Clinical meaning
Atypical (second-generation) antipsychotics exert their therapeutic effects primarily through antagonism at dopamine D2 receptors and serotonin 5-HT2A receptors. Unlike first-generation antipsychotics that are potent D2 blockers, atypicals have a lower D2 binding affinity and faster dissociation from D2 receptors (fast-off theory), which reduces extrapyramidal symptoms (EPS). Serotonin 5-HT2A antagonism in the nigrostriatal pathway disinhibits dopamine release, further mitigating EPS risk. In the mesocortical pathway, 5-HT2A blockade enhances dopamine transmission and may improve negative symptoms and cognitive deficits in schizophrenia. Individual atypicals differ substantially in their receptor binding profiles: clozapine has high affinity for D4, 5-HT2A, muscarinic M1, and histamine H1 receptors; olanzapine binds broadly to D2, 5-HT2A, H1, muscarinic, and alpha-1 receptors (explaining its metabolic side effects); risperidone is a potent D2 and 5-HT2A antagonist with dose-dependent EPS; aripiprazole is a partial D2 agonist (stabilizes dopamine activity rather than blocking it), reducing both hyperdopaminergia in the mesolimbic pathway and hypodopaminergia in the mesocortical pathway. The metabolic side effects (weight gain, dyslipidemia, insulin resistance) are primarily mediated through H1 and 5-HT2C receptor antagonism, with clozapine and olanzapine carrying the highest metabolic...