The effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine on presynaptic dopamine uptake sites in the mouse striatum

Effect of 1,2,3,4,-tetrahydroisoquinoline administration under conditions of CYP2D inhibition on dopamine metabolism, level of tyrosine hydroxylase protein and the binding of [3H]GBR 12,935 to dopamine transporter in the rat nigrostriatal, dopaminergic system

Current concepts of Parkinson's disease (PD) postulate that interaction between neurotoxins and specific genetic background may play an important role in pathogenesis of PD. Therefore, the effect of multiple administration of 1,2,3,4-tetrahydroisoquinoline (TIQ) under conditions of CYP2D blockade on the expression of key markers of PD was studied in the rat striatum (STR) and substantia nigra (SN). TIQ administered alone (50 mg/kg i.p. twice daily for 14 days) markedly decreased the level of tyrosine hydroxylase protein (TH) in the STR; however, this effect was not accompanied by reduction of dopamine (DA) concentration and [(3)H]GBR 12,935 binding to dopamine transporter (DAT). Administration of CYP2D inhibitor, quinine, jointly with TIQ lowered the levels of TH and DA in that structure, but slightly increased DAT binding. In the SN, treatment with TIQ alone did not change TH level although it enhanced DA content and decreased [(3)H]GBR 12,935 binding to DAT in the substantia nigra pars compacta (SNc). Neither the TH level nor DA concentration was affected by the combined treatment, although DAT binding was still reduced in the SN. TIQ did not change the total DA catabolism in the STR, but caused its inhibition in the SN. It strongly depressed the levels of intraneuronal DA metabolite DOPAC and enhanced that of extraneuronal 3-MT in either structure. TIQ more weakly affected the levels of both DA metabolites in the presence of quinine. Our results suggest that endogenous TIQ may act rather as neuromodulator but not as parkinsonism-inducing neurotoxin in the rat brain.

Evaluation of neurotoxicity of TIQ and MPTP and of parkinsonism-preventing effect of 1-MeTIQ by in vivo measurement of pre-synaptic dopamine transporters and post-synaptic dopamine D(2) receptors in the mouse striatum

Parkinsonism-inducing neurotoxicity of 1,2,3,4-tetrahydroisoquinoline (TIQ), as contrasted to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), and parkinsonism-preventing effect of 1-methyl-1,2,3,4-tetrahydroisoquinoline (1-MeTIQ) have been investigated in mice by measuring their effects on the in vivo binding of radioligand to pre-synaptic dopamine transporters (DATs) or to dopamine D(2) receptors (D2R) in the striatum. A significant reduction of the ligand-DATs binding was found in the mice treated with MPTP, but not with TIQ, under the dosage inducing behavioral abnormality and loss of tyrosine hydroxylase-positive cells in the substantia nigra. A slight decrease in the ligand-DATs binding was observed in the mice given a larger dose of TIQ. Compensatory up-regulation in the post-synaptic D2Rs was found in the MPTP-treated mice. Pre-treatment with (S)-enantiomer, but not (R)-enantiomer, of 1-MeTIQ prevented the degeneration of DATs to some extent. We concluded that the TIQ-induced parkinsonism model is different from the MPTP-induced model as evaluated by the radioligand-DATs binding and that (S)-1-MeTIQ has a preventing effect for the degeneration of the DATs to a certain extent.

Tyrosine hydroxylase mRNA expression by dopaminergic neurons in culture: effect of 1-methyl-4-phenylpyridinium treatment

To enable us to study expression of tyrosine hydroxylase [TH; tyrosine 3-monooxygenase; L-tyrosine tetrahydropteridine:oxygen oxidoreductase (3-hydroxylating); EC 1.14.16.2] as a measure of dopaminergic neuron function in future experiments, methods were developed to quantify TH mRNA levels in cultures of dopaminergic mesencephalic cells. The model of selective dopaminergic toxicity of 1-methyl-4-phenylpyridinium (MPP+) was used to verify the specificity of our methods. Fetal (embryonic day 15) rat ventral mesencephalic cell cultures were treated with 15 microM MPP+ for 48 h, conditions previously shown to reduce the number of TH-immunoreactive neurons, TH activity, and dopamine uptake to 5-10% of control values. This treatment decreased the number of neurons labeled by TH in situ hybridization to 9% of untreated controls and caused a strong reduction of the abundance of TH mRNA in Northern blots. Our findings establish TH mRNA expression as a parameter for future studies of toxic and trophic effects on cultured dopaminergic neurons, and they support the view that MPP+ destroys dopaminergic neurons.

Distribution of catecholamine uptake sites in human brain as determined by quantitative [3H] mazindol autoradiography

Because of the importance of the catecholamine system in Parkinson's disease and its relevance to a variety of clinical movement disorders, catecholamine uptake sites were mapped in the human brain using [3H] mazindol autoradiography. Displacement studies with known dopamine (DA) and noradrenaline (NA) uptake blockers showed that binding in the striatum was to dopamine uptake sites; binding in the locus coeruleus was to noradrenergic uptake sites. By using the selective noradrenergic uptake blocker desmethylimipramine (DMI), a comprehensive map of both DA and NA uptake sites was generated. In general, catecholamine uptake sites were better seen in terminals than in cells of origin or axonal projections. In some areas, such as the locus coeruleus, punctate binding could be seen over individual pigmented cells. A variegated pattern of binding was seen in caudate nucleus and putamen and some correspondence of patches of low binding with striosomes was observed in the caudate. The highest levels of binding to DA uptake sites was observed in the striatum, where regional differences in binding occurred. The most dense binding was seen in the ventral striatum, and a rostral-to-caudal decrement in binding levels in caudate nucleus and putamen was evident. Binding was more intense in the putamen compared to the caudate and within the caudate lower values were seen laterally. The highest levels of binding to noradrenergic uptake sites were in the locus coeruleus and dorsal raphé, although these sites may be on terminals from other projections. Whereas uptake sites were more often evident in known catecholamine pathways, [3H] mazindol binding was seen in some areas where catecholamine neurons or terminals had not been identified previously. These maps of the catecholamine uptake system add further information concerning the nature of the distribution of catecholamines in human brain and provide an important baseline for the study of disease and ageing processes.

Catecholamine uptake sites in mouse brain: distribution determined by quantitative [3H]mazindol autoradiography

Because of the importance of the mouse brain catecholamine system in the study of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and because little information is available concerning the chemical neuroanatomy of the mouse, catecholamine uptake sites were mapped in C57 black mouse brain using [3H]mazindol autoradiography. Displacement studies with known dopamine (DA) and noradrenaline (NA) uptake blockers showed that binding in the striatum was entirely to DA uptake sites, while binding in the locus coeruleus was to NA uptake sites only. By using the selective noradrenergic uptake blocker desmethylimipramine (DMI), a complete map of both DA and NA uptake sites was generated. The mesostriatal DA system was the most clearly labelled and uptake sites were seen better in striatal terminals than the substantia nigra. Within the noradrenergic system, highest binding levels were seen over the locus coeruleus, although it was unclear whether these uptake sites were on cell bodies or terminals from the lateral tegmental noradrenergic system. These maps of the catecholamine uptake system in mouse brain provide a baseline for study of newly discovered neurotoxins and ageing processes.

Biochemical mechanism of action of the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)

The various biochemical mechanisms considered to explain the selective dopaminergic neurotoxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) are reviewed. MPTP is metabolized by monoamine oxidase in the brain, ultimately yielding 1-methyl-4-phenylpyridinium cation (MPP+), which is accumulated in dopamine cells by the high-affinity dopamine uptake pump. Cell death appears to reflect a compromise in energy production arising as a result of the Nernstian concentration of MPP+ inside mitochondria and persistent inhibition of Site 1 of the respiratory chain. The structural features underlying each biochemical step involved in the expression of neurotoxicity are described, and the implications of the MPTP phenomenon to efforts aimed at elucidating the pathogenesis of idiopathic parkinsonism are discussed.

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)

Neuropathological study on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine of the crab-eating monkey

Experimental parkinsonism was induced by systemic administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to the crab-eating monkey (Macaca fascicularis). In the acute stage, the substantia nigra showed necrotic nerve cells, extracellular release of pigment granules and histiocytic infiltration. The nerve cells underwent vacuolation of the cytoplasm with chromatin clumping of the nucleus and disintegration of the nucleolus. The striking feature was the presence of variously-shaped inclusion bodies within abnormal mitochondria which sometimes disclosed a distortion of the cristae. Golgi apparatus and endoplasmic reticulum were also dilated. In the locus ceruleus swollen nerve cells were observed with vacuolated cytoplasm and pyknotic nucleus where expanded mitochondria also contained the inclusions. Some of the inclusion bodies are probably insoluble precipitations due to inhibition of mitochondrial oxidation by a certain metabolite of MPTP. In the protracted stage the substantia nigra revealed a considerable loss of the nerve cells associated with melanophagia and astrocytic proliferation. A few surviving nerve cells showed an increased number of Golgi apparatus and rough endoplasmic reticula, and the presence of autophagosomes, dense bodies and intra-mitochondrial inclusion. These changes are interpreted as being a part of the reparative process from the cellular damage.

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.