Novel observations with FDOPA-PET imaging after early nigrostriatal damage

Striatal 6-[18F]fluoro-L-DOPA (FDOPA) kinetic rate constants were measured by positron emission tomography (PET) in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated squirrel monkeys. After scanning, stereological counts of dopaminergic neurons were done in substantia nigra, and dopamine (DA) and metabolite concentrations were determined in the caudate, putamen, and substantia nigra. Graded doses of MPTP produced animals with mild to moderate reductions (10-35%) in dopaminergic neurons, where the percent of cell loss was proportional to the amount of MPTP given. Striatal DA and metabolite concentrations were relatively unchanged in animals given 1.0 and 1.5 mg/kg of MPTP, but were significantly reduced after 2.0 mg/kg of MPTP. All animals injected with a single dose of MPTP showed no overt signs of parkinsonism. In contrast, DA and metabolite concentrations in the substantia nigra were significantly reduced for all MPTP-treated animals. Reduction of dopaminergic indices in the substantia nigra did not parallel reductions in the striatum, indicating differential sensitivity of the nigrostriatal pathway to the neurotoxic effects of MPTP. The percent change in FDOPA uptake (Ki) and decarboyxlation (k3) after MPTP showed significant positive correlations to striatal DA levels, but not to the number of dopaminergic neurons. This suggests that FDOPA is a good index of striatal DA levels.

7-Nitroindazole prevents 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine-induced ATP loss in the mouse striatum

The neurotoxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is dependent upon the MAO-B (monoamine oxidase type B)-catalyzed production of 1-methyl-4-phenylpyridinium ion (MPP(+)) and is likely to involve a perturbation of energy metabolism. Protection against MPTP neurotoxicity has been shown by treating mice with 7-nitroindazole (7-NI), a reversible inhibitor of both MAO-B and neuronal nitric oxide synthase (nNOS) activity. The objective of the present study was to evaluate (i) the relationship between the neuroprotective effect of 7-NI and MPTP-induced energy deficiency, and (ii) the role of nitric oxide production as a potential mechanism for energy perturbation after MPTP exposure. Maximum protection against striatal dopamine depletion and nigral neuronal loss was achieved when 7-NI (50 mg/kg, i.p.) was administered to C57BL/6 mice immediately before and after MPTP (50 mg/kg, s.c.). This short-term regimen of 7-NI administration parallels the time when MPTP exposure causes energy failure. 7-NI also completely prevented the loss of striatal ATP that occurs in mice during the initial hours after MPTP administration. In contrast, N(G)-nitro-L-arginine (two injections of 50 mg/kg each, given i.p. 20 and 4 h prior to MPTP), another NOS inhibitor, failed to affect MPTP-induced ATP depletion. Taken together, data indicate that (i) a temporal and causal relationship exists between the neuroprotective effect of 7-NI and its ability to counteract ATP reduction, and (ii) MAO-B rather than NOS inhibition is the mechanism by which 7-NI counteracts MPTP-induced ATP depletion.

Diethyldithiocarbamate causes nigral cell loss and dopamine depletion with nontoxic doses of MPTP

Although nontoxic when administered alone, diethyldithiocarbamate (DDC) is known to enhance the dopamine-depleting effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in the mouse striatum. The purpose of the present study was twofold: (i) to carefully characterize the effects of DDC on MPTP-induced degeneration of dopaminergic neurons in substantia nigra pars compacta using unbiased, stereological cell counting techniques and (ii) to determine whether or not DDC can convert a nontoxic dose of MPTP into one which is clearly toxic on dopaminergic neurons in the substantia nigra. A single low dose of MPTP (15 mg/kg intraperitoneally (ip)) was used for these studies, which failed to induce any neurochemical or histological effects on the nigrostriatal system of C57BL/6 mice when administered alone. However, when animals were pretreated with DDC (400 mg/kg ip), the same dose of MPTP resulted in a 50% loss of neurons in the substantia nigra pars compacta, as well as a 70% reduction in striatal dopamine (DA). A 31% reduction of DA in the ventral mesencephalon was also seen. This combined regimen of DDC and MPTP was not significantly different from a maximally tolerated "toxic" dose of MPTP alone (15 mg/kg x 4, 1 h apart, ip). As expected, animals receiving DDC alone did not show any dopamine depletion nor nigral neuronal loss. The present study confirms previous work suggesting that DDC enhances MPTP-induced nigral cell loss and shows for the first time that DDC can "unmask" MPTP toxicity. These observations could have implications for theories on the cause of Parkinson's disease.