The mechanism of action of all first-generation antipsychotics (FGAs) appears to be postsynaptic blockade of brain dopamine D2 receptors. Evidence supporting this mechanism includes strong antagonism of D2 receptors in both cortical and striatal areas [ 1 ], a high correlation between D2 receptor binding and clinical potency [ 2 ], and a consistent requirement of 65 percent D2 receptor occupancy for antipsychotic efficacy in functional imaging studies [ 3 ]. The nonspecific localization of FGA dopamine binding throughout the central nervous system is consistent with their risk of movement disorders and prolactinemia. Aside from their common activity as D2 antagonists, each FGA has distinct effects on neuronal 5-HT2a, alpha-1, histaminic, and muscarinic receptors, which generally correspond to their individual side effect profiles, as shown in the table ( table 1 ).
In order to determine whether the diagnosis of a psychotic disorder is warranted, the health care professional has to first consider if a medical illness may be the cause of the behavioral changes. If a medical disease is identified or the psychosis is found to be the result of exposure to a medication or drug, the sufferer is assessed as having psychotic disorder due to a medical condition or psychotic disorder due to toxin exposure or withdrawal, respectively. On the other hand, if a medical cause and toxin exposure have been looked for and not found, a psychotic illness such as schizophrenia could be considered. The diagnosis will best be made by a licensed mental-health professional (like a psychiatrist or clinical psychologist), who can evaluate the patient and carefully sort through the diagnostic criteria for a variety of mental illnesses that might look alike at the initial examination, like schizotypal or schizoid personality disorder or a mood disorder with psychotic features like severe depression , or the mania phase of bipolar disorder. Other health care professionals who may treat psychotic disorders may include licensed social workers, psychiatric nurses and nurse practitioners, mental health physician assistants, and sometimes non-psychiatric physicians.