Relative potency and efficacy of some dopamine agonists with varying selectivities for D1 and D2 receptors in MPTP-induced hemiparkinsonian monkeys

A series of dopamine agonists were studied on contraversive circling behavior in seven 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced hemiparkinsonian monkeys. The compounds selected included [1R,3S]3-(1' adamantyl)-1-aminomethyl-3,4-dihydro-5-6- dihydroxy-1H-2-benzopyran hydrochloride (A-77636), L-3,4-dihydroxyphenylalanine methyl ester hydrochloride ester (L-dopa-methyl ester), (-)-2-[N-propyl-N-(2-thienyl) ethyl-amino-5-hydroxy-tetralin]hydrochloride (N-0923), pergolide, (+)-(4aR)-trans-3,4,4a,5,6,10b-hexahydro-4-propyl-2H-n aphth[1,2-b]-1,2-oxazin-9-ol, naxagolide (PHNO), (+/-)6-chloro-7, 8-dihydroxy-2,3,4,5-tetrahydro-1-phenyl-1H-3-benzazepine hydrobromide, (+-) chloro-PB hydrobromide (SKF-81297) and (+-) 6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benza zep ine hydrobromide, (+-) chloro-APB hydrobromide (SKF-82958). The dose-effect relationship of each of these compounds was determined by measuring contraversive turns/120 min after an i.m. injection. There were marked differences in the potency and efficacy of the various compounds studied. The most potent compounds were the selective D2 agonists PHNO and N-0923. L-dopa methyl ester was equally effective, but much less potent. The D1 agonist A-77636 was equally effective. The D1 agonist SKF-82958 was also effective, but less potent. In the doses studied, the D1 agonist SKF-81297 was ineffective. With the exception of L-dopa methyl ester, the greater the D1/D2 affinity ratio, the greater the 50% of maximal dose to induce contraversive circling (r = 0.974, P < .05).