Type-A Cholecystokinin Binding Sites in Cow Brain: Characterization Using (-)-[3H]L364718 Membrane Binding Assays

(-)-[3H]L364718 membrane binding assays were employed to localize and characterize cholecystokinin (CCK)-A binding sites in rat and cow brain. Specific binding was detected in all brain areas tested, but in all areas of rat brain and most areas of cow brain the level was too low to allow characterization of the ligand binding specificity of these sites. Membranes prepared from cow nucleus accumbens and striatum contained higher levels of (-)-[3H]L364718 specific binding which represented 55-70% of total binding. Characterization of the ligand binding properties of (-)-[3H]L364718 binding sites in cow nucleus accumbens revealed that these sites are similar to CCK-A sites found in pancreatic membranes. Binding of (-)-[3H]L364718 was saturable and had high affinity (Kd = 45 pm). Sites labeled by (-)-[3H]L364718 displayed stereospecificity for the stereoisomers of CR1409. The competition curve for CCK8 was shallow and was steepened and shifted to the right by the presence of the stable GTP analog guanosine 5'-(beta,delta-imido)triphosphate. The potency of CCK8, but not (-)-L36478, was also affected by the buffer in which the assay was conducted. Future use of (-)-[3H]L364718 membrane binding assays using cow nucleus accumbens and/or striatum will help explore the possibility of differences in ligand recognition among CCK-A sites found in brain and peripheral tissues.