Differential pulse voltammetry in vivo--evidence that uric acid contributes to the indole oxidation peak

Previous studies using differential pulse voltammetry have shown that indoleamines contribute to the oxidation peak at +280-300 mV (peak 3) measured in the rat striatum in vivo using carbon fibre electrodes. In this study, using similar techniques, it is shown that 5-hydroxyindoleacetic acid and uric acid oxidize at a similar potential (+270-290 mV) in vitro. Additionally, by microinfusing uric acid or its metabolizing enzyme uricase, it is shown that uric acid oxidation contributes to about 30% of the height of peak 3 measured in the rat striatum in vivo. These results indicate that care needs to be taken in interpreting changes in the height of the in vivo peak 3 since it is not solely due to the oxidation of brain indoleamines.

Measurement of extracellular basal levels of serotonin in vivo using nafion-coated carbon fibre electrodes combined with differential pulse voltammetry

Carbon fibre electrodes combined with differential pulse voltammetry have been used for a number of years to monitor changes in the extracellular concentrations of ascorbic acid, dihydroxyphenylacetic acid, and 5-hydroxyindoleacetic acid. However, the primary objective of in vivo electrochemists has been to monitor changes in the extracellular concentrations of the neurotransmitter amines; dopamine and serotonin rather than their metabolites. In this paper we describe a new chemically- and electrically-pretreated Nafion-coated carbon fibre electrode which can be used to monitor basal levels of serotonin in the extracellular fluid in the frontal cortex and the dorsal raphe nucleus of rat. These electrodes combined with differential pulse voltammetry detect dopamine (Peak A at -70 mV) and serotonin (Peak B at +240 V) oxidation peaks in vitro but not the oxidation of ascorbic acid, dihydroxyphenylacetic acid, 5-hydroxyindoleacetic acid or uric acid, at concentrations up to 10 microM. These electrodes were able to detect serotonin concentration as large as 1 nM in vitro. When used in vivo the oxidation peaks obtained in the frontal cortex and dorsal raphe indicate the basal concentrations of serotonin to be 5 nM and 10 nM respectively. Pharmacological interventions in rats implanted with normal carbon fibre electrodes or with Nafion carbon fibre electrodes further demonstrate that the new Nafion electrodes measure serotonin in vivo. The Nafion-coated electrodes therefore may be a useful tool for the study of serotoninergic systems in vivo with the added advantage that they cause minimal damage due to their small tip size (30 micron).

In vivo evidence that 5-hydroxytryptamine (5-HT) neuronal firing and release are not necessarily correlated with 5-HT metabolism

The relationship between 5-hydroxytryptamine release, metabolism and unit activity has been investigated in the anaesthetized rat. 5-Hydroxytryptamine release and metabolism were monitored in vivo by the measurement of extracellular 5-hydroxytryptamine and 5-hydroxyindoleacetic acid in the frontal cortex using in vivo voltammetry combined with nafion-coated and uncoated electrically pretreated carbon fibre electrodes. The monoamine oxidase inhibitor pargyline (100 mg/kg) increased extracellular 5-hydroxytryptamine and decreased 5-hydroxyindoleacetic acid. The 5-hydroxytryptamine releaser fenfluramine (10 mg/kg i.p.) acutely increased extracellular 5-hydroxytryptamine while having no effect on 5-hydroxyindoleacetic acid and the effect on extracellular 5-hydroxytryptamine was markedly reduced in rats pretreated (four weeks) with 5,7-dihydroxytryptamine. 8-Hydroxy-2-(di-n-propyl-amino) tetralin (10 micrograms/kg i.v.), an agonist at the 5-hydroxytryptamine1A somatodendritic autoreceptor, inhibited 5-hydroxytryptamine neuronal firing in the dorsal raphe nucleus and decreased extracellular 5-hydroxytryptamine during the period when firing was inhibited but did not alter extracellular 5-hydroxyindoleacetic acid. In contrast 5-methoxy-3-(1,2,3,6-tetrahydro-4-pyridin-4-yl) (RU 24969), which is an agonist at the terminal autoreceptor in the rat, had no effect on 5-hydroxytryptamine neuronal firing but decreased 5-hydroxytryptamine and 5-hydroxyindoleacetic acid. The results support the view that extracellular 5-hydroxyindoleacetic acid is not a good index of 5-hydroxytryptamine release and that under specific circumstances 5-hydroxytryptamine neuronal firing, release and metabolism are independent of one another.

Forced swimming test and fluoxetine treatment: in vivo evidence that peripheral 5-HT in rat platelet-rich plasma mirrors cerebral extracellular 5-HT levels, whilst 5-HT in isolated platelets mirrors neuronal 5-HT changes

Low levels of central serotonin (5-HT) have been related to the state of depression, and 5-HT is the major target of the newer antidepressant drugs such as selective serotonin reuptake inhibitors (SSRIs). Neurons and platelets display structural and functional similarities, so that the latter have been proposed as a peripheral model of central functions. In particular, in blood more than 99% of 5-HT is contained in platelets, so that one could consider changes in 5-HT levels in platelets as a mirror of changes in central 5-HT. Here, this hypothesis has been studied via the analysis of the influence of: (1) the forced swimming test (FST, which has been proved to be of utility to predict the clinical efficacy of antidepressants in rodents) and (2) treatment with the SSRI fluoxetine upon 5-HT levels monitored in brain regions and in peripheral platelets by means of electrochemical in vivo and ex vivo measurements. The results obtained confirm that the FST increases immobility; furthermore they show a parallel and significant decrease in cerebral (brain homogenate) and peripheral (in platelet-rich plasma, PRP) voltammetric 5-HT levels following the FST in naive rats. In addition, subchronic treatment with fluoxetine was followed by a significant increase in 5-HT levels in PRP, while the same SSRI treatment performed within the FST resulted in a decrease in the 5-HT levels in PRP. However, this decrease was inferior to that observed without SSRI treatment. These data suggest that there is an inverse relationship between immobility and the levels of 5-HT in PRP and that these peripheral 5-HT levels are sensitive to: (1) the FST, (2) the treatment with fluoxetine and (3) the combination of both treatments, i.e. SSRI + FST. It has been reported that SSRI treatment at first inhibits the 5-HT transporter in brain, resulting in increased extracellular 5-HT, while following sustained SSRI treatments decreased intracellular levels of central 5-HT were observed. Accordingly, the present data show that the initial block of 5-HT reuptake is revealed by the selective increase in 5-HT levels (extracellular content) measured in PRP (not in insulated platelets, IPs) the 1st day of fluoxetine treatment. The initial action of this SSRI upon the 5-HT transporter in brain has also been confirmed by in vivo voltammetric data showing selective increase in the serotonergic signal following local injection of fluoxetine into the brain region studied. Successively, the major effect monitored is a decrease in 5-HT levels, which is more evident in IPs than in PRP. However, it is known that following 2 weeks treatment with an SSRI, 5-HT autoreceptors are desensitized and the serotonin synthesis is restored, together with the intracellular 5-HT levels. The present data showing that the levels of 5-HT in IPs tend to return to control values 12 days after the beginning of chronic fluoxetine treatment suggest that 5-HT levels in IPs (intracellular environment) mirror the influence of SSRI treatment upon the central 5-HT system. On the other hand, at day 12 of the chronic fluoxetine treatment, 5-HT content remains low in PRP. Similarly, low levels of 5-HT have been monitored in brain homogenate of rats chronically treated with fluoxetine. This would support the similarity between PRP preparation and brain homogenate as in both cases cells are disrupted by sample preparation. In conclusion this work supports the literature in proposing platelets as a peripheral model of central functions. In particular, the present data support the idea that peripheral 5-HT platelet levels can reflect the state of the central 5-HT system in conditions of depression. Furthermore, the main outcome of this study is that PRP may mirror central extracellular 5-HT levels, whilst IPs mirror neuronal 5-HT changes.

An improved differential pulse voltammetry technique allows the simultaneous analysis of dopaminergic and serotonergic activities in vivo with a single carbon-fibre electrode

Differential pulse voltammetry has successfully been employed to study either 5-hydroxyindoles, or ascorbic acid and catechols in the brain of anaesthetised or freely moving rats. A new electrochemical pretreatment of pyrolytic carbon-fibre electrodes has been developed, enabling the simultaneous recording of all three compounds in the striatum of anaesthetised rats, using a Tacussel polarography. Furthermore, a fourth peak was recorded at +450 mV. Pharmacological treatments performed to define the nature of the four peaks recorded in the striatum confirmed that peak 1 corresponds to ascorbic acid, peak 2 to dihydroxyphenylacetic acid, peak 3 to 5-hydroxyindoleacetic acid and peak 4 to homovanillic acid.

In vivo voltammetry with micro-biosensors for analysis of neurotransmitter release and metabolism

In vivo voltammetry involves the electrochemical detection of central oxidisable substances in situ. In association with this technique micro carbon fibre electrodes (CFE) are able to separate ascorbic acid (Peak 1) from 3,4-dihydroxyphenylacetic acid (DOPAC) plus dopamine (DA) (Peak 2) and 5-hydroxyindoleacetic acid (5-HIAAA) plus serotonin (5-HT) (Peak 3) in vitro. In vivo these biosensors detect the amine metabolites, due to their high extracellular concentration (microM) compared to the amines (nM). In addition homovanillic acid (HVA) (or 3-methoxytyramine (3-MT) in pargyline-pretreated mice) (Peak 4) and somatostatin (Peak 5) were also measured in vivo. However, potassium-stimulated release of DA has been directly monitored in pargyline pretreated mice. In addition, low concentrations (nM) of DA and 5-HT can now be selectively monitored in vitro with new biosensors coated with Nafion which repels negatively charged species including acid metabolites. In vivo, the combination of the Nafion-CFE and normal CFE allowed simultaneous measurements of release and metabolism of 5-HT, respectively. This permitted the observation that changes in 5-HT release are not necessarily reflected by changes in 5-HIAA levels. At present we are developing a Nafion biosensor to monitor basal extracellular DA. Electron microscope studies have shown radical modifications in the surface and structure of carbon fibres following chemical and electrical pretreatments, which may be involved in the development of sensitivity and selectivity displayed by the pretreated CFE towards electroactive compounds. A new approach for selective detection of neuroamines is the analysis of their stimulated fluorescence using LASER. In vitro, the fluorescence of 5-HT is in fact clearly distinguishable from that of 5-HIAA. The feasibility of this methodology in vivo using fiber optic probes will be explored.

Alteration of tyrosine hydroxylase activity in the locus coeruleus after administration of p-chlorophenylalanine

The time course of the variations in tyrosine hydroxylase activity (THA) and serotonin (5-HT) content were measured in the rat locus coeruleus after parachlorophenylalanine (PCPA) administration. Highly significant decreases in the 5-HT content in LC were found 24-48 h after PCPA treatment (300 mg/kg daily). An increase in THA (in the LC) was found to be significant 4 days after 2 successive injections of PCPA and after 4 successive injections of the drug THA gradually increased, reaching a maximum around 4 days after the last injection. This maximum increase in THA was greatly reduced when 5-HTP was simultaneously administered with PCPA. These results join others which suggest that a serotonin-mediated mechanism could be one of the processes controlling noradrenaline metabolism in the locus coeruleus.

Isolation rearing of rats alters release of 5-hydroxytryptamine and dopamine in the frontal cortex: an in vivo electrochemical study

The effects of rearing hooded Lister rats either in groups of seven or singly on 5-hydroxytryptamine (5-HT) and dopamine (DA) release in the frontal cortex were investigated using in vivo voltammetry together with Nafion coated carbon fibre micro-electrodes. The selective detection of basal extracellular levels of 5-HT with this technique (Peak B) was confirmed with parallel experiments using intracranial microdialysis to measure 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) levels in vivo. The DA voltammetric signal (Peak A) was observed in vivo only following pharmacological or electrical stimulation of DA release. Enhanced efflux of cortical DA and 5-HT in response to local application of KCl and that of 5-HT following parentelar fenfluramine were selectively detected by the association: differential pulse voltammetry (DPV)-Nafion coated microbiosensors, supporting the capability of this electrochemical method to selectively monitor release of these amine neurotransmitters in vivo and in situ. The locomotor behaviour data indicated that isolation rearing resulted in augmented locomotor activity in a novel environment. In addition, the in vivo voltammetric results showed that following KCl or fenfluramine treatment cortical 5-HT release is prolonged while that of DA is increased in rats reared in isolation when compared with socially reared rats. This imbalance between extracellular levels of DA and 5-HT recorded in the frontal cortex of rats exposed to isolated housing conditions may contribute to the behavioural differences reported between isolation and group reared rats.

The role of cholecystokinin (CCK), CCK-A or CCK-B receptor antagonists in the spontaneous preference for drugs of abuse (alcohol or cocaine) in naive rats

A "free choice" two-bottle drinking test paradigm was implemented in naive adult male Wistar rats, resulting in a clear identification of rats drinking mainly water (water-preferring, WP rats) and rats spontaneously drinking also a consistent amount of a solution of cocaine (0.5 mg/ml water, cocaine-drinking, CD rats) or ethanol 10% v/v (ethanol-drinking, ED rats). Low, selective doses (5 micrograms/kg) of the specific cholecystokinin (CCK)-A receptor antagonist L-364,718 largely reduced the intake of ethanol 10% of ED rats only. In contrast, low, selective doses of GV-150013 (5 micrograms/kg) reduced significantly the consumption of cocaine of CD rats only. These results indicate that the CCK-A or B receptors are selectively involved in the modulation of alcohol or cocaine intake, respectively, and suggest an involvement of the CCKergic system in the drug-seeking behavior. WP rats and CD rats were then prepared for ex vivo electro-neurochemical analysis by means of differential pulse voltammetry (DPV) with micro-biosensors to monitor catechol, 5-hydroxyindole and peptidergic oxidation signals in the nucleus accumbens (nAcc). In this area, the peptidergic signal appeared to be related to the oxidation of endogenous CCK, which basal levels resulted higher in ED and CD rats than WP rats. Thus, the hypothesis that the endogenous tone of the CCK system is higher in the ED and CD rats than in the WP rats is proposed, and is supported by the observation that treatment with CCK-5 (CCK receptor agonist) selectively induced the WP rats to drink alcohol or cocaine. The selective effect of the CCK-antagonists on reducing the drug intake of ED or CD rats further supports this view, as it suggests that CCK antagonists may modify the individual sensitivity towards drugs of abuse set by the stimulating effect of high endogenous CCKergic tone over CCK-B or CCK-A receptors in spontaneous ED or CD rats, respectively. Therefore, the present data indicate that: i) Free-choice models may reveal the presence of individual sensitivity to alcohol or cocaine in naive rats; ii) the dopaminergic system is involved within the reward state, while peptidergic (CCKergic) activities modulate the drug-seeking state (craving state); iii) the CCK system could be a new target in the study of the drug dependency phenomenon. In particular, the data imply a CCK-A receptor mechanism in the regulation of individual sensitivity towards ethanol and a CCK-B receptor mechanism in the regulation of individual sensitivity towards cocaine. Thus, a potential therapeutic role for CCK-A antagonists in the treatment of ethanol abuse and for CCK-B antagonists in the treatment of cocaine abuse is proposed.

Effects of idazoxan on dorsal raphe 5-hydroxytryptamine neuronal function

The effects of the alpha 2-adrenoceptor antagonist idazoxan on 5-hydroxytryptamine (5-HT) neuronal firing and release have been investigated. Idazoxan, administered i.v. (10 micrograms/kg and 0.5 mg/kg) increased dorsal raphe nucleus (DRN)-5-HT neuronal firing rate in a dose-dependent fashion. At the higher dose, a voltammetric study revealed increases in extracellular 5-HT and 5-hydroxyindole acetic acid (5-HIAA) levels, there was no effect with the lower dose. Intra-raphe administration of idazoxan (1 ng) also elevated the firing rate of 5-HT neurones in the dorsal raphe, suggesting that idazoxan may produce the increase in firing by a direct effect in the DRN. However, microiontophoretic application of idazoxan did not increase the firing rate of 5-HT neurones in the DRN. Thus the increase in the firing rate of 5-HT neurones in the DRN observed with systemic and local administration of idazoxan is probably not due to a direct action of idazoxan on the 5-HT neurone. Possibly the idazoxan acted at alpha 2-adrenoceptors located on noradrenergic terminals thus stimulating noradrenaline release and consequently increased 5-HT activity. Chronic administration of idazoxan (0.8 mg/kg per h for 14 days), using osmotic mini-pumps, caused an elevation in basal firing rate and an attenuation of the inhibitory response of DRN 5-HT neurones to the 5-HT1A agonist, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OHDPAT) (10 micrograms/kg i.v.). This finding suggests that chronic infusion with idazoxan leads to desensitisation of the 5-HT1A somatodendritic autoreceptor.

Differential pulse voltammetry: simultaneous in vivo measurement of ascorbic acid, catechols and 5-hydroxyindoles in the rat striatum

This paper describes carbon fibre electrodes that can simultaneously monitor changes in ascorbic acid, dihydroxyphenylacetic acid (DOPAC), 5-hydroxyindoleacetic acid (5HIAA) and homovanillic acid (HVA) in vivo in the rat striatum using differential pulse voltammetry. The separation between DOPAC and 5HIAA oxidation is improved and the size of the 5HIAA peak decreased by the removal of uric acid using the enzyme uricase indicating that uric acid oxidation may contribute to the oxidation peak at + 300 mV. Haloperidol (0.5 mg/kg) decreased ascorbic acid and 5HIAA but increased DOPAC and HVA while D-amphetamine (3 mg/kg) increased ascorbic acid, decreased DOPAC and HVA but had no effect on 5HIAA. These electrodes should be a useful means of investigating interactions between dopamine and serotoninergic systems in vivo.

In vivo selective monitoring of basal levels of cerebral dopamine using voltammetry with Nafion modified (NA-CRO) carbon fibre micro-electrodes

The electrochemical technique of differential pulse voltammetry (DPV) with micro-biosensors has been used for a number of years to monitor in vivo and in situ changes in the extracellular concentration of cerebral ascorbic acid, as well as that of the metabolites of dopamine (DA) and serotonin (5-HT). We have recently prepared a carbon fibre micro-electrode (mCFE) which specifically pretreated and coated with Nafion (a negatively charged polymer which repels acids such as 3,4-dihydroxyphenylacetic acid (DOPAC)) allows the direct selective detection of the oxidation of DA and 5-HT in nanomolar concentration in vitro and that of extracellular basal levels of cerebral 5-HT in vivo (peak B at +240 mV). We describe here a modified version of this micro-biosensor now called NA-CRO mCFE as its active tip (30 microns in diameter) is coated with a 50/50 (v:v) mixture of Nafion and dibenzo-18-crown-6 (Aldrich). In vitro this newly reported electrode shows insensitivity to acids (e.g., DOPAC) up to 100 microns and sensitivity to 0.5-1 nM DA. In vivo, in the striatum of anaesthetised rats, a basal oxidation peak at +80 mV (peak A, on average 0.6 nA in height), which corresponds to the oxidation potential of DA in vitro, is consistently detectable with the NA-CRO mCFE (corresponding to an estimated concentration of 1.5 nM). Experiments performed in vivo in anaesthetised rats implanted in the striatum with uncoated (normal) mCFE to measure extracellular DOPAC or with NA-CRO mCFE have been performed in order to analyse the chemical nature of peak A in vivo. It is concluded that the addition of the crown-ether compound to the Nafion coat improves the sensitivity of the micro-biosensor for DA in vitro and allows the detection of its basal extracellular levels in vivo.

Differential pulse voltammetry in brain tissue: III. Mapping of the rat serotoninergic raphe nuclei by electrochemical detection of 5-HIAA

Differential pulse voltammetry using a new type of carbon fiber electrode, electrochemically treated, is described. The working electrode contains 3 pyrolytic carbon fibers, and passes more current, thus giving a greater sensitivity (with the PRG5 Tacussel polarographic system) than the original monofiber electrodes. It is now possible to investigate brain areas where the monofiber electrodes, working near the limit of PRG5 sensitivity, showed too small a signal. These electrodes have, in addition, better mechanical resistance and can be used (after trypsin cleaning and further electrochemical treatment) for several experiments. Electrochemical measurements made in the nucleus raphe dorsalis before and after treatment with p-chlorophenylalanine, reserpine, clorgyline and clorgyline followed by reserpine, suggest that as in the striatum 5-hydroxyindoleacetic acid (5-HIAA) is mainly responsible for peak 3. The map of the raphe system made with this technique is well correlated with the serotoninergic system of the raphe: the highest peak heights are recorded in the raphe dorsalis.

Carbon fibre micro-electrodes for concomitant in vivo electrophysiological and voltammetric measurements: no reciprocal influences

Differential pulse voltammetry and more recently cyclic voltammetry have been successfully used to monitor basal levels of endogenous chemicals by means of treated carbon fibre microbiosensors inserted in specific brain regions. In this study, feasibility of concomitant in vivo recordings of stable electrophysiological signals and basal ascorbate, catecholaminergic and indolaminergic voltammetric peaks at the same cerebral site by means of a single electrically treated carbon fibre micro electrode (microbiosensor) is presented. The results indicate that these two independent techniques can be combined in vivo at a single electrode, and that voltammetric measurements of unstimulated levels of extracellular compounds do not alter concomitant basal cell firing for a period long enough (more than 6 h) to allow pharmacological manipulations.

Differential pulse voltammetric determination of 5-hydroxyindoles in four raphe nuclei of chronic freely moving rats simultaneously recorded by polygraphic technique: physiological changes with vigilance states

Nuclei raphe dorsalis ( RDN ), centralis (RCN), pontis (RPN) and magnus ( RMN ) were separately studied using differential pulse voltammetry ( DPV ) in chronic freely moving rats during the recording of their sleep-waking cycle by polygraphic technique. In each of these nuclei the height of the electrochemical signal appearing at +300 mV (peak 3) was maximum during waking (W), lower during slow-wave sleep (SWS) and minimum during paradoxical sleep (PS). Some pharmacological treatments indicated that in each of these nuclei the peak 3 represents the oxidation of the 5-hydroxyindoles. DPV measurements performed during specific behavioral states (eating, grooming, washing, drinking) called active waking (AW) or manipulations (handling, tail-pinch) demonstrated that this technique enables detection of changes occurring in animals under physiological conditions.

Differential pulse voltammetry: parallel peak 3 changes with vigilance states in raphe dorsalis and raphe magnus of chronic freely moving rats and evidence for a 5-HT contribution to these peaks after monoamine oxidase inhibitors

Nuclei raphe dorsalis (RDN) and magnus (RMN) were simultaneously studied using the differential pulse voltammetry (DPV) technique in chronic freely moving rats during their sleep-waking cycle. Parallel variations in peak 3 (due to 5-hydroxyindoles) were observed in both these areas: the peak 3 heights were maximum in both RDN and RMN during waking (W), decreased in slow wave sleep (SWS) and were minimum in paradoxical sleep (PS). Monoamine oxidase inhibitor (MAOI) treatments induce at first a decrease of peak 3 (-50% compared to the control values), but there was a subsequent increase (+100% compared to the control values). This suggests that 5-hydroxytryptamine (5-HT) can contribute to peak 3 measured in vivo.

In vivo voltammetric detection of neuropeptides with micro carbon fiber biosensors: possible selective detection of somatostatin

The electrochemical activity of catechol- and indoleamines, measured by differential pulse voltammetry (DPV) with specifically electrically pretreated carbon fiber microelectrodes, has been utilized to develop sensitive assays for amine neurotransmitters and metabolites. So far, four oxidation peaks have been recorded in vivo between -200 and +500 mV and are well identified. We now report that by increasing the potential sweep range to +950 mV, a further peak, called Peak 5, was detected at +800 mV in vivo in the striatum of anesthetized rats. Neuropeptides containing tyrosine, tryptophan and/or cysteine appear to be electrochemically active between +600 and +900 mV in vitro in a buffered solution at pH 7.4. The present study investigates the chemical nature of Peak 5 and the possible contribution of electroactive neuropeptides to this in vivo voltammetric signal. Experiments performed in vitro and in vivo with amino acids, neuropeptides, or bacitracin (a potent peptidase inhibitor) support the view that Peak 5 is peptidergic. Furthermore, peripheral administration of cysteamine and intrastriatal injection of specific somatostatin antisera both cause the eventual disappearance of Peak 5, suggesting that somatostatin (which oxidases in vitro at approx +800 mV), or a structurally related peptide, could be the principal component of striatal Peak 5.

Simultaneous in vivo voltammetric measurement of striatal extracellular DOPAC and 5-HIAA levels: effect of electrical stimulation of DA and 5-HT neuronal pathways

Differential pulse voltammetry (DPV) combined with electrically pretreated carbon fibre electrodes has been used to study dopamine (DA)-5-hydroxytryptamine (5-HT) interactions in the striatum of anaesthetised rats. Electrical stimulation of the dorsal raphe (DRN) increased 5HT and decreased DA metabolism in the striatum. Conversely, stimulation of the DA pathway to the striatum was associated with increased DA and decreased 5-HT metabolism. These data support previous studies and demonstrate that there is a functional interaction between DA and 5-HT systems in the striatum. The receptors and other transmitters (if any) involved in this interaction remain to be determined.

Can voltammetry measure nitrogen monoxide (NO) and/or nitrites?

Recently, voltammetry with carbon fibre electrodes (CFE) has been implemented for real time measurement of nitrogen monoxide (NO) indicating that it is oxidised at the potential value of nitrites, approximately +700 mV. In contrast, here we show that modified CFE can monitor NO at oxidation potentials different than that of nitrites, i.e. +550 mV. Indeed, at +550 mV a significant increase of amperometric current levels was obtained when NO but not nitrites, were added to a phosphate buffer saline solution (PBS). Differential pulse voltammetry (DPV) supports these findings as two oxidation peaks were obtained when examining air preserved NO; peak 1 at +550 mV and peak 2 at +700 mV, respectively. In contrast, only peak 2 was monitored when nitrites or a solution of NO oxidised in air was added to PBS. Biological support to these in vitro data comes from the observation that the relaxation of an adrenaline-contracted aortic ring produced via addition of NO is concomitant with peak 1 at +550 mV. The relaxation is almost completed before the appearance of peak 2 at +700 mV. Furthermore, in vivo experiments performed in the striatum of rats show that the amperometric signal monitored at +550 mV is responsive to glutamatergic stimulation or inhibition of NO synthase.

Anaesthesia abolishes the effect of valproate on extracellular 5-HIAA, DOPAC and ascorbate as measured in rat striatum by differential pulse voltammetry

The effect of sodium valproate (VPA, 400 mg kg-1, i.p.) on extracellular ascorbate, 3,4-dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindoleacetic acid (5-HIAA) in the striatum was examined by differential pulse voltammetry in anaesthetized and freely-moving rats. In rats anaesthetized with chloral hydrate (400 mg kg-1, i.p.) pentobarbitone (50 mg kg-1, i.p.) or phenobarbitone (60 mg kg-1, i.p.), VPA produced no significant changes in peak 1 (extracellular ascorbate) or peak 2 (extracellular DOPAC), but produced a slight but statistically significant reduction in the height of peak 3 (extracellular 5-HIAA). In contrast, in freely-moving rats the same dose of VPA greatly reduced extracellular ascorbate and DOPAC concentrations, and increased that of 5-HIAA. These results suggest that VPA may reduce the release or turnover of dopamine, and increase that of 5-hydroxytryptamine in conscious rats. Our data also suggest that caution may be required in the interpretation of the effects of VPA in anaesthetized animals, as the results obtained may not always reflect the situation in the absence of anaesthesia.

Involvement of cholecystokinin within craving for cocaine: role of cholecystokinin receptor ligands

In the brain, cholecystokinin (CCK) has been described to act as a central neurotransmitter or neuromodulator involved in functions such as food consumption, stress and anxiety. Recently, the CCK system has been involved in drug dependence phenomena and proposed to be correlated to a putative state of 'drug preferring' phenotype within free choice tests. CCK exerts its action in the CNS through at least two different G-protein coupled high affinity receptors, CCK1 and CCK2. Various selective CCK receptor agonists and antagonists have been synthesised. In particular, L-364,718 has been demonstrated to be a potent and selective CCK1 receptor antagonist, whereas L-365,260 is a potent and selective CCK2 receptor antagonist. More recently, GV150013 has been reported to be a highly selective CCK2 receptor antagonist. This paper reviews the putative role of the CCK system within drug dependence phenomena. In particular, it analyses the relationship between central CCK activity and the exhibition of spontaneous preference for drugs of abuse, such as cocaine or alcohol. The potential therapeutic role for CCK receptor antagonists is also discussed.

Spontaneous preference for ethanol in naive rats is influenced by cholecystokinin A receptor antagonism

Naive adult male Wistar rats free to choose between water or 10% ethanol (v/v) spontaneously became water-preferring (WP) rats, as they drank mainly water (approximately 35 ml per day), or alcohol-drinking (ED) rats, as they also drank a significant amount of ethanol (approximately 14 ml per day). The selective CCKA receptor antagonist L-364,718 at doses selective for the CCKA receptor (5 micrograms/kg, IP) halved the consumption of alcohol of the ED rats without modifying their total liquid in-take. In contrast, the CCKB antagonists L-365,260 or GV150013 were without effect when used at doses selective for the CCKB receptor. These data indicate that the CCK system could be involved in the modulation of alcohol intake. In particular, they suggest that CCKA receptors could play a role in the ethanol preference.