Central and peripheral cholecystokinin receptors in chickens differ from those in mammals.

Specific binding for the radioligand [3H]CCK-8s has been identified in chicken brain, hypothalamus, pancreas, gallbladder and caecum membranes. This binding was found to be of high affinity, low capacity and saturable, suggesting the presence of specific CCK receptors in these tissues. Scatchard analysis indicated the existence of a single binding site for each tissue. Dissociation constant (kd) values were 0.63 +/- 0.18, 0.73 +/- 0.13, 0.85 +/- 0.12, 1.47 +/- 0.21 and 0.96 nM for brain, hypothalamus, pancreas, caecum and gallbladder, respectively. Binding densities (Bmax) were higher for brain, pancreas and caecum (32.60 +/- 10.70, 30.33 +/- 2.40 and 35.83 +/- 5.10 fmol/mg protein, respectively) than for the other two tissues (9.75 +/- 1.90 and 6.31 fmol/mg protein for hypothalamus and gallbladder, respectively). As in mammals, CCK-4 shows high affinity for CCK receptors located in chicken brain and hypothalamus, and very low affinity for those located in peripheral structures. L-364,718 (a CCK-A antagonist) showed a relative selectivity and a high affinity for those receptors located in central tissues, whereas L-365,260 (a CCK-B antagonist) is almost inactive in all 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.