Characterization of central cholecystokinin receptors using a radioiodinated octapeptide probe

We have developed a binding assay for 125I-Bolton-Hunter-labeled cholecystokinin octapeptide ( 125I-(BH)CCK 8) using mouse cerebral cortex membrane preparations. This ligand interacts with cortical membrane preparations in a saturable, high-affinity manner, satisfying the requirements for specific cholecystokinin receptor labeling. Salt is required for maximal binding and BSA is specifically inhibitory with cerebral cortical but not with pancreatic sites. Cholecystokinin peptides as small as CCK 30–33 displace binding at low nanomolar concentrations. Dissociation of 125I-(BH)CCK 8 is biphasic in both mouse and guinea pig cortex. Pretreatment of membranes at 37°C results in a marked loss of recognition sites, suggesting that the sites may be rapidly metabolized in vivo . After 37°C pretreatment, the loss of CCK recognition sites corresponds to a selective loss of the slow component of dissociation curves. This selective elimination of one dissociation population, as well as the biphasic dissociation kinetics, suggests that at least two distinct CCK receptor subtypes exist in the brain.