Abstract
The effect of a selective agonist of the dopamine D 1 receptor (SKF 38393) and of the D 2 receptor (LY-171555) was tested acutely in normal and in monkeys with a parkinsonian syndrome induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). The D 2 agonist induced a strong locomotor response and lingual dyskinesia in both normal and parkinsonian monkeys. The D 1 agonist however had no locomotor effect by itself but induced tongue protrusions in normal monkeys only. It appeared to potentiate the dyskinetic effect of LY 171555 in MPTP monkeys but it antagonized the locomotor action of the D 2 agonist in both normal and MPTP monkeys. The selective D 1 and D 2 antagonists SCH 23390 and sulpiride were also tested. Both compounds were able to suppress the dyskinetic action of the combined agonists in normal animals but only the D 2 antagonist was effective in the same conditions in MPTP monkeys. These findings emphasize the importance of the D 2 receptor in mediating the locomotor response as well as dyskinesia in monkeys.
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