Background: The circadian rhythms in the suprachiasmatic nucleus (SCN), a central clock, are generated by autoregulatory
network composed of clock genes that encode transcriptional factors. There is a gradual development of clock gene expression
in the SCN during ontogenesis. Moreover, clock genes are expressed in the adult hippocampus with circadian fashion.
Objective: It is of interest to examine daily profiles of the clock gene mRNA and protein expressions in rat hippocampus during
development.
Material and Method: Daily profiles of three clock genes (Per1, Per2, and Bmal1) mRNA, and their protein expressions were
analyzed in the rat hippocampus of pups at postnatal (P) day 4 and 8 (P4 and P8), pre-weaning stage (P16), early pubertal
stage (P32), and adult (P60) by real-time PCR and immunohistochemistry.
Results: The entire studied clock gene mRNAs and proteins did not exhibit circadian rhythm in early postnatal P4-P16.
Rhythmic expression of Per1 and Per2 mRNA started at P32, whereas Bmal1 began at adult. However, their proteins showed
circadian expression together at adult.
Conclusion: The present study suggests that rat hippocampal molecular clock works gradually develop after birth and
slower than that in the central clock SCN. It was possible that ontogenetic development of clock gene in hippocampus was
waiting for central clock synchronization.

Keywords: Hippocampus, Clock gene, Circadian, Postnatal rat