Differential pulse voltammetry in brain tissue. II. Detection of 5-hydroxyindoleacetic acid in the rat striatum

Carbon fibre-based microbiosensors for in vivo measurements of acetylcholine and choline

This report describes technical improvements to the manufacture of a carbon fibre electrode for the stable and sensitive detection of H2O2 (detection limit at 0.5 microM). This electrode was also modified through the co-immobilisation of acetylcholinesterase (AChE) and/or choline oxidase (ChOx) in a bovine serum albumin (BSA) membrane for the development of a sensor for in vivo measurements of acetylcholine and choline. Amperometric measurements were performed using a conventional three-electrode system forming part of a flow-injection set-up at an applied potential of 800-1100 mV relative to an Ag/AgCl reference electrode. The optimised biosensor obtained was reproducible and stable, and exhibited a detection limit of 1 microM for both acetylcholine and choline. However, due to the high operating potential used, the biosensor was prone to substantial interference from other electroactive compounds, such as ascorbic acid. Therefore, in a further step, a mediated electron transfer approach was used that incorporated horseradish peroxidase into an osmium-based redox hydrogel layered onto the active surface of the electrode. Afterwards, a Nafion layer and a coating containing AChE and/or ChOx co-immobilised in a BSA membrane were successively deposited. This procedure further increased the selectivity of the biosensor, when operated in the same flow-injection system but at an applied potential of -50 mV relative to an Ag/AgCl reference electrode. The sensor exhibited good selectivity and a high sensitivity over a concentration range (0.3-100 microM) suitable for the measurement of choline and acetylcholine 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.

Is the nucleus raphe dorsalis a target for the peptides possessing hypnogenic properties?

Several peptides exhibiting hypnogenic properties when administered i.p., i.v. or i.c.v. are now known. No data, however, are available concerning their targets in the brain. In the present work we hypothesize that the nucleus raphe dorsalis (nRD) may be one such target since it contains 2 sleep permissive components that must be influenced for sleep to occur. One of these components is serotoninergic in nature and gates the occurrence of ponto-geniculo-occipital (PGO) waves. The other, of unknown nature, influences tonic sleep phenomena. For hypnogenic peptides, a putative mechanism permitting the triggering and maintenance of sleep might consist of influencing both the above components. In the present work, 3 hypnogenic substances, CLIP (corticotropin-Like intermediate lobe peptide), VIP (vasoactive intestinal polypeptide) and DSIP (delta sleep inducing peptide), were injected into the nRD in order to determine whether these compounds still induce sleep by local administration. To verify that such local injections do not spread outside the nRD, radiolabelled CLIP and VIP were also injected. Autoradiograms obtained with either labeled CLIP or VIP indicate that these compounds, injected in a 0.2 microliter volume, do not spread outside the nRD. The sleep data obtained confirm that CLIP, at a dose of 10 ng, induces an increase in duration of paradoxical sleep (PS); this effect is observed only for injection sites located in the dorsolateral part of the nRD, an area where CLIP immunoreactive (IR) fibers are present. VIP, at a dose of 100 ng, also increases PS duration, whereas at 10 ng, only slow wave sleep duration is increased. In this case, the positive injection sites are scattered throughout the entire nRD as are the VIP-IR fibers. With DSIP, no sleep effect was found whatever the dose used or the site injected; in the same manner, no DSIP-IR fibers have been located in this structure. These data suggest that the nRD is a target for the expression of the hypnogenic properties of CLIP and VIP, but not for DSIP. The nature of the possible mechanisms permitting such expression are discussed.

Real-time monitoring of endogenous noradrenaline release in rat brain slices using fast cyclic voltammetry: 3. Selective detection of noradrenaline efflux in the locus coeruleus

Fast cyclic voltammetry (FCV) at carbon fibre microelectrodes was used to monitor 'real time' endogenous noradrenaline (NA) efflux in superfused slices of rat locus coeruleus (LC) following local electrical stimulation. When stimulated with a standard train (30 pulses, 100 Hz, 0.2 ms, 10 mA, every 5 min), efflux of monoamine was constant over the experimental period (2.5 h): Amine efflux declined by only 16 +/- 5% while uptake half-life lengthened by only 9 +/- 8%. When calibrated in solutions of NA, peak amine efflux corresponded to 0.31 +/- 0.04 microM (mean +/- S.E.M., n = 28) and was removed by uptake with a half-life of 2.93 +/- 0.28 s (n = 16). The released compound was confirmed as NA on the basis of pharmacological and electrochemical criteria. Stimulated monoamine efflux was reversibly reduced by 78% by omission of Ca2+ from the superfusate for 30 min (P < 0.05). Ro 4-1284 (1 microM), a fast-acting reserpine-like drug, decreased amine efflux by 86% (P < 0.05). The monoamine oxidase inhibitor pargyline (2 microM) increased efflux by 30% (P < 0.05). Desipramine (0.05 microM), a selective NA uptake blocker, significantly increased amine efflux (by 96%, P < 0.05) and uptake half-life (by 314%, P < 0.05). Fluvoxamine (0.5 microM), the selective serotonin (5HT) uptake blocker, increased efflux by 59% (P < 0.05) and the uptake half-life by 122% (P < 0.05). Vanoxerine (GBR 12909: 0.3 microM), the dopamine (DA) uptake blocker, had no effect on amine efflux or uptake half-life. The voltammogram of the released amine had single oxidation and reduction peaks.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of RU 24969 on 5-HT metabolism in the medullary dorsal horn as studied by in vivo voltammetry

The effect of i.p. administration of the preferential 5-HT1B agonist 5-methoxy-3(1,2,3,6-tetrahydro-4-pyridinyl)-1H-indole succinate (RU 24969) (10 mg/kg) has been investigated by in vivo 5-hydroxyindole electrochemical (peak 3) detection in the medullary dorsal horn (MDH) of acute anesthetized and unanesthetized freely moving rats. RU 24969 induced a significant decrease in peak 3 in the MDH of anesthetized rats. In freely moving animals, RU 24969 induced a biphasic effect. Thus, after the injection the curve remained above that of the saline group and returned to control levels up to 60 min. Subsequently the curve decayed to below the control values and rapidly plateaued for up to 180 min. The initial increase and the decrease thereafter were both statistically significant vs. saline. With reference to similar in vivo studies demonstrating the responsiveness of ascending serotonergic systems to RU 24969, it is concluded that the 5-HT metabolism in the serotonergic NMR-dorsal horn system is affected by this 5-HT1B agonist. However, the biphasic effect reported here in unanesthetized animals suggests that RU 24969 could act by two different ways on 5-HT metabolism and indicates that there could be a primary interaction of RU 24969 on the 5-HT uptake system (inhibition) which could, at first, prevail over the interaction with terminal autoreceptors.

High sensitivity measurement of brain catechols and indoles in vivo using electrochemically treated carbon-fiber electrodes

The combination of electrochemically treated carbon-fiber electrodes with DPV, DNPV or DPA represents a wide range of possibilities. As shown in this review, the choice of treatment and measurement technique depends on the purpose. As regards in vivo monitoring of 5-HIAA or DOPAC from very small brain nuclei, electrochemically treated carbon-fiber electrodes appear very potent and inexpensive. The main limitation of the established electrochemical techniques, including those discussed here, is that the unequivocal measurement of the basal extracellular neurotransmitter level cannot be achieved unless animals are treated with pargyline. On the other hand, this monitoring is feasible with in vivo dialysis. Therefore, electrochemical techniques, on the one hand, and in vivo dialysis, on the other hand, present different advantages. The former are much more potent than the latter in two respects. First, due to the much smaller size of the sensor, electrochemical techniques are more suitable for studying small brain nuclei. Second, since electrochemical techniques exhibit a better temporal resolution, they are recommended for investigating the relationship between impulse flow and neurotransmitter release. However, when high anatomical or temporal resolution is not required, in vivo dialysis is more suitable for recording the basal monoamine release.

Principles of voltammetry and microelectrode surface states

In vivo voltammetry is approaching the end of its second decade as a technique to explore extracellular concentrations in the brain. The issues of selectivity and sensitivity, which caused considerable discussion and confusion in the early 1980s, are now resolved. It is clear that in vivo voltammetry and dialysis are complimentary tools to understand neurotransmitter dynamics. The two chief advantages of voltammetry compared to dialysis, improved temporal resolution and reduced tissue damage, make this technique exceptionally well suited for providing information which is complementary to that obtained by single-unit recording and is uniquely capable of providing information on the short-term regulation of extracellular levels of biogenic amines.

Effects of tianeptine on 5-hydroxyindoles and on the morphine-induced increase in 5-HT metabolism at the medullary dorsal horn level as measured by in vivo voltammetry in freely moving rats

The present study, by the use of in vivo electrochemical detection of 5-hydroxyindole (peak '3') in the bulbo spinal serotonergic system at the medullary dorsal horn (MDH) level, investigated the effects of the new tricyclic antidepressant (TCA) tianeptine, which has been shown to be a specific serotonin (5-HT) uptake enhancer. It was found that acutely administered tianeptine (10 mg/kg, i.p.) induced a marked significant increase in peak 3 within the dorsal horn, an in vivo observation which is in accordance with the biochemical properties of tianeptine as studied in forebrain structures. In addition, the effect of tianeptine on the morphine-induced increase in 5-HT metabolism was investigated, by comparison with the previous data obtained with the specific 5-HT uptake inhibitor femoxetine in the MDH. It was shown that tianeptine can display additive effect with morphine (10 mg/kg, i.p.) on 5-HT metabolism at the MDH level. These results are discussed in relation to the effects of classical TCAs and the particular properties of tianeptine.

Effect of subcutaneous administration of the chemical algogen formalin, on 5-HT metabolism in the nucleus raphe magnus and the medullary dorsal horn: a voltammetric study in freely moving rats

The effect of subcutaneous administration of the chemical algogen formalin, on serotonin (5-HT) metabolism in the nucleus raphe magnus (NRM) and the medullary dorsal horn (MDH) has been investigated using in vivo 5-hydroxyindole electrochemical (peak '3') detection with treated, multi-carbon fiber electrodes and differential pulse, or normal pulse, voltammetry in freely moving rats. The subcutaneous (s.c.) injection of 50 microliters of 10% formalin in the left forepaw was followed, at the NRM level, by a significant increase in the voltammograms as compared to controls (50 microliters of saline 0.9% s.c. in left forepaw) for about 70 min after the injection, before a return to control values. At the MDH level, the formalin injection induced no significant effect on peak 3, as compared to controls, during the first 70 min. After that, the voltammograms significantly increased and remained above controls for up to 180 min. Thus, the time-courses of NRM and MDH effects appear markedly different. These findings suggest that, depending on the anatomical level (NRM or MDH) and/or the period of observation, one can measure differences in the time-course of the increase in 5-HT metabolism in the NRM-dorsal horn serotonergic system by tonic noxious stimuli, such as the formalin test.

Effect of 5-hydroxy-L-tryptophan on the release of 5-HT in rat hypothalamus in vivo as measured by microdialysis

The effect of systemic administration of the 5-HT precursor, 5-hydroxy-L-tryptophan (5-HTP) on the release of 5-HT in the lateral hypothalamus of the chloral hydrate-anaesthetized rat in vivo was examined using brain microdialysis. Administration of 5-HTP caused an immediate increase of 5-HT in dialysates, which was long lasting (greater than or equal to 140 min) and dose-dependent (30-100 mg/kg i.p.). When calcium was omitted from the perfusion medium, thereby limiting exocytosis, levels of basal 5-HT were significantly decreased and the 5-HTP-induced response of 5-HT was markedly attenuated. Administration of the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino) tetralin (0.25 mg/kg i.p.), which selectively inhibits serotoninergic neuronal activity by activation of the somatodendritic 5-HT autoreceptor, significantly decreased basal levels of 5-HT and markedly attenuated the 5-HTP-induced increase in 5-HT. The data demonstrate that systemic administration of 5-HTP caused an increase in the release of 5-HT in the hypothalamus. Furthermore, this release occurred by a calcium-dependent mechanism (probably exocytosis), was dependent on serotoninergic neuronal activity and predominantly derived from 5-HT neurones. The findings are discussed in relation to the behavioral and neuroendocrine effects of increasing availability of the 5-HT precursor.

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 Ca2+ 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.

Stimulation of 5-hydroxytryptamine nerve cells in dorsal and median raphe nuclei elevates blood glucose in rats

The role played by dorsal or median raphe nuclei in glucoregulation was investigated by stimulating these nuclei in normal rats and in rats with chemical ablation of the hydroxytryptamine (5-HT) nerve cells in these nuclei. Electrical stimulation of either dorsal or median raphe nuclei increased blood glucose or the in vivo voltammetric signal of hypothalamic 5-OH-indole in normal rats; the increase in blood glucose level or the hypothalamic 5-OH-indole release was proportional to the intensity of stimulation. Microinjection of kainic acid or L-glutamate at the same sites also produced hyperglycemia or stimulated the hypothalamic 5-OH-indole release. This stimulation-induced hyperglycemia was significantly reduced by pretreatment of animals with spinal transection or adrenalectomy. In addition, selective destruction of the hypothalamic 5-HT nerve fibers, produced by administration of 5,7-di-hydroxytryptamine (a 5-HT nerve depletor) into both dorsal and median raphe regions, reduced the magnitude of the hyperglycemic responses to electrical stimulation of either dorsal or median raphe nuclei. The data indicate that stimulation of ascending 5-HT pathways in the rat's brain increases the adrenal-sympathetic efferent activity and leads to hyperglycemia.

Stimulation of both D1 and D2 dopamine receptors increases behavioral activation and ascorbate release in the neostriatum of freely moving rats

Electrochemically modified carbon-fiber electrodes were used to assess the effects of indirect (amphetamine and GBR-12909) as well as direct D1 (SKF-38393) and D2 (quinpirole) dopamine agonists on extracellular ascorbate in the neostriatum of awake, behaving rats. Relative to controls, 2.5 mg/kg d-amphetamine and 20.0 mg/kg GBR-12909 produced marked behavioral activation concomitant with a significant increase in ascorbate. Comparable effects were observed following the combined administration of 10.0 mg/kg SKF-38393 and 1.0 mg/kg quinpirole, but not after either of these drugs alone. Thus, behavioral activation and release of neostriatal ascorbate were closely related to the concurrent stimulation of both D1 and D2 dopamine receptors.

Circadian rest-activity rhythms in the anophthalmic, monocular and binocular ZRDCT/An mice. Retinal and serotoninergic (raphe) influences

In the present study, two strains of mice were used: the control C57 Black/6 and the experimental ZRDCT/An, composed of anophthalmic (AN), monocular (mono) and binocular (bi) animals. The circadian rest-activity rhythms of these mice, submitted to a 12 h/12 h light/dark (L/D) cycle, present different characteristics. In C57 Black/6 animals the rhythms are classically synchronized to the L/D cycle with a maximal activity during the dark period, mainly at its beginning. In the AN mice, without exception, the rhythms are free running. In the bi and mono animals they are either synchronized to the L/D cycle, or synchronized but phase shifted in comparison with C57 Black/6, or free running. For the mono and bi animals, the typical or atypical mode of synchronization of their rhythms to the L/D cycle appears to be correlated with the ganglion cell density. It is well known that such a retinal component ensures the retino-hypothalamic connections. In the C57 Black/6 and AN mice, differential pulse voltammetric measurements of 5-hydroxyindoleacetic acid (5-HIAA, peak 3) extracellular concentrations have been also realized together with polygraphic recordings of the sleep/waking cycle. In both strains, the height of the 5-HIAA peak 3 recorded in the cortex is always the highest during active waking, and decreases during slow wave sleep and paradoxical sleep. Further, a circadian variation of this signal is superimposed to that of the waking state amount. Such circadian variations are synchronized to the L/D cycle in C57 Black/6 animals and free running in the AN mice.

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.

Efflux of 5-HIAA from 5-HT neurons: a membrane potential-dependent process

The effect of the membrane potential on the efflux of 5-HIAA from 5-HT neurons was studied in anesthetized (halothane: 1% in gas mixture of N2O: 70% and O2: 30%) cats. The endogenous 5-HT and its metabolite 5-HIAA were measured continuously from the cortex, the thalamus, the hypothalamus and the raphe nuclei using brain microdialysis technique combined with HPLC-ED monoamine measurements. Membrane potential variations were induced by changing the extracellular concentration of potassium through the microdialysis membrane. The levels of the extracellular 5-HIAA varied according to the different regions of the brain, being highest in the hypothalamus and lowest in the cerebral cortex. Increases in the extracellular potassium from 4 to 120 mM invariably produced a decrease of the extracellular 5-HIAA in all the tested brain regions. This decrease was inversely proportional to the logarithm of extracellular potassium concentration. Thus, it is postulated that the 5-HIAA is moved from inside the cell to extracellular space by an active mechanism of transport electrically coupled to the membrane potential.

Electrochemical analysis of hypothalamic serotonin metabolism accompanied by immobilization stress in rats

The effects of stress on serotonin metabolism in the lateral hypothalamic area (LHA) were investigated. Differential pulse voltammetry with a carbon fiber electrode was used to measure 5-hydroxyindoleacetic acid (5-HIAA), the metabolic product of the serotonin (5-HT). High-performance liquid chromatography with electrochemical detection was also used to analyze these compounds in the cerebrospinal fluid (CSF). The peak in voltammogram at an oxidation potential of +230 mV (P3) was identified as 5-HIAA by pharmacological manipulations that are known to affect 5-HT metabolism. A significant increase in 5-HIAA concentration in the LHA was detected after immobilization. An intraperitoneal injection of 5-hydroxytryptophan (5-HTP; a precursor of 5-HT) increased the height of P3, and injection of pargyline prevented the effect of 5-HTP during the course of increasing P3. These results support our previous conclusion that immobilization-induced anorexia might be mediated through activation of serotonergic mechanisms in the LHA.

Measurement of 3-methoxytyramine by in vivo voltammetry: evidence for differences in central dopamine function in BALB/c and CBA mice

Differential pulse voltammetry (DPV) combined with carbon fibre electrodes allows selective detection of electroactive dopamine and serotonin metabolites in vivo. While usually employed in rats, we have now applied this in vivo technique in two inbred strains of mice: BALB/c and CBA. Three distinct oxidation peaks were recorded in vivo in the striatum of either BALB/c or CBA mice with a small shoulder occurring after the third peak at approximately +400 mV. Pargyline (150 mg/kg i.p.) potentiated this voltammetric shoulder into an easily measurable peak (Peak 4). In addition, Peak 4 was 2-3 times larger in BALB/c than in CBA mice. Homovanillic acid (HVA) and 3-methoxytyramine (3-MT), both catabolites of dopamine, oxidised at approximately +400 mV in vitro. Brain tissue levels of HVA and 3-MT, measured by high-performance liquid chromatography (HPLC) with electrochemical detection, demonstrated that pargyline treatment reduced striatal HVA, but increased 3-MT. These results support the view that Peak 4 recorded in the striatum of pargyline-treated mice in vivo is due to the oxidation of extracellular 3-MT. Thus, Peak 4 may be a useful index of dopamine release in situations where dopamine itself cannot be detected. Local infusion of KCl (2 microliters, 0.1 M) further increased the size of Peak 4 in the striatum of both BALB/c and CBA mice. However, the increase was approx. 3 times greater in BALB/c mice, supporting previous evidence of greater dopaminergic function of BALB/c compared with CBA mice. In addition these two inbred strains of mice provide model systems for investigating the comparative functional roles of nigrostriatal pathways.

In vivo measurement of extracellular 5-hydroxytryptamine in hippocampus of the anaesthetized rat using microdialysis: changes in relation to 5-hydroxytryptaminergic neuronal activity

The effect of manipulating the activity of central 5-hydroxytryptamine (5-HT) neurones on extracellular 5-HT in ventral hippocampus of the chloral hydrate-anaesthetized rat was studied using the brain perfusion method, microdialysis. Basal levels of 5-HT in the dialysates were close to the detection limits of our assay using HPLC with electrochemical detection. However, addition of the selective 5-HT reuptake inhibitor citalopram (10(-6) M) to the perfusion medium produced readily measurable amounts of dialysate 5-HT. Citalopram, therefore, was used throughout our experiments. Hippocampal dialysate levels of 5-HT sharply declined over the first hour after dialysis probe implantation, but then became constant. This stable output of 5-HT was reduced by 57% in rats treated 14 days previously with intracerebroventricular injections of the 5-HT neurotoxin 5,7-dihydroxytryptamine. Electrical stimulation (1-ms pulse width, 300 microA, 2-20 Hz) of the dorsal raphe nucleus for 20 min caused a rapid rise in hippocampal 5-HT output, which immediately declined on cessation of the stimulus and was frequency-dependent. Addition of tetrodotoxin (10(-6) M) to the perfusion medium reduced 5-HT levels to 75% of predrug values. Injection of the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (0.5 and 2.5 micrograms) into the dorsal raphe nucleus caused a dose-related fall in hippocampal output of 5-HT compared to saline-injected controls. We conclude from these data that the spontaneous output of endogenous 5-HT into hippocampal dialysates, measured under our experimental conditions, predominantly originates from central 5-HT neurones and changes in accordance with their electrical activity.

Changes in striatal dopamine metabolism measured by in vivo voltammetry during transient brain ischemia in rats

In vivo voltammetry was used in rats with brain ischemia induced by four-vessel occlusion to measure changes in dopamine metabolism via measurement of peak 2 (dopamine compounds) in the striatum. Changes in regional cerebral blood flow in the striatum were also assessed by means of a temperature-controlled thermoelectrical device. Peak 2 increased by 600-900% during 30 minutes of four-vessel occlusion, which may have reflected an ischemia-provoked increase in the release of dopamine and a disturbance in the outward transport of its metabolites. Following reperfusion by discontinuation of carotid occlusion, peak 2 rapidly decreased to below control values and then gradually increased, exceeding control values at 180-210 minutes after reperfusion. REgional cerebral blood flow in the striatum decreased to almost 0 ml/100 g/min during the ischemic period, transiently increased to greater than control values after reperfusion, then gradually decreased during the next 240 minutes. Since dopamine is known to have various effects on cerebral metabolism and blood flow, alterations in its behavior may contribute to changes in cerebral blood flow and to postischemic brain damage. In vivo voltammetry may be useful in the investigation of the pathophysiology of brain ischemia.

Suppressant effects of selective 5-HT2 antagonists on rapid eye movement sleep in rats

The effects of the novel, highly selective serotonin-2 (5-HT2) antagonists, ICI 169,369 and ICI 170,809, on 24 h EEG sleep-wake activity were studied in the rat. Both compounds caused a dose-related increase in the latency to rapid eye movement sleep (REMS) and significantly suppressed cumulative REMS time up to 12 h postinjection. In contrast, neither drug disrupted slow-wave sleep continuity in as much as the latency to non-REMS (NREMS) and cumulative NREMS time were unchanged. However, at the highest dose tested (20 mg/kg) ICI 170,809 did produce a significant increase in total NREMS time during the second half of the sleep-awake cycle. These results demonstrate effects of selective 5-HT2 antagonists on sleep in rats which appear to be specific for REMS behavior, suggesting that the priming influence of serotonin on REMS may involve 5-HT2 receptor subtypes. The relationship between the REMS suppressant actions of these compounds and their consideration as therapeutic agents in depression is discussed.

Changes in serotonin metabolism in the rat raphe magnus and cardiovascular modifications following systemic administration of clonidine and other central alpha 2-agonists: an in vivo voltammetry study

By using the in vivo voltammetry, it was demonstrated that an injection of clonidine induced both cardiovascular modifications (hypotension and bradycardia) and a decrease in the level of 5-hydroxyindolacetic acid (5-HIAA) in the ventromedial B3 serotonergic (5-HT) cell bodies of the medulla oblongata of the rat. The cardiovascular effects of clonidine and of two other imidazolic compounds (detomidine and medetomidine) are likely to be related to their alpha 2 adrenoceptor agonist properties since hypotension and bradycardia were completely antagonized by idazoxan. The decrease in levels of 5-HIAA, induced by these three imidazolic compounds is likely to represent the combination of two additional mechanisms: (i) the stimulation of the alpha 2 adrenoceptors which could contribute to 55% of the decrease observed for the extracellular 5-HIAA and (ii) the interaction with a non-alpha 2 site (through a putative imidazole recognition site), corresponding to the part of the decrease (about 45%) which was not prevented by idazoxan.

Reversibility of para-chlorophenylalanine-induced insomnia by intrahypothalamic microinjection of L-5-hydroxytryptophan

Para-chlorophenylalanine, a blocker of serotonin biosynthesis by inhibiting tryptophan hydroxylase, induced total insomnia which was accompanied in cat by a permanent discharge of ponto-geniculo-occipital activity. L-5-Hydroxytryptophan microinjection (1-4 micrograms/0.5 microliters) in the anterior hypothalamus 72 h after para-chlorophenylalanine administration, restored both slow wave sleep and paradoxical sleep with variable latencies for each state of sleep. On the contrary, ponto-geniculo-occipital activity was never suppressed. The hypnogenic effects of L-5-hydroxytryptophan were always followed by a return of the para-chlorophenylalanine-induced insomnia. On the other hand, the temperature recording did not show any alteration of the cerebral temperature after para-chlorophenylalanine treatment but the subsequent L-5-hydroxytryptophan microinjection was followed by hyperthermia. Using immunohistochemistry for serotonin after intrahypothalamic L-5-hydroxytryptophan microinjection in parachlorophenylalanine-pretreated cat, we defined a restricted region of the anterior hypothalamus possibly responsible for the hypnogenic effect. This region included the lateral hypothalamus and the anterior hypothalamic area. It is suggested that the reversible hypersomnia after L-5-hydroxytryptophan microinjection in the anterior hypothalamus in para-chlorophenylalanine-pretreated cat is due to a neurohormonal action of serotonin: serotonin could act upon the anterior hypothalamus which secondarily inhibits a waking system located in the ventrolateral hypothalamus leading to the appearance of paradoxical sleep.

Morphine increases 5-HT metabolism in the nucleus raphe magnus: an in vivo study in freely moving rats using 5-hydroxyindole electrochemical detection

The purpose of this study was to evaluate in freely moving animals the effect of morphine on the 5-hydroxyindole oxidation current recorded in the nucleus raphe magnus (NRM) which is the origin of serotonergic control systems modulating the transmission of noxious inputs at the spinal level. A current recorded at 270-290 mV (peak 3), characteristic of 5-hydroxyindoleacetic acid (5-HIAA), was measured with treated multi-fiber carbon electrodes, using differential pulse (DPV) or differential normal pulse (DNPV) voltammetry. In control rats the amplitude of the peak remains constant for many hours. Morphine (10 mg/kg i.p.) caused a very significant increase which plateaued between 60 and 80 min (mean increase: 142 +/- 7% of control values); recovery was complete by about 3 h. Simultaneous injection of naloxone (1 mg/kg i.p.) completely abolished the effect of morphine. The peak 3 augmentation was still observed (151 +/- 5%) in rats pretreated with the xanthine oxidase inhibitor, allopurinol (12 mg/kg i.p.), but did not occur when animals were given an anaesthetic dose (450 mg/kg i.p.) of chloral hydrate. It is concluded that morphine clearly increases the metabolism of serotonin (5-HT) in the NRM, and one could speculate that the increase in 5-HIAA results from 5-HT release. Such a release could be due either to 5-HT terminals originating in the periaqueductal gray, or to somato-dendritic mechanisms. Thus the question remains as to the relationship between the activation of 5-HT metabolism in the NRM and previous neurochemical evidence for morphine-induced augmentation of 5-HT metabolism within the terminal area of serotonergic raphe-spinal pathways.

In vivo voltammetry in the B3 group of serotonin neurons of the rat medulla oblongata after drug-induced modifications of arterial pressure

Differential normal pulse voltammetry (DNPV) using an electrochemically treated carbon fiber electrode was applied to the investigation of the in vivo changes in extracellular 5-hydroxyindoleacetic acid (5HIAA) in the B3 group of serotonin neurons during experimental manipulations of arterial pressure. Drug-induced hypertension (phenylephrine infusion) caused, during the infusion, an increase in extracellular 5HIAA concentration which continued to rise, reaching +100% 2 hours after stopping the infusion. In contrast, drug-induced hypotension (sodium nitroprusside infusion) was not associated with any change in extracellular 5HIAA during the infusion while the return to the initial arterial pressure caused a progressive increase in the electrochemical signal, reaching +50% one hour after stopping the infusion. These data show that the extracellular 5HIAA concentration is increased when the arterial pressure increases, a result which suggests that B3 serotonin neurons could have a vasodepressor role in the central regulation of arterial pressure.

Electrocoating carbon fiber microelectrodes with Nafion improves selectivity for electroactive neurotransmitters

A method which improves carbon fiber microelectrode selectivity for cationic amines by electrocoating the fiber with a thin film of the ionic polymer, Nafion, is described. The selectivity and response speed of these electrodes for the detection of electroactive cationic and anionic species found in brain extracellular fluid was evaluated using differential pulse voltammetry and chronoamperometry and compared to uncoated fibers. Carbon fiber microelectrodes electrocoated with Nafion are highly sensitive to cationic amines such as dopamine and serotonin and have minimal sensitivity to anions such as ascorbic acid and uric acid at physiological concentrations.

In vivo electrochemical detection of 5-hydroxyindoles in the dorsal horn of the spinal cord: the contribution of uric acid to the voltammograms

Treated carbon fiber electrodes were used with differential normal pulse voltammetry (DNPV) for in vivo determination of the relative participation of uric acid (UA) to peak 3 derived between 250-300 mV in the dorsal horn of the spinal cord of anesthetized rats. In vitro, treated carbon fiber electrodes respond linearly over a large range of concentrations of UA (oxidation potential around 250 mV) and 5-hydroxyindoleacetic acid (5-HIAA, oxidation potential around 280-290 mV), but are 3 to 4 times more sensitive to 5-HIAA than to UA. In vivo the question remains as to the exact nature of peak 3 because the difference between oxidation potentials of UA and 5-HIAA is not great enough to permit a separate monitoring of the two compounds. In normal rats, administration of the xanthine oxidase inhibitor allopurinol, produced a progressive decrease of the signal, which reached 64.3% of controls at 120 min (35.6% diminution) after injection, and then plateaued around this value for up to 2 h. The administration of the monoamine oxidase inhibitor (MAOI) clorgyline, produced a classical decay in the voltammograms due to a diminution of endogenous 5-HIAA; however, allopurinol injected 3 h after MAOI gave an additional decrease of peak 3 of about 28%. Finally, in rats pretreated with parachlorophenylalanine (pCPA), the residual peak (32.48% as compared to peak 3 of normal rats taken as 100%), the potential of which is shifted to near that of UA, could be decreased by allopurinol to a level of 9.6% of the peak in control animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Nucleus tractus solitarius: an evaluation by in vivo voltammetry

Nucleus tractus solitarius (NTS) is a brainstem nucleus known to play an important role in baroreceptor mediated cardiovascular regulation. As part of our study of the role of monoamines in the function of NTS, we have characterized pharmacologically the in vivo electrochemical signal recorded from the nucleus using carbon paste electrodes and linear sweep voltammetry with semiderivative signal processing in awake, freely moving rats. Two peaks were recorded by these techniques, one at 0.14 V and a second at 0.28 V. The tyrosine hydroxylase inhibitor alpha-methyl-p-tyrosine led to a significant reduction in the peak recorded at 0.14 V whereas it had no effect on the higher potential peak at 0.28 V. The dopamine-beta-hydroxylase inhibitor fusaric acid resulted in a large reduction in the 0.14 V peak and led to a 30% increase in the 0.28 V peak height. Pargyline, a monoamine oxidase inhibitor, did not change the low potential peak but did significantly reduce the 0.28 V peak. Tissue assays provided further support for the interpretation of in vivo electrochemical recordings. Norepinephrine concentration was reduced with fusaric acid. Tissue serotonin was not affected by any of the drugs while the 5-HIAA content was increased with fusaric acid and reduced with pargyline. These experimental findings lead to the conclusion that the first peak in the voltammogram most likely represents norepinephrine with a possible contribution by dopamine but not by DOPAC. The second peak appears to be 5-HIAA.

Does morphine enhance the release of 5-hydroxytryptamine in the rat spinal cord? An in vivo differential pulse voltammetry study

Differential pulse voltammetry used in combination with an electrochemically treated carbon fiber electrode allowed the detection of 5-hydroxyindoles (5-HI) in the dorsal horn of the urethane-anesthetized rat. Voltammograms were recorded every 3 min for up to 4 h. One component of the signal, peak 3, corresponding to 5-HI and uric acid was first identified separately in vitro as well as in vivo, and then further examined by means of systemic L- and D-trytophan administration and by local application of uricase, respectively. It was found that the height of peak 3 was unaffected by systemic morphine. Even following pretreatment with probenecid, the height of peak 3 was increased only 8.6-13.7% over that with saline, by morphine given either intraperitoneally or intracerebrally into the nucleus raphe magnus. However, these increments of peak 3 were not statistically significant. These findings suggest that the serotonin descending system is unlikely to play an important role in morphine analgesia.

Functional meaning of tryptophan-induced increase of 5-HT metabolism as clarified by in vivo voltammetry

Differential pulse voltammetry with carbon fiber electrodes was used to study serotonin (5-HT) metabolism in freely moving rats. The electrodes implanted in the striatum recorded the extracellular 5-hydroxyindoleacetic acid (5-HIAA) oxidation peak after oral tryptophan (150 mg/kg). This 5-HT precursor did not modify the 5-HIAA peak in any rat tested, but it raised 5-HIAA levels determined in total tissue by a classical biochemical method (HPLC). The administration of 5-hydroxytryptophan (5-HTP) (25 mg/kg i.p.) induced an increase of 5-HIAA detectable both in the extracellular medium by voltammetry and in tissue samples. As previously shown, dorsal raphe electrical stimulation raises extracellular 5-HIAA in the striatum and this effect is enhanced by pretreatment with tryptophan. The results suggest that tryptophan in 'normal' conditions enhances 5-HT metabolism without affecting 5-HT release unless such release is stimulated. 5-HTP increases 5-HT metabolism and release.

Modulation of striatal dopamine metabolism by the activity of dorsal raphe serotonergic afferences

Serotonin (5-HT)-dopamine (DA) interaction was studied in the caudate nucleus after electrical stimulation of the dorsal raphe (DR), an area containing 5-HT cell bodies and sending afferences to nigrostriatal dopaminergic neurons. The DR was stimulated by means of a bipolar stainless steel electrode for 16 min (10 Hz, 0.6 ms, 200 microA). 5-HT and DA metabolism were monitored before, during and after stimulation by in vivo differential pulse voltammetry. This electrochemical technique uses carbon fiber electrodes implanted in brain areas to record oxidation peaks corresponding to extracellular 5-hydroxyindolacetic acid (5-HIAA) and dihydroxyphenylacetic acid (DOPAC). Changes in the concentrations of the metabolites were recorded every 2 min in freely moving rats. Both 5-HIAA and DOPAC increased in the first minutes after the beginning of stimulation, the rise lasting 30 min after the end. That DR was closely involved was borne out by the fact that stimulation in the surrounding areas had no effect on either metabolite. Classical biochemical determinations in tissue samples were also used to study the effect on DA release: 3-methoxytyramine (3-MT) levels, measured in basal conditions and after blockade of its degradation by pargyline, were not changed, indicating that DR stimulation, though increasing DA metabolism, does not affect release. However, modulation of DA transmission by 5-HT afferences seems possible in certain circumstances. This 5-HT-DA interaction appears to be presynaptic (on dopaminergic terminals or cell bodies) since it is not prevented by kainic acid degeneration of striatal neurons.

Acute administration of clozapine, thioridazine and metoclopramide increases extracellular DOPAC and decreases extracellular 5-HIAA, measured in the nucleus accumbens and striatum of the rat using in vivo voltammetry

Changes in the extracellular levels of dihydroxyphenylacetic acid (DOPAC) and 5-hydroxy-indoleacetic acid (5-HIAA) after acute administration of clozapine (50 mg/kg s.c.), thioridazine (20 mg/kg s.c.) and metoclopramide (5 mg/kg s.c.), were monitored using in vivo voltammetry with micro-carbon electrodes implanted in the nucleus accumbens and striatum of the rat anaesthetised with halothane/N2O. Both clozapine and thioridazine increased extracellular levels of DOPAC in the striatum and the nucleus accumbens. The maximum increases with clozapine were 60% and 86% in the nucleus accumbens and striatum and 44% and 55% with thioridazine. Both neuroleptics also decreased the extracellular level of 5-HIAA in these regions of the brain. Metoclopramide increased the extracellular level of DOPAC in the nucleus accumbens (42%) and the striatum (57%) and significantly decreased the level of 5-HIAA in the nucleus accumbens. These results suggest that the two so-called atypical neuroleptics, clozapine and thioridazine, do not have selective effects on the metabolism of dopamine in vivo in the nucleus accumbens after acute administration. Furthermore, neuroleptic-induced increases in dopamine metabolism are accompanied by reciprocal decreases in 5-hydroxytryptamine metabolism in vivo.

Differential pulse voltammetry in vivo with working carbon fiber electrodes: 5-hydroxyindole compounds or uric acid detection?

Differential pulse voltammetry was performed in rats chronically implanted with carbon fiber electrodes in the caudate (n.Cd) and raphe dorsalis (n.RD) nuclei. The electrochemical signal obtained at the +300 mV potential (peak 3) in animals implanted for more than one week (long term chronic conditions, greater than 7 days) could be dependent upon the extracellular fraction of 5-hydroxyindolacetic acid (5-HIAA) since a single injection of Pargyline is sufficient to suppress it in n.Cd and n.RD. This result was obtained despite the tendency of Pargyline to increase n.Cd and n.RD endogenous concentrations of Uric Acid (UA) measured by High Performance Liquid Chromatography (HPLC). In contrast, in animals implanted for less than one week (short term chronic conditions, less than 7 days) peak 3 recorded in the same structure could be dependent upon extracellular fractions of 5-HIAA and UA since consecutive injections of Pargyline and Allopurinol are necessary to suppress this signal. The source of extracellular UA measured in brain by voltammetry, in such short term chronic conditions, might result from surgical trauma.

Determination of neurotransmitter release into the caudate nucleus during convulsions induced by pentylenetetrazole using in vivo differential pulse voltammetry

In vivo differential pulse voltammetry using an electrochemically treated carbon fiber electrode was applied to the investigation of the in vivo release of indoleamine and catecholamine within the caudate nucleus of freely moving and immobilized rats during convulsions induced by pentylenetetrazole (PTZ). Two distinct oxidation peaks, on at 130 mV (3,4-dihydroxyphenylacetic acid (DOPAC] and the other at 300 mV (5-hydroxyindoleacetic acid), were observed in voltammograms obtained from the caudate nucleus. In the caudate nucleus of freely moving rats, the in vivo oxidation current that peaked at 300 mV showed almost no change during and after tonic convulsions induced by 60 mg/kg of PTZ i.p. During tonic convulsions, the amplitude of the DOPAC oxidation peak significantly decreased to 75% of the peak height recorded prior to the injection of PTZ, and the minimum lasted for about 30 min; then the mean curve slowly recovered to the control level within 60 min. These results suggest that the release of dopamine (DA) in the caudate nucleus of freely moving rats decreased during tonic convulsions induced by PTZ. In another experiment, the EEGs of immobilized rats were recorded simultaneously, and the changes in the EEG pattern were used as an index of convulsion. In voltammograms from the caudate nucleus of immobilized rat, the peak height of the 130-mV oxidation potential significantly increased during ictal seizures. The increase lasted for 3-6 min after the ictal seizures. The severe electrical activity of the brain during ictal seizures interfered with the recording of some voltammograms.(ABSTRACT TRUNCATED AT 250 WORDS)

Factors affecting in vivo electrochemistry: electrode-tissue interaction and the ascorbate amplification effect

While in vivo electrochemistry has been shown to be useful for discovering new neurophysiological phenomena, there is still considerable controversy about the identity of the compounds being measured and the concentration of those compounds in extracellular fluid in brain. We have found that carbon paste electrodes undergo changes in sensitivity and specificity for dopamine and other compounds after being implanted in brain. We have also examined the effect of ascorbate on the selective enhancement of catecholamine peaks to provide an explanation for the apparently very high concentrations of dopamine measured in the extracellular fluid space. After temporary brain implantation (20 min), carbon paste electrodes tested in vitro showed increased sensitivity and lower oxidation potentials for dopamine, norepinephrine and serotonin. These brain-treated electrodes also detected 3,4-dihydroxyphenylacetic acid (DOPAC) as a distinct peak at +0.16 V, although the electrode sensitivity for DOPAC was some 25 times lower than that for dopamine. Brain treatment did not alter electrode sensitivity or oxidation potential for 5-HIAA. The oxidation current for ascorbic acid when processed as the semiderivative showed no distinct peak in the potential range -0.2 to +0.4V for either untreated or brain-treated electrodes. However ascorbic acid amplified the electrochemical peaks of catechols in direct proportion to the ratio of the concentration of ascorbate to the concentration of the catechol. In the physiologic concentration range of 300 microM ascorbate, the electrochemical signal for 1 microM dopamine was amplified 4250%. While ascorbate amplification improves detectability of dopamine and norepinephrine, it also introduces ambiguity since changing catechol concentrations cannot be distinguished from changing ascorbate concentrations.

Increase of dopamine turnover in bilateral striata after unilateral injection of haloperidol into substantia nigra of unrestrained rats

In order to investigate self-regulation of dopamine (DA) neurons, the effects of intranigrally administered haloperidol (Hal), a DA receptor antagonist, on nigrostriatal DA systems were examined using differential pulse voltammetry with carbon fiber electrode. The measurements were achieved in the bilateral caudate-putamen (CP) of behaving rats, in the region of which DA or 3,4-dihydroxyphenylacetic acid made an oxidative current peak (P2) spontaneously. Unilateral injection of Hal (5 micrograms in 1 microliter) into the substantia nigra of rat increased P2 in a time-dependent manner. This phenomenon was observed in both CP, but a more significant increase was in the ipsilateral side (156 +/- 2% of spontaneous height 2.75 h after injection) than in the contralateral side (129 +/- 7%). These effects enlarged in a dose-dependent manner. The same results were found in tissue homogenates determined by high-performance liquid chromatography with electrochemical detection. In the latter case, however, no significant difference was observed between the left and right sides. The present results suggest that Hal, attaching nigral autoreceptors on the cell bodies and dendrites, blocks inhibitory influence of endogenous DA and then activates the nigrostriatal DA neurons, while the contribution of non-dopaminergic neurons is also possible.

Brain serotonin and blood pressure regulation: studies using in vivo electrochemistry and direct tissue assay

Hypotensive responses to tryptophan and 5-hydroxytryptophan infusions were studied in normotensive male Sprague-Dawley rats. Results showed that 5-hydroxytryptophan but not tryptophan lowered pressure in a dose dependent way in direct relation to the production of brain serotonin and 5-HIAA. Intrinsic release of serotonin from brain was also studied during periods of induced hypertension and hypotension. Brain monoamine responses to blood pressure changes induced by intravenous phenylephrine and nitroprusside were measured in dorsal raphe nucleus and nucleus tractus solitarius by in vivo electrochemistry. Results showed that 5-HIAA was increased during drug induced hypertension and during reflex hypertension which followed a period of hypotension. These changes were blocked by sinoaortic denervation indicating that these central serotonergic neurons are responding to increased pressure sensed by baroreceptors. Therefore, serotonin has a role in blood pressure regulation as a pharmacologic agent and as a neurotransmitter in homeostatic control of pressure.

Factors influencing the properties of voltammetric carbon fibre electrodes: the importance of the pH of the medium used for the electrical treatment and of the resin coating of the fibres

Electrical treatment of resin-coated voltammetric carbon fibre electrodes with triangular voltage at low (1.1) pH resulted in electrodes almost insensitive to ascorbic acid (AA) and dihydroxyphenylacetic acid (DOPAC) while their response to 5-hydroxyindoleacetic acid (5-HIAA) was practically the same as after treatment in the usually employed pH 7.4 medium. Electrodes treated at high pH (12.0), on the other hand, were more sensitive to AA and DOPAC than those treated at pH 7.4 and less sensitive to 5-HIAA. Exposing resin-coated electrodes to the treatment media without electrical treatment was not sufficient to obtain the same results as with the application of the current. Electrodes without resin coating were sensitive to AA without electrical treatment while coated electrodes were not. Electrical treatment increased the sensitivity of non-coated electrodes and rendered coated electrodes even more sensitive than non-coated ones. Treatment of coated electrodes for a maximum sensitivity to 5-HIAA was found to require less time than to obtain maximum sensitivity to the other compounds. Present results suggest that it is possible to prepare selective voltammetric electrodes by choosing the right parameters for their electrical pretreatment.

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.

Effect of short-term swimming stress and diazepam on 3,4-dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindoleacetic acid (5-HIAA) levels in the caudate nucleus: an in vivo voltammetric study

The effects of swimming stress on dopaminergic and serotonergic neurons were studied by an in vivo voltammetry technique. 3,4-Dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindoleacetic acid (5-HIAA) levels in rat striatum were measured by differential pulse voltammetry with an electrochemically treated carbon fiber electrode. Exposure to swimming stress for 1 to 10 min to the animal increased the DOPAC and 5-HIAA peaks, which depended on the length of stress. Pretreatment of the rats with diazepam (10 mg/kg, i.p.) prevented completely the stress-induced increase in DOPAC levels but only partially reduced the increase in 5-HIAA levels.

Simultaneous monitoring of dopamine release in rat frontal cortex, nucleus accumbens and striatum: effect of drugs, circadian changes and correlations with motor activity

Changes in homovanillic acid concentration, recorded in vivo by voltammetry with carbon-paste electrodes, were used as an index of dopamine release. With electrodes implanted in frontal cortex, nucleus accumbens and striatum, the pattern of dopamine release was monitored simultaneously in the three brain regions together with the rats' motor activity. Changes in response to the systemic administration of dopamine-receptor agonists and antagonists were used as an index of feedback control of dopamine release. The relationships between dopamine release and motor activity, as well as that between dopamine release in the different brain regions, were investigated by calculating correlation coefficients for data collected over 24-h periods. The results show that dopamine release in frontal cortex is subject to little feedback regulation, that there is no nocturnal increase and no correlation with motor activity. Dopamine release in accumbens and striatum follows a very different pattern. There was a high correlation between dopamine release in these two regions on both sides of the brain; the correlation between the accumbens and the ipsilateral striatum was the highest. Dopamine release in both regions shows evidence of considerable feedback regulation, a nocturnal increase and a high correlation with motor activity. The importance of the accumbens in relation to the level of motor activity is supported by the finding that the correlation coefficient between motor activity and dopamine release in this structure was significantly higher than that between activity and release in the striatum.