Comparison of the pharmacological characteristics of [3H]raclopride and [3H]SCH 23390 binding to dopamine receptors in vivo in mouse brain.

In vivo binding of the benzamide derivative [3H]raclopride was studied in mouse brain. The binding was saturable, reversible and stereospecific. Non-specific binding was 5-15% of the total binding. Pharmacological characterization of the binding indicated labelling of dopamine D2 receptors since the binding was potently inhibited by compounds with high affinity for this receptor in vitro. On the other hand, compounds with low affinity in vitro i.e., dopamine D1-selective compounds were weak or inactive as inhibitors of [3H]raclopride binding. A comparison of the pharmacological characteristics of [3H]raclopride and [3H]SCH 23390 binding in vivo indicated that compounds with selectivity in vitro retained this selectivity in vivo. Thus, spiroperidol, haloperidol, 1-sulpiride, clebopride, LY 171555 and (-)-NPA ((-)-N-propyl-norapomorphine) were D2 selective while SCH 23390, SKF 38393 and SKF 75670 were D1 selective. Clozapine, tilozepine, cis-flupentixol, chlorpromazine and butaclamol were non-selective both in vitro and in vivo. However, a few compounds changed profile in vivo compared to in vitro. Thus, fluperlapine and fluphenazine had a dual D1-D2 receptor profile in vitro but were D1- or D2-selective in vivo, respectively. Pergolide and molindone which were D2-selective in vitro both had a dual D1-D2 receptor profile in vivo. In conclusion, [3H]raclopride, in vivo, selectively labels the dopamine D2 receptor. Comparison of the pharmacological characteristics of [3H]raclopride and [3H]SCH 23390 binding in vivo supported the that the dopamine D1 receptor is an important target for a variety of neuroleptics, especially of the clozapine type. This may indicate that blockade of the dopamine D1 receptor conveys antipsychotic action.