Comparison of the functional potencies of ropinirole and other dopamine receptor agonists at human D2(long), D3 and D4.4 receptors expressed in Chinese hamster ovary cells

Abstract

1. The aim of the present study was to characterize functional responses to ropinirole, its major metabolites in man (SKF-104557 (4-[2-(propylamino)ethyl]-2-(3H) indolone), SKF-97930 (4-carboxy-2-(3H) indolone)) and other dopamine receptor agonists at human dopamine D2(long) (hD2), D3 (hD3) and D4.4 (hD4) receptors separately expressed in Chinese hamster ovary cells using microphysiometry. 2. All the receptor agonists tested (ropinirole, SKF-104557, SKF-97930, bromocriptine, lisuride, pergolide, pramipexole, talipexole, dopamine) increased extracellular acidification rate in Chinese hamster ovary clones expressing the human D2, D3 or D4 receptor. The pEC50s of ropinirole at hD2, hD3 and hD4 receptors were 7.4, 8.4 and 6.8, respectively. Ropinirole is therefore at least 10 fold selective for the human dopamine D3 receptor over the other D2 receptor family members. 3. At the hD2 and hD3 dopamine receptors all the compounds tested were full agonists as compared to quinpirole. Talipexole and the ropinirole metabolite, SKF-104557, were partial agonists at the hD4 receptor. 4. Bromocriptine and lisuride had a slow onset of agonist action which precluded determination of EC50s. 5. The rank order of agonist potencies was dissimilar to the rank order of radioligand binding affinities at each of the dopamine receptor subtypes. Functional selectivities of the dopamine receptor agonists, as measured in the microphysiometer, were less than radioligand binding selectivities. 6. The results show that ropinirole is a full agonist at human D2, D3 and D4 dopamine receptors. SKF-104557 the major human metabolite of ropinirole, had similar radioligand binding affinities to, but lower functional potencies than, the parent compound.

Figure 1

Acidification rate traces from human dopamine D₂, D₃ and D₄ clones. (a) Typical time course of agonist response to ropinirole (316 nM) in all three cell lines. (b) Single chamber concentration-response curve to bromocriptine at the hD₂ receptor, representative of the bromocriptine response in all three receptor clones. (c) Single chamber concentration-response curve to lisuride at the hD₃ receptor, representative of the lisuride response in all three receptor clones. Exposure to agonist indicated by open bars.

Figure 1

Acidification rate traces from human dopamine D₂, D₃ and D₄ clones. (a) Typical time course of agonist response to ropinirole (316 nM) in all three cell lines. (b) Single chamber concentration-response curve to bromocriptine at the hD₂ receptor, representative of the bromocriptine response in all three receptor clones. (c) Single chamber concentration-response curve to lisuride at the hD₃ receptor, representative of the lisuride response in all three receptor clones. Exposure to agonist indicated by open bars.

Figure 1

Acidification rate traces from human dopamine D₂, D₃ and D₄ clones. (a) Typical time course of agonist response to ropinirole (316 nM) in all three cell lines. (b) Single chamber concentration-response curve to bromocriptine at the hD₂ receptor, representative of the bromocriptine response in all three receptor clones. (c) Single chamber concentration-response curve to lisuride at the hD₃ receptor, representative of the lisuride response in all three receptor clones. Exposure to agonist indicated by open bars.

Figure 2

Extracellular acidification rate concentration-response curves to agonists in cloned cell lines expressing the human dopamine (a) D₂, (b) D₃ or (c) D₄ receptors. Results are expressed as a percentage of the response to a maximal concentration of quinpirole in each experiment. n=5–9 experiments.

Figure 2

Extracellular acidification rate concentration-response curves to agonists in cloned cell lines expressing the human dopamine (a) D₂, (b) D₃ or (c) D₄ receptors. Results are expressed as a percentage of the response to a maximal concentration of quinpirole in each experiment. n=5–9 experiments.

Figure 2

Extracellular acidification rate concentration-response curves to agonists in cloned cell lines expressing the human dopamine (a) D₂, (b) D₃ or (c) D₄ receptors. Results are expressed as a percentage of the response to a maximal concentration of quinpirole in each experiment. n=5–9 experiments.

Figure 3

Concentration-response curves to ropinirole in hD₂, hD₃ or hD₄ cloned cell lines. Data expressed as a percentage of quinpirole internal standard. Results are shown as mean±s.e.mean from 7–9 experiments.