Antipsychotics: Dopamine, Typicals & Atypicals
Block one transmitter — dopamine — and you can quiet the voices and delusions of psychosis. But dopamine runs in four separate highways of the brain, and a drug can't block just one. Silence dopamine everywhere and you calm the psychosis in one pathway while causing Parkinson-like stiffness in another, and a hormone surge in a third. That single fact — four pathways, one drug — explains everything antipsychotics do, for good and ill.
A young man is brought in by his frightened family. Over months he has become convinced the government is broadcasting into his mind; he hears voices commenting on his every move; his thoughts have become tangled and strange. This is a first episode of psychosis, most likely schizophrenia. Started on an antipsychotic, the voices and paranoia gradually fade over weeks — but then his movements slow and stiffen, as if he'd aged forty years. He got better and developed a new problem from the same drug, and the reason is a map of four dopamine roads.
The dopamine hypothesis and four pathways
Psychosis is linked to too much dopamine in one pathway. The dopamine hypothesis holds that the positive symptoms of psychosis — hallucinations and delusions — come from excess dopamine in the mesolimbic pathway. All antipsychotics block dopamine D2 receptors, and blocking mesolimbic dopamine is what treats those symptoms. The trouble is that dopamine runs in three other pathways, and the drug blocks them too. In the nigrostriatal pathway (movement), blockade causes extrapyramidal side effects — Parkinson-like stiffness, tremors, and abnormal movements. In the tuberoinfundibular pathway, dopamine normally suppresses the hormone prolactin, so blocking it RAISES prolactin (causing breast milk production, breast enlargement, and menstrual changes). And in the mesocortical pathway (thought and motivation), blockade may worsen the negative symptoms. One drug, four roads — that's the whole framework.
Typical (first-generation) antipsychotics
The typical antipsychotics — haloperidol (high potency) and chlorpromazine (low potency) — are strong D2 blockers. They work well against the positive symptoms but, because they block D2 so hard everywhere, they cause the most extrapyramidal side effects and the biggest prolactin rise. The extrapyramidal effects come in a sequence worth knowing: an acute dystonia (a sudden muscle spasm, e.g., of the neck or eyes) in hours to days, Parkinsonism and akathisia (an agonizing inner restlessness) over days to weeks, and — most feared — tardive dyskinesia, involuntary repetitive movements (often of the face and mouth) after months to years, which can be permanent. Low-potency typicals block fewer dopamine receptors but more muscarinic, histamine, and alpha receptors, so they cause more sedation, dry mouth, and low blood pressure instead.
Atypical (second-generation) antipsychotics
The atypicals — risperidone, olanzapine, quetiapine, aripiprazole — block D2 but ALSO block serotonin (5-HT2A) receptors, and that combination gives them fewer extrapyramidal effects and a little help with negative symptoms. That's why they're generally first-line today. But they trade one problem for another: metabolic side effects — weight gain, high blood sugar and diabetes, and raised cholesterol — especially with olanzapine. One atypical stands apart: clozapine is the MOST effective antipsychotic, reserved for treatment-resistant schizophrenia, but it can cause agranulocytosis (a dangerous drop in white cells) — so patients need regular blood-count monitoring — along with seizures, heart inflammation, and heavy drooling. The most powerful drug, gated behind the closest monitoring.
- Antipsychotics block dopamine D2; mesolimbic block treats positive symptoms.
- Nigrostriatal block → extrapyramidal effects; tuberoinfundibular block → ↑ prolactin.
- Typicals (haloperidol) = strong D2 block, most EPS & prolactin.
- Atypicals (risperidone, olanzapine…) block D2 + 5-HT2A → fewer EPS but metabolic effects.
- Clozapine is most effective (treatment-resistant) but risks agranulocytosis — monitor blood counts.
The dangerous reaction: NMS
The most dangerous acute reaction is neuroleptic malignant syndrome (NMS) — rare but life-threatening. From too much dopamine blockade, the patient develops high fever, severe 'lead-pipe' muscle rigidity, autonomic instability (unstable blood pressure and heart rate), confusion, and a very high muscle enzyme (CK) from muscle breakdown. It's a medical emergency: stop the antipsychotic, cool and support the patient, and give dantrolene or the dopamine agonist bromocriptine. Don't confuse it with serotonin syndrome from the antidepressant article — that has hyperreflexia and clonus and comes on fast, while NMS features rigidity and evolves over days. Same 'too much fever and rigidity' picture, different transmitter and different cause.
Notice the beautiful symmetry with the neurotransmitter foundations: psychosis is TOO MUCH dopamine, and Parkinson's is TOO LITTLE. So antipsychotics (which block dopamine) can cause Parkinsonism, and Parkinson's drugs (which boost dopamine) can cause psychosis. The two diseases and their treatments are mirror images across the same transmitter — which is exactly why the next article on Parkinson's is the flip side of this one.
- Expecting no side effects from atypicals. They cause metabolic problems (weight, diabetes, lipids).
- Missing tardive dyskinesia. It appears late and can be permanent — monitor for it.
- Confusing NMS (rigidity, slow) with serotonin syndrome (clonus/hyperreflexia, fast).
- Using clozapine without regular blood counts. Agranulocytosis can be fatal.
An antipsychotic causes Parkinson-like stiffness. Which dopamine pathway's blockade is responsible?
- Psychosis = excess mesolimbic dopamine; antipsychotics block D2 to treat it.
- Four dopamine pathways → blocking them causes EPS (movement) and raised prolactin.
- Typicals (haloperidol) = more EPS; atypicals (risperidone, olanzapine) = metabolic effects.
- Clozapine is most effective but risks agranulocytosis; watch for NMS (fever + rigidity).
- Katzung BG. Basic & Clinical Pharmacology — Antipsychotic Agents.
- Brunton LL, et al. Goodman & Gilman's The Pharmacological Basis of Therapeutics — Pharmacotherapy of psychosis & mania.
- Rang HP, Dale MM, et al. Rang & Dale's Pharmacology — Antipsychotic drugs.
- Whalen K. Lippincott Illustrated Reviews: Pharmacology — Antipsychotic drugs & dopamine pathways.
- Clinical guidelines — Extrapyramidal effects, tardive dyskinesia, NMS & clozapine monitoring.
