Application of glutamate in the cortex of rats: a microdialysis study

Meta-analysis of the effects of inert gases on cerebral ischemia-reperfusion injury

Recently, noble gas has become a hot spot within the medical field like respiratory organ cerebral anemia, acute urinary organ injury and transplantation. However, the shield performance in cerebral ischemia-reperfusion injury (CIRI) has not reached an accord. This study aims to evaluate existing evidence through meta-analysis to determine the effects of inert gases on the level of blood glucose, partial pressure of oxygen, and lactate levels in CIRI. We searched relevant articles within the following both Chinese and English databases: PubMed, Web of science, Embase, CNKI, Cochrane Library and Scopus. The search was conducted from the time of database establishment to the end of May 2023, and two researchers independently entered the data into Revman 5.3 and Stata 15.1. There were total 14 articles were enclosed within the search. The results showed that the amount of partial pressure of blood oxygen in the noble gas cluster was beyond that in the medicine gas cluster (P < 0.05), and the inert gas group had lower lactate acid and blood glucose levels than the medical gas group. The partial pressure of oxygen (SMD = 1.51, 95% CI 0.10 ~ 0.91 P = 0.04), the blood glucose level (SMD = - 0.59, 95% CI - 0.92 ~ - 0.27 P = 0.0004) and the lactic acid level (SMD = - 0.42, 95% CI - 0.80 ~ - 0.03 P = 0.03) (P < 0.05). These results are evaluated as medium-quality evidence. Inert gas can effectively regulate blood glucose level, partial pressure of oxygen and lactate level, and this regulatory function mainly plays a protective role in the small animal ischemia-reperfusion injury model. This finding provides an assessment and evidence of the effectiveness of inert gases for clinical practice, and provides the possibility for the application of noble gases in the treatment of CIRI. However, more operations are still needed before designing clinical trials, such as the analysis of the inhalation time, inhalation dose and efficacy of different inert gases, and the effective comparison of the effects in large-scale animal experiments.

Neurotoxicity mechanism of aconitine in HT22 cells studied by microfluidic chip-mass spectrometry

Aconitine, a common and main toxic component of Aconitum, is toxic to the central nervous system. However, the mechanism of aconitine neurotoxicity is not yet clear. In this work, we had the hypothesis that excitatory amino acids can trigger excitotoxicity as a pointcut to explore the mechanism of neurotoxicity induced by aconitine. HT22 cells were simulated by aconitine and the changes of target cell metabolites were real-time online investigated based on a microfluidic chip-mass spectrometry system. Meanwhile, to confirm the metabolic mechanism of aconitine toxicity on HT22 cells, the levels of lactate dehydrogenase, intracellular Ca²⁺, reactive oxygen species, glutathione and superoxide dismutase, and ratio of Bax/Bcl-2 protein were detected by molecular biotechnology. Integration of the detected results revealed that neurotoxicity induced by aconitine was associated with the process of excitotoxicity caused by glutamic acid and aspartic acid, which was followed by the accumulation of lactic acid and reduction of glucose. The surge of extracellular glutamic acid could further lead to a series of cascade reactions including intracellular Ca²⁺ overload and oxidative stress, and eventually result in cell apoptosis. In general, we illustrated a new mechanism of aconitine neurotoxicity and presented a novel analysis strategy that real-time online monitoring of cell metabolites can provide a new approach to mechanism analysis.

Role of Brain Glycogen During Ischemia, Aging and Cell-to-Cell Interactions

The astrocyte-neuron lactate transfer shuttle (ANLS) is one of the important metabolic systems that provides a physiological infrastructure for glia-neuronal interactions where specialized architectural organization supports the function. Perivascular astrocyte end-feet take up glucose via glucose transporter 1 to actively regulate glycogen stores, such that high ambient glucose upregulates glycogen and low levels of glucose deplete glycogen stores. A rapid breakdown of glycogen into lactate during increased neuronal activity or low glucose conditions becomes essential for maintaining axon function. However, it fails to benefit axon function during an ischemic episode in white matter (WM). Aging causes a remarkable change in astrocyte architecture characterized by thicker, larger processes oriented parallel to axons, as opposed to vertically-transposing processes. Subsequently, aging axons become more vulnerable to depleted glycogen, although aging axons can use lactate as efficiently as young axons. Lactate equally supports function during aglycemia in corpus callosum (CC), which consists of a mixture of myelinated and unmyelinated axons. Moreover, axon function in CC shows greater resilience to a lack of glucose compared to optic nerve, although both WM tracts show identical recovery after aglycemic injury. Interestingly, emerging evidence implies that a lactate transport system is not exclusive to astrocytes, as oligodendrocytes support the axons they myelinate, suggesting another metabolic coupling pathway in WM. Future studies are expected to unravel the details of oligodendrocyte-axon lactate metabolic coupling to establish that all WM components metabolically cooperate and that lactate may be the universal metabolite to sustain central nervous system function.

Enhancement of Brain d-Serine Mediates Recovery of Cognitive Function after Traumatic Brain Injury

Cognitive deficits, especially memory loss, are common and devastating neuropsychiatric sequelae of traumatic brain injury (TBI). The deficits may persist for years and may be accompanied by increased risk of developing early- onset dementia. Past attempts to reverse the neuropathological effects of brain injury with glutamate-N-methyl-d-aspartate (NMDA) antagonists failed to show any benefits or worsened the outcome, suggesting that activation, rather than blockage, of the NMDA receptor (NMDAR) may be useful in the subacute period after TBI and stroke. Activation of the NMDAR requires occupation of the glycine-modulatory site by co-agonists to achieve its synaptic functions. Glycine and d-serine are endogenous ligands/co-agonists of synaptic NMDARs in many areas of the mature brain. The aim of the present study was to evaluate the effect of 6-chlorobenzo(d)isoxazol-3-ol (CBIO), an inhibitor of D-amino acid oxidase (DAAO), which degrades d-serine, on cognitive outcome in a mouse model of TBI. Because treating TBI animals with CBIO elevates the endogenous levels of d-serine, we compared this novel treatment with treatment by exogenous d-serine alone and combined with CBIO. The results show that a single treatment (24 h post-injury) with CBIO in the mouse model of closed head injury significantly improves cognitive and motor function, and decreases lesion volume and the inflammatory response. Moreover, the compound proved to be neuroprotective, as the hippocampal volume and the number of neurons in hippocampal regions increased. Treatment with CBIO boosted the NR1 and phospho- NR1 subunits of the NMDAR and affected the CREB, phospho-CREB, and brain-derived neurotropic factor (BDNF) pathways. These findings render CBIO a promising, novel treatment for cognitive impairment following TBI.

Can lactate serve as an energy substrate for axons in good times and in bad, in sickness and in health?

In the mammalian white matter, glycogen-derived lactate from astrocytes plays a critical role in supporting axon function using the astrocyte-neuron lactate transfer shuttle (ANLTS) system with specialized monocarboxylate transporters (MCTs). A rapid breakdown of glycogen to lactate during increased neuronal activity or low glucose conditions becomes essential to maintain axon function. Therefore astrocytes actively regulate their glycogen stores with respect to ambient glucose levels such that high ambient glucose upregulates glycogen and low levels of glucose depletes glycogen stores. Although lactate fully supports axon function in the absence of glucose and becomes a preferred energy metabolite when axons discharge at high frequency, it fails to benefit axon function during an ischemic episode in white matter. Emerging evidence implies a similar lactate transport system between oligodendrocytes and the axons they myelinate, suggesting another metabolic coupling pathway in white matter. Therefore the conditions that activate this lactate shuttle system and the signaling mechanisms that mediate activation of this system are of great interest. Future studies are expected to unravel the details of oligodendrocyte-axon lactate metabolic coupling to establish how white matter components metabolically cooperate and that lactate may be the universal metabolite to sustain CNS function.

Endocannabinoids and traumatic brain injury

Traumatic brain injury (TBI) represents the leading cause of death in young individuals. It triggers the accumulation of harmful mediators, leading to secondary damage, yet protective mechanisms are also set in motion. The endocannabinoid (eCB) system consists of ligands, such as anandamide and 2-arachidonoyl-glycerol (2-AG), receptors (e.g. CB1, CB2), transporters and enzymes, which are responsible for the 'on-demand' synthesis and degradation of these lipid mediators. There is a large body of evidence showing that eCB are markedly increased in response to pathogenic events. This fact, as well as numerous studies on experimental models of brain toxicity, neuroinflammation and trauma supports the notion that the eCB are part of the brain's compensatory or repair mechanisms. These are mediated via CB receptors signalling pathways that are linked to neuronal survival and repair. The levels of 2-AG, the most highly abundant eCB, are significantly elevated after TBI and when administered to TBI mice, 2-AG decreases brain oedema, inflammation and infarct volume and improves clinical recovery. The role of CB1 in mediating these effects was demonstrated using selective antagonists or CB1 knockout mice. CB2 were shown in other models of brain insults to reduce white blood cell rolling and adhesion, to reduce infarct size and to improve motor function. This review is focused on the role the eCB system plays as a self-neuroprotective mechanism and its potential as a basis for the development of novel therapeutic modality for the treatment of CNS pathologies with special emphasis on TBI.

Metabolic and histologic effects of sodium pyruvate treatment in the rat after cortical contusion injury

This study determined the effects of intraperitoneal sodium pyruvate (SP) treatment on the levels of circulating fuels and on cerebral microdialysis levels of glucose (MD(glc)), lactate (MD(lac)), and pyruvate (MD(pyr)), and the effects of SP treatment on neuropathology after left cortical contusion injury (CCI) in rats. SP injection (1000 mg/kg) 5 min after sham injury (Sham-SP) or CCI (CCI-SP) significantly increased arterial pyruvate (p < 0.005) and lactate (p < 0.001) compared to that of saline-treated rats with CCI (CCI-Sal). Serum glucose also increased significantly in CCI-SP compared to that in CCI-Sal rats (p < 0.05), but not in Sham-SP rats. MD(pyr) was not altered after CCI-Sal, whereas MD(lac) levels within the cerebral cortex significantly increased bilaterally (p < 0.05) and those for MD(glc) decreased bilaterally (p < 0.05). MD(pyr) levels increased significantly in both Sham-SP and CCI-SP rats (p < 0.05 vs. CCI-Sal) and were higher in left/injured cortex of the CCI-SP group (p < 0.05 vs. sham-SP). In CCI-SP rats the contralateral MD(lac) decreased below CCI-Sal levels (p < 0.05) and the ipsilateral MD(glc) levels exceeded those of CCI-Sal rats (p < 0.05). Rats with a single low (500 mg/kg) or high dose (1000 mg/kg) SP treatment had fewer damaged cortical cells 6 h post-CCI than did saline-treated rats (p < 0.05), but three hourly injections of SP (1000 mg/kg) were needed to significantly reduce contusion volume 2 weeks after CCI. Thus, a single intraperitoneal SP treatment increases circulating levels of three potential brain fuels, attenuates a CCI-induced reduction in extracellular glucose while increasing extracellular levels of pyruvate, but not lactate, and can attenuate cortical cell damage occurring within 6 h of injury. Enduring (2 week) neuronal protection was achieved only with multiple SP treatments within the first 2 h post-CCI, perhaps reflecting the need for additional fuel throughout the acute period of increased metabolic demands induced by CCI.

D-cycloserine improves functional outcome after traumatic brain injury with wide therapeutic window

It has been long thought that hyperactivation of N-methyl-D-aspartate (NMDA) receptors underlies neurological decline after traumatic brain injury. However, all clinical trials with NMDA receptor antagonists failed. Since NMDA receptors are down-regulated from 4h to 2weeks after brain injury, activation at 24h, rather than inhibition, of these receptors, was previously shown to be beneficial in mice. Here, we tested the therapeutic window, dose regimen and mechanism of action of the NMDA receptor partial agonist D-cycloserine (DCS) in traumatic brain injury. Male mice were subjected to trauma using a weight-drop model, and administered 10mg/kg (i.p.) DCS or vehicle once (8, 16, 24, or 72h) twice (24 and 48h) or three times (24, 48 and 72h). Functional recovery was assessed for up to 60days, using a Neurological Severity Score that measures neurobehavioral parameters. In all groups in which treatment was begun at 24 or 72h neurobehavioral function was significantly better than in the vehicle-treated groups. Additional doses, on days 2 and 3 did not further improve recovery. Mice treated at 8h or 16h post injury did not differ from the vehicle-treated controls. Co-administration of the NMDA receptor antagonist MK-801 completely blocked the protective effect of DCS given at 24h. Infarct volume measured by 2,3,5-triphenyltetrazolium chloride staining at 48h or by cresyl violet at 28days was not affected by DCS treatment. Since DCS is used clinically for other indications, the present study offers a novel approach for treating human traumatic brain injury with a therapeutic window of at least 24h.

Lactate and glucose as energy substrates and their role in traumatic brain injury and therapy

Traumatic brain injury is a leading cause of disability and mortality worldwide, but no new pharmacological treatments are clinically available. A key pathophysiological development in the understanding of traumatic brain injury is the energy crisis derived from decreased cerebral blood flow, increased energy demand and mitochondrial dysfunction. Although still controversial, new findings suggest that brain cells try to cope in these conditions by metabolizing lactate as an energy substrate ‘on-demand’ in lieu of glucose. Experimental and clinical data suggest that lactate, at least when exogenously administered, is transported from astrocytes to neurons for neuronal utilization, essentially bypassing the slow, catabolizing glycolysis process to quickly and efficiently produce ATP. Treatment strategies using systemically applied lactate have proved to be protective in various experimental traumatic brain injury studies. However, lactate has the potential to elevate oxygen consumption to high levels and, therefore, could potentially impose a danger for tissue-at-risk with low cerebral blood flow. The present review outlines the experimental basis of lactate in energy metabolism under physiological and pathophysiological conditions and presents arguments for lactate as a new therapeutical tool in human head injury.

Inhibition of NR2B phosphorylation restores alterations in NMDA receptor expression and improves functional recovery following traumatic brain injury in mice

Traumatic brain injury (TBI) triggers a massive glutamate efflux, hyperactivation of N-methyl-D-aspartate receptors (NMDARs) and neuronal cell death. Previously it was demonstrated that, 15 min following experimentally induced closed head injury (CHI), the density of activated NMDARs increases in the hippocampus, and decreases in the cortex at the impact site. Here we show that CHI-induced alterations in activated NMDARs correlate with changes in the expression levels of the major NMDARs subunits. In the hippocampus, the expression of NR1, NR2A, and NR2B subunits as well as the GluR1 subunit of the AMPA receptor (AMPAR) were increased, while in the cortex at the impact site, we found a decrease in the expression of these subunits. We demonstrate that CHI-induced increase in the expression of NMDAR subunits and GluR1 in the hippocampus, but not in the cortex, is associated with an increase in NR2B tyrosine phosphorylation. Furthermore, inhibition of NR2B-phosphorylation by the tyrosine kinase inhibitor PP2 restores the expression of this subunit to its normal levels. Finally, a single injection of PP2, prior to the induction of CHI, resulted in a significant improvement in long-term recovery of motor functions observed in CHI mice. These results provide a new mechanism by which acute trauma contributes to the development of secondary damage and functional deficits in the brain, and suggests a possible role for Src tyrosine kinase inhibitors as preoperative therapy for planned neurosurgical procedures.

An endogenous glutamatergic drive onto somatic motoneurons contributes to the stereotypical pattern of muscle tone across the sleep-wake cycle

Skeletal muscle tone is modulated in a stereotypical pattern across the sleep-wake cycle. Abnormalities in this modulation contribute to most of the major sleep disorders; therefore, characterizing the neurochemical substrate responsible for transmitting a sleep-wake drive to somatic motoneurons needs to be determined. Glutamate is an excitatory neurotransmitter that modulates motoneuron excitability; however, its role in regulating motoneuron excitability and muscle tone during natural sleep-wake behaviors is unknown. Therefore, we used reverse-microdialysis, electrophysiology, pharmacological, and histological methods to determine how changes in glutamatergic neurotransmission within the trigeminal motor pool contribute to the sleep-wake pattern of masseter muscle tone in behaving rats. We found that blockade of non-NMDA and NMDA glutamate receptors (via CNQX and d-AP-5) on trigeminal motoneurons reduced waking masseter tone to sleeping levels, indicating that masseter tone is maximal during alert waking because motoneurons are activated by an endogenous glutamatergic drive. This wake-related drive is switched off in non-rapid eye movement (NREM) sleep, and this contributes to the suppression of muscle tone during this state. We also show that a functional glutamatergic drive generates the muscle twitches that characterize phasic rapid-eye movement (REM) sleep. However, loss of a waking glutamatergic drive is not sufficient for triggering the motor atonia that characterizes REM sleep because potent activation of either AMPA or NMDA receptors on trigeminal motoneurons was unable to reverse REM atonia. We conclude that an endogenous glutamatergic drive onto somatic motoneurons contributes to the stereotypical pattern of muscle tone during wakefulness, NREM sleep, and phasic REM sleep but not during tonic REM sleep.

Glutamatergic stimulation of the basal forebrain elevates extracellular adenosine and increases the subsequent sleep

A prolonged period of waking accumulates sleep pressure, increasing both the duration and the intensity of the subsequent sleep period. Delta power, which is calculated from the slow range electroencephalographic (EEG) oscillations (0.1-4 Hz), is regarded as the marker of sleep intensity. Recent findings indicate that not only the duration but also the quality of waking, determines the level of increase in the delta activity during the subsequent sleep period. Elevated levels of extracellular adenosine in the basal forebrain (BF) during prolonged waking have been proposed to act as the molecular signal of increased sleep pressure, but the role of BF neuronal activity in elevating adenosine has not been previously explored. We hypothesized that an increase in neuronal discharge in the BF would lead to increase in the extracellular adenosine and contribute to the increase in the subsequent sleep. To experimentally increase neuronal activity in the rat BF, we used 3 h in vivo microdialysis application of glutamate or its receptor agonists N-methyl-D-aspartate (NMDA) or AMPA. Samples for adenosine measurement were collected during the drug application and the EEG was recorded during and after the treatment, altogether for 24 h. All treatments increased the duration of the subsequent sleep following the application. In contrast, delta power was elevated only if both the waking EEG theta (5-9 Hz) power (which can be regarded as a marker of active waking) and the extracellular adenosine in the BF were increased during the application. These results indicate that increased neuronal activity in the BF, and particularly the type of neuronal activity coinciding with active waking, is one of the factors contributing to the buildup of the sleep pressure.

Simultaneous determination ofmyo-inositol andscyllo-inositol by MEKC as a rapid monitoring tool for inositol levels

A simple and rapid MEKC method was developed for the simultaneous determination of myo-inositol, scyllo-inositol, and glucose. Prior to electrophoretic separation, the nonfluorescent inositols and glucose were derivatized by N-methylisatoic anhydride at 25 degrees C for 10 min so that they could be detected by a fluorescence detector during separation. The good separation with high efficiency by MEKC was achieved in 13 min with a glycine buffer containing SDS and PEG 4000. Several parameters affecting the separation were studied, including the pH of BGE, the concentrations of glycine, SDS, and PEG 4000, and the applied voltage. Using glycerol as an internal standard, the linear ranges of the method for myo-inositol, scyllo-inositol, and glucose were 0.03-10, 0.01-5, and 0.05-20 mM; the concentration LODs of myo-inositol, scyllo-inositol, and glucose were 0.020, 0.0078, and 0.026 mM, respectively. The method was applied to analyze extracellular myo-inositol and glucose in the microdialysates from rat brain cortex of ischemia animal model and intracellular myo-inositol and scyllo-inositol in the rat brain extract.

Simultaneous inhibition of caudal medullary raphe and retrotrapezoid nucleus decreases breathing and the CO2 response in conscious rats

The medullary raphe (MR) and the retrotrapezoid nucleus (RTN) in the ventral medulla are two of many central chemoreceptor sites. We examine their combined function in conscious rats by focal inhibition using microdialysis. Inhibition of RTN neurons with the GABA(A) receptor agonist muscimol, with simultaneous dialysis of artificial cerebrospinal fluid (ACSF) in or near to the caudal MR, causes hypoventilation (decrease in the ratio of minute ventilation to oxygen consumption, V(E)/V(O2)) and reduces the ventilatory response to 7% CO(2) by 24%. Inhibition of caudal MR serotonergic neurons with the 5-HT(1A) receptor agonist (R)-(+)-8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT), with simultaneous dialysis of ACSF in or near to the RTN, causes hypoventilation but has no significant effect on the CO(2) response. Inhibition of both the RTN and the caudal MR simultaneously produces enhanced hypoventilation and a 51% decrease in the CO(2) response. The effects of treatment on the CO(2) response are similar in wakefulness and in non-rapid eye movement sleep. Comparison of the effect of 8-OH-DPAT microdialysed into a more rostral portion of the MR, where the CO(2) response is reduced by 22%, demonstrates heterogeneity within the MR of the function of serotonergic neurons in breathing. We conclude that serotonergic neurons within the caudal MR provide a non-CO(2)-dependent tonic drive to breathe and potentiate the effects of RTN neurons that contribute to a resting chemical 'drive to breathe' as well as the response to added CO(2). These effects of caudal MR serotonergic neurons could be at a chemoreceptor site, e.g. the RTN, or at 'downstream' sites involved in rhythm and pattern generation.

Vagal complex monocarboxylate transporter-2 expression during hypoglycemia

Astrocytic provision of lactate provision to neurons may be a critical indicator of substrate fuel availability in metabolic sensing sites in the brain, including the hindbrain dorsal vagal complex. We examined the hypothesis that vagal complex monocarboxylate transporter protein levels are gender dependent and estrogen dependent, and that estrogen influences adaptation of these protein responses during repeated insulin-induced hypoglycemia. Western blot analyses showed that male and estrogen-treated ovariectomized female rats exhibit opposite changes in monocarboxylate transporter-2 levels after one insulin injection, as well as divergent patterns of adaptation to this metabolic challenge. The data suggest that sex differences in hypoglycemic patterns in vagal complex lactate transport may underlie disparate signaling of cellular energy imbalance.

Respiratory activation of the genioglossus muscle involves both non-NMDA and NMDA glutamate receptors at the hypoglossal motor nucleus in vivo

Brainstem respiratory neurons innervate the hypoglossal motor nucleus which in turn transmits this respiratory drive signal to the genioglossus muscle of the tongue. The mechanism of this transmission is important to help maintain an open airspace for effective breathing, and is thought to rely almost exclusively on non-N-methyl-d-aspartate (non-NMDA) glutamate receptor activation during respiration. However those studies were performed in slices of medulla from neonatal animals in vitro which may have led to an underestimation of the contribution of NMDA glutamate receptors that may normally operate in intact preparations. The current study tests the hypothesis that both NMDA and non-NMDA receptors contribute to respiratory drive transmission at the hypoglossal motor nucleus in vivo. Experiments were performed in urethane-anesthetized and tracheotomized adult Wistar rats in which vagus nerves were either intact or sectioned. In the presence of augmented genioglossus activity produced by vagotomy, microdialysis perfusion of either an NMDA receptor antagonist (D-2-amino-5-phosphonovaleric acid, 0.001-10 mM) or a non-NMDA receptor antagonist (6-cyano-7-nitroquinoxaline-2, 3-dione disodium salt, 0.001-1 mM) to the hypoglossal motor nucleus reduced respiratory-related genioglossus activity in a dose-dependent manner (P < 0.001) indicating that both NMDA and non-NMDA glutamate receptors are necessary for transmission of the respiratory drive signal to genioglossus muscle in vivo. Similar effects were observed in the vagus nerve intact rats. Further experiments demonstrated that each delivered antagonist had effects that were specific to its respective receptor. Regression analysis also revealed that the activity of both NMDA and non-NMDA receptors at the hypoglossal motor nucleus is related to levels of the prevailing respiratory drive. These results show that both NMDA and non-NMDA glutamate receptors at the hypoglossal motor nucleus are involved in transmission of the respiratory drive signal to genioglossus muscle in vivo.

Opposing muscarinic and nicotinic modulation of hypoglossal motor output to genioglossus muscle in rats in vivo

The genioglossus (GG) muscle of the tongue, innervated by the hypoglossal motor nucleus (HMN), helps maintain an open airway for effective breathing. In vitro studies in neonatal rodents have separately characterized muscarinic and nicotinic receptor influences at the HMN but the net effects of combined nicotinic and muscarinic receptor activation and increased endogenous acetylcholine have not been determined in adult animals in vivo. Urethane-anaesthetized, tracheotomized and vagotomised rats were studied. Microdialysis perfusion of acetylcholine into the HMN significantly decreased respiratory-related GG activity (28.5 +/- 11.0% at a threshold dose of 0.1 mm). Application of the cholinergic agonists carbachol and muscarine have similar suppression effects (GG activity was decreased 11.8 +/- 4.3 and 20.5 +/- 5.8%, respectively, at 0.01 microm). Eserine, an acetylcholinesterase inhibitor, also decreased the amplitude of respiratory-related GG activity (36.4 +/- 11.3% at 1.0 microm) indicating that endogenous acetylcholine modulates GG activity. Although these results showed that suppression of GG activity predominates during cholinergic stimulation at the HMN, application of the nicotinic receptor agonist dimethyl-4-phenylpiperazinium iodide significantly increased tonic and respiratory-related GG activity (156 +/- 33% for respiratory activity at 1.0 mm) showing that excitatory responses are also present. Consistent with this, 100 microm carbachol decreased GG activity by 44.2 +/- 7.5% of control, with atropine (10 microm) reducing this suppression to 13.8 +/- 4.0% (P < 0.001). However, the nicotinic receptor antagonist dihydro-beta-erythroidine (100 microm) increased the carbachol-mediated suppression to 69.5 +/- 5.9% (P = 0.011), consistent with a role for nicotinic receptors in limiting the overall suppression of GG activity during cholinergic stimulation. Application of eserine to increase endogenous acetylcholine also showed that inhibitory muscarinic and excitatory nicotinic receptors together determine the net level of GG activity during cholinergic stimulation at the HMN. The results suggest that acetylcholine has mixed effects at the HMN with muscarinic-mediated GG suppression masking nicotinic excitation.

Moderate controlled cortical contusion in pigs: effects on multi-parametric neuromonitoring and clinical relevance

Over the last decade, routine neuromonitoring of ICP and CPP has been extended with new on-line techniques such as microdialysis, tissue oxygen (ptiO(2)), acid-base balance (ptiCO(2), pH) and CBF measurements, which so far have not lead to clear-cut therapy approaches in the neurointensive care unit. This is partially due to the complex pathophysiology following a wide-range of brain injuries, and the lack of suitable animal models allowing simultaneous, clinically relevant neuromonitoring under controlled conditions. Therefore, a controlled cortical impact (CCI) model in large animals (pig) has been developed. After placement of microdialysis, ptiO(2), temperature and ICP catheters, an unilateral CCI injury (2.6-2.8 m/sec velocity, 9 mm depth, 400 ms dwell time) was applied and neuromonitoring continued for 10 h. CCI caused a rapid drop in CPP, ptiO(2) and glucose, whereas ICP, glutamate and lactate increased significantly. Most parameters returned to baseline values within hours. Lactate stayed elevated significantly throughout the experiment, but the lactate-to-pyruvate ratio (LPR) changed only slightly, indicating no severely ischemic CBF. Contralateral parameters were not affected significantly. Evaluation of brain water content and histology (12 h post-CCI) showed ipsilateral brain swelling by 5% and massive cell damage underneath the injury site which correlated with changes of ICP, CPP, glutamate, lactate, and ptiO(2) within the first hours post-CCI. Moderate controlled cortical contusion in pigs induced a complex pattern of pathophysiological processes which led to 'early' histological damage. Thus, this new large animal model will enable us to investigate the effect of therapeutic interventions on multi-parametric neuromonitoring and histological outcome, and to translate the data into clinical practice.

5-HT at hypoglossal motor nucleus and respiratory control of genioglossus muscle in anesthetized rats

Serotonin (5-HT) from medullary raphe neurons excites hypoglossal motoneurons innervating genioglossus (GG) muscle. Since some raphe neurons also show increased activity in hypercapnia, we tested the hypothesis that serotonergic mechanisms at the hypoglossal motor nucleus (HMN) modulate GG activity and responses to CO2. Seventeen urethane-anesthetized, tracheotomized and vagotomized rats were studied. Microdialysis probes were used to deliver mianserin (5-HT receptor antagonist, 0 and 0.1 mM) or 5-HT (eight doses, 0-50 mM) to the HMN during room air or CO2-stimulated breathing. Mianserin decreased respiratory-related GG activity during room air and CO2-stimulated breathing (P<0.001), and also suppressed GG responses to CO2 (P=0.05). In contrast, GG activity was increased by 5-HT at the HMN, and was further increased in hypercapnia (P<0.02). However, 5-HT increased respiratory-related GG activity at levels lower (1 mM) than those eliciting tonic GG activity (10-30 mM 5-HT). The results show that 5-HT at the HMN contributes to the respiratory control of GG muscle.

Glutamate infusion coupled with hypoxia has a neuroprotective effect in the rat

Traumatic brain injury leads to a rise in glutamate, interference with oxygen supply and secondary neuronal death in the region surrounding the primary lesion. In the present experiments we have examined the effect of combining glutamate infusion with hypoxia on both brain metabolism and neuronal death. We have used microdialysis in unanaesthetised rats with a novel dual assay for glucose and lactate to monitor the temporal relation of changes in these metabolites resulting from infusion of 100 mM glutamate alone or combined with a reduction of inspired oxygen to 8%. In a parallel series of experiments we have compared the size of neuronal lesions under the same experimental conditions. We have used MAP2 antibody staining to measure the size of the neuronal lesion. Our results demonstrate that a 30 min glutamate infusion causes an immediate increase in neuronal glucose utilisation and a rise in lactate production. When hypoxia is added during the last 15 min of glutamate infusion there is a small rise in glucose and a large additional increase in lactate. The size of the neuronal lesions produced by infusion of 100 mM glutamate is reduced by the addition of hypoxia.

17beta-Estradiol reduces cortical lesion size in the glutamate excitotoxicity model by enhancing extracellular lactate: a new neuroprotective pathway

Estrogens play an important role in neuronal function and in protecting neurones in the cerebral cortex against pathological conditions. An in vivo model of glutamate excitotoxicity in which glutamate is applied to the cortex of rats through a microdialysis probe has been used to investigate the neuroprotective processes initiated by 17beta-estradiol. Rats were pre-treated with 17beta-estradiol (i.v.) before local application of 100 mM glutamate into the cortex through a microdialysis probe. Pre-treatment with 17beta-estradiol significantly reduced the size of the glutamate-induced cortical lesion. In the cortical microdialysates collected from the probe, a peak of lactate was observed immediately after glutamate application. After 17beta-estradiol pre-treatment this peak of lactate was significantly higher with estradiol than without 120 min after glutamate application, reaching 700% basal level at the end of measurement. The level of extracellular glucose was markedly decreased with and without 17beta-estradiol pre-treatment. Local blockage of neuronal lactate transporters with alpha-cyano-4-hydroxycinnamate (4-CIN) completely abolished the neuroprotective effect of 17beta-estradiol and induced a larger cortical lesion. An accumulation of extracellular lactate was observed after inhibition of the lactate transporters suggesting that transport of lactate into neurones is necessary for the neuroprotective effect of 17beta-estradiol. The anti-estrogen tamoxifen also abolished the neuroprotective effect of 17beta-estradiol on the lesion size and inhibited the production of lactate. These results suggest a new neuroprotective mechanism of 17beta-estradiol by activating glutamate-stimulated lactate production, which is estrogen receptor-dependent.

Microdialysis perfusion of 5-HT into hypoglossal motor nucleus differentially modulates genioglossus activity across natural sleep-wake states in rats

1. Serotonin (5-hydroxytryptamine, 5-HT) excites hypoglossal (XII) motoneurons in reduced preparations, and it has been suggested that withdrawal of 5-HT may underlie reduced genioglossus (GG) muscle activity in sleep. However, systemic administration of 5-HT agents in humans has limited effects on GG activity. Whether 5-HT applied directly to the XII motor nucleus increases GG activity in an intact preparation either awake or asleep has not been tested. 2. The aim of this study was to develop a novel freely behaving animal model for in vivo microdialysis of the XII motor nucleus across sleep-wake states, and test the hypothesis that 5-HT application will increase GG activity. 3. Eighteen rats were implanted with electroencephalogram and neck muscle electrodes to record sleep-wake states, and GG and diaphragm electrodes for respiratory muscle recording. Microdialysis probes were implanted into the XII motor nucleus and perfused with artificial cerebrospinal fluid (ACSF) or 10 mM 5-HT. 4. Normal decreases in GG activity occurred from wakefulness to non-rapid eye movement (non-REM) and REM sleep with ACSF (P < 0.01). Compared to ACSF, 5-HT caused marked GG activation across all sleep-wake states (increases of 91-251 %, P < 0.015). Importantly, 5-HT increased sleeping GG activity to normal waking levels for as long as 5-HT was applied (3-5 h). Despite tonic stimulation by 5-HT, periods of phasic GG suppression and excitation occurred in REM sleep compared with non-REM. 5. The results show that sleep-wake states differentially modulate GG responses to 5-HT at the XII motor nucleus. This animal model using in vivo microdialysis of the caudal medulla will enable the determination of neural mechanisms underlying pharyngeal motor control in natural sleep.

Bicuculline dialysis in the retrotrapezoid nucleus (RTN) region stimulates breathing in the awake rat

Muscimol dialysis in the retrotrapezoid nucleus (RTN) region of awake rats reduces tidal volume during air breathing and decreases chemoreception (Nattie, Li, 2000. J. Appl. Physiol., 89, 153-162). Is there an endogenous GABAergic inhibition of the RTN as for medullary respiratory and pressor neurons? Bicuculline microdialysis (30 min; 1 mM) into the RTN region of awake rats reversibly increased tidal volume by 11-16% over the period from 10 to 60 min (P<0.01; six rats). Ventilation increased but this was significant (P<0.05) only at 5, 20, and 25 min as frequency tended to decrease during dialysis. The ventilatory response to 7% CO(2) was unaffected (six rats); dialysis of vehicle alone over 4 h had no effect (five rats). It was concluded that in the awake rat there is ongoing endogenous modulation of RTN effects on tidal volume by a GABAergic process of unknown origin. The lack of effect on the response to systemic hypercapnia suggests that the RTN provides an ongoing endogenous drive to respiration by a process that is independent of its role in chemoreception.

Muscimol dialysis in the retrotrapezoid nucleus region inhibits breathing in the awake rat

Under anesthesia, inactivation of the retrotrapezoid nucleus (RTN) region markedly inhibits breathing and chemoreception. In conscious rats, we dialyzed muscimol for 30 min to inhibit neurons of the RTN region reversibly. Dialysis of artificial cerebrospinal fluid had no effect. Muscimol (1 or 10 mM) significantly decreased tidal volume (VT) (by 16-17%) within 15 min. VT remained decreased for 50 min or more, with recovery by 90 min. Ventilation (VE) decreased significantly (by 15-20%) within 15 min and then returned to baseline within 40 min as a result of an increase in frequency. This, we suggest, is a compensatory physiological response to the reduced VT. Oxygen consumption was unchanged. In response to 7% CO(2) in the 1 mM group, absolute VE and change in VE were significantly reduced (by 19-22%). In the 10 mM group, the response to dialysis included a time-related increase in frequency and decrease in body temperature, which may reflect greater spread of muscimol. In the awake rat, the RTN region provides a portion of the tonic drive to breathe, as well as a portion of the response to hypercapnia.

Glutamate stimulates ascorbate transport by astrocytes

The concentrations of glutamate and ascorbate in brain extracellular fluid increase following seizure activity, trauma and ischemia. Extracellular ascorbate concentration also rises following intracerebral glutamate injection. We hypothesized that glutamate triggers the release of ascorbate from astrocytes. We observed in primary cultures of rat cerebral astrocytes that glutamate increased ascorbate efflux significantly within 30 min. The half-maximal effective concentration of glutamate was 180+/-30 microM. Glutamate-stimulated efflux of ascorbate was attenuated by hypertonic media. 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid inhibited both Na(+)-dependent glutamate uptake and ascorbate efflux. Two other inhibitors of volume-sensitive organic anion channels (1, 9-dideoxyforskolin and 5-nitro-2-(3-phenylpropylamino) benzoic acid) did not slow glutamate uptake but prevented stimulation of ascorbate efflux. Glutamate also stimulated the uptake of ascorbate by ascorbate-depleted astrocytes. In contrast, glutamate uptake was not affected by intracellular ascorbate, thus ruling out a putative glutamate-ascorbate heteroexchange mechanism. These results are consistent with activation by glutamate of ascorbate-permeant channels in astrocytes.

Glucose and lactate metabolism after severe human head injury: influence of excitatory neurotransmitters and injury type

The survival of traumatized brain tissue depends on energy substrate delivery and consumption. Excitatory amino acids produce a disturbance of ion homeostasis and thus, increase energy demand. In head-injured patients, massive release of glutamate has been reported, especially in patients with focal contusions. Therefore, we studied the interrelationship between glutamate, glucose and lactate in relation to the type of injury. We investigated 37 severely head-injured patients in which a microdialysis probe was placed next to a focal contusion (n = 14) or together with a ventricular catheter in diffusely injured tissue (n = 23). Within-subject Spearman-rank correlation revealed an overall strong relationship between glutamate and lactate (p < 0.001) and glutamate and glucose (p < 0.01), but not between glucose and lactate (n.s.). The interrelationship was more pronounced in diffusely injured brain (normal CT appearance) compared to the contused tissue. The results demonstrate that glutamate clearly influences the release of lactate following injury, supporting the hypothesis that glutamate "drives" glycolysis in astrocytes. The strong positive correlation between glutamate and glucose might indicate an effect of glutamate upon glucose uptake by cells which differs according to the type of injury.

Evidence for time-dependent glutamate-mediated glycolysis in head-injured patients: a microdialysis study

In the brain, lactate is not only a marker of anaerobic glycolysis due to hypoxia/ischemia, but also a neuronal energy source which is provided by glutamate-induced astrocytic glycolysis. In the present study we wanted to investigate the relationship between glutamate release and lactate production during the entire time-course and during three time periods of microdialytic monitoring in 54 severely head injured patients. Within-subject Spearman rank correlations were calculated in each period for glutamate and lactate, for each patient and the mean of all correlation coefficients were analyzed for difference from zero by a one-sample t-test. The results show a strong overall positive relationship between glutamate and lactate. However, during the first 12 hours after injury, there was no significant correlation. Thereafter, good correlation was seen. The splitting of patients into groups with good (Glasgow Outcome Scale; GOS 0-2) and poor outcome (GOS 3-4) showed a similar strong correlation for patients with good outcome, but this was lost for patients with poor outcome. The results clearly indicate that glutamate "drives" astrocytic lactate production in head-injured patients. The contribution of glutamate to overall lactate release is thus time-dependent. During the first 12 hours after injury, factors such as hypoxia, ischemia or edema overshadowed glutamate-induced glycolysis in astrocytes. In addition, the effect of glutamate is more pronounced in patients with good outcome.

Neurotoxicity of glutamate at the concentration released upon spinal cord injury

Damage caused by administering glutamate into the spinal cord was characterized histologically. Glutamate destroyed neurons for several hundred micrometers around the administering microdialysis fiber. At 24 h after treatment, significant (P = 0.036) loss of neurons was observed (75%) relative to control (47%) near the fiber when glutamate was administered for 1 h at a concentration outside the fiber approximating the maximum glutamate released upon spinal cord injury. Significant loss of neurons (P = 0.006, 0.022) was also caused by administering a combination of glutamate at about its average concentration released upon injury over the 1 h period of administration in combination with the maximum aspartate concentration released upon injury. This work provides a direct demonstration that the concentrations of excitatory amino acids released upon spinal cord injury are neurotoxic. The destruction of neurons by exposure to excitatory amino acids when there is also substantial loss of neurons simply from the presence of the microdialysis fiber may reflect sensitization of neurons to excitotoxicity by stress.

TRH microdialysis into the RTN of the conscious rat increases breathing, metabolism, and temperature

Thyrotropin-releasing hormone (TRH) injected into the retrotrapezoid nucleus (RTN) of anesthetized rats produces a large, prolonged stimulation of ventilatory output (C. L. Cream, A. Li, and E. E. Nattie. J. Appl. Physiol. 83: 792-799, 1997). Here we inject or dialyze TRH into the RTN of conscious rats. In 6 of 17 injections (200 nl, 3.1 +/- 1.7 mM), ventilation (VE) increased 31% by 10 min, with recovery by 60 min. With dialysis, each animal of one group (n = 5) received, in random order, 10 mM TRH, 10 mM TRHOH (a metabolite of TRH), and artificial cerebrospinal fluid (aCSF); each animal of a second group (n = 5) received aCSF and 1 mM TRH. TRHOH and aCSF had no sustained effects. TRH (1 mM) increased VE (32%, P < 0.02, by 10 min, with recovery by 60 min), O(2) consumption (VO(2); 19%, P < 0. 03), and body (rectal) temperature (T(re); 0.5 degrees C, P < 0.09). TRH (10 mM) increased VE (78%, P < 0.01, by 10 min, with no recovery at 60 min), VO(2) (48%, P < 0.01), and T(re) (1.0 degrees C, P < 0. 01). TRH also induced arousal. The tissue volume affected in dialysis, estimated by spread of dialyzed fluorescein (332.3 mol wt, mol wt of TRH = 362.4), was 1,580 +/- 256 nl for 10 mM (n = 5) and 590 +/- 128 nl for 1 mM (n = 5). We conclude that 1) the RTN is involved in the integration of VE, VO(2), T(re), and arousal and 2) TRH may establish the responsiveness of RTN neurons.

Cortical extracellular sodium transients after human head injury: an indicator of secondary brain damage?

Animal studies indicate that elevated extracellular sodium can increase glutamate-induced excitotoxicity. Therefore, we investigated the relationship between sodium and glutamate and the effect of changes in sodium concentrations on the outcome of head-injured patients. Thirty-four (34) patients were selected for this study and divided into a group of patients having episodes (> or = 30-min) of high sodium in dialysates (> or = 200 mM; HIGH, n = 11) and a group of patients having no such episodes (NORMAL, n = 23). Levels for sodium (226 +/- 5.7 mM), glutamate (12.53 +/- 2.2 microM) and ICP (32.2 +/- 4.0 mm Hg,) were relatively high during the high sodium episodes. Overall, mean values for glutamate, ICP and outcome did not differ amono both groups. The mean dialysate sodium concentration, however, was significantly higher in the HIGH (178 +/- 6 mM) compared to the NORMAL group (158 +/- 3 mM; p < 0.01). Spearman rank correlation between sodium and glutamate or ICP were not significant. The HIGH sodium group did not have significantly more patients with poor outcome than the NORMAL group. The results indicated sodium concentrations did not affect the outcome of head-injured patients. However, other sodium monitoring techniques are desirable to elucidate these apparent potentially major sodium transients, which we have observed in the human cortex, after severe head injury.

Neurodegeneration induced by reversed microdialysis of NMDA; a quantitative model for excitotoxicity in vivo

This study characterizes a quantifiable in vivo model of excitotoxicity. In halothane anesthetized rats, microdialysis probe was implanted into somatosensory cortex/striatum and perfused by various concentrations (1, 10, 50 and 100 mmol/l) of N-methyl-d-aspartate (NMDA) for 20 min. After 24 h, histological quantification confirmed that NMDA produced a concentration-dependent excitotoxic lesion. With 10 mmol/l NMDA, coadministration of magnesium reduced significantly, and 2-amino-5-phosphonovalerate blocked completely the development of excitotoxic injury.

Altered glutamatergic transmission in neurological disorders: from high extracellular glutamate to excessive synaptic efficacy

This review is a critical appraisal of the widespread assumption that high extracellular glutamate, resulting from enhanced pre-synaptic release superimposed on deficient uptake and/or cytosolic efflux, is the key to excessive glutamate-mediated excitation in neurological disorders. Indeed, high extracellular glutamate levels do not consistently correlate with, nor necessarily produce, neuronal dysfunction and death in vivo. Furthermore, we exemplify with spreading depression that the sensitivity of an experimental or pathological event to glutamate receptor antagonists does not imply involvement of high extracellular glutamate levels in the genesis of this event. We propose an extension to the current, oversimplified concept of excitotoxicity associated with neurological disorders, to include alternative abnormalities of glutamatergic transmission which may contribute to the pathology, and lead to excitotoxic injury. These may include the following: (i) increased density of glutamate receptors; (ii) altered ionic selectivity of ionotropic glutamate receptors; (iii) abnormalities in their sensitivity and modulation; (iv) enhancement of glutamate-mediated synaptic efficacy (i.e. a pathological form of long-term potentiation); (v) phenomena such as spreading depression which require activation of glutamate receptors and can be detrimental to the survival of neurons. Such an extension would take into account the diversity of glutamate-receptor-mediated processes, match the complexity of neurological disorders pathogenesis and pathophysiology, and ultimately provide a more elaborate scientific basis for the development of innovative treatments.