Striatal binding of 2-amino-6,7-[3H]dihydroxy-1,2,3,4-tetrahydronaphthalene to two dopaminergic sites distinguished by their low and high affinity for neuroleptics

Abstract

In order to develop more selective methods for labeling brain dopamine receptors, this study describes in detail the properties of 2-amino-6,7,-[3H]dihydroxy-1,2,3,4,-tetrahydronaphthalene ([3H] ADTN) binding to dopaminergic sites in rat, calf, and human brain. [3H]ADTN labeled two distinct types of dopaminergic binding sites in the brain striatum of the rat, calf, and human. Very low concentrations of dopamine and dopaminergic catecholamines (with IC50 values of 1 to 10 nM) inhibited the binding of [3H]ADTN to both sites. Neuroleptics, however, inhibited the binding of [3H]ADTN in two distinctly separate concentration ranges, with IC50 values of 0.15 to 40 nM at one site and 100 and 50,000 nM at the other site. The site with high affinity for dopamine and low affinity for neuroleptics had binding properties that corresponded to those of the previously characterized D3 site (List, S., M. Titeler, and P. Seeman (1980) Biochem. Pharmacol. 29: 1621-1622). The [3H]ADTN binding site with high affinity for neuroleptics demonstrated binding characteristics similar to a site labeled by 3H-Neuroleptics (Sokoloff, P., M. P. Martres, and J. C. Schwartz (1980) Naunyn Schmiedebergs Arch. Pharmacol. 315: 89-102). [3H]Apomorphine appeared to label the same two sites as [3H]ADTN, while [3H]dopamine labeled only the D3 site. Scatchard analysis of [3H]ADTN or [3H]apomorphine binding, under conditions for selective labeling of the low affinity neuroleptic site (D3) and the high affinity site for neuroleptics, detected a density of 70 fmol/mg of protein for each. The density of the D3 site in the calf striatum (170 fmol/mg of protein) was much greater than that of the high affinity neuroleptic site (50 fmol/mg). In the rat, the dissociation constant (KD) of [3H]ADTN was 2 nM for both sites. [3H]Apomorphine, however, had a higher affinity for the D3 site (KD=1.6 nM) than for the high affinity neuroleptic site (KD=4.2 nM). The present results may explain previously observed species and laboratory differences between the binding of [3H]ADTN, [3H]apomorphine, and [3H]dopamine.