Acute administration of 1-N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a compound producing parkinsonism in humans, stimulates [2-14C]deoxyglucose uptake in the regions of the catecholaminergic cell bodies in the rat and guinea pig brains

The PPAR-γ agonist pioglitazone modulates inflammation and induces neuroprotection in parkinsonian monkeys

BACKGROUND

Activation of the peroxisome proliferator-activated receptor gamma (PPAR-γ) has been proposed as a possible neuroprotective strategy to slow down the progression of early Parkinson's disease (PD). Here we report preclinical data on the use of the PPAR-γ agonist pioglitazone (Actos®; Takeda Pharmaceuticals Ltd.) in a paradigm resembling early PD in nonhuman primates.

METHODS

Rhesus monkeys that were trained to perform a battery of behavioral tests received a single intracarotid arterial injection of 20 ml of saline containing 3 mg of the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Twenty-four hours later the monkeys were assessed using a clinical rating scale, matched accordingly to disability, randomly assigned to one of three groups [placebo (n = 5), 2.5 (n = 6) or 5 (n = 5) mg/kg of pioglitazone] and their treatments started. Three months after daily oral dosing, the animals were necropsied.

RESULTS

We observed significant improvements in clinical rating score (P = 0.02) in the animals treated with 5 mg/kg compared to placebo. Behavioral recovery was associated with preservation of nigrostriatal dopaminergic markers, observed as higher tyrosine hydroxylase (TH) putaminal optical density (P = 0.011), higher stereological cell counts of TH-ir (P = 0.02) and vesicular monoamine transporter-2 (VMAT-2)-ir nigral neurons (P = 0.006). Stereological cell counts of Nissl stained nigral neurons confirmed neuroprotection (P = 0.017). Pioglitazone-treated monkeys also showed a dose-dependent modulation of CD68-ir inflammatory cells, that was significantly decreased for 5 mg/kg treated animals compared to placebo (P = 0.018). A separate experiment to assess CSF penetration of pioglitazone revealed that 5 mg/kg p.o. induced consistently higher levels than 2.5 mg/kg and 7.5 mg/kg. p.o.

CONCLUSIONS

Our results indicate that oral administration of pioglitazone is neuroprotective when administered early after inducing a parkinsonian syndrome in rhesus monkeys and supports the concept that PPAR-γ is a viable target against neurodegeneration.

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.

Variable toxicological response to the loss of OXPHOS through 1-methyl-4-phenylpyridinium-induced mitochondrial damage and anoxia in diverse neural immortal cell lines

Immortal cell lines are used to investigate various aspects of neurodegeneration. These cells display high glycolytic turnover rate and produce an abundant amounts of lactate. Our previous studies indicate that these cells survive the loss of mitochondrial oxidative phosphorylation (OXPHOS) with ample glucose supply. In the current study, we investigate if cell type (w/variation in basal metabolic rate (MR)), can alter glucose utilization patterns which in turn may affect LC(50) for the mitochondrial toxin 1-methyl-4-phenylpyridinium (MPP(+)) in various cell lines. The data obtained indicate that cell lines MRs examined were generally consistent with the average of species adult body weight where mouse N-2A > rat-PC-12 > human SH-SY5Y. A higher MR was associated with accelerated utilization of glucose and earlier cell death with MPP(+): LC(50) mouse = 294 µM, rat = 695 µM, and human = 5.25 mM at 24 h. Cell death appears to be a function of the velocity by which glucose disappears, leading to the failure of glycolysis and subsequent halt of energy production. Similar effects were also observed at higher plating densities where the demand for glucose is amplified. A time-lapse study of MPP(+) toxicity (0-36 h) in N-2A cells indicates that an anaerobic shift occurs as early as 2 h (evidenced by a rise in lactate), followed by a descent in glucose concentrations at 4 h and exhaustion of glucose supplies at 22 h which was associated with the first detectable sign of cell death. It was also noted that MPP(+) toxicity was not associated with the generation of reactive oxygen species (O (2) (-) , H(2)0(2), and NO(2)) and was not attenuated by adding catalase or superoxide dismutase to the media. On the other hand, MPP(+) toxicity was reversed by providing additional supply of glucose, pyruvate ± mitochondrial monocarboxylate transporter blocker (α-cyano-4-HCA), or pyruvate ± pyruvate dehydrogenase inhibitor (octanoyl-CoA), suggesting that the exclusive anaerobic survival compensates for the loss of OXPHOS by MPP(+). To examine if neuroblastoma were capable of surviving the deprivation of O(2) for 24 h, a range of hypoxia to anoxia was established with various concentrations of dithionite. The data suggest that cell lines examined continue to thrive when incubated with high-glucose media (25 mM). In summary, vulnerability of immortal neuroblastoma cell lines to MPP(+) toxicity is dependent upon glucose concentrations within the media and cell MR, which indirectly dominates the velocity of glucose use and its end point disappearance, leading to cell death by ergogenic failure.

Disruption of lateral olivocochlear neurons via a dopaminergic neurotoxin depresses sound-evoked auditory nerve activity

We applied the dopaminergic (DA) neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to the guinea pig cochlear perilymph. Immunolabeling of lateral olivocochlear (LOC) neurons using antibodies against synaptophysin was reduced after the MPTP treatment. In contrast, labeling of the medial olivocochlear innervation remained intact. As after brainstem lesions of the lateral superior olive (LSO), the site of origin of the LOC neurons, the main effect of disrupting LOC innervation of the cochlea via MPTP was a depression of the amplitude of the compound action potential (CAP). CAP amplitude depression was similar to that produced by LSO lesions. Latency of the N1 component of the CAP, and distortion product otoacoustic emission amplitude and adaptation were unchanged by the MPTP treatment. This technique for selectively lesioning descending LOC efferents provides a new opportunity for examining LOC modulation of afferent activity and behavioral measures of perception.

Protective action of the peroxisome proliferator-activated receptor-gamma agonist pioglitazone in a mouse model of Parkinson's disease

We examined the effect of pioglitazone, a peroxisome proliferator-activated receptor-gamma (PPARgamma) agonist of the thiazolidinedione class, on dopaminergic nerve cell death and glial activation in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of Parkinson's disease. The acute intoxication of C57BL/6 mice with MPTP led to nigrostriatal injury, as determined by tyrosine hydroxylase (TH) immunocytochemistry, and HPLC detection of striatal dopamine and metabolites. Damage to the nigrostriatal dopamine system was accompanied by a transient activation of microglia, as determined by macrophage antigen-1 (Mac-1) and inducible nitric oxide synthase (iNOS) immunoreactivity, and a prolonged astrocytic response. Orally administered pioglitazone (approximately 20 mg/kg/day) attenuated the MPTP-induced glial activation and prevented the dopaminergic cell loss in the substantia nigra pars compacta (SNpc). In contrast, there was little reduction of MPTP-induced dopamine depletion, with no detectable effect on loss of TH immunoreactivity and glial response in the striatum of pioglitazone-treated animals. Low levels of PPARgamma expression were detected in the ventral mesencephalon and striatum, and were unaffected by MPTP or pioglitazone treatment. Since pioglitazone affects primarily the SNpc in our model, different PPARgamma-independent mechanisms may regulate glial activation in the dopaminergic terminals compared with the dopaminergic cell bodies after acute MPTP intoxication.

Age-related changes in locomotor behavior induced by MPTP in rats

Effect of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on gait was studied in young (4 weeks) and old (21 months) male rats. No significant alteration in stride length (SL) or stride width (SW) was observed 30 min after single dose MPTP treatment (3 mg/kg, i.p.) in both young and old rats. Multidose treatment with MPTP (3 mg/kg, i.p. daily for 8 days) resulted in a marked decrease in SL in old rats compared to that of old control group, while an increase in SL was found in both MPTP-treated and control young rats. In contrast, multidose MPTP administration caused no changes in SW in either young or old rats. The present result of behavioral analysis suggests that multidose MPTP neurotoxic effect on locomotor activity may be age dependent in rats and that the old rat is an appropriate model for MPTP studies.

Acute effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in a model of rat designated a poor metabolizer of debrisoquine

The relationship between oxidative polymorphisms and the cause of Parkinson's disease is controversial. The drug 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), which induces parkinsonism in humans and in some animal models, is metabolized by cytochrome P450 db1 isozyme (the same enzymatic system implicated in 4-hydroxylation of debrisoquine). In this study, we treated females of three rat species, which differ in their ability to hydroxylate debrisoquine, with MPTP (three doses of 30 mg/kg s.c. at 12-h intervals), and we measured their motor activity and brain monoamine levels. Female dark-adapted rats (poor metabolizers of debrisoquine) showed a more pronounced and more maintained reduction of their motor activity after treatment with MPTP. MPTP-treated, dark-adapted rats also had a depletion of noradrenaline in the diencephalon and a depletion of dopamine and serotonine and their respective metabolites in the limbic system when compared with the other two species. These results suggest that oxidative polymorphism of debrisoquine plays a role in the acute effects of MPTP.

The effects of monoamine oxidase inhibition and dopamine uptake blockade on MPTP-induced increase of 2-[3H]deoxyglucose uptake in specific mesencephalic catecholaminergic nuclei

Intraperitoneal injection in C57 black mice of 30 or 7.5 mg/kg MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) followed 1-2 h later by 0.5 mCi/100 g 2-[3H]deoxyglucose (2-DG) produces an intense increase of 2-DG uptake in the dopaminergic zona compacta (ZC) of the substantia nigra and the adjacent ventral tegmental area (VTA) as seen in autoradiographs. Anesthetized animals also exhibit this reaction. Clorgyline (10 mg/kg), a monoamine oxidase (MAO)-A inhibitor, 30 min before MPTP does not block this reaction. Deprenyl (10 mg/kg), an MAO-B inhibitor, blocks increased 2-DG uptake in ZC and VTA if injected 30 min or 12 h before 7.5 mg/kg MPTP, but has no effect if injected 12 h before 30 mg/kg MPTP. Mazindol (10 mg/kg), a dopamine uptake blocker, is effective in blocking the action of 7.5 mg/kg MPTP, but ineffective against 30 mg/kg MPTP. Heavier doses of MPTP can, in some instances, overcome the actions of MAO inhibitors or dopamine uptake blockers in preventing the increased 2-DG uptake elicited by MPTP. But, essentially, the intensely increased glucose metabolism acutely induced in specific catecholaminergic neurons appears to be another significant pathological feature of the dopaminergic neurotoxicity caused by MPTP which can be prevented by MAO inhibition or dopamine uptake blockade.

Neuropathological changes in the caudate nucleus elicited by MPTP and their prevention by monoamine oxidase inhibition

MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), a neurotoxin producing parkinsonism, causes an obvious reduction in tyrosine hydroxylase (TH) activity in the caudate nucleus. This depletion of TH activity is due to degeneration of TH-positive punctate dopaminergic terminals. The disappearance of these terminals unmasks the presence of long branching, varicose preterminal fibers, which may be sprouting after degeneration of their terminal arborizations. Glial cells, normally sparse with delicate processes, undergo intense increase in number and a robust hypertrophy. All of these changes are prevented completely by administration of deprenyl, a monoamine oxidase inhibitor, before and after MPTP. These neuropathological effects are additional manifestations of the dopaminergic neurotoxicity induced by MPTP. The metabolism of MPTP, which is blocked by monoamine oxidase inhibition, is apparently necessary for the expression of toxicity in the brain by this neurotoxin.

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.

Neurochemical changes in the substantiae nigrae and caudate nuclei following acute unilateral intranigral infusions of N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)

Acute unilateral intranigral infusions of MPTP at doses (200 micrograms) which produce robust contralateral rotation in the rat induced significant neurochemical changes in the ipsilateral as well as contralateral nigrostriatal systems. There were pronounced increases in the levels of dopamine (DA), dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the ipsilateral substantia nigra and a significant decrease in the levels of DA in the ipsilateral caudate nucleus while opposite changes occurred in the contralateral substantia nigra and caudate nucleus. The DOPAC:DA and HVA:DA ratios were significantly higher in the ipsilateral caudate nucleus indicating increased activity of the ipsilateral nigrostriatal DA neurones. The levels of noradrenaline and 4-hydroxy-3-methoxyphenylethyline glycol (MHPG) increased and decreased significantly in the ipsilateral and contralateral substantia nigra, respectively, but there were no significant changes in the caudate nuclei. The levels of serotonin (5-HT) and 5-hydroxyindole acetic acid (5-HIAA) increased significantly in the ipsilateral substantia nigra and caudate nucleus as well as in the contralateral caudate nucleus but did not increase significantly in the contralateral substantia nigra. The 5-HIAA:5-HT ratio was significantly decreased in the contralateral caudate nucleus indicating a reduced activity of the contralateral nigrostriatal 5-HT neurones. The data thus indicate that MPTP applied to one substantia nigra is capable of producing profound neurochemical changes not only locally but also in the ipsilateral striatum as well as in the contralateral nigrostriatal system. Previous neuropharmacological studies have suggested that the rotation induced by intranigral MPTP may be mediated via dopamine released from dendrites in the pars reticulata in response to MPTP.(ABSTRACT TRUNCATED AT 250 WORDS)

Symptoms and behavioral features induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in an old Java monkey [Macaca cynomolgus fascicularis (Raffles)]

The study concerns symptoms and behavioral characteristics induced by MPTP in a 20-year-old Macaca cynomolgus fascicularis, their evolution over 7 months, and the animal's response to 1-dopa treatment. The symptoms which the animal developed include those that have been described earlier in Macaca mulatta and Saimiri sciureus, i.e., rigidity, action tremor, postural tremor, postural flexion, hypokinesia, and bradykinesia. In addition, however, the animal developed a 3.8 Hz resting tremor which in humans is pathognomonic of Parkinson's disease, as well as cogwheeling, the glabellar tap sign, drooling, impaired ability to relax, and many other symptoms. Also unlike previously described MPTP monkeys, the animal's symptoms neither improved spontaneously, nor did they remain stable shortly after MPTP injection. Instead, symptoms steadily progressed to reach a severe status 2 months after MPTP, and further progression was apparent after another 5 months. Therapeutic responses to 1-dopa required accumulation of or kindling by the 100 mg unit doses that were spaced 4 hr apart, were often organized in time as ON episodes that alternated with OFF episodes, and were associated with dyskinesias and bizarre behavior. Of particular interest is that the animal showed kinesia paradoxa which, in humans, constitutes a feature that is unique to Parkinson's disease among the extrapyramidal disorders. In addition to available evidence, the present findings validate the syndrome induced by MPTP in monkey as an animal analogue of Parkinson's disease. Taxonomic category, age, and the occurrence of shock in response to MPTP are discussed as variables that may possibly co-determine the pathology which MPTP may induce in monkey.

N-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine increases acetylcholine and decreases dopamine in mouse striatum: both responses are blocked by anticholinergic drugs

The neurotoxin N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) produces neuropathology and clinical symptoms that resemble Parkinsonism in primates and humans. In mice it induces a long-lasting depletion of neostriatal 3,4-dihydroxyphenylethylamine (dopamine) content. Using the mouse, we found that MPTP induces a fall of dopamine and a rise of acetylcholine in the neostriatum. Both responses to MPTP can be blocked by prior treatment with atropine or trihexyphenidyl.

Intrahypothalamic infusions of a synthetic heroin substitute, N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, potentiate mating behaviour in the female rat

Acute microinfusions of N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) bilaterally into the arcuate-ventromedial area of the hypothalamus (ARC-VMH) produced a dose-dependent facilitation of lordosis behaviour in oestrogen- and oestrogen-progesterone-treated ovariectomized female rats. This facilitation could be blocked by pretreatment of the ARC-VMH site with pargyline but not by a potent antagonist analogue of gonadotropin releasing hormone (GnRH). MPTP was not effective in potentiating lordosis when microinfused into other brain sites which are intimately involved in the neural circuitry for this oestrogen-dependent behavioural response. This action of MPTP in the ARC-VMH may be dependent upon the high MPTP receptor densities which are known to occur in the arcuate and periventricular hypothalamic nuclei. Chronic infusions of MPTP (once daily for 21 days) into the ARC-VMH did not result in any significant cell loss and did not interfere with the ability of a later acute MPTP infusion to potentiate lordosis nor with the ability of oestrogen and progesterone to promote the full display of lordosis behaviour. These results provide the first demonstration of neurally placed MPTP in the regulation of a specific hormone-dependent neuroendocrine response.

Parkinsonism-inducing neurotoxin, N-methyl-4-phenyl-1,2,3,6 -tetrahydropyridine: uptake of the metabolite N-methyl-4-phenylpyridine by dopamine neurons explains selective toxicity

N-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) produces neuropathological and clinical abnormalities in humans, monkeys, and mice that closely resemble idiopathic parkinsonism. N-Methyl-4-phenylpyridine (MPP+), a metabolite of MPTP formed by monoamine oxidase B, is accumulated into striatal and cerebral cortical synaptosomes by the dopamine and norepinephrine uptake systems, respectively, whereas MPTP itself is not accumulated. The potencies of drugs in inhibiting [3H]MPP+ or [3H]dopamine uptake into striatal synaptosomes are very similar, as are potencies in inhibiting [3H]MPP+ or [3H]norepinephrine uptake into cortical synaptosomes. The Km values for [3H]MPP+ uptake are 170 and 65 nM and the Vmax values are 2 and 0.1 nmol/g of tissue per min in rat striatum and cortex, respectively, similar to values for [3H]dopamine uptake, Autoradiography of accumulated [3H]MPP+ in slices of rat brain shows high densities in the caudate-putamen and nucleus accumbens. Furthermore, blockade of dopamine uptake by mazindol prevents MPTP-induced damage to nigrostriatal dopamine neurons, indicating that MPP+ concentration into dopamine neurons explains their selective destruction by MPTP.

New amphibian models for the study of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)

We report the development of two animal models in amphibians (frogs and salamanders) in whom 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) produces the behavioral (neurological) and biochemical equivalents of the human disease and, in addition, a measurable modification in at least one form of pigment-bearing cell from the neural crest, the skin melanocyte. We propose that this new approach can become an inexpensive, easily quantifiable model for the study of the effect of MPTP on the central and peripheral nervous systems. We also demonstrate that the toxic effect of MPTP can be completely abolished in vivo by treatment with a monoamine oxidase inhibitor and potentiated by an inhibitor of catechol-O-methyltransferase. MPTP is catabolised by oxidation into toxic metabolites, but 1-methyl-4-phenylpyridinium ion (MPP+), the proposed end-metabolite, is even more toxic than MPTP in this model, possibly through a different mechanism.

Dopamine D2 receptor agents, but not dopamine D1, modify brain glucose metabolism

The effects of recently described selective dopamine D1 and D2 agonists and antagonists on brain glucose metabolism were studied using the 2-[14C]deoxyglucose autoradiographic technique. The administration of LY-141865 or YM-09151-2, which behave as a specific D2 agonist and antagonist respectively, modified brain glucose metabolism in a manner similar to that previously described for more classical dopaminergic agents, such as apomorphine and haloperidol. In contrast, the administration of SKF 38393 or SCH 23390, a specific D1 agonist and antagonist respectively, was not followed by significant modifications of brain glucose metabolism in any of the brain regions studied. These results indicate that D2 but not D1 dopamine receptors are involved in the regulation of local brain glucose metabolism.

Presynaptic dopaminergic agonists modify brain glucose metabolism in a way similar to the neuroleptics

The effects of the (+) and (-) enantiometers of 3-(3-hydroxyphenyl)-N-n-propylpiperidine (3-PPP), 9,10-didehydro-6-methyl-8 beta-(2-pyridylthiomethyl)ergoline (CF 25-397) and 6,7-dihydroxy-2-dimethyl-aminotetralin (TL 99), three agonists of the postulated presynaptic dopamine receptor, on the pattern of brain glucose metabolism were studied using the autoradiographic technique of Sokoloff et al. [19]. It was found that these drugs modify brain glucose metabolism in a way similar to the neuroleptics but different from postsynaptic agonists. These results support the suggestion that these drugs could represent a new type of neuroleptic.

Acute administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) reduces dopamine and serotonin but accelerates norepinephrine metabolism in the rat brain. Effect of chronic pretreatment with MPTP

Acute administration of MPTP (a synthetic heroin substitute) at 10-30 mg/kg (s.c.) produced 'Straub tail' phenomena, piloerection and reduced pelvis elevation in rats. The same dose decreased the concentrations of dopamine metabolites and reduced the rate of dopamine synthesis in the striatum. MPTP also reduced the metabolism of serotonin but accelerated that of norepinephrine in their corresponding terminal areas. The effects on central monoamines probably were not due to an agonistic action of MPTP on dopaminergic and serotonergic receptors, since MPTP only exhibited micromolar affinity to the corresponding binding sites. Furthermore, MPTP failed to induce rotational behavior in animals with unilateral nigrostriatal lesions. Chronic treatment of rats with MPTP (10 mg/kg s.c., daily for 3 weeks) did not result in massive degenerative changes in the nigrostriatal system. Histochemical analysis showed intact dopaminergic neurons. Striatal dopamine levels only were reduced by 10%. Dopaminergic neurons in rats chronically treated with MPTP responded normally to a pharmacological stimulus increasing their transmitter synthesis. Chronic treatment did not affect their response to an acute injection of MPTP.