DRD2 and schizophrenia: It is worth mentioning that the hyperactivity in the dopaminergic system is tied to positive symptoms of schizophrenia such as verbal hallucination and persecutory delusion (169, 170) since some of the positive symptoms can be induced by dopamine stimulants (e.g., methamphetamine administration) and most of the antipsychotics that act as D2 receptor (D2R) antagonists for positive symptoms.