Abstract

The specific binding characteristics of the novel cholecystokinin (CCK)B/gastrin receptor-selective peptoid antagonist radioligand [3H]PD 140376 were investigated using membrane homogenates prepared from guinea pig cerebral cerebral cortex and gastric fundic mucosa. [3H]PD 140376 (0.01-10 nM) bound to both cerebral cortex and gastric gland homogenates with comparable high affinity (Kd, 0.1-0.2 nM) and to an apparent single population of sites with Bmax values of 119 and 296 fmol/mg of protein, respectively. The level of specific binding, defined as that displaced by unlabeled CCK sulfated octapeptide, was routinely between 60 and 70% in the cortex and between 50 and 60% in the fundic mucosa. Pharmacological characterization of the [3H]PD 140376-labeled binding sites with a series of agonist and antagonist ligands selective for each of the CCK receptor subtypes demonstrated, in both preparations, an affinity profile consistent with that of the CCKB/gastrin receptor. However, Hill slopes for the competition curves for the unlabeled agonist ligands against specific [3H]PD 140376 binding were significantly less than unity, whereas those for the antagonist ligands, including unlabeled PD 140376, were close to unity. The affinity and Hill slope for PD 140376 and the related CCKB/gastrin antagonist CI-988 were unaffected by the presence of the nonhydrolyzable GTP analogue guanylyl-5'-imidodiphosphate. In contrast, guanylyl-5'-imidodiphosphate caused a characteristic decrease in affinity and an increase in the Hill slopes towards unity for the agonist ligands CCK sulfated octapeptide and pentagastrin. The binding characteristics of unlabeled PD 140376 were also unaffected by the presence of the monovalent cation sodium. In conclusion, the present study has demonstrated that [3H]PD 140376 is the most potent and selective antagonist radioligand yet described for the characterization of CCKB/gastrin receptors in the central and peripheral nervous systems.