CENTRAL AND PERIPHERAL CHOLECYSTOKININ RECEPTORS IN CHICKENS DIFFER FROM THOSE IN MAMMALS

Specific binding for the radioligand [H-3]CCK-8s has been identified i n chicken brain, hypothalamus, pancreas, gallbladder and caecum membra nes. This binding was found to be of high affinity, low capacity and s aturable, suggesting the presence of specific CCK receptors in these t issues. Scatchard analysis indicated the existence of a single binding site for each tissue. Dissociation constant (k(d)) values were 0.63 /- 0.18, 0.73 +/- 0.13, 0.85 +/- 0.12, 1.47 +/- 0.21 and 0.96 nM for b rain, hypothalamus, pancreas, caecum and gallbladder, respectively. Bi nding densities (B-max) were higher for brain, pancreas and caecum (32 .60 +/- 10.70, 30.33 +/- 2.40 and 35.83 +/- 5.10 fmol/mg protein, resp ectively) than for the other two tissues (9.75 +/- 1.90 and 6.31 fmol/ mg protein for hypothalamus and gallbladder, respectively). As in mamm als, CCK-4 shows high affinity for CCK receptors located in chicken br ain and hypothalamus, and very low affinity for those located in perip heral structures. L-364,718 (a CCK-A antagonist) showed a relative sel ectivity and a high affinity for those receptors located in central ti ssues, whereas L-365,260 (a CCK-B antagonist) is almost inactive in al l studied tissues. These results give support for the existence of at least two distinct CCK receptors in birds and that these receptors are relatively different from those described in mammals.