Fast Cyclic Voltammetry Research Articles

Voltammetric evidence that subsensitivity to reward following chronic mild stress is associated with increased release of mesolimbic dopamine

Chronic exposure to mild unpredictable stress caused a decrease in rats' consumption of a palatable weak sucrose solution, which was reversed by chronic (5 weeks) administration of imipramine (5 mg/kg/day). Dopamine (DA) release in the nucleus accumbens (NAc) and caudate putamen (CPu) was measured in vivo using fast cyclic voltammetry, following electrical stimulation of the medial forebrain bundle. Experiments were performed under chloral hydrate anaesthesia 48 h after the termination of stress and the final imipramine injection. DA release was increased in the NAc of both stressed and imipramine-treated animals; imipramine did not normalize the increased DA release in stressed animals. In a further experiment, brain slices from stressed animals tended to be subsensitive to the inhibition of DA release in the NAc by quinpirole. No changes were observed in the CPu in any experiment. We discuss the relationship of these effects to stress-induced anhedonia.

Fast cyclic voltammetry can be used to measure stimulated endogenous 5-hydroxytryptamine release in untreated rat brain slices

Fast cyclic voltammetry at a carbon fibre microelectrode was used to monitor the time course of 5-hydroxytryptamine (5-HT) overflow in slices of rat dorsal raphe (DRN) and suprachiasmatic nuclei (SCN), incubated in a brain slice chamber for over 6 h. 5-HT overflow was detected in response to electrical brain stimulation in both regions. Voltammetric evidence showed that the released substance was identical to exogenously applied 5-HT. Overflow was reversibly abolished when Ca 2+ was removed from the incubating medium or when TTX was added. Ro4–1284, a reserpine like agent, irreversibly abolished 5-HT overflow from both nuclei. The 5-HT uptake blockers, citalopram, clomipramine, fenfluramine and fluvoxamine dose dependently increased overflow and slowed the rate of removal of 5-HT from the extracellular space in both regions. Benztropine had no effect on overflow in the DRN and SCN whereas it caused a significant increase in dopamine overflow in slices of caudate putamen (CPu). Xylamine had no effect on 5-HT overflow in the DRN and SCN. This evidence indicates that the release of endogenous 5-HT can be measured reliably for long periods and that FCV can be used in brain slices for quantitative studies of 5-HT release and uptake.

Presynaptic regulation of electrically evoked dopamine overflow in nucleus accumbens: a pharmacological study using fast cyclic voltammetry in vitro

Fast cyclic voltammetry has been used to measure electrically evoked dopamine overflow from slices of rat nucleus accumbens in vitro. The substance detected was shown voltammetrically and biochemically to be dopamine of neuronal origin. Enough dopamine was released by a single electrical pulse to be easily detectable, and under these conditions there was no auto-inhibition by the endogenous transmitter (as demonstrated by the failure of dopamine antagonists to increase the amount released). There was no significant inhibition, or enhancement, of release by agonists at the following receptor types: dopamine D1, 5-hydroxytryptamine, cholinoceptors, alpha 1-, alpha 2-, beta-adrenoceptors, cholecystokinin or neurotensin receptors. However, the dopamine D2 receptor agonist, quinpirole, was capable of totally inhibiting the release; this effect was concentration-dependently antagonized by the D2 antagonists haloperidol, sulpiride, metoclopramide and clozapine, with potencies which corresponded to their affinities for D2 receptors in striatal tissue. The results show that the presynaptic receptors on dopaminergic nerve terminals are of the D2 type and apparently identical to those in the corpus striatum.

Differential effects of dopamine agonists upon stimulated limbic and striatal dopamine release: in vivo voltammetric data

1. Fast cyclic voltammetry at carbon fibre microelectrodes was used in rats anaesthetized with chloral hydrate to monitor dopamine release in the caudate and nucleus accumbens evoked by electrical stimulation of the median forebrain bundle. Stimulation trains (50 Hz sinusoidal current, 100 +/- 10 microA r.m.s., 2s duration) were repeated every 5 min throughout the experiment. 2. The actions of the dopamine agonists quinpirole, pergolide, SKF 38393, bromocriptine, (+)-3-(3-hydroxyphenyl)-N-n-propylpiperidine ((+)-3PPP) and (-)-3PPP were compared in the two nuclei. 3. Bromocriptine (10 mg kg-1, i.p.) did not affect release in either nucleus while SKF 38393 caused a fleeting decrease in limbic but not striatal dopamine release at a high dose (20 mg kg-1, i.p.). 4. Quinpirole and pergolide (both 1 mg kg-1, i.p.) decreased stimulated dopamine release in the nucleus accumbens while in the caudate the drugs each caused a transient, though not quite significant, elevation of stimulated dopamine release followed by decrease in release of the same magnitude as that seen in the nucleus accumbens. 5. The (-)-enantiomer of 3PPP (20 mg kg-1, i.p.), a partial agonist at the dopamine autoreceptor, increased stimulated dopamine release in both nuclei although the action in the caudate was larger and more prolonged. (+)-3PPP (20 mg kg-1, i.p.), a full agonist, decreased release in the nucleus accumbens. A small, transient and not significant increase in the caudate was followed by decreased release. 6. The results are interpreted as being evidence for differences in the dopamine autoreceptor in the two nuclei, possibly in the affinity state of the receptor in each nucleus.

Fast differential ramp voltammetry: a new voltammetric technique designed specifically for use in neuronal tissue

This paper describes details of a new voltammetric technique designed specifically for use in living, especially neuronal, tissue. Neuronal tissue is an “active” electrical environment, and the function of the nerve cells may be disrupted by some voltammetric procedures. The new technique, which is called “Fast Differential Ramp Voltammetry (FDRV)”, has been developed from Fast Cyclic Voltammetry (FCV). FCV can be shown not to interfere with ongoing neuronal activity because the working electrode can be switched to record the electrical activity of the surrounding nerve cells in between voltammetric scans. The charging currents generated in FCV are however affected by local neuronal activity, and in the past this has limited the resolution of FCV in vivo. In FDRV these effects are reduced by a double or quadruple offset scan technique which cancels out most of the variation in signal due to fluctuations in charging currents. The resolution of FDRV should be adequate to study “basal” levels of neurotrans-mitters in vivo. The technique is immune to electrocatalytic effects and can resolve separate peaks for mixtures of re-reducible and non re-reducible electroactive compounds. This enables it to be used for separate measurement of, for example, dopamine and ascorbic acid in mixtures of the two.

Fast cyclic voltammetry: Measuring transmitter release in ‘real time’

Fast cyclic voltammetry (FCV) differs from all other voltammetric methods in the type of waveform used, the speed of measurement and the applications for which it is best suited. The main assets of FCV are its high spatial and temporal resolution, the use of a reduction scan and the ease with which it can be combined with unit activity recording at the same electrode. The drawbacks of the method are the inability to measure basal extracellular amine levels or changes in multicomponent signals, and the modest sensitivity of the technique. Principal applications of FCV are the monitoring of ionophoresed or pressure-ejected monoamines, direct measurement of stimulated amine release and uptake, and the study of brain ascorbic acid function. The high spatial (5 μm) resolution of FCV is such that the anatomical heterogeneity of brain nuclei can be measured in great detail. The temporal resolution (25 ms) means that transmitter release and uptake can be measured essentially in ‘real time’.

Fast cyclic voltammetry: improved sensitivity to dopamine with extended oxidation scan limits

Fast cyclic voltammetry at carbon fibre microelectrodes has been used in vitro and in vivo mainly to monitor the release and uptake of dopamine. The ‘standard’ input voltage waveform (−1000 to +1000 mV, vs. Ag/AgCl, 300 V/s scan rate) has a lower limit of detection for dopamine of about 200 nM. In the present study, the scan rate and cathodic limit (−1000 mV) were kept constant while the range of the anodic scan was altered between 800 and 1400 mV. Extension of the oxidation scan to 1400 mV led to a sevenfold increase in dopamine oxidation current relative to the ‘standard’ waveform. This was accompanied by a decrease in the dopamine oxidation peak potential. Increased reactant adsorption for both dopamine and DOPAC was the likely cause of enhanced sensitivity. In vivo experiments indicate that, when used with the 1400 mV scan, the carbon fibre electrodes do not poison on contact with brain tissue and can measure dopamine concentrations lower than 50 nM.

Evoked dopamine overflow from rut forebrain nuclei measured using fast cyclic voltammetry in vitro

Fast cyclic voltammetry (FCV) at carbon fibre microelectrodes was used to monitor endogenous noradrenaline (NA) efflux in superfused slices of bed nucleus of stria terminalis pars ventralis (BSTV) in ‘real time’. NA efflux was evoked by local electrical stimulation at bipolar tungsten stimulating electrodes. Confirmation of the identity of the released species as NA was made on the basis of anatomica, electrochemical and pharmacological proofs. Firstly, the signal matched the NA innervation density: efflux of monoamine was greater in BSTV than in the pars dorsalis of the nucleus. Secondly, the voltammogram of the released species was indistinguishable from those of the catecholamines NA and dopamine (DA) but dissimilar to that of the indoleamine serotonin (5-hydroxytryptamine, 5-HT). Thirdly, amine efflux was influenced in a predictable fashion by the drugs tested. Tetrodotoxin (10−6 M) or omission of Ca2+ from the superfusate reversibly reduced amine efflux by 90.2 and 88.0% respectively. Ro 4-1284 (10−6 M) decreased amine efflux by 75.8%. Desipramine (5 × 10−8 M), the selective NA uptake blocker, significantly increased amine efflux and uptake half-life (to 214.3 and 389.5% of control respectively). Fluvoxamine (5 × 10−7 M) and GBR 12909 (3 × 10−7 M), blockers of 5-HT and DA uptake respectively, had no effect on amine efflux, although fluvoxamine caused a modest (91.0%) increase in the uptake half-life. Pargyline (2 × 10−6 M) affected neither efflux nor uptake. The combined anatomical, electrochemical and pharmacological data confirm that the monoamine detected in BSTV by local electrical stimulation was NA. Stimulated NA efflux was stable and reproducible over at least 2.5 h (longest period tested). This study demonstrates the ability of FCV to selectively monitor endogenous NA efflux and uptake in ‘real time’ and with high spatial resolution.

Striatal dopamine terminals release serotonin after 5-HTP pretreatment: in vivo voltammetric data

Peripheral administration of 5-hydroxytryptophan (5-HTP) to rats causes ‘wet dog’ shakes and a parallel elevation of brain serotonin (5-HT) levels. The increase in 5-HT concentration does not, however, correlate with the endogenous 5-HT innervation raising the possibility that some 5-HTP is decar☐ylated in non-serotonergic cells. In the present study we used in vivo voltammetry to establish whether 5-HTP treatment led to formation of 5-HT as a ‘false transmitter’ in striatal dopamine (DA) neurons. Fast cyclic voltammetry at carbon fibre microelectrodes (CFMs) was used to monitor striatal monoamine release following electrical stimulation of the median forebrain bundle (MFB). In the absence of any pretreatment DA was the sole compound released by stimulation. However, when DA release was abolished withα-methyl-p-tyrosine (AMPT), 5-HTP administration (after peripheral decar☐ylase inhibition) caused a dose-dependent release of 5-HT, confirmed by the voltammetric characteristics. Central decar☐ylase inhibition prevented release indicating that 5-HTP itself was not released. By monitoring reduction peaks it was possible to record DA and 5-HT release simultaneously at a single CFM. While DA and 5-HT oxidised at the same potential their reduction peaks were separated by approximately 450 mV. It was shown, using this means, that 5-HT was still detectable even when DA release was not abolished by AMPT. DA and 5-HT release showed a significant positive correlation suggesting that they were released from the same nerves. We conclude that, after 5-HTP treatment, 5-HT can be released as a false transmitter from striatal DA neurones.

Fast cyclic voltammetry: measurement of dopamine in the presence of its biological precursors and metabolites

In vivo voltammetry is widely used to measure dopamine concentrations in the extracellular fluid of the animal brain. However, a potentially serious analytical problem is the presence in vivo of other compounds that may interfere with electrochemical measurements of dopamine. The present study examined the effects of physiological concentrations of various biological precursors and metabolites of dopamine on the detection of the parent compound with fast cyclic voltammetry at carbon fibre microelectrodes. No compound affected the potentials of the oxidation or reduction peaks for dopamine. Homovanillic acid caused an apparent increase in the sensitivity of the electrodes to dopamine while ascorbic acid decreased the height of the reduction peak. Effects were small and we conclude that physiological levels of precursors and metabolites do not constitute a hazard to the voltammetric detection of dopamine in vivo using fast cyclic voltammetry.

Application of fast cyclic voltammetry to measurement of electrically evoked dopamine overflow from brain slices in vitro

Fast cyclic voltammetry at a carbon fibre microelectrode was used to monitor the time course of dopamine overflow in slices of rat corpus striatum incubated in a brain slice chamber. Dopamine release occurred in response to electrical stimulation. Electrochemical, physiological and pharmacological evidence indicates that release of endogenous dopamine can be measured reliably for up to 9 h and that fast cyclic voltammetry can be used in brain slices for quantitative studies of dopamine release in the CNS.

Fast cyclic voltammetry at ultramicroelectrodes: Current measurement and ohmic drop positive feedback compensation by means of current feedback operational amplifiers

A current measurer for fast cyclic voltammetry at ultramicroelectrodes based on a current feedback operational amplifier is described. A factor of ca. 10 in bandwidth is thus gained as compared to previously described current measurers based on voltage feedback operational amplifiers, hence allowing the recording of cyclic voltammograms where the possible influence of the apparatus, distortion and even potential shift, is negligible up to several million volts per second. Using the same type of amplifier, an ohmic drop positive feedback compensation device in a two-electrode configuration was also designed, allowing an exact compensation without instrumental distortion of the faradaic response (error in the distance between cathodic and anodic peaks less than 10 mV) up to scan rates as high as 200,000 V s −1 . 500,000 V s −1 can be reached if an error of 30 mV is tolerated, corresponding to an uncertainty of 25% in the determination of a standard rate constant of 4 cm s −1.

Presynaptic regulation of dopamine release in corpus striatum monitored in vitro in real time by fast cyclic voltammetry

Dopamine release was evoked by single electrical pulses in slices of rat corpus striatum, and measured by fast cyclic voltammetry in real time. The magnitude of the release varied in the expected way to agents which modify dopamine storage, release and re-uptake. The presence of functional dopamine D 2 autoreceptors was demonstrated by showing that the release was potently and completely inhibited by the selective agonists quinpirole and N, N-dipropyl-5,6-ADTN. The selective D 1 agonist SKF 38393 was ineffective. The inhibition by quinpirole was competitively antagonised by haloperidol and metoclopramide with potencies which correspond closely to published values at postsynaptic striatal D 2 receptors. Thus, the D 2 autoreceptors on striatal nerve terminals appear to be indistinguishable from those on the postsynaptic neurons.

No mediolateral differences in striatal autoreceptor-mediated modulation of dopamine release: In vivo voltammetric data

Medial and lateral neostriatum differ qualitatively in their sources of dopamine (DA) innervation and in their behavioural functions. The present study sought to ascertain whether these differences were reflected in their response to DA autoreceptor drugs. Fast cyclic voltammetry was used to measure stimulated DA release at medial and lateral carbon fibre microelectrodes. Metoclopramide and (+)-3-PPP were used as autoreceptor antagonist and agonist respectively. No differences were observed in the level of DA release evoked by stimulation or in the agonist and antagonist responses at medial or lateral sites. We therefore conclude that differences between medial and lateral striatal DA function are not evident at the autoreceptor level.

Concentration-dependent actions of stimulated dopamine release on neuronal activity in rat striatum

Voltammetric analysis was combined with single unit recording to measure the effects of endogenous dopamine, released by electrical stimulation of the median forebrain bundle, on neuronal activity in the rat striatum in vivo. Fast differential ramp voltammetry, a more sensitive form of fast cyclic voltammetry, was used to measure extracellular dopamine levels during a 50-ms scan epoch every 500 ms. Using the same carbon fibre microelectrode, neuronal activity was recorded in between the electrochemical epochs. A steady-state electrochemical signal equivalent to about 100 nM dopamine was seen in the unstimulated striatum. The responses of 122 striatal units to stimulated dopamine release were recorded in 37 acute experiments. Ninety-one units which displayed a large spike amplitude ( ≥ 50 μV) were recorded during stimulated release of dopamine initially to levels of between 100 and 500 nM. The majority (49) showed a profound excitation, 23 showed inhibition, and nine units gave complex responses. Only 10 units were unresponsive. All the responses of these large units outlasted the transient increase in dopamine levels, often for more than 1 min. In contrast, all the 31 units which displayed a small spike amplitude (< 50 μV) were powerfully activated by dopamine release within this range. Administration of α-methyl-para-tyrosine (250mg/kg, i.p.) abolished both dopamine release and the response of the five large units and four small units examined, indicating that the neuronal response was directly attributable to dopamine. Dopamine release was increased by increasing the stimulus duration over the range 0.25–10 s. With increasing levels of dopamine release the excitatory response of large units rose to a maximum and then decreased until it was eventually transformed entirely into an inhibition at dopamine levels above 1 μM. In contrast, the excitatory response of small units always increased in magnitude with increasing dopamine release to levels greater than 1 μM. The large units that showed inhibition at low levels of dopamine were also inhibited at high levels. Tail-pinch stimuli excited 21/23 large units and all seven small units tested, although this stimulus did not evoke a detectable rise in dopamine levels. We suggest that the fundamental action of dopamine in the striatum is excitation, whether involving D 1 or D 2 receptors. The small units described here could be inhibitory interneurons which convert the excitatory response of large units into inhibition. Dopamine may regulate striatal function by enhancing particular input-output pathways while also activating lateral inhibitory mechanisms serving to “gate-out” alternative outputs.

Dissociation of the actions of uptake blockers upon dopamine overflow and uptake in the rat nucleus accumbens: In vivo voltammetric data

Fast cyclic voltammetry, at carbon fibre microelectrodes, was used to monitor stimulated overflow and uptake of DA in the nucleus accumbens of the rat. Several recognised blockers of neuronal uptake of DA were examined for their actions on each process. Benztropine, cocaine, GBR 12909 and nomifensine elevated the overflow of DA, while only nomifensine and bupropion blocked uptake. The actions of the drugs upon uptake did not correlate with their effects on overflow of DA. It is therefore concluded that the mechanisms by which the drugs influence overflow and uptake are different. It cannot be assumed that the elevation of extracellular DA is solely the result of blockade of uptake by these drugs.

Dopamine release measured by fast cyclic voltammetry in nucleus accumbens slices : D.R. Bull and M.J. Sheehan, Department of Neuropharmacology, Glaxo Group Research Ltd., Ware, Herts., SG12 0DP, U.K.

Dopamine release measured by fast cyclic voltammetry in nucleus accumbens slices : D.R. Bull and M.J. Sheehan, Department of Neuropharmacology, Glaxo Group Research Ltd., Ware, Herts., SG12 0DP, U.K.

Basic instrumentation for fast cyclic voltammetry

Fast cyclic voltammetry is a new voltammetric technique which has been especially useful for measurements of dopamine release in vivo. This paper describes methods for the construction of a basic fast cyclic voltammetric amplifier, and the associated apparatus necessary to carry out fast cyclic voltammetric experiments.

Electrochemical studies of η 5-cyclopentadienyliron hexafluorophosphates of η 6-substituted arenes and η 6-heterocycles

Polarographic half-wave potentials for two reduction steps of 49 cyclopentadienyliron complexes of substituted arenes or heterocycles in dimethylformamide were determined. A fast cyclic voltammetry (10–40 V/s) was used to study the electron transfer kinetics of some of these complexes. After correction for the double layer effects, the rate constants of all the complexes studied show that the transfer of electrons in the second reduction step occurs significantly faster than in the first one. This was interpreted as a result of greater delocalization of electrons in the 20-electron complexes of iron compared to the 19-electron complexes.

Actions of dopamine antagonists on stimulated striatal and limbic dopamine release: an in vivo voltammetric study.

1. Fast cyclic voltammetry at carbon fibre microelectrodes was used to study the effects of several dopamine antagonists upon stimulated dopamine release in the rat striatum and nucleus accumbens. 2. In both nuclei, stimulated dopamine release was increased by D2-receptor-selective and mixed D1/D2-receptor antagonists. The D1-selective antagonist SCH 23390 had no effect. 3. Striatal and limbic dopamine release were elevated by cis- but not trans-flupenthixol. 4. The 'atypical' neuroleptics (clozapine and thioridazine) did not cause a selective elevation of dopamine release in the limbic terminal region, whereas the non-antipsychotic drug metoclopramide increased dopamine release more in striatum than nucleus accumbens. 5. We conclude from this study that striatal and limbic dopamine release are under the control of a stereoselective dopamine D2-autoreceptor on the nerve terminal and that atypical neuroleptics do not show a limbic-selective effect at this receptor after acute administration.