Aims: 

In mammals, the circadian oscillator within the suprachiasmatic nuclei (SCN) entrains circadian clocks in numerous peripheral tissues. Central and peripheral clocks share similar molecular core clock mechanism. The aim of the present work was to elucidate when during ontogenesis expression of clock gene in the rat SCN and liver starts to be rhythmic and entrained by photic and maternal cues.

Methods: 

Daily profiles of clock gene mRNA levels were analyzed in the SCN and liver of fetuses at embryonic day 20, of pups at different postnatal ages from P1 till P30 and in adults by in situ hybridization and/or real-time RT-PCR.

Results: 

The circadian rhythmicity in clock gene expression develops gradually from prenatal to postnatal period both in the SCN and in peripheral organs. The maternal SCN sets the phase of the developing rhythmicity in the fetal and early postnatal SCN clock. The external light-dark cycle starts to reset the rat SCN clock only after the first postnatal week. Full adjustment to the day-length, i.e., to the photoperiod, is accomplished only around the time of weaning. The rhythmicity of the newborn liver clock is entrained by maternal feeding regime and starts to be fully entrained by the SCN only around weaning.

Conclusion: 

The results demonstrate gradual postnatal development of the molecular mechanism responsible for generation of rhythmicity within the central and peripheral circadian clocks. Maternal entrainment of the developing clocks prevails during the first week after birth and, thereafter, photic and photoperiodic entrainment starts to dominate.