Rat hippocampal mossy fibers contain cholecystokinin‐like immunoreactivity

BACKGROUND: The possibility that mossy fiber endings in the rat hippocampal formation may contain cholecystokinin (CCK) was reexamined. METHODS: For this, CCK-immunoreactivity was examined by light and electron microscopy using the avidin-biotin complex method. RESULTS: At the light level, the topographical distribution of perikarya and processes with CCK-like immunoreactivity (CCK-LI) was similar to that previously described by others. Ultrastructural analysis of the dentate gyrus and CA3 region of the hippocampus revealed that some mossy fiber terminals contained CCK-LI most often affiliated with large, dense-core vesicles (DCV). Quantitative analysis revealed that 4-8% of the mossy terminal profiles examined (n = 350) contained CCK-labeled DCVs, which corresponded to 0.03-0.2 labeled DCVs per 100 microns2 of neuropil. CONCLUSIONS: The presence of CCK-LI within mossy fibers in the rat suggests that there is less species variability in peptide expression in this pathway than formerly believed.