Received for publication November 16, 2006.
Revised February 7, 2007.
Accepted for publication February 7, 2007.

Protean agonism at the dopamine D₂ receptor: S-3-(3-hydroxyphenyl)-N-propylpiperidine is an agonist for activation of G_o1 but an antagonist/ inverse agonist for G_i1, G_i2 and G_i3

Abstract

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 G_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 [³⁵S]GTPS onto each of G_i1, G_i2, G_i3 and G_o1. By contrast, S-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine was a partial agonist when G_o1 was the target G protein but an antagonist/inverse agonist at G_i1, G_i2, G_i3. 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 where the D2 receptor was fused to G_i1, G_i2 or G_i3. However, it bound to distinct high and low affinity states of the D2 receptor-G_o1 fusion with the high affinity state being eliminated by guanine nucleotide. Similarly, although dopamine identified guanine nucleotide-sensitive high affinity states of the D2 receptor when expression of pertussis toxin-resistant forms of each of G_i1, G_i2, G_i3 and G_o1 was induced, S-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine identified a high affinity site only in the presence of G_o1. p-tyramine displayed a similar profile as a protean ligand as 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.

Key words: Dopamine, Gi family, Anti-psychotics

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