Chronic levodopa impairs the recovery of dopamine agonist-induced rotational behavior following neural grafting

The effect of chronic levodopa treatment on the function of embryonic mesencephalic tissue grafts was assessed in rats by monitoring rotational behavior elicited by dopamine (DA) agonists before and after neural grafting. Rats were given unilateral 6-hydroxydopamine (6-OHDA) lesions of the nigrostriatal pathway and baseline measures of rotational behavior induced by D1 receptor stimulation, D2 receptor stimulation, or amphetamine were determined. Subsequently, DA grafts were implanted into the lesioned striatum and chronic regimens of either saline or levodopa began one day after neural grafting and were continued for 7 weeks. Rotational behavior elicited by the D1 agonist, SKF 38393, was completely attenuated throughout the six week-period following the commencement of levodopa treatment, regardless of the absence or presence of a DA graft. Conversely, rotational behavior elicited by the D2 agonist, quinpirole, was significantly elevated in ungrafted animals receiving chronic levodopa. Grafted animals receiving chronic levodopa did not show a significant reduction in rotational behavior, whereas grafted animals receiving chronic saline showed a significant 67% reduction in quinpirole-induced rotational behavior. Amphetamine-induced rotational behavior was reduced in both levodopa and saline treated grafted animals, however grafted animals receiving chronic levodopa treatment showed a reduction of rotational behavior that was uncharacteristic and less compensatory than that observed in grafted animals receiving chronic saline treatment. Morphology of grafts indicate that there were areas of impaired neurite outgrowth of TH-positive fibers in animals treated with levodopa. The results of the present study suggest that the impaired recovery in quinpirole and amphetamine-induced rotational behavior in grafted animals receiving chronic levodopa treatment may be related to (1) impaired graft function, (2) an alteration in pre- and postsynaptic mechanisms in the host DAergic system, or (3) a combined effect of (1) and (2).