Levodopa induces long-lasting modification in the functional activity of the 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 (18)F-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 7weeks 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 11months and then followed by a washout period of 6months. (18)F-DOPA positron emission tomography (PET) scans were performed at baseline, after MPTP intoxication, following 11months of treatment, and after a washout period of 1, 3 and 6months. Monkeys were sacrificed 6months after concluding either placebo or levodopa treatment and immediately after the last (18)F-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 (18)F-DOPA PET studies performed at 3 and 6months 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.
Department of Neurology, Clínica Universidad de Navarra, Pamplona, Spain; Laboratory of Regenerative Therapy, Center for Applied Medical Research (CIMA), Pamplona, Spain.
Mario
M
Riverol
Laboratory of Regenerative Therapy, Center for Applied Medical Research (CIMA), Pamplona, Spain.
Cristina
C
Ordóñez
Small Animal Imaging Research Unit, Center for Applied Medical Research (CIMA), Clínica Universidad de Navarra, Pamplona, Spain; Department of Nuclear Medicine, Clínica Universidad de Navarra, Pamplona, Spain.
María
M
Collantes
Laboratory of Regenerative Therapy, Center for Applied Medical Research (CIMA), Pamplona, Spain.
Carla
C
DiCaudo
Small Animal Imaging Research Unit, Center for Applied Medical Research (CIMA), Clínica Universidad de Navarra, Pamplona, Spain; Department of Nuclear Medicine, Clínica Universidad de Navarra, Pamplona, Spain.
Iván
I
Peñuelas
Department of Nuclear Medicine, Clínica Universidad de Navarra, Pamplona, Spain.
Javier
J
Arbizu
Laboratory of Regenerative Therapy, Center for Applied Medical Research (CIMA), Pamplona, Spain.
Irene
I
Marcilla
Department of Neurology, Clínica Universidad de Navarra, Pamplona, Spain; Laboratory of Regenerative Therapy, Center for Applied Medical Research (CIMA), Pamplona, Spain. Electronic address: rluquin@unav.es.
María R
MR
Luquin
D000818:Animals; D000978:Antiparkinson Agents / Q000494:pharmacology*; D001142:Aromatic-L-Amino-Acid Decarboxylases / Q000378:metabolism; D003342:Corpus Striatum / Q000000981:diagnostic imaging / Q000187:drug effects* / Q000378:metabolism; D004295:Dihydroxyphenylalanine / Q000031:analogs & derivatives; D050483:Dopamine Plasma Membrane Transport Proteins / Q000378:metabolism; D059290:Dopaminergic Neurons / Q000187:drug effects* / Q000378:metabolism; D007980:Levodopa / Q000494:pharmacology*; D008252:Macaca fascicularis; D008279:Magnetic Resonance Imaging; D008297:Male; D009434:Neural Pathways / Q000187:drug effects; D020734:Parkinsonian Disorders / Q000000981:diagnostic imaging / Q000378:metabolism*; D049268:Positron-Emission Tomography; D013378:Substantia Nigra / Q000187:drug effects* / Q000378:metabolism
D000978:Antiparkinson Agents; D050483:Dopamine Plasma Membrane Transport Proteins; C043437:fluorodopa F 18; D007980:Levodopa; D004295:Dihydroxyphenylalanine; D001142:Aromatic-L-Amino-Acid Decarboxylases
(18)F-DOPA positron emission tomography; Dopaminergic marker; Levodopa; MPTP; Parkinson's disease