Cholecystokinin receptors: biochemical demonstration and autoradiographical localization in rat brain and pancreas using [3H] cholecystokinin8 as radioligand.

Since cholecystokinin8 (CCK8) seems to be the physiological ligand of CCK receptors in the brain, it would be the most suitable probe for the characterization of CCK receptors in radioligand binding studies. [3H]CCK8 was synthetized with a specific radioactivity sufficient for the detection of high affinity binding sites. [3H]CCK8 binds saturably and reversibly to distinct sites in rat brain and pancreas with nanomolar affinity. While the C-terminal tetrapeptide of CCK is the minimal structure required for nanomolar affinity in the brain, the entire octapeptide sequence is required for binding affinity in pancreas. Desulfated CCK8 and several gastrin-I peptides, which are likewise unsulfated, show virtually no affinity to the binding sites in pancreas but high affinity in cerebral cortex. The ligand specificity of the CCK peptides corresponds to their electrophysiological potency in the brain and their stimulation of secretion in pancreas, respectively. Autoradiographically, high densities of [3H]CCK8 binding sites were found in cerebral cortex and olfactory bulb, medium levels in nucleus accumbens, hippocampus, dentate gyrus, and striatum with virtually no labeling in cerebellum. This pattern is similar to the distribution of CCK-like immunoreactivity in the brain. In pancreas, equally high levels of [3H]CCK8 labeling were found in the exocrine and endocrine region. [3H]CCK8 binding sites differ from those identified previously with [125I]Bolton-Hunter-CCK33 by their sensitivity to guanyl nucleotides in the brain, their ion dependency in the brain, and pancreas, and their different autoradiographical localization in some parts of the brain. The distribution of CCK binding sites labeled with [3H]CCK8 appears to correlate better with the CCK immunoreactivity than those labeled with [125I]Bolton-Hunter-CCK33. Thus, [3H]CCK8 appears to be the radioligand of choice for the investigation of CCK receptors.