Although partial efficacy dopamine D1 receptor agonists have little therapeutic benefit in parkinsonism, the first high potency, full efficacy dopamine D1 receptor agonist dihydrexidine recently has been shown to have profound antiparkinsonian effects. One reason for the greater antiparkinsonian effects of dihydrexidine vs. SKF38393 might be that SKF39393, while a partial dopamine D1 receptor agonist in rodent striatal preparations, has virtually no agonist activity in monkey striatum (Pifl et al., 1991, Eur. J. Pharmacol. 202, 273). To explore this hypothesis, we compared the dopamine D1 receptor affinity and efficacy of dihydrexidine and SKF38393 in striatum from rat and monkey. In vitro binding studies using membranes from putamen of adult rhesus monkeys demonstrated that dihydrexidine competed for dopamine D1 receptors (labeled with [3H]SCH23390) with high potency (IC50 = 20 nM vs. ca. 10 nM in rat brain). SKF38393 was about 4-fold less potent than dihydrexidine in both monkey and rat brain. The in vitro functional activity of these drugs was assessed by their ability to stimulate adenylate cyclase activity in tissue homogenates. Dihydrexidine was of full efficacy (relative to dopamine) in stimulating cAMP synthesis in both monkey and rat. SKF38393 was only a partial efficacy agonist in both rat striatum and monkey putamen, but contrary to the original hypothesis, it had the same efficacy (ca. 40% relative to dihydrexidine) in membranes from both species. Interestingly, greater between-subject variation was found in the stimulation produced by SKF38393 in primate compared to rat brain, although the basis for this variation is unclear. The present data demonstrate for the first time that dihydrexidine is a full efficacy dopamine D1 receptor agonist in primate brain. Moreover, these data indicate that the partial efficacy dopamine D1 receptor agonist SKF38393 causes the same relative response (compared to dopamine) in rat and monkey dopamine D1 receptors. Together, this information suggests that the antiparkinsonian effect of dihydrexidine vs. the relative inactivity of SKF38393 is not due to the fact that primate brains are simply unresponsive to SKF38393.
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