Characterization and Pharmacological Responsiveness of Dopamine Release Recorded by Microdialysis in the Substantia Nigra of Conscious Rats

The extracellular concentration of dopamine (DA) and 3,4-dihydroxyphenylacetic acid in the substantia nigra (SN) and striatum was estimated by microdialysis. The dialysate content of DA from the SN was recorded during infusion of a DA uptake blocker (nomifensine; 5 mumol/L) dissolved in the perfusion fluid. Perfusion of tetrodotoxin (1 mumol/L) produced a virtually complete disappearance of nigral and striatal DA release. Dendritic as well as terminal release of DA was inhibited for several hours when the nerve impulse flow in dopaminergic neurons was blocked by systemic administration of gamma-butyrolactone (750 mg/kg, i.p.). The systemic administration (0.3 mg/kg, i.p.) as well as infusion (1 mumol/L) of the D2 agonist (-)-N-0437 [2-(n-propyl-N-2-thienylethylamino)-5-hydroxytetralin] produced a significant decrease in the release of DA in both the striatum and the SN. DA levels were recorded in the striatum both with and without addition of nomifensine to the perfusion fluid. The decrease in the striatum after (-)-N-0437 was suppressed in the presence of nomifensine. Infusion (1 mumol/L) as well as systemic administration (40 mg/kg) of sulpiride caused a similar increase in the release of striatal DA; this increase was, in both experiments, potentiated by nomifensine coinfusion. Sulpiride administration induced a small increase in the release of nigral DA. Infusion of (-)-N-0437 or (-)-sulpiride into the nigra caused a moderate decrease and increase, respectively, of striatal DA level.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of Selective Inhibition of Monoamine Oxidase A or B on Striatal Metabolism of l-DOPA in Hemiparkinsonian Rats

Abstract: The effect of selective inhibition of monoamine oxidase (MAO) subtypes A and B on striatal metabolism of DOPA to dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), and 4-hydroxy-3-methoxyphenylacetic acid (homovanillic acid; HVA) was studied in halothane-anesthetized rats 3 weeks after unilateral 6-hydroxydopamine lesion of the substantia nigra. Implantation of bilateral microdialysis probes allowed simultaneous quantitation of metabolite production on lesioned and control sides. The DOPA was administered as a 15-min bolus of 1 m M solution in the striatal microdialysate. Rats were pretreated with the selective MAO-A inhibitor clorgyline, or the selective MAO-B inhibitors deprenyl or TVP-101 [2,3-dihydro- N -2-propynyl-1 H -inden-1-amine-(1 R )-hydrochloride]. Intrastriatal infusion of DOPA caused an increased efflux of DA, DOPAC, and HVA, which was greater on the intact side. Clorgyline, but not deprenyl or TVP-101, increased post-DOPA DA efflux on both intact and lesioned sides. Clorgyline also caused a marked suppression of post-DOPA DOPAC and HVA effluxes, whereas only mild effects were produced by the MAO-B inhibitors. There was no evidence for a differential effect of MAO-B inhibition on efflux of DA or metabolites in the lesioned as compared with the control striatum. The results indicate a major role for MAO-A in DA metabolism both intra- and extraneuronally in the rat striatum.     

Stress-induced increase in 3,4-dihydroxyphenylacetic acid (DOPAC) levels in the cerebral cortex and in n. accumbens: reversal by diazepam

The effect of electrical foot shock stress on dopamine and DOPAC levels was examined in the frontal cortex, nucleus accumbens, striatum, substantia nigra and medial basal hypothalamus of rats. DA content did not change after stress in any of the structures analyzed except in the substantia nigra in which DA level decreased by about 35% following 20, 60 or 180 min of stress. DOPAC level did not change in the striatum, medial basal hypothalamus and substantia nigra, but increased in the frontal cortex and in n. accumbens by about 75% and 40%, respectively. Pretreatment with diazepam, but not with pentobarbital, prevented stress-induced increased in DOPAC levels.     

Cholinergic effect on the dopamine turnover in the rat corpus striatum]

The peripheral administration of oxotremorine caused a significant increase in dihydroxyphenylacetic acid (DOPAC) in the striatum of rats, dopamine (DA) level was unaffected. Injection of oxotremorine into the substantia nigra failed to change the content of dopamine and its acid metabolites homovanillic acid (HVA) and DOPAC in striatum. Injection of oxotremorine or carbachol into the substantia nigra or into the caudate nucleus did not significantly influence the DA-turnover. The partly inconsistent results are discussed in connection with literature data in regard to the existence of excitatory as well as inhibitory cholinergic systems, which are located differently and are involved in the regulation of DA-turnover.     

d-Fenfluramine Increases Striatal Extracellular Dopamine In Vivo Independently of Serotonergic Terminals or Dopamine Uptake Sites

Abstract: The effect of various doses of the serotonin (5-HT) release-inducing agent d -fenfluramine ( d -fenf) on extracellular dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), and 5-hydroxyindoleacetic acid (5-HIAA) was studied in vivo in the striatum of halothane-anesthetized rats, following systemic and local administration. At 5 and 10 but not 2.5 mg/kg, d -fenf administered intraperitoneally significantly increased DA extracellular concentration and reduced DOPAC outflow. A concentration-dependent enhancement of DA dialysate content was also found following intrastriatal application (5, 10, 25, and 50 µ M ). The bilateral administration of 5,7-dihydroxytryptamine into the dorsal raphe nucleus, which markedly depleted 5-HT in the striatum, did not modify the effect on extracellular DA concentration of 25 µ M d -fenf locally applied into the striatum. The enhancement of extracellular DA level induced by 25 µ M d -fenf was slightly but significantly reduced by the local application of 25 µ M citalopgram. The blockade of DA uptake sites by nomifensine (0.1, 0.3, and 1 µ M ) did not modify significantly the effect of d -fenf. The rise of DA outflow induced by 25 µ M d -fenf was strongly reduced in the presence of 1 µ M tetrodotoxin (TTX) or by the removal of Ca²⁺ from the perfusion medium. The results obtained show that d -fenf increases the striatal extracellular DA concentration by a Ca²⁺-dependent and TTX-sensitive mechanism that is independent of striatal 5-HT itself or DA uptake sites.     

In Vivo Mechanisms Underlying Dopamine Release from Rat Nigrostriatal Terminals: II. Studies Using Potassium and Tyramine

The brain microdialysis technique has been used to examine the in vivo effects of potassium and tyramine on dopamine (DA) release and metabolism in the striatum of halothane-anaesthetised rats. Increasing the concentration of potassium perfusing the dialysis probe (30-120 mM) induced a dose-related efflux of DA. A dose-related release of DA was also observed following addition of tyramine (1-100 microM) to the perfusing buffer. High concentrations of potassium were found to reduce the dialysate content of the DA metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid and the serotonin metabolite 5-hydroxyindoleacetic acid. No such effect was observed even when using the highest concentration of tyramine tested. Potassium-evoked DA release was facilitated by pretreatment with the DA uptake inhibitor nomifensine, was inhibited by depletion of extracellular calcium, and was not significantly affected by tetrodotoxin (TTX). The effect of tyramine on DA efflux was inhibited by nomifensine and was insensitive to both TTX and calcium depletion. These data suggest that potassium and tyramine induce release of DA via different mechanisms. Potassium-induced DA release involves a carrier-independent process and may utilise an exocytotic release mechanism. On the other hand, tyramine-induced DA release would appear to involve a carrier-dependent process. Depletion of vesicular stores of DA by pretreatment with reserpine did not significantly affect potassium-induced DA release, whereas a marked inhibition of the effects of tyramine was noted. However, in reserpinised animals the potassium-induced release of DA was inhibited by nomifensine, a result suggesting that a carrier-dependent release mechanism operates in the absence of vesicular DA.(ABSTRACT TRUNCATED AT 250 WORDS)     

Differential effects of carboxyfullerene on MPP+/MPTP-induced neurotoxicity

The effects of carboxyfullerene on a well-known neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and its active metabolite 1-methyl-4-phenyl-pyridinium (MPP+) were investigated. In chloral hydrate-anesthetized rats, cytosolic cytochrome c was elevated in the infused substantia nigra 4 h after an intranigral infusion of MPP+. Five days after local application of MPP+, lipid peroxidation (LP) was elevated in the infused substantia nigra. Furthermore, dopamine content and tyrosine hydroxylase (TH)-positive axons were reduced in the ipsilateral striatum. Concomitant intranigral infusion of carboxyfullerene abolished the elevation in cytochrome c and oxidative injuries induced by MPP+. In contrast, systemic application of carboxyfullerene did not prevent neurotoxicity induced by intraperitoneal injection of MPTP. In mice, systemic administration of MPTP induced a dose-dependent depletion in striatal dopamine content. Simultaneous injection of carboxyfullerene (10 mg/kg) actually potentiated MPTP-induced reduction in striatal dopamine content. Furthermore, systemic administration of carboxyfullerene (30 mg/kg) caused death in the MPTP-treated mice. An increase in the striatal MPP+ level and reduction in hepatic P450 level were observed in the carboxyfullerene co-treated mice. These data showed that systemic application of carboxyfullerene appears to potentiate MPTP-induced neurotoxicity while local carboxyfullerene has been suggested as a neuroprotective agent. Furthermore, an increase in striatal MPP+ level may contribute to the potentiation by carboxyfullerene of MPTP-induced neurotoxicity.     

YM-09151-2 but Not l-Sulpiride Induces Transient Dopamine Release in Rat Striatum via a Tetrodotoxin-Insensitive Mechanism

Abstract: The effects of the selective dopamine D₂ receptor antagonists YM-09151-2 and l -sulpiride on the in vivo release of dopamine (DA), l -3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) in rat striatum were investigated. The drugs were injected into the striatum through a microinjection needle attached to a dialysis probe. YM-09151-2 (0.1 or 1.0 μg/0.5 μl) injected into the striatum produced a dramatic rapid-onset transient increase in striatal DA release in a dose-dependent manner. However, the DA increase induced by l -sulpiride (15 or 75 ng/0.5 μl) was small and of slower onset. An increase of DOPAC levels by YM-09151-2 was biphasic: The first peak occurred at 40 min, followed by a delayed-onset gradual increase. Slower-onset gradual increases were also found in DOPAC levels after l -sulpiride injection and in HVA levels after injections of both YM-09151-2 and l -sulpiride. The infusion of tetrodotoxin (TTX; 2 μM) revealed two different types of DA release mechanisms: The rapid-onset transient DA release induced by YM-09151-2 was TTX insensitive, whereas the slower-onset DA release induced by l -sulpiride was TTX sensitive. Moreover, the rapid-onset transient DA release was Ca²⁺ independent and was not affected by pre-treatment with l -sulpiride or nomifensine. Therefore, it is concluded that YM-09151-2 injected into the striatum produced a transient striatal DA release that is independent of D₂ receptors and the action potential.     

A possible role of nitric oxide in the regulation of dopamine transporter function in the striatum.

Brain microdialysis and high-performance liquid chromatography with electrochemical detection were used to study the effect of the nitric oxide synthase inhibitor Nomega-nitro-L-arginine methyl ester (L-NAME) on striatal dopamine (DA) release in the anesthetized rat. Systemic administration of L-NAME (10 mg/kg, i.p.) significantly decreased the resting release of DA. The peak effect (23% decrease) was reached 45 min after injection. The inactive enantiomer D-NAME (10 mg/kg, i.p.) or the vehicle (saline, 5 ml/kg i.p.) had no effect on the striatal DA level. Neither treatment altered significantly the concentration of dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). To investigate the possible involvement of the DA uptake system L-NAME was injected also in the presence of the DA uptake inhibitor nomifensine. Local application of nomifensine (10 microM in the dialysate medium) increased the extracellular concentration of DA to about eight-fold of the basal value and stabilized it at this higher level. Under these conditions L-NAME (10 mg/kg, i.p.) was not able to alter the striatal DA level. Neither nomifensine nor L-NAME caused any change in the level of DOPAC and HVA. Our data suggest that endogenously produced nitric oxide may influence the activity of the DA transporter which effect may have special importance in the regulation of extracellular transmitter concentration in the striatum.     

3-Methoxytyramine Formation Following Monoamine Oxidase Inhibition Is a Poor Index of Dendritic Dopamine Release in the Substantia Nigra

Abstract: This study was undertaken, using microdialysis, to compare the extracellular concentration of 3-methoxytyramine and dopamine in dialysate from the striatum and substantia nigra, after pargyline (75 mg/kg), after pargyline plus amphetamine (3 mg/kg), and after pargyline plus reserpine (5 mg/kg) administration. Treatment with pargyline alone increased the extracellular dopamine concentration by 70% in the striatum and by 140% in the substantia nigra and induced in both regions a time-dependent accumulation of 3-methoxytyramine. The addition of d-amphetamine to pargyline increased the extracellular dopamine concentration, compared with pargyline-treated controls, to the same extent in both the substantia nigra (maximally by 360%) and the striatum (maximally by 400%), but the concomitant increase of 3-methoxytyramine accumulation in the dialysate was relatively smaller in the substantia nigra compared with the striatum. Reserpine treatment decreased the extracellular dopamine concentration in both regions below the detection level (<10% of basal value). When pargyline was added to reserpine, the striatal extracellular dopamine concentration increased to 50% of pargyline-treated controls and the striatal 3-methoxytyramine accumulation was less than in pargyline-treated controls. However, in the substantia nigra, the addition of pargyline to reserpine resulted in dopamine concentrations as high as after pargyline only and the 3-methoxytyramine accumulation was not changed compared with pargyline-treated controls. In summary, our results indicate that dopamine in the substantia nigra is released from reserpine-sensitive storage sites and that pargyline-induced 3-methoxytyramine accumulation is a poor indicator of the local dopamine release. The latter observation may be explained by the fact that the dopamine-metabolizing enzyme, catechol-O-methyltransferase, is located inter alia in the dopamine-containing cell bodies/dendrites in the substantia nigra, in contrast to the situation in the terminals in the striatum where catechol-O-methyltransferase is located only in nondopaminergic cells.     

Effects of iptakalim on extracellular glutamate and dopamine levels in the striatum of unilateral 6-hydroxydopamine-lesioned rats: a microdialysis study

In a previous study, we demonstrated that iptakalim (Ipt) significantly ameliorated hypolocomotion and catalepsy induced by haloperidol and rotenone in rats. In order to further understand the mechanism(s), using a rat model of Parkinson's disease (PD) established by unilateral 6-hydroxydopamine (6-OHDA) administration to the substantia nigra pars compacta (SNpc) and reverse microdialysis techniques with high performance liquid chromatography (HPLC), we investigated the effects of Ipt on extracellular levels of glutamate, dopamine (DA) and its metabolite dihydroxyphenylacetic acid (DOPAC) in the striatum of conscious and freely moving rats. The results indicated that unilateral 6-OHDA-lesioned rats have a significantly higher level of extracellular glutamate and a lower level of extracellular DOPAC in the lesioned-side of the striatum, and a lower level of extracellular DA in both sides of the striatum compared to the striatum of control rats. Ipt reduced extracellular glutamate levels in both sides of striatum of the lesioned and control rats in a concentration-dependent manner. Ipt, at lower concentrations (0.01, 0.1, 1 microM), enhanced extracellular DA levels in the lesioned-side striatum of the unilateral 6-OHDA-lesioned rats, while causing no significant changes in the intact side striatum, and even a significant decline in striatum of control rats at higher concentrations of Ipt (10, 100 microM). In addition, Ipt also caused a significant decline in the extracellular DOPAC levels in the lesioned-side striatum of unilateral 6-OHDA-lesioned rats. These data suggest that the major mechanism underlying the ameliorative effects of Ipt on the behavior in 6-OHDA-lesioned rats is the alteration of levels of extracellular neurotransmitters, such as glutamate and DA in the striatum of unilateral 6-OHDA-lesioned rats.     

Nigral and Striatal Comparative Study of the Neurotoxic Action of 1-Methyl-4-Phenylpyridinium Ion: Involvement of Dopamine Uptake System

Abstract: Microdialysis was used in a comparative study of the neurotoxic action of MPP⁺ in the absence or presence of nomifensine (20 µM) in the striatum and substantia nigra. Three different concentrations of MPP⁺ (1, 2.5, and 5 mM) were perfused for 15 min at 24 (day 1) and 48 h (day 2) after surgery. The dopamine basal value in the striatum was ～17 fmol/min. Nomifensine (20 µM) stimulated dopamine release to ～170 fmol/min. The increase of dopamine extracellular output in the striatum after MPP⁺ perfusion on day 1 was independent of the concentration of MPP⁺ perfused and of the absence or presence of nomifensine (20 µM), being ～2,500 fmol/min. The dopamine basal value in the substantia nigra was below the detection limit of our HPLC equipment. Nomifensine (20 µM) stimulated dopamine release to ～6.3 fmol/min. The increase of dopamine extracellular output in the substantia nigra was MPP⁺ dose-dependent (1 mM, 75 fmol/min; 2.5 mM, 150 fmol/min; and 5 mM, 250 fmol/min) and independent of the presence or absence of nomifensine. On day 2, the presence of nomifensine on day 1 produced a total protection against MPP⁺ (1 mM) perfusion in the striatum, which was not observed against MPP⁺ (5 mM). MPP⁺ (1 mM) did not produce any neurotoxic action in the substantia in the absence or presence of nomifensine. The MPP⁺ (2.5 mM) effect on dopamine extracellular output in the absence of nomifensine (20 µM) in the substantia nigra on day 2 was similar to that of MPP⁺ (1 mM) in the striatum. The presence of nomifensine (20 µM) partially prevented the neurotoxic effect of MPP⁺ (2.5 mM) on dopaminergic cell bodies/dendrites in the substantia nigra. The MPP⁺ (5 mM) effect on dopamine extracellular output was similar in both structures studied in the absence or presence of nomifensine on day 2. These results suggest that terminals in the striatum are more sensitive to the neurotoxicity of MPP⁺ than cell bodies/dendrites in the substantia nigra.     

Central depletion of dopamine in rats by 1-methyl-4-phenylpyridine

Effects of 1-methyl-4-phenylpyridine(MPP+), a putative neurotoxic metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine(MPTP), on the contents of dopamine were examined in the various regions of the rat brain. Under anesthesia with pentobarbital sodium and flunitrazepam, MPP+ 150 micrograms/rat was intracerebroventricularly infused for 5 hours, at 30 micrograms/100 microliters/hr. Seven days later, the contents of dopamine, but not those of noradrenaline and activities of choline acetyl transferase in the brain were found to be significantly decreased, as compared to findings in the respective controls. The MPP+-induced depletion of dopamine was most evident in the striatum (38% of control). Contents of dopamine in the substantia nigra and ventral tegmental area were not significantly affected by MPP+. These results are interpreted to mean that intracerebroventricular continuous infusion of MPP+, in a relatively low concentration, induces a moderate but relatively specific disruption of central dopaminergic nerve terminals in rats, presumably by the selective accumulation of this neurotoxic agent into these nerve terminals.     

Striatal dopamine levels after unilateral lesions of the substantia nigra: evidence for a contralateral decrease

Unilateral electrolytical and chemical (6-hydroxydopamine) lesions in the substantia nigra (SN) of rats were followed 7 days later by considerable bilateral decreases of neostriatal dopamine (DA) levels. Similarly, the DA content of the substantia nigra decreased not only ipsilaterally but contralaterally as well. Positive correlations were found between ipsi- and contralateral nigral DA levels, ipsi- and contralateral striatal DA and between the DA level of the SN and the striatum of the corresponding side both ipsi- and contralaterally to the lesion.     

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.     

6-羟基多巴胺损毁大鼠内侧前脑束早期黑质-纹状体系统的铁水平与多巴胺能神经元退变的关系

应用快速周期伏安法(fast cyclic vohammetry,FCV)、原子吸收分光光度法及免疫组织化学方法,观察了6-羟基多巴胺(6-hydroxydopamine,6-OHDA)单侧损毁大鼠内侧前脑束(medial forebrain bundle,MFB)早期黑质(sub-stantia nigra,sN)铁水平与多巴胺(dopamine,DA)神经元损伤的变化,以及纹状体(striatum,Str)的DA释放。结果如下：6-OHDA单侧损毁大鼠MFB1d和3d后,SN的酪氨酸羟化酶(tyrosine hydroxylase,TH)阳性细胞分别下降了45％和66％;与正常鼠和未损毁侧相比,损毁侧SN的铁染色增强,铁浓度增加,而Str的DA释放量不变;6-OHDA损毁后1d与3d组相比,损毁侧的铁染色、铁浓度及DA释放量差别无显著性。上述结果表明,6-OHDA单侧损毁大鼠MFB的早期阶段,SN的DA能神经元数目中等程度减少时,铁染色及铁浓度即有增加,由于DA能神经系统有强大的代偿功能,使得Str的DA释放量仍趋于正常。     

Semichronic Inhibition of Glutathione Reductase Promotes Oxidative Damage to Proteins and Induces both Transcription and Translation of Tyrosine Hydroxylase in the Nigrostriatal System

We have evaluated the effect of N,N-bis (2-chloroethyl)-N-nitrosourea (BCNU), an inhibitor of glutathione reductase (GR), on the oxidative status along with the integrity of the nigrostriatal dopaminergic system of the rat. The oxidative status was studied by the quantification of carbonyl groups coupled to protein homogenates. Moreover, the specific oxidations in glial fibrillary acidic protein (GFAP) and neurofilament-200 (NF-200) were also measured. The results show that oxidative damage in proteins in the nigrostriatal system is confined to the striatum. Specific carbonyl groups coupled to native NF-200 and GFAP were also increased. These changes were accompanied by reactive astrocytosis in striatum but not in substantia nigra. In substantia nigra, decreased levels of dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) were observed following BCNU treatment. In contrast, DA levels were increased in the striatum along with an overall decrease in the ratios of DA metabolites to DA. We also studied the mRNA levels for tyrosine hydroxylase (TH) and the dopamine transporter (DAT) by in situ hybridization. TH mRNA but not DAT mRNA was significantly induced in substantia nigra following BCNU treatment, which was consistent with significant elevations in TH enzyme amount and activity and unchanged DA uptake in striatum. All these results support the DA free radical hypothesis and the key role of the striatal glutathione system in protecting the striatal system against oxidative stress.     

Semichronic inhibition of glutathione reductase promotes oxidative damage to proteins and induces both transcription and translation of tyrosine hydroxylase in the nigrostriatal system

We have evaluated the effect of N,N-bis (2-chloroethyl)-N-nitrosourea (BCNU), an inhibitor of glutathione reductase (GR), on the oxidative status along with the integrity of the nigrostriatal dopaminergic system of the rat. The oxidative status was studied by the quantification of carbonyl groups coupled to protein homogenates. Moreover, the specific oxidations in glial fibrillary acidic protein (GFAP) and neurofilament-200 (NF-200) were also measured. The results show that oxidative damage in proteins in the nigrostriatal system is confined to the striatum. Specific carbonyl groups coupled to native NF-200 and GFAP were also increased. These changes were accompanied by reactive astrocytosis in striatum but not in substantia nigra. In substantia nigra, decreased levels of dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) were observed following BCNU treatment. In contrast, DA levels were increased in the striatum along with an overall decrease in the ratios of DA metabolites to DA. We also studied the mRNA levels for tyrosine hydroxylase (TH) and the dopamine transporter (DAT) by in situ hybridization. TH mRNA but not DAT mRNA was significantly induced in substantia nigra following BCNU treatment, which was consistent with significant elevations in TH enzyme amount and activity and unchanged DA uptake in striatum. All these results support the DA free radical hypothesis and the key role of the striatal glutathione system in protecting the striatal system against oxidative stress.     

Effects of Intranigral Substance P and Neurokinin A Injections on Extracellular Dopamine Levels Measured with Microdialysis in the Striatum and Frontoparietal Cortex of Rats

Extracellular levels of dopamine (DA) and its metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), in the striatum and frontoparietal (sensorimotor) cortex in halothane-anesthetized rats were analyzed simultaneously using in vivo microdialysis. Basal DA levels, measured from the microdialysis perfusate, were 6.4 +/- 0.8 nM (n = 15) in the striatum and 0.9 +/- 0.1 nM (n = 15) in the frontoparietal cortex. Subcutaneous injections of d-amphetamine (2 mg/kg) increased DA levels 10-fold in the striatum and fivefold in the cortex. Injections of substance P (0.07 nmol/0.2 microliters) into the substantia nigra pars reticulata (SNR) increased DA and DOPAC levels approximately 30% in the ipsilateral striatum and approximately 50% in the ipsilateral frontoparietal cortex. Injections of neurokinin A (0.09 nmol/0.2 microliter) into the SNR increased DA and DOPAC levels approximately 30% in the ipsilateral striatum but did not significantly affect DA levels in the ipsilateral frontoparietal cortex, although DOPAC levels were increased by approximately 50%. It is suggested that striatal and cortical DA release is regulated differently by nigral substance P and neurokinin A terminals.     

Protective effect of histidine on potassium chloride depolarization enhances 1-methyl-4-phenylpyridinium ion-induced hydroxyl radical generation in the rat striatum

The present study examined the antioxidant effect of histidine on extracellular potassium ion concentration, [K+]o-induced depolarization enhances 1-methyl-4-phenylpyridinium ion (MPP+)-induced hydroxyl radical (*OH) generation in the rat striatum. Rats were anesthetized and sodium salicylate in Ringer's solution (0.5 nmol/M microl/min) was infused through a microdialysis probe to detect the generation of *OH as reflected by the nonenzymatic formation of 2,3-dihydroxybenzoic acid (DHBA) in the striatum. Induction of [K+]o (20, 70 and 140 mM) significantly increased the level of 2,3-DHBA by the action of MPP+ (5 mM) in a concentration-dependent manner. However, histidine (25 mM) reduced the [K+]o-induced *OH formation. Although the level of MPP+-induced dopamine (DA) and 2,3-DHBA formation after [K+]o (70 mM) treatment increased, [K+]o failed to increase either the level of MPP+-induced DA and 2,3-DHBA in the reserpinized group. When iron (II) was administered to [K+]o (70 mM)-pretreated rats, iron (II) clearly produced a dose-dependent increase in the level of 2,3-DHBA, as compared with MPP+-only treated rats. However, in the presence of histidine (25 mM), the effect of [K+]o was abolished. These results indicated that histidine may reduce the [K+]o-induced depolarization enhanced *OH formation by the action of MPP+ in the rat striatum.