Binding specificity of the mouse cerebral cortex receptor for small cholecystokinin peptides

Prior studies have shown that the cerebral cortex cholecystokinin (CCK) receptor can bind CCK and gastrin analogs with high affinity. In the present work the brain CCK receptor had approximately a three times greater affinity for CCK₈ than its C-terminal tetrapeptide (CCK₄) while the C-terminal tripeptide (CCK₃) was 1000-fold less potent than CCK₄. Thus the C-terminal tetrapeptide appears to be the minimal C-terminal CCK sequence required for high affinity binding. Since brain membranes degrade various peptides including CCK, we also evaluated the stability of CCK analogs under the conditions used to measure receptor binding by the following three methods: (1) Studies of degradation-resistant analogs in binding assays; (2) analysis of analog degradation by high performance liquid chromatography (HPLC); and (3) determination of the change in potency of CCK analogs in competitive binding studies subsequent to preincubation with brain membranes. These studies indicated that degradation of analogs by the brain membranes although significant did not account for the differences in potency of analogs in competitive binding studies. Therefore, the observed differences in potencies of the analogs tested are due to the receptor affinity and not sensitivity of the analog to degradation.