In vivo potencies of antipsychotic drugs in blocking alpha 1 noradrenergic and dopamine D2 receptors: implications for drug mechanisms of action.

In addition to being dopamine antagonists, all antipsychotic drugs are potent antagonists of alpha-1 noradrenergic receptors. Nevertheless, the contribution of alpha blockade to the clinical therapeutic effects of the antipsychotic drugs has never attracted extensive study. In particular, the relative alpha-1 noradrenergic antagonist potency of antipsychotic drugs has rarely been determined in vivo during extended treatment, although such treatment would provide a better model of clinical drug effects than the determination of potencies in in vitro systems, such as assays of competition for binding sites in tissue homogenates, as is most often done. To estimate the physiological efficacy of antipsychotic drugs as dopamine and alpha adrenergic antagonists, we treated rats for four weeks with daily IP injections of the following antipsychotic drugs: Fluphenazine, 1 mg/kg; haloperidol, 1 mg/kg; chlorpromazine, 25 mg/kg; thioridazine, 25 mg/kg; and clozapine, 25 mg/kg. Effective antagonism should lead to an increase in density of the relevant receptors. After two drug-free days, rats were sacrificed and the affinity and density of dopamine D2 and alpha-1 noradrenergic receptors were determined in striatum and brain exclusive of striatum, respectively. Alpha 1 noradrenergic receptor density but not dopamine receptor density was increased after all treatments. Thus, preliminary experiments with this in vivo model suggest that all antipsychotic drugs are effective antagonists at alpha 1 noradrenergic receptors, while not all are effective antagonists at dopamine D2 receptors.