Analysis of the behaviour of selected CCK sub(B)/gastrin receptor antagonists in radioligand binding assays performed in mouse and rat cerebral cortex

The previously described complex behaviour of the CCK sub(B)/gastrin receptor antagonist, L-365,260, in radioligand binding assays could be explained by a variable population of two binding sites. We have investigated whether other CCK sub(B)/gastrin receptor ligands (PD134,308, PD140,376, YM022 and JB93182) can distinguish between these sites. In the mouse cortex assay, Hill slopes were not different from unity and the ligand pK sub(1) values did not differ when either [ super(125)I]-BH-CCK-8S or [ super(3)H]-PD140,376 was used as label as expected for a single site (G sub(2)). In the rat cortex, where previous analysis of replicate (n = 48) L-365,260 data indicated the presence of two CCK sub(B)/gastrin sites (G sub(1) and G sub(2)), the competition data for PD134,308, PD140,376, YM022 and JB93182 could be explained by a homogeneous population of CCK sub(B)/gastrin sites because the Hill slope estimates were not significantly different from unity. However, the estimated affinity values for JB93182 and YM022 were significantly higher and that for PD134,308 was significantly lower than those obtained in the mouse cortex when the same radioligand was used. In view of our previous data obtained with L-365,260, the rat cortex data were also interpreted using a two-site model. In this analysis, SR27897 expressed similar to 9 fold, PD134,308 similar to 13 fold and PD140,376 similar to 11 fold selectivity for the G sub(2) site. In contrast, JB93182 expressed similar to 23 fold and YM022 similar to 4 fold selectivity for the G sub(1) site. If the two-site interpretation of the data is valid then, because of its reverse selectivity to L-365,260, JB93182 has been identified as a compound which if radiolabelled could provide a test of this receptor subdivision.