Distinctions between ligand-binding sites for [3H]dopamine and D2 dopaminergic receptors characterized with [3H]spiroperidol.

The binding of [3H]Hspiroperidol to D2 dopaminergic receptors in rat striatum was compared to the binding of [3H]dopamine to its binding sites. Both radioligands labeled apparently homogeneous populations of high affinity, stereoselective, saturable sites, determined from analysis of saturation isotherms. [3H]Spiroperidol bound to more than twice as many sites as [3H]dopamine, and antagonist/[3H]spiroperidol competition data were consistent with a single population of receptors. Antipsychotic drugs competed with both high and low affinities for two fractions of [3H]dopamine-binding sites, but for most drugs, their potencies at even the high affinity component were significantly less than their affinities at D2 receptors. The [3H]dopamine-binding sites were altered by kainic acid lesions of the striatum, phenoxybenzamine treatment of tissue homogenates, or reserpine pretreatment of rats. These changes were different from previous reports of alterations in either D2 or D1 dopaminergic receptors. These and other differences in binding properties suggest that [3H]dopamine binds to sites distinct from either D1 or D2 receptors.