Cyclic cholecystokinin analogues with high selectivity for central receptors (original) (raw)

1988, Proceedings of the National Academy of Sciences

Taking as a model the N-terminal folding of the cholecystokinin tyrosine-sulfated octapeptide [CCK-8; Asp-Tyr(SO3H)-Met-Gly-Trp-Met-Asp-Phe-NH2] deduced from conformational studies, two cyclic cholecystokinin (CCK) analogues were synthesized by conventional peptide synthesis: Boc-D-Asp-Tyr(SO3H)-Ahx-D-Lys-Trp-Ahx-Asp-Phe-NH2 [compound I (Ahx, 2-aminohexanoic acid)] and Boc-y-D-Gfu-Tyr(SO3H)-Ahx-D-Ly's-Trp-Ahx-Asp-Phe-NH2 (compound II). The binding characteristics of these peptides were investigated on brain cortex membranes and pancreatic acini of guinea pig. Compounds I and II were competitive inhibitors of [3H]Boc[Ahx28s31JCCK-(27-33) binding to central CCK receptors and showed a high degree of selectivity for these binding sites (compound I: K; for pancreas/K, for brain, 179; compound H: K; for pancreas/K1 for brain, 1979). This high selectivity was associated with a high affinity for central CCK receptors (compound I: KI, 5.1 nM; compound II: KI, 0.49 nM). Similar affinities and selectivities were found when 12'I Bolton-Hunter-labeled CCK-8 was used as a ligand. Moreover, these compounds were only weakly active in the stimulation of amylase release from guinea pig pancreatic acini (EC50 > 10,000 nM) and were unable to induce contractions in the guinea pig ileum (to 10-6 M). The two cyclic CCK analogues, therefore, appear to be synthetic ligands exhibiting both high affinity and high selectivity for central CCK binding sites. These compounds could help clarify the respective role of central and peripheral receptors for various CCK-8-induced pharmacological effects. The C-terminal octapeptide of cholecystokinin, CCK-8 [Asp-Tyr(SO3H)-Met-Gly-Trp-Met-Asp-Phe-NH2], is a hormonal regulator of pancreatic secretion (1-3) that also can induce gallbladder contraction (1, 4, 5) and increase gut motility (4). Moreover, CCK-8 is present in high concentrations in the mammalian brain (6-9), where it could be a neurotransmitter or neuromodulator, as suggested from various biochemical and pharmacological studies (10-16). Extensive binding studies have clearly shown that the ligand specificities differ greatly for central and peripheral receptors (17-20), and irreversible labeling experiments have revealed entities with different molecular weights for pancreas and brain binding sites (21). Nevertheless, the respec