A Combination of Mild Hypothermia and Sevoflurane Affords Long-Term Protection in a Modified Neonatal Mouse Model of Cerebral Hypoxia-Ischemia

Sevoflurane alleviates oxygen-glucose deprivation/reoxygenation-induced damage in HT22 cells by activating the Keap1/Nrf2/ARE pathway to inhibit oxidative stress

Objective: This study aimed to investigate the impact of sevoflurane on oxygen-glucose deprivation/reoxygenation-induced damage in HT22 cells and its associated mechanisms. Methods: HT22 cells were treated with sevoflurane, and an oxygen-glucose deprivation/reoxygenation injury model was established. The HT22 cells were randomly divided into the control group, oxygen-glucose deprivation/reoxygenation group, sevoflurane low-dose group, sevoflurane medium-dose group, and sevoflurane high-dose group. The proliferation of HT22 cells was assessed using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The apoptosis rate and mitochondrial membrane potential of HT22 cells were determined by flow cytometry. Protein expression levels of B-cell lymphoma-2-associated X protein (Bax), B-cell lymphoma-2 (Bcl-2), nuclear factor erythroid 2-related factor 2 (Nrf2), Kelch-like ECH-associated protein 1 (Keap1), and heme oxygenase-1 (HO-1) in HT22 cells were examined using Western blot. Reactive oxygen species (ROS) levels were measured with 2',7'-dichlorofluorescin diacetate (DCFH-DA). Malondialdehyde (MDA), glutathione peroxidase (GSH-Px) levels, and superoxide dismutase (SOD) enzyme activity in HT22 cells were determined using assay kits. Results: Compared to controls, OGD/R group had reduced cell viability, mitochondrial potential, Bcl-2, nuclear Nrf2, HO-1, GSH-Px levels, and SOD enzyme activity (p < 0.05), with increased apoptosis, Bax, cytoplasmic Nrf2, ROS, and MDA levels. Sevoflurane groups showed opposite trends (p < 0.05). Conclusion: Sevoflurane can mitigate oxygen-glucose deprivation/reoxygenation-induced damage in HT22 cells, and its mechanism may be related to the activation of the Keap1/Nrf2/ARE pathway to inhibit oxidative stress.

Unanswered questions of anesthesia neurotoxicity in the developing brain

PURPOSE OF REVIEW

This article reviews recent advances and controversies of developmental anesthesia neurotoxicity research with a special focus on the unanswered questions in the field both from clinical and preclinical perspectives.

RECENT FINDINGS

Observational cohort studies of prenatal and early childhood exposure to anesthesia have reported mixed evidence of an association with impaired neurodevelopment. Meta-analyses of currently available studies of early childhood exposure to anesthesia suggest that, while limited to no change in general intelligence can be detected, more subtle deficits in specific neurodevelopmental domains including behavior and executive function may be seen. Several studies have evaluated intraoperative blood pressure values and neurocognitive outcomes and have not found an association. Although many animal studies have been performed, taking into consideration other peri-operative exposures such as pain and inflammation may help with translation of results from animal models to humans.

SUMMARY

Advances have been made in the field of developmental anesthetic neurotoxicity over the past few years, including the recognition that anesthetic exposure is associated with deficits in certain cognitive domains but not others. Although the most important question of whether anesthetic agents actually cause long-term neurodevelopmental effects in children has still not been answered, results from recent studies will guide further studies necessary to inform clinical decision-making in children.

Changes in breathing pattern during severe hypothermia and autoresuscitation from hypothermic respiratory arrest in anesthetized mice

Some evidence suggests that both hypothermia and anesthesia can exert similar effects on metabolism and ventilation. This study examined the synergistic effects of anesthesia and hypothermia on ventilation in spontaneously breathing adult mice under three different conditions, that is, (1) pentobarbital group (n = 7) in which mice were anesthetized with intraperitoneal pentobarbital of 80 mg/kg, (2) sevoflurane-continued group (n = 7) in which mice were anesthetized with 1 MAC sevoflurane, and (3) sevoflurane-discontinued group (n = 7) in which sevoflurane was discontinued at a body temperature below 22˚C. We cooled mice in each group until breathing ceased and followed this with artificial rewarming while measuring changes in respiratory variables and heart rate. We found that the body temperature at which respiration arrested is much lower in the sevoflurane-discontinued group (13.8 ± 2.0˚C) than that in the sevoflurane-continued group (16.7 ± 1.2˚C) and the pentobarbital group (17.0 ± 1.4˚C). Upon rewarming, all animals in all three groups spontaneously recovered from respiratory arrest. There was a considerable difference in breathing patterns between sevoflurane-anesthetized mice and pentobarbital-anesthetized mice during progressive hypothermia in terms of changes in tidal volume and respiratory frequency. The changes in the respiratory pattern during rewarming are nearly mirrored images of the changes observed during cooling in all three groups. These observations indicate that adult mice are capable of autoresuscitation from hypothermic respiratory arrest and that anesthesia and hypothermia exert synergistic effects on the occurrence of respiratory arrest while the type of anesthetic affects the breathing pattern that occurs during progressive hypothermia leading to respiratory arrest.

Temperature-sensitive bone mesenchymal stem cells combined with mild hypothermia reduces neurological deficit in rats of severe traumatic brain injury

BACKGROUND

To explore the combined influences of temperature-sensitive bone mesenchymal stem cells (tsBMSCs) and mild hypothermia (MH) on neurological function and glucose metabolism in rats with severe traumatic brain injury (TBI).

METHODS

SD rats were randomly divided into sham, TBI, TBI + MH, TBI + BMSCs and TBI + MH +tsBMSCs groups. Then, the brain water content, serum-specific proteins (S100β, NSE, LDH, and CK), and blood glucose at different time points were measured. Furthermore, GLUT-3 expression was detected by Western blotting, and apoptotic rate was determined by TUNEL staining.

RESULTS

After TBI rat establishment, the brain injury resulted in significant increases in mNSS scores and brain water content, and upregulations in serum levels of S100β, NSE, LDH and CK, and blood glucose, with the elevated cell apoptotic rate in the injured cortex. However, these changes were reversed by MH alone, BMSCs alone, or combination treatment of MH and tsBMSCs in varying degrees, and the combination treatment was superior to the treatment with BMSCs or MH alone.

CONCLUSION

Combination therapy of tsBMSCs and MH can reduce the neuronal apoptosis in severe TBI rats, with the suppression of serum biomarkers and hyperglycemia, contributing to the recovery of neurological functions.

ABBREVIATIONS

tsBMSCs: temperature-sensitive bone mesenchymal stem cells; MH: mild hypothermia; TBI: traumatic brain injury; mNSS: modified Neurological Severity Score.

Neuroprotective Role of Hypothermia in Hypoxic-ischemic Brain Injury: Combined Therapies using Estrogen

Hypoxic-ischemic brain injury is a complex network of factors, which is mainly characterized by a decrease in levels of oxygen concentration and blood flow, which lead to an inefficient supply of nutrients to the brain. Hypoxic-ischemic brain injury can be found in perinatal asphyxia and ischemic-stroke, which represent one of the main causes of mortality and morbidity in children and adults worldwide. Therefore, knowledge of underlying mechanisms triggering these insults may help establish neuroprotective treatments. Selective Estrogen Receptor Modulators and Selective Tissue Estrogenic Activity Regulators exert several neuroprotective effects, including a decrease of reactive oxygen species, maintenance of cell viability, mitochondrial survival, among others. However, these strategies represent a traditional approach of targeting a single factor of pathology without satisfactory results. Hence, combined therapies, such as the administration of therapeutic hypothermia with a complementary neuroprotective agent, constitute a promising alternative. In this sense, the present review summarizes the underlying mechanisms of hypoxic-ischemic brain injury and compiles several neuroprotective strategies, including Selective Estrogen Receptor Modulators and Selective Tissue Estrogenic Activity Regulators, which represent putative agents for combined therapies with therapeutic hypothermia.

Hypothermic preconditioning but not ketamine reduces oxygen and glucose deprivation induced neuronal injury correlated with downregulation of COX-2 expression in mouse hippocampal slices

Hypothermic preconditioning is an effective treatment for limiting ischemic injury, but the mechanism is poorly understood. This study was aimed to explore the effect of hypothermic and ketamine preconditioning on oxygen and glucose deprivation (OGD) induced neuronal injury in mouse hippocampal slices, and to investigate its possible mechanism. The population spike (PS) was recorded in the CA1 region of mouse hippocampal slices using extracellular recordings, Na⁺/K⁺ ATPase activity in slices was determined by spectrophotometer and the expression of Cyclooxygenase-2 (COX-2) was measured by Western blot. Ten min of OGD induced a poor recovery of PS in slices after reoxygenation. Hypothermic (33 °C) preconditioning delayed the appearance of transient recovery (TR) of PS and improved the recovery amplitude of PS after reoxygenation. Hypothermic preconditioning also decreased the expression of COX-2 and increased Na⁺/K⁺ ATPase activity in slices. Pretreatment of ketamine, a non-competitive NMDA receptor antagonist at a subanesthetic dose has no effect on OGD induced neuronal injury. Moreover, the protection of hypothermic preconditioning was not added by ketamine. The downregulation of COX-2 expression and the increase of Na⁺/K⁺ ATPase activity may be associated with the effectiveness of hypothermic preconditioning in increasing tolerance to an OGD insult.

Minocycline-Suppression of Early Peripheral Inflammation Reduces Hypoxia-Induced Neonatal Brain Injury

While extensive studies report that neonatal hypoxia-ischemia (HI) induces long-term cognitive impairment via inflammatory responses in the brain, little is known about the role of early peripheral inflammation response in HI injury. Here we used a neonatal hypoxia rodent model by subjecting postnatal day 0 (P0d) rat pups to systemic hypoxia (3.5 h), a condition that is commonly seen in clinic neonates, Then, an initial dose of minocycline (45 mg/kg) was injected intraperitoneally (i.p.) 2 h after the hypoxia exposure ended, followed by half dosage (22.5 mg/kg) minocycline treatment for next 6 consecutive days daily. Saline was injected as vehicle control. To examine how early peripheral inflammation responded to hypoxia and whether this peripheral inflammation response was associated to cognitive deficits. We found that neonatal hypoxia significantly increased leukocytes not only in blood, but also increased the monocytes in central nervous system (CNS), indicated by presence of C-C chemokine receptor type 2 (CCR2⁺)/CD11b⁺CD45⁺ positive cells and CCR2 protein expression level. The early onset of peripheral inflammation response was followed by a late onset of brain inflammation that was demonstrated by level of cytokine IL-1β and ionized calcium binding adapter molecule 1(Iba-1; activated microglial cell marker). Interrupted blood-brain barrier (BBB), hypomyelination and learning and memory deficits were seen after hypoxia. Interestingly, the cognitive function was highly correlated with hypoxia-induced leukocyte response. Notably, administration of minocycline even after the onset of hypoxia significantly suppressed leukocyte-mediated inflammation as well as brain inflammation, demonstrating neuroprotection in systemic hypoxia-induced brain damage. Our data provided new insights that systemic hypoxia induces cognitive dysfunction, which involves the leukocyte-mediated peripheral inflammation response.

Neonatal Hypoxia Ischaemia: Mechanisms, Models, and Therapeutic Challenges

Neonatal hypoxia-ischaemia (HI) is the most common cause of death and disability in human neonates, and is often associated with persistent motor, sensory, and cognitive impairment. Improved intensive care technology has increased survival without preventing neurological disorder, increasing morbidity throughout the adult population. Early preventative or neuroprotective interventions have the potential to rescue brain development in neonates, yet only one therapeutic intervention is currently licensed for use in developed countries. Recent investigations of the transient cortical layer known as subplate, especially regarding subplate's secretory role, opens up a novel set of potential molecular modulators of neonatal HI injury. This review examines the biological mechanisms of human neonatal HI, discusses evidence for the relevance of subplate-secreted molecules to this condition, and evaluates available animal models. Neuroserpin, a neuronally released neuroprotective factor, is discussed as a case study for developing new potential pharmacological interventions for use post-ischaemic injury.

Therapeutic Hypothermia Provides Variable Protection against Behavioral Deficits after Neonatal Hypoxia-Ischemia: A Potential Role for Brain-Derived Neurotrophic Factor

BACKGROUND

Despite treatment with therapeutic hypothermia (TH), infants who survive hypoxic ischemic (HI) encephalopathy (HIE) have persistent neurological abnormalities at school age. Protection by TH against HI brain injury is variable in both humans and animal models. Our current preclinical model of hypoxia-ischemia (HI) and TH displays this variability of outcomes in neuropathological and neuroimaging end points with some sexual dimorphism. The detailed behavioral phenotype of this model is unknown. Whether there is sexual dimorphism in certain behavioral domains is also not known. Brain-derived neurotrophic factor (BDNF) supports neuronal cell survival and repair but may also be a marker of injury. Here, we characterize the behavioral deficits after HI and TH stratified by sex, as well as late changes in BDNF and its correlation with memory impairment.

METHODS

HI was induced in C57BL6 mice on postnatal day 10 (p10) (modified Vannucci model). Mice were randomized to TH (31°C) or normothermia (NT, 36°C) for 4 h after HI. Controls were anesthesia-exposed, age- and sex-matched littermates. Between p16 and p39, growth was followed, and behavioral testing was performed including reflexes (air righting, forelimb grasp and negative geotaxis) and sensorimotor, learning, and memory skills (open field, balance beam, adhesive removal, Y-maze tests, and object location task [OLT]). Correlations between mature BDNF levels in the forebrain and p42 memory outcomes were studied.

RESULTS

Both male and female HI mice had an approximately 8-12% lower growth rate (g/day) than shams (p ≤ 0.01) by p39. TH ameliorated this growth failure in females but not in males. In female mice, HI injury prolonged the time spent at the periphery (open field) at p36 (p = 0.004), regardless of treatment. TH prevented motor impairments in the balance beam and adhesive removal tests in male and female mice, respectively (p ≤ 0.05). Male and female HI mice visited the new arm of the Y-maze 12.5% (p = 0.05) and 10% (p = 0.03) less often than shams, respectively. Male HI mice also had 35% lower exploratory preference score than sham (p ≤ 0.001) in the OLT. TH did not prevent memory impairments found with Y-maze testing or OLT in either sex (p ≤ 0.01) at p26. At p42, BDNF levels in the forebrain ipsilateral to the HI insult were 1.7- to 2-fold higher than BDNF levels in the sham forebrain, and TH did not prevent this increase. Higher BDNF levels in the forebrain ipsilateral to the insult correlated with worse performance in the Y-maze in both sexes and in OLT in male mice (p = 0.01).

CONCLUSIONS

TH provides benefit in specific domains of behavior following neonatal HI. In general, these benefits accrued to both males and females, but not in all areas. In some domains, such as memory, no benefit of TH was found. Late differences in individual BDNF levels may explain some of these findings.

Autophagy activation involved in hypoxic-ischemic brain injury induces cognitive and memory impairment in neonatal rats

Hypoxic-ischemic brain injury (HIBI) in neonates can lead to lifelong cognitive and memory impairment, but protective strategies are lacking at present. It has been demonstrated that autophagy plays a critical role in HIBI, while the function of autophagy in cognitive and memory impairment induced by HIBI in neonates has not been tested. In this study, we tested the impact of autophagy on the impairment of cognitive function and memory in HIBI neonatal rats by using a Morris water maze and investigated its possible mechanisms, which were established as HIBI model by ligating the left common carotid artery in neonatal rats, followed by 2-h hypoxia. The expression of microtubule-associated protein 1 light chain 3 (LC3)-II increased in HI group 24 h after HI in neonatal rats, while Sequestosome 1 (P62/SQSTM1), phosphorylated cAMP-response element-binding protein (p-CREB) decreased (compared with the sham group, p < 0.05), which were shown in the same left hippocampus CA3 region by immunofluorescence analysis. Brain injury of neonatal rats was aggravated significantly at 7 day after HI, coinciding with the results of Morris water maze. An autophagy inhibitor, 3-methyladenine (3-MA) pretreatment significantly attenuated the increase of LC3II and the loss of P62/SQSTM1 and p-CREB, ameliorated neuronal death, and improved the results of Morris water maze. Our results demonstrate that HIBI in neonatal rats induced excessive autophagy flux, which aggravated brain injury and induced cognitive and memory impairment during adolescence. Inhibition of autophagy reversed the results partly and improved the function of spatial learning and memory by attenuating the reduction of p-CREB. The use of autophagy modulators in the immature brain would create new opportunities for protective strategies clinically in the future.

Preserved Nephrogenesis Following Partial Nephrectomy in Early Neonates

Reconstitution of total nephron segments after resection in the adult kidney has not been achieved; however, whether the neonatal kidney can maintain the capacity for neo-nephrogenesis after resection is unknown. We performed partial resection of the kidney in neonatal rats on postnatal days 1 (P1x kidney) and 4 (P4x kidney) and examined morphological changes and relevant factors. The P1x kidney bulged into the newly formed cortex from the wound edge, while nephrogenesis failure was prominent in the P4x kidney. Twenty-eight days post-resection, the glomerular number, cortex area, and collecting duct were preserved in the P1x kidney, whereas these parameters were markedly decreased in the P4x kidney. During normal development, Six2 expression and Six2+ nephron progenitor cells in the cap mesenchyme both rapidly disappear after birth. However, time course analysis for the P1x kidney showed that Six2 expression and Six2+ cells were well preserved in the tissue surrounding the resected area even 2 days after resection. In conclusion, our results indicate that kidneys in early neonate rats retain the capability for neo-nephrogenesis after resection; however, this ability is lost soon after birth, which may be attributed to a declining amount of Six2+ cells.

A systematic review of methodology applied during preclinical anesthetic neurotoxicity studies: important issues and lessons relevant to the design of future clinical research

Preclinical evidence suggests that anesthetic agents harm the developing brain thereby causing long-term neurocognitive impairments. It is not clear if these findings apply to humans, and retrospective epidemiological studies thus far have failed to show definitive evidence that anesthetic agents are harmful to the developing human brain.

AIM

The aim of this systematic review was to summarize the preclinical studies published over the past decade, with a focus on methodological issues, to facilitate the comparison between different preclinical studies and inform better design of future trials.

METHOD

The literature search identified 941 articles related to the topic of neurotoxicity. As the primary aim of this systematic review was to compare methodologies applied in animal studies to inform future trials, we excluded a priori all articles focused on putative mechanism of neurotoxicity and the neuroprotective agents. Forty-seven preclinical studies were finally included in this review.

RESULTS

Methods used in these studies were highly heterogeneous-animals were exposed to anesthetic agents at different developmental stages, in various doses and in various combinations with other drugs, and overall showed diverse toxicity profiles. Physiological monitoring and maintenance of physiological homeostasis was variable and the use of cognitive tests was generally limited to assessment of specific brain areas, with restricted translational relevance to humans.

CONCLUSION

Comparison between studies is thus complicated by this heterogeneous methodology and the relevance of the combined body of literature to humans remains uncertain. Future preclinical studies should use better standardized methodologies to facilitate transferability of findings from preclinical into clinical science.