The theory is that psychosis is caused by high amounts of dopamine, (but the theory is now changing that many other neurotransmitters are involved including glutamate), and this causes your brain to perceive things differently.
The older antipsychotics generally are very strong dopamine (d2) receptor blockers, so although you may still have higher levels, the receptors on the post synaptic cell are not activated, so the effects of having high dopamine are reduced. But this causes movement disorders.
The newer antipsychotics tend to block serotonin (5HT-2) receptors, and block dopamine receptors in a slightly different way to the older ones, but the concept is pretty much the same.
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