Co-operativity in agonist binding at the D2 dopamine receptor: evidence from agonist dissociation kinetics.

There is much evidence to suggest that G protein coupled receptors exist as oligomers but the relevance to their function is unclear. We have, therefore, examined the binding of the radiolabelled agonist [(3)H]NPA to membranes of CHO cells expressing the D(2) dopamine receptor in dissociation rate experiments. When [(3)H]NPA dissociation was started by dilution, the dissociation rate in the absence of sodium ions was unaffected by addition of the antagonist/inverse agonist (+)-butaclamol, but was accelerated by addition of agonists e.g. dopamine, suggesting that the receptor was not behaving as a monomer with a single binding site. The very low efficacy partial agonist, aripiprazole provided an intermediate level of acceleration of dissociation. [(3)H]NPA dissociation experiments started by addition of ligands without dilution gave a similar pattern. [(3)H]NPA dissociation could also be accelerated by GTP. Dissociation of [(3)H]NPA in the presence of GTP and dopamine provided a greater acceleration than for either modulator alone, suggesting synergistic effects related to receptor/G protein interaction. When [(3)H]NPA dissociation experiments were performed in the presence of sodium ions, dissociation was faster than in their absence but the rate still depended on the ligand present in the assay. Overall the data cannot be explained by a ternary complex model and are consistent with an oligomeric receptor in which binding of [(3)H]NPA, as an example of an agonist ligand, can be modulated co-operatively by ligands binding elsewhere in the oligomer. Interactions with G proteins also occurs providing further modulation of [(3)H]NPA binding. Both agonists and G proteins are proposed to modulate the oligomer by switching high affinity agonist binding sites to low affinity sites.