Dopamine receptor binding in bovine intermediate lobe pituitary membranes

[3H]Spiroperidol ([3H]SPIRO) and [3H]N-n-propylnorapomorphine ([3H]NPA), a dopamine antagonist and agonist, respectively, were found to bind stereospecifically to bovine intermediate lobe pituitary membranes. The specific binding was saturable ([3H]SPIRO maximum number of binding sites, 11.2 pmol/g tissue; [3H]NPA maximum number of binding sites, 5.0 pmol/g tissue) and of high affinity affinity ([3H]SPIRO dissociation constant, 0.17 nM; [3H]NPA dissociation constant, 0.35 nM). The rank order of catecholamines, phenothiazines, and related drugs in competing for [3H]SPIRO and [3H]NPA binding is consistent with interactions at a dopamine receptor. Guanine nucleotides were found to selectively decrease agonist but not antagonist affinities for these binding sites. With the addition of 0.1 mM GTP, the ability of agonists to displace [3H]SPIRO binding is decreased 5- to 7-fold, while the dissociation constant for [3H]NPA is increased to 0.8 nM.