Evidence for different states of the dopamine D1 receptor: clozapine and fluperlapine may preferentially label an adenylate cyclase-coupled state of the D1 receptor.

It has been shown previously that typical neuroleptics have higher affinities for 3,4-dihydroxyphenylethylamine (dopamine) D1 receptors as labeled by (R)-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1-N-3-benzazepine- 7-ol ([3H]SCH 23390) than for inhibiting dopamine-stimulated adenylate cyclase. We now report that the atypical neuroleptics, clozapine and fluperlapine, exhibit characteristics opposite to typical neuroleptics, i.e., they have higher affinity for inhibiting dopamine-stimulated adenylate cyclase than [3H]SCH 23390 binding. A variety of compounds, i.e., clozapine, fluperlapine, and dopamine, were tested for their capacity to affect the rate constants of [3H]SCH 23390 binding. Treatment of striatal membranes with phospholipase A2 (PLA2) caused a rapid decrease in the Bmax value of the [3H]SCH 23390 binding with no effect on the KD value. The adenylate cyclase, both the unstimulated, the dopamine-, fluoride-, and forskolin-stimulated activity, was far less sensitive than [3H]SCH 23390 binding to PLA2. Treatment of striatal membranes with filipine and (NH4)2SO4 produced, as did PLA2 treatment, a rapid decline in [3H]SCH 23390 binding. However, opposite to PLA2 treatment, these agents stimulated the adenylate cyclase. In conclusion, a comparison of the pharmacological characteristics of [3H]SCH 23390 binding and dopamine-stimulated adenylate cyclase suggests the existence of two different D1 binding sites. The rate experiments exclude the possibility of allosterically coupled sites. Instead our results favor that the D1 receptor exists in different states/conformations, i.e., both adenylate cyclase-coupled and uncoupled, and further, that the atypical neuroleptics clozapine and fluperlapine may have adenylate cyclase-coupled dopamine D1 receptors as target.