Agonist and Antagonist Effects of Aripiprazole on D(2)-Like Receptors Controlling Rat Brain Dopamine Synthesis Depend on the Dopaminergic Tone

BACKGROUND: The atypical antipsychotic drug aripiprazole binds with high affinity to a number of G protein coupled receptors, including dopamine D(2) receptors, where its degree of efficacy as a partial agonist remains controversial. METHODS: We examined the properties of aripiprazole at D(2)-like autoreceptors by monitoring the changes of dopamine synthesis in adult rat brain striatal minces incubated ex vivo. The effects of the dopaminergic tone on the properties of aripiprazole were assayed by comparing a basal condition (2mM K(+), low dopaminergic tone) and a stimulated condition (15mM K(+), where dopamine release mimics a relatively higher dopaminergic tone). We also used 2 reference compounds: quinpirole showed a clear agonistic activity and preclamol (S-(-)-PPP) showed partial agonism under both basal and stimulated conditions. RESULTS: Aripiprazole under the basal condition acted as an agonist at D(2)-like autoreceptors and fully activated them at about 10nM, inhibiting dopamine synthesis similarly to quinpirole. Higher concentrations of aripiprazole had effects not restricted to D(2)-like autoreceptor activation. Under the stimulated (15mM K(+)) condition, nanomolar concentrations of aripiprazole failed to decrease dopamine synthesis but could totally block the effect of quinpirole. CONCLUSIONS: Under high dopaminergic tone, aripiprazole acts as a D(2)-like autoreceptor antagonist rather than as an agonist. These data show that, ex vivo, alteration of dopaminergic tone by depolarization affects the actions of aripiprazole on D(2)-like autoreceptors. Such unusual effects were not seen with the typical partial agonist preclamol and are consistent with the hypothesis that aripiprazole is a functionally selective D(2)R ligand.