After deep hypothermia (24 degrees C for 2 hours) the animals were rewarmed in a standard, fast, slow, or stepwise manner. Cerebral blood flow (CBF), extracellular glutamate, and lactate were monitored. Hippocampal CA I cell damage was assessed 7 days after induction of ischemia. In animals treated with moderate hypothermia, the rewarming rate had no influence on the number of surviving neurons. However, fast rewarming from deep hypothermia (to 37 degrees C for 30 minutes) failed to provide the neuroprotective effect of hypothermia. Furthermore, this group showed a poor recovery of CBF (p < 0.01) and, consequently, an increase in extracellular glutamate (p < 0.01) and lactate (p < 0.01) in the hippocampus.

CONCLUSIONS

The results of this study indicate a transient uncoupling of CBF and cerebral metabolism during fast rewarming from deep hypothermia, whereas slow and stepwise rewarming periods were found to be useful for protection against uncoupling of CBF and cerebral metabolism during rewarming.

Hypoxic-ischemic brain damage in perinatal age group

Cerebral hypoxia-ischemia in the perinatal period continues to be a major contributor to chronic neurologic impairment in children worldwide. Extensive research conducted in the past several years has led to a better understanding of the mechanisms involved in hypoxic-ischemic brain injury. Based on this understanding, the major potential therapeutic approaches being studied include antagonists of excitatory amino acids, calcium channel antagonists, free-radical scavengers, nitric oxide synthase inhibitors, anti-inflammatory agents, trophic factors, and hypothermia. Several agents are in clinical trial phases in adults. However, safety concerns and close relationship between pathomechanisms of hypoxic-ischemic cerebral injury and normal developmental processes have contributed to the slow pace in the neonatal trials. Large multicenter trials including an adequate number of infants will be needed to evaluate efficacy of therapeutic interventions in this particular age group. A large number of risk factors that predispose to hypoxic ischemic injury have been identified. It is important to control these factors and prevent brain damage in the first place. This is especially true for developing countries where resources for treatment with newer agents (when they become available) are likely to be limited. Recent information regarding mechanisms of injury and potential therapeutic measures related to perinatal age are presented in this paper.

Hypothermic neuroprotection of peripheral nerve of rats from ischemia-reperfusion injury: intraischemic vs. reperfusion hypothermia

The pathophysiology of ischemic fiber degeneration (IFD) is not known, but mechanisms involved during nerve ischemia differ from those during reperfusion. We have previously demonstrated hypothermic neuroprotection of peripheral nerve from IFD. We now evaluate the efficacy of hypothermia in the intraischemic vs. the reperfusion period, using our established model of ischemia-reperfusion injury. Intraischemic hypothermia resulted in significant recovery of all indices (behavior score, electrophysiology and histology, P<0.01 or 0.05) while hypothermia during reperfusion period showed less improvement, significant only for the histological score compared to normothermia group (IFD index, P<0.05). Once hypothermia was applied in the ischemic period, the resultant neuroprotection continued into the reperfusion period, even if nerve temperature was then raised during the reperfusion period. These results indicate that hypothermic neuroprotection is more efficacious during the intraischemic period than during reperfusion, when a lesser degree of neuroprotection ensued.

Adenosine and cerebral ischemia: therapeutic future or death of a brave concept?

Numerous studies have consistently shown that agonist stimulation of adenosine A1 receptors results in a significant reduction of morbidity and mortality associated with global and focal brain ischemia in animals. Based on these observations, several authors have suggested utilization of adenosine A1 receptors as targets for the development of clinically viable drugs against ischemic brain disorders. Recent advent of adenosine A1 receptor agonists characterized by lowered cardiovascular effects added additional strength to this argument. On the other hand, although cardioprotective, adenosine A3 receptor agonists proved severely cerebrodestructive when administered prior to global ischemia in gerbils. Moreover, stimulation of adenosine A3 receptors appears to reduce the efficacy of some of the neuroprotective actions mediated by adenosine A1 receptors. The review discusses the possible role of adenosine receptor subtypes (A1, A2, and A3) in the context of their involvement in the pathology of cerebral ischemia, and analyzes putative strategies for the development of clinically useful strategies based on adenosine and its receptors. It also stresses the need for further experimental studies before definitive conclusions on the usefulness of the adenosine concept in the treatment of brain ischemia can be made.

The effects of temperature and scopolamine on N-methyl-D-aspartate antagonist-induced neuronal necrosis in the rat

The effects of temperature and scopolamine on dizocilpine maleate-induced neuronal necrosis in the rat cingulate/retrosplenial cortex, entorhinal/olfactory cortices and the dentate gyrus were studied. Mild, protracted hypothermia (48 h at a brain temperature of 34 degrees C), induced by a servo-controlled "exposure technique" in the awake female rat, significantly reduced dizocilpine maleate (5.0 mg/kg, i.p.)-induced neuronal death in the cingulate/retrosplenial and entorhinal/olfactory cortices seven days following drug administration. Scopolamine (0.25 mg/kg, i.p.), putatively neuroprotective [Olney J. W. et al. (1991) Science 254, 1515-1518], did not reduce injury in the cingulate/retrosplenial cortex of female rats following one injection, but did following two and three doses. Scopolamine had no significant effect in the other brain regions. A temperature elevation of only 1 degree C above baseline for 48 h in awake female rats increased dizocilpine maleate-induced damage. Finally, the sex differences in N-methyl-D-aspartate antagonist toxicity were replicated and extended to other structures, and found not to be due to temperature differences. Our data show that dizocilpine maleate neurotoxicity is temperature sensitive. Scopolamine treatment needed to be prolonged in order to reduce injury, and even then was only efficacious in one of three brain regions. The results underscore the importance of using neuronal necrosis in several brain regions as the endpoint and for the use of prolonged therapeutic interventions. Furthermore, given the potential hypothermic action of other putative neuroprotective drugs, a mechanistic re-evaluation of N-methyl-D-aspartate antagonist-induced injury is needed, with precise brain temperature measurement.

The pursuit of effective neuroprotection during infant cardiac surgery

Advances in infant cardiac surgery have resulted in a dramatic decline in mortality rates; however, neurological morbidity remains an important concern. The effectiveness of a number of interventional strategies to prevent or minimize brain injury during open heart surgery are currently being investigated. This article provides an overview of two approaches: (1) interventions to enhance intraoperative cerebral oxygenation so as to prevent hypoxic-ischemic insults, and (2) the application of cerebral rescue therapies to attenuate the cascade of brain injury. Infant cardiac surgery provides a controlled environment in which to apply these neuroprotective approaches, so as to optimize the quality of life of these vulnerable children.

Hypothermia: depression of tricarboxylic acid cycle flux and evidence for pentose phosphate shunt upregulation

OBJECT

Hypothermia is used in neurosurgery and other surgical disciplines to reduce tissue injury, but the mechanism of such protection remains elusive. The authors have endeavored to delineate the mechanism of neural protection afforded by hypothermia through a study of glucose metabolism.

METHODS

Nuclear magnetic resonance spectroscopy was used to follow the carbon-13 label from [1-13C]glucose as it was metabolized through the glycolytic and tricarboxylic acid pathways. Male Sprague-Dawley rats were maintained at either 37.5 degrees C or 31 degrees C and infused with labeled glucose for 10, 30, 60, 100, or 200 minutes (five rats were used for each time point and for each temperature). At the end of the infusion period, the rats' brains were subjected to rapid freeze-funnel fixation. Water-soluble metabolites were extracted from samples of the neocortex and hippocampus by using perchloric acid extraction. The fractional enrichment of these metabolites was used to calculate the reaction rate constant of formation and steady-state enrichment for a number of metabolites. Hypothermia resulted in a 30 to 40% depression of metabolism (p < 0.0001) in both the neocortex and hippocampus. Steady-state fractional enrichment of metabolites was also decreased by 20 to 25% with hypothermia (p < 0.0001), implying a loss of label during metabolism.

CONCLUSIONS

The results of this study suggest that an increased fraction of glucose metabolism was shunted through the pentose phosphate pathway in the presence of hypothermia.

Effect of postischaemic hypothermia on the mitochondrial damage induced by ischaemia and reperfusion in the gerbil

In order to test the effect of hypothermia on mitochondrial function damage following cerebral ischaemia/reperfusion, Mongolian gerbils were submitted to 30 min bilateral carotid occlusion and 2 h of reperfusion at 37 degreesC or 30 degreesC. After normothermic (37 degreesC) ischaemia/reperfusion, significant decreases in mitochondrial state 3 (+ADP) oxygen consumption (-42.2%), complex II-III activity in synaptosomes (-31.7%) and complex IV were measured, in both free mitochondria and synaptosomes (-30.3% and -27. 8% respectively). However, following hypothermic (30 degreesC) reperfusion, both respiration rates and all enzyme activities remained at levels not significantly different from those in the sham operated controls.

Adenosine and cerebral ischemia: therapeutic future or death of a brave concept?

Numerous studies have consistently shown that agonist stimulation of adenosine A1 receptors results in a significant reduction of morbidity and mortality associated with global and focal brain ischemia in animals. Based on these observations, several authors have suggested utilization of adenosine A1 receptors as targets for the development of clinically viable drugs against ischemic brain disorders. Recent advent of adenosine A1 receptor agonists characterized by lowered cardiovascular effects added additional strength to this argument. On the other hand, although cardioprotective, adenosine A3 receptor agonists proved severely cerebrodestructive when administered prior to global ischemia in gerbils. Moreover, stimulation of adenosine A3 receptors appears to reduce the efficacy of some of the neuroprotective actions mediated by adenosine A receptors. The review discusses the possible role of adenosine receptor subtypes (A1, A2, and A3) in the context of their involvement in the pathology of cerebral ischemia, and analyzes putative strategies for the development of clinically useful strategies based on adenosine and its receptors. It also stresses the need for further experimental studies before definitive conclusions on the usefulness of the adenosine concept in the treatment of brain ischemia can be made.

The 'pharmacology' of neuronal rescue with cerebral hypothermia

The neuroprotective effects of hypothermia during cerebral ischaemia or asphyxia are well known. Although, in view of this, the possibility of a therapeutic role for hypothermia during or after resuscitation from such insults has been a long standing focus of research, early studies had limited and contradictory results. Clinically and experimentally severe perinatal asphyxial injury is associated with a latent phase after reperfusion, with initial recovery of cerebral energy metabolism but EEG suppression, followed by a secondary phase with seizures, cytotoxic edema, accumulation of cytotoxins, and cerebral energy failure from 6 to 15 h after birth. Recent studies have led to the hypothesis that changes in post-ischaemic cerebral temperature can critically modulate encephalopathic processes which are initiated during the primary phase of hypoxia-ischaemia, but which extend into the secondary phase of cerebral injury. This conceptual framework allows a better understanding of the 'pharmacological' parameters that determine effective hypothermic neuroprotection, including the timing of initiation of cooling, its duration and the depth of cooling attained. Moderate cerebral hypothermia initiated in the latent phase, between one and as late as 6 hours after reperfusion, and continued for a sufficient duration in relation to the severity of the cerebral injury, has been associated with potent, long-lasting neuroprotection in both adult and perinatal species. These encouraging results must be balanced against the adverse systemic effects of hypothermia. Randomised clinical trials are in progress to establish the safety and efficacy of prolonged cerebral hypothermia.

Selective head cooling in newborn infants after perinatal asphyxia: a safety study

AIMS

To determine the practicality and safety of head cooling with mild or minimal systemic hypothermia in term neonates with moderate to severe hypoxic-ischemic encephalopathy.

METHODS

Study group infants >/=37 weeks' gestation, who had an umbilical artery pH </=7. 09 or Apgars </=6 at 5 minutes, plus evidence of encephalopathy. Infants with major congenital abnormalities were excluded. TRAIL DESIGN: Infants were randomized to either no cooling (controls; rectal temperature = 37.0 +/- 0.2 degreesC, n = 10) or sequentially, either minimal systemic cooling (rectal temperature = 36.3 +/- 0.2 degreesC, n = 6) or mild systemic cooling (rectal temperature = 35.7 +/- 0.2 degreesC, n = 6). Head cooling was accomplished by circulating water at 10 degreesC through a coil of tubing wrapped around the head for up to 72 hours. All infants were warmed by servo-controlled overhead heaters to maintain the allocated rectal temperature. The rectal, fontanelle, and nasopharyngeal temperatures were continuously monitored.

RESULTS

From January 1996 to October 1997, 22 term infants were randomized from 2 to 5 hours after birth. All infants showed a metabolic acidosis at delivery, with similar umbilical artery pH in the control group (mean +/- standard deviation, 6.79 +/- 0.25), minimal cooling group (6.98 +/- 0.21), and mild cooling group (6.93 +/- 0.11), and depressed Apgar scores at 5 minutes in the control group (4.5 +/- 2), minimal cooling group, (4.7 +/- 2) and mild cooling group (6.0 +/- 1). In the mild-cooled infants, the nasopharyngeal temperature was 34.5 degreesC during cooling, 1.2 degreesC lower than the rectal temperature. This gradient narrowed to 0.5 degreesC after cooling was stopped. No adverse effects because of cooling were observed. No infants developed cardiac arrhythmias, hypotension, or bradycardia during cooling. Thrombocytopenia occurred in 2 out of 10 controls, 2 out of 6 minimal cooling infants, and 1 out of 6 mild cooling infants. Hypoglycemia (glucose <2.6 mM) was seen on at least one occasion in 2 out of 10 controls, 4 out of 6 minimal cooling infants, and 1 out of 6 mild cooling infants. Acute renal failure occurred in all infants. The metabolic acidosis present in all infants at the time of enrollment into the study progressively resolved despite cooling, even in the mild hypothermia group.

CONCLUSIONS

Mild selective head cooling combined with mild systemic hypothermia in term newborn infants after perinatal asphyxia is a safe and convenient method of quickly reducing cerebral temperature with an increased gradient between the surface of the scalp and core temperature. The safety of mild hypothermia with selective head cooling is in contrast with the historical evidence of adverse effects with greater depths of whole-body hypothermia. This safety study and the strong experimental evidence for improved cerebral outcome justify a multicenter trial of selective head cooling for neonatal encephalopathy in term infants.

Mild hypothermia--a revived countermeasure against ischemic neuronal damages

Although hypothermia as a means of cerebral protection against and resuscitation from ischemic damage has a history of approximately six decades, extensive studies, both in basic and clinical fields, on the mechanisms, effects and methods of mild hypothermia at temperatures no less than 31 degrees C have started only in the last decade. In experiments on rodents, hypothermia in the postischemic period that is introduced up to several hours after reperfusion and is maintained for one day followed by a slow rewarming, significantly protects hippocampal neurons against damage. The mode of action of hypothermia is apparently non-specific and multi-focal in widely progressing cascade reactions in ischemic cells; namely, suppressing: (1) glutamate surge followed by; (2) intraneuronal calcium mobilization; (3) sustained activation of glutamate receptors; (4) dysfunction of blood brain barrier; (5) proliferation of microglial cells; and (6) production of superoxide anions and nitric oxide. In addition, mild hypothermia modulates processes in ischemic condition at the level of cell nucleus, such as the binding of transcription factor AP-1 to DNA, and ameliorates the depression of protein synthesis. This non-specific and widely affecting manner might explain why hypothermia is superior to any medicine developed. Recent clinical trials of mild hypothermia in various individual institutions have revealed significantly beneficial outcomes in some cases, along with an accumulation of practical knowledge of techniques and treatments. Large scale randomized studies involving multiple institutions as well as exchange of informations and ideas are needed for further development of hypothermia treatment.

Neurologic complications after coronary artery bypass grafting

Although outcomes from coronary artery bypass grafting (CABG) surgery have improved in general, there has been little or no improvement in the incidence of postoperative stroke or neurologic dysfunction. Several studies have identified factors that increase the CABG patient's risk for developing a stroke and neurologic complications. It is important to identify those patients at increased risk and differentiate among stroke, delirium, and seizures. Post-CABG patients need to be monitored for neurological dysfunction with appropriate assessments. Neurologic complications must be appropriately managed to optimize patient recovery.