Interactions of dopaminergic agonists and antagonists with dopaminergic D3 binding sites in rat striatum. Evidence that [3H]dopamine can label a high affinity agonist-binding state of the D1 dopamine receptor.

The interactions of dopaminergic agonists and antagonists with 3H-agonist labeled D3 dopaminergic binding sites of rat striatum have been characterized by radioligand-binding techniques. When the binding of [3H]dopamine and [3H]apomorphine to D2 dopamine receptors is blocked by the inclusion of D2 selective concentrations of unlabeled spiroperidol or domperidone, these ligands appear to label selectively the previously termed "D3" binding site. Antagonist/[3H]dopamine competition curves are of uniformly steep slope (nH = 1.0), suggesting the presence of a single D3 binding site. The relative potencies of antagonists to inhibit D3 specific [3H]dopamine binding are significantly correlated with their potencies to block D1 dopamine receptors as measured by the inhibition of both dopamine-stimulated adenylate cyclase and [3H]flupentixol-binding activities. The affinities of agonists to inhibit D3 specific [3H]dopamine binding are also correlated with estimates of these agonists' affinities for the high affinity binding component of agonist/[3H]flupentixol competition curves. Both D3 specific [3H] dopamine binding and the high affinity agonist-binding component of dopamine/[3H]flupentixol competition curves show a similar sensitivity to guanine nucleotides. Taken together, these data strongly suggest that the D3 binding site is related to a high affinity agonist-binding state of the D1 dopamine receptor.