Receptor density as a factor governing the efficacy of the dopamine D 4 receptor ligands, L‐745,870 and U‐101958 at human recombinant D 4.4 receptors expressed in CHO cells

The relationships between the density of dopamine D 4.4 receptors and the agonist efficacies of L‐745,870 (3‐(4‐[4‐chlorophhenyl]piperazin‐1‐yl)‐methyl‐1H‐pyrrolo [2,3‐b]pyridine) and U‐101958 ((1‐benzyl‐piperidin‐4‐yl)‐(3‐isopropoxy‐pyridin‐2‐yl)‐methyl‐amine) were investigated in Chinese hamster ovary (CHO) cells, after treatment with the gene expression enhancer, sodium butyrate. In CHO cells expressing D 4.4 receptors (CHO/D 4 cells), dopamine inhibited forskolin‐stimulated cyclic AMP accumulation (E max 56±1% inhibition, pEC 50 7.4±0.1, n =10). U‐101958 behaved as a partial agonist (39±7% the efficacy of dopamine, pEC 50 8.1±0.3, n =4), whereas L‐745,870 had no detectable agonist effect. Receptor density, as estimated by [ 3 H]‐spiperone saturation binding was 240±30 fmol mg −1 protein ( n =8) in CHO/D 4 cell homogenates. It reached 560±150 ( n =6), 1000±190 ( n =4) and 840±120 ( n =4) fmol mg −1 protein after treatment with sodium butyrate (5 m M ) for 6, 18 and 48 h, respectively. The increase in receptor density was associated with a gradual enhancement of the agonist effects (increased E max and pEC 50 values) of dopamine. The efficacy of U‐101958 (relative to dopamine) doubled and L‐745,870 was turned into a partial agonist (efficacy 49% relative to dopamine, pEC 50 8.6±0.2, n =6, after 48 h treatment with sodium butyrate). These agonist effects of U‐101958 and L‐745,870 could be antagonized by spiperone (0.1 μ M ) but not by raclopride (10 μ M ). The results show that U‐101958 and L‐745,870 are partial agonists at human dopamine D 4.4 receptors expressed in CHO cells. Their efficacy is governed by receptor density. Agonist effects of these two compounds in vivo cannot be excluded under circumstances of increased receptor levels.British Journal of Pharmacology (1999) 128 , 613–620; doi: 10.1038/sj.bjp.0702849