The mechanism of action of MPTP and MPP+ on endogenous dopamine release from the rat corpus striatum superfused in vitro

Administration of tetrahydrobiopterin restored the decline of dopamine in the striatum induced by an acute action of MPTP

Parkinson's disease (PD) is the second common neurodegenerative disorder. Deficit of the nigro-striatal dopaminergic neurons causes the motor symptoms of PD. While the oxidative stress is thought to be deeply involved in the etiology of PD, molecular targets for the oxidative insults has not been fully elucidated. 6R-5,6,7,8-Tetrahydrobiopterin (BH4) is a cofactor for tyrosine hydroxylase (TH), the rate-limiting enzyme for production of dopamine, and easily oxidized to its dihydro-form. In this study, we examined the alteration in the metabolism of BH4 caused by a parkinsonian neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). MPTP reduced the dopamine content and the in vivo activity of TH in the striatum prior to degeneration of the dopaminergic neurons. We found that administration of BH4 could restore the dopamine content and in vivo TH activity in the striatum of MPTP-treated mice. Unexpectedly, when BH4 was administered with MPTP, BH4 contents in the brain were far higher than those injected without MPTP even at 23 h after the last injection. Because MPTP has been shown to increase ROS production in the dopaminergic neurons, we assumed that the increased ROS oxidizes BH4 into its dihydro-form, excreted from the dopaminergic neurons, taken-up by the neighboring cells, reduced back to BH4, and then accumulated in the brain. We also investigated the action of MPTP in mice lacking quinonoid-dihydropteridine reductase (Qdpr), an enzyme catalyzing regeneration of BH4 from quinonoid dihydrobiopterin. The dopamine depletion induced by MPTP was severer in Qdpr-deficient mice than in wild-type mice. The present data suggest that perturbation of the BH4 metabolism would be the cause of early and persistent dopamine depletion in the striatum.

MPP+-Lesioned Mice: an Experimental Model of Motor, Emotional, Memory/Learning, and Striatal Neurochemical Dysfunctions

The neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) induces motor and nonmotor dysfunctions resembling Parkinson's disease (PD); however, studies investigating the effects of 1-methyl-4-phenylpyridinium (MPP⁺), an active oxidative product of MPTP, are scarce. This study investigated the behavioral and striatal neurochemical changes (related to oxidative damage, glial markers, and neurotrophic factors) 24 h after intracerebroventricular administration of MPP⁺ (1.8-18 μg/mouse) in C57BL6 mice. MPP⁺ administration at high dose (18 μg/mouse) altered motor parameters, since it increased the latency to leave the first quadrant and reduced crossing, rearing, and grooming responses in the open-field test and decreased rotarod latency time. MPP⁺ administration at low dose (1.8 μg/mouse) caused specific nonmotor dysfunctions as it produced a depressive-like effect in the forced swim test and tail suspension test, loss of motivational and self-care behavior in the splash test, anxiety-like effect in the elevated plus maze test, and short-term memory deficit in the step-down inhibitory avoidance task, without altering ambulation. MPP⁺ at doses of 1.8-18 μg/mouse increased tyrosine hydroxylase (TH) immunocontent and at 18 μg/mouse increased α-synuclein and decreased parkin immunocontent. The astrocytic calcium-binding protein S100B and glial fibrillary acidic protein (GFAP)/S100B ratio was decreased following MPP⁺ administration (18 μg/mouse). At this highest dose, MPP⁺ increased the ionized calcium-binding adapter molecule 1 (Iba-1) immunocontent, suggesting microglial activation. Also, MPP⁺ at a dose of 18 μg/mouse increased thiobarbituric acid reactive substances (TBARS) and glutathione (GSH) levels and increased glutathione peroxidase (GPx) and hemeoxygenase-1 (HO-1) immunocontent, suggesting a significant role for oxidative stress in the MPP⁺-induced striatal damage. MPP⁺ (18 μg/mouse) also increased striatal fibroblast growth factor 2 (FGF-2) and brain-derived neurotrophic factor (BDNF) levels. Moreover, MPP⁺ decreased tropomyosin receptor kinase B (TrkB) immunocontent. Finally, MPP⁺ (1.8-18 μg/mouse) increased serum corticosterone levels and did not alter acetylcholinesterase (AChE) activity in the striatum but increased it in cerebral cortex and hippocampus. Collectively, these results indicate that MPP⁺ administration at low doses may be used as a model of emotional and memory/learning behavioral deficit related to PD and that MPP⁺ administration at high dose could be useful for analysis of striatal dysfunctions associated with motor deficits in PD.

1-Methyl-4-phenylpyridinium-induced cell death via autophagy through a Bcl-2/Beclin 1 complex-dependent pathway

Several lines of evidence suggest that the mechanism underlying drug-induced neuronal apoptosis is initiated by the increased production of reactive oxygen species (ROS). 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a neurotoxin, has been shown to initiate an apoptotic cascade by increasing ROS in the dopaminergic neurons of the substantia nigra, leading to the morphological and physiological features associated with Parkinson's disease. Recently, it has been reported that autophagy, a type of programmed cell death independent of the apoptotic cascade, also plays a role in neuronal damage. Although autophagy is negatively regulated by the mammalian target of rapamycin receptor (mTOR), there is some evidence showing a novel function for the anti-apoptotic protein Bcl-2. Bcl-2 is proposed to play a role in negatively regulating autophagy by blocking an essential protein in the signaling pathway, Beclin 1. Nevertheless, it is unclear whether autophagy is also correlated with apoptotic signaling in 1-methyl-4-phenylpyridinium (MPP(+)) toxicity. Therefore, we hypothesized that the MPP(+) toxicity generally associated with initiating the apoptotic signaling cascade also increases an autophagic phenotype in neuronal cells. Using the SK-N-SH dopaminergic cell lines, we demonstrate that MPP(+) increases the expression of microtubule-associated protein light chain 3 (LC3-II), an autophagosome membrane marker and the mTOR signaling pathway, and Beclin 1 while decreasing the Bcl-2 levels. Moreover, these expressions correlate with a decreased binding ratio between Bcl-2 and Beclin 1, in effect limiting the regulation of the downstream autophagic markers, such as LC3-II. Our results indicate that MPP(+) can induce autophagy in SK-N-SH cells by decreasing the Bcl-2/Beclin 1 complex.

Long-lasting transcriptional refractoriness triggered by a single exposure to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyrimidine

Parkinson's disease (PD) is a progressive neurodegenerative disorder whose etiology is thought to have environmental (toxin) and genetic contributions. The neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyrimidine (MPTP) induces pathological features of PD including loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) and striatal dopamine (DA) depletion. We previously described the striatal transcriptional response following acute MPTP administration in MPTP-sensitive C57BL/6J mice. We identified three distinct phases: early (5h), intermediate (24h) and late (72h) and reported that the intermediate and late responses were absent in MPTP-resistant Swiss-Webster (SWR) mice. Here we show that C57BL/6J mice pre-treated with a single 40 mg/kg dose of MPTP and treated 9 days later with 4×20 mg/kg MPTP, display a striatal transcriptional response similar to that of MPTP-resistant SWR mice, i.e. a robust acute response but no intermediate or late response. Transcriptional refractoriness is dependent upon the dose of the priming challenge with as little as 10mg/kg MPTP being effective and can persist for more than 28 days. Priming of SWR mice has no effect on their response to subsequent challenge with MPTP. We also report that paraquat, another free radical producer, also elicits striatal transcriptional alterations but these are largely distinct from those triggered by MPTP. Paraquat-induced changes are also refractory to priming with paraquat. However neither paraquat nor MPTP elicits cross-attenuation. Thus exposure to specific toxins triggers distinct transcriptional responses in striatum that are influenced by prior exposure to the same toxin. The prolonged refractory period described here for MPTP could explain at the molecular level the reported discrepancies between different MPTP administration regimens and may have implications for our understanding of the relationship between environmental toxin exposure and PD.

WldS but not Nmnat1 protects dopaminergic neurites from MPP+ neurotoxicity

BACKGROUND

The WldS mouse mutant ("Wallerian degeneration-slow") delays axonal degeneration in a variety of disorders including in vivo models of Parkinson's disease. The mechanisms underlying WldS -mediated axonal protection are unclear, although many studies have attributed WldS neuroprotection to the NAD+-synthesizing Nmnat1 portion of the fusion protein. Here, we used dissociated dopaminergic cultures to test the hypothesis that catalytically active Nmnat1 protects dopaminergic neurons from toxin-mediated axonal injury.

RESULTS

Using mutant mice and lentiviral transduction of dopaminergic neurons, the present findings demonstrate that WldS but not Nmnat1, Nmnat3, or cytoplasmically-targeted Nmnat1 protects dopamine axons from the parkinsonian mimetic N-methyl-4-phenylpyridinium (MPP+). Moreover, NAD+ synthesis is not required since enzymatically-inactive WldS still protects. In addition, NAD+ by itself is axonally protective and together with WldS is additive in the MPP+ model.

CONCLUSIONS

Our data suggest that NAD+ and WldS act through separate and possibly parallel mechanisms to protect dopamine axons. As MPP+ is thought to impair mitochondrial function, these results suggest that WldS might be involved in preserving mitochondrial health or maintaining cellular metabolism.

The biochemical and cellular basis for nutraceutical strategies to attenuate neurodegeneration in Parkinson's disease

Future therapeutic intervention that could effectively decelerate the rate of degeneration within the substantia nigra pars compacta (SNc) could add years of mobility and reduce morbidity associated with Parkinson’s disease (PD). Neurodegenerative decline associated with PD is distinguished by extensive damage to SNc dopaminergic (DAergic) neurons and decay of the striatal tract. While genetic mutations or environmental toxins can precipitate pathology, progressive degenerative succession involves a gradual decline in DA neurotransmission/synaptic uptake, impaired oxidative glucose consumption, a rise in striatal lactate and chronic inflammation. Nutraceuticals play a fundamental role in energy metabolism and signaling transduction pathways that control neurotransmission and inflammation. However, the use of nutritional supplements to slow the progression of PD has met with considerable challenge and has thus far proven unsuccessful. This review re-examines precipitating factors and insults involved in PD and how nutraceuticals can affect each of these biological targets. Discussed are disease dynamics (Sections 1 and 2) and natural substances, vitamins and minerals that could impact disease processes (Section 3). Topics include nutritional influences on α-synuclein aggregation, ubiquitin proteasome function, mTOR signaling/lysosomal-autophagy, energy failure, faulty catecholamine trafficking, DA oxidation, synthesis of toxic DA-quinones, o-semiquinones, benzothiazolines, hyperhomocyseinemia, methylation, inflammation and irreversible oxidation of neuromelanin. In summary, it is clear that future research will be required to consider the multi-faceted nature of this disease and re-examine how and why the use of nutritional multi-vitamin-mineral and plant-based combinations could be used to slow the progression of PD, if possible.

Mechanism of 1-methyl-4-phenylpyridinium-induced dopamine release from PC12 cells

The molecular mechanism of 1-methyl-4-phenylpyridinium (MPP+), a Parkinsonism-inducing neurotoxin, has been studied in PC12 cells. The cells treated with MPP+ (100 microM) induced a rapid increase in phosphorylation of tyrosine residues of several proteins, including synaptophysin, a major 38 kDa synaptic vesicle protein implicated in exocytosis. An accelerated release of dopamine by MPP+ correlated with phosphorylation of synaptophysin. Exposing the cells to MPP+ triggered reactive oxygen species (ROS) generation within 60 min of treatment and the said effect was blocked by mazindol, a dopamine uptake blocker. In addition, pretreatment with 50-100 microM of selegiline, a selective MAO-B inhibitor, significantly suppressed MPP+-mediated ROS generation. These effects of MPP+ result in the generation of ROS, which may be involved in neuronal degeneration seen in Parkinson's disease.

1-Methyl-4-phenylpyridinium-induced down-regulation of dopamine transporter function correlates with a reduction in dopamine transporter cell surface expression

The mechanisms whereby 1-methyl-4-phenylpyridinium (MPP(+)) mediates cell death and Parkinsonism are still unclear. We have shown that dopamine transporter (DAT) is required for MPP(+)-mediated cytotoxicity in HEK-293 cells stably transfected with human DAT. Furthermore, MPP(+) produced a concentration- and time-dependent reduction in the uptake of [3H]dopamine. We observed a significant decrease in [3H]WIN 35428 binding in the intact cells with MPP(+). The saturation analysis of the [3H]WIN 35428 binding obtained from total membrane fractions revealed a decrease in the transporter density (B(max)) with an increase in the dissociation equilibrium constant (K(d)) after MPP(+) treatment. Furthermore, biotinylation assays confirmed that MPP(+) reduced both plasma membrane and intracellular DAT immunoreactivity. Taken together, these findings suggest that the reduction in cell surface DAT protein expression in response to MPP(+) may be a contributory factor in the down-regulation of DAT function while enhanced lysosomal degradation of DAT may signal events leading to cellular toxicity.

Manganese mimics the action of 1-methyl-4-phenylpyridinium ion, a dopaminergic neurotoxin, in rat striatal tissue slices

Manganese and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) are known to induce neurological pathologies similar to that of parkinsonism. Previous studies performed in rat striatal slices have shown that MPTP and related compounds inhibit tyrosine hydroxylation, a rate-limiting step of dopamine biosynthesis. Here, we reported that manganese inhibited tyrosine hydroxylation in rat striatal slices. In addition, manganese caused increase in the levels of lactate indicating that aerobic glycolysis was inhibited in striatal slices. This inhibition was unique to manganese since other divalent cations, such as magnesium and zinc, did not increase lactate concentrations. These results suggest that the mechanisms by which manganese produces dysfunction of the nervous system are similar to those of MPTP.

Methyl-4-phenylpyridinium (MPP(+))-evoked dopamine release from rat striatal slices: possible roles of voltage-dependent calcium channels and reverse dopamine transport

We examined the properties of voltage-dependent Ca(2+) channels (VDCCs) mediating 1-methyl-4-phenylpyridinium (MPP(+))-evoked [3H]DA release from rat striatal slices. In some cases, the Ca(2+)-independent efflux of neurotransmitters is mediated by the high-affinity neurotransmitter-uptake systems. To determine whether such a mechanism might be involved in MPP(+)-evoked [3H]DA release. MPP(+) (1,10 and 100 microM) evoked the release of [3H]DA from rat striatal slices in a concentration-dependent manner. In the absence of Ca(2+), MPP(+) (10 and 100 microM)-evoked [3H]DA release was significantly decreased to approximately 50% of control (a physiological concentration of Ca(2+)). In the presence of Ca(2+), nomifensine (0.1,1 and 10 microM) dose-dependently and significantly inhibited the MPP(+)-evoked release of [3H]DA. Nomifensine (1 and 10 microM) also dose-dependently and significantly inhibited the MPP(+)-evoked release of [3H]DA under Ca(2+)-free conditions. MPP(+)-evoked [3H]DA release was partly inhibited by nicardipine (1 and 10 microM), an L-type Ca(2+) channel blocker. On the other hand, the N-type Ca(2+) channel blocker omega-conotoxin-GVIA (omega-CTx-GVIA) (1 and 3 microM) did not affect this release. omega-agatoxin-IVA (omega-Aga-IVA) at low concentrations (0.1 microM), which are sufficient to block P-type Ca(2+) channels alone, also had no effect. On the other hand, MPP(+)-evoked [3H]DA release was significantly decreased by high concentrations of omega-Aga-IVA (0.3 microM) that would inhibit Q-type Ca(2+) channels. In addition, application of the Q-type Ca(2+) channel blocker omega-conotoxin-MVIIC (omega-CTx-MVIIC) (0.3 and 1 microM) also significantly inhibited MPP(+)-evoked [3H]DA release. These results suggest that MPP(+)-evoked [3H]DA release from rat striatal slices is largely mediated by Q-type Ca(2+) channels, and the Ca(2+)-independent component is mediated by reversal of the DA transport system.

Glyceraldehyde-3-phosphate dehydrogenase in neurodegeneration and apoptosis signaling

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a well-studied glycolytic enzyme that plays a key role in energy metabolism. GAPDH catalyzes the conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate in the glycolytic pathway. As part of the conversion, GAPDH converts NAD+ to the high-energy electron carrier NADH. GAPDH has been referred to as a "housekeeping" protein and based on the view that GAPDH gene expression remains constant under changing cellular conditions, the levels of GAPDH mRNA have frequently been used to normalize northern blots. In recent years, that view has changed since GAPDH is now known to contribute to a number of diverse cellular functions unrelated to glycolysis. Normative functions of GAPDH now include nuclear RNA export, DNA replication, DNA repair, exocytotic membrane fusion, cytoskeletal organization and phosphotransferase activity. Pathologically, GAPDH has been implicated in apoptosis, neurodegenerative disease, prostate cancer and viral pathogenesis (see Sirover (1999) for a recent review of GAPDH functions). Most recently, it has been shown that GAPDH is a target for deprenyl related compounds (Carlile et al., 2000; Kragten et al., 1998) and may contribute to the neuroprotection offered by those compounds.

A new in vitro approach for investigating the MPTP effect on DA uptake

Previous studies have shown that dopamine (DA) uptake was decreased after preincubation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) or 1-methyl-4-phenylpyridinium (MPP(+)) in in vitro slice and synaptosome models. The present study, conducted with and without preincubation, attempted to determine whether inhibition results from a direct effect of neurotoxins on neuronal DA transporter or from an alteration of the transporter secondary to other toxic events. DA uptake was inhibited about 50% in the presence of MPTP+O(2) or MPP(+) (0.1, 1 and 5 mM) in rat striatal slices and synaptosomes. Such inhibition was obtained in synaptosomes preincubated for 150 min with MPP(+) and then washed. Inhibition of DA uptake was lower in slices preincubated with MPTP (5 mM)+O(2) and then washed (30%). Experiments in synaptosomes prepared from slices preincubated with MPTP or MPP(+) showed greater inhibition of DA uptake with MPTP. The results suggest that the inhibition of DA uptake in vitro by MPTP or MPP(+) results initially from a direct effect on the transporter during its penetration in nerve endings and subsequently from a transporter alteration related to toxic events. Thus, the preincubation of striatal slices followed by DA uptake measurement in synaptosomes would appear to be a good in vitro model for studying the dopaminergic toxicity of MPTP.

Role of mitochondrial dysfunction and dopamine-dependent oxidative stress in amphetamine-induced toxicity

To define the molecular mechanisms underlying amphetamine (AMPH) neurotoxicity, primary cultures of dopaminergic neurons were examined for drug-induced changes in dopamine (DA) distribution, oxidative stress, protein damage, and cell death. As in earlier studies, AMPH rapidly redistributed vesicular DA to the cytoplasm, where it underwent outward transport through the DA transporter. DA was concurrently oxidized to produce a threefold increase in free radicals, as measured by the redox-sensitive dye dihydroethidium. Intracellular DA depletion using the DA synthesis inhibitor alpha-methyl-p-tyrosine or the vesicular monoamine transport blocker reserpine prevented drug-induced free radical formation. Despite these AMPH-induced changes, neither protein oxidation nor cell death was observed until 1 and 4 days, respectively. AMPH also induced an early burst of free radicals in a CNS-derived dopaminergic cell line. However, AMPH-mediated attenuation of ATP production and mitochondrial function was not observed in these cells until 48 to 72 hours. Thus, neither metabolic dysfunction nor loss of viability was a direct consequence of AMPH neurotoxicity. In contrast, when primary cultures of dopaminergic neurons were exposed to AMPH in the presence of subtoxic doses of the mitochondrial complex I inhibitor rotenone, cell death was dramatically increased, mimicking the effects of a known parkinsonism-inducing toxin. Thus, metabolic stress may predispose dopaminergic neurons to injury by free radical-promoting insults such as AMPH.

The parkinsonism-inducing drug 1-methyl-4-phenylpyridinium triggers intracellular dopamine oxidation. A novel mechanism of toxicity

Uptake of the Parkinsonism-inducing toxin, 1-methyl-4-phenylpyridinium (MPP(+)), into dopaminergic terminals is thought to block Complex I activity leading to ATP loss and overproduction of reactive oxygen species (ROS). The present study indicates that MPP(+)-induced ROS formation is not mitochondrial in origin but results from intracellular dopamine (DA) oxidation. Although a mean lethal dose of MPP(+) led to ROS production in identified dopaminergic neurons, toxic doses of the Complex I inhibitor rotenone did not. Concurrent with ROS formation, MPP(+) redistributed vesicular DA to the cytoplasm prior to its extrusion from the cell by reverse transport via the DA transporter. MPP(+)-induced DA redistribution was also associated with cell death. Depleting cells of newly synthesized and/or stored DA significantly attenuated both superoxide production and cell death, whereas enhancing intracellular DA content exacerbated dopaminergic sensitivity to MPP(+). Lastly, depleting cells of DA in the presence of succinate completely abolished MPP(+)-induced cell death. Thus, MPP(+) neurotoxicity is a multi-component process involving both mitochondrial dysfunction and ROS generated by vesicular DA displacement. These results suggest that in the presence of a Complex I defect, misregulation of DA storage could lead to the loss of nigrostriatal neurons in Parkinson's disease.

Neuroprotective role of estrogen upon methamphetamine and related neurotoxins within the nigrostriatal dopaminergic system

In this report we describe some of the data on the capacity for estrogen to function as a neuroprotectant of the nigrostriatal dopaminergic (NSDA) system. The data show that estrogen (E) can alter two different response characteristics to NSDA neurotoxins. The first being that striatal DA concentrations of ovariectomized rodents treated with E are consistently greater than non-E-treated animals in response to neurotoxins which produce degeneration of the NSDA system. The second being that E significantly reduces the amount of DA output upon initial exposure to the NSDA neurotoxin, 1-methyl-4-phenylpyridium ion (MPP+). At present, it is not known whether these two response characteristics are related. An intriguing possibility is that the E-dependent changes in initial DA output are related to the resultant neurotoxicity (attenuations in DA concentration reductions). So far our incipient findings do not seem to support this eventuality. However, additional testing on this topic is required. The present data suggest that one of the mechanisms by which E can exert these effects is through inhibition of DAT activity. This conclusion results from data which show that E produces: 1) an inhibition of [3H]DA uptake, 2) a reduction in DA clearance rates, and 3) an effect upon DA recovery that is similar to that observed to the putative DA uptake blocker, nomifensine. The capacity and significance for steroid hormones to modulate neurotransmitter transporters has been recently reviewed.

Estrogen reduces acute striatal dopamine responses in vivo to the neurotoxin MPP+ in female, but not male rats

The effects of in vivo estrogen treatment upon MPP(+)-induced dopamine (DA) release were determined using in vivo microdialysis in female and male rats. Ovariectomized female rats were implanted or not with an estrogen pellet (0.1 mg, 17beta estradiol) and subjected to microdialysis 6 days later. After baseline DA release was determined, 5 mM MPP(+) was infused through the microdialysis probe for one 20-min interval. Perfusion resumed with normal medium for the duration of the experiment. A significant attenuation of MPP(+)-induced DA release was obtained in estrogen-treated females. One week later, striatal DA and dihydroxyphenylacetic acid (DOPAC) concentrations were determined for the lesioned and non-lesioned striata of each animal. MPP(+) infusion significantly decreased striatal DA concentrations, however, there was no effect of estrogen treatment on striatal DA depletion. This experiment was repeated using orchidectomized male rats treated with 0, 0.1, or 5 mg estradiol. In contrast to the females, no differences in MPP(+)-induced DA release were seen among these males, and there was no significant effect of the varying estrogen treatments on striatal DA or DOPAC concentrations. These results demonstrate that in vivo estrogen treatment attenuates MPP(+)-induced striatal DA release in gonadectomized female, but not male, rats.

Glucose protection from MPP+-induced apoptosis depends on mitochondrial membrane potential and ATP synthase

MPP+ inhibits mitochondrial complex I and alpha-ketoglutarate dehydrogenase causing necrosis or apoptosis of catecholaminergic neurons. Low glucose levels or glycolytic blockade has been shown to potentiate MPP+ toxicity. We found that MPP+ caused concentration-dependent apoptosis of neuronally differentiated PC12 cells and that glucose, but not pyruvate, supplementation reduced apoptosis. Oligomycin concentrations sufficient to inhibit ATP synthase blocked the decreased apoptosis afforded by glucose supplementation. Laser-scanning confocal microscope imaging of chloromethyl-tetramethylrosamine methyl ester fluorescence to estimate DeltaPsiM showed that MPP+ and atractyloside reduced DeltaPsiM, while cyclosporin A (CSA) and glucose supplementation reversed decreases in DeltaPsiM caused by MPP+. Oligomycin blocked the effect of glucose supplementation on DeltaPsiM. These findings show that (i) MPP+-induced and atractyloside-induced apoptosis are associated with reduced DeltaPsiM; (ii) CSA maintains DeltaPsiM and reduces MPP+-induced apoptosis; and (iii) glucose supplementation maintains DeltaPsiM, likely by glycolytic ATP-dependent proton pumping at ATP synthase and reduces MPP+-induced apoptosis.

Decreases in mouse brain NAD+ and ATP induced by 1-methyl-4-phenyl-1, 2,3,6-tetrahydropyridine (MPTP): prevention by the poly(ADP-ribose) polymerase inhibitor, benzamide

Inhibitors of poly(ADP-ribose) polymerase (PARP), including benzamide, protect against 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine (MPTP)-induced dopamine neurotoxicity in vivo [Cosi et al., Brain Res. 729 (1996) 264-269]. In vitro, the activation of PARP by free radical damaged DNA has been shown to be correlated with rapid decreases in the cellular levels of its substrate nicotinamide adenine dinucleotide (NAD+), and ATP. Here, we investigated in vivo whether MPTP acutely caused region- and time-dependent changes in brain levels of NAD+, ATP, ADP and AMP in C57BL/6N mice killed by head-focused microwave irradiation, and whether such effects were modified by treatments with neuroprotective doses of benzamide. At 1 h after MPTP injections (4x20 mg/kg i.p.), NAD+ was reduced by 11-13% in the striatum and ventral midbrain, but not in the frontal cortex. The ATP/ADP ratio was reduced by 10% and 32% in the striatum and cortex, respectively, but was unchanged in the midbrain. All of these regional changes were prevented by co-treatment with benzamide (2x160 mg/kg i.p.), which by itself did not alter regional levels of NAD+, ATP, ADP or AMP in control mice. In a time-course study, a single dose of MPTP (30 mg/kg i.p.) resulted in maximal and transient increases in striatal levels of MPP+ and 3-methoxytyramine (+540%) at 0.5-2 h, followed by maximal and coincidental decreases in NAD+ (-10%), ATP (-11%) and dopamine content (-39%) at 3 h. Benzamide (1x640 mg/kg i. p., 30 min before MPTP) partially reduced MPP+ levels by 30% with little or no effect on MPTP or MPDP+ levels, did not affect or even slightly potentiated the increase in 3-methoxytyramine, and completely prevented the losses in striatal NAD+, ATP and dopamine content, without by itself causing any changes in these latter parameters in control mice. These results (1) confirm that MPTP reduces striatal ATP levels [Chan et al., J. Neurochem. 57 (1991) 348-351.]; (2) show that MPTP causes a regionally-dependent (striatal and midbrain) loss of NAD+; (3) indicate that the PARP inhibitor benzamide can prevent these losses without interfering with MPTP-induced striatal dopamine release; and (4) provide further evidence to suggest an involvement of PARP in MPTP-induced neurotoxicity in vivo.

Estrogen as a neuromodulator of MPTP-induced neurotoxicity: effects upon striatal dopamine release

The effects of estrogen upon MPTP-induced neurotoxicity were examined using in vitro superfusion. In Experiment 1, striatal tissue from ovariectomized rats was infused with MPP+ (10 microM), a combination of MPP+ and 17beta-estradiol (300 nM), the same dose of estradiol preceding MPP+, or no treatment infusion. The effects of these treatments on dopamine release rates during the infusion periods were determined. Infusion of MPP+ resulted in a significant increase in dopamine release as compared to the control. Estradiol added to the MPP+ infusion significantly attenuated this dopamine (DA) release, while estradiol treatment preceding the MPP+ had no effect. In Experiment 2, three different doses of estradiol (0.3, 3, or 300 nM) were infused simultaneously with the MPP+. Doses of estradiol below 300 nM did not attenuate the DA release. In Experiment 3, estradiol alone (300 nM) was infused, to determine dopamine release rate effects of the hormone itself. There was no difference between estradiol treated and non-infused control groups. These results demonstrate that the gonadal steroid hormone estradiol can modulate responses of striatal dopamine neurons to MPP+ by altering the immediate increase in dopamine release which occurs in response to this neurotoxin. These modulating effects of estradiol are dose-dependent, and represent a direct effect upon striatal neurons, most likely involving a non-genomic mechanism of action. These results implicate that hormonal modulation of nigrostriatal dopaminergic neurotoxicity may represent an important variable responsible for the sex differences which are reported in Parkinson's disease.

Multiple ionic conductances of the human dopamine transporter: the actions of dopamine and psychostimulants

Electrophysiological and pharmacological studies of a cloned human dopamine transporter (hDAT) were undertaken to investigate the mechanisms of transporter function and the actions of drugs at this target. Using two-electrode voltage-clamp techniques with hDAT-expressing Xenopus laevis oocytes, we show that hDAT can be considered electrogenic by two criteria. (1) Uptake of hDAT substrates gives rise to a pharmacologically appropriate "transport-associated" current. (2) The velocity of DA uptake measured in oocytes clamped at various membrane potentials was voltage-dependent, increasing with hyperpolarization. Concurrent measurement of transport-associated current and substrate flux in individual oocytes revealed that charge movement during substrate translocation was greater than would be expected for a transport mechanism with fixed stoichiometry of 2 Na+ and 1 Cl- per DA+ molecule. In addition to the transport-associated current, hDAT also mediates a constitutive leak current, the voltage and ionic dependencies of which differ markedly from those of the transport-associated current. Ion substitution experiments suggest that alkali cations and protons are carried by the hDAT leak conductance. In contrast to the transport-associated functions, the leak does not require Na+ or Cl-, and DAT ligands readily interact with the transporter even in the absence of these ions. The currents that hDAT mediates provide a functional assay that readily distinguishes the modes of action of amphetamine-like "DA-releasing" drugs from cocaine-like translocation blockers. In addition, the voltage dependence of DA uptake suggests a mechanism through which presynaptic DA autoreceptor activation may accelerate the termination of dopaminergic neurotransmission in vivo.

Alteration of dopamine release by rat caudate putamen tissues superfused with alpha 2-macroglobulin

Monoamine-activated alpha-2-macroglobulin (alpha 2M) has been shown to decrease the dopamine concentrations in rat caudate putamen (CP) in vivo as well as inhibit choline acetyltransferase activities in the culture of basal forebrain neurons. In this study, we further investigated the effects of methylamine-activated alpha 2M (MA-alpha 2M) upon striatal dopaminergic function by determining whether a direct infusion of this glycoprotein will alter dopamine (DA) release in vitro from superfused CP tissue fragments. In experiment 1, an infusion of 2.8 microM MA-alpha 2M produced a statistically significant increase in DA release compared with control superfusions. In experiment 2, varying doses (0, 0.7, 1.4, 2.8, 4.1 microM) of MA-alpha 2M were tested for their capacity to alter DA release. Only the 2.8 microM dose of MA-alpha 2M was effective in producing a significant increase of DA release. In experiment 3, the normal form of alpha 2M (N-alpha 2M) at 2.8 microM was compared with the control superfusions. The infusion of N-alpha 2M produced an increase in DA release which was substantially lower than the DA increase induced by MA-alpha 2M, and not significantly different from that of the control superfusion. These results show that MA-alpha 2M, like some other neurotoxins, can markedly alter CP dopaminergic function as indicated by the acute increase in DA release following infusion of this glycoprotein, and these effects are exerted at a relatively narrow range of doses. Taken together, these data suggest that this glycoprotein, if allowed to accumulate in the central nervous system (CNS), may promote some neurodegenerative changes that can occur in disorders like Parkinson's disease.

Effects of systemic administration of 6-hydroxydopamine, 6-hydroxydopa and 1-methyl-4-phenyl-1,2,3,6-tetrahydroxypyridine (MPTP) on tuberoinfundibular dopaminergic neurons in the rat

Using systemic route of administration, the effects of several neurotoxins on hypothalamic tuberoinfundibular dopaminergic neurons were focused in this study. 6-Hydroxydopamine (6-OHDA, 10 or 100 mg/kg b.wt., i.v. or ip) produced a dose (37 vs. 50%)- and time (41 to 29% from day 4 to day 9)-dependent depletion of hypothalamic median eminence dopamine concentrations, and increases of serum prolactin levels in ovariectomized rats. Other central dopaminergic neurons, however, were not significantly affected. Similar treatments with 6-hydroxydopa (6-OHDOPA) were less effective. On the other hand, treatments of 1-methyl-4-phenyl-1,2,3,6-tetrahydroxypyridine (MPTP, 10 mg/kg b.wt./day, ip) for 7 or 14 days produced significant decreases of dihydroxyphenylacetic acid (DOPAC) levels in the median eminence and periventricular regions, and increases in serum prolactin levels. Other central dopaminergic neurons were not significantly affected, though. These results suggest that systemic administration of 6-OHDA, 6-OHDOPA, or MPTP, can produce specific destructive effects on the tuberoinfundibular dopaminergic neurons.

1-Methyl-4-phenylpyridinium (MPP+) but not 1-methyl-4-phenyl-1,2,3,6- tetrahydropyridine (MPTP) serves as methyl donor for dopamine: a possible mechanism of action

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 1-methyl-4-phenylpyridinium (MPP+), the active product of MPTP, caused Parkinson's disease-like symptoms. The mechanism of action of MPP+ is unknown, but analogues of MPTP lacking an N-methyl group were found to be essentially devoid of toxicity, which means that the methyl group of the pyridine ring plays a role in the toxicity. This is of interest because S-adenosylmethionine (SAM), which is the biologic methyl donor and requires a methyl group for its action, also caused MPP(+)-like motor deficits in rodents. Therefore, the requirement of a methyl group by MPTP and MPP+ for their actions suggests that, like SAM, MPP+ and MPTP may serve as methyl donors. This hypothesis was tested by reacting SAM, MPP+, or MPTP with dopamine in the presence of catechol-O-methyltransferase and measuring the methylated product of dopamine produced. Like SAM, MPP+, but not MPTP, methylated dopamine. The methylated product coeluted from chromatographic columns with standard 3-methoxytyramine. Concentrations of 15.6, 62.5, 250, and 1000 nmoles/tube increased the 3-methoxytyramine recovered above controls by 0.0, 6.88, 44.55, 129.47 and 5.8, 13.9, 50.58, 121.31 nmoles for SAM and MPP+, respectively. The dopamine that remained unreacted was dose-dependently decreased. MPTP had no significant effect. The ability of MPP+ to serve as a methyl donor may represent a mechanism for the toxicity of MPP+.

c-fos mRNA in mouse brain after MPTP treatment

The neurotoxin, MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) induces a transient increase of mRNA for the immediate-early gene c-fos in the mouse brain. The c-fos mRNA level is MPTP dose-dependent and is evident in all brain regions tested including striatum, hypothalamus, cortex, hippocampus, cerebellum and midbrain. There are regional differences in the time-course for the rise of c-fos mRNA. Pretreatment with deprenyl, a selective monoamine oxidase B inhibitor, pargyline, a nonselective monoamine oxidase inhibitor, or mazindol, a dopamine uptake transport inhibitor, does not prevent the c-fos mRNA increase, suggesting that the elevation is due to the action of MPTP and not its neurotoxic metabolite MPP+.

Mechanisms of the inotropic actions of MPTP and MPP+ on isolated atria of rat

Besides having toxic actions, MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) and MPP+ (1-methyl-4-phenyl-pyridinium ion) produce the release of catecholamines in the peripheral and central nervous systems. This paper reports on the effects of MPTP and MPP+ on isolated left atria of rats and their mechanisms. MPP+ and MPTP produced a concentration-dependent positive inotropic effect. This action was blocked by propranolol and nomifensine; however, inhibition of monoamine oxidase had no effect on the response. In atria from reserpinized rats, the positive inotropic effect of MPTP was also blocked and a negative inotropic effect was unmasked which continued to increase in magnitude during wash. The negative inotropic effect was markedly reduced by superoxide dismutase and catalase. These results indicate that MPTP and MPP+ produce a catecholamine-mediated positive inotropic effect that is not MAO-dependent, unlike the toxic actions of MPTP. These results also suggest that MPTP may directly damage cardiac muscle by generating free radicals which might explain why high doses of MPTP are lethal to animals.

The action of MPTP on synaptic transmission is affected by changes in Ca2+ concentrations

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) causes a disruption of nigrostriatal dopaminergic function which resembles parkinsonism. On the cellular level, the disruption involves a Ca2+ independent release of dopamine, depletion of nigral and striatal dopamine, and the death of dopaminergic neurons. We have previously reported that MPTP can cause a non-reversible inhibition of neostriatal synaptic transmission. In this study we investigated the effect of altering Ca2+ concentration on MPTP's actions in the mouse nigrostriatal brain slice. We report finding that the MPTP induced non-reversible decrease in N-2 amplitude did not occur if synaptic transmission had been blocked using a low Ca2(+)-high Mg2+ artificial cerebrospinal fluid (ACSF) during MPTP application. Low Ca2(+)-high Mg2+ ACSF did not however alter the decrease in slice dopamine content caused by MPTP. These data provide initial support for the hypothesis that MPTP's ability to alter functional synaptic transmission is Ca2+ dependent whereas its releasing action on dopamine is Ca2+ independent.

MPTP, MPDP+ and MPP+ cause decreases in dopamine content in mouse brain slices

MPTP causes a Parkinson's disease-like syndrome in which the dopamine content of the nigrostriatal system decreases. We have studied the relationship between physiological changes and dopamine content using a brain slice preparation developed for electrophysiological studies of corticostriate and nigrostriatal synaptic transmission. We report that MPTP, MPDP+ and MPP+ cause significant decreases in dopamine content of mouse brain slices. We also report that compounds (pargyline and GBR-12909) which block MPTP's toxicity in vivo and prevent non-reversible changes in synaptic transmission are not able to alter MPTP's ability to decrease slice dopamine contents. This indicates that the dopamine content in slices may not be causally related to the non-reversible decrease in synaptic transmission or in vivo neurotoxicity.

Yawning behavior in male rats is associated with decreases in in vivo DOPAC efflux from the caudate nucleus

Young adult male rats were implanted with a push-pull cannula aimed at the dorsal and rostral areas of the caudate nucleus. Perfusate samples were collected at two-minute intervals for approximately one hour and assayed for DOPAC concentrations. Simultaneously, yawning, penile erections and grooming behavior were recorded. Yawns were induced by systemic prolactin or apomorphine injections. While mean DOPAC efflux was elevated following prolactin (PRL) and apomorphine decreased mean DOPAC efflux as expected, yawns and penile erections induced by both compounds were associated with rapid momentary decreases in DOPAC efflux in these living animals. Although yawning was associated with significant decreases in DOPAC output, not every momentary DOPAC decrease was associated with a yawn, suggesting that the 'yawning generator' most likely requires additional inputs for the expression of a yawn.

Studies of the in vivo catabolism of exogenous dopamine as infused through a push-pull cannula implanted in the rat caudate nucleus

In this report, using a push-pull perfusion technique, we examined in vivo the effects of exogenous dopamine (DA) on the output of neurochemical substances from the caudate nucleus (CN) of freely behaving rats. Exogenous DA, at concentrations of 10(-5) M, 5 x 10(-5) M and 10(-4) M, in modified Krebs-Ringer phosphate medium (KRP) was infused directly into the CN for 15 min each. Exogenous DA at the doses tested elicited increases in 3,4-dihydroxyphenylacetic acid (DOPAC) output in a dose-dependent manner. In addition, the higher two doses of exogenous DA also induced increases in homovanillic acid (HVA) output from the rat CN. The increases in DOPAC output by 5 x 10(-5) M DA was partially blocked by the inclusion of 10(-3) M nomifensine in KRP. Interestingly, exogenous DA-induced increases in HVA output were little affected by the nomifensine treatment. However, the catabolism of exogenous DA was almost completely eliminated by a prior 6-hydroxydopamine lesion in the ipsilateral substantia nigra. Furthermore, infusions of exogenous DA did not change 5-hydroxyindoleacetic acid output from the CN. In conclusion, our results confirm in vivo that (a) DA catabolic pathway via DOPAC intermediate predominates over the alternative pathway via 3-methoxytyramine, (b) increases in extracellular DA will lead to increases in DOPAC and HVA levels in extracellular space and (c) the majority of the DA is oxidized by intradopaminergic monoamine oxidase.

MPP+-induced increases in extracellular potassium ion activity in rat striatal slices suggest that consequences of MPP+ neurotoxicity are spread beyond dopaminergic terminals

MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) produces symptoms similar to idiopathic Parkinson's disease in primates. A metabolite of MPTP, MPP+ (1-methyl-4-phenylpyridinium), is actively accumulated by dopaminergic (DA) terminals and selectively destroys nigrostriatal DA neurons. The mechanism of this effect remains unknown but reports that MPP+ inhibits electron transport in isolated mitochondria and increases oxidation of cytochrome b in striatal slices suggest that depression of ATP production is involved. To relate metabolic effects of MPP+ with tissue electrophysiology, extracellular potassium ion activity [K+]o was measured by microelectrodes simultaneous to optical monitoring of reduction/oxidation (redox) activity of cytochrome b during superfusion of MPP+ onto rat striatal and hippocampal slices. MPP+ increased oxidation of cytochrome b and increased [K+]o in slices of striatum. These increases were greater than expected from a selective effect of MPP+ on DA terminals which likely comprise no more than 3% of the total striatal mass. These effects of MPP+ were slowed by a dopamine uptake inhibitor (mazindol) and did not occur in hippocampal slices. These findings indicate that MPP+ influences ion transport as well as metabolic activity and that these actions require the presence of functioning DA terminals. However, the large amplitudes of the MPP+-induced changes suggest that consequences of MPP+-neurotoxicity are not ultimately confined to DA terminals. Two hypothesis are proposed: that energy failure in DA terminals results in leakage of neurotoxic substances or metabolites altering membrane conductance properties of adjacent cells and thereby placing additional demand upon ion transport pumps and mitochondrial oxidative phosphorylation; or that there is secondary uptake of MPP+ leading to mitochondrial inhibition in cells neighboring DA terminals.

A potent dopamine-releasing factor is present in high concentrations in the rat adrenal gland

The present study shows that a novel, protease-sensitive factor present in a partially purified preparation from the rat adrenal gland selectively stimulates the release of dopamine from the rat striatal tissue superfused in vitro in a dose-dependent manner. Biological activity is also found to be present in much lower concentrations in the neocortex, striatum and cerebellum but absent in liver or spleen. This putative dopamine releasing factor is probably a glycoprotein, resistent to boiling, partially inactivated by trypsin and completely inactivated by the non-specific protease pronase E.

The effects of 1-methyl-4-phenylpyridinium ion (MPP+) on the efflux and metabolism of endogenous dopamine in rat striatal slices

1-Methyl-4-phenylpyridinium ion (MPP+) was shown to accumulate concentration-dependently in slices from rat striatum. At 10 microM, MPP+, the tissue concentration was found to be 118 +/- 9 microM following 75 min of incubation. The accumulation of MPP+ was reduced in the presence of 10 microM of the selective dopamine uptake inhibitor GBR 12909 (-50%) or by destruction of the dopaminergic terminals by complete hemisection of the forebrain 4 days before the experiments (-75%). Accumulation of MPP+ in the catecholamine-poor occipital cortex and cerebellum was only 25% of that obtained in striatum. Reserpine pretreatment of the rats in-vivo did not modify the accumulation of MPP+ in the striatal slices. MPP+ (1-10 microM) increased the net efflux of dopamine and reduced the efflux of the dopamine metabolite DOPAC from the striatal slices. The effect on dopamine was readily diminished if MPP+, after a 15 min incubation, was then omitted from the medium. In contrast, the DOPAC efflux was reduced for 75 min even though MPP+ was present in the incubation medium only for the first 15 min. In the presence of the monoamine oxidase inhibitor, pargyline (350 microM), MPP+ also produced an increase in dopamine efflux. In normal medium, the presence of the dopamine uptake inhibitor GBR 12909 (10 microM), or the absence of calcium, failed to modify the MPP+-induced increase in dopamine efflux. MPP+ also increased dopamine efflux from slices from reserpinized rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective destruction of cultured dopaminergic neurons from fetal rat mesencephalon by 1-methyl-4-phenylpyridinium: cytochemical and morphological evidence

Dopaminergic neurons in cultures of dissociated cells from fetal rat mesencephalon were exposed to the principal metabolite of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), 1-methyl-4-phenyl-pyridinium ion (MPP+), and several of its structural analogues. At concentrations between 0.01 and 0.1 microM, MPP+ inhibited catecholamine accumulation as visualized by cytofluorescence. Between 0.1 and 10.0 microM, MPP+ resulted in disappearance of tyrosine hydroxylase immunoreactivity without affecting other cells in the cultures. At concentrations higher than 10 microM, MPP+ was toxic to all cells present in the cultures. The effect of low concentrations of MPP+ on catecholamine cytofluorescence of the dopaminergic neurons was partially reversible. The intermediate concentrations produced irreversible structural changes of tyrosine hydroxylase-positive cells, resulting in complete disappearance of these neurons. The morphological changes were specific to the dopaminergic neurons and were not evident in other cells viewed with phase contrast microscopy. Of the structural analogues tested, the 1-ethyl analogue of MPP+ was effective in selectively destroying dopaminergic neurons in our culture system. The antioxidants L-acetyl-carnitine, beta-carotene, and alpha-tocopherol failed to protect against MPP+ neurotoxicity when co-incubated with the toxin.

1-Methyl-4-phenylpyridinium (MPP+) increases oxidation of cytochrome-b in rat striatal slices

Effects of 1-methyl-4-phenylpyridinium, (the active metabolite of the neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), on reduction/oxidation activity of mitochondrial cytochromes were studied in rat striatal slices using scanning spectrophotometry. The objective was to test the hypothesis that the neurotoxin alters electron transport in the mitochondrial respiratory chain. Incubation of rat striatal slices with MPP+ (1 microM) produced a time-dependent oxidation of Cytochrome-b in a manner consistent with the concept of a block in electron transport in the intramitochondrial respiratory chain between nicotinamide adenine dinucleotide (NAD) and Cytochrome-b. This effect of MPP+ was decreased by co-incubation with a potent dopamine uptake inhibitor (mazindol), or when studied in a tissue with low dopaminergic innervation (hippocampus). The amplitude of Cytochrome-b oxidation was greater than that expected from a selective effect of MPP+ on dopaminergic neurons suggesting that neighboring cells are influenced secondary to the MPP+ effect on dopaminergic terminals.

Dopamine-releasing action of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 1-methyl-4-phenylpyridine (MPP+) in the neostriatum of the rat as demonstrated in vivo by the push-pull perfusion technique: dependence on sodium but not calcium ions

This study examined the effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and its metabolite, 1-methyl-4-phenylpyridine (MPP+) on the levels of dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) in push-pull perfusates of the striatum in chloral hydrate-anaesthetized rats. In control animals the levels of DA and DOPAC remained stable for at least 6 h and responded rapidly to a depolarizing stimulus of 25 mM K+. This K+-induced DA release was Ca2+-dependent since no stimulation was observed when the striatal sites were perfused with high K+ in a Ca2+-free medium containing 2 mM EGTA thus verifying that the striatal sites were functionally active. MPTP (0.025 and 0.05 microgram/microliter) stimulated DA release and inhibited DOPAC output in a dose-related manner. MPP+ (0.01, 0.025 and 0.05 microgram/microliter) produced a more robust dose-dependent increase in DA levels in the perfusates; however, the level of suppression of DOPAC was similar to that in response to MPTP. The effect of MPP+ on DA release was attenuated by 10(-6) M benztropine, the DA re-uptake blocker and completely inhibited by 10 micrograms/kg i.p. benztropine and 10(-4) M ouabain, the Na+, K+-ATPase (Na pump) inhibitor. However, although these substances prevented the MPP+-induced release of DA, the levels of DOPAC in the perfusates did not recover and remained completely suppressed suggesting that MPP+ may inhibit extraneuronal rather than intraneuronal monoamine oxidase (MAO). Perfusion of the striatal sites with a Ca2+-free medium containing 2 mM EGTA did not prevent the MPP+-induced DA release indicating that MPP+ does not release DA from the striatal DA terminals by the Ca2+-dependent process of exocytosis. The responses of DA and DOPAC to 25 mM K+ were markedly suppressed in animals treated with MPTP and MPP+, these effects being most severe with the highest dose of MPP+. Moreover, this suppression of the K+-induced responses persisted in animals perfused with MPP+ in the presence of benztropine or ouabain, thus suggesting that MPP+ may have potent deleterious membrane effects. These studies have provided the first direct in vivo demonstration of the action of MPTP and MPP+ and the neuropharmacological basis of this action on DA metabolism in the rat striatum. The results show that the elevated levels of DA in the striatal perfusates are due to a direct action of MPTP and MPP+ on the nigrostriatal DA terminals and cannot be fully accounted for solely by their inhibition of MAO activity and/or inhibition of DA re-uptake.(ABSTRACT TRUNCATED AT 250 WORDS)

Prolactin-induced yawning behavior requires an intact nigro-striatal dopamine system

Herein, we evaluate the importance of the nigro-striatal dopamine system in prolactin-, apomorphine-, and physostigmine-induced yawning behavior. Bilateral 6-OH-dopamine lesions of the substantia nigra were performed on male rats (2-4 months old). The lesioned as well as control rats were injected with either physiological saline, physostigmine (200 micrograms/kg), apomorphine (50 micrograms/kg), or ovine prolactin (0.25 micrograms/kg) 72 hours after the surgical procedure. The results show that bilateral lesions of the substantia nigra did not affect physostigmine-induced yawning whereas both apomorphine- and prolactin-induced yawning were reduced by the lesion. Following the observation period the caudate nuclei were removed and analyzed for dopamine (DA) and dihydroxyphenylacetic acid (DOPAC) content. The lesions reduced DA and DOPAC content in all treatment groups compared to the respective intact groups. Also, both DA and DOPAC concentrations were lower in the intact apomorphine and prolactin treated groups compared to intact saline controls, at times that were temporally related to the display of yawning behavior suggesting a decrease in dopamine activity following apomorphine and prolactin treatment. Interestingly, DA and DOPAC concentrations were higher in the lesioned apomorphine group compared to lesioned saline controls; however, in the lesioned prolactin group only the DA concentrations were higher when compared to lesioned saline controls. These results indicate that prolactin- and apomorphine-induced yawning require an intact nigro-striatal dopamine system and that these substances induce yawning by different mechanisms.

Effects on dopamine metabolism of MPTP and MPP+ infused through a push-pull cannula into the caudate nucleus of awake adult male rats

Our previous data demonstrated that both 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 1-methyl-4-phenylpyridinium ion (MPP+) exerted potent inhibition on endogenous 3,4-dihydroxyphenylacetic acid (DOPAC) output and potent stimulation on endogenous dopamine (DA) release from the rat corpus striatum superfused in vitro. In this report, using a push-pull perfusion technique, we examined in vivo the acute effects of MPTP and MPP+ on DA metabolism in the rat caudate nucleus (CN). MPTP or MPP+ in modified Krebs-Ringer phosphate buffer at concentrations of 10(-6), 10(-5) and 10(-4) M was administered directly into the CN for 15 min, each 90 min apart. Thirty minutes after the infusion of 10(-6) M MPP+, DOPAC output was reduced to a significantly lower value and subsequent infusions of high concentrations of MPP+ further decreased DOPAC output. Homovanillic acid (HVA) output was also decreased by MPP+ infusions, however, at higher concentrations. In respect to DA release, 1 of 10, 4 of 10 and 7 of 10 animals responded with significant increases to 10(-6), 10(-5) and 10(-4) M MPP+, respectively. On the other hand, MPTP was effective in reducing DOPAC output only at 10(-4) M and ineffective in altering DA and HVA output at all doses tested. In addition, neither drugs had a significant effect on 5-hydroxyindoleacetic acid. Accompanying the dramatic changes in DA metabolism caused by MPP+, two uncommon behavioral syndromes were also observed; tremor-body twist and body shaking.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute effects of 1-methyl-4-phenylpyridinium ion (MPP+) on dopamine and serotonin metabolism in rat striatum as assayed in vivo by a micro-dialysis technique

The acute effect of 1-methyl-4-phenylpyridinium ion (MPP+), a neurotoxin derived from 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), was examined by the in vivo micro-dialysis technique. A dialysis cannula was implanted into rat striatum, and the changes in the concentrations of dopamine (DA), 3, 4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) in the perfusate every 20 min after administration of MPP+ were determined by high-performance liquid chromatography with electrochemical detection (HPLC-ED). After MPP+ administration the levels of DOPAC, HVA and 5-HIAA were markedly decreased. On the contrary the level of DA was markedly increased and reached a maximum 40 min after beginning of the MPP+ administration. By postmortem analysis of the striatal tissue MPP+ was proved to cause the inhibition of monoamine oxidase (MAO), especially MAO-B. These results suggest that the acute biochemical changes induced by MPP+ in vivo were MAO inhibition and release of DA.

Behavioral and biochemical changes following acute administration of MPTP and MPP+

The acute effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 1-methyl-4-phenylpyridinium ion (MPP+) on mouse locomotor activity and striatal dopamine (DA) and 5-hydroxytryptamine (5-HT) levels were investigated. A single dose of either MPTP (10-30 mg/kg, i.p.) or MPP+ (5-20 ug/mouse, i.c.v.) decreased locomotor activity 10-40 min after injection: this locomotor effect was significantly suppressed by either pretreatment with nomifensine or 1-deprenyl alone, or by the combination of desmethylimipramine and 6-hydroxydopamine. Pretreatment with clorgyline did not suppress this behavior and a single dose of haloperidol enhanced the effect. The striatal levels of DA, 3-methoxytyramine and 5-HT increased in parallel with the decrease in locomotor activity caused by MPTP or MPP+. In contrast, levels of 3,4-dihydroxyphenylacetic acid, homovanillic acid and 5-hydroxyindoleacetic acid were decreased by injection of either MPTP or MPP+. Possible mechanism(s) of the behavioral and biochemical changes caused by the acute actions of MPTP and MPP+ with respect to their neurotoxic effects on the nigrostriatal DA system are discussed.