Effects of chronic levodopa administration on dopaminergic neurons and dopamine transmitter in Parkinsonian rats

Objective To investigate the effects of three-month levodopa administration on dopaminergic neurons and dopamine transmitter in parkinsonian rats. Methods Different doses of levodopa/benserazide were treated orally for three months in rats with 6-OHDA-induced partial or severe injuries of unilateral nigrostriatal pathway. The effects of levodopa on the remaining dopaminergic neurons in the substantia nigra were studied by the methods of rotational behavior, tyrosine hydroxylase (TH) immunohistochemical assays and high performance liquid chromatography (HPLC). Results No obvious behavioral changes were observed after levodopa treatment. No significant difference was found in the number of TH-positive cells in 6-OHDA-lesioned side over the non-lesioned side in substantia nigra between levodopa-treated and non-treated rats (P0.05). The contents of striatal dopamine and DOPAC were far more increased in levodopa-treated rats with a high dose than in levodopa-treated rats with a low dose and in saline-treated rats (P0.01). Conclusion It suggests that a three-month oral levodopa treatment should be not toxic for the remaining dopaminergic neurons in rats with a unilateral 6-OHDA lesion.