The effect of post-ischemic hypothermia following repetitive cerebral ischemia in gerbils

Therapeutic applications of hypothermia in cerebral ischaemia

There is considerable experimental evidence that hypothermia is neuroprotective and can reduce the severity of brain damage after global or focal cerebral ischaemia. However, despite successful clinical trials for cardiac arrest and perinatal hypoxia-ischaemia and a number of trials demonstrating the safety of moderate and mild hypothermia in stroke, there are still no established guidelines for its use clinically. Based upon a review of the experimental studies we discuss the clinical implications for the use of hypothermia as an adjunctive therapy in global cerebral ischaemia and stroke and make some suggestions for its use in these situations.

Hypothermia for hypoxic-ischemic encephalopathy

We are entering an era in which hypothermia will be used in combination with other novel neuroprotective interventions. The targeting of multiple sites in the cascade leading to brain injury may prove to be a more effective treatment strategy after hypoxic-ischemic encephalopathy in newborn infants than hypothermia alone.

Hypothermic neuroprotection

The possibility that hypothermia during or after resuscitation from asphyxia at birth, or cardiac arrest in adults, might reduce evolving damage has tantalized clinicians for a very long time. It is now known that severe hypoxia-ischemia may not necessarily cause immediate cell death, but can precipitate a complex biochemical cascade leading to the delayed neuronal loss. Clinically and experimentally, the key phases of injury include a latent phase after reperfusion, with initial recovery of cerebral energy metabolism but EEG suppression, followed by a secondary phase characterized by accumulation of cytotoxins, seizures, cytotoxic edema, and failure of cerebral oxidative metabolism starting 6 to 15 h post insult. Although many of the secondary processes can be injurious, they appear to be primarily epiphenomena of the 'execution' phase of cell death. Studies designed around this conceptual framework have shown that moderate cerebral hypothermia initiated as early as possible before the onset of secondary deterioration, 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. Two large controlled trials, one of head cooling with mild hypothermia, and one of moderate whole body cooling have demonstrated that post resuscitation cooling is generally safe in intensive care, and reduces death or disability at 18 months of age after neonatal encephalopathy. These studies, however, show that only a subset of babies seemed to benefit. The challenge for the future is to find ways of improving the effectiveness of treatment.

Therapeutic hypothermia: from lab to NICU

The possibility of a therapeutic role for cerebral hypothermia during or after resuscitation from perinatal asphyxia has been a long-standing focus of research. However, early studies had limited and contradictory results. It is now known that severe hypoxia-ischemia may not cause immediate cell death, but may precipitate a complex biochemical cascade leading to the delayed development of neuronal loss. These phases include a latent phase after reperfusion, with initial recovery of cerebral energy metabolism but EEG suppression, followed by a secondary phase characterized by accumulation of cytotoxins, seizures, cytotoxic edema, and failure of cerebral oxidative metabolism from 6 to 15 h post insult. Although many of the secondary processes can be injurious, they appear to be primarily epiphenomena of the 'execution' phase of cell death. This conceptual framework allows a better understanding of the experimental 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 h after reperfusion, and continued for a sufficient duration in relation to the severity of the cerebral injury, has been consistently associated with potent, long-lasting neuroprotection in both adult and perinatal species. The results of the first large multicentre randomized trial of head cooling for neonatal encephalopathy and previous phase I and II studies now strongly suggest that prolonged cerebral hypothermia is both generally safe - at least in an intensive care setting - and can improve intact survival up to 18 months of age. Both long-term followup studies and further large studies of whole body cooling are in progress.

Hypothermia and stroke: the pathophysiological background

Hypothermia to mitigate ischemic brain tissue damage has a history of about six decades. Both in clinical and experimental studies of hypothermia, two principal arbitrary patterns of core temperature lowering have been defined: mild (32-35 degrees C) and moderate hypothermia (30-33 degrees C). The neuroprotective effectiveness of postischemic hypothermia is typically viewed with skepticism because of conflicting experimental data. The questions to be resolved include the: (i) postischemic delay; (ii) depth; and (iii) duration of hypothermia. However, more recent experimental data have revealed that a protected reduction in brain temperature can provide sustained behavioral and histological neuroprotection, especially when thermoregulatory responses are suppressed by sedation or anesthesia. Conversely, brief or very mild hypothermia may only delay neuronal damage. Accordingly, protracted hypothermia of 32-34 degrees C may be beneficial following acute cerebral ischemia. But the pathophysiological mechanism of this protection remains yet unclear. Although reduction of metabolism could explain protection by deep hypothermia, it does not explain the robust protection connected with mild hypothermia. A thorough understanding of the experimental data of postischemic hypothermia would lead to a more selective and effective clinical therapy. For this reason, we here summarize recent experimental data on the application of hypothermia in cerebral ischemia, discuss problems to be solved in the experimental field, and try to draw parallels to therapeutic potentials and limitations.

Is temperature important in delivery room resuscitation?

The possibility that temperature may affect the outcome of resuscitation from severe perinatal asphyxia has been a long-standing focus of research. Experimentally it is now well established that even small changes in temperature during severe hypoxia-ischemia critically modulate outcome. Clinical and experimental studies have now shown that hypoxic-ischemic injury continues to evolve after resuscitation. Experimentally, prolonged mild to moderate hypothermia can dramatically reduce this delayed injury, while mild hyperthermia over the same period worsens injury. Indeed there are data indicating that moderate post-ischemic hyperthermia can be deleterious as late as 24 h after reperfusion. Hypothermia has significant potential adverse effects, and at present its clinical use is restricted to large randomized controlled trials. The present paper reviews evidence suggesting that both primary prevention of maternal pyrexia during labour, and secondary prevention of hyperthermia after neonatal resuscitation, have the potential to significantly reduce the consequences of perinatal hypoxia-ischemia.

Effects of metabolic alterations on dopamine release in an in vitro model of neostriatal ischaemia

Release of neurotransmitters, including dopamine (DA), plays a central role in neuronal death during cerebral ischaemia. We investigated the effects of changes in energy demand and supply on DA release in cerebral ischaemia in vitro. Rat striatal slices were superfused (400 ml/h) with an artificial cerebrospinal fluid at 34 degrees C, unless otherwise stated. Ischaemia were mimicked by removal of O2 and reduction in glucose concentration from 4 to 2 mM. DA release was monitored by voltammetry. The profile of ischaemia-induced DA release was temperature-dependent. Hypothermia (to 24 degrees C) delayed, slowed, and reduced ischaemia-induced DA release relative to 34 degrees C. Pretreatment of the slices for 3 h with creatine (25 mM) delayed and slowed ischaemia-induced DA release. Conversely, blockade of Na+/K+ ATPase with ouabain induced an anoxic depolarisation and rapid DA release similar to ischaemia. In summary, the onset of DA release in this model is controlled by the balance between energy supply and utilisation. Strategies that increase availability of energy substrates for the membrane sodium pump (i.e., pre-incubation with creatine) or decrease their utilisation (hypothermia) slow and delay DA release. Hypothermia may owe part of its neuroprotective effect to a delay and slowing of ischaemia-induced release of DA and/or other neurotransmitters.

Neuroprotection with prolonged head cooling started before postischemic seizures in fetal sheep

OBJECTIVE

Cerebral hypothermia has been shown to reduce damage from experimental hy-poxia-ischemia if started shortly after reperfusion. However, in the newborn infant it may not be feasible to determine prognosis so soon after exposure to asphyxia. The aim of this study was to determine whether head cooling, delayed until shortly before the onset of postasphyxial seizure activity, is neuroprotective.

METHODS

Unanesthetized near-term fetal sheep in utero were subjected to 30 minutes of cerebral ischemia. Later, at 5.5 hours, they were randomized to either cooling (n = 7) or sham cooling (n = 10) for 72 hours. Intrauterine cooling was induced by circulating cold water through a coil around the fetal head. The water temperature was titrated to reduce fetal extradural temperature from 39.1 +/- 0.1 degreesC to between 30 degreesC and 33 degreesC, while maintaining esophageal temperature >37 degreesC.

RESULTS

Cerebral cooling suppressed the secondary rise in cortical impedance (a measure of cytotoxic edema), but did not prevent delayed seizures, 8 to 30 hours after ischemia. Transient metabolic changes including increased plasma lactate and glucose levels were seen with a moderate sustained rise in blood pressure. This severe cerebral insult resulted in depressed residual parietal electroencephalographic activity after 5 days recovery (-14.2 +/- 1.5 decibels), associated with a watershed distribution of neuronal loss (eg, 94 +/- 4% in parasagittal cortex and 77 +/- 4% in the lateral cortex). Hypothermia was associated with better recovery of electroencephalographic activity (-8.9% +/- 1.8 decibels) and substantially reduced neuronal loss in the parasagittal cortex (46 +/- 13%), the lateral cortex (9 +/- 4%), and other regions except the cornu ammonis sectors 1 and 2 of the hippocampus.

CONCLUSIONS

Delayed selective head cooling begun before the onset of postischemic seizures and continued for 3 days may have potential to significantly improve the outcome of moderate to severe hypoxic-ischemic encephalopathy.

Postischemic hyperthermia exacerbates neurologic injury after deep hypothermic circulatory arrest

BACKGROUND

Aggressive surface warming is a common practice in the pediatric intensive care unit. However, recent rodent data emphasize the protective effect of mild (2 degrees - 3 degrees C) hypothermia after cerebral ischemia. This study evaluates different temperature regulation strategies after deep hypothermic circulatory arrest with a survival piglet model.

METHODS

Fifteen piglets were randomly assigned to 3 groups. All groups underwent 100 minutes of deep hypothermic circulatory arrest at 15 degrees C. Brain temperature was maintained at 34 degrees C for 24 hours after cardiopulmonary bypass in group I, 37 degrees C in group II, and 40 degrees C in group III. Neurobehavioral recovery was evaluated daily for 3 days after extubation by neurologic deficit score (0, normal; 500, brain death) and overall performance category (1, normal; 5, brain death). Histologic examination was assessed for hypoxic-ischemic injury (0, normal; 5, necrosis) in a blinded fashion.

RESULTS

All results are expressed as mean +/- standard deviation. Recovery of neurologic deficit score (12.0 +/- 17.8, 47.0 +/- 49.95, 191.0 +/- 179.83; P = .05 for group I vs III), overall performance category (1.0 +/- 0.0, 1.4 +/- 0.6, 2.8 +/- 1.3; P < .05 for group I vs III), and histologic scores (0.0 +/- 0.0, 1.0 +/- 1.2, 2.8 +/- 1.8; P < .05 for group I vs III cortex) were significantly worse in hyperthermic group III. These findings were associated with a significantly lower cytochrome aa3 recovery determined by near-infrared spectroscopy in group III animals (P = .0041 for group I vs III). No animal recovered to baseline electroencephalographic value by 48 hours after deep hypothermic circulatory arrest. Recovery was significantly delayed in the hyperthermic group III animals, with a lower amplitude 14 hours after the operation, which gradually increased with time (P < .05 for group III vs groups I and II).

CONCLUSIONS

Mild postischemic hyperthermia significantly exacerbates functional and structural neurologic injury after deep hypothermic circulatory arrest and should therefore be avoided.

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.

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.

Zonisamide as a neuroprotective agent in an adult gerbil model of global forebrain ischemia: a histological, in vivo microdialysis and behavioral study

Brief periods of global cerebral ischemia are known to produce characteristic patterns of neuronal injury both in human studies and in experimental animal models. Ischemic damage to vulnerable areas such as the CA1 sector of the hippocampus is thought to result from excitotoxic amino acid neurotransmission. The objective of this study was to determine the ability of a novel sodium channel blocking compound, zonisamide, to reduce neuronal damage by preventing the ischemia-associated accumulation of extracellular glutamate. Using a gerbil model, animals were subjected to 5 min ischemic insults. Both pre- and post-ischemic drug administration (zonisamide 150 mg/kg) were studied. Histological brain sections were prepared using a silver stain at 7 and 28 days post ischemia. The animals sacrificed at 28 days also underwent behavioral testing using a modified Morris water maze. In vivo microdialysis was performed on a separate group of animals in order to determine the patterns of ischemia-induced glutamate accumulation in the CA1 sector of the hippocampus. Pyramidal cell damage scores in the CA1 region of the hippocampus were significantly reduced in animals pre-treated with zonisamide compared to saline-treated controls, both at 7 days (drug pre-treated: 0.812 +/- 0.28, n = 8; controls: 1.625 +/- 0.24, n = 8; *P < 0.05) and 28 (drug pre-treated: 0.833 +/- 0.22, n = 12; controls: 1.955 +/- 0.26, n = 11; **P < 0.01) days post ischemia. However, animals receiving zonisamide post-treatment did not display significant differences from controls. Behavioral studies also showed significant preservation of function in drug-treated animals. Microdialysis studies confirmed a reduction in glutamate release in drug-treated animals compared to saline-treated controls. Our data suggest that zonisamide is effective in reducing neuronal damage by a mechanism involving decreased ischemia-induced extracellular glutamate accumulation and interruption of excitotoxic pathways.

Transport of pituitary adenylate cyclase-activating polypeptide across the blood-brain barrier and the prevention of ischemia-induced death of hippocampal neurons

PACAP is a member of the secretin/glucagon/VIP family of peptides and demonstrates neurotrophic and neuroprotective effects at very low concentrations. We have previously shown that PACAP crosses the BBB to a modest degree by way of a saturable transport system. PACAP is transported across the BBB as an intact peptide to enter the parenchymal space of the brain. We tested the possibility that this modest rate of transport would be sufficient to produce the low levels of PACAP needed in the brain to exert a neuroprotective effect against ischemia. We found that PACAP given intravenously could indeed prevent the death of CA1 hippocampal neurons, even if the administration of PACAP was delayed for 24 h after the ischemic event. We suggest that iv PACAP could be neuroprotective after stroke, cardiac arrest, and hypotensive episodes.

Prevention of ischemia-induced death of hippocampal neurons by pituitary adenylate cyclase activating polypeptide

Because neurons in the CA1 region of the hippocampus are vulnerable to forebrain ischemia, this model has been used for evaluating neuroprotective agents. We evaluated the 38-amino-acid variant of pituitary adenylate cyclase activating polypeptide (PACAP38), which had been previously shown to be neuroprotective in vitro against gp120-induced hippocampal neuronal death at concentrations as low as 0.1 pM. Ischemic death of rat CA1 neurons was prevented by infusing PACAP38 either intracerebroventricularly (1 pmol/h) or intravenously (16-160 pmol/h). Intravenous PACAP38 was effective even if the infusion was begun 24 h after ischemia. The results suggest that a concentration of PACAP38 in the brain which prevents the ischemic death of CA1 neurons can be reached by the systemic administration of a low dose of the peptide. The results are compatible with the previous reports that PACAP38 is transported from the circulation to the brain. Although the exact mechanisms remain to be determined, astrocytes in the CA1 subfield activated by ischemia appear to mediate the neuroprotection with PACAP38. These results are in contrast to those with other neuroprotective compounds and should be clinically important.