Autoradiographic localization of cholecystokinin receptors in rodent brain

Cholecystokinin (CCK) receptor binding sites have been localized by autoradiography in the guinea pig and rat central nervous system. [¹²⁵I]CCK-triacontatriapeptide labeled the sites in brain slices with an observed association constant equal to 0.041 min^-1 and a dissociation constant equal to 0.008 min^-1. CCK-triacontatriapeptide (CCK-33) and the C-terminal octapeptide of CCK-33 (CCK-8) potently inhibited [¹²⁵I]CCK-33 binding with K(i)'s of 2 nM, whereas desulfated CCK-8 (CCK8-ds) and the C-terminal tetrapeptide of CCK-33 (CCK-4) were much weaker. Receptors were concentrated in the olfactory bulb, in the superficial laminae of the primary olfactory cortex, in the deep laminae of the cerebral cortex, and in the pretectal area. Substantial numbers of sites were also found in the basal ganglia, in the amygdala, and in the hippocampal formation. [¹²⁵I]CCK-33 binding sites appear to be located on fibers of the optic tract and probably on olfactory tract fibers as well. These results are discussed in terms of physiological functions associated with CCK, presynaptic receptors, and axonal flow of CCK receptors.