Vol. 283, Issue 2, 794-799, 1997

Motor Response to a Dopamine D3 Receptor Preferring Agonist Compared to Apomorphine in Levodopa-Primed 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine Monkeys¹

P. J. Blanchet, S. Konitsiotis and T. N. Chase

Experimental Therapeutics Branch, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland

The profile of dopamine receptor subtype activation contributing to the therapeutic efficacy and motor response complications of levodopa (nonselective pro-agonist) in Parkinson's disease remains unclear. Potent, selective, short-acting dopamine D2 receptor subfamily agonists show good antiparkinsonian efficacy but produce dyskinesias comparable to levodopa. Nonetheless, agonists displaying higher affinity for dopamine receptors other than the D2 subtype may have a better therapeutic index. To clarify this issue, we compared the nonselective dopamine D1/D2 receptor subfamilies agonist apomorphine to the dopamine D3 receptor preferring agonist [R-(+)-trans-3,4,4a,10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano[4,3-b]-1,4-oxazin-9-ol] (PD 128,907) in 6 levodopa-primed , 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned parkinsonian monkeys with reproducible dyskinesias. Single s.c. dosing with the lowest fully effective dose of apomorphine (averaging 27.9 ± 4.5 µg/kg) and PD 128,907 (averaging 41.7 ± 4.4 µg/kg) yielded equivalent antiparkinsonian efficacy on the behavioral scale and portable activity monitoring used. A comparable significant dose-dependent increase in the response magnitude and duration was seen with two higher doses. The severity of dyskinesia was also similar between the two drugs. When the lower dose for each drug was administered six times at a fixed 90-min interval, both drugs remained efficacious with no significant tolerance observed. The D3 receptor preferring antagonist U-99194A significantly reduced the motor effects of both apomorphine and PD 128,907. Thus, increased D3 receptor tone does not acutely ameliorate dyskinesias in levodopa-primed parkinsonian monkeys. Given the reported lack of affinity of PD 128,907 for central D1 receptors, our data support the concept that the pharmacological activation of D1 receptors is not mandatory for relief of parkinsonism and production of dyskinesia.