A comparative study on characterization and distribution of cholecystokinin binding sites among the rat, mouse and guinea pig brain.

Using the in vitro quantitative receptor autoradiographical technique, [propionyl-3H]propionylated cholecystokinin octapeptide ([3H]pCCK-8) binding sites were investigated in tissue sections of rat, mouse and guinea pig brains. In all the tested animals, [3H]pCCK-8 bound very slowly to the tissue sections. Dissociation was also slow, and had a biphasic profile suggesting CCK-8 binding sites are heterogeneous. Dissociation rate constants were, however, unequal among these species. In the saturation binding studies, both Bmax and (Kd)app values varied among the animal species. The autoradiograms revealed marked species differences in [3H]pCCK-8 binding sites in the brain among 3 closely related species of rodents. [3H]pCCK-8 binding sites were undetectable in the nucleus accumbens/caudate-putamen and the amygdaloid complex of the mouse brain, and scarcely found in the ventromedialis of the hypothalamus of the mouse and guinea pig brain. Furthermore, moderate-to-high densities of [3H]pCCK-8 binding sites were observed in the cerebella of the mouse and guinea pig, whereas in the rat cerebellum the binding sites were undetected. The above-mentioned observations suggest the existence of species differences in the binding pattern of CCK-like peptides among closely related animal species. Furthermore, it would appear that CCK-like peptides in the brain may play different physiological roles among animal species.