Levodopa induces long-lasting modification in the functional activity of the nigrostriatal pathway

- Striatal F-DOPA uptake raised during the washout period in levodopa-treated monkeys.
- Placebo and levodopa-treated animals exhibited a similar number of nigral neurons.
- No differences were observed in DAT or AADC levels between both experimental groups.
- TH levels in the posterior putamen were lower in the levodopa-treated animals.
- Levodopa induced modifications in the dopaminergic nigrostriatal pathway.

Much controversy exists concerning the effect of levodopa on striatal dopaminergic markers in Parkinson's disease (PD) and its influence on functional neuroimaging. To deal with this issue we studied the impact of neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and chronic levodopa administration on striatal 18F-DOPA uptake (Ki) in an animal model of PD. The levels of several striatal dopaminergic markers and the number of surviving dopaminergic neurons in the substantia nigra (SN) were also assessed. Eleven Macaca fascicularis were included in the study. Eight animals received weekly intravenous injections of MPTP for 7 weeks and 3 intact animals served as controls. MPTP-monkeys were divided in two groups. Group I was treated with placebo while Group II received levodopa. Both treatments were maintained for 11 months and then followed by a washout period of 6 months. 18F-DOPA positron emission tomography (PET) scans were performed at baseline, after MPTP intoxication, following 11 months of treatment, and after a washout period of 1, 3 and 6 months. Monkeys were sacrificed 6 months after concluding either placebo or levodopa treatment and immediately after the last 18F-DOPA PET study. Striatal dopamine transporter (DAT) content, tyrosine hydroxylase (TH) content and aromatic l-amino acid decarboxylase (AADC) content were assessed. In Group II 18F-DOPA PET studies performed at 3 and 6 months after interrupting levodopa showed a significantly increased Ki in the anterior putamen as compared to Group I. Levodopa and placebo treated animals exhibited a similar number of surviving dopaminergic cells in the SN. Striatal DAT content was equally reduced in both groups of animals. Animals in Group I exhibited a significant decrease in TH protein content in all the striatal regions assessed. However, in Group II, TH levels were significantly reduced only in the anterior and posterior putamen. Surprisingly, in the levodopa-treated animals the TH levels in the posterior putamen were significantly lower than those in the placebo group. AADC levels in MPTP groups were similar to those of control animals in all striatal areas analyzed. This study shows that chronic levodopa administration to monkeys with partial nigrostriatal degeneration followed by a washout period induces modifications in the functional activity of the dopaminergic nigrostriatal pathway.