Clinical Psychopharmacology · Paper A
Antipsychotic receptor pharmacology
Because side effects and clinical use both trace back to specific receptors rather than the crude 'typical vs atypical' label, antipsychotics are best understood receptor by receptor: e.g. one drug fully blocks dopamine receptors, another blocks them only partially, and a third works mainly through serotonin receptors instead. Clinically, matching a specific named drug to its specific receptor mechanism is a recurring exam theme and predicts its side-effect profile.
Typical antipsychotics such as haloperidol are potent full D2 (dopamine receptor) antagonistsA drug that blocks a receptor, stopping it from being activated.Read more →, giving high rates of extrapyramidal side effects (EPSE, drug-induced stiffness and tremor) and raised prolactinA hormone, raised by some antipsychotic drugs, linked to milk production and periods stopping.Read more → (a hormone that, when elevated, can cause breast milk production and sexual side effects). Atypical antipsychotics add strong 5-HT2AA serotonin receptor subtype that many antipsychotic drugs block, thought to help their antidepressant and antipsychotic effect.Read more → (a serotonin receptor) antagonism on top of D2 blockade, which is thought to reduce EPSE risk. Aripiprazole, brexpiprazole and cariprazine are D2 partial agonistsA drug that activates a receptor but produces a smaller effect than a full agonist, even at a high dose.Read more →, sometimes called 'dopamine system stabilisers', acting as an antagonist where dopamine is high and an agonistA drug or chemical that activates a receptor to produce an effect.Read more → where it is low. H1 (histamine) blockade causes sedation and weight gain, M1 (muscarinic) blockade causes anticholinergicBlocking the action of acetylcholine, a nerve chemical, causing effects like a dry mouth and blurred vision.Read more → effects (dry mouth, constipation, blurred vision), and alpha-1 (adrenergic) blockade causes orthostatic hypotension (a drop in blood pressure on standing): e.g. a patient switched from haloperidol to aripiprazole for troublesome stiffness often improves because the partial-agonist mechanism reduces that specific side effect. In practice, knowing which named drug maps to which specific mechanism is more useful than only knowing the typical/atypical label.
A fresh doctor might learn antipsychotics purely as "typical" versus "atypical" and stop there. That label is a historical shortcut, not a mechanism: real prescribing decisions and real side-effect prediction come from knowing which specific receptor a specific drug acts on. Hold the phrase RECEPTOR PROFILE through every section below, because every side effect discussed here traces back to one.
Antipsychotics are medications used primarily to treat psychosis (a state involving , , or disorganised thinking, diagnosed clinically rather than through any laboratory investigation), and their therapeutic and side-effect profiles are best explained by which receptors they block or, in some cases, partially activate. The core receptors involved are D2 (dopamine), (a serotonin receptor), H1 (histamine), M1 (muscarinic, part of the cholinergic system), and alpha-1 (an adrenergic receptor).
A common mistake is assuming "atypical" always means fewer side effects overall. Atypical antipsychotics trade one side-effect profile for another: many, particularly olanzapine and clozapine, carry substantial metabolic risk (weight gain, dyslipidaemia, and new-onset diabetes) driven by H1 and other receptor effects, which can be more clinically significant long-term than the extrapyramidal side effect (EPSE) risk they reduce. The typical/atypical label describes an era of drug development, not a guarantee of a safer side-effect profile.
Imagine three different antipsychotics: one blocks dopamine fully and permanently, one blocks them only partially, and one barely touches dopamine at all and works mainly through serotonin instead. D2 receptor blockade in the brain's is thought to underlie the core antipsychotic effect (reducing and ), but D2 blockade elsewhere in the brain causes most of the classic side effects: this single receptor explains both the benefit and much of the harm.
Typical (first-generation) antipsychotics such as haloperidol are potent, full D2 . Strong D2 blockade in the causes extrapyramidal side effects (EPSE, drug-induced stiffness, tremor, and abnormal movements), and D2 blockade in the causes raised (a hormone that, when elevated, can cause breast milk production, menstrual disturbance, and sexual side effects). Atypical (second-generation) antipsychotics add strong antagonism on top of D2 blockade, which is thought to reduce EPSE risk relative to typical agents at equivalent doses. Aripiprazole, brexpiprazole, and cariprazine are D2 , sometimes called "dopamine system stabilisers": they act as an antagonist where dopamine activity is high and as an where it is low, rather than blocking the receptor completely in every state. For example, a patient switched from haloperidol to aripiprazole for troublesome stiffness often improves specifically because this partial-agonist mechanism produces less complete D2 blockade than a full antagonist. H1 (histamine) blockade causes sedation and weight gain, and is prominent with drugs such as olanzapine and quetiapine. M1 (muscarinic) blockade causes effects: dry mouth, constipation, blurred vision, and urinary retention. Alpha-1 (adrenergic) blockade causes orthostatic (postural) hypotension, a drop in blood pressure on standing that can cause dizziness or falls, particularly in elderly patients.
A full D2 like haloperidol should not be confused mechanistically with a partial D2 like aripiprazole, even though both are labelled antipsychotics and both reduce psychotic symptoms: their behaviour in low-dopamine states differs completely. Sedation and weight gain from H1 blockade should not be confused with EPSE or raised from D2 blockade; these arise from different receptors entirely and require different clinical monitoring. malignant syndrome (a rare, life-threatening reaction to dopamine blockade causing fever, rigidity, and autonomic instability) should not be confused with ordinary EPSE: it is a medical emergency requiring drug cessation and supportive care, not just a dose review.
There is no diagnostic test that identifies a drug's receptor profile at the bedside; instead, a clinician works out which receptor is responsible for a given side effect by matching the pattern of symptoms to the known pharmacology of the prescribed drug. Raised is confirmed with a serum prolactin level when galactorrhoea or menstrual disturbance is reported. EPSE is assessed clinically (observing tremor, rigidity, and abnormal movements, sometimes using a rating scale), and metabolic side effects are monitored with weight, waist circumference, fasting glucose, and lipid profile, following routine physical health monitoring for anyone prescribed an antipsychotic.
Receptor profile guides first-line drug choice and side-effect monitoring in line with NICE and Maudsley Prescribing Guidelines-informed practice: for example, choosing a lower-EPSE-risk atypical agent for a patient especially sensitive to movement side effects, or avoiding a strongly sedating, weight-gain-prone agent in a patient with existing metabolic risk factors. Monitoring for antipsychotic treatment typically includes physical health checks (weight, lipids, glucose, and ) alongside assessment for EPSE, following standard psychiatric prescribing practice. Where a side effect is intolerable, management is usually to switch to an agent with a different receptor profile (for example, from a high-D2- typical agent to a like aripiprazole for troublesome EPSE) rather than simply adding another drug to treat the side effect.
Questions frequently describe a drug that is "an in high-dopamine states but an in low-dopamine states" and expect recognition of a such as aripiprazole specifically, not a generic "atypical" answer. Another recurring trap asks which receptor is responsible for a specific side effect described in a vignette: raised and galactorrhoea point to D2 blockade, sedation and weight gain point to H1 blockade, dry mouth and constipation point to M1 blockade, and postural dizziness points to alpha-1 blockade.
Match each named antipsychotic to its specific receptor mechanism rather than only its typical/atypical label: D2 blockade drives the antipsychotic effect but also extrapyramidal side effects and raised , blockade in atypicals offsets some of that EPSE risk, and D2 like aripiprazole behave differently from full like haloperidol, while H1, M1, and alpha-1 blockade separately explain sedation and weight gain, effects, and postural hypotension. Don't ask "typical or atypical": ask which receptor, because that's what actually predicts the side effect in front of you.