In Vivo Microdialysis Study of the Extracellular 3,4-Dihydroxyphenylacetic Acid in the Rat Locus Ceruleus: Topographical and Pharmacological Aspects

Abstract: In vivo microdialysis coupled with HPLC and electrochemical detection was used to monitor extracellular levels of 3, 4-dihydroxyphenylacetic acid (DOPAC) of the locus ceruleus (LC) in halothane-anesthetized rats. The identity of DOPAC was confirmed by experiments showing that the chromatographic peak was totally suppressed after inhibition of monoamine oxidase by pargyline. Histological examinations allowed to relate the quantity of DOPAC measured in the dialysates with the localization of the probe implantation site. We found that the DOPAC concentration was inversely proportional to the distance between the probe and the lateral border of the LC. Regardless of the caudorostral level of the nucleus, concentrations were maximal when the axis of the probe was 100 μ M from the lateral border of the LC and decreased by 53% when this distance reached 300 μ M . Activation of LC noradrenergic neurons by systemic administration of the α₂-antagonist piperoxane increased by 100% DOPAC concentrations in LC dialysates. These results suggest that the DOPAC measured by microdialysis could be considered an indicator of the functional state of LC noradrenergic neurons.     

Direct Comparison of the Response of Voltammetry and Microdialysis to Electrically Evoked Release of Striatal Dopamine

Abstract: Carbon fiber microelectrodes either were implanted directly into striatal tissue or were mounted into the outlet of microdialysis probes that were implanted into striatal tissue. This allowed voltammetry and microdialysis to be used under identical in vivo experimental conditions to monitor extracellular dopamine levels during electrical stimulation of the medial forebrain bundle both before and after uptake inhibition with nomifensine. The marked differences between the results obtained with each technique cannot be explained on the basis of their inherent analytical attributes (sensitivity, temporal response, etc.). The results demonstrate that the microdialysis recovery factor for endogenous dopamine increases after uptake inhibition, an observation that stands in contradiction to the existing theory and practice of the microdialysis technique. The observations led to the development of a numerical model that rationalizes the observations reported herein and that allows in vivo voltammetry and in vivo microdialysis results to be interpreted within a single theoretical framework.     

Enhancement of Dopamine Release In Vivo from the Rat Striatum by Dialytic Perfusion of 6R-l-erythro-5,6,7,8-Tetrahydrobiopterin

We have previously reported that intracerebroventricular administration of 6R-L-erythro-5,6,7,8-tetrahydrobiopterin (6R-BH4), a cofactor for tyrosine hydroxylase, enhances biosynthesis of 3,4-dihydroxyphenylethylamine (dopamine) in the rat brain. In the present study, we have more precisely examined the effects of 6R-BH4 on dopamine release in vivo from the rat striatum using brain microdialysis. The amount of dopamine collected in striatal dialysates was determined using HPLC with electrochemical detection after purification with an alumina batch method. When the striatum was dialyzed with Ringer solution containing various concentrations of 6R-BH4 (0.25, 0.5, and 1.0 mM), dopamine levels in striatal dialysates increased in a concentration-dependent manner. Biopterin had little effect on dopamine levels in dialysates. The 6R-BH4-induced increase in dopamine levels in dialysates was abolished after pretreatment with tetrodotoxin (50 microM) added to the perfusion fluid, but after pretreatment with nomifensine (100 mg/kg, intraperitoneal injection), an inhibitor of dopamine uptake mechanism, a larger increase was observed. After inhibition of tyrosine hydroxylase by pretreatment with alpha-methyl-p-tyrosine (250 mg/kg, intraperitoneal injection), most of the increase persisted. These results suggest that 6R-BH4 has a dopamine-releasing action, which is not dependent on biosynthesis of dopamine.     

Use of Microdialysis for Monitoring Tyrosine Hydroxylase Activity in the Brain of Conscious Rats

An on-line microdialysis system was developed which monitored the 3,4-dihydroxyphenylalanine (DOPA) formation in the striatum during infusion of a submicromolar concentration of an L-aromatic amino-acid decarboxylase inhibitor (NSD 1015). The absence of DOPA in dialysates of 6-hydroxydopamine-pretreated rats and the disappearance of DOPA after administration of alpha-methyl-p-tyrosine indicated that the dialyzed DOPA was derived from dopaminergic nerve terminals. Next we investigated whether the steady-state DOPA concentration in striatal dialysates could be considered as an index of tyrosine hydroxylase activity. The increase in DOPA output after intraperitoneal administration of haloperidol or gamma-butyrolactone and the decrease in DOPA output after intraperitoneal administration of apomorphine are in excellent agreement with results of postmortem studies, in which a decarboxylase inhibitor was used to measure the activity of tyrosine hydroxylase. The effect of haloperidol on DOPA formation was not visible when a U-shaped cannula (0.80 mm o.d.) was used. Some methodological problems related to microdialysis of the haloperidol-induced increase in DOPA formation are discussed. We concluded that the proposed model is a powerful and reliable in vivo method to monitor tyrosine hydroxylase activity in the brain. The method is of special interest for investigating the effect of compounds which are not able to pass the blood-brain barrier. As an application of the method in the latter situation, we report the effect of infusion the neurotoxin 1-methyl-4-phenylpyridinium ion (10 mmol/L infused over 20 min) on the activity of striatal tyrosine hydroxylase.     

Measurement of Serotonin Turnover Rate in Rat Dorsal Raphe Nucleus by In Vivo Electrochemistry

5-Hydroxytryptamine (5-HT; serotonin) turnover rate in dorsal raphe nucleus of the urethane-anesthetized rat was estimated by using the in vivo electrochemical detector to measure the decay of extraneuronal 5-hydroxyindole acetic acid (5-HIAA) after monoamine oxidase inhibition. Carbon paste electrodes were scanned by semiderivative voltammetry and revealed two peaks: one at +0.15 V and the other at +0.25 V. The higher potential peak is composed primarily of the 5-HT metabolite 5-HIAA. After administration of pargyline, 75 mg/kg i.p., this peak declined exponentially. Regression analysis of these data by an exponential decay model yielded the fractional rate constant 0.82 +/- 0.06 h-1 (mean +/- SEM). This rate constant of 5-HIAA disappearance measured by in vivo electrochemistry is identical to the rate constant found by others measuring 5-HIAA disappearance by direct tissue assay methods. In animals not treated with pargyline, tissue 5-HIAA concentrations in the dorsal raphe nucleus were measured by HPLC with electrochemical detection. The average 5-HT turnover rate calculated as the product of the fractional rate constant and steady-state tissue 5-HIAA concentration was 12.6 nmol/g/h. These results demonstrate that electrochemical detection of extraneuronal 5-HIAA combined with monoamine oxidase inhibition can be used to measure neurotransmitter turnover in vivo in a discrete brain region.     

Microdialysis Monitoring of 3,4-Dihydroxyphenylalanine Accumulation After Decarboxylase Inhibition: A Means to Estimate In Vivo Changes in Tyrosine Hydroxylase Activity of the Rat Locus Ceruleus

Abstract: An on-line microdialysis approach was developed to estimate changes in tyrosine hydroxylase activity in the locus ceruleus noradrenergic neurons of anesthetized rats by measuring the 3,4-dihydroxyphenylalanine (DOPA) acumulation in the extracellular fluid during perfusion of an aromatic amino acid decarboxylase inhibitor through a dialysis probe. The aromatic amino acid decarboxylase inhibitor used was difluoromethyl-DOPA, which was shown to be more stable than NSD 1015 or Ro 4-4602 in the perfusion fluid. A 1-h perfusion of a 10⁻⁴ mol/L of difluoromethyl-DOPA solution induced a linear increase in DOPA concentration in the locus ceruleus dialysates that achieved a steady state within 1 h. The identity of DOPA accumulated in dialysates during aromatic amino acid decarboxylase inhibition was confirmed by the disappearance of the chromatographic peak when DOPA formation was blocked by the administration of α-methyl- p -tyrosine. Systemic administration of the α₂-antagonist piperoxane before difluoromethyl-DOPA perfusion markedly increased the DOPA concentration during both the accumulation and the steady-state periods, showing that the present technique is a suitable in vivo approach to monitor changes in tyrosine hydroxylase activity occurring in the locus ceruleus neurons.     

Simultaneous Measurement of Extracellular Morphine and Serotonin in Brain Tissue and CSF by Microdialysis in Awake Rats

In this report, we describe an HPLC with electrochemical detection assay for the simultaneous measurement of levels of morphine, serotonin, 5-hydroxyindole-3-acetic acid, and homovanillic acid in dialysates of various brain areas and CSF in the awake rat. Morphine could be detected in the dialysates after a single intraperitoneal injection, with doses as low as 1.0 mg/kg. The time course of extracellular morphine content in the lateral hypothalamus, striatum, cerebellum, periaqueductal gray, and dorsal horn of the spinal cord and in CSF, from the ventricles and cisterna magna, was similar. We detected morphine in the first 15-min sample, and levels peaked 45-60 min after injection. Maximal dialysate levels, however, varied with the type of dialysis probe used and the area sampled. The most efficient in vivo recovery was in CSF dialysates from the cisterna magna, presumably because of minimal tissue interference with the dialysis probe. For this reason, the cisterna is an ideal region for sampling CSF. Morphine had no significant effect on the extracellular concentrations of serotonin in any of the areas studied and did not modify or only slightly increased levels of tissue metabolites; however, morphine markedly increased the CSF levels of 5-hydroxyindole-3-acetic acid and homovanillic acid. Because microdialysis in freely moving animals permits assessment of the behavioral effects of morphine while continuously monitoring the drug levels in discrete brain regions, this approach will greatly facilitate future studies of the neurochemical basis of morphine's effects in the brain.     

Study of Dopamine Turnover by Monitoring the Decline of Dopamine Metabolites in Rat CSF After α-Methyl-p-Tyrosine

CSF was continuously withdrawn from the third ventricle of anesthetized rats. CSF 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid concentrations were determined every 15 min by liquid chromatography coupled with electrochemical detection. Acute tyrosine hydroxylase inhibition [with alpha-methyl-p-tyrosine (alpha-MPT)] induced an exponential decline in levels of DOPAC and HVA in CSF. The decline in DOPAC and HVA concentrations was identical in CSF and forebrain but was much slower in the striatum, suggesting that CSF metabolites of 3,4-dihydroxyphenylethylamine (dopamine) reflect whole forebrain metabolites. The decay in CSF DOPAC and HVA levels after dopamine synthesis inhibition was also used as an in vivo index of forebrain dopamine turnover after various pharmacological treatments. Haloperidol pretreatment accelerated this decay, confirming the increase in brain dopamine turnover induced by neuroleptics. After reserpine pretreatment (15 h before), alpha-MPT produced a very sharp decay in levels of DOPAC and HVA. This result indicates that the residual dopamine that cannot be stored after reserpine treatment is very rapidly renewed and metabolized. Nomifensine strongly diminished the slope of DOPAC and HVA level decreases after alpha-MPT, a result which can be explained either by a slower dopamine turnover or by the involvement of storage dopamine pools. These results exemplify the use of monitoring the decay of dopamine metabolites after alpha-MPT administration in the study of the pharmacological action of drugs on the central nervous system of the rat.     

Study of catecholamine secretion by the rat adrenal glands using microdialysis in vivo

Microdialysis technique has been developed to study secretory function of the adrenal gland in anesthesized rats. Concentration of adrenaline and noradrenaline in sequential 20 min microdialysis samples was measured by HPLC with electrochemical detection. The suitability of method was tested by local and central stimulation of catecholamine secretion. In the first case 100 mmol of KCl or 1 mmol of carbachol were added to perfusion medium, in the second one hypovolemic hypotension was applied. All the stimuli used increased catecholamine levels in the adrenal gland dialysates. Institute of Experimental Cardiology of the All-Union.     

Role of striatal serotonin2A and serotonin2C receptor subtypes in the control of in vivo dopamine outflow in the rat striatum

This study investigated, using in vivo microdialysis in the striatum of freely moving rats, the role of striatal serotonin2A (5-HT2A) and 5-HT2C receptor subtypes in the modulation of dopamine (DA) and 3, 4-dihydroxyphenylacetic acid (DOPAC) outflow, both in basal conditions and under activation induced by subcutaneous administration of 0.01 mg/kg haloperidol. The different 5-HT2 agents used were applied intrastriatally at a 1 microM concentration through the microdialysis probe. Basal DA efflux was enhanced (27%) by the 5-HT2A/2B/2C agonist 1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane (DOI) and reduced (-30%) by the 5-HT2B/2C antagonist SB 206553. It was unaffected by infusion of the 5-HT2A antagonist SR 46349B. The effect of DOI was abolished by SB 206553 but not modified by SR 46349B. Haloperidol-stimulated DA efflux (65-70%) was reduced by both SR 46349B (-32%) and the 5-HT2A/2B/2C antagonist ritanserin (-30%) but not affected by SB 206553. Conversely, the effect of haloperidol was potentiated (22%) when DOI was coperfused with SB 206553. Also, haloperidol-stimulated DOPAC outflow (40-45%) was reduced (-20%) by SR 46349B and potentiated (25%) by the combination of SB 206553 with DOI. These results indicate that striatal 5-HT2A receptors, probably through activation of DA synthesis, positively modulate DA outflow only under activated conditions. In contrast, striatal 5-HT2C receptors exert a facilitatory control on basal DA efflux, which appears to be both tonic and phasic.     

Effects of Choline Administration on In Vivo Release and Biosynthesis of Acetylcholine in the Rat Striatum as Studied by In Vivo Brain Microdialysis

The purpose of the present study is to clarify the effects of the administration of choline on the in vivo release and biosynthesis of acetylcholine (ACh) in the brain. For this purpose, the changes in the extracellular concentration of choline and ACh in the rat striatum following intracerebroventricular administration of choline were determined using brain microdialysis. We also determined changes in the tissue content of choline and ACh. When the striatum was dialyzed with Ringer solution containing 10 microM physostigmine, ACh levels in dialysates rapidly and dose dependently increased following administration of various doses of choline and reached a maximum within 20 min. In contrast, choline levels in dialysates increased after a lag period of 20 min following the administration. When the striatum was dialyzed with physostigmine-free Ringer solution, ACh could not be detected in dialysates both before and even after choline administration. After addition of hemicholinium-3 to the perfusion fluid, the choline-induced increase in ACh levels in dialysates was abolished. Following administration of choline, the tissue content of choline and ACh increased within 20 min. These results suggest that administered choline is rapidly taken up into the intracellular compartment of the cholinergic neurons, where it enhances both the release and the biosynthesis of ACh.     

Role of Endogenous Taurine on the Glutamate Analogue-Induced Neurotoxicity in the Rat Hippocampus In Vivo

The glutamate analogues N-methyl-D-aspartate (NMDA), kainic acid (KA), and quisqualic acid (QA), prepared in different hypertonic media, were perfused in vivo in the hippocampal CA1 field of rats using a microdialysis technique. Extracellular taurine levels, estimated after analysis of the taurine content of dialysates, increased during perfusion of all three agonists but varied according to the osmolarity of the medium. The NMDA-induced increase in extracellular taurine content was only slightly inhibited by perfusion of 150 and 300 mM sucrose. The KA-evoked increase was partially dependent on extracellular osmolarity, because addition of 50 and 150 mM sucrose caused a dose-dependent inhibition that was not augmented using higher sucrose concentrations. QA caused a taurine increase that was totally abolished by addition of 50 mM sucrose. These results indicate that the rise in extracellular taurine level elicited by QA and part of the increase elicited by KA are probably due to a release caused by the cellular swelling that these substances evoke, a finding substantiating the previously proposed osmoregulatory role of taurine. However, almost all the increase in extracellular taurine content caused by NMDA and all the osmotically insensitive part of the KA-evoked rise cannot be explained as release triggered by cell swelling and may reflect a function of taurine other than osmoregulation.     

Role of Na+-K+-ATPase in insulin-induced lactate release by skeletal muscle

Hyperinsulinemia increases lactate release by various organs and tissues. Whereas it has been shown that aerobic glycolysis is linked to Na+-K+-ATPase activity, we hypothesized that stimulation by insulin of skeletal muscle Na+-K+-ATPase is responsible for increased muscle lactate production. To test this hypothesis, we assessed muscle lactate release in healthy volunteers from the [13C]lactate concentration in the effluent dialysates of microdialysis probes inserted into the tibialis anterior muscles on both sides and infused with solutions containing 5 mmol/l [U-13C]glucose. On one side, the microdialysis probe was intermittently infused with the same solution additioned with 2.10(-5) M ouabain. In the basal state, [13C]lactate concentration in the dialysate was not affected by ouabain. During a euglycemic-hyperinsulinemic clamp, [13C]lactate concentration increased by 135% in the dialysate without ouabain, and this stimulation was nearly entirely reversed by ouabain (56% inhibition compared with values in the dialysate collected from the contralateral probe). These data indicate that insulin stimulates muscle lactate release by activating Na+-K+-ATPase in healthy humans.     

Effect of Systemic Administration of N-Methyl-d-Aspartic Acid on Extracellular Taurine Level Measured by Microdialysis in the Hippocampal CA1 Field and Striatum of Rats

Abstract: The extracellular concentrations of amino acids in the hippocampal CA1 field and striatum of conscious freely moving rats were monitored simultaneously by in vivo brain microdialysis using HPLC with electrochemical detection. Under basal conditions, aspartate, glutamate, glutamine, glycine, taurine, and alanine were detected, but γ-aminobutyric acid was undetectable in both regions. In-traperitoneal injection of N -methyl- d -aspartic acid (NMDA; 10 mg/kg) caused a significant increase (three-to fivefold) in the taurine concentration in the dialysate obtained from both the hippocampal CA1 and striatum, whereas other amino acids (aspartate, glutamate, and alanine) did not show significant changes. Local application of NMDA (300 γ) to both regions via the dialysis probes also caused a similar increase (three-to fivefold) in both regions. Under infusion of hypertonic Ringer's solution containing 150 m M sucrose, the effect of NMDA on the level of taurine in both the regional dialysates was not affected. The effect of NMDA was totally reduced by intraperitoneal administration of MK-801 (0.3–1.0 mg/kg), a noncompetitive antagonist of NMDA receptors. Continuous infusion of dl -2-amino-5-phosphonovaleric acid (1.0 mM), a competitive antagonist of NMDA receptors, via the dialysis probes completely inhibited the effect of NMDA. These findings suggest that systemic administration of NMDA is effective as well as local administration into the brain and that NMDA receptors might be involved in the regulation of the extracellular taurine level in the brain without dependence on cell swelling.     

Liquid chromatography with amperometric detection using functionalized multi-wall carbon nanotube modified electrode for the determination of monoamine neurotransmitters and their metabolites

The fabrication and application of a novel electrochemical detection (ED) method with the functionalized multi-wall carbon nanotubes (MWNTs) chemically modified electrode (CME) for liquid chromatography (LC) were described. The electrochemical behaviors of dopamine (DA) and other monoamine neurotransmitters at the CME were investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The results indicated that the CME exhibited efficient electrocatalytic effects on the current responses of monoamine neurotransmitters and their metabolites with high sensitivity, high stability and long-life activity. In LC-ED, DA, norepinephrine (NE), 3-methoxy-4-hydroxyphenylglycol (MHPG), 3,4-dihydroxyphenylacetic acid (DOPAC), 5-hydroxytryptamine (5-HT), 5-hydroxyindoleacetic acid (5-HIAA) and homovanillic acid (HVA) had good and stable current responses at the CME. The linear ranges of seven analytes were over four orders of magnitude and the detection limits were 2.5 x 10(-10) mol/l for DA, 2.5 x 10(-10) mol/l for NE, 5.0 x 10(-10) mol/l for MHPG, 3.0 x 10(-10) mol/l for DOPAC, 3.5 x 10(-10) mol/l for 5-HT, 6.0 x 10(-10) mol/l for 5-HIAA, 1.25 x 10(-9) mol/l for HVA. The application of this method coupled with microdialysis sampling for the determination of monoamine neurotransmitters and their metabolites in Parkinsonian patients' cerebrospinal fluid was satisfactory.     

Formation and Clearance of Interstitial Metabolites of Dopamine and Serotonin in the Rat Striatum: An In Vivo Microdialysis Study

In vivo microdialysis was employed in order to characterize the steady-state kinetics of the turnover of specific dopamine and serotonin metabolites in the rat striatum 48 h after surgery. Inhibitors of monoamine oxidase (MAO; pargyline) and catechol-O-methyltransferase (COMT; Ro 40-7592) were administered, either separately or in conjunction, at doses sufficient to block these enzymes in the CNS. In some experiments, the acid metabolite carrier was blocked with probenecid. Temporal changes were then observed in the efflux of interstitial dopamine, 3-methoxytyramine (3-MT), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA). The fractional rate constants for the accumulation or disappearance of the metabolites could be determined after pharmacological blockade of catabolic enzymes or the acid metabolite carrier. Interstitial 5-HIAA was found to be cleared with a half-life of approximately 2 h. After blockade of either MAO or COMT, HVA disappeared with a half-life of 17 min. Experiments employing probenecid suggested that some of the interstitial HVA was cleared by the acid metabolite carrier, the remainder being cleared by a probenecid-insensitive process, possibly conjugation. After MAO inhibition, DOPAC disappeared with an apparent half-life of 11.3 min. The rate of 3-MT accumulation after pargyline indicated that the majority of interstitial HVA (> 95%) is formed from DOPAC rather than 3-MT. The formation of 3-MT from interstitial dopamine, calculated from the accumulation rate of 3-MT after pargyline, appeared to follow first-order kinetics (k = 0.1 min-1).     

Preloading In Vivo: A Rapid and Reliable Method for Measuring γ-Aminobutyric Acid and Glutamate Fluxes by Microdialysis

In vivo microdialysis was used in conjunction with a novel dual-label preloading method, to monitor changes in extracellular levels of gamma-aminobutyric acid (GABA) and glutamate in the striatum of conscious, unrestrained rats. [3H]GABA and [14C]glutamate were applied in the dialysis stream for a preloading period of 30 min, after which dialysis perfusion was continued for up to 6 h, and dialysate samples were collected for scintillation counting. Veratridine (Vtd: 100 microM in the dialysate) caused significant rises in both 3H and 14C content measured in the dialysates, the majority of which remained associated with the preload GABA and glutamate, respectively. The Vtd-stimulated release of GABA and glutamate measured in this way was blocked by tetrodotoxin and was Ca2+ dependent. Thus, by reproducing results obtained using other techniques, we have shown that the preloading method provides a quick and reliable method for measuring the effects of drugs on the release of neurotransmitter GABA and glutamate in vivo by dyalisis. It should enable sample times as low as 1 min to be used, thus allowing resolution of transient stimulated responses taking place over a time course of minutes.     

Microdialysis of dopamine interpreted with quantitative model incorporating probe implantation trauma

Although microdialysis is widely used to sample endogenous and exogenous substances in vivo, interpretation of the results obtained by this technique remains controversial. The goal of the present study was to examine recent criticism of microdialysis in the specific case of dopamine (DA) measurements in the brain extracellular microenvironment. The apparent steady-state basal extracellular concentration and extraction fraction of DA were determined in anesthetized rat striatum by the concentration difference (no-net-flux) microdialysis technique. A rate constant for extracellular clearance of DA calculated from the extraction fraction was smaller than the previously determined estimate by fast-scan cyclic voltammetry for cellular uptake of DA. Because the relatively small size of the voltammetric microsensor produces little tissue damage, the discrepancy between the uptake rate constants may be a consequence of trauma from microdialysis probe implantation. The trauma layer has previously been identified by histology and proposed to distort measurements of extracellular DA levels by the no-net-flux method. To address this issue, an existing quantitative mathematical model for microdialysis was modified to incorporate a traumatized tissue layer interposed between the probe and surrounding normal tissue. The tissue layers are hypothesized to differ in their rates of neurotransmitter release and uptake. A post-implantation traumatized layer with reduced uptake and no release can reconcile the discrepancy between DA uptake measured by microdialysis and voltammetry. The model predicts that this trauma layer would cause the DA extraction fraction obtained from microdialysis in vivo calibration techniques, such as no-net-flux, to differ from the DA relative recovery and lead to an underestimation of the DA extracellular concentration in the surrounding normal tissue.     

Effects of CGP 28014 on the in vivo release and metabolism of dopamine in the rat striatum assessed by brain microdialysis

The effects on rat striatal dopamine (DA) metabolism of systemic and local administration of CGP 28014, an inhibitor of catechol-O-methyl-transferase (COMT), were studied by in vivo microdialysis. CGP 28014 (30 mg/kg i.p.) significantly reduced the levels of homovanillic acid (HVA), but did not modify DA and 3,4-dihydroxyphenylacetic acid (DOPAC). The intrastriatal administration (via the microdialysis probe) of 5, 7.5, 10, and 20 mM of CGP 28014 elicited a concentration-dependent, several-fold increase in extracellular DA but did not alter the levels of HVA and DOPAC. Thus, the effects of CGP 28014 observed after i.p. injection (decrease in HVA levels) are different from those measured after intrastriatal administration (increase in DA release). Therefore, the inhibition of COMT is likely to be due to the action of a metabolite of CGP 28014 formed in the periphery and not in the brain.     

Ketanserin pretreatment attenuates MDMA-induced dopamine release in the striatum as measured by in vivo microdialysis

Systemic administration of the amphetamine analogue, 3,4-methylenedioxymethamphetamine (MDMA) produced a dose-dependent increase in the extracellular concentration of dopamine (DA) in the striatum as measured by in vivo microdialysis in awake, freely-moving rats. The extracellular concentration of the DA metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), was significantly decreased in dialysate samples following the administration of MDMA (10 and 20 mg/kg, i.p.). The serotonin-2 (5-HT2) antagonist ketanserin (3 mg/kg, i.p.) had no effect on the extracellular concentration of DA or DOPAC in the striatum of vehicle- treated rats. The administration of ketanserin (3 mg/kg) 1 hr prior to MDMA (20 mg/kg) significantly attenuated the MDMA- induced increase in the extracellular concentration of DA without affecting the decrease in DOPAC concentrations. These data are suggestive that MDMA administration increases DA release in the striatum of awake, freely-moving rats. In addition, MDMA-induced increase in the extracellular concentration of DA in the striatum is mediated, in part, via 5-HT2 receptor mechanisms.