The D2 receptor partial agonism of aripiprazole means that in a dopamine-depleted (hypo-dopaminergic) state, aripiprazole will:

• A Provide intrinsic agonist activity, partially stimulating D2 receptors ✓
• B Act as a full antagonist and worsen parkinsonian symptoms
• C Have no pharmacological effect as it cannot bind without endogenous dopamine
• D Upregulate D2 receptor expression compensating for the deficit

Explanation

Aripiprazole is a D2 partial agonist: in hyperdopaminergic states (mesolimbic pathway in psychosis), it competes with excess dopamine and acts as a functional antagonist, reducing excessive D2 stimulation. In hypodopaminergic states (mesocortical pathway, nigrostriatal), it provides intrinsic agonist activity, partially stimulating D2 receptors. This functional selectivity is why aripiprazole causes less EPS and hyperprolactinaemia than typical antipsychotics.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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