In vivo voltammetry: monitoring of dopamine metabolites in CSF following release by electrical stimulation

Correlation of 3-mercaptopropionic acid induced seizures and changes in striatal neurotransmitters monitored by microdialysis

OBJECTIVES

The goal of this study was to use a status epilepticus steady-state chemical model in rats using the convulsant, 3-mercaptopropionic acid (3-MPA), and to compare the changes in striatal neurotransmission on a slow (5min) and fast (60s) timescale. In vivo microdialysis was combined with electrophysiological methods in order to provide a complete evaluation of the dynamics of the results obtained.

OBJECTIVE

To compare the effects of a steady-state chemical model pof status epilepticus on striatal amino-acid and amine neurotransmitters contents, as measured via in vivo microdialysis combined with electrophysiological methods. Measurements were performed on samples collected every 60s and every 5min. "Fast" (60s) and "slow" (5min) sampling timescales were selected, to gain more insight into the dynamics of GABA synthesis inhibition and of its effects on other neurotransmitters and on cortical electrical activity.

METHODS

3-MPA was administered in the form of an intra-venous load (60mg/kg) followed by a constant infusion (50mg/kg/min) for min. Microdialysis samples were collected from the striatum at intervals of 5min and 60s and analyzed for biogenic amine and amino acid neurotransmitters. ECoG activity was monitored via screws placed over the cortex.

RESULTS

In the 5min samples, glutamate (Glu) increased and γ-aminobutyric acid (GABA) decreased monotonically while changes in dopamine (DA) concentration were bimodal. In the sixty second samples, Glu changes were bimodal, a feature that was not apparent with the 5min samples. ECoG activity was indicative of status epilepticus.

CONCLUSIONS

This study describes the combination of in vivo microdialysis with electrophysiology to monitor the effect of 3-MPA on neurotransmission in the brain. This led to a better understanding of the chemical changes in the striatum due to the applied 3-MPA chemical model of status epilepticus.

Flow-injection analysis systems with different detection devices and other related techniques for the in vitro and in vivo determination of dopamine as neurotransmitter. A review

Dopamine (DA) is one of the most important catecholamine neurotransmitters in the human central nervous system in the brain and plays a key role in the functioning of the renal, hormonal, and cardiovascular systems. Abnormal levels of dopamine are related to neurological disorders, such as schizophrenia and Parkinson's disease and the control and fluctuations of the amount of dopamine are extremely important in monitoring with analytical systems in the human brain. This review covers the attributes of flow-injection analysis systems with different detection devices and other related techniques for the in vitro and in vivo determination of dopamine as neurotransmitter and points out the advantages and disadvantages in the implementation thereof.

Application of scanning electrochemical microscopy to biological samples

The scanning electrochemical microscope can be used in the feedback mode in two-dimensional scans over biological substrates to obtain topographic information at the micrometer level. In this mode, the effect of distance between a substrate (either conductive or insulating) and a scanning ultramicroelectrode tip on the electrolytic current flowing at the tip is recorded as a function of the tip x-y position. Scans of the upper surface of a grass leaf and the lower surface of a Ligustrum sinensis leaf (which show open stomata structures) immersed in aqueous solution are shown. Scans of the upper surface of an elodea leaf in the dark and under irradiation, where the tip reaction is the reduction of oxygen produced by photosynthesis, demonstrate the possibility of obtaining information about the distribution of reaction sites on the substrate surface.

In vivo [3H]spiperone binding: evidence for accumulation in corpus striatum by agonist-mediated receptor internalization

The processes of receptor internalization and recycling have been well-documented for receptors for hormones, growth factors, lysosomal enzymes, and cellular substrates. Evidence also exists that these processes also occur for beta-adrenergic, muscarinic cholinergic, and delta-opiate receptors in frog erythrocytes or cultured nervous tissue. In this study, evidence is presented that agonist-mediated receptor internalization and recycling occurs at the dopamine receptor in rat corpus striatum. First, the in vivo binding of the dopamine antagonist [3H]spiperone was increased by both electrical stimulation and pharmacologically induced increases of dopamine release. Conversely, depletion of dopamine with reserpine decreased in vivo [3H]spiperone binding, but the same reserpine treatment did not alter its in vitro binding. Second, the rate of dissociation of [3H]spiperone from microsomal membranes prepared from rat striatum following in vivo binding was fivefold slower than its dissociation following in vitro equilibrium binding. Mild detergent treatment, employed to disrupt endocytic vesicle membranes, increased the rate of dissociation of in vivo bound [3H]spiperone from microsomal membranes to values not significantly different from its in vitro bound dissociation rate. Third, treatment of rats with chloroquine, a drug that prevents receptor recycling but not internalization, prior to [3H]spiperone injection resulted in a selective increase of in vivo [3H]spiperone binding in the light microsome membranes. The existence of mechanisms that rapidly alter the number of neurotransmitter receptors at synapses provides dynamic regulation of receptors in response to varied acute stimulation states.

Determination of the endogenous and evoked release of catecholamines from the hypothalamus and caudate nucleus of the conscious and unrestrained rat

The action of catecholamines within the CNS is important for the expression of numerous vegetative and behavioral functions. To understand the role these amines play, it is necessary to measure changes in the levels of these transmitter substances by utilizing new developments and methodology in the behaving animal. Utilizing new developments in methodology, it is possible to measure the release of amines into perfusates obtained from specific sites in the brain of the rat under basal and evoked conditions without prior purification or concentration. Using the push-pull perfusion technique, perfusates were obtained from the hypothalamus and caudate nucleus and analyzed by liquid chromatography with electrochemical detection. It is possible to readily determine basal release of dopamine from the caudate nucleus. Detection of both dopamine and noradrenaline is possible under ephedrine stimulated conditions from both the caudate nucleus and the hypothalamus. Although levels of serotonin (5-HT) were detected in brain perfusates, it may not be of neuronal origin. It may be possible to use these techniques to delineate the roles these amines play in various physiological functions.

In vivo voltammetry: promise and perspective

Dopamine, 5-hydroxytryptamine and noradrenaline are electroactive (oxidisable) neurotransmitters in the mammalian brain. Voltammetry, a technique which can measure the concentration of such compounds by their oxidation at an inert electrode, has been applied in vivo in the hope of measuring the release of these neurotransmitters without recourse to perfusion-based or post-mortem analyses. The measurement of neurotransmitter release is, however, complicated by the presence of high concentrations of other electroactive species (ascorbic and uric acids). Nevertheless, when used properly, with due emphasis on pharmacological identification of electrochemical signals, the technique can measure catechol and indole metabolites in vivo. Under certain circumstances the release of the catecholamines and 5-hydroxytryptamine themselves can be measured. The advantages and drawbacks of the voltammetric methodology are discussed.

Monitoring the stimulated release of dopamine with in vivo voltammetry. II: Clearance of released dopamine from extracellular fluid

Microvoltammetric electrodes implanted in the caudate nucleus of the anesthetized rat have been used to monitor dopamine released following electrical stimulation of the medial forebrain bundle. These electrodes are fabricated from unmodified carbon fibers and have been used with normal pulse voltammetry. Dopamine appears in the vicinity of the electrode when the stimulation is initiated, and disappears almost immediately when the stimulation is terminated. The data suggest that the effective diffusion distance is less than 100 micron. Postmortem analysis using liquid chromatography with electrochemical detection shows that dopamine released in this manner is metabolized to 3,4-dihydroxyphenylacetic acid (DOPAC); however, neither substance is observed electrochemically in the extracellular fluid within seconds after the stimulation. In addition, inhibitors of neuronal uptake of dopamine, amphetamine (1.8 or 15 mg X kg-1) or benztropine (25 mg X kg-1), or of dopamine metabolism, pargyline (150 mg X kg-1) or tropolone (100 mg X kg-1), do not significantly affect the rate at which dopamine disappears from extracellular fluid, although they can affect the amount released. These results suggest that dopamine cannot freely diffuse in the extracellular fluid because an extraneuronal uptake mechanism exists that clears dopamine from extracellular fluid into an extraneuronal pool where metabolism to 3,4-dihydroxyphenylacetic acid occurs. Dopamine can be observed during electrical stimulation of the ascending fibers because neuronal and extraneuronal uptake systems are unable to remove dopamine on these short time scales.

Differential pulse voltammetry in the dorsal horn of the spinal cord of the anesthetized rat: are the voltammograms related to 5-HT and/or to 5-HIAA?

Treated carbon fiber microelectrodes were used with the differential pulse voltammetry method for in vitro and in vivo determination of indoleamines. Under these conditions a peak of oxidation current which is characteristic of 5-hydroxyindoles is recorded at 280-300 mV. Treated carbon fiber microelectrodes respond in vitro linearly over a large range of concentrations of 5-hydroxytryptamine (5-HT) and of 5-hydroxyindoleacetic acid (5-HIAA), but are 5-8 times more sensitive to 5-HT than to 5-HIAA. In vivo, the question remains as to the exact nature of the peak because the oxidation potentials of 5-HT and 5-HIAA are close together and cannot be monitored separately. Pharmacological investigations were hence carried out in order to characterize the electrochemical signal detected at 300 mV in the dorsal horn of the lumbar spinal cord of chloral hydrate-anesthetized rats. Using 250 micron long carbon fiber microelectrodes, the electrochemical signal stabilizes at 30-90 min and the peak remains constant for up to 210 min. Administration of the monoamine oxidase inhibitor (MAOI) clorgyline produced a progressive decrease of the signal which reached a decrease of 33% of control at 180 min after injection. At this time biochemical measures demonstrated a 117% increase in 5-HT and a 32% decrease in 5-HIAA in the dorsal half of the spinal cord. Reserpine provoked an increase of 20% in the electrochemical peak and the 5-HIAA outflow blocker probenecid gave rise to a sustained plateau of about 60% above control values.(ABSTRACT TRUNCATED AT 250 WORDS)

Probenecid sensitive pathway of elimination of dopamine and serotonin metabolites in CSF of the rat

CSF was removed at a constant flow rate of 1 microliter/min from the third ventricle of anesthetized rats. Five microliter CSF samples were directly injected every 15 min into a liquid chromatographic system coupled with an amperometric detector. Mean CSF values for free dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindolacetic acid (5-HIAA) were 1.4, 0.9, and 2.6 X 10(-6)M respectively. High doses of probenecid resulted in a linear increase of acidic metabolite concentrations which gave an index of the fractional turnover rates related to the resorption by the weak organic acid carrier. Accumulation rates were 0.24, 0.87, and 1.58 mumol/l/h for DOPAC, HVA and 5-HIAA respectively. This route of elimination was predominant for 5-HIAA while it represented only a small part of total turnover for DOPAC. A high elimination rate constant for HVA validates the use of control levels of this metabolite as an indication of fractional HVA turnover dependent upon probenecid-sensitive carrier.

Chronoamperometry in vivo: does it interfere with spontaneous neuronal activity in the brain?

To test whether chronoamperometry in vivo interferes with spontaneous neuronal activity, chronoamperometric measurements were combined with electrophysiological recordings in the same preparation. Chronoamperometric measurements (0.5--1.0 V applied for 1 s) were taken in the rat corpus striatum and single unit activity was recorded extracellularly in the same area. With potentials of 0.5 V, chronoamperometric measurements did not interfere with spontaneous activity of the striatal neurons, even of those units situated in close proximity (approximately 100 micrometers) to the tip of the electrochemical working electrode. Chronoamperometric measurements at potentials from 0.6 to 1.0 V accelerated or inhibited the firing rates of part of the striatal neurons, even when the electrophysiological circuit was interrupted during the application of the chronoamperometric pulse.

Homovanillic acid in the cerebrospinal fluid of Parkinsonian patients

Determinations of CSF HVA before and after Probenecid administration were made in 41 patients with Parkinson’s disease. The means of HVA concentration were lower than those of controls but no correlation with clinical data was found. A negative correlation was demonstrated between post-probenecid HVA levels and subsequent score improvement with L-DOPA at 3,6 and 12 months. Post-probenecid HVA levels suggest that there are distinct forms of parkinsonism and could predict the response to L-DOPA therapy.

In vivo voltammetry with electrodes that discriminate between dopamine and ascorbate

Untreated carbon-fiber voltammetric electrodes have been employed as chemical sensors of easily oxidized compounds in the brains of rats anesthetized with chloral hydrate. These electrodes can be used to distinguish dopamine from ascorbate and dihydroxyphenylacetate by the shape of the voltammograms. The electrodes are shown to provide a reproducible response to different neuronal stimuli. The rapid release of dopamine in the caudate nucleus can be measured following a local application of potassium chloride. Intraperitoneal injections of amphetamine also induce an increase of easily oxidized compounds; however, the voltammetry suggests that ascorbic acid, rather than dopamine, is the primary substance detected. Measurements in the cortex or in the caudate nucleus of animals lesioned by prior injection of 6-hydroxydopamine show that a substance with voltammetric properties identical to those of ascorbic acid also increases in concentration in these areas as a result of amphetamine administration.

Linear sweep voltammetry with carbon paste electrodes in the rat striatum

Voltammetry has been widely used in attempts to measure catecholamine release in vivo. The voltammogram recorded in the rat striatum using carbon paste electrodes and linear sweep voltammetry with semidifferentiation consists of a number of separate peaks; changes in the height of the first of these peaks have been attributed to changes in catecholamine release. We have found that ascorbate, either microinjected into the striatum or injected intraperitoneally, increases the height of the first peak without changing its potential. Microinjection of dopamine or 3,4-dihydroxyphenylacetic acid, or intraperitoneal injection of 3,4-dihydroxyphenylalanine, caused a shift in the potential of peak 1 of 25-50 mV in a positive direction. Amphetamine, administered intraperitoneally to freely moving animals, caused an increase in the height of the first peak but did not change its potential. Oxidation potentials in vitro and the effect of other drugs on the voltammogram obtained in vivo were also measured. Peak 1 is caused by the oxidation of both ascorbate and catechols whose oxidation potentials differ by only 50 mV in vivo; the contribution of catechols in control animals is negligible. Shifts in the potential of peak 1 caused by drugs are not due to changes in the oxidation potentials of the components but to a change in their relative contributions. Therefore changes in the height of peak 1 with no change in position do not represent changes in the extracellular concentration of catechols but are due to changes in ascorbate concentration. Changes in the concentration of catecholamine-related compounds can be detected at potentials some 50 mV greater than that of the first peak.

Normal rats trained to circle show asymmetric caudate dopamine release

Caudate catecholamine release was monitored by bilateral in vivo electrochemical electrodes in male Sprague-Dawley rats trained to circle for sucrose/water reward. Baseline release of dopamine was equal from both sides of caudate. When reinforced circling began, 44 +/- 4 percent greater catechol release occurred from the caudate contralateral to the circling direction. As turning subsided, differential release returned to basal levels. Further evidence that the catecholamine metabolism was affected by turning was obtained by direct measurement of caudate dopamine and DOPAC at selected time points. Concentration data showed relative increases in dopamine and DOPAC in the contralateral caudate. These data provide evidence that dopamine is released asymmetrically from caudate in unlesioned rats during voluntary behavior.

Differential voltammetric measurement of catecholamines and ascorbic acid at surface-modified carbon filament microelectrodes

Triple-barrel carbon filament microelectrodes were chemically and electrochemically oxidized to allow resolution of ascorbic acid from catecholamines during differential pulse voltammetric scans. The contribution of current from DOPAC to the catecholamine signal was minimized by this treatment. These electrodes exhibit linearity over a wide range of concentrations and long-term response stability.

Dopamine release in rat striatum after administration of L-dope as studied with in vivo electrochemistry

The time-course of changes in dopamine (DA) release induced by systemic administration of L-DOPA was studied in the rat striatum using electrochemistry in vivo. Recordings were obtained from anesthetized animals in which carbon past microelectrodes were implanted stereotaxically into the striatum. Chronoamperometric measurements were made by applying 0.8 V for 1 sec, with an interval of 5 min between measurements; in some experiments, the potential was scanned every 5 min from 0 to 1.0 V. Administration of L-DOPA (50 mg/kg) after pretreatment with an inhibitor of peripheral aromatic amino acid decarboxylase (AAAD) produced a large rise in chronoamperometric currents, lasting for about 6 h. Different approaches were used to determined whether DOPA, DA or DA metabolites (all of which are oxidized at the potential chosen) were responsible for these DOPA-induced increases. (1) Rats were killed at different times after L-DOPA administration, and their striatal were analyzed biochemically for DOPA, DA, and DA metabolites. Comparison of time-courses indicated that the rise in chronoamperometric responses correlated with elevations in striatal 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) levels rather than with those of DOPA or DA, suggesting that DA metabolites contribute significantly to the increases in chronoamperometric responses observed after administration of L-DOPA. (2) Administration of L-DOPA after pretreatment with an inhibitor of central AAAD, a treatment that results in large increases in striatal DOPA levels but abolishes L-DO DA, suggesting that DA metabolites contribute significantly to the increases in chronoamperometric responses observed after administration of L-DOPA. (2) Administration of L-DOPA after pretreatment with an inhibitor of central AAAD, a treatment that results in large increases in striatal DOPA levels but abolishes L-DO DA, suggesting that DA metabolites contribute significantly to the increases in chronoamperometric responses observed after administration of L-DOPA. (2) Administration of L-DOPA after pretreatment with an inhibitor of central AAAD, a treatment that results in large increases in striatal DOPA levels but abolishes L-DOPA-induced formation of DA and DA metabolites (measured biochemically), failed to increase chronoamperometric responses. This finding indicates that exogenous L-DOPA entering the striatum is not detected with this technique and that it must be decarboxylated to produce the elevations of currents. (3) In animals pretreated with an inhibitor of monoamine oxidase to abolish formation of DOPAC and HVA, L-DOPA administration produced only a small and short lasting increase in chronomaperometric responses, despite sustained and large increases in striatal DA concentrations. By using the potential-scanning technique we showed that these increase in electrochemical responses occur at a potential at which DA but not methylated catechols are oxidized...

Differential pulse voltammetry in brain tissue. I. Detection of 5-hydroxyindoles in the rat striatum

In vitro, differential pulse voltammetry combined with electrochemically treated carbon fiber electrodes enabled detection, in different solution of 5-hydroxyindole compounds, of an oxidation peak 3 at +300 mV. In vivo, a striatal peak 3 was also recorded at this potential. Electrolytic or 5,7-dihydroxytryptamine lesions interrupting the medial forebrain bundle (MFB) were followed by a decrease of 65% and 64% in peak height, but not elimination of the peak. Biochemical determinations were significantly correlated to the peak 3 measurements. The existence of peak 3 as well as hydroxyindole compounds in blood suggested a blood contamination under the experimental conditions employed. This possibility is confirmed both by the complete disappearance of striatal peak 3 in animals with the MFB lesioned and surgically prepared a week before recordings, and by biochemical measurements in parachlorophenylalanine-treated or perfused (phosphate-buffered saline solution) animals.

Homovanilic acid in Huntington's disease and Sydenham's chorea

Homovanilic acid (HVA) was determined in the lumbar CSF of 12 patients with Huntington's disease and 12 with Sydenham's chorea before and after probenecid administration. The means of HVA concentration (basal and after probenecid) were lower in those with Huntington's disease than in controls, and were even lower in a sub-group characterised by increased tone and slowness of voluntary movement. There was no correlation between CSF HVA values and the severity of abnormal movements, nor with length of the illness and age of the patients with Huntington's disease. The mean basal HVA concentration did not differ from controls in those with Sydenham's chorea but the accumulation with probenecid was significantly lower. These results suggest a decrease in cerebral dopamine release in both forms of chorea.

In vivo electrochemical detection of catechols in the neostriatum of anaesthetized rats: dopamine or DOPAC?

Electroanalytical techniques for the in vivo measurement of neurotransmitters in brain tissue have been applied especially to the catecholamines, which are easily oxidizable. Measurements are, however, complicated by the presence of ascorbic acid (AA) in brain tissue. Lane et al. have been able to circumvent this problem, at least in part, by the application of differential pulse voltametry (DPV) to a surface-modified platinum electrode, obtaining distinct oxidation current peaks in recordings from the rat neostriatum which are attributed to AA and to dopamine (DA), respectively, but which are also unstable. We have recently described a new type of electrode, consisting of a pyrolytic carbon fibre 8 micrometers thick and 0.5 mm long. We now report that the DPV method used in conjunction with an electrochemical treatment of this electrode yields stable and reproducible peaks in which catecholamines and AA are resolved from each other. Moreover, pharmacological investigations suggest that the catecholamine peak measured in vivo in the rat neostriatum should be attributed to 3, 4-dihydroxyphenylacetic acid (DOPAC), suggesting that our technique may be a useful means of following dopaminergic activity in vivo.

An automated electrochemical method for in vivo monitoring of catecholamine release

A computer-controlled electrochemical system for the in vivo monitoring of catecholamines and their metabolites is described. The system is composed of a microcomputer, real time interface board, and a commercial electrochemical instrument. The method and theory of the electrochemical technique are explained and details concerning the construction of the electrodes are given. Descriptions of the hardware and software are also provided. Experimental results are presented to demonstrate the use of the system for long-term in vivo monitoring and for detecting the change in the level of oxidizable species after amphetamine administration.