Distribution of cholecystokinin binding sites in the North American opossum cerebellum

Previous studies in our laboratory have reported on the differential distribution of several neuropeptides, including the octapeptide cholecystokinin (CCK 8), in the cerebellar cortex and nuclei of the North American opossum ( Didelphis marsupialis virginiana). The present account reports on the distribution of CCK 8 binding sites as determined from serial autoradiographic images of the cerebellum which were labelled by using [ 125I]Bolton Hunter sulfated CCK 8. Evidence for the limited presence of CCK 8-like immunoreactivity and CCK 8 binding sites in several other species suggests that the distribution of this peptide and its receptor(s) may be species specific. In the opossum, CCK 8-like immunoreactivity is present in mossy fiber terminals that distribute throughout the cerebellar cortex; it has a very limited distribution in climbing fibers (King and Bishop (1990) J. Comp. Neurol. 238, 373–384. CCK 8 binding sites are present throughout all lobules of the cerebellar cortex and the cerebellar nuclei, which correlates well with the distribution of the peptide. CCK 8-like immunoreactivity is located primarily in the granule cell layer, although the greatest density of binding sites is in the molecular layer. The presence of CCK 8 in mossy fiber terminals, coupled with the presence of CCK 8 binding sites in the cerebellar cortex, and the fact that CCK 8 alters the firing rate of Purkinje cells (Madtes et al. (1992) Neurosci. Abstr. 18, 853) indicate this peptide may function as a neuromodulator in the cerebellum of the North American opossum. Moreover, the primary distribution of CCK 8 binding sites in the molecular layer when compared to the distribution of CCK 8 in axon terminals in the granule cell layer suggest the action of CCK 8 could be mediated through volume transmission.