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

Circadian regulation of membrane physiology in neural oscillators throughout the brain

Twenty-four-hour rhythmicity in physiology and behavior are driven by changes in neurophysiological activity that vary across the light-dark and rest-activity cycle. Although this neural code is most prominent in neurons of the primary circadian pacemaker in the suprachiasmatic nucleus (SCN) of the hypothalamus, there are many other regions in the brain where region-specific function and behavioral rhythmicity may be encoded by changes in electrical properties of those neurons. In this review, we explore the existing evidence for molecular clocks and/or neurophysiological rhythms (i.e., 24 hr) in brain regions outside the SCN. In addition, we highlight the brain regions that are ripe for future investigation into the critical role of circadian rhythmicity for local oscillators. For example, the cerebellum expresses rhythmicity in over 2,000 gene transcripts, and yet we know very little about how circadian regulation drives 24-hr changes in the neural coding responsible for motor coordination. Finally, we conclude with a discussion of how our understanding of circadian regulation of electrical properties may yield insight into disease mechanisms which may lead to novel chronotherapeutic strategies in the future.

Short-range differential pulse voltammetry for fast, selective analysis of basal levels of cerebral compounds in vivo

Differential pulse voltammetry (DPV) with pretreated biosensors (carbon fibre microelectrodes (mCFE), 10-30 microns diameter) allows selective in vivo measurement of basal endogenous levels of dopamine (DA), serotonin (5-HT), their metabolites (dihydroxyphenylacetic acid, DOPAC; 5-hydroxyindoleacetic acid, 5-HIAA), and neuropeptides. We have now modified DPV in order to reduce the time of analysis from tens of seconds to 1-2 s without losing selectivity. We call this newly reported method short-range differential pulse voltammetry (SRDPV). Simply, while in DPV the complete oxidation peak is recorded, SRDPV measures only the top of each oxidation peak. For example, to monitor peak 2 which corresponds to the in vivo oxidation of extracellular DOPAC and occurs at approximately +85 +/- 10 mV, the initial (Ei) and final (Ef) potentials applied with DPV were -100 mV and +200 mV, respectively, while they were +75 mV (Ei) and +95 mV (Ef) with SRDPV. At the typical scan range of 10 mV.s-1, the effective time of measurement was 30 s for DPV and 2 s for SRDPV. A similar procedure was performed to analyze peak 3 (5-HIAA, occurring at +230 +/- 11 mV) with Ei + 50 mV and Ef + 350 mV for DPV, or +220 mV and +240 mV for SRDPV. DPV and SRDPV were compared in vitro by quantitating DOPAC and 5-HIAA in solutions of increasing concentrations (chosen on the basis of the suggested in vivo content of these two compounds). Data indicated that similar sensitivity and selectivity were obtained with both methods at all concentrations, supporting the applicability of SRDPV for in vitro studies. In vivo experiments were performed in anesthetized adult male rats prepared for voltammetry by inserting the electrically pretreated biosensor (mCFE) into the striatum. DPV measurements were performed automatically every 3-5 min and were alternated every 10-20 min with a sequence of 5-10 SRDPV scans performed every 10-30 s. Subsequent pharmacological or electrical manipulations of the two biogenic amine systems studied were monitored by alternate use of DPV and SRDPV. The data presented support the capability of SRDPV with pretreated biosensors to measure in vivo electroactive compounds with selectivity and sensitivity comparable to that of DPV, but with improved time resolution.

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.

Femoxetine blocks the morphine-induced increase in 5-HT metabolism, as measured by in vivo voltammetry in the nucleus raphe magnus of freely-moving rats

Tricyclic antidepressants, when administered acutely, are known to potentiate morphine-induced antinociception. Systemic administration of morphine has been shown to increase the metabolism of serotonin (5-HT) at the level of the nucleus raphe magnus, as measured by in vivo electrochemistry, in freely-moving rats. Using a similar electrochemical detection of 5-hydroxyindole (peak "3") in the nucleus raphe magnus, the present study investigated the effect of the specific 5-HT uptake inhibitor, femoxetine, on peak 3 and on changes in the metabolism of 5-HT, induced by morphine. Acutely administered femoxetine (40 mg/kg i.p.) induced a significant decrease in peak 3 and completely abolished the effect of morphine (10 mg/kg i.p.) on the metabolism of 5-HT. These data do not support the contention that potentiation of morphine-induced analgesia, by tricyclic depressants results from an interaction between the tricyclic antidepressants and the morphine-induced increase in metabolism of 5-HT, at the level of the nucleus raphe magnus.

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.

Alterations in acetylcholine release in the rat hippocampus during sleep-wakefulness detected by intracerebral dialysis

Acetylcholine (ACh) release from the dorsal hippocampus was continuously monitored in freely moving rats during a light period using an intracerebral dialysis technique. A dialysate was collected every 6 min and polygraph recordings including cortical and hippocampal electroencephalograms, electromyogram, and electrooculogram were simultaneously made to determine the stage of sleep-wakefulness. The content of ACh was measured by high-performance liquid chromatography with electrochemical detection. ACh output showed profound and state-dependent fluctuations. ACh levels during waking increased approximately 300% compared to slow wave sleep. In contrast, the rate of ACh release during paradoxical sleep was as high as during waking and appeared to be even higher. These results revealed that the intracerebral dialysis technique provides a useful method to monitor changes in spontaneous neurotransmitter release during the sleep-waking cycle.

Differential responsiveness of the rat dorsal and median raphe 5-HT systems to 5-HT1 receptor agonists and p-chloroamphetamine

The dorsal and median raphe 5-HT neurons give rise to projections that differ in axon morphology and in vulnerability to certain amphetamine derivatives. The present study was undertaken to determine if these two 5-HT systems possess different functional properties. To this end, we studied the effects of selective 5-HT1A or 5-HT1A/5-HT1B receptor agonists and of p-chloroamphetamine on extracellular levels of indoleamines, as measured by differential pulse voltammetry with extracellular levels of indoleamines, as measured by differential pulse voltammetry with electrochemically pretreated carbon fiber electrodes, in cell body and nerve terminal regions of these subsets of 5-HT neurons in the rat brain. The selective 5-HT1A agonist 8-OH-DPAT produced a gradual decrease in the height of the 300 mV oxidation peak in the dorsal raphe and in the frontal cortex, reaching a maximum of 60% 3 h after the i.v. injection of 30 micrograms/kg. However, the same dose of 8-OH-DPAT was ineffective in the median raphe and in the dentate gyrus that receives its 5-HT innervation exclusively from the median raphe. A higher dose of 8-OH-DPAT (150 micrograms/kg, i.v.) produced a 60% decrease in the height of the 300 mV oxidation peak in the median raphe, whereas only a 20% decrease was obtained in the dentate gyrus. In contrast, the non-selective 5-HT1 agonist RU 24,969 (10 mg/kg, i.p.) caused a 70% reduction of the 300 mV peak height in both the dorsal and median raphe and a 50% decrease in both the frontal cortex and the dentate gyrus. Moreover, although a high dose of 8-OH-DPAT (150 micrograms/kg, i.v.) given alone reduced by 20% the amplitude of the oxidative peak in the dentate gyrus, subsequent administration of RU 24,969 (10 mg/kg, i.p.) caused a further 30% diminution of the oxidative peak height. The greater responsiveness of dorsal as compared to median raphe 5-HT systems to 5-HT1A receptor agonists was confirmed in two further series of experiments. First, the microiontophoretic application of 8-OH-DPAT directly onto 5-HT neurons was three times more potent in suppressing the firing rate of dorsal raphe 5-HT neurons than that of their median raphe congeners. Second, 8-OH-DPAT and buspirone were ten and four times, respectively, more potent in decreasing 5-HT synthesis in the frontal cortex than in the hippocampus.(ABSTRACT TRUNCATED AT 400 WORDS)

A comparison of the effects of morphine on 5-HT metabolism in the periaqueductal gray, ventromedial medulla and medullary dorsal horn: in vivo electrochemical studies in freely moving rats

The effect of systemic morphine on serotonin (5-HT) metabolism within the dorsal raphe nucleus (DRN) has been investigated by in vivo 5-hydroxyindole electrochemical (peak '3') detection in freely moving rats. Morphine caused a weak and delayed, but naloxone-reversible, increase in peak '3'. This increase was poorly, if at all, correlated with the morphine-induced analgesia. Finally, stress and/or noxious stimulation had no effect on this signal. These results are compared with our previous studies using the same methodological approaches and show that morphine caused a significant and specific increase in 5-HT metabolism at the levels of nucleus raphe magnus (NRM) and medullary dorsal horn. Furthermore, as shown in the present paper, there was also a good correlation between the time course of such increases and the analgesic effect of morphine. These findings are discussed with reference to the involvement of 5-HT mechanisms in the so-called DRN-NRM-dorsal horn 'intrinsic analgesic system'.

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).

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.

Growth hormone-releasing factor modifies dopaminergic but not serotonergic activity in the arcuate nucleus of hypothalamus in the rat, as recorded in vivo by differential pulse voltammetry

Parenteral (i.v.) injection of growth hormone-releasing factor (GRF) increases the height of the 3,4-dihydroxyphenylacetic acid oxidation peak (peak 2) but does not change 5-hydroxyindole extracellular content (peak 3) in the arcuate nucleus of the hypothalamus, both peaks being recorded by the differential pulse voltammetry technique using a single specifically pretreated monopyrolytic carbon fibre electrode. Conversely, no significant changes are observed in the peak 2 and peak 3 heights recorded in the medial or in the lateral nucleus of the hypothalamus. These data suggest a specific interaction between GRF and the dopaminergic system.

In vivo voltammetry: promise and perspective

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

Monitoring of circadian fluctuations of N-acetylserotonin in the rat pineal body by differential pulse voltammetry

Circadian fluctuations of the electrochemical signal appearing at +270 mV (peak 3) recorded from the pineal body of freely moving rats were first monitored for 24 hours using the in vivo voltammetry technique. The peak 3 height increased after injection of pargyline and S-adenosyl-L-homocysteine but decreased after injection of NSD-1015, while probenecid did not cause any change. Under a 12/12 hours light-dark cycle, the peak 3 height represented circadian fluctuations, similar to those of N-acetyltransferase activity, which were higher in the dark than in the light period. These data suggest that the compound responsible for peak 3 in the pineal body is essentially due to extracellular N-acetylserotonin.

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.

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.

Simultaneous monitoring of 3,4-dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindoleacetic acid (5-HIAA) levels in the brains of freely moving rats by differential pulse voltammetry technique

Differential pulse voltammetry with a newly devised carbon fiber electrode was used to study the nature of striatal electrochemical signals. Voltammograms recorded from the striatum of unanesthetized rats usually yielded the combined oxidation peak (1 + 2) and peak 3. Peaks 1 and 2 could be separated by eliminating peak 1 for ascorbate by electrochemical oxidation in the brain to allow clear monitoring of peak 2 at + 120 mV for catechols and peak 3 at + 270 mV for indoles. The changes in the oxidation potentials and the amplitudes of peaks 2 and 3 corresponded to those of 3,4-dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindoleacetic acid (5-HIAA) in vivo because: the oxidation potentials of peak 2 (+ 120 mV) and peak 3 (+ 270 mV) coincided with those of DOPAC and 5-HIAA in vitro; increases in the heights of peaks 2 and 3 were observed after micro-infusion of DOPAC and 5-HIAA, respectively, into the striatum; and peak 2 height increased after injection of haloperidol and gamma-butyrolactone and decreased after amphetamine and pargyline, while peak 3 amplitude increased following injection of gamma-butyrolactone, probenecid and 5-hydroxytryptophan and decreased after pargyline. Thus, the in vivo voltammetry method enabled simultaneous and stable monitoring of the dynamic changes in DOPAC and 5-HIAA levels in the brains of freely moving rats.

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.

Voltammetric carbon paste electrodes monitor uric acid and not 5-HIAA at the 5-hydroxyindole potential in the rat brain

Changes in the height of peak 2 obtained using linear sweep voltammetry and carbon paste electrodes chronically implanted in discrete brain regions of the unrestrained rat were measured under a variety of conditions; in the past this peak has been attributed to the oxidation of 5-hydroxyindoleacetic acid (5-HIAA). Unilateral 5,7-dihydroxytryptamine (5,7-DHT) lesions of the medial forebrain bundle reduced the 5-HIAA content of the striatum and hippocampus to 10% of the unlesioned side, but did not alter the height of peak 2 recorded in these regions. In contrast, microinfusion of uricase beside striatial electrodes reduced the height of peak 2 by 96%; systemic amphetamine-induced increases in the height of the peak were also prevented by this enzyme. These results indicate that uric acid, and not 5-HIAA, is mainly responsible for peak 2, and that changes in the height of this peak reflect changes in the extracellular concentration of uric acid.

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.