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, Gi2, and Gi3

A range of ligands displayed agonism at the long isoform of the human dopamine D-2 receptor, whether using receptor-G protein fusions or membranes of cells in which pertussis toxin-resistant mutants of individual G alpha(i)-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 [S-35] GTP gamma S onto each of G alpha(i1), G alpha(i2), G alpha(i3), and G alpha(o1). By contrast, ( S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine was a partial agonist when G alpha(o1) was the target G protein but an antagonist/inverse agonist at G alpha(i1), G alpha(i2), and G alpha(i3). In ligand binding assays, dopamine identified both high- and low-affinity states at each of the dopamine D-2 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 D-2 receptor was fused to G alpha(i1), G alpha(i2), or G alpha(i3). However, it bound to distinct high- and low-affinity states of the D-2 receptor-G alpha(o1) fusion, with the high- affinity state being eliminated by guanine nucleotide. Likewise, although dopamine identified guanine nucleotide-sensitive high-affinity states of the D-2 receptor when expression of pertussis toxin-resistant forms of each of G alpha(i1), G alpha(i2), G alpha(i3), and G alpha(o1) was induced, (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine identified a high-affinity site only in the presence of G alpha(o1). 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 D-2 receptor and may explain in vivo actions of this ligand.