3H-dopamine binding to rat striatal D-2 and D-3 sites: enhancement by magnesium and inhibition by guanine nucleotides and sodium.

Previous studies have demonstrated high affinity 3H-dopamine binding sites on mammalian striatal membranes. These putative dopamine receptors of unknown physiological significance have been termed D-3 sites. Such studies have failed, however, to demonstrate high affinity 3H-dopamine binding to D-2 sites, which can be labeled by 3H-butyrophenones, and which represent the putative dopamine receptors most strongly implicated in the behavioral correlates of dopaminergic CNS activity. We now know that preincubation of membrane homogenates with Mg++ and inclusion of Mg (1-10mM) or other divalent metal cations during binding allows high affinity D-2 specific 3H-dopamine binding in rat striatal membranes, and that these ions also increase the Bmax of D-3 specific 3H-dopamine binding. GTP, GDP, and GppNHp can completely abolish all D-2 specific 3H-dopamine binding, while only a magnesium-dependent portion of D-3 sites appears to be GTP sensitive. These data are consistent with the hypothesis that the striatal D-2 receptor exists in two agonist affinity states whose interconversion is effected by guanine nucleotides and divalent metal cations. The GTP sensitive/magnesium dependent nature of 3H-dopamine binding to so-called D-3 sites suggests that some such sites may in fact represent a high agonist-affinity state of the D-1 adenylate cyclase stimulating dopamine receptor also found in this tissue.