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Aripiprazole Activation Mechanism — MRCPsych Paper B MCQ

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HardSchizophrenia & PsychosisAripiprazole Activation MechanismMRCPsych Paper B

A 24-year-old man with first-episode schizophrenia starts aripiprazole. During the first week, he develops new subjective inner restlessness, pacing and difficulty initiating sleep. There is no elevated mood, reduced need for sleep or worsening psychosis. Which receptor-level action best reconciles aripiprazole's antipsychotic effect in hyperdopaminergic states with its capacity to have activating effects when endogenous dopamine tone is low?

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Correct answer: C — Partial D2 receptor agonism with lower intrinsic efficacy than dopamine

The best answer is C. Aripiprazole is a D2 partial agonist with lower intrinsic efficacy than dopamine. In a high-dopamine state, it competes with dopamine and reduces net D2 signalling; when dopaminergic tone is low, it can retain net agonist activity. This bidirectional pharmacology best accounts for its potential activating profile. Restlessness, akathisia and insomnia are recognised adverse effects, although the biology of akathisia is multifactorial and should not be attributed specifically to mesolimbic activation. A neutral D2 antagonist cannot generate agonist signalling. H1 inverse agonism and M1 antagonism are associated principally with sedative or anticholinergic effects, and aripiprazole has no appreciable muscarinic affinity. Aripiprazole is not a 5-HT2C agonist; its established serotonergic actions include 5-HT1A partial agonism and 5-HT2A antagonism.

Reference: Electronic Medicines Compendium. Aripiprazole Milpharm 5 mg tablets, Summary of Product Characteristics, sections 4.8 and 5.1. Text revised 2026. https://www.medicines.org.uk/emc/product/7073/smpc