Differential effects of prolonged isoflurane anesthesia on plasma, extracellular, and CSF glutamate, neuronal activity, 125I-Mk801 NMDA receptor binding, and brain edema in traumatic brain-injured rats

Pluripotential GluN1 (NMDA NR1): Functional Significance in Cellular Nuclei in Pain/Nociception

The N-methyl-D-aspartate (NMDA) glutamate receptors function as plasma membrane ionic channels and take part in very tightly controlled cellular processes activating neurogenic and inflammatory pathways. In particular, the NR1 subunit (new terminology: GluN1) is required for many neuronal and non-neuronal cell functions, including plasticity, survival, and differentiation. Physiologic levels of glutamate agonists and NMDA receptor activation are required for normal neuronal functions such as neuronal development, learning, and memory. When glutamate receptor agonists are present in excess, binding to NMDA receptors produces neuronal/CNS/PNS long-term potentiation, conditions of acute pain, ongoing severe intractable pain, and potential excitotoxicity and pathology. The GluNR1 subunit (116 kD) is necessary as the anchor component directing ion channel heterodimer formation, cellular trafficking, and the nuclear localization that directs functionally specific heterodimer formation, cellular trafficking, and nuclear functions. Emerging studies report the relevance of GluN1 subunit composition and specifically that nuclear GluN1 has major physiologic potential in tissue and/or subnuclear functioning assignments. The shift of the GluN1 subunit from a surface cell membrane to nuclear localization assigns the GluN1 promoter immediate early gene behavior with access to nuclear and potentially nucleolar functions. The present narrative review addresses the nuclear translocation of GluN1, focusing particularly on examples of the role of GluN1 in nociceptive processes.

Are GABAergic drugs beneficial in providing neuroprotection after traumatic brain injuries? A comprehensive literature review of preclinical studies

Traumatic brain injuries (TBI) caused by physical impact to the brain can adversely impact the welfare and well-being of the affected individuals. One of the leading causes of mortality and dysfunction in the world, TBI is a major public health problem facing the human community. Drugs that target GABAergic neurotransmission are commonly used for sedation in clinical TBI yet their potential to cause neuroprotection is unclear. In this paper, I have performed a rigorous literature review of the neuroprotective effects of drugs that increase GABAergic currents based on the results reported in preclinical literature. The drugs covered in this review include the following: propofol, benzodiazepines, barbiturates, isoflurane, and other drugs that are agonists of GABA_A receptors. A careful review of numerous preclinical studies reveals that these drugs fail to produce any neuroprotection after a primary impact to the brain. In numerous circumstances, they could be detrimental to neuroprotection by increasing the size of the contusional brain tissue and by severely interfering with behavioral and functional recovery. Therefore, anesthetic agents that work by enhancing the effect of neurotransmitter GABA should be administered with caution of TBI patients until a clear and concrete picture of their neuroprotective efficacy emerges in the clinical literature.

Comparing Imaging Biomarkers of Cerebral Edema after TBI in Young Adult Male and Female Rats

Traumatic brain injury (TBI) is one of the leading causes of death and disability worldwide. Cerebral edema following TBI is known to play a critical role in injury severity and prognosis. In the current study we used multimodal magnetic resonance imaging (MRI) to assess cerebral edema 24 hours after unilateral contusive TBI in male and female rats. We then directly quantified brain water content in the same subjectsex vivo.We found that both males and females had similarly elevated T₂ values after TBI compared with sham controls. Apparent diffusion coefficient (ADC) was more variable than T₂ and did not show significant injury effects in males or females. Brain water was elevated in male TBI rats compared with sham controls, but there was no difference between female TBI and sham groups. Notably, MRI biomarkers of edema were more closely correlated with brain water in male rats; female rats did not show any relationship between brain water and T₂ or ADC. These observations raise questions about the interpretation of radiological findings traditionally interpreted as edema in female TBI patients. A better understanding of sex differences and similarities in the pathophysiology of post-traumatic edema is needed to help improve patient management and the development of effective treatment strategies for men and women.

Glutamate, Glutamine and GABA Levels in Rat Brain Measured Using MRS, HPLC and NMR Methods in Study of Two Models of Autism

The disorders of the glutamatergic neurotransmission have been associated with pathogenesis of autism. In this study we evaluated the impact of the in vivo and ex vivo test methodology on measurements of levels of neurotransmitter amino acids in hippocampus of rats for valproic acid- (VPA) and thalidomide- (THAL) induced models of autism. The main goal was to compare the changes in concentrations of glutamate (Glu), glutamine (Gln) and GABA between both autistic groups and the control, measured in vivo and ex vivo in homogenates. The rat pups underwent three in vivo tests: ultrasonic vocalization (USV), magnetic resonance spectroscopy (MRS) and unilateral microdialysis of the hippocampus. Analyses of homogenates of rat hippocampus were performed using high-performance liquid chromatography (HPLC) and nuclear magnetic resonance (NMR) spectroscopy. For the statistical analysis, we performed univariate and multivariate tests. USV test, which is considered in rodents as an indicator of pathology similar to autism, showed decreased USV in VPA and THAL groups. In vivo MRS studies demonstrated increases of Glu content in male rat's hippocampus in VPA and THAL groups, while the microdialysis, which allows examination of the contents in the extracellular space, detected decreases in the basal level of Gln concentrations in VPA and THAL groups. Ex vivo HPLC studies showed that levels of Glu, Gln and GABA significantly increased in male rat's hippocampus in the VPA and THAL groups, while NMR studies showed increased levels of Gln and GABA in the VPA group. Collectively, these results are consistent with the hypothesis suggesting the role of the glutamatergic disturbances on the pathogenesis of autism. For all methods used, the values of measured changes were in the same direction. The orthogonal partial least square discriminant analysis confirmed that both animal models of autism tested here can be used to trace neurochemical changes in the brain.

Plasma metabolomics profiles in rats with acute traumatic brain injury

Traumatic brain injury (TBI) is a major cause of mortality and disability worldwide. We validated the utility of plasma metabolomics analysis in the clinical diagnosis of acute TBI in a rat model of controlled cortical impact (CCI) using gas chromatography/mass spectrometry (GC/MS). Thirty Sprague-Dawley rats were randomly divided into two groups of 15 rats each: the CCI group and sham group. Blood samples were obtained from the rats within the first 24 h after TBI injury. GC/MS measurements were performed to evaluate the profile of acute TBI-induced metabolic changes, resulting in the identification of 45 metabolites in plasma. Principal component analysis, partial least squares-discriminant analysis, orthogonal partial least square discriminant analysis using hierarchical clustering and univariate/multivariate analyses revealed clear differences in the plasma metabolome between the acute CCI group and the sham group. CCI induced distinctive changes in metabolites including linoleic acid metabolism, amino acid metabolism, galactose metabolism, and arachidonic acid metabolism. Specifically, the acute CCI group exhibited significant alterations in proline, phosphoric acid, β-hydroxybutyric acid, galactose, creatinine, L-valine, linoleic acid and arachidonic acid. A receiver operating characteristic curve analysis showed that the above 8 metabolites in plasma could be used as the potential biomarkers for the diagnosis of acute TBI. Furthermore, this study is the first time to identify the galactose as a biomarker candidate for acute TBI. This comprehensive metabolic analysis complements target screening for potential diagnostic biomarkers of acute TBI and enhances predictive value for the therapeutic intervention of acute TBI.

Intranasal Administration of the Antisecretory Peptide AF-16 Reduces Edema and Improves Cognitive Function Following Diffuse Traumatic Brain Injury in the Rat

A synthetic peptide with antisecretory activity, antisecretory factor (AF)-16, improves injury-related deficits in water and ion transport and decreases intracranial pressure after experimental cold lesion injury and encephalitis although its role in traumatic brain injury (TBI) is unknown. AF-16 or an inactive reference peptide was administrated intranasally 30 min following midline fluid percussion injury (mFPI; n = 52), a model of diffuse mild-moderate TBI in rats. Sham-injured (n = 14) or naïve (n = 24) animals were used as controls. The rats survived for either 48 h or 15 days post-injury. At 48 h, the animals were tested in the Morris water maze (MWM) for memory function and their brains analyzed for cerebral edema. Here, mFPI-induced brain edema compared to sham or naïve controls that was significantly reduced by AF-16 treatment (p < 0.05) although MWM performance was not altered. In the 15-day survival groups, the MWM learning and memory abilities as well as histological changes were analyzed. AF-16-treated brain-injured animals shortened both MWM latency and swim path in the learning trials (p < 0.05) and improved probe trial performance compared to brain-injured controls treated with the inactive reference peptide. A modest decrease by AF-16 on TBI-induced changes in hippocampal glial acidic fibrillary protein (GFAP) staining (p = 0.11) was observed. AF-16 treatment did not alter any other immunohistochemical analyses (degenerating neurons, beta-amyloid precursor protein (β-APP), and Olig2). In conclusion, intranasal AF-16-attenuated brain edema and enhanced visuospatial learning and memory following diffuse TBI in the rat. Intranasal administration early post-injury of a promising neuroprotective substance offers a novel treatment approach for TBI.

Mechanisms Underlying Astrocyte Endfeet Swelling in Stroke

Astrocyte endfeet envelop the cerebral capillaries that form the blood-brain barrier. Swelling of these endfeet occurs early in cerebral ischemia. It is generally hypothesized that such swelling occurs as the result of factors released from parenchymal brain cells during an ischemic stroke (e.g., K(+) and L-glutamate). In this review of mechanisms that can elicit astrocyte swelling in ischemic stroke, we hypothesize that, instead or in addition, such swelling may be a response to blood-brain barrier dysfunction. Astrocyte endfeet swelling may help form a cuff around a damaged vessel that limits the egress of plasma constituents and blood (hemorrhage) into brain.

Prolonged continuous intravenous infusion of the dipeptide L-alanine- L-glutamine significantly increases plasma glutamine and alanine without elevating brain glutamate in patients with severe traumatic brain injury

INTRODUCTION

Low plasma glutamine levels are associated with worse clinical outcome. Intravenous glutamine infusion dose- dependently increases plasma glutamine levels, thereby correcting hypoglutaminemia. Glutamine may be transformed to glutamate which might limit its application at a higher dose in patients with severe traumatic brain injury (TBI). To date, the optimal glutamine dose required to normalize plasma glutamine levels without increasing plasma and cerebral glutamate has not yet been defined.

METHODS

Changes in plasma and cerebral glutamine, alanine, and glutamate as well as indirect signs of metabolic impairment reflected by increased intracranial pressure (ICP), lactate, lactate-to-pyruvate ratio, electroencephalogram (EEG) activity were determined before, during, and after continuous intravenous infusion of 0.75 g L-alanine-L-glutamine which was given either for 24 hours (group 1, n = 6) or 5 days (group 2, n = 6) in addition to regular enteral nutrition. Lab values including nitrogen balance, urea and ammonia were determined daily.

RESULTS

Continuous L-alanine-L-glutamine infusion significantly increased plasma and cerebral glutamine as well as alanine levels, being mostly sustained during the 5 day infusion phase (plasma glutamine: from 295 ± 62 to 500 ± 145 μmol/ l; brain glutamine: from 183 ± 188 to 549 ± 120 μmol/ l; plasma alanine: from 327 ± 91 to 622 ± 182 μmol/ l; brain alanine: from 48 ± 55 to 89 ± 129 μmol/ l; p < 0.05, ANOVA, post hoc Dunn's test).

CONCLUSIONS

High dose L-alanine-L-glutamine infusion (0.75 g/ kg/ d up to 5 days) increased plasma and brain glutamine and alanine levels. This was not associated with elevated glutamate or signs of potential glutamate-mediated cerebral injury. The increased nitrogen load should be considered in patients with renal and hepatic dysfunction.

TRIAL REGISTRATION

Clinicaltrials.gov NCT02130674. Registered 5 April 2014.

Mapping brain metabolic connectivity in awake rats with μPET and optogenetic stimulation

Positron emission tomography (PET) with [(18)F]2-fluoro-2-deoxy-D-glucose was used to measure changes in regional brain glucose metabolism (BGluM) in response to optogenetic stimulation (using the excitatory channelrhodopsin-2) of the nucleus accumbens (NAc) in awake rats. We demonstrated not only increases in BGluM that correlated with c-Fos expression in the region of stimulation, but also BGluM increases in the ipsilateral striatum, periaqueductal gray, and somatosensory cortex, and in contralateral amygdala, ventral pallidum, globus pallidus, and hippocampus, as well as decreases in BGluM in regions of the default mode network (retrosplenial cortex and cingulate gyrus) and secondary motor cortex. Additional exploration of c-Fos expression in regions found to be activated by PET results found corroborating evidence, with increased c-Fos expression in the ipsilateral somatosensory cortex, contralateral amygdala and globus pallidus, and bilateral periaqueductal gray. These findings are consistent with optogenetic excitation of the area of stimulation (NAc), as well as with stimulatory and inhibitory effects on downstream regions. They also confirm the utility of PET imaging to monitor connectivity in the awake rodent brain.

Volatile anesthetics influence blood-brain barrier integrity by modulation of tight junction protein expression in traumatic brain injury

Disruption of the blood-brain barrier (BBB) results in cerebral edema formation, which is a major cause for high mortality after traumatic brain injury (TBI). As anesthetic care is mandatory in patients suffering from severe TBI it may be important to elucidate the effect of different anesthetics on cerebral edema formation. Tight junction proteins (TJ) such as zonula occludens-1 (ZO-1) and claudin-5 (cl5) play a central role for BBB stability. First, the influence of the volatile anesthetics sevoflurane and isoflurane on in-vitro BBB integrity was investigated by quantification of the electrical resistance (TEER) in murine brain endothelial monolayers and neurovascular co-cultures of the BBB. Secondly brain edema and TJ expression of ZO-1 and cl5 were measured in-vivo after exposure towards volatile anesthetics in native mice and after controlled cortical impact (CCI). In in-vitro endothelial monocultures, both anesthetics significantly reduced TEER within 24 hours after exposure. In BBB co-cultures mimicking the neurovascular unit (NVU) volatile anesthetics had no impact on TEER. In healthy mice, anesthesia did not influence brain water content and TJ expression, while 24 hours after CCI brain water content increased significantly stronger with isoflurane compared to sevoflurane. In line with the brain edema data, ZO-1 expression was significantly higher in sevoflurane compared to isoflurane exposed CCI animals. Immunohistochemical analyses revealed disruption of ZO-1 at the cerebrovascular level, while cl5 was less affected in the pericontusional area. The study demonstrates that anesthetics influence brain edema formation after experimental TBI. This effect may be attributed to modulation of BBB permeability by differential TJ protein expression. Therefore, selection of anesthetics may influence the barrier function and introduce a strong bias in experimental research on pathophysiology of BBB dysfunction. Future research is required to investigate adverse or beneficial effects of volatile anesthetics on patients at risk for cerebral edema.

Impact of anesthesia on pathophysiology and mortality following subarachnoid hemorrhage in rats

BACKGROUND

Anesthesia is indispensable for in vivo research but has the intrinsic potential to alter study results. The aim of the current study was to investigate the impact of three common anesthesia protocols on physiological parameters and outcome following the most common experimental model for subarachnoid hemorrhage (SAH), endovascular perforation.

METHODS

Sprague-Dawley rats (n = 38) were randomly assigned to (1) chloral hydrate, (2) isoflurane or (3) midazolam/medetomidine/fentanyl (MMF) anesthesia. Arterial blood gases, intracranial pressure (ICP), mean arterial blood pressure (MAP), cerebral perfusion pressure (CPP), and regional cerebral blood flow (rCBF) were monitored before and for 3 hours after SAH. Brain water content, mortality and rate of secondary bleeding were also evaluated.

RESULTS

Under baseline conditions isoflurane anesthesia resulted in deterioration of respiratory parameters (arterial pCO2 and pO2) and increased brain water content. After SAH, isoflurane and chloral hydrate were associated with reduced MAP, incomplete recovery of post-hemorrhagic rCBF (23 ± 13% and 87 ± 18% of baseline, respectively) and a high anesthesia-related mortality (17 and 50%, respectively). Anesthesia with MMF provided stable hemodynamics (MAP between 100-110 mmHg), high post-hemorrhagic rCBF values, and a high rate of re-bleedings (> 50%), a phenomenon often observed after SAH in humans.

CONCLUSION

Based on these findings we recommend anesthesia with MMF for the endovascular perforation model of SAH.

EFFECTS OF ISOFLURANE ON GLUTAMATE AND TAURINE RELEASES, BRAIN SWELLING AND INJURY DURING TRANSIENT ISCHEMIA AND REPERFUSION

The volatile anesthetic agent isoflurane was thought to provide neuroprotection against ischemic damage; however, this effect remains controversial. Using the middle cerebral artery occlusion model and intracerebral microdialysis, the authors monitored the variations of glutamate and taurine concentrations in the extra-cellular space in male rats anesthetized with pentobarbital or isoflurane. Brain injury and edema were evaluated 24 h after ischemia. Isoflurane prevented the ischemia-induced efflux of glutamate and reduced the release of taurine. No difference in the size of the brain lesions was observed with both anesthetics, and isoflurane induced the formation of a bigger brain edema and reduced taurine release. These results suggest that inhibiting glutamate release during ischemia may not be sufficient to improve brain outcome after transient ischemia.

Effect of disrupting N-methyl-d-aspartate receptor-postsynaptic density protein-95 interactions on the threshold for halothane anesthesia in mice

BACKGROUND

The authors' previous studies have shown that clinically relevant concentrations of inhalational anesthetics dose-dependently and specifically inhibit the PSD-95, Dlg, and ZO-1 (PDZ) domain-mediated protein interactions between postsynaptic density protein 95 (PSD-95) and N-methyl-d-aspartate receptors, and that the knockdown of spinal PSD-95 by intrathecal injection of PSD-95 antisense oligodeoxynucleotide significantly reduces the minimum alveolar anesthetic concentration for isoflurane in rats.

METHODS

The authors constructed a fusion peptide, Tat-PSD-95 PDZ2, comprising the second PDZ domain of PSD-95, which can specifically disrupt PSD-95 PDZ2-mediated protein interactions by binding to its interaction partner. By intraperitoneal injection of this fusion peptide into mice, the authors investigated the effect of disrupting the PSD-95 PDZ2-mediated protein interactions on the threshold for halothane anesthesia.

RESULTS

Systemically injected fusion peptide Tat-PSD-95 PDZ2 was delivered into the central nervous system, disrupted the protein-protein interactions between N-methyl-d-aspartate receptor NR2 subunits and PSD-95, and significantly reduced the minimum alveolar anesthetic concentration and righting reflex EC50 for halothane.

CONCLUSIONS

By disrupting PSD-95 PDZ2 domain-mediated protein interactions, intraperitoneal injection of cell-permeant fusion peptide Tat-PSD-95 PDZ2 dose-dependently reduces the threshold for halothane anesthesia. These results suggest that PDZ domain-mediated protein interactions at synapses in the central nervous system might play an important role in the molecular mechanisms of halothane anesthesia.

Heterogeneous regional and temporal energetic impairment following controlled cortical impact injury in rats

OBJECTIVES

Following traumatic brain injury metabolic stability is impaired. Duration and reversibility of these changes might be important to guide specific interventions.

METHODS

To characterize temporal and regional changes in cerebral metabolism, 68 male Sprague-Dawley rats were subjected to a focal cortical contusion. Lesion progression and mitochondrial impairment were determined by magnetic resonance imaging (MRI) and triphenyl tetrazolium chloride (TTC) staining, respectively. Metabolic alterations were determined at hours 6 and 24 and day 7 by measuring extracellular glucose, lactate and hypoxanthine levels with microdialysis catheters placed adjacent and distant to the contusion and by quantifying changes in tissue ATP, lactate and glucose using bioluminescence imaging.

RESULTS

The cortical lesion reached its maximal extent at hour 24 and remained confined to the ipsilateral hemisphere. In microdialysate, at hour 6, extracellular hypoxanthine and lactate reached maximal values, thereafter hypoxanthine normalized while lactate remained increased. Extracellular glucose reached the highest values at hour 24 and remained elevated. Bioluminescence imaging revealed heterogeneous changes in areas distant to the contusion. No significant changes were found in ATP content. Slightly elevated tissue glucose until 24 hours in the ipsilateral hemisphere was observed. Following a continuous increase, lactate levels were the highest by 6 hours in the ipsilateral cortex and hippocampus.

DISCUSSION

CCI is associated with disturbances in energetic metabolism. Metabolic perturbation is not restricted to the early phase and the contusional region following focal cortical contusion, but also involves hippocampus and primarily uninjured parts of the hemisphere.

Consensus meeting: monosodium glutamate - an update

OBJECTIVE

Update of the Hohenheim consensus on monosodium glutamate from 1997: Summary and evaluation of recent knowledge with respect to physiology and safety of monosodium glutamate.

DESIGN

Experts from a range of relevant disciplines received and considered a series of questions related to aspects of the topic.

SETTING

University of Hohenheim, Stuttgart, Germany.

METHOD

The experts met and discussed the questions and arrived at a consensus.

CONCLUSION

Total intake of glutamate from food in European countries is generally stable and ranged from 5 to 12 g/day (free: ca. 1 g, protein-bound: ca. 10 g, added as flavor: ca. 0.4 g). L-Glutamate (GLU) from all sources is mainly used as energy fuel in enterocytes. A maximum intake of 6.000 [corrected] mg/kg body weight is regarded as safe. The general use of glutamate salts (monosodium-L-glutamate and others) as food additive can, thus, be regarded as harmless for the whole population. Even in unphysiologically high doses GLU will not trespass into fetal circulation. Further research work should, however, be done concerning the effects of high doses of a bolus supply at presence of an impaired blood brain barrier function. In situations with decreased appetite (e.g., elderly persons) palatability can be improved by low dose use of monosodium-L-glutamate.

Energy metabolic changes in the early post-injury period following traumatic brain injury in rats

Impaired cerebral energy metabolism may be a major contributor to the secondary injury cascade that occurs following traumatic brain injury (TBI). To estimate the cortical energy metabolic state following mild and severe controlled cortical contusion (CCC) TBI in rats, ipsi-and contralateral cortical tissues were frozen in situ at 15 and 40 min post-injury and adenylate (ATP, ADP, AMP) levels were analyzed using high-performance liquid chromatography (HPLC) and the energy charge (EC) was calculated. At 15 min post-injury, mildly brain-injured animals showed a 43% decrease in cortical ATP levels and a 2.4-fold increase in AMP levels (P < 0.05), and there was a significant reduction of the ipsilateral cortical EC when compared to sham-injured animals (P < 0.05). At 40 min post-injury, the ipsilateral adenylate levels and EC had recovered to the values observed in the sham-injury group. In the severe CCC group, there was a 51% decrease in ipsilateral cortical ATP levels and a 5.3-fold increase in AMP levels with a significant reduction of cortical EC at 15 min post-injury (P < 0.05). At 40 min post-injury, a 2.6-fold ipsilateral increase in AMP levels and an 11% and 44% decrease in EC and ATP levels, respectively, remained (P < 0.05). A 37-38% reduction of the total adenylate pool was observed ipsilaterally in both CCC severity groups at the early time-point, and a 19% and 28% decrease remained in the mild and severe CCC groups, respectively, at 40 min post-injury. Significant contralateral ATP and EC changes were only observed in the severe CCC group at 40 min post-injury (P < 0.05). The energy-requiring secondary injury cascades that occur early post-injury do not challenge the brain tissue to the extent of ATP depletion and may provide a window of opportunity for therapeutic intervention.