Evidence for Different States of the Dopamine Dl Receptor: Clozapine and Fluperlapine May Preferentially Label an Adenylate Cyclase-Coupled State of the Dl Receptor

Abstract

It has been shown previously that typical neuroleptics have higher affinities for 3,4-dihydroxyphenyl-ethylamine (dopamine) Dl 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; these experiments revealed no effect of any tested compound on on-rate or off-rate of [³H]SCH 23390 binding. Treatment of striatal membranes with phospholipase A₂ (PLA₂) caused a rapid decrease in the B_max value of the [³H]SCH 23390 binding with no effect on the K_d 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 (NH₄SO₄ 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 Dl binding sites. The rate experiments exclude the possibility of allosterically coupled sites. Instead our results favor that the Dl 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 Dl receptors as target.