Abstract

Atypical antipsychotic drugs, which are distinguished from typical antipsychotic drugs by a lower incidence of extra-pyramidal side effects and less propensity to elevate serum prolactin levels (e.g., clozapine, olanzapine, risperidone, quetiapine, ziprasidone), have become the most widely used treatments for schizophrenia, although their precise mechanism of action remains controversial. It has been suggested that this group of atypical antipsychotic drugs is characterized by preferentially high affinities for 5-hydroxytryptamine (5-HT)_2A serotonin receptors and relatively low affinities for D2-dopamine receptors. It has recently been proposed that these atypical antipsychotic drugs may also be distinguished from typical antipsychotic drugs (e.g., haloperidol, fluphenazine, chlorpromazine, and so on) by inverse agonist actions at the 5-HT_2C-INI RNA edited isoform of the human 5-HT_2C receptor transiently expressed in COS-7 cells. We have examined the relationship among 5-HT_2C inverse agonist potency, efficacy, and atypical antipsychotic drug status in HEK-293 cells of a large number of typical and atypical antipsychotic drugs using human embryonic kidney (HEK)-293 cells stably transfected with the h5-HT_2C-INI receptor. Inverse agonist actions at h5-HT_2C-INI receptors were measured for both typical and atypical antipsychotic drugs. Thus, some typical antipsychotic drugs (chlorpromazine, mesoridazine, fluphenazine, and loxapine) were efficient inverse agonists, whereas several clinically effective atypical antipsychotic drugs (remoxapride, quetiapine, sulpiride, melperone, amperozide) were not. Additionally, several drugs without significant antipsychotic actions (M100907, ketanserin, mianserin, ritanserin, and amitriptyline) were potent inverse agonists at the 5-HT_2C-INI isoform expressed in HEK-293 cells. Taken together, these results demonstrate that both typical and atypical antipsychotic drugs may exhibit inverse agonist effects at the 5-HT_2C-INI isoform of the human 5-HT_2C receptor and that no relationship exists between inverse agonist actions and atypicality.