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

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.

The 5-HT1 receptor agonist RU-24969 decreases 5-hydroxytryptamine (5-HT) release and metabolism in the rat frontal cortex in vitro and in vivo

K+-stimulated release of [3H]-5-hydroxytryptamine ( [3H]-5-HT) from rat frontal cortex slices was decreased by the 5-HT receptor agonists 5-methoxy-n1N-dimethyltryptamine and 5-methoxy-3(1,2,3,6,-tetrahydro-4-pyrindinyl)-1H-indole (RU-24969) (1 X 10(-5)M). RU-24969 (10 mg kg-1, i.p.) decreased extracellular 5-HT and its metabolite 5-hydroxyindoleacetic acid measured in vivo by use of intracerebral dialysis combined with high performance liquid chromatography and electrochemical detection. The decrease in extracellular 5-hydroxyindoleacetic acid in vivo after RU-24969 (10 mg kg-1, i.p.) was also observed by in vivo voltammetry. The non-selective 5-HT antagonist metergoline prevented the RU-24969-induced decrease in 5-HT release and metabolism in vivo while the 5-HT2 receptor antagonist R-55669 (ritanserin) did not. The results support the view that RU-24969 stimulates a 5-HT1 receptor that is involved in the autoregulation of 5-HT release and metabolism.

Effects of D- and L-amphetamine on dopamine metabolism and ascorbic acid levels in nucleus accumbens and olfactory tubercle as studied by in vivo differential pulse voltammetry

Differential pulse voltammetry used together with electrochemically pretreated carbon fibre microelectrodes allowed us to detect in vivo two well-separated peaks in nucleus accumbens and olfactory tubercle. The two peaks situated at -50 mV (peak 1) and + 100 mV (peak 2) correspond, respectively, to the oxidation current of the ascorbic acid and to the oxidation current of the 3,4-dihydroxyphenylacetic acid (DOPAC). The experiments were carried out on anesthetized rats. Voltammograms were recorded in nucleus accumbens and olfactory tubercle every minute alternately in each structure. In control conditions, peak 1 height was greater in olfactory tubercle than in nucleus accumbens and peak 2 height was greater in nucleus accumbens than in olfactory tubercle. Both isomers of amphetamine induced a decrease of the peak 2 height in the two structures. The decrease was greater in olfactory tubercle. Higher doses of L-amphetamine were required to induce peak 2 height decrease of the same extent. Both isomers induced a marked increase of the peak 1 height in nucleus accumbens whereas peak 1 height in olfactory tubercle was slightly augmented. D-amphetamine was more effective than L-amphetamine in increasing peak 1 height.

Measurement of 5-HIAA levels in ventricular CSF (by LCEC) and in striatum (by in vivo voltammetry) during pharmacological modifications of serotonin metabolism in the rat

The relationship between the concentrations of 5-hydroxyindoleacetic acid (5-HIAA) in the CSF and in the striatum has been evaluated in the rat by measuring the levels of this metabolite in ventricular CSF (by liquid chromatography coupled with electrochemical detection) and in the striatal extracellular fluid (by in vivo voltammetry) after administration of inhibitors of serotonin synthesis or degradation. Pargyline, NSD 1015 and alpha-propyldopacetamide all caused an exponential decline of 5-HIAA in both CSF and striatum. For a given drug, the rate constants for 5-HIAA disappearance were identical in the CSF and in the striatal extracellular fluid. These results confirm the view that CSF 5-HIAA may serve as a good index of brain serotonin turnover.

Detection of homovanillic acid in vivo using microcomputer-controlled voltammetry: simultaneous monitoring of rat motor activity and striatal dopamine release

Linear sweep voltammograms recorded with carbon paste electrodes in the striatum of the unanaesthetised, unrestrained rat show three separate peaks. The effect on peak 3 of either unilateral 6-hydroxydopamine lesion of the substantia nigra or intraperitoneal administration of alpha-methyl-paratyrosine, supports our earlier conclusion that peak 3 is due to the dopamine metabolite homovanillic acid. Administration of gamma-butyrolactone, which inhibits firing of dopaminergic nigrostriatal neurones, produces an immediate decrease in striatal homovanillic acid, followed by a prolonged increase. Dopamine-receptor agonists and antagonists produce changes in the extracellular concentration of homovanillic acid which are predicted by their effects on dopamine release. Simultaneous monitoring of total motor activity and homovanillic acid show significant correlation between these two parameters. The usefulness of this technique for monitoring dopamine release is critically evaluated in the light of these results.

In vivo electrochemical detection of 5-hydroxyindoles in the dorsal hippocampus of anesthetized rats treated with idebenone (CV-2619)

The effect of idebenone (CV-2619), 6-(10-hydroxydecyl)-2,3-dimethoxy-5-methyl-1,4-benzoquinone, on 5-hydroxyindoles in the dorsal hippocampus of rats was investigated by using in vivo differential pulse voltammetry. CV-2619 (100 mg/kg, i.p.) produced a large increase in oxidation current at 280 mV (peak 2) in the dorsal hippocampus. The increase was statistically significant at 20 and 30 min after the administration. 5-Hydroxytryptophan (150 mg/kg, i.p.) also caused a marked increase in peak 2, which was clearly inhibited by the treatment with pargyline (50 mg/kg, i.p.). Thus, the increase in peak 2 induced by CV-2619 seems to be associated with an increase in the 5-HT metabolite 5-hydroxyindoleacetic acid and also partly with an increase in 5-HT.

In vivo electrochemical detection of 5-hydroxyindoles in rat cerebral cortex and spinal cord: differential effects of p-chloroamphetamine, probenecid and clorgyline

Differential pulse voltammetry associated with carbon fiber microelectrodes was used to detect the 300 mV signal which is known to reflect the concentration of 5-hydroxyindoles in the spinal cord and cerebral neocortex of rats anesthetized with urethane or chloral hydrate. The intraperitoneal injection of p-chloroamphetamine resulted in an increase in the amplitude of the signal in the neocortex but not in the spinal cord. Administration of clorgyline did not consistently modify the signal monitored in the neocortex whereas it decreased in the spinal cord. Probenecid induced a larger increase in 5-hydroxyindoles in the neocortex than in the spinal cord. These results demonstrate that different parts of the serotonergic system might be differentially sensitive to drugs affecting serotonin metabolism.

Changes in monoamine metabolites measured by simultaneous in vivo differential pulse voltammetry and intracerebral dialysis

A direct comparison has been made of the drug-induced changes in extracellular levels of 3,4-dihydroxyphenylacetic acid and 5-hydroxyindoleacetic acid measured using intracerebral dialysis and differential pulse voltammetry with carbon fibre electrodes. The comparison was carried out in chloral hydrate anaesthetized rats with a pretreated carbon fibre electrode implanted in one striatum and an intracerebral dialysis loop in the contralateral striatum. 3,4-Dihydroxyphenylacetic acid and 5-hydroxyindoleacetic acid in the dialysis perfusates were assayed by high pressure liquid chromatography with electrochemical detection. d-Amphetamine (2 mg/kg) decreased extracellular 3,4-dihydroxyphenylacetic acid and the height of the 3,4-dihydroxyphenylacetic acid oxidation peak, while haloperidol (0.5 mg/kg) increased 3,4-dihydroxyphenylacetic acid levels measured in the perfusates and the height of the 3,4-dihydroxyphenylacetic acid oxidation peak. In these experiments there were parallel changes in 3,4-dihydroxyphenylacetic acid levels and peak height and a close correlation between these changes. Tranylcypromine (10 mg/kg) produced an almost parallel decrease in extracellular 5-hydroxyindoleacetic acid (dialysis) and the height of the 5-hydroxyindoleacetic acid oxidation peak, with similar percentage changes and good correlation values being obtained. However, while 5-hydroxy-L-tryptophan (25 mg/kg) increased both the 5-hydroxyindoleacetic acid levels and the height of the 5-hydroxyindoleacetic acid oxidation peak, 5-hydroxyindoleacetic acid in the dialysis perfusates showed a greater increase than the oxidation peak. The results show a close correlation between changes in extracellular 3,4-dihydroxyphenylacetic acid and its respective voltammetric peak and strongly support the use of in vivo differential pulse voltammetry for monitoring dopamine metabolism.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo voltammetric measurement of extracellular 5-hydroxyindoleacetic acid in the denervated striatum after transplantation of mesencephalic raphe neurons

In vivo differential pulse voltammetry with carbon fiber electrodes was used to monitor extracellular 5-hydroxyindoleacetic acid (5-HIAA) levels in a serotonergic denervated target area, the striatum, before and after transplantation of mesencephalic raphé nuclei into the lateral ventricle in the rat. The amplitude of the electrochemical signal recorded in the striatum on the transplanted side was found to be comparable to that measured in control striatum whereas the signal measured from the contralateral nontransplanted striatum was negligible. These findings suggest that mesencephalic raphé nuclei transplants establish connections with the host tissue.

Inhibitory GABAergic influence on striatal serotonergic transmission exerted in the dorsal raphe as revealed by in vivo voltammetry

In vivo differential pulse voltammetry with electrochemically treated carbon fiber microelectrodes has been used to investigate the anatomical nature of the GABAergic influence on striatal serotonergic transmission in the rat. Lesion studies and pharmacological treatments demonstrated that the electrochemical signal recorded at 300 mV in the striatum probably corresponds to the oxidation of extracellularly released 5-hydroxyindoleacetic acid. Thus, dorsal raphé lesions or systemic administration of alpha-propyldopacetamide, NSD 1015, pargyline and MK212 decreased, whereas reserpine injection increased the amplitude of the signal. Moreover, L-5-hydroxytryptophan administration caused an increase in the signal which was almost completely prevented by pargyline pretreatment. Acute administration of dipropylacetamide (150 mg/kg i.p.) reduced the amplitude of the signal from the striatum, while injection of gamma-acetylenic GABA (200 mg/kg i.p.) was without effect. Repeated (but not acute) treatment with the GABA receptor agonist, progabide (400 mg/kg i.p.b.i.d. for 14 days), led to a pronounced decrease in the amplitude of the signal from the striatum. A similar effect was observed after intradorsal raphé infusion of GABA (10 and 100 micrograms), gamma-vinyl GABA (100 micrograms) and SL 75102 (10 micrograms), a principal metabolite of progabide. In contrast, local injection of the GABA receptor antagonists, bicuculline (1 and 10 micrograms) or R5135 (0.05 microgram), failed to affect the peak amplitude in the striatum. When infused into the dorsal raphé, R5135 (0.05-0.1 microgram) antagonized the diminution of the signal induced by intradorsal raphé infusion of GABA (100 micrograms) or SL 75102 (10 micrograms). Finally, electrolytic lesion of the habenular nuclei completely blocked the diminution of the signal from striatum induced by an intradorsal raphé infusion of GABA (100 micrograms). These results indicate that the inhibitory GABAergic control of striatal serotonergic transmission is exerted at the level of the dorsal raphé cells and depends upon the integrity of the habenulo-dorsal pathway.

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.

Negative feedback control of serotonin release in vivo: comparison of 5-hydroxyindolacetic acid levels measured by voltammetry in conscious rats and by biochemical techniques

All evidence that serotonin release from central neurones is controlled by a negative feedback mechanism comes from in vitro studies. To study this problem in vivo we performed differential pulse voltammetry in conscious rats, in which carbon fibre electrodes had been implanted 2-15 weeks previously. The effects of monoamine oxidase inhibition (which decreases the amount of 5-hydroxyindoleacetic acid), as well as that of probenecid (which increases 5-hydroxyindoleacetic acid), suggests that 5-hydroxyindoleacetic acid rather than serotonin is measured. Blockade of the presynaptic serotoninergic autoreceptors by methiothepin, metergoline or quipazine led to an increase in differential current of the peak attributed to 5-hydroxyindoleacetic acid in hippocampus, hypothalamus and striatum. Stimulation of these receptors by m-chlorophenylpiperazine, MK-212 or LSD decreased the signal attributed to 5-hydroxyindoleacetic acid. A decrease in the signal was also seen with cinanserin. Stimulation of presynaptic alpha 2-adrenoreceptors by clonidine decreased the signal. Metergoline, quipazine and cinanserin showed biphasic effects, and no effect was observed with methysergide. In general, a reasonable agreement with the results of Baumann & Waldmeier obtained in vitro with electrically stimulated [3H]serotonin prelabelled cortex slices was achieved with differential pulse voltammetry. Only partial agreement with the results of voltammetry was obtained if 5-hydroxyindoleacetic acid was determined biochemically under comparable conditions. Qualitatively, the effects observed with methiothepin, m-chlorophenylpiperazine, clonidine and LSD were in good agreement with those measured with voltammetry as well as with the in vitro effects obtained in electrically stimulated cortex slices. No, or only partial correlation with the results obtained with voltammetry was found with MK 212, cinanserin, metergoline and quipazine. It is concluded that voltammetry preferentially measures extraneuronal 5-hydroxyindoleacetic acid rather than overall changes of this metabolite.

Temperature-dependent variations of 5-hydroxyindoles in ventricular cerebrospinal fluid--an in vivo voltammetric study

The in vivo voltammetric signal of 5-OH-indoles increased or decreased in the ventricular cerebrospinal fluid when body temperature was artificially increased or decreased in rats in acute experiments. The increase of the signal occurring after raising the body temperature was facilitated by premedication with the monoamine oxidase inhibitor pargyline. It was not abolished by pretreatment with the serotonin synthesis inhibitor para-chlorophenylalanine, although the latter caused an 85% decrease of ventricular 5-OH-indole level. No similar effects were demonstrable in the 5-OH-indole signal of the caudate nucleus recorded simultaneously. An increase of the 5-OH-indole peak was found also in animals chronically implanted with voltammetric electrodes when the ambient temperature was increased in their cages. According to the results of in vitro experiments, a direct physical effect of changes in brain temperature on the sensitivity of the voltammetric electrodes cannot account for changes found in the cerebrospinal fluid. These data, therefore, demonstrate that the increase of body temperature is followed by an increase of 5-OH-indole concentration in the ventricular cerebrospinal fluid. The in vivo voltammetric technique appears to be a suitable method for studying the serotonergic mechanisms involved in thermoregulatory processes.

Voltammetric measurements of 5-hydroxyindole compounds in the suprachiasmatic nuclei: circadian fluctuations

This study concerns the voltammetric signal appearing at a +300 mV potential (peak 3) recorded from the suprachiasmatic nuclei (SCN) of rats under acute and chronic conditions. In acute conditions, and in order to accurately localize the working electrode, a topographical study of the peak 3 height was first realized in the frontal plane containing the SCN by use of differential pulse voltammetry and monocarbon fiber electrodes. In the same conditions, the effects of clorgyline and reserpine were studied. Clorgyline decreased peak 3 while reserpine increased it. Contrary to 5-HT, 5-HIAA contents of the SCN, measured with high performance liquid chromatography, demonstrate analogous variations. All these data suggest that it is essentially 5-HIAA which is responsible for peak 3 from the SCN. In unanesthetized, freely moving rats, under a 12/12 h light-dark cycle, spontaneous and circadian variations of the SCN's peak 3 were studied, during the rest-activity cycle. Multifiber working electrodes were used for this purpose. During spontaneous and successive periods of rest and activity peak 3 height was always found to be higher during activity. Further, these spontaneous variations were superimposed with a circadian variation exhibiting its acrophasis during the dark period. Our data suggest that the release and catabolism of serotonin is greater in waking than in sleeping animals. They also suggest, that the rostral raphe system, phase locked with the SCN, plays an important role in the circadian variations measured.

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

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

In vivo electrochemical detection of 5-hydroxyindoles in rat somatosensory cortex: effect of the stimulation of the serotonergic pathways in normal and pCPA-pretreated animals

Differential pulse voltammetry was used for the detection of 5-hydroxyindoles in the cerebral cortex of rats anaesthetized with urethane. The stimulation of the lateral hypothalamus or of the dorsal raphe nucleus induced a 10-40% increase in the amplitude of the signal. The signal recorded from p-chlorophenylalanine (pCPA)-pretreated animals was much smaller than in normal animals and could be increased by 5-HTP administration. The stimulation of the serotonergic pathways was ineffective in the pCPA-pretreated animals.

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.

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.

Nomifensine antagonizes the ouabain-induced increase in dopamine metabolites in cerebrospinal fluid of the rat

The effect of the Na+, K+-ATPase inhibitor, ouabain, on cerebrospinal fluid dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindolacetic acid (5-HIAA) levels was studied in pentobarbitone-anesthetized rats. An intracerebroventricular injection of ouabain (100 nmol) dramatically increased DOPAC and HVA levels. A dopamine uptake inhibitor, nomifensine (10 mg/kg i.p.) injected prior to ouabain, completely abolished the effect of ouabain on dopamine metabolites. These results are in agreement with the ouabain-induced increase in dopamine efflux described in vitro and favour the role of a carrier-mediated cytosolic release of dopamine.

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

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

Laminar distribution of serotonergic innervation in rat somatosensory cortex, as determined by in vivo electrochemical detection

In vivo differential pulse voltammetry was used for the detection of indoleamines during vertical electrode penetrations in rat first somatosensory cortex, for studying the laminar distribution of serotonin and/or its metabolites in that part of the cortex. The peak of current corresponding to 5-hydroxyindoles was maximum in the most superficial part of the cortex and diminished gradually in the deeper layers. These results suggest that the cortical serotonergic innervation is predominant in the superficial layers.

Increase of serotonin metabolism within the dorsal horn of the spinal cord during nucleus raphe magnus stimulation, as revealed by in vivo electrochemical detection

Carbon fiber microelectrodes were used with the differential pulse voltammetry method for in vivo determination of indolamines within the extracellular space of the dorsal horn of the spinal cord or chloral hydrate-anesthetized rats. Under these conditions a peak of oxidation current which is characteristic of 5-hydroxyindoles is recorded at 280-300 mV. Stimulation of the nucleus raphé magnus (NRM) with stimulation parameters comparable to those used to elicit analgesia in freely moving animals produced marked alterations in the voltammograms: (1) stimulation of the NRM for 10 min induced an immediate and sustained increase in the peak amplitude; (2) post-effects of variable duration were observed; (3) the increase in the 5-hydroxyindolaminergic signal was significantly reduced during a second series of NRM stimulations indicating some degree of tolerance to central stimulation. The accuracy of these observations is strengthened by the fact that the basal 5 hydroxyindolaminergic signal is strongly depressed after pretreatment of the animal with p-chlorophenylalanine; in addition, under these conditions, NRM stimulation is totally inefficient. We suggest that these results reflect the in vivo modification of 5-HT metabolism. This represents the first evidence for an in vivo release of 5-HT during stimulation of brain stem areas which induces powerful analgesia in freely moving animals.