Agonist action at D-2(long) dopamine receptors: ligand binding and functional assays

1 The activities of a range of agonists at D-2(long) dopamine receptors expressed in CHO cells have been determined in ligand binding and in a functional assay, the stimulation of [S-35]-GTP gamma S binding. 2 For several agonists (apomorphine, dopamine, pergolide, quinpirole, NPA, ropinirole, talipexole) binding in the absence of added guanine nucleotides was best described in terms of interaction at higher and lower affinity states, whereas for other agonists (bromocriptine, DHEC, lisuride, 3-PPP) a one binding site model was a good description of the data. In the presence of GTP (100 mu M) all agonist binding data were best described by a one site model. 3 All of the agonists tested increased [S-35]-GTP gamma S binding above the basal level and the maximal effects and potencies of the agonists in this test were different. There was no clear relation betwen the ability of an agonist to stabilize the formation of the ternary complex of agonist/receptor/G-protein and the maximal activity of the agonist or the amplification factor (ratio of dissociation constant for binding to receptor to EC50 in functional assay). 4 A comparison was made between the profiles of the D-2(short) and D-2(long) receptor isoforms in these assays.