Extracellular glutamate and dopamine measured by microdialysis in the rat striatum during blockade of synaptic transmission in anesthetized and awake rats

Dysregulation of Decision Making Related to Metabotropic Glutamate 5, but Not Midbrain D3, Receptor Availability Following Cocaine Self-administration in Rats

BACKGROUND

Compulsive patterns of drug use are thought to be the consequence of drug-induced adaptations in the neural mechanisms that enable behavior to be flexible. Neuroimaging studies have found evidence of robust alterations in glutamate and dopamine receptors within brain regions that are known to be critical for decision-making processes in cocaine-dependent individuals, and these changes have been argued to be the consequence of persistent drug use. The causal relationships among drug-induced alterations, cocaine taking, and maladaptive decision-making processes, however, are difficult to establish in humans.

METHODS

We assessed decision making in adult male rats using a probabilistic reversal learning task and used positron emission tomography with the [¹¹C]-(+)-PHNO and [¹⁸F]FPEB radioligands to quantify regional dopamine D_2/3 and metabotropic glutamate 5 (mGlu5) receptor availability, respectively, before and after 21 days of cocaine or saline self-administration. Tests of motivation and relapse-like behaviors were also conducted.

RESULTS

We found that self-administration of cocaine, but not of saline, disrupted behavior in the probabilistic reversal learning task measured by selective impairments in negative-outcome updating and also increased cortical mGlu5 receptor availability following 2 weeks of forced abstinence. D_2/3 and, importantly, midbrain D₃ receptor availability was not altered following 2 weeks of abstinence from cocaine. Notably, the degree of the cocaine-induced increase in cortical mGlu5 receptor availability was related to the degree of disruption in negative-outcome updating.

CONCLUSIONS

These findings suggest that cocaine-induced changes in mGlu5 signaling may be a mechanism by which disruptions in negative-outcome updating emerge in cocaine-dependent individuals.

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.

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.

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.

Mu opioid receptor modulation of somatodendritic dopamine overflow: GABAergic and glutamatergic mechanisms

Mu opioid receptor (MOR) regulation of somatodendritic dopamine neurotransmission in the ventral tegmental area (VTA) was investigated using conventional microdialysis in freely moving rats and mice. Reverse dialysis of the MOR agonist DAMGO (50 and 100 microm) into the VTA of rats produced a concentration-dependent increase in dialysate dopamine concentrations. Basal dopamine overflow in the VTA was unaltered in mice lacking the MOR gene. However, basal gamma-aminobutyric acid (GABA) overflow in these animals was significantly increased, whereas glutamate overflow was decreased. Intra-VTA perfusion of DAMGO into wild-type (WT) mice increased dopamine overflow. GABA concentrations were decreased, whereas glutamate concentrations in the VTA were unaltered. Consistent with the loss of MOR, no effect of DAMGO was observed in MOR knockout (KO) mice. These data provide the first direct demonstration of tonically active MOR systems in the VTA that regulate basal glutamatergic and GABAergic neurotransmission in this region. We hypothesize that increased GABAergic neurotransmission following constitutive deletion of MOR is due to the elimination of a tonic inhibitory influence of MOR on GABAergic neurons in the VTA, whereas decreased glutamatergic neurotransmission in MOR KO mice is a consequence of intensified GABA tone on glutamatergic neurons and/or terminals. As a consequence, somatodendritic dopamine release is unaltered. Furthermore, MOR KO mice do not exhibit the positive correlation between basal dopamine levels and the glutamate/GABA ratio observed in WT mice. Together, our findings indicate a critical role of VTA MOR in maintaining an intricate balance between excitatory and inhibitory inputs to dopaminergic neurons.

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.

Functionalized conducting polymer as an enzyme-immobilizing substrate: an amperometric glutamate microbiosensor for in vivo measurements

The functionalized conducting polymer (CP) of 5, 2':5', 2' '-terthiophene-3'-carboxylic acid on a platinum microelectrode was prepared through the electropolymerization process using cyclic voltammetry and was used as a substrate for the immobilization of enzymes. The nanoparticles of the CP were obtained at a high scan rate in the cyclic voltammetric experiment. A needle-type amperometric glutamate microbiosensor based on the covalent immobilization of glutamate oxidase (GlOx) onto the CP layer was fabricated for in vivo measurements. The surfaces of the CP/Pt and GlOx/CP/Pt were characterized by QCM, ESCA, and AFM. The biosensor efficiently detected glutamate through the oxidation of enzymatically generated H2O2 at approximately +0.45 V versus Ag/AgCl. Various experimental parameters, such as pH, temperature, and the applied potential in the detection step were optimized. The interference effects from other biological compounds were examined, and ascorbate and dopamine interferences were observed, which were completely minimized by coimmobilizing ascorbate oxidase and by coating the sensor surface with a cationic polymer, polyethyleneimine. A linear calibration plot for glutamate was obtained between 0.2 and 100 microM with a detection limit of 0.1 +/- 0.03 microM. The proposed glutamate microbiosensor was successfully used for in vivo monitoring of the extracellular glutamate released by cocaine stimulation.

Purinergic modulation of extracellular glutamate levels in the nucleus accumbens in vivo

In the present study, the P2 receptor-mediated modulation of the extracellular glutamate concentration was investigated by microdialysis in the nucleus accumbens (NAc) of freely moving rats. Because of the known interference of dopaminergic and glutamatergic mechanisms in this area the experiments were performed with animals intra-accumbally treated with 6-hydroxydopamine (6-OHDA) to deplete dopamine pools. Perfusion of the NAc with the prototypic P2 receptor agonist 2-methylthioadenosine 5'-triphosphate (2-MeSATP, 0.1, 1 and 10mM) concentration-dependently increased the extracellular level of glutamate in this area. Pretreatment with the P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 0.1mM) decreased the basal extracellular glutamate concentration and inhibited the 2-MeSATP-induced outflow of glutamate. In rats treated with 6-OHDA, 2-MeSATP increased the total extracellular glutamate to an extent about fivefold larger than in sham-lesioned rats. The perfusion of the dopamine-depleted NAc with the D(2)/D(3) dopamine receptor agonist quinpirole (0.1mM) diminished the basal concentration of glutamate and reduced the effect of 2-MeSATP on the extracellular glutamate. These results provide evidence that the stimulation of P2 receptors is involved in the increase of accumbal extracellular glutamate in vivo. This behaviourally relevant mechanism depends on a dopamine D(2) receptor-mediated tone in the nucleus accumbens. Furthermore, the inhibition of P2 receptors may prevent, at least partly, glutamate-mediated neurodegeneration.

Isoflurane induces dopamine transporter trafficking into the cell cytoplasm

Previous investigations have shown that the binding of a selective hydrophilic positron emission tomography radiotracer for the dopamine transporter (DAT) (2beta-carbomethoxy-3beta-(4-chlorophenyl)-8-(2-18F-fluoroethyl)nortropane) decreased in monkey striatum during deep isoflurane anesthesia. Immunohistochemistry experiments suggested but did not prove that isoflurane induced a decrease in cell surface DAT. The present investigation was undertaken to demonstrate quantitatively the isoflurane-induced internalization of DAT using a rapid and relatively uncomplicated biochemical technique in human embryonic kidney (HEK-293) cells stably expressing the human DAT (h-DAT) protein. Biotinylation followed by Western blot analysis was used to determine the extent of change in cell surface expression of the DAT under control conditions and in the presence of a clinically relevant concentration of isoflurane. Isoflurane treatment for 30 min resulted in a highly significant decrease in the amount of h-DAT on the cell surface (21 +/- 15% of control; P < 0.01) (mean +/- SD; n = 4). These data are consistent with the hypothesis that isoflurane results in internalization of DAT from the cell membrane and further validate our qualitative results reported previously. In addition, the current results confirm the hypothesis that biotinylation can be used to quantitate the extent of disappearance of DAT from the cell surface making dose-response studies and comparisons of DAT internalization with other general anesthetics practical.

Long-term potentiation in the dentate gyrus of the anaesthetized rat is accompanied by an increase in extracellular glutamate: real-time measurements using a novel dialysis electrode

We have used a glutamate-specific dialysis electrode to obtain real-time measurements of changes in the concentration of glutamate in the extracellular space of the hippocampus during low-frequency stimulation and following the induction of long-term potentiation (LTP). In the dentate gyrus, stimulation of the perforant path at 2 Hz for 2 min produced a transient increase in glutamate current relative to the basal value at control rates of stimulation (0.033 Hz). This activity-dependent glutamate current was significantly enhanced 35 and 90 min after the induction of LTP. The maximal 2 Hz signal was obtained during post-tetanic potentiation (PTP). There was also a more gradual increase in the basal level of extracellular glutamate following the induction of LTP. Both the basal and activity-dependent increases in glutamate current induced by tetanic stimulation were blocked by local infusion of the N-methyl-D-aspartate receptor antagonist D-APV. In areas CA1 and CA3 we were unable to detect a 2 Hz glutamate signal either before or after the induction of LTP, possibly owing to a more avid uptake of glutamate in the pyramidal cell fields. These results demonstrate that LTP in the dentate gyrus is associated with a greater concentration of extracellular glutamate following activation of potentiated synapses, either because potentiated synapses release more transmitter per impulse, or because of reduced uptake by glutamate transporters. We present arguments favouring increased release rather than decreased uptake.

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.

Real-time monitoring of brain energy metabolism in vivo using microelectrochemical sensors: the effects of anesthesia

Rats were implanted in the striatum with a Pt/Ir electrode for measurement of regional cerebral blood flow (rCBF) (H(2) clearance technique), a carbon paste electrode for monitoring tissue oxygen and a glucose biosensor for monitoring extracellular glucose. Changes in all three parameters were recorded in response to the intraperitoneal (i.p.) administration of the anesthetics chloral hydrate (350 mg/kg), sodium pentobarbitone (60 mg/kg) and ketamine (200 mg/kg). An i.p. injection of normal saline, given as a control for the injection of the anesthetics, produced a parallel increase in rCBF and tissue oxygen accompanied by a brief decrease in extracellular glucose. Changes in tissue oxygen reflected the changes in rCBF; there was a decrease in both after sodium pentobarbitone, a decrease followed by a rebound after ketamine and a transient increase after chloral hydrate. All three anesthetics produced a decrease in extracellular glucose. The disparity between the changes in glucose and the changes in rCBF and oxygen suggests that during anesthesia, the reduction in extracellular glucose is not due to a reduction in the direct delivery of glucose from the blood vascular system. These results also indicate that levels of enzymatic substrates and mediators, which are intrinsic to the design and operation of amperometric biosensors, are clearly altered in a complex manner by anesthesia and suggest that caution should be exercised in extrapolating data from acute anesthetized experiments to normal physiology.

Glutamate regulates the spontaneous and evoked release of dopamine in the rat striatum

Resting and evoked extracellular dopamine levels in the striatum of the anesthetized rat were measured by fast-scan cyclic voltammetry in conjunction with carbon fiber microelectrodes. Identification of the substance detected in vivo was achieved by inspection of background-subtracted voltammograms. Intrastriatal microinfusion of kynurenate, a broad-spectrum antagonist of ionotropic glutamate receptors, caused a decrease in the resting extracellular level of dopamine. The kynurenate-induced decrease was unaffected by systemic pretreatment with pargyline, an inhibitor of monoamine oxidase, but was significantly attenuated by systemic pretreatment with alpha-methyl-p-tyrosine, an inhibitor of tyrosine hydroxylase. Although glutamate by itself did not affect resting extracellular dopamine levels, glutamate did attenuate the kynurenate-induced decrease. Kynurenate decreased dopamine release in response to electrical stimulation of the medial forebrain bundle, an effect that was also attenuated by glutamate. These results suggest that both spontaneous and evoked dopamine release in the rat striatum are under the local tonic excitatory influence of glutamate. Interactions between central dopamine and glutamate systems that have been implicated in the etiologies of Parkinson's disease, schizophrenia, stress, and substance abuse. The precise nature of those interactions, however, remains a matter of some controversy.

Pharmacological evidence for the selectivity of in vivo signals obtained with enzyme-based electrochemical sensors

Carbon fiber microelectrodes that support enzyme-containing redox polymer gels permit the amperometric detection of glutamate, choline, and glucose. These devices are of interest for in vivo neurochemical monitoring because their small dimensions may permit highly localized measurements within small brain nuclei. In vitro calibration procedures confirm that the sensors respond in a selective fashion towards their respective target analyte. In the current work, the selectivity of the in vivo response of the microsensors during pharmacological manipulations is considered. The response of choline and glucose microsensors during the local infusion of tetrodotoxin and neostigmine in rat striatum is reported. The results of this study support the conclusion that these microsensors respond selectively to their respective targets under in vivo conditions.

Monitoring glutamate and ascorbate in the extracellular space of brain tissue with electrochemical microsensors

This paper describes electrochemical microsensors for the in vivo measurement of glutamate and ascorbate in the extracellular space of brain tissue. To prepare glutamate microsensors, carbon fiber microelectrodes (10 microns in diameter and 300-400 microns long) were modified with a cross-linked redox polymer film containing enzymes. The microsensors were coated with a thin Nafion film before use. The glutamate microsensors were both selective and sensitive toward glutamate, with detection limits in the low micromolar range. Physiologically relevant concentrations of several electroactive compounds found in brain tissue produced no response at the glutamate microsensors and also did not affect their glutamate response, the only exception being glutamine, for which a small response was observed in the absence, but not in the presence, of glutamate. The ascorbate microsensors were used in conjunction with cyclic voltammetry. They were sensitive and selective toward ascorbate, but did exhibit a small sensitivity toward the dopamine metabolite, dihydroxyphenylacetic acid. The in vivo measurements performed establish the ability of the glutamate microsensors to monitor the component of the basal extracellular glutamate level that is derived from the neuronal activity of brain tissue.

Lactic acid-induced increase of extracellular dopamine measured by microdialysis in rat striatum: evidence for glutamatergic and oxidative mechanisms

Striatal lactacidosis was induced by direct lactic acid perfusion to obtain a local pH as close as possible to that observed in ischemia. In a previous study we showed that such lactacidosis produces a diphasic increase in extracellular dopamine (DA). The present work investigated whether DA accumulation is related to a glutamatergic mechanism and/or production of reactive oxygen species (ROS) in the striatum. Concentrations of extracellular DA, glutamate and hydroxyl radicals ((.)OH) were measured in the presence or absence of an N-methyl-D-aspartate (NMDA) receptor blocker (dizocilpine, MK-801) or an antioxidant (Trolox). Measurements were performed using high-performance liquid chromatography (HPLC) with electrochemical and fluorimetric detection on samples obtained by an in vivo microdialysis perfusion technique and stored at -80 degrees C. The increase in lactic acid-induced DA was entirely suppressed by MK-801 and Trolox. Lactacidosis also induced an increase in extracellular glutamate and (.)OH concentrations at the same time as the first DA accumulation, as well as another (.)OH accumulation which preceded and accompanied the second DA concentration peak. Glutamate release was totally inhibited by MK-801 or Trolox. The first peak of (.)OH production was completely suppressed by MK-801 and Trolox, but the second one was only suppressed by Trolox. These data showed that the increase in DA induced by lactic acid was related to glutamatergic excitotoxicity and ROS production, suggested that the kinetic of events was different for the two DA accumulations.

Measurement of cholecystokinin release in vivo in the rat spinal dorsal horn

The microdialysis technique, used to monitor extracellular levels of transmitter substances in the central nervous system of laboratory animals as a reflection of transmitter release, is based on the ability of neurotransmitters to diffuse in the extracellular fluid from the site of release and to cross a semipermeable dialysis membrane. Even though the surgical procedure is not very complicated, the detection of released substances in the recovered dialysate may be difficult. Especially, the measurement of neuropeptide release is limited by the low extracellular concentration and of low recovery as compared to, for example, monoamines. Thus, for example, cholecystokinin (CCK), which is the most abundant neuropeptide in the central nervous system, is found at concentrations that are several orders of magnitude lower than those of classical transmitters. Therefore a highly sensitive detection method is of utmost importance. In the dorsal horn of the spinal cord CCK is found mainly in interneurons and in terminals of descending fibers. CCK seems to be involved in nociceptive transmission and CCK attenuates morphine-induced antinociception. We here describe in vivo microdialysis in the lumbar dorsal horn of the rat with subsequent quantification of the level of CCK-like immunoreactivity (-LI) by a highly sensitive radioimmunoassay.