Receptor density as a factor governing the efficacy of the dopamine D₄ receptor ligands, L‐745,870 and U‐101958 at human recombinant D_4.4 receptors expressed in CHO cells

Abstract

1 The relationships between the density of dopamine D 4.4 receptors and the agonist e cacies 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. 2 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 e cacy of dopamine, pEC 50 8.1+0.3, n=4), whereas L-745,870 had no detectable agonist e ect. 3 Receptor density, as estimated by [ 3 H]-spiperone saturation binding was 240+30 fmol mg 71 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 71 protein after treatment with sodium butyrate (5 mM) for 6, 18 and 48 h, respectively. 4 The increase in receptor density was associated with a gradual enhancement of the agonist e ects (increased E max and pEC 50 values) of dopamine. The e cacy of U-101958 (relative to dopamine) doubled and L-745,870 was turned into a partial agonist (e cacy 49% relative to dopamine, pEC 50 8.6+0.2, n=6, after 48 h treatment with sodium butyrate). These agonist e ects of U-101958 and L-745,870 could be antagonized by spiperone (0.1 mM) but not by raclopride (10 mM). 5 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 e cacy is governed by receptor density. Agonist e ects of these two compounds in vivo cannot be excluded under circumstances of increased receptor levels.

Cloning, expression, functional coupling and pharmacological characterization of the rat dopamine D 4 receptor

“…This may be a peculiarity of rat, as opposed to human dopamine D 4 receptors. Even though we and others have reported agonist activities of U-101958 at human dopamine D 4 receptors (Gazi et al 1998(Gazi et al , 1999bSharma et al 1999), the similar activities of PD168077 and U-101958 are noteworthy. This finding shows that data on the human dopamine D 4 receptor should be extrapolated with caution to the rat equivalent.…”

“…At the rat dopamine D 4 receptor, these compounds acted as partial agonists (relative efficacy around 30%). Here again, although all three have been presented as antagonists (Patel et al 1997;Seeman et al 1997;Tallman et al 1997), we have found evidence that they can behave, at least under some circumstances, as partial agonists at the human dopamine D 4 receptor (Gazi et al 1998(Gazi et al , 1999a(Gazi et al , 1999b. For example, an increase in the expression of human dopamine 562 Fig.…”

“…7 Stimulation of SRE-driven luciferase expression by dopamine in CCL39-SRE-Luci-rat-D4-3 cells. Data are the means ± SEM of triplicates from a representative experiment D 4 receptors led to a gradual enhancement of the agonist effects of L-745,870 (and of U-101958) in Chinese hamster ovary cells (Gazi et al 1999b). Even though it may be argued that receptor density (B max ) was particularly high in CCL39-SRE-Luci-rat-D4-3 cells, given the lack of native D 4 receptor model, it is difficult to predict to which extent such compounds will behave as agonists in animal tests.…”

The dopamine D4 receptor: biochemical and signalling properties

“…[177][178][179]; development of PET radioligands [180]; in D4 receptor-overexpressing cell systems [52,181]; tests for antipsychotic activity [120,182] L-741,742 3.5 In vivo administration to mice for anxiety testing [183] ABT 724 *45-65…”

FAUC 213, a highly selective dopamine D4 receptor full antagonist, exhibits atypical antipsychotic properties in behavioural and neurochemical models of schizophrenia

“…Now, after several of these compounds have been tested and published, the issue remains controversial Patel et al 1997;Mansbach et al 1998). For instance U-101958, a prominent D 4 selective antagonist, was found to exhibit some agonistic activity (Gazi et al 1998(Gazi et al , 1999. In contrast, 2-[4-(4-chlorophenyl)piperazin-1-ylmethyl]pyrazolo[1,5-a]pyridine (FAUC 213) was shown to be not only highly D 4 selective (Table 1), but also a complete antagonist with less than 3% agonistic activity compared with the full agonist quinpirole (Löber et al 2001).…”