Long-term Levodopa Treatment Accelerates The Circadian Rhythm Dysfunction In A 6-hydroxydopamine Rat Model Of Parkinson's Disease

Objective: Parkinson's disease(PD)patients with long-term levodopa(L-DOPA)treatment are suffering from severe circadian dysfunction.However,it is hard to distinguish that the circadian disturbance in patients is owing to the disease progression itself,or is affected by L-DOPA replacement therapy.This study was to investigate the role of L-DOPA on the circadian dysfunction in a rat model of PD.Methods: The rat PD model was constructed by a bilateral striatal injection with 6-hydroxydopamine(6-OHDA),followed by administration of saline or 25mg/kg L-DOPA for 21 consecutive days.Rotarod test,footprint test and open field test were carried out to evaluate the motor function.Suprachiasmatic nucleus(SCN),striatum,cortex,liver and plasma were collected at 6:00,12:00,18:00,and 24:00.Quantitative real-time PCR(QPCR)was used to examine the expression of clock genes.Enzyme-linked immunosorbent assay(ELISA)was used to determine the secretion level of cortisol and melatonin.High performance liquid chromatography(HPLC)was used to measure the neurotransmitters.Western blot was used to detect the expression of TH and D1,D2 receptors.Analysis of variance(ANOVA)was used for data analysis.Results: L-DOPA alleviated the motor deficits induced by 6-OHDA lesions in the footprint and open field test(P<0.01,P<0.001,respectively).After L-DOPA treatment,Bmal1 decreased in the SCN compared with 6-OHDA group at 12:00(P<0.01)and 24:00(P<0.001).In the striatum,the expression of Bmal1,Ror? was lower than that in the 6-OHDA group at 18:00(P<0.05)and L-DOPA seemed to delay the peak of Per2 to 24:00.But the amplitude of Clock was elevated in the cortex at four time points in L-DOPA group.In liver,L-DOPA did not affect the rhythmicity and expression of these clock genes(P>0.05).In addition,the cortisol secretion was increased(P>0.05),but melatonin was further inhibited after L-DOPA treatment at 6:00(P<0.01).Furthermore,D2 receptor expression was decreased in the striatum(P<0.05)while D1 receptor in the cortex was unaltered in 6-OHDA lesioned rats.L-DOPA treatment had no effect on the expression of these dopamine receptors.Conclusion: Our results indicated that in the circadian system of advanced PD rat models,circadian dysfunction is not only contributed by the degeneration of the disease itself,but long-term L-DOPA therapy may further aggravate it.