Acetyl-L-carnitine releases dopamine in rat corpus striatum: an in vivo microdialysis study.

The effect of acetyl-L-carnitine, a compound reported to be beneficial for senile patients, on the release of dopamine (DA) from the striatum was studied by using in vivo brain dialysis in anesthetized rats coupled with HPLC-electrochemical detection. Striatal infusion of acetyl-L-carnitine increased the efflux of DA with no apparent changes in efflux of DA metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and 4-hydroxy-3-methoxyphenylacetic acid (HVA). The DA-releasing effect of acetyl-L-carnitine was concentration- and Ca(2+)-dependent, and was abolished by omega-conotoxin fraction GVIA and tetrodotoxin, inhibitors of the voltage-dependent Ca2+ and Na+ channels, respectively. Nomifensine, an inhibitor of DA reuptake did not alter the DA-releasing property of acetyl-L-carnitine. DA released from the striatum by acetyl-L-carnitine was decreased by reserpine pretreatment whereas the d-amphetamine-evoked DA outflow was not affected. In contrast to acetyl-L-carnitine, d-amphetamine reduced the extracellular concentrations of DOPAC and HVA. We conclude from the present data that acetyl-L-carnitine evokes DA release from the vesicular pools of the nigrostriatal dopaminergic neurons by a Ca(2+)-dependent, exocytotic process.     

Effects of morphine on release of acetylcholine in the rat striatum: an in vivo microdialysis study

Summary We examined the effect of morphine on the release of acetylcholine (ACh) in the striatum of freely moving rats using the in vivo microdialysis method. The basal level of ACh was 3.01 ± 0.51 pmol/30 l/15 min in the presence of neostigmine (10 M). Tetrodotoxin (1 M), a selective blocker of voltage-dependent Na⁺ channels, rapidly decreased the release of ACh in the striatal perfusates. Morphine at a dose of 10 mg/kg (i.p.) caused a reduction of ACh release in the striatum at 90–150 min. However, a lower dose of morphine (5 mg/kg, i.p.) did not affect ACh release in the striatum. The reduction following intraperitoneal administration of morphine was abolished by naloxone (1.0 mg/kg).After microinjection of the neurotoxin 6-hydroxydopamine (6 g/3 l, 7 days before) in the substantia nigra, the morphine (10 mg/kg)-induced decrease of ACh was attenuated, and a similar result occurred following reserpine (2 mg/kg, i.p.) 24 h before combined with -methyl-p-tyrosine (300 mg/kg, i. p.) 2.5 h before.These findings indicate that morphine exerts an inhibitory influence on striatal ACh release in freely moving rats and that this inhibitory effect is mediated by the nigro-striatal dopaminergic system.Correspondence to K. Taguchi at the above address     

Effects of halothane anaesthesia on extracellular levels of dopamine,dihydroxyphenylacetic acid,homovanillic acid and 5-hydroxyindolacetic acid in rat striatum: a microdialysis study

Summary The effects of halothane anaesthesia on striatal extracellular levels of dopamine, dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindolacetic acid (5HIAA) were investigated in microdialysis experiments. Induction of anaesthesia was accompanied by a rapid increase in dopamine levels and a slower increase in DOPAC and HVA. 5HIAA was not affected. The reduction of dopamine levels induced by apomorphine 0.05 mg/kg appeared with a shorter latency in conscious rats than in anaesthetised rats but the maximum decrease was unaffected by anaesthesia. The decreases in dopamine and DOPAC induced by -methyl-p-tyrosine 50 mg/kg were affected in opposite directions by halothane: the dopamine reduction was more pronounced while the DOPAC reduction was less pronounced in anaesthetized than in conscious animals. In no case was a qualitative shift in the response observed. It is concluded that halothane may influence the levels of dopamine as well as the response to dopaminergic drugs.Send offprint requests to L. Ståhle at the above address     

Acute effects of bupropion on extracellular dopamine concentrations in rat striatum and nucleus accumbens studied by in vivo microdialysis

This study examined the acute effects of the novel antidepressant drug, bupropion, on extracellular concentrations of dopamine (DA), its metabolites, and the serotonin metabolite 5-HIAA in the striatum and nucleus accumbens using on-line microdialysis in freely moving rats. Bupropion HCl (10, 25, and 100 mg/kg intraperitoneally) increased extracellular striatal DA in a dose- and time-dependent manner; 1 mg/kg did not affect extracellular DA. The maximal response occurred within the first 20 minutes (+76%, +164%, and +443% for each dose, respectively) followed by a gradual decrease to a stable but elevated level for the next 2 hours. This neurochemical response was strongly associated with bupropion-induced stereotyped behavior during the first hour but not during the subsequent 2 hours. Bupropion decreased DOPAC concentrations, increased 5-HIAA, and had variable effects on homovanillic acid (HVA) (decreases with 10 mg/kg and increases with 25 and 100 mg/kg). The increase in extracellular DA after bupropion (25 mg/kg) was blocked by tetrodotoxin and was therefore action-potential-dependent. Bupropion produced similar neurochemical responses in the striatum and the nucleus accumbens. These results suggest that increases in DA transmission contribute to the behavioral effects of bupropion and are consistent with a role for DA in the antidepressant effects of this drug. The partial dissociation between DA release and stereotyped behavior suggests that the relationship between neurotransmitter release and behavior may be complex.     

Local modulation of the 5-HT release in the dorsal striatum of the rat: an in vivo microdialysis study

Using in vivo microdialysis in freely moving rats, we examined the involvement of major striatal transmitters on the local modulation of the 5-HT release. Tetrodotoxin reduced the striatal 5-HT output to 15–20% of baseline. The selective 5-HT_1B receptor agonist CP 93129 (50 μM) reduced (50%) and the 5-HT_2A/2C receptor agonist DOI (1–100 μM) increased (220%) the 5-HT output. Neither GABA nor baclofen (100 nM–100 μM) altered the 5-HT output. The glutamate reuptake inhibitor -trans-PDC (1–4 mM) raised 5-HT to 280% of baseline. This effect was not antagonized by the NMDA receptor antagonist MK-801 (0.5 mg/kg i.p.). Local MK-801 (10–100 μM) did not significantly alter the 5-HT output. Finally, neither carbachol (10–100 μM) nor quipirole (10 μM–1 mM) affected 5-HT. These data suggest that the striatal 5-HT release is influenced by local serotonergic and glutamatergic (but not GABAergic) inputs.     

Cholinergic activity in the rat hippocampus, cortex and striatum correlates with locomotor activity: an in vivo microdialysis study

The possible relationship between behavioral arousal and acetylcholine release in the striatum, hippocampus and frontal cortex was investigated in rats. In vivo microdialysate concentrations of acetylcholine and choline from these brain structures, and photocell beam interruptions (as a measure of behavioral arousal), were measured simultaneously under three conditions: after injections of 1) vehicle or 2) scopolamine (0.4 mg/kg), and 3) before and after the beginning of the rats' night cycle. Dialysate concentrations of ACh in all 3 brain structures and locomotor activity were increased after scopolamine and the onset of the lights out condition. Vehicle injections transiently increased ACh in the hippocampus and cortex and caused short-lasting increases in locomotor activity. Under all conditions, the release of ACh from each of the 3 brain structures correlated with the level of locomotor activity.     

Electroconvulsive shock produces large increases in interstitial concentrations of dopamine in the rat striatum: an in vivo microdialysis study.

This study examined the effects of electroconvulsive shock (ECS) on interstitial concentrations of dopamine (DA) and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA), and the purine metabolite uric acid, in the striatum using on-line microdialysis in freely moving rats. Interstitial striatal DA increased to 1310% of baseline when the ECS was administered 18 to 24 hours after implantation of the dialysis probe. DOPAC (+ 19%), HVA (+ 30%), 5-HIAA (+10%), and uric acid (+111%) were increased to a smaller extent. The ECS-induced increase in DA was derived from a Ca++ sensitive pool since perfusion of a modified solution in which Ca++ had been replaced with Mg++ blocked this effect.     

Enantiomeric differences in the effects of 3,4-methylenedioxymethamphetamine on extracellular monoamines and metabolites in the striatum of freely-moving rats: an in vivo microdialysis study.

The effects of (+) and (-) 3,4-methylenedioxymethamphetamine (MDMA) and racemic p-chloroamphetamine (PCA) on extracellular dopamine and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), as well as the metabolite of 5-hydroxytryptamine (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), were determined in dialysates of the striatum conscious rats by using intracerebral dialysis and high performance liquid chromatography with electrochemical detection (HPLC-EC). The (+) and (-)MDMA isomers (5, 10 mg/kg, s.c.) and PCA (2.5, 5 mg/kg, s.c.) caused a rapid increase of extracellular levels of dopamine and decreased extracellular levels of DOPAC and HVA immediately after administration in dialysates of striatum. The order of potency for this effect was PCA greater than (+)MDMA greater than (-)MDMA. The levels of 5-HIAA also decreased after the administration of drugs, but the effect had a slower time course than DOPAC and HVA and did not exhibit an enantiomeric difference. The data indicate that, although these drugs are thought to affect the 5-HT neuronal system preferentially, they also affect dopamine systems and by a mechanism in which the (+) isomer was more potent than the (-).     

Pharmacokinetic and pharmacodynamic analyses of trazodone in rat striatum by in vivo microdialysis

The aim of this study was to investigate the brain pharmacokinetics and pharmacodynamics of trazodone. Sensitive microbore high-performance liquid chromatographic methods with electrochemical detection (LC-ED) were developed for the determination of trazodone, serotonin (5-HT), and their respective metabolites. The feasibility of microdialysis coupled with LC-ED system for direct analysis of these compounds in the rat striatum was investigated. Striatal dialysates were automatically injected onto a cyano microbore column, through an on-line injector, for the determination of trazodone and its metabolite or onto a reversed phase microbore column for the determination of 5-HT and its metabolite. A monophase phenomenon with a first-order elimination rate constant was observed for trazodone. The brain pharmacokinetics of trazodone appear to conform to a one-compartment model. Surprisingly, no significant changes in striatal 5-HT or its metabolite were observed following the same dosage and time course. The present results suggest that brain microdialysis methods may be applicable to pharmacokinetic and pharmacodynamic studies of psychotrophic agents.     

Fluvoxamine preferentially increases extracellular 5-hydroxytryptamine in the raphe nuclei: an in vivo microdialysis study.

The effects of systemic administration of fluvoxamine on extracellular serotonin (5-hydroxytryptamine, 5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) concentrations in the frontal cortex and raphe nuclei of freely moving rats were examined. Fluvoxamine significantly increased extracellular 5-HT concentrations in both regions at the two doses used (1 and 10 mg/kg i.p.). However, the increase in the raphe nuclei was several-fold that in the frontal cortex. Dialysate 5-HIAA concentrations decreased after treatment with fluvoxamine. These results confirm that 5-HT uptake inhibitors preferentially increase extracellular concentrations of 5-HT in the vicinity of cell bodies and dendrites of serotonergic neurones.     

D-serine modulates the NMDA receptor/nitric oxide/cGMP pathway in the rat cerebellum during in vivo microdialysis

Several lines of investigation indicate that D-serine may be an endogenous ligand for the glycine site of N-methyl-D-aspartate (NMDA) receptors in some CNS regions. We here studied the in vivo effects of D-serine on the NMDA receptor/nitric oxide/cGMP pathway by monitoring extracellular cGMP in the cerebellum of freely-moving rats subjected to transcerebral microdialysis. Local application of NMDA (200, 500 μM) through the dialysis probe for 20 min evoked transient, concentration-dependent cGMP responses which peaked in the fraction of drug administration, the nucleotide levels returning to basal values after 40 min. The NMDA-induced elevation of the extracellular nucleotide was completely inhibited by the selective receptor channel blocker dizocilpine (MK-801) locally co-perfused at the concentration of 10 μM. The non-competitive antagonist had no effect on its own suggesting that endogenous glutamic acid does not tonically activate NMDA receptors. The effect of 200 μM NMDA was largely attenuated by 30 μM 7-chloro-kynurenic acid and completely abrogated when the concentration of the strychnine-insensitive glycine receptor antagonist was raised to 100 μM. D-serine (300 μM), perfused in the presence of 7-chloro-kynurenate (30 μM), was able to fully restore the NMDA (200 μM)-induced increase of cGMP extracellular levels. On the other hand, the D-amino acid directly potentiated in a concentration-dependent manner (0.3, 1 and 10 mM) the NMDA (200 μM)-evoked cGMP production whereas it was inactive on its own. These data show that in vivo the activation of the strychnine-insensitive glycine site is essential for the functioning of the NMDA receptor complex and can be activated by the selective agonist D-serine. They also confirm that cerebellar NMDA receptors do not have their glycine sites saturated. Received: 25 June 1996 / Accepted: 23 September 1996     

Effects of subchronic treatment with selegiline on L-DOPA-induced increase in extracellular dopamine level in rat striatum

Selegiline is used an adjunct to L-DOPA therapy. We investigated extracellular striatal dopamine (DA) level in awake rats treated with L-DOPA and/or selegiline using a microdialysis method. Rats given 10 mg/kg, i.p. per day selegiline for 7 days were administered with a single dose of 100 mg/kg, i.p. L-DOPA 0 (3 h), 1, 3, 7, 14, 21, or 28 days after the last selegiline treatment. Carbidopa was administered 0.5 h before L-DOPA administration. The significant increase in basal DA level before L-DOPA treatment persisted until 1 day after the last selegiline treatment, and the significant decrease in basal DOPAC level persisted for more than 28 days. Thus, selegiline affected DA catabolism for more than 28 days. Total monoamine oxidase (MAO) and MAO-B activities at day 0 decreased by 22% and 5.7%, respectively. The significant enhancement of L-DOPA-induced increase in DA level was observed until 3 days after the last selegiline treatment. Next, the effects of reducing L-DOPA dose by 25% were examined 3 h after the last selegiline treatment. A dose-dependent decrease in DA level was observed, indicating that DA level in selegiline-treated rats can be controlled by L-DOPA dose.     

Dopaminergic regulation of striatal cholinergic interneurons: an in vivo microdialysis study

Summary In vivo microdialysis was used to study the putative inhibitory effects of dopamine on cholinergic interneurons in the striatum of conscious rats. The dopamine receptor agonists apomorphine (0.3 and 3 mg/kg, s.c.) and (±)N-0437 (1.4 mg/kg, s.c.) decreased interstitial concentrations of acetylcholine while increasing those of choline. In contrast, the dopamine receptor antagonists haloperidol (0.1 and 1 mg/kg, i.p.) and (±)sulpiride (20 mg/kg, i.p.) enhanced striatal acetylcholine output but had little effect on choline. Previously, a lack of effect of these drugs on striatal acetylcholine was reported. The main methodological difference between these studies was that the calcium concentration of the microdialysis perfusion solution was 3.4 mM in the former study versus 1.2 mM in the present experiments. The results of this study reemphasize the importance of the calcium concentration in determining the effects of drugs on central neurotransmitter release, and confirm a role of dopamine in the regulation of striatal cholinergic interneurons.Send offprint requests to G. Damsma at the present address     

GABAergic modulation of hippocampal glutamatergic neurons: an in vivo microdialysis study

We have demonstrated the effects of activation of presynaptic gamma-aminobutyric acid (GABA) receptors on glutamate release using in vivo brain microdialysis. A dialysis probe inserted into the hippocampus CA2 area of freely moving rats was perfused with Ringers solution containing 100 mM potassium chloride (KCl) or 0.05 mM veratridine for 20 min. Extracellular concentrations of amino acids were monitored by measuring their levels in dialysates by high performance liquid chromatography (HPLC) fluorometry. Perfusion with depolarizing agents, such as KCl or veratridine, increased extracellular glutamate levels in the hippocampus. Pretreatment with 1 mM GABA, before perfusion with depolarizing agents, significantly suppressed the depolarizing agent-induced increase in glutamate levels. The GABA(B) receptor agonist baclofen (1 mM) also significantly inhibited the depolarizing agent-induced increase in glutamate levels, whereas the GABA(A) receptor agonist, muscimol, had no affect. Similarly, baclofen (0.5 mM) decreased the KCl (13.5 mM)-induced 45Ca(2+) influx into cortical synaptosomes to 57% of the level induced in the absence of baclofen. On the other hands, GABA did not affect the increases in glycine and taurine level by depolarizing agents. These results suggest that GABA modulates depolarization-evoked glutamate release in the hippocampus by inhibiting Ca(2+) entry into neurons, an effect mediated by presynaptic GABA(B) receptors.     

Anin vivo study of dopamine metabolism in hyperglycemic rat striatum

The changes in the levels of the extracellular dopamine metabolites and the responses to various dopamine agents were studied by using microdialysis in hyperglycemic rat striatum. The hyperglycemia were induced by the administration of streptozotocin (40 mg/kg, i.p. for 3 days.). The basal levels of striatal dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were significantly decreased in hyperglycemic rat striatum. After the administration of D-1 and D-2 receptor antagonists, SCH-23390 and (?)sulpiride, to rats 14 days after the last administration of STZ, the increased rates in DOPAC levels were higher in hyper- than in normoglycemic rats. However after the administration of dopamine autoreceptor agonist, 3(?)PPP, the levels of the extracellular HVA were increased in normoglycemic rats, but those were not altered in hyperglycemic rats. The results indicate that the striatal dopamine activities were decreased in the hyperglycemic rats and suggest that the release of dopamine may be decreased in hyperglycemic rats. Furthermore it suggest that the increase in the levels of the extracellular dopamine metabolites by dopamine antagonists might be due to the increased sensitivities of the dopamine receptors in hyperglycemic state.     

Excitatory and inhibitory amino acid changes during repeated episodes of ethanol withdrawal: an in vivo microdialysis study

Changes in amino acid levels in the hippocampus during repeated ethanol withdrawal were studied. Wistar rats were made ethanol-dependent by 4-week vapour inhalation. After this first cycle of chronic ethanol treatment, rats underwent repeated and alternate cycles of 24 h of withdrawal followed by 1 week of chronic ethanol treatment. The microdialysis technique was used together with high-performance liquid chromatography and electrochemical detection to separate and quantify different amino acids such as aspartate, glutamate, arginine, taurine, alanine and gamma-aminobutyric acid. During the first cycle of ethanol withdrawal, increases in glutamate, taurine and alanine levels were observed. During the third withdrawal period, further increases were detected in aspartate, glutamate, taurine and alanine in the comparison with levels in the control group. However, the arginine level during the third withdrawal period decreased when compared to that after the first withdrawal period. The results of this study demonstrate that excitatory amino acid levels increased with subsequent withdrawal periods.     

Acute perfusion of BMAA in the rat's striatum by in vivo microdialysis

The present study is concerned with the hypothetical toxicity of beta-N-methylamino-L-alanine (BMAA), a compound that has been hypothesized to produce amyotrophic lateral sclerosis/Parkinson-dementia complex. We have used the microdialysis technique to perfused different concentrations of BMAA in the rat's striatum 24h after the implantation of a microdialysis probe (day 1). BMAA perfusion produced a dose-response increase in the extracellular output of dopamine. Forty-eight hours after implantation of the probe (day 2), we have perfused MPP+ 1 mM to check the integrity of the dopaminergic terminals present around the cannula. Only the highest concentration of BMAA studied, 50mM, produced a clear decrease in the extracellular output of dopamine after MPP+ perfusion. However, this decrease was very similar, even smaller, to that obtained in a previous study carried out by us with MPP+ 1 mM, a dose much lower than that used for BMAA. Our model to study toxicity in the striatal dopaminergic terminal did not show that acute perfusion of BMAA at high doses produces a clear damage to the dopaminergic terminals.     

A partial noradrenergic lesion induced by DSP-4 increases extracellular noradrenaline concentration in rat frontal cortex: a microdialysis study in vivo

The effect of systemic administration of the selective neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) on noradrenaline efflux in the frontal cortex was studied in freely-moving rats using microdialysis in vivo. Five days after treatment with DSP-4 (40 mg/kg IP), the noradrenaline content of the frontal cortex was reduced by 75%. Yet, noradrenaline efflux in the frontal cortex was nearly two-fold greater in DSP-4 treated rats than in saline-injected controls. Local infusion of the noradrenaline-selective uptake blocker, desipramine (5 μM), via the microdialysis probe, increased noradrenaline efflux in rats from both groups. Perfusion of Ringer’s solution, containing 80 mM K⁺, also increased noradrenaline efflux in both groups, but the increase after DSP-4 pretreatment was greater than in the controls. In contrast, removal of Ca²⁺ from the infusion medium reduced noradrenaline efflux in both treatment groups. These results indicate that, at this dose, DSP-4 increases the extracellular concentration of noradrenaline in rat frontal cortex despite causing a partial lesion of noradrenergic neurones. This is due to an increase in the release of noradrenaline, although reduced clearance is also likely. These data challenge the assumption that depletion of noradrenaline content after treatment with DSP-4 invariably translates into diminished noradrenergic transmission. Received: 4 September 1997/Final version: 7 September 1997