The effect of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on striatal and limbic catecholamine neurones in white and black mice. Antagonism by monoamine oxidase inhibitors

Quantitative assessment of gait and neurochemical correlation in a classical murine model of Parkinson's disease

Background

Gait deficits are important clinical symptoms of Parkinson’s disease (PD). However, existing behavioral tests for the detection of motor impairments in rodents with systemic dopamine depletion only measure akinesia and dyskinesia, and data focusing on gait are scarce. We evaluated gait changes in the methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced C57BL/6 murine model of PD by using a computer-assisted CatWalk system. Correlations of gait parameters with tyrosine hydroxylase (TH) protein levels in the substantia nigra (SN) were also investigated.

Results

The gait readouts, including the walking duration, variation of walking speed, step cycle, duty cycle, stance, initial dual stance, terminal dual stance, three- and four-point supports, and the base of support between hind limbs was noted to increase significantly one week after MPTP injection. In contrast, values of the stride length, cadence, swing speed, and diagonal dual support decreased substantially following MPTP treatment (p < 0.05). All of these changes lasted for three weeks after the last MPTP administration. Except for the stance in the fore limbs and the swing speed in the hind limbs, the gait variability in the PD mice showed a closer correlation with the protein levels of TH in the SN than the walking distances in the conventional open field test. Coordination parameters of the regularity index and step pattern were not affected in mice treated with MPTP.

Conclusion

Data of the study suggest that the computer-assisted CatWalk system can provide reliable and objective criteria to stratify gait changes arising from MPTP-induced bilateral lesions in C57/BL6 mice. The extent of gait changes was noted to correlate with the expression of the biomarker for dopaminergic neurons. This novel analytical method may hold promise in the study of disease progression and new drug screening in a murine PD model.

The effects of piroxicam in the attenuation of MPP+/MPTP toxicity in vitro and in vivo

Several lines of evidence support the neuroprotective action of cyclooxygenase-2 (COX-2) inhibitors in various models of Parkinson's disease (PD). In the current study, we investigated the neuroprotective properties of several COX inhibitors against 1-methyl-4-phenylpyridinium (MPP+) in neuroblastoma Neuro 2A (N-2A) cells in vitro and the protection against degeneration of substantia nigra pars compacta (SNc) dopaminergic (DA) neurons after the administration of 1-methyl 4-phenyl 1,2,3,6-tetrahydropyridine (MPTP) in C57/BL6 male mice. The data obtained demonstrate a lack of protective effects observed by COX 1-2 inhibitors ibuprofen and acetylsalicylic acid against MPP+ toxicity in N-2A, where piroxicam was protective in a dose dependent manner (MPP+ control: 15 +/- 2% MPP+ piroxicam: 5 mM 89 +/- 4%). The data also indicate a drop in mitochondrial oxygen (O(2)) consumption and ATP during MPP+ toxicity with no restoration of mitochondrial function concurrent to a heightened concentration of somatic ATP during piroxicam rescue. These findings indicate that the neuroprotective effects of COX inhibitors against MPP+ are not consistent, but that piroxicam may work through an unique mechanism to propel anaerobic energy metabolism. On the other hand, using mice, piroxicam (20 mg/kg) was effective against MPTP-induced dopaminergic degeneration in the (SNc) and loss of locomotive function in mice. Administering a 3 day pre-treatment of piroxicam (20 mg/kg) was effective in antagonizing the losses in SNc tyrosine hydroxylase protein expression, SNc DA concentration and associated anomaly in ambulatory locomotor activity. It was concluded from these findings that piroxicam is unique among COX inhibitors in providing very significant neuroprotection against MPP+ in vitro and in vivo.

Swim-test as a function of motor impairment in MPTP model of Parkinson's disease: A comparative study in two mouse strains

Parkinson's disease (PD) is a common neurodegenerative disease that exhibits motor dysfunctions, such as tremor, akinesia and rigidity. In the present study, we investigated whether swim-test could be used as one of the behavioural monitoring techniques to study motor disability in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced parkinsonism in two mouse strains, Balb/c and C57BL/6. Mice were treated with different doses of MPTP (10, 20 and 30 mg/kg, twice, 16 h apart), and were subjected to swim-test on the third day of the first MPTP injection. MPTP-induced tremor was monitored at 30 min, and akinesia and rigidity developed were studied 3 h after the second MPTP treatment. While tremor and akinesia produced were dose-dependent and the intensity of tremor was comparable in the two strains of mice studied, the latter response in C57BL/6 was significantly lesser than that observed in Balb/c. Rigidity exhibited in Balb/c mice were dose-dependent, but not in C57BL/6. There was observed an inverse relationship between swim-score and the doses of MPTP in both the strains. MPTP caused a significant and dose-dependent reduction in striatal dopamine level in both the strains of mice, when assayed on the fourth day employing an HPLC with electrochemical detector. A significant positive correlation existed (r = 0.94 for Balb/c and r = 0.82 for C57BL/6) for the striatal dopamine-depletion and the swim-score in the MPTP-treated mice. While swim deficit and striatal dopamine loss were long lasting (till the third week) in C57BL/6, in Balb/c mice the motor deficit showed recovery by the second week. In these animals, a significant attenuation in striatal dopamine loss was observed by the third week. These results indicate that swim ability is directly proportional to striatal dopamine content, and suggest that swim-test could be used as a major technique to monitor motor dysfunction in experimental animals.

Acetyl-L-carnitine cytoprotection against 1-methyl-4-phenylpyridinium toxicity in neuroblastoma cells

Acetyl-L-carnitine (ALCAR) plays an integral role in the transport of long chain fatty acids across the inner mitochondrial membrane for oxidative phosphorylation. In non-human primates, administration of ALCAR was reported to prevent 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced neurological injury to the substantia nigra. The present study investigates the effects of ALCAR against the toxicity of 1-methyl-4-phenylpyridinium (MPP(+)), the neurotoxic metabolite of MPTP, in murine brain neuroblastoma cells. MPP(+), a potent mitochondrial toxin, induced a dose-dependent reduction in mitochondrial oxygen consumption and cell viability, corresponding to an accelerated rate of cellular glucose utilization. Treatment with ALCAR, but not L-carnitine, prevented MPP(+) toxicity and partially restored intracellular ATP concentrations, but did not reverse the MPP(+)-induced loss of mitochondrial oxygen consumption. These data indicate that protective effects are independent of oxidative phosphorylation. ALCAR had a substantial glucose sparing effect in both controls and MPP(+)-treated groups, demonstrating a potential role in enhancing glucose utilization through glycolysis. Antagonizing the entry of fatty acids into the mitochondria, with either insulin or malonyl CoA, did not interfere with ALCAR protection against MPP(+). On the contrary, insulin potentiated the protective effects of ALCAR. In conclusion, these data indicate that ALCAR protects against MPP(+) toxicity, independent of mitochondrial oxidative capacity or beta-oxidation of fatty acids. In contrast, the protective effects of ALCAR appear to involve potentiation of energy derived from glucose through anaerobic glycolysis.

Tyrosinase: a developmentally specific major determinant of peripheral dopamine

L-3,4-dihydroxyphenylalanine, the immediate precursor of dopamine, can be formed by two enzymes: tyrosine hydroxylase (TH) in catecholamine-producing neurons and chromaffin cells and tyrosinase in melanocytes. In this study we examined whether tyrosinase contributes to production of dopamine. Deficiency of TH caused marked reductions in norepinephrine in albino and pigmented 15-day-old mice. In contrast, peripheral levels of dopamine were reduced only in albino TH-deficient mice and were higher in pigmented than in albino mice, regardless of the presence or absence of TH. We next examined age-related changes in dopamine and cutaneous expression of tyrosinase and melanin in albino and pigmented TH wild-type mice. We found that the differences in peripheral dopamine between pigmented and albino mice disappeared with advancing age following changes in expression and function of tyrosinase. In young animals, tyrosinase was present in epidermis but did not produce detectable melanin. With advancing age, tyrosinase was localized only around hair follicles, melanin synthesis became more pronounced, and dopamine synthesis decreased. The data reveal a previously unrecognized TH-independent major pathway of peripheral dopamine synthesis in young, but not adult, mice. The transient nature of this source of dopamine reflects a developmental switch in tyrosinase-dependent production of dopamine to production of melanin.

D-(+)-glucose rescue against 1-methyl-4-phenylpyridinium toxicity through anaerobic glycolysis in neuroblastoma cells

The active neurotoxin of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), 1-methyl-4-phenylpyridinium (MPP+), exerts its lethal effect by inhibiting Complex I of the electron transport chain (ETC). MPP+ shuts down aerobic oxidative phosphorylation and ETC-mediated ATP synthesis. The present investigation examines anaerobic survival during MPP+ toxicity in murine neuroblastoma cells Neuro 2-A (N2-A). MPP+ addition to the cells resulted in a reduction in cell viability, mitochondrial O(2) consumption (MOC) and ATP concentration in a dose-dependent manner. However, the addition of 10 mM of D-(+)-glucose prevented MPP+ toxicity, attenuated the loss of ATP, but did not reverse the complete inhibition of MOC, indicating substrate level phosphorylation and explicit anaerobic survival. Glucose addition prevented MPP+-mediated drop in DeltaPsim, endoplasmic reticulum and intracellular organelle membrane potential tantamount to an increase of cell viability. Secondly, we examined the metabolic regulation of pyruvate dehydrogenase (PDH) and carnitine palmitoyl transferase (CPT) activities during glucose rescue. These enzymes exert control over acetyl CoA reservoirs in the mitochondria during aerobic metabolism. DL-6,8-Thioctic acid (PDH prosthetic group) and insulin slightly augmented metabolic rate, resulting in enhanced vulnerability to MPP+ in a glucose-limited environment. Additional glucose prevented these effects. Amiodarone (CPT inhibitor) and glucagon did not hamper or potentiate glucose rescue against MPP+. These data support strict anaerobic glucose utilization in the presence of toxic levels of MPP+. Moreover, the findings indicate that MPP+ exerts two distinct modes of toxicity (fast and slow death). With MPP+ (<1 mM), anaerobic glycolysis is operational, and toxicity is strictly dependent upon glucose depletion. MPP+ (1-10 mM) initiated acute metabolic collapse, with failure to sustain or switch to anaerobic glycolysis. In conclusion, overcoming energy failure against MPP+ may involve targeting rate-limiting controls over anaerobic energy pathways.

The role of glycolysis and gluconeogenesis in the cytoprotection of neuroblastoma cells against 1-methyl 4-phenylpyridinium ion toxicity

1-Methyl-4-phenylpyridinium (MPP+) is a mitochondrial Complex I inhibitor and is frequently used to investigate the pathological degeneration of neurons associated with Parkinson's disease (PD). In vitro, extracellular concentration of glucose is one of the most critical factors in establishing the vulnerability of neurons to MPP+ toxicity. While glucose is the primary energy fuel for the brain, central nervous system (CNS) neurons can also take up and utilize other metabolic intermediates for energy. In this study, we compared various monosaccharides, disaccharides, nutritive/non-nutritive sugar alcohols, glycolytic and gluconeogenic metabolic intermediates for their cytoprotection against MPP+ in murine brain neuroblastoma cells. Several monosaccharides were effective against MMP+ (500 microM) including glucose, fructose and mannose, which restored cell viability to 109 +/- 5%, 70 +/- 5%, 99 +/- 3% of live controls, respectively. Slight protective effects were observed in the presence of 3-phosphoglyceric acid and glucose-6-phosphate; however, no protective effects were exhibited by galactose, sucrose, sorbitol, mannitol, glycerol or various gluconeogenic and ketogenic amino acids. On the other hand, fructose 1,6 bisphosphate and gluconeogenic energy intermediates [pyruvic acid, malic acid and phospho(enol)pyruvate (PEP)] were neuroprotective against MPP+. The gluconeogenic intermediates elevated intracellular levels of ATP and reduced propidium iodide (PI) nucleic acid staining to live controls, but did not alter the MPP(+)-induced loss of mitochondrial O2 consumption. These data indicate that malic acid, pyruvic acid and PEP contribute to anaerobic substrate level phosphorylation. The use of hydrazine sulfate to impede gluconeogenesis through PEP carboxykinase (PEPCK) inhibition heightened the protective effects of energy substrates possibly due to attenuated ATP demands from pyruvate carboxylase (PC) activity and pyruvate mitochondrial transport. It was concluded from these studies that several metabolic intermediates are effective in fueling anaerobic glycolysis during mitochondrial inhibition by MPP+.

Behavioral phenotyping of the MPTP mouse model of Parkinson's disease

In mice, the systemical or intracranial application of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) can lead to severe damage to the nigrostriatal dopaminergic system. This can result in a variety of symptoms concerning motor control resembling those in human Parkinson's disease, such as akinesia, rigidity, tremor, gait and posture disturbances. The aim of this work is to review a variety of behavioral paradigms for these and other symptoms, which have been used to characterize behavioral changes in mice after MPTP treatment. Main results are summarized, and general influential factors as well as potential problems in the experimental procedures are discussed, which should be taken into account when conducting behavioral analyses in mice with parkinsonian symptoms. Since there is reliable evidence (e.g. from strain comparisons) that the susceptibility of the nigrostriatal pathway to neurodegeneration is probably genetically influenced, relevant genes can be expected to be identified in the future. Therefore, the points discussed here will be useful not only for further applications in the MPTP mouse model, but also more generally for the behavioral characterization of future mouse models of PD, e.g. mice with a manipulation of genes relevant to the function of the basal ganglia.

Strain-dependent recovery of open-field behavior and striatal dopamine deficiency in the mouse MPTP model of Parkinson's disease

The neurotoxin MPTP can damage dopamine systems in the brains of rodents, cats, or monkeys, and is therefore widely used to model degenerative processes that underlie human Parkinson's disease. Here, we investigated the relationships between behavioral and neurochemical effects of systemic MPTP treatment in C57Bl/6 and Balb/c mice. Initially, different doses of MPTP were used to determine which of them might be useful to establish severe striatal dopamine depletions. These data showed that four injections of 20mg/kg at two hour intervals, were more efficient than 10 or 15mg/kg per injection. However, this dose was not usable due to its severe lethality in females. In contrast, 4x 15mg/kg had a low risk of lethality and led to substantial dopamine depletions, which were more severe in the neostriatum than the ventral striatum, and more severe in C57 than in Balb mice. In the first open field test, which was performed two hours after the last injection, this treatment led to severe behavioral inactivation in all parameters taken (distance and speed of locomotion, peripheral activity, frequency and duration of rearing). This effect was seen in both strains and gender. Thereafter, recovery differed between strains, since Balb mice, which had sustained the smaller lesions, had completely recovered on the subsequent day, whereas similar recovery took longer in C57 mice. On the fourth day, all groups appeared largely normal; however, the measure of rearing behavior still showed a deficit in C57 mice. This deficit on day 4 was correlated with neostriatal dopamine depletion; that is, the larger the lesion, the less the number and duration of rearings. Interestingly, these relationships were also observed with respect to ventral striatal dopamine damage, which was correlated with the rearing deficit not only on day 4, but also on day 1. These data will be discussed with respect to mechanisms of toxicity, functional recovery, and the function of striatal dopamine systems.

Resistance of golden hamster to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine: relationship with low levels of regional monoamine oxidase B

Effects of acute and chronic administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) were investigated for dopamine (DA) and its metabolites, 3,4-dihydroxyphenylacetic acid and 4-hydroxy-3-methoxyphenylacetic acid, in nucleus caudatus putamen (NCP), limbic system, and substantia nigra (SN) of golden hamster and BALB/c and C57/BL mice to obtain a clue for the variance of MPTP toxicity between the strains and species. Regional differences in the levels of monoamine oxidase (MAO) and the in vitro effects of MAO inhibitors were also determined and correlated with MPTP neurotoxicity. Concentrations of MPTP in the brains of mice and golden hamster at 10 min were comparable. Golden hamster was found to be resistant to the administration of MPTP as indicated by a lack of any alteration from the normal content of DA in NCP, limbic system, and SN. Both strains of mice exhibited > 50% and > 75% depletion of DA (C57/BL and BALB/c, respectively). The metabolites-to-DA ratios were decreased and increased in golden hamster and mouse strains, respectively, after acute or chronic treatment. Whereas the content of total MAO in golden hamster was one-third to one-sixth of any nuclei or mitochondria of both strains of mice, the ratio of MAO A to B was significantly higher in the former species.(ABSTRACT TRUNCATED AT 250 WORDS)

Selegiline can mediate neuronal rescue rather than neuronal protection

Selegiline [(-)-deprenyl] has been reported to slow the progression of disabling deficits in Parkinson's disease (PD) and cognitive decline in Alzheimer disease (AD). The apparent slowing has been proposed to be based on either symptomatic improvement due to increased dopaminergic neurotransmission or alternately on protection of neurons from damage caused by toxic oxidative radicals. Both mechanisms are hypothesized to result from the inhibition of monoamine oxidase type B (MAO-B) activity. Our experiments in two animal models have shown that selegiline has a second, previously unsuspected action. That is, selegiline can rescue neurons after they have sustained lethal damage and the rescue is independent of MAO-B inhibition. It was previously shown that the coadministration of selegiline with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) could protect dopaminergic substantia nigra neurons (dSNns) from damage by blocking conversion of MPTP to its active radical N-methyl-4-phenylpyridinium (MPP+) by inhibiting MAO-B. In the first model, we treated C57BL mice with MPTP but delayed selegiline treatment for 72 h after the MPTP treatment to allow for complete conversion of MPTP to MPP+ and for maximal dSNn damage by MPP+. The delayed selegiline treatment rescued approximately 69% of the dSNns that had not died by the time the treatment began but were found to die with saline treatment. Selegiline doses that were too small to cause inhibition of MAO-B substrate oxidation rescued the MPTP-damaged dSNns. The second model was based on previous work showing that immature (14-day-old) rat facial motoneurons die after axotomy because of a loss of trophic support from the muscle they innervate. Selegiline treatment increased the number of motoneurons surviving axotomy from 24 to 52%, showing that selegiline can rescue neurons by partially compensating for the loss of target-derived trophic support. This "trophic-like" action of selegiline might account for the reported slowing of the progression of PD and AD and suggests that selegiline therapy may be of value with acute nervous system damage, particularly damage caused by trauma.

Striatal subregions are differentially vulnerable to the neurotoxic effects of methamphetamine

Methamphetamine (m-AMPH) or saline was repeatedly administered to rats. One week later, the caudate-putamen of the m-AMPH-treated rats revealed a decrease in both [3H]mazindol-labeled dopamine uptake sites and tissue dopamine content. Moreover, the resulting pattern of decline in these measures was regionally heterogeneous. The ventral caudate-putamen displayed the greatest decrease in both [3H]mazindol binding and dopamine content while the neighboring nucleus accumbens and the dorsal caudate-putamen remained relatively intact. These results indicate a regional difference in the susceptibility of striatal dopaminergic terminals to the neurotoxic effects of methamphetamine.

Genetic factors in neurotoxicology and neuropharmacology: a critical evaluation of the use of genetics as a research tool

Animals have evolved a detoxication system to enable them to survive in a hostile chemical environment in which foods contain many non-nutrient chemicals. Detoxication depends on enzymes which are often genetically polymorphic. As a result, inter-individual variation is common, and in humans several Mendelian loci have been identified. However, most variation in response is probably due to the action of several genes. Genetic variation in response to the neurotoxin MPTP and to chemically and physically-induced seizures is reviewed. In the former case, differences between pigmented and white mouse strains have been noted which are consistent with the hypothesis that humans are more sensitive than mice or rats because of the presence of melanin in human brains. However, variation in sensitivity probably also depends on other genes. In the case of audiogenic seizures, a single locus has been identified and mapped, but its relationship with seizures induced by other agents is not clear. Genetic variation in response to alcohol is also discussed. The failure of most toxicologists to consider genetic variation as a potentially confounding variable, and as a powerful research tool, is discussed critically in relation to non-repeatability of research on the neurotoxic effects of lead, and in relation to the genetic variation in MPTP, seizures, and alcohol response already noted. It seems clear that genetic methods provide a powerful research tool which is largely being ignored by toxicologists.

Selective vulnerability of hippocampal CA3 neurones after microinfusion of paraquat into the rat substantia nigra or into the ventral tegmental area

The neuropathological effects of various doses of paraquat, a widely used herbicide, given directly into different areas of the rat brain, were studied. Paraquat, microinfused into the pars compacta of the substantia nigra (3.2, 16, 32 and 160 nmol), i.e. concentrations of 3.2 to 160 mmol l-1 applied at 1 microliter min-1 for 1 min, produced dose-dependent neuropathological lesions culminating in neuronal necrosis. A particular feature of paraquat neurotoxicity after its microinfusion into the substantia nigra (3.2 mmol per 1 at 1 microliter per min for 1 min) or into the ventral tegmental area (1.6 mmol per 1 at 1 microliter per min for 1 min), but not into other areas of the brain, was a selective vulnerability of hippocampal CA3 neurones consisting initially of a significant decrease of dendritic spines and later in neuronal degeneration and cell loss. No damage occurred after microinfusion of paraquat into other areas of the brain near or distant from the infusion sites. In addition, similar neuropathological alterations occurred in other non-dopaminergic areas, such as the locus coeruleus and some raphe nuclei after direct microinfusion of paraquat into these sites. In conclusion, the above neuropathological findings show that paraquat possesses marked neurotoxicity which, despite its chemical similarity to MPTP, is not selective for dopaminergic neurones.

Evaluation of a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated C57 black mouse model for parkinsonism

We evaluated neurochemically, behaviorally, and neuropathologically the availability of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated C57 black (BL) mice as a model for Parkinson's disease. The dopamine and 3,4-dihydroxyphenyl acetic acid content in the striatum, measured by high-performance liquid chromatography with an electrochemical detector, decreased by 70% at 10 and 20 days after the withdrawal of MPTP (30 mg/kg, i.p. twice daily for 5 days). During these days, the mice showed a decrease in locomotor activity and exhibited akinesia in both pole and traction tests. Light microscopically, 44% of the MPTP-treated mice showed neuronal degeneration in the substantia nigra 1 month after the withdrawal (damaged group), and 56% showed no change (undamaged group). Morphometric analysis revealed that the number of neurons in the substantia nigra decreased by 33% on the average in both groups. Electron microscopically, an electron-dense degeneration of most neurons was seen in the substantia nigra of the damaged group, and even in the undamaged group, loss of rough endoplasmic reticulum and mitochondrial deformity were seen in 50-70% of the neurons. Electron-dense bodies were seen in the striatum of both groups. These results show the validity of the MPTP-treated C57 BL mice as a suitable model for parkinsonism, including Parkinson's disease.

MPTP treatment combined with ethanol or acetaldehyde selectively destroys dopaminergic neurons in mouse substantia nigra

We have previously reported that ethanol and acetaldehyde potentiate 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) neurotoxicity in mice, enhancing dopamine (DA) depletion in the striatum. The present study was designed to determine whether such enhancement of neurotoxicity was specific for the nigro-striatal DA pathway. In 5-week-old mice acetaldehyde treatment did not enhance DA depletion seen 7 days after MPTP treatment. In 8-week-old animals, however, acetaldehyde or ethanol given with MPTP decreased striatal DA content to about 10% of controls, whereas the depletion was to 43% of controls when MPTP was given alone. In acetaldehyde or ethanol and MPTP-treated mice, changes in DA levels were observed only in the striatum. DA contents in the hypothalamus, olfactory bulb and frontal cortex were similar to that in controls. Contents of norepinephrine and serotonin in striatum, hypothalamus, olfactory bulb and cerebral cortex were not affected by any of the treatments. Three months after MPTP alone, striatal DA recovered to 74% of controls in 8-week-old mice, whereas no recovery occurred in acetaldehyde and MPTP-treated mice. Moreover, both tyrosine hydroxylase (TH) immunocytochemistry and Cresyl violet staining showed an extensive and selective cell loss in the pars compacta of the substantia nigra (SNc) of the mice treated with acetaldehyde or ethanol and MPTP, whereas MPTP alone caused only a limited cell degeneration.

Selective visualization of rodent locus ceruleus by a radiolabeled N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine analog

The neurotoxic metabolite of N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), 1-methyl-4-phenylpyridinium, selectively accumulates in dopaminergic neurons via the dopamine reuptake system. Consequently, nontoxic radiolabeled MPTP analogs may be potentially useful for visualizing catecholaminergic neurons in vivo. N-Methyl-4-(4-hydroxy-3-[125I]iodobenzyl)-1,2,3,6-tetrahydropyridine [( 125I]MHTP), an analog of the nontoxic N-methyl-4-benzyl-1,2,3,6-tetrahydropyridine, has been studied in rats and mice. After intravenous administration of [125I]MHTP to rodents, the initial accumulation of radioactivity within the brain was found to be comparable to that of radiolabeled MPTP. Following intravenous administration of [125I]MHTP, in vivo autoradiographic visualization of the rodent brain revealed selective accumulation of [125I]MHTP-derived radioactivity within the locus ceruleus; there was no accumulation of the radiotracer within dopaminergic fibers and cell bodies. The accumulation of radioactivity within the locus ceruleus was blocked by pretreatment with pargyline, a result suggesting that an MHTP metabolite formed by monoamine oxidase was responsible for the localization of the radiotracer within this structure. The anatomical distribution of the radiolabel demonstrates selective accumulation of this metabolite within noradrenergic cell bodies and those fibers making up the locus ceruleus. These findings further suggest that nontoxic metabolites of MPTP may become useful for in vivo labeling of selected populations of catecholaminergic neurons.

The neurotoxicant MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) increases glial fibrillary acidic protein and decreases dopamine levels of the mouse striatum: evidence for glial response to injury

The neurotoxicant MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), administered to male or female mice, decreased striatal dopamine content and increased the levels of the astrocyte intermediate filament protein, glial fibrillary acidic protein (GFAP). The rise in GFAP was evident as early as two days following the last dose of MPTP, was maximal 7 days after the toxicant and returned to control levels by two months, post MPTP. Striatal dopamine content was decreased post-MPTP administration, showing a slight recovery between one and two months after the toxicant. No differences were observed among male and female mice in their responses to the toxicant. Hippocampal noradrenaline content was not affected by the toxicant, neither was the GFAP content altered by MPTP in this structure. Additionally, pargyline pretreatment prevented both the rise in GFAP and the decrease in dopamine in striatum. MPTP produced a smaller elevation in GFAP levels within a midbrain section of tissue containing the substantia nigra, without significantly decreasing the dopamine content of this structure, suggesting neurotoxic involvement at the level of dopamine perikarya. The toxicant did not affect the molecular radius of the protein detected by the antibody to GFAP, as determined by immunoblot analysis.

Behavioural and electrocortical changes induced by paraquat after injection in specific areas of the brain of the rat

The behavioural and electrocortical effects of paraquat were studied after its administration into the substantia nigra, pars compacta, an area where dopamine-(DA) containing cell bodies are present, into the caudate nucleus, where DA-containing nerve endings of the DA nigro-striatal system project, into the locus coeruleus, an area containing noradrenaline cell-bodies and into the n. raphe dorsalis or into the n. raphe medianus, two nuclei containing serotonin (5-HT) cell bodies. The intraventricular administration of paraquat (10 and 50 micrograms) produced an intense pattern of behavioural stimulation and an increase in locomotor activity, circling and the wet-dog syndrome. This symptomatology was accompanied by desynchronization of the electrocorticogram (ECoG) and the appearance of bilateral high voltage epileptogenic spikes, culminating in clonic convulsions. The infusion of paraquat into the s. nigra produced contralateral head and neck deviation, behavioural and motor stimulation, these effects being observed also with smaller doses (1 and 5 micrograms), than those used intraventricularly. The ECoG activity was desynchronized and characterized by high voltage spike discharges. A similar behavioural, postural and ECoG pattern was also observed after infusion of paraquat into the caudate nucleus (10, 25 and 50 micrograms). In addition, paraquat, infused into the locus coeruleus or into the raphe nuclei (5 and 10 micrograms), produced circling, escape responses, jumping and clonic convulsions accompanied by ECoG desynchronization and epileptic phenomena. In conclusion, the present experiments showed that paraquat was able to produce central neurotoxicological effects which did not seem to be specific, at least for the doses used, for the DA nigro-striatal system.

Acute ultrastructural and behavioral effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in mice

Acute actions of MPTP on behavior and on neostriatal ultrastructure were examined in young C57 Black mice. Autonomic, motor, and toxic effects of MPTP exhibited dependence on dose (20-40 mg/kg) and time during the first 4 h after subcutaneous injection. The ultrastructure of the neostriatum was altered very quickly (2-24 h) after single injections of MPTP. Darkened glial processes were found within 2-8 h, followed by dark degeneration of synaptic boutons, especially those making small symmetric synapses. More rarely, swollen axons and postsynaptic degeneration were also observed.

Regional and temporal effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine on dopamine uptake sites in mouse brain

When the regional effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on brain dopamine uptake sites in C57 Black mice were studied using [3H]mazindol autoradiography, marked regional differences in effect were seen: the mesolimbic system was less affected than the nigrostriatal tract and within each system the effect was more severe in the terminal fields of the striatum than in the cells of origin. Within the striatum itself there was inhomogeneity of effect, with relative sparing of the dorsomedial aspect compared to the remainder. Complete recovery of [3H]mazindol binding to striatal membranes occurred over 12 months, while dopamine levels recovered more slowly. This supports the concept that MPTP has a highly selective effect within dopaminergic systems and that the initial effect is more pronounced on distal terminals compared to cell bodies. The possibility that recovery of mazindol binding with time may be associated with terminal regrowth needs to be investigated further.