G-Protein Sensitivity of Ligand Binding to Human Dopamine D2 and D3 Receptors Expressed in Escherichia coli: Clues for a Constrained D3 Receptor Structure

Human dopamine D2 and D3 receptors were expressed in Chinese hamster ovary (CHO) and Escherichia coli cells to compare their ligand binding properties in the presence or absence of G-proteins and to analyze their ability to interact with Gi/o-proteins. Binding affinities of agonists (dopamine, 7-OH-DPAT, PD128907, lisuride) and antagonists/inverse agonists (haloperidol, risperidone, domperidone, spiperone, raclopride, nemonapride), measured using [125I]iodosulpride and [3H]7-OH-DPAT, were similar for hD3 receptors in E. coli and CHO cell membranes. Both agonists and antagonists showed 2- to 25-fold lower binding affinities at hD2 receptors in E. coli versus CHO cell membranes (measured with [3H]spiperone), but the rank order of potencies remained similar. Purported inverse agonists did not display higher affinities for G-protein-free receptors. In CHO membranes, GppNHp decreased high affinity agonist ([3H]7-OH-DPAT) binding at hD2 receptors but not at hD3 receptors. Also, [3H]7-OH-DPAT (nanomolar concentration range) binding was undetectable at hD2 but clearly measurable at hD3 receptors in E. coli membranes. Addition of a Gi/o-protein mix to E. coli membranes increased high affinity [3H]7-OH-DPAT binding in a concentration-dependent manner at hD2 and hD3 receptors; this effect was reversed by addition of GppNHp. The potency of the Gi/o-protein mix to reconstitute high affinity binding was similar for hD2 and hD3 receptors. Thus, agonist binding to D3 receptors is only slightly affected by G-protein uncoupling, pointing to a rigid receptor structure. Furthermore, we propose that the generally reported lower signaling capacity of D3 receptors (versus D2 receptors) is not due to its lower affinity for G-proteins but attributed to its lower capacity to activate these G-proteins.