Characterization of the binding of a novel radioligand to CCK B /gastrin receptors in membranes from rat cerebral cortex

We have investigated the binding of a novel radiolabelled CCK B /gastrin receptor ligand, [ 3 H]‐JB93182 (5[[[(1S)‐[[(3,5‐dicarboxyphenyl)amino]carbonyl]‐2‐phenylethylamino]‐carbonyl]‐6‐[[(1‐adamantylmethyl) amino]carbonyl]‐indole), to sites in rat cortex membranes. The [ 3 H]‐JB93182 was 97% radiochemically pure as assessed by reverse‐phase HPLC (RP‐HPLC) and was not degraded by incubation (150 min) with rat cortex membranes. Saturation analysis indicated that [ 3 H]‐JB93182 labelled a homogeneous population of receptors in rat cortex membranes (pK D =9.48±0.08, B max =3.61∼plusmn;0.65 pmol g −1 tissue, n H =0.97±0.02, n =5). The pK D was not significantly different when estimated by association‐dissociation analysis (pK D =9.73±0.11; n =10). In competition studies, the low affinity of the CCK A receptor antagonists, L‐364,718; SR27897 and 2‐NAP, suggest that, under the assay conditions employed, [ 3 H]‐JB93182 (0.3 n M ) does not label CCK A receptors in the rat cortex. The affinity estimates obtained for reference CCK B /gastrin receptor antagonists were indistinguishable from one of the affinity values obtained when a two site model was used to interpret [ 125 I]‐BH‐CCK8S competition curves obtained in the same tissue ( Harper et al ., 1999 ). This study provides further evidence for the existence of two CCK B /gastrin sites in rat cortex. [ 3 H]‐JB93182 appears to label selectively sites previously designated as gastrin‐G 1 and therefore it may be a useful compound for the further discrimination and characterization of these putative receptor subtypes.British Journal of Pharmacology (1999) 126 , 1504–1512; doi: 10.1038/sj.bjp.0702447