Hypothermia decreases excitatory neurotransmitter release in bacterial meningitis in rabbits

A New Vision for Therapeutic Hypothermia in the Era of Targeted Temperature Management: A Speculative Synthesis

Three decades of animal studies have reproducibly shown that hypothermia is profoundly cerebroprotective during or after a central nervous system (CNS) insult. The success of hypothermia in preclinical acute brain injury has not only fostered continued interest in research on the classic secondary injury mechanisms that are prevented or blunted by hypothermia but has also sparked a surge of new interest in elucidating beneficial signaling molecules that are increased by cooling. Ironically, while research into cold-induced neuroprotection is enjoying newfound interest in chronic neurodegenerative disease, conversely, the scope of the utility of therapeutic hypothermia (TH) across the field of acute brain injury is somewhat controversial and remains to be fully defined. This has led to the era of Targeted Temperature Management, which emphasizes a wider range of temperatures (33–36°C) showing benefit in acute brain injury. In this comprehensive review, we focus on our current understandings of the novel neuroprotective mechanisms activated by TH, and discuss the critical importance of developmental age germane to its clinical efficacy. We review emerging data on four cold stress hormones and three cold shock proteins that have generated new interest in hypothermia in the field of CNS injury, to create a framework for new frontiers in TH research. We make the case that further elucidation of novel cold responsive pathways might lead to major breakthroughs in the treatment of acute brain injury, chronic neurological diseases, and have broad potential implications for medicines of the distant future, including scenarios such as the prevention of adverse effects of long-duration spaceflight, among others. Finally, we introduce several new phrases that readily summarize the essence of the major concepts outlined by this review—namely, Ultramild Hypothermia, the “Responsivity of Cold Stress Pathways,” and “Hypothermia in a Syringe.”

Heat Shock Factor 1 Deficiency Affects Systemic Body Temperature Regulation

INTRODUCTION

Heat shock factor 1 (HSF1) is a ubiquitous heat-sensitive transcription factor that mediates heat shock protein transcription in response to cellular stress, such as increased temperature, in order to protect the organism against misfolded proteins. In this study, we analysed the effect of HSF1 deficiency on core body temperature regulation.

MATERIALS AND METHODS

Body temperature, locomotor activity, and food consumption of wild-type mice and HSF1-deficient mice were recorded. Prolactin and thyroid-stimulating hormone levels were measured by ELISA. Gene expression in brown adipose tissue was analysed by quantitative real-time PCR. Hypothalamic HSF1 and its co-localisation with tyrosine hydroxylase was analysed using confocal laser scanning microscopy.

RESULTS

HSF1-deficient mice showed an increase in core body temperature (hyperthermia), decreased overall locomotor activity, and decreased levels of prolactin in pituitary and blood plasma reminiscent of cold adaptation. HSF1 could be detected in various hypothalamic regions involved in temperature regulation, suggesting a potential role of HSF1 in hypothalamic thermoregulation. Moreover, HSF1 co-localises with tyrosine hydroxylase, the rate-limiting enzyme in dopamine synthesis, suggesting a potential role of HSF1 in the hypothalamic control of prolactin release. In brown adipose tissue, levels of prolactin receptor and uncoupled protein 1 were increased in HSF1-deficient mice, consistent with an up-regulation of heat production.

CONCLUSION

Our data suggest a role of HSF1 in systemic thermoregulation.

Use of induced hypothermia for neuroprotection: indications and application

Therapeutic temperature regulation has become an exciting field of interest. Mild-to-moderate hypothermia is a safe and feasible management strategy for neuroprotection and control of intracranial pressure in neurological catastrophies such as traumatic brain injury, subarachnoid and intracerebral hemorrhage, and large hemispheric stroke. Fever is associated with worse neurological outcome in patients with brain injury, normothermia may be of benefit in this patient population. The efficacy of mild-to-moderate hypothermia has been proven for neuroprotection after cardiac arrest with ventricular fibrillation as initial rhythm, and after neonatal asphyxia. Application of hypothermia and fever control in neurocritical care, available cooling technologies and systemic effects and complications of hypothermia will be discussed.

Hypothermic preconditioning reduces Purkinje cell death possibly by preventing the over-expression of inducible nitric oxide synthase in rat cerebellar slices after an in vitro simulated ischemia

We showed that hypothermic preconditioning (HPC) increased survival of Purkinje neurons in rat cerebellar slices after oxygen-glucose deprivation (OGD). HPC also reduced the OGD-increased expression of high mobility group I (Y) proteins, a transcription factor that can enhance inducible nitric oxide synthase (iNOS) expression. iNOS is a putatively damaging protein that contributes to ischemic brain injury. Heat shock proteins (HSPs) can be induced by various stimuli to protect cells. We hypothesize that HPC induces neuroprotection by reducing the expression of putatively damaging proteins such as iNOS and/or by increasing the expression of putatively protective proteins such as HSPs. Cerebellar slices were prepared from adult male Sprague-Dawley rats and incubated in circulating artificial cerebrospinal fluid. OGD was for 20 min at 37 degrees C and was followed by a 5-h recovery at 37 degrees C before slices were used for morphological, immunohistochemical and Western analyses. HPC was performed by incubating slices at 33 degrees C for 20 min at 1 h before the OGD. HPC and aminoguanidine, an iNOS inhibitor, prevented OGD-induced Purkinje cell death/injury. OGD increased the expression of iNOS and nitrosylated proteins. These increases were abolished by aminoguanidine and HPC. Interestingly, the expression of HSP70 was increased by OGD but not by HPC. Our results suggest that an increased iNOS expression contributes to the pathophysiology of OGD-induced Purkinje neuronal death in our model. Our results also suggest the involvement of inhibiting the expression of the putatively damaging iNOS proteins in the HPC-induced neuroprotection. HSP70 may not contribute to the HPC-induced neuroprotection.

Elevated cerebrospinal fluid levels of glutamate in children with bacterial meningitis as a predictor of the development of seizures or other adverse outcomes

OBJECTIVE

Evaluation of elevated cerebrospinal fluid levels of glutamate in children with bacterial meningitis as a predictor of seizures or other adverse outcomes.

DESIGN

Prospective cohort study with controls.

SETTING

A 36-bed pediatric intensive care unit and primary pediatric referral center.

PATIENTS

From 1999 to 2001, a total of 55 patients, between the ages of 0 and 18 yrs, with lumbar punctures performed for suspected meningitis.

MEASUREMENTS AND MAIN RESULTS

A total of 23 patients had bacterial meningitis confirmed by cerebrospinal fluid/blood culture and elevated cerebrospinal fluid white blood cell counts, and 32 patients, who tested negative, were included as controls. The median age for the patients with meningitis was 1.0 yr (range, 0.0-15.2 yrs), and in the culture-negative group (control group), the median age was 0.3 yrs (range, 0.0-17.0 yrs). The average cerebrospinal fluid white blood cell count was 2707 +/- 3897 in the group with bacterial infection, whereas in the control group, the average was 148 +/- 259 (p < .01). Patients with bacterial meningitis had a mean cerebrospinal fluid glutamate level of 60.5 +/- 88.4 mol/L, whereas the mean cerebrospinal fluid glutamate level in the control group was 4.9 +/- 11.0 mol/L (p < .01). However, only 10 of 23 children with bacterial meningitis had a second lumbar puncture performed during the study. There was no correlation between the cerebrospinal fluid white blood cell count and cerebrospinal fluid glutamate levels in either the study or control patients. None of the control patients developed seizures or neurologic deficits, despite some patients having elevated glutamate levels. However, four patients with bacterial meningitis developed seizures after admission to the hospital, and ten were discharged with at least some neurologic sequelae attributable to their infection. Two out of the three who developed seizures and had a repeat lumbar puncture demonstrated persistent elevation of cerebrospinal fluid glutamate levels. In addition, 70% of patients (7 of 10) with Streptococcus pneumoniae meningitis developed neurologic complications (p = .04).

CONCLUSIONS

Bacterial meningitis in children causes an increase in cerebrospinal fluid glutamate that in many cases persists over time. However, in this limited study, neither higher nor persistent elevation of cerebrospinal fluid glutamate levels is predictive of which patients might develop seizures or other apparent immediate adverse outcomes after invasive infection. The responsible organism seems to have far more significance in predicting the development of adverse sequelae.

Modulation of nuclear factor-kappaB activation and decreased markers of neurological injury associated with hypothermic therapy in experimental bacterial meningitis

OBJECTIVE

This study was designed to evaluate the use of moderate hypothermia in a model of meningitis-induced brain injury and its effect on the activation of nuclear factor-kappaB, biological markers of neuronal injury, and neurobehavioral performance.

DESIGN

Randomized, prospective animal study.

SETTING

University research laboratory.

SUBJECTS

Male Wistar rats.

INTERVENTIONS

Animals underwent a basilar cistern tap receiving either sterile saline as a placebo or an equivalent volume of a group B streptococcal suspension. Sixteen hours after inoculation, animals were stratified by their clinical severity score, were randomized to either hypothermic (32-34 degrees C) or normothermic (37-39 degrees C) conditions, and received antibiotics. Hypothermic animals were kept under these temperature conditions for 6 hrs before rewarming. Two protocols were used. For the first protocol, changes in nuclear factor-kappaB activation and heat shock protein induction at 24 hrs and 48 hrs after inoculation were evaluated. In the second protocol, serum C-tau concentrations at 5 days and neurobehavioral performances at 3 wks were assessed.

MEASUREMENTS AND MAIN RESULTS

Meningitis triggered a >50% increase in cerebral nuclear factor-kappaB activation. The addition of a 6-hr period of hypothermia reduced nuclear factor-kappaB activation by 32% when measured at the end of the hypothermic period. At 48 hrs, this decrease in nuclear factor-kappaB activation was no longer apparent, but there was a significant decrease in the heat shock response. Serum C-tau concentrations at 5 days postinjury, a biomarker of brain injury, were reduced by 69% in hypothermic treated animals. Furthermore, hypothermia reduced the brain water content of infected animals. However, hypothermia did not improve the animals' neurobehavioral performance.

CONCLUSION

The findings from this study suggest that hypothermia produces a transitory attenuation of nuclear factor-kappaB activation in meningitic brain injury and improvement in some biomarkers of neuronal injury. The consequence of intermittent suppression of nuclear factor-kappaB activation by inducing specific periods of hypothermia requires further study.

New therapies and vaccines for bacterial meningitis

Acute bacterial meningitis remains an important cause of morbidity and mortality worldwide. There have recently been major advances in the prevention of the major causes of bacterial meningitis following improvements in vaccinology. The success of immunisation against Haemophilus influenzae type b infection is being mirrored with serogroup C conjugated meningococcal vaccine and pneumococcal conjugate vaccine. However, there remain major challenges, notably, serogroup B meningococcal infection and shifts in epidemiology caused by vaccine introduction. In addition, much of the world's population is unvaccinated. Therefore, improvements in management of acute bacterial meningitis are vital. In this review we attempt to summarise important advances in both prevention and treatment of acute bacterial meningitis.

Effect of hypothermia on bilirubin-induced alterations in brain cell membrane function and energy metabolism in newborn piglets

The aim of this study was to evaluate the effects of hypothermia on bilirubin-induced alterations in brain cell membrane function and energy metabolism in the developing brain. Thirty-seven newborn piglets were divided randomly into four groups: normothermic control (NC, n=9); hypothermic control (HC, n=7); normothermic bilirubin infusion (NB, n=11); and hypothermic bilirubin infusion (HB, n=10) groups. In bilirubin infusion groups (NB and HB), a loading dose of bilirubin (35 mg/kg) was given over 5 min, followed by a continuous infusion (25 mg/kg/h) for 4 h. The control groups (NC, HC) received a bilirubin-free buffer solution. Sulfadimethoxine was administered to animals in all experimental groups. Rectal temperature was maintained between 38.0 and 39.0 degrees C in normothermic groups, and between 34.0 and 35.0 degrees C in hypothermic groups for 4 h after the start of bilirubin infusion. The final blood and brain bilirubin concentrations in the bilirubin infusion groups (NB and HB) were not significantly different. Decreased cerebral cortical cell membrane Na(+),K(+)-ATPase activity and increased lipid peroxidation products observed in the NB group, indicative of bilirubin-induced brain damage, were significantly attenuated in the HB group. Hypothermia also significantly improved the bilirubin-induced reduction in brain ATP and phosphocreatine levels and increase in blood and brain lactate levels. In summary, hypothermia significantly attenuated the bilirubin-induced alterations in brain cell membrane function and energy metabolism in the newborn piglet. These findings suggest the possibility that hypothermia could be a good neuroprotective therapeutic modality in neonatal bilirubin encephalopathy.

Serum cleaved Tau protein and neurobehavioral battery of tests as markers of brain injury in experimental bacterial meningitis

Brain injury due to bacterial meningitis affects multiple areas of the brain with a heterogeneous distribution generating a challenge to assess severity. Tau proteins are microtubular binding proteins localized in the axonal compartment of neurons. Brain injury releases cleaved Tau proteins (C-tau) into the extracellular space where they are transported to the cerebral spinal fluid. We hypothesized that C-tau crosses the blood-brain barrier during inflammation and that it can be detected in serum. The correlation between serum C-tau levels and the extent of the meningitic insult was examined. Furthermore, we studied whether the use of a subset of neurobehavioral tasks can assess the extent of brain injury after meningitis. The tests were chosen primarily for their ability to detect deficits in the acoustic system, low brain, reflexive responding, as well as for impaired motor coordination and the higher brain functions of learning and memory. A rat model of group B streptococcal meningitis with variable severity was utilized. At five days after bacterial inoculation followed by antibiotic therapy neurobehavioral tests were performed and serum C-tau and histologic samples of the brain were obtained. Our study shows that during meningitis C-tau appears in serum and reflects the extent of neurologic damage. Neurobehavioral performance was altered after bacterial meningitis and could be correlated with histologic and biochemical markers of neurologic sequelae. We conclude that serum C-tau and a composite of neurobehavioral tests could become useful markers for assessing the severity of neurological damage in experimental bacterial meningitis.

Hypothermia attenuates beta1 integrin expression on extravasated neutrophils in an animal model of meningitis

Brain injury in meningitis occurs in part as a consequence of leukocyte migration and activation. Leukocyte integrins are pivotal in the inflammatory response by mediating adhesion to vascular endothelium and extracellular matrix proteins. We have demonstrated that moderate hypothermia early in the course of meningitis decreases leukocyte sequestration within the brain parenchyma. This study examines whether hypothermia alters neutrophil integrin expression in a rabbit model of bacterial meningitis. Prior to the induction of meningitis, peripheral blood samples were obtained and the neutrophils isolated. Sixteen hours after inducing group B streptococcal meningitis, animals were treated with antibiotics, i.v. fluids, and mechanically ventilated. Animals were randomized to hypothermia (32-33 degrees C) or normothermia conditions. After 10 hours of hypothermia or normothermia, neutrophils were isolated from the blood and cerebral spinal fluid (CSF), stained for beta1 and beta2 integrins, and analyzed using flow cytometry. Cerebral spinal fluid neutrophil beta1 integrin expression was significantly decreased in hypothermic animals. Beta-1 integrins can assume a higher affinity or "activated" state following inflammatory stimulation. Expression of "activated" beta1 integrins was also significantly decreased in hypothermic animals. Beta2 CSF neutrophil integrin expression was decreased in hypothermic animals, but failed to reach significance. These data suggest hypothermia may attenuate extravasated leukocyte expression of both total and "activated" beta1 integrins.

Effect of hypothermia on brain cell membrane function and energy metabolism in experimental Escherichia coli meningitis in the newborn piglet

We evaluated the anti-inflammatory and neuroprotective effects of hypothermia during the early phase of experimental Escherichia coli meningitis in the newborn piglet. Hypothermia significantly attenuated the meningitis-induced acute inflammatory responses such as increased intracranial pressure, decreased glucose level, increased lactate concentration, increased tumor necrosis factor-alpha level and leukocytosis in the cerebrospinal fluid. Decreased cerebral cortical cell membrane Na+,K+-ATPase activity and increased lipid peroxidation products, indicative of meningitis-induced brain damage, were significantly improved with hypothermia. Hypothermia also significantly improved the meningitis-induced reduction in brain ATP and phosphocreatine levels. In summary, hypothermia significantly attenuated the acute inflammatory responses and the ensuing brain injury in experimental neonatal bacterial meningitis.

Hypothermia as an adjunctive treatment for severe bacterial meningitis

Brain injury due to bacterial meningitis results in a high mortality rate and significant neurologic sequelae in survivors. The objective of this study was to determine if the application of moderate hypothermia shortly after the administration of antibiotics would attenuate the inflammatory response and increase in intracranial pressure that occurs in meningitis. For this study we used a rabbit model of severe Group B streptococcal meningitis. The first component of this study evaluated the effects of hypothermia on blood-brain barrier function and markers of inflammation in meningitic animals. The second part of the study evaluated the effects of hypothermia on intracranial pressure, cerebral perfusion pressure and brain edema. This study demonstrates that the use of hypothermia preserves CSF/serum glucose ratio, decreases CSF protein and nitric oxide and attenuates myeloperoxidase activity in brain tissue. In the second part of this study we show a decrease in intracranial pressure, an improvement in cerebral perfusion pressure and a decrease in cerebral edema in hypothermic meningitic animals. We conclude that in the treatment of severe bacterial meningitis, the application of moderate hypothermia initiated shortly after antibiotic therapy improves short-term physiologic measures associated with brain injury.

New Advances in the Pathogenesis and Pathophysiology of Bacterial Meningitis

Acute bacterial meningitis continues to be a significant health concern, with a fatality rate of more than 30% in some studies. Although the face of bacterial meningitis has changed substantially over the past 15 years, this disease still causes significant mortality (particularly in underdeveloped countries) and neurological sequelae. Our understanding of the pathophysiology of bacterial meningitis continues to develop. Our understanding of the mechanisms of neuronal injury now includes the concept that many of the pathological changes are only secondary to the infection and that the human immune system contributes to the majority of the neuronal death. A complicated series of interactions among immune, vascular and central nervous system cells, cytokines and chemokines, matrix metalloproteinases and free radical molecules are ultimately responsible for many bacterial meningitis changes. We hope that a complete understanding of these processes will ultimately lead to better diagnostic techniques and improved treatments.

Induced hypothermia in experimental pneumococcal meningitis

Pneumococcal meningitis resulting from Streptococcus pneumoniae has a death rate of 28% in adults. In severe head injury and stroke, inflammatory changes and intracranial hypertension are improved by induced hypothermia, which also is neuroprotective. We hypothesized that moderate hypothermia ameliorates inflammatory changes in experimental pneumococcal meningitis. Wistar rats were cooled systemically, and meningitis was induced by pneumococcal cell wall components. The increase of regional cerebral blood flow in the meningitis animals was blocked by hypothermia at 6 hours. The reduction of intracranial pressure correlated with temperature. The influx of leukocytes into the cerebrospinal fluid and levels of tumor necrosis factor alpha in the cerebrospinal fluid were decreased. Cooling the animals 2 hours after meningitis induction to 30.5 degrees C was also protective. We conclude that hypothermia is a new adjuvant approach to reduce meningitis-induced changes, in particular intracranial pressure, in the early phase of the disease.