Distribution of cholecystokinin-8-like immunoreactivity in the frog brain and spinal cord

The distribution of cholecystokinin-8 (CCK8) containing structures in the central nervous system of the frog (Rana esculenta) was investigated with the aid of the peroxidase immunohistochemical method. In the forebrain, immunoreactive perikarya were found in the main olfactory bulb, septum, amygdala, and ventromedial thalamic nuclei. Networks of immunoreactive varicose fibers were observed in the septum, the basal area, and the infundibulum, and fiber bundles were found in the marginal and axial optic tracts. In the midbrain, immunoreactive cell bodies were seen in the mesencephalic reticular nucleus, and networks of fibers were observed in the optic tectum and the tegmentum. In the hindbrain, immunoreactive neurons were found in the nucleus of the solitary tract, and varicose fibers were observed in the superior vestibular nucleus and the subventricular area. The gray matter of the spinal cord was interlaced by immunoreactive fibers, and a few intensely stained perikarya were seen in the base of the dorsal horn. These findings indicate that CCK8 is widely distributed in the central nervous system of the frog, like in mammals. The CCK8 immunoreactivity is particularly high in the limbic structures and some diencephalic and mesencephalic areas (infundibulum, optic tectum), whereas the pallium is devoid of this peptide in the frog.