Rapid Communication: Attenuation of Methamphetamine-Induced Neurotoxicity in Copper/Zinc Superoxide Dismutase Transgenic Mice

Abstract: Administration of methamphetamine (METH) to rats and nonhuman primates causes loss of terminals in the nigrostriatal dopaminergic system. The mechanism by which METH causes its neurotoxicity is not known. To evaluate further the role of oxyradicals in METH-induced neurotoxicity, we have tested its effects in CuZn superoxide dismutase (SOD) transgenic (Tg) mice, which express the human CuZnSOD gene. In non-Tg mice, acute METH administration causes significant decreases in levels of dopamine (DA) and 3, 4-dihydroxyphenylacetic acid (DOPAC) in the striata and cortices of non-Tg mice. In contrast, there were no significant decreases in cortical or striatal DA in the SOD-Tg mice. The effects of METH on DOPAC were also attenuated in both structures of these SOD-Tg mice. Chronic METH administration caused decreases in levels of striatal DA and DOPAC in the non- Tg mice, whereas the SOD-Tg mice were not affected. These results suggest that METH-induced dopaminergic toxicity in mice may be secondary to increased production of reactive oxygen species such as the superoxide radical.     

Propentophylline increases striatal dopamine release but dampens methamphetamine-induced dopamine dynamics: A microdialysis study

While there are currently no medications approved for methamphetamine (METH) addiction, it has been shown that propentofylline (PPF), an atypical methylxanthine, can suppress the rewarding effects of methamphetamine (METH) in mice. This experiment studied the interactions of PPF with METH in striatal dopaminergic transmission. Herein, the impact of PPF (10–40 mM, intrastriatally perfused (80 min) on the effect of METH (5 mg/kg, i.p.) on striatal dopamine (DA) release was evaluated using brain microdialysis in Sprague–Dawley adult rats. METH was injected at the 60 min time point of the 80 min PPF perfusion. The extracellular levels of DA and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were determined using high performance liquid chromatography with electrochemical detection (HPLC-ED). PPF induced a concentration-dependent increase in DA release beginning 30 min after the onset of PPF perfusion. DA peak levels evoked by 40 mM PPF were similar to those induced by 5 mg/kg METH i.p. Only the highest concentration of PPF decreased the METH-induced DA peak (circa 70%). The significant decreases in extracellular levels of DOPAC and HVA evoked by METH were partially blocked by 10 and 20 mM PPF. Although 40 mM of PPF also partially blocked the METH-induced DOPAC decrease, it completely blocked HVA depletion after a transient increase in HVA levels in METH-treated rats. Data indicates for the first time that while PPF increases presynaptic striatal DA dynamics it attenuates METH-induced striatal DA release and metabolism.     

Prolonged Alterations in Canine Striatal Dopamine Metabolism Following Subtoxic Doses of l-Methyl-4-Phenyl- 1,2,3,6-Tetrahydropyridine (MPTP) and 4''-Amino-MPTP Are Linked to the Persistence of Pyridinium Metabolites

Single toxic doses of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP).HCl (2.5 mg/kg i.v.) and 4'-amino-MPTP.2HCl (22.5 mg/kg) induce loss of striatal dopamine (DA) and tyrosine hydroxylase (TH) activity and of nigral DA neurons in the dog. To examine the subacute neurochemical changes induced by low doses of MPTP and 4'-amino-MPTP, dose-response studies of these compounds were carried out in the dog, using 6- and 3-week survival times for these two compounds, respectively. Low single doses of MPTP (1.0, 0.5, and 0.1 mg/kg i.v.) and 4'-amino-MPTP (15, 7.5, and 3.75 mg/kg i.v.) did not cause depletion of canine striatal DA or TH or a loss of nigral neurons. However, levels of the DA metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were decreased in a dose-related fashion, with significant loss of DOPAC being evident 6 weeks after the lowest administered dose of MPTP and 3 weeks after 4'-amino-MPTP. This selective loss of DA metabolites following nontoxic doses of MPTP and 4'-amino-MPTP led to a shift in the ratio of DA to DOPAC or HVA, which was characteristic for each compound. The measurement of striatal 1-methyl-4-phenylpyridinium (MPP+) and 4'-amino-MPP+ levels revealed that high concentrations (up to 150 microM) persist in the striatum for weeks following administration of a single nontoxic dose of MPTP or 4'-amino-MPTP. A causal relationship between the striatal concentration of MPP+ or 4'-amino-MPP+ and the change in DA metabolism as reflected in the DA/DOPAC ratio is suggested by a significant correlation between these measures. It is suggested that presynaptic sequestration and retention of MPP+ and 4'-amino-MPP+ by striatal DA terminals result in the inhibition of the monoamine oxidase contained within these terminals.     

Genetic or pharmacological blockade of noradrenaline synthesis enhances the neurochemical, behavioral, and neurotoxic effects of methamphetamine

N -(2-chloroethyl)- N -ethyl-2-bromobenzylamine (DSP-4) lesions of the locus coeruleus, the major brain noradrenergic nucleus, exacerbate the damage to nigrostriatal dopamine (DA) terminals caused by the psychostimulant methamphetamine (METH). However, because noradrenergic terminals contain other neuromodulators and the noradrenaline (NA) transporter, which may act as a neuroprotective buffer, it was unclear whether this enhancement of METH neurotoxicity was caused by the loss of noradrenergic innervation or the loss of NA itself. We addressed the specific role of NA by comparing the effects of METH in mice with noradrenergic lesions (DSP-4) and those with intact noradrenergic terminals but specifically lacking NA (genetic or acute pharmacological blockade of the NA biosynthetic enzyme dopamine β-hydroxylase; DBH). We found that genetic deletion of DBH (DBH−/− mice) and acute treatment of wild-type mice with a DBH inhibitor (fusaric acid) recapitulated the effects of DSP-4 lesions on METH responses. All three methods of NA depletion enhanced striatal DA release, extracellular oxidative stress (as measured by in vivo microdialysis of DA and 2,3-dihydroxybenzoic acid), and behavioral stereotypies following repeated METH administration. These effects accompanied a worsening of the striatal DA neuron terminal damage and ultrastructural changes to medium spiny neurons. We conclude that NA itself is neuroprotective and plays a fundamental role in the sensitivity of striatal DA terminals to the neurochemical, behavioral, and neurotoxic effects of METH.     

Evaluation of the effects of alpha-phenyl-N-tert-butyl nitrone pretreatment on the neurobehavioral effects of methamphetamine

A relationship between formation of reactive oxygen species (ROS) and energy depletion has been proposed to play an important role in mediating methamphetamine (METH)-induced neurotoxicity. To evaluate this relationship, we examined the effect of the spin-trap agent, alpha-phenyl-N-tert-butyl nitrone (PBN) on hyperthermia and self-injurious behavior (SIB) and striatal dopamine (DA) depletion produced by METH (4 injections of 4 mg/kg, 2 hr intervals, s.c.) in BALB/c mice. Repeated administration of METH induced hyperthermia, incidence of SIB and striatal DA depletion (84% after 3 days). Pretreatment with PBN (4 injections of 60 or 120 mg/kg, i.p.) reduced METH-induced hyperthermia, but did not significantly attenuate METH-induced SIB or the striatal DA depletion. On the other hand, pretreatment with high doses of PBN (4 injections of 180 or 240 mg/kg, i.p.) protected against METH-induced hyperthermia and SIB, and PBN (180 mg/kg) also completely protected against the acute striatal DA depletion 60 min after the last injection of the drug. However, the long-lasting striatal DA depletion was only attenuated by 52 or 56%, respectively. These results indicate that METH-induced hyperthermia contributes to, but is not solely responsible for METH-induced neurotoxicity, and supports a role for formation of ROS and other mechanisms in the generation of METH-induced striatal dopaminergic neurotoxicity. In addition, the difference in the efficacy of PBN to protect against the acute or long-lasting striatal DA depletion induced by METH may indicate that both ROS formation and other mechanisms are required for METH-induced neurotoxicity to develop.     

Rat-1 Fibroblasts Engineered with GAD65 and GAD67 cDNAs in Retroviral Vectors Produce and Release GABA

Abstract: The effects of the adenosine A₁ agonist N ₆-cyclohexyladenosine (CHA) on MPTP-induced dopamine (DA) depletion in the striatum of C57BL/6 mice were studied. Twenty hours after a single injection of MPTP (30 mg/kg, s.c.), the toxin caused 62% depletion of striatal DA. CHA (0.2–3 mg/kg, s.c.), when given together with MPTP, prevented the toxin-induced DA depletion in a dose-dependent manner. This protective action was apparently mediated by the A¹ receptors, because this effect was selectively antagonized by pretreating the animals with the A₁ antagonist 8-cyclopentyl-1,3-dipropylxanthine (25 mg/kg, i.p.) but not with the A₂ antagonist 1,3-dipropyl-7-methylxanthine (25 mg/kg, i.p.). When CHA (3 mg/kg) was injected 5 h after MPTP administration, at which point striatal DA levels were already reduced significantly, a rapid and complete recovery of the striatal DA levels occurred. These neurochemical data suggest that the A₁ agonist CHA is potentially useful as a neuroprotective agent against MPTP-induced toxicity.     

Evidence against an essential role of endogenous brain dopamine in methamphetamine-induced dopaminergic neurotoxicity

The present studies examined the role of endogenous dopamine (DA) in methamphetamine (METH)-induced dopaminergic neurotoxicity while controlling for temperature-related neuroprotective effects of the test compounds, reserpine and alpha-methyl-p-tyrosine (AMPT). To determine if the vesicular pool of DA was essential for the expression of METH-induced DA neurotoxicity, reserpine (3 mg/kg, given iintraperitoneally 24-26 h prior to METH) was given prior to a toxic dose regimen of METH. Despite severe striatal DA deficits during the period of METH exposure, mice treated with reserpine prior to METH developed long-term reductions in striatal DA axonal markers, suggesting that vesicular DA stores were not crucial for the development of METH neurotoxicity, but leaving open the possibility that cytoplasmic DA might be involved. To evaluate this possibility, cytoplasmic DA stores were depleted with AMPT prior to METH administration. When this study was carried out at 28 degrees C, complete neuroprotection was observed, likely due to lingering effects on core temperature because when the same study was repeated at 33 degrees C (to eliminate AMPT's hypothermic effect in METH-treated animals), the previously observed neuroprotection was no longer evident. In the third and final set of experiments, mice were pretreated with a combination of reserpine and AMPT, to deplete both vesicular and cytoplasmic DA pools, and to reduce striatal DA levels to negligible values during the period of METH administration (< 0.05%). When core temperature differences were eliminated by raising ambient temperature, METH-induced DA neurotoxic changes were evident in mice pretreated with reserpine and AMPT. Collectively, these findings bring into question the view that endogenous DA plays an essential role in METH-induced DA neurotoxicity.     

Treatment with GM1 Ganglioside Restores Striatal Dopamine in the 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine-Treated Mouse

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), 30 mg/kg i.p. daily for 7 days, was administered to mice. This dosage regimen resulted in an approximately 50% reduction of striatal dopamine (DA) level. Chronic administration of GM1 ganglioside (II3NeuAc-GgOse Cer), beginning between 1 to 4 days after terminating MPTP dosing, resulted in partial restoration of the striatal DA level. From dose- and time-response studies, it appeared that 30 mg/kg i.p. of GM1 administered daily for approximately 23 days resulted in an approximately 80% restoration of the DA level and complete restoration of the 3,4-dihydroxyphenylacetic acid (DOPAC) content. This dosage of GM1 also restored the turnover rate of DA in the striatum to near normal. Discontinuing GM1 treatment resulted in a fall of DA and DOPAC levels to values found in mice treated with MPTP alone. There was no evidence for regeneration of nerve terminal amine reuptake in the GM1-treated mice as evaluated by DA uptake into synaptosomes. Our biochemical findings in animals suggest that early GM1 ganglioside treatment of individuals with degenerative diseases of dopaminergic nigrostriatal neurons might be fruitful.     

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) neurotoxicity in mice is enhanced by ethanol or acetaldehyde

Persistent neurochemical changes consistent with parkinsonism have been reported in brains of mice treated with repeated high doses of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). We now report that ethanol or acetaldehyde potentiate MPTP-induced damage to mouse striatum. One hour after the combined treatments (ethanol and MPTP or acetaldehyde and MPTP), the animals exhibited a marked and long-lasting catatonic posture and then returned gradually to apparently normal locomotion. Seven days after MPTP administration, depletion of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in mouse striatum were further potentiated in the group of animals treated with ethanol. This effect was more evident when the treatment was repeated twice and was dose-dependent. Acetaldehyde was more potent than ethanol in enhancing MPTP neurotoxicity. A single exposure to acetaldehyde before and during MPTP treatment produced a very consistent fall of DA, DOPAC and HVA but not serotonin (5HT) or 5-hydroxyindoleacetic acid (5HIAA) in the striatum. This suggests that ethanol effects on MPTP neurotoxicity might be related to acetaldehyde formation.     

4-Phenylpyridine (4PP) and MPTP: the relationship between striatal MPP+ concentrations and neurotoxicity

Because of the chemical and structural similarity between 4-phenylpyridine (4PP) and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), the effects of 4PP alone and in combination with MPTP on striatal dopamine (DA) concentrations were studied in mice. 4PP did not deplete striatal DA, even when given in maximally tolerated doses (five times that required for MPTP neurotoxicity). However, when 4PP was administered prior to MPTP, it provided significant protection against the DA-depleting effects of MPTP. Additional experiments showed that 4PP pretreatment reduced striatal concentrations of 1-methyl-4-phenylpyridinium ion (MPP+) - the putative toxic biotransformation product of MPTP, and that the concentration of this metabolite closely mirrored striatal DA depletion in MPTP-treated mice. In vitro studies established that 4PP probably lowers MPP+ concentrations by inhibiting the biotransformation of MPTP to MPP+. These observations could be of clinical interest in view of the lower incidence of cigarette smoking among Parkinson's disease patients, and the fact that 4PP is known to be present in cigarettes.     

The newly synthesized pool of dopamine determines the severity of methamphetamine-induced neurotoxicity

The neurotransmitter dopamine (DA) has long been implicated as a participant in the neurotoxicity caused by methamphetamine (METH), yet, its mechanism of action in this regard is not fully understood. Treatment of mice with the tyrosine hydroxylase (TH) inhibitor α-methyl- p -tyrosine (AMPT) lowers striatal cytoplasmic DA content by 55% and completely protects against METH-induced damage to DA nerve terminals. Reserpine, by disrupting vesicle amine storage, depletes striatal DA by more than 95% and accentuates METH-induced neurotoxicity. l -DOPA reverses the protective effect of AMPT against METH and enhances neurotoxicity in animals with intact TH. Inhibition of MAO-A by clorgyline increases pre-synaptic DA content and enhances METH striatal neurotoxicity. In all conditions of altered pre-synaptic DA homeostasis, increases or decreases in METH neurotoxicity paralleled changes in striatal microglial activation. Mice treated with AMPT, l -DOPA, or clorgyline + METH developed hyperthermia to the same extent as animals treated with METH alone, whereas mice treated with reserpine + METH were hypothermic, suggesting that the effects of alterations in cytoplasmic DA on METH neurotoxicity were not strictly mediated by changes in core body temperature. Taken together, the present data reinforce the notion that METH-induced release of DA from the newly synthesized pool of transmitter into the extracellular space plays an essential role in drug-induced striatal neurotoxicity and microglial activation. Subtle alterations in intracellular DA content can lead to significant enhancement of METH neurotoxicity. Our results also suggest that reactants derived from METH-induced oxidation of released DA may serve as neuronal signals that lead to microglial activation early in the neurotoxic process associated with METH.     

3-Nitropropionic Acid Neurotoxicity Is Attenuated in Copper/Zinc Superoxide Dismutase Transgenic Mice

Abstract: The mitochondrial toxin 3-nitropropionic acid (3-NP) produces selective striatal lesions in both experimental animals and humans. The pathogenesis of the lesions involves secondary excitotoxicity that may then lead to free radical generation. To test this further we examined the effects of 3-NP in both transgenic (Tg) mice that carry the complete sequence for the human copper/zinc superoxide dismutase (SOD) gene as well as non-Tg littermate controls. The Tg-SOD mice showed a pronounced attenuation of Nissl-stained striatal lesions compared with non-Tg mice. Systemic administration of 3-NP resulted in production of hydroxyl free radicals as assessed by the conversion of salicylate to 2,3- and 2,5-dihydroxybenzoic acid. This production was attenuated significantly in Tg-SOD mice. In a similar way, 3-NP produced significant increases in 3-nitrotyrosine/tyrosine, a marker for peroxynitrite-mediated damage, which were significantly attenuated in Tg-SOD mice. These results support that oxygen free radicals and peroxynitrite play an important role in the pathogenesis of 3-NP neurotoxicity.     

A rapid oxidation and persistent decrease in the vesicular monoamine transporter 2 after methamphetamine

Methamphetamine (METH) produces long-term decreases in markers of dopamine (DA) terminals in animals and humans. A decrease in the function of the vesicular monoamine transporter 2 (VMAT2) has been associated with damage to striatal DA terminals caused by METH; however, a possible mechanism for this decrease in VMAT2 function has not been defined. The current study showed that METH caused a rapid decrease to 68% of controls in VMAT2 protein immunoreactivity of the vesicular fraction from striatal synaptosomes within 1 h after a repeated high-dose administration regimen of METH. This decrease was associated with a 75% increase in nitrosylation of VMAT2 protein in the synaptosomal fraction as measured by nitrosocysteine immunoreactivity of VMAT2 protein. The rapid decreases in VMAT2 persisted when evaluated 7 days later and were illustrated by decreases in VMAT2 immunoreactivity and DA content of the vesicular fraction to 34% and 51% of control values, respectively. The decreases were blocked or attenuated by prior injections of the neuronal nitric oxide synthase inhibitor, S-methyl-l-thiocitrulline. These studies demonstrate that METH causes a rapid neuronal nitric oxide synthase-dependent oxidation of VMAT2 and long-term decreases in VMAT2 protein and function. The results also suggest that surviving DA terminals after METH exposure may have a compromised capacity to buffer cytosolic DA concentrations and DA-derived oxidative stress.     

Sodium diethyldithiocarbamate protects against the MPTP-induced inhibition of immune responses in mice

The effects of 1-methyl-4-phenyl - 1,2,3,6-tetrahydropyridine (MPTP) on immune parameters, and the restorative influence of sodium diethyldithiocarbamate (DTC) or deprenyl were evaluated in mice. The concentrations of dopamine (DA), 3-methoxytyramine (3-MT), 3-4-dihydroxyphenyl acetic acid (DOPAC), and homovanillic acid (HVA), were concomitantly measured in the striatum. MPTP depressed T-cell responses. DTC restored these responses as well as the concentration of striatal DA. Deprenyl had no effect on the concentrations of DA and its metabolites, yet it modified the immune responses alike MPTP. The findings suggest a dopamine pathway could be involved in the brain-controlled immunostimulation afforded by DTC.     

Striatal and Urinary DOPAC/DA Ratio May Indicate a Long-Lasting DA Release Enhancement by MPP+ and MPTP

The DOPAC/DA ratio in mouse striatum, in striatal synaptosomes, and in rat urine after MPP⁺ and MPTP neurotoxin administrations to the animals was followed temporally. The neurotoxins were given intraperitoneally and, in some experiments, to enhance the sensitivity, the animals were subsequently reserpinized before either sacrifice or 24 hour urine collection. MPP⁺ treatment, followed by saline, weakly lowered mouse striatal DOPAC/DA ratio up to 6 hours; in reserpinized animals, however, the neurotoxin reduced striatal ratio potently and for longer periods. Similarly, MPP⁺ reduced rat (saline treated) urinary DOPAC level and DOPAC/DA ratio in the short term (1.0 hr) while the neurotoxin effects could still be detected following longer periods up to 27 days in reserpinized animals. A single MPTP treatment (90 min.), followed by preparation of striatal synaptosomal fraction and its incubation (37°C) with or without reserpine, also led to a reduced DOPAC/DA ratio. Although mainly the pooled peripheral effect is directly indicated by urinary DOPAC/DA ratio, MPP⁺ may reduce DA oxidation in the CNS and may similarly affect the amine oxidation in the peripheral tissues. The CNS and peripheral effects differ, however, in respect to dose-sensitivity and time course. The similarities between the CNS and peripheral effects suggest that a blunted rise of urinary DOPAC/DA ratio after reserpine challenge could be utilized as a peripheral marker of MPP⁺ action in the CNS, a marker that is not currently available.     

Parity decreases methamphetamine-induced striatal dopaminergic perturbation

The role of parity upon methamphetamine-induced neurotoxicity of the striatal dopaminergic system was assessed. Female CD-1 mice either remained nulliparous or underwent one or three complete pregnancies and were designated as the 0, 1 or 3 pregnancy groups. The mice were then treated with a neurotoxic regimen of methamphetamine (MA - 40 mg/kg) or its saline vehicle (control) and striatal dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) levels were measured at 7-days post-MA. Basal levels of striatal DA, DOPAC and the DOPAC/DA ratio were similar among the saline (control) 0, 1 and 3 pregnancy groups. In response to MA, striatal DA and DOPAC were significantly decreased in the 0 and 1 pregnancy as compared with the control group. Mice with 3 pregnancies showed DA and DOPAC levels that did not differ from controls and were significantly greater than the 0 pregnancy group. The DOPAC/DA ratios of the 0 pregnancy group were significantly greater than all other groups (control, 1 and 3 pregnancy) which failed to differ among each other. These results demonstrate that parity decreases MA-induced striatal dopaminergic neurotoxicity, and the degree of this neuroprotection is related to the number of pregnancies experienced.     

Methamphetamine-induced neurotoxicity and microglial activation are not mediated by fractalkine receptor signaling

Methamphetamine (METH) damages dopamine (DA) nerve endings by a process that has been linked to microglial activation but the signaling pathways that mediate this response have not yet been delineated. Cardona et al. [Nat. Neurosci. 9 (2006), 917] recently identified the microglial-specific fractalkine receptor (CX3CR1) as an important mediator of MPTP-induced neurodegeneration of DA neurons. Because the CNS damage caused by METH and MPTP is highly selective for the DA neuronal system in mouse models of neurotoxicity, we hypothesized that the CX3CR1 plays a role in METH-induced neurotoxicity and microglial activation. Mice in which the CX3CR1 gene has been deleted and replaced with a cDNA encoding enhanced green fluorescent protein (eGFP) were treated with METH and examined for striatal neurotoxicity. METH depleted DA, caused microglial activation, and increased body temperature in CX3CR1 knockout mice to the same extent and over the same time course seen in wild-type controls. The effects of METH in CX3CR1 knockout mice were not gender-dependent and did not extend beyond the striatum. Striatal microglia expressing eGFP constitutively show morphological changes after METH that are characteristic of activation. This response was restricted to the striatum and contrasted sharply with unresponsive eGFP-microglia in surrounding brain areas that are not damaged by METH. We conclude from these studies that CX3CR1 signaling does not modulate METH neurotoxicity or microglial activation. Furthermore, it appears that striatal-resident microglia respond to METH with an activation cascade and then return to a surveying state without undergoing apoptosis or migration.     

In vitro and in vivo evidences that antioxidant action contributes to the neuroprotective effects of the neuronal nitric oxide synthase and monoamine oxidase-B inhibitor, 7-nitroindazole

The neuronal nitric oxide synthase (nNOS) inhibitor, 7-nitroindazole (7-NI) is neuroprotective against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced parkinsonism. Monoamine oxidase (MAO)-B inhibitory action partially contributes to this effect. We tested the hypothesis that 7-NI could be a powerful hydroxyl radical (OH) scavenger, and interferes with oxidative stress caused by MPTP. We measured OH, reduced glutathione (GSH), as well as superoxide dismutase (SOD) and catalase activities in the nucleus caudatus putamen and substantia nigra of Balb/c mice following MPTP and/or 7-NI administration. The nNOS inhibitor caused dose-dependent inhibition in the production of OH in (i) Fenton-like reaction employing ferrous citrate in a cell-free system in test tubes, (ii) in isolated mitochondrial preparation in presence of MPP+, and (iii) in the striatum of mice systemically treated with MPTP. An MPTP-induced depletion of GSH in both the nuclei was blocked by 7-NI, which was dose-dependent (10-50mg/kg), but independent of MAO-B inhibition. The nNOS-mediated recovery of GSH paralleled attenuation of MPTP-induced depletion of striatal dopamine. MPTP-induced increase in the activities of striatal or nigral SOD and catalase were significantly attenuated by 7-NI treatment. These results suggest potent antioxidant action of 7-NI in its neuroprotective effects against MPTP-induced neurotoxicity.     

Immunity of Fetal Mice to Prenatal Administration of the Dopaminergic Neurotoxin 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine

Abstract: Subcutaneous injection of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) HC1 (25 mg/kg) in pregnant female mice at the 17th day of gestation markedly depleted striatal dopamine (DA) concentrations in the mothers 24 h later and at 24 h and 28 days after delivery. By contrast, in the offspring of the female mice exposed to MPTP during pregnancy, fetal brain DA concentrations at 24 h after injection and at 24 h after birth and striatal DA levels at 14 and 28 days postnatally were unaffected and identical to those in age-matched controls. The postnatal ontogenesis of striatal DA levels was identical in offspring of control vehicle- and MPTP-treated pregnant mice. Also, prenatal challenge with MPTP did not make nigrostriatal DA neurons more vulnerable to a second postnatal treatment with the toxin. Striatal DA depletions were identical in 6-week-old mice given MPTP, whether they were exposed to MPTP or to vehicle in utero. Monoamine oxidase (EC 1.4.3.4; MAO) type B activity was extremely low in the fetal brain and, relatively, much lower than that of MAO-A. Prenatal MPTP administration reduced maternal striatal and also embryonal brain MAO-B activity at 24 h post treatment but did not alter the normal postnatal development of striatal MAO-A and -B activities in the offspring. Study suggests that resistance of fetal DA neurons to the DA-depleting effect of MPTP may be due, at least in part, to an absence in the embryonal brain of adequately developed MAO-B activity required for the conversion of MPTP to its toxic metabolite, 1-methyl-4-phenylpyridinium ion.     

Dopamine turnover is upregulated in the caudate/putamen of asymptomatic MPTP-treated rhesus monkeys

In Parkinson's disease (PD) and experimental parkinsonism, losses of up to 60% and 80%, respectively, of dopaminergic neurons in substantia nigra, and dopamine (DA) in striatum remain asymptomatic. Several mechanisms have been suggested for this functional compensation, the DA-mediated being the most established one. Since this mechanism was recently challenged by striatal DA analysis in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys, we present data on several DAergic parameters in three groups of rhesus monkeys: MPTP-treated asymptomatic animals; symptomatic MPTP-treated animals with stable parkinsonism; and untreated sex and age matched controls. We determined ratios of striatal and nigral 3,4-dihydroxyphenyl acetic acid (DOPAC) to DA levels and tyrosine hydroxylase (TH) enzyme activity to DA levels, in addition to the commonly used homovanillic acid (HVA)/DA ratios which, as such, might be less reliable under the conditions of partial denervation. We found that in the asymptomatic MPTP monkeys the DOPAC/DA ratios in putamen and caudate nucleus were shifted with high statistical significance 1.9-5.8-fold, as compared to controls, the shifting of the ratios being in the same range as the 2.6-5.4-fold shifts in the symptomatic animals. Also TH/DA ratios were significantly increased in both, the asymptomatic and the symptomatic MPTP-treated monkeys, with shifts in the putamen and caudate nucleus of 3- and 2.7-7.0-fold, respectively. In the substantia nigra, DOPAC levels and TH activity were strongly decreased after MPTP (-77 to -97%), but the ratios DOPAC/DA and TH/DA were not changed in this brain region. Collectively, our findings support the concept of DAergic compensation of the progressive striatal DA loss in the presymptomatic stages of the parkinsonian disease process.