Isoflurane impairs immature astroglia development in vitro: the role of actin cytoskeleton

Astrocyte atrophy induced by L-PGDS/PGD2/Src signaling dysfunction in the central amygdala mediates postpartum depression

BACKGROUND

Postpartum depression (PPD) is a serious psychiatric disorder that has significantly adverse impacts on maternal health. Metabolic abnormalities in the brain are associated with numerous neurological disorders, yet the specific metabolic signaling pathways and brain regions involved in PPD remain unelucidated.

METHODS

We performed behavioral test in the virgin and postpartum mice. We used mass spectrometry imaging (MSI) and targeted metabolomics analyses to investigate the metabolic alternation in the brain of GABAAR Delta-subunit-deficient (Gabrd-/-) postpartum mice, a specific preclinical animal model of PPD. Next, we performed mechanism studies including qPCR, Western blot, immunofluorescence staining, electron microscopy and primary astrocyte culture. In the specific knockdown and rescue experiments, we injected the adeno-associated virus into the central amygdala (CeA) of female mice.

RESULTS

We identified that prostaglandin D2 (PGD2) downregulation in the CeA was the most outstanding alternation in PPD, and then validated that lipocalin-type prostaglandin D synthase (L-PGDS)/PGD2 downregulation plays a causal role in depressive behaviors derived from PPD in both wild-type and Gabrd-/- mice. Furthermore, we verified that L-PGDS/PGD2 signaling dysfunction-induced astrocytes atrophy is mediated by Src phosphorylation both in vitro and in vivo.

LIMITATIONS

L-PGDS/PGD2 signaling dysfunction may be only responsible for the depressive behavior rather than maternal behaviors in the PPD, and it remains to be seen whether this mechanism is applicable to all depression types.

CONCLUSION

Our study identified abnormalities in the L-PGDS/PGD2 signaling in the CeA, which inhibited Src phosphorylation and induced astrocyte atrophy, ultimately resulting in the development of PPD in mice.

Potential Risk of Choline Alfoscerate on Isoflurane-Induced Toxicity in Primary Human Astrocytes

OBJECTIVE

Isoflurane, a widely used common inhalational anesthetic agent, can induce brain toxicity. The challenge lies in protecting neurologically compromised patients from neurotoxic anesthetics. Choline alfoscerate (L-α-Glycerophosphorylcholine, α-GPC) is recognized for its neuroprotective properties against oxidative stress and inflammation, but its optimal therapeutic window and indications are still under investigation. This study explores the impact of α-GPC on human astrocytes, the most abundant cells in the brain that protect against oxidative stress, under isoflurane exposure.

METHODS

This study was designed to examine changes in factors related to isoflurane-induced toxicity following α-GPC administration. Primary human astrocytes were pretreated with varying doses of α-GPC (ranging from 0.1 to 10.0 μM) for 24 hours prior to 2.5% isoflurane exposure. In vitro analysis of cell morphology, water-soluble tetrazolium salt-1 assay, quantitative real-time polymerase chain reaction, proteome profiler array, and transcriptome sequencing were conducted.

RESULTS

A significant morphological damage to human astrocytes was observed in the group that had been pretreated with 10.0 mM of α-GPC and exposed to 2.5% isoflurane. A decrease in cell viability was identified in the group pretreated with 10.0 μM of α-GPC and exposed to 2.5% isoflurane compared to the group exposed only to 2.5% isoflurane. Quantitative real-time polymerase chain reaction revealed that mRNA expression of heme-oxygenase 1 and hypoxia-inducible factor-1α, which were reduced by isoflurane, was further suppressed by 10.0 μM α-GPC pretreatment. The proteome profiler array demonstrated that α-GPC pretreatment influenced a variety of factors associated with apoptosis induced by oxidative stress. Additionally, transcriptome sequencing identified pathways significantly related to changes in isoflurane-induced toxicity caused by α-GPC pretreatment.

CONCLUSION

The findings suggest that α-GPC pretreatment could potentially enhance the vulnerability of primary human astrocytes to isoflurane-induced toxicity by diminishing the expression of antioxidant factors, potentially leading to amplified cell damage.

Dexmedetomidine improves the circulatory dysfunction of the glymphatic system induced by sevoflurane through the PI3K/AKT/ΔFosB/AQP4 pathway in young mice

Multiple sevoflurane exposures may damage the developing brain. The neuroprotective function of dexmedetomidine has been widely confirmed in animal experiments and human studies. However, the effect of dexmedetomidine on the glymphatic system has not been clearly studied. We hypothesized that dexmedetomidine could alleviate sevoflurane-induced circulatory dysfunction of the glymphatic system in young mice. Six-day-old C57BL/6 mice were exposed to 3% sevoflurane for 2 h daily, continuously for 3 days. Intraperitoneal injection of either normal saline or dexmedetomidine was administered before every anaesthesia. Meanwhile the circulatory function of glymphatic system was detected by tracer injection at P8 and P32. On P30-P32, behavior tests including open field test, novel object recognition test, and Y-maze test were conducted. Primary astrocyte cultures were established and treated with the PI3K activator 740Y-P, dexmedetomidine, and small interfering RNA (siRNA) to silence ΔFosB. We propose for the first time that multiple exposure to sevoflurane induces circulatory dysfunction of the glymphatic system in young mice. Dexmedetomidine improves the circulatory capacity of the glymphatic system in young mice following repeated exposure to sevoflurane through the PI3K/AKT/ΔFosB/AQP4 signaling pathway, and enhances their long-term learning and working memory abilities.

General anesthetic agents induce neurotoxicity through astrocytes

ABSTRACT

Neuroscientists have recognized the importance of astrocytes in regulating neurological function and their influence on the release of glial transmitters. Few studies, however, have focused on the effects of general anesthetic agents on neuroglia or astrocytes. Astrocytes can also be an important target of general anesthetic agents as they exert not only sedative, analgesic, and amnesic effects but also mediate general anesthetic-induced neurotoxicity and postoperative cognitive dysfunction. Here, we analyzed recent advances in understanding the mechanism of general anesthetic agents on astrocytes, and found that exposure to general anesthetic agents will destroy the morphology and proliferation of astrocytes, in addition to acting on the receptors on their surface, which not only affect Ca2+ signaling, inhibit the release of brain-derived neurotrophic factor and lactate from astrocytes, but are even involved in the regulation of the pro- and anti-inflammatory processes of astrocytes. These would obviously affect the communication between astrocytes as well as between astrocytes and neighboring neurons, other neuroglia, and vascular cells. In this review, we summarize how general anesthetic agents act on neurons via astrocytes, and explore potential mechanisms of action of general anesthetic agents on the nervous system. We hope that this review will provide a new direction for mitigating the neurotoxicity of general anesthetic agents.

Fer-1 Protects against Isoflurane-Induced Ferroptosis in Astrocytes and Cognitive Impairment in Neonatal Mice

Early and prolonged exposure to anesthetic agents could cause neurodevelopmental disorders in children. Astrocytes, heavily outnumber neurons in the brain, are crucial regulators of synaptic formation and function during development. However, how general anesthetics act on astrocytes and the impact on cognition are still unclear. In this study, we investigated the role of ferroptosis and GPX4, a major hydroperoxide scavenger playing a pivotal role in suppressing the process of ferroptosis, and their underlying mechanism in isoflurane-induced cytotoxicity in astrocytes and cognitive impairment. Our results showed that early 6 h isoflurane anesthesia induced cognitive impairment in mice. Ferroptosis-relative genes and metabolic changes were involved in the pathological process of isoflurane-induced cytotoxicity in astrocytes. The level of GPX4 was decreased while the expression of 4-HNE and generation of ROS were elevated after isoflurane exposure. Selectively blocking ferroptosis with Fer-1 attenuated the abovementioned cytotoxicity in astrocytes, paralleling with the reverse of the changes in GPX4, ROS and 4-HNE secondary to isoflurane anesthesia. Fer-1 attenuated the cognitive impairment induced by prolonged isoflurane exposure. Thus, ferroptosis conduced towards isoflurane-induced cytotoxicity in astrocytes via suppressing GPX4 and promoting lipid peroxidation. Fer-1 was expected to be an underlying intervention for the neurotoxicity induced by isoflurane in the developing brain, and to alleviate cognitive impairment in neonates.

Angelicin Alleviates Maternal Isoflurane Exposure-Induced Offspring Cognitive Defects Through the Carbonic Anhydrase 4/Aquaporin-4 Pathway

An increasing number of studies reveal the deleterious effects of isoflurane (Iso) exposure during pregnancy on offspring cognition. However, no effective therapeutic strategy for Iso-induced deleterious effects has been well developed. Angelicin exerts an anti-inflammatory effect on neurons and glial cells. This study investigated the roles and mechanism of action of angelicin in Iso-induced neurotoxicity in vitro and in vivo. After exposing C57BL/6 J mice on embryonic day 15 (E15) to Iso for 3 and 6 h, respectively, neonatal mice on embryonic day 18 (E18) displayed obvious anesthetic neurotoxicity, which was revealed by the elevation of cerebral inflammatory factors and blood-brain barrier (BBB) permeability and cognitive dysfunction in mice. Angelicin treatment could not only significantly reduce the Iso-induced embryonic inflammation and BBB disruption but also improve the cognitive dysfunction of offspring mice. Iso exposure resulted in an increase of carbonic anhydrase (CA) 4 and aquaporin-4 (AQP4) expression at both mRNA and protein levels in vascular endothelial cells and mouse brain tissue collected from neonatal mice on E18. Remarkably, the Iso-induced upregulation of CA4 and AQP4 expression could be partially reversed by angelicin treatment. Moreover, GSK1016790A, an AQP4 agonist, was used to confirm the role of AQP4 in the protective effect of angelicin. Results showed that GSK1016790A abolished the therapeutic effect of angelicin on Iso-induced inflammation and BBB disruption in the embryonic brain and on the cognitive function of offspring mice. In conclusion, angelicin may serve as a potential therapeutic for Iso-induced neurotoxicity in neonatal mice by regulating the CA4/AQP4 pathway.

Prolonged exposure of neonatal mice to sevoflurane leads to hyper-ramification in microglia, reduced contacts between microglia and synapses, and defects in adult behavior

Background

Prolonged exposure to general anesthetics during development is known to cause neurobehavioral abnormalities, but the cellular and molecular mechanisms involved are unclear. Microglia are the resident immune cells in the central nervous system and play essential roles in normal brain development.

Materials and methods

In the study, postnatal day 7 (P7) C57BL/6 mice were randomly assigned to two groups. In the sevoflurane (SEVO), mice were exposed to 2.5% sevoflurane for 4 h. In the control group, mice were exposed to carrier gas (30% O2/70% N2) for 4 h. Fixed brain slices from P14 to P21 mice were immunolabeled for ionized calcium-binding adapter molecule 1 (IBA-1) to visualize microglia. The morphological analysis of microglia in the somatosensory cortex was performed using ImageJ and Imaris software. Serial block face scanning electron microscopy (SBF-SEM) was performed to assess the ultrastructure of the microglia and the contacts between microglia and synapse in P14 and P21 mice. The confocal imaging of brain slices was performed to assess microglia surveillance in resting and activated states in P14 and P21 mice. Behavioral tests were used to assess the effect of microglia depletion and repopulation on neurobehavioral abnormalities caused by sevoflurane exposure.

Results

The prolonged exposure of neonatal mice to sevoflurane induced microglia hyper-ramification with an increase in total branch length, arborization area, and branch complexity 14  days after exposure. Prolonged neonatal sevoflurane exposure reduced contacts between microglia and synapses, without affecting the surveillance of microglia in the resting state or responding to laser-induced focal brain injury. These neonatal changes in microglia were associated with anxiety-like behaviors in adult mice. Furthermore, microglial depletion before sevoflurane exposure and subsequent repopulation in the neonatal brain mitigated anxiety-like behaviors caused by sevoflurane exposure.

Conclusion

Our experiments indicate that general anesthetics may harm the developing brain, and microglia may be an essential target of general anesthetic-related developmental neurotoxicity.

Putative Roles of Astrocytes in General Anesthesia

General anesthetics are a mainstay of modern medicine, and although much progress has been made towards identifying molecular targets of anesthetics and neural networks contributing to endpoints of general anesthesia, our understanding of how anesthetics work remains unclear. Reducing this knowledge gap is of fundamental importance to prevent unwanted and life-threatening side-effects associated with general anesthesia. General anesthetics are chemically diverse, yet they all have similar behavioral endpoints, and so for decades, research has sought to identify a single underlying mechanism to explain how anesthetics work. However, this effort has given way to the 'multiple target hypothesis' as it has become clear that anesthetics target many cellular proteins, including GABAA receptors, glutamate receptors, voltage-independent K+ channels, and voltagedependent K+, Ca2+ and Na+ channels, to name a few. Yet, despite evidence that astrocytes are capable of modulating multiple aspects of neural function and express many anesthetic target proteins, they have been largely ignored as potential targets of anesthesia. The purpose of this brief review is to highlight the effects of anesthetic on astrocyte processes and identify potential roles of astrocytes in behavioral endpoints of anesthesia (hypnosis, amnesia, analgesia, and immobilization).

Astrogliosis in juvenile non-human primates 2 years after infant anaesthesia exposure

BACKGROUND

Infant anaesthesia causes acute brain cell apoptosis, and later in life cognitive deficits and behavioural alterations, in non-human primates (NHPs). Various brain injuries and neurodegenerative conditions are characterised by chronic astrocyte activation (astrogliosis). Glial fibrillary acidic protein (GFAP), an astrocyte-specific protein, increases during astrogliosis and remains elevated after an injury. Whether infant anaesthesia is associated with a sustained increase in GFAP is unknown. We hypothesised that GFAP is increased in specific brain areas of NHPs 2 yr after infant anaesthesia, consistent with prior injury.

METHODS

Eight 6-day-old NHPs per group were exposed to 5 h isoflurane once (1×) or three times (3×), or to room air as a control (Ctr). Two years after exposure, their brains were assessed for GFAP density changes in the primary visual cortex (V1), perirhinal cortex (PRC), hippocampal subiculum, amygdala, and orbitofrontal cortex (OFC). We also assessed concomitant microglia activation and hippocampal neurogenesis.

RESULTS

Compared with controls, GFAP densities in V1 were increased in exposed groups (Ctr: 0.208 [0.085-0.427], 1×: 0.313 [0.108-0.533], 3×: 0.389 [0.262-0.652]), whereas the density of activated microglia was unchanged. In addition, GFAP densities were increased in the 3× group in the PRC and the subiculum, and in both exposure groups in the amygdala, but there was no increase in the OFC. There were no differences in hippocampal neurogenesis among groups.

CONCLUSIONS

Two years after infant anaesthesia, NHPs show increased GFAP without concomitant microglia activation in specific brain areas. These long-lasting structural changes in the brain caused by infant anaesthesia exposure may be associated with functional alterations at this age.

Anesthetic neurotoxicity in the pediatric population: a systematic review of the clinical evidence

Background: Exposure to general anesthesia (GA) in early life is known to be neurotoxic to animals. Objectives: To evaluate the risk of GA inducing long-term neurodevelopmental deficits in human children. Design: Systematic review. Methods: We included observational and randomized studies that compared the long-term neurodevelopment of postnatal children exposed to GA to the long-term neurodevelopment of children not exposed to GA. We searched MEDLINE, Embase and Web of Science for relevant studies published in the year 2000 or later. We screened all the identified studies on predetermined inclusion and exclusion criteria. A risk of bias assessment was made for each included study. We identified 9 neurodevelopmental domains for which a sub-analysis was made: intelligence; memory; learning; language/speech; motor function; visuospatial skills; development/emotions/behavior; ADHD/attention; autistic disorder. Results: We included 26 studies involving 605.391 participants. Based on AHRQ-standards 11 studies were of poor quality, 7 studies were of fair quality and 8 studies were of good quality. The major causes of potential bias were selection and comparability bias. On 2 neurodevelopmental domains (visuospatial skills and autistic disorder), the available evidence showed no association with exposure to GA. On 7 other neurodevelopmental domains, the available evidence showed mixed results. The 4 studies that used a randomized or sibling-controlled design showed no association between GA and neurodevelopmental deficits in their primary endpoints. Limitations: The absence of a meta-analysis and funnel plot. Conclusions: Based on observational studies, we found an association between GA in childhood and neuro-developmental deficits in later life. Randomized and sibling-matched observational studies failed to show the same association and therefore no evidence of a causal relationship exists at present. Since GA seems to be a marker, but not a cause of worse neurodevelopment, we argue against delaying or avoiding interventional or diagnostic procedures requiring GA in childhood based on the argument of GA-induced neurotoxicity.

Neurotoxicity of Anesthesia in Children: Prevention and Treatment

PURPOSE OF REVIEW

The purpose of this review is to summarize the current evidence regarding the impact of the exposure to anesthetic and sedative agents on neurodevelopment during the period of rapid brain growth in the first 3 years of life. Though much of the definitive data demonstrating anesthesia-induced neurotoxicity has come from studies in young animals, the focus of this review is on emerging human data.

RECENT FINDINGS

In 2016, the first prospective trials investigating the neurodevelopmental impact of early anesthetic exposure (GAS and PANDA studies) were published, both showing no significant impact on IQ from a single brief anesthetic. More recent population cohort analyses have shown varying, but minimal, impacts from early anesthetic exposure on academic performance and IQ, much smaller than that of maternal education and other environmental factors. Animal and human data document that post-anesthetic neurotoxicity is a genuine phenomenon, but its long-term clinical significance is uncertain. Most experts would agree that a single, brief anesthetic likely has no significant impact on neurodevelopment, but it is yet to be determined whether longer exposures or multiple anesthetics are associated with subsequent learning issues. Future research is aimed at determining the mechanisms of neuronal injury from exposure to anesthetic and sedative agents, adjunctive medications that may prevent or ameliorate this injury, and therapeutic approaches such as early intervention that can enhance recovery. While these studies are underway, it is recommended that exposure to anesthetic and sedative agents be minimized in young children and consideration be given to alternative methods of immobilization for nonpainful procedures such as radiologic imaging.

Early Developmental Exposure to General Anesthetic Agents in Primary Neuron Culture Disrupts Synapse Formation via Actions on the mTOR Pathway

Human epidemiologic studies and laboratory investigations in animal models suggest that exposure to general anesthetic agents (GAs) have harmful effects on brain development. The mechanism underlying this putative iatrogenic condition is not clear and there are currently no accepted strategies for prophylaxis or treatment. Recent evidence suggests that anesthetics might cause persistent deficits in synaptogenesis by disrupting key events in neurodevelopment. Using an in vitro model consisting of dissociated primary cultured mouse neurons, we demonstrate abnormal pre- and post-synaptic marker expression after a clinically-relevant isoflurane anesthesia exposure is conducted during neuron development. We find that pharmacologic inhibition of the mechanistic target of rapamycin (mTOR) pathway can reverse the observed changes. Isoflurane exposure increases expression of phospho-S6, a marker of mTOR pathway activity, in a concentration-dependent fashion and this effect occurs throughout neuronal development. The mTOR 1 complex (mTORC1) and the mTOR 2 complex (mTORC2) branches of the pathway are both activated by isoflurane exposure and this is reversible with branch-specific inhibitors. Upregulation of mTOR is also seen with sevoflurane and propofol exposure, suggesting that this mechanism of developmental anesthetic neurotoxicity may occur with all the commonly used GAs in pediatric practice. We conclude that GAs disrupt the development of neurons during development by activating a well-defined neurodevelopmental disease pathway and that this phenotype can be reversed by pharmacologic inhibition.

Effect of the volatile anesthetic agent isoflurane on lateral diffusion of cell membrane proteins

The volatile anesthetic isoflurane (ISO) has previously been shown to increase the fluidity of artificial lipid membranes, but very few studies have used biological cell membranes. Therefore, to investigate whether ISO affects the mobility of membrane proteins, fluorescence‐labeled transferrin receptor (TfR) and glycosylphosphatidylinositol (GPI)‐anchored protein were expressed in human embryonic kidney 293T cells and neural cells and lateral diffusion was examined using fluorescence recovery after photobleaching. Lateral diffusion of the TfR increased with ISO treatment. On the other hand, there was no effect on GPI‐anchored protein. We also used GC/MS to confirm that there was no change in the concentration of ISO due to vaporization during measurement. These results suggest that ISO affects the mobility of transmembrane protein molecules in living cells.

Does pediatric anesthesia cause brain damage? - Addressing parental and provider concerns in light of compelling animal studies and seemingly ambivalent human data

Anesthesia facilitates surgery in millions of young children every year. Structural brain abnormalities and functional impairment observed in animals have created substantial concerns among clinicians, parents, and government regulators. Clinical studies seemed ambivalent; it remains unclear whether differential species effects exist towards anesthetic exposure. The current literature search and analysis attempts to unify the available clinical and animal studies, which currently comprise of > 530 in vivo animal studies and > 30 clinical studies. The prevalence of abnormalities was lowest for exposures < 1 hour, in both animals and humans, while studies with injurious findings increased in frequency with exposure time. Importantly, no exposure time, anesthetic technique, or age during exposure was clearly identifiable to be entirely devoid of any adverse outcomes. Moreover, the age dependence of maximum injury clearly identified in animal studies, combined with the heterogeneity in age in most human studies, may impede the discovery of a specific human neurological phenotype. In summary, animal and human research studies identify a growing prevalence of injurious findings with increasing exposure times. However, the existing lack of definitive data regarding safe exposure durations, unaffected ages, and non-injurious anesthetic techniques precludes any evidence-based recommendations for drastically changing current clinical anesthesia management. Animal studies focusing on brain maturational states more applicable to clinical practice, as well as clinical studies focusing on prolonged exposures during distinct developmental windows of vulnerability, are urgently needed to improve the safety of perioperative care for thousands of young children requiring life-saving and quality of life-improving procedures daily.

General Anesthesia and Young Brain: What is New?

Considering that growing population of very young children is exposed to general anesthesia every year, it is of utmost importance to understand how and whether such practice may affect the development and growth of their very immature and vulnerable brains. Compelling evidence from animal studies suggests that an early exposure to general anesthesia is detrimental to normal brain development leading to structural and functional impairments of neurons and glia, and long-lasting impairments in normal emotional and cognitive development. Although the evidence from animal studies is overwhelming and confirmed across species examined from rodents to non-human primates, the evidence from human studies is inconsistent and not conclusive at present. In this review we focus on new developments in animal studies of anesthesia-induced developmental neurotoxicity and summarize recent clinical studies while focusing on outcome measures and exposure variables in terms of their utility for assessing cognitive and behavioral development in children.

Neurocognitive Adverse Effects of Anesthesia in Adults and Children: Gaps in Knowledge

Numerous preclinical and clinical studies investigating the neurodevelopmental and neurocognitive effects of exposure to anesthesia and the combination of anesthesia and surgery have demonstrated histopathological and both temporary and long-term cognitive and behavioral effects at the extremes of the human age spectrum. Increasing coverage in the lay press for both our youngest and oldest patient populations has led to heightened concerns regarding the potential harmful side effects of almost all commonly used anesthetic drug regimens. Although the majority of information regarding anesthetic risks in the developing brain derives from preclinical work in rodents, research involving the aged brain has identified a well-defined postoperative cognitive phenotype in humans. While preclinical and clinical data appear to support some association between anesthesia and surgery and the development of detrimental cognitive changes in both the developing and the aged brain, correlation between anesthesia and surgery and poor neurological outcomes does not imply causation. Given this information, no single anesthetic or group of anesthetics can be recommended over any other in terms of causing or preventing negative neurocognitive outcomes in either population. This review summarizes the growing body of preclinical and clinical literature dedicated to the detrimental effects of anesthesia on both the developing and the aging brain.

TREK-1 mediates isoflurane-induced cytotoxicity in astrocytes

Background

There are growing concerns that anaesthetic exposure can cause extensive apoptotic degeneration of neurons and the impairment of normal synaptic development and remodelling. However, little attention has been paid to exploring the possible cytotoxicity of inhalation anaesthetics, such as isoflurane, in astrocytes. In this research, we determined that prolonged exposure to an inhalation anaesthetic caused cytotoxicity in astrocytes, and we identified the underlying molecular mechanism responsible for this process.

Methods

Astrocytes were exposed to isoflurane, and astrocytic survival was then measured via LDH release assays, MTT assays, and TUNEL staining. TWIK-related potassium (K⁺) channel-1 (TREK-1) over-expression and knockdown models were also created using lentiviruses. The levels of TREK-1 and brain-derived neurotrophic factor (BDNF) were measured via Western blot and qRT-PCR.

Results

Prolonged exposure to isoflurane decreased primary astrocytic viability in a dose- and time-dependent manner. Moreover, with prolonged exposure to isoflurane, the TREK-1 level increased, and the BDNF level was reduced. TREK-1 knockdown promoted the survival of astrocytes and increased BDNF expression following isoflurane exposure.

Conclusions

Overdoses of and prolonged exposure to isoflurane induce cytotoxicity in primary astrocytes. TREK-1 plays an important role in isoflurane-induced cultured astrocytic cytotoxicity by down-regulating the expression of BDNF.

Early postnatal exposure to isoflurane causes cognitive deficits and disrupts development of newborn hippocampal neurons via activation of the mTOR pathway

Clinical and preclinical studies indicate that early postnatal exposure to anesthetics can lead to lasting deficits in learning and other cognitive processes. The mechanism underlying this phenomenon has not been clarified and there is no treatment currently available. Recent evidence suggests that anesthetics might cause persistent deficits in cognitive function by disrupting key events in brain development. The hippocampus, a brain region that is critical for learning and memory, contains a large number of neurons that develop in the early postnatal period, which are thus vulnerable to perturbation by anesthetic exposure. Using an in vivo mouse model we demonstrate abnormal development of dendrite arbors and dendritic spines in newly generated dentate gyrus granule cell neurons of the hippocampus after a clinically relevant isoflurane anesthesia exposure conducted at an early postnatal age. Furthermore, we find that isoflurane causes a sustained increase in activity in the mechanistic target of rapamycin pathway, and that inhibition of this pathway with rapamycin not only reverses the observed changes in neuronal development, but also substantially improves performance on behavioral tasks of spatial learning and memory that are impaired by isoflurane exposure. We conclude that isoflurane disrupts the development of hippocampal neurons generated in the early postnatal period by activating a well-defined neurodevelopmental disease pathway and that this phenotype can be reversed by pharmacologic inhibition.

The United States Food and Drug Administration has recently warned that exposure to anesthetic and sedative drugs during the third trimester of prenatal development and during the first 3 years of life may cause lasting impairments in cognitive function. The mechanisms by which this undesirable side effect occurs are unknown. In this manuscript, we present evidence in mice that early developmental exposure to isoflurane, a canonical general anesthetic, disrupts the appropriate development of neurons in the hippocampus, a brain region associated with learning and memory. Isoflurane also causes up-regulation of the mechanistic target of rapamycin (mTOR) pathway, a signaling system that has been associated with other neurodevelopmental cognitive disorders. Treatment with an inhibitor of the mTOR pathway after isoflurane exposure normalizes neuronal development and also ameliorates the impairments in learning induced by isoflurane. We conclude that early exposure to isoflurane can cause learning deficits via actions on the mTOR pathway, and that this mechanism represents a potentially druggable target to minimize the side effects of anesthetics on the developing brain.

Anaesthetic considerations for surgery in newborns

Almost 30 years ago, the American Academy of Pediatrics Committee on Fetus and Newborn coauthored a policy statement strongly advocating for the use of anaesthesia in all neonates stating 'local or systemic pharmacologic agents now available permit relatively safe administration of anesthesia or analgesia to neonates undergoing surgical procedures and that such administration is indicated according to the usual guidelines for the administration of anesthesia to high-risk, potentially unstable patients'. With current techniques and advanced monitoring, preterm and full-term infants routinely undergo surgical procedures under general anaesthesia to repair congenital defects that were lethal in years past. Recent research in immature animal models, however, has shown evidence of enhanced neuroapoptosis and other signs of neurotoxicity with all of the currently used anaesthetic agents. There is also increasing concern about the potential adverse effects of perioperative hypotension and hypocapnia on neurocognitive development in infants. This review outlines the most recent animal and human evidence regarding the effects of general anaesthesia and anaesthetic-related haemodynamic changes on the developing brain of newborns.

A holistic approach to anesthesia-induced neurotoxicity and its implications for future mechanistic studies

The year 2016 marked the 15th anniversary since anesthesia-induced developmental neurotoxicity and its resulting cognitive dysfunction were first described. Since that time, multiple scientific studies have supported these original findings and investigated possible mechanisms behind anesthesia-induced neurotoxicity. This paper reviews the existing mechanistic literature on anesthesia-induced neurotoxicity in the context of a holistic approach that emphasizes the importance of both neuronal and non-neuronal cells during early postnatal development. Sections are divided into key stages in early neural development; apoptosis, neurogenesis, migration, differentiation, synaptogenesis, gliogenesis, myelination and blood brain barrier/cerebrovasculature. In addition, the authors combine the established literature in the field of anesthesia-induced neurotoxicity with literature from other related scientific fields to speculate on the potential role of non-neuronal cells and to generate new future hypotheses for understanding anesthetic toxicity and its application to the practice of pediatric anesthesia.

The effect of anaesthesia on the infant brain

Pre-clinical studies have consistently found that most general anaesthetics produce accelerated apoptosis in the developing brain. The effect has been seen in species ranging from the nematode to the non-human primate. A variety of other effects are also seen. There is also some evidence that animals exposed to anaesthesia are at increased risk of deficits in memory and learning. The effects are only seen with prolonged exposure. There are numerous problems in translating these findings to human clinical scenarios. Several human cohort studies have found an association between surgery in infancy and increased risk of poorer neurobehavioural outcome; however the possibility of confounding factors such as co-morbidity and surgery itself make it impossible to determine if these associations are due to anaesthesia toxicity. A recent trial and cohort studies suggest that an exposure of less than an hour does not increase the risk of poor outcome.

From the Cover: Volatile Anesthetics Transiently Disrupt Neuronal Development in Neonatal Rats

Volatile anesthetics can cause neuronal and glial toxicity in the developing mammalian brain, as well as long-term defects in learning and memory. The goals of this study were to compare anesthetics using a clinically relevant exposure paradigm, and to assess the anesthetic effects on hippocampal development and behavior. Our hypothesis was that volatile anesthetics disrupt hippocampal development, causing neurobehavioral defects later in life. Bromodeoxyuridine (BrdU) was administered to rats on postnatal day (P)1, and the rats were exposed to volatile anesthetics (isoflurane, sevoflurane, or desflurane) for 2 h on P2. On days P7 and P14, the BrdU-labeled cells were quantified in the hippocampal dentate gyrus using immunohistochemical assays and fluorescent microscopy. Caspase-3 positive cells were quantified on P2 to evaluate apoptosis. The remaining animals underwent behavioral testing at ages 6 weeks and 6 months, using the Morris Water Maze. Significantly fewer BrdU-positive cells were detected in the hippocampal dentate gyrus in both isoflurane and desflurane-treated animals compared with controls at P7, but there were no changes in cell numbers after sevoflurane exposure. Cell counts for all three anesthetics compared with controls were equivalent at P14. Isoflurane or desflurane exposure yielded slight differences in the behavioral tests at 6 weeks, but no differences at 6 months post-exposure. We conclude that a single 2-h exposure at P2 to either isoflurane or desflurane causes a transient disruption of hippocampal neuronal development with no significant detectable long-term effects on learning and memory, whereas the same exposure to sevoflurane has no effects.

A lasting effect of postnatal sevoflurane anesthesia on the composition of NMDA receptor subunits in rat prefrontal cortex

Sevoflurane is widely used in pediatric anesthesia and studies have shown that it is capable of inducing neurodegeneration and subsequent cognitive disorders in the developing brain. However, the evidence that anesthetics are toxic to the human brain is insufficient. N-Methyl-d-aspartate (NMDA) receptors, critical for learning and memory, display expression changes with age and can be modulated by inhalation anesthetics. Generally, NMDA receptor (NR) type 1 is expressed at birth, peaks around the third postnatal week, and then declines slightly to adult levels. NR2Bs slowly decrease and NR2As gradually increase during postnatal development. These developmental switches of NMDA receptor subunits composition mark the transition from immature to adult neural processing and allow for the final maturation of associative learning abilities. In this study, we aimed to evaluate the effect of repeated sevoflurane anesthesia on NMDA receptor subunits composition in the developing rat brain and related behavioral disorders. Six-day-old male Sprague Dawley rats were randomly allocated into either a control group (group con) or a sevoflurane group (group sevo). Group sevo inhaled 2.1% sevoflurane carried by 70% oxygen for 2h each day from postnatal day (PND) 6 to PND 8. The same procedure, without applying the sevoflurane, was executed in group con. The membrane protein expression of NR1, NR2A and NR2B in the prefrontal cortex (PFC) and hippocampus was assessed at the end of the three days of anesthesia and at PND 21. An open field test was carried out to assess spontaneous locomotion on PNDs 21, 28 and 35. Y maze performance was used to assess attention and working memory on PND 28. Sevoflurane induced upregulation of NR1 and NR2B in the PFC at the end of anesthesia. On PND 21, NR1 and NR2B receptors were significantly increased whereas NR2A receptors were significantly decreased in the PFC in group sevo. Sevoflurane-treated rats showed hyper-locomotion and impairment of working memory in the behavior tests. These results indicate that repeated sevoflurane anesthesia at early stage of life can induce a long lasting effect of interfering with NMDA receptor subunits composition in rat PFC. These changes may contribute to the effects of sevoflurane on neuronal development and subsequent neurobehavioral disorders.

Astrocytes Protect against Isoflurane Neurotoxicity by Buffering pro-brain-derived Neurotrophic Factor

BACKGROUND

Isoflurane induces cell death in neurons undergoing synaptogenesis via increased production of pro-brain-derived neurotrophic factor (proBDNF) and activation of postsynaptic p75 neurotrophin receptor (p75). Astrocytes express p75, but their role in neuronal p75-mediated cell death remains unclear. The authors investigated whether astrocytes have the capacity to buffer increases in proBDNF and protect against isoflurane/p75 neurotoxicity.

METHODS

Cell death was assessed in day in vitro (DIV) 7 mouse primary neuronal cultures alone or in co-culture with age-matched or DIV 21 astrocytes with propidium iodide 24 h after 1 h exposure to 2% isoflurane or recombinant proBDNF. Astrocyte-targeted knockdown of p75 in co-culture was achieved with small-interfering RNA and astrocyte-specific transfection reagent and verified with immunofluorescence microscopy. proBDNF levels were assessed by enzyme-linked immunosorbent assay. Each experiment used six to eight replicate cultures/condition and was repeated at least three times.

RESULTS

Exposure to isoflurane significantly (P < 0.05) increased neuronal cell death in primary neuronal cultures (1.5 ± 0.7 fold, mean ± SD) but not in co-culture with DIV 7 (1.0 ± 0.5 fold) or DIV 21 astrocytes (1.2 ± 1.2 fold). Exogenous proBDNF dose dependently induced neuronal cell death in both primary neuronal and co-cultures, an effect enhanced by astrocyte p75 inhibition. Astrocyte-targeted p75 knockdown in co-cultures increased media proBDNF (1.2 ± 0.1 fold) and augmented isoflurane-induced neuronal cell death (3.8 ± 3.1 fold).

CONCLUSIONS

The presence of astrocytes provides protection to growing neurons by buffering increased levels of proBDNF induced by isoflurane. These findings may hold clinical significance for the neonatal and injured brain where increased levels of proBDNF impair neurogenesis.

Toxic and protective effects of inhaled anaesthetics on the developing animal brain: systematic review and update of recent experimental work

BACKGROUND

Accumulating preclinical data indicate that neonatal exposure to general anaesthetics is detrimental to the central nervous system. Some studies, however, display potential protective effects of exactly the same anaesthetic agents on the immature brain. The effects of inhaled anaesthetics on the developing brain have received close attention from researchers, clinicians and the public in recent decades.

OBJECTIVES

To summarise the preclinical evidence reported in the last 5 years on both the deleterious effects and the neuroprotective potential in special indications, of inhaled anaesthetics on the developing brain.

DESIGN

A systematic review.

DATA SOURCES

PubMed search performed in June 2013.

ELIGIBILITY CRITERIA

Search terms included brain, development, inhaled anaesthetic, toxicity and protection within the scope of the last 5 years with animals. The reference lists of relevant articles and recent reviews were also hand-searched for additional studies. The type, dose and exposure duration of anaesthetics, species and age of animals, histopathologic indicators, outcomes and affected brain areas, neuro developmental test modules and outcomes, as well as other outcomes and comments were summarised.

RESULTS

Two hundred and nineteen relevant titles were initially revealed. In total, 81 articles were identified, with 68 articles assessing the detrimental effects induced by inhaled anaesthetics in the immature brain along with possible treatments. The remaining 13 articles focused on the protective profile of inhaled anaesthetics on perinatal hypoxic-ischaemic brain injury. Administration of inhaled anaesthetic agents to the immature brain was shown to be deleterious in several preclinical studies. In perinatal hypoxic-ischaemic brain injury models, pre- and postconditioning of inhalational anaesthetics exerted neuroprotective effects.

CONCLUSION

The majority of studies have linked inhaled anaesthetics to toxic effects in the neonatal brain of rodents, piglets and primates. Only a few studies, however, could demonstrate long-lasting cognitive impairment. The results of inhalational anaesthetic-induced neuroprotection in perinatal hypoxic-ischaemic brain injury are a promising basis for more research in this field. In general, prospective clinical trials are needed to further differentiate the effects of inhaled anaesthetics on the immature brain.

microRNA Expression Profiling of Propofol-Treated Developing Rat Hippocampal Astrocytes

Although propofol exerts toxic effects on the developing central nervous system (CNS), it remains a first-choice anesthetic in the pediatric population. Astrocytes represent a major glial cell population whose role in CNS development is widely appreciated and that has been recently shown to be mediated in large part by microRNAs (miRNAs). In contrast, relatively little is known about the roles of miRNAs in developing astrocytes during propofol treatment. Here, miRNA microarray was used to profile fluctuations in miRNA expression in immature hippocampal astrocytes in response to propofol treatment, and results were subsequently validated using quantitative real-time polymerase chain reaction. Predictive analysis of genes targeted by propofol-regulated miRNAs indicated enrichment of genes in the gene ontology (GO) nervous system development and differentiation category, and in the Kyoto encyclopedia of genes and genomes (KEGG) apoptotic pathway category. A total of 24 (10 short-term dosage and 14 long-term dosage) miRNAs were significantly regulated, one of which was rno-miR-665. Ectopic overexpression and silencing of rno-miR-665 demonstrated its role in the neurotoxic effects of propofol on hippocampal immature astrocytes. We present evidence that the role of rno-miR-665 in anesthesia-induced disturbances in astroglia development may involve direct downregulation of the anti-apoptotic gene Bcl2l1, and subsequent increased caspase-3-mediated apoptosis. Our results shed light on the anesthetic mechanism of propofol and have implications for its use in the clinical setting.

Early Exposure to General Anesthesia Disrupts Spatial Organization of Presynaptic Vesicles in Nerve Terminals of the Developing Rat Subiculum

Exposure to general anesthesia (GA) during critical stages of brain development induces widespread neuronal apoptosis and causes long-lasting behavioral deficits in numerous animal species. Although several studies have focused on the morphological fate of neurons dying acutely by GA-induced developmental neuroapoptosis, the effects of an early exposure to GA on the surviving synapses remain unclear. The aim of this study is to study whether exposure to GA disrupts the fine regulation of the dynamic spatial organization and trafficking of synaptic vesicles in presynaptic terminals. We exposed postnatal day 7 (PND7) rat pups to a clinically relevant anesthetic combination of midazolam, nitrous oxide, and isoflurane and performed a detailed ultrastructural analysis of the synaptic vesicle architecture at presynaptic terminals in the subiculum of rats at PND 12. In addition to a significant decrease in the density of presynaptic vesicles, we observed a reduction of docked vesicles, as well as a reduction of vesicles located within 100 nm from the active zone, in animals 5 days after an initial exposure to GA. We also found that the synaptic vesicles of animals exposed to GA are located more distally with respect to the plasma membrane than those of sham control animals and that the distance between presynaptic vesicles is increased in GA-exposed animals compared to sham controls. We report that exposure of immature rats to GA during critical stages of brain development causes significant disruption of the strategic topography of presynaptic vesicles within the nerve terminals of the subiculum.

Isoflurane impairs the capacity of astrocytes to support neuronal development in a mouse dissociated coculture model

BACKGROUND

There is growing concern that pediatric exposure to anesthetic agents may cause long-lasting deficits in learning by impairing brain development. Most studies to date on this topic have focused on the direct effects of anesthetics on developing neurons. Relatively little attention has been paid to possible effects of anesthetics on astrocytes, a glial cell type that plays an important supporting role in neuronal development.

METHODS

Astrocytes were exposed to isoflurane and then cocultured with unexposed neurons to test for astrocyte-specific toxic effects on neuronal growth. Axon length was measured in the cocultured neurons to assess neuronal growth.

RESULTS

We found that neurons cocultured with astrocytes exposed to isoflurane exhibited a 30% reduction in axon outgrowth. Further experimentation showed that this effect is likely due to reduced levels of brain-derived neurotrophic factor in the coculture media.

CONCLUSIONS

Isoflurane interferes with the ability of cultured astrocytes to support neuronal growth. This finding represents a potentially novel mechanism through which general anesthetics may interfere with brain development.

Review: effects of anesthetics on brain circuit formation

The results of several retrospective clinical studies suggest that exposure to anesthetic agents early in life is correlated with subsequent learning and behavioral disorders. Although ongoing prospective clinical trials may help to clarify this association, they remain confounded by numerous factors. Thus, some of the most compelling data supporting the hypothesis that a relatively short anesthetic exposure can lead to a long-lasting change in brain function are derived from animal models. The mechanism by which such changes could occur remains incompletely understood. Early studies identified anesthetic-induced neuronal apoptosis as a possible mechanism of injury, and more recent work suggests that anesthetics may interfere with several critical processes in brain development. The function of the mature brain requires the presence of circuits, established during development, which perform the computations underlying learning and cognition. In this review, we examine the mechanisms by which anesthetics could disrupt brain circuit formation, including effects on neuronal survival and neurogenesis, neurite growth and guidance, formation of synapses, and function of supporting cells. There is evidence that anesthetics can disrupt aspects of all of these processes, and further research is required to elucidate which are most relevant to pediatric anesthetic neurotoxicity.

S6 inhibition contributes to isoflurane neurotoxicity in the developing brain

Postnatal isoflurane exposure leads to neurodegeneration and deficits of spatial learning and memory in the adulthood. However, the underlying mechanisms remain unclear. Ribosomal protein S6 is demonstrated to play a pivotal role in control of cell survival, protein synthesis and synaptogenesis for brain development. In this study, the possible role of S6 and its upstream signaling pathways in the developmental neurotoxicity of isoflurane was evaluated using models of primary cultured hippocampal neurons and postnatal day 7 rats. We found that isoflurane decreased IGF-1 level and suppressed activation of IGF-1 receptor, sequentially inhibiting S6 activity via IGF-1/MEK/ERK and IGF-1/PI3K/Akt signaling pathways. S6 inhibition enhanced isoflurane-induced decreased Bcl-xL and increased cleaved caspase-3 and Bad, also reduced PSD95 expression and aggravated deficits of spatial learning and memory. S6 activation could reverse the damages above. These results indicate that S6 inhibition, led by suppression of upstream IGF-1/MEK/ERK and IGF-1/PI3K/Akt signaling pathways, is involved in the neuroapoptosis, synaptogenesis impairment and spatial learning and memory decline caused by postnatal isoflurane exposure. S6 activation may exhibit protective potential against developmental neurotoxicity of isoflurane.

Isoflurane affects the cytoskeleton but not survival, proliferation, or synaptogenic properties of rat astrocytes in vitro

BACKGROUND

More than half of the cells in the brain are glia and yet the impact of general anaesthetics on these cells is largely unexamined. We hypothesized that astroglia, which are strongly implicated in neuronal well-being and synapse formation and function, are vulnerable to adverse effects of isoflurane.

METHODS

Cultured rat astrocytes were treated with 1.4% isoflurane in air or air alone for 4 h. Viability, proliferation, and cytoskeleton were assessed by colorimetric assay, immunocytochemistry, or a migration assay at the end of treatment or 2 days later. Also, primary rat cortical neurones were treated for 4 days with conditioned medium from control [astrocyte-conditioned media (ACM)], or isoflurane-exposed astrocytes (Iso-ACM) and synaptic puncta were assessed by synapsin 1 and PSD-95 immunostaining.

RESULTS

By several measures, isoflurane did not kill astrocytes. Nor, based on incorporation of a thymidine analogue, did it inhibit proliferation. Isoflurane had no effect on F-actin but reduced expression of α-tubulin and glial fibrillary acidic protein both during exposure (P<0.05 and P<0.001, respectively) and 2 days later (P<0.01), but did not impair astrocyte motility. ACM increased formation of PSD-95 but not synapsin 1 positive puncta in neuronal cultures, and Iso-ACM was equally effective.

CONCLUSIONS

Isoflurane decreased expression of microtubule and intermediate filament proteins in astrocytes in vitro, but did not affect their viability, proliferation, motility, and ability to support synapses.

Anaesthetic neurotoxicity and neuroplasticity: an expert group report and statement based on the BJA Salzburg Seminar

Although previously considered entirely reversible, general anaesthesia is now being viewed as a potentially significant risk to cognitive performance at both extremes of age. A large body of preclinical as well as some retrospective clinical evidence suggest that exposure to general anaesthesia could be detrimental to cognitive development in young subjects, and might also contribute to accelerated cognitive decline in the elderly. A group of experts in anaesthetic neuropharmacology and neurotoxicity convened in Salzburg, Austria for the BJA Salzburg Seminar on Anaesthetic Neurotoxicity and Neuroplasticity. This focused workshop was sponsored by the British Journal of Anaesthesia to review and critically assess currently available evidence from animal and human studies, and to consider the direction of future research. It was concluded that mounting evidence from preclinical studies reveals general anaesthetics to be powerful modulators of neuronal development and function, which could contribute to detrimental behavioural outcomes. However, definitive clinical data remain elusive. Since general anaesthesia often cannot be avoided regardless of patient age, it is important to understand the complex mechanisms and effects involved in anaesthesia-induced neurotoxicity, and to develop strategies for avoiding or limiting potential brain injury through evidence-based approaches.

Anesthetics interfere with the polarization of developing cortical neurons

Numerous studies from the clinical and preclinical literature indicate that general anesthetic agents have toxic effects on the developing brain, but the mechanism of this toxicity is still unknown. Previous studies have focused on the effects of anesthetics on cell survival, dendrite elaboration, and synapse formation, but little attention has been paid to possible effects of anesthetics on the developing axon. Using dissociated mouse cortical neurons in culture, we found that isoflurane delays the acquisition of neuronal polarity by interfering with axon specification. The magnitude of this effect is dependent on isoflurane concentration and exposure time over clinically relevant ranges, and it is neither a precursor to nor the result of neuronal cell death. Propofol also seems to interfere with the acquisition of neuronal polarity, but the mechanism does not require activity at GABAA receptors. Rather, the delay in axon specification likely results from a slowing of the extension of prepolarized neurites. The effect is not unique to isoflurane as propofol also seems to interfere with the acquisition of neuronal polarity. These findings demonstrate that anesthetics may interfere with brain development through effects on axon growth and specification, thus introducing a new potential target in the search for mechanisms of pediatric anesthetic neurotoxicity.

Preclinical research into the effects of anesthetics on the developing brain: promises and pitfalls

Every year millions of children are treated with anesthetics and sedatives to alleviate pain and distress during invasive procedures. Accumulating evidence suggests the possibility for deleterious effects on the developing brain. This has led to significant concerns among pediatric anesthesiologists and to the formation of the Pediatric Anesthesia NeuroDevelopmental Assessment (PANDA) group and its biannual symposium. Not surprisingly, the majority of the data in this field have thus far been derived through laboratory research. Accordingly, this review summarizes the current state of animal research in this field, introduces some of the findings presented at the PANDA symposium, and addresses some of the difficulties in translating these findings to pediatric anesthesia practice, as discussed during the symposium. The symposium participants' consensus was that significant preclinical and clinical research efforts are still needed to investigate this important concern for child health.

Neuropathological sequelae of developmental exposure to antiepileptic and anesthetic drugs

Glutamate (Glu) and γ-aminobutyric acid (GABA) are major neurotransmitters in the mammalian brain which regulate brain development at molecular, cellular, and systems level. Sedative, anesthetic, and antiepileptic drugs (AEDs) interact with glutamate and GABA receptors to produce their desired effects. The question is posed whether such interference with glutamatergic and GABAergic neurotransmission may exert undesired, and perhaps even detrimental effects on human brain development. Preclinical research in rodents and non-human primates has provided extensive evidence that sedative, anesthetic, and AEDs can trigger suicide of neurons and oligodendroglia, suppress neurogenesis, and inhibit normal synapse development and sculpting. Behavioral correlates in rodents and non-human primates consist of long-lasting cognitive impairment. Retrospective clinical studies in humans exposed to anesthetics or AEDs in utero, during infancy or early childhood have delivered conflicting but concerning results in terms of a correlation between drug exposure and impaired neurodevelopmental outcomes. Prospective studies are currently ongoing. This review provides a short overview of the current state of knowledge on this topic.

Mechanistic insights into neurotoxicity induced by anesthetics in the developing brain

Compelling evidence has shown that exposure to anesthetics used in the clinic can cause neurodegeneration in the mammalian developing brain, but the basis of this is not clear. Neurotoxicity induced by exposure to anesthestics in early life involves neuroapoptosis and impairment of neurodevelopmental processes such as neurogenesis, synaptogenesis and immature glial development. These effects may subsequently contribute to behavior abnormalities in later life. In this paper, we reviewed the possible mechanisms of anesthetic-induced neurotoxicity based on new in vitro and in vivo findings. Also, we discussed ways to protect against anesthetic-induced neurotoxicity and their implications for exploring cellular and molecular mechanisms of neuroprotection. These findings help in improving our understanding of developmental neurotoxicology and in avoiding adverse neurological outcomes in anesthesia practice.

Propofol neurotoxicity is mediated by p75 neurotrophin receptor activation

BACKGROUND

Propofol exposure to neurons during synaptogenesis results in apoptosis, leading to cognitive dysfunction in adulthood. Previous work from our laboratory showed that isoflurane neurotoxicity occurs through p75 neurotrophin receptor (p75(NTR)) and subsequent cytoskeleton depolymerization. Given that isoflurane and propofol both suppress neuronal activity, we hypothesized that propofol also induces apoptosis in developing neurons through p75(NTR).

METHODS

Days in vitro 5-7 neurons were exposed to propofol (3 μM) for 6 h and apoptosis was assessed by cleaved caspase-3 (Cl-Csp3) immunoblot and immunofluorescence microscopy. Primary neurons from p75(NTR-/-) mice or wild-type neurons were treated with propofol, with or without pretreatment with TAT-Pep5 (10 μM, 15 min), a specific p75(NTR) inhibitor. P75(NTR-/-) neurons were transfected for 72 h with a lentiviral vector containing the synapsin-driven p75(NTR) gene (Syn-p75(NTR)) or control vector (Syn-green fluorescent protein) before propofol. To confirm our in vitro findings, wild-type mice and p75(NTR-/-) mice (PND5) were pretreated with either TAT-Pep5 or TAT-ctrl followed by propofol for 6 h.

RESULTS

Neurons exposed to propofol showed a significant increase in Cl-Csp3, an effect attenuated by TAT-Pep5 and hydroxyfasudil. Apoptosis was significantly attenuated in p75(NTR-/-) neurons. In p75(NTR-/-) neurons transfected with Syn-p75(NTR), propofol significantly increased Cl-Csp3 in comparison with Syn-green fluorescent protein-transfected p75(NTR-/-) neurons. Wild-type mice exposed to propofol exhibited increased Cl-Csp3 in the hippocampus, an effect attenuated by TAT-Pep5. By contrast, propofol did not induce apoptosis in p75(NTR-/-) mice.

CONCLUSION

These results demonstrate that propofol induces apoptosis in developing neurons in vivo and in vitro and implicate a role for p75(NTR) and the downstream effector RhoA kinase.

Isoflurane does not cause neuroapoptosis but reduces astroglial processes in young adult mice

BACKGROUND

Isoflurane, a volatile anesthetic widely used clinically, has been implicated to be both neuroprotective and neurotoxic. The claim about isoflurane causing neural apoptosis remains controversial. In this study, we investigated the effects of isoflurane exposures on apoptotic and anti-apoptotic signals, cell proliferation and neurogenesis, and astroglial processes in young adult mouse brains.

METHODS

Sixty 6-week-old mice were randomly assigned to four anesthetic concentration groups (0 as control and 0.6%, 1.3%, and 2%) with four recovery times (2 h and 1, 6, and 14 d) after 2-h isoflurane exposure. Immunohistochemistry measurements of activated caspase-3 and Bcl-xl for apoptotic and anti-apoptotic signals, respectively, glial fibrillary acidic protein (GFAP) and vimentin for reactive astrocytosis, doublecortin (Dcx) for neurogenesis, and BrdU for cell proliferation were performed.

RESULTS

Contrary to the previous conclusion derived from studies with neonatal rodents, we found no evidence of isoflurane-induced apoptosis in the adult mouse brain. Neurogenesis in the subgranule zone of the dentate gyrus was not affected by isoflurane. However, there is a tendency of reduced cell proliferation after 2% isoflurane exposure. VIM and GFAP staining showed that isoflurane exposure caused a delayed reduction of astroglial processes in the hippocampus and dentate gyrus.

CONCLUSION

Two-hour exposure to isoflurane did not cause neuroapoptosis in adult brains. The delayed reduction in astroglial processes after isoflurane exposure may explain why some volatile anesthetics can confer neuroprotection after experimental stroke because reduced glial scarring facilitates better long-term neuronal recoveries.

Mechanisms of general anesthetic action: Focus on the cellular network

The discovery of general anesthetics had a tremendous impact on development of surgery and medicine in general, during the last century. Despite the widespread use of general anesthetics, the mechanisms by which they produce their effects in the central nervous system are still poorly understood. Over the past decade, several new findings have contributed significantly to a better understanding of general anesthetic mechanisms. The current review summarizes recent data on different anesthetic neuronal targets that might be involved in the mechanism of action of general anesthetics, giving special attention to the importance of binding pockets for anesthetics within transmembrane receptors and cellular signaling leading to morphological changes of neuronal cells. Several lines of evidence suggest that disruption in brain network connectivity is important for anaesthesia-induced loss of consciousness and this is discussed in relation to morphological changes.