Similar binding of 3H-ADTN and 3H-apomorphine to calf brain dopamine receptors.

The binding of 3H(+/-)-ADTN (of high specific activity; 7.6 Ci/mmole) to homogenates of calf striatum was investigated. The dissociation constant (KD) for the specific, saturable binding of 3H-(+/-)-ADTN was 1 nM and the density of specific sites was 100 fmoles/mg protein. The IC50 values (nM concentrations inhibiting specific binding by 50%) were 0.9 for (+/-)-N-propyl-norapomorphine, 3.0 for dopamine, 7 for (--)-adrenaline, 60 for (--)-noradrenaline and 4000 for isoproterenol, a series of potencies compatible with properties for a dopaminergic site. The (+)-enantiomer of ADTN was 10 times more potent than (--)-ADTN in competing for 3H-(+/-)-ADTN, while the (--)-enantiomer of 5-OH-dipropyl-ATN was 40 times more potent than the (+)-isomer. The IC50 values for various agonists against 3H-(+/-)-ADTN were similar to those against 3H-apomorphine or 3H-dopamine in the calf striatum. A comparison of these 3H-(+/-)-ADTN data to those for 3H-spiperone suggests that the two 3H-ligands label different receptor sites.