Characterisation of extracellular amino acids in striatum of freely moving rats by in vivo microdialysis

Collection of nanoliter microdialysate fractions in plugs for off-line in vivo chemical monitoring with up to 2 s temporal resolution

An off-line in vivo neurochemical monitoring approach was developed based on collecting nanoliter microdialysate fractions as an array of "plugs" segmented by immiscible oil in a piece of Teflon tubing. The dialysis probe was integrated with the plug generator in a polydimethlysiloxane microfluidic device that could be mounted on the subject. The microfluidic device also allowed derivatization reagents to be added to the plugs for fluorescence detection of analytes. Using the device, 2 nL fractions corresponding to 1-20 ms sampling times depending upon dialysis flow rate, were collected. Because axial dispersion was prevented between them, each plug acted as a discrete sample collection vial and temporal resolution was not lost by mixing or diffusion during transport. In vitro tests of the system revealed that the temporal resolution of the system was as good as 2 s and was limited by mass transport effects within the dialysis probe. After collection of dialysate fractions, they were pumped into a glass microfluidic chip that automatically analyzed the plugs by capillary electrophoresis with laser-induced fluorescence at 50 s intervals. By using a relatively low flow rate during transfer to the chip, the temporal resolution of the samples could be preserved despite the relatively slow analysis time. The system was used to detect rapid dynamics in neuroactive amino acids evoked by microinjecting the glutamate uptake inhibitor l-trans-pyrrolidine-2,4-dicarboxylic acid (PDC) or K(+) into the striatum of anesthetized rats. The resulted showed increases in neurotransmitter efflux that reached a peak in 20 s for PDC and 13 s for K(+).

Changes in extracellular amino acid concentrations in the rat hippocampus after in vivo actin depolymerization with latrunculin A

The effect of latrunculin A microperfusion on hippocampal extracellular concentrations of glutamate, aspartate, glycine and GABA, as measured by in vivo microdialysis, was investigated. Latrunculin A (4 microg/ml) was perfused for three consecutive days (8h a day) to promote in vivo F-actin depolymerization. Intrahippocampal latrunculin A microdialysis induced seizures during the second and third day of perfusion, and the animals started showing spontaneous seizures 1 month after lartrunculin A administration. Hippocampal glutamate levels were significantly increased during the first day of latrunculin A microperfusion without significant changes during the second and third day of perfusion. Aspartate levels were significantly increased during the first and second days of treatment. The rise on glutamate and asparate levels was partially reversed by perfusion of NMDA antagonist MK-801. Glycine concentrations were significantly increased during the 3 days of latrunculin A microdialyis, but no significant effect was observed on baseline GABA levels. One month after latrunculin A microperfusion, no significant differences in glutamate and aspartate extracellular concentrations were detected as compared to controls, however, significant increases in glycine and GABA extracellular concentrations were observed. The immediate increases in glutamate, aspartate and glycine levels indicate a modulatory effect of the F-actin cytoskeleton on extracellular concentrations of glutamate, aspartate and glycine. The chronic elevations in GABA and glycine levels are more likely to be related with long-term epileptogenesis processes. Our results suggest that the in vivo biochemical study of actin-dependent processes seems to be a promising approach to the neuropathology and neuropharmacology of epileptic seizures.

The effects of systemically administered taurine and N-pivaloyltaurine on striatal extracellular dopamine and taurine in freely moving rats

The second most abundant cerebral amino acid, taurine, is widely consumed in the so-called "energy drinks". Therefore, its possible actions on the brain are of great interest. In the present experiments taurine was given intraperitoneally to rats in order to study if it can be administered systemically in large enough amounts to alter cerebral dopaminergic transmission or to induce hypothermia. In addition, the effects of subcutaneously administered lipophilic taurine analogue, N-pivaloyltaurine, were studied. The extracellular striatal taurine and dopamine concentrations were estimated using in vivo microdialysis in awake and freely moving rats, and the rectal temperatures were measured. Taurine at the total dose of 45 mmol/kg i.p. led to a maximally 8-fold increased striatal extracellular taurine concentration, induced a long-lasting hypothermia, and significantly reduced the striatal extracellular dopamine concentration. The latter effect was strengthened by co-treatment with reuptake inhibitor nomifensine. N-pivaloyltaurine (15 mmol/kg in total, s.c.) only slightly elevated the striatal extracellular taurine concentration, failed to alter the rectal temperature, and in contrast to taurine somewhat elevated the striatal extracellular dopamine concentration suggesting a different mechanism or locus of action from that of taurine. Finally, our experiments using brain microdialysis confirmed the earlier findings that taurine is slowly eliminated from the brain. The results clearly indicate that systemically given taurine enters the brain in concentrations that induce pharmacological effects.

D-Serine differently modulates NMDA receptor function in rat CA1 hippocampal pyramidal cells and interneurons

The organization of the neuronal hippocampal network depends on the tightly regulated interaction between pyramidal cells (PCs) and interneurons (Ints). NMDA receptor (NMDAR) activation requires the binding of glutamate and co-activation of the 'glycine site'. It has been reported that D-serine is a more potent endogenous agonist than glycine for that site. While many studies have focused on NMDAR function in PCs, little is known regarding the modulation of NMDARs in Ints. We studied the modulatory effect of D-serine on NMDAR EPSCs in PCs and in stratum radiatum Ints using whole-cell patch-clamp recording in rat acute hippocampal slices. We found that D-serine enhances NMDAR function and differently modulates NMDAR currents in both cell types. The augmentation of NMDAR currents by D-serine was significantly larger in PCs compared with Ints. Moreover, we found differences in the kinetics of NMDAR currents in PCs and Ints. Our findings indicate that regulation of NMDAR through the 'glycine site' depends on the cell types. We speculate that the observed differences arise from assemblies of diverse NMDAR subunits. Overall, our data suggest that D-serine may be involved in regulation of the excitation-inhibition balance in the CA1 hippocampal region.

Glutamate and aspartate do not exhibit the same changes in their extracellular concentrations in the rat striatum after N-methyl-D-aspartate local administration

To determine whether glutamate (Glu) and aspartate (Asp) undergo a similar regulation of their extracellular levels, Glu and Asp were simultaneously monitored in the striatum of anesthetized rats after local N-methyl-D-aspartate (NMDA) receptor stimulation, using 1-min in vivo microdialysis coupled to capillary electrophoresis with laser-induced fluorescence detection. Application of NMDA (10 min, 10(-3) M) through the dialysis probe induced 1) an increase (+50%) in Asp during the NMDA administration and 2) a surprising biphasic effect on Glu, with a rapid increase (+30%) and a return to baseline before the end of NMDA application, followed by a second increase (+40%) occurring after and linked to the end of NMDA administration. When studied in the presence of 10 microM tetrodotoxin (TTX) or 0.1 mM Ca(2+), the increase in Asp was partially TTX-dependent, and the early increase in Glu appeared to be partially TTX and Ca(2+) dependent, whereas the second increase in Glu was not. The second increase in Glu level was still present when NMDA antagonists (AP5 or MK-801) were administered at the end of NMDA application. Finally, only extracellular Asp was increased through application of lower NMDA concentrations (10(-4) M, 10(-5) M), whereas extracellular Glu was not affected. In conclusion, these results suggest a differential control of Glu and Asp extracellular levels in rat striatum by distinct mechanisms linked to NMDA receptors and involving neuronal or nonneuronal release.

Long-lasting enhancement of corticostriatal neurotransmission by taurine

Taurine occurs at high concentrations in the forebrain and its distribution varies with (patho)physiological conditions; however, its role in neural function is poorly understood. We have now characterized its effects on corticostriatal synaptic transmission. Bath application of taurine (10 mm) to slices obtained from mice and rats exerted a biphasic action on corticostriatal field potentials. The fast and reversible inhibition by taurine was accompanied by a depolarization and conductance increase in medium spiny neurons and was sensitive to gamma-aminobutyric acid (GABA)A and glycine receptor (GlyR) antagonists. A long-lasting enhancement (LLETAU) of field potentials was recorded after taurine withdrawal. The LLETAU was not prevented by N-methyl-d-aspartate (NMDA)- or by GABAA receptor-antagonists, but was sensitive to the GlyR-antagonist strychnine and blocked by the competitive taurine uptake inhibitor guanidinoethylsulphonate (GES, 1 mm). GES at 10 mm evoked an enhancement of field potentials similar to LLETAU. LLETAU depended on protein kinase C activation as it was blocked by chelerythrine, but was unaffected by trifluoperazine, and thus independent of calmodulin. LLETAU was significantly smaller in juvenile than in mature rodents. Activation of GlyRs and the specific taurine transporter by taurine evoke a long-lasting enhancement of corticostriatal transmission.

Feeding-induced decrease in extracellular glutamate level in the rat nucleus accumbens: dependence on glutamate uptake

In vivo microdialysis combined with high-performance liquid chromatography and electrochemical detection was used to monitor extracellular glutamate levels in the medial nucleus accumbens of Sprague-Dawley rats during their feeding behaviour. Consumption of a palatable new diet or a diet to which rats were previously exposed caused a decrease in extracellular level of glutamate in the nucleus accumbens during and after feeding. The presentation of an inedible object (a piece of rubber) instead of the expected food caused a marked increase in extracellular glutamate levels. In contrast, if the piece of rubber was presented to rats that did not expect food delivery, the extracellular level of glutamate remained unchanged during the rubber presentation. The feeding-induced decrease in the extracellular glutamate level did not depend on food deprivation and was completely prevented by intraaccumbal infusions through the dialysis probe of 10 mM D,L-threo-beta-hydroxyaspartate (a glutamate uptake inhibitor). Intraaccumbal infusions of 10 microM S-(-)-raclopride L-tartrate (a D2/D3 dopamine receptor antagonist) or 1 microM tetrodotoxin (a voltage-dependent Na(+) channel blocker) also completely reversed the decrease in extracellular glutamate level in response to food intake. The D1/D5 dopamine receptor antagonist SCH-23390 (10 microM) administered into the nucleus accumbens had no significant effect on the feeding-induced decrease in extracellular glutamate level. From the data obtained we suggest that the decrease in the extracellular level of glutamate in the medial nucleus accumbens in response to feeding appears to arise from a temporal increase in glutamate uptake that is probably operated by dopamine inputs to the nucleus accumbens via D2/D3 receptors. Our findings also suggest that the dissociation between the expected biological value of a presented object and the reality might be an important determinant for regulation of glutamate release in this brain area during feeding behaviour.

Automated capillary liquid chromatography for simultaneous determination of neuroactive amines and amino acids

A method for the separation and quantitative determination of neuroactive amino acids (aspartate, glutamate, citrulline, arginine, glycine, taurine, gamma-aminobutyric acid) and neuroactive amines (noradrenaline, dopamine and serotonin) in a single chromatographic analysis is presented. The method is based on pre-column derivatization with o-phthalaldehyde and tert.-butyl thiol, on-column preconcentration and separation using 50 microns I.D. packed capillary columns, and detection by amperometry. Mass limits of detection are 80-900 amol for all neurotransmitters with RSDs of 0.71 and 4.6% or better for retention time and peak area, respectively. The method was demonstrated by application to the determination of neurotransmitters in microdialysis samples collected from striatum of live rats and tissue samples extracted from butterfly brains.

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.

The nucleus paragigantocellularis and opioid withdrawal-like behavior

Participation of the nucleus paragigantocellularis (PGi) in mediation of opioid withdrawal was examined in conscious, unrestrained, non-opioid-dependent rats, using electrical stimulation of the PGi. A characteristic series of behaviors, which resembled those seen during naloxone-precipitated withdrawal from dependence on the opioid agonist, butorphanol, was elicited during 30 min of PGi stimulation. Thus, the behavioral syndrome has been termed opioid withdrawal-like. Simultaneous microdialysis measurement of glutamate within the locus ceruleus indicated a positive correlation between extracellular glutamate concentrations and behavioral responses. Behavioral responses were inhibited by 50% during reverse dialysis perfusion of the locus ceruleus with the glutamate receptor antagonist, kynurenic acid, without any effect on glutamate concentrations. Thus, increases in locus ceruleus glutamate partially mediate opioid withdrawal-like behavior. Intracerebroventricular (i.c.v.) injections of the opioid antagonist, naloxone, or of the mu-selective (beta-funaltrexamine) or the delta-selective (naltrindole) opioid antagonists decreased, but did not abolish, stimulation-induced behavioral responses. Similar i.c.v. injections of the kappa-selective antagonist, nor-binaltorphimine, had no effect on behavioral responses to PGi stimulation. Activation of the PGi by electrical stimulation can elicit behaviors similar to those observed during opioid withdrawal. Moreover, additional levels of complexity are evident in the neuropharmacology of PGi stimulation-induced opioid withdrawal-like behavior.

Trace-level amino acid analysis by capillary liquid chromatography and application to in vivo microdialysis sampling with 10-s temporal resolution

A sensitive method was developed to determine 16 amino acids, including all the neurotransmitter amino acids and neuromodulators, in physiological samples. Samples were derivatized with o-phthalaldehyde/tert-butyl thiol followed by two scavenging reactions that reduced the chemical background caused by excess derivatization reagent by approximately 90%. A total of 250 nL of the derivatized sample was injected and concentrated onto a 50-micron-inner diameter capillary column packed with 5-micron reversed-phase particles and separated using gradient elution. Analytes were detected amperometrically at a cylindrical 9-micron carbon fiber microelectrode. The combination of on-column concentration, scavenging reactions after derivatization, high sensitivity electrochemical detection, and protocols to minimize amine contamination allowed detection limits of 90-350 pM (20-80 amol) for all the amino acids tested. This method was used to analyze in vivo microdialysate samples from probes implanted in the striatum of anesthetized rats. Probes were perfused at 1.2 microL/min and fractions collected every 10 s. The 200-nL fractions were diluted to 2 microL to facilitate sample handling for off-line analysis. The suitability of this method for simultaneous monitoring of all the major amino acid neurotransmitters with 10-s temporal resolution under basal conditions, during potassium stimulation, and during selective uptake inhibition of gamma-aminobutyric acid is demonstrated.

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.

Contribution of glutamatergic systems in locus coeruleus to nucleus paragigantocellularis stimulation-evoked behavior

The role of extracellular glutamate, within the locus coeruleus, in mediation of the behavioral signs elicited by electrical stimulation of the nucleus paragigantocellularis (PGi) was investigated in conscious, opioid-naive rats. Each rat was prepared with a chronically implanted unilateral electrode within the PGi and a microdialysis guide cannula directed at the ipsilateral locus coeruleus. Opioid withdrawal-like behaviors (rearing, teeth-chattering, wet-dog shakes, etc.) and increases in extracellular glutamate concentrations within the locus coeruleus were evoked, in a frequency-dependent (0.5-50 Hz) manner, during PGi stimulation. Reverse dialysis perfusion of the locus coeruleus with the nonspecific glutamate receptor antagonist, kynurenic acid (0.1, 1 mM), reduced the intensity of stimulation-induced behaviors by roughly 50%, but had no effect on the corresponding increases in glutamate concentrations. Perfusion of the locus coeruleus with the glutamate transporter inhibitor, L-trans-pyrrolidine dicarboxylic acid, at 1, but not at 0.1, mM significantly increased glutamate levels in dialysates. Neither concentration of the transporter inhibitor altered the behavioral score. The results indicate that the opioid withdrawal-like behaviors elicited by electrical stimulation of the brainstem at the site of the PGi are positively correlated with locus coeruleus levels of glutamate, and suggest further that the behaviors are partially mediated by release of glutamate within the locus coeruleus or its immediate vicinity.

Simultaneous determination of basal and evoked output levels of aspartate, glutamate, taurine and 4-aminobutyric acid during microdialysis and from superfused brain slices

A HPLC method, involving pre-column derivatisation with o-phthalaldehyde and fluorescence detection, is described. It allows the resolution of aspartate, glutamate, taurine and GABA, in a single run with detection limits of 3.2, 1.7, 1.4 and 2 fmol/microl of perfusate, respectively. It is sufficiently sensitive and rapid (15 min) for the determination "on line" of the four amino acids in perfusates obtained during in vivo microdialysis experiments. The procedure has been used to determine basal, K+ - or veratridine-stimulated release of these amino acids in different brain areas during microdialysis and from perfused tissue slices.

Time-dependent changes in striatal glutamate synapses following a 6-hydroxydopamine lesion

The goal of this study was to investigate changes in glutamatergic synapses in the striatum of rats at two different time-points following a unilateral injection of 6-hydroxydopamine into the medial forebrain bundle. One month following this lesion of the nigrostriatal pathway, there was an increase (70%) in the mean percentage of asymmetrical synapses within the dorsolateral striatum containing a discontinuous, or perforated, postsynaptic density, possibly suggesting an increase in glutamatergic activity. This was correlated, in the same brain region, with a decrease (44%) in the density of glutamate immunoreactivity within nerve terminals associated with all asymmetrical synapses and also with those terminals associated with a perforated postsynaptic density. These morphological changes were consistent with an increase (>two-fold) in the basal extracellular level of striatal glutamate, as measured by in vivo microdialysis. The density of GABA immunolabeling within symmetrical nerve terminals was increased (25%) at this one month time-period. Dopamine levels within the lesioned striatum were >99% depleted. However, at three months, while an increase in the mean percentage of striatal perforated synapses was maintained, a significant increase (50%) in the density of striatal nerve terminal glutamate immunolabeling within all asymmetrical synapses and those associated with a perforated postsynaptic density was observed. This was correlated with a small, but significant, decrease (32%) in the basal extracellular level of striatal glutamate. The density of GABA immunolabeling within nerve terminals associated with a symmetrical contact remained elevated at this three month time-period, while striatal dopamine levels remained depleted. While the density of nerve terminal GABA immunolabeling remained elevated at both the one and three month time-periods, there appeared to be a differential effect on glutamatergic synapses. The in vivo microdialysis data suggest that glutamate synapses were more active at a basal level at one month and become less active compared to the control group at the three month time-period. These data suggest that there are compensatory changes in glutamatergic synapses within the striatum following a 6-hydroxydopamine lesion that appear to be independent of the level of striatal dopamine or GABA. We propose that changes in the activity of the thalamo-cortico-striatal pathway may help to explain the differential time-course change in striatal glutamatergic synaptic activity.

Locally infused taurine, GABA and homotaurine alter differently the striatal extracellular concentrations of dopamine and its metabolites in rats

We studied in vivo the effects of locally infused taurine (50, 150, and 450 mM) on the striatal dopamine and its metabolites in comparison with those of GABA and homotaurine, a GABAA receptor agonist, in freely moving rats. The extracellular dopamine concentration was elevated maximally 2.5-, 2- and 4-fold by taurine, GABA and homotaurine, respectively. At 150 mM concentration, at which the maximum effects occurred, homotaurine increased the extracellular dopamine more than taurine or GABA. When taurine and GABA were infused simultaneously with tetrodotoxin the output of dopamine did not differ from that in the presence of tetrodotoxin alone. In comparison, tetrodotoxin did not inhibit the increase in extracellular dopamine caused by homotaurine. Furthermore, omission of calcium from the perfusion fluid inhibited the increase of extracellular dopamine caused by GABA. However, it did not block the increase of dopamine caused by taurine or homotaurine. The present study suggests that the effects of intrastriatal taurine, GABA and homotaurine on the striatal extracellular dopamine differ. Thus, these amino acids seem to affect the striatal dopaminergic neurons via more than one mechanism.

Regional differences in the effects of glutamate uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylic acid on extracellular amino acids and dopamine in rat brain: an in vivo microdialysis study

The effects of the glutamate transporter inhibitor, trans-PDC, on extracellular amino acids, were investigated in the frontal cortex, striatum, hippocampus and cerebellum of rats using in vivo microdialysis. Trans-PDC infusion (0.1, 1, 10 mM) dose-dependently increased Glu and Asp levels, and these increases in the cerebellum were smaller than those in other brain regions. A small but significant dose-dependent increase was observed for Gly and Tau. However, high extracellular Glu induced by trans-PDC was not sufficient to increase extracellular DA in the striatum and frontal cortex.

Effect of precipitated withdrawal on extracellular glutamate and aspartate in the nucleus accumbens of chronically morphine-treated rats: an in vivo microdialysis study

Excitatory amino acids release during morphine or naloxone administration was studied in rats. Microdialysis in freely moving animals and capillary electrophoresis with laser-induced fluorescence detection were used to measure several amino acids including glutamate and aspartate in the extracellular fluid at the nucleus accumbens. Perfusion with a calcium-free Ringer's solution decreased glutamate and aspartate in nucleus accumbens dialysates to 35% of its baseline levels, suggesting partial synaptic origin of these amino acids. The first morphine injection decreased glutamate and aspartate to 50% of its baseline level. After repeated morphine injections this effect disappeared, suggesting tolerance. Naloxone injections to morphine-dependent rats increased 300% glutamate and aspartate release; these experiments suggest that excitatory amino acid release in the nucleus accumbens might play a role in morphine withdrawal.

Evidence for a striatal NMDA receptor modulation of nigral glutamate release. A dual probe microdialysis study in the awake freely moving rat

Dual probe microdialysis was employed to characterize dialysate glutamate levels from the substantia nigra pars reticulata of awake freely moving rats, and to test its sensitivity to alterations in striatal neurotransmission including striatal N-methyl-D-aspartic acid (NMDA) receptor stimulation and blockade. Intranigral perfusion with low (0.1 mM) Ca2+ medium (60 min) did not affect nigral glutamate levels, whereas intranigral perfusion with tetrodotoxin (10 microM, 60 min) increased nigral glutamate levels. Perfusion of KCI (100 mM, 10 min) in the dorsolateral striatum transiently stimulated nigral glutamate levels (maximal increase + 60%), whereas intrastriatal perfusion (60 min) with low Ca2+ medium and tetrodotoxin gradually increased nigral glutamate levels. Intrastriatal perfusion with NMDA (0.1-100 microM, 10 min) dose-dependently stimulated glutamate levels in the substantia nigra pars reticulata. The NMDA (1 microM)-induced increase in nigral glutamate release was transient and maximal (+60% within 20 min), whereas that for NMDA (10 microM) had a slow onset but was long lasting (+35% after 60 min). Lower (0.1 microM) and higher (100 microM) NMDA concentrations were ineffective. The effect of intrastriatal NMDA (1 microM) was prevented by coperfusion with MK-801 (1 microM). Intrastriatal MK-801 (10 microM) alone gradually increased glutamate levels up to +50% after 60 min of perfusion. The present results suggest that glutamate levels in the substantia nigra pars reticulata are sensitive to changes in neuronal transmission in the dorsolateral striatum, and that striatal NMDA receptors regulate nigral glutamate release in both a tonic and phasic fashion.

On the release of glutamate and aspartate in the basal ganglia of the rat: interactions with monoamines and neuropeptides

Using highly sensitive analytical procedures, glutamate (Glu), aspartate (Asp) and several putative neurotransmitters and metabolites can be monitored simultaneously in the extracellular space of neostriatum, substantia nigra and cerebral cortex of the rat by in vivo microdialysis. Glu and Asp are found at sub-micromolar concentrations in all investigated brain regions. In order to ascertain their neuronal origin, we have extensively studied the sensitivity of extracellular Glu and Asp levels to: (i) K(+)-depolarization, (ii) Na(+)-channel blockade, (iii) removal of extracellular Ca2+, (iv) depletion of presynaptic vesicles, and (v) integrity of neuronal pathways. The relevance of these criteria for several neurotransmitters monitored simultaneously or in parallel experiments has also been examined. The functional interactions among different neuronal pathways in the basal ganglia are studied by using selective pharmacological treatments, administered systemically, or locally via intracerebral injections or the microdialysis perfusion medium. Immunohistochemical evidence for the existence of Glu and/or Asp neuronal pathways in the basal ganglia of the rat is presented, discussing especially new findings indicating the existence of a Glu-independent Asp system, intrinsic to the neostriatum of the rat. The clinical relevance of these interactions is discussed, focusing on the implications for the treatment of neurodegenerative disorders affecting the basal ganglia.

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

We investigated the effect of high dose tetrodotoxin (TTX) on microdialysis measurements of extracellular striatal glutamate and dopamine in normal female rats. Both halothane-anesthetized rats with acutely implanted microdialysis probes and awake rats with microdialysis probes implanted for 24 h were tested. Glutamate levels in awake rats were 45% higher than those of anesthetized rats. Extracellular glutamate remained TTX-insensitive regardless of TTX concentration, anesthesia, or time lapsed after probe implantation. In contrast, TTX reduced dialysate dopamine in all TTX concentrations tested. We speculate that the lower glutamate levels in anesthetized rats reflect the effect of anesthesia. Because glutamate is involved, either as a reactant or a product in a variety of reactions critical to intermediary metabolism in the brain, basal dialysate glutamate levels might indirectly reflect brain metabolism. Further, we conclude that extracellular glutamate collected during non-stimulated conditions is TTX-insensitive. The fact that glutamate levels are TTX-independent does not rule out that glutamate is synaptic in origin but rather demonstrates that it is not nerve impulse-dependent. However, the brain interstitial glutamate pool accessible to the microdialysis probe during control conditions is most likely isolated from the synapse, and therefore does not impose a neurotoxic potential.

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

1. Glutamate in the extracellular compartment of the striatum of freely moving rats was monitored at 5 min intervals using microdialysis and an enzyme-based assay. 2. Basal levels of dialysate glutamate were 3.6 +/- 0.5 microM. Local infusion through the dialysis probe of tetrodotoxin (TTX), cadmium chloride or magnesium chloride produced no reduction in basal levels of glutamate; with the latter two there was, instead, an increase. 3. Neuronal activation stimulated by induced grooming was accompanied by an increase in total glutamate efflux of 47.5 +/- 25.0% above basal level; this increase was not reduced by local infusion of TTX. 4. We propose that the TTX-insensitive release of glutamate in response to physiological stimulation is derived from glial cells and is a Ca(2+)-dependent mechanism triggered by a receptor-mediated release of Ca2+ from internal stores that spreads through the network of astrocytes.

Taurine infused intrastriatally elevates, but intranigrally decreases striatal extracellular dopamine concentration in anaesthetised rats

In the present study we infused taurine (50, 150 or 450 mM, 2 microliters/min for 4h) into the dorsal striatum or into the substantia nigra via microdialysis probe and estimated the extracellular concentrations of dopamine and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), in the dorsal striatum of anaesthetised rats. Intrastriatal infusion of taurine elevated striatal dopamine at all concentrations studied. At the 450 mM concentration taurine elevated the extracellular dopamine 10-fold, but only in the first 30 min sample after starting the taurine infusion. At 50 and 150 mM taurine elevated dopamine throughout the 4h infusion maximally up to 3-4-fold the control level. Extracellular DOPAC was increased by 150 and 450 mM taurine (up to about 150-160% of the control level), whereas at all three concentrations taurine decreased HVA to about 85% of the control; however, the decrease caused by 450 mM taurine was short-lasting. At all three concentrations taurine infused into the substantia nigra decreased the extracellular dopamine in the ipsilateral striatum to about 40-50% of the control, and increased extracellular DOPAC and HVA maximally to about 150% and 170% of the control, respectively. These results show that the effects of taurine on the concentrations of extracellular dopamine and its metabolites depend on its administration site on nigrostriatal dopaminergic neurons. It elevates the extracellular dopamine when given into the striatum, but when given into the cell body region of the nigrostriatal dopaminergic pathway it decreases the extracellular dopamine in the ipsilateral striatum.

Behavioural and neurochemical effects of OPC-14597, a novel antipsychotic drug, on dopaminergic mechanisms in rat brain

OPC-14597 is a new antipsychotic drug with a unique pharmacological profile. In a behavioural study in rats OPC-14597 did not show cataleptogenic activity even at the highest dose (40 mg/kg, i.p.), whereas it antagonized apomorphine-induced stereotypy dose-dependently (0.5-40 mg/kg). In vivo microdialysis showed that extracellular dopamine (DA) in the striatum was decreased significantly after OPC-14597 administration at higher doses of 10 and 40 mg/kg. Similar results were obtained in extracellular dopamine concentration in the frontal cortex, although the changes in DOPAC and HVA concentrations were smaller than those in the striatum. OPC-14597 also antagonized DA increase induced by the DA autoreceptor antagonist (+)-AJ76. These results that OPC-14597 acts either as an antagonist at postsynaptic dopamine receptors or as an agonist at presynaptic dopamine autoreceptors.