[Effects of different doses of levodopa on paraquat-induced neurotoxicity: an experiment with mice].

Objective: To investigate the effects and mechanisms of different doses of levodopa on paraquat-induced neuro-toxicity.

Methods: 72 C57BL mice were divided into 2 equal groups: acute experiment group and chronic experiment groups. The acute experiment group was re-divided into 2 subgroup: subgroup A to be injected with levodopa of the doses of 0 (distilled water instead), 10 mg/kg, or 100 mg/kg and then paraquat 30 mg/kg (levodopa + paraquat), and then killed 90 minutes after; and subgroup B, to be injected with paraquat 30 mg/kg and then levodopa 0, 10 mg/kg, or 100 mg/kg (paraquat + levodopa), and then killed 2 hours after. The chronic experiment group was re-divided into 2 subgroups to be injected with levodopa + paraquat or paraquat + levodopa once a week for 3 weeks, and then killed 24 hours after the injection. Fluorescent microscopy was used to observe the fluorescent staining of paraquat in the substantia nigra in the acute experiment group and the fluorescent staining of tyrosine hydroxylase (TH) in the substantia nigra in the chronic experiment group. in the chronic experiment group Western blotting was used to examine the protein expression of TH; thioflavine double labeling was used to observe the alpha-synuclein aggregation by immunofluorescence staining and Western blotting. The slices of substantia nigra of the mice in the chronic experiment group treated with distilled water + paraquat were inoculated with or without 250 micromol/L levodopa and then underwent thioflavine staining to observe the alpha-Syn aggregation.

Results: The paraquat staining was strongly positive in the substantia nigra of the mice in Group A-1, and was decreased gradually in the group A-2 and A-3. The paraquat staining was strongly positive in the substantia nigra of Group B-1 without a significant difference between Group A-1 and Group B-1, and was not remarkable in Group B-2 and B-3. The TH staining and protein expression in the substantia nigra of Group A-2 were significantly stronger than that of Group A-1 (P < 0.05), and the TH staining was remarkably weaker in Group A-3 (P < 0.05), as shown by immunofluorescence staining and Western blotting. There was no significant difference in TH staining and protein expression in the substantia nigra among Group A-1, Group B-1, and Group B-2 (all P > 0.05). However, the TH staining was remarkably weaker in Group B-3 (P < 0.05). The thioflavine and alpha-Syn double staining was significantly weaker in Groups A-2 and A-3 in comparison with Group A-1. There was no significant difference in the double staining among Group A-1, Group B-1, and Group B-2 (all P > 0.05). However, the double staining was remarkably weaker in Group B-3 (P < 0.05). The thioflavine positive staining in the tissue slices inoculated with levodopa was significantly weaker in comparison with those un-inoculated.

Conclusion: Pre-treatment with lower dose L-dopa before the paraquat administration is neuroprotective by preventing paraquat from access into central nervous system through a blood-brain barrier competitive uptake mechanism, while higher dose L-dopa shows neurotoxicity through disaggregating alpha-synuclein deposits in Parkinsonian mice.