Protean agonism at the dopamine D2 receptor: (S)-3-(3-hydroxyphenyl)-N-propylpiperidine is an agonist for activation of Go1 but an antagonist/inverse agonist for Gi1, G i2, and Gi3

A range of ligands displayed agonism at the long isoform of the human dopamine D2 receptor, whether using receptor-G protein fusions or membranes of cells in which pertussis toxin-resistant mutants of individual GIi-family G proteins could be expressed in an inducible fashion. Varying degrees of efficacy were observed for individual ligands as monitored by their capacity to load [35S]GTP gS onto each of GI1,Gi2,GIi3, and GIo1. By contrast, (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine was a partial agonist when GIo1 was the target G protein but an antagonist/inverse agonist at GIi1,GIi2, and GIi3. In ligand binding assays, dopamine identified both high- and low-affinity states at each of the dopamine D2 receptor-G protein fusion proteins, and the high-affinity state was eliminated by guanine nucleotide. (S)-(-)-3-(3-Hydroxyphenyl)-N-propylpiperidine bound to an apparent single state of the constructs in which the D2 receptor was fused to GIi1,GIi2, or GIi3. However, it bound to distinct high- and low-affinity states of the D2 receptor-GIo1 fusion, with the high-affinity state being eliminated by guanine nucleotide. Likewise, although dopamine identified guanine nucleotide-sensitive high-affinity states of the D2 receptor when expression of pertussis toxin-resistant forms of each of GIi1, GIi2, GIi3, and GIo1 was induced, (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine identified a high-affinity site only in the presence of GIo1. p-Tyramine displayed a protean ligand profile similar to that of (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine but with lower potency. These results demonstrate (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine to be a protean agonist at the D2 receptor and may explain in vivo actions of this ligand.