Early L-dopa, but not pramipexole, restores basal ganglia activity in partially 6-OHDA-lesioned rats

- Early-start L-dopa induces a lower level of dyskinesia than delayed-start L-dopa.
- Early-start L-dopa, but not pramipexole, normalizes 6-OHDA-induced molecular changes.
- Dyskinesias are not associated with an increase in D1-D3 heteromers.

The most appropriate time for the initiation of dopaminergic symptomatic therapy in Parkinson's disease remains debatable. It has been suggested that early correction of basal ganglia pathophysiological abnormalities may have long-term beneficial effects. To test this hypothesis, we investigated the early and delayed actions of L-dopa and pramipexole, using a delayed-start protocol of treatment. The effects of early and delayed administration of these drugs on motor response, development of dyskinesias, neurogenesis and molecular markers in basal ganglia were studied in rats with a unilateral and partial 6-hydroxydopamine-induced nigrostriatal lesion. Ten days after lesioning, rats received treatment with: a) L-dopa methyl ester (25 mg/kg with 6.25 mg/kg of benserazide, i.p., twice a day); b) pramipexole (0.5 mg/kg, sc, twice a day) or c) saline for 4 weeks. Four weeks after treatment initiation, rats from the saline group were distributed in three groups that then received the following treatments: d) L-dopa, e) pramipexole or f) saline, for 4 weeks more. Three animals in each treatment arm received 5-bromo-2-deoxyuridine injections (200 mg/kg) 3 days before starting treatment. When compared with delayed-start L-dopa, early-start L-dopa treatment induced a lower rotational response (p < 0.01), an improvement in limb akinesia (p < 0.05), a lower level of dyskinesia (p < 0.01) and a normalization of lesion-induced molecular changes in basal ganglia. When compared with delayed-start pramipexole, early-start pramipexole induced a higher rotational response (p < 0.01), but did not improve limb akinesia, induce dyskinesia nor normalize lesion-induced molecular changes. Neither significant modifications of striatal dopamine D1-D3 receptor heteromerization nor subventricular zone neurogenesis was found after any L-dopa or pramipexole treatments. Our data support a possible restoration of basal ganglia physiological mechanisms by early-start L-dopa therapy.