L-DOPA-elicited abnormal involuntary movements in the rats damaged severely in substantia nigra by 6-hydroxydopamine

Background: Dyskinesia of rat models can occur in several conditions: acute levodopa (L-DOPA) administration provided that the drug dose is sufficiently high and/or that the nigrostriatal dopamine (DA) pathway is seriously damaged, and repeated L-DOPA administration which could cause a reduction of the dyskinesia-threshold dose, a progressive aggravation and an increasing incidence of dyskinesia. Therefore, if the damage of the nigrostriatal DA pathway is extremely severe, what abnormal movements can be elicited by first injecting L-DOPA or other dopaminergic agonists? The problem deserves exploring. Methods: Rat models with damage of varying severity were divided into three groups: the serious lesion [induced by 40 mu g 6-hydroxydopamine (6-OHDA), two injected coordinates including substantia nigra (SN) and medial forebrain bundle], the moderate lesion (20 mu g 6-OHDA, a coordinate in SN) and the control. Three weeks after lesion, the Rota Rad test and Cylinder test were performed to assess the motor activities of rat models, the abnormal involuntary movements (AIMs) elicited by L-DOPA or apomorphine (APO) were observed, and the dopaminergic degeneration in SN and striatum was determined. Results: Both seriously lesioned rats and the moderately were observed to exhibit a significant decrease in motor activities. In the rats with a serious lesion, scarcely any dopaminergic neurons were present in the SN, tissue DA level decreased by 99% in the striatum, and both L-DOPA and APO could elicit AIMs and rotational movements. In the rats with the moderate lesion, only rotation movements could be elicited. The rotation speed of moderately lesioned rats was 9 turns/min, but that of seriously was only 4.5 turns/min elicited by APO. Conclusions: Both dyskinesia and rotation movement are the specific expressions elicited by L-DOPA or APO in rats whose SN is damaged by 6-OHDA. Dyskinesia reflects more severe damage than rotation movement.