Analysis of 3-morpholinosydnonimine and sodium nitroprusside effects on dopamine release in the striatum of freely moving rats: role of nitric oxide, iron and ascorbic acid

The effects of intrastriatal infusion of 3-morpholinosydnonimine (SIN-1) or sodium nitroprusside (SNP) on dopamine (DA), 3-methoxytyramine (3-MT), dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), L-dihydroxyphenylalanine (L-DOPA), ascorbic acid and uric acid concentrations in dialysates from the striatum of freely moving rats were evaluated using microdialysis. SIN-1 (1 mM) infusion for 180 min increased microdialysate DA and 3-MT concentrations, while L-DOPA, DOPCA+HVA, ascorbic acid and uric acid levels were unaffected. Co-infusion with ascorbic acid (0.1 mM) inhibited SIN-1-induced increases in DA and 3-MT dialysate concentration. SNP (1 mM) infusion for 180 min increased greatly the dialysate DA concentration to a peak (2950% of baseline) at the end of the infusion, while increases in 3-MT were negligible. In addition, SNP decreased ascorbic acid and L-DOPA but increased uric acid concentration in the dialysate. Co-infusion with deferoxamine (0.2 mM) inhibited the late SNP-induced increase in DA dialysate concentration, but did not affect the decrease in ascorbic acid and increase uric acid dialysate concentrations. SNP (1 mM) infusion for 20 min moderately increased uric acid, DA and 3-MT, but decreased L-DOPA levels in the dialysate. Ascorbic acid concentration increased at the end of SNP infusion. Co-infusion with ascorbic acid (0.1 mM) inhibited the SNP-induced increase in DA and 3-MT, but did not affect the decrease in L-DOPA and increase in uric acid dialysate concentrations. These results suggest that NO released from SIN-1 may account for the increase in the dialysate DA concentration. NO released following decomposition of SNP may account for the early increase in dialysate DA, while late changes in microdialysate composition following SNP may result from an interaction between NO and the ferrocyanide moiety of SNP. Exogenous ascorbic acid inhibits the effect of exogenous NO on DA release probably by scavenging NO, suggesting that endogenous ascorbic acid may modulate the NO control of DA release from 300 striatal dopaminergic terminals.     

Analysis of S-nitroso-N-acetylpenicillamine effects on dopamine release in the striatum of freely moving rats: role of endogenous ascorbic acid and oxidative stress

1. We showed previously that interaction between NO and iron(II), both released following decomposition of sodium nitroprusside (SNP), accounted for the late SNP-induced dopamine (DA) increase in dialysates from the striatum of freely moving rats. 2. In this study, intrastriatal infusion of the NO-donor S-nitroso-N-acetylpenicillamine (SNAP) (0.2 mM for 180 min) induced a moderate increase in dialysate DA and decreases in ascorbic acid dialysate concentrations; in contrast, SNAP 1 mM infusion induced a long-lasting decrease in both DA and ascorbic acid dialysate concentrations. 3-Methoxy-tyramine (3-MT), dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and uric acid levels were unaffected. 3. Co-infusion of ferrous sulphate [iron(II), 1 mM for 40 min] with SNAP either 1 or 0.2 mM (for 180 min), produced a significant increase in both DA and 3-MT dialysate concentrations, but it did not affect decreases in dialysate ascorbic acid levels. All other dialysate neurochemicals were unaffected. 4. Co-infusion of ascorbic acid (0.1 mM) with SNAP (1 mM) for 180 min did not modify SNAP-induced decreases in dialysate DA levels. In contrast, co-infusion of uric acid (1 mM) reversed SNAP-induced decreases in dialysate DA; co-infusion of a superoxide dismutase mimetic delayed SNAP-induced DA decreases for a short period, while co-infusion of the antioxidant N-acetylcysteine (NAC, 0.1 mM) significantly increased dialysate DA. 5. The results of this study show that SNAP induces concentration-related changes in DA dialysate levels. At higher concentrations, SNAP induces non-enzymatic DA oxidation, which is inhibited by uric acid and NAC; ascorbic acid failed to protect dialysate DA from oxidation, probably owing to its promoting effect on SNAP decomposition; exogenous iron(II) may react with NO generated from SNAP decomposition, with a consequent increase in dialysate DA and 3-MT, therefore mimicking SNP effects on striatal DA release.     

On the mechanism of d-amphetamine-induced changes in glutamate, ascorbic acid and uric acid release in the striatum of freely moving rats

1. The effects of systemic, intrastriatal or intranigral administration of d-amphetamine on glutamate, aspartate, ascorbic acid (AA), uric acid, dopamine (DA), dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA) concentrations in dialysates from the striatum of freely-moving rats were evaluated using microdialysis. 2. d-Amphetamine (2 mg kg-1) given subcutaneously (s.c.) increased DA, AA and uric acid and decreased DOPAC + HVA, glutamate and aspartate dialysate concentrations over a 3 h period after d-amphetamine. 5-HIAA concentrations were unaffected. Individual changes in glutamate and AA dialysate concentrations were negatively correlated. 3. d-Amphetamine (0.2 mM), given intrastriatally, increased DA and decreased DOPAC + HVA and aspartate dialysate concentrations, but failed to change those of glutamate, AA uric acid or 5-HIAA, over a 2 h period after d-amphetamine. Haloperidol (0.1 mM), given intrastriatally, increased aspartate concentrations without affecting those of glutamate or AA. 4. d-Amphetamine (0.2 mM), given intranigrally, increased AA and uric acid dialysate concentrations and decreased those of glutamate, aspartate and DA; DOPAC + HVA and 5-HIAA concentrations were unaffected. 5. These results suggest that d-amphetamine-induced increases in AA and uric acid and decreases in glutamate concentrations are triggered at nigral sites. The changes in aspartate levels may be evoked by at least two mechanisms: striatal (mediated by inhibitory dopaminergic receptors) and nigral (activation of amino acid carrier-mediated uptake).     

Detection of levodopa, dopamine and its metabolites in rat striatum dialysates following peripheral administration of L-DOPA prodrugs by mean of HPLC-EC

A high performance liquid chromatography (HPLC) method was developed to detected simultaneously L-dihydroxyphenylalanine (L-DOPA), dopamine (DA), dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in rat striatum dilaysates following oral administration of L-DOPA or its prodrugs. The chromatographic system uses a reversed-phase C18 column with electrochemical detection at +0.30 V. Mobile phase consisted of 0.05 M citric acid, sodium EDTA 50 microM, sodium octylsulphonate 0.4 nM at pH of 2.9 and 8% methanol (v/v) at a flow rate of 1 ml/min. The calibration curves were linear over the concentration range of 10nm to 100 microM and the lower limits of detections were 125 fmol for L-DOPA, 50 fmol for DOPAC, 250 fmol for DA and 150 fmol for HVA at signal noise to ratio of 3. The repeatability (or intra-day precision), expressed by the relative standard deviation, were better than 4%. The construction of microdialysis probes has been described. The in vitro relative recoveries of each microdialysis probe were evaluated and the results show that they are similar and reproducible for all the analytes with CVs from 1 to 4%. The HPLC-EC method was applied to detect the extracellular levels of L-DOPA, DA, DOPAC and HVA in the striatum dialysates of freely moving rats after oral administration of six new potential L-DOPA prodrugs.     

BIA 3-202, a novel catechol-O-methyltransferase inhibitor, enhances the availability of L-DOPA to the brain and reduces its O-methylation.

1-[3,4-Dihydroxy-5-nitrophenyl]-2-phenyl-ethanone (BIA 3-202) is a new long-acting catechol-O-methyltransferase (COMT) inhibitor with limited access to the brain. The present study evaluated the interference of BIA 3-202 upon levels of L-3,4-dihydroxyphenylalanine (L-DOPA) and metabolites in plasma (3-O-methyl-L-DOPA) and brain [3-O-methyl-L-DOPA, dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA)] in rats orally treated with L-DOPA (20 mg/kg) plus benserazide (30 mg/kg). At different time points (1, 3 and 6 h) after the administration of BIA 3-202 (0, 3, 10 and 30 mg/kg) or L-DOPA plus benserazide, rats were sacrificed and the right striatum was quickly dissected out and stored for the assay of L-DOPA, 3-O-methyl-L-DOPA, dopamine and amine metabolites. Levels of L-DOPA, 3-O-methyl-L-DOPA, dopamine, DOPAC and HVA in the striatum in L-DOPA plus benserazide-treated rats were higher than in vehicle-treated rats. However, this increase in striatal L-DOPA, dopamine, DOPAC and HVA was, in a dose- and time-dependent manner, even higher (P<0.05) in rats given BIA 3-202 (3, 10 and 30 mg/kg). This effect was accompanied by a marked decrease in 3-O-methyl-L-DOPA levels in the striatum of L-DOPA plus benserazide-treated rats. Increases in levels of L-DOPA and decreases in 3-O-methyl-L-DOPA levels in plasma also accompanied the administration of BIA 3-202. BIA 3-202 did not significantly affect levels of DOPAC and HVA in the striatum in vehicle-treated rats. It is concluded that administration of BIA 3-202 enhances the availability of L-DOPA to the brain by reducing its O-methylation in the periphery, which may prove beneficial in parkinsonian patients treated with L-DOPA plus an aromatic amino acid decarboxylase inhibitor.     

Monitoring of dopamine metabolism by in vivo voltammetry and high performance liquid chromatography in L-dopa-treated rats

Following L-DOPA administration, DA metabolites were measured by both in vivo voltammetry and HPLC-ECD. In vivo, the amplitudes of the catechol oxidation currents in both the striatum and frontal cortex increased and reached a plateau from 60 min to 240 min after L-DOPA treatment. L-DOPA, DA, DOPAC and HVA levels measured by HPLC-ECD after L-DOPA treatment were increased in both regions, and the occurrence of the peak of each compound was dependent on its order in the metabolic pathway, i.e., 15 min for DA, 45 min for DOPAC and 60-180 min for HVA. However, NE remained unchanged. No indication of a rapid DA increase was apparent in the catechol oxidation current at 15 min; and thus, DA is unlikely to be a main contributor to the catechol oxidation current in the brain of rats treated with L-DOPA.     

The metabolism of systemically-administered L-dihydroxyphenylalanine, by intact and dopamine-denervated striata, as revealed by brain microdialysis

Idiopathic Parkinson's Disease arises from the progressive loss of dopamine (DA)-utilizing neurons of the nigrostriatum and responds to the replacement of DA with L-dihydroxyphenylalanine (L-DOPA). In awake rats, with unilateral lesions induced with 6-hydroxydopamine (6-OHDA) of the DA-utilizing nigrostriatal pathway, treatment with L-DOPA causes the rapid onset of brisk contralateral turning behaviour. In urethane-anesthetized rats with identical unilateral lesions of the nigrostriatum, dialysis of the striatum, performed before and after the systemic administration of L-DOPA (25 mg/kg i.p.), did not demonstrate any alteration in extracellular DA in the striatum which was DA-deprived compared to intact striata. After treatment with L-DOPA extracellular levels of the metabolites of DA. DOPAC and HVA increased several fold. These results suggest: (a) DA neurons surviving after extensive lesions with 6-OHDA can compensate for loss of DA in the striatum and maintain extracellular fluid (and presumably synaptic) concentrations of DA; (b) in striata with extensive depletion of DA L-DOPA undergoes rapid decarboxylation to DA, followed by catabolism to DOPAC and HVA; and (c) in urethane-anesthetized animals, DA formed from DOPA does not appear to enter a releasable pool.     

EFFECTS OF ALLOPURINOL ON STRIATAL DOPAMINE, ASCORBATE AND URIC ACID DURING AN ACUTE MORPHINE CHALLENGE: EX VIVO AND IN VIVO STUDIES

In the present studyin vivoandex vivoexperiments were combined to evaluate the effects of allopurinol on the neurochemical changes induced by an acute morphine challenge (2 mg kg⁻¹, s.c.). In samples from rat striatum, levels of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), 3-methoxytyramine (3-MT), ascorbate (AA), dehydroascorbate (DHAA), hypoxanthine, xanthine and uric acid (UA) were measured. Brain microdialysis experiments were carried out in freely moving rats. Striatal dialysate levels were assayed for DA, DOPAC+HVA, AA and UA using liquid chromatography followed by electrochemical detection. Morphine administration increased the striatal levels of DA metabolites, UA and DHAA and the extracellular concentrations of DA, DOPAC+HVA, UA and AA. Allopurinol (50 mg kg⁻¹by gavage), an inhibitor of xanthine oxidase which catalyses oxidation of xanthine to UA, decreased basal UA and AA concentrations and the morphine-induced increase in DA metabolites and AA oxidation. Since oxidation of DA and xanthines generates reactive oxygen species (ROS) and AA and UA are the main cellular antioxidants, these findings suggest that: (a) single morphine administration increases DA and xanthine oxidative metabolism with a consequent increase in ROS production, which may account for changes in concentrations of extracellular AA and tissue DHAA; (b) allopurinol decreases morphine-induced DA and xanthine oxidation; (c) UA and AA may act in concert to regulate levels of ROS in the brain.     

Release of dopamine is reduced by diazepam more in the nucleus accumbens than in the caudate nucleus of conscious rats

The effects of 1-20 mg/kg diazepam were studied on the extracellular concentrations of dopamine (DA), dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the nucleus accumbens and striatum of conscious rats, using intracerebral microdialysis. Five, but not 1 mg/kg diazepam significantly reduced extracellular DA, DOPAC and HVA in the nucleus accumbens. Twenty mg/kg diazepam significantly reduced extracellular DA, DOPAC and HVA in the striatum. A significant effect on striatal DOPAC, but not on DA and HVA, was seen with 10 mg/kg diazepam, while no changes were found with 5 mg/kg diazepam. The results suggest that diazepam reduces the release and metabolism of DA in the nucleus accumbens more than in the striatum.     

A study on the role of nitric oxide and iron in 3-morpholino-sydnonimine-induced increases in dopamine release in the striatum of freely moving rats

1. We showed previously that interaction between NO and iron (II), both released following the decomposition of sodium nitroprusside (SNP), accounted for the late SNP-induced dopamine (DA) increase in dialysates from the striatum of freely moving rats; in addition, we showed that co-infusion of iron (II) with the NO-donor S-nitroso-N-acetylpenicillamine mimicked SNP effects on striatal DA release. 2. In the present study, intrastriatal co-infusion of iron (II) (given as FeSO(4), 1 mM for 40 min) with the NO-donor and potential peroxynitrite generator 3-morpholinosydnonimine (SIN-1) (0.2, 0.5, 1.0 or 5.0 mM for 180 min), potentiated the SIN-1-induced increase in DA concentration in dialysates from the striatum of freely moving rats. Neither alone nor associated with iron (II) did SIN-1 induce changes in dialysate ascorbic acid or uric acid concentrations. 3. Neither co-infusion of a superoxide dismutase mimetic nor uric acid affected SIN-1-induced increases in dialysate DA concentration. 4. Infusion of the iron chelator deferoxamine (0.2 mM for 180 min) decreased dialysate DA and attenuated SIN-1-induced increases in dialysate DA concentrations. 5. These results suggest that iron plays a key role in SIN-1-induced release of striatal DA and do not support any role for either peroxynitrite or superoxide anion in SIN-1-induced release of striatal DA.     

Effects of Acute Lethal Cyanide Intoxication on Central Dopaminergic Pathways

Abstract: In rats treated with sodium cyanide (NaCN), 20 mg/kg intraperitoneally, the striatal dopamine (DA) level was decreased within 60 sec. compared to controls injected with NaCl 0.9%. Treatment with NaCN also increased the naturally occuring L-DOPA in the striatum, but not in the other brain regions studied. Decreased DA levels but increased L-DOPA accumulation were also seen in cyanide-treated animals after inhibition of neuronal L-aromatic amino decarboxylase. In rats given a non-lethal dose of NaCN, 2.5 mg/kg intraperitoneally, 30 min. before sacrifice and L-DOPA, 100 mg/kg intraperitoneally, 25 min. before sacrifice, regional L-DOPA levels were not significantly changed, but the striatal DA levels were diminished compared to controls. Decreased DA levels might indicate that cyanide inhibits the synthesis of brain DA. However, both increased L-DOPA and increased accumulation of L-DOPA after neuronal decarboxylase were observed after cyanide. Furthermore, we have earlier reported that lethal doses of NaCN decreased the DA metabolite HVA in the striatum but did not significantly change the oxidatively deaminated metabolite of DA, DOPAC. Inhibition of L-aromatic amino acid decarboxylase appears to play a minor role in causing decreased striatal DA levels. However, our findings might be compatible with cyanide-produced inhibition of the energy-demanding granular uptake and/or release of DA.     

Mexidol potentiates antiparkinsonian effect of L-DOPA in MPTP-induced parkinsonism model]

Preliminary peroral administration of mexidol into mice with (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine)-induced parkinsonism model prevented the formation of malonaldehyde in striatum. In combination with small doses of L-DOPA (2-mg/kg), mexidol decreased oligokinesia and muscular rigidity manifestations and increased the levels of dopamine and its metabolites--dioxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in striatum. The combined (mexidol + L-DOPA) drug administration produced a 3-5 h increase in the time of the parkinsonism syndrome inhibition as compared to that upon the administration of a greater L-DOPA dose (100 mg/kg) without mexidol. It is suggested that mexidol, possessing antioxidant properties, is capable of preventing the damage and loss of the nigrostriatial dopaminergic neurons.     

Effects of acute lethal cyanide intoxication on central dopaminergic pathways.

In rats treated with sodium cyanide (NaCN), 20 mg/kg intraperitoneally, the striatal dopamine (DA) level was decreased within 60 sec. compared to controls injected with NaCl 0.9%. Treatment with NaCN also increased the naturally occurring L-DOPA in the striatum, but not in the other brain regions studied. Decreased DA levels but increased L-DOPA accumulation were also seen in cyanide-treated animals after inhibition of neuronal L-aromatic amino decarboxylase. In rats given a non-lethal dose of NaCN, 2.5 mg/kg intraperitoneally, 30 min. before sacrifice and L-DOPA, 100 mg/kg intraperitoneally, 25 min. before sacrifice, regional L-DOPA levels were not significantly changed, but the striatal DA levels were diminished compared to controls. Decreased DA levels might indicate that cyanide inhibits the synthesis of brain DA. However, both increased L-DOPA and increased accumulation of L-DOPA after neuronal decarboxylase were observed after cyanide. Furthermore, we have earlier reported that lethal doses of NaCN decreased the DA metabolite HVA in the striatum but did not significantly change the oxidatively deaminated metabolite of DA, DOPAC. Inhibition of L-aromatic amino acid decarboxylase appears to play a minor role in causing decreased striatal DA levels. However, our findings might be compatible with cyanide-produced inhibition of the energy-demanding granular uptake and/or release of DA.     

Effects of 3-<i>O</i>-Methyldopa, <small>L</small>-3,4-Dihydroxyphenylalanine Metabolite, on Locomotor Activity and Dopamine Turnover in Rats

It has been well known that 3-O-methyldopa (3-OMD) is a metabolite of L-3,4-dihydroxyphenylalanine (L-DOPA) formed by catechol O-methyltransferase (COMT), and 3-OMD blood level often reaches higher than physiological level in Parkinson's disease (PD) patients receiving long term L-DOPA therapy. However, the physiological role of 3-OMD has not been well understood. Therefore, in order to clarify the effects of 3-OMD on physiological function, we examined the behavioral alteration in rats based on locomotor activity, and measured dopamine (DA) and its metabolites levels in rats at the same time after 3-OMD subchronic administration. The study results showed that repeated administrations of 3-OMD increased its blood and the striatum tissue levels in those rats, and decreased locomotor activity in a dose dependent manner. Although 3-OMD subchronic administration showed no significant change in DA level in the striatum, DA metabolite levels, such as 3,4-dihydroxyphenylacetic acid (DOPAC), 3-methoxytyramine (3-MT), and homovanillic acid (HVA) were significantly decreased. After 3-OMD washout period (7?d), locomotor activity and DA turnover in those rats returned to normal levels. Furthermore, locomotor activity and DA turnover decreased by 3-OMD administration were recovered to normal level by acute L-DOPA administration. These results suggested that 3-OMD affect to locomotor activity via DA neuron system. In conclusion, 3-OMD itself may have a disadvantage in PD patients receiving L-DOPA therapy.     

Dopamine in the basal ganglia and benzodiazepine-induced sedation

The effects of the anxiolytic benzodiazepine flurazepam on motor activity and the turnover of dopamine were measured in rats. Changes in motor activity were measured using a doppler-shift device; changes in extracellular homovanillic acid (HVA), monitored by linear sweep voltammetry with carbon paste electrodes implanted in the striatum and nucleus accumbens and ex vivo measurements of changes in 3,4-dihydroxyphenylacetic acid/dopamine (DOPAC/DA) ratios in the striatum and nucleus accumbens were used as indices of changes in the turnover of dopamine. Injection of vehicle increased the nocturnal rise in the concentration of HVA and the ex vivo DOPAC/DA ratio in the nucleus accumbens. Injection of flurazepam decreased the nocturnal rise in HVA and DOPAC/DA ratio in the nucleus accumbens below control levels. There was also a decrease in the nocturnal rise in motor activity. Neither injection of vehicle nor injection of flurazepam caused changes in either the concentration of HVA or the DOPAC/DA ratio in the striatum. The correlation coefficient for motor activity compared to concentration of HVA remained high for the nucleus accumbens but was reduced for the striatum after administration of flurazepam. The results suggest that the sedative effect of flurazepam may be due to an action on the mesolimbic but not the nigrostriatal dopaminergic pathway.     

Comparison of the effects of epibatidine and nicotine on the output of dopamine in the dorsal and ventral striatum of freely-moving rats

The effects of the nicotinic acetylcholine receptor (nAChR) agonist epibatidine on the extracellular concentrations of dopamine (DA) and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the dorsal (caudate-putamen) and the ventral striatum (nucleus accumbens) of freely-moving male Wistar rats were studied by in vivo microdialysis. In the dorsal striatum, epibatidine (3.0 microg/kg s.c.) significantly elevated the extracellular concentrations of DA, DOPAC and HVA. In contrast, epibatidine did not alter the extracellular DA concentration in the ventral striatum, but elevated significantly the concentration of DOPAC and also tended to elevate that of HVA. In parallel experiments, nicotine (0.5 mg/kg s.c.) significantly increased DA output in the ventral striatum whereas only a modest and non-significant increase of extracellular DA concentration was found in the dorsal striatum. Earlier studies have shown that the doses of epibatidine and nicotine used in the present study are about equieffective at least with respect to the analgesia-producing or hypothermic effects of the drugs. Comparison of the effects of epibatidine and nicotine suggests that the responses of the mesolimbic and nigrostriatal dopaminergic systems to the two nicotinic receptor agonists differ. Epibatidine, in contrast to nicotine, preferentially stimulates the nigrostriatal vs. the mesolimbic dopaminergic system. Therefore, novel nicotinic AChR ligands structurally related to epibatidine may have low abuse potential.     

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.     

Modification of the striatal dopaminergic neuron system by carbon monoxide exposure in free-moving rats,as determined by in vivo brain microdialysis

Acute carbon monoxide (CO) intoxication in humans results in motor deficits, which resemble those in Parkinson's disease, suggesting possible disturbance of the central dopaminergic (DAergic) neuronal system by CO exposure. In the present study, therefore, we explored the effects of CO exposure on the DAergic neuronal system in the striatum of freely moving rats by means of in vivo brain microdialysis. Exposure of rats to CO (up to 0.3%) for 40 min caused an increase in extracellular dopamine (DA) levels and a decrease in extracellular levels of its major metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), in the striatum depending on the CO concentration. Reoxygenation following termination of the CO exposure resulted in a decline of DA to the control level and an overshoot in the recovery of DOPAC and HVA to levels higher than the control. A monoamine oxidase type A (MAO-A) inhibitor, clorgyline, significantly potentiated the CO-induced increase in DA and completely abolished the subsequent overshoot in the recovery of DOPAC and HVA. Tetrodotoxin, a Na(+) channel blocker, completely abolished both the CO-induced increase in DA and the overshoot of DOPAC and HVA. A DA uptake inhibitor, nomifensine, strongly potentiated the CO-induced increase in DA without affecting the subsequent overshoot of DOPAC and HVA. Clorgyline further potentiated the effect of nomifensine on the CO-induced increase in DA, although a slight overshoot of DOPAC and HVA appeared. These findings suggest that (1) CO exposure may stimulate Na(+)-dependent DA release in addition to suppressing DA metabolism, resulting in a marked increase in extracellular DA in rat striatum, and (2) CO withdrawal and subsequent reoxygenation may enhance the oxidative metabolism, preferentially mediated by MAO-A, of the increased extracellular DA. In the light of the neurotoxicity of DA per se and reactive substances, such as quinones and activated oxygen species, generated via DA oxidation, the significant modification of the striatal DAergic neuronal system by CO exposure might participate in the neurological outcome following acute CO intoxication.     

Effect of conditioning with d-amphetamine on the extracellular concentration of dopamine and its metabolites in the striatum of behaving rats

The effect of classical conditioning with d-amphetamine on the extracellular concentrations of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the striatum of awake, freely moving rats was studied using microdialysis. This was done in order to test, whether there occurred alterations in DA release as conditioned responses in the striatum. The first series of experiments studied the acute effects of d-amphetamine on the concentration of DA and its metabolites DOPAC and HVA. d-Amphetamine (2 mg/kg, s.c.) increased extracellular DA and decreased DOPAC and HVA. Behaviorally, it led to stereotyped locomotor activation and sniffing. In principle, these observations confirmed earlier findings. In a second series, conditioned responses to d-amphetamine were studied. Rats were implanted with guide cannulas prior conditioning experiments started. For conditioning experiments, the rats were divided into three groups: conditioned group, pseudoconditioned group and a drug-naive control group. After 7 daily training sessions with d-amphetamine (2 mg/kg), on the 8th day, the test day, rats were injected with saline and exposed to the conditional stimuli, while they were observed for their stereotyped, conditioned response. Additionally, microdialysis was performed in order to observe possible changes in the extracellular transmitter or metabolite concentrations. Conditioning with amphetamine led to conditioned stereotypic behavior. In comparison with the pseudoconditioned rats, there was an increase in DA release as conditioned response to amphetamine. In pseudoconditioned rats. DOPAC and HVA were slightly higher than in both other groups. DOPAC and HVA were lower in rats conditioned to d-amphetamine when compared with the pseudoconditioned ones. The results suggest that with regard to DA release, the conditioned responses to d-amphetamine mimicked the acute pharmacological responses. The same is valid for the DA metabolites, although in the opposite direction — they mimicked decreases. Furthermore, the conditioned DA responses to d-amphetamine might contribute to conditioned behavioral responses observed in these experiments. Correspondence to: K. Kuschinsky at the above address     

Effect of α-methyl fluorodopa on dopamine metabolites: Importance of conjugation and egress

The effects of D,L-α-monofluoromethyldopa (MFMD), an inhibitor of aromatic L-amino acid decarboxylase, on the metabolism of dopamine (DA) and 5-hydroxytryptamine was investigated using rat brain and cerebrospinal fluid (CSF). Vehicle or MFMD (100 mg/kg) was given p.o. and 16 h later probenecid (200 mg/kg i.p.). CSF was sampled and the rats killed immediately or after 1 h. Vehicle treated rats showed regional differences of percentage conjugation of DA metabolites: 3,4-dihydroxyphenylacetic acid (DOPAC), striatum 10%, rest of brain 45%, CSF 67%; homovanillic acid (HVA), striatum 20%, rest of brain 35%, CSF 53%. These differences and the proportionately greater increases of conjugates than of free acids after probenecid vitiate regional comparisons of DA metabolism if conjugates are not included. MFMD alone decreased neither 5HIAA (except in the striatum) nor the free DA metabolites but decreased both conjugates in CSF and conjugated DOPAC in rest of brain. The inhibitory effects of MFMD on 5HT and DA synthesis were most evident when measured by the accumulation of 5HIAA or total (DOPAC + HVA) after giving probenecid. MFMD may also inhibit amine metabolite egress and the conjugation of DOPAC and HVA.