The source of physiologically stimulated glutamate efflux from the striatum of conscious rats

Glutamate homeostasis and dopamine signaling: Implications for psychostimulant addiction behavior

Cocaine, amphetamine, and methamphetamine abuse disorders are serious worldwide health problems. To date, there are no FDA-approved medications for the treatment of these disorders. Elucidation of the biochemical underpinnings contributing to psychostimulant addiction is critical for the development of effective therapies. Excitatory signaling and glutamate homeostasis are well known pathophysiological substrates underlying addiction-related behaviors spanning multiple types of psychostimulants. To alleviate relapse behavior to psychostimulants, considerable interest has focused on GLT-1, the major glutamate transporter in the brain. While many brain regions are implicated in addiction behavior, this review focuses on two regions well known for their role in mediating the effects of cocaine and amphetamines, namely the nucleus accumbens (NAc) and the ventral tegmental area (VTA). In addition, because many investigators have utilized Cre-driver lines to selectively control gene expression in defined cell populations relevant for psychostimulant addiction, we discuss potential off-target effects of Cre-recombinase that should be considered in the design and interpretation of such experiments.

Translating the atypical dopamine uptake inhibitor hypothesis toward therapeutics for treatment of psychostimulant use disorders

Medication-assisted treatments are unavailable to patients with cocaine use disorders. Efforts to develop potential pharmacotherapies have led to the identification of a promising lead molecule, JJC8-091, that demonstrates a novel binding mode at the dopamine transporter (DAT). Here, JJC8-091 and a structural analogue, JJC8-088, were extensively and comparatively assessed to elucidate neurochemical correlates to their divergent behavioral profiles. Despite sharing significant structural similarity, JJC8-088 was more cocaine-like, increasing extracellular DA concentrations in the nucleus accumbens shell (NAS) efficaciously and more potently than JJC8-091. In contrast, JJC8-091 was not self-administered and was effective in blocking cocaine-induced reinstatement to drug seeking. Electrophysiology experiments confirmed that JJC8-091 was more effective than JJC8-088 at inhibiting cocaine-mediated enhancement of DA neurotransmission. Further, when VTA DA neurons in DAT-cre mice were optically stimulated, JJC8-088 produced a significant leftward shift in the stimulation-response curve, similar to cocaine, while JJC8-091 shifted the curve downward, suggesting attenuation of DA-mediated brain reward. Computational models predicted that JJC8-088 binds in an outward facing conformation of DAT, similar to cocaine. Conversely, JJC8-091 steers DAT towards a more occluded conformation. Collectively, these data reveal the underlying molecular mechanism at DAT that may be leveraged to rationally optimize leads for the treatment of cocaine use disorders, with JJC8-091 representing a compelling candidate for development.

Adaptation of Microelectrode Array Technology for the Study of Anesthesia-induced Neurotoxicity in the Intact Piglet Brain

Every year, millions of children undergo anesthesia for a multitude of procedures. However, studies in both animals and humans have called into question the safety of anesthesia in children, implicating anesthetics as potentially toxic to the brain in development. To date, no studies have successfully elucidated the mechanism(s) by which anesthesia may be neurotoxic. Animal studies allow investigation of such mechanisms, and neonatal piglets represent an excellent model to study these effects due to their striking developmental similarities to the human brain. This protocol adapts the use of enzyme-based microelectrode array (MEA) technology as a novel way to study the mechanism(s) of anesthesia-induced neurotoxicity (AIN). MEAs enable real-time monitoring of in vivo neurotransmitter activity and offer exceptional temporal and spatial resolution. It is hypothesized that anesthetic neurotoxicity is caused in part by glutamate dysregulation and MEAs offer a method to measure glutamate. The novel implementation of MEA technology in a piglet model presents a unique opportunity for the study of AIN.

Perfusion with carbon monoxide does not affect extracellular glutamate in dialysates of the hippocampus of freely moving mice

Carbon monoxide (CO) produces several neurological effects, including cognitive, mood, and behavioral disturbance. Glutamate is thought to play a particularly important role in learning and memory. Thus, the present study was aimed at investigating the local effect of CO on the glutamate level in the hippocampus of mice using in vivo reverse microdialysis. Mice were perfused with Ringer's solution (control) or CO (60-125 μM) in Ringer's solution into the hippocampus via microdialysis probe. Dialysate samples were collected every 20 min, and then analyzed with high-performance liquid chromatography coupled to an electrochemical detector. The result revealed that the perfusion with CO had no significant effect on glutamate levels (p = 0.316) as compared to the control group. This finding does not support a local CO rise as the cause of the increased glutamate level in the hippocampus of mice.

An electrochemiluminescent method for glutamate measurement in small microdialysate samples in asphyxiated young rats

Glutamate (Glu) quantification has been performed by a combination of intracerebral microdialysis through which the samples are obtained and analyzed by high performance liquid chromatography (HPLC); its measurement requires a large expenditure of time (15-30 min per sample) and special training. Therefore, an alternative method is presented here, based on the electrochemiluminescence produced by the use of an enzymatic reactor, containing glutamate-oxidase, mixed and incubated with microdialysate from dorsal striatum (DS) and prefrontal cortex (PFC) of young rats asphyxiated during the neonatal period, under a global asphyxia model in order to test this method. Using this approach, we found high extracellular Glu concentration in the DS of asphyxiated animals, but only during K⁺ stimulation, while in the PFC, only a delay in the rise of Glu after K⁺ stimulation was observed, without any difference in extracellular Glu content when compared with controls. This new method permitted a fast measurement of Glu in brain dialysate samples, it significantly reduces the cost of the analysis per sample, since only a single device and pump are needed without using columns and high pressure inside the system or complex hardware and software to control pumps, detector, fraction collector or any other peripheral used in HPLC.

The Amino Acid Transporter JhI-21 Coevolves with Glutamate Receptors, Impacts NMJ Physiology, and Influences Locomotor Activity in Drosophila Larvae

Changes in synaptic physiology underlie neuronal network plasticity and behavioral phenomena, which are adjusted during development. The Drosophila larval glutamatergic neuromuscular junction (NMJ) represents a powerful synaptic model to investigate factors impacting these processes. Amino acids such as glutamate have been shown to regulate Drosophila NMJ physiology by modulating the clustering of postsynaptic glutamate receptors and thereby regulating the strength of signal transmission from the motor neuron to the muscle cell. To identify amino acid transporters impacting glutmatergic signal transmission, we used Evolutionary Rate Covariation (ERC), a recently developed bioinformatic tool. Our screen identified ten proteins co-evolving with NMJ glutamate receptors. We selected one candidate transporter, the SLC7 (Solute Carrier) transporter family member JhI-21 (Juvenile hormone Inducible-21), which is expressed in Drosophila larval motor neurons. We show that JhI-21 suppresses postsynaptic muscle glutamate receptor abundance, and that JhI-21 expression in motor neurons regulates larval crawling behavior in a developmental stage-specific manner.

Reward related neurotransmitter changes in a model of depression: An in vivo microdialysis study

OBJECTIVES

The self-medication hypothesis assumes that symptoms related to potential monoaminergic deficits in depression may be relieved by drug abuse. The aim of this study was to elucidate the neurotransmitter changes in a rat model of depression by measuring their levels in the nucleus accumbens shell, which is typically involved in the drug of abuse acquisition mechanism.

METHODS

Depression was modelled by the olfactory bulbectomy (OBX) in Wistar male rats. In vivo microdialysis was performed, starting from the baseline and following after a single methamphetamine injection and behaviour was monitored. The determination of neurotransmitters and their metabolites was performed by high-performance liquid chromatography combined with mass spectrometry.

RESULTS

OBX animals had lower basal levels of dopamine and serotonin and their metabolites. However, γ-aminobutyric acid (GABA) and glutamate levels were increased. The methamphetamine injection induced stronger dopamine and serotonin release in the OBX rats and lower release of glutamate in comparison with sham-operated rats; GABA levels did not differ significantly.

CONCLUSIONS

This study provides an evidence of mesolimbic neurotransmitter changes in the rat model of depression which may elucidate mechanisms underlying intravenous self-administration studies in which OBX rats were demonstrated to have higher drug intake in comparison to intact controls.

A cellular perspective on brain energy metabolism and functional imaging

The energy demands of the brain are high: they account for at least 20% of the body's energy consumption. Evolutionary studies indicate that the emergence of higher cognitive functions in humans is associated with an increased glucose utilization and expression of energy metabolism genes. Functional brain imaging techniques such as fMRI and PET, which are widely used in human neuroscience studies, detect signals that monitor energy delivery and use in register with neuronal activity. Recent technological advances in metabolic studies with cellular resolution have afforded decisive insights into the understanding of the cellular and molecular bases of the coupling between neuronal activity and energy metabolism and point at a key role of neuron-astrocyte metabolic interactions. This article reviews some of the most salient features emerging from recent studies and aims at providing an integration of brain energy metabolism across resolution scales.

Central and peripheral contributions to dynamic changes in nucleus accumbens glucose induced by intravenous cocaine

The pattern of neural, physiological and behavioral effects induced by cocaine is consistent with metabolic neural activation, yet direct attempts to evaluate central metabolic effects of this drug have produced controversial results. Here, we used enzyme-based glucose sensors coupled with high-speed amperometry in freely moving rats to examine how intravenous cocaine at a behaviorally active dose affects extracellular glucose levels in the nucleus accumbens (NAc), a critical structure within the motivation-reinforcement circuit. In drug-naive rats, cocaine induced a bimodal increase in glucose, with the first, ultra-fast phasic rise appearing during the injection (latency 6–8 s; ~50 μM or ~5% of baseline) followed by a larger, more prolonged tonic elevation (~100 μM or 10% of baseline, peak ~15 min). While the rapid, phasic component of the glucose response remained stable following subsequent cocaine injections, the tonic component progressively decreased. Cocaine-methiodide, cocaine's peripherally acting analog, induced an equally rapid and strong initial glucose rise, indicating cocaine's action on peripheral neural substrates as its cause. However, this analog did not induce increases in either locomotion or tonic glucose, suggesting direct central mediation of these cocaine effects. Under systemic pharmacological blockade of dopamine transmission, both phasic and tonic components of the cocaine-induced glucose response were only slightly reduced, suggesting a significant role of non-dopamine mechanisms in cocaine-induced accumbal glucose influx. Hence, intravenous cocaine induces rapid, strong inflow of glucose into NAc extracellular space by involving both peripheral and central, non-dopamine drug actions, thus preventing a possible deficit resulting from enhanced glucose use by brain cells.

An update of the classical and novel methods used for measuring fast neurotransmitters during normal and brain altered function

To understand better the cerebral functions, several methods have been developed to study the brain activity, they could be related with morphological, electrophysiological, molecular and neurochemical techniques. Monitoring neurotransmitter concentration is a key role to know better how the brain works during normal or pathological conditions, as well as for studying the changes in neurotransmitter concentration with the use of several drugs that could affect or reestablish the normal brain activity. Immediate response of the brain to environmental conditions is related with the release of the fast acting neurotransmission by glutamate (Glu), γ-aminobutyric acid (GABA) and acetylcholine (ACh) through the opening of ligand-operated ion channels. Neurotransmitter release is mainly determined by the classical microdialysis technique, this is generally coupled to high performance liquid chromatography (HPLC). Detection of neurotransmitters can be done by fluorescence, optical density, electrochemistry or other detection systems more sophisticated. Although the microdialysis method is the golden technique to monitor the brain neurotransmitters, it has a poor temporal resolution. Recently, with the use of biosensor the drawback of temporal resolution has been improved considerably, however other inconveniences have merged, such as stability, reproducibility and the lack of reliable biosensors mainly for GABA. The aim of this review is to show the important advances in the different ways to measure neurotransmitter concentrations; both with the use of classic techniques as well as with the novel methods and alternant approaches to improve the temporal resolution.

Physiological fluctuations in brain temperature as a factor affecting electrochemical evaluations of extracellular glutamate and glucose in behavioral experiments

The rate of any chemical reaction or process occurring in the brain depends on temperature. While it is commonly believed that brain temperature is a stable, tightly regulated homeostatic parameter, it fluctuates within 1-4 °C following exposure to salient arousing stimuli and neuroactive drugs, and during different behaviors. These temperature fluctuations should affect neural activity and neural functions, but the extent of this influence on neurochemical measurements in brain tissue of freely moving animals remains unclear. In this Review, we present the results of amperometric evaluations of extracellular glutamate and glucose in awake, behaving rats and discuss how naturally occurring fluctuations in brain temperature affect these measurements. While this temperature contribution appears to be insignificant for glucose because its extracellular concentrations are large, it is a serious factor for electrochemical evaluations of glutamate, which is present in brain tissue at much lower levels, showing smaller phasic fluctuations. We further discuss experimental strategies for controlling the nonspecific chemical and physical contributions to electrochemical currents detected by enzyme-based biosensors to provide greater selectivity and reliability of neurochemical measurements in behaving animals.

Modeling secondary messenger pathways in neurovascular coupling

Neurovascular coupling is the well-documented link between neural stimulation and constriction or dilation of the surrounding vasculature. Glial cells mediate this response via their unique anatomy, which connects neurons to arterioles. It is believed that calcium transients and the release of secondary messengers by these cells influence the vascular response. We present a model of intracellular calcium dynamics in an astrocyte (glial cell) and show that stable oscillatory behaviour is possible under certain conditions. We then couple this to a novel model for the relationship between calcium concentration and the production of vasoactive secondary messengers through a fatty-acid intermediate. The two secondary messengers modelled are epoxyeicosatrienoic and 20-hydroxyeicosatetraenoic acids (EET and 20-HETE, respectively). These secondary messengers are produced on different time scales, and we show how this supports the observation that the vasculature dilates rapidly in response to neural stimulation, before returning to baseline levels on a slower time scale.

Electrochemically deposited iridium oxide reference electrode integrated with an electroenzymatic glutamate sensor on a multi-electrode array microprobe

An implantable micromachined multi-electrode array (MEA) microprobe modified for utilization as a complete electrochemical biosensor for rapid glutamate detection is described. A post-fabrication method for electrochemical deposition of an iridium oxide (IrOx) film onto a designated microelectrode enabled incorporation of an IrOx reference electrode (RE) on the microprobe. The on-probe IrOx RE provides an alternative to the commonly utilized Ag/AgCl wire RE, which has been shown to be unstable and to cause an inflammatory response in living tissue. The IrOx film electrodeposited onto a platinum site was tested as part of a complete chemical sensing system that included a platinum counter electrode and enzymatic glutamate sensing electrodes all on a single silicon-based MEA platform. The thin film IrOx was mechanically robust enough to endure conditions of repeated heating and wetting during the MEA fabrication process. The pH dependence of the IrOx open circuit potential (OCP) was measured at -77±0.4 mV/pH and remained stable over a two-week period. The on-probe IrOx RE was tested in a two- and three-electrode system with glutamate biosensors. The biosensors were shown to detect a physiologically relevant range of glutamate concentrations and to reject the interferents, dopamine and ascorbic acid. By incorporating all of the electrodes onto a single device, baseline noise was reduced by an average of ∼61% in vitro and ∼71% in vivo with reduced tissue damage, since only a single probe needed to be implanted.

Lactate as a biomarker for sleep

STUDY OBJECTIVES

An ideal biomarker for sleep should change rapidly with sleep onset, remain at a detectably differential level throughout the sleep period, and exhibit a rapid change with waking. Currently, no molecular marker has been identified that exhibits all three properties. This study examined three substances (lactate, glucose, and glutamate) for suitability as a sleep biomarker.

DESIGN

Using amperometric biosensor technology in conjunction with electroencephalograph (EEG) and electromyograph (EMG) monitoring, extracellular concentrations of lactate and glucose (Cohort 1) as well as lactate and glutamate (Cohort 2) were recorded over multiple sleep/wake cycles.

PATIENTS OR PARTICIPANTS

There were 12 C57Bl/6J male mice (3-5 mo old).

INTERVENTIONS

Sleep and waking transitions were identified using EEG recordings. Extracellular concentrations of lactate, glucose, and glutamate were evaluated before and during transition events as well as during extended sleep and during a 6-h sleep deprivation period.

MEASUREMENTS AND RESULTS

Rapid and sustained increases in cortical lactate concentration (approximately 15 μM/min) were immediately observed upon waking and during rapid eye movement sleep. Elevated lactate concentration was also maintained throughout a 6-h period of continuous waking. A persistent and sustained decline in lactate concentration was measured during nonrapid eye movement sleep. Glutamate exhibited similar patterns, but with a much slower rise and decline (approximately 0.03 μM/min). Glucose concentration changes did not demonstrate a clear correlation with either sleep or wake.

CONCLUSIONS

These findings indicate that extracellular lactate concentration is a reliable sleep/wake biomarker and can be used independently of the EEG signal.

Thinking outside the cleft to understand synaptic activity: contribution of the cystine-glutamate antiporter (System xc-) to normal and pathological glutamatergic signaling

System x(c)(-) represents an intriguing target in attempts to understand the pathological states of the central nervous system. Also called a cystine-glutamate antiporter, system x(c)(-) typically functions by exchanging one molecule of extracellular cystine for one molecule of intracellular glutamate. Nonvesicular glutamate released during cystine-glutamate exchange activates extrasynaptic glutamate receptors in a manner that shapes synaptic activity and plasticity. These findings contribute to the intriguing possibility that extracellular glutamate is regulated by a complex network of release and reuptake mechanisms, many of which are unique to glutamate and rarely depicted in models of excitatory signaling. Because system x(c)(-) is often expressed on non-neuronal cells, the study of cystine-glutamate exchange may advance the emerging viewpoint that glia are active contributors to information processing in the brain. It is noteworthy that system x(c)(-) is at the interface between excitatory signaling and oxidative stress, because the uptake of cystine that results from cystine-glutamate exchange is critical in maintaining the levels of glutathione, a critical antioxidant. As a result of these dual functions, system x(c)(-) has been implicated in a wide array of central nervous system diseases ranging from addiction to neurodegenerative disorders to schizophrenia. In the current review, we briefly discuss the major cellular components that regulate glutamate homeostasis, including glutamate release by system x(c)(-). This is followed by an in-depth discussion of system x(c)(-) as it relates to glutamate release, cystine transport, and glutathione synthesis. Finally, the role of system x(c)(-) is surveyed across a number of psychiatric and neurodegenerative disorders.

Rapid changes in extracellular glutamate induced by natural arousing stimuli and intravenous cocaine in the nucleus accumbens shell and core

Glutamate (Glu) is a major excitatory neurotransmitter, playing a crucial role in the functioning of the nucleus accumbens (NAc), a critical area implicated in somatosensory integration and regulation of motivated behavior. In this study, high-speed amperometry with enzyme-based biosensors was used in freely moving rats to examine changes in extracellular Glu in the NAc shell and core induced by a tone, tail pinch (TP), social interaction with a male conspecific (SI), and intravenous (iv) cocaine (1 mg/kg). To establish the contribution of Glu to electrochemical signal changes, similar recordings were conducted with null (Glu(0)) sensors, which were exposed to the same chemical and physical environment but were insensitive to Glu. TP, SI, and cocaine, but not a tone, induced relatively large and prolonged current increases detected by both Glu and Glu(0) sensors. However, current differentials revealed very rapid, much smaller, and transient increases in extracellular Glu levels, more predominantly in the NAc shell than core. In contrast to monophasic responses with natural stimuli, cocaine induced a biphasic Glu increase in the shell, with a transient peak during the injection and a slower postinjection peak. Therefore, Glu is phasically released in the NAc after exposure to natural arousing stimuli and cocaine; this release is rapid, stimulus dependent, and structure specific, suggesting its role in triggering neural and behavioral activation induced by these stimuli. This study also demonstrates the need for multiple in vitro and in vivo controls to reveal relatively small, highly phasic, and transient fluctuations in Glu levels occurring under behaviorally relevant conditions.

Transient extracellular glutamate events in the basolateral amygdala track reward-seeking actions

The ability to make rapid, informed decisions about whether or not to engage in a sequence of actions to earn reward is essential for survival. Modeling in rodents has demonstrated a critical role for the basolateral amygdala (BLA) in such reward-seeking actions, but the precise neurochemical underpinnings are not well understood. Taking advantage of recent advancements in biosensor technologies, we made spatially discrete near-real-time extracellular recordings of the major excitatory transmitter, glutamate, in the BLA of rats performing a self-paced lever-pressing sequence task for sucrose reward. This allowed us to detect rapid transient fluctuations in extracellular BLA glutamate time-locked to action performance. These glutamate transients tended to precede lever-pressing actions and were markedly increased in frequency when rats were engaged in such reward-seeking actions. Based on muscimol and tetrodotoxin microinfusions, these glutamate transients appeared to originate from the terminals of neurons with cell bodies in the orbital frontal cortex. Importantly, glutamate transient amplitude and frequency fluctuated with the value of the earned reward and positively predicted lever-pressing rate. Such novel rapid glutamate recordings during instrumental performance identify a role for glutamatergic signaling within the BLA in instrumental reward-seeking actions.

The effects of thioperamide on extracellular levels of glutamate and GABA in the rat prefrontal cortex

RATIONALE

Histamine H3 receptors (H3R) are presynaptic heteroreceptors that negatively modulate the release of histamine and other neurotransmitters such as acetylcholine. Blocking H3 receptors with antagonists/inverse agonists has been shown to be procognitive and this effect has often been associated with increases in acetylcholine transmission. H3 receptors are abundantly expressed in the prefrontal cortex, an area associated with cognitive performance. While the procognitive effects of H3 receptor antagonists/inverse agonists may depend on alterations to acetylcholine or histamine release, other transmitters involved in cognitive processing such as glutamate and gamma-aminobutyric acid (GABA) may also be involved.

OBJECTIVE

The purpose of the present study was to examine the effects of thioperamide, an H3 receptor antagonist, on extracellular levels of glutamate and GABA in the prefrontal cortex.

MATERIALS AND METHODS

By means of in vivo microdialysis on freely moving Sprague Dawley rats, samples were collected and assayed via high-performance liquid chromatography coupled to electrochemical detection.

RESULTS

Replacement of calcium with magnesium revealed that the release of GABA, but not glutamate, was calcium-dependent. Thioperamide (10-20 mg/kg) did not affect basal glutamate or GABA release. Perfusion with a high concentration of potassium (100 mM) increased GABA, but not glutamate, release and thioperamide (20 mg/kg) attenuated the effects of high potassium on GABA release.

CONCLUSION

These data indicate that H3 receptors in the prefrontal cortex can enhance stimulated GABA release, but do not regulate basal levels of glutamate or GABA.

Computational model of extracellular glutamate in the nucleus accumbens incorporates neuroadaptations by chronic cocaine

Chronic cocaine administration causes instability in extracellular glutamate in the nucleus accumbens that is thought to contribute to the vulnerability to relapse. A computational framework was developed to model glutamate in the extracellular space, including synaptic and nonsynaptic glutamate release, glutamate elimination by glutamate transporters and diffusion, and negative feedback on synaptic release via metabotropic glutamate receptors (mGluR2/3). This framework was used to optimize the geometry of the glial sheath surrounding excitatory synapses, and by inserting physiological values, accounted for known stable extracellular, extrasynaptic concentrations of glutamate measured by microdialysis and glutamatergic tone on mGluR2/3. By using experimental values for cocaine-induced reductions in cystine-glutamate exchange and mGluR2/3 signaling, and by predicting the down-regulation of glutamate transporters, the computational model successfully represented the experimentally observed increase in glutamate that is seen in rats during cocaine-seeking. This model provides a mathematical framework for describing how pharmacological or pathological conditions influence glutamate transmission measured by microdialysis.

Time-resolved microdialysis for in vivo neurochemical measurements and other applications

Monitoring changes in chemical concentrations over time in complex environments is typically performed using sensors and spectroscopic techniques. Another approach is to couple sampling methods, such as microdialysis, with chromatographic, electrophoretic, or enzymatic assays. Recent advances of such coupling have enabled improvements in temporal resolution, multianalyte capability, and automation. In a sampling and analysis method, the temporal resolution is set by the mass sensitivity of the analytical method, analysis time, and zone dispersion during sampling. Coupling methods with high speed and mass sensitivity to microdialysis sampling help to reduce some of these contributions to yield methods with temporal resolution of seconds. These advances have been primarily used in monitoring neurotransmitters in vivo. This review covers the problems associated with chemical monitoring in the brain, recent advances in using microdialysis for time-resolved in vivo measurements, sample applications, and other potential applications of the technology such as determining reaction kinetics and process monitoring.

Effect of eslicarbazepine acetate (BIA 2-093) on latrunculin A-induced seizures and extracellular amino acid concentrations in the rat hippocampus

PURPOSE

Eslicarbazepine acetate (ESL, BIA 2-093) is a novel antiepileptic drug endowed with an anticonvulsant potency similar to that of carbamazepine, and shares with carbamazepine and oxcarbazepine the capability to inhibit voltage-gated sodium channels. ESL is efficacious against maximal electroshock seizure-induced seizures, protects against picrotoxin-induced seizures in mice and rats, and prevents development of kindling in rats. In vivo, latrunculin A microperfusion in the rat hippocampus induces acute epileptic seizures and long-term biochemical changes leading to decreased picrotoxin seizure threshold and spontaneous seizures. We have tested the effect of ESL on latrunculin A-induced seizures, and its effect on the changes in extracellular amino acid levels induced by latrunculin A.

METHODS

Rat hippocampus was continuously perfused with a latrunculin A solution (4 microM) through CMA/12 microdialysis probes at a flow rate of 2 microl/min during 8 h with continuous EEG and videotape recording for 3 consecutive days. The same protocol was repeated after oral administration of ESL (3, 10 and 30 mg/kg). Samples from the microdialysate were collected and analyzed by HPLC using pre-column derivatization with 6 aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC) and fluorescence detection.

RESULTS

After the administration of 3 mg/kg of ESL, seizures were completely suppressed in the 66.7% of the rats. 10 and 30 mg/kg of ESL did completely suppressed seizures in the 100% of the animals studied. Hippocampal extracellular levels of glutamate, glycine and aspartate were significantly increased during latrunculin A microperfusion, while GABA levels remained unchanged. At the doses studied, ESL reversed the increases in extracellular glutamate and aspartate concentrations to basal levels and significantly reduced glycine levels.

CONCLUSIONS

ESL, at oral doses of 3, 10 and 30 mg/kg, shows an excellent anticonvulsant effect against seizures induced by latrunculin A microperfusion in the rat, and prevents the increases in glutamate and aspartate induced by latrunculin A.

Microdialysis of GABA and glutamate: analysis, interpretation and comparison with microsensors

GABA and glutamate sampled from the brain by microdialysis do not always fulfill the classic criteria for exocytotic release. In this regard the origin (neuronal vs. astroglial, synaptic vs. extrasynaptic) of glutamate and GABA collected by microdialysis as well as in the ECF itself, is still a matter of debate. In this overview microdialysis of GABA and glutamate and the use of microsensors to detect extracellular glutamate are compared and discussed. During basal conditions glutamate in microdialysates is mainly derived from non-synaptic sources. Indeed recently several sources of astrocytic glutamate release have been described, including glutamate derived from gliotransmission. However during conditions of (chemical, electrical or behavioral) stimulation a significant part of glutamate might be derived from neurotransmission. Interestingly accumulating evidence suggests that glutamate determined by microsensors is more likely to reflect basal synaptic events. This would mean that microdialysis and microsensors are complementary methods to study extracellular glutamate. Regarding GABA we concluded that the chromatographic conditions for the separation of this transmitter from other amino acid-derivatives are extremely critical. Optimal conditions to detect GABA in microdialysis samples--at least in our laboratory--include a retention time of approximately 60 min and a careful control of the pH of the mobile phase. Under these conditions it appears that 50-70% of GABA in dialysates is derived from neurotransmission.

Evaluation of hydrogel-coated glutamate microsensors

Glutamate microsensors form a promising analytical tool for monitoring neuronally derived glutamate directly in the brain. However, when a microsensor is implanted in brain tissue, many factors can diminish its performance. Consequently, a thorough characterization and evaluation of a microsensor is required concerning all factors that may possibly be encountered in vivo. The present report deals with the validation of a hydrogel-coated glutamate microsensor. This microsensor is constructed by coating a carbon fiber electrode (10-microm diameter; 300-500 microm long) with a five-component redox hydrogel, in which L-glutamate oxidase, horseradish peroxidase, and ascorbate oxidase are wired via poly(ethylene glycol) diglycidyl ether to an osmium-containing redox polymer. A thin Nafion coating completes the construction. Although this microsensor was previously used in vivo, information concerning its validation is limited. In the present study, attention was given to its selectivity, specificity, calibration, oxygen dependency, biofouling, operating potential dependency, and linear range. In addition, successful microsensor experiments in microdialysate, in vitro (in organotypic hippocampal slice cultures), and in vivo (in anesthesized rats) are shown.

In vivo monitoring of extracellular glutamate in the brain with a microsensor

Recent discoveries have revealed that glutamatergic neurotransmission in the central nervous system is mediated by a dynamic interplay between neurons and astrocytes. To enhance our understanding of this process, the study of extracellular glutamate is crucial. At present, microdialysis is the most frequently used analytical technique to monitor extracellular glutamate levels directly in the brain. However, the neuronal and physiological origin of the detected glutamate levels is questioned as they do not fulfil the classical release criteria for exocytotic release, such as calcium dependency or response to the sodium channel blocker tetrodotoxine (TTX). It is hypothesized that an analytical technique with a higher spatial and temporal resolution is required. Glutamate microsensors provide a promising analytical solution to meet this requirement. In the present study, we applied a 10 micro m diameter hydrogel-coated glutamate microsensor to monitor extracellular glutamate levels in the striatum of anesthetized rats. To explore the potential of the microsensor, different pharmacological agents were injected in the vicinity of the sensor at an approximate distance of 100 micro m. It was observed that KCl, exogenous glutamate, kainate and the reuptake inhibitor DL-threo-beta-benzyloxyaspartate (DL-TBOA) increased the extracellular glutamate levels significantly. TTX decreased the basal extracellular glutamate levels approximately 90%, which indicates that the microsensor is capable of detecting neuronally derived glutamate. This is one of the first studies in which a microsensor is applied in vivo on a routine base, and it is concluded that microsensor research can contribute significantly to improve our understanding of the physiology of glutamatergic neurotransmission in the brain.

Oxygen tolerance of an implantable polymer/enzyme composite glutamate biosensor displaying polycation-enhanced substrate sensitivity

Biosensors were fabricated at neutral pH by sequentially depositing the polycation polyethyleneimine (PEI), the stereoselective enzyme L-glutamate oxidase (GluOx) and the permselective barrier poly-ortho-phenylenediamine (PPD) onto 125-microm diameter Pt wire electrodes (Pt/PEI/GluOx/PPD). These devices were calibrated amperometrically at 0.7 V versus SCE to determine the Michaelis-Menten parameters for enzyme substrate, l-glutamate (Glu) and co-substrate, dioxygen. The presence of PEI produced a 10-fold enhancement in the detection limit for Glu (approximately 20 nM) compared with the corresponding PEI-free configurations (Pt/GluOx/PPD), without undermining their fast response time (approximately 2 s). Most remarkable was the finding that, although some designs of PEI-containing biosensors showed a 10-fold increase in linear region sensitivity to Glu, their oxygen dependence remained low.

Application of rapid-sampling, online microdialysis to the monitoring of brain metabolism during aneurysm surgery

OBJECTIVE

To introduce rapid-sampling microdialysis for the early detection of adverse metabolic changes in tissue at risk during aneurysm surgery.

METHODS

A microdialysis catheter was inserted under direct vision into at-risk cortex at the start of surgery. This monitoring was sustained throughout the course of the operation, during which intraoperative events, for example, temporary arterial occlusion or lobe retraction, were precisely documented. A continuous online flow of dialysate was fed into a mobile bedside glucose and lactate analyser. This comprises flow-injection dual-assay enzyme-based biosensors capable of determining values of metabolites every 30 seconds.

RESULTS

Eight patients underwent clipping or wrapping of intracranial aneurysms and were monitored. Time between events and detection: 9 minutes. Mean change in metabolite value +/- standard deviation: temporal lobe retraction lactate, +656 +/- 562 micromol/L (n = 7, P < 0.05); glucose, -123 +/- 138 micromol/L (n = 6, P = 0.08). Glucose intravenous bolus infusion glucose, +512 +/- 244 micromol/L (n = 5, P < 0.01); peak at mean time after bolus, 16 minutes. Temporary proximal clip lactate, +731 +/- 346 micromol/L (n = 6, P < 0.01); glucose, -139 +/- 96 micromol/L (n = 5, P < 0.05); mean clip time, 8.6 minutes.

CONCLUSION

The technique detects changes 9 minutes after intraoperative events occur (limited only by probe-to-sensor tubing length and dialysate flow rate). This provides reliable information to the surgeon and anesthetist promptly. It is a useful method for monitoring glucose and lactate in dialysate, particularly when rapid, transient changes in brain analyte levels need to be determined and the alternative offline methodology would be inadequate.

The role of lactate in brain metabolism

According to the astrocyte-neurone-lactate shuttle (ANLS) hypothesis, activated neurones use lactate released by astrocytes as their energy substrate. The hypothesis, based largely on in vitro experiments, postulates that lactate is derived from the uptake by astrocytes of synaptically released glutamate. The time course of changes in lactate, derived from in vivo experiments, is incompatible with the ANLS model. Neuronal activation leads to a delayed rise in lactate followed by a slow decay, which greatly outlasts the period of neuronal activation. The present review proposes that the uptake of stimulated glutamate release from astrocytes, rather than synaptically released glutamate, is the source of lactate released following neuronal activation. This rise in lactate occurs too late to provide energy for neuronal activity. Furthermore, there is no evidence that lactate undergoes local oxidative phosphorylation. In conclusion, under physiological conditions, there is no evidence that lactate is a significant source of energy for activated neurones.

In vivo neurochemical monitoring and the study of behaviour

In vivo neurochemical monitoring techniques measure changes in the extracellular compartment of selected brain regions. These changes reflect the release of chemical messengers and intermediates of brain energy metabolism resulting from the activity of neuronal assemblies. The two principal techniques used in neurochemical monitoring are microdialysis and voltammetry. The presence of glutamate in the extracellular compartment and its pharmacological characteristics suggest that it is released from astrocytes and acts as neuromodulator rather than a neurotransmitter. The changes in extracellular noradrenaline and dopamine reflect their role in the control of behaviour. Changes in glucose and oxygen, the latter a measure of local cerebral blood flow, reflect synaptic processing in the underlying neuronal networks rather than a measure of efferent output from the brain region. In vivo neurochemical monitoring provides information about the intermediate processing that intervenes between the application of the stimulus and the resulting behaviour but does not reflect the final efferent output that leads to behaviour.

Improving Glutamate Microsensors by Optimizing the Composition of the Redox Hydrogel

Amperometric hydrogel-coated glutamate microsensors form a promising concept to detect glutamate levels directly in brain tissue. These microsensors are constructed by coating a carbon fiber electrode (CFE) (10 microm diameter; 300-500 microm long) with a five-component redox-hydrogel, in which L-glutamate oxidase, horseradish peroxidase, and ascorbate oxidase are wired via poly(ethylene glycol) diglycidyl ether to an osmium-containing redox polymer. Coating with a thin Nafion film completes the construction. Prior to use in vivo, a reliable and reproducible construction of microsensors with a high performance is required. For an optimal microsensor performance, the balance between the five individual hydrogel components is critical. However, due to their small size, hydrogel application to CFE's need to be performed by dip-coating. Dip-coating is a difficult procedure to control and does not allow individual application of hydrogel constituents. To improve the microsensor construction and to better control the dip-coating procedure, we have recently developed an automated device. Throughout this study, automatic dip-coating was performed with premixed solutions, in which the amount of a single component was varied. This allowed us to optimize the hydrogel composition, which resulted in a significant improvement of the microsensor properties in terms of sensitivity, current density, linearity, detection limit, and interference by ascorbic acid.

Improving the reproducibility of hydrogel-coated glutamate microsensors by using an automated dipcoater

Hydrogel-coated microsensors based on carbon fiber electrodes (CFEs) are promising tools for in vivo analysis of endogeneous compounds such as glutamate. However, their construction generally depends on manual fabrication, which often results in poor reproducibility. The aim of this study was to improve the reproducibility and performance of glutamate microsensors. CFEs (10 microm diameter, 300-500 microm long) were coated with a cross-linked redox-polymer hydrogel containing l-glutamate oxidase, horseradish peroxidase and ascorbate oxidase. Various parameters that are likely to influence the reproducibility of the glutamate microsensors were studied. It appeared that the most crucial step in determining the microsensor performance is the manual hydrogel-application procedure. To control this procedure an automated dipcoater was constructed, which allowed mechanical application of the hydrogel on the CFE under standardized conditions. Significant improvements in performance were seen when the CFEs were dipcoated for 10 min at 37 degrees C. Further improvements were obtained when the automated hydrogel application was combined with other cross-link methods, such as electrodeposition and electrostatic complexation. A crucial factor in determining the microsensor performance is the hydrogel thickness. Microscopic observations revealed that, despite the use of an automated dipcoater, the layer thickness was not constant. By combining the automated dipcoat technique with amperometry, the layer thickness could be indirectly monitored and controlled, which resulted in significant improvements of the reproducibility of the sensors.

Voltammetric study of the control of striatal dopamine release by glutamate

The central dopamine systems are involved in several aspects of normal brain function and are implicated in a number of human disorders. Hence, it is important to understand the mechanisms that control dopamine release in the brain. The striatum of the rat receives both dopaminergic and glutamatergic projections that synaptically target striatal neurons but not each other. Nevertheless, these afferents do form frequent appositional contacts, which has engendered interest in the question of whether they communicate with each other despite the absence of a direct synaptic connection. In this study, we used voltammetry in conjunction with carbon fiber microelectrodes in anesthetized rats to further examine the effect of the ionotropic glutamate antagonist, kynurenate, on extracellular dopamine levels in the striatum. Intrastriatal infusions of kynurenate decreased extracellular dopamine levels, suggesting that glutamate acts locally within the striatum via ionotropic receptors to regulate the basal extracellular dopamine concentration. Infusion of tetrodotoxin into the medial forebrain bundle or the striatum did not alter the voltammetric response to the intrastriatal kynurenate infusions, suggesting that glutamate receptors control a non-vesicular release process that contributes to the basal extracellular dopamine level. However, systemic administration of the dopamine uptake inhibitor, nomifensine (20 mg/kg i.p.), markedly decreased the amplitude of the response to kynurenate infusions, suggesting that the dopamine transporter mediates non-vesicular dopamine release. Collectively, these findings are consistent with the idea that endogenous glutamate acts locally within the striatum via ionotropic receptors to control a tonic, impulse-independent, transporter-mediated mode of dopamine release. Although numerous prior in vitro studies had suggested that such a process might exist, it has not previously been clearly demonstrated in an in vivo experiment.

Discovery and Neurochemical Screening of Peptides in Brain Extracellular Fluid by Chemical Analysis of in Vivo Microdialysis Samples

Endogenous peptides from brain extracellular fluid of live rats were analyzed using capillary liquid chromatography (LC)-tandem mass spectrometry (MS2). A 4-mm-long microdialysis probe perfused at 0.6 microL/min implanted into the striatum of anesthetized male rats was used to collect 3.6 microL dialysate fractions that were injected on-line into the capillary LC-MS2 system for analysis. A total of 3349 MS2 spectra were collected from 13 different animals under basal conditions and during localized depolarization evoked by infusion of a high-K+ solution through the microdialysis probe. Subtractive analysis revealed a total of 859 MS2 spectra that were observed only during depolarization. From these spectra, 29 peptide sequences (25 were peptides not previously observed) from 6 different protein precursors were identified using database searching software. Proteins identified include precursors to neuropeptides, synaptic proteins, blood proteins, and transporters. The identified peptides represent candidates for neurotransmitters, neuromodulators, and markers of synaptic activity or brain tissue damage. A screen for neuroactivity of novel proenkephalin fragments that were found was performed by infusing the peptides into the brain while monitoring amino acid neurotransmitters by microdialysis sampling combined with capillary electrophoresis. Three of the six tested peptides evoked significant increases in various neuroactive amino acids. These results demonstrate that this combination of methods can identify novel neurotransmitter candidates and screen for potential neuroactivity.

Haloperidol impairs auditory filial imprinting and modulates monoaminergic neurotransmission in an imprinting-relevant forebrain area of the domestic chick

In vivo microdialysis and behavioural studies in the domestic chick have shown that glutamatergic as well as monoaminergic neurotransmission in the medio-rostral neostriatum/hyperstriatum ventrale (MNH) is altered after auditory filial imprinting. In the present study, using pharmaco-behavioural and in vivo microdialysis approaches, the role of dopaminergic neurotransmission in this juvenile learning event was further evaluated. The results revealed that: (i) the systemic application of the potent dopamine receptor antagonist haloperidol (7.5 mg/kg) strongly impairs auditory filial imprinting; (ii) systemic haloperidol induces a tetrodotoxin-sensitive increase of extracellular levels of the dopamine metabolite, homovanillic acid, in the MNH, whereas the levels of glutamate, taurine and the serotonin metabolite, 5-hydroxyindole-3-acetic acid, remain unchanged; (iii) haloperidol (0.01, 0.1, 1 mm) infused locally into the MNH increases glutamate, taurine and 5- hydroxyindole-3-acetic acid levels in a dose-dependent manner, whereas homovanillic acid levels remain unchanged; (iv) systemic haloperidol infusion reinforces the N-methyl-d-aspartate receptor-mediated inhibitory modulation of the dopaminergic neurotransmission within the MNH. These results indicate that the modulation of dopaminergic function and its interaction with other neurotransmitter systems in a higher associative forebrain region of the juvenile avian brain displays similar neurochemical characteristics as the adult mammalian prefrontal cortex. Furthermore, we were able to show that the pharmacological manipulation of monoaminergic regulatory mechanisms interferes with learning and memory formation, events which in a similar fashion might occur in young or adult mammals.

Continuous monitoring of intracerebral glutamate levels in awake monkeys using microdialysis and enzyme fluorometric detection

A technique for continuous on-line detection of glutamate using brain microdialysis in awake primates is described. The method is based on an enzymatic assay using fluorescence detection of glutamate. The time resolution of the continuous fluorescent readout compares favorably with that of most published studies, which have used standard high-pressure liquid chromatography detection methods for glutamate. Exposure of the system to other amino acids (GABA, aspartate, glutamine, ascorbate, taurine, valine, alanine, and D-glutamate) revealed that this method is highly specific for L-glutamate. In vitro, the system detects reliably glutamate levels as low as 0.5 micromol/l. In vivo testing in the striatum of Rhesus macaques showed that glutamate levels were enhanced after reverse microdialysis with a glutamate uptake blocker. Stimulation with high potassium increased substantially the levels of glutamate, an effect that was calcium-dependent. Glutamate levels were also increased when the microdialysis solution contained the blocker of voltage-gated potassium channels, 4-aminopyridine. This technique effectively detects short-term changes in glutamate levels evoked by physiologic or pharmacologic manipulations in the primate brain.

Changes in dialysate concentrations of glutamate and GABA in the brain: an index of volume transmission mediated actions?

Brain microdialysis has become a frequently used method to study the extracellular concentrations of neurotransmitters in specific areas of the brain. For years, and this is still the case today, dialysate concentrations and hence extracellular concentrations of neurotransmitters have been interpreted as a direct index of the neuronal release of these specific neurotransmitter systems. Although this seems to be the case for neurotransmitters such as dopamine, serotonin and acetylcholine, the extracellular concentrations of glutamate and GABA do not provide a reliable index of their synaptic exocytotic release. However, many microdialysis studies show changes in extracellular concentrations of glutamate and GABA under specific pharmacological and behavioural stimuli that could be interpreted as a consequence of the activation of specific neurochemical circuits. Despite this, we still do not know the origin and physiological significance of these changes of glutamate and GABA in the extracellular space. Here we propose that the changes in dialysate concentrations of these two neurotransmitters found under specific treatments could be an expression of the activity of the neurone-astrocyte unit in specific circuits of the brain. It is further proposed that dialysate changes of glutamate and GABA could be used as an index of volume transmission mediated actions of these two neurotransmitters in the brain. This hypothesis is based firstly on the assumption that the activity of neurones is functionally linked to the activity of astrocytes, which can release glutamate and GABA to the extracellular space; secondly, on the existence of extrasynaptic glutamate and GABA receptors with functional properties different from those of GABA receptors located at the synapse; and thirdly, on the experimental evidence reporting specific electrophysiological and neurochemical effects of glutamate and GABA when their levels are increased in the extracellular space. According to this concept, glutamate and GABA, once released into the extracellular compartment, could diffuse and have long-lasting effects modulating glutamatergic and/or GABAergic neurone-astrocytic networks and their interactions with other neurotransmitter neurone networks in the same areas of the brain.

Effects of ethanol on the accumbal output of dopamine, GABA and glutamate in alcohol-tolerant and alcohol-nontolerant rats

Effects of ethanol on the accumbal extracellular concentrations of dopamine, as well as of the amino acid transmitters gamma-amino butyric acid (GABA), glutamate and taurine, were studied in the alcohol-insensitive (alcohol-tolerant, AT) and alcohol-sensitive (alcohol-nontolerant, ANT) rats selected for low and high sensitivity to ethanol-induced motor impairment. Ethanol (2 or 3 g/kg ip) enhanced the output of dopamine and its metabolites in freely moving rats of both lines as measured by in vivo microdialysis. The effect of ethanol on the metabolites of dopamine tended to be stronger in the ANT rats. The smaller dose of ethanol decreased the output of GABA only in the AT rats, whereas the larger dose of ethanol decreased the output of GABA in rats of both lines to a similar degree. Ethanol at the dose of 2 g/kg slightly, but statistically, significantly decreased the output of glutamate in rats of both lines, but the larger dose of ethanol decreased the output of glutamate only in the AT rats. Ethanol at the dose of 2 g/kg induced a small transient increase in the output of taurine within 2 h after its administration in rats of both lines, but the larger dose of ethanol was without significant effect. These results confirm the previous findings that ethanol suppresses the release of GABA more in the AT than ANT rats. Thus, among the neurotransmitter systems we studied, the effects of ethanol might be the most relevant on GABAergic transmission regarding the sensitivity towards ethanol. However, our findings suggest that glutamate is also involved in this respect.

The origin and neuronal function of in vivo nonsynaptic glutamate

Basal extracellular glutamate sampled in vivo is present in micromolar concentrations in the extracellular space outside the synaptic cleft, and neither the origin nor the function of this glutamate is known. This report reveals that blockade of glutamate release from the cystine-glutamate antiporter produced a significant decrease (60%) in extrasynaptic glutamate levels in the rat striatum, whereas blockade of voltage-dependent Na+ and Ca2+ channels produced relatively minimal changes (0-30%). This indicates that the primary origin of in vivo extrasynaptic glutamate in the striatum arises from nonvesicular glutamate release by the cystine-glutamate antiporter. By measuring [35S]cystine uptake, it was shown that similar to vesicular release, the activity of the cystine-glutamate antiporter is negatively regulated by group II metabotropic glutamate receptors (mGluR2/3) via a cAMP-dependent protein kinase mechanism. Extracellular glutamate derived from the antiporter was shown to regulate extracellular levels of glutamate and dopamine. Infusion of the mGluR2/3 antagonist (RS)-1-amino-5-phosphonoindan-1-carboxylic acid (APICA) increased extracellular glutamate levels, and previous blockade of the antiporter prevented the APICA-induced rise in extracellular glutamate. This suggests that glutamate released from the antiporter is a source of endogenous tone on mGluR2/3. Blockade of the antiporter also produced an increase in extracellular dopamine that was reversed by infusing the mGluR2/3 agonist (2R,4R)-4-aminopyrrolidine-2,4-dicarboxlylate, indicating that antiporter-derived glutamate can modulate dopamine transmission via mGluR2/3 heteroreceptors. These results suggest that nonvesicular release from the cystine-glutamate antiporter is the primary source of in vivo extracellular glutamate and that this glutamate can modulate both glutamate and dopamine transmission.

The origin and neuronal function of in vivo nonsynaptic glutamate

Basal extracellular glutamate sampled in vivo is present in micromolar concentrations in the extracellular space outside the synaptic cleft, and neither the origin nor the function of this glutamate is known. This report reveals that blockade of glutamate release from the cystine-glutamate antiporter produced a significant decrease (60%) in extrasynaptic glutamate levels in the rat striatum, whereas blockade of voltage-dependent Na+ and Ca2+ channels produced relatively minimal changes (0-30%). This indicates that the primary origin of in vivo extrasynaptic glutamate in the striatum arises from nonvesicular glutamate release by the cystine-glutamate antiporter. By measuring [35S]cystine uptake, it was shown that similar to vesicular release, the activity of the cystine-glutamate antiporter is negatively regulated by group II metabotropic glutamate receptors (mGluR2/3) via a cAMP-dependent protein kinase mechanism. Extracellular glutamate derived from the antiporter was shown to regulate extracellular levels of glutamate and dopamine. Infusion of the mGluR2/3 antagonist (RS)-1-amino-5-phosphonoindan-1-carboxylic acid (APICA) increased extracellular glutamate levels, and previous blockade of the antiporter prevented the APICA-induced rise in extracellular glutamate. This suggests that glutamate released from the antiporter is a source of endogenous tone on mGluR2/3. Blockade of the antiporter also produced an increase in extracellular dopamine that was reversed by infusing the mGluR2/3 agonist (2R,4R)-4-aminopyrrolidine-2,4-dicarboxlylate, indicating that antiporter-derived glutamate can modulate dopamine transmission via mGluR2/3 heteroreceptors. These results suggest that nonvesicular release from the cystine-glutamate antiporter is the primary source of in vivo extracellular glutamate and that this glutamate can modulate both glutamate and dopamine transmission.

Activation of skeletal muscle afferents evokes release of glutamate in the subretrofacial nucleus (SRF) of cats

The subretrofacial nucleus (SRF) is a region of the rostral ventrolateral medulla known to play a crucial role in sympathoexcitation. SRF neurons send direct projections to the intermediolateral cell columns of the spinal cord where they form synaptic contact with preganglionic sympathetic motor neurons. Activation of this neural pathway increases sympathetic outflow to the heart and blood vessels affecting cardiac function and vasomotor tone. Previous studies utilizing electrophysiological recording techniques and c-Fos expression have established that the activity of SRF neurons is increased during skeletal muscle contraction. However, the excitatory neurotransmitter mediating this increased activity remains in question. In the present study, static contraction of the triceps surae was induced by electrical stimulation of L7 and S1 ventral roots in anesthetized cats (n=12). Endogenous release of glutamate (Glu) from the SRF was recovered by microdialysis and measured by HPLC. Static muscle contraction for 4 min increased mean arterial pressure (MAP) 38+/-4 mmHg from a control level of 102+/-12 mmHg (P< 0.05). During muscle contraction the extracellular concentration of Glu recovered from the SRF increased from 623+/-117 to 1078+/-187 nM (P<0.05). To determine the effect of muscle contraction on Glu release in the absence of synaptic input from other reflexogenic areas, contraction was repeated following acute sinoaortic denervation and vagotomy. Following this denervation, muscle contraction increased MAP 41+/- 4 mmHg (P < 0.05) and Glu concentration from 635+/-246 to 1106+/-389 nM (P < 0.05). Muscle paralysis prevented the increases in MAP and Glu concentration during ventral root stimulation. These results suggest that: (i) Glu is released in the SRF during activation of contraction-sensitive skeletal muscle afferent fibers in the cat; and (ii) synaptic input from other reflexogenic areas appears to be ineffective in modulating the release of Glu in the SRF during static muscle contraction.

Microdialysis coupled to online enzymatic assays

In vivo sampling of interstitial fluid by using microdialysis fibers has become a standard and accepted procedure. This sampling method is generally coupled to offline analysis of consecutive dialysate samples by high-performance liquid chromatography or capillary electrophoresis, but this combination is not the best approach for some applications, especially those which require high temporal resolution and rapid data collection. The purpose of this review is to provide information on enzyme-based online assays, i.e., continuous analysis of the dialysate as it emerges from the outlet of the sampling device. We have focused on methods developed specifically for the analysis of solutions perfused at a very slow flow rate, i.e., a feature of microdialysis and ultrafiltration techniques. These methods include flow enzyme-fluorescence assays, flow enzyme-amperometric assays, and sequential enzyme-amperometric detection. Each type of assay is discussed in terms of principle, applications, advantages, and limitations. We also comment on implantable biosensors, an obvious next step forward for in vivo monitoring of molecules in neuroscience.

Effect of ionotropic glutamate receptors antagonists on the modifications in extracellular glutamate and aspartate levels during picrotoxin seizures: a microdialysis study in freely moving rats

Our previous studies have shown a local decrease in glutamate and aspartate levels during seizures, induced by picrotoxin microdialysis in the hippocampus of chronic freely moving rats. In this paper, we study the effect of continuous hippocampal microperfusion of the NMDA, AMPA and kainate glutamate receptor inhibitors 5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5, 10-imine (MK-801); 6,7-dinitroquinoxaline-2,3-dione (DNQX), and 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride (GYKI 52466). We also examine the action of L(-)-threo-3-hydroxyaspartic acid (THA), a glutamate and aspartate reuptake blocker, on the modification of extracellular glutamate and aspartate levels induced by picrotoxin, using the microdialysis method in freely moving rats. We found that changes in extracellular hippocampal concentrations in both amino acids are prevented by NMDA, AMPA and kainate receptor inhibitors. Seizures elicited under DNQX also induce a transient increase in aspartate extracellular levels coincident with seizure time. L(-)-threo-3-hydroxyaspartic acid increased the basal extracellular concentrations of both amino acids, but did not prevent the seizure-related decrease. Our results suggest that glutamate, the major neurotransmitter at the synaptic level, may also play an important role in non-synaptic transmission during seizures.

Parabrachial nucleus induces suppression of baroreflex bradycardia by the release of glutamate in the rostral ventrolateral medulla of the rat

The involvement of glutamatergic neurotransmission in the rostral ventrolateral medulla (RVLM) in the suppression of baroreflex bradycardia by the parabrachial nucleus (PBN) was investigated. Repeated electrical activation of the PBN increased the concentration of glutamate in the dialysate collected from the RVLM. The same stimulation also suppressed baroreflex bradycardia in response to transient hypertension evoked by phenylephrine (5 microg/kg, intravenously). Microinfusion of L-glutamate (10, 50 or 100 microM) via the microdialysis probe into the RVLM dose-dependently elicited a significant inhibition of baroreflex bradycardia that paralleled the concentration and time course of the PBN-elicited elevation in extracellular glutamate in the RVLM. The suppression of baroreflex bradycardia elicited by microinjection of L-glutamate (1 nmol) into the RVLM was appreciably reversed by coinjection of the NMDA receptor antagonist, dizocilpine (500 pmol), or the non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2, 3-dione (50 pmol). These results suggest that an increase in the extracellular concentration of glutamate and activation of both NMDA and non-NMDA receptors in the RVLM may mediate the suppression of baroreflex bradycardia by activation of the PBN.

Excitotoxicity in neurological disorders--the glutamate paradox

Beneficial effects of glutamate-receptor antagonists in models of neurological disorders are often used to support the notion that endogenous excitotoxicity (i.e. resulting from extracellular accumulation of endogenous glutamate) is a major contributor to neuronal death associated with these conditions. However, this interpretation conflicts with a number of robust and important experimental evidence. Here, emphasis is placed on two key elements: (i) very high extracellular levels of glutamate must be reached to initiate neuronal death, far above those measured in models of neurological disorders; and (ii) changes in extracellular glutamate as measured by microdialysis are not related to changes in the synaptic cleft, i.e. the compartment where neurotransmitter glutamate interacts with its receptors. It has become clear that the diversity and complexity of glutamate-mediated processes allow for a wide range of potential abnormalities (e.g. loss of selectivity of glutamate-operated ion channels, abnormal modulation of glutamate receptors). In addition, as neuronal death subsequent to ischemia and other insults is likely to result from multifactorial processes that may be inter-related, inhibition of glutamate-mediated synaptic transmission may be neuroprotective by increasing the resistance of neurons to other deleterious mechanisms (e.g. inadequate energy supply) that are not directly related to glutamatergic transmission.

High extracellular glutamate and neuronal death in neurological disorders. Cause, contribution or consequence?

In models of neurological disorders, increased extracellular glutamate and beneficial effects produced by glutamate-receptor antagonists are consistently taken as supporting evidence of excitotoxicity. This systematic interpretation is over-simplified and potentially misleading. High extracellular glutamate is not a reliable indicator of endogenous excitotoxicity, i.e., the intrinsic, potential neurotoxicity of endogenous glutamate whenever it accumulates extracellularly. Firstly, because the extracellular levels of glutamate necessary to produce depolarization and death in vivo, are far above those measured in models of neurological disorders. Secondly, because changes in the concentration of glutamate in the synaptic cleft (i.e., the relevant compartment for endogenous excitotoxicity) are not reflected extracellularly. Protection by glutamate-receptor antagonists does not necessarily imply inhibition of excitotoxic abnormalities. Indeed, neuronal death initiated by insults such as ischemia results from multifactorial processes that may be interrelated. Therefore, beneficial effects resulting from an interaction with glutamate-mediated transmission may actually render the cell more resistant to other deleterious mechanisms (e.g., mitochondrial injury, oxidative stress).

Presence of NMDA-type glutamate receptors in cingulate corticostriatal terminals and their postsynaptic targets

The glutamatergic projection from the anterior cingulate cortex to the medial caudate-putamen nucleus (CPN) has been implicated in motor and cognitive functions, many of which are potently modulated by activation of N-methyl-D-aspartate subtype of glutamate receptors (NMDARs). To determine the functional sites for NMDAR activation within this circuitry, we combined anterograde transport of biotinylated dextran amine (BDA) from deep layers of the rat anterior cingulate cortex with immunogold labeling of NMDAR subunit, NMDAR1, in the dorsomedial CPN. BDA-containing axons were seen in patch-like clusters in a neuropil that showed more uniform immunogold-silver labeling for NMDAR1. Electron microscopy of these regions showed that BDA-labeling was present exclusively in axons and terminals, 23% (98 of 421) of which also contained NMDAR1-immunoreactivity (IR). BDA-labeled terminals often apposed NMDAR1-immunoreactive neuronal and glial profiles. These terminals also formed asymmetric excitatory-type synapses with dendritic spines. Of 155 anterogradely labeled axon terminals forming asymmetric synapses, 34% were with NMDAR1-labeled, and 66% with unlabeled dendritic spines. These results provide ultrastructural evidence for the involvement of NMDARs in presynaptic regulation of glutamate transmission, and in postsynaptic modulation of the excitability of spiny neurons in patch-like compartments of the dorsomedial CPN. These dual NMDAR-mediated actions are likely to play a major role in the acquisition of new behaviors and reward-related processes that have been associated with cortical input to the striatal patch compartments.

The role of astrocytes and noradrenaline in neuronal glucose metabolism

In the classical model the energy requirements during neuronal activation are provided by the delivery of additional glucose directly into the extracellular compartment that results from the increase in local cerebral blood flow (rCBF). The present review proposes that astrocytes play a key role in the response to neuronal activation. Arginine for the synthesis of NO, which has a major role in the increase in rCBF, is released from astrocytes in response to stimulation of astrocytic glutamate receptors. The increased delivery of glucose by the blood stream enters astrocytes, where some of it is converted to glycogen. During neuronal activation there is a decrease in extracellular glucose owing to increased utilization followed by a delayed increase; this results from stimulation of astrocytic beta-adrenergic receptors, which leads to a breakdown of glycogen and the export of glucose.