Antipsychotics are medications primarily used to manage psychotic disorders such as schizophrenia and bipolar disorder through blockade of dopamine and serotonin receptors. They are classified into first-generation and second-generation agents based on their receptor binding profiles and adverse effect patterns, which differ substantially between the two groups.

What Are Antipsychotics?

Psychotic disorders are characterized by delusions, hallucinations, disorganized thinking, and negative symptoms such as social withdrawal and apathy. The dopamine hypothesis of schizophrenia proposes that mesolimbic dopamine hyperactivity mediates positive symptoms, while mesocortical dopamine hypoactivity contributes to negative and cognitive symptoms. Antipsychotics are categorized by their relative affinities for dopamine, serotonin, and other receptors.

Mechanism of Action

All antipsychotics share the property of dopamine D2 receptor antagonism, which is essential for their antipsychotic effect and also responsible for their extrapyramidal adverse effects. The degree of D2 occupancy correlates with therapeutic response, with approximately sixty-five to eighty percent occupancy required for optimal effect.

First-generation antipsychotics such as haloperidol and chlorpromazine are primarily D2 antagonists with varying degrees of alpha-1 adrenergic, histamine H1, and muscarinic receptor blockade. Haloperidol is a high-potency agent with strong D2 affinity and minimal anticholinergic effects, resulting in a high risk of extrapyramidal symptoms. Chlorpromazine is a low-potency agent with broader receptor blockade, causing more sedation and anticholinergic effects but fewer extrapyramidal symptoms.

Second-generation antipsychotics such as clozapine, olanzapine, and risperidone combine D2 antagonism with serotonin 5-HT2A receptor antagonism. The serotonin-dopamine antagonist profile is thought to reduce extrapyramidal symptoms and improve negative and cognitive symptoms compared to first-generation agents. Clozapine is uniquely effective for treatment-resistant schizophrenia but carries risks of agranulocytosis, myocarditis, and metabolic effects requiring stringent monitoring. Olanzapine is highly effective but associated with significant weight gain and metabolic adverse effects.

Therapeutic Uses

Antipsychotics are first-line therapy for schizophrenia, schizoaffective disorder, and psychotic episodes in bipolar disorder. They are also used for acute agitation, treatment-resistant depression as augmentation, Tourette syndrome, and behavioral symptoms in dementia, though the latter carries black box warnings for increased mortality in elderly patients.

Adverse Effects

Extrapyramidal symptoms including acute dystonia, parkinsonism, akathisia, and tardive dyskinesia are most common with first-generation antipsychotics, particularly high-potency agents. Second-generation antipsychotics have lower risk of extrapyramidal symptoms but higher risk of metabolic adverse effects including weight gain, diabetes, and dyslipidemia. Hyperprolactinemia occurs with agents that strongly block D2 receptors without pituitary-sparing properties. QT interval prolongation is a concern with certain agents. Neuroleptic malignant syndrome is a rare but potentially fatal adverse effect of all antipsychotics.

Contraindications

Antipsychotics are contraindicated in patients with severe central nervous system depression and should be used cautiously in patients with Parkinson disease due to worsening of motor symptoms. Caution is required in patients with prolonged QT interval, history of seizures, and narrow-angle glaucoma.

Conclusion

Antipsychotics are essential in managing psychotic disorders, with the choice between first-generation and second-generation agents guided by the balance between extrapyramidal and metabolic adverse effects. Clozapine remains the gold standard for treatment-resistant schizophrenia despite its monitoring requirements.