Pharmacological characterization and autoradiographic localization of dopamine receptor sites in the caudal artery of the rat.

The existence and the localization of dopamine DA-1 and DA-2 receptors in the main ventral artery of the rat tail were studied using combined radioligand assay and autoradiographic techniques. [3H]-SCH 23390 was used as a ligand of DA-1 receptors, whereas [3H]-spiroperidol was used as a ligand of DA-2 receptors. No specific binding of [3H]-SCH 23390 was detectable in frozen sections of the caudal artery, neither in receptor binding nor in autoradiography experiments. In contrast, the ligand was specifically bound to sections of the rat brain including the neostriatum. [3H]-Spiroperidol was bound to sections of the caudal artery in a manner consistent with the labeling of DA-2 receptor sites. The dissociation constant value was 3.37 nM. The number of binding sites was 90 fmol/mg protein. Autoradiography demonstrated the localization of specific [3H]-spiroperidol binding sites in the adventitia and in the adventitia-media border. These sites disappeared after 6-hydroxydopamine sympathectomy, suggesting their prejunctional nature. The present results suggest the existence of DA-2 but not of DA-1 receptors in the rat caudal artery. These prejunctional DA-2 receptors are probably involved in the inhibition of noradrenaline release caused by DA-2 receptor agonists.