Aripiprazole's mechanism of action is different from those of the other FDA-approved atypical antipsychotics (., clozapine , olanzapine , quetiapine , ziprasidone , and risperidone ).         Rather than acting as a pure antagonist of the dopamine D 2 receptor , aripiprazole shows functional selectivity at the D 2 receptor, acting as a silent antagonist of some subpopulations of D 2 receptors but as a high-efficacy partial agonist ( intrinsic activity = 75%) of other D 2 -receptor subpopulations.  It appears to show predominantly antagonist activity on postsynaptic D 2 receptors and partial agonist activity on presynaptic D 2 receptors.  Aripiprazole is also a partial agonist of the D 3 receptor .  In healthy human volunteers, D 2 and D 3 receptor occupancy levels are high, with average levels ranging between approximately 71% at 2 mg/day to approximately 96% at 40 mg/day.   Most atypical antipsychotics bind preferentially to extrastriatal receptors, but aripiprazole appears to be less preferential in this regard, as binding rates are high throughout the brain.