Antipsychotics primarily affect dopamine, the neurotransmitter most directly linked to psychotic symptoms like hallucinations and delusions. Every conventional antipsychotic on the market works by reducing dopamine activity at a specific receptor called D2. But dopamine is only part of the story. Newer antipsychotics also target serotonin, and the full list of neurotransmitters these drugs interact with includes histamine, acetylcholine, and norepinephrine.
Dopamine: The Core Target
The connection between dopamine and psychosis dates back to the earliest antipsychotic drugs developed in the 1950s. These first-generation (or “typical”) antipsychotics work by blocking D2 receptors on brain cells, essentially preventing dopamine from delivering its signal. This is effective because psychotic symptoms are associated with excess dopamine activity in certain brain pathways.
Brain imaging studies have pinpointed a narrow therapeutic window for this blockade. Antipsychotics need to occupy roughly 65% to 80% of D2 receptors to effectively reduce psychotic symptoms. Below that range, the drug doesn’t do enough. Above 80%, the risk of movement-related side effects (stiffness, tremors, involuntary muscle contractions) rises sharply. That leaves a surprisingly tight band where the drug works well without causing problems. Older antipsychotics like haloperidol bind very tightly to D2 receptors and often push past that 80% threshold, which is why movement side effects were so common with those medications.
Serotonin: What Makes Newer Drugs Different
Second-generation (or “atypical”) antipsychotics, introduced starting with clozapine in the 1990s, changed the equation by also targeting serotonin receptors, particularly 5-HT2A. The defining feature of these newer drugs is that they actually bind more strongly to serotonin 5-HT2A receptors than to dopamine D2 receptors. This combination allows them to reduce psychotic symptoms while causing fewer movement side effects, because serotonin blockade in certain brain areas loosens up dopamine signaling in the pathways responsible for motor control.
The serotonin effects go further than just offsetting dopamine blockade. Atypical antipsychotics also interact with 5-HT2C receptors, and this activity appears to help with the “negative” symptoms of schizophrenia, things like emotional flatness, social withdrawal, and difficulty thinking clearly, which older dopamine-only drugs barely touched. Some atypical antipsychotics, like aripiprazole, also partially activate 5-HT1A receptors, adding another layer of serotonin modulation.
Histamine and Weight Gain
Many antipsychotics block histamine H1 receptors, the same receptors targeted by allergy medications that make you drowsy. This histamine blockade is the primary driver of two common side effects: sedation and weight gain. The correlation between a drug’s H1 receptor binding strength and the amount of weight gain it causes is remarkably consistent. Clozapine and olanzapine bind most strongly to H1 receptors and cause the most weight gain. Aripiprazole, ziprasidone, and haloperidol have minimal H1 binding and cause little to no weight gain.
Acetylcholine: Side Effects and a New Frontier
Antipsychotics that block muscarinic acetylcholine receptors cause the classic “anticholinergic” side effects: dry mouth, constipation, blurry vision, and urinary retention. Blocking a specific subtype called M3, which sits on insulin-producing cells in the pancreas, also disrupts blood sugar regulation. Olanzapine and clozapine have the strongest M3 binding, which helps explain why they carry the highest risk of triggering type 2 diabetes among all antipsychotics.
But the acetylcholine story has a fascinating twist. Clozapine, widely considered the most effective antipsychotic for treatment-resistant schizophrenia, turns out to work differently from every other antipsychotic at muscarinic receptors. Rather than simply blocking them, clozapine partially activates the M4 subtype. Researchers now believe this partial activation is what gives clozapine its unique effectiveness, not its dopamine or serotonin binding, which is unremarkable compared to other drugs in its class.
This discovery led to an entirely new type of antipsychotic. In 2024, the FDA approved a combination drug (xanomeline-trospium, sold as Cobenfy) that directly stimulates M1 and M4 muscarinic receptors without blocking dopamine at all. Xanomeline crosses into the brain and activates these acetylcholine receptors, while trospium stays outside the brain and blocks muscarinic receptors in the rest of the body to prevent side effects like nausea and excess salivation. It represents the first approved antipsychotic that works through a completely non-dopamine mechanism.
Norepinephrine and Blood Pressure
Several antipsychotics block alpha-1 adrenergic receptors, which are part of the norepinephrine system and play a key role in controlling blood vessel tone. When these receptors are blocked, blood vessels relax, and blood pressure drops. This is why some people feel dizzy or lightheaded when standing up after starting an antipsychotic, a reaction called orthostatic hypotension. Clozapine, chlorpromazine, and quetiapine are among the strongest alpha-1 blockers, which matters especially for people already taking blood pressure medication or those who are physically unwell.
Glutamate: An Indirect Influence
Glutamate is the brain’s main excitatory neurotransmitter, and disruptions in glutamate signaling are increasingly recognized as part of what goes wrong in schizophrenia, particularly for cognitive and negative symptoms. Antipsychotics don’t directly target glutamate receptors, but both first- and second-generation drugs have been shown to indirectly modify how the glutamate system works, altering the expression of glutamate receptors and the way glutamate is transported between brain cells. This modulation may contribute to their therapeutic effects in ways that aren’t yet fully understood.
Third-Generation Drugs: Partial Agonists
Aripiprazole, brexpiprazole, and cariprazine represent a third generation of antipsychotics that relate to dopamine differently from their predecessors. Instead of simply blocking D2 receptors, they act as partial agonists, meaning they occupy the receptor and produce a weaker version of dopamine’s signal. In brain areas where dopamine is overactive, they dampen the signal by competing with dopamine. In areas where dopamine is underactive, they provide a mild boost. This “stabilizing” action reduces the risk of both movement side effects and the emotional blunting that full D2 blockade can cause. These drugs also interact with serotonin receptors, partially activating 5-HT1A and blocking 5-HT2A.
Why Multi-Receptor Binding Matters
No antipsychotic hits just one neurotransmitter system. Even haloperidol, often described as a pure dopamine blocker, has some serotonin activity. The full receptor profile of each drug determines not only how well it controls symptoms but which side effects a person is likely to experience. A drug with strong histamine binding will be sedating and promote weight gain. One with strong muscarinic blocking will cause dry mouth and raise diabetes risk. One with heavy alpha-1 binding will drop blood pressure.
Understanding these profiles helps explain why switching from one antipsychotic to another can produce such different experiences, even though both drugs are “antipsychotics.” The dopamine blockade provides the core antipsychotic effect, but everything else the drug touches shapes what it feels like to take it day to day. The newest generation of drugs, including muscarinic agonists and compounds targeting trace amine receptors, suggests the field is moving toward treatments that may not need to touch dopamine directly at all.