Changes in monoamine metabolites measured by simultaneous in vivo differential pulse voltammetry and intracerebral dialysis

Microdialysis in the rat striatum: effects of 24 h dexamethasone retrodialysis on evoked dopamine release and penetration injury

The power of microdialysis for in vivo neurochemical monitoring is a result of intense efforts to enhance microdialysis procedures, the probes themselves, and the analytical systems used for the analysis of dialysate samples. Our goal is to refine microdialysis further by focusing attention on what happens when the probes are implanted into brain tissue. It is broadly acknowledged that some tissue damage occurs, such that the tissue nearest the probes is disrupted from its normal state. We hypothesize that mitigating such disruption would refine microdialysis. Herein, we show that the addition of dexamethasone, an anti-inflammatory drug, to the perfusion fluid protects evoked dopamine responses as measured by fast-scan cyclic voltammetry next to the probes after 24 h. We also show that dexamethasone stabilizes evoked dopamine responses measured at the probe outlet over a 4-24 h postimplantation interval. The effects of dexamethasone are attributable to its anti-inflammatory actions, as dexamethasone had no significant effect on two histochemical markers for dopamine terminals, tyrosine hydroxylase and the dopamine transporter. Using histochemical assays, we confirmed that the actions of dexamethasone are tightly confined to the immediate, local vicinity of the probe.

Elevated cortical extracellular fluid glutamate in transgenic mice expressing human mutant (G93A) Cu/Zn superoxide dismutase

Transgenic mice expressing a mutated (G93A) human Cu/Zn superoxide dismutase (SOD1) develop motor neuron pathology and clinical symptoms similar to those seen in patients with amyotrophic lateral sclerosis. Loss of motor neurons is most prominent in lumbar, followed by cervical cord and then brainstem. No significant cell death has been reported in motor cortex. The integrity of the cortical glutamate reuptake systems was evaluated using intracerebral microdialysis and western immunoblot assays for the glutamate transporters GLT-1, GLAST, and EAAC1. The basal extracellular fluid levels of aspartate, glutamate, glutamine, 3,4-dihydroxyphenylacetic acid, and 5-hydroxyindole-3-acetic acid were evaluated by HPLC. The extraction fraction of L-3H]glutamate, corrected with [14C]mannitol, was also evaluated. GLT-1, EAAC1, and GLAST protein levels were determined by semiquantitative chemiluminescence immunoblot of proteins from membrane-enriched fractions. The relative optical density of film was translated into relative protein level by comparison with a standard control mouse. The SOD1 mutant mice demonstrated a significant (p < 0.05) increase in basal levels of extracellular aspartate and glutamate. In addition, when the glutamate extraction fraction was challenged with exogenous unlabeled glutamate (500 microM) by reversed microdialysis, the glutamate extraction fraction in the mutant SOD1 mice was decreased significantly from control levels. The SOD1 mutant mice demonstrated no difference in the cortical protein levels of the glutamate transporter subtypes. This study demonstrates that in areas of no visible pathology and no loss of glutamate transporter proteins, SOD1 mutant mice have elevated extracellular fluid aspartate and glutamate levels and a decreased capacity to clear glutamate from the extracellular space.

Quantitation of in vivo measurements with carbon fiber microelectrodes

Fast-scan cyclic voltammetry (FSCV) at carbon fiber disk microelectrodes and quantitative microdialysis were used to measure striatal concentration changes of N-acetyl-p-aminophenol (APAP, acetaminophen) following an intraperitoneal injection of 75 mg/kg APAP in rats. The goal of this work was to determine which in vitro calibration procedure, precalibration or postcalibration, gave the most accurate results when using carbon fiber microelectrodes in vivo. Voltammetric detection of APAP in vivo was complicated with normal electrodes by interference from pH changes. An electrode treatment was used to minimize electrode sensitivity to pH and this allowed successful APAP detection. In vitro calibrations of the treated carbon fiber disk microelectrodes before and after the in vivo experiment were used to calculate APAP concentration changes measured in vivo and compared to microdialysis results. The maximal striatal APAP concentration determined by microdialysis, adjusted for in vitro recovery, was 23.1 microM. The electrochemical results were approximately two times greater (postcalibration) or smaller (precalibration) than the microdialysis result.

Intracerebral microdialysis study of glutamate reuptake in awake, behaving rats

The central nervous system has high-affinity uptake systems for the clearance of amino acid transmitters. These systems are found in both neurons and astrocytes. Previous studies have shown that the uptake of amino acid transmitters by astrocytes in culture can be modulated by adrenergic agents. The objectives of this study were to develop a methodology that evaluates the brain's reuptake capacity for glutamate in awake, behaving animals and to determine whether glutamate reuptake is under alpha-adrenergic regulation in the intact central nervous system. Male Sprague-Dawley rats weighing 250-450 g were used in this study. The extraction fraction of L-[3H]glutamate with [14C]mannitol as a reference was measured. The cortical extraction fraction of L-[3H]glutamate corrected for [14C]mannitol (EL-glu) reaches steady state rapidly and is both stable and repeatable. EL-glu is a measure of L-glutamate reuptake and not metabolism. EL-glu is decreased in a dose-dependent manner by the addition of the glutamate reuptake blocker D,L-threo-beta-hydroxyaspartic acid or unlabeled L- glutamate. In addition, EL-glu is increased in a dose-dependent manner by the alpha1-adrenergic agonist phenylephrine, and this increase is blocked by the alpha-adrenergic antagonist phentolamine.

Microelectrodes for the measurement of catecholamines in biological systems

Many of the molecules involved in biological signaling processes are easily oxidized and have been monitored by electrochemical methods. Temporal response, spatial considerations, and sensitivity of the electrodes must be optimized for the specific biological application. To monitor exocytosis from single cells in culture, constant potential amperometry offers the best temporal resolution, and a low-noise picoammeter improves the detection limits. Smaller electrodes, with 1-micron diameters, provided spatial resolution sufficient to identify the locations of release sites on the surface of single cells. For the study of neurotransmitter release in vivo, larger cylindrical microelectrodes are advantageous because the secreted molecules come from multiple terminals near the electrode, and the greater amounts lead to a larger signal that emerges from the Johnson noise of the current amplifier. With this approach, dopamine release elicited by two electrical stimulus pulses at 10 Hz was detected with fastscan cyclic voltammetry in vivo. Nafion-coated elliptical electrodes have previously been shown to be incapable of detecting such concentration changes without extensive signal averaging. In addition, we demonstrate that high-pass filtering (200 Hz) of cyclic voltammograms recorded at 300 V/s decreases the background current and digitization noise at these microelectrodes, leading to an improved signal. Also, high-pass filtering discriminated against ascorbic acid, DOPAC, and acidic pH changes, three common interferences in vivo.

On the significance of brain extracellular uric acid detected with in-vivo monitoring techniques: a review

The concentration of uric acid [UA] in the extracellular fluid (ECF) estimated with in-vivo voltammetry and microdialysis data is compared for probes of different diameters from the day of implantation (acute) to several days (chronic) or even months after surgery. For small probes (diameter < 160 microns) the acute [UA] of ca. 5 microM decreased significantly to ca. 1 microM under chronic conditions. For larger probes (e.g., 320-microns diameter) the acute [UA] was also ca. 5 microM, but this value significantly increased to ca. 50 microM under chronic conditions. Associated with this difference in [UA], there were parallel differences in the extent of gliosis around the probes. These findings are discussed in terms of possible sources of extracellular UA and their implications for in-vivo monitoring techniques in behaving animals.

Effects of selective dopamine depletion in medial prefrontal cortex on basal and evoked extracellular dopamine in neostriatum

In this study, we demonstrate that 6-hydroxydopamine (6-OHDA) can be used to produce a lesion of dopamine (DA) terminals in medial prefrontal cortex (mPFC) while sparing the noradrenergic innervation in this region. Furthermore, we determined the impact of these lesions on both extracellular DA in neostriatum, using in vivo microdialysis, and locomotor activity. Our results demonstrate that, whereas higher doses of 6-OHDA (> or = 4 micrograms) depleted both DA and norepinephrine (NE) in mPFC, 1 micrograms 6-OHDA produced a depletion of DA (-79%) without significantly affecting NE content (-13%). Selective depletion of DA content in mPFC did not alter basal levels of extracellular DA in neostriatum determined 14 days after the lesion. The lesion also did not alter the ability of acute tail pressure (30 min) to increase extracellular DA in neostriatum or to stimulate locomotor activity. Depletion of DA in mPFC did not alter the ability of d-amphetamine (1.5 mg/kg, i.p.) to increase intracellular DA in neostriatum. In contrast, the maximum amphetamine-induced increase in locomotor activity was attenuated in lesioned rats as compared with control rats (670 and 280 locomotor counts/15 min, respectively). These data suggest that in the intact system, DA terminals in mPFC do not regulate extracellular DA in neostriatum. In addition, these data confirm that DA terminals in mPFC can influence stimulant-induced locomotion.

In vivo voltammetry and microdialysis monitoring of monoamine metabolism in the rat brainstem neurons

Two "in vivo" techniques allow the monitoring of extracellular levels of monoamines and related compounds in selected rat brainstem regions: voltammetry and microdialysis. "In vivo" voltammetry has a high regional selectivity: for example, we have been able to perform a subregional study and to show that the increase in extracellular DOPAC induced by 30 min-hypotension was twice as larg in the rostral as in the caudal rat locus coeruleus. The spatial resolution, as expressed by x 1/2 (see text), is 4 times better for voltammetry (50 microns) than for microdialysis (190 microns). Another advantage of voltammetry is its excellent time resolution. However, microdialysis has a much better biochemical specificity than voltammetry. Furthermore it allows some enzymatic activities, such as tyrosine hydroxylase, to be measured almost continuously in catecholaminergic brain nuclei. From a functional point of view, the results of our experiments (alpha 2 ligand administration, arterial hypotension or stress) illustrate the respective interest and complementarity of these two "in vivo" techniques. Their current developments will lead to a better temporal and biochemical resolution combined with an increase in the number of substances analyzed "in vivo", including peptides and nitric oxide.

Effect of probe size on the concentration of brain extracellular uric acid monitored with carbon paste electrodes

We have investigated further the anomalously high concentration of brain extracellular uric acid detected with in vivo sampling probes reported recently. The contribution by uric acid and 5-hydroxyindoleacetic acid (5-HIAA) to peak 2 recorded in rat striatum with chronically implanted carbon paste electrodes (CPEs) of different sizes was estimated by comparing peak current densities and the effect of the monoamine oxidase inhibitor pargyline. The concentration of uric acid in the extracellular fluid was some 50 times greater for 320-microns-diameter CPEs than for 160-microns-diameter electrodes, where the urate level was estimated at approximately 1 microM. The concentration of 5-HIAA was similar for 320-, 260-, and 160-microns-diameter CPEs. These data provide an explanation for the previously observed differences in 5-HIAA/urate ratios recorded with 320-microns-diameter CPEs and smaller carbon fibre electrodes. The results also indicate that chronically implanted sampling probes of diameter > 160 microns perturb the surrounding tissue, which produces uric acid by a mechanism yet unknown, although preliminary histological data suggest that glial cells may be involved.

Peripheral nerve stimulation increases serotonin and dopamine metabolites in rat spinal cord

Extracellular serotonin (5-HT), dopamine (DA), and their metabolites, 5-hydroxyindoleacetic acid (5-HIAA), 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA), were assessed in the rat lumbar spinal cord (L3-4) by in vivo microdialysis with high performance liquid chromatography and electrochemical detection (HPLC-ECD). Under urethane-chloralose anesthesia, basal levels of 5-HT and DA in the dialysates were approximately 1.0-1.2 pg/22 microliters sample, 5-HIAA, DOPAC, and HVA were constant at 322.6 +/- 14.9, 8.6 +/- 0.7, and 10.4 +/- 0.4 pg/22 microliters sample (mean +/- SE), respectively. Local application of 100 mM KCl via the dialysis probe increased the 5-HT and DA. Peripheral nerve stimulation that selectively excited the large (A-beta) or small (A-delta) myelinated fibres increased the metabolites. Excitation of the A-beta fibers increased the levels of 5-HIAA to 138%, DOPAC to 155%, and HVA to 143% of the controls. Stimulation of the A-delta fibers increased 5-HIAA to 121%, DOPAC to 120%, and HVA to 124% of the controls. The results suggest that non-nociceptive peripheral nerve stimulation may activate the descending 5-HT and DA systems in the spinal cord.

The dopamine D3 receptor and autoreceptor preferring antagonists (+)-AJ76 and (+)-UH232; a microdialysis study

The in vivo neurochemical profiles of haloperidol, raclopride and the dopamine D3 and autoreceptor preferring dopamine receptor antagonists (+)-UH232 and (+)-AJ76 on dopamine release and metabolism in the dorsal striatum and in the nucleus accumbens are described. It is shown that both (+)-UH232 and especially (+)-AJ76 have different effects on brain dialysate dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) as compared to haloperidol or raclopride. It is suggested that the relative increase in dialysate dopamine over the relative increase in DOPAC is a neurochemical fingerprint, unique for different dopamine receptor antagonists. As a consequence the increased release and metabolism of dopamine after systemic administration of dopamine receptor antagonists may be controlled by different receptors and different dopamine antagonists can partly distinguish between these receptors. This may be due to their different interactions with different dopamine D2 type receptors. It is finally concluded that (+)-UH232 and especially (+)-AJ76 seem to prefer release regulating autoreceptors at the level of the axon terminals.

Overview of chemical sampling techniques

Although many of the ideas for sampling the chemical microenvironment of the brain were present, at least in nascent form, three decades ago or more, the last 10 years have witnessed a particularly spectacular surge of development, refinement, and use. We are now able to measure virtually any endogenous brain chemical in vivo at commendable levels of sensitivity, selectivity, and speed. The long-dreamt-of goal of being able to correlate neurochemical events with ongoing behavior and/or presentation of salient environmental cues and stimuli has already been largely achieved. Further refinements of existing techniques may well lead to levels of analysis inconceivable even a few years ago. The implications for theory-building and hypothesis-testing are enormous, particularly within such essentially virgin domains as behavioral neuroscience and biological psychiatry. These are truly exciting times.

Sensor-tissue interactions in neurochemical analysis with carbon paste electrodes in vivo

Characterization of voltammetric signals recorded with microelectrodes in the living brain is fraught with difficulties. In addition to being anatomically complicated, brain tissue presents the analytical electrochemist with a complex chemical environment that includes surfactants (lipids), electrode poisons (proteins), electrocatalysts such as glutathione and ascorbic acid, and a tissue matrix that both restricts mass transport to the electrode surface and reacts physiologically to the presence of the probe. Identification of electrochemical signals recorded in vivo with carbon paste electrodes is discussed in the context of these problems. This examination shows that modification of both the electrode surface by tissue, and of the tissue environment by the electrode have important implications for voltammetric signal analysis in vivo. Despite these problems, valuable data on the relationship between behaviour and chemical changes in the brain can be obtained using in vivo electrochemical techniques.

Plasma microdialysis. A technique for continuous plasma sampling in freely moving rats

Microdialysis provides a means of continuous plasma sampling without repeated blood drawing. We report here the use of a specially designed and constructed microdialysis probe to sample plasma glucose, protein, and luteinizing hormone from the right atrium of a freely moving rat. Our probe has a unique side-arm tubing, which can be used to draw blood for in vivo probe calibration and infuse heparin continuously to prevent blood clotting. Glucose recovery rate (18%) of the probe remained relatively stable in continuously heparinized rats over 24 hr, but it dropped rapidly to 1% in nonheparinized rats. The concentration of plasma glucose was significantly underestimated, when it was converted from the perfusate concentration based on the in vitro, but not in the in vivo, recovery rate of the probe. The recovery of plasma protein was only 0.07% initially and rapidly declined to about 0.03%. Luteinizing hormone was not detected in the perfusates from either normal or luteinizing hormone-releasing hormone-stimulated rats. These results indicate that continuous heparinization and in vivo probe calibration are essential for successful plasma microdialysis, and our current dialysis membrane can be used to sample non-protein-bound molecules in the plasma.

H(+)-ATPase and transport of DOPAC, HVA, and 5-HIAA in monoamine neurons

The effects of N-methylmaleimide (N-MtM), a vacuolar H(+)-ATPase inhibitor, were evaluated in the putamen of the cat to study the in vivo transport mechanisms of dopamine (DA), 5-hydroxytryptamine (5-HT), and their metabolites 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindolacetic acid (5-HIAA), using the brain focal microdialysis technique combined with HPLC. The addition of N-MtM to the perfusate altered invariably the flux of the DOPAC, HVA, and 5-HIAA in a similar pattern, resulting in a decrease of the extracellular levels of such metabolites, its extent being N-MtM concentration dependent, thus indicating that the mechanism(s) of such a decrease is (are) related most likely to decreased transport from the intracellular to the extracellular space as the consequence of the inhibition of the vacuolar H(+)-ATPase of DA and 5-HT neurons by the N-MtM. Furthermore, N-MtM masked the release of DA and 5-HT produced by KCl 120 mmol/l. Indeed, N-MtM increased the extracellular levels of such transmitters to values exceeding 4 to 6 times of those produced by KCl 120 mmol/l alone, which suggests that vacuolar H(+)-ATPase is probably involved also in the retention and/or reuptake process of DA and 5-HT.

Voltammetric monitoring of brain extracellular levels of serotonin, 5-hydroxyindoleacetic acid and uric acid as assessed by simultaneous microdialysis

We have previously developed a microcomputer-assisted curve-fitting method for measuring the components of the mixed electrochemical signals recorded by differential normal pulse voltammetry in the living brain. It was initially used for resolution of the dopamine and dihydroxyphenylacetic acid components of the catechol signal (peak 2). This report shows how it can be applied to analysis of the indoleamine/uric acid (UA) components of the more complex peak 3. The voltammogram is modeled as a mixture of 3 normal curves of known parameters corresponding to the oxidation of UA, 5-hydroxyindoleacetic acid and serotonin, which is solved by non-linear iterative procedures. Performance was assessed by treatments with drugs having well-known effects on the substances monitored, pargyline and allopurinol, and by the chromatographic analysis of microdialysates collected simultaneously from the contralateral side.

Intracerebral infusion of DOPAC decreases striatal dopamine

The purpose of this study was to determine whether elevated levels of 3,4-dihydroxyphenylacetic acid (DOPAC), the major metabolite of dopamine (DA) in the brain, could decrease the DA content in the striatum. Levels of DA were determined by high pressure liquid chromatography with electrochemical detection (HPLC-EC) in the striatum of male rats 24 h following a single intracerebral administration of DOPAC into the right striatum. DOPAC at 16.8 micrograms reduced the DA content of the infused side by 17%, p = 0.01. In contrast, infusion of 1.68 micrograms of DOPAC or the vehicle had no effect on striatal DA levels. Coapplication of the antioxidant, ascorbic acid, at 0.2 mg/ml with 16.8 micrograms of DOPAC prevented the decrease in DA content. Furthermore, infusion of 18.2 micrograms of homovanillic acid (HVA), the product of DOPAC methylation, had no effect on striatal DA. These results indicate that DOPAC may undergo autoxidation in vivo to produce neurotoxic species which may result in reduction of striatal DA. Formation of such an autoxidation product(s) of endogenous DOPAC was verified in the extracellular fluid of striatal slices in vitro.

Serotonin denervation enhances responsiveness of presynaptic dopamine efflux to acute clozapine in nucleus accumbens but not in caudate-putamen

Clozapine alters mesolimbic dopamine (DA) function but spares nigrostriatal DA function in laboratory animals, but the underlying mechanism is unknown. In the present study, acute intraperitoneal injection of clozapine (5-40 mg/kg) increased extracellular DA levels in nucleus accumbens (Acb) and caudate-putamen (CPu) of awake, freely moving rats as measured by in vivo brain microdialysis, without anatomic selectivity. However, in serotonin (5HT)-denervated rats acute clozapine preferentially enhanced DA levels in Acb as compared to CPu. Since (i) up-regulation of 5HT receptors on DA neurons may result from 5HT denervation, (ii) clozapine has potent anti-5HT action, and (iii) 5HT receptors are more dense in Acb than CPu, these data appear to add additional weight to previous suggestions that a serotonergic mechanism may partly underlie clozapine's mesolimbic selectivity.

Muscarinic antagonists attenuate neurotensin-stimulated accumbens and striatal dopamine metabolism

The effect of scopolamine and atropine upon the increase in extracellular 3,4-dihydroxyphenylacetic acid induced by central injection of neurotensin was examined in the nucleus accumbens and the striatum of anaesthetized rats using in vivo differential pulse voltammetry with carbon fibre electrodes. Scopolamine (1 and 3 mg/kg, i.p.) and atropine (20 micrograms, i.c.v.) did not alter the 3,4-dihydroxyphenylacetic acid level in the nucleus accumbens or the striatum, measured for 60 min after administration. Neurotensin (10 micrograms, i.c.v.) increased the 3,4-dihydroxyphenylacetic acid peak height in both regions. Pretreatment with scopolamine (1 mg/kg) 15 min before neurotensin injection blocked the increase in extracellular 3,4-dihydroxyphenylacetic acid in the striatum but not in the nucleus accumbens whilst scopolamine (3 mg/kg) partially attenuated the effect of neurotensin in the nucleus accumbens and blocked the increase in 3,4-dihydroxyphenylacetic acid in the striatum. Atropine partially attenuated the effect produced by neurotensin in the nucleus accumbens and blocked the increase in 3,4-dihydroxyphenylacetic acid induced by the peptide in the striatum. However, the increase in extracellular 3,4-dihydroxyphenylacetic acid induced by haloperidol (1 mg/kg, s.c.) was not altered by scopolamine (1 mg/kg) or atropine. Also, the increase in dopamine metabolism in the nucleus accumbens and the striatum after centrally injected haloperidol (10 micrograms, i.c.v.) was not altered by atropine (20 micrograms, i.c.v.). Together, the results demonstrate a functional interaction between muscarinic antagonists and neurotensin on in vivo dopamine metabolism in the nucleus accumbens and the striatum but with a greater effect in the latter region.

Effects of acute and chronic clozapine on dopaminergic function in medial prefrontal cortex of awake, freely moving rats

We previously showed that chronic administration of the clinically atypical and clinically superior antipsychotic drug clozapine selectively reduces dopamine (DA) release in the nucleus accumbens but not neostriatum, and that this effect appears mediated by anatomically selective mesolimbic DA depolarization blockade. The present study extends that research to another mesocorticolimbic DA locus, the medial prefrontal cortex. Acute clozapine challenge (5-40 mg/kg i.p.) produced dose-dependent increased extracellular levels of DA and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), in the medial prefrontal cortex of awake, free-moving rats as measured by in vivo brain microdialysis. Chronic clozapine treatment (20 mg/kg/day for 21 days) did not significantly change basal extracellular levels of DA, DOPAC or HVA. Acute clozapine challenge on day 22 in the chronic clozapine-treated animals produced no significant differences in medial prefrontal cortex DA, DOPAC or HVA as compared to chronic vehicle-treated animals, indicating that tolerance to clozapine does not develop in the mesocortical DA system, in contrast to the mesolimbic system. The DA agonist apomorphine (100 micrograms/kg) produced decreased basal extracellular levels of DA, DOPAC and HVA in medial prefrontal cortex of both chronic clozapine-treated and chronic vehicle-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of amfonelic acid on the haloperidol- and clozapine-induced increase in extracellular dopac in the nucleus accumbens and the striatum

This study compares the effects of the nonamphetamine stimulant amfonelic acid on the increase in extracellular 3,4-dihydroxyphenylacetic acid (DOPAC) induced by haloperidol and clozapine in the nucleus accumbens and the striatum of anaesthetized rats. DOPAC was simultaneously recorded in both regions using differential pulse voltammetry with electrically pretreated carbon fibre electrodes. Amfonelic acid (2.5 mg/kg s.c.) did not alter basal striatal DOPAC but produced a significant reduction in extracellular DOPAC in the nucleus accumbens. Haloperidol (1 mg/kg s.c.) increased extracellular DOPAC in both regions. When amfonelic acid was injected 5 min before haloperidol, the increase in DOPAC was potentiated in both the nucleus accumbens and the striatum but with a greater effect in the striatum. Clozapine (30 mg/kg i.p.) increased extracellular DOPAC in both regions, an effect partially attenuated by amfonelic acid in both regions but to a greater extent in the striatum. When ritanserin (5 mg/kg i.p.), a serotonergic antagonist (5-HT-2), was co-administered with haloperidol, the potentiation by amfonelic acid of the increase in extracellular DOPAC induced by haloperidol was attenuated in both the nucleus accumbens and the striatum. The present results confirm that amfonelic acid can be used to discriminate neurochemically between haloperidol and clozapine in vivo. The effects of amfonelic acid on the neuroleptic-induced changes in extracellular DOPAC were greater in the striatum than the nucleus accumbens. These results further demonstrate that both neuroleptics increase dopamine metabolism in the two brain regions but by different mechanisms, supporting the view that the regulation of dopamine metabolism differs in the two regions.(ABSTRACT TRUNCATED AT 250 WORDS)

Mathematical resolution of mixed in vivo voltammetry signals. Models, equipment, assessment by simultaneous microdialysis sampling

A microcomputer-assisted curve-fitting procedure was developed for the quantitative estimation of the components of the mixed "catechol peak" recorded with differential normal pulse voltammetry (DNPV) at electrochemically pretreated carbon fiber microelectrodes in the living brain. The contribution of each of the relevant electroactive species is fitted by a normal probability function, the parameters of which are previously determined in vitro for each electrode and substance. The voltammogram is thus modeled as a mixture of normal curves corresponding to the individual oxidizable substances plus a low order polynomial approximating the baseline. In a former approach the function was solved by linear least squares techniques. As a further improvement, we now propose a non-linear model of the voltammogram and a Gauss-Newton iterative algorithm with stepwise regression for parameter estimation. This report shows the application of the method for the resolution of the dopamine (DA) and dihydroxyphenylacetic acid (DOPAC) components of the DNPV signal recorded from the striatum of freely moving animals in response to amphetamine and pargyline. The method was validated by the chemical assay of contralateral microdialysates collected simultaneously. The changes detected by both methodologies were closely parallel, with highly significant correlation coefficients (0.87 and 0.99 for DA and DOPAC, respectively, P less than 0.001). This study further illustrates that the in vivo voltammetry methodology can be improved substantially by incorporating a suitable mathematical treatment of the electrochemical signals.

Muscarinic antagonists attenuate the increase in accumbens and striatum dopamine metabolism produced by clozapine but not by haloperidol

1. The effect of the muscarinic antagonists, scopolamine and atropine, were examined on the increase in extracellular 3,4-dihydroxyphenylacetic acid (DOPAC) in the nucleus accumbens and the striatum induced by haloperidol and clozapine by use of in vivo differential pulse voltammetry with carbon fibre electrodes in anaesthetized rats. 2. Animals received saline (1 ml kg-1, s.c.), scopolamine (1 mg kg-1, o.p.) or atropine (20 micrograms, i.c.v.) followed 15 min later by saline (10 microliters, i.c.v.), haloperidol (1 mg kg-1, s.c.) or clozapine (30 mg kg-1, i.p.) and extracellular DOPAC was simultaneously recorded in the nucleus accumbens and the striatum every 5 min for 60 min after drug administration. 3. Scopolamine or atropine alone had no effect on the DOPAC peak height but attenuated the increase in extracellular DOPAC induced by clozapine in both brain regions. Neither scopolamine nor atropine altered the haloperidol-induced increase in accumbens or striatal extracellular DOPAC. 4. The present results demonstrate that muscarinic antagonists attenuate the increase in accumbens and striatal dopamine metabolism in vivo produced by the atypical neuroleptic clozapine but not the haloperidol-induced increase in dopamine metabolism. The results indicate that central muscarinic receptors are involved in the actions on dopaminergic function of clozapine but not haloperidol.

Strain-specific facilitation of dopamine efflux by delta 9-tetrahydrocannabinol in the nucleus accumbens of rat: an in vivo microdialysis study

This study tests the hypothesis that delta 9-tetrahydrocannabinol (delta 9-THC) has a strain-specific facilitatory effect on dopamine (DA) efflux in rat nucleus accumbens, a crucial forebrain convergence of reward-relevant DA neural fibers that has been implicated as a focal brain locus mediating the euphorigenic properties of drugs of abuse. The dependent variable is presynaptic DA efflux measured by in vivo microdialysis in the nucleus accumbens. The independent variables are: (1) intraperitoneal injections of delta 9-THC at 0.0 (vehicle), 0.5 and 1.0 mg/kg; (2) Sprague-Dawley vs Lewis strain rat. Results show that delta 9-THC produces a dose-dependent, strain-specific enhancement of basal DA efflux in Lewis strain rats. These results suggest that genetic variation influences drug abuse vulnerability.

Anomalously high concentrations of brain extracellular uric acid detected with chronically implanted probes: implications for in vivo sampling techniques

The height of peak 2, h2, recorded using linear sweep voltammetry with 350-micron-diameter carbon paste electrodes in rat striatum was measured from the day of implantation (day 0) to 4 months after surgery. The value of h2 was at a minimum on day 0 (0.6 +/- 0.2 nA; n = 20), rose sharply to a maximum on day 2 (6.3 +/- 0.9 nA; n = 12), and decreased to a stable level by day 7 (3.3 +/- 0.7 nA; n = 16), which lasted for 4 months (3.2 +/- 0.6 nA; n = 9). These changes were shown by microinfusion of uricase to be due to variations in the concentrations of extracellular uric acid, although h2 appears to have a small baseline contribution of approximately 0.3 nA from 5-hydroxyindoleacetic acid. The stable value of h2 recorded under chronic conditions was estimated to correspond to a minimal uric acid concentration of 50 mumol/L, which represents a 10-fold increase in the extracellular level of this purine metabolite compared with the initial (acute) value. Very similar results were obtained using a microdialysis technique that detected uric acid directly. These estimates of striatal uric acid concentration are in marked contrast to those obtained using 40-micron diameter carbon fiber electrodes, which showed a decrease from the acute preparation to less than 1 mumol/L under chronic conditions. Large values of h2 were also recorded with chronically implanted paste electrodes in the hippocampus and frontal cortex.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuropeptides can interfere with the recording properties of voltammetric carbon fibre electrodes

The effects of neuropeptides on the recording properties of carbon fibre micro-electrodes used with differential pulse voltammetry were examined both in vitro and in vivo. The in vitro voltammetric signal recorded in a solution containing 3,4-dihydroxyphenylacetic acid (10(-4) M) was attenuated after the addition of thyrotropin releasing hormone, neuromedin N and neurotensin (10(-5) M). The administration of neurotensin (1 or 3 micrograms/microliters) into the nucleus accumbens adjacent to the carbon fibre electrode produced a decrease in the extracellular 3,4-dihydroxyphenylacetic acid peak. However, evidence was obtained that peptides alter the recording properties of the microelectrodes when present at sufficient concentrations and such an effect may explain the result obtained in vivo. In contrast, neurotensin administered into the ventral tegmental area increased the 3,4-dihydroxyphenylacetic acid peak in the nucleus accumbens as previously reported. Thus, attenuation of the voltammetric signal was caused by an effect of the peptide on the recording properties of the electrode rather than real diminution of the free oxidisable amine around the electrode. Caution should be taken when investigating the effects of peptides on extracellular amines or their metabolites using in vivo voltammetry with carbon fibre electrodes if the peptides are administered in high concentrations close to the electrode.

Use of amfonelic acid to discriminate between classical and atypical neuroleptics and neurotensin: an in vivo voltammetric study

Previous ex vivo studies have shown that the non-amphetamine stimulant amfonelic acid potentiates the increase in DOPAC induced by classical but not by atypical neuroleptics. In the present study, we have demonstrated that this neurochemical model can be used to discriminate typical from atypical neuroleptics in vivo using differential pulse voltammetry with carbon fibre electrodes. The study also compared the effect of intracerebroventricular (i.c.v.) administration of neurotensin, on extracellular striatal DOPAC following amfonelic acid, with the effects of both classical and atypical neuroleptics. Saline or amfonelic acid (2.5 mg/kg s.c.) were administered; followed 5 min later by the classical neuroleptics haloperidol, perphenazine, or the atypical neuroleptics clozapine, thioridazine, or by neurotensin. After drug administration extracellular striatal DOPAC was recorded every 5 min for 90 min. Amfonelic acid did not alter basal striatal DOPAC but potentiated the increase in DOPAC induced by haloperidol (1.0 and 0.05 mg/kg s.c.) and perphenazine (10 mg/kg s.c.). Both clozapine (30 mg/kg i.p.) and thioridazine (20 mg/kg s.c.) increased extracellular DOPAC, but pretreatment with amfonelic acid prevented the increase in DOPAC produced by both drugs. Neurotensin (10 micrograms, i.c.v.), in a similar manner to the atypical neuroleptics, increased extracellular DOPAC in the striatum and the effect was prevented by amfonelic acid. The present study demonstrates that pretreatment with amfonelic acid is a valuable tool to discriminate between classical and atypical neuroleptics in vivo. The results also indicate that neurotensin in the presence of amfonelic acid has a profile similar to the atypical neuroleptics.

Activation of 5-HT3 receptor by 1-phenylbiguanide increases dopamine release in the rat nucleus accumbens

The serotonin-3 (5-HT3) agonist 1-phenylbiguanide (0.1-1.0 mM in perfusate) caused a robust, dose-dependent enhancement of extracellular dopamine content in nucleus accumbens as measured by in vivo microdialysis. This action was antagonized by co-perfusion of the 5-HT3 antagonists zacopride and GR38032F (1 mM in perfusate). Similar effects were observed in 5-HT-denervated rats. These findings suggest that there is a potent modulation of dopamine (DA) release in the nucleus accumbens mediated via 5-HT3 receptors, which appear to be located presynaptically on DA terminals of the mesolimbic DA pathway.

Chronic treatment with clozapine selectively decreases basal dopamine release in nucleus accumbens but not in caudate-putamen as measured by in vivo brain microdialysis: further evidence for depolarization block

As measured using in vivo brain microdialysis in conscious freely-moving rats, chronic treatment (20 mg/kg/day i.p. for 21 days) with the clinically atypical neuroleptic clozapine selectively reduced basal dopamine (DA) release in the nucleus accumbens (Acb) but not in caudate-putamen (CPu). Apomorphine (100 micrograms/kg s.c.) enhanced presynaptic Acb DA release in clozapine-treated rats, but reduced Acb DA release in vehicle-treated rats. These findings provide further evidence that depolarization block of mesolimbic DA neurons projecting to Acb but not of nigrostriatal DA neurons projecting to CPu may underlie clozapine's unusual clinical efficacy and its lack of production of extrapyramidal motoric effects.

RIA-linked microdialysis sampling in the awake rat: application to free-drug pharmacokinetics of hydrocortisone

The purpose of this research was to combine microdialysis sampling techniques with a highly sensitive radioimmunoassay (RIA) to study the in vivo kinetic response of pharmacologically important substances. This technique allowed for a dense sampling regimen from an awake, free-roaming experimental animal with no loss of blood and with rapid analysis of the dialysate. An important methodological criterion for accurate quantitation of a test drug in the extracellular space was knowledge of the relative recovery of the sampling system at the time of experimentation. Accordingly, the factors which influenced the recovery of drug during dense in vivo microdialysis sampling were examined and an analytical technique was developed to measure the instantaneous recovery of drug from the extracellular space. This information was applied to in vivo (iv) sampling experiments on anaesthetized and awake, free-roaming rats following bolus and multiple long-term iv administrations of the highly protein bound steroid (i.e. greater than 90%), hydrocortisone-21-phosphate. These studies indicated that unbound hydrocortisone levels as determined by the RIA-linked microdialysis (RIALM) technique fluctuated rapidly between each 2-min sampling interval, but nevertheless decreased to predose endogenous concentrations in a first-order fashion (t1/2 = 17-29 min). The rapid fluctuations of unbound hydrocortisone may reflect real pharmacokinetic or pharmacodynamic phenomena, attributed, perhaps, to reequilibration of the unbound drug pool with proteins and tissues in the blood.

Correlation of functional recovery after a 6-hydroxydopamine lesion with survival of grafted fetal neurons and release of dopamine in the striatum of the rat

Female rats were lesioned with 6-hydroxydopamine in the left substantia nigra. At least two weeks later they were tested with amphetamine (5 mg/kg, s.c.) and apomorphine (0.25 mg/kg, s.c.). A cell suspension from the ventral mesencephalon of rat embryos was distributed in three sites in a triangular fashion in the center of the denervated striatum. The amphetamine test was then repeated every month for six months. The pattern of circling to amphetamine before the graft was strictly ipsiversive in all animals. From the first month we observed a progressive change and three patterns of rotation could be observed. In 21% of animals, the total number of ipsiversive turns in 90 min actually increased but during the first 20 min the animals turned contralaterally to the lesion (and to the graft). In 38% of animals, the total number of turns switched from ipsiversive to contraversive with the animals turning initially toward the intact side and during the second half of the test toward the lesion. Finally 41% of rats progressively switched to turning only toward the intact side. In all cases, maximal contraversive turning occurred during the initial 20 min. In these rats, tyrosine hydroxylase-positive cells were detected mainly in the dorsal striatum with a few in the central portion. Moreover there was a strong correlation between the number of surviving grafted neurons and the growth of their fiber into the host striatum and the extent of recovery.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurotensin causes a greater increase in the metabolism of dopamine in the accumbens than in the striatum in vivo

Differential pulse voltammetry with carbon fibre electrodes was used to study the effect of central administration of neurotensin on the extracellular level of 3,4-dihydroxyphenylacetic acid (DOPAC) in the nucleus accumbens and the striatum in anaesthetised rats. Intracerebroventricular injection of neurotensin (10 micrograms) increased the peak height for DOPAC 20 min after administration in the nucleus accumbens but only after 40 min in the striatum. The maximum increase was similar in both regions, with 30% and 27% above the pre-injection basal level, respectively. Neurotensin (1 micrograms) however increased the extracellular level of DOPAC in the nucleus accumbens alone. Neurotensin (0.1, 1.0 and 3.0 micrograms/0.5 microliter), injected into the ventral tegmental area, induced a potent and long-lasting elevation of the peak height for DOPAC in the nucleus accumbens, while the same doses in the substantia nigra produced effects on the metabolism of dopamine in the striatum of smaller amplitude and shorter duration. The maximum effect of each dose was about 2.5 times greater in the mesolimbic, compared to the nigrostriatal system. Amphetamine (2 mg/kg, s.c.) decreased the extracellular level of DOPAC with a similar magnitude, both in the nucleus accumbens (52%) and the striatum (47%). Intracerebroventricular administration of neurotensin (1 micrograms), 5 min after amphetamine, did not alter the effect of amphetamine on the extracellular level of DOPAC either in the nucleus accumbens or the striatum. However, neurotensin (10 micrograms) partially reversed the effect of amphetamine in the nucleus accumbens and had a similar but smaller and delayed effect in the striatum. The results from the present study, together with previous neurobehavioural studies, suggest that neurotensin has a relatively selective action on the mesolimbic dopaminergic system in the rat.

Active transport pumps of HVA and DOPAC in dopaminergic nerve terminals

The effect of the membrane potential on the efflux of HVA and DOPAC from DA neurons was studied in anesthetized (1% halothane in gas mixture of 70% N2O and 30% O2) cats. Extracellular DA, HVA and DOPAC were measured continuously from the putamen, the hypothalamus, the thalamus, the raphe nuclei and the cortex using brain microdialysis technique combined with HPLC-ED monoamine measurements. HVA and DOPAC concentrations were highest in the putamen and lowest in the cerebral cortex. Extracellular HVA levels exceed those of the DOPAC. Increases in the extracellular potassium from 4 to 120 mM invariably produced decreases of the extracellular HVA and DOPAC in all the tested brain regions. These decreases were inversely proportional to the extracellular potassium concentration. Thus, it is concluded that the HVA and the DOPAC are extruded from inside the cell to the extracellular space by active mechanisms of transport similar to that reported for 5-HIAA in serotonergic neurons.

In vivo neurochemical and behavioural effects of intracerebrally administered neurotensin and D-Trp11-neurotensin on mesolimbic and nigrostriatal dopaminergic function in the rat

The effects of intracerebral application of neurotensin on the behavioural responses to peripheral dopamine agonist administration to rats, and on the levels of DOPAC voltammetrically determined in the striatum and nucleus accumbens of the anaesthetized rat have been examined. Bilateral application of neurotensin to the nucleus accumbens, like the neuroleptic haloperidol, inhibited the hyperactivity response to the dopamine agonist, n,N-propylnorapomorphine, but, unlike haloperidol, its bilateral intrastriatal application failed to reduce the degree of stereotyped behaviour induced by peripheral apomorphine injection. In the halothane-anaesthetized rat, neurotensin, when applied to the ventral tegmental area, stimulated DOPAC production in the ipsilateral nucleus accumbens, while its application to the substantia nigra did not affect striatal DOPAC levels significantly. Following its intracerebroventricular injection, however, DOPAC levels were considerably enhanced in both regions. The 11-D-tryptophan-substituted analogue of neurotensin potently mimicked the effects of the peptide itself in all studies. The findings of these investigations further confirm in vivo that the functional antagonism of dopamine by neurotensin is selective for the mesolimbic system, and that the effects of neurotensin can be correlated with reports on the regional distribution of its high-affinity binding sites in the rat brain.

Differential effects of forced locomotion, tail-pinch, immobilization, and methyl-beta-carboline carboxylate on extracellular 3,4-dihydroxyphenylacetic acid levels in the rat striatum, nucleus accumbens, and prefrontal cortex: an in vivo voltammetric study

In vivo voltammetry with carbon fiber electrodes was used to assess extracellular 3,4-dihydroxyphenylacetic acid (DOPAC) levels in striatum, nucleus accumbens, and anteromedial prefrontal cortex of freely moving rats subjected to altered motor activity or anxiogenic stimuli. Forced locomotion on a rotarod for 40 min caused an increase in extracellular DOPAC levels in the striatum and to a lesser extent in the nucleus accumbens but not in the prefrontal cortex. Subcutaneous injection of the anxiogenic agent methyl-beta-carboline carboxylate (10 mg/kg) increased extracellular DOPAC levels to a similar extent in prefrontal cortex and nucleus accumbens. Immobilization for 4 min augmented dopamine (DA) metabolism preferentially in the nucleus accumbens and to a lesser extent in the prefrontal cortex. Tail-pinch caused a selective activation of DA metabolism in the nucleus accumbens. None of these stimuli altered extracellular striatal DOPAC levels. These results confirm the involvement of dopaminergic systems projecting to the striatum and nucleus accumbens in motor function and suggest that mesolimbic and mesocortical dopaminergic systems can be specifically activated by certain kinds of anxiogenic stimuli; the relative activation of either of these latter systems could depend primarily on the nature (sensory modality, intensity) of the acute stressor.

In vivo monitoring of catecholaminergic metabolism in the C1 region of rat medulla oblongata: a comparative study by voltammetry and intracerebral microdialysis

In vivo voltammetry or microdialysis was used to monitor catecholaminergic metabolism in the C1 region of the ventrolateral medulla oblongata of anesthetized rats. In vivo voltammetry allowed the recording of a catechol oxidation current (CA.OC) peak in this region. This CA.OC was suppressed after inhibition of monoamine oxidase by pargyline or after inhibition of tyrosine hydroxylase by alpha-methyl-p-tyrosine and was markedly increased after blockade of dopamine-beta-hydroxylase by FLA 63. Similar results were found when intracerebral microdialysis coupled with HPLC and electrochemical detection was used to measure the concentration of 3,4-dihydroxyphenylacetic acid (DOPAC) in the dialysates obtained from the C1 region: The changes in CA.OC and DOPAC concentration in the dialysates exhibited very similar kinetic characteristics in the three pharmacological experiments. These results support the involvement of DOPAC as a major component of the electrochemical signal recorded by voltammetry in the C1 group of adrenergic neurons.

Physical and chemical considerations in the in vitro calibration of microdialysis probes for biogenic amine neurotransmitters and metabolites

The object of the present study was to examine the effects of temperature, oxidation, and pH on in vitro relative recovery of catecholamine and indoleamine neurotransmitters and their metabolites using microdialysis probes. Relative recovery of norepinephrine (NE), dihydroxyphenylacetic acid (DOPAC), 5-hydroxyindoleacetic acid (5HIAA), dopamine (DA), homovanillic acid (HVA), and 5-hydroxytryptamine (5HT) increased with temperature from 0 to 46 degrees C. For each compound, the increase in the amount recovered with increasing temperature was different. The stability of norepinephrine and dopamine was not affected at any temperature using deoxygenated calibration standard solutions containing ascorbic acid but was greatly reduced when exposed to ambient air without antioxidant treatment; catecholamine metabolites and the indole compounds were less affected. No change for in vitro relative recovery was observed by varying the pH of the perfusing solution from 6 to 8. Thus, temperature control in probe calibration as well as analyte stability using antioxidant treatment are important in reducing the error when estimating extracellular concentrations of neurotransmitter and metabolites.

Delta 9-tetrahydrocannabinol produces naloxone-blockable enhancement of presynaptic basal dopamine efflux in nucleus accumbens of conscious, freely-moving rats as measured by intracerebral microdialysis

This study examined the effects of acute administration of delta-9-tetrahydrocannabinol (delta 9-THC), the psychoactive ingredient in marijuana, on extracellular efflux of dopamine (DA) and its metabolites as measured by in vivo microdialysis in nucleus accumbens of conscious, freely-moving rats. delta 9-THC, at low doses (0.5-1.0 mg/kg), which significantly enhance brain stimulation reward (intracranial self-stimulation), significantly increased DA efflux in nucleus accumbens. Augmentation of DA efflux by delta 9-THC was abolished by removal of calcium (Ca++) ions from the perfusion fluid, indicating a Ca(++)-dependence of delta 9-THC's action. Augmentation of DA efflux by delta 9-THC was either totally blocked or significantly attenuated by doses of naloxone as low as 0.1 mg/kg. Given the postulated role of mesocorticolimbic DA circuits in mediating and/or modulating brain stimulation reward, the present data raise the possibility that marijuana's rewarding effects, and hence its euphorigenic effects and abuse potential, may be related to pharmacological augmentation of presynaptic DA mechanisms. Additionally, the DA mechanisms enhanced by marijuana appear to be modulated by an endogenous opioid peptide system.

Biochemical and in vivo voltammetric evidence for differences in striatal dopamine levels in inbred strains of mice

High performance liquid chromatography with electrochemical detection and differential pulse voltammetry were used to provide a direct measurement of tissue content of dopamine and its metabolites and extracellular dopamine levels, respectively, in the striata of BALB/c and CBA inbred strains of mice. We found that levels of striatal dopamine and its metabolite, dihydroxyphenylacetic acid, were significantly higher in the CBA strain than in the BALB/c strain, whereas levels of homovanillic acid were not significantly different between the strains. Levels of the dopamine metabolite 3-methoxytyramine, on the other hand, were higher in the BALB/c mice. Dopamine turnover rates were significantly higher in the CBA strain when the homovanillic acid/dopamine ratio was used as an index of dopamine activity. Voltammetric recording showed that the local infusion of K+ in pargyline-treated mice resulted in the immediate appearance of a peak at +85 mV, which has been shown to correspond to extracellular dopamine in the rat. The mean height of this peak detected in vivo following K+ stimulation corresponds to in vitro dopamine concentrations of 25 +/- 8 microM for BALB/c mice and 7 +/- 2 microM for CBA mice. K(+)-stimulated dopamine release in the BALB/c mice could be evoked every 10-15 min with similar magnitude. In contrast, very little dopamine release in CBA mice could be evoked after the first stimulation. Since striatal dopamine levels are higher in CBA mice, these data suggest that (a) BALB/c strain may have more dopamine in the readily releasable pool, whereas the CBA mice have a larger storage pool of dopamine, and/or (b) that dopamine uptake in the CBA mice is much more avid than in BALB/c.

Difference in the effects of the antidepressant tianeptine on dopaminergic metabolism in the prefrontal cortex and the nucleus accumbens of the rat. A voltammetric study

The effects of the new tricyclic antidepressant tianeptine were investigated on dopaminergic (DAergic) metabolism in the anteromedian prefrontal cortex and the nucleus accumbens of the rat. DAergic metabolism was assessed by the measurement of DOPAC, the main presynaptic metabolite of dopamine, using in vivo voltammetry in rats ventilated with halothane (0.5-0.75% in air). Acute treatment with tianeptine (10 mg/kg, 20 mg/kg) only increased significantly DOPAC levels in the anteromedian prefrontal cortex. After chronic treatment with tianeptine (15 days, 2 times/day) the increases in DOPAC levels in this structure were altered and less pronounced with the 20 mg/kg dose. Previous studies led to suggest that both acute and chronic effects on DAergic terminals in the anteromedian prefrontal cortex may be involved in the therapeutic action of this new antidepressant.

Post-mortem dopamine dynamics assessed by voltammetry and microdialysis

The effects of total brain ischemia (decapitation) on striatal extracellular levels of dopamine (DA) dihydroxyphenyl acetic acid (DOPAC) and ascorbic acid (AA) in chloral hydrate anesthetized rats were monitored at 1-min intervals by differential normal pulse voltammetry (DNPV) with numerical deconvolution of the catechol peak. Changes in pH were assessed by the shift of AA oxidation potential and incorporated into the computational procedure. The AA peak showed a sharp, short-lived (less than 15 min) postdecapitation rise, followed by a slower secondary increase. The DA signal increased 100-fold in the first 20 min followed by a slow decline. DOPAC levels fell 80% within 15 min after death. The post-mortem changes in extracellular DA and DOPAC were verified by a similar experiment using microdialysis. These observations probably reflect massive release and impaired uptake of DA combined with reduced monoamine oxidase activity. Changes in membrane permeability to DOPAC as a consequence of a post-mortem drop in pH may also contribute to the decline in extracellular DOPAC levels.