Dopamine D3 receptor binding by D3 agonist 7-OH-DPAT (7-hydroxy-dipropylaminotetralin) and antipsychotic drugs measured ex vivo by quantitative autoradiography

Because the dopamine D3 receptor is primarily expressed in regions of the limbic system of brain, it was proposed that it may represent a target for antipsychotic drugs that is free of extrapyramidal side effects. An ex vivo receptor binding technique employing 3H]7-OH-DPAT was used to evaluate in vivo occupancy of dopamine D3 receptors in the rat nucleus accumbens by selective D3 agonist 7-OH-DPAT (7-hydroxy-dipropylaminotetralin) and various antipsychotic drugs. With an ID50 value of 0.07 mg/kg, the selective D3 agonist (+)-7-OH-DPAT had the most potent inhibitory effect on ex vivo binding of 3H]7-OH-DPAT among all drugs tested. Clinical doses of phenothiazine drugs, such as chlorpromazine and levomepromazine, induce binding to D3 receptors in vivo, while atypical antipsychotic drugs, such as clozapine, pimozide, and sulpiride, are very weak in inhibiting ex vivo binding of 3H]7-OH-DPAT, indicating that the role of D3 receptors as targets of antipsychotic drugs free of extrapyramidal side effects may not be important.