The effects of l-carnitine on the combination of, inhalation anesthetic-induced developmental, neuronal apoptosis in the rat frontal cortex

Impact of anaesthetics and surgery on neurodevelopment: an update

Accumulating preclinical and clinical evidence suggests the possibility of neurotoxicity from neonatal exposure to general anaesthetics. Here, we review the weight of the evidence from both human and animal studies and discuss the putative mechanisms of injury and options for protective strategies. Our review identified 55 rodent studies, seven primate studies, and nine clinical studies of interest. While the preclinical data consistently demonstrate robust apoptosis in the nervous system after anaesthetic exposure, only a few studies have performed cognitive follow-up. Nonetheless, the emerging evidence that the primate brain is vulnerable to anaesthetic-induced apoptosis is of concern. The impact of surgery on anaesthetic-induced brain injury has not been adequately addressed yet. The clinical data, comprising largely retrospective cohort database analyses, are inconclusive, in part due to confounding variables inherent in these observational epidemiological approaches. This places even greater emphasis on prospective approaches to this problem, such as the ongoing GAS trial and PANDA study.

Preclinical assessment of ketamine

Ketamine is used as a general anesthetic, and recent data suggest that anesthetics can cause neurodegeneration and/or neuroprotection. The precise mechanisms are not completely understood. This review is to examine the work on ketamine and to address how developmental biology may be utilized when combined with biochemical, pathological, and pharmacokinetic assessments to produce a bridging model that may decrease the uncertainty in extrapolating preclinical data to human conditions. Advantages of using preclinical models to study critical issues related to ketamine anesthesia have been described. These include the relationships between ketamine-induced neurotoxicity/protection and the preclinical models/approaches in elucidating mechanisms associated with ketamine exposure. The discussions focus on the following: (1) the doses and time-course over which ketamine is associated with damage to, or protection of, neural cells, (2) how ketamine directs or signals neural cells to undergo apoptosis or necrosis, (3) how such exposures can trigger mitochondrial dysfunction, (4) how antioxidants and knockdowns of specific transcription modulators or receptors affect neurotoxicity induced by ketamine, and (5) whether the potential neural damage can be monitored after ketamine exposure in living animals using positron emission tomography.

Inhalation Anesthesia-Induced Neuronal Damage and Gene Expression Changes in Developing Rat Brain

Nitrous Oxide (N2O), an N-methyl-D-aspartate (NMDA) receptor antagonist, and isoflurane (ISO), which acts on multiple receptors including postsynaptic gamma-aminobutyric acid (GABA) receptors, are frequently used inhalation anesthetics, alone or as a part of a balanced anesthetic regimen administered to pregnant women and to human neonates and infants requiring surgery. The current study investigated histological features and gene expression profiles in response to prolonged exposure to N2O or ISO alone, and their combination in developing rat brains. Postnatal day 7 rats were exposed to clinically-relevant concentrations of N2O (70%), ISO (1.0%) or N2O plus ISO (N2O + ISO) for 6 hours. The neurotoxic effects were evaluated and the brain tissues were harvested for RNA extraction 6 hours after anesthetic administration. The prolonged exposure to N2O + ISO produced elevated neuronal cell death as indicated by an increased number of TUNEL-positive cells in frontal cortical levels compared with control. No significant neurotoxic effects were observed in animals exposed to N2O or ISO alone. DNA microarray analysis revealed gene expression changes after N2O, ISO or N2O + ISO exposure. Differentially expressed genes (DEGs) from the N2O + ISO group were significantly associated with 45 pathways directly related to brain functions. Although the gene expression profiles from animals exposed to N2O or ISO alone were remarkably different from those of the control group, the pathways of these genes involved were not closely associated with neurons. These findings provide novel insights into the mechanisms by which N2O + ISO cause neurotoxicity in the developing brain, suggesting multiple factors are involved in the neuronal cell death-inducing effects (cascades) of N2O + ISO.

MicroPET/CT Imaging of [18F]-FEPPA in the Nonhuman Primate: A Potential Biomarker of Pathogenic Processes Associated with Anesthetic-Induced Neurotoxicity

Background. The inhalation anesthetics nitrous oxide (N2O) and isoflurane (ISO) are used in surgical procedures for human infants. Injury to the central nervous system is often accompanied by localization of activated microglia or astrocytosis at the site of injury. The tracer that targets to the peripheral benzodiazepine receptor (PBR), [18F]N-2-(2-fluoroethoxy)benzyl)-N-(4-phenoxypyridin-3-yl)acetamide ([18F]-FEPPA), has been reported as a sensitive biomarker for the detection of neuronal damage/inflammation. Methods. On postnatal day (PND) 5 or 6 rhesus monkey neonates were exposed to a mixture of N2O/oxygen and ISO for 8 hours and control monkeys were exposed to room air. MicroPET/CT images with [18F]-FEPPA were obtained for each monkey 1 day, one week, three weeks, and 6 months after the anesthetic exposure. Results. The radiotracer quickly distributed into the brains of both treated and control monkeys on all scan days. One day after anesthetic exposure, the uptake of [18F]-FEPPA was significantly increased in the temporal lobe. One week after exposure, the uptake of [18F]-FEPPA in the frontal lobe of treated animals was significantly greater than that in controls. Conclusions. These findings suggest that microPET imaging is capable of dynamic detection of inhaled anesthetic-induced brain damage in different brain regions of the nonhuman primate.

Ketamine-induced neuronal damage and altered N-methyl-D-aspartate receptor function in rat primary forebrain culture

Ketamine, a noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist, is frequently used in pediatric general anesthesia. Accumulating evidence from animal experiments has demonstrated that ketamine causes neuronal cell death during the brain growth spurt. To elucidate the underlying mechanisms associated with ketamine-induced neuronal toxicity and search for approaches or agents to prevent ketamine's adverse effects on the developing brain, a primary nerve cell culture system was utilized. Neurons harvested from the forebrain of newborn rats were maintained under normal control conditions or exposed to either ketamine (10 µM) or ketamine plus L-carnitine (an antioxidant; 1-100 µM) for 24h, followed by a 24-h withdrawal period. Ketamine exposure resulted in elevated NMDA receptor (NR1) expression, increased generation of reactive oxygen species (ROS) as indicated by higher levels of 8-oxoguanine production, and enhanced neuronal damage. Coadministration of L-carnitine significantly diminished ROS generation and provided near complete protection of neurons from ketamine-induced cell death. NMDA receptors regulate channels that are highly permeable to calcium, and calcium imaging data demonstrated that neurons exposed to ketamine had a significantly elevated amplitude of calcium influx and higher intracellular free calcium concentrations ([Ca(2+)]i) evoked by NMDA (50 µM), compared with control neurons. These findings suggest that prolonged ketamine exposure produces an increase in NMDA receptor expression (compensatory upregulation), which allows for a higher/toxic influx of calcium into neurons once ketamine is removed from the system, leading to elevated ROS generation and neuronal cell death. L-Carnitine appears to be a promising agent in preventing or reversing ketamine's toxic effects on neurons at an early developmental stage.

Anesthesia-induced developmental neurodegeneration: the role of neuronal organelles

Exposure to general anesthetics (GAs) and antiepileptics during critical stages of brain development causes significant neurotoxicity to immature neurons. Many animal, and emerging human studies have shown long-term functional sequelae manifested as behavioral deficits and cognitive impairments. Since GAs and antiepileptic drugs are a necessity, current research is focused on deciphering the mechanisms responsible for anesthesia-induced developmental neurotoxicity so that protective strategies can be devised. These agents promote massive and wide-spread neuroapoptosis that is caused by the impairment of integrity and function of neuronal organelles. Mitochondria and endoplasmic reticulum are particularly vulnerable. By promoting significant release of intracellular calcium from the endoplasmic reticulum, anesthetics cause an increase in mitochondrial calcium load resulting in the loss of their integrity, release of pro-apoptotic factors, functional impairment of ATP synthesis, and enhanced accumulation of reactive oxygen species. The possibility that GAs may have direct damaging effects on mitochondria, resulting in the impairment of their morphogenesis, also has been proposed. This review will present evidence that neuronal organelles are critical and early targets of anesthesia-induced developmental neurotoxicity.

Neuroprotective effects of L-carnitine against oxygen-glucose deprivation in rat primary cortical neurons

Purpose

Hypoxic-ischemic encephalopathy is an important cause of neonatal mortality, as this brain injury disrupts normal mitochondrial respiratory activity. Carnitine plays an essential role in mitochondrial fatty acid transport and modulates excess acyl coenzyme A levels. In this study, we investigated whether treatment of primary cultures of rat cortical neurons with L-carnitine was able to prevent neurotoxicity resulting from oxygen-glucose deprivation (OGD).

Methods

Cortical neurons were prepared from Sprague-Dawley rat embryos. L-Carnitine was applied to cultures just prior to OGD and subsequent reoxygenation. The numbers of cells that stained with acridine orange (AO) and propidium iodide (PI) were counted, and lactate dehydrogenase (LDH) activity and reactive oxygen species (ROS) levels were measured. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay and the terminal uridine deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling assay were performed to evaluate the effect of L-carnitine (1 µM, 10 µM, and 100 µM) on OGD-induced neurotoxicity.

Results

Treatment of primary cultures of rat cortical neurons with L-carnitine significantly reduced cell necrosis and prevented apoptosis after OGD. L-Carnitine application significantly reduced the number of cells that died, as assessed by the PI/AO ratio, and also reduced ROS release in the OGD groups treated with 10 µM and 100 µM of L-carnitine compared with the untreated OGD group (P<0.05). The application of L-carnitine at 100 µM significantly decreased cytotoxicity, LDH release, and inhibited apoptosis compared to the untreated OGD group (P<0.05).

Conclusion

L-Carnitine has neuroprotective benefits against OGD in rat primary cortical neurons in vitro.

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.

Anesthetic neurotoxicity

Concerns for toxic effects of anesthesia to the brains of the young and the elderly are mounting. While experimental evidence for such effects in the developing brain is strong, the underlying mechanisms are less well understood and debate continues as to whether young humans are at risk for anesthetic neurotoxicity. The phenomenon of postoperative cognitive deterioration in the elderly remains controversial. Time course, severity, and whether or not it persists long term are under debate. For both patient groups, today's evidence is not sufficient to guide change in clinical practice. Well-designed research is therefore imperative to tackle this critical issue.

The abolishment of anesthesia-induced cognitive impairment by timely protection of mitochondria in the developing rat brain: the importance of free oxygen radicals and mitochondrial integrity

Early exposure to general anesthesia (GA) causes developmental neuroapoptosis in the mammalian brain and long-term cognitive impairment. Recent evidence suggests that GA also causes functional and morphological impairment of the immature neuronal mitochondria. Injured mitochondria could be a significant source of reactive oxygen species (ROS), which, if not scavenged in timely fashion, may cause excessive lipid peroxidation and damage of cellular membranes. We examined whether early exposure to GA results in ROS upregulation and whether mitochondrial protection and ROS scavenging prevent GA-induced pathomorphological and behavioral impairments. We exposed 7-day-old rats to GA with or without either EUK-134, a synthetic ROS scavenger, or R(+) pramipexole (PPX), a synthetic aminobenzothiazol derivative that restores mitochondrial integrity. We found that GA causes extensive ROS upregulation and lipid peroxidation, as well as mitochondrial injury and neuronal loss in the subiculum. As compared to rats given only GA, those also given PPX or EUK-134 had significantly downregulated lipid peroxidation, preserved mitochondrial integrity, and significantly less neuronal loss. The subiculum is highly intertwined with the hippocampal CA1 region, anterior thalamic nuclei, and both entorhinal and cingulate cortices; hence, it is important in cognitive development. We found that PPX or EUK-134 co-treatment completely prevented GA-induced cognitive impairment. Because mitochondria are vulnerable to GA-induced developmental neurotoxicity, they could be an important therapeutic target for adjuvant therapy aimed at improving the safety of commonly used GAs.

Erythropoietin attenuates isoflurane-induced neurodegeneration and learning deficits in the developing mouse brain

OBJECTIVES

To examine whether recombinant erythropoietin (rEPO) attenuates neurodegeneration and the learning disability induced by isoflurane with the postnatal day 7 (P7) mice.

BACKGROUND

Some of general anesthetic agents induce neurodegeneration in developing brain. Several drugs, but not rEPO, were reported as candidates for the prevention of or treatment for neurodegeneration.

METHOD AND MATERIALS

We divided P7 mice into three groups at random. One group (IE group) was exposed to 6-h isoflurane (1.0%) after 50,000 IU·kg(-1) rEPO administered subcutaneously. The second group (I) was exposed to isoflurane in the same manner as IE group except saline instead of rEPO. The third group (E) was exposed to air after rEPO administered. The mice were assigned to the radial arm maze on four consecutive days from P56 (day 1) to P59 (day 4). We divided the number of errors each day by that of day 1 to establish each-day performance ratio. After the test, neurodegenerative change in the hilus of dentate gyrus was assessed using Nissl staining.

RESULTS

In radial maze test, the performance ratios of day 3 (mean ± sd) were 0.3 ± 0.2 (P < 0.05, vs I group), 0.8 ± 0.5, and 0.6 ± 0.2 in IE, I, and E groups, respectively, while those of day 4 were 0.3 ± 0.1 (P < 0.05), 0.8 ± 0.5, and 0.3 ± 0.2 (P < 0.05), respectively. The histopathological study revealed that in IE group the degenerative neuronal change was attenuated compared with I group.

CONCLUSIONS

These results suggested that rEPO attenuated isoflurane-induced neurodegeneration.

Protective effect of L-carnitine against H(2)O(2)-induced neurotoxicity in neuroblastoma (SH-SY5Y) cells

OBJECTIVES

4-N-trimethylammonium-3-hydroxybutyric acid (L-carnitine) is an endogenous mitochondrial membrane compound and some studies have reported that L-carnitine could effectively protect various cells against oxidative injury both in vitro and in vivo. In the present study, we used the human neuroblastoma SH-SY5Y cell line as an in vitro model and assessed the effect of L-carnitine on hydrogen peroxide (H(2)O(2))-mediated oxidative stress and neurotoxicity.

METHODS

Cells in culture were treated with different concentrations of H(2)O(2) alone or pretreated with L-carnitine. 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays, transmission electron microscopy, agarose gel electrophoresis, biochemical methods, and Western blotting were employed in the present study.

RESULTS

Pretreatment with L-carnitine for 3 hours inhibited H(2)O(2)-induced cell viability loss, morphological changes, intracellular reactive oxygen species generation, and lipid peroxidation in a concentration-dependent manner. Endogenous anti-oxidant defense components including total anti-oxidative capacity, glutathione peroxidase, catalase, and superoxide dismutase were also promoted by L-carnitine. Meanwhile, H(2)O(2)-induced down-regulation of Bcl-2, up-regulation of Bax, and DNA damage and apoptosis were also inhibited in the presence of L-carnitine.

DISCUSSION

Taken together, these results suggest that L-carnitine may function as an anti-oxidant to inhibit H(2)O(2)-induced oxidative stress as well as regulation of Bcl-2 family and prevent the apoptotic death of neuronal cells, which might be beneficial for the treatment of oxidative stress in neurodegenerative diseases.

L-Carnitine rescues ketamine-induced attenuated heart rate and MAPK (ERK) activity in zebrafish embryos

Ketamine, an antagonist of the N-methyl-D-aspartate (NMDA)-type glutamate receptors, is a pediatric anesthetic. Ketamine has been shown to be neurotoxic and cardiotoxic in mammals. Here, we show that after 2 h of exposure, 5 mM ketamine significantly reduced heart rate in 26 h old zebrafish embryos. In 52 h old embryos, 1 mM ketamine was effective after 2 h and 0.5 mM ketamine at 20 h of exposure. Ketamine also induced significant reductions in activated MAPK (ERK) levels. Treatment of the embryos with the ERK inhibitor, PD 98059, also significantly reduced heart rate whereas the p38/SAPK inhibitor, SB203580, was ineffective. Ketamine is known to inhibit lipolysis and a decrease of ATP content in the heart. Co-treatment with l-carnitine that enhances fatty acid metabolism effectively rescued ketamine-induced attenuated heart rate and ERK activity. These findings demonstrate that l-carnitine counteracts ketamine's negative effects on heart rate and ERK activity in zebrafish embryos.

L-carnitine is an osmotic agent suitable for peritoneal dialysis

Excessive intraperitoneal absorption of glucose during peritoneal dialysis has both local cytotoxic and systemic metabolic effects. Here we evaluate peritoneal dialysis solutions containing L-carnitine, an osmotically active compound that induces fluid flow across the peritoneum. In rats, L-carnitine in the peritoneal cavity had a dose-dependent osmotic effect similar to glucose. Analogous ultrafiltration and small solute transport characteristics were found for dialysates containing 3.86% glucose, equimolar L-carnitine, or combinations of both osmotic agents in mice. About half of the ultrafiltration generated by L-carnitine reflected facilitated water transport by aquaporin-1 (AQP1) water channels of endothelial cells. Nocturnal exchanges with 1.5% glucose and 0.25% L-carnitine in four patients receiving continuous ambulatory peritoneal dialysis were well tolerated and associated with higher net ultrafiltration than that achieved with 2.5% glucose solutions, despite the lower osmolarity of the carnitine-containing solution. Addition of L-carnitine to endothelial cells in culture increased the expression of AQP1, significantly improved viability, and prevented glucose-induced apoptosis. In a standard toxicity test, the addition of L-carnitine to peritoneal dialysis solution improved the viability of L929 fibroblasts. Thus, our studies support the use of L-carnitine as an alternative osmotic agent in peritoneal dialysis.

Neuronal death and oxidative stress in the developing brain

The developing brain is particularly vulnerable to reactive oxygen and reactive nitrogen species-mediated damage because of its high concentrations of unsaturated fatty acids, high rate of oxygen consumption, low concentrations of antioxidants, high content of metals catalyzing free radical formation, and large proportion of sensitive immature cells. In this review, we outline the dynamic changes of energy resources, metabolic requirements, and endogenous free radical scavenging systems during physiologic brain development. We further discuss the involvement of oxidative stress in the pathogenesis of neuronal death after exposure of the infant brain to hyperoxia, hypoxia/ischemia, sedative drugs, ethanol, and mechanical trauma. Several approaches have been developed to combat oxidative stress, but neuroprotective treatment strategies are limited in the clinical setting.

Repeated but not single isoflurane exposure improved the spatial memory of young adult mice

BACKGROUND

Exposure to general anesthesia damages the spatial memory of aged rodents, but its role in the young adult has not been well investigated. This study was conducted to determine the effects of single and repeated isoflurane exposure on spatial memory in young adult mice.

METHODS

The experimental design comprised three series of experiments. In the first series, animals (2 months old) were randomly exposed to 1.4% isoflurane for 2 or 6 h or vehicle gas for 6 h (n=15 in each group). The Morris water maze (MWM) test was performed at 48 h after anesthetic exposure to evaluate spatial memory. In the second part, animals were randomly exposed to 1.4% isoflurane (isoflurane group, n=15) or vehicle gas only (control group, n=15) for 2 h/day on 5 consecutive days. Forty-eight hours later, the MWM was performed. In the third part, animals were randomized into an isoflurane group (n=15) and a control group (n=15) as in the second part of the experiment. Two weeks later, the MWM was performed.

RESULTS

The mean escape latency time and the mean pathway length measured at 48 h after repeated isoflurane exposure decreased significantly compared with the control. However, there was no difference between the two groups when tested at 2 weeks after repeated isoflurane exposure. A single exposure to isoflurane lasting 2 or 6 h had no effect on the performance of mice in the MWM as compared with the control.

CONCLUSION

Repeated but not single isoflurane exposure temporarily improved the spatial memory of young adult mice.

Ketamine anesthesia during the first week of life can cause long-lasting cognitive deficits in rhesus monkeys

Previously our laboratory has shown that ketamine exposure (24h of clinically relevant anesthesia) causes significant increases in neuronal cell death in perinatal rhesus monkeys. Sensitivity to this ketamine-induced neurotoxicity was observed on gestational days 120-123 (in utero exposure via maternal anesthesia) and on postnatal days (PNDs) 5-6, but not on PNDs 35-37. In the present study, six monkeys were exposed on PND 5 or 6 to intravenous ketamine anesthesia to maintain a light surgical plane for 24h and six control animals were unexposed. At 7 months of age all animals were weaned and began training to perform a series of cognitive function tasks as part of the National Center for Toxicological Research (NCTR) Operant Test Battery (OTB). The OTB tasks used here included those for assessing aspects of learning, motivation, color discrimination, and short-term memory. Subjects responded for banana-flavored food pellets by pressing response levers and press-plates during daily (M-F) test sessions (50 min) and were assigned training scores based upon their individual performance. As reported earlier (Paule et al., 2009) beginning around 10 months of age, control animals significantly outperformed (had higher training scores than) ketamine-exposed animals for approximately the next 10 months. For animals now over 3 and one-half years of age, the cognitive impairments continue to manifest in the ketamine-exposed group as poorer performance in the OTB learning and color and position discrimination tasks, as deficits in accuracy of task performance, but also in response speed. There are also apparent differences in the motivation of these animals which may be impacting OTB performance. These observations demonstrate that a single 24-h episode of ketamine anesthesia, occurring during a sensitive period of brain development, results in very long-lasting deficits in brain function in primates and provide proof-of-concept that general anesthesia during critical periods of brain development can result in subsequent functional deficits. Supported by NICHD, CDER/FDA and NCTR/FDA.

Evidence for the need for anaesthesia in the neonate

Neonates are both capable of experiencing pain and memory formation, albeit implicit memory. During surgical procedures, insufficient ablation of the stress response and possible implicit memory formation of intra-operative events might result in adverse early and long-term outcomes. Neonates deserve the same respect as adult patients. It is thus the responsibility of the anaesthetist to provide sufficient anaesthesia for neonates undergoing surgery. A critical approach in weighing the risks and benefits of exposing a neonate to anaesthesia is prudent, and truly elective surgery should be delayed.

Effects of exogenous agents on brain development: stress, abuse and therapeutic compounds

The range of exogenous agents likely to affect, generally detrimentally, the normal development of the brain and central nervous system defies estimation although the amount of accumulated evidence is enormous. The present review is limited to certain types of chemotherapeutic and "use-and-abuse" compounds and environmental agents, exemplified by anesthetic, antiepileptic, sleep-inducing and anxiolytic compounds, nicotine and alcohol, and stress as well as agents of infection; each of these agents have been investigated quite extensively and have been shown to contribute to the etiopathogenesis of serious neuropsychiatric disorders. To greater or lesser extent, all of the exogenous agents discussed in the present treatise have been investigated for their influence upon neurodevelopmental processes during the period of the brain growth spurt and during other phases uptill adulthood, thereby maintaining the notion of critical phases for the outcome of treatment whether prenatal, postnatal, or adolescent. Several of these agents have contributed to the developmental disruptions underlying structural and functional brain abnormalities that are observed in the symptom and biomarker profiles of the schizophrenia spectrum disorders and the fetal alcohol spectrum disorders. In each case, the effects of the exogenous agents upon the status of the affected brain, within defined parameters and conditions, is generally permanent and irreversible.

[Effects of general anaesthetics on the developing brain]

OBJECTIVE

To expose the current knowledge about the anaesthetic effects on the developing brain.

DATA SOURCES

Publications (original articles and reviews) in English and in French language from 1980 were obtained from the Medline database using alone or in combination following keywords: anaesthetics, developing brain, neurodevelopment, neurogenesis, synaptogenesis, neurotoxicity, apoptosis.

DATA SYNTHESIS

Several lines of evidence resulting from animal experiments conducted in rodents and non-human primates have suggested that exposing the developing brain to anaesthetic drugs may elicit an increase a physiological programmed neuronal death (i.e. apoptosis). This neuronal death is not only seen at the cellular level but also results in alterations in some behavioural abilities in the adult animal. However, the vast majority of experiments reported have been conducted in animals not exposed to any surgical or painful stimulation. Moreover, the literature raises contradictory results, some authors not confirming this neurotoxic effect of anaesthetic drugs. Last, available clinical data are scarce and do not allow to claim that exposure to general anaesthesia definitely alters the cognitive development of children.

CONCLUSION

This review raises the question of the innocuity of anaesthetic agents on the developing brain; further clinical trials are required in order to test this effect on human babies.

Metabolism of acetyl-L-carnitine for energy and neurotransmitter synthesis in the immature rat brain

Acetyl-L-carnitine (ALCAR) is an endogenous metabolic intermediate that facilitates the influx and efflux of acetyl groups across the mitochondrial inner membrane. Exogenously administered ALCAR has been used as a nutritional supplement and also as an experimental drug with reported neuroprotective properties and effects on brain metabolism. The aim of this study was to determine oxidative metabolism of ALCAR in the immature rat forebrain. Metabolism was studied in 21-22 day-old rat brain at 15, 60 and 120 min after an intraperitoneal injection of [2-(13)C]acetyl-L-carnitine. The amount, pattern, and fractional enrichment of (13)C-labeled metabolites were determined by ex vivo(13)C-NMR spectroscopy. Metabolism of the acetyl moiety from [2-(13)C]ALCAR via the tricarboxylic acid cycle led to incorporation of label into the C4, C3 and C2 positions of glutamate (GLU), glutamine (GLN) and GABA. Labeling patterns indicated that [2-(13)C]ALCAR was metabolized by both neurons and glia; however, the percent enrichment was higher in GLN and GABA than in GLU, demonstrating high metabolism in astrocytes and GABAergic neurons. Incorporation of label into the C3 position of alanine, both C3 and C2 positions of lactate, and the C1 and C5 positions of glutamate and glutamine demonstrated that [2-(13)C]ALCAR was actively metabolized via the pyruvate recycling pathway. The enrichment of metabolites with (13)C from metabolism of ALCAR was highest in alanine C3 (11%) and lactate C3 (10%), with considerable enrichment in GABA C4 (8%), GLN C3 (approximately 4%) and GLN C5 (5%). Overall, our (13)C-NMR studies reveal that the acetyl moiety of ALCAR is metabolized for energy in both astrocytes and neurons and the label incorporated into the neurotransmitters glutamate and GABA. Cycling ratios showed prolonged cycling of carbon from the acetyl moiety of ALCAR in the tricarboxylic acid cycle. Labeling of compounds formed from metabolism of [2-(13)C]ALCAR via the pyruvate recycling pathway was higher than values reported for other precursors and may reflect high activity of this pathway in the developing brain. This is, to our knowledge, the first study to determine the extent and pathways of ALCAR metabolism for energy and neurotransmitter biosynthesis in the brain.

Developmental neurotoxicity of sedatives and anesthetics: a concern for neonatal and pediatric critical care medicine?

OBJECTIVE

To evaluate the currently available evidence for the deleterious effects of sedatives and anesthetics on developing brain structure and neurocognitive function.

DESIGN

A computerized, bibliographic search of the literature regarding neurodegenerative effects of sedatives and anesthetics in the developing brain.

MEASUREMENTS AND MAIN RESULTS

A growing number of animal studies demonstrate widespread structural damage of the developing brain and long-lasting neurocognitive abnormalities after exposure to sedatives commonly used in neonatal and pediatric critical care medicine. These studies reveal a dose and exposure time dependence of neuronal cell death, characterize its molecular pathways, and suggest a potential early window of susceptibility in humans. Several clinical studies document neurologic abnormalities in neonatal intensive care unit graduates, usually attributed to comorbidities. Emerging human epidemiologic data, however, do not exclude prolonged or repetitive exposure to sedatives and anesthetics in early childhood as contributing factors to some of these abnormalities.

CONCLUSIONS

Neuronal cell death after neonatal exposure to sedatives and anesthetics has been clearly demonstrated in developing animal models. Although the relevance for human medicine remains speculative, the phenomenon's serious implications for public health necessitate further preclinical and clinical studies. Intensivists using sedatives and anesthetics in neonates and infants need to stay informed about this rapidly emerging field of research.

Nitrative and oxidative stress in toxicology and disease

Persistent inflammation and the generation of reactive oxygen and nitrogen species play pivotal roles in tissue injury during disease pathogenesis and as a reaction to toxicant exposures. The associated oxidative and nitrative stress promote diverse pathologic reactions including neurodegenerative disorders, atherosclerosis, chronic inflammation, cancer, and premature labor and stillbirth. These effects occur via sustained inflammation, cellular proliferation and cytotoxicity and via induction of a proangiogenic environment. For example, exposure to the ubiquitous air pollutant ozone leads to generation of reactive oxygen and nitrogen species in lung macrophages that play a key role in subsequent tissue damage. Similarly, studies indicate that genes involved in regulating oxidative stress are altered by anesthetic treatment resulting in brain injury, most notable during development. In addition to a role in tissue injury in the brain, inflammation, and oxidative stress are implicated in Parkinson's disease, a neurodegenerative disease characterized by the loss of dopamine neurons. Recent data suggest a mechanistic link between oxidative stress and elevated levels of 3,4-dihydroxyphenylacetaldehyde, a neurotoxin endogenous to dopamine neurons. These findings have significant implications for development of therapeutics and identification of novel biomarkers for Parkinson's disease pathogenesis. Oxidative and nitrative stress is also thought to play a role in creating the proinflammatory microenvironment associated with the aggressive phenotype of inflammatory breast cancer. An understanding of fundamental concepts of oxidative and nitrative stress can underpin a rational plan of treatment for diseases and toxicities associated with excessive production of reactive oxygen and nitrogen species.

Neonatal NMDA receptor antagonist treatments have no effects on prepulse inhibition of postnatal day 25 Sprague-Dawley rats

Glutamate activation of the NMDA receptor is essential for neuronal differentiation, migration, and survival. Treatment with NMDA receptor antagonists, such as ketamine (KET) or phencyclidine (PCP), can trigger apoptosis in neonatal rats. However, L-carnitine (LC) treatment appears to prevent glutamate-induced toxicity in the developing CNS. Previously, we described altered preweaning behaviors (i.e., abnormal home cage, slant board and forelimb hang behaviors) resulting from neonatal PCP and KET treatment. Those adverse effects of KET were somewhat ameliorated by LC [Boctor SY, Wang C, Ferguson SA. Neonatal PCP is more potent than ketamine at modifying preweaning behaviors of Sprague-Dawley rats. Toxicol Sci 2008;106:172-9]. Here, a portion of those subjects were evaluated for prepulse inhibition (PPI) of the acoustic startle response at postnatal day (PND) 25 since previous reports described PCP-induced effects on this response. Rats were subcutaneously treated with: saline; 10 mg/kg PCP (1x/day) on PNDs 7, 9 and 11; 20 mg/kg KET (6 injections every 2h on PND 7); or a similar regimen of ketamine and 250 mg/kg LC on PND 7, with a single injection of 250 mg/kg LC on PNDs 8-11 (KLC). Male and female rats were assessed using a standard PPI paradigm with prepulses of 68, 78 and 82 dB. Body weight was decreased 17-21% and whole brain weight was decreased 10% in PCP-treated rats. Specifically, cerebellar weight was significantly less in PCP-treated rats relative to control. Despite the magnitude of those PCP-induced changes, startle response in normal pulse only trials and percent of PPI in PCP-, KET-, and KLC-treated groups were comparable to controls. Average latency to maximum startle was 2.6 ms less in females than males (p<0.007); there were no other significant sex effects. The lack of neonatal PCP treatment on later PPI is similar to that reported by Rasmussen et al. [Rasmussen BA, O'Neil J, Manaye KF, Perry DC, Tizabi Y. Long-term effects of developmental PCP administration on sensorimotor gating in male and female rats. Psychopharmacology (Berl) 2007; 190: 43-9.], and indicates that neonatal PCP-induced effects on PPI [Wang C, McInnis J, Ross-Sanchez M, Shinnick-Gallagher P, Wiley JL, Johnson KM. Long-term behavioral and neurodegenerative effects of perinatal phencyclidine administration: implications for schizophrenia. Neuroscience 2001; 107: 535-50.] appear difficult to replicate.

Neonatal PCP is more potent than ketamine at modifying preweaning behaviors of Sprague-Dawley rats

Treatment with N-methyl-D-aspartate (NMDA) receptor antagonists, such as ketamine (KET) or phencyclidine (PCP), can trigger apoptotic neurodegeneration in neonatal rodents; however, little is known about the behavioral alterations resulting from such treatment. Here, rats were sc treated with saline; 10 mg/kg PCP on postnatal days (PNDs) 7, 9, and 11; 20 mg/kg KET (six injections every 2 h on PND 7); or a regimen of ketamine and 250 mg/kg L-carnitine (KLC) both administered on PND 7 with additional 250 mg/kg doses of L-carnitine given on PNDs 8-11. Postinjection, the home cage behavior of each pup was categorized on PNDs 7-11. Slant board and forelimb hang behaviors were examined on PNDs 8-11 and 12-16, respectively. The initial KET or KLC injections on PND 7 elevated abnormal home cage activity (i.e., paresis and paddling); however, KLC pup behavior returned to normal by the fourth injection, indicating the protective effects of L-carnitine against NMDA antagonist toxicity. PCP treatment caused substantial abnormal home cage activity on each injection day (PNDs 7, 9, and 11). Latencies to turn on the slant board were significantly longer on PND 8 for KET- and PCP-treated pups and PND 10 for PCP-treated pups. On PND 12, the forelimb hang time of PCP-treated pups was significantly shorter. Body weight was decreased on PNDs 8-18 in PCP-treated pups and PNDs 8-10 in KET-treated pups. These data indicate that developmental NMDA antagonist treatment causes short-term behavioral alterations which appear related to motor coordination and may be cerebellar in nature. Furthermore, single PCP injections appear more potent at altering behavior than multiple injections of KET.